A shallow origin for diamonds in ophiolitic chromitites: COMMENT

Abstract

Farré-de-Pablo et al. (2018) present microdiamonds in chromite pods hosted by serpentinized ultramafic rocks at the Mexican municipality of Tehuitzingo. As these rocks experienced pressures (~1 GPa at ~600 °C) significantly below those of the diamond-graphite transition, the authors concluded that the diamonds had formed metastably. If such metastable formation of diamond occurs, indeed, in nature, it could, in principle, mean that all nanoand micro-diamonds in crustal rocks found to date (e.g., Gilotti, 2013) do not unequivocally point to ultrahigh-pressure (UHP) conditions. The consequence would be that there is no clear proof of deep continental subduction because coesite, the other frequently cited UHP mineral, typically occurs in eclogites witnessing the subduction of oceanic crust only. A few occurrences of preserved coesite in other rock types, such as the so-called pyrope-quarzite from the Dora Maira Massif of the western Alps (Chopin, 1984), could be related to former sediments on top of the subducted oceanic crust. I (Massonne, 2014) already demonstrated that metapelites, in which nanodiamonds were previously found by Ruiz Cruz (2013), had experienced a clearly lower peak pressure (1.3 GPa at 700 °C) than necessary for the formation of diamond in its stability field. Ruiz Cruz, however, thought of two reasons for this discrepancy: metastable diamond formation, as Farré-de-Pablo et al. state, and the artificial introduction of diamond during the preparation of thin sections from the “diamondiferous” rock. The latter reason must be excluded for the chromitites at Tehuitzingo, Mexico, in order to prove metastable diamond formation. Nasdala and Massonne (2000) verified microdiamonds completely enveloped by the host mineral inside a thin section with Raman microspectroscopy. In fact, Farré-de-Pablo et al. applied this analytical method but only to identify diamond being exposed to the thin section surface. The displayed five grains (Farré-de-Pablo et al.’s figure 1) are clearly xenomorphic. Microdiamonds that have crystallized from supercritical COH silicate fluids (e.g., Stöckhert et al., 2001) are usually idiomorphic. In fact, it could be that carbonic fluids penetrating through cracks precipitate diamonds with other shapes than those formed from such silicate fluids, but why they are not at least hypidiomorphic? A possible answer could be that diamonds, being part of the abrading material used during the preparation of even surfaces of chromitite specimens, were trapped in pores opened by the polishing procedure. If this were true, diamonds pressed into the open pores could be fixed there, whereas other finer-grained polishing debris, which was not found by Farré-de-Pablo et al., besides the diamonds, could have been easily removed (in an ultrasonic bath?). From the discussion above I conclude that more evidence must be presented in order to demonstrate that the diamonds of the Tehuitzingo chromitite were not artificially introduced and, thus, point to metastable diamond formation.