Beryllium in Antarctic Ultrahigh-Temperature Granulite-Facies Rocks and its Role in Partial Melting of the Lower Continental Crust

0087235Grew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，支持一个研究铍在下地壳部分熔融事件中的作用的项目。地壳深部部分熔融形成花岗质液体是当今火成岩和变质岩石学研究的主要课题之一。这一研究领域的一个方面是该过程的开始，特别是熔体离开源区之前熔体和源岩之间的地球化学相互作用。受地壳深部条件影响的变质岩中的深熔作用的一个例子是南极洲东部恩德比地太古代低温麻粒岩相纳皮耶杂岩中的伟晶岩。这种麻粒岩相变质作用的峰值条件估计达到了近1100摄氏度和11帕，即太古代地球下地壳的条件。拟议的研究是对纳皮耶杂伟晶岩的研究，重点是铍的矿物和地球化学。据估计，这种元素在地球上地壳中的含量为百万分之三，但在受到地球下地壳条件影响的变质岩中很少发现这种元素。将进行结构，年代学和矿物学的研究，以测试的假设，铍伟晶岩造成的变质寄主岩石在超高温变质事件在太古代晚期。将分析寄主岩石的主要和微量元素。矿物的主要成分将用电子微探针分析，包括氟，锂、铍和硼则用离子微探针分析。分析数据将用于确定铍和其他微量成分是如何在超高温条件下从寄主岩石中提取出来，随后浓缩在花岗岩熔体中，最终在伟晶岩中结晶出来，成为铍石和Khmaralite，这些矿物在其他地方的伟晶岩中没有发现。矿物成分和组合将被用来确定伟晶岩及其寄主岩石的结晶和重结晶的演化和条件，在超高温事件后的变质事件。独居石将分析铅，钍和铀，以确定这些事件的年龄。由于氟在动员铍中起着重要作用，因此一名本科生将研究伟晶岩中的氟磷酸镁瓦氏体，以估计熔体中的氟活性，作为高级项目的一部分。本项目的结果将提供关于熔化过程的重要见解，特别是关于铍在地球下地壳特有的高温和低水活动情况下的地球化学行为。

0087235GrewThis award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports a project to investigate the role of beryllium in lower crustal partial melting events. The formation of granitic liquids by partial melting deep in the Earth's crust is one of the major topics of research in igneous and metamorphic petrology today. One aspect of this sphere of research is the beginning of the process, specifically, the geochemical interaction between melts and source rocks before the melt has left the source area. One example of anatexis in metamorphic rocks affected by conditions found deep in the Earth's crust is pegmatite in the Archean ultrahigh temperature granulite-facies Napier Complex of Enderby Land, East Antarctica. Peak conditions for this granulite-facies metamorphism are estimated to have reached nearly 1100 Degrees Celsius and 11 kilobar, that is, conditions in the Earth's lower crust in Archean time. The proposed research is a study of the Napier Complex pegmatites with an emphasis on the minerals and geochemistry of beryllium. This element, which is estimated to constitute 3 ppm of the Earth's upper crust, is very rarely found in any significant concentrations in metamorphic rocks subjected to conditions of the Earth's lower crust. Structural, geochronological, and mineralogical studies will be carried out to test the hypothesis that the beryllium pegmatites resulted from anatexis of their metapelitic host rocks during the ultrahigh-temperature metamorphic event in the late Archean. Host rocks will be analyzed for major and trace elements. Minerals will be analyzed by the electron microprobe for major constituents including fluorine and by the ion microprobe for lithium, beryllium and boron. The analytical data will be used to determine how beryllium and other trace constituents were extracted from host rocks under ultrahigh-temperature conditions and subsequently concentrated in the granitic melt, eventually to crystallize out in a pegmatite as beryllian sapphirine and khmaralite, minerals not found in pegmatites elsewhere. Mineral compositions and assemblages will be used to determine the evolution and conditions of crystallization and recrystallization of the pegmatites and their host rocks during metamorphic episodes following the ultrahigh-temperature event. Monazite will be analyzed for lead, thorium and uranium to date the ages of these events. Because fluorine is instrumental in mobilizing beryllium, an undergraduate student will study the magnesium fluorphosphate wagnerite in the pegmatites in order to estimate fluorine activity in the melt as part of a senior project. The results of the present project will provide important insights on the melting process in general and on the geochemical behavior of beryllium in particular under the high temperatures and low water activities characteristic of the Earth's lower crust.