The Search for Subducted Components in the Mantle: A Boron and Lithium Isotope, and Fluid-Mobile Element Study of Mount Erebus

寻找地幔中俯冲的成分：硼和锂同位素以及埃里伯斯山的流体移动元素研究

• 批准号: 9980421
• 负责人: Jeffrey Ryan
• 金额: $ 3.96万
• 依托单位: University of South Florida
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2000
• 资助国家: 美国
• 起止时间: 2000-02-15 至 2003-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9980421&HistoricalAwards=false
• 关键词: Search; Subducted; Components; Mantle; Boron

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports research examine volcanic rocks from Mount Erebus, Antarctica, for evidence of previously subducted materials that might be mixed into the mantle source region beneath this volcano. Many workers have, over the years, suggested that the radiogenic isotopic signatures of basaltic volcanic rocks from intraplate volcanic centers reflect the development of heterogeneities in the Earth's mantle brought about largely by the chemical processes of subduction at convergent plate margins.Generally, however, data for incompatible trace elements, which should reflect to some degree the variability recorded by the isotopes, suggests little heterogeneity in intraplate mantle sources relative to one another, or to the mantle source of mid-ocean ridge basalts (MORBs). While part of the difference between the isotopic and trace element record of intraplate sources may certainly relate to the time factor involved in "growing-in" a radiogenic isotope signature, it is reasonable to expect that given the unique chemical signatures of subducted materials, and the profound changes that basaltic crust and sediments on the slab undergo during subduction metamorphism, that some recognizable "fingerprint" of subduction-related material recycling should be left on some portion of the mantle. A problem in seeing such a signature may lay in the evolutionary history of intraplate lavas, as trace element signatures in particular may be obscured by crystallization and assimilatory processes occurring as intraplate magmas evolve in crustal magma chambers.This project is a focused study that uses new and sensitive tracers of subduction effects - boron and lithium isotopes, and the abundances of "fluid-mobile" elements (B, Cs, Rb, Pb, U, As, Sb) - to examine the volcanic products of a single intraplate site - Mount Erebus, in Antarctica - as a means of discovering the influence that subduction has had on the development of intraplate mantle sources. Mount Erebus is a well characterized intraplate volcano of HIMU isotopic affinities, and its different lava series are documented as recording the effects of both closed and open system evolution of parental magmas. Erebus lavas ranging from primitive basanites to evolved phonolites and trachytes, along with a selection of associated anorthoclase phenocrysts are available for study.Boron isotope and B abundance systematics during subduction-zone processes are well understood, and intraplate lavas record both boron abundance and B isotope anomalies, reflecting a mantle source distinct from that of ocean ridge basalts or lavas from volcanic arcs. The systematics of Li during subduction are well understood also, and Li isotopes, while a relatively new system, show great potential as a means of resolving among mantle sources, especially those with subduction zone affinities. Both to, we shall conduct High precision B and Li isotope and abundance measurements, and high precision analyses of "fluid-mobile" elements will be undertaken in order to characterize the mantle source region of Erebus and to search for distinctions between the mantle source of Erebus and the mantle source of other basaltic volcanoes. These data should allow the effects of magma differentiation and assimilation in Erebus lavas to be determined so that source signatures can be identified and key questions can be addressed about whether or not subduction processes (and/or subducted materials) were involved in the development of this intraplate mantle source region.This project will contribute to the training of undergraduate students and will contribute to a greater understanding of the sources of basaltic magmas in general.

该奖项由极地项目办公室的南极地质和地球物理项目提供，支持对南极洲埃里伯斯火山的火山岩进行研究，以寻找以前俯冲物质可能混合到该火山下地幔源区域的证据。多年来，许多工作者提出，来自板内火山中心的玄武岩火山岩的放射性成因同位素特征反映了地幔非均质性的发展，这种发展主要是由收敛板块边缘俯冲的化学过程引起的。然而，一般来说，不相容微量元素的数据在一定程度上反映了同位素记录的变异性，表明板内地幔源之间或洋中脊玄武岩（MORBs）地幔源之间的非均质性很小。虽然板块内来源的同位素和微量元素记录之间的部分差异肯定与放射性成因同位素特征“渐近”所涉及的时间因素有关，但考虑到俯冲物质的独特化学特征，以及板块上玄武岩地壳和沉积物在俯冲变质过程中所经历的深刻变化，我们有理由期望，与俯冲有关的物质循环的一些可识别的“指纹”应该留在地幔的某些部分。看到这种特征的一个问题可能在于板内熔岩的进化史，因为在地壳岩浆室中，当板内岩浆演化时，微量元素的特征可能会被结晶和同化过程所掩盖。该项目是一项重点研究，利用新的、敏感的俯冲作用示踪剂——硼和锂同位素，以及“流体流动”元素（B、Cs、Rb、Pb、U、As、Sb）丰度——来研究南极洲埃里伯斯火山（Mount Erebus）一个板块内地点的火山产物，作为发现俯冲作用对板块内地幔源发育的影响的一种手段。埃里伯斯火山是一座极具HIMU同位素亲和特征的板内火山，其不同的熔岩系列记录了母岩浆封闭和开放体系演化的影响。从原始玄武岩到演化的空云母岩和粗叶岩的黑云熔岩，以及相关的斜长石斑晶可供研究。俯冲带过程中的硼同位素和B丰度系统得到了很好的了解，板内熔岩记录了硼丰度和B同位素异常，反映了不同于海脊玄武岩或火山弧熔岩的地幔源。在俯冲过程中，Li同位素的分系统也得到了很好的认识，虽然Li同位素是一个相对较新的体系，但它作为地幔源，特别是具有俯冲带亲缘关系的地幔源之间的分辨手段显示出很大的潜力。我们将进行高精度的B和Li同位素和丰度测量，并进行高精度的“流体流动”元素分析，以确定埃里伯斯地幔源区的特征，并寻找埃里伯斯地幔源与其他玄武岩火山地幔源之间的区别。这些数据将有助于确定埃里巴斯熔岩的岩浆分化和同化作用，从而识别源特征，并解决俯冲过程（和/或俯冲物质）是否参与了该板内地幔源区发育的关键问题。这个项目将有助于培养本科生，并有助于更好地了解玄武岩岩浆的一般来源。