High Sulfur in Primitive Arc Magmas, Its Origin and Implications

Vadim S Kamenetsky,Alkiviadis Kontonikas-Charos,Nikolai Nekrylov,Michael Zelenski

doi:10.3390/min12010037

Abstract

Sulfur contents in 98.5% of melt inclusions (MI) from calc-alkaline subduction basalts do not exceed 4000 ppm, whereas experimentally established limits of sulfur solubility in basaltic melts with high fO2 (characteristic of subduction zones, e.g., QFM + 2) surpass 14,000 ppm. Here we show that primitive (Mg# 62-64) subduction melts may contain high sulfur, approaching the experimental limit of sulfur solubility. Up to 11,700 ppm S was measured in olivine-hosted MI from primitive arc basalt from the 1941 eruption of the Tolbachik volcano, Kamchatka. These MI often contain magmatic sulfide globules (occasionally enriched in Cu, Ni, and platinum-group elements) and anhydrite enclosed within a brown, oxidized glass. We conclude that the ubiquitous low sulfur contents in MI may originate either from insufficient availability of sulfur in the magma generation zone or early magma degassing prior to inclusion entrapment. Our findings extend the measured range of sulfur concentrations in primitive calc-alkaline basaltic melts and demonstrate that no fundamental limit of 4000 ppm S exists for relatively oxidized subduction basalts, where the maximum sulfur content may approach the solubility limit determined by crystallization of magmatic anhydrite.

Highlights

Dumańska-Słowik, Beata Naglik, The behavior of sulfur in magmatic systems is complex and not fully understood due to the fact that sulfur has several valence states from −2 to +6 and may form solid, liquid, or gaseous compounds, which are simultaneously present in magma
The Mg# reaches 62–64, i.e., it can be characterized as primitive high-K magnesian basalt
High sulfur concentrations in the range of 4000–11700 ppm were measured in naturally quenched melt inclusions in olivine from the 1941 Tolbachik eruption, Kamchatka

Summary

Introduction

Dumańska-Słowik, Beata Naglik, The behavior of sulfur in magmatic systems is complex and not fully understood due to the fact that sulfur has several valence states from −2 to +6 and may form solid, liquid, or gaseous compounds, which are simultaneously present in magma. Spectroscopic methods show that silicate melt may contain sulfate (S6+ ), sulfide (S2− ), or both sulfur species [1]. According to the experimental data, sulfur concentration in silicate melts is limited by the exsolution of a sulfur-containing phase. For more than half a century, empirical, semi-empirical, and thermodynamic models have attempted to estimate the solubility of sulfide and sulfate in magma [2] based on melt composition, f O2 , temperature, pressure, the presence of an aqueous fluid, and other parameters, e.g., [5,6,7,8,9]. The sulfur content in silicate melt in equilibrium with sulfide melt (SCSS, sulfur content at sulfide saturation) typically does not exceed 1300 ppm S in a reducing environment (f O2 ≤ QFM [3], and references therein). With an increase in oxygen fugacity, the solubility of sulfur in the silicate melt increases, and at f O2 ~ QFM + 1 . . . 2 is determined by equilibrium with magmatic anhydrite

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Discussion

Conclusion