Redox Equilibria Involving Chromium Minerals in Aqueous Fluids in the Deep Earth - Implications for Diamond Formation

Diamonds are often associated with inclusions of garnet that are characteristically Cr-rich and Ca-poor, suggesting metasomatic reactions involving fluids [1]. To investigate these reactions, we developed a thermodynamic characterization of Cr-bearing minerals and integrated it with our database for the thermodynamic properties of aqueous Cr-species [2]. We retrieved thermodynamic properties of picrochromite (MgCr2O4), and knorringite (Mg3Cr2Si3O12) consistent with minerals in the Berman (1988) using calorimetric data and experimental phase equilibria involving the reactions: MgCr2O4 + SiO2 = Cr2O3 + MgSiO3 [2] and MgCr2O4 + 4MgSiO3 = Mg3Cr2Si3O12 + Mg2SiO4 [3], respectively.At high temperatures and pressures, neutral pH and FMQ, the predicted solubilities of eskolaite and knorringite equilibrium with Cr2+ in a pure water system are very low. However, we found that complexes of Cr2+ and Cl- could increase the solubilities of chromium minerals significantly. At 500°C and 0.2 - 1.0 GPa, we retrieved the CrCl(OH)0 neutral complex from experiments on the solubility of Cr2O3 in HCl solutions [4]. At 1,000°C and 4.0 GPa, we retrieved the properties of a CrCl3- complex from experiments on the solubility of Cr2O3 in KCl solutions [5]. The predicted solubility of a garnet containing 23 mole% of knorringite in equilibrium with CrCl3- in a peridotitic diamond-forming fluid is 22 millimolal (1,144 ppm). This result suggests that a redox reaction relating to diamond formation might involveMg3Al2Si3O12 + 0.5CO2(aq) + 2 CrCl3- + 2H+ = Mg3Cr2Si3O12 + 0.5C-Diamond + 2Al3+ + 6Cl-. In this way, high temperature and pressure fluids containing Cr(II)-complexes might promote the mobility of chromium and be involved in metasomatic reactions and diamond formation.[1]Boyd et al. (1993)[2] Hao et al. (submitted to Geochem. Persp. Letters)[3] Berman (1988)[4] Klemme et al. (2000)[5] Klemme et al. (2004)[6] Watenphul et al. (2014)[7] Klein-BenDavid et al. (2011)