Geochemical Evolution of the Mantle

[9804891]这项以成果为基础的更新工作的总体目标是了解几个已知的非均质地幔域的位置、成因、演化和岩性性质，以及这些地幔域的熔融和熔体提取动力学。其中两个域的锇同位素组成尚未得到充分表征。为了解决这一问题，将对被认为来自EM1地幔域的海洋岛屿玄武岩（皮特凯恩岛和圣诞岛）和来自世界所有主要扩张脊系统的深海橄榄岩（MORB域）中的难熔铬铁矿进行锇同位素研究。一些地幔域被认为是岩石圈再循环的结果，但这些物质的岩性表达和熔融动力学尚不清楚。这些问题将通过两个途径进行探讨。离子探针对不同洋岛玄武岩矿物（斑晶）熔体包裹体Pb同位素组成和微量元素特征的研究，有助于揭示其熔体岩性和熔体输运网络。第二种方法将利用Th和Ra同位素数据与加拉帕戈斯群岛一系列历史熔岩的熔体包裹体研究相结合。从单一地幔源的大范围熔融程度来看，这些岛屿的明显衍生性将允许对批量熔融与部分熔融、渗透熔融与集中熔体输送、连续熔体再平衡与高速管道逃逸、均匀源岩性与小规模（脉状）非均质性等问题施加限制。

9804891HartThis accomplishment-based renewal has, as its overarching goals, the understanding of the placement, genesis, evolution and lithologic nature of the several known heterogeneous mantle domains, and the dynamics of melting and melt extraction from these mantle domains. Two of these domains have yet to be adequately characterized for their osmium isotopic composition. In order to address this, osmium isotope studies will be conducted on ocean island basalts (Pitcairn and Christmas Islands) thought to be derived from the EM1 mantle domain and on refractory chromites in abyssal peridotites (MORB domain) from all of the major spreading ridge systems of the world. Several of Earth's mantle domains are thought to be a result of lithospheric recycling, yet the lithologic expression and melting dynamics of this material are not understood. These issues will be explored via two avenues. Ion microprobe studies of the Pb isotope composition and trace element signatures of mineral (phenocryst) melt inclusions from various ocean island basalts will shed light on the melting lithologies and melt transport networks involved. The second approach will utilize Th and Ra isotope data in conjunction with melt inclusion studies for a series of historic lavas from the Galapagos Islands. The apparent derivation of a number of these islands by a wide range in degree of melting from a single mantle source will allow constraints to be placed on issues of batch versus fractional melting, percolative versus focused melt transport, continuous melt re-equilibration versus escape in high velocity conduits, and homogenous source lithology versus small-scale (veined) heterogeneities.