Ruthenium isotope signatures of terrestrial and lunar impact structures

ntroduction: Impact-related rocks and deposits from terrestrial and lunar impact structures are characterized by elevated concentrations of highly-siderophile elements (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, Au) compared to crustal target rocks [1-10]. Consequently, HSE systematics such as inter-element ratios and 187Os/188Os isotope compositions were used to deduce the nature of the meteoritic component in terrestrial and lunar impact rocks. Here, we show that in addition to relative HSE abundances, ruthenium isotope compositions of impact rocks and deposits are very distinctive for the type of meteoritic material in terrestrial and lunar impact rocks. This is because different groups of meteorites can be distinguished based on their Ru isotopic compositions [e.g., 11, 12]. Owing to very low Ru concentrations in crustal target rocks, the presence of minute amounts (>0.5 %) of impactor component already shift the Ru isotopic composition of crustal rocks towards the meteoritic end-member composition. Hence, the Ru isotope signature of a given impact rock or deposit much more directly corresponds to the extraterrestrial component because the Ru in these rocks will almost quantitatively be derived from the impactor. Results: We have determined the Ru isotope compositions of impact rocks and deposits from several terrestrial impact structures including the K-Pg boundary layer, Brent, Clearwater East, Popigai, Rochechouart, Morokweng, as well as Archean spherule layer samples from Barberton (South Africa). Our ongoing investigations are focused on additional terrestrial impact structures (e.g., Luna crater, India) and lunar impact rocks from Apollo 14, 16 and 17. Terrestrial impact structures: Our still limited Ru isotope data for terrestrial impact structures in conjunction with previously reported HSE concentration data [1-6] reveal that the projectiles for most Phanerozoic impacts derive from a population of bodies that formed from an inner solar system reservoir, best represented by ordinary chondrites. So far, among the investigated Phanerozoic impact structures, the only exception from this composition are samples from the K-Pg boundary layer that exhibit carbonaceous chondrite-like Ru isotope compositions. Archean spherule layers exhibit Ru isotope signatures consistent with ordinary and CI carbonaceous chondrite compositions. Lunar impact rocks: In case of lunar impact rocks, previous studies revealed that according to their relative HSE abundances different impactor components can be identified in lunar impactites. For instance, inter-element HSE and 187Os/188Os variations of many lunar impact rocks can be explained by binary mixing of two prominent meteoritic end-member compositions. One end-member is defined by lunar granulitic and aphanitic impact rocks from Apollo 16 and 17, which have carbonaceous chondrite-like HSE compositions [8, 9]. The other end-member is most promi- nent at Apollo 16 and has more fractionated HSE pattern with supra-chondritic Pd/Ir and Ru/Ir similar to some IVA and IIIA iron meteorites [9, 10]. Outlook: Like in the case of K-Pg boundary layer samples, the occurrence of carbonaceous chondrite-like and iron meteorite-like impactor components on the Moon can be tested with high-precision Ru isotope data. Until now, only a very limited number of lunar impact rocks were analyzed for their Ru isotope compositions [13]. The range of observed HSE signatures in lunar impactites suggests that they relate to impactors deriving from distinct populations of inner solar system and outer solar system bodies. Hence, further investigation of lunar samples covering the whole range of observed impactor compositions are needed to assess if Ru isotopic variations that were imparted by diverse impactor populations could be identified on the lunar surface.