Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29s.447b&link_type=abstract
Meteoritics, vol. 29, no. 4, page 447
Other
Carbonaceous Chondrites, Clasts, El-Chondrites, Fractionation, Kaidun, Metal, Mineralogy, Siderophile Elements, Trace Elements
Scientific paper
Kaidun is a complex breccia containing clasts of at least four different lithologies [1,2]. We have analyzed magnetic fractions (MKD4-6) of three different grain sizes from most reduced lithology of Kaidun, the Kaidun IV sample (EL3). Fractions MKD4 (fine-grained), MKD5, and MKD6 (coarse-grained) contain both isolated metal grains and metal with varying amounts of attached silicate-rich matrix. Metal grains are present as pure metal and as multiphase mineral assemblages containing variable amounts of troilite, schreibersite, graphite, enstatite, and silica. Some rounded metal nodules contain enstatite crystals of euhedral shape. The compact coarse-grained silicate-rich matrix consists mainly of enstatite. Averaged compositions of low-Ni metal (most abundant, present in all fractions) and high-Ni metal (not detected in MKD4) are (7.2-7.7 wt% Ni, 0.32 Co, 0.34-0.38 Si, 0.15-0.18 P), and (12.2-12.6 Ni, 0.2 Co, 0.38 Si, 0.09 P), respectively. According to the analyses obtained so far, there seems to be a continuous range in metal composition. However, kamacite (6-9 wt% Ni) clusters around a mean composition with a roughly chondritic Ni/Co ratio whereas high-Ni metal (10-16 wt% Ni) exhibits an anti-correlation between Ni and Co. The composition of schreibersite enclosed by metal is highly variable for different grains (7.5-18.0 wt% Ni, 0.10-0.28 Co) but is almost constant within grains. Troilite compositions are relatively uniform (on average 2.18 wt% Cr, .37 Mn, 0.37 Ti). Alabandite, Mn0.68Fe0.24Mg0.08S, occurs only outside metal in association with troilite and schreibersite. Randomly selected enstatites from MKD4 are highly variable in their minor element contents. The ranges for FeO, MnO, CaO, and Al2O3 are 0.24-0.88 wt%, <0.02-1.44, 0.10-0.50, and 0.08-0.77, respectively. Samples MKD4-6 and MKD3(non-magnetic fraction) have been analyzed by INAA. Siderophile element abundances of the magnetic fractions normalized to CI are shown in the Fig. 1. In general, compositions of most phases are in good agreement with those reported in [1] except for Si in kamacite (0.63 wt% in [1]) and Cr in troilite (4.4 wt% in [1]). Metal seems to contain more Ni then common in other E-chondrites. However, the Eagle EL6-chondrite contains metal with up to 11 wt% Ni [4]. Alabandite plots close to the EL-field in the ternary MgS-MnS-FeS diagram as given by [5]. The overall abundance pattern of siderophile elements (Fig. 1) exhibits two distinctive groupings. The refractory siderophile elements show parallel fractionation trends (W>Re>Os>Ir) for the different magnetic fractions with MKD6 having the lowest abundances. Molybdenum, Co, Fe, and the moderately volatile elements (Au to Sb) covary with Ni. Iron is depleted with respect to Ni and Co, whereas Au is slightly enriched, a feature typical of E-chondrites (e.g., [6,7]). The pattern in the Figure is very similar to patterns of the E chondrites Abee and Hvittis [8], confirming the close relationship of Kaidun IV to E-chondrites. References: [1] Ivanov A. V. et al. (1986) Meteoritika, 45, 3-19. [2] Ivanov A. V. (1989) Geochem. Int. 26, 84-91. [3] Palme H. et al. (1981) In Landoldt-Boerstein (K. Scheifers and H. H. Voigt, eds.), 2, 257-273. [4] Olsen E. J. et al. (1988) Meteoritics, 23, 379-380. [5] Zhang Y. et al. (1993) Meteoritics, 28, 468-469. [6] Hertogen J. et al. (1983) GCA 47, 2241-2255. [7] Kallemeyn G. W. and Wasson J. T. (1986) GCA, 50, 2153-2164. [8] Rambaldi E. and Cendales M. (1980) EPSL, 48, 325-334. Fig. 1., which appears here in the hard copy, shows Orgueil-normalized siderophile element abundances in magnetic separates from Kaidun IV.
Brandstatter Frans
Ivanov Alexei V.
Kurat Gero
Palme Herbert
Spettel Bernhard
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