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

南极超高温麻粒岩相岩石中的铍及其在下陆壳部分熔融中的作用

• 批准号: 0087235
• 负责人: Edward Grew
• 金额: $ 9.81万
• 依托单位: University of Maine
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-01-15 至 2004-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0087235&HistoricalAwards=false
• 关键词: Beryllium; Antarctic; Ultrahigh; Temperature; Granulite

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.

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。地壳深处部分熔融形成花岗岩液体是当今火成岩和变质岩岩石学研究的主要课题之一。该研究领域的一个方面是该过程的开始，特别是在熔体离开源区之前熔体与烃源岩之间的地球化学相互作用。受地壳深处条件影响的变质岩深熔作用的一个例子是南极洲东部恩德比地太古代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峰值条件已达到近 1100 摄氏度和 11 千气压，即太古宙地球下地壳的条件。拟议的研究是对纳皮尔复杂伟晶岩的研究，重点是铍的矿物和地球化学。据估计，该元素占地球上地壳的 3 ppm，但在受地球下地壳条件影响的变质岩中很少发现显着浓度的元素。将进行结构、年代学和矿物学研究，以检验铍伟晶岩是由晚太古代超高温变质事件期间变泥质母岩深熔形成的假设。将分析主岩的主量元素和微量元素。将通过电子微探针分析矿物质的主要成分，包括氟，并通过离子微探针分析锂、铍和硼。分析数据将用于确定如何在超高温条件下从主岩中提取铍和其他微量成分，并随后浓缩在花岗岩熔体中，最终在伟晶岩中结晶为铍蓝宝石和克马拉长石，这是伟晶岩中未发现的矿物别处。矿物成分和组合将用于确定超高温事件后变质过程中伟晶岩及其主岩的结晶和重结晶的演化和条件。将分析独居石中的铅、钍和铀，以确定这些事件的年龄。由于氟有助于调动铍，因此作为高级项目的一部分，本科生将研究伟晶岩中的氟磷酸镁硅镁石，以估计熔体中的氟活性。本项目的结果将为了解一般熔化过程以及铍的地球化学行为（特别是在地球下地壳的高温和低水活动特征下）提供重要的见解。