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Xerox University Microfilms 300 North Zeeb Road Ann Arbor, Michigan 48106 75-3162

OWEN, Lawrence Barry, 1942- AGE DETERMINATIONS BY THE LUTETIUM- 176/HAFNIUM-176 METHOD.

The Ohio State University, Ph.D., 1974 Geochemistry

Xerox University Microfilms Annr Arbor, Michigan 48106 AGE DETERMINATIONS BY THE LUTETIUM-176/HAFNIUM-176 METHOD

DISSERTATION

Presented in Partial Fulfillment of the Requirements for

the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

Lawrence B. Owen, A.A.S., B.A. , M .A,

*****

The Ohio State University 1974

Approved By

Reading Committee:

Dr. David Elliot r Adviser Department of Dr. Gunter Faure Geology and Mineralogy

Dr. John Sutter ACKNOWLEDGMENTS This project was initiated at the suggestion of Profes­

sor Gunter Faure. Without his advice, encouragement, and continuing assistance, its successful completion would not have been possible.

Drs. D.H. Elliot, and J. Sutter critically read the manuscript, and made many helpful suggestions.

Samples from the Lackner Lake Carbonatite Complex in

Northern Ontario, Canada, were made available through the courtesy of the Ontario Department of Mines and Northern

Affairs. I would especially like to thank Mr. Phil Thurston and Mr. Ron Sage of the department of mines for their assistance. The manuscript was typed by Miss Jerina F. Copsey, and photographs of the mass spectra of lutetium, hafnium ahd zirconium were prepared by Mr. Robert Wilkinson. Mr. Robert

Markley assisted with the fabrication of laboratory equip­ ment and the reproduction of the manuscript. This research was supported by the National Science Foundation through Grant GA-21126.

Finally, I would like to express my gratitude to my wife Jill for her patience and assistance. VITA January 16 , 1942...... Born - Brooklyn, New York.

1965...... A.A.S., Suffolk County Community College, Selden, New York.

1967...... B.A., State University of New York at Buffalo, Buffalo, New York.

Summer, 1967...... Field Assistant, Iron Ore Com­ pany of Canada, Schefferville, Quebec.

1967-1968 ...... Teaching Assistant, State University of New York at Buffalo, Buffalo, New York.

1969 ...... M.A. , State University of New York at Buffalo, Buffalo, New York. 1969...... Teaching Assistant, The Ohio State University, Columbus, Ohio.

1970-1971...... Teaching Assistant, The Ohio State University, Columbu,s, Ohio. Summer, 197 0...... Senior Field Assistant, Ontario Department of Mines and Northern Affairs. Summer, 1971...... Senior Field Assistant, Ontario Department of Mines and Northern Affairs. 1972-1974 ...... Graduate Research Associate, The Ohio State University, Columbus, Ohio

iii PUBLICATIONS

Owen, L .B. , Hodge, D.S., and Smithson, S.B., 19 69, Quantitative geophysical study of anorthosite petrogenesis: Trans. Amer. Geophysical Union (Abstract), V.50, p.333.

Owen, L .B., 1971, A rapid sample preparation method for powder diffraction cameras: Amer. Mineralogist, V. 56.

Hodge, '.S., Owen, L.B., and Smithson, S.B., 1973, Gravity interpretation of the Laramie Anorthosite Complex, Wyoming: Geol. Soc. Amer. Bull, V.84, p.1451. Owen, L B., and Faure, G., 197 3, Dating of rocks and minerals by the Lu/Hf method: Abstract, Annual Meeting, Geol. Soc. Amer., V.5, No.7, p.761.

Owen, L B., and Faure, G., 1974, Simultaneous determination of hafnium and zirconium in silicate rocks by isotope dilution: Anal. Chem., V.46, No.9, p.1323. Faure, ., Owen, L.B., and Elliot, D.H., Zirconium concentrations and initial 87sr/86sr ratios, Kirkpatrick Basalt, Storm Peak, Queen Alexandra Range: In Press, Ant. J.U.S.

iv TABLE OF CONTENTS Page ACKNOWLEDGMENTS...... ii VITA...... iii

PUBLICATION...... iv

TABLES...... ix FIGURES...... xiii

CHAPTER

I. INTRODUCTION...... 15

1. Statement of Objectives...... 15 2. Rare Earth Geochronometers...... 15

3. Previous Work...... 18 4. Applications for a Lutetium/Hafnium Geochronometer...... 19

II .GEOCHEMISTRY OF THE RARE EARTH ELEMENT...... 22

1. Properties of the Rare Earth Elements...... 22

2. Lanthanide Abundances in the Solar System...... 35 3. Rare Earth Element Concentration Data...... 37

4. The Concentrations of Rare Earth Elements in Rocks...... 38 5. The Abundance of the Rare Earth Elements in the Earth's Crust...... 44

6. Rare Earth Concentrations in Minerals...... 47 7. The Role of Minerals in Lanthanide Fractionation...... 50

v CHAPTER Page III. GEOCHEMISTRY OP LUTETIUM...... 63

1. Isotopes of Lutetium...... 63 2. Half-Life of Lutetium-176...... 63

3. Lutetium Concentrations in Rocks...... 69

4. Lutetium Concentrations in Minerals.... 79 IV. THE GEOCHEMISTRY OF HAFNIUM...... 88

1. Chemical Properties of Hafnium...... 88 2. Isotopes of Hafnium...... 89 3. Solar, Cosmic, and Chondritic Abundances of Hafnium and Zirconium..... 93

. 4. Hafnium and Zirconium Determinations...... 9 3

5. Hafnium Concentrations in Rocks...... 95 6. The Abundance of Hafnium in Igneous Rocks of the Oceanic and Continental Crust 108

7. Hafnium Isomorphism...... 112 8. The Hafnium Content of Minerals...... 112

9. Fractionation of Hafnium and Zirconium...... 120

10. Fractionation Mechanisms...... 122 11. Empirical Relationship Between Hafnium and Zirconium Concentrations...... 126 12. Lutetium/Hafnium Ratios in Rocks and Minerals...... 131 V. LUTETIUM DETERMINATIONS...... 139

1. Introduction...... 13 9 2. Chemical Extraction of Lutetium from Rocks and Minerals...... 141

vi Page 3. Lutetium-176 Spike Calibrations...... 157

4 „ Lutetium Mass Spectrometry...... 161

5. Calculation of Lutetium Concentrations...... 163 6. Lutetium Blank Determinations...... 166

7. Testing of the Lutetium Procedure...... 166

VI. MEASUREMENT OF THE CONCENTRATION OF HAFNIUM AND ZIRCONIUM, AND OF THE 176Hf/177Hf RATIO. 167

1. Introduction...... 167 2. Experimental...... 170

3. Spike Calibrations...... 192

4. Mass Spectrometry...... 195 -5. Calculation of Zirconium Concentrations...... 207

6. Calculation of Hafnium Concentrations...... 212 7. Calculation of Spike-Corrections for l7^Hf/l77Hf Isotopic Ratios...... 213

8. Blank Determinations...... 215 9. Results...... 222

10. Separation of Hafnium from Zirconium...... 232

VII. MEASUREMENT OF THE CONCENTRATIONS OF RUBIDIUM AND STRONTIUM, AND OF THE 87sr/86sr RATIO... 238

1. Rubidium and Strontium Determinations...... 238 2. Determination of the Isotopic Composition of Strontium...... 245 VIII. AGE DETERMINATIONS...... 249 1. Introduction...... 249

2. Geology of the Lackner Lake Carbonatite Complex...... 24 9 3. Sample Descriptions...... 255 vii Page 4. Sample Preparation...... 256

5. Rubidium/Strontium Geochronology of the Lackner Lake Carbonatite Complex...... 264

6. Lutetium/Hafnium Geochronology of the Lackner Lake Carbonatite Complex...... 268

IX. ISOTOPE GEOCHEMISTRY OF HAFNIUM...... 279

1. Introduction...... 279 2. Hafnium Development Diagram...... 280

3. Models of Possible Evolution of Hafnium in the Earth...... 283

X. CONCLUSIONS...... 286

BIBLIOGRAPHY...... 289

viii TABLES Page 1. Potential Rare Earth Element Geochronometers..... 17

2. Properties of the Group IIIB Elements...... 24

3. Properties of the Rare Earth Metals...... 25 4. Synoptic Summary of Discoveries of the Rare Earth Elements...... 29 5. Solubility Product Constants of the Trivalent Rare Earth Hydroxides...... 32

6. Atomic Abundances of the Lanthanides in the Sun and in Chondritic Meteorites...... 36 7. The Composition of Representative Composite Samples of Basalt, Intermediate Igneous Rocks, Granite, and Sedimentary Rocks...... 4 0 8. The Average Concentrations (PPM) of Lanthanides in Rocks...... 41 9. The Lanthanide Abundance Ratios (Rock/ Chondrite) for Average Basalt, Intermediate Igneous Rocks, Granite, and Sedimentary Rocks...... 4 43 10. The Concentrations of the Rare Earth Elements in the Continental Crust...... 4 8 11. The Main Rare Earth Minerals . 51

12. Lanthanide Concentrations (PPM), Including Yttrium, in Minerals...... 55 13. Elements with Ionic Radii Similar to Those of the Lanthanides...... 59

14. Naturally Occurring Isotopes of Lutetium...... 64

15. Half-Life Determinations of Lutetium-176...... 65

ix Page 16. The Concentrations of Lutetium (PPM) in Various Kinds of Material...... 70

17. Average Lutetium Concentrations in Rocks...... 75

18. Lutetium Isomorphism...... 81 19. Lutetium Concentrations in Minerals...... 82

20. Properties of Hafnium and Zirconium...... 90 21. Naturally Occurring Isotopes of Hafnium and Zirconium...... 92

22. Solar, Cosmic, and Chondritic Atomic Abundances of Hafnium and Zirconium (atoms per 10^ silicon atoms)...... 94

23. The Concentration (PPM) of Hafnium and Zirconium in Meteorites...... 96

24. Concentrations (PPM) of Hafnium in Lunar Rocks from Mare Tranquillitatis...... 99 25. Hafnium Concentrations in Ultra-Basic Rocks...... 100 26. Hafnium Concentrations in Basalt and Gabbro...... 101 27. Hafnium Concentrations (PPM) in Igneous Rocks of Intermediate Composition...... 103

28. Hafnium Concentrations (PPM) in Granitic * Rocks...... 105 29. Hafnium Concentrations (PPM) in Alkali Granites v. . . 107 30. Hafnium Concentrations (PPM) in Sediments and Sedimentary Rocks...... 109

31. Average Hafnium Concentrations (PPM) in Rocks 110 32. Abundance of Hafnium in the Oceanic and Continental Crust...... Ill

33. Elements with Ionic Radii Similar to Hafnium..... 113

x Page 34. Hafnium-Bearing Minerals...... i.. H 6

35. Concentrations (PPM) of Hafnium in Rocks and Minerals of the Kangerdlugssuaq Alkaline Intrusion and the Skaergaard Intrusion, East Greenland...... 118

36. Lutetium/Hafnium Ratios in Rocks...... 13 2

37. Coefficients at Different Normalities of Hydrochloric Acid for AG50W-X8 Cation Exchange Resin ...... 14 6 33. Coefficients at Different Normalities of Nitric Acid for AG50W-X8 Cation Exchange Resin...... 148

39. Kj Coefficients at Different Normalities of Sulfuric Acid for AG50W-X8 Cation Exchange Resin...... 150 40. Isotopic Composition of the Lutetium-176 Spike.... 158

41. Calibration of the Lutetium-176 Spike Solution.... 159

42. Isotopic Composition of Normal Lutetium...... 160 43. Neutron Activation of Hafnium Oxide...... 17 2

44. Isotopic Compositions of Hafnium and Zirconium Spikes...... 193

45. Spike Calibrations...... 194 46. Hafnium Blank Determinations ...... 217 47. Isotopic Analyses of Mixtures of ISljjf Tracer and Hafnium Shelf Solution...... 220 48. Concentrations (PPM) of Hafnium and Zirconium in Rock Standards...... 223

49. Summary of Hafnium Shelf Solution (17 6/177) Isotope Ratio Determinations...... 233 50. Rubidium and Strontium XRF Calibration Standards...... 241

xi Page 51. Operating Parameters for Determination of Rubidium and Strontium by X-Ray Fluorescence...... 244 52. Description of Samples from the Lackner Lake Carbonatite Complex...... 257 53. Rubidium and Strontium Analytical Data for the Lackner Lake Carbonatite Complex...... 2 66 54. Lutetium and Hafnium Analytical Data for the Lackner Lake Carbonatite Complex...... 270

55. Analytical Data for the Lackner Lake Complex 176Lu/177Hf Isochron...... 272

56. Average Lutetium-176 Half-Life Estimates...... 276

57. -Comparison of Results for Lutetium-176/ Hafnium-17 6 and Rubidium-87/ Strontium-87 Age Determinations for the Lackner Lake Carbonatite Complex...... 277

xii FIGURES

The Lanthanide Contraction...... 23

Rare Earth Abundance Patterns in Rocks..... 34 Relative Fractionation of the Rare Earth Elements in Rocks...... 45 Rare Earth Element Comparison Plot for Minerals of the San Marcos Gabbro of Southern California...... 61 Histogram of Published Lutetium-176 Half-Lives...... 68 Histograms of Lutetium Concentration (PPM) in Rocks...... 77 Lutetium Concentrations (PPM) in U.S.G.S. Rock Standards...... 85 Distribution of Lutetium Between Minerals of the Same Rock...... 86 Hafnium and Zirconium Concentrations in U.S.G.S. Rock Standards...... 98 Linear Relationship Between Hafnium and Zirconium Concentrations in Rocks..... 128 Expanded Scale Plot of Hafnium and Zirconium Concentrations in Rocks..... 129 Average Concentrations of Hafnium and Zirconium in Rocks...... 130 Lutetium/Hafnium and Rubidium/Strontium Ratios in U.S.G.S. Rock Standards..... 134 Lutetium/Hafnium and Rubidium/Strontium Ratios in Igneous Rocks...... 135 Coefficients for Elements in Hydrochloric Acid...... 152 xiii Page 16. Coefficients for Elements in Nitric Acid 153

17. Coefficients for Elements in Sulfuric Acid... 154

18. Lutetium Mass Spectra...... 162

19. Sample - -*-^Lu spike Hyperbolic Mixing Curve.... 165

20. Zirconium and Hafnium Coefficients at Different Normalities of Nitric Acid for AG50W-X8 Resin ...... 178

21. Zirconium Mass Spectra...... 197

22. Hafnium Mass Spectra...... 202 23. Sample - ^-*-Zr Spike Hyperbolic Mixing Curves.... 209 24. Sample - ^^Hf Spike Hyperbolic Mixing Curve.... 214

25.' XRF Calibration Curve for Rubidium and Strontium...... 240

26. Time-Dependent Peak Intensity Fluctuation Corrections for XRF Analyses...... 243 27. Carbonatite Complexes in Eastern Ontario...... 251

28. Geologic Map Of the Lackner Lake Carbonatite Complex...... 252 4 29. Composition of Important Alkaline Silicate Rocks...... 254

30. Purification of Apatite and Magnetite Concentrates...... 259 31. Rubidium/Strontium Isochron for the Lackner Lake Carbonatite Complex, Northern Ontario, Canada...... 265

32. Lutetium/Hafnium Isochron for the Lackner Lake Carbonatite Complex, Northern Ontario, Canada...... 274

33. Hafnium Development Diagram...... 281

34. Models of the Isotopic Evolution Of Hafnium in the Earth...... 285 xiv CHAPTER I

INTRODUCTION

1. Statement of Objectives The purpose of this study is to demonstrate the feasi­ bility of dating rocks and minerals by the lutetium/hafnium isochron method, based on the decay of naturally occurring

176lu to stable ^-^Hf. To accomplish this it is necessary to identify those materials that are most suitable for dating, to develop analytical techniques for the determina­ tion of low-level lutetium and hafnium concentrations and hafnium isotopic compositions, and to confirm the half-life of 176I,u.

2. Rare Earth Geochronometers

Several of the fourteen naturally occurring rare earth elements (REE) have long-lived radioactive isotopes. How­ ever, only 138La/ 147sm, and l^^Lu are potentially useful as geochronometers. The other radioactive REE are not suit­ able for age determinations because their half-lives, which range between 10-^ to 10^® years, are too long. The decay ojE a radioactive nuclide to a stable daugh­ ter can be used for age determinations, if the half-life of the parent is known, and if the isotopic composition of the daughter and concentrations of the parent and daughter can be determined. Mass spectrometry is the only available 15 analytical method that can yield both the concentration and isotope compositions of an element or elements in a sample.

Decay schemes for the potentially useful rare earth geo­ chronometers are shown in Table 1. The last column of Table 1, labeled Isobaric Interferences, refers to mass spectrometric analyses, and indicates interferences by spe­ cies with isotopes of the same mass number as the element being determined„ A ■*-4'7sm/].'ON£ geochronometer has the distinct advan­ tage of being free of isobaric interferences. Its primary disadvantages, however, are the relatively long half-life of l^Sm, and the similarity in geochemical behavior of parent and daughter elements. 3-33La belongs to the isobar triplet -^38ga_138La-138ce, and undergoes branches decay (Rankama, 1963) . The branch­

ing ratio is about 0.5, and approximately 33.3% of 138La decays by electron capture to l33Ba which is the most abun­ dant barium isotope. The extremely low relative abundance of (0.089%), its long half-life, and the high relative abundance of l33Ba (71.66%) all suggest that a 138La/138ga geochronometer is impractical. However, a l38La/l33Ce geochronometer is an interesting possibility. Although the B“ decay of 133La to 138ce occurs with a slightly longer half-life than that of the electron capture branch, the low relative abundance of normal -*-3^Ce (0 .250%) suggests that small amounts of radiogenic 138ce may be de- Table 1. Potential Rare Earth Element Geochronometers

Abundance Abundance Mode of Half-Life Isobaric Parent (atom %) Daughter (atom %) Decay (x 10!0y) Interferences

138Ba 71.66 e .c. 21 138La 0.089 Ba, La, Ce 138Ce 0.250 B~ 24

147Sm 15.07 143Nd 12.20 10.7 NONE

17 6Hf 5.21 B- 2-4 176Lu 2.60 Lu, Yb, Ta, W 176Yb 12.73 e .c. (negligible)

Data are from Rankama (1963). 18 tectable in old (greater than 1 billion year) lanthanum- bearing samples. In addition, lanthanum/cerium ratios may be variable in nature as a result of the oxidation of Ce+^ to Ce+4 . The analytical difficulties resulting from iso­ baric interferences can be resolved by suitable chemical techniques (Schnetzler et al, 1967; Gast et al, 1970).

The decay of l ^ L u to offers the best possibili­ ties for a geochronometer because of a favorable half-life (significantly shorter than the half-life of *^Rb) f an(j ke_ cause of variations of lutetium/hafnium ratios that result from differences in chemical properties of lutetium and hafnium. The sources of isobaric interferences are not serious, and can be removed by rather simple chemical pro­ cedures. Although l ^ L u undergoes branched decay, the branching ratio is less than 3%, and may be neglected. The most serious problems impeding development of a lutetium/ hafnium geochronometer are the analytical difficulties as­ sociated with determining the radiogenic ^ " ^ H f in samples with hafnium concentrations of a few parts per million or less, and the uncertainty (about 30%) in the half-life of

-*-^Lu. Although, about twenty determinations of the half- life have been reported, the values tend to fall into one of two groups of about 20 to 25 and 30 to 40 billion years.

3. Previous Work

Information describing development of a samarium/ neodymium or a lanthanum/cerium geochronometer is unavail­ able. However, in a summary of the natural radioactivity of lanthanum, Rankama (1963) noted that attempts to measure excess radiogenic TOO -JOBa in a one-billion-year old Norwegian allanite were not successful, and, therefore, the develop­ ment of a lanthanum/barium geochronometer did not appear likely. The earliest indication that the lutetium/hafnium method could be used for age determinations was obtained by Heyden and Wefelmeyer in 1938 when they made the first physical measurement of the half-life of 176l u> Subsequently, Rankama (1954) and Arnold (1954) also suggested that lutetium/hafnium age determinations may be possible. In 1958, Herr and his coworkers succeeded in determining a model date for the mineral gadolinite [FeY2Be2 (SiOg)2] by the lutetium/hafnium method. However, four kilograms of sample were required for the hafnium analyses. The analy­ tical requirements for dating by the lutetium/hafnium method have been discussed by Mukoto and Sadaya (1966),

Boudin (1967) , and Boudin and Dehon (1969) . Subsequently, Boudin and Deutsch (197 0) reported lutetium/hafnium model dates for the minerals gadolinite and priorite [Y,Er,Ca,Fe,

Th) (Ti,Nb)20gl obtained with an isotope dilution procedure. 4. Applications for a Lutetium/Hafnium Geochronometer Development of a rare earth geochronometer is important for several reason: 1) Dating samples which are enriched in REE, but which are not datable by other methods; 2) In- 20 dependent confirmation of age determinations by other methods; 3) Use of radiogenic isotopes as geologic tracers - Previously reported lutetium/hafnium model dates were based on the assumption that the initial isotopic composi­ tion of hafnium extracted from a sample was identical to the isotopic composition of present day reagent hafnium. If this assumption is incorrect, a model date may be in error as its accuracy is a function of the uncertainty of the assumed intial isotopic composition. Accurate model dates can only be calculated when either the initial isotopic composition of hafnium is known, or when the lutetium/ hafnium ratio of a sample is high. For example, the gado­ linite and priorite dated by Boudin and Deutsch (1970) had lutetium/hafnium ratios of 71 and 363 respectively. Most common rocks and minerals, however, have lower lutetium/ hafnium ratios and are more reliably dated by the isocron method that is employed for dating cogenetic samples by the rubidium/strontium method. An important application of a lutetium/hafnium geo­ chronometer is the dating of common accessory minerals that have favorable lutetium/hafnium ratios. The ideal minerals for this application are those which permit substitution by REE, but exclude hafnium. A family of minerals most likely to satisfy these conditions are the phosphates of which apatite is the most common. The wide distribution of 21 apatite and other accessory minerals in igneous, metamorphic, and sedimentary rocks may make the lutetium/hafnium method of dating applicable to the study of a wide range of geolog­ ic problems.

Faure and Hurley (1963) suggested that variations in the isotope composition of stronium in igneous rocks could be used to study geochemical differentiation of the mantle and the evolution of the continental crust. They correctly predicted that igneous rocks derived from mantle or remo- bolized sialic sources could be distinguished on the basis of their strontium isotope compositions (Faure and Powell,

197 2). Analogous interpretations of variations in hafnium isotope composition, expressed as the ^ ^ H f/l^Hf isotopic ratio, are also possible.

Direct measurement of initial hafnium isotopic composi­ tions can be made either by establishing lutetium/hafnium isochrons, or by analyzing hafnium-enriched, lutetium de­ ficient samples of known age. Alternatively, if an isochron is established for cogenetic samples of known age, the half- life of -*-7 6Lu can be confirmed. At the present time no data are available concerning the variations of initial l^^Hf abundances, nor has a lutetium/hafnium isochron ever been reported in the literature. CHAPTER II

GEOCHEMISTRY OF THE RARE EARTH ELEMENTS

1. Properties of the Rare Earth Elements

The lanthanides or rare earths consist of a group of fifteen elements with atomic numbers 57 to 71. Of these, all are naturally occurring except promethium which has only short-lived radioactive isotopes, all of which have decayed since the time of nucleosynthesis. Ionic radii of the rare earths decrease as a function of atomic number. This phenomenon was called the lanthanide contraction of Goldschmidt (1954), and is a result of the electronic con­ figuration of these elements (Fig. 1).

The lanthanides are Group III B elements formed by addition of electrons to inner 4f and 5d orbitals. They are moderately electronegative, and their most common oxi­ dation state is +3 (Table 2). Samarium and europium can be reduced to the +2 state, and cerium, praseodymium, and terbium may be oxidized to the +4 state. However, the only valence changes from the +3 state of geochemical importance are the reduction of europium and oxidation of cerium. The rare earth elements (REE) all have completed 6s orbitals, and, since it is these electrons that control chemical be­ havior, they have similar chemical properties (Table 3). The similarity of properties of the REE is primarily re- 22 iue . h ataie contraction lanthanide The 1.Figure °< IONIC RADII 0-65 0-90 1 1-25 0-95 1-00 1-05 1-1O 1-15

-20 864 58 60 62 TMC NUMBER ATOMIC 66 68 70 72 23 Table 2. Properties of the Group III B Elements

Lanthanides Actinides Properties Sc Y La - Lu Ac

Atomic Number 21 39 57 71 89 Atomic Weight^ (Based on ^C) 44 . 96 88 .91 138.91 174.97 227

Ionic Radius (A)^ CM 1—1 1—1 00

(6-Fold Co-ordination) 0.83 0.98 1.13 0.94 • Radius Ratio (0"2 = 1:32 A) 0.63 0.74 0.86 0.71 0.84 (2)

Expected Coordination 6 8 8 6 8

Coordination in Ionic Crystals 6 6 8 6 -

Electronegativity^ 1.3 1.2 1.1 1.2 -

Percent Ionic Character of Bond with q 2 65 74 77 76 - * Reference: 1. Except where noted otherwise, Krauskopf (1967); 2. Weast (1973); 3. Whittaker and Muntus (1970); 4. Pauling (I960). Table 3. Properties of the Rare Earth Metals

Part A

Ionic Radii(A)3 First Ioni- Atomic Atomic Electronic Valence zation Po- Name Symbol Number Weight Configuration3- > 2 + 2 +3 +4 tential(ev)'

Lanthanum La 57 138 .91 [Xe]-4f°5d16s2 1.13 5.55

Cerium Ce 58 140.12 -4f25d°6s2 1.09 0.88 5.65

Praseodymium Pr 59 140.907 -4f35d°6s2 1.08 0.86 5.42

Neodymium Nd 60 144 .24 -4f45d°6s2 1.06 5.49

Promethium Pm 61 - - - - -4f55d°6s2 1.04 5.55

Samarium Sm 62 150.35 -4f^5d^6s^ 1.04 5.63

Europium Eu 63 151.96 -4f75d°6s2 1.25 1.03 5 .68

Gadolinium Gd 64 157 .25 -4f75d16s2 1.02 6.16

Terbium Tb 65 158.924 -4f95d°6s2 1.00 0.84 5 .85

Dysprosium Dy 66 162.50 -4f105d°6s2 0.99 5.93

Holmium Ho 67 164 .930 -4f i;L5d06s2 0.98 6.02 Table 3 - Continued

Ionic Radii(A) First Ioni­ Atomic Atomic Electronic Valence zation Po­ Name Symbol Number Weight Configuration^- ' 2 +2 +3 +4 tential (ev)'

Erbium Er 68 167.26 -4f125d°6s2 0.97 6.10 (5.95)

Thulium Tm 69 168 . 934 -4f135d°6s2 0.96 6.18 (6.03)

Ytterbium Yb 70 173.04 -4f ^-45d^6s2 0.95 6.25 (6.04)

Lutetium Lu 71 174.97 -4f145d16s2 0.94 6.15 (5.32)

Part B

M.P. B.P. Density M.P. B.P. Density Element °C °C g/cm2 Element °C °C g/cm3

La 921 3457 6.145 Gd 1313 3266 7.900

Ce 799 3426 6.767 Tb 1356 3123 8 .227 Table 3 - Continued

M.P. B.P. Density M.P. B.P. Density Element °C °C g/cm3 Element °C OC g/cm 3

Pr 931 3512 6.773 Dy 1412 2562 8 .550 Nd 1021 3068 7 .007 Ho 1474 2695 8.795

Pm 1168 2700 — Er 1529 2863 9.066

Sm 1077 1791 7 .520 Tm 1545 1947 9.321

Eu 822 1597 5.243 Yb 819 1194 6.965

Lu 1663 3395 9.840

References and Notes: Unless otherwise noted, Weast (1973); 1. Heydemann (1969); 2. [Xe] refers to the electronic configuration of xenon: Is22s^2p^3s23p^3d^04s24p°4dl05s25p6 ; 3. Effective ionic radii for ions in 6 fold coordination by Whittaker and Muntus (1970); 4. Reader and Sugar (1966). Values in brackets are from Hertel (1968) . His values for Ce-Tb, determined by the surface ionization comparison technique, agree within experimental limits with values calculated from relative energies of various vj electronic configurations by Reader and Sugar. 28

sponsible for the fact that it took over 100 years before

they were all discovered (Table 4). The physical and chemical properties of the lanthanides

have been summarized by Vlasov (1966) and by Ryabchikov and

Ryabukhin (1970). The lanthanides are lithophile elements that form non-volatile and refractory compounds of ionic

character. The densities and melting points of the lan­

thanide metals increase from lanthanum to lutetium, and the group forms basic oxides. In terms of basicity, the

light rare earth oxides are similar to calcium, the heavier

rare earth oxides are similar to magnesium, and the relative

basicity of the group decreases from lanthanum to lutetium

(Table 5). Carbonate, fluoride, and other complexes of the

heavier lanthanides are most stable than the analogous com­ plexes of the lighter REE.

The name rare earths is a misnomer. They are not rare,

and, as a group, are actually more abundant than cobalt, nickel, copper, and zinc. The geochemical behavior of yttrium is similar to that of the heavier REE because of

the similarity of ionic radii, and, as a result, yttrium is usually determined with the lanthanides. The REE are well dispersed, and are found as minor constituents in rocks

and minerals of all types. The number of principal rare earth minerals is not large. Although it was formerly be­ lieved that as a consequence of their very similar proper­

ties the REE tended to behave as a coherent group in nature Table 4. Synoptic Summary of Discoveries of the Rare Earth Elements

Author and place Year Element Source of discovery Origin of name

17 94* Yttrium Ytterbite Gadolin (Finland In honor of the village of (gadolinite) Ytterby where the mineral was found

1839 Lanthanum Cerium oxide Mosander (Sweden) From the Greek "lanthanos" (hidden)

18 03 Cerium Cerite Berzelius,Hisinger, After the asteroid Ceres Klaproth,(Sweden, Germany)

188 5 Praseody- Didymium oxide Auer von Welsbach From the Greek "prasios"(pale- mium (rare-earth oxide) (Austria) green) and "didymos"(twin)

1885 Neodymium Didymium oxide Auer von Welsbach From the Greek "neos"(new) and (Austria) "didymos"

187 9 Samarium Didymium oxide Lecog de Boisbau- After the mineral samarskite dran (France)

19 01** Europium Samarium oxid^e Demarcay (France) In honor of Europe to 1886***Gadolini- Samarskite Lecoq de Boisbau- In honor of Gadolin um dran (France) Table 4 - Continued

Author and place Year Element Source of discovery Origin of name

1843 Terbium Yttrium oxide Mosander (Sweden) From Ytterby

1886 Dysprosium Terbium oxide Lecoq de Boisbau- From the Greek "dysprosos" dran (France) (inaccessible)

1879 Holmium Erbium oxide Cleve (Sweden) From the ancient name of Stockholm (Holmia)

1843 Erbium Yttrium oxide Masander(Sweden) From Ytterby

1879 Thulium Erbium oxide Cleve (Sweden) From the ancient name Thule de­ noting a legendary country on the northernmost fringe of Europe

1878 Ytterbium Erbium oxide Marignac(France) From Ytterby

1907 Lutecium Ytterbium oxide Urbain(France), From the ancient name of Paris Auer von Welsbach (Lutecia) (Austria),Ch.James (USA) U> Table 4 - Continued

Reference: Trifonov (1966).

* Yttrium has been included in the table since its history is close­ ly connected with rare-earth elements; the table does not include promethium, which was artificially obtained in 1947 by Marinsky, Glendenin and Coryell (USA), 2 2 fisson product of U235.

** The existence of europium was suspected as early as 1892 (Lecoq de Boisbaudran) and in 1896 (Demarcay).

***Marignac postulated the existence of gadolinium in 1880. Table 5. Solubility Product Constants (25°C) of the Trivalent Rare Earth Hydroxides

Solubility Symbol Product For Comparison

La l.OxlO"19

Ce 1 .5xl0”20

Pr 2.7x10 -20 Nd 1.9x10 -21 A1(OH)3 3.7xl0-15 Sm 6 .8xl0-22 Ca(OH)2 5.47xl0“6 (18°C) Eu 3.4xl0"22 Fe(OH)2 4.8xl0-16 (18°C) Gd 2.lxlO-22 Fe (OH)3 3.8xl0-38 (18°C) Er 1 .3xl0“23

Tm 3.3xl0“24

Yb 2.9x10 -24

Lu 2.5x10 -24 NJu> Reference: Herrmann (1970) 33

(Rankama, 1950; Goldschmidt, 1954), it is now known that they may be fractionated by a variety of processes. The relative abundances of the REE decrease exponenti­ ally from lanthanum to lutetium, and the rare earths with even atomic numbers are more abundant than adjacent REE with odd atomic numbers, a classic example of the Oddo-

Harkins rule. Superimposed on this pattern are the fluctu­ ations in rare earth abundances caused by fractionation processes. Unless the Oddo-Harkins effect is removed, sub­ tle differences in REE abundances caused by fractionation may be obscured. For this reason, lanthanide concentrations are usually normalized by dividing the sample REE concen­ trations by the corresponding REE concentrations in chondri- tic meteorites because it has been demonstrated that the lanthanide distribution in this class of meteorites is nearly constant (Haskin and Frey, 1966) . If the normalized # # *r concentration ratios are then plotted as functions of rare earth atomic number, smooth curves can be drawn which greatly simplify the identification of fractionation pat­ terns . In summaries of the geochemistry of the lanthanides compiled by Haskin and Frey (1966) , Haskin et al (1966) ,

Vlasov (1966) , and Herrmann (1970) many examples of REE fractionation are given. However, only two basic patterns emerge (Fig. 2). Crustal sialic rocks and sedimentary rocks are enriched in light REE (lanthanum to gadolinium), ROCK/CHOND RITE 100 50 10 5 1 iue . aeerhaudnepten inrocks. Rareearthabundancepatterns 2.Figure a e r d m u d b y o r m b Lu Yb Tm Er Ho Dy Tb Gd Eu Sm Nd Pr Ce La i r 1 1 r 1 i X X 0 NEMDAE GEU ROCKS IGNEOUS INTERMEDIATE GRANITE ----- L J y -—x .-~— AE EARTHS RARE BASALT 0 SDMNAY ROCKS SEDIMENTARY x ------x i 1 I ------1 ------1 ------r OJ while the relative abundances of the other REE are usually

similar to those in chondrites. On the other hand, oceanic tholeiitic basalts have rare earth abundance patterns simi­

lar to chondrites with the exception of lanthonum to prase­

odymium and europium. Heavy rare earth enrichment some­ times occurs in acidic rocks and is though to result from

the higher stability of heavy REE complexes in resdiual magmas or hydrothermal fluids and from the formation of minerals that concentrate REE. The REE patterns of sedi­

mentary rocks are explicable on the basis of mixing of sialic and basaltic components. However, in sedimentary rocks, the REE may be fractionated subsequently by weather­

ing processes. In general, acidic rocks have greater con­ centrations of the REE than do basic or ultra-basic rocks.

These relationships have been interpreted as evidence that the Earth has an average chondritic rare earth abun­

dance pattern modified by the chemical differentiation of the crust (Haskin and Frey, 1966) .

2. Lanthanide Abundances in the Solar System The solar abundances of the REE have been determined

by Grevesse and Blanquet (1969) . Their determinations were based on measurements taken from a high-resolution tracing

'i-.of the solar spectrum and are thought to be superior to

previous determinations. A compilation of the solar abun-

dances of the REE is given in Table 6 relative to 10 hy- 36

Table 6. Atomic Abundances of the Lanthanides in the Sun and in Chondritic Meteorites*

Atomic Solar Atomic Atomic Abundance in Element Number Abundance Chondritic Meteorites

La 57 1.81+0.27 1.11

Ce 58 1 .88+0.21 1.62 Pn 59 1.63+0.12 0.78

Nd 60 1.82+0.12 1.44

Sm 62 1 .66+0.21 0.91 Eu 63 0.49+0.14 0 .51

Gd 64 1.12+0.15 1.15

Dy 66 1.11+0.14 1.11

Er 68 0.76 0.87

Tm 69 0.43+0.20 0.09 Yb 70 0.81 0.87

Lu 71 0.84 0.09

Reference: Grevesse and Blanquet (1969).

