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

“这项奖励是根据2009年美国复苏和再投资法案（公法111-5）资助的。”知识价值。了解高P-T麻粒岩相地体的主要挑战是确定原始原岩及其最初沉积的构造环境。本项目主要研究暴露在南极洲东部Prydz湾Larsemann Hills的粒状片麻岩。这些岩石被认为是变质沉积岩（副沉积岩），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形成具有重要意义。岩石独特的富b特征为了解沉积的原始环境以及随后的构造和变质历史提供了可能。由于B对含水流体和硅酸盐熔体的相对亲和力，B在这类高品位变质岩中通常被强烈贫化；有些仍然极富b族（电气石含量高达20%）的事实是非常神秘的。在这项研究中，B同位素组成将用于研究硼富集的初始原因以及变质作用过程中硼的重新分配过程。硼有两种原子量差别很大的同位素：10B和11B。作为B同位素测量判别能力的一个例子，非海洋蒸发物的特征是10B相对于11B异常枯竭，而典型的海洋沉积物具有高11B/10B（即继承自海水）。B同位素组成将使用次级离子质谱法（SIMS）对2003-2004年野外季节在Larsemann Hills获得的样品中的电气石和其他硼硅酸盐矿物进行原位测量。B同位素数据将用于约束原岩的组成，并结合岩石学研究结果来约束变质作用、深熔作用、脱挥发作用对富B片麻岩形成的影响。通过与澳大利亚布罗肯山的类似岩石进行类比，他提出研究将验证以下假设：拉尔森山原岩起源于富含B的非海相蒸发，原始B部分被海底热液与碎屑沉积物和火山碎屑岩反应而调动，形成含电气石的岩石（原岩）；在变质作用和深熔作用中，电气石等硼硅酸盐矿物的保存使片麻岩的富b特征得以保留。更广泛的影响。该项目将支持一名研究生，他将分析拉尔森山片麻岩中的硼硅酸盐和伴生矿物，作为硕士论文项目。该学生将学习如何(1)使用电子探针测量化学成分，(2)熟悉用离子探针测量锂、铍和硼，(3)将化学成分与地质环境联系起来，(3)根据化学和结构数据进行反应，(4)在同行面前口头展示她的结果，(5)写一篇科学论文发表在同行评审的期刊上。

"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.