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. δ11B ranges from −2.8 to −14.4‰ in tourmaline, from −9.6 to −17.8‰ in prismatine and from-2.8 to −8.7‰ in grandidierite (weighted uncertainties mostly ±1-2‰ per sample). In most cases, average δ11B 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 Δ11BA-B (= δ11BA − δ11BB) represents equilibrium: Δ11BTur-Prs = + 5.0 ± 1.4‰ (5 of 12 pairs), Δ11BTur -Gdd = −3.3 ± 0.8‰ (3 of 5 pairs), and Δ11BGdd-Prs = + 7.2 ± 1.3‰ (2 of 2 pairs), consistent with the preference of 10B for tetrahedral sites (prismatine) and 11B for trigonal sites (tourmaline, grandidierite). Ab initio computations of B isotope fractionation factors, Δ11BTur-Prs = + 6.4 ± 1.3‰, Δ11BTur -Gdd = −1.8 ± 1.1‰, and Δ11BGdd-Prs = + 8.2 ± 1.1‰, provide confirmation of the measured fractionations. The computed, relaxed ionic structure of grandidierite gave a shorter (1.368 Å) B-O bond length than in case of dravite (1.385 Å), which explains the measured enrichment of grandidierite in 11B relative to tourmaline. The precursor of the B-rich rock least changed by metamorphism, tourmaline metaquartzite (δ11B(Tur) = −8.6‰ to −5.9‰), is interpreted to be a product of pre-metamorphic, hydrothermal B-metasomatism. Boron sources consistent with this δ11B 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.