Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.299t&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 299
Other
2
Scientific paper
A new meteorite find from West Texas, U.S.A., is a CV3 carbonaceous chondrite. The provisional name of Red Bluff has been proposed to the Nomenclature Committee by R. Farrell. Red Bluff consists of chondrules (56.4%), CAIs (8.3%), amoboid olivine aggregates (0.6%), mineral fragments (0.8%), and Ca-Al chondrules (0.4%) in a fine-grained, clay-rich matrix (33.1%). Chondrules in Red Bluff are spherical to irregular in shape, and from 0.25-3.5 mm diameter in thin section; the average diameter is 0.95 mm, with standard deviation of 0.6 mm (69 chondrules). Three of the chondrules are distinctly larger than the rest; without these three, the average diameter is 0.86 mm (s.d. 0.4 mm). The chondrules are almost all of type I (Fe-poor), as shown by cathodoluminescence and chemical analyses; most are also rich in opaques. Compositions of chondrule olivines average Fa1.9+-1.2 (s.d.); compositions of chondrule pyroxenes average Fs3.4+-3.3 (s.d.). Chondrule varieties include porphyritic olivine, microporphyritic olivine, granular olivine, macroporphyritic olivine, barred olivine, and rare extracentroradial pyroxene (0.25 mm diam.) [1,2]. Two calcium-aluminum chondrules were observed. The largest, 1.5 mm diam., contains spinel, plagioclase, and fassaite, and includes a circular spinel palisade [3]. The other Ca-Al chondrule is within a coarse-grained CAI, and could also be a well-developed spinel palisade [3]. Fine- and coarse-grained CAIs are present but have been studied little; most appear to be type B (melilite+pyroxene+plagioclase). Red Bluff's matrix is composed of fine-grained clay, with minor olivine, "limonite," troilite, and Fe metal. Alignment of grains and oxide-rich streaks in the matrix mark a planar fabric that wraps around chondrules and inclusions. Chondrules are commonly surrounded by shells of dark red alteration, darker than the bulk of matrix material. Red Bluff is weathered. It is stained red by oxidized iron minerals, which are most common as veinlets (after Fe metal or troilite?) and as rinds around Fe metal and troilite. The matrix clays may have formed during weathering of an olivine matrix similar to that of Allende. However, Fe metal and troilite remain common, both occluded within silicates and as discrete grains in the matrix. Cracks in Red Bluff are partially filled by minute, euhedral carbonate crystals. Red Bluff's exterior is coated with desert varnish (light-brown anisotropic film composed of many fine laminae); the varnish fills void space in one carbonate-rich veinlet. Classification Red Bluff's petrography, mineralogy, and mineral chemistry fall within the range of known CV3s [4]. Most characteristic are the proportion of matrix (33%) and the size of chondrules (avg ~1 mm), which are consistent with CV and inconsistent with known CI, CO, CM, CR, and CK chondrites. The proportions of CAIs, Ca-Al chondrules, and mineral fragments are also consistent with known CVs. The presence of type 1 and type 2 chondrules, and the variations in olivine compositions among type 1 chondrules implies that Red Bluff is of metamorphic grade 3. Compared to other CV3s, Red Bluff has rather little matrix and rather magnesian silicate minerals [4]. Red Bluff does not appear to be paired with known CV3s, and in fact none are known from West Texas [5]; the closest CV3 fall or find is Leoville (KS) [5]. This work was conducted under an NRC Fellowships to Treiman (senior) and DeHart (regular). We are grateful to R. Farrell for the meteorite sample, data on its source, and its proposed name. [1] Scott E. R. D. and Taylor G. J. (1983) Proc. 14 LPSC, B275. [2] Jones R. (1992) GCA 56, 467. [3] Wark D. A. and Lovering J. F. (1982) GCA 46, 2595. [4] McSween H. Y. Jr. (1977) GCA 71, 1777. [5] Graham et al. (1985) Catalog of Meteorites.
Dehart John M.
Treiman Allan H.
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