Petrography and mineral chemistry of the anhydrous component of the Tagish Lake carbonaceous chondrite

Abstract

Abstract-Most studies of Tagish Lake have considered features that were either strongly affected by or formed during the extensive hydrous alteration experienced by this meteorite. This has led to some ambiguity as to whether Tagish Lake should be classified a CI, a CM, or something else. Unlike previous workers, we have focused upon the primary, anhydrous component of Tagish Lake, recovered through freeze-thaw disaggregation and density separation and located by thin section mapping. We found many features in common with CMs that are not observed in CIs. In addition to the presence of chondrules and refractory forsterite (which distinguish Tagish Lake from the CIs), we found hibonite-bearing refractory inclusions, spinel-rich inclusions, forsterite aggregates, Cr-, Al-rich spinel, and accretionary mantles on many clasts, which clearly establishes a strong link between Tagish Lake and the CM chondrites. The compositions of isolated olivine crystals in Tagish Lake are also like those found in CMs. We conclude that the anhydrous inclusion population of Tagish Lake was, originally, very much like that of the known CM chondrites and that the inclusions in Tagish Lake are heavily altered, more so than even those in Mighei, which are more heavily altered than those in Murchison.

Re-Os isotopic systematics and platinum group element composition of the Tagish Lake carbonaceous chondrite

“…The oxygen isotopic compositions are intermediate between CI and CM chondrites (Brown et al, 2000). Simon and Grossman (2003) found that the anhydrous inclusions in Tagish Lake were most similar to those found in CM chondrites, although the abundances were lower (13%, compared with 39%-56% in CM chondrites). In contrast, Grady et al (2002) suggested that Tagish Lake was most similar to CI chondrites in terms of carbon and nitrogen isotope compositions and carbon contents, but could be intermediate between CI and CM in classification.…”

Comparing Wild 2 particles to chondrites and IDPs

“…About one-fourth of these Mn-and Cr-rich olivines contain <1% FeO. Olivines with enrichments in these elements have been reported in carbonaceous chondrites, micrometeorites, and chondritic IDPs, though they are rare (Klöck et al 1989;Rietmeijer 1998;Gounelle et al 2002;Simon and Grossman 2003;Weisberg et al 2004). The wide Mg-Fe composition range of Wild 2 olivine is similar to that for anhydrous chondritic IDPs.…”

The oxygen-isotope composition of chondrules and isolated forsterite and olivine grains from the Tagish Lake carbonaceous chondrite

“…It is composed of chondrules, isolated forsterite, isolated (more Fe-rich) olivine crystals, and rare Calcium Aluminum Inclusions (CAIs) contained in a matrix dominated by phyllosilicates, carbonates and magnetite (Zolensky et al, 2002;Simon and Grossman, 2003;Izawa et al, in press). The Tagish Lake meteorite is currently classified as 'C2-ungrouped' but its further designation continues to be debated.…”