Hafnium Isotopic Approaches to Precambrian Crust-Mantle Evolution

该项目的主要目的是评估已报道的几种早期太古宙片麻岩的初始（Nd）值的巨大变化。为了确定这些初始Nd值是真实的还是由Sm-Nd系统中的干扰产生的，对早期地球演化模型进行认真和彻底的讨论是至关重要的。该项目的初步工作已经完成，并表明这些早期片麻岩中锆石的Hf同位素组成与其全岩Nd同位素组成一致，从而证实了早期地幔不相容元素的衰竭。P.I.s还不能分析那些初始Nd值为负（低至-4.0）和高正（高达+4.0）的片麻岩中的锆石。侦探这个项目的基础工作已经证明了这些岩石的复杂性，解决这个问题只能通过分析精心挑选的样本来实现。P.I.s提出了三项研究：(1)调查格陵兰片麻岩中报道的高阳性（Nd）值；(2)评估已知Sm-Nd干扰样品中Lu-Hf和U-Pb的干扰；(3)评价了阿卡斯塔片麻岩的Nd值范围（-4 ~ +4）。

9526536 Patchett The primary aim of this project is to evaluate the large variability in initial (Nd values that have been reported for several early Archean gneisses. It is essential for a serious and thorough discussion of models of early Earth evolution to determine whether these initial (Nd values are real or if they are produced by disturbances in the Sm-Nd system. Preliminary work on this project has been completed and has shown that the Hf isotopic compositions of zircons from these early gneisses are consistent with their whole-rock Nd isotopic compositions and as such verify the early incompatible-element depletion of the mantle. The P.I.s have not yet been able to analyze zircons from those gneisses with negative (as low as -4.0) and highly positive (up to +4.0) initial (Nd values. The P.I.'s groundwork on this project has demonstrated the complexity of these rocks, and that a solution to this problem can only be achieved by analysis of carefully selected samples. The P.I.s proposes three studies: (1) investigation of highly positive (Nd values reported for Greenland gneisses; (2) evaluation of disturbances in Lu-Hf and U-Pb in samples with known Sm-Nd disturbances; (3) evaluation of the range of Nd values (-4 to +4) in the Acasta gneisses.