Experimental Investigation on the Transport of Sulfide Driven by Melt-Rock Reaction in Partially Molten Peridotite

Extraction of sulfides from the partially molten mantle is vital to elucidate the cycling of metal and sulfur elements between different geochemical circles but has not been investigated systematically. Using laboratory experiments and theoretical calculations, this study documents systematical variations in lithologies and compositions of silicate minerals and melts, which are approximately consistent with the results of the thermodynamically-constrained model. During a melt-peridotite reaction, the dissolution of olivine and precipitation of new orthopyroxene generate an orthopyroxene-rich layer between the melt source and peridotite. With increasing reaction degree, more melt is infiltrated into and reacts with upper peridotite, which potentially enhances the concomitant upward transport of dense sulfide droplets. Theoretical analyses suggest an energetically focused melt flow with a high velocity (similar to 170.9 mu m/hr) around sulfide droplets through the pore throat. In this energic melt flow, we, for the first time, observed the mechanical coalescence of sulfide droplets, and the associated drag force was likely driving upward entrainment of fine mu m-scale sulfide. For coarse sulfide droplets whose sizes are larger than the pore throat in the peridotite, their entrainment through narrow constrictions in crystal framework seems to be physically possible only when high-degree melt-peridotite reaction drives high porosity of peridotite and channelized melt flows with extremely high velocity. Hence, the melt-rock reaction could drive and enhance upward entrainment of mu m- to mm-scale sulfide in the partially molten mantle, potentially contributing to the fertilization of the sub-continental lithospheric mantle and the endowment of metal-bearing sulfide for the formation of magmatic sulfide deposits. Plain Language Summary Sulfides are a pivotal potential reservoir for sulfur and economically important metals. Their transport in the Earth's mantle plays a vital role in understanding many crucial geological and environmental processes, especially the formation of mineral deposits, and the environmental damage and health hazards related to volcanic eruptions. This work proposes a new driving force for the upward transport of dense sulfide drops in the upper mantle that experiences partial melting. The reaction between melt and rock potentially leads to focused melt flow in new-forming channels with three orders of magnitude higher velocity than that of melt flowing among the crystal framework of peridotite. This energetic melt flow drives the upward transport of tiny mu m-scale sulfide droplets in peridotite and may also facilitate the amalgamation of droplets contacting each other. Coarse sulfide droplets could be possibly entrained upward through narrow pore throats, especially when a high-degree melt-peridotite reaction drives fast-flowing melt in the mantle with high porosity.