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

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"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.