Experimental study of coupled redox equilibria in a peridotite–metasediment–eclogite mélange and implications for diamond formation

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Diamond formation in the mantle occurs by redox reactions between carbon-bearing species and the silicate matrix. It forms most readily when a mobile carbonatitic melt passes through peridotite containing a metallic phase. However, the presence of metal is not a prerequisite. In fact, redox reactions can also occur in Fe-free systems. For example, the carbonation reaction for peridotite is known as EMOG or EMOD buffer depending on the presence of graphite or diamond:Mg2SiO4 (olivine) + C (diamond) + O2 = MgSiO3 (pyroxene) + MgCO3 (magnesite) (1)The maximum ƒO2 of this oxygen buffer reaction is lower than that of analogous equilibria in eclogites or sediments such as: CaMgSi2O6 (pyroxene) + 2C (diamond) + 2O2 = CaMg(CO3)2 (dolomite) + 2SiO2 (2)Thus, juxtaposed carbonated eclogitic or sedimentary lithologies should become reduced solely by the exchange of oxygen with adjacent peridotite. Such situations are expected to be widespread in mélange zones in a subduction setting. Experiments will be conducted in the simple system CaO-MgO-Al2O3-SiO2-C-O2. Pure, synthetic forsterite will be the endmember for the carbonation reaction for peridotite and appropriate mixtures of CaCO3, MgCO3, SiO2, and Al2O3 will be used as starting materials to produce model sedimentary and eclogite compositions. The different starting materials will be physically separated with a graphite disk to limit chemical exchange to that of oxygen transfer. The experiments will be performed within the diamond stability field in a multianvil apparatus at 10 GPa over a range in temperature (1300 -1500°C). The results will have applications to possible mechanisms of diamond formation in the upper mantle. They may also explain the seeming paradox that the majority of mineral inclusions in diamond belong to the peridotite suite while diamondiferous eclogites are much more common than diamond-bearing peridotites. Carbonate-bearing metasediments or metabasalts can be transformed to diamond-bearing carbonate-free eclogites, while the adjacent diamondiferous peridotites will be carbonated in the mélange zone above a subducting slab.