Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.479a&link_type=abstract
Meteoritics, vol. 30, no. 5, page 479
Other
Chondrites, Ordinary, Chondrules, Condensation, Matrix, Rims
Scientific paper
It has long been thought that chondrule rims and interchondrule matrix are amongst the most primitive materials in chondrites. Indeed, they are known to contain presolar grains [1]. However, most of the components in rims and matrix are Solar System in origin and may include nebular condensates [2], chondrule condensates [3] and chondrule fragments [4]. Discerning the relative importance of these possible sources has proved problematical. Both rims and matrix do contain chondrule fragments and the concentration of chondrule glass in the matrix could explain the general Al-enrichment of matrix in many UOCs [4], but in other meteorites, such as the CO3 ALHA 77307 [5], chondrule fragments are only a minor constituent. TEM observations show that rims and matrix do not contain significant amounts of equilibrium condensates. In the UOCs and CO3s, the rims and matrix appear to be composed of amorphous material, mineral fragments (mostly chondrule minerals) and secondary minerals that grew in the solid state, probably during metamorphism [2,4,5]. These and other observations prompted Brearley et al. [2] to suggest that rims and matrix formed from amorphous nebular condensates rather than crystalline condensates or chondrule glass. More recently it has been suggested that rims are composed, at least partially, of material that was volatilized during chondrule formation which then recondensed onto chondrules during cooling. Rims, but not matrix, in UOCs show correlated enrichments in FeO, Si, Mn and other moderately volatile elements compared to refractory elements like Al or Ti [3]. The abundances of Fe, Si etc. in rims range from matrix-like to highly enriched. The lack of enrichments in matrix suggests that, if volatilization occurred during chondrule formation, recondensation was confined mainly to chondrule rims. The fine-grained matrix, with its large surface area, was presumably not present during recondensation. Although, since matrix-like compositions form one endmember of the rim trend, matrix-like material must have been present during chondrule formation and was, perhaps, efficiently accreted by the chondrules. Two possible explanations for the Al enrichment of matrix are that it is rich either in refractory amorphous condensates or chondrule glass. If volatile losses of even only a few percent were common for chondrules during their formation, there is a third possible explanation. To a first approximation, the fractional mass loss rate of a chondritic particle during evaporation is inversely proportional to its radius. Thus, if the dust (1-10 micrometers) in the chondrule forming region experienced similar conditions to chondrules (100-1000 micrometers), evaporative losses from dust would have been severe. As a result, the dust would have been enriched in refractory elements like Al, perhaps explaining some of the fine-grained corundum and hibonite found in the CAI-poor OCs. If some of this dust was fractionated from the gas and chondrules before much recondensation took place and later mixed in with unprocessed material, the Al enrichment of matrix could be understood. Recondensation and subsequent redistribution in the parent body would explain alkali abundances in matrix. Secondary redistribution may have also produced the fractionation of many refractory elements from Al in UOC and CO3 rims and matrix [4]. The ordinary and enstatite chondrites have bulk compositions that indicate they lost a fraction of their refractory elements. This has always been explained as being due to the loss of refractory nebular condensates. However, evaporative residues of chondritic material can have very similar compositions to condensates. Consequently, if volatilization during chondrule formation was an important process and some of the finer grained material was lost prior to complete recondensation it may be possible to explain rim, matrix and bulk meteorite refractory and common lithophile compositions References: [1] Huss G. R. (1990) Nature, 347, 159-162. [2] Brearley A. J. et al. (1989) GCA, 53, 2081-2094. [3] Alexander C. M. O'D. (1995) GCA, in press. [4] Alexander C. M. O. et al. (1989) EPSL, 95, 187-207 [5] Brearley A. J. (1993) GCA, 57, 1521-1550.
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