Date of Award

8-2012

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Edward S. Grew

Second Committee Member

Christopher C. Gerbi

Third Committee Member

Martin G. Yates

Abstract

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 of melting and crystallization from melt together did not further fractionate B isotopes. The average δ11B increases in a given sample, prismatine < tourmaline < grandidierite, with two exceptions. This fits expectations that 10B prefers the tetrahedrally coordinated crystallographic sites in prismatine, and 11B for trigonally coordinated crystallographic sites in tourmaline and grandidierite. The equilibrium distribution of B isotopes between coexisting borosilicates is reported as Δ11B = difference in δ11B between two coexisting minerals. The equilibrium distributions reported here are Δ11BTourmaline-Prismatine = 5.46±0.25 ‰, Δ11B Tourmaline-Grandidierite = 3.13±0.72 ‰, and give Δ11BGrandidierite-Prismatine = 7.48±0.88 ‰. The tourmaline compositions of tourmaline quartzites are used to constrain possible sources of B in the precursor. The tourmaline quartzite δ11B values are used to extrapolate back to infer the initial hydrothermal fluid composition using experimentally determined tourmaline-fluid fractionation. The δ11B of pre-metamorphic fluids is constrained to be –4.9 to –0.4 ‰ (200 to 300 °C) assuming no B loss during metamorphism. However, it is likely that devolatilization decreased tourmaline δ11B, so the pre-metamorphic fluid likely exceeded 0‰. The most plausible source of B is nonmarine evaporite with contribution from classic sediments, which is consistent with the range of δ11B inferred for tourmaline and the hydrothermal fluid from which it precipitated.

Share