REDUCED CHROMIUM IN OLIVINE GRAINS FROM LUNAR BASALT 15555 - X-RAY ABSORPTION NEAR EDGE STRUCTURE (XANES)

The oxidation state of Cr in 200 mum regions within individual lunar olivine and pyroxene grains from lunar basalt 15555 was inferred using X-ray Absorption Near Edge Structure (XANES). Reference materials had previously been studied by optical absorption spectroscopy and included Cr-bearing borosilicate glasses synthesized under controlled oxygen fugacity and Cr-doped olivines. The energy dependence of XANES spectral features defined by these reference materials indicated that Cr is predominantly divalent in the lunar olivine and trivalent in the pyroxene. These results coupled with the apparent f(O2)-independence of partitioning coefficients for Cr into olivine imply that the source magma was dominated by divalent Cr at the time of olivine crystallization.