^1,2P. Bonnand, ^1,3A. N. Halliday
Nature Geoscience (in Press) Link to Article [doi:10.1038/s41561-018-0128-2]
¹Department of Earth Sciences, University of Oxford, Oxford, UK
²Laboratoire Magmas et Volcans, Université Clermont Auvergne, CNRS, Clermont-Ferrand, France
³The Earth Institute, Columbia University, New York, NY, USA

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¹Yuki Hibiya et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.04.031]
¹Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
Copyright Elsevier

Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and 187Re–187Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En55-57Wo3-4Fs40-42; Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa50-53; Fe/Mn = 1.1–2.1), chromite (Cr# ∼ 0.94), awaruite, sulfides, plagioclase (Ab92An5Or3) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1–102°C/hr), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high super-saturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re–Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O–Ti isotope systematics (Δ17O = −1.052 ± 0.002, 1 SD; ε50Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = −2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

^1,2C.K. Shearer, ³S. Messenger, ²Z.D. Sharp, ¹P.V. Burger,^3,4 A.N. Nguyen, ³F.M. McCubbin 
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.04.034]
¹Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131
²Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131
³NASA Johnson Space Center, Mailcode XI, 2101 NASA Parkway, Houston, Texas 77058
⁴Jacobs, NASA Johnson Space Center, Houston, Texas 77058
Copyright Elsevier

The δ37Cl from different generations of apatite in martian meteorite Chassigny has a range of ≈10‰ and is almost as great as measurements made on all martian meteorites (≈14‰). This range represents the mixing of distinct Cl isotope reservoirs during the formation of Chassigny: (1) an isotopically light-Cl mantle reservoir (δ37Cl=-4 to -6‰) that exhibits limited variability and (2) an isotopically heavy Cl crustal reservoir (δ37Cl>0) that exhibits significant variability. The mantle component documented in Chassigny melt inclusions that host a solar noble gas composition are derived from pristine, martian mantle. The incompatible element depleted and enriched shergottite sources as defined by radiogenic isotope systematics and trace element concentration ratios have very similar Cl isotopic signatures and suggest that both are derived from the martian mantle. The enrichment of isotopically heavy Cl in the crust resulted from protracted loss of 35Cl to space that started early in the history of Mars. The Cl isotopic signature of the martian mantle is different from the Earth, Moon, and many primitive meteorites (δ37Cl=0), suggesting that these differences represent distinct Cl sources in the solar nebula. The low δ37Cl source represents the primordial Solar System composition from which Mars accreted. The higher δ37Cl values observed for the Earth, Moon, and many chondrites are not primordial, rather they represent the later incorporation of 37Cl-enriched HCl-hydrates into accreting material.