I: Isotopic Systematics in Archean Rocks, West Greenland. II: Mineralogic and Petrologic Investigations of Lunar Rock Samples

Citation

Gancarz, Alexander John, Jr. (1976) I: Isotopic Systematics in Archean Rocks, West Greenland. II: Mineralogic and Petrologic Investigations of Lunar Rock Samples. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/S5C5-9E60. https://resolver.caltech.edu/CaltechETD:etd-07062006-115658

Abstract

Pb isotopic abundances and U-Th-Pb concentrations are reported for feldspar megacrysts from the 3.59 AE old Amitsoq Gneiss, 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 appears to be either the initial Pb in the Amitsoq Gneiss or the initial Pb only slightly modified by subsequent metamorphism in a low 238U/204Pb environment. 238U/204Pb in the feldspars is low and the corrections for in situ produced Pb are only 0.4 per cent for 207Pb/206Pb and 0.6 per cent for 204Pb/206Pb. The mean corrected isotopic abundances are 204Pb/206Pb=0.08720, 207Pb/206Pb = 1.1513, and 208Pb/206Pb = 2.7309. The feldspars contain a very small amount of easily leachable radiogenic Pb which is strongly 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 dyke cutting across the gneiss. Assuming that the Amitsoq 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 age of some meteorites. If we assume that the earth originally formed at 4.6 AE and assume that it underwent major, large-scale differentiation at a time T(D), we use the same observed data to calculate that the time of differentiation is approximately 4.4 AE and that µ for the total earth is approximately 1.6 while µ for the mantle and crustal rocks is approximately 9.5. Pb and Sr isotopic data and K-Rb-Sr-U-Th-Pb concentration data are presented for three anorthosite complexes in West Greenland. The three complexes are the Majorqap Qava outcrop of the Fiskenaesset Anorthosite Complex, the Storo Anorthosite Complex, Godthaab District, and the Ivnajaugtoq Anorthosite Complex, Godthaab District. The Pb isotopic data yield an age of 2.8 AE for the Majorqap Qava Anorthosite and Storo Anorthosite. The Ivnajaugtoq data are consistent with a 2.8 AE age. We interpret this as the time of igneous crystallization and metamorphic recrystallization. U-Pb data indicate substantial U redistribution within the last ~100 m.y. These three complexes have very different initial Sr isotopic abundances. These differences indicate that either the anorthosites crystallized at 2.8 AE from melts with different µ's which were derived and fractionated at ~3.6AE from "normal" (i.e. µ~9) mantle material and remained isolated until 2.8 AE or the anorthosites crystallized at 2.8 AE from melts derived at 2.8 AE from "normal" mantle material and which were variably contaminated during emplacement with Pb like that in the Amitsoq Gneiss. Included are a series of published articles which are primarily concerned with the mineralogic and petrographic characterization of lunar highland samples.

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