Combined Microscale Investigations of Hydrogen Isotopes and Iron Valence in Dish Hill Kaersutites: Implications for the Petrogenesis of Martian Meteorites

We are investigating microscale variations in D/H ratios, water abundances and Fe valence in carefully characterized terrestrial samples with the goal of understanding the variability in hydrogen isotope compositions and redox within and among the martian meteorites. A large suite of kaersutite megacrysts was collected from the Dish Hill alkali basalt volcano in southeastern California. Of these, 17 were analyzed with the electron microprobe. These megacrysts span a range in Mg# with systematically correlated minor and trace element abundances. Bulk water contents and D/H ratios were previously determined for these 17 kaersutites by vacuum line manometry and gas source IRMS (Bell and Hoering, 1994). In this study, we selected a subset of these kaersutites (DSH 34, 40, 41, 53, 54 and 55) for co-located microscale analyses of hydrogen isotope compositions via secondary ion mass spectrometry (SIMS) at ASU and iron valence using synchrotron micro-XANES at the Advanced Photon Source at ANL.

The water (H₂O) abundances measured via SIMS in the DSH kaersutites range from 0.7 to 1.8 wt.% while the δD values vary from -168 ± 84 ‰ to +538 ± 155 ‰ and are anti-correlated. The Fe valence, defined as (Fe³⁺/ΣFe + 2), in most kaersutites analyzed here (DSH 40, 41, 54 and 55) is identical (2.43 ± 0.02; 2SD), so the variation in water content and δD in these samples may be attributed largely to subsolidus diffusion and loss of H₂O. However, the Fe valence in DSH 34 is higher (2.52 ± 0.04), while that in DSH 53 is on average still higher and shows resolvable variation (2.50 ± 0.01 to 2.58 ± 0.02). Petrologic and geochemical evidence suggests that the higher Fe valence in DSH 34 may be from wall-rock assimilation. In contrast, the Fe valence in DSH 53 is anti-correlated with H₂O content and correlated with δD, indicative of dehydrogenation of its parent magma. These results suggest that combined H₂O-δD-Fe³⁺/ΣFe systematics are likely to provide valuable insights into the petrogenesis of the martian meteorites, some of which may also have experienced similar processes (e.g., assimilation, dehydrogenation, and subsolidus diffusion) on Mars.

Bell, D. R., Hoering, T.C. (1994) D/H ratios and H₂O contents of mantle-derived amphibole megacrysts from Dish Hill, California. Conference on Deep Earth and planetary volatiles, Pasadena, CA, LPI Contribution: p.4.