Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.549m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 549
Other
Chondrites, Ordinary, Clasts, Fractional Crystallization, Meteorites, Bovedy, Farmington, Hedjaz, Parnallee, Rare Earth Elements
Scientific paper
Silica-rich igneous-textured clasts are found in OC [1-6]. The SiO2-bearing clasts found in the Parnallee (LL3.6) and Farmington (L5) chondrites are isotopically unique [4-6]. They plot on a mixing line defined between UOC chondrules and an 16(sub)O-depleted end member in the oxygen three isotope diagram. We analyzed trace elements including REE by MSID technique for SiO2-bearing clasts (CB1, CB4, CB7, and CB8) from Parnallee. Some major and minor elements of CB8 were determined by AA or ICP-AES. The CI-chondrite normalized REE patterns of the clasts are shown in Fig. 1. CB8 has a high Si/Mg ratio (2.5), although its bulk Mg/(Mg + Fe) and Fe/Mn ratios (0.79 and 51, respectively) are within the range of chondritic values. Refractory elements Ca and Al are highly fractionated in CB8; the clast is enriched in Ca (3 x CI) but depleted in Al (0.7 x CI). CB1, CB4, and CB7 also show Ca enrichment (3-7 x CI). CB8 is depleted in moderately volatile lithophiles Mn, Na, K, and Rb (0.18-0.71 x CI), siderophile elements Fe, Co, and Ni (0.0041-0.39 x CI), and chalcophile element Zn (0.076 x CI). The SiO2-bearing clasts analyzed exhibit a gradual depletion from LREE to HREE (CI-normalized La/Lu ratios vary from 1.6 to 18) and a large positive Eu anomaly (Eu/Eu*=2.4-14) along with a depletion of La. This LREE/HREE fractionation is inversely correlated with SiO2 contents of the clasts. Abundance of Sr is parallel to that of Eu in CB8. However, Sr is depleted compared with Eu in the other clasts. These abundance patterns are quite different from those of typical ferromagnesian chondrules in UOCs, SiO2-bearing pyroxene-rich clast in Hedjaz (L3.7) [3], and silica-rich orthopyroxenite Bo-1 in Bovedy (L3) [7]. Absence of metal and sulfide, low abundances of siderophile and chalcophile elements in the clasts imply that metal and sulfide were removed from precursor material before or during clast formation. General REE patterns of SiO2-bearing clasts from Parnallee suggest that they were produced by igneous fractionation controlled by the plagioclase component. Extensive olivine extraction followed plagioclase crystallization from the residua may be able to explain the REE pattern of CB8 [5, 6]. However, simple fractional crystallization or extraction of partial melting liquid from a chondritic source could explain neither large fractionation of La/Lu nor enrichment of Eu relative to Sr observed in CB1, CB4, and CB7. Detailed fractionation processes occurred on a parent body still remain unsolved. References: [1] Olsen E. J. et al. (1981) EPSL, 56, 82-88. [2] Brigham C. A. et al. (1986) GCA, 50, 1655-1666. [3] Nakamura N. et al. (1990) EPSL, 99, 290-302. [4] Bridges J. C. et al. (1993) Meteoritics, 28, 329-330. [5] Bridges J. C. et al. (1994) Meteoritics, 29, 448-449. [6] Bridges J. C. et al. (1995) Meteoritics, submitted. [7] Ruzicka A. et al. (1995) Meteoritics, 30, 57-70.
Bridges John C.
Hutchison Robert
Kanazawa Mitsunori
Misawa Kazuhiko
Nakamura Nobuhiro
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