Sulfur in basaltic magmas

Abstract

The concentration of S in basaltic magmas at 1 atm pressure is strongly dependent on temperature, the fugacities of oxygen ( ƒ O 2 ) and sulfur ( ƒ S 2 ), and bulk composition. Microprobe analyses of total S in rapidly quenched, submarine basalt glasses, used in conjunction with wet chemical analyses of Fe 2O 3 FeO and its relationship to ƒ o 2 , allow direct calculation of ƒ s 2 using an expression which relates dissolved sulfur content to sulfur fugacity. The relationship between S fugacity and dissolved S in a silicate liquid at 1 bar total pressure can be represented by the expression ln X s = a ln ƒ s 2 − b ln ƒ o 2 + c ln X FeO + d T + e + Σƒ iX i , where X s is the mole fraction of dissolved S, a through ƒ i are experimentally calibrated regression coefficients, and the summation is over melt components, i. Back calculation of the input data yields a standard error of 0.026 wt% S for a magma with 0.1 wt% of S. Prediction of the immiscible Fe-S-O liquid saturation surface for basaltic liquids in T-X-ƒ o 2 -ƒ s 2 space is made through consideration of the heterogeneous equilibrium 1 2 S 2(gas) + FeO (silicate melt) = FeS (sulfide melt) + 1 2 O 2(gas) , using standard state thermodynamic data. Data from natural basalt glasses demonstrate that during the differentiation and Fe enrichment of basaltic magmas, the increases in S content which are observed require the ratio ƒ o 2 ƒ s 2 to decrease by 3 log 10 units over the temperature range 1260 to 1050°C. This decrease is equivalent to the enthalpy change of the above reaction. Application to basalts and gases from Kilauea volcano demonstrates that during ascent, S is depleted in the magma as a result of shallow effervescence, and the gases which are evolved are in equilibrium with the magma during fire fountaining.