NON-TRADITIONAL STABLE ISOTOPE VARIATIONS IN THE IMPACTITES OF THE ROCHECHOUART IMPACT STRUCTURE: TRACING IMPACT VOLATILIZATION, MELTING, MIXING, AND HYDROTHERMAL OVERPRINTING

Introduction: The Rochechouart structure is a deeply eroded impact crater formed ~ 207 Myr ago and that no longer displays any impact-related topography [1]. Its surface is currently at the level of the crater floor, with a di-ameter of about 20-25 km, based on morphological, geophysical, and structural reconstructions [2,3]. Despite its high erosion degree, the Rochechouart impact structure displays a preserved suite of impactites (impactoclastites, suevites, impact melt rocks, breccias, granite and gneiss from basement), with a large proportion of melted material. These lithologies were sampled during the 2017 drilling campaign, held by the CIRIR in 2017 [4] (funded by the Réserve Naturelle Nationale de Rochechouart-Chassenon). This campaign resulted in 18 drill holes (with a cumula-tive length of ~ 540 m) located at 8 sites along two 10-km radial transects across the center of the structure [5].

Samples and methods: Nineteen samples from 6 drilling sites from across the Rochechouart impact structure have been selected for this study. The selected samples are from the cores SC1, SC2, SC3, SC7, SC11, SC15, SC16, and SC17. Three of them are from the basement (1 granite and 2 gneiss), 8 from impact melt rocks, 3 from suevites, 3 from impact breccias, and 2 from impactoclastites intervals. The nineteen samples selected for this study have been selected to complement the first petrographic and geochemical studies held. The aim is to trace a meteoritic component thanks to germanium [Ge] isotopic variations (CRPG-CRNS Nancy) and highly/moderately siderophile elements concentrations among the different lithologies selected. Volatilization may also be traced by Ge isotopic variations. Iron, zinc, and copper isotope systematics are also complementing the Ge isotope data to study other syn- and post-impact processes (melting, mixing of target rocks, hydrothermal alteration).

Results: The Ge isotope results for the 19 selected impactites and target lithologies are the first obtained within any impact structure. They display a strong varia-tion in d74/70Ge (from ~ 0.1 to 1‰). The observed Ge isotope compositions, in combination with their Ge con-centrations, put forward 2 groups among the impactite samples, distinct from the basement samples. One group (mainly impactoclastites and suevites) exhibits similar Ge concentrations as the basement samples but at higher d74/70Ge isotopic signatures, while the other group (mostly impact melt rocks) displays comparable or lower Ge isotopic signatures relative to the basement samples, but at higher Ge elemental concentrations.

Discussion and conclusions: These results imply at least 2 distinct processes affecting the rocks sampled in the drill cores: (1) heavy d74/70Ge isotopic signatures pos-sibly indicating impact induced volatilization, (2) signa-tures that would reflect secondary alteration. Moreover, data for Fe, Cu, Zn isotopes and HSE concentrations are currently collected for the same nineteen samples, in order to constrain the geochemical and isotopic signatures of the various lithologies of the Rochechouart impact structure. Along with geochemical and petrographic parameters, the nature of these syn- and post-impact processes, including melting and mixing of target rocks, volatilization, meteoritic contribution, and hydrothermal alteration, may be traced and refined.

References: [1] Rasmussen C. et al. (2020) Geochimica and Cosmochimica Acta 273, 313-330. [2] Lambert P. (1977) Earth and Planetary Science Letter 35, 258-268. [3] Koeberl C. et al. (2007) Earth and Planetary Science Letter 256, 534-546. [4] Lambert P. et al. (2016) Meteoritics & Planetary Science, A399. [5] Lambert P. et al. (2018) LPSC XXXIX, Abstract #1954. [6] Luais B. (2012) Chemical Geology 334, 295-311.