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.

HARTEAR-0125917赠款具有其总体目标，即对几个已知的异质地幔域的放置，起源，进化和岩性性质的理解，以及从这些地幔域中熔化和融化的动力学的动力学。在我们上次续签期间，有几个范式“扭曲”正在进行中。越来越多的证据表明，中尺度的地幔异质性（静脉，豆荚，层）对于在地幔衍生的熔体中观察到的一些同位素异质性可能很重要。反过来，这需要更好地了解熔化和融化运输过程。第二个概念是，最深的地幔可以从上覆的地幔（大约1700 km的边界）对流分离。反过来，这需要更好地了解地幔羽的化学和岩性构成，因为它们可能会提供我们对深幔的最直接采样。几个同位素定义的地幔结构被认为是岩石圈回收的结果，但是我们对这种材料的亲父体系或岩性表达或其熔化动力学几乎没有理解。我们将探索两种途径，以此来解决这个问题。第一个是对萨摩亚热点（最极端的EM2地幔羽流）的深入研究。萨摩亚羽流的高87SR/86SR（〜0.7089）及其独特的化学作用通常归因于含有陆质沉积物成分的古代岩石圈的回收。但是，尚无模型令人满意地解释了EM2热点的许多令人困惑的地球化学特征（例如，富集的87SR/86SR的共存，重型D18O和High 3He/4he）。我们建议从萨摩亚以西的较旧海峡，西部Upolu和Savai'i（萨摩亚最古老的萨摩亚火山的最古老的山地）以及来自萨摩亚以东的两个年轻海峡的玄武岩的火山进行分析研究。所有这些位置的样品套件都在手里。这项工作将包括这些火山的40AR/39AR日期，以测试年龄的假设以及痕量元素和同位素工作，以阐明Samoa中EM2成分的谱系。第二大道是对近2和Em1和Em2的近2个基于近2的同位素组成和痕量元素的研究的研究激光燃料ICP/MS技术）。通常，熔体夹杂物中捕获的地球化学多样性远远超过了在爆发的宿主熔岩中观察到的。这些熔体夹杂物的性质，可能代表预处理的熔体，将对涉及EM热点火山的融化岩性和熔融传输网络阐明。我们应该能够区分大规模和小规模的地幔异质性，以及镁铁质和超镁铁质岩性。这应该允许对EM2羽流的再生岩石圈模型进行牢固的测试。