BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. ee PP. 1131-1148 SEPTEMBER 1955 ISOTOPIC COMPOSITION AND DISTRIBUTION OF LEAD, URANIUM, AND THORIUM IN A PRECAMBRIAN GRANITE By GEORGE R. TILTON, CLAIRE PATTERSON, HARRISON BROWN, MARK INGHRAM, RICHARD HAYDEN, DAVID HESS, AND ESPER LARSEN, JR. ABSTRACT The isotopic compositions and concentrations of lead and uranium have been deter- mined in some separated minerals and the composite of a granite from Monmouth town- ship, Haliburton County, Ontario. The chemical and mass spectrometric methods that were used are described. The age of the zircon from the granite is 1050 million years. Much of the lead, uranium, and thorium exists in chemically unstable and presumably interstitial phases of the granite. A comparison of the observed amounts of uranium, thorium, and lead in the various minerals with those amounts that should have been pres- ent, had these three elements existed within the minerals as closed systems, shows a non- balance of these elements in every case. It appears that the granite as a whole has closely approximated a closed system since it was formed with respect to uranium and its decay products, but has been an open system with respect to thorium and its decay products. Interpretations concerning the relationship of these data to lead ores are discussed. CONTENTS TEXT Page Conclusions 1147 Page References cited 1147 Introduction 1131 Acknowledgments 1132 TABLES Rock description 1132 Table Page Chemical procedures 1134 1 Chemical analysis, norm, and mode of Contamination 1134 granite from near Tory Hill, Ontario.. 1133 Reagents and apparatus 1135 2. Semiquantitative spectrographic analysis Isolation of lead 1135 of granite from Tory Hill, Ontario, and Isolation of uranium 1136 of three minerals separated from the Isolation of thorium 1137 granite 1134 Mass spectrometry of uranium and lead 1137 3. Uranium, thorium, and lead concentration General 1137 and isotopic compositions of lead in vari- Lead procedure 1138 ous constituents of the granite 1140 Uranium procedure 1139 4. Comparison of lead and thorium values ob- Accuracy 1139 tained by different methods 1141 Results 1139 5. Ages for the granite and the Wilberforce Discussion 1141 pegmatite 1142 Uranium-lead age of granite 1141 6. Contribution by each mineral to the ura- Potassium-argon age of the granite 1142 nium, thorium, and lead content of the Loss or gain of uranium, thorium, and lead total granite 1145 in the granite 1143 7. Comparisons between the observed and Distribution of uranium, thorium, and lead calculated amounts of radiogenic leads, in in the granite 1144 various constituents of the granite 1145 Material balance 1145 8. Calculated isotopic compositions of lead in Internal transfer of uranium, thorium, and the granite at various times 1146 lead 1145 9. Average composition of lead ores at various Relation to lead ores 1146 times 1147 INTRODUCTION in amounts of but a few parts per million, the It has long been recognized that if techniques range of materials amenable to precise radio- could be developed to determine precisely the active dating would be enormously increased. isotopic compositions and concentrations of lead In addition, such techniques would permit more and uranium, when these elements are present detailed studies of the time and space relations 1131 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/66/9/1131/3431742/i0016-7606-66-9-1131.pdf by guest on 27 September 2021 1132 TILTON ET AL—LEAD, URANIUM, AND THORIUM IN A GRANITE of radioactive elements and their decay ACKNOWLEDGMENTS products. The isotope dilution technique of quantita- We wish to express our gratitude that the tive analysis, first described by Hayden, Rey- facilities of the Argonne National Laborato- nolds, and Inghram (1949, p. 1500) and later ries, the Department of Terrestrial Magnetism applied (Inghram et al., 1950, p. 916) to the of the Carnegie Institution, the U. S. Geologi- study of calcium and argon in Stassfurt sylvite, cal Survey, the Institute for Nuclear Studies has been applied in this work to lead, thorium, of the University of Chicago, and the California and uranium. The techniques described in this Institute of Technology could be combined to paper make possible both the precise determi- make these studies possible. We are indebted nation of as little as 0.01 part per million of to The Geological Society of America for a uranium and thorium and 0.1 part per million grant which has supported some of this work. of lead in silicates, and the determination of The portion of the work undertaken at the the isotopic composition of lead totaling only a California Institute of Technology was sup- few micrograms in a sample. ported by the U. S. Atomic Energy Commis- Briefly, the