Carbon isotope fractionation during diamond growth in depleted peridotite: Counterintuitive insights from modelling water-maximum CHO fluids as multi-component systems

摘要

Because of the inability of depleted cratonic peridotites to effectively buffer oxygen fugacities when infiltrated by CHO or carbonatitic fluids, it has been proposed recently (Luth and Stachel, 2014) that diamond formation in peridotites typicallY does not occur by rock-buffered redox reactions as previously thought but by an oxygen-conserving reaction in which minor coexisting CH4 and CO2 components in a water-rich fluid react to form diamond (CO2 + CH4 = 2C + 2H(2)O). In such fluid-buffered systems, carbon isotope fractionation during diamond precipitation occurs in the presence of two dominant fluid carbon species. Carbon isotope modelling of diamond precipitation from mixed CH4- and CO2-bearing fluids reveals unexpected fundamental differences relative to diamond crystallization from a single carbon fluid species: (1) irrespective of which carbon fluid species (CH4 or CO2) is dominant in the initial fluid, diamond formation is invariably associated with progressive minor (<1 parts per thousand) enrichment of diamond in C-13 as crystallization proceeds. This is in contrast to diamond precipitation by rock-buffered redox processes from a fluid containing only a single carbon species, which can result in either progressive C-13 enrichment (CO2 or carbonate fluids) or C-13 depletion (CH4 fluids) in the diamond. (2) Fluid speciation is the key factor controlling diamond delta C-13 values; as X-CO2 (X-CO2 = CO2/[CO2 + CH4]) in the initial fluid increases from 0.1 to 0.9 (corresponding to an increase in fO(2) of 0.8 log units), the carbon isotope composition of the first-precipitated diamond decreases by 3.7 parts per thousand. The tight mode in delta C-13 of -5 +/- 1%o for diamonds worldwide places strict constraints on the dominant range of X-CO2 in water-rich fluids responsible for diamond formation. Specifically, precipitation of diamonds with delta C-13 values in the range -4 to -6%o from mantle-derived fluids with an average delta C-13 value of -5 parts per thousand (derived from evidence not related to diamonds) requires that diamond-forming fluids were relatively reduced and had methane as the dominant carbon species (X-CO2 = 0.1-0.5).