Watson 002--The First CK/Type 3 Chondrite

Details Watson 002--The First CK/Type 3 Chondrite Watson 002--The First CK/Type 3 Chondrite

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.352g&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 352

Mathematics

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Carbonaceous Chondrites, Ck Chondrites, Classification, Minerals, Oxygen Isotopes, Petrography

Scientific paper

The CK chondrites studied so far are all of petrologic types 4-6. In 1991 Watson 002, a petrologic type 3 carbonaceous chondrite, was found in the Nullarbor Region, South Australia and was classified as "CK3 anomalous" [1]. The supplement "anomalous" was added because it shows some features that do not fit into the mineralogical [2] or into the bulk chemical characteristics [3] of the CK chondrite group. Here we report on the petrography and mineralogy as well as on the bulk chemistry and on the oxygen isotopic composition of Watson 002. Petrograpy and mineralogy: The piece we studied has a 3-5 mm wide margin that shows a less compacted texture, which seems to be caused by terrestrial weathering. Watson 002 has a chondritic texture characterized by abundant chondrules, inclusions, and fragments embedded in a matrix. Matrix is the most comprehensive textural unit making up 70 vol% of the meteorite. Chondrules, inclusions, and fragments (grain size >100 micrometers) are present in modal portions of about 10 vol% each determined by point counting methods. The mean apparent diameter of 43 detected chondrules is 870 +- 380 micrometers ranging from 160 to 2100 micrometers. The mineral chemistry of olivines, pyroxenes, and plagioclases is comparable to other CK chondrites. Olivines and pyroxenes in some chondrules and fragments are not equilibrated. Almost pure forsterites (FaO.3) and enstatites (Fs1.6) occur in the core. Olivines at the edge contain ~20 mol% Fa. Like in other CK chondrites chondrules are frequently rimmed by magnetite. Matrix olivines have a mean Fa content of 34.4 mol% (range 31-38 mol%), and contain an average of 0.35 wt% NiO. Pyroxenes are less equilibrated than olivines; the composition of the clinopyroxenes vary from 6.7 to 19.9 mol% Fs. Two low-Ca pyroxenes with Fs 23.1 and 24.4 were found. Plagioclases occur with An contents from 26.8 to 50.1 mol%. Unlike the other known CK chondrites, Watson 002 contains abundant CAIs that consist of An-rich plagioclase, olivine, and green pleonast-spinel as the main phases. Similar spinel phases were reported from Maralinga 001 [4], and Karoonda [5]. The pleonasts are containing small amounts of Cr and Ti (0.25 wt% Cr203 and TiO2, respectively). Like in other CK chondrites the most abundant opaque phase is magnetite containing about 3.1 wt% Cr203 and 1 wt% TiO2, but we did not find the exsolution of ilmenite and spinel [6]. Sulfides are rare--only a few micron-sized pentlandite grains were found. Bulk chemistry and oxygen/isotopes: Watson 002 shows an unusual enrichment of Na, K, Ba, and light REE. Different INA-analyses of the core and the weathered margin show enrichment of these elements in the margin up to 50 x CI, whereas the core shows a "normal" CK pattern. Therefore we conclude that the high abundance of these elements in the weathered margin is due to terrestrial weathering. Besides that, there is also a strong enrichment of Ca (4.7 x CI) and Ti (~9 x CI) in the core, which can be explained in the case of Ca by the high abundance of CAls compared to other CK chondrites. Ti is predominantly located in magnetite and spinels. The oxygen isotopic composition differs from that of other CK chondrites. Previously analyzed CK chondrites plot within the range of the CO chondrites [7] but Watson plots at the uppermost end of the CV chondrite range and the lowermost end of the CM range. The oxygen isotopic ratios are delta^18O = +5.12 and delta^17O = -0.55. The composition of a single chondrite was found to be delta^18O = +0.56,delta^17O = -3.31, which lies within the range of CV chondrules, and is unequilibrated with the bulk meteorite. References: [1] Wlotzka F. (1993) Met. Bull., 74, Meteoritics 28, 1. [2] Geiger T. and Bischoff A. (1991) Meteoritics, 26, 4, 337. [3] Kallemeyn et al. (1991) GCA, 55, 881-892. [4] Keller L. P. et al. (1991) Meteoritics, 27, 1, 87-91. [5] MacPherson G. J. and Delaney J. S. (1985) LPSC XVI, 515-516. [6] Geiger T. and Bischoff A. (1990) LPSC XXI, 409-410. [7] Clayton R. N. and Mayeda T. K. (1989) LPSC XX, 169-170.

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