isotope dilution method of anal- sion (Contract #AT-11-1-208). We sincerely thank Kenneth Jenson of the Argonne National ysis operates as follows: If the uranium content Laboratories for the careful and precise potas- constituting about one part per million of a sium analyses, Claude Waring of the U. S. mineral is to be determined, approximately 5 Geological Survey for making available the re- grams of the mineral are dissolved and a few 235 sults of the spectrographic analyses for lead micrograms of U containing but a small frac- to which he has devoted considerable time and 238 236 tion of U are added to the solution. The U effort, and H. S. Armstrong of McMasters Uni- becomes mixed with the uranium from the un- versity for providing the sample of pegmatite known which is primarily U238. The mixture of perthite. We are grateful for the advice and uranium from the unknown and the uranium criticism of our colleagues, in particular Gunnar tracer is extracted from the solution, and the Kullerud of the Mineralogisk-Geologisk Mu- new ratio of U235 to U238 is determined. Knowing seum, Oslo, Norway, and Leon Silver, Cali- the initial sample weight, the weight of U236 fornia Institute of Technology. tracer added, and the original and final ratios of U235 to U238, one can readily compute the ROCK DESCRIPTION concentration of uranium in the unknown. The It was agreed that the granite to be used for application of the technique to lead, although this investigation should be of Precambrian age more complicated, is essentially the same. and should fulfill the following requirements: The main obstacles to be overcome in this it should not have been altered or recrystal- work were contamination during the isolation lized, but it should be moderately radioactive; of such small quantities of material and diffi- it should contain a moderate amount of lead culty in measuring accurately the isotopic com- and at least average amounts of the accessory position of extremely small samples. Therefore minerals zircon, apatite, sphene, and magne- extreme care was taken to control contamina- tite; its potash feldspar should be rich in lead tion and a combination of surface ionization so that the isotopes of the lead of the original and electron multiplier detection techniques magma could be determined. The late Dr. H. were used in the mass spectrometer. V. Ellsworth supplied granite that satisfied The techniques developed for uranium, tho- most of these requirements from a source he rium, and lead have been applied to a study described as follows: of their distribution in a Precambrian granite. "On Map No. S2a, Haliburton area, Province of The studies were not exhaustive, but were un- Ontario, (Satterly, 1943) the area between Tory Hill and Essonville marked with the symbol 'Ra' dertaken primarily to test the method and represents roughly the area from which the granite demonstrate its applicability to geochemical samples were taken and the symbols probably the actual openings. This granite is from the Brower- and geochronological problems. Simpson property." Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/66/9/1131/3431742/i0016-7606-66-9-1131.pdf by guest on 27 September 2021 ROCK DESCRIPTION 1133 The geology of the area between Tory Hill Semiquantitative spectrographic analyses of and Essonville is shown on a map by Adams the granite and of apatite, sphene, and zircon and Barlow (1910) and on a modified map by separated from the granite are given in Table 2. TABLE 1.—CHEMICAL ANALYSIS, NORM, AND MODE or GRANITE FROM NEAR TORY HILL, ONTARIO Chemical analysis by A. M. Sherwood, U.S. Geological Survey, Washington, D.C. Chemical Analysis Norm Mode* SiO2 72.55 Q 24.24 Quartz 24 ± 1 TiO2 0.25 or 30.02 Perthite 52 ± 2 A1203 14.29 ab 41.39 Plagioclase 20 ± 1 Fe203 1.41 an 0.83 Hornblende 2 ± 1 FeO 0.54 C 0.41 Magnetite 0.4 ± 0.2 MnO 0.02 mt 1.16 Zircon 0.04 ± 0.01 MgO 0.06 il 0.46 Apatite 0.02 ± 0.01 CaO 0.17 hm 0.64 Sphene 0.4 ± 0.1 Na20 4.92 hy 0.20 Pyrite 0.02 K2O 5.06 Calcite trace H2O- 0.09 + H2O 0.14 P206 0.01 U 0.001 Total 99.50 * The mode was estimated from the chemical analysis, thin sections, and the amounts of separated minerals. Satterly (1943); both, on a scale of 2 miles to About 50 pounds of the granite were ground the inch, show metasedimentary rocks of the by hand to pass 15 mesh and sized. The miner- Grenville series at Tory Hill and Essonville als were separated by using bromoform, meth- and granite on the road between the two vil- ylene iodide, and the Frantz Isodynamic sepa- lages, with the contact between the Grenville rator with special precautions to avoid lead and the granite about a mile east of the road. contamination (Larsen, Keevil and Harrison, In October 1952, Larsen investigated the excel- 1952, p.