Shuo Ding and Rajdeep Dasgupta
Abstract: Magmatism at mid ocean ridges is one of the main pathways of S outflux from deep Earth to the surface reservoirs and is a critical step in the global sulfur cycle, yet our understanding of the behavior of sulfide during decompression melting of the upper mantle is incomplete. In order to constrain the sulfur budget of the mantle and reconcile the sulfur and chalcophile element budget of mantle partial melts parental to primitive mid-ocean ridge basalts (MORBs), here we developed a model to describe the behavior of sulfide and Cu during decompression melting by combining the pMELTS thermodynamic model and empirical sulfur contents at sulfide concentration (SCSS) models, taking into account the effect of the presence of Ni and Cu in sulfides on SCSS of mantle-derived melts. Calculation of SCSS along melting adiabat at mantle potential temperature of 1380 °C with variable initial S content in the mantle indicates that the complete consumption or partial survival of sulfide in the melting residue depends on initial S content and degree of melting. Primitive MORBs (Mg#>60) with S and Cu mostly concentrated in 800–1000 ppm and 80–120 ppm are likely mixture of sulfide undersaturated high degree melts and sulfide saturated low degree melts derived from depleted peridotite containing 100–200 ppm S. Model calculations to capture the effects of variable mantle potential temperatures (1280–1420 °C) indicate that for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle owing to the effect of temperature in enhancing sulfide solubility in silicate melt, and higher mantle temperature stabilizing partial melt with higher FeO⁎ and lower SiO2 and Al2O3, all of which generally enhance sulfide solubility. However, sulfide can still be exhausted by ∼10–15% melting with bulk S of 100–150 ppm in the mantle when TP is as low as 1300 °C. We also show that although variation of and initial Cu in the mantle can all affect the Cu concentration of primitive MORBs, 100–200 ppm S in the MORB source mantle can satisfy both S and Cu geochemistry of partial melts parental to ocean floor basalts.
Ding, S. & Dasgupta, R. (2017). The fate of sulfide during decompression melting of peridotite – implications for sulfur inventory of the MORB-source depleted upper mantle. Earth and Planetary Science Letters. doi:10.1016/j.epsl.2016.11.020