Revisiting the B isotope fractionation and partitioning systematics in synthetic MORB

Boron isotope systematics allow tracing of e.g. fluid-mediated mobilisation of subducted material in subduction zones. The associated B isotope fractionation factors are constrained by e.g. analysis of synthetic samples.

Here we investigate the effect of temperature, and water content as a function of pressure, on the B isotope composition of a basaltic melt and coexisting fluid. The MORB-like starting material was doped with a ~ 450 µg/g aliquot of the NIST951a B isotope reference material. Synthetic MORB are produced using internally heated pressure vessels (IHPV). Experiments were performed between 1250 and 1000 °C and 1 and 2.5 kbar, (fO2 ~ ΔQFM +3.3).

Boron isotope compositions were determined using fs-LA MC-ICP MS, intermediate precision of NIST610 is δ(11B/10B)NIST951a, hence δ11B = –1.43 ± 0.82 ‰ (2 s.d., average of 5 sessions, total average (n =134) is –1.46 ± 0.16 ‰, 95% c.i.). Our MORB glass yields an intermediate precision of δ11B = +0.05 ± 0.11 ‰ (2 s.d. of 5 sessions, total average (n = 153) = +0.01 ± 0.02 ‰ (95% c.i.).

Boron melt-fluid partition coefficients are below unity (Dmelt/fluid < 1), δ11B decreases from ± 0 to –1.5 ‰ with increasing total water content. Our results show that B will be concentrated in the fluid at magmatic temperatures. Moreover, it is shown that the degassing of fluid from a basaltic melt likely leads to measurable B isotope fractionation (tentative Δ11Bmelt-fluid = –2.2 ± 0.6 ‰), with potential implications to our interpretation of the δ11B of degassed, water-rich basaltic magmas.