Diamond forms during low pressure serpentinisation of oceanic lithosphere Pujol Solà, N. García Casco, Antonio González Jiménez, J.M. Sánchez Navas, Antonio 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 CH4-bearing fluids after olivine alteration by reaction with infiltrating fluids. Here we report the first ever observed in situ diamond within olivine-hosted, CH4-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 logfO2 (MPa) = −45.3 (ΔlogfO2[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. 2020-11-23T11:32:01Z 2020-11-23T11:32:01Z 2020-09-10 info:eu-repo/semantics/article Pujol-Sola, N., Garcia-Casco, A., Proenza, J. A., Gonzalez-Jimenez, J. M., del Campo, A., Colas, V., ... & Roque-Rosell, J. (2020). Diamond forms during low pressure serpentinisation of oceanic lithosphere. GEOCHEMICAL PERSPECTIVES LETTERS, 15, 19-24. [doi: 10.7185/geochemlet.2029] http://hdl.handle.net/10481/64436 10.7185/geochemlet.2029 eng http://creativecommons.org/licenses/by-nc-nd/3.0/es/ info:eu-repo/semantics/openAccess Atribución-NoComercial-SinDerivadas 3.0 España European Assoc Geochemistry