In search of geochemically discriminating tracers during subducting slab devolatilization, melting, and/or dissolution into supercritical fluid – SlabFlux

The project aims to experimentally determine a series of trace element equilibrium partition coefficients between mineral phases in mantle and metabasalt and silicate melts or aqueous fluids at the condition relevant to so-called mantle wedge and slab of subduction zones, with specific emphasis on aqueous fluid immobile elements and halogen elements. The motive is simple: we intend to determine a new set of trace element partition coefficients that can clearly discriminate physicochemical character of the element-transporting agent from subducting slab to the region of arc magma generation. From March 2008, we have launched the experiments of "task 1" with one-year financial support from Centre National de la Recherche Scientifique ' Institute National de Sciences de l'Univers (CNRS-INSU) under program SEDIT. In following sections, we present our recent findings as the demonstration of our competence and feasibility of the project. This proposal was already submitted to ANR programme blanc in 2008. We have responded to the two comments concerning (1) the behavior of halogen before the eruption, by presenting OUR RECENT DATA from natural samples and (2) the lack of manpower by doubling the manpower by adding ONE POSTDOC and ONE PHD STUDENTSHIP to the proposal. Over the years, many research projects have been conducted on the theme of subduction zones, and have reported a vast body of observations and models. To our view, the current consensus is that arc magma is generated by partial melting of the mantle beneath the arc after interaction with an unknown quantity of chemical flux from the subducting slab. The determination of the quantity and character of this flux has been the key question of the subduction zone research. In fact, the geochemical correlations among ratios of isotopes and trace elements have proven to be successful in deciphering the potential sources of the slab flux. As we outline our reasons in the later section, while isotopes effectively identify their sources, the character of the agent which carries the isotopic signature is far from conclusive (is it a fluid, a melt'), and this limits the comprehension of element fractionations in a subduction zone system. Given the complexity and variety of arc settings, we view that the current convention relies too heavily on lithophile trace element fractionations and this limits the ability to decode the complex geochemical signals of arcs. In recent years, with advances of micro analytical techniques, there are increasing number of observations describing primary (i.e. mantle equilibrated) composition of arc lava with specific emphasis on volatile elements. We think that these volatile elements are the most promising candidate to discriminate trace elements of subduction flux, while little is known about their partitioning behavior during magma generation. To our knowledge, we are presenting first experimentally determined halogen partition coefficients between mantle phases and basalt at the Fall meeting of American Geophysical Union, 2008. Considering the growing interests on the behavior of volatile elements in solid earth geochemistry, and the growing data set of volatile concentrations in primitive arc lavas, we believe this study is timely and needed. Furthermore, there is an ongoing effort at the Laboratoire Magmas et Volcans to identify volatile abundances in primary melts (ANR young scientist VOLMELT 2006, PI:Rose-Koga: LMV Clermont-Ferrand). The expected results from this project will be immediately applied to the data of such studies, and contributes to the further comprehension of subduction zone geodynamics. The core of this project is motored by a group of researchers from the Laboratoire Magmas et Volcans in Clermont-Ferrand, while some state-of-art analyses, and experimental approach unavailable to our group, are realized through partners and collaborators in France, Switzerland, and the US. Because more than 75% of the manpower is within a single organization, this project will be realized with tight communication of participants and smooth coordination. The project is organized to work towards tightly defined common goals and should permit active interactions of relatively young participants. The study of volatile element behavior in the magma source should provide a useful set of fundamental data applicable to the comprehension of volcanic fumaroles and ejected material compositions. Upon the completion of the project, we hope to bring a new set of data useful from volcanic processes to mantle recycling of subducted material.