Boron Isotopic Study of the Borosilicates Tourmaline, Prismatine and Grandidierite in Granulite Facies Paragneisses, from the Larsemann Hills, Prydz Bay, East Antarctica
The granulite-facies paragneisses in the Larsemann Hills include several units containing up to 20 000 ppm B, much greater than boron concentrations in most granulite-facies rocks. This locality provides a unique opportunity to study the distribution of B isotopes between the borosilicate minerals t...
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| Format: | Text |
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DigitalCommons@UMaine
2012
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| Online Access: | https://digitalcommons.library.umaine.edu/etd/1795 https://digitalcommons.library.umaine.edu/cgi/viewcontent.cgi?article=2824&context=etd |
| Summary: | The granulite-facies paragneisses in the Larsemann Hills include several units containing up to 20 000 ppm B, much greater than boron concentrations in most granulite-facies rocks. This locality provides a unique opportunity to study the distribution of B isotopes between the borosilicate minerals tourmaline, prismatine and grandidierite and to use the isotopic data to constrain the source of B. Research for the thesis included petrographic study of microstructures, chemical analyses using the electron microprobe at the University of Maine, and in situ B isotopic analysis using secondary ionization mass spectrometry with the ion microprobe at the University of Edinburgh. The dominant B-rich metamorphic rock types found in the Larsemann Hills are tourmaline quartzites, borosilicate-bearing leucogneisses, and borosilicate-bearing gneisses. Microstructural evidence suggests that grandidierite and tourmaline crystallized in a three-stage evolution with early, peak, and post-peak generations during metamorphism. In contrast, prismatine apparently crystallized only at peak metamorphic conditions. A regular distribution of major elements is observed among associated borosilicates. Associated tourmaline-prismatine displays two trends, KD (Fe-Mg) = [(Mg/Fe Tourmaline)÷(Mg/Fe Prismatine)]= 0.927, and 1.076. The KD (Fe-Mg) of tourmaline-grandidierite is 0.6474, and the grandidierite-prismatine KD (Fe-Mg) is 1.47. Disequilibrium microstructures are commonly associated with outliers from these KD values. Boron isotope composition of tourmaline in tourmaline quartzites exhibit the narrowest range (δ11B = –5.9 to –8.8‰ where δ11B (= {[(sample11B/10B) / (standard SRM 95111B/10B)] – 1} × 1000), whereas B isotopic composition of tourmaline in leucogneiss is lighter (–9.6 to –14.3‰). Boron isotopic composition of tourmaline in borosilicate gneisses shows the greatest range (δ11B = –3.0 to –11.8‰). δ11B values in the anatectic pegmatites fall within the corresponding ranges in the metamorphic rocks, which implies that the processes ... |
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