The Oxidation State of Hawaiian Magmas

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The oxygen fugacity of basaltic magma is a fundamental intensive variable that controls the iron redox state (Fe2O3/FeO + Fe2O3) 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. More importantly, 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. The oxygen fugacity of Hawaiian magmas 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 other Hawaiian volcanoes, and those that do exist may be suspect. The lavas may well have undergone subaerial or near-surface oxidation during eruption, with the consequence that the reported values may be too high. That is, the oxygen fugacities may be lower than FMQ and closer to MW (magnetite-wustite buffer). This research will attempt to clarify this situation. Carefully selected, rapidly quenched samples will be analyzed for FeO and Fe2O3 in order to estimate the oxygen fugacity. Major elements, trace elements and sulfur abundances will also be determined on the same samples. Sulfur will be measured to test the hypothesis that sulfur degassing results in the reduction of the magma. The samples will include quenched lavas, spatter, hyaloclastites, and glassy pillow margins from Mauna Loa, Kilauea, Mauna Kea and Loihi volcanoes. This data should provide a firm base for establishing the oxygen fugacity of Hawaiian magmas and show whether or not there are significant differences in oxygen fugacity of lavas from different volcanoes. It will place limits on the oxidation state of the Hawaiian plume, and contribute to the vigorous debate on the oxidation state of the mantle. Over the last twenty five years there has been a vigorous debate on the oxidation state of the earth's mantle, partly because it has relevance to the origin of the earth's atmosphere through volcanic degassing, and therefore ultimately to the development and evolution of life. Basaltic magmas, which are produced by melting of the earth's mantle are thought to provide information on the oxidation state of their mantle source. Those erupted on Hawaiian volcanoes are thought to have their origins in a hot, mantle plume that may have originated as deep as the core-mantle boundary. It is speculated that this plume material is a mixture of primitive mantle and re-cycled crustal material from subduction zones along the earth's plate margins. An understanding of its oxidation state is therefore of some importance. By precisely measuring the amounts of ferric and ferrous iron in basaltic lava it is possible to estimate the oxidation state of the magma, and from this arrive at inferences concerning the oxidation state of the mantle plume. The work done to date, suggests that Hawaiian magmas are relatively oxidized, more so than magmas erupted along the Earth's spreading mid-ocean ridges. These results may be in error. This is because basaltic magmas are prone to oxidation during eruption and transportation in lava flows. Our preliminary results indicate that the oxidation of a lava is critically dependent on how, when and where it is sampled. Lavas that have cooled slowly, or have traveled some distance from the vents are invariably oxidized. Only lavas that have been quenched rapidly, and/or have been sampled at the eruptive vent, retain their original oxidation state, which is much lower than has previously been supposed. This research will focus on determining the oxidation state of rapidly quenched lavas from Mauna Loa, Kilauea, Mauna Kea and Loihi volcanoes in an attempt to rectify this situation.