¹D.P. Moriarty III,²C. M. Pieters
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005364]
¹Planetary Geology, Geophysics, and Geochemistry Laboratory, NASA GSFC, Greenbelt, MD
²Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI
Published by arrangement with John Wiley & Sons

Using Moon Mineralogy Mapper data, we characterize surface diversity across the enormous South Pole – Aitken Basin (SPA) by evaluating the abundance and composition of pyroxenes, which are overwhelmingly the most abundant mafic mineral in the region. Although SPA exhibits significant complexity due to billions of years of geologic processes subsequent to formation, the basin has retained regular patterns of compositional heterogeneity across its structure. Four distinct, roughly concentric zones are defined: (1) a central SPA compositional anomaly (SPACA), which exhibits a pervasive elevated Ca,Fe-rich pyroxene abundance, (2) a Mg-Pyroxene Annulus, which is dominated by abundant Mg-rich pyroxenes, (3) a Heterogeneous Annulus, which exhibits localized pyroxene-rich areas spatially mixed with feldspathic materials, and (4) the SPA Exterior, which is primarily feldspathic. Pyroxene compositions in the Heterogeneous Annulus are similar to those in the Mg-Pyroxene Annulus, and Mg-rich pyroxenes also underlie the more Ca,Fe-rich pyroxene surface material across SPACA. The establishment of these four distinct compositional zones across SPA constrains future basin evolution models serves to guide potential sample return (and other science) targets.

¹M. Boyet, ²A. Bouvier, ¹P. Frossard, ¹T. Hammouda, ³M. Garçon, ¹A. Gannoun
Earth and Planetary Science Letters 488, 68-78 Link to Article [https://doi.org/10.1016/j.epsl.2018.02.004https://doi.org/10.1016/j.epsl.2018.02.004]
¹Laboratoire Magmas et Volcans, Université Clermont Auvergne, France
²Department of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, London, Canada
³Department of Earth Sciences, ETH Zurich, Switzerland
Copyright Elsevier

The 146Sm–142Nd extinct decay scheme (146Sm half-life of 103 My) is a powerful tool to trace early Earth silicate differentiation. Differences in 142Nd abundance measured between different chondrite meteorite groups and the modern Earth challenges the interpretation of the 142Nd isotopic variations found in terrestrial samples because the origin of the Earth and the nature of its building blocks is still an ongoing debate. As bulk meteorites, the enstatite chondrites (EC) have isotope signatures that are the closest to the Earth value with an average small deficit of ∼10 ppm in 142Nd relative to modern terrestrial samples. Here we review all the Nd isotope data measured on EC so far, and present the first measurements on an observed meteorite fall Almahata Sitta containing pristine fragments of an unmetamorphosed enstatite chondrite belonging to the EL3 subgroup. Once 142Nd/144Nd ratios are normalized to a common chondritic evolution, samples from the EC group (both EL and EH) have a deficit in 142Nd but the dispersion is important (μ142Nd=−10±12 (2SD) ppm). This scatter reflects their unique mineralogy associated to their formation in reduced conditions (low fO2 or high C/O). Rare-earth elements are mainly carried by the sulfide phase oldhamite (CaS) that is more easily altered than silicates by weathering since most of the EC meteorites are desert finds. The EL6 have fractionated rare-earth element patterns with depletion in the most incompatible elements. Deviations in Nd mass independent stable isotope ratios in enstatite chondrites relative to terrestrial standard are not resolved with the level of analytical precision achieved by modern mass spectrometry techniques. Here we show that enstatite chondrites from the EL3 and EL6 subgroups may come from different parent bodies. Samples from the EL3 subgroup have Nd (μ142Nd=−0.8±7.0, 2SD) and Ru isotope ratios undistinguishable from that of the Bulk Silicate Earth. EL3 samples have never been analyzed for Mo isotopes. Because these enstatite chondrites are relatively small in size and number, they are usually not available for destructive isotopic measurements. Average values based on the measurement of EL6 samples should not be considered as representative of the whole EL group because of melting and thermal metamorphism events affecting the Sm/Nd ratios and prolonged open-system history. The EL3 chondrites are the best candidates as the Earth’s building blocks. These new results remove the need to change the composition of refractory incompatible elements early in Earth’s history.