¹Paolo A. Sossi, ²Frédéric Moynier
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2017.04.029]
¹Institut de Physique du Globe de Paris, Université Paris Diderot, Université Sorbonne Paris Cité, CNRS UMR 7154, 1 rue Jussieu, 75238 Paris Cedex 05, France
²Institut Universitaire de France, Paris, France
Copyright Elsevier

The Moon and the Earth’s mantle share many chemical and isotopic traits, leading to the prevailing theory that they were formed from similar material. Iron is one element that shows apparent differences between the two bodies, with models for the composition of the Moon having ≈1.5 times more FeO (12–14 wt.%), relative to the Earth’s mantle (8 wt.%). This difference is mirrored in their isotope compositions, where lunar mare basalts have δ57Fe (per mille deviation of the 57Fe/54Fe ratio from the IRMM-014 standard) 0.1–0.2‰ higher than peridotitic rocks representative of Earth’s mantle, a feature initially attributed to loss of isotopically light Fe following a giant impact. However, whether basaltic rocks are suitable analogues for the Moon’s composition is debatable in the light of their distinct source regions that reflect the extensive lithological stratification of the lunar mantle. Here, we evaluate the iron isotope composition of the bulk Moon through the study of igneous cumulate rocks of the lunar highlands Magnesium Suite (Mg Suite). The δ57Fe of Mg Suite rocks spans a limited range, from 0.05‰ to 0.10‰, with an average (+0.07±0.02‰+0.07±0.02‰) that overlaps with Earth’s mantle (+0.05±0.01‰+0.05±0.01‰), similarities that extend to their Mg#s, where both reach 0.9. Numerical modelling of iron isotope fractionation during lunar magma ocean crystallisation shows that the Mg Suite should accurately reflect the composition of the bulk Moon, which is therefore +0.07±0.02‰+0.07±0.02‰, indistinguishable from Earth’s mantle but heavier than chondrites (−0.01±0.01‰−0.01±0.01‰). Iron thus behaves coherently with other elements that condense at temperatures higher than Li in showing no isotopic difference between the Earth and Moon, suggesting element depletion on the Moon affected only the more volatile elements. Therefore, there is no cosmochemical basis for iron enrichment or depletion in the bulk Moon relative to the Earth’s mantle, whose composition is an analogue for that of the Moon.

¹Zaicong Wang, ¹Harry Becker
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.024]
¹Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstrasse 74-100, 12249, Berlin, Germany
Copyright Elsevier

Silver and Cu show very similar partitioning behavior in sulfide melt-silicate melt and metal-silicate systems at low and high pressure-temperature (P-T) experimental conditions, implying that mantle melting, fractional crystallization and core-mantle differentiation have at most modest (within a factor of 3) effects on Cu/Ag ratios. For this reason, it is likely that Cu/Ag ratios in mantle-derived magmatic products of planetary bodies reflect that of the mantle and, in some circumstances, also the bulk planet composition. To test this hypothesis, new Ag mass fractions and Cu/Ag ratios in different groups of Martian meteorites are presented and compared with data from chondrites and samples from the Earth’s mantle.

Silver contents in lherzolitic, olivine-phyric and basaltic shergottites and nakhlites range between 1.9 and 12.3 ng/g. The data display a negative trend with MgO content and correlate positively with Cu contents. In spite of displaying variable initial Ɛ143Nd values and representing a diverse spectrum of magmatic evolution and physiochemical conditions, shergottites and nakhlites display limited variations of Cu/Ag ratios (1080±320, 1s, n=14). The relatively constant Cu/Ag suggests limited fractionation of Ag from Cu during the formation and evolution of the parent magmas, irrespectively of whether sulfide saturation was attained or not. The mean Cu/Ag ratio of Martian meteorites thus reflects that of the Martian mantle and constrains its Ag content to 1.9±0.7 ng/g (1s).

Carbonaceous and enstatite chondrites display a limited range of Cu/Ag ratios of mostly 500-2400. Ordinary chondrites show a larger scatter of Cu/Ag up to 4500, which may have been caused by Ag redistribution during parent body metamorphism. The majority of chondrites have Cu/Ag ratios indistinguishable from the Martian mantle value, indicating that Martian core formation strongly depleted Cu and Ag contents, but probably did not significantly change the Cu/Ag ratio of the mantle compared to bulk Mars. Bulk Mars is richer in moderately volatile elements than Earth, however, the Martian mantle displays a much stronger depletion of the moderately volatile elements Cu and Ag, e.g., by a factor of 15 for Cu. This observation is consistent with experimental studies suggesting that core formation at low P-T conditions on Mars led to more siderophile behavior of Cu and Ag than at high P-T conditions as proposed for Earth. In contrast, Cu/Ag ratios of the mantles of Mars and Earth (Cu/AgEarth=3500±1000) display only a difference by a factor of 3, which implies restricted fractionation of Cu and Ag even at high P-T conditions. The concentration data support the notion that siderophile element partitioning during planetary core formation scales with the size of the planetary body, which is particularly important for the differentiation of large terrestrial planets such as Earth. Collectively, the Ag and Cu data on magmatic products from the mantles of Mars and Earth and the data on chondrites confirm experimental predictions and support the limited fractionation of Cu and Ag during planetary core formation and high-temperature magmatic evolution, and probably also in early solar nebular processes.