Impact-generated hydrothermal circulation and metasomatism of the rochechouart astrobleme: mineralogy and major and trace element distribution

The energy released during a hypervelocity impact on Earth can generate high temperatures in the target rock. There are currently 170 known impact structures worldwide, of which over one-third contain fossil hydrothermal systems [1]. Results from the analysis of these hydrothermal systems have many implications for the study of the origin of life on Earth and potential thereof on Mars. Hypervelocity impacts are also of particular economic interest as they may produce, expose or concentrate high commodity resources such as hydrocarbons, precious metals and ore minerals

Impact fracturing and aqueous alteration of the CM carbonaceous chondrites

Aqueous alteration of the CM carbonaceous chondrites has produced a suite of secondary minerals, and differences between meteorites in their abundance defines a progressive alteration sequence [e.g. 1, 2]. The means by which this water gained access to the original anhydrous constituents of the meteorites is the subject of considerable debate. Studies of rock texture, mineralogy and bulk chemical composition have concluded that solutions were generated by the melting of water ice in situ, and remained essentially static as a consequence very low intergranular permeabilities [e.g. 3, 4]. By contrast, results of oxygen isotope work and modelling have suggested that the fluids moved considerable distances within the parent body [5, 6]. Given the intergranular permeability of the CMs, an extensive fracture network would be required to support such flow. Clues to how the two very different models for aqueous alteration of the CMs can be reconciled have been recently provided by Rubin [7]. He recognised a good correlation between the magnitude of impact-induced compaction of CM meteorites and their degree of aqueous processing, with the more highly deformed meteorites being more altered. Here we have asked whether compaction was accompanied by the development of fracture networks that could have provided the conduits for aqueous solutions that mediated all or some of the alteration

Evidence for an impact-induced biosphere from the δ34S signature of sulphides in the Rochechouart impact structure, France

The highly eroded 23 km diameter Rochechouart impact structure, France, has extensive evidence for post-impact hydrothermal alteration and sulphide mineralization. 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

Evidence for Localized High Temperature Hydrothermal Fluid Flow within the Sub-Crater Environment of the Rochechouart Impact Structure: Observations from a Polymict Breccia Dike

Hypervelocity impacts into volatilebearing terrestrial targets can initiate hydrothermal circulation for a finite period of time; evidence for this is preserved in approximately one-third of impact structures on Earth [1, 2]. Hydrothermal environments can host extremophile life, and microbial communities have been found to colonize impact craters [3, 4]. The majority of impact structures on Earth have yet to be studied in great detail; many aspects of the post-impact environment such as the extent and duration hydrothermal circulation with respect to location within the structure as well as crater diameter, target composition and external influences, (paleogeography) are not fully understood. <p></p>We present evidence for high temperature hydrothermal fluid circulation within the sub-crater environment of the highly eroded, 23km diameter, Mesozoic Rochechouart impact structure located in west-central France [5]. This evidence is a new impact lithology that was found during a recent field campaign at a collection site located approximately 7.5km north-east of the structure's center. It is a highly porous, polymict lithic impact breccia dike containing carbonate mineralization found below the transient crater floor. Secondary hydrothermal mineral assemblages are diagnostic of a range of temperatures (>100°C to low temperature diagenetic).<p></p&gt

Impact-generated hydrothermal circulation and metasomatism of the rochechouart astrobleme: mineralogy and major and trace element distribution

The energy released during a hypervelocity impact on Earth can generate high temperatures in the target rock. There are currently 170 known impact structures worldwide, of which over one-third contain fossil hydrothermal systems [1]. Results from the analysis of these hydrothermal systems have many implications for the study of the origin of life on Earth and potential thereof on Mars. Hypervelocity impacts are also of particular economic interest as they may produce, expose or concentrate high commodity resources such as hydrocarbons, precious metals and ore minerals

Impact fracturing and aqueous alteration of the CM carbonaceous chondrites

Aqueous alteration of the CM carbonaceous chondrites has produced a suite of secondary minerals, and differences between meteorites in their abundance defines a progressive alteration sequence [e.g. 1, 2]. The means by which this water gained access to the original anhydrous constituents of the meteorites is the subject of considerable debate. Studies of rock texture, mineralogy and bulk chemical composition have concluded that solutions were generated by the melting of water ice in situ, and remained essentially static as a consequence very low intergranular permeabilities [e.g. 3, 4]. By contrast, results of oxygen isotope work and modelling have suggested that the fluids moved considerable distances within the parent body [5, 6]. Given the intergranular permeability of the CMs, an extensive fracture network would be required to support such flow. Clues to how the two very different models for aqueous alteration of the CMs can be reconciled have been recently provided by Rubin [7]. He recognised a good correlation between the magnitude of impact-induced compaction of CM meteorites and their degree of aqueous processing, with the more highly deformed meteorites being more altered. Here we have asked whether compaction was accompanied by the development of fracture networks that could have provided the conduits for aqueous solutions that mediated all or some of the alteration

The recent and rapid spread of Themeda triandra

Tropical savannas cover over 20% of land surface. They sustain a high diversity of mammalian herbivores and promote frequent fires, both of which are dependent on the underlying grass composition. These habitats are typically dominated by relatively few taxa, and the evolutionary origins of the dominant grass species are largely unknown. Here, we trace the origins of the genus Themeda, which contains a number of widespread grass species dominating tropical savannas. Complete chloroplast genomes were assembled for seven samples and supplemented with chloroplast and nuclear ITS markers for 71 samples representing 18 of the 27 Themeda species. Phylogenetic analysis supports a South Asian origin for both the genus and the widespread dominant T. triandra. This species emerged ~1.5 Ma from a group that had lived in the savannas of Asia for several million years. It migrated to Australia ~1.3 Ma and to mainland Africa ~0.5 Ma, where it rapidly spread in pre-existing savannas and displaced other species. Themeda quadrivalvis, the second most widespread Themeda species, is nested within T. triandra based on whole chloroplast genomes, and may represent a recent evolution of an annual growth form that is otherwise almost indistinguishable from T. triandra. The recent spread and modern-day dominance of T. triandra highlight the dynamism of tropical grassy biomes over millennial time-scales that has not been appreciated, with dramatic shifts in species dominance in recent evolutionary times. The ensuing species replacements likely had profound effects on fire and herbivore regimes across tropical savannas

Rochechouart 2017-Drilling Campaign: First Results

No abstract available

The JCMT Legacy Survey of the Gould Belt: a first look at Orion B with HARP

‘The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical Society.The Gould Belt Legacy Survey will survey nearby star-forming regions (within 500 pc), using Heterodyne Array Receiver Programme (HARP), Submillimetre Common-User Bolometer Array 2 and Polarimeter 2 on the James Clerk Maxwell Telescope. This paper describes the initial data obtained using HARP to observe 12CO, 13CO and C18O J= 3 → 2 towards two regions in Orion B, NGC 2024 and NGC 2071. We describe the physical characteristics of the two clouds, calculating temperatures and opacities utilizing all the three isotopologues. We find good agreement between temperatures calculated from CO and from dust emission in the dense, energetic regions. We determine the mass and energetics of the clouds, and of the high-velocity material seen in 12CO emission, and compare the relative energetics of the high- and low-velocity material in the two clouds. We present a clumpfind analysis of the 13CO condensations. The slope of the condensation mass functions, at the high-mass ends, is similar to the slope of the initial mass function.Peer reviewe

Stable isotope studies of post impact hydrothermal deposits within the sub-crater environment of the Rochechouart structure

No abstract available