Boron in Antarctic granulite-facies rocks: under what conditions is boron retained in the middle crust?

南极麻粒岩相岩石中的硼：硼在什么条件下保留在中地壳中？

• 批准号: 0228842
• 负责人: Edward Grew
• 金额: --
• 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: 2003
• 资助国家: 美国
• 起止时间: 2003-06-01 至 2009-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0228842&HistoricalAwards=false
• 关键词: Boron; Antarctic; granulite; facies; rocks

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports a project to investigate the role and fate of Boron in high-grade metamorphic rocks of the Larsemann Hills region of Antarctica. Trace elements provide valuable information on the changes sedimentary rocks undergo as temperature and pressure increase during burial. One such element, boron, is particularly sensitive to increasing temperature because of its affinity for aqueous fluids, which are lost as rocks are buried. Boron contents of unmetamorphosed pelitic sediments range from 20 to over 200 parts per million, but rarely exceed 5 parts per million in rocks subjected to conditions of the middle and lower crust, that is, temperatures of 700 degrees C or more in the granulite-facies, which is characterized by very low water activities at pressures of 5 to 10 kbar (18-35 km burial). Devolatization reactions with loss of aqueous fluid and partial melting with removal of melt have been cited as primary causes for boron depletion under granulite-facies conditions. Despite the pervasiveness of both these processes, rocks rich in boron are locally found in the granulite-facies, that is, there are mechanisms for retaining boron during the metamorphic process. The Larsemann Hills, Prydz Bay, Antarctica, are a prime example. More than 20 lenses and layered bodies containing four borosilicate mineral species crop out over a 50 square kilometer area, which thus would be well suited for research on boron-rich granulite-facies metamorphic rocks. While most investigators have focused on the causes for loss of boron, this work will investigate how boron is retained during high-grade metamorphism. Field observations and mapping in the Larsemann Hills, chemical analyses of minerals and their host rocks, and microprobe age dating will be used to identify possible precursors and deduce how the precursor materials recrystallized into borosilicate rocks under granulite-facies conditions. The working hypothesis is that high initial boron content facilitates retention of boron during metamorphism because above a certain threshold boron content, a mechanism "kicks in" that facilitates retention of boron in metamorphosed rocks. For example, in a rock with large amounts of the borosilicate tourmaline, such as stratabound tourmalinite, the breakdown of tourmaline to melt could result in the formation of prismatine and grandidierite, two borosilicates found in the Larsemann Hills. This situation is rarely observed in rocks with modest boron content, in which breakdown of tourmaline releases boron into partial melts, which in turn remove boron when they leave the system. Stratabound tourmalinite is associated with manganese-rich quartzite, phosphorus-rich rocks and sulfide concentrations that could be diagnostic for recognizing a tourmalinite protolith in a highly metamorphosed complex where sedimentary features have been destroyed by deformation. Because partial melting plays an important role in the fate of boron during metamorphism, our field and laboratory research will focus on the relationship between the borosilicate units, granite pegmatites and other granitic intrusives. The results of our study will provide information on cycling of boron at deeper levels in the Earth's crust and on possible sources of boron for granites originating from deep-seated rocks.An undergraduate student will participate in the electron microprobe age-dating of monazite and xenotime as part of a senior project, thereby integrating the proposed research into the educational mission of the University of Maine. In response to a proposal for fieldwork, the Australian Antarctic Division, which maintains Davis station near the Larsemann Hills, has indicated that they will support the Antarctic fieldwork.

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，支持一个研究硼在南极洲拉尔森丘陵地区高品位变质岩中的作用和命运的项目。微量元素为沉积岩在埋藏过程中随着温度和压力的增加而发生的变化提供了宝贵的信息。其中一种元素，硼，对温度升高特别敏感，因为它对含水流体有亲和力，当岩石被掩埋时，含水流体就会消失。未变质的泥质沉积物的硼含量从百万分之20到百万分之200以上不等，但在中下地壳条件下（即麻粒岩相温度为700摄氏度或更高）的岩石中，硼含量很少超过百万分之5，其特征是在5至10千巴的压力下（18-35公里的埋藏）水活动非常低。在麻粒岩相条件下，含水流体损失的脱挥发反应和熔体去除的部分熔融反应被认为是硼耗损的主要原因。尽管这两种作用都很普遍，但在麻粒岩相中局部发现了富硼岩石，即在变质过程中存在保留硼的机制。南极洲Prydz湾的Larsemann Hills就是一个典型的例子。在50平方公里的区域内，有20多个含有4种硼硅酸盐矿物的透镜体和层状体，因此非常适合研究富硼麻粒岩相变质岩。虽然大多数研究人员都集中在硼流失的原因上，但这项工作将研究硼在高变质作用期间是如何保留的。在Larsemann Hills的实地观察和制图、矿物及其寄主岩石的化学分析和微探针年龄定年将用于确定可能的前体，并推断前体物质如何在麻粒岩相条件下重新结晶成硼硅酸盐岩石。工作假设是，高初始硼含量促进了变质过程中硼的保留，因为超过一定阈值的硼含量，一种机制“启动”，促进了变质岩石中硼的保留。例如，在含有大量硼硅酸盐电气石的岩石中，如层状电气石，电气石的分解融化可能会形成棱柱石和辉绿石，这是在拉尔森山发现的两种硼硅酸盐。这种情况在硼含量适中的岩石中很少观察到，其中电气石的分解将硼释放成部分熔体，当它们离开系统时，这些熔体又将硼除去。层控电气石与富锰石英岩、富磷岩石和硫化物浓度相关，可用于在沉积特征已被变形破坏的高度变质杂岩中识别电气石原岩。由于部分熔融在变质过程中对硼的命运起着重要作用，我们的野外和实验室研究将集中在硼硅酸盐单元与花岗岩伟晶岩和其他花岗岩侵入岩之间的关系上。我们的研究结果将为地壳深部硼的循环提供信息，并为源自深部岩石的花岗岩提供硼的可能来源。作为高级项目的一部分，一名本科生将参与monazite和xenotime的电子显微探针年龄测定，从而将拟议的研究纳入缅因大学的教育使命。负责维护拉斯曼山附近戴维斯站的澳大利亚南极部在回应实地考察的提议时表示，他们将支持南极实地考察。