Petrography of refractory inclusions in CM2.6 QUE 97990 and the origin of melilite-free spinel inclusions in CM chondrites

Abstract

Queen Alexandra Range (QUE) 97990 (CM2.6) is among the least-altered CM chondrites known. It contains 1.8 vol% refractory inclusions; 40 were studied from a single thin section. Inclusion varieties include simple, banded and nodular structures as well as simple and complex distended objects. The inclusions range in mean size from 30 to 530 micrometers and average 130 +/- 90 m. Many inclusions contain 25 +/- 15 vol% phyllosilicate (predominantly Mg-Fe serpentine); several contain small grains of perovskite. In addition to phyllosilicate, the most abundant inclusions in QUE 97990 consist mainly of spinel-pyroxene (35%), followed by spinel (20%), spinel-pyroxene-olivine (18%), pyroxene (12%), pyroxene-olivine (8%) and hibonite spinel (8%). Four pyroxene phases occur: diopside, Al-rich diopside (with 8.0 wt% Al2O3), Al-Ti diopside (i.e., fassaite), and (in two inclusions) enstatite. No inclusions contain melilite. Aqueous alteration of refractory inclusions transforms some phases (particularly melilite) into phyllosilicate; some inclusions broke apart during alteration. Melilite-free, phyllosilicate-bearing, spinel inclusions probably formed from pristine, phyllosilicate-free inclusions containing both melilite and spinel. Sixty-five percent of the refractory inclusions in QUE 97990 appear to be largely intact; the major exception is the group of spinel inclusions, all of which are fragments. Whereas QUE 97990 contains about 50 largely intact refractory inclusions/cm2, estimates from literature data imply that more-altered CM chondrites have lower modal abundances (and lower number densities) of refractory inclusions: Mighei (CM ~ 2.3) contains roughly 0.3-0.6 vol% inclusions (~10 largely intact inclusions/cm2); Cold Bokkeveld (CM2.2) contains ~0.01 vol% inclusions (on the order of 6 largely intact inclusions/cm^2).