¹J.J.Bellucci,^1,2A.A.Nemchin,²M.Grange,^3,4K.L.Robinson,⁵G.Collins,¹M.J.Whitehouse,^1,6J.F.Snape,⁷M.D.Norman,^3,4D.A.Kring
Earth and Planetary Science Letters 510, 173-185 Link to Article [https://doi.org/10.1016/j.epsl.2019.01.010]
¹Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
²Department of Applied Geology, Curtin University, Perth, WA 6845, Australia
³Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Blvd., Houston, TX 77058, United States of America
⁴Center for Lunar Science and Exploration, NASA Solar System Exploration Research Virtual Institute, United States of America
⁵Department of Earth Science & Engineering, Imperial College London, Kensington, London SW7 2AZ, UK
⁶Department of Earth Sciences, Faculty of Science, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands
⁷Research School of Earth Sciences, The Australian National University, 142 Mills Road, Acton ACT, 2601, Australia
Copyright Elsevier

A felsite clast in lunar breccia Apollo sample 14321, which has been interpreted as Imbrium ejecta, has petrographic and chemical features that are consistent with formation conditions commonly assigned to both lunar and terrestrial environments. A simple model of Imbrium impact ejecta presented here indicates a pre-impact depth of 30–70 km, i.e. near the base of the lunar crust. Results from Secondary Ion Mass Spectrometry trace element analyses indicate that zircon grains recovered from this clast have positive Ce/Ce^⁎ anomalies corresponding to an oxygen fugacity +2 to +4 log units higher than that of the lunar mantle, with crystallization temperatures of 771±88 to 810 ± 37 °C (2σ) that are unusually low for lunar magmas. Additionally, Ti-in-quartz and zircon calculations indicate a pressure of crystallization of 6.9±1.2 kbar, corresponding to a depth of crystallization of 167±27 km on the Moon, contradicting ejecta modelling results. Such low-T, high-fO₂, and high-P have not been observed for any other lunar clasts, are not known to exist on the Moon, and are broadly similar to those found in terrestrial magmas.

The terrestrial-like redox conditions inferred for the parental magma of these zircon grains and other accessory minerals in the felsite contrasts with the presence of Fe-metal, bulk clast geochemistry, and the Pb isotope composition of K-feldspar grains within the clast, all of which are consistent with a lunar origin. The dichotomy between redox conditions and the depth of origin inferred from the zircon compositions compared to the ejecta modelling necessitates a multi-stage petrogenesis. Two, currently unresolvable hypotheses for the origin and history of the clast are allowed by these data. The first postulates that the relatively oxidizing conditions were developed in a lunar magma, possibly by fractional crystallization and enrichment of incompatible elements in a fluid-rich, phosphate-saturated magma, at the base of the lunar crust to form the zircon grains and their host felsite. Subsequent excavation by the Imbrium impact introduced more typical lunar features to the clast but preserved primary chemical characteristics in zircon and some other accessory minerals. However, this hypothesis fails to explain the high P of crystallization. Alternatively, the felsite and its zircon crystallized on Earth at a modest depth of 19±3 km in the continental crust where oxidizing, low-T, fluid-rich conditions are common. Subsequently, the clast was ejected from the Earth during a large impact, entrained in the lunar regolith as a terrestrial meteorite with the evidence of reducing conditions introduced during its incorporation into the Imbrium ejecta and host breccia.

¹Fang Huang,¹Chen Zhou,²Wenzhong Wang, ¹Jinting Kang,²Zhongqing Wu
Earth and Planetary Science Letters 510, 153-160 Link to Article [https://doi.org/10.1016/j.epsl.2018.12.034]
¹CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China
²Laboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China
Copyright Elsevier

Calcium is a major element of the Earth, the Moon, terrestrial planets, and rocky meteorites. Here we present equilibrium Ca isotope fractionation factors of Ca-bearing minerals using the first-principles calculations based on density functional theory (DFT). The sequence of minerals from the isotopically heaviest to the lightest in Ca is forsterite > orthopyroxene (opx) > grossular ∼ pigeonite > diopside > anorthite > oldhamite. Overall, the equilibrium fractionation of Ca isotopes is mainly controlled by the average bond lengths. Although oldhamite is enriched in light Ca isotopes relative to silicate minerals in equilibrium, natural oldhamite of enstatite chondrites are isotopically heavier than coexisting silicate materials. This implies that enstatite chondrites oldhamites should have been formed during solar nebular gas condensation instead than during parent body processing.
Following previous models for crystallization of the Lunar Magma Ocean (LMO), we simulated Ca isotopic fractionation of the LMO based on our calculated equilibrium Ca isotope fractionation factors. It shows that the δ44/40Ca of the lunar anorthositic crust should be lower than the average of the bulk Moon by 0.09–0.11‰. Considering that the lunar mantle might have overturned and mixed after solidification of the LMO, we further predict that the lunar mantle should be isotopically heavier than the bulk Moon by 0.17–0.26‰ if the mantle was fully overturned, or only by 0.06–0.08‰ for the case of fully mixing. Therefore, we predict that the potential offset of Ca isotopic composition between the anorthositic crust and the lunar mantle can be used to test LMO evolution models.

¹Romain Tartèse,^2,3Mahesh Anand,²Ian A.Franchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.01.024]
¹School of Earth and Environmental Sciences, The University of Manchester, Manchester, M13 9PL, UK
²School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
³Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK
Copyright Elsevier

The paired achondrites Graves Nunataks (GRA) 06128 and 06129 are samples of an asteroid that underwent partial melting within a few million years after the start of Solar System formation. In order to better constrain the origin and processing of volatiles in the early Solar System, we have investigated the abundance of H, F and Cl and the isotopic composition of H and Cl in phosphates in GRA 06128 using secondary ion mass spectrometry. Indigenous H in GRA 06128, as recorded in magmatic merrillite, is characterised by an average δD of ca. -152 ± 330‰, which is broadly similar to estimates of the H isotope composition of indigenous H in other differentiated asteroidal and planetary bodies such as Mars, the Moon and the angrite and eucrite meteorite parent bodies. The merrillite data thus suggest that early accretion of locally-derived volatiles was widespread for the bodies currently populating the asteroid belt. Apatite formed at the expense of merrillite around 100 million years after the differentiation of the GRA 06128/9 parent body, during hydrothermal alteration, which was probably triggered by an impact event. Apatite in GRA 06128 contains 5.4-5.7 wt.% Cl, 0.6-0.8 wt.% F, and ∼20 to 60 ppm H₂O, which is similar to the H₂O abundance in merrillite from which apatite formed. The apatite δD values range between around +100‰ and +2000‰ and are inversely correlated with apatite H₂O contents. The Cl isotope composition of apatite appears to be homogeneous across various grains, with an average δ³⁷Cl value of 3.2 ± 0.7‰. A possible scenario to account for the apatite chemical and isotopic characteristics involves interaction of GRA 06128/9 with fumarole-like fluids derived from D- and HCl-rich ices delivered to the GRA 06128/9 parent-body by an ice-rich impactor.