Meteorite Ablation Rinds as Analogs for the Origin of Rims on Chondrules

Details Meteorite Ablation Rinds as Analogs for the Origin of Rims on Chondrules Meteorite Ablation Rinds as Analogs for the Origin of Rims on Chondrules

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..334b&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 334

Physics

Abalation, Chondrules, Rims, Unequilibrated Chondrites

Scientific paper

Conventional wisdom holds that UOC chondrule rims were formed in the nebula by dust accretion. Following the accretion stage, some investigators suggest that these porous rims were subjected to thermal alteration that ranged from sintering to melting [e. g., 1-3]. To understand the evolutionary history of chondrules we need to ask: (1) What nebular mechanism(s) concentrated the dust for rapid accretion? (Addressed in a companion paper at this meeting [4]). (2) What thermal event(s) welded or melted the dust? (3) Is this dust solely responsible for the rim composition, or are some rims composed, in part, of the parent chondrule? Production and/or modification of rims during atmospheric entry onto a parent body is a scenario that is testable by examination of ablation rinds produced on meteorites during entry into Earth's atmosphere. Comparison of ablation rind features with opaque rims on UOC chondrules will indicate whether this is a viable method for the production of chondrule rims. Terrestrial ablation rinds on UOCs and carbonaceous chondrites have been examined both texturally and chemically. Ablation rinds have these distinct characteristics: (1) The bulk composition of the rind is a reflection of the bulk chemistry of the host object, including Na, K, and P, but with the exception of much lower S. (2) Boundaries between unmelted bulk meteorite and rind silicates are physically sharp over distances of microns, similar to boundaries between rims and their chondrules. However, compositional transition zones extend inward from the boundaries for 10s of microns. (3) Melted meteorite matrix in the rind is compositionally similar to unmelted matrix and is texturally and chemically similar to rims. (4) Mineral texture and chemistry at chondrule/rim and meteorite/rind interfaces indicate significant thermal processing has occurred. For example, sulfides show high concentrations of included, more refractory phases at the melt interface with a corresponding loss of S. Overall, the comparison of ablation rinds with rims strongly suggests that opaque rims formed by melting of dusty accretion mantles. This melting event may have continued into the outer margins of host chondrules, or may be restricted to the accreted dust. SEM examination of the boundaries between chondrules and rims indicate that both cases probably occur. The major and minor element composition of opaque rims is similar to "accretionary" rims on objects in CM meteorites [5]. We suggest that both types of rims were formed from the same basic anhydrous dust, although CM rims acquired more O^16-bearing component than UOCs. From here, their evolutionary paths diverged: Opaque rims were thermally processed and CM rims were aqueously altered. Calculations of rim melting due to entry into a transient atmosphere of low scale height [6] indicate that encounter velocities in the range 2-4 km/sec are sufficient to melt the outer parts of chondrules. If the thermal conductivity of porous accretionary rims is as low as that of powdered chondrite [7], gas dynamic deceleration can produce totally or partially melted rims on chondrules without melting the chondrule itself. References: [1] Rubin A. and Wasson J. (1987) GCA, 51, 1923-1937. [2] Podolak et al. (1990) Icarus, 84, 254-260. [3] Bunch T. et al (1991) Meteoritics, 26, 326. [4] Cuzzi J. and Dobrovolskis A. (1993) this meeting. [5] Metzler et al. (1992) GCA, 56, 2873- 2898. [6] Podolak et al. (1993) Icarus, in press. [7] Wechsler A. E. and Glaser P. E. (1965) Icarus, 4, 335.

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