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MAPPING the K, Th, U DISTRIBUTION at the ROCHECHOUART IMPACT STRUCTURE: IN- SIGHT INTO IMPACT-RELATED and POST-IMPACT PROCESSES

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MAPPING the K, Th, U DISTRIBUTION at the ROCHECHOUART IMPACT STRUCTURE: IN- SIGHT INTO IMPACT-RELATED and POST-IMPACT PROCESSES Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5019.pdf MAPPING THE K, Th, U DISTRIBUTION AT THE ROCHECHOUART IMPACT STRUCTURE: IN- SIGHT INTO IMPACT-RELATED AND POST-IMPACT PROCESSES. D. Baratoux1, C.A.B. Niang1,2,3, J. Lofi4, P. Rochette5, W.U. Reimold6, P. Lambert7, 1Géosciences Environnement Toulouse, University of Toulouse, CNRS & IRD, 14, Avenue Edouard Belin, 31400, Toulouse, France. 2Département de Géologie, Université Cheikh Anta Diop, Dakar, Senegal, [email protected], 3Institut Fondamental d’Afrique Noire Cheikh Anta Diop, Dakar, Senegal. 4 Geosciences Montpellier, Université de Montpellier Campus Triolet cc060Place Eugène Bataillon 34095 Montpellier, France. 5Centre Européen de Recherche et d’Enseignement des Géosciences et de l’Environnement, Aix-Marseille Université, CNRS, IRD, CEREGE UM34, Aix en Provence, France. 6Institute of Geosciences, Laboratory of Geodynamics, Geochronology and Environmental Science, University of Brasília, Bra- sília, Brazil.7Center for International Research & Restitution on Impacts and on Rochechouart, 87600 Rochechouart, France. Introduction: Radiometric data can be processed ducted in parallel with remote sensing and field studies into distribution maps for K – Th - U concentrations of of K, Th, U signatures at various impact structures in surface materials. These three elements are incompati- Africa, and Australia. ble - K is mobile under aqueous alteration, whereas the Methodology: We aim at mapping the distribution mobilities of Th and U are comparatively much lower of K, Th, and U concentrations on surface and at depth and depend on pH and degree of oxidation, respective- at various scales (from the size of mineral grains to the ly. Therefore, impact processes (shock metamorphism, scale of the impact structure). For this purpose, we melting and vaporization of target rocks), post-impact plan to combine different techniques, including a) hydrothermal, and surface processes may all be re- ground-measurements using a portable radiospectrom- sponsible for redistribution of these elements within eter (RS-BGO-230), b) airborne radiometric data ac- the impact-affected crust.The radiometric signature of quisition (planned in 2020 by the Comissariat à terrestrial impact craters has not been well studied. l’Energie Atomique), c) gamma-ray logging using the Only two cases were formally documented in peer- different drill holes of the Rochechouart drilling cam- reviewed publications, Lake Bosumtwi in Ghana [1,2] paign, and d) autoradiography of thin sections (to in- and Serra da Cangalha in Bresil [3], wheareas anoma- vestigate the distribution of K, Th, U at thin sec- lies appear to be also common for a number of Austral- tion/mineral scale). We report here preliminary results ian meteoritic impact structures [4]. The importance of based on ground-based measurements, evaluation of the respective processes potentially responsible for vertical core profiles, and geostatistical analysis of such anomalies is not understood. Moreover, the redis- these data. tribution of heat-producing elements in the planetary Surface mapping of K, Th, U concentrations at crust in relation to impact processes has fundamental Rochechouart: Ground-based measurements of K, implications for planetary evolution, especially during Th, and U concentrations were achieved during a field the early phases of planetary evolution characterized campaign from 25 September to 15 November 2017. A by intense impact bombardment as well as much more total of 520 ground-based measurements were intense heat production from the decay of 40K. It is achieved during this period at outcrops representing possible that radiometric anomaly patterns could also the diversity of lithologies affected or generated by the be useful in the search for new impact structures or impact (granitoids, gneiss, impact breccias, and cata- may together with other geophysical data, orovide clasites). The measurements reveal a wide range of constraints of the size of known impact structures. concentrations (K from 2 to more than 10 wt%, Th In this context, we would like to use the from 5 to 40 ppm, and U from 2 to 16 ppm), which Rochechouart impact structure and the opportunity represents a favorable context to investigate potential