¹Ian S. Sanders,²Edward R. D. Scott
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13742]
¹Department of Geology, Trinity College, Dublin 2, Ireland
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, 96822 USA
Published by arrangement with John Wiley & Sons

The remarkable complementary isotopic relationship in the Allende chondrite between chondrules (depleted in s-process molybdenum and tungsten) and matrix (enriched in these nuclides) has been interpreted as evidence that the anomalies were established during chondrule formation, and that chondrules were, therefore, not made by planetesimal collisions. We question this interpretation, and to better understand the complementary relationship, we review nucleosynthetic isotopic variations of Mo and W in bulk carbonaceous chondrites, their components, and acid leachates extracted from them. Mo isotopic data almost always track a mixing line between pure s-process Mo and s-process-depleted Mo (i.e., with excess p-process and r-process Mo in a fixed ratio). Tungsten data track an equivalent mixing line. Guided by our review, we develop a model suggesting how the isotopic variations in Allende’s chondrules and matrix could be attributable to hydrothermal alteration in the parent body. In our model, anomalous Mo and W, both depleted in s-process isotopes, are easily leached from their carriers in the matrix, then transported in solution and precipitated preferentially in water-deficient components, such as chondrules, where the aqueous solvent is consumed. The model operates after accretion so does not inform chondrule-forming mechanisms. It also goes some way to explaining variations of Mo and W isotopes in Ca-Al-rich inclusions in Allende, and variations of s-process Mo in bulk carbonaceous chondrites.

¹Fridolin Spitzer,¹Christoph Burkhardt,²Francis Nimmo,¹Thorsten Kleine
Earth and Planetary Science Letters 576, 117211 Link to Article [https://doi.org/10.1016/j.epsl.2021.117211]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA, 95064, USA
Copyright Elsevier

The 182Hf-182W chronology of iron meteorites provides crucial information on the timescales of accretion and differentiation of some of the oldest planetesimals of the Solar System. Determining accurate Hf-W model ages of iron meteorites requires correction for cosmic ray exposure (CRE) induced modifications of W isotope compositions, which can be achieved using in-situ neutron dosimeters such as Pt isotopes. Until now it has been assumed that all Pt isotope variations in meteorites reflect CRE, but here we show that some ungrouped iron meteorites display small nucleosynthetic Pt isotope anomalies. These provide the most appropriate starting composition for the correction of CRE-induced W isotope variations in iron meteorites from all major chemical groups, which leads to a ∼1 Ma upward revision of previously reported Hf-W model ages. The revised ages indicate that core formation in non-carbonaceous (NC) iron meteorite parent bodies occurred at ∼1–2 Ma after CAI formation, whereas most carbonaceous (CC) iron meteorite parent bodies underwent core formation ∼2 Ma later. We show that the younger CC cores have lower Fe/Ni ratios than the earlier-formed NC cores, indicating that core formation under more oxidizing conditions occurred over a more protracted timescale. Thermal modeling of planetesimals heated by 26Al-decay reveals that this protracted core formation timescale is consistent with a higher fraction of water ice in CC compared to NC planetesimals, implying that in spite of distinct core formation timescales, NC and CC iron meteorite parent bodies accreted about contemporaneously within ∼1 Ma after CAI formation, but at different radial locations in the disk.