Stephen M. Elardo^1,*, Charles K. Shearer Jr.¹, Amy L. Fagan^2,3, Lars E. Borg⁴, Amy M. Gaffney⁴, Paul V. Burger¹, Clive R. Neal², Vera A. Fernandes⁵, Francis M. McCubbin¹

¹Department of Earth & Planetary Sciences, Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, USA
²Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, Notre Dame, Indiana, USA
³Lunar & Planetary Institute, USRA, Houston, Texas, USA
⁴Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, California, USA
⁵Museum für Naturkunde- Berlin, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany

Northwest Africa (NWA) 4734 is an unbrecciated basaltic lunar meteorite that is nearly identical in chemical composition to basaltic lunar meteorites NWA 032 and LaPaz Icefield (LAP) 02205. We have conducted a geochemical, petrologic, mineralogic, and Sm-Nd, Rb-Sr, and Ar-Ar isotopic study of these meteorites to constrain their petrologic relationships and the origin of young mare basalts. NWA 4734 is a low-Ti mare basalt with a low Mg* (36.5) and elevated abundances of incompatible trace elements (e.g., 2.00 ppm Th). The Sm-Nd isotope system dates NWA 4734 with an isochron age of 3024 ± 27 Ma, an initial ε_Nd of +0.88 ± 0.20, and a source region ¹⁴⁷Sm/¹⁴⁴Nd of 0.201 ± 0.001. The crystallization age of NWA 4734 is concordant with those of LAP 02205 and NWA 032. NWA 4734 and LAP 02205 have very similar bulk compositions, mineral compositions, textures, and ages. Their source region ¹⁴⁷Sm/¹⁴⁴Nd values indicate that they are derived from similar, but distinct, source materials. They probably do not sample the same lava flow, but rather are similarly sourced, but isotopically distinct, lavas that probably originate from the same volcanic complex. They may have experienced slightly different assimilation histories in route to eruption, but can be source-crater paired. NWA 032 remains enigmatic, as its source region ¹⁴⁷Sm/¹⁴⁴Nd definitively precludes a simple relationship with NWA 4734 and LAP 02205, despite a similar bulk composition. Their high Ti/Sm, low (La/Yb)_N, and Cl-poor apatite compositions rule out the direct involvement of KREEP. Rather, they are consistent with low-degree partial melting of late-formed LMO cumulates, and indicate that the geochemical characteristics attributed to urKREEP are not unique to that reservoir. These and other basaltic meteorites indicate that the youngest mare basalts originate from multiple sources, and suggest that KREEP is not a prerequisite for the most recent known melting in the Moon.

Reference
Elardo SM, Shearer CK, Fagan AL, Borg LE, Gaffney AM, Burger PV, Neal CR, Fernandes VA and McCubbin FM (in press) The origin of young mare basalts inferred from lunar meteorites Northwest Africa 4734, 032, and LaPaz Icefield 02205. Meteoritics & Planetary Science
[doi:10.1111/maps.12239]
Published by arrangement with John Wiley & Sons

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Ann M. Ollila¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Princeton University, Princeton, NJ 08540, USA

The ChemCam instrument package on the Mars rover, Curiosity, provides new capabilities to probe the abundances of certain trace elements in the rocks and soils on Mars using the laser-induced breakdown spectroscopy technique. We focus on detecting and quantifying Li, Ba, Rb, and Sr in targets analyzed during the first 100 sols, from Bradbury Landing Site to Rocknest. Univariate peak area models and multivariate partial least squares models are presented. Li, detected for the first time directly on Mars, is generally low (<15 ppm). The lack of soil enrichment in Li, which is highly fluid mobile, is consistent with limited influx of subsurface waters contributing to the upper soils. Localized enrichments of up to ~60 ppm Li have been observed in several rocks but the host mineral for Li is unclear. Bathurst_Inlet is a fine-grained bedrock unit in which several analysis locations show a decrease in Li and other alkalis with depth, which may imply that the unit has undergone low-level aqueous alteration that has preferentially drawn the alkalis to the surface. Ba (~1000 ppm) was detected in a buried pebble in the Akaitcho sand ripple and it appears to correlate with Si, Al, Na, and K, indicating a possible feldspathic composition. Rb and Sr are in the conglomerate Link at abundances >100 ppm and >1000 ppm, respectively. These analysis locations tend to have high Si and alkali abundances, consistent with a feldspar composition. Together, these trace element observations provide possible evidence of magma differentiation and aqueous alteration.

Reference
Ann M. Ollila et al. (in press) Trace element geochemistry (Li, Ba, Sr, and Rb) using Curiosity’s ChemCam: Early results for Gale crater from Bradbury Landing Site to Rocknest. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004517]
Published by arrangement with John Wiley & Sons

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Nancy L. Chabot^a, E. Alex Wollack^b, Rachel L. Klima^a and Michelle E. Minitti^a

^aThe Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
^bPrinceton University, Princeton, NJ 08540, USA

The recent discovery of high S concentrations on the surface of Mercury by spacecraft measurements from the MESSENGER mission provides the potential to place new constraints on the composition of Mercuryʼs large metallic core. In this work, we conducted a set of systematic equilibrium metal–silicate experiments that determined the effect of different metallic compositions in the Fe–S–Si system on the S concentration in the coexisting silicate melt. We find that metallic melts with a range of S and Si combinations can be in equilibrium with silicate melts with S contents consistent with Mercuryʼs surface, but that such silicate melts contain Fe contents lower than measured for Mercuryʼs surface. If Mercuryʼs surface S abundance is representative of the planetʼs bulk silicate composition and if the planet experienced metal–silicate equilibrium during planetary core formation, then these results place boundaries on the range of possible combinations of Si and S that could be present as the light elements in Mercuryʼs core and suggest that Mercuryʼs core likely contains Si. Except for core compositions with extreme abundances of Si, bulk Mercury compositions calculated by using the newly determined range of potential S and Si core compositions do not resemble primitive meteorite compositions.

Reference
Chabot NL, Wollack EA, Klima RL and Minitti ME (2014) Experimental constraints on Mercuryʼs core composition. Earth and Planetary Science Letters 390:199–208.
[doi:10.1016/j.epsl.2014.01.004]
Copyright Elsevier

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