HR: 0800h
AN: V31B-0615    [Abstracts]
TI: Complications in Determining Oxygen Fugacities From Olivine-Melt Equilibrium Illustrated by the Pu'u'O'o Lavas, Hawaii
AU: * McCann, V E
EM: mccann.110@osu.edu
AF: The Ohio State University, 275 Mendenhall Laboratory 125 South Oval Mall, Columbus, OH 43201 United States
AU: Barton, M
EM: barton.2@osu.edu
AF: The Ohio State University, 275 Mendenhall Laboratory 125 South Oval Mall, Columbus, OH 43201 United States
AU: Thornber, C R
EM: cthornber@usgs.gov
AF: David A. Johnston Cascades Volcano Observatory U.S. Geological Survey, 1300 SE Cardinal Court, BLDG 10, Vancouver, WA 98683 United States
AB: We have shown previously that oxygen fugacities calculated from olivine-melt equilibrium using rim compositions agree well with those calculated from analyzed Fe³⁺/σFe for MORB and for Icelandic OIB (MORB - average ΔFMQ -0.72 versus -0.70, Iceland average ΔFMQ -0.49 versus -0.58). The agreement between oxygen fugacities calculated from Ol-melt equilibrium and those calculated from analyzed Fe³⁺/σFe is excellent for individual samples of MORB from the FAMOUS region and Blanco Trough ( difference in calculated ΔFMQ < 0.30). Published analyses of Fe³⁺/σFe yield oxygen fugacilties of ΔFMQ= -0.72±0.43 for lavas from Kilauea and Mauna Loa, and ΔFMQ=0.91±0.72 for Loihi, very close to those for MORB. Oxygen fugacities determined using carefully selected Ol-melt analyses (ΔFMQ= -0.43±0.32) are in reasonable agreement with those determined from Fe³⁺/σFe for Kilauea and Mauna Loa, and agreement between fO₂ calculated from Ol-melt equilibrium (ΔFMQ= -0.26) and that calculated from Fe³⁺/σFe (ΔFMQ= -0.64) is excellent for one sample from Kilauea. However, olivine-melt pairs from some samples, including those from the Pu'u'O'o lavas, yield anomalously high or low estimates of fO₂ (average ΔFMQ =-0.6907, range -4.07 to +0.34). We suggest that these anomalous values reflect the complex history of olivines in these lavas, in particular the effects of magma mixing. Some olivines in the Pu'u'O'o lavas clearly have rims that are anomalously rich in Fa, whereas others have rims that are anomalously rich in Fo and do not appear to have equilibrium compositions. Given the dependence of olivine composition on melt Fe³⁺/σFe, there is no simple method to determine the equilibrium composition of olivine for a particular melt. However, detailed zoning profiles and analyses of microphenocrysts allow probable equilibrium compositions to be identified. We suggest that the average fO₂ of Hawaiian lavas lies close to (ΔFMQ= -1) based on published results using spinel-melt equilibrium and Fe-Ni exchange between olivine and sulfide liquid. Olivine-glass inclusion data for Koolau samples also suggest (ΔFMQ= ~-1).
DE: 1025 Composition of the mantle
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
DE: 3699 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005