David A. McKeown^1,*, Andrew C. Buechele¹, Ryan Tappero², Timothy J. McCoy³ and Kathryn G. Gardner-Vandy³

¹Vitreous State Laboratory, The Catholic University of America, 620 Michigan Avenue NE, Washington, D.C. 20064, U.S.A.
²Photon Sciences Department, Brookhaven National Laboratory, Upton, New York 11793, U.S.A.
³Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560-0119, U.S.A.

Chromium K-edge X-ray absorption spectra were collected to characterize Cr in forsterite (Mg₂SiO₄) as well as sulfides within the MAC 88136 EL3 chondrite to determine Cr valence and to see whether forsterite within this meteorite can be used as a Cr²⁺-silicate standard. Spectra were measured on several areas within a nearly pure 100 × 200 μm forsterite grain containing 0.13 wt% Cr. XANES findings indicate highly reduced Cr²⁺ species, with no clear evidence of Cr³⁺ or Cr⁶⁺. EXAFS data indicate an average 2.02 Å Cr-O nearest-neighbor distance, consistent with Cr-O distances found in square-planar Cr²⁺O₄ sites observed in synthetic crystalline silicates, and an average 2.69 Å Cr-Si second-nearest neighbor distance, consistent with Cr²⁺substituting for Mg²⁺ in the forsterite M(1) site. Nearest-neighbor Debye-Waller factor and coordination number parameters indicate Cr²⁺ is likely entering forsterite in disordered sites that are possible intermediates between M(1) and square-planar Cr²⁺O₄ configurations. Preliminary Cr XAS measurements on sulfides within this meteorite also indicate Cr²⁺ in CrS₆octahedra.

Reference
McKeown DA, Buechele AC, Tappero R, McCoy TJ and Gardner-Vandy KG (2014) X-ray absorption characterization of Cr in forsterite within the MacAlpine Hills 88136 EL3 chondritic meteorite. American Mineralogist 99:190-197.
[doi:10.2138/am.2014.4508]
Copyright: The Mineralogical Society of America

Link to Article

Nachiketa Rai and Wim van Westrenen

Faculty of Earth and Life Sciences, VU University Amsterdam, 1081 HV, Amsterdam, The Netherlands

Analyses of Apollo era seismograms, lunar laser ranging data and the lunar moment of inertia suggest the presence of a small, at least partially molten Fe-rich metallic core in the Moon, but the chemical composition and formation conditions of this core are not well constrained. Here, we assess whether pressure–temperature conditions can be found at which the lunar silicate mantle equilibrated with a Fe-rich metallic liquid during core formation. To this end, we combine measurements of the metal–silicate partitioning behavior of siderophile elements with the estimated depletion due to core formation in those elements in the silicate mantle of the Moon. We also explore how the presence of the light element sulfur (suggested by seismic models to be present in the core at concentrations of up to 6 wt%) in the lunar core affects core formation models.
We use published metal–silicate partitioning data for Ni, Co, W, Mo, P, V and Cr in the lunar pressure range (1 atm–5 GPa) and characterize the dependence of the metal/silicate partition coefficients (D) on temperature, pressure, oxygen fugacity and composition of the silicate melt and the metal. If the core is assumed to consist of pure iron, core–mantle equilibration conditions that best satisfy lunar mantle depletions of five siderophile elements—Ni, Co, W, Mo and P—are a pressure of 4.5(±0.5) GPa and a temperature of 2200 K. The lunar mantle depletions of Cr and V are also consistent with metal–silicate equilibration in this pressure and temperature range if 6 wt% S is incorporated into the lunar core. Our results therefore suggest that metal–silicate equilibrium during lunar core formation occurred at depths close to the present-day lunar core–mantle boundary. This provides independent support for both the existence of a deep magma ocean in the Moon in its early history and the presence of significant amounts of sulfur in the lunar core.

Reference
Rai N and Westrenen W (2014) Lunar core formation: New constraints from metal–silicate partitioning of siderophile elements. Earth and Planetary Science Letters 388:343–352.
[doi:10.1016/j.epsl.2013.12.001]
Copyright Elsevier

Link to Article