Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27s.290s&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 290
Mathematics
Logic
1
Scientific paper
Terrestrial weathering is an important process that can significantly alter the elemental and isotopic character of meteorites (e.g., SOCKI et al., 1991). JULL et al. (1988) demonstrated that alteration and subsequent formation of the hydrated Mg-carbonates, nesquehonite and hydromagnesite, can occur in geologically short time frames (<40 A). KARLSSON et al. (1991) showed that a large portion of the carbonate material in seven Antarctic meteorites either underwent extensive isotopic exchange with atmospheric CO2 or formed recently in the Antarctic environment. To further constrain the effects of terrestrial weathering on Antarctic meteorites, we have characterized isotopic exchange equilibria for the hydrated Mg-carbonate nesquehonite {Mg(HCO3 x OH) x 2H2O}. To this end, mineralogically pure nesquehonite was grown from fixed-temperature solutions under controlled isotopic composition to monitor the partitioning of hydrogen and oxygen isotopes during the growth of the solid phase. These results are used to interpret the isotopic composition of water extracted from nesquehonite occurring on the surface of LEW85320. Following the procedures of MING and FRANKLIN (1985), mineralogically pure nesquehonite was synthesized at 10 degrees and 25 degrees C from solutions of constant hydrogen and oxygen isotopic composition. After filtration, the mineralogy of oven-dried precipitate was confirmed by X-ray diffraction (XRD) to be pure nesquehonite. Differential scanning calorimetry (DSC) heating curves from the synthetic product match those of the salt scraped from LEW85320. This salt was identified by XRD as nesquehonite by GOODING et al. (1988). Approximately 250 microliters of water was extracted from synthetic nesquehonite by cryogenic trapping during heating of the solid to 625 degrees C in vacuum. The delta^18O of extracted water was determined following the procedure of SOCKI et al. (1992). A second water extraction was condensed into a pyrex tube containing zinc. The tube was heated to 450 degrees C for one half hour, and the resultant H2 was analyzed for D/H. Solution water was also analyzed for hydrogen and oxygen isotopes. Hydrogen isotope data for water extracted from synthetically grown nesquehonite (-29o/oo SMOW) is virtually identical to the isotopic composition of the precipitating solution at both 10 degrees and 25 degrees C, suggesting that structural water can be used as a proxy for the ambient solution in which the hydrated carbonate formed. This observation has important ramifications for the study of meteorites and associated weathering products. For example, debate exists as to the processes responsible for the isotopic modification of solution water that formed nesquehonite on LEW85320 (GOODING et al., 1988). GRADY et al. (1989) suggest that nesquehonite formed from a water enriched in ^18O (-17o/oo) compared with the ambient environment (~-30o/oo). This ^18O-enriched water cannot be generated in the Antarctic environment without significant exchange of oxygen with silicates in underlying bedrock, or by extensive evaporation of ambient waters. D/H isotopes, extracted from the hydration water of alteration products, can be used to distinguish between these two processes as they relate to the origin of weathering products on Antarctic meteorites. References. Gooding, J.L., Jull, A.J.T, Cheng, S., and Velbel, M.A. (1988) Lunar Planet. Sci. (abst.) 19, 397-398; Grady, M.M., Gibson, E.K., Jr., Wright, I.P. and Pillinger, C.T. (1989) Meteoritics 24, 1-7; Jull, A.J.T., Cheng, S., Gooding, J.L. and Velbel, M.A. (1988) Science 242, 417-419; Karlsson, H.R., Jull, A.J.T., Socki, R.A. and Gibson, E.K., Jr. (1991) Lunar Planet. Sci. (abst.) 22, 689-690; Ming, D.W. and Franklin, W.T. (1985) Soil Sci. Soc. Am. J. 49, 1303-1308; Socki, R.A, Gibson, E.K., Jr., Jull, A.J.T. and Karlsson, H.R. (1991) Meteoritics 26, 396- 397; Socki, R.A., Karlsson, H.R. and Gibson, E.K., Jr. (1992) Analytical Chem. 64, 829-831.
Allton Judith H.
Gibson Everett K. Jr.
Romanek Christopher S.
Socki Richard A.
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