Secular Evolution in the Composition of the Mantle

当今地幔成分的变化是由于在地质年代中发生的一系列地壳-地幔和地幔内部分异过程。这些古老的过程中有一些与今天发生的过程明显不同。大陆地壳的形成对确定现今地幔的组成有最大的影响，但这究竟是如何以及何时发生的仍有争议。提出了对现代和古代基性到超基性熔岩及其熔融包裹体进行一系列高质量的地球化学测量，将对地幔成分的长期演化产生新的认识。在这些研究中，最重要的将是利用微束技术对古熔岩中含有橄榄石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。从广泛不同的时间和空间关联将调查含有橄榄石的熔融包裹体的科马长岩样品。这些微光束技术还将用于确定现代海洋玄武岩玻璃部分的组成，以便确定熔岩及其源区中不相容元素的“恒定比例”的变化。描述地幔组成的长期演化特征将有助于加强对地壳-地幔和地幔内分化过程及其发生速度的理解。

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.