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Evidence for anorthositic crust formed on an inner solar system planetesimal

P. Frossard1,

1Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France

M. Boyet1,

1Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France

A. Bouvier2,3,

2Department of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, Ontario, Canada
3Bayerisches Geoinstitut, Universität Bayreuth, Germany

T. Hammouda1,

1Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France

J. Monteux1

1Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Frossard, P., Boyet, M., Bouvier, A., Hammouda, T., Monteux, J. (2019) Evidence for anorthositic crust formed on an inner solar system planetesimal. Geochem. Persp. Let. 11, 28–32.

European Research Council H2020 Grant No.682778 – ISOREE to M. Boyet.
NSERC Discovery Grant, Canada Research Chair, and the Canada Foundation for Innovation JELF programs to A. Bouvier.
Laboratory of Excellence ClerVolc.

Geochemical Perspectives Letters v11  |  doi: 10.7185/geochemlet.1921
Received 18 March 2019  |  Accepted 14 August 2019  |  Published 7 October 2019
Copyright © The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0




Figure 1 Trace element composition of NWA 8486 normalised to CI chondrites (Anders and Grevesse, 1989

Anders, E., Grevesse, N. (1989) Abundances of the elements: Meteoritic and solar. Geochimica et Cosmochimica Acta 53, 197-214.

). All fractions exhibit Eu and Sr positive anomalies. NWA 7325 has a lower content for most incompatible elements compared to NWA 8486 owing to its modal enrichment in plagioclase. Lunar anorthosites are reported for comparison (data from Haskin et al., 1973

Haskin. L. A., Helmke P. A., Blanchard D. P., Jacobs J. W., Telander K. (1973) Major and trace elements abundances in samples from lunar highlands. Proceedings of the Lunar Science Conference 4, 1275-1296.

and Norman et al., 2003

Norman, M.D., Borg, L.E., Nyquist, L.E., Bogard, D.D (2003) Chronology, geochemistry, and petrology of a ferroan noritic anorthosite clast from Descartes breccia 67215: Clues to the age, origin, structure, and impact history of the lunar crust. Meteoritics and Planetary Science 38, 645-661.

).
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Figure 2 Eu/Sm and Sr/Nd ratios of achondrites and NWA 8486 normalised to CI chondrites (Anders and Grevesse, 1989

Anders, E., Grevesse, N. (1989) Abundances of the elements: Meteoritic and solar. Geochimica et Cosmochimica Acta 53, 197-214.

). Overlap between the different fields is due to whole rocks mainly composed of pyroxene or plagioclase (e.g., aubrites, ureilites, lunar anorthosites). For each field group, NWA 8486 exhibits higher EuN/SmN and SrN/NdN. Only lunar anorthosites are similar to NWA 8486 whole rock, but they contain much more plagioclase than NWA 8486. See Supplementary Information for data sources.
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Figure 3 REE composition of liquids in equilibrium with NWA 8486 minerals (in grey dashed lines) compared to liquids modelled with compositions of the cumulate eucrite Moore County and the pyroxene-rich lunar anorthosite Apollo 62236, normalised to CI chondrites (Anders and Grevesse, 1989

Anders, E., Grevesse, N. (1989) Abundances of the elements: Meteoritic and solar. Geochimica et Cosmochimica Acta 53, 197-214.

). A non-modal melting is considered, in agreement with petrological constraints, of eutectic proportions of 42 % plagioclase and 58 % pyroxene (Osborn, 1942

Osborn, E.F. (1942) The system CaSiO3-diopside-anorthite. American Journal of Science 240, 751-788.

). The range of composition of the liquids from 1 % to 50 % degree of melting is represented for each source composition. NWA 8486 whole rock composition is shown (black line) for comparison. See Supplementary Information for details on the model.
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Figure 4 Time for plagioclase crystal ascent during the late stage of a magma ocean. This time is calculated for a range of viscosities (0.1 to 10 Pa.s), crystal diameters (a; 100 µm to 1 cm) and density contrasts (ρ; 50 to 500 kg/m3) between the crystal and the liquid phase. The time for a crystal to reach the surface of the magma ocean is represented with a colour scale ranging between 10 kyr (blue) and 2 Myr (red). Both panels show that the time of ascent of the crystal is a few tens of thousands of years in magma ocean conditions, except for small crystals around 100 µm and high viscosities of >10 Pa.s for which time of ascent is longer.
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