OXYGEN FUGACITIES OF BASALTIC MAGMAS IN THE EGERSUND DIKE SWARM, SW. NORWAY; NO EVIDENCE FOR A SECULAR CHANGE IN MANTLE REDOX STATE OVER THE PAST ~650 MA

Oxygen fugacities (fO₂) of basaltic magmas potentially provide information about the redox states of their mantle source regions. Estimates of fO₂for basalts erupted at different times in earth history can therefore be used to monitor possible secular changes in the redox state of the mantle. We have calculated fO₂ for basaltic magmas that intruded to form the Egersund dikes in the Rogaland/Vest Agder region of SW Norway at ~650Ma. The magmas include olivine tholeiites, tholeiites, transitional basalts and trachybasalts. The rocks in most dikes are remarkably well preserved, and some chilled margins contain fresh olivine and plagioclase set in a glassy matrix. The olivine tholeiites have major oxide, trace element and isotopic compositions very similar to those of plume-related OIB and E-MORB (eg. olivine tholeiites from Kilauea) and this is interpreted to indicate formation from a plume during the early stages of breakup of the supercontinent Rodinia. Values of fO₂ were calculated from olivine-melt equilibrium using microprobe analyses of olivine rims and coexisting glasses. Calculated oxygen fugacities are similar for all magma-types preserved in the dike system: ΔFMQ=-0.2 to -0.7 for the olivine tholeiites, ΔFMQ=-0.2 to -0.4 for the tholeiites, ΔFMQ=-0.45 for the transitional basalt, and ΔFMQ=-0.2 to -0.3 for the trachybasalts. These results indicate that fO₂ remained essentially constant during magma evolution. Moreover, values of fO2 calculated for the dikes agree well with those obtained using the same method for MORB (ΔFMQ=-0.71±0.64) and for magmas erupted from some modern plumes (eg. Hawaii, ΔFMQ=-0.67±0.71; Iceland, ΔFMQ=-0.97±47). This indicates that the magmas in the Egersund dikes were ultimately derived from a mantle source region with a redox state similar to that of the mantle source regions modern MORB and some OIB. Consequently, the results provide no evidence for a change in the redox state of the mantle source regions of basaltic magma over the past ~600-650 Ma. It follows that volcanic degassing cannot be responsible for the increase in concentration of atmospheric O2 inferred to have occurred at the end of the Neoproterozoic. It seems likely that any increase in atmospheric O2 levels at that time is related to biogenic processes or to an increase in the rate of carbon burial.