Experimental Investigation on the Transport of Sulfide Driven by
Melt-rock Reaction in Partially Molten Peridotite
Abstract
Extraction of sulfide liquid from 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
the reaction couple method of 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
thermodynamically-constrained model. During melt-peridotite reaction,
dissolution of olivine and precipitation of new orthopyroxene produce an
orthopyroxene-rich layer between 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
energetical focused melt flow with a high velocity (~
170.9 μm/h) around sulfide droplet through pore throat. In this energic
melt flow, we, for the first time, observed the mechanical coalescence
of sulfide droplets, and produced drag force was likely driving upward
entrainment of fine μm-scale sulfide. For coarse sulfide droplets whose
sizes are larger than the pore throat in partially molten 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 some channelized melt
flows with extremely high velocity. Hence, melt-rock reaction could
drive and enhance upward entrainment of μm- 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.