Initial Pb of the Amîtsoq gneiss, West Greenland, and implications for the age of the Earth

Abstract

Pb isotopic abundances and U-Th-Pb concentrations are reported for feldspar megacrysts from the 3.59 AE old Amîtsoq gneisses, Godthaab District, West Greenland. The distinctive Pb in the feldspars is the most primitive terrestrial Pb so far observed. It is observed in feldspars which are from different geographic localities and which exhibit varying degrees of deformation and recrystallization. This appear to be either the initial Pb in the Amîtsoq gneiss or the initial Pb only slightly modified by subsequent metamorphism in a low ^(238)U/^(204)Pb environment. ^(238)U/^(204)Pb in the feldspars is low and the corrections for in situ produced Pb are only 0.4% for ^(207)Pb/^(206)Pb and 0.6% for ^(204)Pb/^(206)Pb. The mean corrected isotopic abundances are ^(204)Pb/^(206)Pb = 0.08720, ^(207)Pb/^(206)Pb = 1.1513, and ^(208)Pb/^(206)Pb = 2.7350. The feldspars contain a very small amount of easily leachable radiogenic Pb which is correlated with U and which indicates the formation of U-rich phases at about 2.7 AE. The matrix surrounding the feldspar megacrysts contains Pb which is much evolved relative to the megacrysts and this matrix does not appear to have behaved as a simple closed system. Element redistribution and open system behavior at about 2.7 AE is also suggested by Pb in feldspar from a dike cutting across the gneiss. Assuming that the Amîtsoq gneiss feldspar Pb corrected for in situ U decay was the initial Pb in the gneiss at 3.59 AE (Baadsgaard, 1973), a single-stage "age of the earth" is determined as 4.47 ± 0.05 AE and μ is 8.5. This is indistinguishable from the single-stage age for modern rocks and is distinctly younger than the 4.55 AE age of some meteorites. If the feldspar Pb was modified by metamorphism at 2.7 AE the model age of the earth is calculated as 4.53 AE which is similar to the 4.55 AE age of some meteorites. Two-stage models using the nominal 3.59 AE initial Pb indicate that if the earth is ~4.55 AE old then μ values were low in the early Earth's history and differentiation occurred within a few hundred million years after the planet formed.

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© 1977 Pergamon Press. Received 21 December 1976; accepted in revised form 21 April 1977. F. TERA has generously given his time in teaching one of us (A.J.G.) the skills necessary to do U-Th-Pb analyses. He has also spent many hours discussing U-Th-Pb systematics and the importance of initial Pb. We thank G. R. TILTON for his critical review of the manuscript. The samples used in this study were collected by Project Oldstone. The project was initiated and organized at California Institute of Technology for the purpose of collecting samples of "ancient" anorthositic rocks from West Greenland for direct comparison with lunar anorthositic rocks. The Oldstone project, which collected not only the anorthositic rocks in West Greenland but also the Amîtsoq gneisses and other associated rocks, was financed by NSF Grant GA-31729. This grant was originally issued for the absolute age dating of rocks 50,000 to 100,000 years old, but through the efforts of B. FRENCH the funds were permitted to be used for the study of somewhat older rocks. We thank A. L. ALBEE for the enormous effort he put into organizing and participating in Project Oldstone and for his encouragement of this research. V. R. McGREGOR of the Greenland Geological Survey (GGU) was most helpful in arranging contacts for us in Godthaab and escorting us in the fjords, teaching us to "recognise" Amîtsoq gneisses and teaching us to swim. D. BRIDGWATER of the GGU also escorted us in the Godthaad region and helped in sample collecting. The immediate members of the Project Oldstone are J. ALBEE, A. L. ALBEE, R. F. DYMEK, R. KIECKHEFER, C. W ASSERBURG, R. BURGHARDT and J. 0STERGAARD. We also thank L. RANCITELLI for generously providing the γ-ray data. This work was supported by NSF grants DES-75-03417 and PHY-76-02724.

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Initial Pb of the Amîtsoq gneiss, West Greenland, and implications for the age of the Earth

Abstract

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