Effect of sulfate on the basaltic liquidus and Sulfur Concentration at Anhydrite Saturation (SCAS) of hydrous basalts- Implications for sulfur cycle in subduction zones

Proteek Chowdhury, Rajdeep Dasgupta

To add to our understanding of sulfur cycle in subduction zones in general and constrain the effects of slab-released sulfate (SO₄²⁻) on the magma genesis and sulfur transport from sub-arc mantle to the arc volcanoes in particular we carried out an experimental study. High pressure-temperature, piston-cylinder experiments were carried out in Au-Pd capsules at 0.5–3 GPa and 1050–1325 °C to investigate (a) the effect of variable sulfur concentration from 0 to ~2 wt%, dissolved as SO₄²⁻ on the stability field of a primary arc basalt with ~4 wt% H₂O and determine (b) the sulfur content at anhydrite saturation (SCAS) of hydrous mafic magmas. Speciation of sulfur in the silicate melt was confirmed to be SO₄²⁻ by S Kα X-ray peak position using electron microprobe. S-free hydrous clinopyroxene liquidus at 2 GPa is ~25 °C hotter than the hydrous clinopyroxene liquidus with ~0.1 wt% S in the liquidus melt as SO₄²⁻ and the liquidus depression with further S-enrichment to anhydrite saturation (~2 wt% S) can be fitted by a power function ΔT(°C) = 26.52(±3.48)(S_melt in wt%)^0.24(±0.06). Anhydrite-saturated experiments show that SCAS increases with increasing temperature and CaO content of melt and decreases with increasing SiO₂ content of the melt. Previous SCAS models based mostly on lower P–T experiments and/or on silicic melt compositions can’t capture our new experimental SCAS data. A new SCAS parameterization was developed using previous and our new experimental data. Calculations using our new parameterization and assuming 200–500 ppm S in the arc mantle show that <10% hydrous melting of mantle wedge would exhaust anhydrite, if present. Therefore, anhydrite even if present is expected to be exhausted by partial melting in the mantle wedge and parental basaltic melt will extract similar amount of S via mantle melting irrespective of the presence of sulfide or sulfate at subsolidus conditions. The S content, as dissolved SO₄²⁻, of hydrous arc basalts produced by 10–30% melting will be 500–4000 ppm, which is comparable to the melt inclusion S contents from various arcs. The sulfate undersaturated basalts may assimilate crustal sulfate and lead to high observed SO₂ flux at the arcs, known as the “excess S”.

Chowdhury, P. and Dasgupta, R., 2019. Effect of sulfate on the basaltic liquidus and Sulfur Concentration at Anhydrite Saturation (SCAS) of hydrous basalts–Implications for sulfur cycle in subduction zones. Chemical Geology. https://doi.org/10.1016/j.chemgeo.2019.05.020