Target rock mixing, meteoritic contribution, and hydrothermal alteration recorded in the 2017 drill core material of the Rochechouart impact structure

Abstract This work presents a detailed petrographic, geochemical, and isotopic characterization of 49 samples from the Rochechouart impact structure (Limousin, France) recovered during the 2017 drilling campaign. The aims of this study were to constrain the processes that affected the target lithologies and led to the formation of the melt-rich impactites preserved in the Rochechouart impact structure today. Formed on a gneissic and granitic target, the current lithologies observed in the Rochechouart impact structure and in the drilled cores range from the fractured target to the impact melt rocks, forming whole impactite sequences with breccias, suevites, and impactoclastites. Two different types of impact melt rocks (described as red and yellow in this study) are found. The dichotomy in impact melt rocks has previously been proposed to be linked to the two major target lithologies in the region, with the red impact melt rocks originating mainly from the gneiss lithologies and the yellow type dominated by a granitic precursor. Impact-induced hydrothermal alteration affected all recovered samples but is particularly pronounced in the case of the yellow impact melt rocks, as highlighted by the increases in fluid-mobile element concentrations, with up to four times the concentration of K2O compared to basement lithologies. The light rare earth elements (La, Yb) also record hydrothermal alteration. However, for the other lithologies, the level of alteration remains limited and allows the meteoritic component to be traced among the melt-rich lithologies, recorded by the increases in siderophile elements (moderate increases in Ni, Cr, and Co and high increases in Os, Ir, Ru, Pt, Pd, and Re) in all characterized red impact melt rocks. The Os isotopic compositions of the red impact melt rocks show meteoritic contributions up to 0.5% CI chondrite.