¹Stein B. Jacobsen
¹Gang Yu
¹Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA

Heterogeneities in terrestrial samples for 182W/183W and 142Nd/144Nd are only preserved in Hadean and Archean rocks while heterogeneities in 129Xe/130Xe and 136Xe/130Xe persist to very young mantle-derived rocks. In contrast, meteorites from Mars show that the Martian mantle preserves heterogeneities in 182W/183W and 142Nd/144Nd up to the present. As a consequence of the probable “deep magma ocean” core formation process, we assume that the Earth and Mars both had a very early two-mantle-reservoir structure with different initial extinct nuclide isotopic compositions (different 182W/183W, 142Nd/144Nd, 129Xe/130Xe, 136Xe/130Xe ratios). Based on this assumption, we developed a simple stochastic model to trace the evolution of a mantle with two initially distinct layers for the extinct isotopes and its development into a heterogeneous mantle by convective mixing and stretching of these two layers. Using the extinct isotope system 182Hf-182W, we find that the mantles of Earth and Mars exhibit substantially different mixing or stirring rates. This is consistent with Mars having cooled faster than the Earth due to its smaller size, resulting in less efficient mantle mixing for Mars. Moreover, the mantle stirring rate obtained for Earth using 182Hf-182W is consistent with the mantle stirring rate of ~500 Myr constrained by the long-lived isotope system, 87Rb-87Sr and 147Sm-143Nd. The apparent absence of 182W/183W isotopic heterogeneity in modern terrestrial rocks is attributed to very active mantle stirring which reduced the 182W/183W isotopic heterogeneity to a relatively small scale (~83 m for a mantle stirring rate of 500 Myr) compared to the common sampling scale of terrestrial basalts (~30 or 100 km). Our results also support the “deep magma ocean” core formation model as being applicable to both Mars and Earth.

Reference
Jacobsen SB, Yu G (2015) Extinct isotope heterogeneities in the mantles of Earth and Mars: Implications for mantle stirring rates. Meteoritics&Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12426]

Published by arrangement with John Wiley & Sons

¹J.A. Barrat,²O. Rouxel,³K. Wang,^4,5F. Moynier,^6,7A. Yamaguchi,⁸A. Bischoff,⁹J. Langlade
¹Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer, CNRS UMR 6538, Place Nicolas Copernic, 29280 Plouzané, France
²IFREMER, centre de Brest, 29280 Plouzané, France
³Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
⁴Institut de Physique du Globe de Paris, Institut Universitaire de France, Université Paris Diderot, Sorbonne Paris Cité, 1 rue Jussieu, 75238 Paris Cedex 05, France
⁵Institut Universitaire de France, Paris, France
⁶National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
⁷Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tachikawa, Tokyo 190-8518, Japan
⁸Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
⁹CNRS UMS 3113, I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France

Ureilite meteorites are achondrites that are debris of the mantle of a now disrupted differentiated asteroid rich in carbon. They provide a unique opportunity to study the differentiation processes of such a body. We analyzed the iron isotopic compositions of 30 samples from the Ureilite Parent Body (UPB) including 29 unbrecciated ureilites and one ureilitic trachyandesite (ALM-A) which is at present the sole large crustal sample of the UPB. The δ56Fe of the whole rocks fall within a restricted range, from 0.01 to 0.11‰, with an average of +0.056±0.008‰+0.056±0.008‰, which is significantly higher than that of chondrites. We show that this difference can be ascribed to the segregation of S-rich metallic melts at low degrees of melting at a temperature close to the Fe–FeS eutectic, and certainly before the onset of the melting of the silicates (View the MathML source<1100°C), in agreement with the marked S depletions, and the siderophile element abundances of the ureilites. These results point to an efficient segregation of S-rich metallic melts during the differentiation of small terrestrial bodies.

Reference
Barrat JA, Rouxel O,Wang K, Moynier F,Yamaguchi A,Bischoff A, Langlade J (2015) Early stages of core segregation recorded by Fe isotopes in an asteroidal mantle. Earth and Planetary Science Letters, 419, 93–100
Link to Article [http://dx.doi.org/10.1016/j.epsl.2015.03.026]

Copyright Elsevier

¹Thomas B. McCord,²Jennifer E.C. Scully
¹The Bear Fight Institute, Winthrop WA 98862
²Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, 595 Charles Young Drive East, Box 951567, Los Angeles, CA 90095-1567

Vesta’s surface composition has been of special interest since early, disk-integrated telescopic spectral observations indicated that it is basaltic, differentiated and similar to the HED (howardite-eucrite-diogenite) class of meteorites. The Dawn mission, orbiting Vesta, provided a large and varied set of unique observations on the detailed mineralogy, molecular and elemental composition, and their distributions in association with surface features and geology. The set of articles contained in this special issue is the first treatment of the entire surface composition of Vesta using the complete Dawn Vesta data set and the calibrations from the entire campaign. Most articles treat a region of Vesta within the context of the entire body, but there are several articles that treat global or technical topics. As a whole, these articles provide a current and comprehensive view of Vesta’s composition using all the relevant data that is available. Vesta’s surface composition is consistent with the upper layer being created by igneous processes, while a more mafic lithology generally associated with a mantle is surprisingly limited. There is evidence of contamination by low velocity infall of several types of objects: dark hydrated/hydroxolated material, and probably Fe/Mg silicates differing from Vesta’s. Isolated blocks of differing compositions, seen especially in crater walls, could indicate incomplete melting and mixing during the differentiation process, and retention of some evidence of the original building blocks of the accreted Vesta. This lead article introduces and provides the context for the following articles, presents a summary of the various findings, and integrates them into overall conclusions.

Reference
McCord TB, Scully JEC (2015) The Composition of Vesta from the Dawn Mission. Geochimica et Cosmochimica Acta (in Press)
Link to Article [http://dx.doi.org/10.1016/j.icarus.2015.03.022]

Copyright Elsevier