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

• Main Author: Grew, Edward
• Format: Text
• Language: unknown
• Published: DigitalCommons@UMaine 2013
• Subjects: Granulitic gneisses; Metasediments; Geochemistry; Geology; East Antarctica; Prydz Bay; Larsemann Hills; Antarc*; Antarctica
• Online Access: https://digitalcommons.library.umaine.edu/orsp_reports/431; https://digitalcommons.library.umaine.edu/cgi/viewcontent.cgi?article=1437&context=orsp_reports

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 ...