*Abundances are given relative to 10-*-2 hydrogen atoms. 37 drogen atoms. Abundance data for chondritic meteorites

are included for comparison. The use of hydrogen rather

than silicon as the basis for calculating relative atomic

abundances is advantageous because of the extremely low abundance of silicon in the solar spectrum. In general, there is good agreement between solar and meteoritic rare earth atomic abundances. The deviations may result from poorly known constants used in calculating solar abundances, and in the case of Lu, inaccurate measure­ ment of the solar spectrum caused by a blended Lu line

(Grevesse and Blanquet, 1969). The similarities between solar and meteoritic rare earth abundances supports the general hypothesis that REE abundances in chondritic meteorites reflects the abundances of these elements in the primordial matter from which the solar system formed.

3. Rare Earth Element Concentration Data

Concentrations of the REE in various kinds of naturally occurring materials have been available since 1935 (Goldschmidt, 1954). However, in many cases analyses are

incomplete in that concentrations are not reported for all of the REE. This problem is especially acute in the case of lutetium, because the concentration of lutetium is usual­

ly, but not always, a small fraction of the total rare earths present. The first high-precision REE analyses became

available in the early 19 60's with the development of neutron activation and mass spectrometric methods (isotope 38 dilution). Since then, most of the REE concentrations reported in the literature have been obtained by these two techniques. In general, REE determinations by isotope di­

lution or neutron activation are accurate to ± 5 %. While

similar accuracy might be achieved for the lighter REE by other methods, determinations of some of the heavier rare earths may be no better than ± 50 to 100 %. The beast es­ timates of the concentrations of lutetium in terrestrial rocks are available for basalts, granites, and shales, be­ cause both individual rocks and composites consisting of large numbers of samples have been analyzed. Relatively few lutetium analyses are available for other kinds of terrestrial rocks. However, the lutetium concentrations in meteorites and lunar rocks are well known.

4. The Concentrations of Rare Earth Elements in Rocks

A characteristic feature of chondritic meteorites is the absence of significant fractionation of lanthanide el­ ements. Differences in REE concentrations in chondrites are more a function of the effects of the Oddo-Harkins rule than of operation of fractionation processes either prior or subsequent to the formation of these meteorites. Aver­ age chondritic REE concentrations have been computed based on 26 determinations of 22 individual meteorites, and a composite sample consisting of 9 chondrites (Herrmann,

1970) . The average concentrations of REE in basalts, inter- 39 mediate igneous rocks, granites, and sedimentary rocks were estimated from REE analyses of composite samples of the ap­ propriate composition (Herrmann, 1970). Composite samples from which average REE concentrations were derived are given in Table 7, and the average lanthanide concentrations are given in Table 8 .

Compared to chondrites, the lanthanides are enriched in terrestrial rocks by about 20 to 80 times, or more. REE abundance ratios (rock/chondrite) suggest that in addition to enrichment, the lanthanides have also undergone exten­ sive fractionation in terrestrial rocks (Table 9). Ba­ salt, while enriched in lanthanides about 20 times relative to chondrites, usually show, with the exception of europium, little evidence of fractionation. However, in more acidic igneous rocks, and in sedimentary rocks, relative enrich­ ment in both total rare earth content and in the lighter rare earths is common.

Vlasov(1966) summarized the geochemistry of the lan­ thanides as follows: Ultra-basic and basic igneous rocks are deficient in total REE, but they are relatively enrich­ ed in intermediate and heavy REE. Basic alkali and ultra- basic rocks have greater rare earth contents than similar non-alkaline varieties, but they are relatively enriched in the light REE. Granitic rocks have the highest rare earth contents, and they are usually enriched in the light REE. An illustration of the relative fractionation of the 40 Table 7. The Composition of Basalt, Intermediate Igneous Rocks, Granite, and Sedimentary Rocks

Average Rock Type Composition

Basalt 70% Oceanic tholeiite 30% continental tholeiite

Intermediate 85 igneous rocks with 60% Si02

Granite 80% granite with 70% Si02 10% Precambrian granites 10% standard granite G-l

Sedimentary* 77% European Paleozoic shale 15% graywacke 8% limestone

*Assumed that the clay residue of limestone is equivalent to 1% paleozoic shale, and that sand­ stone equals graywacke. Table 8 . The Average Concentration (PPM) of Lanthanides in Rocks

Intermediate Sedimentary Element Chondrites Basalts Igneous Rocks Granites Rocks

La 0.32 6.1 31 55 40

Ce 0.94 16 60 104 80

Pr 0.12 2.7 7.4 12 9.5

m 0.60 14 31 47 37

Sm 0.20 4.3 6.2 8 6.4

Eu 0.073 1.5 1.3 1.1 1.3

Gd 0.31 6.2 6.8 7.4 5.5

Tb 0.050 1.1 1.1 1.1 0.9

Dy 0.31 5.9 6.1 6.2 5.2 Ho 0 .073 1.4 1.5 1.5 1,4

Er 0.21 3.6 3.9 4.2 3.4 Table 8 - Continued

Intermediate Sedimentary Element Chondrites Basalts Igneous Rocks Granites Rocks

Tm 0.033 0.60 0.65 0.69 0.6

Yb 0.19 3.2 3.8 4.3 3.3

Lu 0.031 0.55 0.62 0.68 0.6

Total REE 3.46 67 161 252 195

Reference: Herrmann (1970).

NJ 43 Table 9. The Lanthanide Abundance Ratios (Rock/ Chondrite) for Average Basalt, Intermediate Igneous Rocks, Granite, and Sedimentary Rocks

Element Basalt Intermediate Granitic Sedimentary

La 19.06 96 .88 171.88 125.00

Ce 17.02 63 .83 110.64 85.11

Pr 22.50 61.67 100.00 79.17 Nd 23 .33 51.57 78 .33 61.67 Sm 21.50 31.00 40.00 32.00

Eu 20.55 17 .81 15.07 17.81

Gd 20.00 21.94 23 .87 17.74

Tb 22.00 22 .00 22.00 18.00

Dy 19 . 03 19 .68 20.00 16.77 Ho 19.18 20.55 20.55 19.18

Er 17.14 18 .57 20.00 16.19 Tm 18.18 19.70 20.91 18.18

Yb 16.84 20.00 22.63 17.37

Lu 17.74 20.00 21.94 19.36

Reference: Hermann (1970). 44 lanthanides in rocks is provided in Fig. 3. The heavier

REE, represented by the concentration of lutetium, have

been plotted as a function of the light and intermediate

rare earths, represented by the sum of concentrations of

lanthanum and gadolinium. The data are from Table 8 , and

have been recalculated to relative proportions where the concentrations of lanthanum plus gadolinium plus lutetium

are equal to 100 per cent. The average concentrations of lanthanumand of lutetium

in chondrites are low. The lutetium concentration (0.031

ppm) is only about 1/10 of the concentration of lanthanum

(0.32 ppm). Lutetium concentrations in most terrestrial rocks are usually much higher than the average lutetium

content of chondrites. However, when compared to the total

lanthanide content, lutetium is found to be relatively en­ riched in chondrites and basalts, but relatively depleted

in intermediate igneous rocks, granites, and sedimentary rocks.

5. The Abundance of the Rare Earth Elements in the Earth's Crust According to Taylor (1964), the composition of the continental crust can be represented by a mixture of equal parts of basic and acidic igneous rocks. Taylor noted that

the relative abundance patterns of the REE are similar in different kinds of sedimentary rocks and granites, but dif­ fer from patterns observed for basalts. He calculated that

the ratio of basic to acidic rare-earth concentrations re- rare earths in rocks. Data are fromTableare Data inearthsrarerocks. RELATIVE PER CENT LUTETIUM 5 4 1 0 2 3 95 EAIE E CN LNHNM GADOLINIUM + LANTHANUM CENT PER RELATIVE iue . eaie fractionationthe Relativeof 3.Figure CHONDRITES BASALT 96 SEDIMENTARY SEDIMENTARY 97 GEU ROCKS IGNEOUS INTERMEDI A T E U. E T A INTERMEDI ROCKS e t i n a r g

^ 8 " 999 6 . 46 quired to yield the observed sedimentary pattern for shale, volumetrically, the most important sedimentary rock type in

the Earth's crust, was a 1:1 mixture of basic and acidic rocks. This led him to conclude that the rare earth abun­ dance pattern of shale is representative of the abundance of REE in the continental crust.

Haskin and Frey (1966) suggested, on the basis of more extensive data than were available to Taylor, that a mix­ ture of 60% granite and 4 0% basalt could account more ade­ quately for the observed average sedimentary REE distribu­ tion pattern. They noted, however, that the sensitivity of the method is low because of the similarity in REE abun­ dances in the three rock types, and the divergence of the rare earth abundance patterns for the heavy rare earths in sedimentary and basaltic rocks. According to them, all that can be said about the REE pattern of sedimentary rocks is that it is consistent with the view that it is the re­ sult of some mixture of basaltic and granitic material. They also concluded that estimates of the chemical composi­ tion of the whole crust, derived by Taylor's method, may be incorrect because the REE pattern of sedimentary rocks is derived from rocks exposed to weathering processes which may alter their total lanthanide content. The REE abundances in igneous rocks of the upper con­ tinental crust have also been estimated by utilizing REE analyses of representative rock types. Herrmann (1970) 47 assumed that the following proportions of igneous rocks ap­ proximate the composition of the continental crust: 4 5% granite, 35% granodiorite, 5% diorite, and 15% continental tholeiite. He calculated a weighted average of REE in sed­ imentary rocks that agreed very closely with rare earth abundances derived from igneous rocks. Horn and Adams

(1966) made a computer-derived estimate of the average REE concentrations in igneous rocks of the crust exposed to weathering processes, based on 1964 and pre-1964 rare earth determinations for a large number of samples. The crustal abundance data are summarized in Table 10. Agreement be­ tween the various estimates is, in general, good, but

Taylor's values tend to be lower than those of other inves­ tigators. The short-comings of Taylor's method have been discussed above.

6 . Rare Earth Concentrations in Minerals

54 minerals, exclusive of varieties, can be classified as being rare earth minerals (Vlasov, 1966). These are grouped as follows: 1 sulphate, 3 fluorides, 6 phosphates,

11 oxides, 13 carbonates, and 20 silicates. The REE are also dispersed as trace constituents in almost all minerals. Vlasov listed 237 mineralspecies in which the total rare earth content .exceeds 0.01 weight per cent. In general, all of the lanthanides occur together, but the concentra­ tions of individual rare earths may be negligible. A com­ pilation of the total REE concentrations in the primary Table 10. The Concentrations of the Rare Earth Elements in the Continental Crust

Weighted Average of Sedimentary Rocks Element Taylor(1964) Horn and Adams(1966) Herrmann(1970) Herrmann (1970)

Ld 30 48 .1 42 40 Ce 60 130 81 80

Pr . 8.2 17.7 10 9.5 Nd 28 56.5 40 37

Sm 6.0 15.5 7.6 6.4

Eu 1.2 2.3 1.5 1.3

Gd 5.4 9.9 7.2 5.5

Tb 0.90 1.8 1.1 0.9

Dy 3.0 9.8 5.8 5.2

Ho 1.2 2.4 1.4 1.4

CO Table 10 - Continued

Weighted Average of Sedimentary Rocks Element Baylor(1964) Horn and Adams(1966) Hermann(1970) Herrmann (1970)

Er 2.8 3.6 3.9 3.4

Tm 0.48 0.9 0.62 0.6 Yb 3.0 4.8 3.7 3.3

Lu 0.50 1.1 0.61 0.6

MD 50 lanthanide minerals, and in some common rock-forming min­ erals is given in Tables 11 and 12 respectively. It can be seen that the rare earth concentrations tend to be higher in acidic igneous rocks than in basic rocks. It is also apparent that, while accessory minerals such as monazite and apatite may contain high total rare earth contents, the bulk of the rare earths reside in the common rock-forming minerals.

REE most commonly occur in minerals as minor or trace constituents, probably replacing other cations of similar ionic radii by processes of isovalent and heterovalent iso­ morphism. They may also be present as adsorbed elements on mineral surfaces, associated with crystal defects, or pre­ sent as local concentrations of one or more REE (Haskin and

Frey, 19 66). The heavier REE are undoubtably favored for inclusion in crystal lattice interstices because they are smaller, and are able to fit a wider range of available sites than the lighter REE. In some cases, the bulk of the rare earth content of a mineral may be present in inclu­ sions. For example, most of the rare earth content of the St. Peter Sandstone is probably present in inclusions in quartz grains (Haskin, Wildeman et al, 1966). The major part of the REE content of biotite may also be in inclu­ sions such as zircon and apatite.

7. The Role of Minerals in Lanthanide Fractionation Rare earth minerals can be divided into distinct Table 11. The Main Rare Earth Minerals

o , Concentration, 'O Genetic type Mineral Chemical Composition (Cer)3°3* (Yttr)203** Ln20 3 of deposit

Complex Oxides

Knopite (Ca,Ce)Ti03 6.81 Contact- Metasomatic, magmatic

Loparite (Na,Ca,Ce)2 (Ti,Nb)20g 31-33 -- Magmatic

Pyrochlore NaCaNb206F 4.36-5.90 0.46 4 .36-6.36 Pegmatitic

Koppite NaCaNb206F 9.83 - - Contact- metasomatic

Fergusonite ’(Y,Fe,Ce)(Nb,Ta#Ti)04 0.2-4.0 28-40 31-40 Pegmatitic (granites)

Euxenite (Y,Ca,Ce,U,Th)(Nb,Ta,Ti)2Og 0.4-2.4 24-28 25-30 Pegmatitic (granites)

Polycrase (YrCa,Ce,UfTh) (Ti,Nb,Ta)20g 0.6-2.6 25.27 26.29 Pegmatitic (granites) {£ Table 11 - Continued

Concentration, % Genetic type Mineral Chemical Composition (Cer^O-^* (Yttr^O^** 1 ^ 0 3 of deposit

Aeschynite (Ce,Ca,Fe^+ ,Th)(Ti,Nb)20g 19.50 4.53 25.0 Pegmatitic (nephelinic syenites)

Priorite (Y,Er,Ca,Fe^+ ,Th)(Ti,Nb)2^^ 2-4.3 17-29 21-30 Pegmatitic (granites)

Samarskite (Y,Er)(Nb,Ta)o0^ 0.9-4.2 8-17 10-19 Pegmatitic ^ 6 (granites)

Khlopinite (Y,U,Th)(Nb,Ti,Fe)03 17.65 - Pegmatitic (granites)

Viikite (Y,U,Fe,Ca) (Nb,Ta,Ti)2 (0,0H)6 0.5-8.6 0.8-29 3-33 Pegmatitic (granites)

Breunnerite (U,Ca,Fe)TiOg(?) 0.3-7.3 1.8-4.3 up to 7.35 Pegmatitic, contact- metasomatic

Uraninite (U,Th)02 *mU0 3 *nPbO up to 4.4 Pegmatitic (granites) On Broggerite (Contains Th and REE) 6.16 tvj Cleveite (Contains REE and Th) 15.0 Table 11 - Continued

Concentration/ % Genetic type Mineral Chemical Composition (Cer)2°3* (Yttr)203** Ln203 of deposit'

Carbonates

Parisite (Ce,La)2Ca(C03 )3F2 55-61 0.0-7.86 5.5-61.0 Pegmatitic, hydro thermal.

Bastnaesite (Ce,La) (CO3)F 73-76 Contact- metasomatic hydro thermaL

Phosphates

Xenotime YPC>4 0 .9-2.1 57-68 57-68 Pegmatitic Monazite (Ce,La,Y,Th)OP^ 52-74 1.1-5.0 56-75 Alluvial

Rhabdophanite (Ce/YjPO^ 55-62 0-8.5 62-64 Hypergene- tic

Apatite Ca^(PO4)3 (F,C1,OH) 0.7-4.9 Magmatic Silicates

Yttrialite (Y,Th,U,Fe) 2Si20y 3.3-8.2 43.4-49.3 49-51 Pegmatitic (granites) Table 11 - Continued

Concentration, % Genetic type Mineral Chemical Composition (Cer)203* (Yttr)203** Ln203 of deposit

Orthite (Ca,Ce)2 (A1,Fe)3Si301 2 (0,0H) 3.3-8.2 0.1-6.1 11-23.3 Magmatic, pegmatitic, sedimental- metamorphic

Cyrtolite ZrSiO^ *nH20 up to 1.16 up to 8.93 ip to 10.1 Pegmatitic

Rincolite Na2Ca4CeTi(Si4Ol5)(F,OH) 13.7-14.4 0.9-1.8 15.5-19.1 Pegmatitic (nepheline syenite)

Lovchorrite Na2Ca4CeTi(Si4015)(F,OH)3 11-15 1.3-3.4 14-17 Pegmatitic (nepheline syenite)

Gadolinite Y2Fe2+Be2Si2O10 5-32 22-50 - Pegmatitic (granites)

Reference: Ryabehikov and Ryabukhin (1970).

* Oxides of cei'ium earths. ui **Oxides of yttrium earths. Table 12. Lanthanide Concentrations (PPM) , Including Yttrium, in Minerals*

Source Olivine Pyroxene Amphibole Biotite Plag. K-Field. Apatite Garnet Monazite R

Ankaramite- 1.34 59.53 1 Oceanite 28 .38 11.06 1

Eclogite 30 43 8

Aklaki- Basalt 40.96 2

Gabbro 85 130 9.5 2,600 3

Norite 29 26 4

Syenite Pegmatite 1,800 180 6

Leuco- Granite 1,300 60 18.01 3

Granite 5,400 250 500,000 7

Reference: Herrmann (1910). Reference numbers (R) are those listed by Herrmann (p. 57-71-D-6 to D-8 ). *Total REE concentrations are minimum estimates because complete rare earth analyses are not always available. 56

classes on the basis of the most abundant lanthanide(s)

present. Lanthanide-selective minerals are defined when

the ratio (MAXIMUM REE CONCENTRATION/MINIMUM REE CONCENTRA­

TION) of the even atomic number lanthanides exceeds 50

(Herrmann, 1970). If the ratio is lower than 50 the rare

earth assemblage is termed complex. The characteristics of

the three most common lanthanide-selective groups that have been recognized are:

1. Cerium-Selective:

Minerals in which cerium and the other light REE

(La to Pr) predominate. Cations are present in 10, 11, or

12-fold coordination. Monazite is an example of a cerium selective mineral.

2. Yttrium-Selective: Minerals in which yttrium and the heavier REE (Er to Lu) predominate. Yttrium, although not a rare earth, is included in this group because it always occurs with the heavier REE. This group is called yttrium-selec­ tive because the concentration of yttrium usually exceeds the concentration of any one of the heavy REE. Cations are most commonly present in 6-fold coordination in this group. Xenotime is an example of a typical yttrium-selective min­ eral . 3. Complex Assemblages: Minerals in which all of the REE are present in similar amounts. Cations in this group have coordination 57 numbers that are intermediate, that is, between 6 and 10- fold. Gadolinite is an example of a complex assemblage mineral.

The distribution of rare earths in minerals is not as simple as implied above. Other rare earth-selective miner­ al classes have been recognized. It has also been demon­ strated that lanthanide distributions may be controlled by paragenesis. For example, Fleischer and Altschuler (1969) found that REE concentrations in monazite, sphene, and apatite varied as a function of rock type. The light REE were enriched in the above minerals separated from alkali and mafic rocks, but the heavier REE were enriched in the same minerals separated from granitic rocks. Graham and

Nicholls (1969) found, that in the olivine tholeiite series of basalts, the lighter REE were preferentially enriched in silica undersaturated rocks while the heavier REE were en­ riched in silica saturated rocks.

The existence of lanthanide-selective minerals implies that the REE may be fractionated as a result of the differ­ ences in their ionic radii. Adams (1969) has discussed how crystal chemistry, and relative differences in rare earth basicity may cause fractionation of the REE. He emphasized, however, that both mechanisms ultimately depend upon the small differences in lanthanide ionic radii.

A direct relationship exists between the crystallochem- ical properties and the mole fraction of lanthanides. Unit 58 cell dimensions are less dependent upon REE ionic radii when the total concentration of REE is low, resulting in a wider range of REE substitutions. Thus minerals such as monazite and xenotime, with high REE contents, are highly selective, while minerals such as euxenite and eschynite, with lower REE contents, are much less selective (Haskin and Frey, 1966).

Khomyakov (196 3) found that the total rare earth con­ tent of mineral species, and non-uniform changes in effec­ tive ionic radii of coordinating cations when changes in coordination number occur, may be important factors in lan­ thanide fractionation. For example, in 6-fold coordination,

Ca"1^ has an effective ionic radius of 1.80 A° , but in 8- fold coordination it has an effective ionic radius of 1.20 A° (Table 13). The smaller radius is similar to those of the intermediate (Sm to Dy) and the light (La to Nd) rare earths in 6-fold coordination. The similarity of ionic radii of all the lanthanides suggests that any of them might be able to replace calcium in 6-fold coordination. However, according to the rules of isomorphous replacement, if two ions can occupy the same lattice position, the one that forms stronger bonds with adjacent ions is the one with the greater charge/volume ratio. For this reason, the smaller REE might be expected to preferentially substitute for calcium. In addition, radius ratio considerations also indicate preferential replacement of calcium by the heavier 59

Table 13. Elements v/ith Ionic Radii Similar to Those of the Lanthanides

6-Fold 8-Fold 10 Fold Ions Coordination Coordination Coordination

Lu+3 0.94 1.05

Gd+3 1.02 1.14

La+3 1.13 1.26

Ca+2 1.08 1.20

Mg+2 0.80 0.97

Sr+2 1.21 1.33

Sc+3 0.83 0.95 yt 3 0.98 1.10 y + 3 1.12

Mn+2 0.91 1.01

Fe+2 0.86

Pb+2 1.26 1.37

Hf+4 0.79 0.91

Th+4 1.08 1.12 u+4 1. 08

Zr+4 0.80 0.92

Reference: Whittaker and Muntus (1970). 60

REE. In 8-fold coordination, Ca + 3 has an effective ionic

_1. O radius which is almost identical to that of La . The dif­ ferences in effective ionic radii of the heavier rare earths and calcium in this case approaches 15 per cent, and prefer- substitution by the light REE is favored.

According to Ringwood (1955a), the rules of isomorphous replacement must be modified to take into account the in­ fluence of electronegativity. Ringwood (1955a) formulated the following rule which governs the behavior of univalent, divalent, and some larger trivalent ions in silicate melts: "Whenever diadochy in a crystal is possible between two el­ ements possessing appreciably different electronegativities the element with the lower electronegativity will be pref­ erentially incorporated because it forms a stronger and more ionic bond than the other." The lanthanides do not replace calcium in early-formed minerals on a large scale. Instead, they become concen­ trated in residual magmas. Ringwood attributes this be­ havior to the fact that the electronegativities of the REE are slightly higher (about 0.1 to 0.2) than the electroneg­ ativity of calcium (1.0). REE distribution coefficents (mineral REE content/ whole-rock REE content) when plotted as a function of REE ionic radii can be used to demonstrate how minerals can produce fractionation of REE in igneous rocks (Fig. 4). In plot for minerals of the San Marcos gabbro of Sangabbro theMarcos of for minerals plot otenClfri (Haskin 1966)andFrey, California Southern MINERAL REE / WHOLE ROCK REE 005 0-01 0-5 1.0 0-1 Figure 4. Rare earth element comparison elementcomparison earth Rare 4.Figure La AE AT INC RADIUS(A) IONIC EARTH RARE e Nd Ce HOLE-ROCK W m d b o rm Lu ErTm Ho Tb Gd Sm f AUGITE PLAGIOCLASE HORNBLENDE atite pa a rocks of the San Marcos gabbro of the batholith of Southern

California apatite is enriched in REE, but the relative abundance of this mineral is negligible. The bulk of the REE are associated with hornblende and augite, and the feldspars. The mafic minerals are enriched in the heavy- rare earths, while the light REE are concentrated in the feldspars. Plagioclase has a well-defined positive europium anomaly, but the other minerals are deficient in this el­ ement. Positive europium anomalies are usually ascribed to the prevalence of reducing conditions that result in reduc­ tion of Eu+3 an<3 subsequent replacement of Ca+^ by Eu+2.

However, it has been suggested that lattice energy available to europium in feldspars may be sufficent to cause its re­ duction (Haskin and Frey, 1966; Schnetzler and Philpotts,

1968). It is clear, therefore, that simple crystallization of the common rock-forming minerals, especially if accom­ panied by crystal settling, can change the REE distribution in residual liquids, and, as a consequence, cause the REE to be fractionated. CHAPTER III GEOCHEMISTRY OF LUTETIUM

1. Isotopes of Lutetium There are sixteen isotopes of lutetium. Of these, only two occur in nature, the remainder can be produced artifical- 1 ly and are all radioactive (Table 14). Lu is part of the isobar triplet -*-7^Hf - l ^ L u - -*-^Yb and undergoes branched decay to -*-7^Hf by beta emission and to ^7^Yb by electron capture. The branching ratio has a maximum value of only about 3 per cent, and as a consequence, beta emission may be regarded as the principal decay mode of ^7^Lu (Dixon et al, 1954; Sakamoto, 1967). A generalized equation for the beta decay of -^^Lu is

Tl/2 176Lu 176Hf + 3" + v + Q 4 (1) Where:

3~ = negative beta particle v = neutrino Q = maximum decay energy Tj_/2 = half-life

2. Half-Life of Lutetium-176

The correct value of the half-life of ^7^Lu is uncer­ tain. Determinations have been made by various methods since

1938, but published values vary between 21 and 73 billon years (Table 15). A histogram of published half-lives is 63 64

Table 14. Naturally Occurring Isotopes of Lutetium

Atomic Atom % Atomic Number Symbol Mass Half-Life-*- Abundance Weight

71 Lu 175 Stable 97.4 174.94080

176 3.53 x 1010 2.6 175.94271 years

Mean Atomic Weight 174.97

Reference: Holden and Walker (197 2).

^See text.

4 Table 15. Half-Life Determinations of Lutetium-176

Year Reference Method T1/2 (1010yr)

1938 Heyden and Wefelmeyer 6 counting 4

1939 Libby 8 counting 7.3 + 2

1943 Flammersfeld and Mattauch 8 counting 2.4

1954 Arnold Y counting, Nal crystal 2.15+0.1

1954 Dixon et al. 2tt 3 proportional counting 4.56+0.3

1957 Glover and Watt 2ir6 proportional counting >2.8

1957 Glover and Watt Y counting, Nal crystal 2.1+0.2

1958 Herr et al. ■^^Lu/l^Hf age determination 2.17+0.35

1961 McNair recalculation of Dixon et al. 4.1+0.2

1961 McNair Y counting, Nal crystal: Sum peak 3.6+0.1

1961 McNair 4it 8 proportional counting 3.2 1961 McNair Total absorption, Nal crystal 3.57+0.02

1964 Donhoffer 4tt8 liquid scintillation counting 2.18+0.06 Table 15 - Continued

Year Reference Method T^/2 (lO^yr)

1965 Brinkman et al. 0-Y coincidence counting 3.54+0.05 1965 Brinkman et al. Y counting, Nal crystal 3.50+0.14

1965 Brinkman et al. Y counting, Nal crystal: Sum peak 3.68+0.0 6

1967 Sakamoto Y counting, Nal crystal: Sum peak 5.0+0.3

1968 Winters Y~Y coincidence; 3-y coincidence 2.17+0.023

1969 Prodi et al. 4tt8 liquid scintillation & B-y coincidence 3.27+0.05

1970 Boudin and Deutsch. '^^Lu/^'^Hf from two age determinations 3.3+0.5

CTi 67 shown in Fig. 5. The distribution is bimodal with the most frequently occurring values grouped in the ranges 20 to 25 and 30 to 40 billon years. Half-life determinations within each group have been determined both directly by measure­ ments of the specific decay rate and indirectly by analysis of lutetium-bearing minerals of known age.

The accuracy of lutetium half-life determinations ob- - tained by various kinds of physical counting techniques has been discussed by Prodi et al (1969). They suggested the following reasons for disagreement of the published half- lives :

(1) Radioactive contamination in lutetium samples, especially from the thorium series, results in half-lives that are too low.

(2) Errors in counting low-specific activity lutetium samples result in half-life values that are too high.

Previous indirect half-life determinations were based on analysis of samples of known age and on the assumption that the initial hafnium isotopic composition of the sample was the same as that of hafnium reagent. Examination of

Table 15 indicates a significant discrepency between geologic half-life determinations by Herr et al (1958) and Boudin and Deutsch (1970), although both are based on gadolinites from pegmatites of the same region in Norway, and presumably have the same age.

It was suggested by Boudin and Deutsch (197 0) that the discrepency exists because the sample analyzed by Herr and FREQUENCY 1-5 Figure 5. Histogram of published Lutetium-176 Histogramhalf-lives.publishedof 5.Figure

2

0

2-5 AFLF O LTTU-7(1 yrs) LUTETIUM-176(x10 OF HALF-LIFE

3

0

35

40

4-5

5

0

55

60

10 6-5

70

75 CO CTl 69

his co-workers had not remained a closed system to uranium

and lead since the time of its crystallization. The 207Pb/206Pb, 207Pb/235U , and 206/238U dates determined by

Boudin and Deutsch are concordant and in good agreement with dates of 880 to 930 millon years reported by others

for pegmatites from the same region. However, the age of

determinations by Herr et al (1958) on the gadolinite were

discordant and probably too low. Two other possible expla­ nations for the discrepency are analytical error and in­

correct choice of the initial isotopic composition of hafni­

um. The uncertainty in the choice of the initial isotopic composition, expressed as the ^78Hf/^77Hf isotopic ratio,

can be substantially reduced by analysis of two or more co-

genetic samples. This evaluation of the available data suggests that the

half-life of x/DLu17 6 is most likely between 30 to 40 billon

years, and an acceptable working approximation to the correct value might be 3.53 x 10^8 years which is the average of the

nine determinations from Table 15 that fall within this range.

3. Lutetium Concentrations in Rocks Concentrations of lutetium in various kinds of naturally- occurring materials are given in Table 16. Average lutetium concentrations in important rock types are summarized in Table 17. For comparison, weighted averages computed by

Herrmann (1970) , based on lutetium analyses of igneous rocks Table 16. The Concentrations of Lutetium (PPM) in Various Kinds of Material

Number of Range of Lu Average Lu Material Sample Lu Analyses Concentration Concentration References

Alkali intrusives nepheline 3 0.27-0.32 0.29 1 syenite

carbonatite 6* 0.165-2.4 1.21 1,13 Basic Rocks oceanic tholeiitic basalt 19 0.057-0.92 0.44 1,2 oceanic island and tholeiitic basalts 19 0.12-0.94 0.50 1,2,3

average tholeiitic basalt 38 0.057-0.94 0.47

alkali olivine basalt 4 0.25-0.6 0.43 1,2 nephelinites 3 0.6-0.8 0.67 1

andesites 2 0.37-2.29 1.33 1,4 v* * phonolite 1 — 0.5 1

gabbro 5 0.156-0.60 0.34 1,5 Table 16 - Continued

Number of Range of Lu Average Lu Material Sample Lu Analyses Concentration Concentration References

Biological recent sea shells 1 _ — 0.003 1

Paleozoic fish remains 1 — 18 1

Russian soils 1 — <0.7 1

coal 1 — 0.072 1

Glass tektites 2 0.36-0.47 0.42 1

obsidian 1 -- 2.6 1

Granitic Rocks granite 12* 0.02-0.78 0.36 1,5,6,7,15

granodiorite 3* 0.17-0.51 0.30 1,15

quartz monzonite 1* 0.23-0.58 0.42 15

quartz diorite 1 — 0.07 1

tonalite *' 1 — 0.30 1

rhyolite 1 — 0.72 1 Table 16 - Continued

Number of Range of Lu Average Lu Material Sample Lu Analyses Concentration Concentration References

rhyolitic ash 4* 0.39-1.33 0.86 14

pegmatite 1 1.7 1

aplite 1* 0.07-0.20 0.12 15

Lunar Rocks Apollo 11 basalts, gabbros,breccias, and soils 36 1.17-2.87 1.88 4,8,9,10,11

Manganese Nodule recent 1 < 1.2 1

Meteorites chondrites 25 0.019-0.043 0.031 1

Ap-achondrites 4 0.231-0.50 0.35 1

Metamorphic marble 3 0.025-0.15 0.11 12,13

metamorphic shale 1 0.41 1

Pegmatite granitic pegmatite 1 1.7 1 tvj Phosphorites concretions and nodules 3 1.16-3.0 1.82 1 Table 16 - Continued

Number of Range of Lu Average Lu Material Sample Lu Analyses Concentration Concentration References

Sedimentary Rocks shale 5 0.33-0.93 0.57 1,12

graywacke 4 0.24-0.61 0.40 1

limestone 5 0.013-0.52 0.16 12

sandstone 4 0.033-0.13 0.081 12

recent carbonate sediment 1 0.063 12 average sedimentary rocks 15 0.081-0.57 0.50 **

Ultra-Basic Peridotite 7 0.0148-0.072 0.038 1 Rocks eclogite 3 0.16-0.32 0.25 1

kimberlite 2 0.16-0.19 0.175 1 Water Pacific (100 meter depth) 1 0.00012ppb 1

Pacific (4000 meter depth) 1 — 0.00064ppb 1 Table 16 - Continued

Number of Range of Lu Average Lu Material Sample Lu Analyses Concentration Concentration References

N.Atlantic (deep water) 1 — 0.00015ppb 1

Gulf of Mexico (surface water) 1 — 0.0046ppb 1 well water (Washington,D.C.) 1 — 0.3 1

Reference: 1. Herman (1970); 2. Graham and Nicholls(1970); 3. Fleischer (1969); 4. Haskin et al (1970); 5. Haskin and Haskin (1968); 6. Flanagan (1969); 7. Nagasawa (1970); 8. Morrison (1970); 9. Philpotts and Schnetzler (1970); 10. Gast and Hubbard (1970); 11. Wanke et al (1970); 12. Haskin, Wildeman et al (1966); 13. Loubet et al (1972); 14. Borchardt et al (1972); 15. Condie and Lo (1971).

*Includes averages of several analyses, of different samples, from the same source.

**Computed average Lu content in sedimentary rocks based on the following proportions of sedimentary rocks in the earth's crust: 77% shale; 15% sandstone; 8% limestone. 75

Table 17. Average Lutetium Concentrations (PPM) in Rocks

Rock Type Herrmann( 1 9 7 0 ) This Study

chondrites 0.031 0.031 lunar rocks (Apollo 11) 1.88 tholeiitic basalts 0.55 0.47 intermediate igneous rocks 0.62 0.22 granites 0.68 0.37 sedimentary rocks 0.6 0. 50

Reference: 1. Table 8; 2. Table 16. 76

representative of the continental crust, are also given. In general, both estimates indicate that the variability of lutetium concentrations in rocks is small. Averages compiled from data presented in Table 16 indicate that basalts contain about 0.10 ppm more lutetium than granite.

Histograms of lutetium concentrations in rocks are shown in Fig. 6. The data include composite samples consisting of large numbers of a particular rock type, and average con­ centrations based on several analyses of individual samples.

Most rocks have lutetium contents that fall within the range of zero to one ppm. Lunar rocks are an important exception, as their average lutetium content is significantly higher (1.88 ppm). The largest number of lutetium analyses avail­ able in the literature are for basalts, meteorites, and lunar rocks. Relatively few lutetium analyses have been pub­ lished for other types of rocks. It seems incredible that more high-precision lutetium analyses are available for lunar rocks than for some common terrestrial rocks.

Meteorites The lutetium analyses include chondrites, calcium-rich achondrites, and calcium-poor achondrites. However, almost 7 0 per cent of them are for chondrites. The concentration of lutetium in all chondrites analyzed to date fall in the range of 0 to 0.1 ppm.

Perldotites The histogram includes data for high temperature intru- NUMBER OF ANALYSES 40 201 20i 20 20i 10H 10 10 0 0 0 10 0 0 0 50 40 30 1.0 20 0 02 - 0.6 1-0 0-8 0-2 0-4 0 Figure 6. Histograms of lutetiumconcentrations(ppm) Histogramsof 6.Figure 02 - 0-6 1.0 0-8 0-20-4 0 02 - 06 8 1-0 0-4 0-6 08 0-2 0 - OTNNA THOLEIITES CONTINENTAL CAI ILN and ISLAND OCEANIC AOL 11) (APOLLO RNTC ROCK?; GRANITIC CAI THOLEIITES OCEANIC UA ROCKS LUNAR

i —i i- i— i— -i— UEIM OCNRTO (PPM) CONCENTRATION LUTETIUM

0 2 10 '— i— i— i— i— i— — i— i— r n O 0 0 06 8 1.0 08 0-6 04 02 0 - 0 0 06 - 1-0 0.6 0-8 04 02 0 0 04 6 10 8 0 06 0-4 02 0 EIETR ROCKS SEDIMENTARY TORITES R ETEO M PERIDOT ITES PERIDOT —i —i i i— i— i— 1— 78

sive types, mylonitized peridotites, and one nodule from

a kimberlite. Peridotites are similar to chondrites in

that their lutetium content is less than 0,1 ppm. Tholeiitic Basalts

All of the analyses of oceanic tholeiites are for in­ dividual rock samples. However, one analysis of a composite sample consisting of 282 continental tholeiitic basalts is

included in the histogram of oceanic island and continental basalts. The variation of lutetium in basalts is between 0 to 1 ppm.

