Diamond forms during low pressure serpentinisation of oceanic lithosphere

Abstract

Diamond is commonly regarded as an indicator of ultra-high pressure conditions in Earth System Science. This canonical view is challenged by recent data and interpretations that suggest metastable growth of diamond in low pressure environments. One such environment is serpentinisation of oceanic lithosphere, which produces highly reduced CH 4-bearing fluids after olivine alteration by reaction with infiltrating fluids. Here we report the first ever observed in situ diamond within olivine-hosted, CH 4-rich fluid inclusions from low pressure oceanic gabbro and chromitite samples from the Moa-Baracoa ophiolitic massif, eastern Cuba. Diamond is encapsulated in voids below the polished mineral surface forming a typical serpentinisation array, with methane, serpentine and magnetite, providing definitive evidence for its metastable growth upon low temperature and low pressure alteration of oceanic lithosphere and super-reduction of infiltrated fluids. Thermodynamic modelling of the observed solid and fluid assemblage at a reference P-T point appropriate for serpentinisation (350°C and 100 MPa) is consistent with extreme reduction of the fluid to logfO 2 (MPa) = −45.3 (ΔlogfO 2 [Iron-Magnetite] = −6.5). These findings imply that the formation of metastable diamond at low pressure in serpentinised olivine is a widespread process in modern and ancient oceanic lithosphere, questioning a generalised ultra-high pressure origin for ophiolitic diamond.

Metapelite from the high‐ to ultrahigh‐pressure terrane of the Eastern Alps (Pohorje Mountains, Slovenia)—New pressure, temperature and time constraints on a polymetamorphic rock

“…Recently, Farré‐de‐Pablo et al (2019) reported diamond enclosed in chromite from a serpentinite body and argued for metastable formation of diamond. Although this conclusion was questioned by Massonne (2019) because of the exposure of xenomorphic diamond in opened voids, a very similar occurrence of diamond enclosed in olivine of a basic body in a Cuban ophiolite was reported by Pujol‐Solà et al (2020). These authors could demonstrate by Raman spectroscopy that xenomorphic diamond is already in voids in olivine before these were opened.…”

“…This diamond formed metastably at significantly lower pressures than necessary for stable formation of diamond. In case of both diamond occurrences in basic to ultrabasic rocks, Farré‐de‐Pablo et al (2019) and Pujol‐Solà et al (2020) argued that the voids containing xenomorphic diamond were previously filled with a CO 2 + CH 4 fluid which can be extremely reducing. Such a fluid, that has filled the inclusions in garnet in which diamond was found by Janak et al (2015), was reported by Hurai et al (2010).…”

“…Such a fluid, that has filled the inclusions in garnet in which diamond was found by Janak et al (2015), was reported by Hurai et al (2010). After opening, these inclusions showed xenomorphic diamond, cavities (after the fluid has escaped) and, in some instances, moissanite (SiC) which could have formed because of the extremely reducing behaviour of the CO 2 + CH 4 fluid (Pujol‐Solà et al, 2020). Given the similarity (fluid, xenomorphic diamond) with the diamond‐bearing inclusions reported by Farré‐de‐Pablo et al (2019) and Pujol‐Solà et al (2020), we consider it possible that the diamond in the studied metapelite body of the Pohorje Mts.…”

“…After opening, these inclusions showed xenomorphic diamond, cavities (after the fluid has escaped) and, in some instances, moissanite (SiC) which could have formed because of the extremely reducing behaviour of the CO 2 + CH 4 fluid (Pujol‐Solà et al, 2020). Given the similarity (fluid, xenomorphic diamond) with the diamond‐bearing inclusions reported by Farré‐de‐Pablo et al (2019) and Pujol‐Solà et al (2020), we consider it possible that the diamond in the studied metapelite body of the Pohorje Mts. also formed metastably and thus cannot be used to emphatically constrain the metamorphic peak pressure to the diamond field.…”

“…The diamond-graphite and coesite-quartz transition curves are from Day (2012) and Massonne (1999) respectively. Note that the retrograde paths for metapelites were partially drawn by the present authors using P-T information (crosses) given by the corresponding authors mentioned in the legend [Colour figure can be viewed at wileyonlinelibrary.com] which could have formed because of the extremely reducing behaviour of the CO 2 + CH 4 fluid (Pujol-Solà et al, 2020). Given the similarity (fluid, xenomorphic diamond) with the diamond-bearing inclusions reported by Farré- de-Pablo et al (2019) and Pujol-Solà et al (2020), we consider it possible that the diamond in the studied metapelite body of the Pohorje Mts.…”