^1,2Thomas S. Kruijer, ¹Thorsten Kleine
Earth and Planetary Science Letters 475, 15-24 Link to Article [https://doi.org/10.1016/j.epsl.2017.07.021]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany
²Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
Copyright Elsevier

The giant impact model of lunar origin predicts that the Moon mainly consists of impactor material. As a result, the Moon is expected to be isotopically distinct from the Earth, but it is not. To account for this unexpected isotopic similarity of the Earth and Moon, several solutions have been proposed, including (i) post-giant impact Earth–Moon equilibration, (ii) alternative models that make the Moon predominantly out of proto-Earth mantle, and (iii) formation of the Earth and Moon from an isotopically homogeneous disk reservoir. Here we use W isotope systematics of lunar samples to distinguish between these scenarios. We report high-precision 182W data for several low-Ti and high-Ti mare basalts, as well as for Mg-suite sample 77215, and lunar meteorite Kalahari 009, which complement data previously obtained for KREEP-rich samples. In addition, we utilize high-precision Hf isotope and Ta/W ratio measurements to empirically quantify the superimposed effects of secondary neutron capture on measured 182W compositions. Our results demonstrate that there are no resolvable radiogenic 182W variations within the Moon, implying that the Moon differentiated later than 70 Ma after Solar System formation. In addition, we find that samples derived from different lunar sources have indistinguishable 182W excesses, confirming that the Moon is characterized by a small, uniform ∼+26 parts-per-million excess in 182W over the present-day bulk silicate Earth. This 182W excess is most likely caused by disproportional late accretion to the Earth and Moon, and after considering this effect, the pre-late veneer bulk silicate Earth and the Moon have indistinguishable 182W compositions. Mixing calculations demonstrate that this Earth–Moon 182W similarity is an unlikely outcome of the giant impact, which regardless of the amount of impactor material incorporated into the Moon should have generated a significant 182W excess in the Moon. Consequently, our results imply that post-giant impact processes might have modified 182W, leading to the similar 182W compositions of the pre-late veneer Earth’s mantle and the Moon.

¹F.P. Leitzke, ¹R.O.C. Fonseca, ²P. Sprung, ³G. Mallmann, ¹M. Lagos, ¹L.T. Michely, ²C. Münker
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2017.07.009]
¹Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Rheinische Friedrich-Wilhelms-Universität Bonn, 53115 Bonn, Germany
²Institut für Geologie und Mineralogie, Universität zu Köln, 50674 Köln, Germany
³Research School of Earth Sciences, Australian National University, ACT 2601, Canberra, Australia
Copyright Elsevier

We present results of high-temperature olivine-melt, pyroxene-melt and plagioclase-melt partitioning experiments aimed at investigating the redox transition of Mo in silicate systems. Data for a series of other minor and trace elements (Sc, Ba, Sr, Cr, REE, Y, HFSE, U, Th and W) were also acquired to constrain the incorporation of Mo in silicate minerals. All experiments were carried out in vertical tube furnaces at 1 bar and temperatures ranging from ca. 1220 to 1300 °C. Oxygen fugacity was controlled via CO–CO2 gas mixtures and varied systematically from 5.5 log units below to 1.9 log units above the fayalite–magnetite–quartz (FMQ) redox buffer thereby covering the range in oxygen fugacities of terrestrial and lunar basalt genesis. Molybdenum is shown to be volatile at oxygen fugacities above FMQ and that its compatibility in pyroxene and olivine increases three orders of magnitude towards the more reducing conditions covered in this study. The partitioning results show that Mo is dominantly tetravalent at redox conditions below FMQ-4 and dominantly hexavalent at redox conditions above FMQ. Given the differences in oxidation states of the terrestrial (oxidized) and lunar (reduced) mantles, molybdenum will behave significantly differently during basalt genesis in the Earth (i.e. highly incompatible; average View the MathML source) and Moon (i.e. moderately incompatible/compatible; average View the MathML source). Thus, it is expected that Mo will strongly fractionate from W during partial melting in the lunar mantle, given that W is broadly incompatible at FMQ-5. Moreover, the depletion of Mo and the Mo/W range in lunar samples can be reproduced by simply assuming a primitive Earth-like Mo/W for the bulk silicate Moon. Such a lunar composition is in striking agreement with the Moon being derived from the primitive terrestrial mantle after core formation on Earth.