Granites

The histogram includes analyses for the following com­ posite samples:

(1) 213 samples with greater than 70 per cent SiO^.

(2) 191 samples with 60 to 70 per cent SiC>2 -

(3) 85 samples with less than 60 per cent SiC>2 (4) Several western U.S. Precambrian granites. The lutetium content of granitic rocks varies between 0 to

0.8 ppm. Several Finnish granites, however, have been de­ scribed with lutetium contents up to 8.8 ppm (Herrmann, 197 0). These rocks were not included in the histogram be­ cause the lutetium determinations were obtained by x-ray analysis, and are not considered to be as reliable as lute­ tium determinations done by neutron activation or isotope dilution. 79

Sedimentary Rocks

The distribution of lutetium in sedimentary rocks is bimodal. The lowest concentrations are observed in lime­

stones and sandstones, and the highest concentrations are found in shales. This is probably a reflection of the pri­ mary sources of REE available to these rocks. Shales are

assumed to be representative samples of the crust because they are derived primarily as weathering products of crust-

al rocks. The main source of REE in limestone is sea water. The concentration of lutetium in both sea water and ground water is exceedingly low, less than 1 ppb. The REE content of sandstone may be localized in quartz inclusions, or in heavy minerals such as zircon, monazite, apatite, etc. The histogram of sedimentary rocks includes the following com­ posite samples:

(1) 36 European Shales

(2) 4 0 North American Shales Lunar Rocks

The histogram of lutetium concentrations in lunar rocks is based on the analysis of basalts, gabbros, breccias, and soils from the Sea of Tranquillity. The range of lutetium concentrations in these rocks is 0.5 to 3.0 ppm. The dis­ tribution is symmetrical, and the mode is 1.5 to 2.0 ppm.

4. Lutetium Concentrations in Minerals

The large number of REE carrier minerals is an indica- 80 tion of the high degree of lanthanide isomorphism. Some ex­ amples of lutetium isovalent and heterovalent isomorphism are presented in Table 18. The lutetium content of common and accessory minerals of igneous rocks is extremely vari­ able, ranging from less than 0.1 ppm (olivine) to levels exceeding 2000 ppm (gadolinite). Average lutetium concen­ trations in minerals are given in Table 19. An attempt has been made to organize the data to illustrate how lutetium concentrations in minerals vary as a function or rock type.

Although the available data are limited, it is apparent that the lutetium content of feldspars is low. The highest concentrations of lutetium occur in accessory minerals like zircon, garnet, and apatite, or in relatively rare minerals such as gadolinite. In general, the lutetium content of a given mineral is higher in acidic rocks than in basic rocks, a reflection of the lithophile nature of lanthanides. An important exception to this rule are the mafic minerals pyroxene and amphibole. The general formula for pyroxene is given by Berry and Mason (1959) as (W,X,Y) 2Z2°6' an<^ ^or amphibole as (W,X,Y) 7_g (Z40-j_-]_) 2 (OH) 2 , where:

W = Large cations with 6-fold coordination (i.e. Ca+^, Na+1, and K+1). X = Medium-sized bivalent cations in 6-fold coordina­

tion (i.e. Mg+^, and Fe*^) .

Y = Medium-sized trivalent and tetravalent cations in

i O _i_ O 6-fold coordination (i.e. Al ,Fe , and titanium). 81

Table 18- Lutetium Isomorphism

Structural Mineral REE Analog Isomorphism fersmite euxenite Lu+3+Ti+4 Ca+2+Nb+5 zircon xenotime Lu+3+P+5 Zr+4+Si+4

Fluorite yttrofluorite Lu +3+0“2 Ca+2+F_1 epidote yttrium-orthite Lu+3+Fe+2 Ca+2+Al+3 spessartite Lu +3+A1+3 (Mn,Fe)2+Si uraninite Lu+3 U4O"2 calci.te Lu 3+B3 Ca2+C4 galena Lu 3+S"2 Pb2 0 .5 gypsum churchite Lu 3 +P 6 Ca2+S6 homolite gadolinite Lu3+Be2 Ca 2 +B 3

Reference: Vlasov (1966). Table 19. Lutetium Concentrations (PPM) ip Minerals

-

MINERAL Eclogite Basalt Gabbro Andesite Dacite Granite Pegm. Fe-Ore Reference

Olivine 0.0094 1

Pyroxene 0.012 0.319 0.54 0.057 0.021 1 Amphibole 1. 07 0.117 6.8 1

Plagioclase 0.008 0.029 0.057 0.021 0.13 0.17 1

K-feldspar 0.07 1 Biotite 2.7 1

Garnet 0.4 9 10.1 4 1,2

Apatite 3.1 26.2 24.9 about 10 1,3,4

Zircon 133 154 3

Gadolinite 2,134

Reference: 1. Herrmann (1970); 2. Schnetzler and Philpotts (1970); 3. Nagasawa (1970); 4. Young et al (1969); 5. Boudin and Deutsch (1970). 83

Z = Small cations in 4-fold coordination (i.e. Si+^,

and Al+^).

Pyroxenes and amphiboles have the potential of incorporating lutetium in W, X, and Y sites.

In basic rocks such as basalt and gabbro, pyroxenes are the chief mafic minerals. Table 19 indicates that the lutetium content of pyroxene separates from these rock types may be as much as 20 to 50 times greater than in co-existing plagioclase, and actually approaches the average whole-rock lutetium content of tholeiitic basalt. The lutetium con­ tent of plagioclase is suprisingly low as one might expect the lanthanides to freely substitue for Ca + 7 .

Berry and Mason (1959) give the general formula of feldspars as W Al(Al,Si) Si20g, where W may be sodium, po­ tassium, calcium, or other monovalent or bivalent cations. The Si:Al ratio is one in anorthite and 3 in albite. In anorthite Ca+^is in 7- or 8-fold coordination, and in "albite

Na+^ is in 6-fold coordination (Bragg, 1937). Thus, replace­ ment of Ca+2 by the light REE is favored in plagioclase of increasing anorthite content. The primary lutetium carriers in acidic rocks are the mafic and accessory minerals, but their relative abundance in these rocks is low. Quartz is an important constituent of granitic rocks, but its lutetium content is probably due to the presence of inclusions. Whole-rock lutetium analyses for acidic rocks tend, therefore, to reflect the contribu­ 84 tion of lutetium primarily by feldspars and quartz, and the resulting lutetium determinations are lower than those ob­ tained from basalts. These relationships are illustrated in Figs. 7 and 8. The highest concentrations of lutetium are found in minerals and rocks that represent the end-stages of igneous activity. This is illustrated in Table 19 by the data pre­ sented for pegmatite minerals. An excellent example of the effect of differentiation on lutetium concentrations was obtained by Haskin and Haskin (1968) in their study of the distribution of REE in rocks of the Skaergaard intrusion.

They found that no significant alteration cf the relative

REE abundance pattern occurred during fractional crystal­ lization and final solidifcation of the Skaergaard magma. The primary magma of the intrusion is believed to be repre­ sented by chilled marginal gabbro, and picrite gabbro. The last differentiate is represented by the upper C zone, and the transgressive hedenbergite granophyre. Between these two extremes both the lutetium content and the total rare earth concentrations increase by at least a factor of 10. This analysis of the lutetium content of rock and min­ erals leads to the following conclusions: 1. The lutetium content of most minerals increases

in more differentiated rocks.

2. Whole-rock lutetium concentrations in basalts

are slightly higher (by a few tenths of a ppm) LUTETIUM (PPM) 0-4 0-5 02 03 0-1 0 0 5 0 5 0 75 70 65 60 55 50 iue . ueimcnetain i .... rock U.S.G.S. in concentrations Lutetium 7. Figure W-1 standards. 2 i0 S PER CENTWEIGHT BCR-1 AGV-1 GSP-1 G-2

LUTETIUM CONCENTRATION (PPM) 0-5 4-0 30 0 3 2-0 2.5 3.5 1.0 A - ) LNE (An-20) BLENDE (An-5 5) Figure 8. Distribution of lutetium between minerals of the same rock same the of minerals between lutetium of Distribution 8. Figure SAN MARCOS GABBRO MARCOS SAN AUGITE Twl e a, 1965). al, et (Towell AHLT O SOUTHERN OF BATHOLITH HORN-APATITE PLAG WHOLE-ROCK CALIFORNIA K-FELD RUBIDOUX MT. LEUCOGRANITE PLAG WHOLE - ROCK 0 0 5 WHOLE - ROCK BIOTITE 4.0 10 1-5 20 2-5 3.5

o\ 0 0 than lutetium concentrations in granites CHAPTER IV THE GEOCHEMISTRY OF HAFNIUM

1. Chemical Properties of Hafnium The geochemically coherent pair zirconium-hafnium is one of the best examples of the effect of the lanthanide contraction on properties of isovalent elements separated by the lanthanide group (Goldschmidt, 1954) . The similarity in properties of hafnium and zirconium, especially the a- tomic dimensions, are a result of their similar outer elec­ tronic configurations. Hafnium is almost always present in minerals and rocks that contain zirconium, and the most important hafnium-bearing mineral is zircon. In 1789, Klaproth analyzed zircon and discovered the element zirconi­ um. Over 100 years later, in 1923, von Hevesy and Coster discovered hafnium by x-ray analysis of zircon ( Hevesy, 1936). The name hafnium was derived from the latin name for Copenhagen, Hafnia, the city where the discovery was made.

Hafnium belongs to Group IVB of the periodic table, and is a lithophile element that forms highly refractory compounds. Its most common valence is +4. Pure hafnium metal is silver-gray in color and displays high plasticity down to the temperature of liquid nitrogen (Vlasov,1966).

88 It is highly resistant to corrosion and is not appreciably

affected by water, hydrochloric acid, nitric acid, dilute sulfuric acid, or alkali solutions. However, hafnium is readily soluble in hydrofluoric acid. The compounds of

hafnium are easily hydrolized. Hafnium forms stable com­

plexes , but the zirconium halogen complexes are more stable

than the hafnium analogs. Salts of hafnium can combine with (CO^) -2 ions to form hafnium carbonates that are solu­ ble in all acids stronger than carbonic acid. The basicity of hafnium compounds is greater than that of similar zir­ conium compounds.

Excellent summaries of the physical and chemical pro­ perties, and analytical chemistry of zirconium and hafnium have been compiled by Elinson and Petrov (1969)r and

Mukherji (197 0). Some important properties of hafnium are presented in Table 20. In the following discussion, data will be presented for zirconium, where pertinent, because of the close similarity in behavior of both elements.

2. Isotopes of Hafnium

There are six naturally occurring stable hafnium iso­ topes (Table 21). Ten short-lived radioactive isotopes of hafnium can be produced artifically, but no naturally oc- curring radioactive hafnium isotopes are known. 17 f* Hf is produced as a result of the beta decay of 17 Lu. Therefore, the relative abundances of all stable isotopes of hafnium Table 20. Properties of Hafnium and Zirconium

Property Hafnium Zirconium

Electronic Configuration [Xe]4F15d26s2 [Kr]4d25s2

Atomic Number 72 40

Atomic Weight 178.49 91.22

Most Common Oxidation State +4 +4

Ionic Radius (+4) (6-Fold coordination)^ 0.79 0.80

Expected Coordination 6 6

Observed Coordination^ 6 6 Electronegativity^ 1.3 1.4

% Ionic Character*2 70 65 Table 20 - Continued

Property Hafnium Zirconium

Melting Point (°C)3 2222+30 1830+40 Boiling Point (°C)^ 3100 2900

O O Density (g/cm ) 13.01-13.09 6.49

Average Thermal Neutron Capture Cross- Section (Barnes)3 115+5 0.18+0.02

Reference: 1. Whittaker and Muntus (1970); 2. Krauskopf (1967); 3. Elinson and Petrov (1969).

VO (— 1 92

Table 21. Naturally Occurring Isotopes of Hafnium and Zirconium

Atomic Atom % Atomic Number Symbol Mass Abundance Weight

40 Zr 90 51.4 89.904711

91 11.2 90.905643

92 17.1 91.905039 94 17.5 93 .906320

96 2.8 95.90829

Mean Atomic Weight 91.22

72 Hf 174 0.17 173.94014 176 5.2 175.94143

177 18.5 176.94325 178 27.2 177 .94372

179 13 .8 178.9458,4

180 35.1 179.94658 Mean Atomic Weight 178.49

Reference: Holden and Walker (1972). 93 are changing with time.

3. Solar, Cosmic, and Chondritic Abundances of Hafnium and Zirconium Data for solar, cosmic, and chondritic atomic abun­ dances are summarized in Table 22. The solar abundance of

zirconium has been obtained directly by analysis of the solar spectrum (Aller, 1965; Urey, 1967), but the solar a- bundance of hafnium was estimated from the observed distri­ bution of elements in the solar spectrum (Clayton and Fowler,

1961) . 28 chondritic meteorites have been analyzed for hafnium and zirconium (Ehmann and Rebagay, 1970). The agreement between measured and calculated abundances is generally good. However, Cameron's (1968) calculated value for the abundance of zirconium appears to be too high by a factor of about two. The good agreement between solar and chondritic zirconium/hafnium atomic ratios suggests that these elements were not fractionated at the time of the formation of chondrites.

4. Hafnium and Zirconium Determinations Summaries of the geochemistry of hafnium compiled by

Goldschmidt (1954) , Rankama and Sahama (1950) , and Vlasov (1966) relied primarily on analytical data obtained by means of x-ray, optical spectrographic, and wet chemical tehcniques. The accuracy of these methods is poor in the range of hafnium concentrations most frequently encountered Table 22. Solar, Cosmic, and Chondritic Atomic Abundances of Hafnium and Zirconium (atoms per 10^ silicon atoms)

Solar Cosmic Chondritic Element (1) (2) (3) (4) (5) (6)

hafnium — 0.18 0.16 0.19 13 zirconium 14 14 30 13 0.19

Zr/Hf 78 188 68 68

Reference: 1. Aller (1965); 2. Urey (1967); 3. Clayton and Fowler (1961); 4. Cameron (1968); 5. Seeger et al (1965); 6 . Ehmann and Rebagay (1970) . 95

in rocks (0 to 20 ppm). At the present time, highly pre­

cise and accurate determinations can be made for low-level

concentrations of hafnium by means of neutron activation. Simialr determinations of zirconium are not as good, because

it has a low neutron capture cross-section. As a result, zirconium determinations by neutron activation are not as

accurate as those for hafnium even though zirconium con­

centrations may be 100 times or more higher than hafnium

concentrations in a given sample. Highly accurate x-ray procedures for zirconium (Dunn, 1962), and for zirconium

and hafnium (Luke, 1968) have been reported, but they are not directly applicable to whole-rock analyses.

5. Hafnium Concentrations in Rocks Meteorites

The average hafnium content of 35 stony meteorites has been determined by neutron activation and is 0.23 ppm (Table 23). The observed variation in concentration was from 0.01 to 1.06 ppm. Calcium-rich achondrites have the highest average hafnium concentrations (0.78 ppm), while calcium-poor achondrites have the lowest average hafnium concentration (0.02 ppm). The Zr/Hf ratios are highest in calcium-poor achondrites and lowest in chondrites, but the average Zr/Hf ratio for all 35 stony meteorites is 43, and agrees closely with the estimated average Zr/Hf ratio of the Earth (44-Vlasov, 1966; 41-Horn and Adams, 1966; 55- Taylor, 1964). Table 23. The Concentration (PPM) of Hafnium and Zirconium in Meteorites

Calcium-Rich Calcium-Poor All Stony Element Chondrites Achondrites Achondrites Meteorites

Hafnium 0.19 0.78 0.02 0.23

Zirconium 6.7 46 1.2 9.95

Zr/Hf 35 59 60 43

No. of Samples 28 3 4 35

Range of Zr 2.6-17 17-64 0.64-2.2 0.64-64

Range of Hf 0.06-0.38 0.61-1.06 0.01-0.05 0.01-1.06

Reference: Ehmann and Rebagay (1970) .

VO O'! 97

Lunar Rocks Hafnium concentrations in various kinds of lunar rocks

from Mare Tranquillitatis are summarized in Table 24. The highest hafnium concentrations, up to 20 ppm, occur in basalts. Hafnium and zirconium are enriched in lunar ba­

salt relative to terrestrial tholeiites by factors of 5 or

6 . However, the average Zr/Hf ratios of these rocks are very similar to the terrestrial average. Igneous Rocks Hafnium concentrations in various kinds of igneous rocks are presented in Tables 25 to 29. Ultra-basic rocks have hafnium concentrations of 1.0 ppm or less. The high­ est hafnium concentrations (up to about 100 ppm) are found in granitic rocks derived from residual magmas that develop­ ed by fractional crystallization, and especially in alkali granites, syenites and pegmatites (Goldschmidt, 1954). In general, both hafnium and zirconium concentrations increase as a function of increasing degree of differentiation

(Fig. 9) . Sedimentary Rocks

There are insufficent data available at the present time to permit direct estimates of the concentrations of hafnium in various kinds of sedimentary rocks. However, estimates have been made on the basis of proportions of igneous and sedimentary rocks in the crust, and their av­ erage hafnium content (Turekian and Wedepohl, 1961; Horn iue . anu n zirconiumconcentrations and Hafnium 9.Figure Hf (PPM) 25 20 10 15 055 50 W-1 BCR-1 ar, 1974)Faure, . nUSGS rc tnad (Owenand standards rockin U.S.G.S. AGV-1 60 C>2 2 > iC S Jit (w t t (w 65 GSP-1 Zr °/o) G G - 2 70 98 G-1 75 100 200 400 600 300 500

Zr (PPM) 99

Table 24. Concentration (PPM) of Hafnium in Lunar Rocks from Mare Tranquillitatis.

Rock Number of Hafnium Average Average Type Samples Variation Hafnium Zr/Hf

Basalt 8 11-20.0 17 .2 36

Gabbro 10 11-13.2 11.5 36

Breccia 21 8.9-15.4 12.2 37

Soil 2 9-11 10 39 All Rocks 41 8.9-20.0 12.7 35

Reference: Morrison et al (1970); Goles et al (1970); Turekian and Kharkar (197 0). Table 25. Hafnium Concentrations (PPM) in Ultra-Basic Rocks

Rock Type Source Hf Zr/Hf Reference

Peridotite Garabal Hill-Glen Fyne complex,Scotland 1.0 26 1

PCC-1 periodtite U.S.G.S. Standard 0.06 117 2

DTS-1 dunite U.S.G.S. Standard 0.01 300 2

Reference: 1. Brooks (1970); 2. Flanagan (1973). Table 26. Hafnium Concentrations (PPM) in Basalt and Gabbro

Rock Type Source Hf Zr/Hf Reference

Tholeiite Hawaiian Islands 3.3 57 1

Quartz dolerite Hawaiian Islands 2.7 53 1

Oceanic baslat Carlsberg Ridge 1.9 51 1

Oceanic basalt Carlsberg Ridge 2.9 28 1

Oceanic basalt Mid-Atlantic Ridge 2.1 44 1 CO Oceanic basalt Mid-Atlantic Ridge • 48 1

W-L Diabase U.S.G.S. Standard 2.67 39 2

BCR-1 Continental Th.U.S.G.S. Standard 4.7 40 2

Dolerite (6) Central-zone, Great Lake Intrusion, Tasmania 2.2 53 3

Dolerite (6) Lower-zone, Great Lake Intrusion Tasmania 0.85 69 3

Diabase (5) Dillsburg, Pa. 1.7 54 3

Diabase GSJ-JB-1 (standard) 3.5 86 2 Table 26 - Continued

Rock Type Source Hf Zr/Hf Reference

Basalt ZGI-BM (standard) 3.1 34 2

Gabbro (2.) Garabal Hill-Glen Fyne Complex, Scotland 1.3 41 1

Gabbro (2) Skaergaard Intrusion,Greenland 2.0 39 4

Norite NIM-N (standard) 5 5 2

Alkali Olivine basalt Hawaiian Islands 2.9 61 1

Nepheline basalt Hawaiian Islands 3.5 67 1

Melilite Nepheline basalt Hawaiian Islands 6.6 65 1

Mawaiite Hawaiian Islands 10.1 72 1

Ankaramite Hawaiian Islands 4.1 59 1

AGV-1 andesite U.S.G.S. Standard 5.2 43 2

Trachyte Hawaiian Islands 17.7 85 1 102 Reference: 1. Brooks (1970); 2. Flanagan (1973); 3. Gottfried et al (1968) ; 4 . Brooks (1969) • Table 27. Hafnium Concentrations in Igneous Rocks of Intermediate Composition

Rock Type Source Hf Zr/Hf Reference

Pyroxene-mica diorite Garabal Hill-Glen Fyne Complex, Scotland 3.5 50 1

Medium granodiorite Garabal Hill-Glen Fyne Complex, Scotland 4.2 29 1

Porphyritic granodiorite Garabal Hill-Glen Fyne Complex, Scotland 4.4 43 1

Ferrodiorite Isle of Skye, Scotland 12.1 38 1 .

Granodiorite (17 samples) Louis Lake Batholith, Wyoming 5.9 56 2

Quartz Monzonite (4 samples) Louis Lake Batholith, Wyoming 7.5 46 2

Ferrodiorite Layered Series, Skaergaard Intrusion 1.8 29 3 Ferrodiorite Layered Series, Skaergaard Instrusion 25.1 50 3 103 Table 27 - Continued

Rock Type Source Hf Zr/Hf Reference

GSP-1 granodiorite U.S.G.S. Standard 14.7 41 4 FI tonalite Geologic Survey, Tanganyika 6.3 - 5

Granodiorite GSJ-JG-1 (standard) 3.5 46 4

Reference: 1. Brooks (1970); 2. Condie and Lo (1971); 3. Brooks (1969); 4. Flanagan (1973); 5. Brooks (1968). Table 28. Hafnium Concentrations (PPM) in Granitic Rocks

Rock Type Source Hf Zr/Hf Reference

G-l, granite U.S.G.S. Rock standard 5.2 40 1

G-2, granite U.S.G.S. Rock standard 7.35 41 1

Epigranite (4 samples) Isle of Skye, England 17 37 2

Biotite-granite Cornwall, S.W. England 5.9 29 3 00 1—1

Altered granite Cornwall, S.W. England 40 3 O C ■3*

Granite Geevor, S.W. England • 21 3

Biotite-granite Malaya 2.5 28 3

Biotite-granite Malaya 12 21 3

Granite (4 samples) Louis Lake Batholith,Wyoming 6.5 42 4

Aplite (6 samples) Louis Lake Batholith, Wyoming 2.6 41 4

Pegmatite Garabal Hill-Glen Fyne Complex, Scotland 0.8 32 2 105

Porphyritic Felsite Isle of Skye, Scotland 11.3 33 2 Table 28 - Continued

Rock Type Source Hf Zr/Hf Reference

Marscoite Isle of Skye, Scotland 10.7 47 2

Granophvre Dillsburg, Pa. 5.9 47 5

Granophyre Great Lake Intrusion,Tasmania 6.5 33 5

Granite NIM-G (standard) 12 25 1

Granite ZGI-GM (standard) 4.7 31 1

Syenite S-l (standard) 66.1 48 6

Granophyre Skaergaard Intrusion,Greenland 43.6 64 7

Reference: 1. Flanagan (1973); 2. Brooks (1970); 3. Butler and Thompson (1965); 4. Condie and Lo (1971); 5. Gottfried et al (1968); 6 . Owen and Faure (1974); 7. Brooks (1969). Table 29. Hafnium Concentrations (PPM) in Alkali Granites

Rock Type Source Hf Zr/Hf Reference

Granite (6) Liruei, Northern Nigeria 27 22 1

Granite (6) Amo, Northern Nigeria 24 24 1

Granite New Hampshire, (Mt. Tremont) 23 27 1

Granite Ascension Island 28.8 64 2

Comendite Liruei, Northern Nigeria 63 37 1

Rhyolite (2) Liruei, Northern Nigeria 64 35 1

Reference: 1. Butler and Thompson (1965); 2. Brooks (1970). 108 and Adams, 1966). These data are summarized in Table 30.

The average concentrations of hafnium in important rock types are summarized in Table 31 (See Fig. 12). Meteorites and ultra-basic rocks tend to have low and rather uniform hafnium contents. Other types of igneous rocks, and presumably sedimentary rocks, have both higher hafnium contents and greater variability in hafnium concentrations.

6 . The Abundance of Hafnium in Igneous Rocks of the Oceanic and Continental Crust The average concentration of hafnium in the oceanic crust can be approximated from the average hafnium content of oceanic tholeiitic basalts (Taylor, 1964). According to Herrmann (1970), the average concentration of an element in igneous rocks of the upper continental crust may be cal­ culated from analyses of representative rock types assuming the following composition for the crust: 45% granite, 35% granodiorite, 5% diorite, and 15% continental tholeiite. Various estimates of the crustal abundance of hafnium are presented in Table 32. The abundance of hafnium in the oceanic crust (2.2 ppm) was calculated from data pre­ sented in Table 26 for oceanic tholeiitic basalts, and is in good agreement with Taylor's estimate as both were com­ puted in the same way. However, there is a significant discrepency between previous estimates and this author's value for the abundance of hafnium in the upper continental crust. An average concentration of 6.1 ppm was computed, Table 30. Hafnium Concentrations (PPM) in Sediments and Sedimentary Rocks.

Lithologic Hafnium Zr/Hf Catagory Concentrations Ratios Reference

Shale 3.14 45.2 1

Sandstone 2.99 68 .2 1

Limestone 0.234 77.4 1

Oceanic Clay 4.63 32.4 1

Oceanic Carbonate 1.13 74 .7 1

Limestone 0.8 37.5 2

Reference: 1. Horn and Adams (1966); 2. Flanagan (1973). Table 31. Average Hafnium Concentrations (PPM) in Rocks

Rock Type Number of Samples Hafnium Zr/1-If

Peridotite 2 0.53 72 gabbro 4 1.65 40 basalt 11 2.44 49 intermediate igneous rocks 7 8 .09 39 granites 15 6.93 36 late-stage alkali granites 4 25.7 34 lunar basalt 8 17.2 36 lunar gabbro 10 11.5 36 chondrites 28 0.19 35

sedimentary rocks * 2.91 49

Data are from Tables 23 to 30. *Hafnium and zirconium concentrations were calculated, using data in Table 30, for an average sedimentary rock assuming that the fraction of sediment mass on the continents is equivalent to a mixture consisting of 77% shale, 15% sand­ stone, and 8% limestone (Herrmann, 1970). Ill

Table 32. Abundance of Hafnium in the Oceanic and Continental Crust

Horn and Adams Taylor This (1966) (1964) Work

Oceanic Crust - 2 2.2 Continental Crust 3.9 3 6.1 Zr/Hf Ratio (Oceanic Crust) - 75 46 Zr/Hf Ratio (Continental Crust) 41 55 39 112 by Herrmann's method, with data from Tables 26, 27 and 28.

The discrepancy is probably a reflection of the greater amount of hafnium data now available for igneous rocks of granitic and intermediate composition. Taylor's estimates of the Zr/Hf ratio in the oceanic and continental crust are probably too high.

7. Hafnium Isomorphism

Various mechanisms for isovalent and heterovalent sub­ stitutions of zirconium for major cations have been suggest­ ed (Vlasov, 1966) . Presumably, similar mechanisms control hafnium substitutions. Some examples are given below. The hafnium content of pyroxenes and amphiboles may result from the following types of coupled substitutions:

Ca+2 + Mg+2 + Si+4 Na+1 + Hf+4 + (Al, Fe)+3

Mg+ 2 + Si+4 Hf+4 + Be+2

Replacement of Ca+2 by Hf+4 may occur in titaniferous gar­ nets and sphenes as follows:

Ca+2 + Ti+4 Hf+4 + (Mg+2, Fe+2)

Hf+4 can also substitute for Ti+4. Evidence for this type of substitution is the almost invariable presence of zir­ conium and hafnium in titanium minerals (Vlasov, 1966). A tabulation of elements with ionic radii similar to hafnium is given in Table 33.

8 . The Hafnium Content of Minerals

Hafnium occurs in a large number of minerals, the most Table 33. Elements with Ionic Radii Similar to Hafnium

Ionic Radii (A) Ionic Radii (A) Ionic Radii (A) Element for +4 Ions for +3 Ions for +2 Ions

Hf 0.79

Zr 0.80

Ti 0.69 0.94

V 0.67 0.87

Cr 0. 63 0.81

Mn 0.62 0.75

Mo 0.73 0.75

Nb 0.77 0 .79

U+6 0.81

Th 1.08

Fe 0.63 0.69 113 REE (La-Lu) 1.13-0.94 Table 33 - Continued

Ionic Radii (A) Ionic Radii (A) Ionic Radii (A) Elements for +4 Ions for +3 Ions for +2 Ions

Sc 0.83

Y 0.98

Mg 0.80

Ca 1.08

Sr 1.21

Co 0 . 61 0.65

Ni 0.64 0.77

Cu 0.81

Zn 0.83

Reference: Whittaker and Muntus (1970).

Note: Ionic radii are for elements in 6-fold coordination. 115

important of which is zircon (Table 34). According to

Vlasov (1966), most of the main hafnium-bearing minerals, are found in alkali rocks. Ubiquitous zircon, however, is an exception. The concentration of hafnium is all known hafnium-bearing minerals, with the exception of some occur­ rences of the scandium silicate thortveitite, is exceeded by the concentration of zirconium. There are no known occurrences of hafnium-bearing minerals that do not also contain zirconium. With the exception of thortveitite, independent hafnium minerals do not occur in nature because of the great chemical affinity of zirconium and hafnium, and not because of the realtively low abundance of hafnium (Vlasov, 1966).

Relatively few hafnium analyses for minerals in common igneous rocks have been published. Brooks (1969) analyzed a suite of rocks and minerals from alkaline intrusions of East Greenland. His results indicated that sodium-rich pyroxene and amphibole contain 264 ppm and 74 ppm hafnium respectively, while the concentration of hafnium in co­ existing biotite (about 1 ppm) was very low. Brooks (1969) also reported relatively high hafnium concentrations for ilmenite (12.5 ppm) and for pyroxene (3.6 ppm) separated from rocks of the Skaergaard intrusion (Table 35). The geochemical affinity of zirconium and hafnium al­ lows estimates of hafnium concentrations to be made, if the zirconium content of the sample is known. Hafnium may Table 34. Hafnium-Bearing Minerals

Weight Per Cent Type Mineral Formula Hf0 2 Zr/Hf

Oxides Baddeleyite Zr02 1.1 -2.45 35-81

Polymignite (C2 ,Fe,Ce) (Zr,TiNb)^ Og 0.6 -0.9 32-48

Zirkelite (Zr,Ca,Ti,Fe+2 ,Mg,REE,U 7Th) 05 .25-1.2 43-144

Silicates Zircon and its varieties Zr (Si04) 0.30-31 12-217

(a) Astrophyllite (K ,Na,Ca)(Fe,Mn)4 (Ti,Zr)

(OH,F)2Si4Oi4 (?) 0.20 58

Sphene Ca Ti Si 05 0.0012 —

Thortveitite (Sc,Y)2Si20 7 0.5 -3.2 0.6-4.0

Eudialyte (Na,Ca)r(Zr,Fe,Mn) (Si601?)

(0,0H,C1) 0.13-0.70 20-115 116 Zirfesite MZr02-nFe202-pSi02~qH20 0.87-0.90 37-55

Keldyshite Na2Zr(Si20^) 0.72-0.81 47-51 Table 34 - Continued

Weight Per Cent Type Mineral Formula Zr/Hf

Rosenbuschite (Na,Ca)3 (Zr,Fe,Ti) 0 (Si20^)F 0.3 66

Seidoserite Na^ ,Mn (Zr ,Ti) 9Ti (8120-7)202

(F,OH)2 0.30-0.40 56-70

Catapleiite Na2Zr (Si30 )-2 H?0 0.20-0.65 46-153

Lovozerite (Na,Ca)2 (Zr,Ti)(Sig0^3)(OH)^

-3H20 0.14-0.25 67-78

Vlasovite Na2Zr (Si40ll) 1.7 15

Elpidite Na2Zr (Si6015).3 H20 0.2 -0.4 52.-101

Wadeite K2 Zr (Si^O^) 1.6 15

Wohlerite Na Ca2 (Zr,Nb )0 (Si207) F 0.5 -0.7 25-31

Ti-lavenite (Na,Ca,Mn)3 (Zr,Ti,Fe)0 (Si207)F 0.27-0.34 47-56

Lomonosovite Anhydrous and phosphorous-bearing 117 Ti ,Nb , silica te of sodium ___ --______52-53 Reference: Vlasov(1966) unless noted otherwise; (a) Brooks (1969) . Table 35. Concentrations (PPM) of Hafnium in Rocks and Minerals of the Kangerdlugssuaq Alkaline Intrusion and the Skaergaard Intrusion, East Greenland

Variation of Intrusion Sample Hafnium Hafnium Zr/Hf

Kangerdlugssuaq Rocks 3.9-7 .5 5.8 40

Aegirine 51.9-264 .3 128 37

Arfvedsonite 7.5-73.9 33 37

Biotite 0 .9-1.1 1.0 110

Eudialyte --- 1186 128

Astrophyllite --- 1999 58

Sphene ---- 12.4 __

Melanite --- 54.1 47 118 Table 3 5 - Continued

Variation of Intrusion Sample Hafnium Hafnium Zr/Hf I— 00 CM LO i 2 —1 1 ■ I

Skaergaard Layered Series Rock 10.5 37

Ilmenite (Layered Series) 12.4-12.6 12.5 37

Pyroxene (Layered Series) 3.2-3.9 3.6 30

Reference: 1. Brooks (1970); 2. Brooks (1969). 119 120 be calculated from the empirical relationship Hf (ppm) =

(Zr (ppm) - 14.266)/(34.707) (See Section 11). Mason and Graham (197 0) measured zirconium concentrations in chondritic pyroxene (90 ppm) and plagioclases (5 ppm), and in a peridotite inclusion (60 ppm) from a continental ba­ salt. These zirconium concentrations correspond to calcu­ lated hafnium concentrations of approximately 2 , 0 , and

1 .5 respectively.

Cruft (19 66) reported maximum zirconium concentrations of 134 ppm for metamorphic apatite, and 1000 ppm for igne­ ous apatite. These concentrations correspond to hafnium concentrations of about 3 and 28 ppm respectively.

9. Fractionation of Hafnium and Zirconium It was previously believed that zirconium and hafnium did not undergo significant fractionation relative to each other because of the similarity in their atomic and ionic dimensions. In fact, Rankama and Sahama (1950) claimed that no separation of hafnium from zirconium occurs in nature. This view is incorrect. Characteristic changes in Zr/Hf ratios for rocks belonging to particular dif­ ferentiation series have been interpreted as being indica­ tive of fractionation of these elements.

Igneous rocks ranging in composition from andesite to rhyolite and from diorite to granite belong to the calc- alkaline differentiation series (Verhoogen et al, 1970). 121

The general trend for rocks derived from magmas of calc- alkaline or tholeiitic (silica saturated) composition is a progressive decrease of the Zr/Hf ratios in the order basalt to granite. A convincing example of this type of variation has been provided by Kosterin et al (1958) . They measured Zr/Hf ratios in zircons from igneous rocks of

Northern Kirgizia, U.S.S.R., and found that the ratios de­ creased linearly as follows: gabbro (71) - granodiorite (58) - granite (46) - hydrothermal veins (29). Subsequent investigations have substantiated this trend (Gottfried and Waring, 1964; Vlasov, 1966; Gottfried et al, 1968; Condie and Lo, 1971). In general, Zr/Hf ratios in basic rocks are expected to be high (about 50), while in granitic rocks they are expected to be lower (3 0 to 40). However, at least one exception to this rule has been reported. Brooks (1969) measured Zr/Hf ratios in a complex consisting of ultra- basic, intermediate, and acid plutonics in Scotland and found no direct relationship between degree of differenti­ ation and Zr/Hf ratios.

