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

“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。“智力优势。在理解高P-T麻粒岩相岩石的主要挑战是识别原始原岩和它们最初沉积的构造环境。 该项目的重点是暴露在拉斯曼山，普里兹湾（东南极洲）麻粒片麻岩。这些岩石被认为是变质沉积物（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提供了对沉积的原始环境以及随后的构造和变质历史的潜在见解。由于其对含水流体和硅酸盐熔体的相对亲和力，B在这种高级变质岩中通常被强烈地耗尽;一些仍然非常富含B（高达20%的电气石）的事实是非常神秘的。在这项研究中，B同位素组成将被用来研究硼富集的初始原因，以及在变质过程中重新分配硼的过程。硼有两种原子量不同的同位素：10 B和11B。作为B同位素测量的鉴别能力的一个例子，非海洋蒸发物的特征是10 B相对于11 B异常贫化，而典型的海洋沉积物具有高的11 B/10 B（即，从海水中继承而来）。B同位素组成将使用二次离子质谱法（西姆斯）的电气石和其他硼硅酸盐矿物的样品中获得的Larsemann山在2003-2004年领域的季节在现场测量。B同位素数据将被用来限制原岩的成分，这些数据结合岩石学研究的结果，以限制变质作用，深熔作用，挥发分的影响，导致富B片麻岩的形成。通过与澳大利亚布罗肯希尔的类似岩石进行类比，他提出研究将测试以下假设：拉斯曼山原岩起源于富含B的非海洋蒸发物，原始B部分被海底热液流体动员，与碎屑沉积物和火山岩反应形成含电气石的岩石（原岩），而在变质和深熔作用期间，经由电气石和其他硼硅酸盐矿物的保存，片麻岩的富硼特征得以保存。该项目将支持一名研究生，他将分析拉斯曼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）熟悉使用离子探针测量锂，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开发反应（4）在同龄人面前口头展示她的结果，（5）写一篇科学论文，在同行评审的期刊上发表。

"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.