Geochemical and mineralogical studies of meteorite impact-generated hydrothermal systems on Earth and tracing sources of water: observations from the Rochechouart impact structure, France

Simpson, Sarah Lauren (2015) Geochemical and mineralogical studies of meteorite impact-generated hydrothermal systems on Earth and tracing sources of water: observations from the Rochechouart impact structure, France. MSc(R) thesis, University of Glasgow.

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Research within the last decade has revealed that meteorite impacts on our planet have the potential to create transient hydrothermal environments that are conducive to the development of extremophile microbial communities. Impact products also record the hydrologic and environmental history of a terrestrial body’s surface, making them attractive targets in the search for water and exolife on other planets and satellites. The interpretation of geologic and surface processes observed on other terrestrial bodies is based on ground-truths we recognize on Earth. Due to the recycling of crust and an active surface environment, the direct study of hydrothermal activity in craters on our planet is often obscured by pre-impact fluid episodes and non-preservation at ambient surface conditions. As a result, detailed studies focusing on water-rock interactions during the post-impact cooling period are lacking. Many mysteries still surround the development and sustainability of impact-hydrothermal activity, such as the role of target composition and paleofluid reservoirs.
This study investigates post-impact hydrothermal activity within the complex, 23km diameter, marginal marine, late Triassic Rochechouart impact structure located in west-central France. Previous studies have noted a pervasive K-metasomatic overprint attributed to post-impact hydrothermal circulation; however, Rochechouart is highly weathered, and the target consists primarily of Variscan quartzofeldspathic igneous and amphibolite facies metamorphic rocks which show evidence for aqueous alteration during pre-impact events. This thesis aims to overcome the complications arising from multiple fluid event overprinting in Rochechouart by documenting secondary alteration throughout all impactites using detailed petrography, scanning electron microscopy, energy dispersive X-ray and Raman analysis combined with carbon, oxygen and sulphur stable isotope techniques to place temperature and fluid constraints on secondary carbonate and sulphide minerals within lithic impactites.
Results from this study reveal that the intensity of alkali metasomatism directly correlates with host rock melt content; relatively impermeable melt rocks and melt-bearing breccias show evidence for pervasive K-metasomatism, with late-stage carbonate-sulphide mineralization concentrated in lithic breccias and autochthonous impact fractures. No pre-impact hydrothermal carbonates were detected isotopically. Isotopic data also supports the hypothesis that seawater, and possibly meteoric water, mixed with metamorphic fluids, were the contributing fluid reservoirs fuelling hydrothermal circulation within the structure; this conclusion is further supported by the strongly biological signatures of secondary sulphide minerals and the structure’s marginal marine paleogeography.
The sequence of hydrothermal alteration within Rochechouart follows the following scheme; early stage alkali-saturated waters sourced from the quartzofeldspathic, impermeable melt rocks were responsible for the pervasive K-metasomatic overprint in all lithologies; intermediate stage alteration was dominated by chlorite-smectite assemblages with continued K-metasomatism, which decreases in intensity in melt-poor rocks; and relatively cool carbonate-sulphide assemblages dominate lithic impactites and basement fractures where hydrothermal fluid flow is concentrated well into the late stages of cooling. Results also reveal that the porous network of parautochthonous breccias and autochthonous impact fractures within the sub-crater environment of complex craters with a majority-crystalline target may host hydrothermal fluid circulation for relatively long periods of time. These sites may also provide an environment conducive to the development of thermophilic microbial communities.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Keywords: Impact crater, impact-hydrothermal, Rochechouart, stable isotopes, mineralogy, geochemistry
Subjects: Q Science > Q Science (General)
Q Science > QE Geology
Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences > Earth Sciences
Supervisor's Name: Lee, Professor Martin R.
Date of Award: 2015
Depositing User: Ms Sarah L Simpson
Unique ID: glathesis:2015-6949
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 07 Dec 2015 11:13
Last Modified: 05 Jan 2016 09:22

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