Carbon Isotopic Measurements of Third-Generation Salts from LEW 85320

Details Carbon Isotopic Measurements of Third-Generation Salts from LEW 85320 Carbon Isotopic Measurements of Third-Generation Salts from LEW 85320

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.341d&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 341

Other

Antarctic Meteorites, Carbon, H Chondrites, Isotopic Variation, Weathering

Scientific paper

Preterrestrial salts associated with SNC meteorites indicate that water-based chemical activity probably took place on the parent body of these samples. One way of trying to understand the nature of these weathering processes is to look at situations on Earth where analogous effects might occur. For an initial investigation, an ordinary chondrite collected from Antarctica, LEW85320 (H5), has been selected. When this sample was found it was extensively coated with carbonate deposits (subsample ,39). After their removal, a second generation of minerals grew while the sample was in storage at Houston under an atmosphere of dry nitrogen sample ,15. Subsequently, a third crop has formed (subsample ,103). Subsamples ,39 and ,15 have been identified as the magnesium carbonate nesquehonite with (<10%) hydromagnesite and barringtonite [1,2]. The carbonates in LEW85320 have been analyzed for their carbon content and stable isotopic compositions using stepped heating analysis and static mass spectrometry. The first generation carbonate (,39) was found to contain 9.7 wt% carbon, which is close to the theoretical value for pure nesquehonite (Mg (HCO3) (OH).2H2O). The carbon release profile over the temperature range 200-600 degrees C shows two well resolved peaks. The first (300- 425 degrees C) reached a maximum yield at 375 degrees C with a delta^13C = +4.4 per mil, while the second (450-550 degrees C) had a delta^13C of +5.4 per mil. A similar experiment with the second generation carbonate (,15), affords what appears to be a single release of carbon, although a small partially resolved second component may be present (400-550 degrees C). The overall release seen in subsample ,15 reaches a delta^13C = +3.6 per mil and accounts for 9.2 wt% of the sample. The third generation of carbonate (,103), which formed only after several years of storage, has an overall carbon content of 6.8 wt%. Upon stepped heating a double release of carbon is observed, similar to ,39, but with the highest delta^13C value observed being -0.8 per mil. The double carbon release seen in ,39 and ,103 may be associated with dehydration of the nesquehonite followed by decrepitation of the anhydrous salt, or it may be related to the salt's formation conditions since both ,39 and,103 formed over a period of years in fairly dry conditions, while ,15 was formed over a period of months in what could have been a more water-rich environment. Assuming the carbon in the carbonate derives from atmospheric CO2 (-7 to -8.0 per mil) and using the fractionation factor for calcite (no value is known for nesquehonite) carbonate formation temperatures can be calculated [2]. Results obtained in the current study give temperatures of 8.7 +- 4 degrees C, 24.5 +- 4 degrees C, and 57 +- 5 degrees C respectively for the three generations of carbonate. The first two temperatures, corresponding to formation in Antarctica and Houston respectively, although high, are believable while the third, also for Houston, is not. During prolonged storage in a dry nitrogen atmosphere, the CO2 source could have been something other than atmospheric and its availability could have been variable. The weathering products observed in this study are of a complex nature and may be more complicated by collection and storage conditions. Analyses on other weathered samples from different collection sites, both on Antarctica and elsewhere, together with a study of material stored in an environment controlled for the specific purpose of investigating the above problems are necessary. The understanding of different weathering regimes could ultimately lead to better interpretation of the martian climate, past and present, through the study of SNC meteorites. References: [1] Gooding (1992) Icarus, 99, 28-41 [2] Grady et al. (1989) Meteoritics, 24, 1-7 [3] Velbel et al. (1990) GCA, 55, 67- 76.

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