A shallow origin for diamonds in ophiolitic chromitites: COMMENT

Farré-de-Pablo et al. (2019) report a new occurrence of microdiamonds in ophiolitic chromitites of the Tehuitzingo serpentinite of southern Mexico, and suggest that the microdiamonds, along with serpentine, quartz, and chlorite formed “in sealed fractures” in chromite during serpentinization. We discuss here strong evidence supporting formation of ophiolite-hosted diamond under ultrahigh-pressure conditions. The chromite sealing the fracture (Farré-de-Pablo et al.’s figure 1B) shows no boundary with the chromite of the “fresh core”. The authors kindly identified for us the locations of the EPMA analyses (their table DR2), which are plotted Figure 1. These show that the chromite of the healed fractures has essentially the same composition as that of unaltered magmatic cores, but is significantly different from the ferritchromite of the grain rims. Therefore, it is difficult to understand how the diamondbearing fracture could have been healed by magmatic chromite after “serpentinization”.