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

该奖项由极地项目办公室的南极地质与地球物理项目提供，用于支持一个研究铍在地壳下部部分熔融事件中的作用的项目。地壳深处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这一过程的开始，具体地说，是熔体与烃源岩在熔体离开烃源区之前的地球化学相互作用。在变质岩中发现的受地壳深处条件影响的深熔岩的一个例子是南极洲东部恩德比地的太古代超高温麻粒岩相Napier杂岩中的伟晶岩。据估计，这种麻粒岩相变质作用的峰值条件达到了近1100摄氏度和11千巴，即太古宙时期地球下地壳的条件。提出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，这种元素在地球上地壳中占3ppm，在受地球下地壳条件影响的变质岩中很少发现任何显著浓度的元素。将进行构造、年代学和矿物学研究，以验证铍伟晶岩是在太古宙晚期的超高温变质事件中由其变质岩为主岩的深熔形成的假设。将分析宿主岩石的主要和微量元素。矿物将通过电子显微探针分析包括氟在内的主要成分，并通过离子显微探针分析锂、铍和硼。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主岩石中提取出来的，然后集中在花岗岩熔体中，最终结晶成伟晶岩，形成绿柱石蓝宝石和红晶石，这些矿物质在其他地方的伟晶岩中找不到。矿物组成和组合将用于确定超高温变质期伟晶岩及其寄主岩的结晶和再结晶的演化和条件。将对独居石进行铅、钍和铀的分析，以确定这些事件的年代。由于氟有助于调动铍，本科生将研究伟晶岩中的氟磷酸镁镁矿，以估计熔体中氟的活性，这是高级项目的一部分。本项目的结果将为一般熔融过程和铍的地球化学行为提供重要的见解，特别是在地球下地壳的高温和低水活动特征下。

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.