OXYGEN FUGACITIES OF MAFIC MAGMAS ERUPTED IN THE SANTORINI VOLCANIC FIELD, AEGEAN SEA, GREECE

Oxygen fugacities (fO2)of magmas provide information about the redox states of their mantle source regions. Therefore, estimates of fO2 for magmas erupted in different tectonic environments can be used to map variations in the redox state of the underlying mantle. We have calculated fO2 for mafic calc-alkaline magmas erupted from four volcanic centers (Akrotiri, Micro Profitis Ilias, Megalo Vouno, and Skaros) on Santorini, Greece, over the past 600,000 years. Values of fO2 were calculated from olivine-melt equilibrium using microprobe analyses of olivine rims and coexisting groundmass. We consider the results accurate to ± 0.3 log-bar units based on comparison with experimental data. The oxygen fugacities calculated for 12 samples lie in a fairly narrow range (ΔFMQ=1.06±0.18), and are consistent results obtained from olivine-augite-magnetite assemblages in the same samples. Combination of the results with those obtained for other samples (based on mineral equilibria), indicates that pre-eruptive oxygen fugacities of the majority of Santorini magmas (including that erupted in the well-known Minoan event) lie slightly above, but close to, values defined by the NNO buffer (ΔFMQ=1.13±0.26; n=86). A small number of silicic lavas erupted in the Akrotiri region early in the history of the volcanic field evolved at higher fO2 (ΔFMQ=2.11±0.15). The reason for this difference is not understood at present. Nevertheless, the data indicate that the redox states of mafic magmas erupted on Santorini have remained constant over ~400,000 to 600,000 years. This implies that the redox state of mantle source region has remained constant over this time interval, and/or that processes such as mixing homogenize the mantle-derived basalts in chambers located at the base of the crust (~22 km). This conclusion is surprising given the subduction rate (indicating subduction of ~24-30 km of slab over the timespan of volcanism) and the lithological variablility of sediments on the African plate, but is consistent with other geochemical evidence indicating that the composition of parental basalts erupted on Santorini has remained nearly constant over time. Additional work os required to establish the effects of degassing on the oxygen fugacities of arc magmas before firm conclusions can be drawn about the redox states of sub-arc mantle.