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Carbonates in the Martian meteorite Allan Hills 84001 formed at 18?±?4?°C in a near-surface aqueous environment

  1. John M. Eiler
  1. Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125
  1. Edited by Mark H. Thiemens, University of California San Diego, La Jolla, CA, and approved September 2, 2011 (received for review June 10, 2011)

  1. Fig. 1.

    Major element composition of the ALH84001 carbonates digested in the three steps, estimated from their δ18O and δ13C values and correlations between major element and isotopic composition (see text).

  2. Fig. 2.

    Comparison of Δ47, δ18O, and δ13C values (see SI Text) with models for the isotopic variation in ALH84001 carbonates. (A) Relationship between δ13C and δ18O. The solid black line and the broken black curves are a weighted total least squares linear fit to the data and a 95% confidence bound on the fit, respectively. The diagonal gray line shows the equilibrium relationship between δ13C and δ18O over the range of temperatures required to explain the range of Graphic observed in microprobe studies (approximately 5–30‰). A hypothesis of evolving temperature can be rejected on the basis of the misfit between these lines. (B) Relationship between Δ47 and δ18O. The solid black line and the broken black curves are a weighted total least squares linear fit to the data and a 95% confidence bound on the fit, respectively. The diagonal gray line shows the equilibrium relationship between Δ47 and δ18O over the range of temperatures required to explain the observed Graphic range (approximately 5–30‰). The horizontal gray line shows the isotopic evolution due to evaporation at a constant temperature. The temperature of 21.2?°C is the average temperature of the two leftmost data points. The light and dark orange wedges are model predictions for the relationship between Δ47 and δ18O that is expected from rapid degassing and associated kinetic isotope effects (38). The confidence bounds on the fit to the data are most consistent with constant temperature, less consistent with an evolving temperature (which can be ruled out by the relationship between δ13C and δ18O) and only marginally include kinetic isotope effects due to rapid degassing (see text).

  3. Fig. 3.

    Model for carbonate formation in ALH84001. (A) Physical model of a shallow subsurface aquifer. The depth of carbonate formation is constrained by a combination of the cosmic ray exposure pattern of ALH84001 and Martian meteorite ejection models (48, 49). (B) Reduction of the physical model to a geochemical model of carbonate precipitation and CO2 degassing driven by gradual evaporation of water in a confined volume, coupled to loss of the vapor phase.

  4. Fig. 4.

    Comparison of observed and modeled isotopic composition. Gray crosses are combinations of δ18O and δ13C microprobe data correlated by Mg content, including uncertainty in the correlation (SI Text). The black curves show a weighted total least squares linear fit through these data (solid) and 95% confidence bounds on the fit (broken). The black triangles are measurements from this study. The gray rectangle brackets the highest values of Graphic achievable by water–rock oxygen isotope exchange, given the uncertainty on temperature and on the measured δ18O of the ALH84001 silicates (SI Text). The light orange field shows the coevolution of δ18O and δ13C during 42% evaporation and diffusion-limited water loss from a subsurface reservoir, accompanied by carbonate precipitation and CO2 degassing and diffusion, at a temperature of 17.5?±?5?°C (SI Text). The darker orange field is for 92% evaporative water loss and accompanying carbonate precipitation if transport to the surface is not rate-limited by diffusion. The gray line with a negative slope shows the δ18O and δ13C coevolution if the water slowly freezes instead of evaporating. The initial pCO2 in the calculations was 600?millibar and the nonprecipitating solute concentrations were 0?mol?liter-1. The results are relatively insensitive to this choice (SI Text and Fig.?S6).

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