Abstract
The highly eroded 23 km diameter Rochechouart impact structure, France, has extensive evidence for post-impact hydrothermal alteration and sulphide mineralisation. The sulphides can be divided into four types on the basis of their mineralogy and host rock. They range from pyrites and chalcopyrite in the underlying coherent crystalline basement to pyrites hosted in the impactites. Sulphur isotopic results show that δ34S values vary over a wide range, from −35.8‰ to +0.4‰. The highest values, δ34S −3.7‰ to +0.4‰, are recorded in the coherent basement, and likely represent a primary terrestrial sulphur reservoir. Sulphides with the lowest values, δ34S −35.8‰ to −5.2‰, are hosted within locally brecciated and displaced parautochthonous and autochthonous impactites. Intermediate δ34S values of −10.7‰ to −1.2‰ are recorded in the semi-continuous monomict lithic breccia unit, differing between carbonate-hosted sulphides and intraclastic and clastic matrix-hosted sulphides. Such variable isotope values are consistent with a biological origin, via bacterial sulphate reduction, for sulphides in the parautochthonous and autochthonous units; these minerals formed in the shallow subsurface and are probably related to the post impact hydrothermal system. The source of the sulphate is likely to have been seawater, penecontemporaneous to the impact, as inferred from the marginal marine paleogeography of the structure. In other eroded impact craters that show evidence for impact-induced hydrothermal circulation, indirect evidence for life may be sought isotopically within late-stage (≤120 °C) secondary sulphides and within the shocked and brecciated basement immediately beneath the transient crater floor.
Highlights
Detailed descriptions follow (Figs. 2–6), but they can broadly be described : Type 1 sulphides are pyrites within the semi-continuous monomict lithic breccia, and are further subdivided based on geologic context into Type 1A, 1B and 1C; Type 2 sulphides, pyrite and marcasite, are fracture-coating and vein forming massive sulphides and occur within the locally brecciated and fractured parautochthonous and autochthonous basement; Type 3 sulphides, pyrite, are those found within coherent blocks of crystalline basement gneiss showing no macro- or microscopic shock features, and are further subdivided into Type 3A, 3B and 3C (Section 3.1.3); Type 4 sulphides, pyrite and chalcopyrite, are those found within the mineralised basement fractures of unknown origin, and are subdivided into Type 4A and 4B (Section 3.1.4)
Type 1 sulphides The Champagnac-type monomict breccias are one of several discontinuous parautochthonous impactite bodies that occur throughout the Rochechouart structure (Fig. 1)
The distribution of hydrothermal sulphides and their corresponding levels of sulphur isotope fractionation in impactites from the Champagnac quarry directly correlate with host rock porosity and permeability
Summary
Meteorite impacts on rocky planetary bodies have the potential to initiate transient hydrothermal systems if: (i) the target contains sufficient volatiles (e.g. liquid water or ice), and (ii) a substantial heat source is generated (e.g. melt sheet, raised geothermal gradient) (Abramov and Kring, 2004; Naumov, 2005; Osinski et al, 2013; Koeberl, 2014) These impact-hydrothermal environments can host bacterial life, associated craters are promising targets in the search for evidence of water and exolife on other planets and satellites in our Solar System (Cockell and Bland, 2005; Naumov, 2005; Parnell et al, 2010b, 2012; Osinski et al, 2013; Sapers et al, 2014a). Impact fractures are likely to be optimum localities to search for evidence of life because convective heat transfer and hydrothermal fluid flow is focused within the fracture system as a result of the generally low permeability (
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
