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，但是，由于它们可能代表了熔体形成、源区运输、岩浆室演化和喷发过程中获得的混合贡献，因此这些过程的化学特性和相对作用是有争议的话题。基性熔岩斑晶中熔体包裹体的同位素和化学研究提供了一种方法，可以在外来熔体在熔体聚集和岩浆储存过程中混合之前，检查和量化这些过程的贡献。本文的工作重点是在加利福尼亚东部西马火山带0-8 Ma碱性玄武岩中研究这些贡献。熔岩中包含的地幔包体表明，西马地下浅层地幔的成分是不均匀的，熔岩在地壳室中的停留时间有限。因此，在西马套岩浆中观测到的铅同位素组成的巨大变化很可能反映了它们在形成和通过地幔运输过程中获得的特征，而不是来自大陆地壳的同化。拟议的研究将结合原位技术（如电子和离子探针）来表征基性（MgO 6 wt. %）西玛熔岩中橄榄石和其他相的熔体和矿物包裹体的主要、痕量和Pb同位素组成。我们的研究结果将解决大陆熔岩中熔体包裹体的成分和铅同位素变化是否比包含它们的熔岩套件中观察到的更大，由此我们可以更好地界定比捕虏体取样更深层次的地幔中成分域的性质和起源。该项目的结果可以与目前正在进行的海洋玄武岩斑晶中熔体包裹体同位素和成分变化的研究相结合，以提高我们对玄武岩岩浆生成过程的理解。

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.