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

该奖项由极地计划办公室南极地质和地球物理计划提供，支持研究检查来自南极洲埃里伯斯山的火山岩，以寻找可能混合到该火山下方地幔源区的先前俯冲物质的证据。多年来，许多研究者认为，来自板内火山中心的玄武质火山岩的放射性同位素特征反映了地幔中的不均匀性的发展，这种不均匀性主要是由会聚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，表明板内地幔源相对于彼此或洋中脊玄武岩（MORB）的地幔源的异质性很小。虽然板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同位素特征“生长”中所涉及的时间因素有关，但有理由预期，鉴于俯冲物质的独特化学特征，以及板片上的玄武质地壳和沉积物在俯冲变质作用期间经历的深刻变化，一些可识别的“指纹”的俯冲有关的材料回收应留在地幔的某些部分。观察这种特征的一个问题可能在于板内熔岩的演化历史，特别是微量元素的特征可能会被地壳岩浆房中板内岩浆演化时发生的结晶和同化过程所掩盖。该项目是一项重点研究，使用新的和敏感的俯冲效应示踪剂--硼和锂同位素，以及“流体流动”元素的丰度（B、Cs、Rb、Pb、U、As、Sb）--检验一个单一板内地点--南极洲的埃里伯斯山--的火山产物，作为发现俯冲作用对板内地幔源发展的影响的一种手段。埃里伯斯山是一个很好的特点板内火山的HIMU同位素亲和力，其不同的熔岩系列记录为封闭和开放系统的母岩浆演化的影响。Erebus熔岩范围从原始的玄武质到演化的响岩和粗面岩，沿着与相关的斜长岩斑晶的选择是可供研究的。俯冲带过程中的硼同位素和B丰度系统是很好的理解，板内熔岩记录硼丰度和B同位素异常，反映了地幔源不同于大洋中脊玄武岩或火山弧熔岩。Li在俯冲过程中的系统学也得到了很好的理解，Li同位素虽然是一个相对较新的系统，但作为解决地幔来源，特别是与俯冲带密切相关的地幔来源的一种手段，显示出巨大的潜力。为此，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“流体活动”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区与其他玄武质火山地幔源区的区别。这些数据应该能够确定埃里伯斯熔岩中岩浆分异和同化的影响，以便能够识别源特征，并解决俯冲过程是否（和/或俯冲物质）参与了这个板内地幔源区的开发。这个项目将有助于培养本科生，并将有助于更好地了解玄武岩岩浆的来源。

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.