Terrestrial alteration of carbonate in a suite of Antarctic CM chondrites: Evidence from oxygen and carbon isotopes

Temperatures of aqueous alteration and evidence for methane generation on the parent bodies of the CM chondrites

“…Most of the available measurements of δ 13 C (Grady et al, 1988) and ∆ 17 O (Clayton and Mayeda, 1984;Benedix et al, 2003) in carbonates from the CM chondrites were made on separate sample aliquots and thus do not clearly test this prediction (although the ranges of their variations are broadly consistent with our predicted trend; Figure 1-3). The only exception is a recent study on paired Antarctic CM chondrites (Tyra et al, 2007), which observed an inverse correlations between δ 13 C and ∆ 17 O, as predicted. However, this study also observed that both δ 13 C and ∆ 17 O correlate with 14 C content in the carbonates, and thus the correlation between δ 13 C and ∆ 17 O could be caused or modified by terrestrial weathering (i.e., mixing or replacement of primary carbonate with lower δ 13 C, higher ∆ 17 O and 14 C terrestrial carbonates) (Tyra et al, 2007).…”

“…The only exception is a recent study on paired Antarctic CM chondrites (Tyra et al, 2007), which observed an inverse correlations between δ 13 C and ∆ 17 O, as predicted. However, this study also observed that both δ 13 C and ∆ 17 O correlate with 14 C content in the carbonates, and thus the correlation between δ 13 C and ∆ 17 O could be caused or modified by terrestrial weathering (i.e., mixing or replacement of primary carbonate with lower δ 13 C, higher ∆ 17 O and 14 C terrestrial carbonates) (Tyra et al, 2007).…”

An oxygen isotope dichotomy in CM2 chondritic carbonates—A SIMS approach

“…Guo and Eiler (2007) used d 13 C, d 18 O, d 17 O and D 17 O of bulk CM2 carbonate to determine temperatures of carbonate formation, to constrain Eh and pH during aqueous alteration conditions, and to infer that methane production was likely. Tyra et al (2007) examined petrography and measured O and C isotopic compositions of bulk carbonate from paired Antarctic CM chondrites EET (Elephant Moraine) 96006, 6; EET 96016, 9; EET 96017, 6; and EET 96019, 6 and suggested the presence of a terrestrially-precipitated carbonate component in these meteorites. They identified three main calcium-carbonate types: (1) small (<50 lm) inclusion-free blocky grains interspersed throughout the matrix, (2) large rounded aggregates (50-250 lm) that contain some mineral inclusions, and (3) vein-infill carbonate.…”

“…They identified three main calcium-carbonate types: (1) small (<50 lm) inclusion-free blocky grains interspersed throughout the matrix, (2) large rounded aggregates (50-250 lm) that contain some mineral inclusions, and (3) vein-infill carbonate. Tyra et al (2007) referred to type 1 grains as "clean" and type 2 grains as "mottled." Figure 1 shows these three carbonate types; see Tyra et al (2007) for more detailed descriptions.…”

Micro-spectroscopic characterization of organic and hydrous components in weathered Antarctic micrometeorites

“…The weathering effects on organic materials during residence in Antarctic ice (Tyra et al, 2007) are not yet well understood. Sephton et al (2004) suggested that Antarctic weathering may have similar effects as the parent body aqueous alteration by removing ether bonds from organic material and alkyl side chains from its constituent units (Sephton et al, 2004).…”