MINOR ELEMENT PARTITIONING AND SECTOR ZONING IN SYNTHETIC AND METEORITIC ANORTHITE

Regular, geometric, apparently crystallographically-controlled, sector zoning has been produced in synthetic anorthite crystallized from a l iquid of Type B Ca,Al-rich inclusion (CAI) composition. Cathodolumines cence (CL) provides a rapid method for observing the sectors. Sharp di scontinuities 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 relat ionship, and Mg concentration steps range from as little as 5% to almo st a factor of 2 for different CL boundaries. When measurable, Na also tends to anti-correlate with CL brightness in a given crystal and thu s correlates with Mg. The partition coefficients for Mg and Na can var y by up to a factor of 2 depending on the particular sector and thus a re not constant for CAI anorthite which formed by fractional crystalli zation. In contrast, Ti (either Ti+3 Or Ti+4) concentration is not a f unction of sector zoning and thus does not affect the CL intensity. Th e sector zoning and correlated chemical variations for synthetic anort hite are similar to those in natural anorthite from Type B CAIs. The C L intensity, boundary sharpness, and minor element zoning in synthetic anorthite are not affected by annealing at 850 degrees C for three we eks. Overall, our observations suggest that the observed CL patterns a nd minor element zoning in Type B anorthite are igneous in origin and have not been modified by subsolidus reequilibration or alteration. Th is leads to the conclusion that the Mg-26-Al-26 systematics in CAIs (b ased almost entirely on anorthite) have not been modified by subsolidu s reequilibration or alteration. Copyright (C) 1997 Elsevier Science L td.