Minor element partitioning and sector zoning in synthetic and meteoritic anorthite

Abstract

Regular, geometric, apparently crystallographically-controlled, sector zoning has been produced in synthetic anorthite crystallized from a liquid of Type B Ca,Al-rich inclusion (CAI) composition. Cathodoluminescence (CL) provides a rapid method for observing the sectors. Sharp discontinuities in Mg concentration always accompany sharp changes in CL intensity at sector boundaries. Although CL intensity anti-correlates with Mg concentration, there is not a well-defined quantitative relationship, and Mg concentration steps range from as little as 5% to almost a factor of 2 for different CL boundaries. When measurable, Na also tends to anti-correlate with CL brightness in a given crystal and thus correlates with Mg. The partition coefficients for Mg and Na can vary by up to a factor of 2 depending on the particular sector and thus are not constant for CAI anorthite which formed by fractional crystallization. In contrast, Ti (either Ti^+3 or Ti^+4) concentration is not a function of sector zoning and thus does not affect the CL intensity. The sector zoning and correlated chemical variations for synthetic anorthite are similar to those in natural anorthite from Type B CAIs. The CL intensity, boundary sharpness, and minor element zoning in synthetic anorthite are not affected by annealing at 850°C for three weeks. Overall, our observations suggest that the observed CL patterns and minor element zoning in Type B anorthite are igneous in origin and have not been modified by subsolidus reequilibration or alteration. This leads to the conclusion that the ^(26)Mg-^(26)Al systematics in CAIs (based almost entirely on anorthite) have not been modified by subsolidus reequilibration or alteration.

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