Magma Genesis Beneath Continents

大陆玄武岩为大陆岩石圈的性质和起源提供了最广泛的见解，但由于它们可能代表了熔体形成、源区迁移、岩浆房演化和喷发过程中所获得的贡献的混合物，因此这些过程的化学特性和相对作用是争论的主题。 镁铁质熔岩斑晶中熔体包裹体的同位素和化学研究提供了一种手段，以检查和量化的贡献，从这些过程之前，外来熔体已混合在熔体聚集和岩浆储存。拟议的工作的重点是调查这些贡献的0-8马碱性玄武岩的西马火山场东部加州。 地幔捕虏体内的熔岩表明，在Cima下面的浅层次的地幔成分是不均匀的，熔岩有一个有限的停留时间在地壳室。 因此，在岩浆中观察到的西马套件的铅同位素组成的大的变化最有可能反映他们的形成和通过地幔运输过程中获得的签名，而不是从大陆地壳同化。 拟议的研究将结合联合收割机现场技术（例如，电子和离子微探针），以表征主要，微量和铅同位素组成的熔体和矿物包裹体中的橄榄石和其他阶段的镁铁质（MgO 6 wt. %）火山熔岩。 我们的研究结果将解决是否被困在大陆熔岩中的熔融包裹体的成分和铅同位素的变化，甚至大于在包含它们的熔岩套件中观察到的，从中我们可以更好地划定组成域的性质和起源在地幔中的水平比那些采样的捕虏体更深。从这个项目的结果可以结合正在进行的研究中的同位素和成分的变化被困在大洋玄武岩斑晶熔融包裹体，以提高我们的理解过程中显着的玄武岩岩浆的生成。

BryceEAR-0207939Continental basalts provide the broadest insights into the nature and origin of the continental lithosphere but, because they may represent a mixture of contributions acquired during melt formation, transport from the source region, evolution in a magma chamber, and eruption, the chemical identities and the relative roles of these processes are topics of debate. Isotopic and chemical studies of melt inclusions in phenocrysts of mafic lavas provide a means to examine and quantify contributions from these processes before exotic melts have been mixed away during melt aggregation and magma storage. The focus of the proposed work is to investigate these contributions in the 0-8 Ma alkali basalts of the Cima volcanic field of eastern California. Mantle xenoliths contained within the lavas demonstrate that the mantle at shallow levels beneath Cima is compositionally heterogeneous and that the lavas had a limited residence time in crustal chambers. Accordingly, the large variations in Pb isotopic compositions observed in magmas of the Cima suite most likely reflect signatures acquired during their formation and transport through the mantle, rather than from assimilation of continental crust. The proposed study will combine in situ techniques (e.g., electron and ion microprobe) to characterize the major, trace, and Pb isotopic compositions of melt and mineral inclusions in olivine and other phases from mafic (MgO 6 wt. %) Cima lavas. Our results will address whether the compositional and Pb isotopic variety of melt inclusions trapped in continental lavas is even greater than that observed in the suite of lavas that contain them, from which we can better delimit the nature and origin of compositional domains in the mantle at levels deeper than those sampled by xenoliths. The results from this project can be combined with ongoing studies of the isotopic and compositional variability of melt inclusions trapped in oceanic basaltic phenocrysts to improve our understanding of the processes significant in the generation of basaltic magmas.