Concentrations of siderophile elements in nonmagnetic fractions of Antarctic H‐ and L‐chondrites: A quantitative approach on weathering effects

Abstract

Abstract-The nonmagnetic separates of Antarctic H-and L-chondrite finds fiom the Lewis Cliff stranding area are enriched in Fe and siderophile elements such as Ni, Co, Cu, As, Ir, and Au, relative to those of nonAntarctic falls. The siderophile enrichments are caused by the oxidation of metallic Fe-Ni to nonmagnetic hydrous Fe-Ni oxides. The concentrations of Fe, Ni, and Co in each sample were used to calculate the amount of oxidized metal, which serves as a quantitative measure of the degree of weathering. The amount of oxidized metal is generally between 1-10 wt?? and shows a rough correlation with the qualitative A-B-Cweathering classification for increasing rustiness. No clear correlation was found between weathering and terrestrial age, although meteorites older than 0.2 Ma contain on the average -50% more oxidized metal. Also, no correlation was found between the degree of weathering and the natural thermoluminescence (TL) level, which is dominated by the thermal history in space rather than by the thermal history in or on the Antarctic ice. The only significant parameter for the degree of weathering seems to be the location of find, with the most weathered specimens being found in a part of the stranding area where melt-water ponds are known to occur. This observation confirms that the occasional presence of liquid water plays an important role in the Antarctic weathering process and explains the poor correlation of the degree of weathering of Antarctic meteorites with terrestrial age and surface exposure age.

Noble gases and cosmogenic radionuclides in the Gold Basin L4 chondrite shower: Thermal history, exposure history, and pre‐atmospheric size

“…The measured concentrations of Fe (16.4-21.4 wt%) and Ni (0.17-0.80 wt%) in the stone fraction (Table 2) are significantly higher than the average literature values of 16.0 ± 0.6 wt% Fe (Jarosewich 1990) and 0.05 ± 0.02 wt% Ni for L chondrite falls (Rambaldi, Cendales, and Thacker 1978). Based on equations given in Welten (1999), we calculate that the stone fraction of the Gold Basin samples contains between 1.4 wt% (UA-1217) and 12.3 wt% (UA-682) of oxidized metal (Table 1), which is here defined as a mixture of hydrated Fe and Ni oxides. These values cannot directly be compared with the Mössbauer results of Kring et al (2001), but confirm their observation that there is considerable variation in the degree of oxidation from sample to sample.…”

Exposure history and terrestrial ages of ordinary chondrites from the Dar al Gani region, Libya

“…Therefore, we normalized the 41 Ca concentrations in the same way as the 36 Cl concentrations, assuming a production rate ratio of 41 Ca from Ca relative to that from Fe, P( 41 Ca)Ca/P( 41 Ca)Fe = 6 (Vogt et al 1991). In addition, we corrected for the dilution with oxidized metal, which is up to 15% for H chondrites but <10% and <5% for L and LL chondrites, respectively (Welten 1999).…”

Comparative stable isotope geochemistry of Ni, Cu, Zn, and Fe in chondrites and iron meteorites

“…Ordinary chondrites contain between 1% and 12% metal (Zanda et al, 2006) with a concentration of $10% Ni. The fairly low Ni contents of silicates (<500 ppm, (Rambaldi et al, 1978;Welten, 1999)) mean that more than 95% of the Ni present in chondrites reside in the metal phase. The isotope composition of the metal-hosted Ni therefore can be considered representative of the chondritic material as a whole, assuming that the Ni isotopic fractionation between silicate and metal is no more than a few permil as observed for other transition elements (Luck et al, 2005;Poitrasson et al, 2005).…”