Evidence for anorthositic crust formed on an inner solar system planetesimal

Abstract

doi: 10.7185/geochemlet.1921 During the first million years of solar system history, planetesimals experienced extensive melting powered by the radioactive decay of 26Al (Lee et al., 1977). To date, the only known anorthositic crust on a solar system body is that of the Moon, formed by plagioclase flotation on top of the magma ocean (Wood et al., 1970). Here we show evidence from the ungrouped achondrite meteorite Northwest Africa (NWA) 8486 that an anorthositic crust formed on a planetesimal very early in solar system history (<1.7 Ma). NWA 8486 displays the highest anomalies in Eu and Sr found in achondrites so far and, for the first time, this characteristic is also identified in clinopyroxene. Elemental modelling, together with calculated timescales for crystal settling, show that only the melting of an anorthosite can produce NWA 8486 within the first 5 million years of solar system history. Our results indicate that such a differentiation scenario was achievable over short timescales within the inner solar system, and must have contributed to the making and elemental budget of the terrestrial planets. Received 18 March 2019 | Accepted 14 August 2019 | Published 7 October 2019 1. Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France 2. Department of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, Ontario, Canada 3. Bayerisches Geoinstitut, Universität Bayreuth, Germany * Corresponding author (email: [email protected]) Introduction Over 80 ungrouped achondrites have been found in the past 20 years, enriching our collections with new types of meteoritic samples. Northwest Africa (NWA) 8486 is one of several paired stones, along with the ungrouped achondrites NWA 7325 and 8014, that have been found in the Sahara desert (Ruzicka et al., 2017). They are plagioclase-rich cumulative gabbros that experienced remelting and fast cooling (Yang et al., 2019), with a very peculiar calcic and magnesian mineralogy. NWA 8486 formed under reduced conditions, with an oxygen fugacity ( fO2) of 3.2 log units below the iron-wüstite (IW) buffer (Sutton et al., 2017), and therefore likely originates from the inner solar system. Based on Cr, Ti and O isotopic compositions, NWA 7325/8486 have affinities with both acapulcoite-lodranite and ureilite groups (Barrat et al., 2015; Weber et al., 2016; Goodrich et al., 2017). The Pb-Pb isochron age of 4563.4 ± 2.6 Ma, Al–Mg age of 4563.09 ± 0.26 Ma and initial Mg isotopic composition for NWA 7325 indicate that the parent material of this meteorite had to be formed within 1-2 Myr after Solar System formation (Koefoed et al., 2016). Eu and Sr Anomalies in NWA 7325/8486 We investigated major and trace element composition of NWA 8486 through in situ and in solution analyses. The mineral compositions of NWA 8486 are consistent with those of NWA 7325 (Barrat et al., 2015; Weber et al., 2016; Goodrich et al., 2017), with An88.7±3.0Ab11.2±3.0 plagioclase, Wo45.4±0.5En53.4±0.5Fs1.2±0.1 clinopyroxene and Fo97.1±0.3 olivine. The modal compositions reported for different fragments of NWA 7325 vary (Barrat et al., 2015; Weber et al., 2016; Goodrich et al., 2017), illustrating the heterogeneous distribution of the minerals at the centimetre scale. The fragment of NWA 8486 studied here contains the highest pyroxene modal content at 52 %, the lowest plagioclase content at 44 %, with the remaining 4 % consisting of olivine, sulphides and metal. The range reported for NWA 7325 is 30-44 % for pyroxene, 54-60 % for plagioclase and 2-15 % for olivine (Barrat et al., 2015; Weber et al., 2016; Goodrich et al., 2017). We report trace element abundances in mineral ( plagioclase, pyroxene and olivine) and a whole rock powder of NWA 8486. Minerals were analysed either in situ or in solution after mechanical separation (Supplementary Information). The whole rock composition slightly differs from that of NWA 7325 from Barrat et al. (2015) as a consequence of different modal compositions (Fig. 1). Incompatible and moderately volatile elements are depleted in this meteorite, below 0.5 × CI chondrites for the whole rock with the exceptions of Eu and Sr. Positive Eu and Sr anomalies (Eu/Eu* and Sr/Sr*) are present in each mineral phase with different magnitudes, from 1.8 to 6.5 in pyroxene and 450 to 1039 in plagioclase (Supplementary Information). Their amplitude in the whole rock is much higher than that measured in lunar anorthosites (Fig. 1).