A shallow origin for diamonds in ophiolitic chromitites

Abstract

Recent findings of diamonds in ophiolitic peridotites and chromitites challenge our traditional\nnotion of Earth mantle dynamics. Models attempting to explain these findings involve\nincorporation of diamonds into chromite near the mantle transition zone. However, the\noccurrence of metastable diamonds in this context has not been considered. Here, we report\nfor the first time in situ microdiamonds in chromite from ophiolitic chromitite pods hosted\nin the Tehuitzingo serpentinite (southern Mexico). Here, diamonds occur as fracture-filling\ninclusions along with quartz, clinochlore, serpentine, and amorphous carbon, thus indicating\na secondary origin during the shallow hydration of chromitite. Chromite chemical variations\nacross the diamond-bearing healed fractures indicate formation during the retrograde evolution\nof chromitite at temperatures between 670 °C and 515 °C. During this stage, diamond\nprecipitated metastably at low pressure from reduced C-O-H fluids that infiltrated from the\nhost peridotite at the onset of serpentinization processes. Diamond was preserved as a result\nof fracture healing at the same temperature interval in which the chromite alteration began.\nThese mechanisms of diamond formation challenge the idea that the occurrence of diamond\nin ophiolitic rocks constitutes an unequivocal indicator of ultrahigh-pressure conditions.