Boron isotope composition of tourmaline, prismatine and grandidierite in granulite-facies boron-rich rocks from the Larsemann Hills, Prydz Bay, East Antarctica: no evidence for a non-marine evaporite precursor

摘要: In contrast to most granulite-facies rocks, tourmaline (Tur) quartzite, prismatine (Prs)-bearing leucogneiss and grandidierite-bearing (Gdd) borosilicate gneiss in the Larsemann Hills, East Antarctica, have retained B in amounts up to 20 000 ppm. To constrain possible sources of B in the precursor, Tur, Prs and Gdd were analyzed in situ for B isotopes using secondary ion mass spectrometry with a Cameca ims 4f ion microprobe. δ 11 B (={[sample 11 B/10 BSRM 951 11 B/10 B]–1}× 1000) averaged over several grains per sample ranges from–3.0 to–14.3‰ in Tur, from–9.6 to 18.1‰ in Prs and from–1.9 to–8.7‰ in Gdd (1s mostly 1-2‰ per sample). Tur in Tur quartzite is isotopically homogeneous from grain to grain in each sample and average Tur δ 11 B varies little from sample to sample (6 samples:–5.9 to–8.8‰). Average leucogneiss Tur composition is lighter (3 samples, δ 11 B=–9.6 to–14.3‰), whereas average borosilicate-gneiss Tur varies more (6 samples, δ 11 B=–3.0 to–11.8‰). With two exceptions, average δ 11 B increases in a given sample Prs< Tur< Gdd with Prs B 4.8±1.6‰(9 pairs) lighter and Gdd B 2.8±1.9‰(6 pairs) heavier than Tur B. This regularity suggests that the three borosilicates were in near isotopic equilibrium. In the anatectic pegmatites, δ 11 B ranges from–4.8 to–12.1‰ in Tur (4 samples),–12.5 to–14.0‰ in Prs (2 samples) and–6.6 to–6.8‰ in Gdd (2 samples). These ranges lie within the corresponding ranges in the metamorphic rocks, ie, the processes of melting and crystallization from melt together had little effect on B isotopic composition. Boron in Larsemann Hills Tur quartzite is at least 8‰ heavier than B in …