Geochemical Evolution Of The Mantle

HartEAR-0125917这项赠款的首要目标是了解几个已知的非均质地幔区域的位置、成因、演化和岩性，以及从这些地幔区域熔融和熔体提取的动力学。在我们最后一次更新期间，几个范式“扭曲”一直在进行中。一个是越来越多的证据表明，中尺度地幔的不均一性(矿脉、豆荚、层)可能是造成在地幔熔体中观察到的某些同位素不均一性的重要因素。这反过来又要求更好地了解熔化和熔体输送过程。第二种观点认为，最深的地幔可能与上覆的地幔对流隔离(边界约为1700公里)。这反过来又要求更好地了解地幔热柱的化学和岩性组成，因为它们可能为我们提供对深部地幔最直接的采样。一些同位素定义的地幔区域被认为是岩石圈再循环的结果，但我们对这种物质的亲缘关系或岩性表现，或其熔融动力学知之甚少。我们将探索两种途径来应对这一问题。第一个是对萨摩亚热点(最极端的EM2地幔热柱)的深入研究。萨摩亚岩柱的高87Sr/86Sr(~0.7089)及其独特的化学组成，传统上归因于含有陆源沉积组分的古代岩石圈的再循环。然而，至今还没有一个模型能令人满意地解释EM2热点的许多令人困惑的地球化学特征(例如，富87Sr/86Sr、重d18O和高3He/4He共存)。我们提议对萨摩亚以西较老的海山的火山岩、乌波卢西部和萨瓦伊(被认为是萨摩亚最古老的空中火山)西部的盾状熔岩以及萨摩亚以东两座较年轻的海山的玄武岩进行分析研究。所有这些地点的样品套房都在手中。这项工作将包括对这些火山的40Ar/39Ar测年，以检验年龄演化假说，以及微量元素和同位素工作，以阐明萨摩亚EM2组分的谱系。第二条途径是研究EM1和EM2玄武岩斑晶中熔融包裹体的同位素组成和微量元素特征(使用原位离子探针和激光烧蚀电感耦合等离子体质谱技术)。总的来说，在熔体包裹体中捕获的地球化学多样性远远超过在喷发的宿主熔岩中观察到的。这些熔体包裹体的性质可能代表了预聚的熔体，这将对EM热点火山活动所涉及的熔融岩性和熔体传输网络提供重要的信息。我们应该能够区分大规模和小规模的地幔不均质性，以及镁铁质和超镁铁质岩性；这应该允许对EM2柱的循环岩石圈模型进行坚定的测试。

HartEAR-0125917This grant has as its overarching goals the understanding of the placement, genesis, evolution and lithologic nature of the several known heterogeneous mantle domains, and the dynamics of melting and melt extraction from these mantle domains. Several paradigm "warps" have been in progress during the period of our last renewal. One is the increasing evidence that mesoscale mantle heterogeneities (veins, pods, layers) may be important in creating some of the isotopic heterogeneity observed in mantle-derived melts. This in turn demands a better understanding of melting and melt transport processes. The second is the concept that the deepest mantle may be convectively isolated from the overlying mantle (with a boundary at circa 1700 km). This in turn demands a better understanding of the chemical and lithologic make-up of mantle plumes, as they may provide our most direct sampling of the deep mantle. Several of the isotopically-defined mantle domains are thought to be a result of lithospheric recycling, and yet we have little understanding of the parentage or lithologic expression of this material, or of its melting dynamics. We will explore two avenues as a means to confront this problem. The first is an in-depth study of the Samoa hotspot (the most extreme EM2 mantle plume). The high 87Sr/86Sr (~ 0.7089) of the Samoan plume, and its distinctive chemistry, is conventionally ascribed to recycling of ancient lithosphere containing a terrigenous sediment component. However, no model has as yet satisfactorily accounted for many of the puzzling geochemical characteristics of EM2 hotspots (e.g. the coexistence of enriched 87Sr/86Sr, heavy d18O, and high 3He/4He). We propose to undertake analytical studies of volcanics from the older seamounts west of Samoa, of shield lavas from western Upolu and Savai'i (putatively the oldest subaerial volcano in Samoa), and of basalts from two younger seamounts east of Samoa. Sample suites from all of these locations are in hand. This work will include 40Ar/39Ar dating of these volcanoes, to test the age-progression hypothesis, and trace element and isotopic work to elucidate the pedigree of the EM2 component in Samoa.The second avenue is a study of the isotopic composition and trace element signatures of melt inclusions in phenocrysts from EM1 and EM2 basalts (using in-situ ionprobe and laser-ablation ICP/MS techniques). In general, the geochemical diversity captured in melt inclusions far exceeds that observed in the erupted host lavas. The nature of these melt inclusions, which likely represent pre-aggregated melts, will shed significant light on the melting lithologies and melt transport networks involved in EM hotspot volcanism. We should be able to discriminate between large-scale and small-scale mantle heterogeneities, and between mafic and ultramafic lithologies; this should allow a firm test of the recycled lithosphere model for EM2 plumes.