Zr/Hf ratios of rocks derived by differentiation of an alkaline magma increase as a function of increasing de­ gree of differentiation. According to Vlasov (1966), the

Zr/Hf ratios in rocks of tbeLovozero alkali massif increase from 38 for early-formed rocks to 60 for late-stage rocks.

Brooks (1970) reported a rise in Zr/Hf ratios from 36 to 47 122

in early to late differentiation products, respectively,

from the Kangerdlugssuaq alkaline intrusion, East Greenland. He also reported thatZr/Hf ratios rise from 59 to 85 in

Hawaiian rocks belonging to an alkali basalt differentiation series. The Zr/Hf ratios in rocks of the Skaergaard intru­ sion, East Greenland, increase slightly with increasing degree of differentiation (Brooks, 1969). Rocks derived by crystallization from hydrothermal fluids are usually found to have low (less than 30) Zr/Hf ratios. Vlasov (1966) reported that Zr/Hf ratios of late- stage minerals decreased to 31, in the case of alkali-rich pegmatites, and to 1.7, in the case of granitic pegmatites.

Butler and Thompson (1965) reported that Zr/Hf ratios of late-stage granites, that were not associated with pegma­ tites, were low and varied between limits of 29 to 21.

They also reported low Zr/Hf ratios (33 to 11) for late- stage alkali granites.

10. Fractionation Mechanisms Although generally accepted explanations for systematic variations of Zr/Hf ratios in rocks have not been offered, several possibilities have been suggested. Butler and

Thompson (1965) postulated that late-stage hafnium enrich­ ment may occur during crystallization of a magma, because of the large difference in atomic weight of hafnium and zirconium. In aqueous fluoride or chloride systems, 123

zirconium and hafnium may become separated by gaseous dif­

fusion processes resulting from differences in molecular weight of the volatile phases. Zirconium is selectively

fractionated into the vapor phase, and,-as a result, may be lost from the system. As a consequence, hafnium concen­

trations in the residual solid phases may be significantly

increased. Zr/Hf ratios would tend to decrease, therefore, during the late-stages of differentiation regardless of the initial chemical affinities of the magma.

Increases in Zr/Hf ratios observed for rocks derived

from alkali-rich magmas can be attributed to differences in basicity of zirconium and hafnium (Vlasov, 1966) . During differentiation of silica-depleted, alkali-rich magmas, zirconium becomes concentrated in the liquid phase as haf­ nium is more likely to be removed in early-formed solid phases. Crystal-chemistry is thought to play an important role in the fractionation of hafnium and zirconium. Brooks

(1969) suggested that high Zr/Hf ratios observed in biotite from the Kangerdlugssuaq alkaline intrusion are caused by a crystallochemical peculiarity of the biotite structure that causes it to discriminate against hafnium. Condi and Lo (1971) argued that decreases in the concentration of hafnium and Zr/Hf ratios with differentiat±nnay be controlled by zircon. According to them, some poorly understood fac­ tor causes hafnium to be preferentially excluded from 124 octahedral sites in zircon during early stages of fraction­

al crystallization. The crystallochemical control of Zr/

Hf ratios has also been emphasized by Vlasov (1966) .

While it is probable that changes in Zr/Hf ratios may be related to crystall chemistry of mineral species, it is not obvious why a particular mineral species discriminates

against one or the other element considering that both have the same valence and almost identical ionic radii.

One possible explanation might be the slightly greater ten­ dency of hafnium to form ionic ompounds as its electro­ negativity (1.3) is slightly greater than the electronega­ tivity of zirconium (1.4). It is also possible that the hafnium and zirconium content of many minerals is controlled by the composition of solid or liquid inclusions. However, the ultimate cause of changes in Zr/Hf ratios, during crystallization of a magma, may be related to properties of hafnium and zirconium complex ions. Ringwood (1955 b) has considered the behavior of ele­ ments with charges greater than three in magmas. He con­ cluded that these elements form stable complexes during the course of crystallization, and that it is the properties of the complex, rather than those of the central cation, that govern their geochemical behavior. In a silicate melt, SiO^ -4 tetrahedra may exist either as discrete enti­

ties, or linked in a framework by sharing of oxygen anions by adjacent tetrahedra. According to Ringwood, the preva­ 125 lance of linked tetrahedra (degree of polymerization) in the melt is governed by its (Si + Al)/0 ratio, and its volatile content (primarily water, fluorine, and chlorine). The volatiles tend to decrease the degree of polymerization of the melt as does an increase in the relative amounts of univalent, divalent, and some trivalent ions.

The stability of cation complexes is indicated by the magnitude of their ionic potentials, defined as the ratio of cation charge to ionic radius (Ringwood, 1955 b). The ionic potentials of Zr+^ (5.08) and Hf+^ (5.12) are rela­ tively low, and indicate that their complexes, of the type

MO4 , are less stable than analogous complexes of silica or carbon whose ionic potentials are 9.52 and 25.00 respec­ tively .

Two factors govern the behavior of hafnium and zirconi­ um in a magma (Ringwood, 1955 b). As a consequence of the relatively low stability of their complexes, they may exist in the free state as +4 ions, and replace other cations, such as Ca +2 , m crystallizing phases. Ringwood notes, however, that this process is of secondary importance, be­ cause both elements have a higher tendency to exist as com­ plexes .

The solubility of accessory minerals containing multi- valent elements such as hafnium and zirconium increases as the degree of polymerization of a magma decreases. As a result, concentrations of zirconium and hafnium tend to be 126

higher in rocks that have crystallized from an acidic melt, because the degree of polymerization of a basaltic magma is lower.

The systematic decrease of Zr/Hf ratios that is charac­

teristic of differentiation of a melt of calc-alkaline af­ finities may be a result of the slightly higher stability of hafnium complexes. Zirconium would have a greater ten­ dency to exist in the free state as +4 ions, and, as a consequence, would be partially removed from the melt by crystallization of the common rock forming minerals. Haf­ nium, however, would have a greater tendency to remain in the liquid phase until the degree of polymerization of the melt was high enough to permit precipitation of hafnium- bearing accessory minerals.

11. Empirical Relationship Between Hafnium and Zirconium Concentrations Although Zr/Hf ratios in nature are variable, concen­ trations of these elements in whole-rocks and in minerals are linearly related over an extended range. This type of distribution suggests that hafnium concentrations can be computed empirically when zirconium has been previously determined, or vice versa. Since, in most cases, zirconium can be determined relatively rapidly, a simple, semi- quantitative method is thus available for estimating the hafnium concentration. Hafnium and zirconium concentrations in rocks of vari­ 127 ous types (Data from Tables 23 to 30) were plotted in Fig.

10. The distribution of points is approximately linear, but the degree of scatter increases at higher concentration levels. The region enclosed by dashed lines, which includes all points with hafnium and zirconium concentrations of up to 20 ppm and 700 ppm respectively, was replotted at an ex­ panded scale in Fig. 11. The best line through this array of points was obtained by least-squares regression, and yield­ ed the following expression:

Zr(ppm) = (Hf(ppm)) (34.707) + (14.266) (1) To evaluate whether a similar relationship between hafnium and zirconium concentrations exists for mineral species, a least-squares regression line was fitted to data from Table 35. The resulting distribution was also approxi­ mately linear: Zr(ppm) = (Hf(ppm)) (33.612) + 18.130 (2) The small differences in slopes and intercepts for whole- rock and mineral data are probably a result of the combined effects of fractionation and the smaller number of analyses used to calculate the mineral regression line. The dif­ ferences are, therefore not significant.

Average hafnium and zirconium concentrations, calcu­ lated for specific rock types, are also related by a linear function. This can be demonstrated by computing average concentrations of these elements and plotting them as shown in Fig. 12. The distribution is directly related to rock n zroim ocnrtos n rocks. in concentrations zirconium and ZIRCONIUM (PPM) 1500 2000 3000 1000 2500 3500 500 Figure 10. Linear relationship between hafnium between relationship Linear 10. Figure 0 10 20 30 ANU (PPM) HAFNIUM 40 50 60 7Q 128 ocnrtos n rocks. in concentrations ZIRCONIUM (PPM) 500 600 400 200 700 300 100 0 iue 1 Epne sae lto hfimad zirconium and hafnium of plot scale Expanded 11. Figure 5 ANU (PPM) HAFNIUM 10 15

20 129 ZIRCONIUM (PPM) 800 300 400 200 600 500 iue 2 Aeae ocnrtos f anu ad icnu i rocks. in zirconium and hafnium of concentrations Average 12. Figure 700 100 PERIDOTITE GABBRO tholeutic CHONDRITES

basalt granite ANU (PPM) HAFNIUM NTERMED1ATE IGNEOUS ROCKS 10 UA GABBRO LUNAR L LNR ROCKSALL LUNAR • • J=U.NAJi. LT. A S A B ALKALI GRANITE 20

25 130 131

type. Although individual rocks of a particular type may

very widely in hafnium and zirconium content, averages for major groups appear to be characteristic. Chondrites, peridotites, basalts, and granites are clearly delineated.

A similar relationship was obtained by Kosterin et al (1953) for zircons separated fromrocks varying in composition from basalt to granite. The distributions probably reflect the

effects of igneous differentiation to some degree. Con­ sidered in this way, the position of lunar rocks on the curve is consistent v/ith the view that they represent end- products of lunar differentiation.

12. Lutetium/Hafnium Ratios in Rocks and Minerals The utility of a lutetium/hafnium geochronometer is dependant in part on the variability of lutetium/hafnium concentration ratios in nature. In general, observed ratios are highest in basic igneous rocks and chondritic meteorites, and lowest in acidic igneous rocks (Table 36).

From a consideration of the available data, it appears like­ ly that lutetium/hafnium ratios in whole-rocks will seldom be found to exceed about 0.3, and will most commonly vary between 0.2 (basic rocks) and 0.01 (granitic rocks). For comparison, rubidum/strontium ratios most frequently en­ countered in whole-rocks vary between 0,1 to 20.

The maximum estimated change in average lutetium con­ centrations in igneous rocks ranging in composition from Table 36. Lutetium/Hfanium Ratios in Rocks

Rock Type Lutetium (ppm) Hafnium (ppm) Lu/Hf Ratics chondrites 0.031 0.19 0.16 lunar rocks 1.88 12.7 0.15 peridotites 0.038 0.54 0.07 gabbro 0.34 1.65 0.21 basalt 0.47 2.46 0.19 intermediate igneous rocks 0.21 8.75 0.02 granites 0.37 7.46 0.05 sedimentary rocks'*' 0.17 2.91 0.17

Data are from CHAPTERS III and IV.

1. Based on the following proportions of sedimentary rocks in the crust:

77% shale, 15% sandstone, and 8% limestone. 132 133 basalts to granites is about 0.3 ppm. The corresponding changes in average hafnium concentrations, however, are from about 2.0 to 26 ppm or greater. Clearly, changes in lutetium/hafnium ratios are controlled to a large degree by variations in hafnium concentrations. Although lutetium is a lithophile element, whole-rock lutetium concentrations in acidic igneous rocks are lower than in basalts as a result of the lack of suitable luteti­ um sites in major mineral phases of these rocks. Instead, lutetium is concentrated by the accessory minerals such as monazite, zircon, apatite, sphene, etc., whose relative abundances may be negligible. In basic igneous rocks, how­ ever, most of the lutetium is probably associated with pyroxene, amphibole, garnet, and in the case of instrusives, the fine-grained or glassy matrix. The ultimate result is that basic rocks tend to have higher lutetium/hafnium ratios than acidic rocks. The variations of lutetium/hafnium and of rubidium/strontium ratios in igneous rocks are summarized in Figs. 13, and 14. The sense of the variation is in op­ posite directions. Lutetium/hafnium ratios decrease, but rubidium/strontium ratios increase in more acidic rocks be­ cause rubidium is concentrated in the crust and strontium is concentrated in the mantle. Although analytical data for concentrations of both lutetium and hafnium in the same samples are not abundant, conclusions regarding the expected variability of lutetium/ 134

W-1

BCR-1

0-10

M— X D _I AGV-1 0-05

G-1

GSP-1 G - 2

50 55 60 65 70 75

Figure 13. Lutetium/hafnium ratios in U.S.G.S. rock standards. 135

0.5

0-4

0-3 n

X) 0 2

• Lu/Hf

O Rb/Sr

0-1

BASALT INTERMEDIATE GRANITE ROCK TYPE

Figure 14. Lutetium/ha£nium and rubidium/ strontium ratios in igneous rocks. 136

hafnium ratios in rocks and minerals may be drawn on the

basis of geochemical considerations:

1. REE do not freely substitute for Ca+^ in ferro- magnesians or feldspars because their electronegativities

are too high. Hence, the total concentration of lanthanides in these minerals is low. However, lutetium is relatively enriched in ferromagnesian minerals because of the availa­ bility of 6-fold coordination sites. Lutetium is relatively deficent in calcium-rich plagioclase because calcium is in 7-fold or 8-fold coordination in these feldspars. Most of

the lutetium in K-feldspar may be associated with liquid of solid inclusions.

2. High hafnium concentrations occur in ferromagnesian minerals crystallized from alkalie-rich magmas. The concen­ trations of hafnium in ferromagnesian minerals derived from melts of other compositions are probably significantly lower as evidenced by the low hafnium content of tholeiitic basalts. Ultra-mafic rocks generally have hafnium contents of less than 1 ppm.

3. The highest concentrations of lutetium occur in secondary accessory minerals that are most abundant in acidic rocks. However, certain sedimentary rocks such as bone beds may be enriched in lutetium possibly as the re­ sult of biological activity. Someminerals, such as calcite, apatite, and possibly biotite, may discriminate against hafnium but not lutetium. 137

4. Whole-rock hafnium concentrations are lowest in

basic rocks and highest in acidic rocks as a result of the properties of hafnium complex ions in silicate melts. The

relatively high hafnium content of lunar basalt is consis­ tent with the view that hafnium concentrations increase as a function of increasing degree of differentiation.

5. Lutetium/hafnium ratios in basic rocks are higher than in acidic rocks. Lutetium/hafnium ratios in mantle

source regions of basaltic magma are probably also higher than in the crust. As a result, the abundance of radiogenic 176 Hf in the mantle may be increasing significantly with time.

6. The low lutetium/hafnium ratios characteristic of acidic rocks suggests that once a mantle derived magma is 7 fi intruded into the crust, the abundance of radiogenic 1 Hf tends to remain constant.

7. The lutetium content of basic rocks may be oftly

of secondary importance, as the highest concentrations of lutetium are associated with secondary accessory minerals. These minerals may have the highest lutetium/hafnium ratios, and, as a consequence, the highest concentrations of radi­ ogenic 176Hf.

8. Of importance may be old bone bed deposits, which, by virtue of biological accumulations of lutetium, may have lutetium/hafnium ratios in excess of 100. Hafnium determi­ nations have not been reported for this type of material, 138 but, based on data for carbonates, hafnium concentrations will probably not exceed 1 ppm. 9. The relatively high concentrations of lutetium reported for biotite and garnet suggests that rocks enrich­ ed in these minerals may also have high lutetium/hafnium ratios. CHAPTER V

LUTETIUM DETERMINATIONS 1. Introduction

The REE can be determined by a variety of methods in­ cluding neutron activation, x-ray fluorescence, spark source mass spectrometry, and isotope dilution- Eby (1972) devel­ oped a simply x-ray fluorescene procedure which involved group separation of the rare earths by cation exchange chro­ matography prior to x-ray analysis. However, the accuracy at the ppm level was only 10 to 30%. Rey et al (1970) dis­ cussed several available analytical methods for REE deter­ minations. They concluded that radiochemical neutron acti­ vation analysis, involving a chemical procedure for the quantitative separation of individual rare earths, was neces­ sary for rapid and accurate low-level determinations of all the REE.

Quantitative separation of individual REE is difficult. The standard procedure involves cation exchange chromato­ graphy with an organic complexing agent as the elutant (a-hydroxyisobutyric acid or 2-methyl lactic acid) under carefully controlled conditions (Eugster et al, 1970) . Im­ proved separations are achieved when gradient elution tech­ niques are employed (Massart and Hoste, 1963; Rey et al, 1970; Wolfsberg, 1962). Gradient elution is a procedure 139 140

whereby the composition of the elutant is varied systema­ tically throughout the chromatographic separation (Bock

and Nan-Sing Ling, 1954).

A much simpler procedure for the determination of REE

by isotope dilution v;as reported by Schnetzler et al, (1967). This method involved measuring the change in isotopic com­ position of an element, with two or more stable or long-

lived isotopes, induced by equilibration of a sample with

a spike. The isotopic composition of the spike is artifi­ cially enriched in a particular isotope of the same element.

Since quantitative recovery is unnecessary after sample- spike isotopic equilibration, Schwetzler et al, (1967), were

able to separate the REE into two groups consisting of light

(La-Eu), and heavy (Gd-Lu) REE by cation excahnge chroma­ tography using 6N hydrochloric acid as the elutant. The heavy REE were removed from the column first. Similar pro­ cedures have been successfully employed for the determina­

tion of REE in a variety of samples (Masuda, 1967; Nagasawa, 1970; Gast et al, 1970; Loubet et al, 1972) . Principal disadvantages of this technique when employed specifically for lutetium determinations are the necessity of a correc­ tion for isobaric interference of htterbium at mass 17 6, and the suppression of lutetium emission in the presence of

large amounts of ytterbium. 141

2. Chemical Extraction of Lutetium from Rocks and Minerals In this study, a procedure was developed for lutetium determinations by stable isotope dilution using a mass spectrometer. Initially, the REE were concentrated prior to the chromatographic separation of lutetium by precipita­ tion of the hydroxides with ammonia, followed by ether ex- 4-3 traction of Fe (Boudin and Deutsch, 1970) . This operation was subsequently superceded by a much more efficent chro- matrographic separation. The procedure finally adopted for lutetium analyses consisted of the following steps:

1. Dissolution and isotopic equilibration of the sample powder (200 mesh or finer) with 17 6 Lu

spike, and 6.71 day 1 77 ''Lu tracer.

2. Preliminary separation of the REE as a group by

cation exchange chromatography. 3. Partial separation of lutetium from ytterbium by

cation exchange chromatography. 4. Final purification of the lutetium concentrate

by selective evaporation of ytterbium in the mass spectrometer. Reagents and Apparatus Double-distilled and demineralized water was used ex­ clusively. Dilute hydrochloric acid (2.2N) was prepared in large quantities by bubbling hydrogen chloride gas through a tank filled with about 4 5 liters of water. More concen­ trated solutions of hydrochloric acid were prepared as needed by dilution of a stock of reagent-grade hydrochloric acid that had been purified by distillation from a 2 liter pyrex boiling flask connected to a quartz water-cooled con­ denser. Nitric acid solutions were prepared from a stock of distilled reagent-grade acid. Reagent-grade sulfuric acid, hydro-fluoric acid, ammonium hydroxide, perchloric acid (double-distilled from Vycor, 70-73%) , and ether were used without further purification. Analytical-grade filter paper was employed for removal of precipitates and insoluble residues from solutions. The concentrations of acid solu­ tions were established by means of specific gravity deter­ minations obtained with hydrometers.

Chromatographic separations were performed on BIO-RAD AG50W-X8 sulfonated polystyrene cation exchange resin (200-

400 mesh). Pyrex columns (I.D.=2.54 cm) were used for the first-stage REE group separation. The height of the resin columns following cleaning with 300 ml of 6N hydrochloric acid and re-equilibration with 4 00 ml of 2.2N hydrochloric acid was 20 cm. The second-stage separation of lutetium was carried out in Pyrex columns (I.D.= 2.0 cm) fitted with 250 ml Pyrex elutant resevoirs. After cleaning with 200 ml of 6N hydrochloric acid and re-equilibration with 300 ml of

2.2N hydrochloric acid, the height of the resin columns was 20 cm. All columns were fitted with glass wool plugs to prevent breakage of their fritted glass discs resulting from expansion or contraction of resin when elutant compo- 143 sitions were changed.

Sample Dissolution

Several methods (discussed in detail in Chapter VI) were employed for sample dissolution. Spiked whole-rock sample powders, weighing between 0.2 and 0.5 grams, and 177 Lu tracer were treated with concentrated hydrofluoric acid and 50% sulfuric acid in a ratio of 4:1. The solutions were heated overnight in covered, 100 ml Teflon dishes. The residues, following expulsion of excess hydrofluoric acid by heating at high temperatures, were re-disolved with di­ lute hydrochloric acid (2.2N). Insoluble sulfates were removed by filtration, and the solutions were adjusted to volumes of 50 ml with water.

Non-silicate mineral powders were treated with 6N hydrochloric acid and a small amount of 8N nitric acid (10 to 15 ml) in 250 ml Vycor beakers. The samples were spiked, and 177 Lu tracer was added before acid dissolution. They were heated overnight to volumes of about 10 ml to insure isotopic equilibration with spikes, and tracer. Insoluble residues were removed by filtration and weighted. The final solution volumes were adjusted to 50 ml by addition of water as required.

Hafnium and zirconium determinations, in most cases, required sample dissolution by fusion. Corresponding lutetium concentrations were also obtained by fusion. Spiked mineral samples, and tracer were fused with a flux consisting of a 144 mixture of 5 grams of sodium carbonate and 0.1 grams sodi­ um peroxide in platinum crucibles. The fusion cakes were dissolved with hydrochloric acid in 250 ml Vycor beakers, and following filtration, the solutions were diluted to

250 ml with water in preparation of the first-stage chro­ matographic separation. Spiked whole-rock samples, and tracer were fused with a mixture of 60% lithium fluoride and 40% boric acid crystals in platinum crucibles. The fusion cakes were dissolved with concentrated sulfuric acid (97%) and 6N hydrochloric acid in 100 ml Teflon dishes. For this reason, boric acid- lithium fluoride fusion of calcium-rich samples was not feasible, as calcium forms insoluble sulfates. Following filtration, the normality of the solutions were adjusted to about 2, with respect to the sulfate anion, by dilution with water. Cation Exchange Chromatography The behavior of an element on a cation exchange resin can be predicted if equilibrium distribution coefficients (Kd) are known (Strelow, 1960). Kd is defined as follows:

AMOUNT OF ION ON RESIN VOLUME OF AQUEOUS PHASE Kd=------X ------AMOUNT OF ION IN ACUEOUS PHASE RESIN DRY WEIGHT (4) Kd is not a constant. It is controlled by the composition of the aqueous phase, size and charge of the cation, the cation/resin weight ratio, the resin type, and, to a lesser degree, by temperature and pressure (Strelow, 1960). Ions 145 are selectively fractionated from the aqueous phase and re­ tained by the resin as values increase. An element can be eluted rapidly from the resin when is 10 or less. At higher values (20 or more) the amount of elutant required to quantitatively recover an element from the resin is markedly increased. Good separations are achieved by vary­ ing the composition of the elutant in stages as required to effect the desired result. In other words, a procedure can be developed for the separation of one or more elements by maximizing differences in coefficients for desired and undesired sample constituents (Strelow, 1959; Strelow et al, 1969). Values of for different elements on AG50W-X8 resin in hydrochloric acid, nitric acid and sul­ furic acid are given in Tables 37 to 39. In order to faci­ litate identification of the optimum elutant(s) for the separation of the REE as a group, and for the subsequent partial separation of lutetium from ytterbium, coeffici­ ents have been plotted as functions of acid concentration in Figs. 15, 16, and 17. Lanthanides, thorium, zirconium, and hafnium are the elements with the highest coefficients in hydrochloric acid and nitric acid. In sulfuric acid coefficients are highest for the REE. the position of thorium, hafnium, and zirconium have shifted because of the complexing effect of the sulfate ion. These data suggest that the separation of the REE as a group can be accomplished easily by cation 146

Table 37. K-, Coefficients at Different Normalities or Hydrochloric Acid for AG50W-X8 Cation Exchange Resin

Cation 0.1 0.2 0.5 1.0 2.0 3.0 4.0

Zr(IV) >10 5 >105 ~105 7250 489 61 14 .5 Th (IV) >105 >105 ~105 2049 239 114 67 La(III) >105 >105 2480 265.1 48 18.8 10.4 CE(III) >10 5 >105 2460 264 .8 48 18.8 10.5 Y (III) >10 5 >10^ 1460 144 .6 29.7 13.6 8.6 Ba (II) >104 2930 590 126.9 36 18.5 11.9 Hg (I) >10^ 7600 640 94.2 33 19.2 13.6 Al(III) 8200 1900 318 60.8 12. 5 4.7 2.8 Sr (II) 4700 1070 217 60.2 17.8 10 .0 7.5 Ga (III) >10 4 3036 260 42.58 7.75 3.2 0.36 Ca (II) 3200 790 151 42.29 12. 2 7.3 5.0 Pb (II) >10^ 1420 183 35.66 9.8 6.8 4.5 Fe(III) 9000 3400 225 35.45 5.2 3.6 2.0 Cr(III) 1130 262 73 26.69 7.9 4.8 2.7 T1(I) 173 91 41 22.32 9.9 5.8 3.3 Ni (II) 1600 450 70 21.85 7.2 4.7 3.1 Co(II) 1650 460 72 21.29 6.7 4.2 3.0 Mg(II) 1720 530 88 20.99 6.2 3.5 3.5 Mn(II) 2230 610 84 20.17 6.0 3.9 2.5 Fe(II) 18 20 370 66 19.77 4.1 2.7 1.8 Cs (I) 182 99 44 19.41 10.4 • • • • • • UO (II) 5460 860 102 19.20 7.3 4.9 3.3 Ag (I) 156 83 35 18.08 7 . 9 5.4 4.0 Cu (II) 1510 420 65 17.50 4.3 2.8 1.8 Hg(II) 4700 1090 121 16.85 5.9 3.9 2.8 Zn(II) 1850 510 64 16.03 3.7 2.4 1.6 Rb (I) 120 72 33 15.43 8.1 • • • • • • K (I) 106 64 29 13.87 7.4 • • • • • • Be (II) 255 117 42 13.33 5.2 3.3 2.4 Ti(IV) >10^ 297 39 11.86 3.7 2.4 1.7 V (IV) 230 44 7.20 « • « • • • • « • Na (I) 52 28. 3 12 5.59 3.6 • • • • • • Li(I) 33 18. 9 8.1 3.83 2.5 • • • • • • Sn(IV) ~104 45 6.2 1.60 1.2 • • • • • • Cd (II) 510 84 6.5 1.54 1.0 0.6 • • • V(V) 13. 9 7. 0 5.0 1.10 0.7 0.2 0.3 Mo (V) 10. 9 4.5 0.3 0.81 0 . 2 0.4 0 . 3 Se(IV) 1. 1 0.6 00.8 .63 1.0 0.7 Bi(III) Ppt. Ppt. <1.0 1.0 1.0 1.0 1. 0 As(III) 1. 4 1. 6 2.2 3.81 2 . 2 • • • . ♦ . 147

Table 37 - Continued

Cation 0.1 0.2 0.5 1.0 2.0 3.0 4.0

Sb (III) Ppt. Ppt. Ppt. Ppt. 2.8 • * • Pt(IV) • • « • • • • • • 1.4 • • • • • • • • • Au (IV) 0.5 0.1 0.4 0.84 1.0 0.7 0.2 Hg (II) 1.6 0.9 0.5 0.28 0.3 0.2 0.2

Reference: Strelow (1960) 148

Table 38. Coefficients at Different Normalities of Nitric Acid for AG50W-X8 Cation Exchange Resin

Cation 0 .IN 0.2N 0.5N 1. ON 2. ON 3. ON 4 . ON

Zr(IV) >104 >10^ > 10^ 6500 652 112 30.7 Hf(IV) >10^ >10^ >10^ 2400 166 61 20,8 Th(IV) >10'+ >10^ > 104 1180 123 43.0 24 .8 La(III) >10*+ >10^ 1870 267 47.3 17.1 9.1 Ce (III) >10^ >10^ 1840 246 44.2 15.4 8.2 Yb (III) >10"+ >10^ 1150 193 41.3 16.0 9.0 Er (III) >10'+ >10^ 1100 182 38 .2 14 .9 8.0 Y (III) >10^ >10^ 1020 174 35 .8 13.9 10.0 Sm(III) >10^ >10^ 1000 168 29.8 10.9 7.2 Gd (III) >10^ >104 1000 167 29 .2 10.8 6.9 In(III) >10*+ >104 680 118 23.0 10.1 5.8 Sc (III) >10'+ 3300 500 116 23.3 11.6 7.6 Cr (III) 5100 1620 418 112 27.8 19.2 10 .9 Hg (I) >10^ 7600 640 94 33.5 19.2 13.6 Ga(III) >10^ 4200 445 94 20.0 9.0 5.8 Al (ill) >10^ 3900 392 79 16 .5 8.0 5.4 Fe (III) >10^ 4100 362 74 14.3 6.2 3.1 Ba (II) 5000 1560 271 68 13 .0 6.0 3.6 Sr(II) 3100 775 146 39.2 8.8 6.1 4.7 Pb (II) >10^ 1420 183 35.7 8.5 5.5 4.5 Ca(II) 1450 480 113 35.3 9.7 4.3 1.8 Cd(II) 1500 392 91 32.8 10.8 6.8 3.4 Co (II) 1260 392 91 28 .8 10 .1 6.1 4.7 Mn(II) 1240 389 89 28.4 11.4 7.1 3.0 Ni(II) 1140 384 91 28 .1 10.3 8.6 7.3 Cu(II) 1080 356 84 26.8 8.6 4.8 3.1 Zn(II) 1020 352 83 25.2 7.5 4.6 3.6 Bi(III) 893 305 79 25.0 7.9 3.7 3.0 U(VI) 659 262 69 24.4 10.7 7.4 6.6 Mg (I I) 794 295 71 22.9 9.1 5.8 4.1 T1 (I) 173 91 41.0 22.3 9.9 5.8 3.3 Ag(I) 156 86 36.0 18.1 7.9 5.4 4.0 Hg(II) 4700 1090 121 16.9 5.9 3.9 2.8 Cs (I) 148 81 34 .8 16 .8 7.6 4.7 3.4 Be (II) 553 183 52 14 .8 6.6 4.5 3.1 Ti(IV) 1410 461 71 14 .6 6.5 4 . 5 3.4 V(IV) 495 157 35.6 14.0 4.7 3.0 2.5 Rb (I) 118 68 29.1 13 .4 6.6 4.1 2.9 K(I) 99 59 26.2 11.4 5.7 3.5 2.6 Te(IV) 40.3 19.7 8.5 5.0 2.4 0.6 0.2 Pd (II) 97 62 23.5 9.1 3.4 2.7 2.5 149

Table 3 8 - Continued

Cation 0 . IN 0. 2N 0 . 5N 1. ON 2 . ON 3. ON 4 .ON

Rh(III) 78 44.7 19.5 7.8 4.1 2.1 1.0 Na (I) 54 29.4 12.7 6.3 3.4 2.0 1.3 Li (I) 33 .1 18 . 6 8.0 3.9 2.6 1.7 1.1 V(V) 20.0 10.9 4.9 2.0 1.2 0.8 0.5 Mo (VI) Ppt. 5.2 2.9 1.6 1.0 0.8 0.6 Nb (V) 11.6 6.3 0.9 0.2 0.1 0.1 0.1 Se (IV) <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 As (III) <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1

Reference: Srelow et al (1965).

4 150

Table 39. Coefficients at Different Normalities of Sulfuric Acid for AG50W-X8 Cation Exchange Resin

Cation 0. IN 0. 2N 0.5N 1. ON 2. ON 3. ON 4 .ON

La (III) >1 0 ^ >10^ 1860 329 68 24.3 12.1 Ce(III) >10 >10^ 1800 318 66 23.8 11.8 Sm (III) >10 *♦ >1 0 ^ 1460 269 56 20.1 10.0 Y (III) >1 0 ^ >1 0 ^ 1380 253 49.9 18.0 9.4 Yb (III) >1 0 4 >1 0 ^ 1330 249 48.1 17.3 8.8 Gd (III) >1 0 ^ >1 0^ 1390 246 46. 6 17.9 8.9 Er (III) >1 0 ^ >1 0 ^ 1300 242 48.6 16.7 8.5 Bi (III) >1 0 4 >1 0 ^ 6800 235 32.3 11.3 6.4 Ga (III) >1 0 ^ 3500 618 137 26.7 10.0 4.9 A1 (III) >10 *♦ 8300 540 126 27.9 10.6 4.7 Hg (II) 7900 1790 321 103 34.7 16.8 12.2 In(III) >1 0 4 3190 376 87 17 .2 6.5 3.8 M n (II) 1590 610 165 59 17.4 8.9 5.5 Fe (III) >104 2050 255 58 13.5 4.6 1.8 Cr (III) 198 176 126 55 18.7 0.9 0.2 Th(IV) >1 0 ^ 3900 263 52 9.0 3.0 1.8 T1(I) 452 236 97 49.7 20.6 11.6 8.7 Tl (III) 6500 1490 205 47.4 12.0 7.2 5.2 V(IV) 1230 490 140 46.6 11.5 2.4 0.4 Ni (II) 1390 590 140 46.0 16.5 6.1 2.8 Fe (II) 1600 560 139 46.0 15.3 9.8 6.6 Cd (II) 1420 540 144 45.6 14.8 6.6 4.3 Zn(II) 1570 550 135 43.2 12.2 4.9 <4.0 Co (II) 1170 433 126 42.9 14.2 6.2 5.4 Cu (II) 1310 505 128 41.5 13.2 5.7 3.7 Mg (II) 1300 484 124 41.5 13.0 5.6 3.4 Sc(III) 5600 1050 141 34.9 8.5 4.4 3.4 Be(II) 840 305 79 27.0 8.2 3.9 2.6 Cs(I) 175 108 52 24 .7 9.1 4.8 3.5 Rb (I) 14 8 91 43. 8 21.3 8.3 4.4 3.1 K(I) 138 86 41. 1 19.4 7.4 3.7 2.9 Rh(III) 80 49. 3 28 .5 16.2 4.5 2.2 1.3 Pd(II) 109 71 32. 5 13 .9 6.0 3.8 2.7 Hf (IV) 2690 1240 160 12.1 1.7 1.0 0.7 U(VI) 596 118 29. 2 9.6 3.2 2.3 1.8 Ti(IV) 3 95 225 45. 8 9.0 2.5 1.0 0.4 Na (I) 81 47 .7 2 0 .1 8.9 3.7 2.6 1.7 Li (I) 48.0 28 .2 1 1 .7 5.8 3 . 0 1.6 1.1 Te(IV) Ppt. 30. 8 9. 8 5.2 2.6 0.6 0.3 Zr(IV) 546 474 98 4.6 1.4 1.2 1.0 151

Table 39 - Continued

Cation 0 . IN 0 . 2N 0.5N 1. ON 2. ON 3 . ON 4 .ON

V(V) 27.1 15.2 6.7 2.8 1.2 0.7 0.4 Nb (V) 14.2 7.4 4.0 1.9 0.7 0.5 0.3 Mo (VI) Ppt. 5.3 2.8 1.2 0.5 0.3 0.2 Se(IV) <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 As(III) <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1

Reference: Strelow et al (1965). 152

Ce Sr

100

50

Fe

10

Sr

Fe

0 1 2 3 4 NORMALITY (HCI)

Figure 15. coefficients for elements in hydrochloric acid. 153

100 -

50 -

Yb 10 -

5 - Ba

NORMALITY (HNO3)

Figure 16. coefficients for elements in nitric acid. 154

100

50

K d Yb

10

Fe4

Hf

NORMALITY (hLSO„ )

Figure 17. coefficients for elements in

sulfuric acid. 155 exchange chromatography. However, quantitative separations of individual REE with these acids as elutants is not pos­ sible, because of the similarity of their coefficients. A suitable chromatographic procedure was developed for lutetium with the aid of ^ S r an(j 17?lu radioactive tracers, as described below.

Cation Exchange Chromatography-First-Stage Separation

The procedure adopted for the separation of the REE was dictated by the method employed to bring samples into solution, as the behavior of an element on cation exchange resions is governed by the composition of the aqueous phase.

Whole-rock solutions obtained by fusion with lithium flu­ oride and boric acid crystals were percolated through resin, columns. The resin was washed with 200 ml of 2.2N hydro­ chloric acid to remove calcium, aluminum, iron and other elements with lower coefficients. Barium and strontium were then removed with 20 0 ml of 2.5N nitric acid. The REE were recovered in 150 ml of 6N hydrochloric acid. Dilute hydrochloric acid solutions obtained by acid dissolution of samples were percolated through resin columns, followed by elution of 200 ml of 2 .2N hydrochloric acid, 200 ml of 2.5N nitric acid, and recovery of the REE in

150 ml of 6N hydrochloric acid. The migration of lutetium through the column was monitored with -^^Lu tracer.

