Geochemical Constraints on Mantle Sources and Melting Conditions in Pacific Back-Arc Basins

Abstract

The effect of water in the mantle has been well studied and has well-known effects on the behavior and properties of magmas and the mantle including an increase in the extent of melting, suppression of plagioclase during crystallization, and a general reduction of seismic parameters and viscosity. At mid-ocean ridges, magmatic and mantle H2O contents are relatively low, and a reasonable understanding of the behavior of H2O has been obtained (Dixon and Stolper, 1995; Dixon et al., 1995; Dixon et al., 2002; Asimow and Langmuir, 2003). However, at back-arc basins, H2O can also be added to the mantle source by the subducting slab, changing the melting behavior and the mantle source composition (Stolper and Newman, 1994; Taylor and Martinez, 2003). Quantitative constraints on these factors lag behind, and this thesis will test hypotheses related to the role of volatiles in these three major processes at back arcs: (1) tracing mantle source compositions and flow vectors, (2) refining mantle melting models, and (3) constraining the origin of back-arc slab-derived fluids. Tracing mantle source compositions is done best in places where mantles of starkly contrasting compositions are juxtaposed, as in the case of plume-ridge interaction. The NW Lau Basin, a back-arc with little influence from the slab, provides an ideal setting to address mantle flow where the mantle source contrast is potentially well made with the interaction of relatively depleted mantle with the Samoan plume. Geochemical tests of the interaction between the Samoan Plume and the Lau Basin mantle have relied on one tracer (3He/4/He), but the addition of volatiles (H2O, CO2), trace elements (e.g., La, Nb), and other radiogenic isotopes (Sr, Nd, Pb, Hf) provides further constraints on tracing the enriched Samoan mantle composition. Our new data suggest two-component mixing of MORB-like mantle with an enriched mantle source, similar to Samoa, although consideration of a complete regional data set suggests there may be other sources of heterogeneity in the mantle beneath NW Lau. Aside from tracing mantle flow, volatiles and trace elements provide constraints on mantle melting as H2O has an effect on where and how much melt can be made in the mantle, recorded in incompatible trace element signatures (e.g., Ti, Nb). Observations based on geochemical data suggest two possibilities: mixing of end-member melts or a continuous melting regime, but most models of mantle melting are restricted to isobaric-isothermal conditions and offer unrealistic tests of the competing hypotheses of back-arc