¹F. N. Lindsay et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13637]
¹Department of Chemistry & Chemical Biology, Rutgers University, New Brunswick, New Jersey, 08854 USA
Published by arrangement with John Wiley & Sons

The Martian breccias NWA 7034, NWA 7533, and paired meteorites record events ranging in age from 4.47 Ga to <200 Ma. Published ages indicate a period of major disturbance at ~1.4 Ga, examined in detail here through 40Ar/39Ar dating of handpicked grains and two small chips. Argon diffusion parameters were obtained for six samples. Also presented are He, Ne, Ar, Kr, and Xe contents of two small (<100 µg), handpicked mineral separates, a felsic “Light” sample and a mafic/pyroxene‐rich “Dark” sample. The 40Ar/39Ar ages of five samples, four containing >1 wt% K and thought to be rich in feldspar and one containing <~1 wt% K, cluster near 1.4 Ga. The 40Ar/39Ar ages of nine grains with low K contents have a wide range of apparent ages from 0.3 ± 0.1 Ga to 2.9 ± 0.1 Ga for individual temperature steps, and from 0.74 ± 0.06 Ga to ~2.1 Ga for plateau ages. Isochron ages are less precise, but generally agree with plateau ages. Only two isochrons have the significantly positive intercepts expected in the presence of terrestrial or Martian atmospheric argon. At higher release temperatures, activation energies for diffusion obtained from 39Ar data for six samples are generally 160–200 kJ mol−1, consistent with published values for feldspathic minerals. For three of these samples, lower temperature data on Arrhenius plots are best fit with a much lower activation energy of <100 kJ mol−1. We attribute the low values to the effects of varying degrees of shock on feldspathic minerals and/or the presence of phases in vitrophyric spherules produced by hydrothermal alteration. The low activation energies place an upper limit of ~14 ka on the terrestrial age of NWA 7034. Much lower concentrations of cosmogenic (c) 3He and 21Ne in the Light than in the Dark separate indicate substantial losses concurrent with or postdating cosmic ray irradiation. A one‐stage, cosmic ray exposure (CRE) age for the Dark separate from NWA 7034 is estimated to be between 7 and 10 Ma from the concentrations of 3Hec and 38Arc, and of close to 15 Ma from the concentration of 21Nec. Most of the 40Ar/39Ar and noble gas data are compatible with (1) a heating and alteration event ~1.40 Ga caused by contact metamorphism, an impact, and/or the infiltration of hydrothermal fluids; and (2) at least one later event at lower temperatures that led to either loss of He and Ar from phases with low activation energies, or to gain of K. Most of the 40Ar/39Ar ages are consistent with the assembly of NWA 7034 1.4 Ga ago or perhaps earlier followed more recently by selective alteration. A more recent time of assembly is also consistent with these ages provided that the temperature stayed low. The five most precise 40Ar/39Ar ages of the samples analyzed are all ~1.4 Ga, a value seen frequently in other NWA 7034 chronometers and very similar to crystallization ages of nakhlites and chassignites (NC). Some CRE ages based on noble gases in NWA 7034 agree within their considerable uncertainties with those of NC. These two chronometric coincidences suggest that the NWA 7034 clan and the NC share a launch date on Mars. We propose that K‐rich fluids derived from the nakhlite source area interacted with proto‐NWA 7034 and modified the K/Ar ratios and ages of previously shocked feldspar grains, with the degree of modification depending on the degree of shock. The NWA 7034 clan may therefore be considered components from a metamorphic aureole around a nakhlite massif.

^1,2F. Jourdan,^2,3L. Forman,^1,2T. Kennedy,¹G. K. Benedix,⁴E. Eroglu,¹C. Mayers
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13640]
¹Western Australian Argon Isotope Facility, John de Laeter Centre, TIGeR, Curtin University, Perth, 6845 Western Australia, Australia
²Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Perth, 6845 Western Australia, Australia
³Department of Earth & Planetary Sciences, Western Australian Museum, Locked Bag 49, Welshpool DC, Perth, 6986 Western Australia, Australia
⁴School of Molecular and Life Sciences, Curtin University, Perth, 6845 Western Australia, Australia
Published by arrangement with John Wiley & Sons

Asteroid 4 Vesta is the only largely preserved differentiated asteroid and thus is an excellent proxy to study early magmatism occurring on planets and moons. In this study, we focus on eucrite Pecora Escarpment (PCA) 82502, a medium‐ to fine‐grained eucrite which chemical analyses suggest belongs to the main howardite–eucrite–diogenite clan, albeit with some peculiarities. We carried out backscattered electron and electron backscattered diffraction microscopy analyses of the meteorite along with step‐heating 40Ar/39Ar dating analyses of various types of groundmass aliquots. We show that Pecora Escarpment 82502 is composed of medium‐grained igneous crystalline clasts and smaller fractured satellite clasts surrounded by approximately 50 µm wide impact melt veins of the same composition. Our results show that the large crystalline clasts and fine‐grained veins both display little evidence of shock processes. Six groundmass aliquots from large crystalline clasts returned concordant plateau (>70% of 39Ar) or mini‐plateau (50–70% of 39Ar) 40Ar/39Ar ages with a weighted mean of 4531 ± 6 Ma (P = 0.67). Thermodynamic cooling and 40Ar diffusion models suggest that the K/Ar system recorded and preserved the igneous age despite subsequent infiltration of hot and quickly quenched melt veins. Our new igneous age, combined with evidence for four other young volcanic and plutonic eucrites of similar age, shows that Vesta was still magmatically active around 4531 Ma. The lack of younger ages suggests that this age might well represent the end of the magmatic activity in the upper crust of Vesta. When combined with existing paleomagnetic constraints, our data suggest that 4 Vesta had an active dynamo that was still active ~35 Ma after accretion.