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The Origin of the Circular K Anomalies at the Bosumtwi Impact Structure

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The Origin of the Circular K Anomalies at the Bosumtwi Impact Structure Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5008.pdf THE ORIGIN OF THE CIRCULAR K ANOMALIES AT THE BOSUMTWI IMPACT STRUCTURE. C.A.B. Niang1,2,3, D. Baratoux3, D.P. Diallo1, R. Braucher4, P. Rochette4, C. Koeberl5, M.W. Jessell6, W.U. Reimold7, D. Boamah8, G. Faye9, M.S. Sapah10, O. Vanderhaeghe3, S. Bouley11. 1Département de Géologie, Université Cheikh Anta Diop, Dakar, Senegal, [email protected], 2Institut Fon- damental d’Afrique Noire Cheikh Anta Diop, Dakar, Senegal. 3Géosciences Environnement Toulouse, University of Toulouse, CNRS & IRD, 14, Avenue Edouard Belin, 31400, Toulouse, France. 4Centre Européen de Recherche et d’Enseignement des Géosciences et de l’Environnement, Aix-Marseille Université, CNRS, IRD, CEREGE UM34, Aix en Provence, France, 5Department of Lithospheric Research, University of Vienna, 1090 Vienna, Austria, and Natural History Museum 1010 Vienna, Austria. 6Centre for Exploration Targeting, The University of Western Aus- tralia, 35 Stirling Highway, Crawley, WA 6009, Australia. 7Institute of Geosciences, Laboratory of Geodynamics, Geochronology and Environmental Science, University of Brasília, Brasília, Brazil. 8Geological Survey Department, Accra, Ghana. 9Institut des Sciences de la Terre, Université Cheikh Anta Diop de Dakar, Sénégal.10Department of Earth Science, University of Ghana, Accra, Ghana. 11GEOPS - Géosciences Paris Sud, Univ. Paris-Sud, CNRS, Université Paris-Saclay, Rue du Belvédère, Bât. 504-509, 91405 Orsay, France Introduction: Radiometric data are commonly to weathering, erosional and transport processes. Sev- used for geological mapping in mineral exploration, eral samples were also taken at surface and combined particularly in tropical regions where outcrop condi- with samples from shallow boreholes used for cosmo- tions are poor. This technique uses the emission of genic nuclide analysis. This part of the project has the gamma rays from the decay of the naturally occurring objective to quantify the exposure age and erosion rate radioisotopes 40K, 232Th, and 238U. Thus, airborne radi- of Bosumtwi impactites. ometric data are used to produce maps of surface con- centrations of K, Th, and U, at different scales, de- pending on the parameters of acquisition (flight alti- tude, line spacing). Airborne radiometric data have been rarely used to date to investigate meteorite impact structures, in comparison with other geophysical data sets, such as gravity or aeromagnetic anomaly maps, or electric methods, which have been commonly used to investigate impact structures buried under sedimentary layers. To date, K, Th, and U radiometric signatures have been documented for only two impact structures: Bosumtwi in Ghana [1] and Serra da Cangalha in Bra- zil [2]. Both structures are characterized by an annular high-K anomaly, whereas Serra da Cangalha is also marked by annular low-Th and low-U anomalies. Bo- sumtwi is a 1.07 Ma old complex impact structure 10.5 km diameter, which shows a double-ring K anomaly. The first anomaly, 10 – 12 km from the center, corre- Fig. 1 – RGB representation of K, Th, and U concen- sponds more or less to the crater rim section (ring I). trations based on the airborne radiometric data with The second one has a diameter of about 18 km (ring II) locations of two shallow boreholes (BH1 and BH3) [3, 4]. (modified after [4]). To the southeast of the structure, Data and method: To decipher the origin of the K the hills forming the Obuom Range are composed of anomalies, we combined morphological analysis based basic intrusive and Birimian metasediments [5]. on 30 m/pixel SRTM data and field observations (from Results: The morphological analysis reveals the a 4-day field campaign in November 2017). The latter presence of an annular plateau, which is terminated by includes a series of ground-based gamma-ray meas- a distal ridge. The distal ridge indicates fluidized em- urements, using a portable gamma-ray spectrometer placement of ejecta [4,5]. K, Th, and U concentrations (RS BGO 230). The objective of this analysis was to were