¹S. Yamamoto et al. (>10)*
¹Center for Environmental Measurement and Analysis, National Institute for Environmental Studies, Tsukuba, Japan
*Find the extensive, full author and affiliation list on the publishers website

We report the global distribution of areas exhibiting no absorption features (featureless or FL) on the lunar surface, based on the reflectance spectral data set obtained by the Spectral Profiler onboard Kaguya/SELENE. We found that FL sites are located in impact basins and large impact craters in the Feldspathic Highlands Terrane (FHT), while there are no FL sites in the Procellarum regions nor the South Pole–Aitken basin. FL sites in each impact basin/crater are mainly found at the peak rings or rims, where the purest anorthosite (PAN) sites are also found. At the local scale, most of the FL and PAN points are associated with impact craters and peaks. Most of the FL spectra show a steeper (redder) continuum than the PAN spectra, suggesting the occurrence of space weathering effects. We propose that most of the material exhibiting a FL spectrum originate from space weathered PAN. Taking into account all the occurrence trends of FL sites on the Moon, we propose that both the FL and PAN materials were excavated from the primordial lunar crust during ancient basin formations below the megaregolith in the highlands. Since the FL and PAN sites are widely distributed over the lunar surface, our new data may support the existence of a massive PAN layer below the lunar surface.

Reference
Yamamoto S et al.(2015) Featureless spectra on the Moon as evidence of residual lunar primordial crust. Journal of Geophysical Research, Planets (in Press)
Link to Article [doi: 10.1002/2015JE004935]
Published by arrangement with John Wiley&Sons

^1,2J. Labidi, ¹A. Shahar, ¹C. Le Losq, ¹V.J. Hillgren, ¹B.O. Mysen, ²J. Farquhar
¹Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, USA.
²Department of Geology, University of Maryland, College Park MD, 20740, USA

The Earth’s mantle displays a subchondritic 34S/32S ratio. Sulfur is a moderately siderophile element (i.e. iron-loving), and its partitioning into the Earth’s core may have left such a distinctive isotope composition on the terrestrial mantle. In order to constrain the sulfur isotope fractionation occurring during core-mantle differentiation, high-pressure and temperature experiments were conducted with synthetic mixtures of metal and silicate melts. With the purpose to identify the mechanism(s) responsible for the S isotope fractionations, we performed our experiments in different capsules – namely, graphite and boron nitride capsules – and thus at different fO2, with varying major element chemistry of the silicate and metal fractions.

The S isotope fractionations Δ34Smetal-silicate of equilibrated metal alloys versus silicate melts is +0.2±0.1‰ in a boron-free and aluminum-poor system quenched at 1-1.5 GPa and 1650 ˚C. The isotope fractionation increases linearly with increasing boron and aluminum content, up to +1.4±0.2‰, and is observed to be independent of the silicon abundance as well as of the fO2 over ∼ 3.5 log units of variations explored here. The isotope fractionations are also independent of the graphite or nitride saturation of the metal. Only the melt structural changes associated with aluminum and boron concentration in silicate melts have been observed to affect the strength of sulfur bonding. These results establish that the structure of silicate melts has a direct influence on the S2- average bonding strengths.

These results can be interpreted in the context of planetary differentiation. Indeed, the structural environments of silicate evolve strongly with pressure. For example, the aluminum, iron or silicon coordination numbers increase under the effect of pressure. Consequently, based on our observations, the sulfur-bonding environment is likely to be affected. In this scheme, we tentatively hypothesize that S isotope fractionations between the silicate mantle and metallic core of terrestrial planetary bodies would depend on the average pressure at which their core-mantle differentiation occurred.

Reference
Labidi J, Shahara A, Le Losq C, Hillgren VJ, Mysen BO, Farquhar J (2015) Experimentally determined sulfur isotope fractionation between metal and silicate and implications for planetary Differentiation. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.12.001]
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