Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..357g&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 357-358
Other
Breccia, Clasts, Fabric, Fragments, Lithic, Regolith
Scientific paper
The matrices of CM2 chondrites are a complex assemblage of high- and low-temperature components, some of which may have formed in a nebular environment, others by reprocessing in an asteroidal regolith. A necessary first step in identifying the primitive components is to understand the processes by which they were modified following incorporation into their parent bodies. Here we report the results of a textural investigation of Cold Bokkeveld. This work follows an earlier study [1] that had identified a planar fabric within Cold Bokkeveld, defined by the alignment of the long axes of various macroscopic objects. However, on sectioning the meteorite it was realized that it is composed of a more diverse range of lithic material than had been previously recognized. The nature and origin of these lithic fragments have therefore been examined in some detail. Method: To study the structure and fabric of Cold Bokkeveld a single fusion-crusted stone (maximum diameter 8cm) was cut along three directions at right angles and a series of slices removed. The stone was photographed before and after cutting to record the relationships between the slices and to document the major structural features. A polished section from each of the orthogonal cuts was prepared (total area 9 cm^2) and these were photographed using a Hitachi S2500 SEM. Montages of back- scattered electron images (x30 magnification), covering the full area of each section, were assembled. Results: Cold Bokkeveld is an inhomogeneous breccia comprising lithic fragments enclosed in a matrix of comminuted clastic material. Two end-member lithic fragment-types are present, fine- grained dark clasts and lighter-colored, coarse-grained fragments. Dark clasts are up to 1.2 cm diameter and consist predominantly of fine-grained Mg-phyllosilicate-rich material with a variable Fe-Ni sulphide content; coarser-grained, anhedral olivine grains (Fo(sub)98.1-99.5) are sometimes present. Raster- beam analysis of the four largest dark clasts examined indicates that they have a major element composition similar to dust mantles [2]. Light-colored, coarse-grained lithic fragments are up to 1.3 cm diameter, consist of abundant high-temperature objects (chondrules, etc.) enclosed by dust mantles. Features present on cut surfaces and on back-scattered montages demonstrate clearly that Cold Bokkeveld possesses a weakly- developed planar fabric defined by the alignment of the long axes of most components. Dark clasts are generally more deformed than light-colored fragments, a feature that presumably reflects the higher phyllosilicate content of dark clasts. In general the fabric within individual lithic fragments is parallel to that in the meteorite as a whole, however, in a few cases foliations are present, which show a marked discordance to that in the host. Discussion: The results of this and previous studies [2] indicate that clastic matrix in CM2 chondrites is produced within a parent body regolith by disaggregation of lithic fragments. Since it has been shown that clastic matrix in Cold Bokkeveld and Murchison is the host to interstellar silicon carbide [3] it is clearly important to identify the full range of lithic material that contributed to its formation. It remains a possibility that presolar grains may be present in one lithic component and not others. It has been proposed by [2] that clastic matrix in CM2s was formed from only a single lithic component termed by them 'primary accretionary rock' and equivalent to the light-colored lithic fragments described here. However, our evidence suggests that at least two lithic components are required to produce clastic matrix, namely i) fine-grained phyllosilicate dark clasts and ii) coarse-grained light colored fragments. References: [1] Greenwood R. C. et al. (1991) Meteoritics, 26, 340. [2] Metzler K. et al. (1992) GCA, 56, 2873-2897. [3] Alexander C. M. O'D. et al. (1990) Nature, 348, 715-717.
Greenwood Richard C.
Hutchison Robert
Jones Gregory C.
No associations
LandOfFree
The Structure and Evolution of a CM2 Regolith: A Three-dimensional Study of Cold Bokkeveld does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with The Structure and Evolution of a CM2 Regolith: A Three-dimensional Study of Cold Bokkeveld, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and The Structure and Evolution of a CM2 Regolith: A Three-dimensional Study of Cold Bokkeveld will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1072341