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

与像U-Pb或K-(Ar)-Ar这样的绝对计时器相比，半衰期为几百万年的短寿命衰变系统可以更精确地检测陨石之间的相对年龄差异，它们的成分（即球粒，矿物相）和陨石群作为一个整体。与所有测年方法一样，各自的年龄理想地代表了所讨论的陨石中某种元素系统的温度依赖保留的开始。这样，就有可能建立一个太阳系早期小天体形成和演化的相对时间尺度。在计划项目中应用半衰期为15.7 Ma的129I-129Xe-和半衰期为3.7 Ma的53mn - 53cr -计时器，将有助于我们对顽辉石球粒陨石母体热演化的理解。顽辉石球粒陨石分为EH亚群和EL亚群，它们在化学、矿物学和岩石学演化方面存在差异。例如，在当前项目中获得的新的i - xe年龄证实了EH和EL球粒陨石之间已知的年龄序列（EH大约早2-6 Ma）。然而，这种二分法是分散的，观察到的年龄变化可能是撞击引起的I-Xe系统部分重置的结果。这一假设得到了I-Xe和K-(Ar)-Ar体系中不同年龄的多个等时线的支持，EH球粒陨石中39Ar-40Ar年龄明显降低，即在一个更容易受热扰动影响的同位素体系中。这可能同样适用于EL球粒陨石，但干扰似乎与EL母体仍在进行的变质作用同时发生。为了更好地了解母体过程（即变质作用、撞击事件）所涉及的时间尺度，在拟议的项目中计划对尚未或仅研究较少的岩石学类型（EL3-5, EH6）进行额外的I-Xe分析。此外，锰铬计时器将作为附加计时器应用于顽辉石球粒陨石的测年。由于各自的载流子相的保留率不同（在一个元素系统内和与其他元素系统相比），如果保留温度足够好，（至少）两个辉化辉石球粒陨石母体的热演化可能会被重建。

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.