Cation Exchange Chromatography-Second-Stage Separation

The REE solutions obtained from the first-stage 156 separation were evaporated in 100 ml Teflon dishes to a vol­ ume of about 10 ml. Following filtration, the solutions were diluted to 50 ml with water and percolated through resin columns. Lu was eluted with 3.ON hydrochloric acid, between 150 and 2 00 ml being required to recover all of the

177Lu activity. Fractions of 15 ml were collected following elution of the first 100 ml of acid. The 15 ml fraction with the highest activity as measured with a gamma ray detector was evaporated to dryness in a 30 ml Vycor evapora­ tion dish. In cases where two 15 ml fractions had about equal levels of 177 Lu activity, both were evaportaed to dryness. The residues were dissolved with 2.2N hydrochloric acid and transfered to 5 ml beakers. Two or three drops of concentrated perchloric acid v/ere added to oxidize small amounts of resin lost from the columns, and the solutions were then evaporated to dryness.

Final Purification of the Lutetium Concentrate

Sample residues were recovered from the 5 ml beakers in a drop of dilute nitric acid (about 0.5N) with a Vycor- tipped syringe, and deposited on a tantelum filament. The filament was then mounted in the source of a mass spectro­ meter (Nuclide Corp. Model 6-60-S). Ytterbium and lutetium are not quantitatively separated by the cation exchange chromatography. However, ytterbium ion beams can be gener­ ated at significantly lower temperatures than required for lutetium. This permits selective evaporation of ytterbium 157 from the filament with minimal loss of lutetium. The length of time required for complete removal of ytterbium depends on the Yb/Lu ratio of the sample, and on the efficiency of the chromatographic separation, but in most cases only

15 to 30 minutes are required.

3. Lutetium-176 Spike Calibrations A lutetium spike enriched in 17 f iLu was obtained from the Oak Ridge National Laboratory in the form of the oxide

(LU20^). The spike was dissolved in 2N hydrochloric acid and the solution was diluted to 250 ml with water. The isotopic composition of the spike was measured and found to be in excellent agreement with Oak Ridge (Table 40).

The spike concentration was calibrated with a shelf solution prepared gravim.etrically from Johnson Mathy spec- pure LU2O3 (Table 41). The shelf solution was used for spike calibrations over a period of several years. The isotopic composition of the shelf solution was determined and is in good agreement with lutetium abundances specified by Holden and Walker (1972) (see Table 42). The isotopic composition of lutetium was also determined in a mixture of lutetium and ytterbium oxides (Table 42). In this instance a correction for ^^®Yb was necessary, and was applied as follows: 176 peak height = 176Lu + 176Yb 158

Table 40. Isotopic Composition of the Lutetium-17 6 Spike

Source Abs.175 Abs.17 6 176/175

This Study 0.2839 0.7161 2.522+0.003

Oak Ridge 0.284 0.716 2.521 Table 41. Calibration of the Lutetium-176 Spike Solution

Weight (g) Weight (g) Spike Date (176/175)m Shelf Solution Spike Solution Concentration (ug/g)

4/29/69 0.9285 0.6102 1.7336 3.727

5/29/69 0.5472 0.8338 1.1014 3.734

4/ 9/70 0.6124 4 .5948 7.1043 3 .712

8/14/72 0.4070 2.2002 1.9964 3.708

Average 3.7 20

Concentration of Lu in the shelf solution: 5.431 ug/g 160

Table'42. Isotopic Composition of Normal Lutetium

Source Compound 175/176

This Study 37.3 0+0.4 6

This Study Lu203+Yb203 38.21+0.7 5

Holden and Walker (1972) 37.46 161

176Yb/174Yb = 0.39937

176Yb = (17 4yb) (0.39937)

■^■7^Lu = 176 peak height - -*-7^Yb

In order to apply this correction it was necessary to scan 174Yb thus increasing the time required for the analysis. In addition, the corrected lutetium isotopic abundances differed slightly from those obtained in the absence of ytterbium thus illustrating the desirability of quantita­ tive removal of ytterbium prior to determination of luteti­ um .

4. Lutetium Mass Spectrometry

All lutetium determinations were made on a Nuclide Model 6-90-S, single filament, solid source, 6-inch, 60° sector mass spectrometer. The instrument was equipped with a vibrating reed electrometer, 1 0 ^ ohm collector re­ sistor, and a solid state strip chart recorder. New tanta­ lum filaments were used for each run. The filaments were not pre-cleaned as it was demonstrated that lutetium emis­ sion was undetectable from new filaments heated to tempera­ tures in excess of those normally required to generate a lutetium signal from a sample.

Mass spectrums of normal and of spike lutetium, and of a sample plus spike mixture are shown in Fig. 18. The resolution of ^7^Lu and ^7^Lu is satisfactory although the valley between the two peaks does not reach the baseline. 162

176 175

f ! Normal

hfrtr

#2-t

1

Mixture

176 175

Figure 18. Lutetium Mass Spectra 163

However, the excellent agreement of data for the isotopic abundances of lutetium in both spike and shelf solutions with those reported by Oak Ridge and Holden and Walker

(1972) respectively suggests that the measured -^^Lu/^^Lu ratio is not affected in a significant way by resolution problems. During a typical Lu-ID run a ytterbium spectrum is first observed at filament current setting of from 1.4 to

1.8 amps D.C. Mass 17 6 is scanned at a magnet current of about 164 ma. Initially, the presence of ytterbium almost completely suppresses lutetium emmission. Shortly before complete removal of ytterbium the signal undergoes explo­ sive growth. ^^Yb ion beam intensities may increase up to

30 times in an interval of a few seconds, immediately fol­ lowed by the disappearance of the ytterbium signal and the growth of a stable lutetium ion beam. At this point, the filament current is decreased as necessary to prevent rapid loss of lutetium from the filament. The spectrum is then scanned 4 0 times or more to assure adquate statistics for estimating the precision of the calculated lutetium con­ centration .

5. Calculation of Lutetium Concentrations The concentration of lutetium in a sample can be cal­ culated from the following expression: R = (0.02588N + .71606S)/(0 .9741N + 0 .28393S) (5) 164

Where: R = Measured ^^Lu/^^Lu ratio

N = Number of normal lutetium atoms

S = Number of spike lutetium atoms Coefficients of N and S are atomic abundances of normal and spike lutetium isotopes. This expression can be solved for the (N/S) ratio as follows:

(N/S)w t = (f)[(0.71606 - 0.28393R)/(0.9741R - 0.02588] (6) The (N/S) atomic ratio is converted to a weight ratio by the weight factor (f) which is defined as:

(Atomic Weight Normal Lu) (f) ------0.99608 (7) (Atomic Weight Spike Lu)

Since all parameters except N 1, the weight of lutetium (ug) in the sample are known or can be measured, N' can be cal­ culated and expressed in ppm by dividing it by the weight of sample in grams.

In order to achieve the highest possible accuracy, the

(N/S) ratio must be set within fairly narrow limits prior to the mass spectrometric analysis. The reason for this requirement is illustrated by Pig. 19 where (N/S) weight ratios have been plotted as functions of the measured

1 "7 d I n r Lu/ Lu ratios. The resulting hyperbolic mixing curve indicates that (N/S) weight ratios between about 0.4 to 1.2 produce relatively large fluctuations in measured -^^Lu/

17!5lu ratios. Beyond these limits, the curve rapidly flattens, and changes in ^Lu ratios become insensi- 40 0 3 (N/S)wt2-0 Figure 19.Figure 0 UE SAMPLE PURE 05 u L > ^ ^ - - e l p m a S 10 6 7 1 ,175Lu u L spikehyperboliccurve. mixing . 2-5 1.5 UE SPIKE PURE 20

30 165 166

tive to changes in (N/S) weight ratios.

6. Lutetium Blanks

Two lutetium procedural blanks were determined by pro-

cessing x/DLu17 f \ spike through the entire chemical procedure. The average blank was 0.019 + 0.042 mg where the error is given as half the difference between the two determinations. The 17 7 Lu tracer was not carrier free. A tracer blank was determined by isotope dilution using the lutetium shelf solution as a standard. The tracer blank was 0.0086 ug/ drop. All analyses were corrected for both procedural and tracer blanks.

7. Testing of the Lutetium Procedure

The chemical procedure developed for extraction of lutetium from silicate rocks was tested by analysis of the U.S.G.S. rock standard BCR-1 (a continental tholeiite from the Columbia River Plateau). A lutetium concentration of

0.434 ppm was determined by isotope dilution, and is in satisfactory agreement with the recommended concentration of lutetium (0.55 ppm) in this standard (Flanagan, 1973). CHAPTER VI

MEASUREMENT OF THE CONCENTRATION OF HAFNIUM AND ZIRCONIUM

AND OF THE 176Hf/177Hf RATIO

1. Introduction

One of the primary objectives of this study was to de­ velop a suitable chemical procedure for extraction of haf­ nium from rocks and minerals for subsequent determination of concentrations (by isotope dilution) and l7^Hf/l’77Hf i- sotopic ratios. Development of a ^-7^Lu/176Hf geochronom­ eter has been impeded primarily by the analytical difficul­ ties associated with measurement of the isotopic composi­ tion of hafnium Most rocks and minerals have hafnium contents of be­ tween 1 to 20 ppm, but corresponding zirconium concentra- tions may be as much as 50 to 100 times, or more, greater.

As a result of their similar chemical properties, zirconi­ um is recovered when hafnium is extracted from a sample un­ less special techniques are employed to prevent this from occurring. Since the isotopic composition of hafnium can only be measured by mass spectrometry, the presence of large amounts of zirconium is undesirable. Diffusion of hafnium ions through a dense cloud of ionized zirconium can result in drastic reduction of hafnium signal intensities

167 168

and resolution of adjacent isotopes can be impaired. Thus,

in the design of a suitable chemical procedure, it is also important to consider how zirconium/hafnium ratios might be decreased.

Highly precise and accurate determinations of low- level hafnium concentrations can be obtained routinely by

radiochemical neutron activation techniques. This is pos­

sible because hafnium has a high neutron capture cross-

section. After irradiation of the sample, contamination during subsequent chemical processing can not occur. Suit­ able procedures have been reported for analysis of silicate

rocks by Brooks (1968) , Hahn (1972) , Rebagay and Ehmann (1970b), and others (see: Fleischer, 1969; Flanagan, 1969; Butler and Kniseley, 1973). Determinations of zirconium by neutron activation are not as good, because zirconium has a very low neutron capture cross-section.

Chemical procedures for the separation of hafnium, de­ veloped expressly for neutron activation determinations, can be adopted, with some modifications, for mass spectro- metric analyses. Hafnium is usually concentrated either by precipitation with mandelic acid, or its derivatives (Oesper arid Klingenberg, 1949), or by liquid-liquid extraction with

TTA (Moore, 1956; Marsh et al, 1961). Brooks (1968) dis­ cussed the relative merits of both methods and concluded that precipitation of mandelates was superior to extraction. Both mandelic acid and TTA are specific for zirconium and hafnium. Irradiated samples are usually equilibrated with large amounts of a non-irradiated zirconium carrier to insure quantitative recovery of trace amounts of hafnium.

However, use of a zirconium carrier for hafnium determi­ nations by isotope dilution is not feasible, because even ‘spec-pure’ zirconium reagent contains some hafnium. In addition, its use would only complicate the already serious problem of sample zirconium interference with hafnium ion emission in the mass spectrometer.

An isotope dilution procedure for hafnium has been described by Boudin and Deutsch (1970). They separated hafnium and zirconium by extraction with TTA, but the pre­ cision of the method for total hafnium and radiogenic hafni urn concentrations was no better than * 10%. They ascribed the poor precision to ion beam instability, REE isobaric interference, and low enrichment of the spike they used. In other words, extraction with TTA failed to quanti tatively remove the heavy REE, and the original zirconium ratios of their samples were probably unchanged.

The principal requirements for hafnium determinations by isotope dilution are isotopic equilibration of samples with hafnium spike, and subsequent recovery of sufficient hafnium, of adequate purity, to permit isotope analyses in the mass spectrometer. Determination of 17^Hf/177nf iso­ topic ratios of hafnium extracted by a similar procedure is more difficult, because of the low relative abundances of 170

176pif ancj 177nf, and, in the absence of spike, the total

amount of hafnium recovered for the analyses is lower. In

the following section, development of a suitable hafnium procedure is discussed in detail.

2. Experimental Reagents and Apparatus

Oxalic acid solutions were prepared from reagent-grade

oxalic acid crystals. Other acid solutions were prepared as described in CHAPTER V. p-bromomandelic acid was ob­

tained from VWR Scientific, Columbus, Ohio. Tri-n-octyl- amine (97%) was supplied by the Aldrich Chemical Company, Milwaukee, Wisconsin. The G.F. Smith Chemical Company, Columbus, Ohio supplied cupferron reagent and lithium meta­

borate flux. Ion exchange chromatography was carried out

in Pyrex columns with BIO-RAD AG50W-X8 (200-400 mesh) cat­

ion exchange resin, and BIO-RAD AG-2X8 (100-200 mesh) anion

exchange resin. All other reagents were of reagent-grade quality, and were obtained from Laboratory Stores, The Ohio

State University. Fusions were carried out in 30 ml platin­ um crucibles. Cupferron precipitates were collected by fil­ tration on MF-Millipore filter discs (Type HA; pore size:

0.45 pxn; diameter: 47 mm) employing an all-glass Millipore suction filtration apparatus. HA filters were used because they are unstable in mineral acid solutions, and are, there­ fore, readily destroyed by a simple wet-ashing procedure. 100 ml Pyrex boiling flasks, connected to a small (18 171

cm long) water cooled condenser by an adaptor fitted with

ground glass joints, were used for the wet-ashing of filter

discs and cupferron precipitates (Smith, 1965; Hillebrand,

1953). The same apparatus was employed for the destruction of hafnium and zirconium oxalate complexes. The boiling

flasks were heated in a small electric mantle regulated by a powerstat auto-transformer.

Production of Hafnium-181 Tracer

During the initial stages of this investigation, the

need for a radioactive tracer to monitor hafnium became evident. However, at the time, commercial -^ljjf tracer,

prepared once a year and distributed by the Oak Ridge

National Laboratory, was not available. Therefore, a small quantity of ISlHf was prepared by neutron irradiation of hafnium oxide at the reactor facility of Battelle Laborato­ ries, Columbus, Ohio.

The radioactive nuclides and ISljjf are produced by (n,2r ) nuclear reactions as follows: l ^ H f (n,V) 17 5jjf. -^^Hf (n,?f) 183-Hf. if hafnium oxide of normal isotopic composition is irradiated by a thermal neutron flux, the re­ sulting activity is due primarily to the decay of ISlnf, be­ cause the relative abundance of ^^Hf ^ow (' 0,17 atom j.oer cent) (Table 43) . Activity resulting from decay of a radioactive nuclide produced by neutron irradiation is given by the relationship (Mapper, 1960) : Table 43. Neutron Activation of Hafnium Oxide

Atom % Neutron-Capture Half-Life Abundance of Cross-Section of Daughter Reaction Parent Nuclide (Burns) Nuclide

^ ( n , y) -*-^Hf 0.17 4xl02 70 day

180Hf(n,y)181Hf 35.1 14.04xl02 4 2.4 day

180(n,y)190 0.205 0.00016 26.9 sec

Reference: Holden and Walker (1972). 173

A = I'l a fv c (1 - e"**' ) (e~**x) (8) where:

A = The disintegration rate in counts/second if

are in reciprocal seconds and if t is in seconds. Activi­ ties are expressed as millicuries (1 me = 3.7 x 10^ cts/ sec) .

N = Number of parent nuclide atoms initially present,

a = Isotopic abundance of the parent nuclide, f = Thermal neutron flux density (n/cm^/sec).

KT" = Thermal neutron capture cross-section in Barns (1 Barn = 10-^ cm^) .

c = Counter efficiency (assumed equal to 100%) .

A = Decay constant of radioactive daughter nuclide, t^ = Irradiation inteval.

t2 = Decay time following irradiation.

The weight of hafnium oxide (0.01 grams) required to pro- duce about 2 me of 181 Hf activity was calculated by means of equation (8) to prevent exposure of laboratory personnel to dangerous levels of radioactivity when using the tracer.

I O 1 Procedure for Production of Hf Tracer:

About 0.1 grams of hafnium oxychloride was dissolved with water in a 10 ml Teflon beaker and converted to the oxide by evaporation and prolonged heating of the residue. A portion of the residue, weighing 0.015 grams, was sepa­ rated and sealed in a silica ampoule, which was subsequent- 174 ly irradiated by a thermal neutron flux of 1 x 10^^ n/cm^/ sec for a period of one hour. Following irradiation, the ampoule was stored for three days to permit initially high levels of radioactivity to decrease.

The hafnium oxide was dissolved in a loo ml Teflon dish with 5 ml of concentrated hydrofluoric acid and about 25 ml of water. Fluoride was removed by fuming with 1 ml of 50% sulfuric acid. The residue was dissolved with water and transfered to a 500 ml polyethylene bottle where its final volume was adjusted to 250 ml by dilution with water. The -^ljjf tracer, produced as described, was not carrier-free, as it contained a significant amount of 'dead1 hafnium of altered isotopic composition. Although the tracer was employed solely as an aid during procedure devel­ opment, actual hafnium determinations could have been made if the effective hafnium content of the tracer had been measured by isotope dilution. A blank correction of this type was necessary during the subsequent use of a commercial -*-^^Hf tracer. In both cases, however, hafnium isotope anal­ yses of samples equilibrated with tracer were not possible because the isotopic composition of the tracer was not known.

Development of a Hafnium Procedure

The aqueous chemistry of hafnium (and zirconium) is complicated by complex formation with inorganic ligands

(Connick and Me Vey, 1949) . An additional problem is the 175 fact that in many rocks and minerals, a significant frac­ tion of the total hafnium may be associated with mineral phases or inclusions, such as zircon, that are not at­ tacked to any appreciable extent by mineral acids. To overcome these difficulties, it was ultimately necessary to fuse samples and spikes to insure both complete sample dissolution, and isotopic equilibration with the spike. Initial attempts to extract hafnium from whole-rocks were based on acid dissolution of samples with a mixture of hydrofluoric acid (20 ml) and sulfuric acid (5 ml). Samples were equilibrated with -^^Hf spike and -^^-Hf tracer at the time of dissolution. An attempt was then made to precipitate the hafnium and zirconium tetramandelates directly with 50 ml of 0.1 p-bromomandelic acid (Oesper and

Klingenberg, 1949). Tracer experiments indicated that the method was not effective, probably because of interference by sulfate and fluoride anions.

It has been noted that fluorides, phosphates, oxalates, nitrates, sulfates, and strong oxidizing agents prevent quantitative precipitation of the tetramandelates (Hahn,

1972; Elinson and Petrov, 1969). This problem can be cir­ cumvented by alkali fusion of the sample and subsequent dissolution of the fusion cake with hot water (Brooks,1968).

The filtrate contains interfering anions and is rejected. The insoluble residue contains hafnium and zirconium which can be precipitated by mandelic acid following dissolution 176 with 2N hydrochloric acid. Further experimentation with mandelic acid was not attempted.

Cation exchange chromatographic procedures for zirco­ nium have been described (Strelow, 1959; Strelow, 1960;

Strelow et al, 1965; Strelow et al, 1969). Hafnium is re­ covered with zirconium. The separation is based on the fact that equilibrium distribution coefficients (K^) for zirconium (and hafnium) are higher, in dilute nitric acid or dilute hydrochloric acid than for other elements on

AG50W-X8 cation exchange resin (see: CHAPTER V).

The basic procedure, as applied to silicates, consists of sample dissolution, either by fusion with sodium carbon­ ate and subsequent removal of silica, or acid dissolution with hydrofluoric acid and sulfuric acid, followed by di­ lution of resulting solutions to volumes of up to 600 ml with water. The purpose of the dilution is to insure that sulfate anion concentrations are below 0.1N to prevent for­ mation of neutral or negative sulfate complexes. Strelow et al (1959) note that sulfate concentrations in excess of 0.1N will result in loss of zirconium when dilute hydro­ chloric acid sample solutions (up to 0.3N) are loaded on resin columns.

Once the samples have been loaded on the resin columns, aluminum, iron, titanium, and other cations with similar or lower coefficients are removed by elution with 2N hydro­ chloric acid. Zirconium is recovered by elution with 5N 177

hydrochloric acid.

The procedure can be modified by incorporation of a second ion exchange step to effect a partial separation of

hafnium from zirconium. A plot of coefficients for zir­ conium and hafnium on AG50W-X8 resin at various nitric acid concentrations is shown in Fig. 20. A partial separation of these two elements appears possible by elution with 4.ON nitric acid. The exact amount of elutant required can be established by monitoring the separation with ^^H f tracer.

Under these conditions, hafnium should be removed from the column before zirconium. The procedure outlined above was adopted, and subsequently tested by analysis of the U.S.G. S. granodiorite rock standard, GSP-1.

Initial Hafnium Procedure:

Whole-rock powders(less than 200 mesh), weighing up to 0.5 grams, were dissolved with 20 ml of concentrated hydrofluoric acid and 5 ml of 50% sulfuric acid in 100 ml 1 7 q i o i Teflon dishes. -L/;7Hf spike and OJ-Hf tracer were added at the time of sample dissolution in an attempt to isotopical- ly equilibrate the sample with these reagents. The solu­ tions were covered and heated for 24 hours. The residues were heated with fuming sulfuric acid to remove fluoride ion, and then redissolved with 30 ml of 2.2N hydrochloric acid. The solutions were filtered and then diluted to about 600 ml with water. 178

Hf Zr

100

50

K.

10

NORMALITY (NITRIC ACID)

Figure 20. Zirconium and hafnium coef­ ficients at different normalities of nitric acid for AG50W-X8 resin. 179

Cation exchange chromatography was carried out with resin columns that were about 26 cm high following clean­ ing with 300 ml of 6N hydrochloric acid, and re-equilibra­ tion with 400 ml of 2. 2N hydrochloric acid. The tops of the columns were fitted with ground glass joints to permit attachment of 500 ml Pyrex resevoirs. Sample solutions were loaded into the resevoirs, and when they had drained to the level of the resin, the apparatus was rinsed with

0.IN hydrochloric acid to wash adhering cations onto the resin.

Aluminum, iron, and other cations were eluted with 500 ml of 2.2N hydrochloric acid, and zirconium and hafni­ um were recovered by elution of 150 ml of 6N hydrochloric acid. This fraction was evaporated to a volume of about 10 ml, and one or two drops of 50% sulfuric acid (less than 0.5 ml) were added. The solutions were then heated to fumes of sulfuric acid. Treatment with sulfuric acid was recommended by Strelow and Bothma (1967) to destroy hafni- um-zirconium polymers of the type:

X X I I - Zr - O - Hf - I I X X where X is an OH~ group or another monovalent anion. Following this treatment, the solutions were diluted to 100 ml with 2.2N hydrochloric acid and immediately passed through resin columns of dimensions similar to those outlined above. The columns were rinsed with about

20 ml of 2.2N hydrochloric acid, followed by elution of 4 00 ml of 4N nitric acid to remove remaining strontium, calcium, barium, and REE. Hafnium was then eluted, in 50 ml fractions, with additional 4N nitric acid. Approxi­ mately 700 ml (total) of nitric acid were required to elute -^^Hf activity from the columns. The three 50 ml fractions with the highest activity were combined and evap­ orated to a volume of about 1 ml in Teflon dishes. Follow­ ing transfer to a 5 ml beaker and addition of a few drops of concentrated perchloric acid, the solutions were evap­ orated to dryness. The residues were loaded on rhenium filaments in a drop of 0.5N nitric acid and analyzed in the mass spectrometer.

Results:

Mass spectrometric analyses of the sample residues in­ dicated that zirconium and hafnium concentrates of suffi­ cient purity to generate stable ion beams were obtained. Interestingly, zirconium emission was obtained at a signif­ icantly lower filament temperature than required to produce hafnium emission. This immediately suggested the possibil­ ity of obtaining both zirconium and hafnium concentrations during the same run. A zirconium spike enriched in ^ Z r was obtained for this purpose, and a suitable procedure 181 was subsequently developed (Owen and Faure, 1974). Preliminary determinations of the hafnium concentra­ tions of a rock standard (GSP-1, Flanagan, 1973) suggested non-equilibration of sample-spike mixtures, and failure to achieve complete sample dissolution with mineral acids.

As a result, an investigation of suitable fusion techniques was initiated as it was thought that sample fusion might overcome these problems.

Fusion Techniques

Compilations of fusion methods suitable for zirconium and hafnium analyses are given by Elinson and Petrov (1969), and Mukherji (1970). The disadvantages of several of these methods, when applied to the dissolution of sili­ cates, has been discussed by Sill (1961), who proposed his own method based on potassium fluoride and sodium pyrosul- fate fusions. However, his procedure was lengthy, and in­ volved the use of large quantities of alkali reagents. Its principal advantages were effectiveness and the potential for fusion of samples weighing up to 10 grams by increasing sample-flux ratios accordingly. Applications for lithium metaborate fusion in silicate analysis have been discussed by Suhr and Ingamells (1966) ,

Yule and Swanson (1969) , Medlin et al (1969) , Ingamells (1970), and Keller and Parsons (1970). The fusion melts do not wet the sides of graphite crucibles, and, as a result, may be poured quantitatively into dilute nitric acid for 182

rapid dissolution. Unfortunately, the method is imprac­

tical for samples weighing much more than 0.1 grams as the

subsequent dissolution of fusion products becomes difficult. In addition, samples cannot be spiked prior to fusion as

graphite crucibles are porous.

Boudin and Deutsch (1970) employed borax fusions of samples and spikes for their hafnium analyses. However, borax flux must be pre-fused prior to use, as it contains water of hydration. Failure to observe this precaution may result in the explosive loss of sample when the mixture is heated. The method does not result in removal of silica, the presence of which, may interfere with the subsequent extraction of hafnium by formation of soluble complexes. Lithium tetraborate flux was recommended for silicate analysis by Biskupsky (1965). The flux is formed by heat­ ing a mixture of sample, lithium fluoride, and boric acid in platinum crucibles to temperatures of about 800 0 C.'

Silica is evolved during the fusion in the form of silica tetrafluoride, and subsequent treatment of the fusion cake with concentrated (97%) sulfuric acid removes boron and excess fluoride as BF^. Advantages claimed for this method were its effectiveness, simplicity, and rapidity. This procedure was adopted for dissolution of silicate rocks.

Phosphate minerals (apatite) were fused with sodium car­ bonate and sodium peroxide in platinum crucibles following a procedure recommended by Cruft et al (1965). 183

Final Hafnium Procedure

Separation of hafnium by cation exchange chromatog­

raphy requires prior removal of interfering anions. This

was accomplished by precipitation of the cupferron group (Hf, Zr, Ti, V, Nb, Mo, Ta, W, Fe+3 , Pd, Ga, Sn, Sb, Bi,

and Po) with a freshly prepared, cold solution of 6% cup­

ferron (Smith, 1938). Iron and titanium, usually the most abundant members of the cupferron group in silicate rocks,

act as carriers for microgram amounts of hafnium and zir­ conium. The REE will not be co-precipitated from solutions of pH less than 0.5 (Howell et al, 194 9; Popov and

Wend'landt, 1954). Hox^ever, REE may be entrained by bulky

precipitates. Quantitative removal of REE was assured by

subsequent cation exchange with 0.5N oxalic acid. Under

these conditions, the cupferrates form anion oxalate com­ plexes and are not retained by cation exchange resin. The

REE do not form complexes and are strongly retained by the

resin.

Apatite almost invariably contains inclusions. Cruft (1966) listed 47 igneous and metamorphic apatites with in­

clusions of various types. If inclusions are predominantly zircon, or other silicates with relatively high hafnium concentrations, they could account for a significant frac­

tion of the total hafnium content. Hydrochloric acid treatment of apatite does not result in dissolution of such silicate inclusions. Therefore, if separate apatite frac­ 184 tions are dissolved by fusion and by acid treatment, two sets of data can be obtained that might potentially define two points on an isochron diagram. Isotopic equilibration of apatite samples and hafnium spike is difficult to achieve because hafnium and zirconium form insoluble phosphates in mineral acid solutions. The problem can be overcome either by fusion of samples and spikes, or by acid dissolution followed by treatment with dilute oxalic acid, as phosphate precipitates are soluble in this reagent.

For apatite dissolution by fusion, advantage was taken of the fact that insoluble phosphate precipitates form when fusion cakes are dissolved. Collection of these precipi­ tates by filtration, followed by re-dissolution with dilute oxalic acid and subsequent removal of interfering anions (oxalate and phosphate) by precipitation of cupferrates was

Outlines of procedures developed for hafnium and zir­ conium analyses are given below:

Silicate Rocks: (1) Fusion of sample and spikes with a mixture consisting of 60% lithium fluoride and 40% boric acid.

(2) Precipitation of cupferrates. (3) Cation exchange with 0.5N oxalic acid to remove REE. (4) Cation exchange with 4N nitric acid to partially sepa- 185 rate hafnium and zirconium.

Phosphate Minerals (Apatite)-Fusion: (1) Fusion of sample and spikes with 5 grams of sodium car­ bonate and 0.1 grams of sodium peroxide.

(2) Collection, by filtration, of insoluble zirconium and hafnium phosphates.

(3) Dissolution of precipitates by washing the filter paper with 50 ml of 0.5N oxalic acid.

(4) Precipitation of cupferrates.

(5) Partial separation of hafnium and zirconium by cation exchange with 4N nitric acid as the elutant.

Phosphate Minerals (Apatite)-Acid Dissolution:

(1) Acid dissolution of sample, spikes, and -^^Hf tracer with 6N hydrochloric acid.

(2) Isotopic equilibration of sample-spike-tracer mixtures in 0.5N oxalic acid. *

(3) Precipitation of cupferrates. (4) Separation of hafnium and zirconium by cation exchange with 0.5N oxalic acid and 4N nitric acid as described above for silicate whole-rocks.

Final Hafnium Procedure-Silicate Rocks: 1. The desired amounts of hafnium and zirconium spikes were weighed into a 30 ml platinum crucible. About 10 p.c of -^^Hf tracer was added, and the mixture was slowly evap­ orated to dryness with an infra-red lamp. 186

2. Samples weighing up to 0.75 grams were fused with

6.2 grams of a flux consisting of a mixture of 40% boric acid and 60% lithium fluoride. One third of the flux was poured into the crucible and evenly distributed by tapping the crucible bottom on a flat surface. An evenly distrib­ uted layer of sample (200 mesh or finer) was added, and then covered with a smooth layer of the remaining flux.

3. A six inch square of asbestos board, with a small diameter hole cut in its center, was placed on a ring stand, two sides of which were covered with a layer of aluminum foil. The covered crucible, supported by a pair of stiff tongs, was placed in the cut-out with only its lower one third protruding. This was done in order to keep the upper portion of the crucible cool during the fusion and thereby prevent loss of sample by creep of the melt. The smallest flame obtainable from a Meeker burner was used to heat the crucible for five minutes. The temperature of the flame was then increased, in three minute intervals, until the sample was melted, followed by application of the full flame of the burner for an additional fifteen minutes. 4. The molton fusion mixture was quenched by immersion of the crucible bottom in a water-filled beaker. The fusion cake was dissolved by addition of 20 ml of concen­ trated sulfuric acid and heating carefully with a Meeker burner. After a period of about thirty minutes a clear solution was usually obtained. Quantitative removal of 187

fluorine was assured by increasing the burner temperature

to the point where fumes of SC>2 were evolved and heating for five minutes. It was not unusual for some solutions to become opaque at this stage. 5. The hot solution was poured into a 100 ml Teflon dish and allowed to cool. The crucible was rinsed with about 20 ml of 6N hydrochloric acid and heated. The sample mush in the Teflon dish was dissolved by carefully adding 50 ml of water. The hot 6N hydrochloric acid in the cruci­ ble was transfered to the Teflon dish, and the mixture was heated for thirty minutes. 6. Following filtration with analytical-grade filter paper to remove insoluble sulfates, the solution was di­ luted to 350 ml with water. 20 ml of concentrated (97%) sulfuric acid was added, and the final volume was adjusted to 4 00 ml by addition of water, as necessary.

7. An ice bath was used to cool the solution to^6 to 7 °C. The cupferron group was then precipitated by addi­ tion of 5 to 10 ml of a freshly prepared 6% solution of cold (6 to 7 °C) cupferron. With constant stirring, the reagent was added until a white flash occurred indicating addition of excess precipitant. After standing in an ice bath for 10 minutes, the cupferrates were collected on a Millipore filter disc by vacuum filtration. The precipi­ tate was washed with cold (6 to 7 °C) v/ater. 8. Employing the apparatus previously described, the 188

precipitate and filter disc were wet-ashed with 5 ml of

concentrated perchloric acid and 40 ml of 7N nitric acid. The powerstat transformer was initially set to 80 volts,

and the flask and adaptor were covered with aluminum foil. Best results were obtained by setting the flask on its side

in the heating mantle with its neck at about 30 degrees to

the horizontal. Initially, the solution was black in color and viscous. After boiling gently for about 30 minutes, its color changed to reddish brown and then to yellow. The

distillation was continued until fumes of perchloric acid

were generated. The current to the heating mantle was in­ creased as necessary to keep the solution boiling during the course of the distillation. The entire process re­

quired about two hours to complete. 9. 3 0 ml of 0.5N Oxalic acid were added to the cool

solution in the flask, and the mixture was heated for 30 minutes. Care was taken to avoid boiling the solutioii as

oxalate complexes may be destroyed.

10. Following filtration, the solution was adjusted to a volume of 50 ml by addition of 0.5N oxalic acid as re­ quired, and was then percolated through a cation exchange

resin column (Pyrex: I.D. = 2.54 cm). The height of the resin column, following cleaning with 3 00 ml of 6N hydro­

chloric acid, and re-equilibration with 400 ml of 2.2N hydrochloric acid was about 15 cm. All ■LOJ_Hf1 ft 1 activity was removed from the columns by elution of an additional 100 ml 189

of 0.5N oxalic acid.

11. The 150 ml fraction eluted from the column was com­

bined with 10 ml of 7N nitric acid and 1 ml of concentrated perchloric acid in a Teflon dish and evaporated to a volume

of about 20 ml. The solution was transferred to a 100 ml boiling flask, diluted with 30 ml of 7N nitric acid, and

distilled as before to fumes of perchloric acid in.order to destroy oxalate complexes. Treatment with boiling perchlor­ ic acid also destroys zirconium-hafnium polymers.

12. About 25 ml of 2.2N hydrochloric acid were added to the cool solution in the boiling flask, and the mixture was heated for fifteen minutes. After cooling, the solution was adjusted to 25 ml by addition of 2.2N hydrochloric acid and was percolated through a cation exchange column. The dimensions of thecolumn were the same as described previ­ ously for the second-stage hafnium separation. Hafnium was eluted with 400 ml of 4.ON nitric acid, followed by collec- 1 Q I txon of additional 50 ml fractions until all xo Hf activity had been recovered (approximately 700 ml total of eluted 4.ON nitric acid). 13. The three 50 ml fractions containing the highest activity were combined and evaporated to a volume of about 2 ml in a Teflon dish. The solution was transferred to a 5 ml beaker, and 1 ml of dilute sulfuric acid (about 0.5N) and a few drops of concentrated perchloric acid were added.

The solution was evaporated to dryness, and the residue was 190 loaded in a drop of 0.5N nitric acid on the side filaments of the mass spectrometer.

Procedure-Apatite (Fusion):

1. A mixture of sample powder (finer than 20 0 mesh), weighing up to one gram, plus zirconium and hafnium spikes, and 181 Hf tracer (10uc) were fused, in a 30 ml platinum cru­ cible, with a flux consisting of 5 grams of sodium carbon­ ate and 0.1 grams of sodium peroxide.

2. The fusion cake was partially digested in the cru­ cible with 2.2N hydrochloric acid. Final dissolution was accomplished by transferring the contents of the crucible to a 250 ml Vycor beaker and adding 6N hydrochloric acid as necessary until reaction ceased. The volume of the so­ lution was adjusted to about 150 ml by addition of 2.2N hydrochloric acid. Fluoride was evolved as BFd by addition of 2 grams of boric acid crystals. The solution was bjeated several hours to insure quantitative precipitation of phosphates.

3. Insoluble phosphates were collected on analytical- grade filter paper. The fitrate was rejected. After wash­ ing the filter paper several times with dilute hydrochloric acid (2.2N), hafnium and zirconium phosphates were recover­ ed by washing the filter paper with 50 ml of 0.5N oxalic acid. The oxalic acid fraction was diluted to 300 ml by addition of 220 ml of water and 3 0 ml of concentrated sul­ furic acid. 191

4. Hafnium and zirconium were separated by precipita­ tion of cupferrates, and cation exchange chromatography as described previously for silicate rocks. Cation exchange with 0.5N oxalic acid as the elutant was not necessary as the REE were effectively removed by precipitation of phos­ phates and cupferrates. Procedure-Apatite (Acid Dissolution): 1. Up to 3 grams of sample powder (finer than 200 mesh) , hafnium and zirconium spikes, and 181 Hf tracer were treated with 200 ml of 6N hydrochloric acid in 250 ml Vycor beakers. The resulting solutions were evaporated to volumes of about

50 ml.

2. Spikes, tracer, and sample were isotopically equili­ brated by addition of 150 ml of 0.5N oxalic acid and pro­ longed heating (24 hours). Additional oxalic acid was added periodically to maintain the volume of the solutions at about 200 ml. 3. Following filtration to remove acid insoluble re­ sidues, the solutions were diluted to 300 ml by addition of 30 ml of concentrated sulfuric acid (97%), and 2. 2N hydro­ chloric acid as necessary. 4. Hafnium and zirconium were separated by precipitation of cupferrates and cation exchange chromatography as des­ cribed previously for silicate rocks. 192

3. Spike Calibrations

Spikes enriched in ^^Hf and ^ Z r were obtained in the

form of oxides from the Oak Ridge National Laboratory. They

were dissolved with concentrated hydrofluoric acid in 10 ml

Teflon beakers. Following removal of fluoride ion by fuming with concentrated perchloric acid (hafnium spike) and 50%

sulfuric acid (zirconium spike), the spike residues were re-dissolved and diluted to volumes of 250 ml with 2.2N

hydrochloric acid.

The isotopic composition of both spikes were determined independently and in duplicate (Table 44). Results for the

hafnium spike are in good agreement with Oak Ridge. How­

ever, relative to Oak Ridge, the zirconium spike was found to be slightly enriched in ^-*-Zr (0.7%). This discrepency

can not be a result of contamination with normal zirconium

as ^ Z r is the enriched isotope. Spike isotope compositions as determined by the author were used for all hafnium and

zirconium analyses.

The concentrations of hafnium (9.93+0.11 ug/g) and of zirconium (93.4+0.4 ug/g) in the spike solutions were estab­ lished independently by replicate isotope dilution analyses using shelf solutions prepared from Johnson, Matthey, "spec-pure" hafnium and zirconium oxychlorides of normal

isotopic composition (Table 45). Each oxychloride reagent was carefully weighed into a separate 10 ml Telfon beaker, and dissolved with dilute (0.1N) hydrochloric acid. The 193

Table 44. Isotopic Compositions of Hafnium and Zirconium Spikes (Atom Percent) Part A: Hafnium Spike (^^Hf enriched) Oak Ridge Nat. Lab., Series NC 159180-159181, Sample 159101

Isotope This Study* Oak Ridge

176 0.165 + 0.005 0.18 + 0.05

177 1.10 + 0.01 1.03 + 0.05

178 3.27 + 0.005 3.26 + 0.05 179 87.07 + 0.015 86.98 + 0.10 180 8.41 + 0.04 8.55 + 0.10

Part B: Zirconium Spike(^^Zr enriched) Oak Ridge Nat. Lab., Series MZ, Sample 157526

Isotope This Study* Oak Ridge

90 5.84 + 0 . 01 6.16 + 0.13 91 90.01 + 0. 035 89.31 + 0.15

92 2.93 + 0.02 3.10 + 0.05 94 1.15 + 0.015 1.28 + 0.03

96 0 .12 + 0.035 0.15 + 0.02

*The errors are half the difference between duplicate determinations. 194

Table 45. Spike Calibrations

Part A: Hafnium-179 Spike Calibrations

Date (179/180) jyj Hafnium (PPM)

11-7-72 2.3932 9.78

11-9-72 2.7436 9.84

3-12-73 1.9656 9.95

3-13-73 1.2509 9.91 10-9-73 1.4209 10.09 10-15-73 1.5822 9.99

Average 9.93 + 0.11 (la)

Part B: Zirconium-91 Spike Calibrations

Date (91/90)M Zirconium (PF>M)

9-19-72 2.7074 93.84

9-27-72 2.7990 92.98 Average 93.4 + 0.4 195 solutions were quantitatively transferred to volumetric flasks, and, following dilution to exactly 250 ml with 2. 2N hydrochloric acid, they were stored in sealed poly­ ethylene bottles.

4. Mass Spectrometry

The isotopic analyses were performed with the Nuclide Corp. (Model 12-90-S) solid source mass spectrometer de­ scribed previously (CHAPTER V). The ion signals were am­ plified by a vibrating reed electrometer (VRE) employing a 1 0 ^ ohm input resistor with critically clamped response.

Signal intensities were recorded on a strip chart. The samples were deposited in rhenium side filaments 0.001 inches thick and 0.0 20 inches wide. The central ionizing filament consisted of a rhenium ribbon of the same thickness as the side filaments, but 0.030 inches wide. New filaments were used for each analysis without precleaning as it was demon- strated several times that neither hafnium nor zirconium emission occurred at filament temperatures in excess of those normally required during analysis of samples. Hafnium and zirconium were determined from the same filament loading when samples had been spiked for both ele- ments. This was possible, because Zr +1 ion beams were gen­ erated at lower filament currents than required for emission of Hf+1 ions. Simultaneous determinations of zirconium, hafnium, and hafnium (176/177) ratios were not attempted, 196 however/ since the zirconium spike contained a small, but measurable quantity of hafnium of unknown isotopic com­ position (see Blank Determinations). In addition, hafnium signal intensities were drastically reduced in the presence of large amounts of zirconium when the primary objective

■j **7 / ■ *1 * 7 was to measure the ( Hf/ Hf) ratio, samples were spiked for hafnium, but not for zirconium. In this way, hafnium concentrations and (176/177) ratios were obtained simultane­ ously, and the chemical processing of the sample was sim­ plified somewhat, because the spike acted as a hafnium carrier.

Zirconium Normal, spike, and normal-spike zirconium spectrums are shown in Fig. 21. Zirconium concentrations were calcu­ lated on the basis of the average value of the (91zr/90zr) ratio. The emitted ion species was Zr+^. A ZrO+ spectrum was never observed. There are no sources of isobaric*inter­ ference at either ^ Z r or ^Zr, but ^ Z r and ^£>Zr are iso_ baric with the corresponding isotopes of molybdenum.

Initially, a molybdenum spectrum was always present during zirconium isotope analyses. This was rather sur­ prising as it should have been quantitatively separated during chemical processing of samples. Subsequently, the original stock of rhenium ribbon used for fabrication of center filaments was exhausted, and replaced by a new sup­ ply. No trace of this element has since been observed 197

Figure 21. Zirconium mass spectra. A. Normal zir­ conium spectrum; B. Sample - Zirconium-91 spike mixtures.

4 NORMAL ZIRCONIUM

r{ * ? • &«■ » - i> hr ;/ , ^ * 'Si 'Si , ,S-^WV'-z i V v v V . . > > . , , , , i. e A ; " V “J- ■'1‘, \ ^ £ w> ’ ,^St ‘ ■'1‘, .'„, ■

FIGURE 21-A 8 6 L 8 6 SAMPLE - SPIKE MIXTURES

! ;t>c: ro :2Lf■"ei -■ :Q, ! .f^jl M Q f“j r | p ” j'j |l~j' : I-''

FIGURE 21 - B 200

suggesting that molybdenium was present as an impurity in

the original stock of rhenium ribbon.

A monoisotopic niobium spectrum is almost always ob­

served during zirconium isotope analyses. The presence of

niobium was also surprising since it should have been re­ moved by cation exchange chromatography with 4.ON nitric,

acid. Niobium signal intensities are initially high, and then decay to low levels, suggesting that it is not present

as an impurity in the filaments. If it were, signal inten­ sities would tend to remain constant or would increase at

any particular filament current setting. Subsequent high

temperature analysis in the mass spectrometer confirmed that niobium was not present as an impurity in the rhenium fila­ ment ribbons. One possible explanation for the presence of niobium is the formation of hydrated Zr-Nb polymers during sample processing of the type: OH OH I I — Zr ---- 0 Nb — I I OH OH

Hafnium

Hafnium analyses were usually performed on the URE 100 MU scale. Typically, ion beam currents produced by 1 to 20 ug of hafnium varied from 1.5 x 10“-*-^ to 1 x lO--*-^ amps. When large amounts of reagent hafnium were analyzed greater sig­ nal intensities were recorded. 50 ug or more of hafnium oxychloride produced in beam currents in excess of 201

3 x 1 0 ~ ^ ‘‘ amps. Hafnium, mass spectrums are shown in

Fig. 22, and it can be seen that resolution of adjacent peaks is excellent.

Hf+1 is the most common species emitted. An oxide spectrum (HfO+) was not observed during analyses of samples with low (less than 20 ug/g) concentrations of hafnium.

However, oxide spectra were observed when large amounts of reagent hafnium (50 ug or more) were analyzed. The pro­ duction of an oxide spectrum is undesirable as fractiona­ tion of hafnium isotopes between Hf+ and HfO+ ionic species may occur. Isobaric interference from -^^Ta and 18 0^ is possible, but was never encountered, permitting determination of hafnium concentrations from the average ) ratio without interference. However, isobaric interference from lutetium and/or ytterbium at mass 176 was a far more seri­ ous possibility. In their presence 17 6 peak height cor­ rections would be required resulting in lower precision and accuracy of (-^^Hf/^^Hf) ratio determinations. In addi­ tion, the presence of even small amounts of these elements at the high filament currents necessary for hafnium analysis could impair resolution of hafnium isotopes as the REE are very efficient ion emitters. Therefore, quantitative re­ moval of lutetium and ytterbium by chemical techniques was required. In almost all cases, this goal v/as achieved as heavy REE signals were seldom observed during hafnium 202

Figure 22. Hafnium mass spectra. A. Hf+ / Re+ , and HfO+ mass spectra; B. Normal hafnium and normal. - -^^Hf spike mixtures. Hf Spectrum (x 10:174x100) Re+ Spectrum 187

178 ; a

HfO Spectrum (x100)

THE OXIDE SPECTRUM IS LABLED RELATIVE TO THE CORRESPONDING Hf* SPECIES.

Figure 22 - A 3 0 2 NORMAL HAFNIUM

r r n NORMAL - SPIKE MIXTURE

Nm ft

~ar CO to

co- ar N s n K -!«- N 10 r S' n._y a... o :

L 204

FIGURE 22-B 205

analyses.

Isotope Analysis Procedure

In the following discussion, a general operational pro­ cedure is outlined for obtaining, and maintaining stable

zirconium and hafnium ion emission in the mass spectrometer. 1. The source focus controls should be pre-set prior to the beginning of the analysis: (a) Ionizing Bias...... 0 to +4 (b) Sample filament (SF) #1 Bias..... 0 to +5 (c) Sample filament (SF) #2 Bias..... -1 to +5 (d) J2...... 6 to 8.5 (e) J3...... 5.5 to 6 (f) J4...... 7 (g) J5...... 6 to 10 .(h) J4-J5...... 4.5 to 5.0 (i) Y-Deflect; Z-Deflect + ; 12-o'clock 2. The Center Filament (CF) current should be slowly raised to 4.0 amps in 0.5 samp increments, over a period of several hours (3 to 4 hours). The source pressure should not be allowed to rise above 1 x 10-^ Torr. Usually, one- half hour is required for the pressure to drop to 5 or 6 x _7 10 Torr after each increase ofthe CF current. 3. When the CF current has been adjusted to 4.0 amps, and the source pressure has stabilized at 5 or 6 x 10“^ Torr, SF current is increased, in 0.5 amp increments, to 1.5 amps. The amount of time required to reach this level is dependent on source pressure. Typically, no difficulty was experienced in maintaining the source at 7 x 10“^ Torr or less.

4. The 10^-2 ohm input resistor and VRE 100 MV scale were selected. 206

5. The source was pre-focused with an ion signal.This

was usually accomplished with either rubidium or strontium as both elements are present on new filaments. If niether

rubidium nor strontium signals were observed, the source was focused with -^K.

6. Zirconium emission begins when the SF current has been raised to 1.7 amps. If a strontium signal is avail­

able, the location of and ^-*-Zr may be inferred by noting the peak-to-peak distance between ^ S r and ®^Sr as

the magnet is swept up mass.

7. Once the source had been focused on either 9 0 Zr or 9^Zr (whichever has the greater intensity) , only slight ad­

justments in SF current were required. The majority of zirconium analyses were made with SF currents of 1.8 to

1.95 amps using the CRE 300-MV scale. The runs were termi­ nated when 4 0 to 50 scans had been obtained.

8. SF current was then set at 2.0 amps, and the mag­ net regulator current was increased to 0.455 Tesla (about

9.5 amps). 9. A rhenium spectrum produced by emission from the CF, was found by sweeping the magnet down mass. The source focus was checked on -^"^Re. The location of the hafnium spectrum was inferred by noting the peak-to-peak distance between ^^^Re and ^®^Re as the magnet was swept down mass.

The absence of a rhenium signal indicates that the source is not properly focused. It should be emphasized that 207 hafnium spectrums were never observed, if rhenium emission had not first been obtained. 10. Initially low hafnium signal intensities were not immediately increased by raising filament current as this usually resulted in loss of the sample. Adequate signal intensities were obtained by gradually raising CF current to 4.2 to 4.6 amps and SF current to 2.1 to 2.3 amps, without allowing source pressure to increase beyond 1.5 x

10 ® Torr. In many instances, source pressures were main-

_7 tamed at 4 x 10 Torr or lower. The run was terminated after accumulation of 40 to 50 -^^Hf and -*-^Hf scans.

11. When the primary objective of the analysis was the determination of ) isotopic ratios, the opera­ tional procedure was modified slightly. The source was pre-focused with -^^Re when the SF had been raised to 1.0 amps. The concentration of hafnium was based on only 10 1 o n i 7 n to 12 scans of Hf and in order to obtain as many scans of -^^Hf and l^Hf as possible. Baseline was estab­ lished below ^ ® H f . In order to minimize the possibility of isobaric interference by ^-^^Lu Or ^"^Yb, the analyses were made at the lowest possible filament temperatures.

5. Calculation of Zirconium Concentration

Zirconium concentrations v/ere computed from the rela­ tionship : 0.1123N + 0.9001 S R = (9) 0.514 6N + 0.0 58 4 S 208

Where: R = The average (^Zr/^Zr) isotopic ratio

N = The number of normal zirconium atoms

S = The number of spike zironcium atoms. The (N/S) weight ratio was calculated from:

9.9001-0.0584R (N/S) = (f) (------) (10) 0.514 6R-0.1123 where (f) is a conversion factor which changes (N/S) atomic

ratios to weight'ratios. (f) is given by:

Atomic weight normal zirconium (f) = (11) Atomic weight spike zirconium

91.2237 (f) = ------= 1 .0029 (12) 90.9624

The zirconium content of the sample is expressed in units of ug/g (ppm) as follows:

N zirconium (ppm) -- = (S) (N/S)WT (13) N1 where N-^ is the weight of sample in grams taken for the analysis. The average (^Zr/^Zr) ratio may also be used as the basis for computing zirconium concentration:

0.1123N + 0.9001S R = ( Zr/ Zr) = ------(14) 0.1711N + 0.0293S Hyperbolic mixing curves illustrating the relationship between (N/S) weight ratios and R are shown in Fig. 23. The ideal (N/S)WT ratio for determining zirconium concentra­ tions, based on the average value of the (^Zr/^Zr) ratio 209

Figure 23. Sample - ^ Z r spike hyperbolic mixing curves. A. Concentrations of zirconium based on the value of the 9-*-Zr/:30zr isotopic ratio; B. Concentrations of zirconium based on the value of the ^^Zr/^^Zr isotopic ratio. 210

PURE SAMPLE

10-0

5-0

PURE SPIKE

0 1-0 2-0 3-0 4.05-0 6 0

91Z r / 90Zr

Figure 23-A. (N/S) 10-0 15-0 50 0 PURE SAMPLE iue 23-B. Figure 5 PURESPIKE Zr 92 Zr 10 211 212

is about 2. The corresponding value of R is 3, indicating

that 91-Zr peak intensities will be three times greater than Q O ^AZr. In practical terms, this means that the analysis would have to be performed on different scales of the VRE necessitating separate baseline corrections for each iso­

tope. As a result, the overall precision and accuracy of

the determination will suffer. If, however, the computed zirconium concentration is based on the average (^Zr/^Zr) ratio, peak intensities of both isotopes will be similar, and the analysis can be carried out on the same VRE scale.

6. Calculation of Hafnium Concentration

The following relationships are used in computing hafnium concentrations: 0.1375N + 0.8707S R ------(15) 0.3522N + 0.0841S Where: R = The average value of the (-^9jjp/180Hf) ratio

N = Number of normal hafnium atoms

S = Number of spike hafnium atoms.

0.8707 - 0.0841R (N/S)w_ = (f) (------) (16) 1 0.3522R - 0.1375 Where: Atomic weight normal hafnium (f) (17) Atomic weight spike hafnium 213

178.4957 (f) = ------= 0.9972 (18) 179.0059

N Hafnium (ppm) = — - = (s) (N/S)wrT, (19) N1

Where: is the weight, in grams, of the sample. The hyperbolic mixing relationship for normal hafnium

and a ^"^Hf spike is shown in fig. 24. An ideal (N/S)WT ratio of about 2 corresponds to a measured (l^Hf/^^Hf) ratio of 1.4. Both isotopes are normally recorded on the

same VRE scale, and baseline is taken above mass 180.

7. Calculation of Spike-Corrections for (-^^Hf/^^Hf) Isotopic Ratios

When a sample is equilibrated with an aliquote of ■^^Hf spike, its hafnium isotopic composition is altered.

The average (^"^^Hf/^^^Hf) ratio obtained in the course of an isotope dilution analysis may be spike-corrected as follows: n176 + s176 + R' R = ------(20) N177 + s177 Where: R = Measured (^^ ^Hf/^^Hf) ratio

n17 6 = Number of normal -^^Hf atoms

n177 = Number of normal ^^Hf atoms

^17 6 = Number of spike ^®Hf atoms curve.

(N/S) 100 50 iue 4 Sample- 179Hf spikeFigure 24.hyperbolic mixing 1-0 PURE SAMPLE 20 179

3-0 Hf I /180 Hf 4.0

PURE SPIKE 5-0 214

7-0 215

s177 = Number of spike ^77Hf atoms R' = Number of radiogenic -*-^^Hf atoms.

For any analysis, R can be measured, Ng7g and ^ 7 7 can be calculated, and S]_7 g and are known. As a consequence, the number of 176nf radiogenic atoms present in the sample may be found from the expression: R- = R(N177 + S177) - (Nl76 + S176) (21) and the corrected value of the measured (^-^^Hf/^^^Hf) ratio is then given by:

Nl76 + R ' (176Hf/177Hf)corr> = ------(22) n177

8 . Blank Determination Hafnium

Several sources of hafnium contamination were identi­ fied. They were:

(1) Procedural Blank - contamination occurring during

chemical processing of samples

(2 ) ^ Z r spike contamination

(3) -^^Hf tracer contamination A procedural blank was determined in the absence of both 181 Hf tracer and 91 Zr spike by processing 179 Hf spike solutions through the entire chemical procedure. Subsequent­ ly the catio exchange columns were identified as the princi­ ple source of hafnium contamination by isotope analysis of a ^-7^Hf spike solution which had only been eluted through a resin column, with 4.ON nitric acid, prior to isotope analysis. The blank thus determined was experimentally indistinguisable from the average total procedural blank (Table 46). All hafnium analyses were subsequently correct­ ed for a total hafnium procedural blank of 0.134 ug/g.

The isotopic composition of the hafnium contaminant in the ^ Z r spike was not known, but its effective concen­ tration was determined by isotope dilution analysis employ­ ing the hafnium shelf solution as a standard. The hafnium concentration found in this way was 0.04 ug per gram of 9-*-Zr spike solution added.

When hafnium and zirconium were determined simultane- ously, the weight of 91 Zr spike solution equilibrated with the sample was always recorded. Hence, a simple correction for hafnium contributed by the ^ Z r spike could be applied:

Hafnium (ug) = Hafnium (ug)™ - Hafnium (ug)Q1 (24) corr. -total Zr *Spike l8iHf tracer, supplied by Oak Ridge National Laboratory is not carrier-free as its radiochemical purity is only about 90%. As a result, a correction for the effective amount of ’dead’ hafnium contributed by equilibration of "JOT Hf tracer with samples was calculated as follows: 217

Table 46. Hafnium Blank Determinations

Run No. Hafnium (ug)

1 0.0679 2 0.2409

3 0.1171 4* 0.1102

Average 0.1340

^Determined by analysis of an 179 Hf spike solution that had been eluted through a cation exchange column with 4.ON nitric acid, but that had not been treated with any other reagents. 218 N N'' =(-) (S’’) - T (26) S WT

Where:

N 11 = Weight of hafnium (ug) in the sample

S 1' = Weight of hafnium (ug) in the spike 181 T = Effective weight of hafnium in the Hf tracer N Normal Hafnium (-) = (------) weight ratio (27) S ^rp Spike Hafnium

i oi The effective hafnium concentration of the Hf tracer was determined by isotope dilution analysis using the

U.S.G.S. rock standard BCR-1 as the hafnium standard. The hafnium concentration of BCR-1 was determined in triplicate

by isotope dilution in the absence of tracer, and corrected

for the procedural blank. The average concentration of hafnium (4.62 + 0.04 ppm) was in excellent agreement with

the recommended value of 4.7 ppm (Flannagan, 1973). Two 181 separate batches of Hf tracer were employed in this in­ vestigation, and their effective hafnium concentrations were: Batch Number 1 : 0.0496 ug/drop of tracer Batch Number 2 : 0.0552 ug/drop of tracer The weight of tracer hafnium (T) added during an analysis was obtained by simply multiplying the hafnium concentration of the tracer by the number of drops of tracer taken for

the analysis. 219

A serious problem would have been encountered had the 181 Hf tracer been used when samples were processed for subsequent determination of their (-^^Hf) ratios, because the isotopic composition of the tracer was not known, nor could it readily be determined. The potential effect of the 181 Hf tracer on the isotopic composition of hafnium in a sample was indicated by analysis of tracer- shelf solution mixtures (Table 47). In addition to hafnium, the tracer was also contaminated with lutetium as evidenced by the intense -^^Lu peak observed during these analyses. It may be of interest to note here, that the absence of lutetium emission during analyses of samples which had been equilibrated with 181 Hf tracer was an excellent demonstra­ tion of the effectiveness of the chemical procedure in quantitatively removing the heavy REE.

1 "7 / - "1 *7 C For °Lu/ Hf age determinations, hafnium concentra­ tions and (-^^Hf/l^Hf) isotope ratio measurements were ob­ tained in the absence of both ^^ H f tracer and ^ Z r spike. In this way determinations of the highest possible pre­ cision and accuracy were obtained, by completely eliminating two serious sources of error. Zirconium Samples were corrected for a procedural blank, and for zirconium contributed by the -^^Hf spike. The procedural blank (8.7 ug) was determined by analysis of a mixture of

^ Z r spike and zirconium shelf solution, of known concentra- 220

Table 47. Isotopic Analyses of Mixtures of l^Hf Tracer and Hafnium Shelf Solution

Isotopic Holden and 8 0 ug Shelf 20 ug Shelf Ratio Walker(1972) 10 Drops Tracer 4 0 Drops Tracer

176/180 0.1477 0.1472 0.1139 177/180 0.5256 0.5229 0.4366 178/180 0.7699 0.7805 0.8315

179/180 0.3921 0.3912 0.4252 176/177 0.2811 0.2814 0.2607

*17 6 peak height corrected for presence lutetium assuming lutetium of normal isotopic composition. 221 tion, processed through the chemical procedure. The.zir- comum content of the 1 7 ' ^HfQ spike was obtained by isotope dilution, and v/as found to be 0.09 ug per gram of spike added.

Analysis of 91 Zr spike solutions that had been eluted through resin columns with 4.ON nitric acid, identified the columns as the principle source of zirconium contamina­ tion. By virture of the large quantity used for each sam­ ple analysis, lithium fluoride reagent was assumed to be a potential source of zirconium contamination. However, XRF analysis indicated that its zirconium content is negli­ gible . Procedural blanks for both hafnium and zirconium may be substantially reduced by washing the resin columns with hydrofluoric acid - sulfuric acid mixtures to remove any insoluble precipitates that may have formed during sample elutions. This type of pre-treatment requires columns made of Teflon or some other acid insoluble plastic. Rela­ tively large samples (up to 0.75 grams) are required for the simultaneous determination of hafnium and zirconium. As a result, the zirconium blank is much greater than the blank observed for hafnium because zirconium concentrations may be as much as 100 times or more greater than hafnium.

If zirconium were determined alone, the procedural blank could be reduced by simply decreasing sample weights taken for analysis to between 0.1 to 0.2 grams. 222

9. Results Hafnium and Zirconium Determinations

The chemical procedure was tested by analysis of a

suite of rock standards (Table 48). All determinations were corrected for procedural blanks, and spike and 181 Hf tracer blanks as required. The procedural blank corrections constituted from 1.0 to 10% of the total hafnium concentra­

tions of the standards, while corresponding zirconium blanks represented from 1.4 to 16.8% of their zirconium contents.

Previous determinations of hafnium in these standards were obtained by instrumental or radiochemical neutron activation, or by spark source mass spectrometry (Fleischer, 1969;

Flanagan, 1969; Rebagay and Ehmann, .1969; Borchardt et al, 1972; Hahn, 1972). Zirconium concentrations in these stan­ dards have been determined by a variety of methods including x-ray fluorescence, neutron activation, spark-source mass

spectrometry, optical spectroscopy and colorimetry (Fleischer, 1969; Flanagan, 1969). However, determinations of hafnium and/or zirconium by isotope dilution have never been report­ ed for these standards. Leary et al (1973) determined Zr/ Hf ratios in GSP-1, and in S-l by electron impact mass spec­ trometry of volatile chelates.

Replicate analyses for W-l, BCR-1, GSP-1, and S-l in­ dicated the reproducibility of hafnium and zirconium deter­ minations. The average variability of replicate hafnium analyses, expressed as one standard deviation, ranged from Table 48. Concentrations (PPM) of Hafnium and Zirconium in Rock Standards

Part A: Hafnium Concentrations (PPM)

This Previous Determinations Standards Study Average Range References

G-l (Granite) 5.87* 6.0 5.1-6.0 Fleischer(1969)

5.2 --- Flanagan(1973)

5.8 5.48-6.05 Brooks(1968)

4.2 --- Rebagay and Ehmann (1970) 5.9 5.8-6.0 Hahn(1972)

G-2 (Granite) 7.47* 7.5 6.1-7.8 Flanagan(1969)

7.30 7.35 --- Flanagan(1973)

Average: 7.39 + 0.07 7.7 7.58-7.91 Brooks(1968)

5.8 --- Rebagay and Ehmann (1970) Table 48 - Continued

This Previous Determinations Standards Study Average Range References

GSP-1 (Granodiorite) 14.4 12.4 9.7-15.0 Flanagan(1969) 14 .5* 15.9------Flanagan(1973)

14.4* 16.7 16.12-17.38 Brooks(1968)

15.3* 17------Rebag ay and Ehmann(197 0) Average: 14.7 + 0.4

AGV-1 (Andesite) 4.69* 5.1 3.7-6.6 Flanagan(1969)

5.65 5.2 -- Flanagan(1973)

Average: 5.17 + 0.48 6.5 6.41-6.61 Brooks(1968) 4.1 Rebagay and Ehmann(1970) Table 4 8 - Continued

This Previous Determinations Standards Study Average Range References

W-l (Diabase) 3.05 2.0 0.93-3.0 Fleischer(1969)

2.83 2.67 -- Flanagan(1973)

2.75* 2.2 2.18-2.21 Brooks(1968) 2.25* 3.0 -- Rebagay and Ehmann(1970) Average: 2.7 2 + 0.34 2.45 2.6-2.3 Hahn(1972)

BCR-1 (Basalt) 4.63 4.4 3.3-5.4 Flanagan(1969)

4.57 4.7------Flanagan(1973) 4.65 5.4 5.27-5.52 Brooks(1968)

Average: 4.62 + 0.04 2.8------Rebagay and Ehmann(1970) 225 Table 48 - Continued

This Previous Determinations Standards Study Average Range References

S — 1 (Syenite) 66.5* 94.5 “■ * Brooks(1968) 66.3*

65.5*

Average: 66.1 + 0.5

PCC-1 (Peridotite) 0.04 0.06 0.05-0.08 Flanagan(1969)

0.06 Flanagan(1973)

0.08 0.06-0.09 Brooks(1968)

0.03 Rebagay and Ehmann(1970)

* Corrected for presence of hafnium in the -^l^f tracer. Errors reported for duplicate determinations are half the difference of the two measurements. All

other errors are one standard deviation. 226 Table 48 - Continued Part B: Zirconium Concentrations (PPM)

This Previous Determinations Standards Study Average Range References

G-l (Granite) 206 210 101-220 Fleischer(1969)

216 201-231 Hahn(1972)

219 Rebagay and Ehmann(1970)

G-2 (Granite) 324 316 250-400 Flanagan(1969) 297 300 --- Flanagan(1973)

Average: 311 + 13.5 393 --- Rebagay and Ehmann(1970)

GSP-•1 (Granodiorite) 598 544 270-685 Flanagan(19 6 9)

621 500 --- Flanagan(1973)

570 645 — — Rebagay and Ehmann(1970)

590 LZZ

Average: 595 + 21 Table 48 - Continued

This Previous Determinations Standards Study Average Range References

AGV-1 (Andesite 221 227 186-315 Flanagan(1969)

228 225 Flanagan(1973)

Average:225 + 4 213 Rebagay and Ehmann(1970)

W-l (Diabase) 101 100 34-120 Fleischer(1969)

109 105 Flanagan(1973)

101 104.5 103-107 Hahn(1972)

Average:104 + 5 110 Rebagay and Ehmann(1970)

BCR-1 (Basalt) 178 185 144-275 Flanagan(1969)

185 190 Flanagan(1973)

182 184 Rebagay and Ehmann(197 0) Average: 182 + *3.5 Table 48 - Continued

This Previous Determinations Standards Study Average Range References

S-l (Syenite) 3145 3030 2444-3900 Sine et al(1969)

Errors reported for duplicate determinations are half the difference of the two determinations. All other errors are one standard deviation. 229 230

0.8% for S-l to 12.5% for W-l, while corresponding errors for zirconium were 1.9 5% for BCR-1 to 4.4% for W-l. In

the case of both elements, errors were inversely related to concentration as would be expected.

An estimate of the accuracy of hafnium and zirconium determinations was obtained by comparing averages for

replicate analyses with previous determinations for G-l, G-2, AGV-1, W-l, and BCR-1 reported in the literature.

For these standards, the accuracy, represented by the per­

cent deviation between previous determinations adn those obtained during this study were less than 1.4% for hafnium and less than 2.2% for zirconium

The greatest discrepency between previously reported concentrations of hafnium and zirconium in these standards and those reported here were noted for GSP-1 and S-l. The preferred hafnium content of GSP-1 is 15.9 ppm (Flanagan, 1973). However, in an earlier compilation, Flanagan (1969) recommended a hafnium concentration of only 12.4 ppm based in part on a radiochemical neutron activation (RNA) deter­ mination of 9.7 ppm privately communicated by Brooks. Subsequently, Brooks (1968) reported 16.7 ppm hafnium in GSP-1. Rebagay and Ehmann (19 69) reported 17 ppm hafnium in GSP-1 by radiochemical neutron activation, but their hafnium determinations for G-l, G-2, AGV-1, and BCR-1 are considerably lower than the presently recommended values

(Flanagan, 1973), and similar determinations by Brooks 231

(1968). It is concluded, therefore, that previous hafnium determinations of GSP-1 are not definitive. Flanagan (1969) listed zirconium determinations for

GSP-1 by 20 investigators. Although the values ranged from 27 0 to 63 5 ppm, and the average of determinations in excess of 500 ppm (15) was 560 ppm, Flanagan (1973) recommends a zirconium concentration of only 500 ppm. The basis for this latest compilation is not known as individual analyses were not tabulated.

Considering the accuracy achieved for other rock stan­ dards by isotope dilution, it is concluded that concentra­ tions of hafnium and zirconium reported here for GSP-1 are the most reliable presently available. Hafnium data for the syenite S-l are apparently limited to the single determination by Brooks (1968) who reported 94.5 ppm. This standard was analyzed three times by iso- tope dilution, and the mean of 66.1+0.5 ppm was significant­ ly lower than the value of Brooks (1968). The reason for this discrepency is not obvious, but it may be a reflection of the inhomogeniety of the original sample powder distri­ buted by the Canadian Society of Applied Spectroscopy (Sine et al, 1969). A compilation of zirconium determina­ tions by Sine et al (1969) exhibit a 38% variation about their mean, but the concentration of zirconium reported here is within 4% of that value.

It is concluded, on the basis of the foregoing dis- 232 cussion, that hafnium and zirconium concentrations can be determined by isotope dilution analysis with precision and accuracy at least equivalent to that obtainable by other available methods. A paper reporting these results is in press in Analytical Chemistry (Owen and Faure, 1974).

Determinations of ) Isotopic Ratio The precision of (■^^Hf/^'^Hf) isotopic ratio measure­ ments was estimated from replicate analyses of reagent- grade hafnium oxychloride (Table 49). On the basis of six determinations, the precision of the measurement is better than + 1% (la). The accuracy of these determinations is indicated by the excellent agreement of the average (l^Hf/ 17 7Hf) ratio of 0.2.816 with values reported by other inves­ tigators .

10. Separation of Hafnium from Zirconium

Originally, it was intended to effect partial separa­ tions of hafnium and zirconium by cation exchange chromato- 1 Q1 graphy with 4.ON nitric acid, using Hf tracer to identify the hafnium elution peak. However, after the magnitude of the tracer hafnium blank was established, its use was dis­ continued. Although subsequent hafnium isotope analyses, in the presence of zirconium, were adequate, hafnium ion beam intensities could be increased by removal of zirconium.

Simple modifications of the hafnium procedure, which can result in the almost quantitative removal zirconium are discussed below. Table 49. Summary of Hafnium Shelf Solution (176/177) Isotope Ratio Determinations

G.E. Chart of Boudin and White et Ohio State University the Nuclides (1972) Deutsch (1970) al (1956)

0.2823 0.2821

0.2771

0 . 2828

0.2848

*0.2806

0.2816 + 0.0026 (la) 0.2811 0.2817 + 0.0022 0.2807 + 0.001

*Calculated from a ^^Hf spike calibration run. 233 234

Separation of hafnium and zirconium can be effected either by ion exchange chromatography or liquid-liquid extraction. Compilations of available methods are given in Elinson and Petrov (1969), and in Mukheiji (1970). The most efficient techniques developed to date are: 1. Anion exchange in sulfuric acid media with

AG1-X8 resin (Strelow and Bothma, 1967),

2. Cation exchange in a nitric acid - citric acid media with AG50W-X8 resin (Benedict et al, 1953),

3. Anion exchange in hydrochloric acid media with Dowex-2 resin (Huffman et al, 1951),

4. Extraction with Tri-n-octylamine (TNOA) (Cerrai and Testa, 1959a, 1959b).

The procedure developed by Strelow and Bothma is in­ sensitive to anion interferences of the type previously discussed. The other procedures are very sensitive to anion interference, and, as a result, require careful control of sample solution compositions. The principal advantage of ex­ traction with TNOA is that 75% of the zirconium present is transferred from the aqueous phase to the organic phase af­ ter a contact period of only 15 minutes I Removal of zir­ conium by anion exchange with sulfuric acid elutant, or by

TNOA extraction can be incorporated into the hafnium pro­ cedure with little difficulty. However, of the two methods, extraction is recommended for future work, because this procedure is simple, rapid and effective, and relatively in­ 235 sensitive to contamination as new reagent is used for.each analysis.

Hafnium Procedure Modification

Anion Exchange Chromatography:

1. Proceed, as described previously for silicate rocks, to destruction of cupferrates.

2. Wet ash the filter disc and cupferrates with a mix­ ture consisting of 5 ml of 50% sulfuric acid, and 40 ml of 7N nitric acid by distillation to fumes of SC^ •

3. Dilute the solution to 75 ml with water, and immedi­ ately pass through a column of BIO-RAD AG1-X8 anion resin

(200-400 mesh). Suitable column dimensions, and cleaning and re-equilibration data are given by Luke (1968).

4. Elute hafnium with 3% sulfuric acid (Luke, 1968) . Initially, it may be necessary to calibrate columns with 181 x Hf tracer, but subsequent separations should not require tracer as elution peaks for hafnium and zirconium are high­ ly resolved. Alternatively, a method employed by Luke

(1968) , involving cupferron precipitations of eluted hafni­ um and zirconium carrier solutions, may be utilized for column calibration. REE are not retained by the resin, and are, therefore, quantitatively removed prior to re­ covery of hafnium.

5. Precipitate hafnium with cupferron in the presence of iron carrier, as recommended by Luke (1968).

6. Destroy cupferrates as usual, and remove iron by 236 cation exchange chromatography with 2.2N hydrochloric acid.

Extraction with TNOA: 1. Proceed, as described previously for silicate rocks, to filtration of fusion cake solutions.

2. Dilute filtrate to 200 ml with water and precipitate phosphates by addition of a freshly prepared 10% solution of disubstituted ammonium phosphate (Elinson and Petrov, 1969, page 55). Co-precipitation of Ti+^ , Nb+5 f and Ta+5 is desirable as these elements will serve as hold-back carriers for hafnium during teh TNOA extraction.

3. Collect phosphate precipitates by filtration, re­ dissolve with 0.5N oxalic acid, and precipitate cupferrates as described previously for analysis of apatite.

4. Wet ash the filter disc and cupferrates with 50 ml of 6N hydrochloric acid and 3 ml of 7N nitric acid by dis­ tillation to a volume of about 5 ml. * . 5. Dilute the solution to 100 ml by addition of 8N hydrochloric acid, and cool in an ice bath to 6 to 7°C. 6. Prepare 100 ml of 0.2N TNOA in cyclohexane and cool, in an ice bath of 6 to 7°C. 7. Extract zirconium into the organic phase by a 15 minute contact of the sample solution and TNOA-cyclohexane mixture in a sepatory funnel. Discard the organic phase.

8. Evaporate the aqueous phase to 10 ml, dilute to

50 ml with water, and separate Ti + 4 by cation exchange 237 with 2.2N hydrochloric acid.

4 CHAPTER VII

MEASUREMENT OF THE CONCENTRATIONS OF RUBIDIUM AND STRONTIUM, AND OF THE 87s R/86s r RATIO 1. Rubidium and Strontium Determinations

Concentrations of rubidium and strontium were obtain­ ed simultaneously by a rapid, non-destructive, x-ray fluores­ cence (XRF) procedure similar to those described by Eastin

(1970) and Gunner (1971). The relationship between inten­ sity of the characteristic x-radaition and concentration of an element (z) was given by Reynolds (1963) as:

MXI7K = Z (ppm) K (28) a Where:

vX ~ Mass-absorption coefficient for wavelength of Compton scattered portion of incident radiation,

I„K = intensity (cps) of secondary characteristic ka a spectral line, Z(ppm) = concentration, in parts per million, of element Z,

K = instrumental proportionality constant. Reynolds demonstrated that the mass absorption coeffi­ cient is inversely proportional to the intensity of the in- coherantly scattered (Compton radiation) portion of the in­ cident x-ray beam. Therefore, the relationship between concentration and intensity of an elemental Ka spectral line, excited by molybdenum Ka radiation, may bp re-written 238 239 as :

(29)

The corresponding expressions for rubidium and strontium are:

^RbK /IMoK ) = (Rb(ppm)) (Kj) (30) a ac

^ S r K ^ M o K ^ = (Sr(ppm)) (K2) (31)

These equations are in the form of straight lines, with slopes equal to the instrumental constant K.

Concentrations of rubidium and strontium were deter­ mined by comparing intensity ratios (IZk ^ M o K ^ unknowns a ac with intensity ratios of standards for which concentrations of these elements have been established by other methods. This was accomplished by means of calibration curves derived from XRF analyses of a suite of U.S.G.S. rock standards

(Table 50). Plots of intensity ratios as functions of ru- bidium or strontium concentrations in these standards are linear, and least-squares regression analysis, yields the value of the instrumental constant K, and the value of the Y-intercept (Fig. 25). The linear equations thus derived permit calculation of rubidium and strontium concentrations for the unknowns. X-Ray Fluorescence Procedure Pellets, consisting of 3 grams of sample powder (less than 200 mesh) and a backing consisting of 7 grams of AGV-1

BCR-1

GSP-1

GSP-1 CALIBRATION DATA Date KRb KSr G-2 8/16/73 0-00122 000164 9/26/73 0-00135 0-00175

r AGV-1 BCR-1 0 100 200 300 400 500 600 700

CONCENTRATION (PPM) 240

Figure 25. XRF calibration curve for rubidium and strontium. 241

Table 50. Rubidium and Strontium XRF Calibration Standards

Standard Description Strontium(PPM) Rubidium(PPM)

GSP-1 (B) Granodiorite 233 254

AGV-1 (C) Andesite 657 67

BCR-1(C) Basalt 330 46.6

G-2(B) Granite 479 168

Reference: Flanagan (1973). 1. Several pellets of each standard were available. The bracketed suffix identified the particular pellet employed for all analyses. 242 reagent-grade boric acid crystals were formed in a cylindri­ cal mold by compression with a London hydraulic press. A pressure of 12 tons per square inch, applied for 2 minutes, produced pellets of uniform dimensions (3.2 cm diameter;

0.8 cm thick). Similar pellets of rock standards were used to establish calibration curves.

XRF analyses were made with a Diano Corp. XRD-6 spec­ trometer. The instrument was equipped with a pulse height discriminator, lithium fluoride analyzing crystal cut parallel to 220, and a molybdenum target x-ray tube. The operating parameters are summarized in Table 51. One hour prior to the start of the analyses, the various electronic systems were turned on and stabilized. The detector was peaked on SrK radiation (35.85° 20) generated by a limestone pellet containing a high strontium concentration. Rubidium and strontium intensity ratios were obtained for each pellet in duplicate. Samples and standards were analyzed alter­ nately. Variations in measured peak intensities resulting from instrument drift were controlled by designating one sample as the internal standard, and re-analyzing it peri­ odically. The time-dependent peak intensity fluctuations observed for the internal standard were used to correct similar variations in the other pellets to a nominal level

(Fig. 26). The average precision of rubidium and strontium concentrations, as estimated from replicate analyses of in­ ternal standards, and expressed as one standard deviation, 243

SrKoc RbK MoK MoK <*C 1-5 SAMPLE D jih 1-4b -rr^.---- o — ------O--- —- o- 0-60 o 1.3 0-50 Sr 12 0-40

1-1 0-30

1-0 0-20

2 8 0.50

2-7 SAMPLE E 0.40 _RJ2_ 1 _ - Q ------Q ----- O — ■O — -o — 2-6 0.30

2-5 0-20 Sr ------— Q — — O- -o - - -o 2-4 - 0-10 O •V 2-3 _L 2 3 4

TIME (HOURS)

Figure 26. Time-dependent peak intensity fluc­ tuation corrections for XRF analysis. Table 51. Operating Parameters for Determination of Rubidium and Strontium by X-Ray Fluorescence

Parameter Description

Target Tube Molybdenum-operated at 65 KV and 55 Ma

Incident X-radiation Mo Ka Analyzing Crystal Lithium Fluoride(220); 2d = 2.848°A

Colimator 0.010 Soller Slit Detector Thallium activated, sodium iodide scintillation crystal

peaked on SrKa (35.85°26)

Pulse Height Discriminator e= 0.30V; Ae = 8 .6V

Compton Peak MoKac at 30 . OO°20

Strontium: Peak 200 second count at 35.85°26

Baseline 100 second count at 35.20° & 36.50°29

Rubidium: Peak 200 second count at 37.S9°26 «?- 244 Baseline 100 second count at 37.40° & 38.58°20 245 were + 3.7%, and + 0.8% respectively. These errors include drift which was removed from the other samples. .The re­ producibility of strontium concentrations by this method, expressed as one standard deviation, has been estimated to be better than + 3% at the 90% confidence level (Boger, personal communication).

2. Determination of the Isotopic Composition of Strontium

A cation exchange chromatographic procedure was employed to extract strontium from samples for isotope analysis by solid source mass spectrometry. The procedure consisted of the following steps:

1. Acid dissolution of sample powders (finer than

200 mesh).

2. Extraction of strontium by cation exchange monitor- 8 9 ed with 50.5 day Sr tracer. 3. Automated strontium isotope analysis on a solid- source mass spectrometer, described later.

Reagents and Apparatus

Cation exchange chromatography was carried out in Pyrex columns (I.D. = 2.54 cm) with BIO-RAD AG50W-X8 sulfonated polystyrene resin. After washing with 300 ml of 6N hydro­ chloric acid and re-equilibration with 400 ml of 2.2N hydro­ chloric acid, the height of the resin columns was about

27 cm. All reagents were prepared as described in CHAPTER

V. 246

Sample Dissolution

Silicate sample powders (finer than 200 mesh), weigh­ ing about 0.2 grams, were dissolved with 20 ml of concen­ trated hydrofluoric acid and 5 ml of 50% sulfuric acid in

100 ml Teflon dishes. The solutions were covered and heated overnight. Following expulsion of excess hydro­ fluoric acid, the residues were re-dissolved with about

20 ml of 2.2N hydrochloric acid. Insoluble sulfates were removed by filtration, and a few drops of 8 9 Sr tracer were added to the filtrates. The final volume of the sample solutions were adjusted to 50 ml by dilution with water as necessary.

Non-silicate sample powders (finer than 200 mesh), weighing about 0.2 grams, were dissolved in 250 ml Vycor beakers with 6N hydrochloric acid, and a small amount of

8N nitric acid (about 10 ml). Carbonate samples were first partially digested with 0.IN' hydrochloric acid in order to minimize splattering. The solutions were evaporated to volumes of about 10 ml, and following removal of insoluble 8 9 residues by filtration, a few drops of Sr tracer were added to the filtrates. The final volume of mineral solu­ tions were adjusted to 50 ml by dilution with water.

Cation Exchange Chromatography Strontium concentrates of adequate purity were obtain­ ed by passing sample solutions through the cation exchange resin columns twice. Approximately 650 ml of 2.2N hydro- 247 chloric acid were required to elute all ^ S r activity- from the columns. On the first pass, 525 ml of 2.2N hydrochloric acid was eluted followed by collection of 15 ml fractions until all activity had been recovered. The six fractions with the highest activities were combined, and evaporated to dryness in Vycor beakers. The residues were re-dissolved with 15 ml of 2.2N hydrochloric acid, diluted to 50 ml with water, and percolated through clean resin columns. Stron­ tium was eluted as described above. The 15 ml fraction with the highest activity, and the fraction immediately following were combined, and evaporated to dryness in 30 ml Vycor dishes. The residues were transferred to 5 ml beakers with dilute hydrochloric acid, and evaporated to dryness. Organic residues were oxidized by heating with a few drops of concentrated perchloric acid.

Mass Spectrometry Strontium isotope analyses were performed in a Nuclide

Corp. Model 12-90-S, triple filament, solid source mass spectrometer. The instrument was equipped with a 1 0 ^ ohm collector resistor, and a vibrating reed electrometer. The samples were loaded on pre-cleaned rhenium filaments in a drop of dilute (0.5N) nitric acid. Data collection was con­ trolled by an automated system consisting of a General Auto­ mation Inc. Model SPC-12 computer, and Nuclide Corp. DACS-

III interfacing and software. During typical strontium isotope analyses, strontium ion beam currents of 10-^ amps were generated at source

pressures of about 5 x 10”^ Torr with center and side fila­ ments currents of 3.2 to 3.8 amps, and 0.2 amps, respective­ ly. The computer, controlling the magnet regulator, swept

the strontium mass spectrum, and outputed, via a teletype, O Q peak intensities, expressed relative to °Sr, corrected

for baseline, and for isotope fractionation assuming the

86Sr/88Sr ratio is equal to 0.1194.

The average value of the corrected 8^Sr/88Sr ratio for

each sample was based on a minimum of 2 80 scans of the strontium spectrum. The precision of the measured ratios,

expressed as one standard deviation, was in most cases better than + 0.00010. The reproducibility of 87sr/88Sr

ratio determinations is indicated by the results for 11 analyses of the Eimer and Amend SrCO^ standard of 0.7 0794 + 0.00053 (la) summarised by Ikpeama et al (1974). CHAPTER VIII AGE DETERMINATIONS

1. Introduction

Carbonatite complexes are favorable targets for -^®Lu/ 176Rf age determinations, because they are unusually enrich­

ed in REE. For example, the Mountain Pass complex of Cali­

fornia is the world's largest source of rare earth metals (Heinrich, 1966). Methods developed for the determination of lutetium and hafnium were used to date a suite of rocks

and minerals from the Lackner Lake Carbonatite Complex lo­ cated in Northern Ontario, Canada. The only previously re­ puted age determination for this complex was a K/Ar date cf

1090 M.Y. by Lowdon et al (1963). It was based on analysis of biotite separated from nepheline syenite. The time of

emplacement of Lockner Lake was confirmed by the Rb/Srv iso- chron method to permit the accuracy of ^^^Lu/^-^^Hf age

determination, and the value of the half-life of ^ ® L u to be estimated.

2. Geology of the Lackner Lake Carbonatite Complex At least four distinct periods of carbonatite emplace­ ment have been recognized in Eastern Ontario at 125, 565,

1075, and 1700 million (Gittins et al, 1967). Lackner Lake belongs to the 1075 M.Y. group. The locations of carbonatite

249 250

complexes in Eastern Ontario are shown in Fig. 27. The

Lackner Lake Complex is located in portions of McNaught and Lackner Townships, Sudbury Mining District, near Chapleau

in northern Ontario. It is about 14 mile east of Chapleau, and 7 miles north of the Nemogas station on the Canadian Pacific Railway. Access is by unimproved roads beginning

at Nemogas. The complex is a prominant circular topographic

feature that rises several hundred feet above the surround­ ing Precambrian terrain.

The geology of the complex has been described by

Hodder (1961) , and Parsons (1961). It is a ring-complex

consisting of circular and inward dipping sheets of foliated

alkaline silicate rocks enclosed in nepheline syenite, and intruded into gneissic country rock of Superior Province

age (about 2500 M.Y.). A smaller circular mass of yolite, about 4000 feet in diameter, is located adjacent to the

northeastern portion of the Lackner Complex, and is known as the Portage Complex. A generalized geologic map, after

Parsons (1961) is shown in Fig. 28. The Lackner Lake Complex has been investigated by sev­ eral mining companies as a possible source of niobium (pyro- chlore), phosphate (apatite), and iron (magnetite). A con­

siderable amount of diamond drilling and quarrying was carried out, but all activities were abandoned by 1968. The general composition of the most important alkaline

silicate rocks characteristic of niobium-bearing complexes 251

Timmins \

Lackner L. Superior Lake •5^ L. Nipissing Ottawa' Montreal Huron

Toronti miles

Figure. 27. Carbonatite complexes in Eastern Ontario .(Gittins et al, 1967). Carbonatite complexes are denoted by the symbol (*). IOOO 2000, 5000 « n « -i - i t

- . - . 1 ~-

* r

.■S'-'.’/./yA/.y-J

LACKNER LAKE vV,V •.*. » ••.’ ■•••I1

•—

\ + | CARBONATITE NEPHELINE SYENITE fw il IJOLITE, MALIGNITE, GNEISS L^ 1 SYENITE (FOLITED) • •• * • • •• IJOLITE, IJOLITE |/^ | FAULTS

5RECCIA 252 GITTENS (1966) A FTER PARSONS (1961) Figure 28. Geologic map of the Lackner Lake Carbonatite Complex 253 of Eastern Ontario are shown in Fig. 29. According to

Parsons (1961), the sequence of events resulting in the for­ mation of these complexes were:

1. Drilling of a central diatreme by explosive gases, with possible concurrent ejection of volcanic rocks.

2. Alkaline metasomatism of the country rock. 3. Carbonatization of volcanic rocks in the neck, and/ or emplacement of carbonatites in the central diatreme and ring-like adjacent zones. 4. Emplacement of nepheline syenite. Hodder (1961) in his more detailed study of Lackner

Lake, proposed a somewhat different sequence of events.

1. Formation of ring fractures, and metasomatic altera­ tion of country rock by release of alkali fluids.

2. Intrusion of nepheline syenite. 3. Metasematism of country rock, trapped between con­ centric intrusions of nepheline syenite, to malignite.* 4. Metasomatism of nepheline syenite to ijolite by instrusion of fluids, enriched in iron, and carbonate, that ultimately crystallized as magnetite-apatite ore bodies, and carbonatite. Hodder (1971) discussed several hypotheses that have been proposed to account for the formation of alkali com­ plexes : 1. Fractionation of basaltic or peridotitc magmas. 2. Desilication of a silica saturated magma by ossimi- 254

MAFIC MINERALS MOSTLY Na-Fe A V PYROXENE) f

NEPHELINE

Figure 29. Composition of important alkaline silicate rocks (Parsons, 1961). 255

lation of limestone. 3. Intrusion of a carbonate magma contaminated by desilication of country rock. As there are no known occurrences of Precambrian lime­ stone in the region, Hodder (1971) concluded that the magma, which gave rise to the alkali rocks of the Lackner Lake Com­ plex was formed by fractionation of a parent magma of either basaltic or granitic affinities. He suggested that frac­ tionation may have lead to firmation of an aqueous liquid consisting of an upper phase enriched in water, and calcium, an intermediate layer enriched in iron, and a basal alkali-rich layer. Strontium isotopic evidence sug­ gests that the parental magma was derived from a mantle source as the initial ^ S r / ^ S r ration of carbonatite from

Lackner Lake is 0.7032 (Powell, 1965). A more precise ini­ tial ratio of 0.7 0274 was determined from a Rb/Sr isochron obtained during this study.

3. Sample Descriptions

Core samples of carbonatite, magnetite-apatite ore, ijolite, and various types of nepheline syenite were col­ lected by the author at the remains of the Multi Minerals core shack located adjacent to Camp Lake. These samples were unalteredA and, in the case of carbonatite, represent the only source of material as the principle occurrences of carbonatite at Lackner Lake are subsurface dikes. Un­ altered samples of magnetite-apatite ore, and ijolized 256 nepheline syenite were obtained from outcrops exposed by the quarrying operation in the No. 6 orebody. A brief descrip­ tion of samples analyzed is provided in Table 52.

4. Sample Preparation

Whole-Rocks Samples weighing approximately 100 grams were crushed in an iron mortar, and subsequently reduced to a fine pow­ der in a Spex ball mill. Each sample was then screened through 200 mesh sieves to insure uniformity of grain size, transferred to glass bottles, and thoroughly homogenized on an electrically operated roller table.

Mineral Concentrates

Apatite-Magnetite: High purity apatite and magnetite concentrates were prepared by a combination of magnetic and heavy liquid separations. A flow chart of the procedure is shown in

Fig. 30. Approximately 10 pounds of one inch-diameter mag­ netite-apatite core were cut into discs of about one inch thickness with a water-cooled saw. The discs were washed with dimineralized water, allowed to dry, and then coarsely crushed in an iron mort r. This material was subsequently reduced to about 100 mesh size with a mechanical grinder, and sieved. Material finer than 120 mesh was discarded, and material coarser than 8 0 mesh was re-ground.

An initial separation of magnetite from non-magnetic Table 52. Sample Descriptions

Sample Description

(A) Apatite Concentrates were prepared from massive magnetite-apatite ore. Samples were obtained in the form of one inch diameter core, and from outcrops exposed by quarrying at the No. 6 ore body. Green apatite grains, averaging about 0.5 mm in diameter, were identified as carbonate species by x-ray dif­ fraction analysis [Caqo (PO4)g(CO3)H2O]. Microscopic exami­ nation indicated inclusions were abundant, but only zircon could be positively identified from its characteristic habit, Intergrowths of magnetite made purification of apatite con­ centrates difficult.

(B) Carbonatite Massive, white two-inch diameter core samples, containing more than 95% calcite, with grain sizes exceeding 2 cm, and minor magnetite, apatite, pyroxene, and black garnet (possibly melanite as suggested by Hodder, 19 61) .

(C) Magnetite Samples were as described previously for apatite. The magne­ tite was massive or intergrown with apatite, and concentrates contained trace amounts of pyrrhotite, pyrite, chalcopyrite, and possibly bornite.

(D) Ijolitic Nepheline Samples were obtained from outcrops exposed at the open pit

Syenite on the No. 6 ore body. The rock was mesocratic and contained 257 blebs of nepheline-alkali feldspar intergrowths, ranging in diameter from 4 to 20 mm, and comprising from 35 to 40% of the rock. The dark gray matrix was fine-grained and ijolitic Table 52 - Continued

Sample Description

in composition. This sample may be called ijolitic or trans­ itional nepheline syenite as it belongs to a sequence which varies in composition from magnetite veins - to - ijolite - to - ijolitic nepheline sytenite - to - nepheline syenite (Hodder, 1961).

(Eq) Nepheline Syenite The sample was obtained as two-inch diameter core, and is similar to Parsons' (1961) type 6c nepheline sytenite. It is a coarse-grained, inequigranular, almost leucocratic rock of juvitic composition. Laths of alkali feldspar, up to 1.5 cm in length, and euhedral, rectangular nepheline, up to 6 mm long, are set in a fine-grained matrix of intergrown nepheline and alkali feldspar. The rock contains about 10% aegerinaugite in the form of irregular patches up to 3 mm long. The nepheline has been altered along grain boundaries and fractures to cancrinite, and feldspar are altered to sericite at grain boundaries. Less than about 0.5% magnetite.

(Ep) Alkali Feldspar Separated from Eq by magnetic and heavy liquid techniques. Concentrate Partial separation of feldspar and nepheline was verified by x-ray diffraction analysis. (E3) Nepheline Concentrate 258 (F) Ij olite The sample was obtained as two-inch diameter core. It is an unaltered melanocratic, fine-grained rock consisting of about equal amounts of nepheline and aegerinaugite with minor a- mounts of feldspar laths up to 1.5 mm long (see Hodder,1961). 259

Figure 30. Purification of apatite and magnetite concentrates. 260

SAMPLE

PULVERIZED IN AN IRON MORTAR

MECHANICALLY GROUND TO 100 MESH SIZE

INITIAL SEPARATION WITH HAND MAGNET

APATITE CONCENTRATE MAGNETITE CONCENTRATE

MAGNETIC SEPARATOR REDUCED TO 20 0 MESH SIZE IN BALL MILL DISCARD MAGNETIC FRACTION CONCENTRATED WITH HAND MAGNET MAGNETIC SEPARATOR

HOMOGENIZED TF HEAVY LIQUID SEPARATION

DISCARD INTERMEDIATE FRACTION

APATITE HEAVY FRACTION

WASHED HAND MAGNET

SULFIDES MAGNETITE MAGNETIC SEPARATOR

MAGNETIC FRACTION DISCARDED

MAGNETIC SEPARATOR NON-MAGNETIC FRACTION DISCARDED REDUCED TO 200 MESH SIZE IN BALL MILL

HOMOGENIZED 261

material was accomplished with a small magnet. A sleeve,

seven inches in length, was cut from a plastic bag. The

magnet, held inside the sleeve, was repeatedly swept over small aliquots (about 10 ml) of the ground sample until a-

bout 1500 ml of magnetite, and about 500 ml of apatite were

contaminated primarily with magnetite, in the form of inter­

growth, and with calcite. The primary impurity in the mag­ netite fraction was intergrown apatite.

Approximately 10 0 grams of the initial magnetite con­ centrate was crushed in a Spex ball mill and screened through

a 200 mesh sieve. Small aliquotes were repeatedly swept

with a hand magnet to remove the non-magnetic fraction. The final concentrate was transferred to a glass bottle and

homogenized on a roller table. The apatite concentrate was passed through a Franz mag­

netic separator operated with a forward slope of 30°, a side slope of +18°, and a field current of 0.05 amps. After

completion of the first pass, the magnetic fraction was discarded. The Field current was increased to 0.1 amp, and

the non-magnetic fraction was reintroduced into the separa­

tor. At the end of the second pass the magnetic fraction was discarded, and the non-magnetic fraction was set aside

for heavy liquid separations. A water-soluble heavy liquid, Thallium Formate (TF) (density 3.4), was used to separate apatite (density 3.2)

from magnetite (density 4.2). The highly toxic nature of 262

TF requires stringent safety measures when it is used.

Heavy liquid separations were performed under a fume hood.

Rubber gloves were worn to prevent direct absorption of TF through the skin. About 40-50 ml of TF was poured into a separatory funnel. A small portion of sample (10-15 ml) was added and the mixture was stirred well with a glass rod. The sample was not disturbed for several hours at the end of which time three well-defined layers had formed in the funnel. The lower heavy layer was black and consisted of magnetite. The intermediate layer was comprised of dis­ persed grains of apatite and magnetite. The upper floating layer was green and was essentially pure apatite. The three fractions were isolated by draining the TF through separate filters.

The intermediate and heavy fractions were discarded, and the light fraction was transferred to a porcelain dish.

Enough sample was processed in this manner to yield over

100 grams of purified apatite concentrate. The concentrate was rinsed thoroughly with demineralized water to remove all traces of TF, and dried. Final puri­ fication was accomplished by passing the concentrate through the Franz separator. Two passes were made. For the first cycle, the forward slope was set at 30°, the side slope at

+20°, with a field current of 0.4 5 amps. The magnetic fraction was discarded. The non-magnetic fraction was re­ introduced into the separator at a field current of 1.20 263

amps to separate minor amounts of non-magnetic calcite. The

apatite concentrate was pulverized in a Spex bs.ll mill,

sieved through a 200 mesh screen, aid thoroughly homogenized. Carbonatite:

A three-inch disc was cut from a length of two inch di­ ameter carbonatite core in a water-cooled saw. After being washed with demineralized water and dried, the disc xvas crushed in an iron motar. Following removal of magnetite with a hand magnet, the powdered sample was brushed through a 200 mesh screen with a camel's hair brush. Advantage was taken of the excellent cleavage of calcite and its relative softness, as compared to the impurities which remained

(i.e., pyroxene, apatite, and garnet). The brushing action broke down grains of calcite to sizes small enough to pass through the screen without concurrent reduction of grain sizes of the impurities. About 100 grams of calcite concen­ trate was prepared in this way.

Nepheline and K-Feldspar:

The quantitative separation of nepheline and K-Feldspar is difficult because of the overlap of their densities, and similarity of behavior in a Franz magnetic separator. An aiiquote of homogenized whole-rock nepheline syenite (60-120 mesh size fraction) was split off for the separation of these minerals. A coarse split was made by heavy liquid techniques using Bromoform adjusted to a density of about

2.60. The feldspar-rich fraction floated while the 264

nepheline-rich fraction sank. Both concentrates were re­

covered and subsequently purified by passage through a Franz

separator. The samples were then reduced to 200 mesh size

or finer in a Spex ball mill, and thoroughly homogenized.

The purity of the concentrates was checked by x-ray diffrac­ tion. Based on the relative intensities of nepheline and

K-Feldspar diffraction peaks it was determined that nepheline- rich and K-feldspar-rich concentrates had been obtained.

5. Rubidium/Strontium Geochronology of the Lackner Lake Carbonatite Complex In order to establish the accuracy of lutetium/hafnium age determinations, the age of the Lackner Lake Complex was confirmed by the rubidium/strontium isochron method. A five

point ^ R b / ^ S r isochron, based on analysis of four whole- rocks and one mineral concentrate, was established by least-

squares regression (York, 1966). Analytical data are pre­ sented in Table 53. The complex was emplaced 1094 + 8 M.Y.

ago, and its initial ^ S r / ^ S r isotopic ratio was 0.70274 +

0.00006 (Fig. 31). Powell (1965) reported initial ^^Sr/^^Sr ratios for carbonatite complexes in Ontario (including Lackner Lake) varied between 0.7037 and 0.7019, and concluded that their

low initial ratios indicated that they may have been derived from the mantle. Results of this study also suggest that

the Lackner Lake Complex was formed by intrusion and sub­ sequent crystallization of a mantle-derived magma. The S p S 6 NEPHELINE SYENITE

0.7300

0-7200 IJOLITE

NEPHELINE SYENITE t = 1094 ± 8m-y- 0 7100 K-FELQ 87 = 0-70277 1 000004 8 6

_X-a-PATITE- CARBONATITE

0.7000 0.5 15 2-0 265

Figure 31. Rubidium/strontium isochron for the Lackner Lake Carbonatite Complex. Table 53. Rubidium and Strontium Analytical Data for the Lackner Lake Complex

Sample Sr(PPM) Rb(PPM) 87Rb/86Sr (87Sr/86sr) Corr.*

A 9475.5 0 0 0.70337 + 0.00022

B 20521.8 0 0 0.70279 + 0.00010

D 750.5 485.5 1.872 0.73142 + 0.00010

E1 1381.9 243 .8 0.511 0.71055 + 0.00010

e 2 2108.6 340.5 0.467 0.70982 + 0.00010

e3 1513.1 261.7 0.500 Not Determined F 989.3 323 .2 0.945 0.71731 + 0.00005

*Strontium isotopic ratios have been corrected for fractionation assuming (86Sr/88Sr) = 0.1194.

Errors are reported as half the difference between duplicate determinations. 266 267 error (+ 8 M.Y.) associated with the age of the Lackner

Lake Complex, calculated from the uncertainty of the slope of the regression line, indicates that the Complex has re­ mained a closed system to the migration of rubidium and/or strontium since the time of its emplacement.

Apatite does not fit the isochron. It lies slightly above the isochron indicating the presence of a small incre­ ment of excess radiogenic ^Sr. Apatite apparently remains an open system with respect to migration of strontium. Riley (197 0) reported that complex lithian-rich pegmatites, representing five continents and of widely varying geologic age, all contained phosphate mineals with anomolously high radiogenic ^ S r contents. Apatite is an acid soluble miner­ al which might exchange strontium with percolating ground water. Alternatively, the data can be interpreted as being indicative of a somewhat younger age for magnetite-apatite ore formation, or co-genetic crystallization of apatite from a magma or aqueous fluid whose source region was relatively enriched in rubidium. The data can also be interpreted as evidence for an older age for apatite, because it lies to the left of the isochron. However, geologic relationships indicate that massive magnetite-apatite bodies were formed relatively late in the history of the complex (Hodder,

1961; Parson, 1961) . 268

6. Lutetium/Hafnium Geochronology of the Lackner Lake Carbonatite Complex The growth of radiogenic as a result of the de­ cay of -*-7^lu in a rock or mineral is given by:

(176Hf/177Hf)M=(176Hf/177Hf)0+176Lu/177Hf(eAt-l) (32)

Where: (l76Hf/l77Hf) = The value of this ratio at the time of

the analysis,

(17^Hf/l77Hf)q = Initial isotopic ratio at the time the sample formed,

^•7^Lu/^-77Hf = Ratio of these isotopes in the sample at the time of analysis,

t = Time elapsed since the sample became closed to lutetium and hafnium,

X = Decay constant of ^7®Lu

(1.964 x 10“-^ yrs.”■*■). The age of a lutetium-bearing rock or mineral, that has remained a closed system with respect to both lutetium and hafnium, can be computed from equation (3 2) if the present day values of the ^ ^Hf/^^Hf and ^^Lu/^^Hf iso­ topic ratios can be determined, and by assuming an appropri­ ate value for initial ^®Hf/^77Hf isotopic ratio. Age determinations of this type are called "model dates," and

their accuracy is a function of the uncertainty of the assumed initial ^7®Hf/-^77Hf ratio. This method has been

employed successfully by Boudin and Deutsch (1970) to date minerals having a high lutetium/hafnium ratio (gadolinite:71; 269

priorite: 363).

Most common rocks and minerals have lower lutetium/ hafnium ratios, and are, therefore, more reliably dated

by the "isochron method" routinely employed for dating oo­

genetic rocks or minerals by the ^ R b / ^ S r method (see: Section 5). Equation (32) represents a family of straight

lines in coordinates of ('*'^®Hf/^^^Hf)M and ^^^Lu/^^^Hf whose

slopes are proportional to t. Therefore, systems having a common age, identical initial ^^^Hf/-^^Hf ratios, and which

have remained closed to migration of both lutetium and hafnium since the time of their formation, will satisfy equation (32), and will lie on straight-line isochrons whose

slope is a function of the decay time given by: M = eAt-l (33) where M is the slope of the isochron, X is the decay constant

for -^^Lu, and t is the age of the system. An advantage of the isochron method of dating is that both the common age of a suite of co-genetic samples and their initial

-^?Hf isotopic ratio can be determined simultaneously.

Lutetium and hafnium analytical data for a suite of rocks and minerals from the Lackner Lake Complex are given in Table 54. The data used to establish a three point -^^Lu/^^Hf isochron are summarized in Table 55. Replicate determinatiohs indicate that the average precision of lu­ tetium and hafnium concentrations for whole-rocks and apatite were bettern than + 5%. Duplicate determinations of the Table 54. Lutetium and Hafnium Analytical Data for the Lackner Lake Carbonatite Complex

Sample Lu(PPM) Hf(PPM) * Zr(PPM) 176Lu/177Hf 176Hf/177Hf

Apatite 3.76 0.424** 1.2370 0.3255 3 .59

Average: 3.67 + 0.08

Apatite 1.07

0.98

Average: 1.025 + 0.045

Carbonatite 1.14

1.22

1.18 + 0.04

Magnetite 0.133 0.092 270 Average: 0.113 + 0.021 Table 54 - Continued

Sample Lu(PPM) Hf(PPM) * Zr(PPM) 176Lu/177Hf 176Hf/177Hf

Ijolitic 0.405 5.62 470 0.0099 0 .3031 Nepheline Syenite 0.359 5.22 487 0.3002

0.381* 5.32 Average: 4 7 8.5 + 8.5 Average: 0.30165 +

Average: 0.382 + 0.023 5.70 0.00302 6.05

5.23

Average : 5 . 52 + 0.33

Ijolite 0.281 5.62 387 0.0067 0.3003

0 .261 5.50

0.245 Average: 5.56 + 0.06

Average: 0.262 + 0.018 271

*Indicates sample dissolution by fusion. **Indicates acid dissolution of sample. Table 55. Analytical Data for the Lackner Lake Complex -^^Lu/l^Hf isochron.

Sample Lu(PPM) Hf(PPM) 176Lu/177Hf 176Hf/177Hf

Apatite 3.67 0.424 1.2370 0.3255

Ijolitic Nepheline 0.382 5 .52 0.0099 0 .30165 Spark

Ijolite 0 .262 5.56 0.0067 0.3003 272 273

ratio for the nepheline syenite and for reagent

hafnium oxychloride (0.2816 + 0.0026) indicate that the ■^^Hf/^^Hf ratios were measured with a precision of at least

+ 1%.

The lutetium/hafnium isochron for the Lackner Lake

Complex is shown in Fig. 32. As a result of the uncertainty in the measured -^^Hf/^^Hf ratio, the two whole-rock samples can not be resolved. They are, therefore, equivalent to a single point. A regression line was fitted to the data by

the least-squares method of York (1966). Its slope and in­ tercept were 0.01997 + 0.00170 and 0.3008 + 0.0007, respec­ tively. The intercept, which is the initial ^^^Hf/^^^Hf isotopic ratio, is significantly higher than the nominal crustal value of 0.2811. The isochron indicates a date of 1009 + 77 M.Y. for the Lackner Lake Complex assuming the half-life of -^6Lu is 3.53 x 10^® years.

The date derived from a lutetium/hafnium isochrorf’ is dependent upon the choice of a half-life for ^®Lu. However, the correct value of the half-life is uncertain, as publish­ ed values tend to group around either 35 or 22 billion years (Table 15). Although the age of the Lackner Lake

Complex is known, the uncertainty of the slope of the luteti­ um/hafnium isochron (+8.5%) is too great to permit calcula­ tion of an accurate half-life. However, the analytical data are of sufficient accuracy and precision to establish whether the helf-life of -^^Lu is in the range 30 to 40 or 20 to 25 (176/177)

0-330

0-320

0-310 NEPHELINE SYENITE t = 1009 ! 77 my- 0-300 IJOLITE T1/2 = 3-53 x 1010 yrs-

(176/177)0= 0-3008 * 0-0007 0-290

0-280 0 0-1 0-2 0-3 0-4 0-5 0-6 0-7 0-8 0-9 1-0 1-1 1-2 176 Lu 177 Hf 274 Figure 32. Lutetium/hafnium isochron for the Lackner Lake Carbonatite Complex, Northern Ontario, Canada.

•v 275

billion years.

