Preliminary Results on Fractionation of H, C, S and Cl Isotopes by Thermal Diffusion in Silicate Melts

Molten silicates containing dissolved volatiles (H2O, CO2, SO2 or Cl) were placed in a thermal gradient and the resulting 'hot-to-cold' fractionation of H, C, S and Cl isotopes was characterized in the quenched samples by ion microprobe. The specific melt compositions were tailored to the volatile species of interest: natural obsidian for H-D, Fe-free haplobasalt for C, natural MORB basalt for S, and synthetic andesite for Cl. The SIMS analyses were facilitated by adding volatiles (H2O, Na2CO3, and FeS) that were markedly enriched in the (rare) heavy isotopes of interest (D, 13C, 36S). The ~1-cm long samples were run vertically in the piston-cylinder apparatus (hot end up), with the basalt and andesite contained in graphite and the other two melts in Pt. The Cl, S and C experiments were run for ~24h with a 'hot end' temperature of 1500°C (ΔT ~ 150°C) and the H-D experiment was run for ~16h with a 'hot-end' T of 1200°C (ΔT ~ 300°C). Short-duration (30-60 min) duplicates of all experiments were run as controls. Isotope ratios of Cl (37/35), S (34/32 and 36/32) and C (13/12) were characterized in the quenched glasses using the Cameca 1280 ion microprobe at WHOI; H/D was measured at ASU using the Cameca ims 6f. 'Hot-to-cold' fractionation of Cl isotopes along the quenched glass samples was not detectable above analytical uncertainty, but all other isotope ratios (and dissolved volatile concentrations) conformed in some way to the thermal field of the diffusion samples. In the case of sulfur, 34S variation across the sample was barely above analytical uncertainty, but 36S showed ~1% variation, with the heavy isotope (and total S) enriched at the 'cold' end. Carbon isotopes (13/12) are more strongly fractionated, with the 'cold' end depleted in 13C by ~20%. The obsidian sample showed enrichment in D at the 'cold' end by approximately a factor of ~2. Taken together, these preliminary results indicate that sustained temperature gradients in magmas are capable of producing substantial isotopic fractionations of volatile-forming elements.