¹Alison C. Hunt, ¹David L. Cook, ^2,3Tim Lichtenberg, ¹Philip M. Reger,¹ Mattias Ek, ⁴Gregor J. Golabek, ¹Maria Schönbächler
Earth and Planetary Science Letters 482, 490-500 Link to Article [https://doi.org/10.1016/j.epsl.2017.11.034]
¹Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
²Institute of Geophysics, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland
³Institute for Astronomy, ETH Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
⁴Bayerisches Geoinstitut, University of Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany
Copyright Elsevier

The short-lived ¹⁸²Hf–¹⁸²W decay system is a powerful chronometer for constraining the timing of metal–silicate separation and core formation in planetesimals and planets. Neutron capture effects on W isotopes, however, significantly hamper the application of this tool. In order to correct for neutron capture effects, Pt isotopes have emerged as a reliable in-situ neutron dosimeter. This study applies this method to IAB iron meteorites, in order to constrain the timing of metal segregation on the IAB parent body.

The ε¹⁸²W values obtained for the IAB iron meteorites range from −3.61 ± 0.10 to −2.73 ± 0.09. Correlating εⁱPt with ε¹⁸²W data yields a pre-neutron capture ε¹⁸²W of −2.90 ± 0.06. This corresponds to a metal–silicate separation age of 6.0 ± 0.8 Ma after CAI for the IAB parent body, and is interpreted to represent a body-wide melting event. Later, between 10 and 14 Ma after CAI, an impact led to a catastrophic break-up and subsequent reassembly of the parent body. Thermal models of the interior evolution that are consistent with these estimates suggest that the IAB parent body underwent metal–silicate separation as a result of internal heating by short-lived radionuclides and accreted at around 1.4±0.1 Ma after CAIs with a radius of greater than 60 km.

¹Nigel M.Kelly, ^1,2Rebecca M.Flowers, ¹James R.Metcalf, ^1,2,3Stephen J.Mojzsis
Earth and Planetary Science Letters 482, 222-235 Link to Article [https://doi.org/10.1016/j.epsl.2017.11.009]
¹Department of Geological Sciences, University of Colorado, 2200 Colorado Avenue, UCB 399, Boulder, CO 80309-0399, USA
²Collaborative for Research in Origins (CRiO), The John Templeton Foundation – FfAME Origins Program, USA
³Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 45 Budaörsi Street, H-1112 Budapest, Hungary
Copyright Elsevier

We conducted zircon (U–Th)/He (ZHe) analysis of lunar impact-melt breccia 14311 with the aim of leveraging radiation damage accumulated in zircon over extended intervals to detect low-temperature or short-lived impact events that have previously eluded traditional isotopic dating techniques. Our ZHe data record a coherent date vs. effective Uranium concentration (eU) trend characterized by >3500 Ma dates from low (≤75 ppm) eU zircon grains, and ca. 110 Ma dates for high (≥100 ppm) eU grains. A progression between these date populations is apparent for intermediate (75–100 ppm) eU grains. Thermal history modeling constrains permissible temperatures and cooling rates during and following impacts. Modeling shows that the data are most simply explained by impact events at ca. 3950 Ma and ca. 110 Ma, and limits allowable temperatures of heating events between 3950–110 Ma. Modeling of solar cycling thermal effects at the lunar surface precludes this as the explanation for the ca. 110 Ma ZHe dates. We propose a sample history characterized by zircon resetting during the ca. 3950 Ma Imbrium impact event, with subsequent heating during an impact at ca. 110 Ma that ejected the sample to the vicinity of its collection site. Our data show that zircon has the potential to retain 4He over immense timescales (≥3950 Myrs), thus providing a valuable new thermochronometer for probing the impact histories of lunar samples, and martian or asteroidal meteorites.