Boron Isotopic Study of the Borosilicates Tourmaline, Prismatine and Grandidierite in Granulite Facies Paragneisses, from the Larsemann Hills, Prydz Bay, East Antarctica

Boron Isotopic Study of the Borosilicates Tourmaline, Prismatine and Grandidierite in Granulite Facies Paragneisses, from the Larsemann Hills, Prydz Bay, East Antarctica

By JohnRyan MacGregor

Thesis Advisor: Dr. Edward S. Grew

A Lay Abstract of the Thesis Presented

in Partial Fulfillment of the Requirements for the

Degree of Master of Science

(in Earth Sciences)

August, 2012

Keywords: Boron isotopes, Larsemann Hills, Borosilicates, Tourmaline, Prismatine, Grandidierite

Mid to lower crustal rocks that have experienced elevated temperatures and pressures, a process called metamorphism, are often depleted in the chemical element boron. Rocks in the Larsemann Hills, Prydz Bay, East Antarctica however, have retained anomalously high concentrations of boron despite metamorphism. These rocks contain three uncommon boron-bearing minerals, tourmaline, prismatine and grandidierite. The element boron consists of two isotopes, the same element but with different atomic weights, which makes boron a suitable candidate for isotopic analysis. This locality provides a unique opportunity to study (1) how boron was retained by the metamorphic rocks, (2) how the two boron isotopes partition between the coexisting boron-bearing minerals, and (3) what the original source of boron for the rocks was.

Boron retention in the metamorphosed rocks of the Larsemann Hills was facilitated by the abundance of tourmaline prior to metamorphism, and by the crystallization of grandidierite and prismatine as a product from tourmaline breakdown. This allowed high concentrations of boron to be retained during metamorphism, as opposed to being lost from the system.

The partitioning of the two boron isotopes between the coexisting boron-bearing minerals indicate that the lightest isotope prefers to go into the mineral prismatine, and the heaviest isotope prefers to go into tourmaline and grandidierite. Prismatine contains the greatest concentration of the lightest isotope, and grandidierite contains the greatest concentration of the heaviest isotope, where tourmaline is intermediate.

The ratio of the heaviest to the lightest boron isotope in tourmaline from the Larsemann Hills is compared to the same ratio in tourmaline from other rocks that have inferred boron sources. This direct comparison puts constraints on potential sources of boron for the Larsemann Hills rocks. The data in this study supports a dominantly non-marine evaporite boron source, with minor input from sediments that have been weathered from pre-existing rock. The proposed model suggests boron from the non-marine evaporite source was leached by waters with elevated temperatures that circulated through the sediments, which crystallized the mineral tourmaline in the sediments prior to the rocks being metamorphosed and driven into the mid to lower crust due to plate tectonics.