Secular Evolution in the Composition of the Mantle

9506517雅各布森现今地幔的成分变化是在地质时期内若干次壳-幔和地幔内分异作用的结果。这些古老的过程中，有许多与今天发生的过程有明显的不同。陆壳的产生对确定当今地幔的组成产生了最大的影响，但具体是如何以及何时发生的仍存在争议。建议对现代和古代镁铁质熔岩-超镁铁质熔岩及其熔体包裹体进行一系列高质量的地球化学测量，这将为地幔成分的长期演化提供新的见解。在这些研究中，最重要的将是利用微束技术对古熔岩中以橄榄石为宿主的熔体包裹体进行研究，以确定原始熔岩及其源区的成分特征。将对含有橄榄石寄主熔体包裹体的科马提岩样品进行调查，这些包裹体来自不同的时间和空间组合。这些微束技术还将用于确定现代大洋玄武岩的玻璃部分的成分，以便确定熔岩及其源区中不相容元素的“恒定比例”的变化。确定地幔成分的长期演化特征将有助于更好地了解壳-地幔和地幔内的分异过程及其发生的速率。

9506517 Jacobsen The compositional variation of the present-day mantle results from a number of crust-mantle and intra-mantle differentiation processes operating over geologic time. Some of these ancient processes many have been markedly different from those occurring today. The production of continental crust has had the greatest influence in establishing the composition of the present-day mantle, but exactly how and when this occurred is controversial. It is proposed to conduct a series of high quality geochemical measurements of modern and ancient mafic to ultramafic lavas and their melt inclusions that will yield new insights into the secular evolution in the composition of the mantle. Foremost among these studies will be the use of microbeam techniques on olivine-hosted melt inclusions in ancient lavas to determine the compositional characteristics of the primary lavas and their source regions. Komatiitic samples containing olivine-hosted melt inclusions from widely varying temporal and spatial associations will be investigated. These microbeam techniques will also be applied to determine the compositions of glassy portions of modern oceanic basalts in order to establish variations in "constant ratios" of incompatible elements in the lavas and their source regions. Characterizing the secular evolution of the composition of the mantle will lead to a enhanced understanding of crust-mantle and intra-mantle differentiation processes and the rate at which they occurred.