The Activity of Chromite in Multicomponent Spinels: An Experimental Study with Implications for the Metamorphic History of Equilibrated Ordinary Chondrites

An experimental technique for evaluating the activity of chromite in multicomponent spinels was developed by equilibrating the spinel of interest with a Pt-alloy under a controlled temperature and oxygen fugacity. The thermodynamic properties of the ternary Pt-Fe-Cr system was evaluated, such that activities of Cr and Fe in an equilibrated Pt-alloy can be used to calculate the activity of chromite in the spinel of interest. The ternary activity model formulation used is based on the characterization of each bounding binary system, Pt-Fe, Pt-Cr, Fe-Cr, with the addition of ternary interaction terms. The Pt-Fe and Pt-Cr systems are described as asymmetric regular solutions with interaction parameters of W_(PtFe)=-138.0±3.3, W_(FePt)=-90.8±24.0, and W_(PtCr)=-129.1±1.2, W_(CrPt)=-80.9±4.4, and D_(PtCr)=+94.4±2.5 kJ/mol (1σ), respectively. Combined with literature thermodynamic properties for the Fe-Cr system, the ternary interaction parameters in the Pt-Fe-Cr system were found to be C_(Cr)=0, C_(Pt)=+ll5.7, and C_(Fe)=-68.6 kJ/mol. Using this technique, the metamorphic history of equilibrated ordinary chondrites was evaluated by examining the compositions and textures of olivines, pyroxenes, spinels, and alloys. Equilibrium temperatures based on Fe-Mg exchange between olivine and spinel exhibit a range of 680-796°C in H, L, and LL ordinary chondrites spanning petrographic type 4 to 6. Type 4 chondrites in all groups record variable temperatures that are lower than or equal to those of types 5 and 6 chondrites, implying decoupling of metamorphic temperature from petrographic type. Cooling rates near 800°C were found to be 1-3 K/Ma, slow enough to allow continous re-equilibration of spinel grains from peak metamorphic temperatures to the olivine-spinel equilibration temperature. The temperature-oxygen fugacity relationships in equilibrated H chondrites were constrained from spinel-alloy and olivine-pyroxene-alloy phase assemblages based on the mineral compositions and activity-composition models. Log_(10)f_(02) values based on the assemblage olivine-pyroxene-alloy are -1.75±0.02 log units below Iron-Wüstite (IW) buffer, regardless of petrographic type. The log_(10)f_(02) values calculated based on the spinel-alloy coexistence are at least ~1.5 log units more oxidizing than those based on the olivine-pyroxene-alloy if olivine-spinel equilibration temperatures (728-820°C) are assumed. This probably indicates that closure for spinel-alloy equilibria occurred under retrograde conditions at temperatures below 700°C.