measured at 46 outcrops (Fig. 2). These meas- determine the nature of K-poor and K-rich material urements indicate that variations in K are not primarily observed in the airborne radiometric and determine related to the compositions of metasediments and other whether or not variations in K are related to lithologi- lithologies. However, K-poor material corresponds to cal properties (presence of different types of rocks) or Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5008.pdf laterite surfaces, which are present outside of the struc- whereas a new laterite surface developed within the ture and within the annular depression (or moat) be- moat. The annular K-rich anomaly is therefore the tween the crater rim and the distal ridge, whereas K- consequence of post-impact erosion and alteration rich material corresponds to fresh rock exposures, processes in the equatorial climate. which occur at the crater rim and distal ridge. Fig. 2 – Illustration of ground-based radiometric meas- urements of K, Th, and U concentrations. Fig. 3 – Scenario of formation of the annular K-rich Table1: Mean values of K, Th, and U concentrations anomalies at Bosumtwi. measured at Bosumtwi (see Fig. 2) with a handheld spectrometer. Conclusions: This study reveals that the enigmatic Rock type K Stdv Th Stdv U Stdv circular K anomalies result from post-impact surface (wt%) (ppm) (ppm) processes. This was controlled by the topography of Metasediments 2.09 0.5 4.27 1.5 1.38 0.7 the impact structure is not a direct consequence of the Intrusive (granite) 1.3 0.2 3.4 1.1 1.03 0.2 impact process. Considering the context and presence Breccias 1.9 0.6 4.5 1.2 1.3 0.4 of laterite surfaces in Central Brazil, we suggest that Saprock 1.22 0.6 5 1.8 1.6 0.5 the circular enrichment in K observed at Serra da Saprolite 1.18 0.6 4.1 1.1 1.2 0.3 Cangalha may represent an analogous situation to Preliminary analyses of cosmonuclides (10Be and Bosumtwi. Other circular radiometric anomalies at 26Al) on surface samples and collected to a depth of 15 impact structures in Australia are being investigated to m (shallow boreholes BH1 and BH2 into the ejecta, capture the diversity of processes responsible for radi- see Fig. 1, [1]) suggest that the laterite surface within ometric anomalies associated with meteorite impact the moat developed since emplacement of Bosumtwi structures. ejecta at ca. 1 Myr, whereas the laterite surface outside References: [1] Boamah, D., Koeberl, C. 2002. In the crater, the ejecta, and the surface beneath the ejecta Impact in Precambrian Shields. Springer, Berlin Hei- are much older. The in-situ produced 10Be concentra- delberg. Eds J. Plado, L. J. Pesonen, pp. 211–255. [2] tions measured on purified quartz extracted from the Vasconcelos, M.A.R. et al.., 2012. Geophys. Res. Lett., samples varies between 0.01 and 2 Mat/g. This is much 39, L04306, doi:10.1029/2011GL050525. [3] Pesonen, lower than concentrations previously measured from L.J., et al., 2003. Airborne geophysical survey of the total superficial material [6], in the 100-400 Mat/g Lake Bosumtwi meteorite impact structure (Southern range, because the latter are mainly controlled by at- Ghana) – Geophysical maps with description. Techni- mospheric 10Be deposited in the soil [7]. cal Report. Jhrb. Geol. Bundesanstalt, Wien, ISSN Based on these results, we propose the following 0016-7800. [4] Baratoux, D. et al. (2019) Meteorit. scenario for the formation of the K-rich circular anom- Planet. Sci. doi: 10.1111/maps.13253. [5] Koeberl C. alies (Fig. 3). The impact crater formed about 1 Myr and Reimold W. U. 2005. Geological map of the Bo- ago on a well-developed laterite surface (locally at sumtwi impact crater. Map Supplement to the Jahrbuch least 15-20 m thick), which covered K-rich metasedi- der Geologischen Bundesanstalt, Vienna, Yearbook of ments, and which is commonly observed in this part of the Austrian Geological Survey. [6] Wulf, G. et al Ghana. Since crater formation, the distal ejecta ridge (2019) Earth Planet. Sci. Lett. 506, 209-220. and the crater rim have been continuously eroded. This doi:10.1016/j.epsl.2018.11.009. [7] Serefiddin et al. resulted in exhumation of K-rich metasediments, (2007) Geochem. Cosmoch. Acta, 71, 1574–1582. .
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