Alteration and metamorphism of CO3 chondrites: Evidence from oxygen and carbon isotopes

Abstract

Alteration and metamorphism of CO3 chondrites:Evidence from oxygen and carbon isotopes The results of this study indicate that the oxygen and carbon isotopes in CO3 finds have been significantly disturbed by terrestrial weathering processes. Conclusions based on the isotopic composition of such weathered finds may be significantly flawed. In particular, the Antarctic meteorite ALH A77307 (3.0), suggested as being close in composition to CO-CM chondrite precursor material, has experienced significant terrestrial contamination. Oxygen isotope data for CO3 falls indicates that there is a subtle increase in ∆ 17 O values with increasing metamorphic grade for sub-types 3.1 to 3.4. This increase does not persist to higher sub-types, i.e., Warrenton (3.6). These relationships are explicable in terms of the progressive formation of phyllosilicates, coupled with loss of primary phases such as melilite, and suggest that an aqueous fluid phase was present during metamorphism. Carbon abundance and δ 13 C values of CO3 falls decrease with increasing metamorphic grade. These trends reflect progressive changes in the nature of the organic macromolecular component during metamorphic heating and lend additional support to the evidence that CO3 chondrites are part of a metamorphic series.The most likely setting for metamorphism was on the CO3 parent body. The "Ornans paradox," whereby Ornans (3.3) should belong to a higher sub-type based on chemical compared to petrographic evidence, may result from local-scale redox differences on the CO3 parent body. A wide variety of classification schemes have been proposed for CO3 chondrites. In view of its simplicity and applicability, the scheme of Scott and Jones (1990) is regarded as the most useful in assigning sub-types to new CO3 samples.

A mutli-technique search for the most primitive CO chondrites

“…The isotopic composition of the purified O gas is then analyzed using a Thermo Fisher MAT 253 dual inlet mass spectrometer (mass resolving power ~200). Analytical precision (1σ) for the system, based on replicate analyses of an internal obsidian standard (n=39) As demonstrated by Greenwood and Franchi (2004), significant shifts in the O isotope composition of Antarctic COs can occur as a consequence of terrestrial weathering. In order, to evaluate the extent of this problem, a subset of primitive COs were leached in a solution of ethanolamine thioglycollate (EATG), which has proved to be efficient at removing terrestrial weathering products from a range of meteorite types, without significantly disturbing their primary O isotope composition (Greenwood et al, 2012 This preparation minimizes the tendency for preferred alignment of platy minerals and allows us to collect random powder diffraction patterns that are representative of the bulk powder.…”

“…Position sensitive detector X-ray diffraction (PSD-XRD) has proven to be a very useful tool both for determining bulk mineral abundances and for classification Menzies et al, 2005;Dunn et al, 2010;Howard et al, 2010Howard et al, , 2011Howard et al, 2015). Lastly, bulk O isotopes are an indispensable tool for classifying meteorites and for providing constraints on the influence of fluids during parent body processing (e.g., Clayton and Mayeda, 1999;Young et al, 1999;Greenwood and Franchi, 2004;Schrader et al, 2011).…”

Fe‐Ni metal in primitive chondrites: Indicators of classification and metamorphic conditions for ordinary and CO chondrites

“…There is also no reason to assume that chondrites classified as type 3 have escaped aqueous alteration. Hydrous minerals are well known in type 3 chondrites like Semarkona (e.g., Hutchison et al 1987), and there is evidence for aqueous alteration in CO chondrites (e.g., Rubin 1998;Greenwood and Franchi 2004;Tomeoka and Itoh 2004).…”

Molecular study of insoluble organic matter in Kainsaz CO3 carbonaceous chondrite: Comparison with CI and CM IOM

“…Several studies have dealt with the determination of the petrographic type by using various proxies as mineralogical associations (Scott and Jones 1990;Chizmadia et al 2002), noble gas and presolar grain content (Newton et al 1992a;Huss et al 2002), organic matter structure, or isotope ratios (Greenwood and Franchi 2004;Bonal et al 2007 and references therein). These studies revealed that Kainsaz has a moderate to intermediate metamorphic grade (3.2 to 3.6) depending on the technique used.…”