A shallow origin for diamonds in ophiolitic chromitites: REPLY

Abstract

We are grateful to Massonne (2019) and Yang et al. (2019) for their constructive Comments to our recent publication (Farré-de-Pablo et al., 2018). We appreciate their interest in discussing our results and interpretations, which confirm that our work is a benchmark for future research. We are pleased to have the opportunity to address in further detail their concerns on the natural origin of the reported microdiamonds in Tehuitzingo, Mexico, chromitites (Massonne, 2019), as well as their geological meaning (Yang et al., 2019).We (Farré-de-Pablo et al., 2018) provided many lines of evidence (e.g., detailed inspection by transmission electron microscopy [TEM] of focused ion beam [FIB] prepared foil) supporting the presence of natural diamonds in the Tehuitzingo chromitites. However, Massonne (2019) argues against a natural origin for the diamonds reported by us, stating that microdiamonds crystallized from supercritical C-O-H fluids are usually idiomorphic. We should stress that Dobrzhinetskaya et al. (2004) noted that diamonds formed from this type of fluid may also exhibit irregular morphologies. Further, there are additional lines of evidence for the natural origin of microdiamonds in our case study. First, all the diamonds were found along sealed fractures within chromite crystals, and none were found in micrometric open pores or spaces of the altered rims of chromite grains (our figure 1). This observation is incompatible with artificially (i.e., mechanically) incorporated diamonds into chromite. Second, the Tehuitzingo microdiamonds are often associated with amorphous carbon (our figure 2), a phase that commonly occurs in microtextural association with natural ophiolitic diamond (e.g., in situ diamonds in chromitites from the Ray-Iz ophiolite, Polar Urals, Russia;Yang et al., 2015). Third, our diamonds exhibit similar morphologies and sizes to those of natural diamonds reported as negative crystals included in garnet from metasedimentary rocks (e.g., Janák et al., 2015). These diamonds also exhibit cohesive contacts with the host mineral, and hollows interpreted as the result of fluid escape, similar to our case study (our figures 1 and 2). Fourth, we provided high-resolution transmission electron microscopy images evidencing the complete lack of polishing debris and epoxy in the chromite-hosted diamond inclusions from the Tehuitzingo chromitites (our figure 2), as the presence of this material would be expected if diamonds were incorporated during the polishing process. This is a key observation that has been used by other authors (Dobrzhinetskaya et al., 2014) to rule out the natural origin of microdiamonds, such as those included in zircons from the Jack Hills (Australia) conglomerate (Menneken et al., 2007). The TEM images provided by Dobrzhinetskaya et al. (2014) clearly show that open pores in zircon were filled with abundant debris and nanometric fragments of diamonds with sharp corners and acute fins. These authors concluded that the diamond grains were fragmented during polishing and were incorporate...

Diamond forms during low pressure serpentinisation of oceanic lithosphere

“…Recently, the finding of in situ diamond in chromite-hosted fluid inclusions from ophiolitic chromitites by Farré- de-Pablo et al, (2019a) provided the first evidence for empirical (Simakov et al, 2015(Simakov et al, , 2020, experimental (Simakov et al, 2008) and theoretical (Manuella, 2013;Simakov, 2018) work on low pressure growth of diamond. However, the debate on the natural origin of diamond continued (e.g., Farré-de-Pablo et al, 2019b;Massonne, 2019;Yang et al, 2019). In this regard, Litasov et al (2019a,b) have recently claimed that most diamonds, if not all, from ophiolitic rocks are not natural but instead have a synthetic origin, and emphasised the need to identify diamond below the polished surface of the host mineral.…”

Diamond and Other Exotic Mineral-Bearing Ophiolites on the Globe: A Key to Understand the Discovery of New Minerals and Formation of Ophiolitic Podiform Chromitite

“…The controversial genetic interpretation of diamond, chromitite and their host peridotite in various ophiolites not only includes the aforementioned four main models but also involves other viewpoints, e.g., UHP and highly reduced mineral assemblages possibly formed by lightning strikes [145], contamination by artificial abrasive materials [146], or formed during serpentinization under low pressure [39]. These issues have recently been hotly debated [147][148][149][150]. It is noted that Farré-de-Pablo et al (2018) reported in situ microdiamonds (diameter = 1-8 µm) in chromite from the Tehuitzingo chromitite in the Acatlán Orogenic Belt in southern Mexico.…”

“…Farré-de-Pablo et al (2019) replied that they had not found UHP minerals other than diamond in the Tehuitzingo chromitites, serpentinized peridotites and other rocks from the Acatlán Complex and claimed that the metamorphism of the Acatlán Complex did not even reach the stability of coesite. They argued that the aforementioned abundant so-called UHP and highly reduced minerals or oriented lamellae of Cpx in chromite developed during cooling and serpentinization rather than under UHP conditions [147].…”