Hydrothermally altered impact melt rock and breccia: Contributions to the soil of Mars

Samples of melt rock and breccia from 12 terrestrial impact craters were examined to identify alteration minerals and their conditions of formation. The dominant assemblage in most cases is clay‐silica‐K feldspar‐zeolite, indicating hydrothermal alteration at low pressures and temperatures of 100°–300°C. The clays are predominantly Fe‐chlorites and smectites, in most cases depleted in Al and enriched in Fe and Mg relative to their source rocks. Alteration of impact glass is often complete, while alteration of crystalling melt rock is limited to a few percent of the rock volume. Impact breccia is altered to only a small extent compared to the alteration of glass. Impact‐induced hydrothermal alteration is to be expected at Martian impact sites if significant quantities of ground ice and/or water are present. The alteration products are likely to resemble the clays found at terrestrial craters. Al/Fe fractionation similar to that observed on Earth is strongly suggested by the Viking soil analyses. The quantitative contribution of hydrothermal clays to the soil of Mars is estimated, based on a model which predicts the generation of the equivalent of a 61‐m‐thick blanket of impact melt planet‐wide. The inefficiency of melt production in small craters and the inefficiency of alteration in large craters limit the expected production of clay to a few percent of this figure. Clays produced by impact‐induced hydrothermal alteration may be expected to contribute to the fines on Mars, but this contribution is probably not dominant relative to other suggested sources.