HR: 1340h
AN: V43A-1417    [Abstracts]
TI: Ferrous/Ferric Ratios in 1984 Mauna Loa Lavas: A Contribution to Understanding the Oxidation State of Hawaiian Magmas
AU: * Rhodes, J M
EM: jmrhodes@geo.umass.edu
AF: Dept. of Geosciences, University of Massachusetts, Amherst, MA 01003 United States
AU: Vollinger, M J
EM: mikev@geo.umass.edu
AF: Dept. of Geosciences, University of Massachusetts, Amherst, MA 01003 United States
AB: The oxygen fugacity of basaltic magma is a fundamental intensive variable that controls the iron redox state of the melt, and has a strong influence on the sequence and composition of minerals that crystallize from a cooling magma, and therefore on the composition of a fractionated melt. Additionally, the oxygen fugacity of basaltic magma is thought to reflect the oxygen fugacity of the mantle source or, at the very least, to place an upper limit on that of the source. In Hawaiian magmas, the oxygen fugacity is widely accepted as being close to the FMQ (fayalite-magnetite-quartz) buffer. This assumption is based largely on analyses of lavas from Kilauea volcano. There are very few published measurements on lavas from the other Hawaiian volcanoes. Ferric/ferrous ratios have been used to estimate the oxygen fugacity of lavas erupted in 1984 on Mauna Loa Volcano, Hawaii. Rapidly quenched lavas erupted close to vents are less oxidized than rapidly quenched lavas scooped from lava flows away from the vents, or samples that have cooled slowly. These results demonstrate that sampling is of critical importance in determining the oxidation state of a lava. The oxidation state of the vent lavas are below, or at, MW (magnetite-wstite), and are significantly lower than that previously reported for Hawaiian lavas (about FMQ). Similarly, rapidly quenched lavas from the ongoing Kilauea eruption and Loihi seamount, all have oxygen fugacities that are close to MW and on the low side of the range previously reported for Hawaiian lavas. From this we conclude that the initial oxygen fugacity of parental Hawaiian magmas is close to MW, not FMQ, and that previous estimates of the oxidation state of Hawaiian lavas may be suspect, having undergone subaerial or near-surface oxidation during eruption. The implications are that the plume source of these magmas is also at or below MW, but not as reduced as the mantle source of mid-ocean ridge basalts. Additionally, Mauna Loa lavas appear to be slightly more reduced than Kilauea or Loihi lavas, perhaps indicating heterogeneous oxidation within the Hawaiian plume.
DE: 8439 Physics and chemistry of magma bodies
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting