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

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岩流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过程的开始，具体地说，就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。变质岩中受地壳深处条件影响的深熔作用的一个例子是东南极恩德比地太古宙超高温麻粒岩相纳皮尔杂岩中的伟晶岩。据估计，这种麻粒岩相变质作用的顶峰条件已达到近1100摄氏度和11千巴，即太古代地球下部地壳的条件。拟议的研究是对纳皮尔杂岩伟晶岩的研究，重点是铍的矿物和地球化学。据估计，这种元素占地球上地壳的3ppm，在受地球下地壳条件制约的变质岩中很少有明显的浓度。将进行构造、年代学和矿物学研究，以验证铍伟晶岩是晚太古代超高温变质事件期间变质寄主岩石深熔的结果。将对主岩进行常量和微量元素分析。矿物的主要成分将通过电子微探针进行分析，包括氟和锂、铍和硼的离子微探针进行分析。这些分析数据将被用来确定如何在超高温条件下从寄主岩石中提取铍和其他微量成分，然后浓缩到花岗岩熔体中，最终在伟晶岩中结晶出铍蓝宝石和红柱石，这两种矿物在其他地方的伟晶岩中找不到。矿物组成和组合将被用来确定伟晶岩及其寄主岩石在超高温事件后的变质阶段的演化和结晶重结晶的条件。独居石将被分析铅、钍和铀，以确定这些事件的年龄。由于氟有助于动员铍，一名本科生将研究伟晶岩中的氟磷酸盐镁，以估计熔体中的氟活度，作为高级项目的一部分。本项目的结果将对一般的熔化过程，特别是在地球下部地壳高温和低水活动的情况下铍的地球化学行为提供重要的见解。

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.