Using 226Ra-230Th-238U Disequilibria to Test the Hypothesis of Peridotite-Pyroxenite Melt Mixing at Hawaiian Shield Volcanoes

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Intellectual merit. It has long been recognized that incorporation of ancient, recycled lithospheric materials (e.g., subducted oceanic crust and/or sediment) into the deep source regions of mantle plumes is an important process in creating the chemical and isotopic heterogeneity observed in ocean-island basalts. An important component of this "recycling hypothesis" is the idea that many mantle plumes contain a significant amount of pyroxenite or eclogite (the metamorphic equivalent of the recycled oceanic crust) in addition to the garnet or spinel peridotite that is generally thought to make up most of the Earth's mantle. Geochemical and isotopic analyses of lavas from Hawaiian volcanoes have figured prominently in this debate. An important observation is that some tholeiitic basalts from Hawaiian shield volcanoes have unusually high SiO2 contents (at a given MgO value) that cannot be due to shallow-level crystal fractionation. It has been proposed that these high-SiO2 basalts might result from the partial melting of a modest amount (5% by weight) of an ancient, recycled quartz-bearing eclogite. More recently, studies have identified additional major- and trace-element signatures of a pyroxenitic or eclogitic component in Hawaiian lavas, such as Ni abundances and CaO contents that are too high and too low (respectively) to be in equilibrium with a peridotite source. These studies are notable for two reasons. First, they require the presence of an enormous amount of pyroxenite or eclogite within the source regions of Hawaiian volcanoes (up to 100%). Second, they require an unusual two-stage melting process in which deep melts of eclogite react with the surrounding peridotite matrix to create a solid, secondary pyroxenite. Both the secondary pyroxenite and leftover, unreacted peridotite rise to shallower depths and melt to create a mixed "peridotite-pyroxenite" magma. If correct, this model would represent a fundamental change in our thinking about the nature of source heterogeneity and the mechanism of melt generation beneath Hawaiian shield volcanoes, and may even have global geodynamic implications. We propose to test the hypothesis that the Hawaiian mantle plume contains two distinct lithologies (peridotite vs. pyroxenite) that contribute melts (that mix in variable proportions) to create the voluminous tholeiitic basalts making up the bulk of Hawaiian shield volcanoes. The analytical approach involves detailed, high-precision study of the 226Ra-230Th-238U disequilibria of well-characterized lavas from Mauna Loa and Kilauea, the two most active Hawaiian shield volcanoes. U-series isotopes are ideally suited to investigate this issue because the 226Ra-230Th-238U disequilibria of melt extracted from a clinopyroxene- and garnet-dominated pyroxenite source vs. an olivine- and orthopyroxene-dominated peridotite source is predicted to differ greatly. This approach represents an independent test of the peridotite-pyroxenite melt mixing model because it is not based on major or compatible trace elements.Broader impacts. Located in a large and diverse metropolitan area bordering Mexico and the Pacific Rim, San Diego State University (SDSU) is an important contributor to the scientific and technical resources of this region. This study will provide educational opportunities to our diverse student population, many of who are first-generation college students. At least one talented undergraduate student will be involved in the sample preparation, analytical work, and interpretation via a Senior Thesis project. In addition, the geochemical and isotopic analyses, and subsequent interpretation, will form a Masters Thesis for a graduate student at SDSU. The proposed study will fully utilize the SDSU isotope laboratory, ensuring that this multi-user facility continues to be valuable site for advanced technological mentoring of students in the Geosciences. To broaden dissemination of our results to the general public, Pietruszka has volunteered to conduct regular scientific presentations, incorporating aspects of his active research projects, at the Reuben H. Fleet Science Center in San Diego to school groups and senior citizens.