Application of Boron Isotopes to Constrain the Depositional Environment of the Precursors to Proterozoic Granulite-Facies Borosilicate Paragneisses, Larsemann Hills, Antarctica

应用硼同位素约束南极拉尔斯曼山元古界麻粒岩相硼硅酸盐副片麻岩前驱体的沉积环境

• 批准号: 0837980
• 负责人: Edward Grew
• 金额: $ 19.07万
• 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: 2009
• 资助国家: 美国
• 起止时间: 2009-08-01 至 2013-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0837980&HistoricalAwards=false
• 关键词: Application; Boron; Isotopes; Constrain; Depositional

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."Intellectual Merit. Major challenges in understanding high P-T granulite facies terranes are identifying the original protolith rocks and the tectonic environment in which they were originally deposited. This project focuses on granulitic gneisses exposed in the Larsemann Hills, Prydz Bay (East Antarctica). These rocks are considered to be metasediments (paragneisses), but, despite more than 20 years of intense study, the tectonic framework of deposition of the original rocks is still debated. However the origin of these rocks has important implications for determining the assembly of the ancient Gondwana continent. The unique B-rich character of the rocks offers potential insight into the original environment of deposition as well as the subsequent tectonic and metamorphic history. Because of its relative affinity for aqueous fluids and silicate melts, B typically is strongly depleted in such high-grade metamorphic rocks; the fact that some remain extremely B-rich (with up to 20% tourmaline) is highly enigmatic. In this study, B isotopic composition will be used to investigate the initial cause of boron enrichment as well as processes that redistribute boron during metamorphism. Boron has two isotopes that differ significantly in atomic weight: 10B and 11B. As an example of the discriminating power of B isotopic measurements, non-marine evaporates are characterized by unusually depleted in 10B relative to 11B, whereas typical marine sediments have high 11B/10B (i.e., inherited from seawater). B isotopic composition will be measured in situ using secondary ion mass spectroscopy (SIMS) of tourmaline and other borosilicate minerals from samples obtained in the Larsemann Hills during the 2003-2004 field season. B isotope data will be used to constrain compositions of the protolith rocks, and these data combined with results of petrologic studies to constrain the effects of metamorphism, anatexis, devolatilization leading to formation of B-rich gneisses. By analogy with similar rocks from Broken Hill, Australia, he proposed research will test the hypothesis that the Larseman Hills protolith originated as B-rich non-marine evaporates, that the original B was partly mobilized by submarine hydrothermal fluids that reacted with clastic sediments and volcaniclastic rocks to form tourmaline-bearing rocks (the protolith), and that the B-rich character of the gneisses was preserved via preservation of tourmaline and other borosilicate minerals during metamorphism and anatexis.Broader Impacts. The project will support one graduate student, who will analyze borosilicate and associated minerals in the Larseman Hills gneisses as a MA thesis project. This student will learn how to (1) use the electron microprobe to measure chemical composition, (2) become acquainted with measurement of lithium, beryllium and boron with the ion microprobe, (3) relate chemical composition to geologic environment, (3) develop reactions based on chemical and textural data (4) present her results orally before her peers, and (5) write a scientific paper for publication in a peer-reviewed journal.

“该奖项是根据 2009 年美国复苏和再投资法案（公法 111-5）资助的。”智力优点。了解高 P-T 麻粒岩相地体的主要挑战是识别原始原岩及其最初沉积的构造环境。 该项目的重点是普里兹湾（南极洲东部）拉尔斯曼山中暴露的粒状片麻岩。这些岩石被认为是变沉积岩（副片麻岩），但是，尽管经过 20 多年的深入研究，原始岩石沉积的构造框架仍然存在争议。然而，这些岩石的起源对于确定古冈瓦纳大陆的组装具有重要意义。岩石独特的富硼特征为了解原始沉积环境以及随后的构造和变质历史提供了潜在的见解。由于其对水性流体和硅酸盐熔体的相对亲和力，B 在此类高品位变质岩中通常会严重贫化；事实上，有些仍然富含 B（电气石含量高达 20%），这一事实非常神秘。在本研究中，B同位素组成将用于研究硼富集的初始原因以及变质过程中硼的重新分配过程。硼有两种原子量显着不同的同位素：10B 和 11B。作为 B 同位素测量的区分能力的一个例子，非海洋蒸发物的特点是 10B 相对于 11B 异常减少，而典型的海洋沉积物具有较高的 11B/10B（即从海水继承）。 B 同位素组成将使用二次离子质谱 (SIMS) 对电气石和其他硼硅酸盐矿物进行原位测量，这些样品取自 2003-2004 年野外季节在 Larsemann Hills 获得的样品。 B同位素数据将用于限制原岩的成分，并将这些数据与岩石学研究结果相结合，以限制变质作用、深熔作用、脱挥发分的影响，从而导致富含B的片麻岩的形成。通过与澳大利亚布罗肯山的类似岩石进行类比，他提出研究将检验以下假设：拉斯曼山原岩起源于富含 B 的非海相蒸发物，原始 B 部分是由与碎屑沉积物反应的海底热液流动的，并且火山碎屑岩形成含电气石的岩石（原岩），片麻岩的富含 B 的特征通过电气石和其他硼硅酸盐的保存得以保留变质作用和深熔过程中的矿物。更广泛的影响。该项目将支持一名研究生，作为硕士论文项目，他将分析拉尔斯曼山片麻岩中的硼硅酸盐和伴生矿物。该学生将学习如何 (1) 使用电子微探针测量化学成分，(2) 熟悉使用离子微探针测量锂、铍和硼，(3) 将化学成分与地质环境联系起来，(3) 开发基于化学和结构数据的反应（4）在同行面前口头展示她的结果，以及（5）撰写一篇科学论文以在同行评审的期刊上发表。