Physics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.228g&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 228-229
Physics
Scientific paper
Refractory spherules in CM2 meteorites are small, <300 micrometers in diameter, inclusions composed predominantly of spinel, with accessory hibonite and perovskite (Macdougall 1981). On the basis of their chondrule-like morphology, and the inward-radiating habit of hibonite in some inclusions, it has been suggested that refractory spherules formed from liquid droplets (Macdougall 1981; MacPherson et al. 1983). Since many spherules are composed purely of spinel, Macdougall (1981) estimated that their 1-atm melting temperature might have been as high as 2135 degrees C. Melt temperatures in excess of 1550 degrees C were estimated by MacPherson et al. (1983) for the spinel-hibonite spherule BB1. Refractory spherules are a minor component of the Ca-Al rich inclusions (CAIs) found in CM2s. Of 345 CAIs located in the CM2 Cold Bokkeveld only 4 are refractory spherules (study in collaboration with M. Lee, University of Essex). Textural evidence from Cold Bokkeveld demonstrates that CAIs in CM2s are highly fragmented and must have been derived by disruption of larger objects (Greenwood et al. 1991). That this is also the case for refractory spherules is clearly demonstrated by MSP1, an anhedral, spinel-bearing inclusion (300 mmicrometer longest dimension) located in situ in Murchison (CM2). It comprises a rounded core (110 micrometers in diameter) of Fe-free spinel (V2O3 0.5wt%) surrounded by a rim of pyroxene (15-25 micrometers thick), in turn enclosed by a zone of olivine (Fo 99.7) and Mg-rich phyllosilicate. The spinel core contains 15% void space (estimated). The pyroxene rim is zoned outwards from fassaite to diopside. Blocky crystals of olivine <20 micrometers in diameter form a discontinuous rim to pyroxene and occur as isolated grains enclosed by Mg-phyllosilicate. The inclusion has an irregular outline and a sharp contact with surrounding matrix, indicating that it is a fragment of a larger, now disrupted CAI. In CV3 meteorites refractory spinel-rich spherules, similar to the Murchison example, occur within a number of different inclusion-types. Nodules, 5-300 micrometers in diameter, composed of spinel, melilite, perovskite, and pyroxene are common constituents of amoeboid olivine aggregates (Hashimoto and Grossman 1987). Melilite is also present in some Murchison spherules (MacPherson et al. 1983), and prior to aqueous alteration may have been an important constituent in many of these objects. Spherical clumps of spinel crystals, termed "framboids" by El Goresy et al. (1979) are common constituents of type B2 coarse-grained CAIs (Wark and Lovering 1982). One B2 CAI in Vigarano contains a 160-micrometer-diameter framboid with a 10-20-micrometer-thick rim of spinel enclosing a touching framework of rounded grains (5-15 micrometers in diameter). Melilite, present in the bulk inclusion, forms an outer rind to the framboid 5-10 micrometers thick and may be contiguous with crystals (angstrom k(sub)16.5) interstitial to spinel within the framboid. Individually rimmed spinel nodules, up to 300 micrometers in diameter, are also an important component of "fluffy" type A inclusions (MacPherson and Grossman 1984). The structure of the Murchison inclusion MSP1 indicates that at least some CM2 refractory spherules were components of larger inclusions. In CV3s, experimental evidence suggests that spinel spherules represent residual solid material that acquired a rounded form during partial melting (Wark and Lovering 1982). The comparison made between chondrules and spherules may therefore be misleading and results in erroneously high estimates of the temperatures experienced by these objects. El Goresy A., Nagel K., and Ramdohr P. (1979) Proc. Lunar Planet. Sci. Conf. 10th, 833-850. Greenwood R.C., Hutchison R., and Cressey G. (1991) Meteoritics (abstract) 26, 340. Hashimoto A. and Grossman L. (1987) Geochim. Cosmochim. Acta 51, 1685-1704. Macdougall J.D. (1981) Geophys. Res. Lett. 8, 966-969. MacPherson G.J. and Grossman, L. (1984) Geochim. Cosmochim. Acta 48, 29-46. MacPherson G.J., Bar-Matthews M., Tanaka T., Olsen E., and Grossman L. (1983) Geochim. Cosmochim. Acta 47, 823-839. Wark D.A. and Lovering J.F. (1982) Geochim. Cosmochim. Acta 46, 2581-2594.
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