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

该奖项由极地方案办公室的南极地质和地球物理方案提供，支持对南极埃里巴斯火山的火山岩进行研究，以寻找以前被俯冲的物质可能混合到这座火山下面的地幔源区的证据。多年来，许多工作人员认为，板内火山中心玄武岩的放射性同位素特征反映了地幔中不均质性的发展，这主要是由会聚的板块边缘俯冲的化学过程引起的。然而，一般而言，不相容的微量元素数据在一定程度上反映了同位素记录的可变性，表明板内地幔来源相对于彼此或大洋中脊玄武岩(MORBs)的地幔来源几乎没有不均一性。虽然板内源的同位素和微量元素记录之间的部分差异肯定与“生长”放射性同位素特征所涉及的时间因素有关，但有理由预计，鉴于俯冲物质的独特化学特征，以及板块上的玄武岩结壳和沉积物在俯冲变质过程中经历的深刻变化，与俯冲有关的物质再循环的一些可辨认的“指纹”应该留在地幔的某些部分。看到这种迹象的问题可能存在于板内熔岩的进化史中，因为随着板内岩浆在地壳岩浆室中的演化，特别是微量元素的特征可能被结晶和同化过程所掩盖。该项目是一项重点研究，使用新的和灵敏的俯冲效应示踪剂--硼和锂同位素，以及“流体可移动”元素(B、Cs、Rb、Pb、U、As、Sb)的丰度--来研究单一板内地点--南极洲的埃里伯斯山--的火山产物，以此作为发现俯冲对板内地幔来源发展的影响的一种手段。埃里巴斯火山是一座具有HIMU同位素亲和力的板内火山，其不同的熔岩系列记录了母岩浆闭合和开放系统演化的影响。从原始玄武岩到演化的方英岩和粗面岩，以及一些伴生的斜长石斑晶，都可供研究。俯冲带过程中的硼同位素和B丰度系统学已被很好地了解，板内熔岩记录了B丰度和B同位素异常，反映了不同于洋脊玄武岩或火山弧熔岩的地幔来源。Li在俯冲过程中的系统学也得到了很好的了解，Li同位素虽然是一个相对较新的系统，但在地幔来源之间，特别是那些具有俯冲带亲和力的来源之间显示出巨大的潜力。为此，我们将进行高精度的B和Li同位素和丰度测量，并将进行高精度的“流体活动”元素分析，以确定Erebus的地幔源区的特征，并寻找Erebus的地幔来源与其他玄武岩火山的地幔来源之间的区别。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，以便确定源区特征，并解决关于板块内地幔源区是否涉及俯冲过程(和/或俯冲物质)的关键问题。该项目将有助于本科生的培训，并将有助于从总体上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.