¹Lars E. Borg, ^1,2Gregory A. Brennecka, ³Steven J.K. Symes
¹Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L-231, Livermore CA 94550 USA
²Institut für Planetologie, Westfälische Wilhelms-Universität Munster, Wilkhekm-Klemm-Str. 10, 48149 Münster Germany
³Department of Chemistry, University of Tennessee-Chattanooga, Chattanooga, TN 37403, USA

High precision Sm−Nd isotopic analyses have been completed on a suite of 11 martian basaltic meteorites in order to better constrain the age of silicate differentiation on Mars associated with the formation of their mantle sources. These data are used to evaluate the merits and disadvantages of various mathematical approaches that have been employed in previous work on this topic. Ages determined from the Sm−Nd isotopic systematics of individual samples are strongly dependent on the assumed Nd isotopic composition of the bulk planet. This assumption is problematic given differences observed between the Nd isotopic composition of Earth and chondritic meteorites and the fact that these materials are both commonly used to represent bulk planetary Nd isotopic compositions. Ages determined from the slope of 146Sm−142Nd whole rock isochrons are not dependent on the assumed 142Nd/144Nd ratio of the planet, but require the sample suite to be derived from complementary, contemporaneously-formed reservoirs. In this work, we present a mathematical expression that defines the age of formation of the source regions of such a suite of samples that is based solely on the slope of a 143Nd−142Nd whole rock isochron and is also is independent of any a priori assumptions regarding the bulk isotopic composition of the planet. This expression is also applicable to mineral isochrons and has been used to successfully calculate 143Nd−142Nd model crystallization ages of early refractory solids as well as lunar samples. This permits ages to be obtained using only Nd isotopic measurements without the need for 147Sm/144Nd isotope dilution determinations. When used in conjunction with high-precision Nd isotopic measurements completed on martian meteorites this expression yields an age of formation of the martian basaltic meteorite source regions of 4504 ± 6 Ma. Because the Sm−Nd model ages for the formation of martian source regions are commonly interpreted to record the age at which large scale mantle reservoirs formed during planetary differentiation associated with magma ocean solidification, the age determined here implies that magma ocean solidification occurred several tens of millions of years after the beginning of the Solar System. Recent thermal models, however, suggest that Mars-sized bodies cool rapidly in less than ∼5 Ma after accretion ceases, even in the presence of a thick atmosphere. Assuming these models are correct, an extended period of accretion is necessary to provide a mechanism to keep portions of the martian mantle partially molten until 4504 Ma. Late accretional heating of Mars could either be associated with protracted accretion occurring at a quasi-steady state or alternatively be associated with a late giant impact. If this scenario is correct, then accretion of Mars-sized bodies takes up to 60 Ma and is likely to be contemporaneous with the core formation and possibly the onset of silicate differentiation. This further challenges the concept that isotopic equilibrium is attained during primordial evolution of planets, and may help to account for geochemical evidence implying addition of material into planetary interiors after core formation was completed.

Reflectance
Borg LE, Brennecka GA, Symes SJK (2015) Accretion Timescale and Impact History of Mars Deduced from the Isotopic Systematics of Martian Meteorites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.12.002]
Copyright Elsevier

¹A. Nathues et al. (>10)*
¹Institute for Solar System Research, Goettingen, Germany
*Find the extensive, full author and affiliation list on the publishers Website

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Reference
Nathues A et al. (2015) Sublimation in bright spots on (1) Ceres. Nature 528, 237–240
Link to Article [doi:10.1038/nature15754]

^1,2Pierre Bonnand, ^1,3Ian J. Parkinson, ^4,5Mahesh Anand
¹Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
²Department of Earth Sciences, University of Oxford, South Parks Roads, Oxford, OX1 3AN, United Kingdom
³School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Clifton BS8 1RJ, Bristol, United Kingdom
⁴Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
⁵Department of Earth Sciences, The Natural History Museum, London SW7 5DB, United Kingdom

We present the first stable chromium isotopic data from mare basalts in order to investigate the similarity between the Moon and the Earth’s mantle. A double spike technique coupled with MC-ICP-MS measurements was used to analyse 19 mare basalts, comprising high-Ti, low-Ti and KREEP-rich varieties. Chromium isotope ratios (δ53Cr) for mare basalts are positively correlated with indices of magmatic differentiation such as Mg# and Cr concentration which suggests that Cr isotopes were fractionated during magmatic differentiation. Modelling of the results provides evidence that spinel and pyroxene are the main phases controlling the Cr isotopic composition during fractional crystallisation. The most evolved samples have the lightest isotopic compositions, complemented by cumulates that are isotopically heavy. Two hypotheses are proposed to explain this fractionation: (i) equilibrium fractionation where heavy isotopes are preferentially incorporated into the spinel lattice and (ii) a difference in isotopic composition between Cr2+ and Cr3+ in the melt. However, both processes require magmatic temperatures below 1200˚C for appreciable Cr3+ to be present at the low oxygen fugacities found in the Moon (IW -1 to -2 log units). There is no isotopic difference between the most primitive high-Ti, low-Ti and KREEP basalts, which suggest that the sources of these basalts were homogeneous in terms of stable Cr isotopes. The least differentiated sample in our sample set is the low-Ti basalt 12016, characterised by a Cr isotopic composition of -0.222 ± 0.025 ‰, which is within error of the current BSE value (-0.124 ± 0.101 ‰). The similarity between the mantles of the Moon and Earth is consistent with a terrestrial origin for a major fraction of the lunar Cr. This similarity also suggests that Cr isotopes were not fractionated by core formation on the Moon.

Reference
Bonnand P, Parkinson IJ, Anand M (2015) Mass dependent fractionation of stable chromium isotopes in mare basalts: implications for the formation and the differentiation of the Moon. Geochimica et Cosmochicmica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.11.041]
Copyright Elsevier