FERRIC ION IN PERIDOTITES AND MANTLE OXIDATION-STATES

Fe-57 Mossbauer spectroscopy is employed to estimate Fe3+/SIGMAFe rati os and the Fe2O3 content of 28 well characterized pyroxenite and perid otite xenoliths from central France, southeastern Australia, northern Tanzania, southern Africa and Siberia. The Fe2O3 contents of various m antle regions were assessed, and found to range from 0.1 to 0.4 wt%. W ith an appraisal of the likely mantle inventories of C, H and S, the r esults demonstrate that these volatile elements and Fe all have simila r buffering capacities for oxygen in peridotites. For this reason, the oxidation states of upper mantle peridotites should not generally be controlled by any particular buffer system. New estimates for the oxid ation states of cratonic garnet peridotites indicate that the upper ma ntle beneath cratons is within 1 log unit of the fayalite-magnetite-qu artz (FMQ) buffer, but may have been progressively oxidized since its original formation in the early Archean.