MAGNESIUM AND TITANIUM PARTITIONING BETWEEN ANORTHITE AND TYPE B CAI LIQUID: DEPENDENCE ON OXYGEN FUGACITY AND LIQUID COMPOSITION

Abstract

Experiments were conducted in air and at low oxygen fugacity (fO2) to evaluate Mg and Ti partitioning between anorthite and liquid (DMg and DTi) in a synthetic composition similar to that of a Type B Ca, Al-rich inclusion (CAI). The starting material showed a range of compositions, which allowed assessment of the composition dependence of DMg and DTi in this system. Additional experiments using a homogeneous split of the same material investigated the effect of oxygen fugacity on the partitioning of Ti3+ and Ti4+ between anorthite and liquid. The low fO2 charges were purple, consistent with the presence of significant amounts of Ti3+. This was verified by electron spin resonance (ESR) spectra, and quantitative estimates of Ti3+ contents were obtained using ESR. The Ti and Mg partition coefficients in the air run using the homogeneous starting material are similar (0.034 and 0.036, respectively) and consistent with those determined in other studies. However, DTi at low fO2 is slightly greater than DTi in the air experiments. Using Ti3+/total Ti from the ESR measurements, DTi3+ is calculated to be about 0.040.The range of compositions reveal a clustering of DMg and DTi within charges, but a wide range ofD s between charges of different composition. A well-defined inverse correlation exists between DMg andDTi . This variation is not due to temperature-dependence, but is instead due to the dependence of DMg and DTi on liquid composition (Si and Al in particular). DMg correlates positively with Si content and negatively with Al content, while DTi shows the opposite correlations.The results of these experiments have interesting implications for the petrogenesis of Type B CAIs and for substitution mechanisms of Mg, Ti4+, and Ti3+ into anorthite. Crystallization models for Type B CAIs permit certain predictions concerning trace element systematics in plagioclase. The Mg and Ti systematics are best explained by a fractional crystallization model where plagioclase crystallizes very late (>95% crystallization), and DTi3+ is equal to DTi4+. The results from our experiments support this model for the relative partitioning of Ti4+ and Ti3+ between plagioclase and liquid. In addition, the dependence of DMg and DTi on the Si content of a Type B CAI liquid helps explain systematics expected during late-stage crystallization of plagioclase. The composition dependence of DMg and DTi also allows assessment of substitution mechanisms in anorthite using a crystallization reaction approach. Using these methods, a plausible mechanism for Mg involves substitution for tetrahedral Al by the reaction Mg2+ + Si4+ = 2Al3+, consistent with that proposed by previous workers. The systematics are also consistent with Ti4+ and Ti3+ substitution for tetrahedral Si4+ by the reactions 2Al3+ + Ti4+ = Ca2+ + 2Si4+ and Al3+ + Ti3+ = Ca2+ + Si4+, respectively.