offered by the results of the CIRIR drilling campaign redistribution of K, Th and U by impact and/or post- to develop a better understanding of the mechanisms of impact processes. Granitoids have distinct and elevated redistribution of K, Th and U in relation to impact Th/K and Th/U ratios, whereas the Th/K ratio is lower (shock metamorphism, deformation) and post-impact to similar in gneiss and impact breccias. These meas- processes (hydrothermal alteration and surface pro- urements will be completed in August 2019 to estimate cesses This knowledge may be used to develop strate- the frequency distribution of K, Th and U concentra- gies for the exploration of new impact structures as tions and of the K/Th and U/Th ratios for each of the well as characterization of known impact structures lithologies exposed on surface. based on the analysis of K, Th and U maps derived Vertical distribution of K, Th and U concentra- from airborne surveys on the Earth, and from orbiting tions at Rochechouart: Vertical K, Th, and U con- satellites equiped with gamma-ray sensors on other centration profiles were acquired for 6 drill sites. One rocky bodies. This research at Rochechouart is con- measurement is available for every 5 cm of core. For Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5019.pdf each of these profiles (e.g., Fig.1), we analyzed the (Fig. 3), with K-rich material exclusively found near- one-to-one variation of the elements as a function of surface. The distributions of Th and U are generally depth (e.g. Fig. 2), the frequency distribution for each log-normal (gaussian shape in log-scale) rather than element along linear and log scales (e.g., Fig. 3), and normal (gaussian shape in linear scale) - as generally we also evaluated the 1-D variogram. observed fo igneous rocks [5]. Fig. 2 – Frequency distribution and geostastitical parameters for the concentrations of K, Th and U along the SC2 drill core. Upper part: frequency distri- butions for K, Th and U concentrations estimated at a linear scale. Bottom: frequency distributions of K, Th and U concentrations estimated at log-scale. Fig. 1 – Extract of the vertical profiles for the K, Th Conclusion: The wide range of concentrations of and U concentrations for the SC2 drill core from K (0 to 10 wt%), Th (0 to ~100 pm), and U (0 to more Chassenon. Thin lines correspond to original data, than 100 ppm) provides a favorable context to examine whereas the thicker line represents the variation after the potential re-distribution of these elements associat- smoothing with a 1 m wide Savitzky-Golay filter [6]. ed with impact-related or post-impact processes. Fur- The data are relatively noisy, and therefore the ther work will focus on the comparison of K, Th, U original analyses were smoothed over 1 meter intervals profiles with images and lithological description of the (thick lines in Fig. 1). drill cores to further elucidate the processes controlling the 3D distribution of these elements at Rochechouart. Acknowledgments: The authors the National Re- serve and the local communities (Porte Océane du Limousin with support of the State and EU) for fund- ing the drillings and for supporting the CIRIR facilities and means on site. References: [1] Boamah, D., Koeberl, C. 2002. Geochemistry of soils from the Bosumtwi impact structure, Ghana, and relationship to radiometric air- borne geophysical data. In Impact in Precambrian Shields. Springer, Berlin-Heidelberg. Eds J. Plado, L. J. Pesonen, pp. 211–255. [2] Baratoux, D. et al. (2019) Meteorit Planet Sci. doi:10.1111/maps.13253. [3] Wulf et al (2019) Earth Planet. Sci. Lett. 506, 209-220 doi: Fig. 2 – K versus Th concentrations as a function of /10.1016/j.epsl.2018.11.009. [3] Vasconcelos, M.A.R. depth for the SC2 drilling site (Chassenon). et al., 2012, Geophys. Res. Lett., 39, L04306, Discussion: Comparison of concentrations between doi:10.1029/2011GL050525. [4] Hawke, P.J. 2004, surface measurements and vertical profiles for the SC2 Ph.D. Dissertation, The University of Western Austral- drilling indicates that surface material is relatively ia. [5] M. Fall et al. (2018) J. Afr. Earth Sci, 148, 30- enriched in K, and the range of variation for Th and U 51, doi: 10.1016/j.jafrearsci.2018.03.026. [6] concentrations is wider at depth than on surface (Fig. A. Savitzky and M. J. E. Golay (1964) Analytical o 2). The vertical profile also reveals an enhanced de- Chemistry, 8, n 36, 1627–1639, crease in K concentrationswith depth below ~ 15 to 20 doi :10.1021/ac60214a047. m. This is reflected by a bi-modal distribution of K .
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