^1,2Martin Schmieder,²Fred Jourdan,^1,Eric Tohver,³Edward A. Cloutis
¹School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
²Western Australian Argon Isotope Facility, Department of Applied Geology and JdL Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
³Planetary Spectrophotometer Facility, Department of Geography, University of Winnipeg, Canada

New 40Ar/39Ar dating of impact-melted K-feldspars and impact melt rock from the ∼40 km Lake Saint Martin impact structure in Manitoba, Canada, yielded three plateau ages and one mini-plateau age in agreement with inverse isochron ages for the K-feldspar melt aliquots and a minimum age for a whole-rock impact melt sample. A combination of two plateau ages and one isochron age, with a weighted mean of 227.8±0.9 Ma227.8±0.9 Ma [±1.1 Ma; including all sources of uncertainty] (2σ ; MSWD = 0.52; P=0.59P=0.59), is considered to represent the best-estimate age for the impact. The concordant 40Ar/39Ar ages for the melted K-feldspars, derived from impact melt rocks in the eastern crater moat domain and the partially melted Proterozoic central uplift granite, suggest that the new dates accurately reflect the Lake Saint Martin impact event in the Carnian stage of the Late Triassic. With a relative error of ±0.4% on the 40Ar/39Ar age, the Lake Saint Martin impact structure counts among the most precisely dated impact structures on Earth. The new isotopic age for Lake Saint Martin significantly improves upon earlier Rb/Sr and (U–Th)/He results for this impact structure and contradicts the hypothesis that planet Earth experienced the formation of a giant ‘impact crater chain’ during a major Late Triassic multiple impact event.

Reference
Schmieder M, Jourdan F, Tohver E, Cloutis EA (2014) 40Ar/39Ar age of the Lake Saint Martin impact structure (Canada) – Unchaining the Late Triassic terrestrial impact craters. Earth and Planetary Science Letters (in Press)
Link to Article [DOI: 10.1016/j.epsl.2014.08.037]

Copyright Elsevier

^1,3W.M. Farrell,^2,3D.M. Hurley,^2,3M.I. Zimmerman
¹NASA/Goddard Space Flight Center, Greenbelt, MD
²Johns Hopkins University/Applied Physics Laboratory, Laurel, MD
³NASA’s Solar System Exploration Research Virtual Institute, NASA/Ames Research Center, Moffett Field, CA, 08101414

Solar wind protons are implanted directly into the top 100 nanometers of the lunar near-surface region, but can either quickly diffuse out of the surface or be retained, depending upon surface temperature and the activation energy, U, associated with the implantation site. In this work, we explore the distribution of activation energies upon implantation and the associated hydrogen-retention times; this for comparison with recent observation of OH on the lunar surface. We apply a Monte Carlo approach: for simulated solar wind protons at a given local time, we assume a distribution of U values with a central peak, Uc and width, Uw, and derive the fraction retained for long periods in the near-surface. We find that surfaces characterized by a distribution with predominantly large values of U (> 1 eV) like that expected at defect sites will retain implanted Hs (to likely form OH). Surfaces with the distribution predominantly at small values of U (< 0.2 eV) will quickly diffuse away implanted Hs. However, surfaces with a large portion of activation energies between 0.3 eV < U < 0.9 eV will tend to be H-retentive in cool conditions but transform into H-emissive surfaces when warmed (as when the surface rotates into local noon). These mid-range activation energies give rise to a diurnal effect with diffusive loss of H at noontime.

Reference
Farrell WM,Hurley DM, Zimmerman MI (2014) Solar wind implantation into lunar regolith: Hydrogen retention in a surface with defects. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.09.014]

Copyright Elsevier

¹Maud Boyet,²Richard W. Carlson,³Lars E. Borg,²Mary Horan
¹Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, UMR 6524, 5 rue Kessler 63038 Clermont-Ferrand, France
²Department of Terrestrial Magnetism 5241 Broad Branch Road, NW Washington, DC 20015-1305 USA
³Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L-231, Livermore CA 94550 USA

We have measured Sm-Nd systematics, including the short-lived 146Sm-142Nd chronometer, in lunar ferroan anorthositic suite (FAS) whole rocks (15415, 62236, 62255, 65315, 60025). At least some members of the suite are thought to be primary crystallization products formed by plagioclase flotation during crystallization of the lunar magma ocean (LMO). Most of these samples, except 62236, have not been exposed to galactic cosmic rays for a long period and thus require minimal correction to their 142Nd isotope composition. These samples all have measured deficits in 142Nd relative to the JNdi-1 terrestrial standard in the range -45 to -21 ppm. The range is -45 to -15 ppm once the 62236 142Nd/144Nd ratio is corrected from neutron-capture effects. Analyzed FAS samples do not define a single isochron in either 146Sm-142Nd or 147Sm-143Nd systematics, suggesting that they either do not have the same crystallization age, come from different sources, or have suffered isotopic disturbance. Because the age is not known for some samples, we explore the implications of their initial isotopic compositions for crystallization ages in the range of 50-300 Ma after the beginning of accretion, a timing interval that covers all the ages determined for the ferroan anorthositic suite whole rocks as well as different estimates for the crystallization of the LMO. 62255 has the largest deficit in initial 142Nd and does not appear to have followed the same differentiation path as the other FAS samples. The large deficit in 142Nd of FAN 62255 may suggest a crystallization age around 60-125 Ma after the beginning of solar system accretion. This result provides essential information about the age of the giant impact forming the Moon. The initial Nd isotopic compositions of FAS samples can be matched either with a bulk-Moon with chondritic Sm/Nd ratio but enstatite-chondrite-like initial 142Nd/144Nd (e.g. 10 ppm below modern terrestrial), or a bulk-Moon with superchondritic Sm/Nd ratio and initial 142Nd/144Nd similar to ordinary chondrites.

Reference
Boyet M, Carlson RW, Borg LE, Horan M (2014) Sm-Nd systematics of lunar ferroan anorthositic suite rocks: Constraints on lunar crust Formation. Geochimica et Cosmochimica Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.09.021]

Copyright Elsevier