The effects of Cr solubility in chlorites and its implications for Cr-clinochlore stability

Chlorite is an ubiquitous hydrous phase in a variety of rock compositions involved in subduction processes when aluminum is a major chemical component. Because of its high H2O/Al2O3 ratio (close to 3/4), even small amounts of Al2O3 translates in large quantities of water potentially released at chlorite phase transformations. The chlorite stability field does not exceed 760-780 °C in a model NCFMASH peridotite, up to 4 GPa, but chlorite was found to extend to more than 850 °C at approx. 3 GPa in more complex chemical systems (Grove et al., 2006), in which the addition of Cr2O3 leads to Cr-chlorite with up to 1.4 wt.% Cr2O3. On the other hand, natural chlorite included in garnet porphyroblasts from ultra-high pressure ultramafic bodies may well exceed 2.3 wt.% Cr2O3. In order to discriminate whether Cr exerts such a profound effect on chlorite breakdown reactions, the thermodynamic properties as a function of different Cr content need to be explored. End-loaded piston cylinder synthesis experiments have been performed at 2.0 GPa, 800 °C in the model Cr2O3-MgO-Al2O3-SiO2-H2O system. Assuming that Cr is substituting for Al in the octahedral site of a clinochlore stoichiometry Mg10Al2(Si6Al2)O20(OH)16, three compositions JF1, JF2, and JF3 have been considered with Cr/(Cr+AlVI) = 0.15, 0.5 and 1.0 respectively. Gels were used as starting materials. In order to avoid the occurrence of metastable phases, as found in earlier experiments at the same PT conditions, 5 wt.% of natural clinochlore (Val Malenco, Italy) was added to the gels. Run products have been first identified by X-ray powder diffractometry, and analysed by EMPA. Cr-clinochlore has been successfully synthesized from JF1 seeded composition at 2.0 GPa, 800°C, 172 hrs. X-ray powder diffraction pattern suggests a single phase assemblage, with the basal peaks 001 and 002 at 6.2 and 12.4 degrees of 2 Theta respectively. Synthetic Cr chlorite has been analyzed by electron microprobe corroborating the X-ray powder diffraction observation. However, microprobe analysis revealed traces of orthopyroxene: however, coexisting orthopyroxenes contain no more than 0.02 Cr a.p.f.u. and Cr-clinochlore analyses are well within the expected composition (0.3 Cr a.p.f.u.). Crystal chemistry and cell parameters of synthetic Cr-clinoclore with different Cr contents will provide insights on the thermodynamic properties of chlorite and new constraints on phase equilibria in Cr bearing ultramafic systems.