Geochemical Evolution Of The Mantle

地幔的地球化学演化

• 批准号: 0125917
• 负责人: Stanley Hart
• 金额: $ 49.95万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-12-01 至 2005-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0125917&HistoricalAwards=false
• 关键词: Geochemical; Evolution; Mantle

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.

该基金的总体目标是了解几个已知的非均质地幔域的位置、成因、演化和岩性性质，以及这些地幔域的熔融和熔体提取动力学。在我们上一次更新期间，已经进行了几次范式“扭曲”。一个是越来越多的证据表明，中尺度地幔非均质性（脉状、豆荚状、层状）可能对形成在地幔衍生熔体中观察到的一些同位素非均质性很重要。这反过来又要求对熔化和熔体输运过程有更好的了解。第二个概念是最深的地幔可能与上覆的地幔（在大约1700公里处有一个边界）对流隔离。这反过来又要求我们更好地理解地幔柱的化学和岩性组成，因为它们可能为我们提供最直接的深部地幔样本。一些同位素确定的地幔域被认为是岩石圈再循环的结果，但我们对这些物质的来源或岩性表达，或其熔融动力学知之甚少。我们将探索两种途径来解决这个问题。第一个是对萨摩亚热点（EM2最极端的地幔柱）的深入研究。萨摩亚羽流的高87Sr/86Sr（~ 0.7089）及其独特的化学成分通常被认为是古代岩石圈的再循环，其中含有陆源沉积物成分。然而，目前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，富集的87Sr/86Sr、重d18O和高3He/4He的共存）。我们建议对萨摩亚西部较老的海底山的火山岩、西乌波卢和萨瓦伊（据推测是萨摩亚最古老的陆上火山）的盾状熔岩以及萨摩亚东部两个较年轻的海底山的玄武岩进行分析研究。所有这些地点的样本套房都在手中。这项工作将包括对这些火山的40Ar/39Ar定年，以测试年龄进展假说，以及微量元素和同位素工作，以阐明萨摩亚EM2成分的谱系。第二个途径是研究EM1和EM2玄武岩斑晶中熔体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ICP/MS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很可能代表了预聚合的熔体，这将为EM热点火山活动中涉及的熔体岩性和熔体运输网络提供重要的信息。我们应该能够区分大尺度和小尺度的地幔非均质性，以及基性和超基性岩性；这将允许对EM2羽流的再循环岩石圈模型进行可靠的测试。