HR: 0830h
AN: V51A-05    [Abstracts]
TI: Olivine-Melt Equilibrium and the Oxygen Fugacities of Lavas from Hawaii.
AU: * McCann, V E
EM: mccann.110@osu.edu
AF: The Ohio State University, Department of Geological Sciences 275 Mendenhall Laboratory 125 South Oval Mall, Columbus, OH 43210 United States
AU: Barton, M
EM: barton.2@osu.edu
AF: The Ohio State University, Department of Geological Sciences 275 Mendenhall Laboratory 125 South Oval Mall, Columbus, OH 43210 United States
AB: Oxygen fugacities of magmas provide information about the redox states of their mantle source regions. Therefore, oxygen fugacities for magmas erupted in different tectonic environments can be used to map variations in the redox state of the underlying mantle. Previous data have been interpreted to indicate that the mantle source of MORB is reduced relative to that of OIB, implying that plumes are oxidized relative to MORB mantle. However, oxygen fugacities calculated from analyzed Fe₂O₃/FeO for lavas from Hawaii (ΔQFM=0.363 to -1.812, ave= -0.705) and Loihi (ΔQFM=0.293 to -2.121, ave= -0.994) fall in the range of those for MORB (ΔQFM=0.402 to -2.732, ave=-1.304). This suggests that the mantle source of Hawaiian magmas is similar to that of MORB. However, there is some dispute about the validity of oxygen fugacities calculated from analyzed Fe₂O₃/FeO of lavas. Therefore, we have also calculated oxygen fugacities for lavas from Hawaii, and for MORB samples using olivine-melt equilibrium. For Hawaiian samples we obtain ΔQFM=0.586 to -2.059, ave=-0.779, values similar to those obtained from analyzed Fe₂O₃/FeO providing confidence that the values accurately represent magmatic oxygen fugacities. For MORB samples (mostly from the EPR and MAR) we obtain ΔQFM=0.190 -2.594 ave=-0.713, values that are also similar to those obtained from analyzed Fe₂O₃/FeO. These results therefore indicate that the redox state of the mantle source of Hawaiian magmas is virtually identical to that of the MORB source. This confirms results obtained from Icelandic lavas, and strongly suggests that the redox states of the mantle sources of OIB and MORB are essentially identical at the depths of magma segregation. It follows that there is no evidence that the redox state of the lower mantle (or deep mantle) differs from that of the upper mantle if OIB are derived from deep mantle plumes. Unless the redox states of plumes are affected by exchange with surrounding the mantle during ascent, this may provide evidence that the lower mantle is relatively oxidized.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly