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Rb -Sr87 AGE of STONY METEORITES by A.B

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Rb -Sr87 AGE of STONY METEORITES by A.B THE Rb -Sr87 AGE OF STONY METEORITES INST. TEc by ROBERT McCORMICK SHIELDS LINDGREN A.B., Dartmouth College (1960) SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY at the MASSACHUSETTS Ii\STITUTE OF TECHNOLOGY February, 1965 Signature of Author Department Qf Geology and Geophysics, Novemder 12, 1964 Certified by- 7Th This 5 visor I./ I I 0' Accepted by_ Chairman,oDepartmental Committee on Graduate Students 2 ABSTRACT The Rb87-Sr87 Age of Stony Meteorites Robert M. Shields Submitted to the Department of Geology and Geophysics on November 12, 1964 in partial fulfillment of the requirements for the degree of Doctor of Philosophy. ABSTRACT - Refined Rb87-Sr 8 7 age measurements have been made on three stony meteorites: Pasamonte, a eucrite; Bath, an olivine-bronzite chondrite; and Bruderheim, an olivine-hypersthene chondrite. Each of these meteorites is a "fall' and was selected for its freshness of appear- ance and contamination free history. The analyses of these three meteorites have been combined with the analyses of three other fresh meteorites - Murray, a Type II carbonaceous chondrite analyzed by Beiser (1964), and Nakhla, a nakhlite, and Estherville, a mesosiderite, analyzed by Pinson et al., (lP63) - for the construction of a highly precise meteorite isochron. These six s ecimens possess a wide spread of Rb 8 /Sr86 and Sr /Sr 6 ratios and yield a 4 45 + .0 x 109 years isochron (for X18 = 1.39 x 10-1i years-1) with an initial Sr 8/Srgbratio of .6982. If one assumes that the Pb-Pb and Rb-Sr techniques give the same age for meteorites, a decay cons ant of 1.36 x 10-11 years-l can be calculated for Rb6 7 by comparing the Rb-Sr data with the Pb-Pb age of 4.55 x 109 years (Patterson, 1956). Insofar as the sampling has been carried out and within the precision of the measurements, all stony meteorites are of identical age. The major contribution of this work lies in the achievement of a high degree of anal- ytical precision. Improvements in both the chemical and mass spectrometric techniques of analysis have made possible the precise measurement of exceedingly small quantities of Rb and Sr such as are found in meteorites and some ultramafic rocks. Several phases were also separated from the Bjurble chondrite for Rb-Sr analyses. The results, however, indicate that this chondrite is severely con- taminated. Thesis Supervisor: William H. Pinson Title: Associate Professor of Geology 3 TABLE OF CONTENTS ABSTRACTA................................. LIST OF TABLES....00..................,..,..., LIST OF FIGURES...... ........... .. .... ... 12 ACKNOWLEDGEMENTS..... .......... o......... 0 0oo 16 PART I (Papers to be submitted for publication) THE Rb87-Sr87 AGE OF STONY METEORITES Abstract.... .. .. .. .. .. .. 19 Introduction...................... 19 Experimental Techniques................. 22 31 50 Acknowledgements.......... -- --- --.. - 58 References. .. ... ... .------------------. 59 Rb-Sr ANALYSES OF THE BJURBILE CHONDRITE 64 64 Experimental Techniques................. 64 69 71 Acknowledgements..............0000004090 76 References.o............,................. 77 PART II (Complete description of thesis investigation) CHAPTER I INTRODUCTION 1.1 - Introductory Meteoritics.......... 80 1.2 - Fundamental Principles of the Rb 87 - Sr Age Measurement Technique.... 98 1.3 - Previous Rb-Sr Age Investigations of Meteorites..................... 117 CHAPTER II EXPERIMENTAL TECHNIQUES 2.1 - Introduction...................... 131 2.2 - Changes Instituted in the Existing Chemical Procedure................ 132 2.21 - Reagent Purification....... 132 2.22 - Elimination of Pyrex....... 137 2.23 - Miscellaneous Improvements. 139 2.3 - Present Chemical Procedure........ 140 2.31 - Preparation of Samples..... 142 2.32 - Dissolving of Samples...... 143 2.33 - Removal of Insoluble Residue 144 2.34 - Ion Exchange Techniques.... 146 2.35 - FinalSteps................ 149 2.4 - Mass Spectrometry................. 150 CHAPTER III SPIKES AND 3HELF SOLUTIONS 3.1 - Preparation of Shields' Rb Shelf Solution.......... 154 3.2 - Preparation of Shields' Srt.......... Shelf Solution.......... 157 3.3 - Rubidium Spikes........., 16o 3.31 - Rb 8 DiluteSp 31.26 p s Rb/ml) 160 3.32 - New Rb Spike (2.7 pgms Rb/ml) ....... ..... a... 163 3.33 - Rb8 5/Rb87 Ratio o f Normal Rb 165 3.4 - Strontium Spikes........ 171 3.41 - Sr86 Dilute Spike (2.2 pgmsSrm)........ 171 3.42 - Sr-Sr84_S 86 Double Spike (A.-:1.05 pgns S/ i . 172 3.43 - Calculation of Sr8 /Sr8 6 Ratio in Original Sample from Sr Isotope Dilution Analysis......... 176 CHAPTER IV COMPUTER PROGRAMS 4.1 - Introduction...................... 182 4.2 - A Computer Program to Calculate a Least Squares Isochron and Assoc- 4.2 - (continued) iated Errors...................... 183 4.21 - Introduction............... 183 4.22 - What the Program Computes.. 183 4.23 - How to Use the Program..... 192 4.24 - Notation Used in the Program 198 4.3 - A Computer Program to Calculate Rb87/Sr86 Ratios.................. 201 4.31 - Introduction............... 201 4.32 - How to Use the Program..... 205 4.33 - Derivation of Equations Used to Calculate Rb Sr6 Ratios...... 207 4.34 - Notation Used in the Program 216 CHAPTER V DESCRIPTION OF SAMPLES 5.1 - Introduction............ ........ 218 5.2 - The Bruderheim Chondrite.......... 222 5.3 - The Bath Chondrite................ 225 5.4 - The Pasamonte Achondrite.......... 228 5.5 - The Bjurbole Chondrite............ 234 5.6 - The Murray Carbonaceous Chondrite. 239 5.7 - The Nakhla Achondrite............. 244 5.8 - The Estherville Mesosiderite...... 247 CHAPTER VI RESULTS 6.1 - Blanks 254 6.2 - SrCO 3 Isotope Ratio Standard...... 254 6.3 - Rb-Sr Analyses of Bruderheim, Bath and Pasamonte and the Meteorite Isochron.............. .... *........ 257 6.4 - Rb-Sr Analyses of Bjurbdle........ 272 6.5 - Errors and Improvements in Analytical 285 CHAPTER VII DISCUSSION 7.1 - The Meteorite Isochron and Other Recent Rb-Sr Meteorite Age Investi- 303 7.2 - Rb-Sr vs. the Pb-Pb Age of 314 7.3 - Rb-Sr vs. Gas Retention Ages of 325 7.4 - The Bjurbole Chondrite............ 329 7.5 - Leaching and the Use of Weathered Meteorites for Rb-Sr Analysis..... 333 CHAPTER VIII SUMY AND CONCLUSIONS......................, 341 CHAPTER IX SUGGESTIONS FOR FUTURE WORK.................. 344 BIBLIOGRAPHY................................. BIOGRAPHY ..... ............................... 364 9 LIST OF TABLES PART I Ia Rb and Sr Blank Determinations (Pinson et al., 1963, 10)..-...... 33 Ib Rb and Sr Blank Determinations (this wok..............34 Ic Reproducibility of SrCO 3Isotope Ratio Standard........................ 36 Id RubidiumAnalyses..................... 37 Ie Strontium Isotope Dilution Analyses... 39 If Strontium Isotope Ratio Analyses...... 40 Ig Summary of Rb-Sr Analyses............. 42 Ih Rb67/Sr86 and (Sr8 /Sr86 )* Ratiosfor Murray, Nakhla and Estherville........ 44 I1 Comparison of Overall Standard Devia- ti on s... .. 49 II Summary of Bjurble Chondrite Rb-Sr Analyse s. .. .. .. .. .... 7 0 PART II 3.31 Summary of Spike Calibration Runs on Rb8 7 Dilute Spike.......................... 161 3.32a Isotope Ratio Analyses of New Rb87 Spike................................. 00 0 00* 0 164 3.32b Spike Calibration Runs on New Rb8 7 Spike...............................6 166 3.41 Spike Calibration Runs on Sr 86 Dilute Sp ike- . .00- 000- a00 - - #0 -- #0 0*173 3.42a Spike Calibration Runs on Sr84-Sr86 Double Spike. .............. .. .. ... ... .. 174 3.42b Isotope Ratio Analyses on Sr -Sr86 Double Spike---------------* -*-*--- --- 175 3.43 Comparison of Measured Sr /Sr86 with Calculated Sr /Sr 86 181 5.la Reference Data for Meteorite Samples.. 219 5.lb. Chemical Analyses of Meteorite 220 6.1a Rb and Sr Blank Determinations (Pinson et al., 1963, 10)) ................... 255 6.1b Rb and Sr Blank Determinations (this work).................................... 256 6.2 Reproducibility of SrCO3 Isotope Ratio S tanda rd.. 0#0 . 0 .00,* 00. 9a258.. , 6.3a Rubidium Analyses..................... 260 6.3b Strontium Isotope Dilution Analyses... 261 6.3c Strontium Isotope Ratio Analyses...... 263 6.3d Summary of Rb-Sr Analyses............. 266 6.3e Rb87/Sr86 and (Sr8 7/Sr 86 )* Ratios for Murray, Nakhla and Estherville........ 268 Ii 6. 4 a BjurbUle Chondrite Rubidium Analyses.. 278 6.4b Bjurblle Chondrite Strontium Isotope Dilution Analyses..................... 280 6.4c Bjurbdle Chondrite Strontium Isotope Ratio Analyses.... 283 6.4d Summary of BjurbZle Chondrite Rb-Sr 284 6.5a Comparison of Overall Standard Deviations..*................... 295 6.5b Standard Deviations of Rb/Sr Ratios (Pinson et al., 1963)................. 297 6.5c Standard Deviations of Rb/Sr Ratios (this work).. ................ .... 298 7.3 Gas RetentionAges.................... 327 12 LIST OF FIGURES PART I Ia Schematic Diagram of Ion Exchange Column 27 Ib Sample of Mass Spectrometer Scan of Sr Spectrum.............................. 32 Ic Shields,' Meteorite Isochron............. 46 Id Computer Output Data for Shields' Meteorite I sochroni.. ........................ 47 le Pinson et al., (1963) Meteorite Isochron 51 Ila Chondrules Separated from Bjurb01e...... 66 IIb Photograph of Eight Largest BjurbUle Chondrules............................ 67 IIc Shields' Meteorite Isochron Including Bjurbble Analyses..................... 72 PART II 1.la Relationship of Fe in FeO to Fe in FeS Plus Metallic Fe in Chondrites.......... 83 Partial Phase Diagram for Fe-Ni System.. 92 Subsolidus Relationships in Fe-Ni Phase Diagram... .. .. .. ...
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