^1,2Steven J.Jaret, ^2,3Sidney R.Hemming, ¹E. Troy Rasbury, ⁴Lucy M.Thompson, ¹Timothy D.Glotch, ⁵Jahandar Ramezani, ⁴John G.Spray
Earth and Planetary Science Letters 501, 78-89 Link to Article [https://doi.org/10.1016/j.epsl.2018.08.016]
¹Department of Geosciences, Stony Brook University, Stony Brook, NY 11794, USA
²Lamont-Doherty Earth Observatory, Palisades, NY 10964, USA
³Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA
⁴Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada
⁵Department of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Copyright Elsevier

As an analog for interpretations of the ages of martian shergottite meteorites, we have conducted an argon isotopic study of plagioclase feldspars exhibiting varying levels of shock from in and around the Manicouagan impact structure, Canada. Plagioclase from the impact melt sheet at Manicouagan yields an age of 215.40 ± 0.16 Ma, which indicates the time of impact. Plagioclase from a clast within melt-bearing breccias of the melt sheet and a hornfels adjacent to the melt sheet yield ages of 216 ± 3 Ma and 218 ± 7 Ma, respectively, which are interpreted to have been reset by contact metamorphism from the impact melt. Country rocks that were unaffected by the impact gives ∼849 Ma ages, consistent with the known Grenvillian target rock history. Maskelynite (amorphous plagioclase, which has been transformed in the solid state) yields an age of 567 ± 6 Ma. This age is geologically meaningless because it is not consistent with the target age, the impact age, or regional metamorphic ages at Manicouagan. Our results show that maskelynite argon ages are not meaningful, and that context is critical for proper interpretation of impact-affected argon ages.

¹Jeremy W.Boyce, ¹Sarah A.Kanee, ¹Francis M.McCubbin, ¹Jessica J.Barnes, ²Hayley Bricker, ³Allan H.Treiman
Earth and Planetary Science Letters 500, 205-214 Link to Article [https://doi.org/10.1016/j.epsl.2018.07.042]
¹NASA – Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, United States of America
²Department of Physics and Astronomy, UCLA, 475 Portola Plaza, Los Angeles, CA, 90095-1547, United States of America
³Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, United States of America
Copyright Elsevier

The relative abundances of chlorine isotopes measured in low-Ti basalts from the Moon appear to reflect mixing between two reservoirs: One component representing the urKREEP—the final product of the crystallization of the lunar magma ocean—with δ37Cl=+25‰(relative to Standard Mean Ocean Chlorine), the other representing either a mare basalt reservoir or meteoritic materials with
δ37Cl∼0‰. Using the abundances of other KREEP-enriched elements as proxies for the abundance of Cl in low-Ti mare basalts—which is difficult to constrain due to magmatic processes such as fractional crystallization and degassing—we find that the urKREEP contains ∼28 times higher Cl abundance (25–170 ppm Cl) as compared to the low-δ37 Cl end member in the observed mixing relationship. Chlorine—with an urKREEP/C.I. ratio of 0.2 to 1.5—is 500 to 3400 times less enriched than refractory incompatibles such as U and Th, and is consistent with incomplete loss of Cl species taking place during or prior to the magma ocean phase. The preservation of multiple, isotopically distinct reservoirs of Cl can be explained by: 1) Incomplete degassing pre- or syn-giant impact, with preservation of undegassed chondritic Cl and subsequent formation of an enriched and isotopically fractionated reservoir; or 2) Development of both high-concentration, high-δ37Cland low-concentration, low-δ37Cl reservoirs during the degassing and crystallization of the lunar magma ocean. A range of model bulk lunar Cl abundances from 0.3–0.6 ppm allows us to place Cl in the context of the rest of the elements of the periodic table, and suggests that Cl behaves as only a moderately volatile element during degassing. Chlorine isotope fractionation resulting from loss syn- or pre-magma ocean is characterized by 1000•ln⁡[α]=−3.96 to −4.04. Abundance and isotopic constraints are consistent with the loss of Cl being limited by vaporization of mixtures of Cl salts such as HCl, ZnCl2, FeCl2, and NaCl. These new constraints on the chlorine abundance and isotopic values of urKREEP make it a well-constrained target for dynamic models aiming to test plausible conditions for the formation of the Earth–Moon system.

¹Jean-Pierre Lorand, ^2,3R.H.Hewins, ⁴M.Humayun,²L.Remusat, ²B.Zanda, ¹C.La, ²S.Pont
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.08.041]
¹Laboratoire de Planétologie et Géodynamique à Nantes, CNRS UMR 6112, Université de Nantes, 2 Rue de la Houssinère, BP 92208, 44322 Nantes Cédex 3, France
²Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC) – Sorbonne Université- Muséum National d’Histoire Naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD UMR 206, 61 rue Buffon, 75005 Paris, France
³Department of Earth & Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA
⁴Department of Earth, Ocean & Atmospheric Science and National High Magnetic, Field Laboratory, Florida State University, Tallahassee, FL 32310, USA
Copyright Elsevier

Unlike other martian meteorites studied so far, Martian regolith breccia NWA 7533 and paired meteorites that have sampled 4.4 Ga-old impact lithologies show only sulfides of hydrothermal origin (mostly pyrite (<1 vol.%) and scarce pyrrhotite). NWA 7533 pyrite has been analyzed for 25 chalcophile-siderophile trace elements with laser ablation-inductively coupled plasma mass spectrometer (LA-ICPMS). Micronuggets of highly siderophile elements-HSE (Os, Ir, Pt, Ru, Rh) along with occasional detection of Mo and Re were observed in half of the 52 analyzed crystals as random concentration spikes in time-resolved LA-ICPMS data. These nuggets are interpreted as variably altered remnants from repeated meteorite bombardment of the early martian crust, as are chondritic Ni/Co ratios of pyrite (10-20). Pyrite displays superchondritic S/Se (54,000 to 3,300) and Te/Se (0.3 – >1). The reasonably good positive correlation (R²=0.72) between Se and Ni reflects a temperature control on the solubility of both elements. Apart from the chalcogens S, Se and Te, pyrite appears to be a minor contributor (<20%) to the whole-rock budget for both HSE (including Ni and Co) and chalcophile metals Ag, As, Au, Cu, Hg, Pb, Sb, Tl and Zn. This deficit can result from i) high (>400°C) temperature crystallization for NWA 7533 pyrite, as deduced from its Se and Ni contents, ii) magmatic sulfide-depletion of brecciated early martian crust, iii) precipitation from near neutral H₂S-HS-H₂O-rich hydrothermal fluids that did not provide halogen ligands for extensive transport of chalcophile-siderophile metals. It is suggested that the 1.4 Ga lithification event that precipitated hydrothermal pyrite left the chalcophile-siderophile element budget of the early martian crust nearly unmodified, except for S, Se and Te.