Melt infiltration and redox heterogeneity in the upper mantle: Evidence from Fe-FeO-Fe3C assemblages in Luobusa chromitite (Southern Tibet)

Abstract

Chromitites from the Luobusa ophiolite (southern Tibet) preserve nanoscale to microscale evidence for melt immiscibility and redox heterogeneity in the supra-subduction mantle. Focused ion beam transmission electron microscopy (FIB-TEM), SEM, and EELS reveal spheroidal Fe0-FeO droplets, faceted Fe3C, Ti-rich magnetite rims, and amorphous silicate films along oxide-metal boundaries, with HFSE-bearing nanophases (Nb-Ti-Zr-V) concentrated at oxide-glass interfaces. Magnetite and spinel display Fe-Ti-Cr zoning and nanoporous core-rim contacts consistent with dissolution-reprecipitation and volatile-rich melt infiltration. The co-occurrence of Fe0, FeO, and Fe3C, together with quenched interfacial glasses, suggests stabilization of immiscible metal-oxide-carbonaceous melts at upper-mantle conditions (∼1300–1450°C, low fO2) during chromite growth, followed by Ti-bearing metasomatism, serpentinization (∼400–500°C), and a minor low-temperature (<300°C) oxidative-hydrative overprint. The absence of high-pressure breakdown products of chromite supports an upper-mantle origin and takes off the need to invoke transition-zone conditions. These results demonstrate that highly reduced microdomains (≪ΔIW) can coexist with more oxidized surroundings at the grain scale, trapping Fe-FeO-Fe3C droplets and generating Ti-enriched magnetite and interfacial glass by rapid quenching. The study refines a physically consistent, four-stage model for chromitite evolution in subduction-modified mantle and provides microstructural and chemical criteria for recognizing detained immiscibility and metasomatic overprint in other supra-subduction zone chromitite deposits.