Fine-grained Chondrule Rims in the Murchison CM2 Chondrite: Compositional and Mineralogical Systematics

Details Fine-grained Chondrule Rims in the Murchison CM2 Chondrite: Compositional and Mineralogical Systematics Fine-grained Chondrule Rims in the Murchison CM2 Chondrite: Compositional and Mineralogical Systematics

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..328b&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 328

Mathematics

Logic

2

Aqueous Alteration, Cm2 Chondrites, Rims

Scientific paper

The CM2 carbonaceous chondrites contain numerous chondrules, silicate grains, CAIs, etc, which are mantled by rims of fine- grained material [1]. These rims often consist of two or more layers that can be distinguished on chemical and textural criteria. The formation mechanisms of rims, and the timing and location (i.e., nebular vs planetary) of aqueous alteration of rim materials is the subject of some controversy. We are examining, in detail, the relationships between the mineralogy and bulk composition of individual rims in the Murchison CM2 chondrite in order to place constraints on their mechanisms of formation and alteration histories. We have carried out detailed SEM, electron microprobe and TEM studies of a number of rim sequences in Murchison. Our data provide further confirmation of some of the observations of [1]. For example, we have found that on any given chondrule, inner rims are almost invariably more Mg-rich than outer rims. However, when the entire population of rims is considered it is evident that the compositional field for inner rims overlaps that for outer rims in terms of Mg/Fe ratio. Na and K are also consistently enriched in inner rims, but all the other analyzed elements show variable behavior. Elemental ratio diagrams for rims show some variation in their shape, but in most cases are relatively flat. The elements that consistently show exceptions are Ca and S are frequently depleted relative to CI values in both inner and outer rims. We have also examined the interelement variations in inner and outer rims. One of the surprising results of this study is that some elements may be correlated in inner rims, but not in outer rims and vice versa. Fe and S show a strong positive correlation in outer rims, but have no correlation in inner rims. The reverse is true of Na and S. Our TEM studies of the fine-grained mineralogy of 5 rims have, so far, revealed consistent relationships between rim composition and mineralogy. All the inner rims studied consist dominantly of microcrystalline Mg-rich serpentine, rare platy cronstedtite crystals and poorly crystalline pentlandite and pyrrhotite. The sulfides are disseminated throughout the regions of microcrystalline serpentine. Tochilinite has not been found. The compositions of serpentine and cronstedtite in the different rims studied are very similar and define distinct, tightly clustered compositional groups on Si-Fe-Mg ternary diagrams. Outer rims are mineralogically distinct. For intermediate Mg/Fe ratios, outer rims are dominated by relatively coarse-grained platy cronstedtite, a minor amorphous component and sulfides, whereas the most Fe-rich outer rims contain tochilinite and minor cronstedtite. Our present data indicate clear relationships between bulk rim composition and mineralogy, which would appear to support a parent body location for aqueous alteration, rather than nebular. In addition, the evidence that some elements show variable correlations in inner and outer rims, indicates that there must be mineralogical controls on the major and minor element chemistry of rims. This may reflect variations in the mineralogy of the precursor components of rims, or the mineralogical constraints imposed on elemental mobility between rims and chondrules during alteration. The depletions in Ca and S in rims may be a reflection of the high mobility of these elements during alteration, as is certainly the case in CI chondrites. Finally the textural characteristics of inner rim materials appear to be inconsistent with alteration of a crystalline precursor, because there is no evidence of pseudomorphic replacement of phases. Many of the textures are similar to those produced during low temperature alteration of basaltic glass [2]. The possibility that the precursor was an amorphous material, perhaps of the type observed in ALH A77307 [3] and several of the least equilibrated ordinary chondrites [4], should be considered. Funded by NASA grant NAGW-3347 to J. J. Papike (P.I.). References: [1] Metzler K. et al. (1992) GCA, 65, 2873-2897. [2] Tazaki K. et al. (1989) Clays Clay Minerals, 37, 348-354. [3] Brearley A. J. (1993) GCA, 57, 1521-1550. [4] Alexander C. M. et al. (1989) EPSL, 95, 187-207.

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