Published in Meteoritics, 29, p. 547.

We report the yield and D/H of water released by stepped heating of bulk Shergotty, Zagami, Chassigny, and the newest martian meteorite, ALH 84001 [1]. For comparison, we also report data from Nakhla using the same procedure since the heating steps in this study are slightly different than our previously reported nakhlite analyses [2]. With this work, we have completed a survey of D/H in bulk samples representative of all SNC mineralogical types. Assuming these meteorites are martian samples [3,4], the D/H of the water contained in SNC hydrous phases might be a useful tracer of water that once resided in the martian atmosphere (currently delta-D_SMOW ~ 4000 [5]) or has isotopically exchanged with it.

A heating procedure similar to Karlsson et al.’s [6] was used to facilitate direct comparison to their O data in which high values of Delta¹⁷O relative to bulk SNCs were observed in water released from some samples at high temperatures. Samples were heated under vacuum (pyrolysis) to temperatures of 150°, 350°, 600°, and 1000°C for 1-1.5 hr per temperature step. Our gas collection and analysis procedure has been previously described [2].

Hydrogen isotopic compositions of water released at each temperature step and total water yields for each sample are reported in Table 1. Yields generally agree with those of Karlsson et al. [6] except in the case of Shergotty, which was found to contain approximately one-third less water. Similar to earlier results for the nakhlites [2], isotopic compositions are terrestrial at low temperatures and delta-D values increase with increasing temperature for all samples except Chassigny. This pattern is consistent with mixing between low-temperature-adsorbed or -exchanged terrestrial water and indigenous water released at higher temperatures. Chassigny delta-D values are generally indistinguishable from terrestrial, becoming only slightly D-enriched (delta-D = 26) in the highest temperature step. This result is unexpected given that Chassigny reportedly has similarly anomalous Delta¹⁷O values to the nakhlites [6]. The results are also surprising since we have previously measured high delta-D values (~800 to 1850) of amphiboles in Chassigny by ion microprobe [7,8]. Water released from these amphiboles may be responsible for the slight increase in delta-D at high temperature.

The delta-D of ALH 84001 water is similar to Nakhla, and is consistent with a martian origin for ALH 84001 [1]. ALH 84001 also contains macroscopic carbonates [1]. We have measured the delta¹³C_PBD of the CO₂ released between 350° and 600°C to be 40.9. The yield was 0.13 wt%, representing 86% of the total CO₂ released in the entire experiment. This is the heaviest C ever reported in a martian meteorite sample.

The presence of D-enriched water in Shergotty is consistent with the results of Kerridge [9] who measured delta-D values up to 900 in Shergotty. However, the delta-D of 2061 reported here for the Shergotty 600°-1000°C temperature step is the highest ever measured in a whole-rock SNC. Both this study and our ion microprobe measurements [8] suggest that the shergottites may contain more D-enriched water than other SNCs. Although preliminary until further measurements can be made, this distinction may be reflecting the difference in geologic history or age [10] of Shergotty and Zagami relative to the possibly older and more deeply emplaced nakhlites, Chassigny and ALH 84001.

References:  [1] Mittlefehldt D. W. (1994) Meteoritics, 29, 214-221. [2] Watson L. L. et al. (1994) LPS XXV, 1471-1472. [3] McSween H. Y. and Stolper E. M. (1980) Sci. Am., 242, 54-63. [4] Bogard D. D. and Johnson P. (1983) Science, 221, 651-654. [5] Bjoraker G. L. et al. (1989) in Proc. 4th Int. Conf. Mars, Tucson, 69-70. [6] Karlsson H. R. et al. (1992) Science, 255, 1409-1411. [7] Watson L. L. et al. (1993) LPS XXIV, 1493-1494. [8] Watson L. L. et al., Science, submitted. [9] Kerridge J. F. (1988) LPS XIX, 599-600. [10] Jones J. H. (1986) GCA, 50, 969-977.