Equation of thermobarometer for description of sulfide-silicate liquid immiscibility in basaltic systems

Abstract

In order to describe sulfide-silicate liquid immiscibility, coefficients and constants in sulfide thermobarometer equation X S = EXP(A/T − βP/T − B − CT − $D\log f_{O_2 } $ − ΣJ i X i ) were determined by multidi-mensional statistic analysis of a data set of more than 200 quench experiments on the solubility of sulfide sulfur in dry basaltic melts. Experiments characterize the sulfide-silicate equilibrium in a wide range of compositions, temperatures (1115-1800°C), pressures (1 atm-90 kbar) and oxygen fugacities ( $\log f_{O_2 } $ from −3.7 to −12.2). The average difference between experimental and calculated values of sulfur contents in sulfide-saturated basaltic melts is close to zero (0.0006 mol %), which indicates the absence of a systematic shift. The values of 5% confidence interval are described by equation ±(0.415C S 2 − 0.211C S + 0.038) (C S in mol %) and fall within the range from ±0.012 to ±0.076 mol %. Proposed thermobarometer is presently a single thermobarometer that provides accuracy better than ±10 rel % of sulfur content, not logarithmic contents, within concentration range of 0.1-0.7 mol %. Verification of the thermobarometer on the basis of layered intrusion data showed that the proposed thermobarometer predicts the position of cumulus sulfide in the vertical sections of the Tsipringa and Kivakka intrusions with an accuracy of ±70 meters. At the same time, the reliable local prediction of layering-associated low-sulfide mineralization is impossible in the framework of model with ideal convective magma mixing in a chamber.