TEMPERATURES OF AQUEOUS ALTERATION ON CARBONACEOUS CHONDRITE PARENT BODIES
2. S AMPLES AND M ETHOD 1 Samples
3.3 Isotopic compositions of the alteration fluid
preferred approach (i.e., voltage normalization at 5V). The difference is the biggest for calcite in Tagish Lake: its formation temperature estimated without any voltage correction is 26˚C lower than what estimated from our preferred approach (39˚C v.s.
65˚C). For all the other samples, the differences in estimated formation temperatures from different approaches are less than 16˚C without any systematic trends. These differences in temperature estimations are within or close to the external errors typical of our analyses (±10˚C, 1σ).
Figure 2-4 Oxygen isotope compositions (all values reported vs. VSMOW) of water in equilibrium with carbonaceous chondrite carbonates at their known growth temperatures (Guo and Eiler 2007 and this study). Also shown for comparison are the oxygen isotope compositions of the water in equilibrium with magnetites in CM and CI chondrites and in Tagish Lake (TL) (squares, estimated from δ18OVSMOW of magnetites reported in Rowe et al. 1994 and Engrand et al. 2001b, with an equilibrium fractionation factor of -13‰ between magnetite and water). The terrestrial mass fractionation line ('TMF') and carbonaceous chondrite anhydrous mineral line ('CCAM') are shown for reference.
We estimated δ18OVSMOW values of the fluids from which these carbonates grew by combining the carbonate formation temperatures established by clumped isotope thermometry with the temperature-dependent carbonate-water oxygen isotope fractionations established by previous studies⎯ either experimental (for calcite, Kim and O’Neil 1997; and dolomite, Vasconcelos et al. 2005) or theoretical (for magnesite,
Rosenbaum 1997 and Schauble et al. 2006). These data and experiments constrain the water from which carbonates grew to have δ18OSMOW values of: -12.3‰ to -7.1‰ for calcite in GRO 95577; -4.0‰ for dolomite and -29.7‰ for breunnerite in Orgueil; and 11.8‰ for calcite, 5.8‰ for dolomite and -18.6‰ for breunnerite in Tagish Lake (±2‰, 1σ; Table 2-1, Fig. 2-4). Assuming that the carbonates we analyzed have ∆17O values equal to the averages previously determined by Leshin et al. (2001) and Clayton and Mayeda (1999), our results indicate that the δ17Ovalues of waters from which carbonates grew varied between –14.9‰ and 7.6‰.
Again, our estimated formation temperatures for breunnerite in Orgueil and Tagish Lake (-20˚C and -31˚C, respectively) is beyond the temperature range of theoretically estimated magnesite-water oxygen isotope fractionation relationship (valid ≥0˚C;
Rosenbaum 1997 and Schauble et al., 2006), forcing us to extrapolate that trend to arrive at an estimated δ18O value for water. This extrapolation is justified by the general invariance of temperature-dependence of oxygen isotope fractionations over this temperature range. In addition, our suggestion that breunnerite precipitated from low temperature brines containing divalent cations such as Ca2+ and Mg2+ (section 3.2), raises the possibility of an oxygen isotope ‘salt effect’ (Horita, 2005) during carbonate precipitation (that is, a dependence of the carbonate-water oxygen isotope fractionation on the salinity of the solution). Consideration of these isotope salt effects would tend to increase our estimated oxygen isotope compositions of alteration fluids (Horita, 2005).
The exact magnitude of the effect depends on the cation compositions of the brine (especially concentrations of Ca2+ and Mg2+), and could be as much as several per mil (Horita, 2005). For example, for brines with the same cation composition as the eutectic
point of NaCl-MgCl2-H2O system (1.10mol NaCl and 2.48mol MgCl2 per kg H2O;
Spencer et al., 1990), an oxygen isotope salt effect of -2.7‰ is expected at 25˚C (Horita, 2005).
The matrices of carbonaceous chondrites are also rich in a variety of secondary phases (i.e., aqueous alteration products) other than carbonates, such as magnetite and phyllosilicates (Brearley and Jones, 1998). The oxygen isotope compositions of these phases must also reflect the conditions of aqueous alterations and the δ18O values of alteration fluid, though they may have grown at temperatures different from those of carbonate precipitation, and these phases are less likely than carbonates to fully equilibrate with co-existing fluid. Nevertheless, we can examine the plausibility of the fluid δ18O values we established through carbonate clumped isotope measurements by estimating the oxygen isotope compositions of alteration fluids from which magnetite formed. We adopt previously measured oxygen isotope compositions of magnetites in CI and CM chondrites (Rowe et al., 1994) and in Tagish Lake (Engrand et al., 2001b), and assume an oxygen isotope fractionation of -13‰ between magnetite and water (Rowe et al., 1994). We didn’t include in our discussion the oxygen isotope compositions of meteorite matrix water (extracted through stepped pyrolysis of bulk meteorite samples), due to the large uncertainties associated with the nature of the extracted water and thus with the quantitative interpretations of their isotopic compositions (Baker et al., 2002).
Over all, these estimated δ18O and δ17O for the alteration fluids within each carbonaceous chondrite group are all significantly lower than those estimated for their respective primary water (Clayton and Mayeda, 1999), and decrease in the order of magnetite > calcite > dolomite > breunnerite in CI and CM chondrites and in the order of
calcite > dolomite ~magnetite > breunnerite in Tagish Lake (Fig. 4). The differences in isotopic composition between the model reactant water and that water from which secondary mineral phases grew are consistent with the expectation that water evolves toward lower δ18O, δ17O, and Δ17O as a result of reaction with host rock (Clayton and Mayeda, 1999; Guo and Eiler, 2007).
3.4 Implications for the chemical and thermal evolution of the carbonaceous