A hibonite-spinel-perovskite CAI from an Allende thin section is in many respects similar to other hib-sp-pv inclusions, especially one described in , but also shows several unusual alteration features. The inclusion is about 300 micrometers in long dimension, but with extended curved ends giving an indication of an original spherical object. A single angular 200-micrometer hibonite grain (TiO2: 1.2-1.6; MgO: 0.57-0.77; SiO2: <0.05 FeO: <0.10 Cr2O3 <0.01%), which includes several 20-micrometer perovskites (Al2O3 up to 2%) occurs at the inclusion center and is surrounded by a mantle of spinel zoned with FeO near 1% at hibonite boundary to 12% at inclusion edge. Within the spinel are blades of texturally and compositionally distinct hibonite (TiO2: 4.6-8.2; MgO: 2.4-4.4; SiO2: 0.13-0.35; FeO: 0.26-0.70; Cr2O3: 0.03-0.07%), which appear to have grown from the rim toward the inclusion center. Also within the spinel mantle are numerous micrometer-sized perovskite grains. A rim sequence surrounds the entire inclusion and includes Fe-olivine, diopside, scapolite-nepheline, and hibonite layers from edge toward spinel mantle. No melilite, forsterite, or fassaite is present in inclusion, i.e., no silicates are present. Evidence of low-temperature alteration occurs as (1) replacement of perovskite within spinel mantle and adjacent to rim by Mg-ilmenite (MgO: 3.0-7.8; Al2O3: 0.3-2.0; MnO: 0.25%; V present); (2) exsolution as oriented submicrometer lamellae of Al2O3 from the larger Al-rich perovskites within central hibonite; and (3) veins, especially within hibonite and adjacent to perovskite, of Al2O3 composition (Al2O3: 97.2-79.2; SiO2: 1.5-14.2; Na2O: 0.6-4.9%; minor Fe, Ca, K) and common fiberous texture. All analyses sum to 100%, and Na correlates with Si. The ratio of Na to Si implies a mix of Al2O3 and nepheline. The inclusion is very similar to one previously described , but in place of central melilite, the present inclusion has hibonite. The angular hibonite grain contrasts with the euhedral hibonite blades and generally spherical inclusion. While these features imply an original melt, the angular hibonite suggests a relic grain that may have formed a nucleus for a melt. Special care must be given to interpreting bulk analyses if some CAI contain two or more distinct components. The occurrence of Al2O3 in two textural forms suggests that at least some Al2O3 is secondary either due to exsolution or possibly from breakdown(?) of hibonite. Previous observations of corundum do not necessarily imply that Al2O3 is a primary phase. References:  Bischoff A. et al. (1982) LPSC XVIII, 81-82.
- Pub Date:
- July 1993