ABSTRACT

We used synchrotron radiation Fe micro-XANES (X-ray Absorption Near Edge Structure) analysis to determine the iron valence of maskelynite (shocked-plagioclase glass) in thirteen shergottites, and we compared the Fe³⁺–Fe²⁺ peak-intensity ratios for maskelynite in the shergottites in order to compare the redox states of the shergottites and discuss the evolution of the Martian crust and mantle. We analyzed two geochemically depleted (Dar al Gani 476 and Dhofar 019), four geochemically intermediate (EETA 79001 lithology A, ALH 77005, LEW 88516, and NWA 5029), and seven geochemically enriched (NWA 856, Zagami, Shergotty, RBT 04262, NWA 4468, NWA 1068, and LAR 06319) shergottite samples. The Fe³⁺–Fe²⁺ peak-intensity ratios for maskelynite in the depleted, intermediate, and enriched shergottites were 0.39–0.49, 0.13–0.66, and 0.40–0.81, respectively. LAR 06319, NWA 856, NWA 1068, Zagami, and Shergotty, whose log fO₂s were higher than QFM-2.0. RBT 04262 showed the highest Fe³⁺–Fe²⁺ peak-intensity ratios for maskelynite in the enriched shergottites; therefore, RBT 04262 might have crystallized under a more oxidized environment than the other enriched shergottites. Similarly, NWA 4468 was estimated to have crystallized at log fO₂ higher than QFM-2.3. This is the first report showing that RBT 04262 and NWA 4468 are oxidized samples because no oxybarometer measurements had previously been performed on these samples. We showed that XANES analysis can be applied to any maskelynite-bearing samples in order to discuss the difference between the redox states of Martian meteorites. We also revealed that the Fe³⁺–Fe²⁺ peak-intensity ratios for maskelynite in the intermediate shergottites show a very wide range (0.13–0.66), exceeding the ratios obtained for maskelynite in the depleted and enriched shergottites. When previously reported geochemical characteristics such as the εNd and crystallization ages are included, the wide range of the Fe³⁺–Fe²⁺ peak-intensity ratios obtained for maskelynite in the intermediate shergottites does not support the model that proposes that two distinct mantle reservoirs mix during melting. The results of this study reveal that the redox state of the source of the enriched shergottites is heterogeneous.