Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.595w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 595
Mathematics
Logic
3
Chondrites, Carbonaceous, Inclusions, Calcium Aluminum-Rich, Refractory, Meteorites, Acfer 094, Primitive
Scientific paper
Introduction: The meteorite Acfer 094 is a uniquely primitive carbonaceous chondrite from the Sahara, which was first classified as a CO(CM) chondrite [1,2]. Trace element abundances of Acfer 094 are closely related to those from CM2 chondrites [1,3], but the oxygen isotopes suggest a relationship to C03 chondrites [1]. Studies on stable isotopes indicate that Acfer 094 contains isotopically anomalous carbon and nitrogen [4,5]. Based on mineralogical and petrographical observations, Bischoffand Geiger [6] suggested that Acfer 094 may be the first CM3 chondrite, whereas Newton et al. [5] pointed out that this meteorite is indeed very primitive but may not be directly related to existing types of carbonaceous chondrites. This study on refractory inclusions contribute further informations on Acfer 094 in view of a possible relationship to other meteorites. Results: The modal abundance of refractory or calcium-aluminium-rich inclusions (CAIs) within Acfer 094 is below 2 vol.% and the sizes of the objects range from 40 to 500 micrometers. The most abundant phases within the inclusions are melilite and spinel. Other phases identified within 23 studied inclusions include perovskite, hibonite, Ca-pyroxene (diopsidic to fassaitic), anorthite, grossite (CaA1407), an Al-, Ti-, Zr-, Y-rich phase and metal particles rich in Os, Ir, and Ru (PGEs). Secondary alteration products, such as sodalite, nepheline, or grossular, do not occur. The most common types of CAIs are type A (compact and fluflo and spinel-rich inclusions (together 15 objects). However, some of these inclusions have modal melilite and spinel abundances which indicate a smooth transition between type A (= melilite-rich) and spinel-rich inclusions. Thus, a reasonable distinction between these two inclusion types is not possible in every case. Usually, the melilite- and/or spinel-rich inclusions are rimmed by an outermost layer made of Ca-pyroxene and sometimes by an inner indistinct layer enriched in spinel. Four inclusions are rich in anorthite, three in hibonite. Another CAI with an apparent size of 140 x 140 micrometers consists of an intergrowth of grossite, melilite, perovskite, and hibonite, which is surrounded by a mantle of spinel. This mantle is rimmed by an inner layer of melilite and an outer layer of Ca-pyroxene. Two of the investigated inclusions (one anorthite-rich and one hibonite-rich CAI) are enclosed in a several hundred microns thick aggregate of fine-grained olivines. In addition, 5 objects were found which resemble AOAs. Discussion: Comparing CAIs from Acfer 094 to those from CV, CM, and CH chondrites the following general differences are obvious: Refractory inclusions from CV chondrites usually are larger and often affected by secondary processes, those from CM chondrites do not show the high modal abundance of melilite, and CAIs from CH chondrites contain grossite as a common phase and are rare in PGE-enriched metal particles. Some of the studied objects are restricted to Acfer 094 (e.g. inclusions enclosed in a thick aggregate of olivines), but many CAIs exhibit mineralogical and petrographical features which are similar to those from CO and CR inclusions. Therefore, it seems that the Acfer 094 CAI population is more related to inclusions known from CO and CR chondrites than to CAI populations from other carbonaceous chondrites. However, refractory inclusions occurring in Acfer 094 and other meteorites show such a broad variety and complexity that a standardized CAI classification is impossible. Based on the investigations on CAIs, the question whether Acfer 094 fits into the known classification scheme for carbonaceous chondrites or not remains unsolved. References: [1] Bischoff A. et al. (1991) Meteoritics, 26, 318-319. [2] Wlotzka F. (1991) Meteoritics, 26, 255-262. [3] Spettel B. et al. (1992) Meteoritics, 27, 290-291. [4] Newton J. et al. (1992) Meteoritics, 27, 267-268. [5] Newton J. et al. (1995) Meteoritics, 30, 47-56. [6] Bischoff A. and Geiger T. (1994) LPS XXV, 115-116.
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