I-Xe and Ar-Ar chronology of enstatite chondrite and enstatite achondrite parent bodies in the Early Solar System

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Short-lived decay systems with half-lives of few million years allow a more precise detection of relative age differences between meteorites, their constitutents (i.e., chondrules, mineral phases) and meteorite groups as a whole than absolute chronometers like U-Pb or K-(Ar)-Ar. Common to all dating methods, the respective ages ideally represent the onset of temperature-dependent retention of a certain element system within the meteorite in question. As such, it is possible to establish a relative time-scale of formation and evolution of small bodies in the early Solar System. Within the proposed project application of the 129I-129Xe- with a half-life = 15.7 Ma and 53Mn-53Cr-chronometers with a half-life = 3.7 Ma should enhance our understanding of the thermal evolution of the enstatite chondrite parent bodies. Enstatite chondrites are subdivided in EH and EL subgroups which differ chemically, mineralogically and in their petrologic evolution. For example, new I-Xe-ages obtained within current project confirm the already known age sequence between EH and EL chondrites (EH are about 2-6 Ma older). However, this dichotomy is diffuse and observed variability in ages possibly a consequence of impact-induced partial resetting of the I-Xe system. This hypothesis is supported by multiple isochrones of differing ages in the I-Xe and K-(Ar)-Ar systems and significantly lowered 39Ar-40Ar ages in EH chondrites, i.e., in an isotope system more easily affected by thermal disturbances. This is likely similarly valid for EL chondrites, but disturbance appears coeval with still ongoing metamorphism on the EL parent body. In order to better understand the time-scales involved in parent body processes (i.e., metamorphism, impact events) additional I-Xe analyses of petrologic types which have been not or only poorly investigated (EL3-5, EH6) are planned within the proposed project. Furthermore, the Mn-Cr chronometer as an additional chronometer will be applied for dating of enstatite chondrites. Because respective carrier phases differ in their retentivity (both, within one element system and in comparison to other element systems) thermal evolution of the (at least) two enstatite chondrite parent bodies might be reconstructed, provided retention temperatures are known sufficiently well.