کد مقاله کد نشریه سال انتشار مقاله انگلیسی نسخه تمام متن
9528969 1637784 2005 21 صفحه PDF دانلود رایگان
عنوان انگلیسی مقاله ISI
Evidence from kimberlitic zircon for a decreasing mantle Th/U since the Archean
موضوعات مرتبط
مهندسی و علوم پایه علوم زمین و سیارات ژئوشیمی و پترولوژی
پیش نمایش صفحه اول مقاله
Evidence from kimberlitic zircon for a decreasing mantle Th/U since the Archean
چکیده انگلیسی
A large TIMS, SIMS and LA-ICPMS database of Th and U concentrations for kimberlite-hosted zircon, particularly from Cr-poor megacrystic suites, now exists (Kinny et al., 1989, Berryman et al., 1999, Griffin et al., 2000, Spetsius et al., 2002; Appendix A U-Th-Pb age of Monastery zircon, Appendix B U-Th-Pb age of Ekati, Northwest Territories, zircon, this work). Six suites comprising 10 or more zircon grains with ages between 90 and 2550 Ma reveal consistent patterns when plotted on Th/U vs. U diagrams. We interpret these patterns as resulting from fractional crystallization of a melt with kimberlite affinity presumably derived from the asthenosphere, permitting the extrapolation to an initial Th/U at the time zircon crystallization began. A two-fold decrease is seen in this ratio over the past 2.5 Ga, suggesting that during this time a similar change has occurred in the parent silicate melt. Estimates of Th and U distribution coefficients between zircon and coexisting melt permit calculation of Th/U in the melt, which, for these highly incompatible elements, presumably is the same as for its mantle source rock. Kimberlitic zircon may thus indeed give evidence of a reduction in κ, tentatively calculated as from ∼ 4 to ∼ 2, since the Archean for the depleted asthenosphere.
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Chemical Geology - Volume 220, Issues 3–4, 5 August 2005, Pages 263-283
نویسندگان
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