BORON ISOTOPE COMPOSITIONS IN GRANULITE-FACIES BOROSILICATE MINERALS FROM GRANULITE-FACIES PARAGNEISSES AND ANATECTIC PEGMATITES OF THE LARSEMANN HILLS, PRYDZ BAY, EAST ANTARCTICA – THE INFLUENCE OF ISOTOPIC FRACTIONATION

Jim Thompson told Ed Grew to look for kornerupine in the Eastern Ghats when Ed went to India to study granulites in 1981. Ever since, Ed has continued to pursue kornerupine and its B-dominant analog prismatine, most recently in study of a unique suite of B-rich rocks containing prismatine (Prs) with the borosilicates tourmaline (Tur) and grandidierite (Gdd) in the Larsemann Hills, Antarctica. In situ analyses with a Cameca ims 4f ion microprobe gave sample average δ¹¹B (= {[sample ¹¹B/¹⁰B / SRM 951 ¹¹B/¹⁰B] – 1} × 1000) to be –2.8 to –14.4 ‰ in tourmaline, –9.7 to –17‰ in prismatine and –1.4 to –8.7‰ in grandidierite (errors mostly ±1-2‰ per sample). Whether the observed B-isotope distribution represents equilibrium can be assessed using three criteria: (1) microstructural equilibrium, (2) regular distribution of major constituents (e.g., Mg and Fe), and (3) regular distribution of B isotopes. All three criteria are met in 10 samples (of 22), and we have assumed the measured distribution of isotopes is equilibrium: Δ¹¹B(Tur-Prs) = 5.5±1.8‰ and Δ¹¹B(Tur-Gdd) = –3.2±1.6‰, where Δ¹¹B between two minerals is the difference in their δ¹¹B. The marked fractionation implies that the breakdown of tourmaline to prismatine and grandidierite will change the δ¹¹B of remaining tourmaline, which is confirmed by the narrow range of isotopic composition of tourmaline in tourmaline quartzite containing no other borosilicate (δ¹¹B(Tur) = –8.6 to –5.9 ‰) compared to the range in borosilicate gneiss, which also contains grandidierite ± prismatine (δ¹¹B(Tur) = –11.2 to –2.8 ‰). The ranges of δ¹¹B of tourmaline (–11.9 to –4.6 ‰), prismatine (–13.2to –13.8 ‰) and grandidierite (–7.3 to –6.4 ‰) in anatectic pegmatites lies within their respective ranges in the metamorphic rocks. Because ¹¹B is fractionated into fluid relative to tourmaline, δ¹¹B of tourmaline and prismatine would be expected to decrease in the presence of fluid (Slack et al. 1993 Econ Geol 88, 505-541), and consequently the δ¹¹B ranges observed in the pegmatites imply there was little loss of B to aqueous fluid during anatexis and crystallization from the melt. Low fluid activities would explain the distinctive pegmatitic borosilicate assemblages comprising also boralsilite, werdingite and dumortierite, minerals that are rarely found in granitic pegmatites elsewhere.