Sulfide Melt Wetting Properties in Earth's Mantle: New Constraints From Combined 2D and 3D Imaging

Abstract

Base-metal sulfur liquids (mattes) play a crucial role as metasomatic agents and carriers of highly siderophile elements (HSE) within the Earth's mantle. Prior research has predominantly focused on sulfur-poor metallic liquids involved in core formation scenarios. We conducted high-pressure experiments using a multi-anvil apparatus to investigate the effects of pressure, non-ferrous compounds in mattes, and the mineral composition of the silicate host on matte wetting properties. Specifically, we explored conditions representing both the lithospheric (6 and 7 GPa) and sub-lithospheric Earth's mantle (13 GPa). We characterized the experiments using the distribution of the dihedral angle in backscattered-electron sections and the sphericity and network topology of the mattes in tomography scans. Our findings reveal distinct behaviors: while the matte in olivine-dominated samples exhibited behaviors consistent with previous studies, such as high dihedral angle values (94° and 100°), the majorite-bearing sample run at 13 GPa formed a disseminated network with a mean dihedral angle of 43°, below the connectivity threshold of 60°. Furthermore, in an experiment involving a garnet-bearing silicate host, we observed a decrease in the matte's dihedral angle to 72°. Our results suggest that pressure within mafic hosts contributes to increased matte mobility in the sub-lithospheric Earth's mantle, especially inasmuch as the stability of garnet phases is concerned. Consequently, mattes within subducted oceanic crusts may efficiently transport HSE into surrounding lithologies, while mattes within depleted, more harzburgitic lithologies and the ambient mantle may remain trapped within the silicate host at low melt fractions.