On type B CAI formation: experimental constraints on fO2 variations in spinel minor element partitioning and reequilibration effects

We report data from a series of dynamic crystallization experiments that focus on determining the partition coefficients (D’s) for V and Ti in the spinel + liquid system of an average type B1 CAI bulk composition for three different fO₂ conditions. Partitioning data for Ca and Si are also obtained. We show that the D’s for V and Ti are fO₂ dependent with D_Ti decreasing at low oxygen fugacity due to the presence of Ti³⁺. D_V is essentially 0 in air, rises to 2.2 at the Fe-FeO buffer and drops to 1.4 at the C-CO buffer. This indicates that V³⁺ is highly compatible in spinel and that higher and lower valence states are much less compatible. We also report data from isothermal experiments that determine diffusion times for V and Ti in same system at a temperature close to the T_max for type B1 CAIs. Diffusion of these elements between spinel and liquid is surprisingly rapid, with essentially total equilibration of Ti and V between spinel and liquid in 90 h run duration. Lack of equilibration of Cr, Si, and Ca shows that the Ti and V equilibration mechanism was diffusion and not dissolution and reprecipitation. Our experimental run durations set an upper limit of a few tens of hours on the time that type B1 CAIs were at their maximum temperature. Based on our data we argue that subsolidus reequilibration between spinel inclusion and host-silicate phases within type B CAIs likely explains the observed range of V and Ti concentrations in spinels which are inclusions in clinopyroxene.