Boron isotopic composition of tourmaline, prismatine, and grandidierite from granulite facies paragneisses in the Larsemann Hills, Prydz Bay, East Antarctica: Evidence for a non-marine evaporite source

A unique assemblage of B-rich rocks (<2 wt.% B) containing the borosilicate minerals tourmaline (Tur), prismatine (Prs) and grandidierite (Gdd) is exposed in the ca. 1000 Ma Brattstrand Paragneiss in the Larsemann Hills, East Antarctica. The B isotope composition of these three minerals in 21 paragneisses and 6 anatectic pegmatites were measured in situ using secondary ion mass spectrometry. delta B-11 ranges from -2.8 to -14.4 parts per thousand in tourmaline, from -9.6 to -17.8 parts per thousand in prismatine and from-2.8 to -8.7 parts per thousand in grandidierite (weighted uncertainties mostly +/- 1-2 parts per thousand per sample). In most cases, average delta B-11 increases in a given sample Prs < Tur < Gdd. Whether the observed B-isotope distribution has attained equilibrium were assessed using two criteria: microstructural equilibrium and regular distribution of Mg and Fe. In samples for which the two criteria are met, we assume the measured distribution of isotopes Delta B-11(A-B) (=delta B-11(A) - delta B-11(B)) represents equilibrium: Delta B-11(Tur-Prs) = +5.0 + 1.4 parts per thousand (5 of 12 pairs), Delta B-11(Tur-Gdd) = -3.3 +/- 0.8 parts per thousand(3 of 5 pairs), and Delta B-11(Gdd-Prs) = + 7.2 +/- 1.3 parts per thousand(2 of 2 pairs), consistent with the preference of B-10 for tetrahedral sites (prismatine) and B-11 for trigonal sites (tourmaline, grandidierite). Ab initio computations of B isotope fractionation factors, delta B-11(Tur-Prs) = + 6.4 +/- 1.3 parts per thousand, delta B-11(Tur-Gdd) = -1.8 +/- 1.1 parts per thousand, and delta B-11(Gdd-Prs) = + 8.2 +/- 1.1 parts per thousand, provide confirmation of the measured fractionations. The computed, relaxed ionic structure of grandidierite gave a shorter (1.368 angstrom) B-O bond length than in case of dravite (1.385 angstrom), which explains the measured enrichment of grandidierite in B-11 relative to tourmaline. The precursor of the B-rich rock least changed by metamorphism, tourmaline metaquartzite (delta B-11(Tur) = - 8.6 parts per thousand to -5.9 parts per thousand), is interpreted to be a product of pre-metamorphic, hydrothermal B-metasomatism. Boron sources consistent with this delta B-11 range include oceanic crust, clastic sediments, volcanic rocks and non-marine evaporite borate, but not marine evaporite. However, dominance of metasediments in the Brattstrand Paragneiss, major and trace element compositions of the B-rich rocks and presence of abundant B are consistent with non-marine evaporite being the most important source of B for the B rich lithologies of the Larsemann Hills. A possible scenario for precursors of these rocks is a succession of clastic sediments, in part tuffaceous, intercalated with B-rich evaporite deposits in a continental rift basin, in which circulating hydrothermal fluids leached B from the evaporite and precipitated it as tourmaline in the associated clastic rocks to form the precursors to the tourmaline metaquartzites. (C) 2013 Elsevier Ltd. All rights reserved.