HR: 15:15h
AN: V33F-06    [Abstracts]
TI: Chlorine cycling in subduction zones: Insights from submarine glasses and melt inclusions from arc and back-arc basalts
AU: * Kent, A J
EM: adam.kent@geo.oregonstate.edu
AF: Department of Geology, Oregon State University, Corvallis, OR 97330 United States
AU: Rowe, M C
AF: Department of Geology, Oregon State University, Corvallis, OR 97330 United States
AB: Chlorine is an excellent tracer of contributions of slab-derived fluids to mantle-derived melts produced during subduction zone magmatism, as saline fluids released during slab dehydration are enriched in chlorine by several orders of magnitude over mantle peridotite. Chlorine and other halogens also strongly influence the solubility of many cations in high temperature solutions and thus play an important role in mediating fluid-driven mass transfer within subduction zones. However, quantitative understanding of chlorine systematics in subduction zone magmas is limited by loss of chlorine during high level degassing. This problem may be circumvented by: (i) analyzing samples, typically from back-arc spreading systems, that erupt under sufficient depth of seawater to limit chlorine loss, and (ii) using melt inclusions hosted in early-formed phenocrysts to examine the chlorine contents of melts trapped prior to degassing. Chlorine contents of primitive back-arc basalt glasses range from values that overlap those of MORB and OIB to substantially higher values ($>$ 1000 ppm). Although chlorine contents may be strongly enriched from shallow seawater contamination processes, this is relatively easily discerned from enrichment due to slab inputs as the latter correlates strongly with other indices of slab input. By coupling measured chlorine, and H$_{2}$O contents with simple flux melting models it can be shown that the slab fluids that contribute to back-arc melts have salinities equivalent to ~1-10% NaCl, sufficient to strongly alter cation solubilities. Furthermore, the estimated Cl/H$_{2}$O and Cl/K$_{2}$O of the slab-derived components also decrease systematically with distance from the arc front, consistent with progressive preferential loss of chlorine with respect to these elements. Melt inclusions from arc lavas generally show higher chlorine and Cl/K$_{2}$O than back-arcs, consistent with greater contributions from slab fluids, but within arcs may show complex variations related to source and melting regime. For example melt inclusions from OIB-like lavas from the Cascade Arc range to higher chlorine contents and Cl/K$_{2}$O than evident in CAB and MORB-like samples, suggesting that in this hot and dry arc an enriched source region may produce greater enrichment in chlorine than inputs from slab fluids.
DE: 1030 Geochemical cycles (0330)
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting