The activity of chromite in multicomponent spinels: Implications for T-fO_2 conditions of equilibrated H chondrites

Activities of chromite in multicomponent spinels with compositions similar to those of H chondrites were experimentally determined by equilibrating Pt-alloys with spinel at known temperature and fO_2. Our results are consistent with predictions based on the spinel solid solution model incorporated into the MELTS program. Therefore, we combined literature formulations for the activities of components in spinel, the ferromagnesian silicates, and alloys with measured and literature (bulk alloy) compositions of the meteoritic phases to constrain T-fO_2 conditions for the H-group chondrites Avanhandava (H4), Allegan (H5), and Guareña (H6). Log10fO2 values based on the assemblage of olivine + orthopyroxene + metal are 2.19–2.56 log units below the iron-wüstite (IW) buffer for any equilibration temperature between 740 and 990 °C, regardless of petrographic type. Only lower limits on fO_2 could be determined from spinel + metal equilibria because of the extremely low concentrations of Cr in the alloys of equilibrated H chondrites (≤3 ppb). Log10fO_2 values required by spinel + metal equilibria are inconsistent with those for olivine + orthopyroxene + metal if equilibration temperatures were at or above those inferred from olivine-spinel thermometry. This probably indicates that the closure for spinel + metal equilibria occurred under retrograde conditions at temperatures below ∼625 °C for Allegan and Guareña and below ∼660 °C for Avanhandava