Other
Scientific paper
Jul 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.2785p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 13, pp.2785-2796
Other
8
Scientific paper
Experiments were conducted in air and at low oxygen fugacity ( f O 2 ) to evaluate Mg and Ti partitioning between anorthite and liquid ( D Mg and D Ti in a synthetic composition similar to that of a Type B Ca, AI-rich inclusion (CAI). The starting material showed a range of compositions, which allowed assessment of the composition dependence of D Mg and D Ti in this system. Additional experiments using a homogeneous split of the same material investigated the effect of oxygen fugacity on the partitioning of Ti 3+ and Ti 4+ between anorthite and liquid. The low foe charges were purple, consistent with the presence of significant amounts of Ti 3+ .This was verified by electron spin resonance (ESR) spectra, and quantitative estimates of Ti 3+ 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, D Ti at low f O 2 is slightly greater than DT; in the air experiments. Using Ti 3+ /total Ti from the ESR measurements, D Ti 3+ is calculated to be about 0.040. The range of compositions reveal a clustering of D Mg and D Ti within charges, but a wide range of D s between charges of different composition. A well-defined inverse correlation exists between D Mg and D Ti . This variation is not due to temperature-dependence, but is instead due to the dependence of D Mg and D Ti on liquid composition (Si and Al in particular). D Mg correlates positively with Si content and negatively with Al content, while D Ti 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, Ti 4+ , and Ti 3+ 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 D Ti 3+ . is equal to D Ti 4+ . The results from our experiments support this model for the relative partitioning of Ti 4+ and Ti 3+ between plagioclase and liquid. In addition, the dependence of D Mg , and D Ti on the Si content of a Type B CAI liquid helps explain systematics expected during late-stage crystallization of plagioclase. The composition dependence of D Mg and D Ti 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 A1 by the reaction Mg 2+ + Si 4+ = 2AI 3+ , consistent with that proposed by previous workers. The systematics are also consistent with Ti 4+ and Ti3+ substitution for tetrahedral Si 4+ by the reactions 2Al 3+ + Ti 4+ = Ca 2+ + 2Si 4+ and Al 3+ + Ti 3+ = Ca 2+ + S i4+ , respectively.
Burnett Donald S.
Kim Soon Sam
Peters Mark T.
Shaffer Elizabeth E.
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