Assessment of shock effects on amphibole water contents and hydrogen isotope compositions: 1. Amphibolite experiments

Kaersutitic amphiboles found within a subset of the Martian meteorites have low water contents and variably heavy hydrogen isotope compositions. In order to assess if impact shock-induced devolatilization and hydrogen isotope fractionation were determining factors in these water and isotopic characteristics of the Martian kaersutites, we conducted impact shock experiments on samples of Gore Mountain amphibolite in the Ames Vertical Gun Range (AVGR). A parallel shock experiment conducted on an anorthosite sample indicated that contamination of shocked samples by the AVGR hydrogen propellant was unlikely. Petrographic study of the experimental amphibolite shock products indicates that only ∼ 10% of the shock products experienced levels of damage equivalent to those found in the most highly shocked kaersutite-bearing Martian meteorites (30–35 GPa). Ion microprobe studies of highly shocked hornblende from the amphibolite exhibited elevated water contents (ΔH2O ∼ 0.1 wt.%) and enriched hydrogen isotope compositions (ΔD ∼ + 10‰) relative to unshocked hornblende. Water and hydrogen isotope analyses of tens of milligrams of unshocked, moderately shocked, and highly shocked hornblende samples by vacuum extraction/uranium reduction and isotope ratio mass spectrometry (IRMS), respectively, are largely consistent with analyses of single grains from the ion microprobe. The mechanisms thought to have produced the excess water in most of the shocked hornblendes are shock-induced reduction of hornblende Fe and/or irreversible adsorption of hydrogen. Addition of the isotopically enriched Martian atmosphere to the Martian meteorite kaersutites via these mechanisms could explain their enriched and variable isotopic compositions. Alternatively, regrouping the water extraction and IRMS analyses on the basis of isotopic composition reveals a small, but consistent, degree of impact-induced devolatilization (∼ 0.1 wt.% H2O) and H isotope enrichment (ΔD ∼ + 10‰). Extrapolating the shock signature of the regrouped data to grains that experienced Martian meteorite-like shock pressures suggests that shock-induced water losses and hydrogen isotope enrichments could approach 1 wt.% H2O and ΔD = + 100‰, respectively. If these values are valid, then impact shock effects could explain a substantial fraction of the low water contents and variable hydrogen isotope compositions of the Martian meteorite kaersutites.