AN ISOTOPIC AND PETROLOGIC STUDY OF CALCIUM-ALUMINUM-RICH INCLUSIONS FROM CO3 METEORITES

We have studied the mineralogy and petrology of 229 calcium-aluminum-r ich inclusions (CAIs) from ten CO3 meteorites of petrologic types 3.0- 3.7. Subsets of these inclusions were measured by ion probe for magnes ium, calcium, and titanium isotopes and REE abundances. Most CAIs from CO3 meteorites fall into three major types: (1) melilite-rich inclusi ons, which also contain spinel/hercynite, perovskite, and occasionally hibonite; (2) spinel-pyroxene inclusions: and (3) hibonite-hercynite inclusions. In addition, several isolated hibonite grains, two grossit e (CaAl4O7) bearing CAIs, two hibonite-fassaite microspherules, and on e anorthite-spinel-pyroxene inclusion were found. CAIs from CO3 meteor ites exhibit all of the REE patterns commonly seen in inclusions from CV3 and CM2 chondrites. Most exhibit evidence of Al-26, and many have inferred (Al-26/Al-27)(o) approximate to 5 x 10(-5). The relative abun dances of different types of CAIs in CO3 chondrites differ from those in CV3 and CM2 chondrites. CAIs in CO3 chondrites have experienced con siderable secondary alteration, both before and after accretion. Signa tures of nebular alteration include Wark-Lovering rims and the high Fe contents in spinels from all hibonite/hercynite inclusions. Occasiona lly, melilite and acorthite show evidence of nebular alteration to fel dspathoids and pyroxene. The magnesium-aluminum systematics of some me lilite-rich inclusions were apparently disturbed prior to final accret ion of the parent body. Parent body alteration is indicated by correla tions between CAI characteristics and the petrologic type of the host meteorite. Spinel in melilite-rich and coarse-grained spinel-pyroxene inclusions becomes more Fe rich, with the development of relatively ho mogeneous hercynitic spinel (similar to 50-60 mol%) in CAIs from metam orphic grades >3.4. Perovskite has been converted to ilmenite in types >3.4. Melilite-rich inclusions are abundant in CO3.0-3.3 meteorites, rare in 3.4 meteorites, and absent meteorites of types 3.5-3.7; melili te-rich CAIs are probably replaced by inclusions rich in feldspathoids , pyroxene, and Fe-rich spinel. Isotopic disturbance of the magnesium- aluminum systematics may be more severe in higher petrologic types. Hi bonite seems to be unaffected by this level of metamorphism. Three iso topically unusual inclusions were found. One single-crystal hibonite, Isna SP16, has a REE pattern strongly depleted in Ce and Y. (Al-26/Al- 27)(o) = (2.4 +/- 0.3) x 10(-5), and mass fractionated calcium (F-Ca = +12 +/- 2 parts per thousand/amu), but no resolvable nuclear anomalie s in neutron-rich calcium isotopes. The REE pattern, which is thought to reflect nebular conditions, and mass-fractionated calcium, indicati ve of evaporation, are similar to those of the FUN inclusion, HAL, and related hibonites, indicating similar formation conditions. The absen ce in Isna SP16 of the nuclear anomalies observed in HAL and the diffe rence in (Al-26/Al-27)(o) between HAL and Isna SP16 indicate that the processes that produced HAL-type hibonites operated on diverse materia ls. Two hibonite-bearing microspherules, Colony SP1 and ALH82101 SP15, exhibit nearly flat REE patterns with negative europium anomalies and slightly negative delta(26)Mg. ALH82101 SP15 has resolved excesses of Ca-48 and Ti-50. These characteristics are similar to those of previo usly described microspherules from Murchison and Lance, implying that the microspherules formed via a single process from related, but not i dentical source materials. Copyright (C) 1998 Elsevier Science Ltd.