The slope of an isochron, as noted previously, is re­

lated to the age of a system by the relationship: (eAt _]_) = where t is the age of the system, and M is the slope of the isochron. The decay constant ( ) is related to the half-life by the simple relationship:

A= 0.693/T1y2 (34)

The corresponding expression for the age of the system is therefore:

(T1/2) [In(M+l)] t = ------(35) 0.693

The age of the Lackner Lake Complex was calculated for

-i *7 appropriate values of the half-life of Lu (see Table 56) with equation (35), assuming the value of M was 0.01997 as derived from the Lutetium/Hafnium isochron. A "best" half- life of 3.53 + 0.27 x lolO years is indicated, as it cor­ responds to a date of 1009 + 77 million years which is in­ distinguishable, within the analytical uncertainty, from the known age of the complex (Table 57). A half-life of

2.2 x 1010 years can not be correct as it corresponds to a date (627 + 60 Million years) which is dearly too low.

These results indicate that age determinations by the lutetium/hafnium isochron method are feasible. The stronti­ um isotope data described in Section 5 indicates that the 276

Table 56. Average Lutetium-176 Half-Life Estimates

Half-Life: 3.0 to 4.0 x 1010Yrs 2.0 to 2.5 x 1010 Yrs

4.1 2.171

3.6 2.18

3.2 2.17**+ 0.35 3.57 2.1 3.54 2.15 3.50 2.4 3 .68 3.27

3.30*+ 0.5 3.53 + 0.27 2.195 + 0.104

Data are from Table 15. Errors are given as la.

*Geologic determination by Boudin and Deutsch (1970). **Geologic determination by Herr et al (1958). Table 57. Comparison of Results for Lutetium-17 6/Hafnium-176 and Rubidium-87/ Strontium-87 Age Determinations for the Lackner Lake Carbonatite Complex

Calculated Range 87Rb/87Sr Range Half-Life of 176Lu/176nf Age (Million Isochron Age (Million ^-7^Lu (x 10^-0 Years) (Million Years) Years) (Million Years) Years)

3.53 + 0.27 1009 932-1086 1094 1086-1102 (1090)*

2.195 + 0.104 627 597-657

*K/Ar age determination for biotite from the Lackner Lake Complex by Lowdon et al (1963) . 277 278

Lackner Lake Complex was derived from a deep-seated or mantle source, and, with the exception of apatite, has re­ mained a closed system with respect to both rubidium and strontium since 1094 million years ago. Apatite has ap­

parently remained a closed system with respect to both lutetium and hafniu, as the ^ ^ L u / ^ ^ H f isochron date for

the Lackner Lake Complex is in satisfactory agreement with

age determinations by other methods. Isotope exchange reactions between ground water and phosphate minerals may be responsible for the anomolous radiogenic ^ S r content of apatite. Similar reactions in­ volving lutetium and hafnium may not occur, as the concen­ tration of these elements in ground water is probably negli­ gible. In addition, the close similarity of ionic radii of

Lu+3 and Hf+4, and the high valence of hafnium (+4) suggests that radiogenic hafnium is stable in lutetium sites, and, compared to radiogenic 3^Sr, relatively resistant to remo­ bilization . It is concluded, therefore, that magmas derived from sub-crustal source regions may be characterized by elevated ■^^Hf/^^Hf ratios relative to the nominal crustal value.

This suggests that the mantle may be enriched, relative to crustal sialic rocks, in lutetium. CHAPTER IX

ISOTOPE GEOCHEMISTRY OF HAFNIUM

1. Introduction The relative abundances of isotopes of hafnium in nature change continuously through time as a consequence of the radioactive decay of ^^Lu. In any closed system, radio- genic 1 7 '°Hf accumulates at a rate governed by the decay con­ stant of -^^Lu, and its lutetium/hafnium concentration ratio. If hafnium is separated from its parent system, the rela­ tive abundance of ^^Hf, expressed as the ^"^Hf/^^Hf ratio, may subsequently remain constant, or it may continue to change at a rate controlled by the lutetium/hafnium ratio of its new environment.

Variations in the isotope composition of hafnium in geo­ logical materials can be used to study the geochemical dif­ ferentiation of the Earth's Mantle and the evolution of the continental crust. Mantle-derived magmas contain hafnium whose ‘^^Hf/'^^Hf ratio depends on the lutetium/hafnium ratio of, and the time elapsed since withdrawal from their source regions. From considerations of the geochemical be­ havior of lutetium and hafnium, it was inferred earlier

(CHAPTERS III, IV and VIII) that the mantle may be slightly enriched in lutetium, but depleted in hafnium compared to the crust. Igneous rocks of sialic composition in the crust 279 280 have low lutetium/hafnium ratios by virtue of their high hafnium content. Therefore, the -^^Hf/^^Hf ratios of crus­ tal rocks may change more slowly than those in the mantle in which this ratio may vary measurably as a function of time. Moreover, if the isotopic composition of crustal hafnium tends to remain constant, differentiation between mantle-derived and remobilized sialic igneous rocks may be possible on the basis of observed variations of initial 176Hf/177Hf ratios.

2. Hafnium Development Diagram

To test these ideas, initial isotopic ratios for the Lackn:?r Lake Complex, basalt, and reagent hafnium have been plotted on a hafnium development diagram (Fig.33).

It would be helpful in understanding the evolution of ter­ restrial hafnium if the primeval value of the -^®Hf/177Hf ratio were known. This ratio can be determined by analysis of chondritic meteorites whose age is known. Unfortunately, such analytical data are not yet available. The primeval ratio can, however, be estimated from the present day aver­ age lutetium/hafnium ratio of chondrites (0.163). A line, whose slope is proportional to the average lutetium/hafnium ratio and age of chondrites, when drawn between the present day value of the -*-^®Hf/^^^Hf ratio of reagent hafnium to a point representing 4.5 billion years ago, the age of chondrites, indicates a primeval ^ ^Hf/-^^Hf ratio of about

0.279. (176/177)

0.330

0.320

0-310 BCR-1

LACKNER LAKE 0.300

0290

MANTLE DEVELOPMENT CURVE CRUSTAL DEVELOPMENT CURVE 0280 CHONDRITIC DEVELOPMENT CURVE

0-270 5 0 4 0 3-0 20 10 0 '' TIME (by)

Figure 33. Hafnium development diagram. 282

The primeval ratio is the starting point

for a terrestrial hafnium development diagram. A mantle development curve has been defined by fitting a curve to points representing the Lackner Lake Carbonatite Complex, the diabase, W-l, and the continental tholeiitic basalt,

BCR-1. The slope of the crustal development curve is pro­ portional to the age of the continental crust (assumed to 9 be 3.6 x 10 years) and its average lutetium/hafnium ratio

(0.091). Inferred crustal and chondritic hafnium develop­ ment curves have almost identical slopes. It is apparent that mantle-derived igneous rocks have elevated initial

17^Hf/177Hf ratios relative to hafnium reagent, which is assumed to be representative of the crust, as it is primari­ ly derived from the mineral zircon. The shape of the mantle development curve is controlled by the exponential function that governs the rate of production of radiogenic hafnium by the decay of ^^^Lu [(176LU/177Hf) (e^^-l)].

After a magma is separated from a mantle source region its 17 6 Hf/177 Hf isotopic ratios tend to remain constant, as its new crustal environment is lutetium-deficient. This condition is illustrated by the Lackner Lake Carbonatite

Complex. The present day ^®Hf/-^^Hf ratios of whole-rocks from the complex are very similar to their initial ratio

(0.3008) as indicated by the isochron diagram shown pre­ viously in CHAPTER VIII (Fig 32). 283

3. Models of Possible Evolution of Hafnium in the Earth

The writer now proposes two alternative models to ac­ count for the observed variation of initial hafnium isotopic compositions (Fig 3 4). In Model I, the mantle is assumed to be lutetium-deficient. As a consequence, basalts derived from the mantle at times t^ and t2 have similar initial 176Hf/177nf ratios. The hafnium development lines for the basalts are steeper than the mantle development curve, be­ cause they are inferred to have greater lutetium/hafnium ratios than the source regions from which they were derived.

In Model II, the mantle source regions are assumed to have higher lutetium/hafnium ratios than derived basaltic magmas. Consequently, basalts originating at times t^ and t2 have significantly different initial x/DHf/1 7 £ 177 Hf ratios. In fact, differences in initial ratios may be great enough to permit dating basalts, or other mantle-derived igneous rocks, directly, by simply determining the isotopic com­ position of their hafnium. Although based on limited data, Model II satisfactorily accounts for the observed variation in initial -^^Hf/^-^Hf isotopic ratios shown in Fig 33. A systematic study of oceanic basalts, or other mantle-derived material, of known age is required to fix the lateral homogeneity of the upper mantle with respect to its lutetium/hafnium ratio, and ini­ tial ^^^Hf/^^^Hf isotopic ratios. The analytical methods that have been developed are suitable for a study of this type. 285

176 M O DEL. 1 177

} (176/177) At

MODEL 2

(176/177)

At

4-5 PRESENT TIME (b.y.)

Figure 34. Models of-possible isotopic evolution of hafnium in the Earth. CHAPTER X . CONCLUSIONS

The S~ decay of naturally-occurring to stable

-*-7% f is a potentially useful geochronometer for dating lutetium-bearing rocks and minerals. Published half-life determinations for -^^Lu indicate that it has a value of

either 22 or 30 to 40 billion years. In order to use this geochronometer, lutetium, hafnium, and the radiogenic component ^^Hf, expressed in terms of the

isotopic ratio, must be determined. Development of a

17 6l u /17 6Hf geochronometer has been impeded primarily by the analytical difficulties associated with determining the

isotopic composition of hafnium. Analytical procedures were developed for the determi­ nation of low-level concentrations of lutetium and hafnium

by isotope dilution and solid source mass spectrometry. The hafnium procedure was also used to measure the isotopic

composition of hafnium with sufficient precision (1 1%) to make dating by the ^Lu/^^Hf isochron method feasible.

Lutetium was determined in a 6-inch, single filament solid source mass spectrometer, and hafnium was determined in a

12-inch, triple filament instrument. Separate sample dissolutions were used for these elements to avoid isobaric

286 287 interference during hafnium analyses. Silicate rocks and spikes (-^^Lu or l^Hf) and tracer, in the case of lutetium, were fused with a mixture of 40% boric acid and 60% lithium fluoride. Apatite samples were either dissolved with 6N hydrochloric acid and, in the case of hafnium, equilibrated with spikes by addition of 0.5N oxalic acid, or fused with sodium carbonate. Lutetium was extracted by a simple cation exchange chromatographic procedure involving the use of 6.7 day ^"^Lu tracer. Iso- baric interference from-^^Yb was eliminated in the mass spectrometer by quantitative removal of ytterbium at a filament temperature too low to produce lutetium ion emis­ sion. Hafnium and zirconium were separated from other elements by precipitation of cupferrates, followed by cation exchange chromatography with 4.ON nitric acid. An advantage of this procedure is that hafnium and zirconium may be determined simultaneously if samples are spiked for zirconi­ um (^Zr spike) , because zirconium ion emission occurs at a significantly lower temperature than required for hafnium. Concentrations of hafnium and zirconium were calculated from the measured -*-^^Hf/-*-^^Hf and ^ Z r / ^ Z r isotopic ratios, respectively. The procedures were tested by analysis of a suite of rock standards, and the results were in satisfactory agreement with previous determinations. A three-point ^^Lu/-^ 6jjf isochron was obtained for a suite of co-genetic rocks and minerals from the Lackner 288

Lake Carbonatite Complex, Northern Ontario, Canada. The only previously reported age determination for this complex was 1090 million years, based on a K-Ar age determination of biotite. The age of the Lackner Lake Complex was con­ firmed by a five-point ^ R b / ^ S r isochron which yielded a date and initial ^ S r / ^ S r ratio of 1094 i 8 million years and 0.70277 t 0.00004, respectively.

Apatite, nepheline syenite, and ijolite from the Lackner Lake Complex contain hafnium which is measurably enriched in radiogenic l^ H f compared to "spec-pure" hafni­ um oxychloride reagent. The isochron indicates a date of 1009 1 77 million years assuming a half-life of 3.53 x 1 0 ^ years for ^^^Lu. If a half-life of 2.2 x 10-*-® years is used the date calculated for the complex is too low (627 million years). The initial ratio of 0.3008 is significantly greater than the nominal crustal ^^^Hf/l^^Hf ratio of 0.2811 as indicated by hafnium reagent. As a consequence, it was concluded that the mantle may be enriched in radiogenic hafnium.

High concentrations of lutetium have been reported in the literature for apatite and garnet. This suggests that potentially important applications for a ^^^Lu/^-^^Hf geo­ chronometer are the dating of apatite-bearing rocks, in­ cluding sedimentary phosphate deposits, and metamorphic rocks enriched in garnet. BIBLIOGRAPHY

Adams, J.W., 1969, Distribution of lanthanides in minerals: U.S.G.S. Prof. Paper, No. 650-C, C38.

Aller, L.H., 1965, The abundance of the elements in the solar atmosphere: Adv. Astron. Astrophys., V.3, pp. 1. Arnold, J.R. , 1954 , Energy levels of 17^Lu and ]-^6Hf: Phys. Rev., V.93, p.743. Benedict, J.T., Schumb, W.C., and Coryell, C.D., 1953, Dis­ tribution of zirconium and hafnium between cation- exchange resin and acid solutions. The column separation with nitric acid-critic acid mixture: J. Amer. Chem. Soc . , V.76, p.2036 . Berry, L.G., and Mason, B., 1959, Mineralogy: W.H. Freeman and Company, San Francisco and London, 630 p. Biskupsky, V.S., 1965, Fast and complete decomposition of rocks, refractory silicates and minerals: Anal. Chim. Acta, V.33, p.333.

Bock, R.M., and Nan-Sing Ling, 1954, Devices for gradient elution in chromatography: Anal. Chem., 26_, 1543.

Boger, P.D., 1974, Personal Communication: Laboratory for Isotope Geology and Geochemistry, The Ohio State University, Columbus, Ohio.

Borchardt, G.A., Aruscavage, P.J., and Millard, H.T., Jr., 1972, Correlation of the Bishop Ash, a Pleistocene marker bed, using instrumental neutron activation analysis: J. Sed. Petrol., V.42, p.301.

Boudin, A., 1967, Development de la methode de measure d'age des rockes fondes sur le rapport Lutetium-176/ Hafnium-176: In "Radioactive Dating and Methods of Low-Level Counting," Atomic Energy Agency, p.515. Boudin, A., and Dehon, M., 1969, Methodes d'analyse quanti­ tative du lutetium dans les mineraux: Geochim. Cosmochim Acta., V.33, p. 142. 289 290

Boudin, ,A. , and Deutsch, S., 1970, Geochronology: Recent development in the Lutetium-176/Hafnium-176 dating method: Science, V.168, p.1219.

Bragg, W •L., 1937, Atomic Structure of Minerals: Cornell University Press, Ithaca, N.Y., 292 p.

Brinkman , G.A., Aten, A.H.W., and Veenboer, J.Th., 1965, Natural radioactivity of K-40, Rb-87, and Lu-176: Physica, V.31, p.1305.

Brooks, (3.K., 1968, A radiochemical separation for the determination of zirconium or hafnium in rocks and minerals by neutron activation analysis: Radiochimica Acta, V.9, No. 213, p. 157.

Brooks, (3.K. , 1969, On the distribution of zirconium and hafnium in the skaergaard intrusion, East Green­ land: Geochimica Cosmochimica Acta, V.33, No.3, p.357.

Brooks,

Butler, (I.C., and Kniseley, R.N., 1973, Nonferrous metal­ lurgy. II. Zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten: Anal. Chem., V.45, p.l29R.

Butler, l I.R. , and Thompson, A.J., 1965, Zirconium:hafnium ratios in some igneous rocks: Geochimica Cosmochimica Acta, V.29, No.3, p.167.

Cameron, A.G.W., 1968, A new table of abundances of the elements in the solar system: In: "Origin and Distribution of the Elements," L.H. Ahrens (Ed.), Pergamon Press, p.125. Cerrai, I3. , and Testa, C., 1959a, Extraction and separation of zirconium and hafnium by means of liquid anionic exchangers in a hydrochloric acid medium - Part I: Energia Nucleare, V.6, No.11, p.707. Cerrai, I3., and Testa, C., 1959b, Extraction and separation of zirconium and hafnium by means of liquid anionic exchangers in a hydrochloric acid medium - Part II: Energia Nucleare, V.6, No.12, p.768. 291

Clayton, D.D., and Fowler, W.A. , 1961, Abundances of heavy nuclides: Ann. Phys., V.16, p.51. Condie, K.C., and Lo, H.H., 1971, Trace element geochemistry of the Louis Lake Batholith of early Precambrian age, Wyoming: Geochimica Cosmochimica Acta, V.35, N o .11, p .1099.

Connick, .R.E, and McVey, W.H., 1949, The aqueous chemistry of zirconium: J. Amer.Chem. Soc., V.71, p.3182.

Cruft, E.F., Ingamells, C.O., and Muysoon, J. , 1965, Chemical analysis and the stoichiometry of apatite: Geochimica Cosmochimica Acta, V.29, No.5, p.581.

Cruft, E.F., 1966, Minor elements in igneous and metamor- phic apatite: Geochimica Cosmochimica Acta, V.30, No . 4, p.375.

Dixon, D., McNair, A., and Curran, S.C., 1954, The natural radioactivity of lutetium: Phil. Mag., V.7., No. 4 5, p . 68 3 .

Donhoffer, D., 1964, Bestimmung der halbwertszeiten der in der natur vorkommenden radioaktiven nuklike 147sm ana 17 6l u mittels flussinger szintillatoren: Nucl. Phys., V.50, p.489.

Dunn, H.W., 1962, X-ray absorption edge analysis: Analytical Chemistry, V.34., No.1, p.116. Eby, G.N., 197 2, Determination of rare-earth, yttrium, and scandium abundances in rocks and minerals by an ion exchange-x-ray fluorescence procedure: Anal. Chem., V.44, No.13, P.2137.

Ehmann, W.D., and Rebagay, T.V., 1970a, Zirconium and hafnium in meteorites by activation analysis: Geochimica Cosmochimica Acta, V.34, p.649. Elinson, S.V., and Petrov, K.I., 1969, Analytical Chemistry of Zirconium and hafnium: Israel Program for Scientific Translations, 243 p.

Eugster, 0., Tera, F. , Burnett, D.S., and Wasserburg, G.J., 197 0, Isotopic composition of gadolinium and neutron-capture effects in some meteorites: J. Geophy. Res., 75, 27 53. 292

Faure, G. , and Hurley, P.M., 1963, The isotopic composition of strontium in oceanic and continental basalt: Application to the origin of igneous rocks: J. Petrol., V .4, p.31.

Faure, G., and Powell, J.L., 1972, Strontium Isotope Geology: Springer-Verlag, New York, 188 p.

Flammersfeld, A., and Mattauch, J., 1943, Natural and artificial activity of lutecium. A new case of isomerism: Naturwissenschaften, V.31, p.66.

Flanagan, F.J., 1969, U.S. Geological Survey standards - II: First compilation of data for the new U.S.G.S. rocks: Geochimica Cosmochimica Acta, V.33, No.l, p. 81. Flanagan, F.J., 1973, 1972 values for international geochemical reference samples: Geochimica Cosmochimica Acta, V.37, p.1189. Fleischer, M. , and Altschuler, Z.S-., 1969, The relationship of the rare-earth composition of minerals to geological environments: Geochimica Cosmochimica Acta, V.33, No.6, p.725.

Fleischer, M., 1969, U.S. Geological Survey standards - I: Additional data on rocks G-l and W-l: Geochimica Cosmochimica Acta, V.33, No.l, p.65. Furman, N.H., Mason, W.B., and Pekola, J.S., 1949, Extraction of cupf errates: Anal. Chem., V.2H-, No.11, p.1325. Gast, P.W., and Hubbard, N.J., 1970, Abundance of alkali metals, alkaline and rare earths, and strontium-87/ strontium-86 ratios in lunar samples: Science, V.167, p.485. Gast, P.W., Hubbard, N.J., and Wiesmann, H., 1970, Chemical composition and petrogenesis of basalts from Tranquillity Base: Proc. Apollo II Lunar Sci. Conf., 2, 1143. Grittins, J., Macintyre, R.M., and York, D., 1967, The ages of carbonatite complexes in Eastern Canada: Canadian J. Earth Sci., V.4, p.651. 293

Glover, R.N., and Watt, D.E., 1957, A search for electron capture in 176l u : Phil. Mag., V.8, No.2, p.699. Goldschmidt, V.M., 1954, Geochemistry: Oxford Press, 730 p.

Goles, G.G., Randle, K., Osawa,M., Schmitt, R.A., Wakita, FI., Ehmann, W.D., and Morgan, J.W., 1970, Elemental abundances by instrumental activation analysis in chips from 27 lunar rocks: Proc. Apollo II Lunar Science Conf., V.2.,p.1165.

Gottfried, D., and Waring, C.L., 1964, Hafnium content and Hf/Zr ratios in zircon from the Southern California Batholith: U.S.G.S. Prof. Paper 501-B, p.88. Gottfried, D., Greenland, L.P., and Campbell, E.Y., 1968, Variation of Nb-Ta, Zr-Hf, Th-U, and K-Cs in two diabase-granophyre suites: Geochimica Cosmochimica Acta, V.32, p.925.

Graham, A.L., and Nicholls, G.D., 1969, Mass Spectrographic determinations of lanthanide element contents in basalts: Geochimica Cosmochimica Acta, V.33, No.5, p .555.

Grevesse, N., and Blanquet, G., 1969, Abundances of the rare-earths in the sun: Solar Physics, V.8, No.1, p .5 .

Hahn, R.B., 1972, Radiochemical separation of zirconium and hafnium from other radionuclides: Talanta, V .19, p.1454.

Haskin, L.A., and Frey, F.A., 1966, Dispersed and not-so- rare earths: Science, V.152, p.229. Haskin, L.A., Schmitt, R.A., and Smith, R.H., 1966, Meteoritic, solar, and terrestrial rare earth distributions: In "Physics and Chemistry of the Earth," V.7, Pergamon Press, p.167.

Haskin, L.A. Wildeman, T.R., Frey, F.A., Collins, K .A. , Keedy, C.R., and Haskin, M.A., 1966, Rare earths in sediments: J. Geophys. Res., V.71, No.24, p.6091.

Haskin, L.A., and Haskin, M.A., 1968, Rare-earth elements in the skaergaard intrusion: Geochimica Cosmochimica Acta, V.32, p.433. 294

Haskin/ L.A., Helmke, P.A., and Allan, P.O., 1970, Rare-earth elements in returned lunar samples: Science, V.167, p.487.

Heinrich, E.W., 1966, The Geology of Carbonatites: Rand McNally, New York, 555 p. Hertel, G.R., 1968, Surface Ionization.Ill. The first ioni­ zation potentials of the lanthanides: J. Chemical Physics, V.48, No.5, p.2053. Harr, W. , Merz, E., Eberhardt, P., and Singer, P., 1958, Zur Bestimmung der B-Halbwertszeit des 176lu durch den Nachweis von radiogenem 176nf: Zeit. Naturforsch., A13, p.268.

Herrmann, A.G., 1970, Yttrium and Lanthanides: In "Handbook of Geochemistry," V.II-2, K.H. Wedepohl, Ed., p.39, 57-71-B-l.

Hevesy, G., 1936, Discovery of hafnium: Current Sci., V.5, p .236.

Heydemann, A., 1968, Handbook of Geochemistry: V.I, K.H. Wedepohl, Ed., 442 p. Heyden, M. and Wefelmeyer, W., 1938, Eine naturliche 6- radioaktivitat des cassiopeiums: Naturwiss., V.26, p .612.

Hillebrand, W.F., 1953, Applied Inorganic Analysis: 2nd Edition, J. Wiley and Sons, 1034 p.

Hodder, R.W., 1961, Alkaline Rocks and Niobium Deposits Near Nemegos, Ontario: Geol. Survey Canada, Bull.70, 75 p. Holden, N.E., and Walker, F.W., 1969, The General Electric chart of the Nuclides: Knolls Atomic Power Labora­ tory, Schenectady, N.Y., 10th Edition. Horn, M.K., and Adams, J.A.S., 1966, Computer-derived geo­ chemical balances and element abundances: Geochimica Cosmochimica Acta, V.30, No.3, p.279. Huffman, E.H., Iddings, G.M., and Lilly, R.C., 1951, Anion exchange of zirconium, hafnium, niobium, and tantalum in hydrochloric acid solutions: J. Amer. Chem. Soc., V.73, p.4474. 295

Ikpeama, M.O.U., Boger, P.D., and Faure, G., 1974. A study of strontium in core 119K, Discovery Deep, Red Sea: Chemical Geology, V.13,p.11,

Ingamells, C.O., 1970, Lithium metaborate flux in silicate analysis: Analytics Chimica Acta, V.52, p.323.

Keller, E. and Parsons, M.L., 1970, Determination of tungsten in silicate ores: Atomic Absorption Newsletter, V .9, No.4, p.92.

Khomyakov, A.P., 1967, Chemical and crystallographic factors in the distribution of rare earths: Geochemistry International, V.4, p.127. Kosterin, A.V., Zuev, V.N., and Shevaleevskii, I.D., 1958, Zr/Hf ratios in zircons in some igneous rocks of Northern Kirgizia: Geochemistry, No.l, p.116.

Krauskopf, K.B., 1967, Introduction to Geochemistry: McGraw- Hill , Inc. , 721 p.

Leary, J.J., Tsuge, S., and Isenhour, T.L., 1973, Measurement of zirconium-hafnium ratios in geological samples by electron impact mass spectrometry: Anal. Chem., V .45, No.7, p.1269.

Libby, W.F., 1939, Natural radioactivity of lutecium: Phys. Rev., V.56, p.21. Loubet, M., Bernat, M., Javoy, M., and Allegre, C.J., 1972, Rare earth contents in carbonatites: Earth and Planetary Science Letters, V.14, p.226. Lowdon, J.A., Stockwell, C.H., Tipper, H.W., and Wanless,R.K., 1963, Age determinations and geological studies: Geol. Survey of Canada, Paper 62-17, p.83.

Luke, C.L., 1968, X-ray determination of traces of Hafnium in zirconium metal or traces of zirconium in hafni­ um metal after separation by ion exchange: Analytica Chimica Acta, V.41, p.453.

MacNair, A., 1961, The half-life of long-lived Lutetium-176: Phil. Mag., V.6, p.851.

Makoto, S., and Sadaya, A., 1966, Age dating by the Lu^^^- Hfl76 Method I. Separation of Zr and Hf: Rika Gaku Kenkyusho Hokoku, 42(3), 133. 296

Mapper, D., 1960, Radioactivation analysis: In: Methods in Geochemistry," Interscience Publishers, Inc., 464 p. Marsh, S .F., Maeck, W.J., Booman, G.L., and Rein, J.E., 1961, Improved 2-thenoyltrifluoroacetone extraction method for radiozirconium: Anal. Chem., V.33, p.870.

Mason, B . , and Graham, A.L., 1970, Minor and trace elements in meteoritic minerals: Smithsonian Contributions to the Earth Sciences, No.3, 17 p.

Massart, D.L. , and Hoste, J., 1963 , La separation Lu-Yb-Tm sur echangeur de cations par L'a-IHBA: Anal. Chim. Acta, 28_, 378.

Masuda, . k. , 1967 , Lanthanides in the Norton County achondrite: Geochemical Jour., 2_, 135.

Med1in, .T.H., Suhr, N.H., and Bodkin, J.B., 1969, Atomic absorption analysis of silicates employing LiBC>2 fusion: Atomic Absorption Newsletter, V.8, No.2, p. 25.

Moore, F .L., 1956, Separation of zirconium from other elements bv liquid-liquid extraction: Anal. Chem., V. 28, N o . 6', p.997.

Morrison , G.H., Gerard, J.T., Kashuba, A.T., Gangadharam,E.V., Rothenberg, A.M., Potter, N.M., Miller, G.B., 1970, Multielement analysis of lunar soil and rocks: Science, V. 167, p.505.

Morrison G.H., Gerard, J.T., Kashuba, A.T., Gangadharam,E.V., Rothenberg, A.M., Potter, N.M., and Miller, G.B., 19 70b, Elemental abundances of lunar soil and rocks: Proc. Apollo II Lunar Science Conf., V.2f p .1383. Mukherj i A.K. , 1970 , Analytical Chemistry of Zirconium and hafnium: Pergamon Press, 281 p. Nagasawa H., 197 0, Rare earth concentrations in zircons and apatites and their host dacites and granites: Earth and Planetary Science Letters, V.9, p.359. Oesper, jI.E., and Klingenberg, J.J., 1949, Use of glycolic acid derivatives in determination of zirconium: Anal. Chem. V.21, No.12,p.1509. 297

Owen, L.B., and Faure, G., 1974, Simultaneous determination of hafnium and zirconium in silicate rocks by iso­ tope dilution: Anal. Chem., V.46, No.9, p.1323. Parsons, G.E., 1961, Niobium-Bearing Complexes East of Lake Superior: Ont. Dept. Mines, Geol. Rep. No.3, 7 3 p.

Pauling, L., 1960, The Nature of the Chemical Bond: Cornell University Press, Ithaca, New York.

Philpotts, J.A., and Schnetzler, C.C., 1968, Europium anomalies and the genesis of basalt: Chemical Geology, V.3, No.l, p.15.

Philpotts, J.A., and Schnetzler, C.C., 1970, Potassium, rubidium, strontium, barium, and rare earth concen­ trations in lunar rocks and separated phases: Science, V.167, p.493.

Popov, A.I., and Wendlandt, W.W., 1954, Cupferron and neocupferron complexes of the rare earth elements: Anal. Chem., V.26, No.5, p.883. Powell, J.L. , 1965, Low abundance of Sr®^ j_n Ontario carbonatites: Amer. Min., V.50, p.1075. Prodi, V. , Flynn, K.F., and Glendenin, L.E., 1969, Half- life and beta spectrum of 176l u : Phys. Rev., V.188, Mo.4, p.1930.

Rankama, K., and Sahama, Th. G., 1950, Geochemistry: Univ. Chicago Press, 912 p.

Rankama, K., 1954, Isotope Geology: McGraw-Hill, New York, 535 p. Rankama, K., 1963, Progress in Isotope Geology: Interscience, 705 p. Reader, J., and Sugar, J., 1966, Ionization energies of the neutral rare earths: J. Optical Soc. Amer., V.56, No.9, p.1189. Rebagay, T.V., and Ehrnann, W.D., 1970b, Simultaneous deter­ mination of zirconium and hafnium in standard rocks by neutron activation analysis: J. of Radioanalytical Chemistry, V.5, p.51. 298

Rey, P., Wakita, H., and Schmitt, R.A., 1970, Radiochemical neutron activation analysis of indium, cadmium, yttrium, and the 14 rare earth elements in rocks: Anal. Chim. Acta, 5_1, 163.

Reynolds, R.C., Jr., 1963, Matrix corrections in trace element analysis by x-ray fluorescence: Estimation of the mass absorption coefficient by Compton scattering: Amer. Miner., V.48, p.1133.

Ringwood, A.E., 1955a, The principles governing trace element distribution during magmatic crystallization (Part I): Geochimica Cosmochimica Acta, V.7, p.189. Ringwood, A.E., 1955b, The principles governing trace- element behaviour during magmatic crystallization (Part II): Geochimica Cosmochimica Acta: p.242. Ryabchikov, D.I., and Ryabukhin, V.A., 1970, Analytical Chemistry of Yttrium and the Lanthanide Elements: Israel Program for Scientific Translations, Ann Arbor-Humphrey Science Publishers, 365 p.

Sakamoto, K., 1967, The half-lives of natural 176lu and 180Ta: Nuclear Phys., A 103, p.134. Seeger, P.A., Fowler, W.A., and Clayton, D.D., 1965, Nucleosynthesis of heavy elements by neutron capture: Astrophys. J. Suppl. Ser. II, p.121. Schnetzler, C.C., and Philpotts, J.A., 1970, Partition coefficients of rare-earth elements between igneous matrix material and rock forming mineral phenocrysts - II: Geochimica Cosmochimica Acta, V.34, No.4, p.331.

Sill, C.W., 1961, Decomposition of refractory silicates in ultramicro analysis: Anal. Chem., V.33, No.12, p.1684.

Smith, G.F., 1965, The Wet Chemical Oxidation of Organic Compositions Employing Perchloric Acid: G.F. Smith Chemical Co., Columbus, Ohio, 105 p. Smith, G.F., 1938, Cupferron and Neo-cupferron: G.F. Smith Chemical Co., Columbus, Ohio, 47 p.

Strelow, F.W.E., Liebenberg, C.J., and Toerien, F., von S., 1969, Accurate silicate analysis based on separation by ion-exchange chromatography: Anal. Chim. Acta, 47, p.251. 299

Strelow, F.W.E., and Bothma, C.J.C., 1967, Anion exchange and a selectivity scale for elements in sulfuric acid media with a strongly basic resin: Anal. Chem., V.39, No.6, p.595. Strelow, F.W.E., Rethemeyer, R., and Bothma, C.J.C., 19 65, Ion exchange selectivity scales for cations in nitric acid and sulfuric acid media with a sul- fonated polystyrene resin: Anal. Chem., V.37, No.1, p .106. Strelow, F.W.E., 1960, An ion exchange selectivity scale of cations based on equilibrium distribution coefficients: Anal. Chem., V.32, N.9, p.1185.

Strelow, F.W.E., 1959, Separation of Zirconium from Titanium, ferris iron, aluminum, and other cations by cation exchange chromatography: Anal. Chem., V.31, N .12, p.1974.

Suhr, N. ;., and Ingamells, C.O., 1966, Solution technique for analysis of silicates: Anal. Chem., V.38, N o .6, p.730. Taylor, :.R., 1964, Trace element abundances and the chondritic earth model: Geochimica Cosmochimica Ac ta, V .2 8, p .19 8 9 . Towel1, :1.G., Winchester, J.W., and Spirn, R.V., 1965, Rare earth distributions in some rocks and associated minerals of the batholith of Southern California: J. Geophys. Res., V.70, No.14, p.3485.

Trifonov D.N., 1966, Problems in the study of rare earths: Israel Program for Scientific Translations, 144 p.

Turekian K.K., and Wedepohl, K.H., 1961, Distribution of the elements in some major units of the Earth's crust: Bull. G.S.A., V.72, p.175.

Turekian K.K., and Kharkar, D.P., 1970, Neutron activation analysis of milligram quantities of lunar rocks and soils: Science, V.167, p.507. Urey, H.C., 1967, The abundances of the elements with special reference to the problems of iron abundance: Quart. J. Astron. Soc., V.8,p.23. 300

Verhoogen, J., Turner, F.J., Weiss, L.E., Wahrhaftig, C. , and Fyfe, W.S., 1970, the earth-an introduction to physical geology: Holt, Rinehart and Winston, Inc., 748 p .

Vlasov, K.A. , 1966, Geochemistry and mineralogy of rare elements and genetic types of their deposits: Israel Program for Scientific Translations, V.l (Geochemistry of Rare Elements), 688 p.

Wanke, H., Begemann, F., Vilesek, E., Rieder, R., Teschke,F., Born, W. , Quijano-Rico, M., Voshage, H., Wlotzka,F., 1970, Major and trace elements and cosmic-ray produced radioisotopes in lunar samoles: Science, V.167, p.523. Weast, R.C., 1973, Handbook of Chemistry and Physics: CRC Press, Ohio.

White, F.A., Collins, T.C., and Rouke, F.M., 1956, Search for possible naturally occurring isotopes of low abundance: Phys. Rev., 101, p.1786.

Whittaker, E.J.W., and Muntus, R., 1970, Ionic radii for use in geochemistry: Geochimica Cosmochimica Acta, V .34, No.9, p.945.

Winters, R.R., 1968, Half-life determination of lutetium-176 by three coincidence methods: Dissert.Abstracts, B, V.28, p.5162.

Wolfsberg, K., 1962, Determination of rare earths in fission products by ion exchange at room temperatures: Anal. Chem., 3_4_, p. 518.

York, D., 1966, Least-squares fitting of a straight line: Canadian J. Physics, V.44, p.1079. Young, E.J., Myers, A.T., Munson, E.L., and Conklin,N.M., 1969, Mineralogy and geochemistry of fluorapatite from Cerro De Mercado, Durango, Mexico: U.S.G.S. Prof. Paper, 650-D, p.D84. Yule, J.W., and Swanson, G.A., 1969, A rapid method for decomposition and the analysis of silicates and carbonates by atomic absorption spectroscopy: Atomic Absorption Newsletter, V.8, No.2, p.30.