Collaborative Research: Geochemistry of the Source Regions of Southwest Pacific Volcanic Islands: An Approach Using Ultramafic Xenoliths & Host Basalts

这是斯克里普斯研究所的詹姆斯·纳特兰和麻省理工学院的弗雷德·弗雷合作的项目。它是对来自西南太平洋萨摩亚群岛、社会群岛和马克萨斯群岛的超镁铁质包体及其寄主熔岩的综合结构、矿物学和地球化学研究。捕虏体在现有的收集物中，初步的矿物学和同位素研究表明，它们与这些火山下面的部分熔融和岩浆分异过程有关。这些岛链包含海洋盆地中一些最丰富的玄武岩熔岩，这些熔岩被认为与地幔中广泛存在的、可能是半球状的地球化学异常有关。包体应提供关于地幔源区成分、交代过程在影响或改变源区成分方面的作用，以及洋岛玄武岩源区及其附近的熔体提取和岩浆分异过程的补充信息。到目前为止的研究表明，在所有三个岛链中都存在残余(类型1)和“堆积”(类型2)包体，相当程度的部分熔融可能产生拉斑岩浆，使来自地幔的橄榄石-尖晶石-辉石组合结晶。这样的熔岩可能包括这些火山大量的、淹没的、但仍未取样的部分。应用于捕虏体的主要技术将是：1)岩相学；2)矿物相的电子探针显微分析；3)矿物中微量元素丰度的离子探针分析；以及4)全岩和矿物分离的质谱仪(常规固体源和电感耦合等离子体质谱)、XRF和仪器中子活化分析的组合。步骤1和步骤2将用于选择适当范围的样品进行详细的离子探针、同位素和痕量元素研究。大量的化学、微量元素和同位素测量将在宿主熔岩上进行，这些测量与岛屿上的时间序列有关，这些都是已知的。还将与已发表的关于大陆和夏威夷捕虏体套装的信息进行比较。

This is a collaborative project between James Natland of Scripps Institution, and Fred Frey of MIT. It is an integrated textural, mineralogical, and geochemical study of ultramafic xenoliths and their host lavas from the Samoan, Society, and Marquesas Islands in the southwest Pacific. The xenoliths are in existing collections, and preliminary mineralogical and isotopic work demonstrates that they are related to processed of partial melting and magmatic differentiation beneath these volcanoes. The island chains contain some of the most enriched basaltic lavas in the ocean basins, which are hypothesized to be related to a regionally widespread, perhaps hemispheric, geochemical anomaly in the mantle. Xenoliths should provide complementary information on the compositions of mantle sources, the action of metasomatic processes in influencing or modifying source compositions, and the processes of melt extraction and magmatic differentiation in and near the source regions of ocean-island basalts. Studies so far indicate that both residual (Type 1) and "cumulate" (Type 2) xenoliths are present in all three island chains, and that substantial degrees of partial melting produced possible tholeiitic magmas which crystallized olivine-spinel- pyroxene assemblages en route from the mantle. Such lavas may comprise much of the voluminous, submerged, but still unsampled portions of these volcanoes. Principal techniques to be applied to the xenoliths will be 1) petrography; 2) electron-probe microanalysis of mineral phases; 3) ion-probe analysis of trace-element abundances in minerals, and 4) a combination of mass spectrometry (conventional solid- source and ICP-MS), XRF, and instrumental neutron-activation analysis for whole-rocks and mineral separates. Steps 1 and 2 will be used to select the appropriate range of samples for detailed ion-probe, isotope, and trace-element studies. Bulk chemical, trace-element, and isotopic measurements will be made on host lavas and these related to temporal successions on the islands, which are already known. Comparisons will also be made to published information on continental and Hawaiian xenolith suites.