Project Title: Volatiles (H2O and CO2) in Mariana and Izu Arc Magmas
PI's: T.Plank, BU * T. Grove, MIT * S.Newman / E.Stolper, Caltech
Project Summary (NSF Funded Project)
A fundamental goal of the MARGINS subduction factory initiative is to understand the cycling of volatile compounds through the subduction zone. Water and CO2 in particular affect not only the physics of subduction, but also arc magma evolution and mantle melting itself. Volatiles are also the primary agents for recycling of material from the downgoing plate to the upper plate. Despite their importance, volatile concentrations have been determined in only a limited number of arc basalts, typically by micro-analysis of undegassed melt inclusions in volcanic phenocrysts. Comprehensive studies which link experimental constraints with direct measurements of volatiles and trace elements in melt inclusions do not exist for any arc.
The Mariana-Izu arc is well suited to such a volatile study for several reasons. 1) Existing trace element and isotopic data make clear predictions as to the fluid and sediment contributions from the slab, which can be tested with volatile measurements. 2) Contrasts in the composition of the Mariana and Izu basalts provide a simple test of slab input vs. subduction parameters as along-strike forcing-functions. 3) Landmark volatile studies in the Mariana Trough provide clear models of melt transport and melting to test beneath the arc. 4) Recent drilling during Leg 185 will provide the only estimates of both sediment and Layer 2A volatile inputs to any subduction zone; thus an approach to volatile mass balance is possible here. 5) Prior work with some Marianas materials demonstrates the feasability of this project, in that appropriate melt inclusions exist, some with high H2O (>3 wt%).
This collaborative study aims to attack the volatile problem in three ways. 1) FTIR measurements of H2O and CO2 will be carried out on glassy melt inclusions hosted within the most primitive olivine, clinopyroxene, and plagioclase phenocrysts available for the Mariana and Izu arcs. The emphasis will be on scoria samples from several islands that encompass the range in inferred water-contents based on liquid lines of descent and trace element proxies of slab fluids. 2) Major and comprehensive trace elements will be determined on a wider suite of melt inclusions (including the exact ones used for the FTIR analyses) using laser ablation microprobe-ICP-MS and electron microprobe. This will provide a data set for non-volatile salb tracers (e.g., Ba/La, U/Th, Ce anomalies, Ce/Pb) with which to link the volatile data, and test models of trace element fluid proxies and mantle melting. 3) High pressure water-saturated (100, 200 MPa) and water-undersaturated (800 MPa) experiments will be carried out using TZM pressure vessels and the piston cylinder apparatus in order to provide independent constraints from phase equilibria on the melt inclusion volatile data, and to deveop melt geohygrometers for Mariana-Izu compositions.