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

Harold C. Connolly, D. S. Burnett

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Abstract

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 f O2 conditions. Partitioning data for Ca and Si are also obtained. We show that the D's for V and Ti are f O2 dependent with DTi decreasing at low oxygen fugacity due to the presence of Ti3+. DV 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 Tmax 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.

Original languageEnglish (US)
Pages (from-to)4429-4434
Number of pages6
JournalGeochimica et Cosmochimica Acta
Volume67
Issue number22
DOIs
StatePublished - Nov 15 2003

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All Science Journal Classification (ASJC) codes

  • Geochemistry and Petrology

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