Island Arc Magmatism Eruption of Anatahan Volcano, Northern Mariana Is, June 10, 2003

Subduction Factory

Location of arc volcanoes Open circles = N Pacific Arcs Syracuse & Abers, 2006

Why does melting occur? Decompression Melting? This model includes a backarc spreading center and shows contours of decompression melt production Conder et al., 2002

Amphibole dehydration model (Davies & Stevenson 1992) However: Later experiments suggest Amphibole dehydrates at depths too shallow Also: Geochemical evidence does not support an amphibole-only origin for arc fluids

Improved model – water input from several hydrous phases Hydrous minerals are: lawsonite, zoisite, chloritoid, and talc in the oceanic crust Chlorite and serpentine in the mantle They dehydrate over a large depth range, to at least 150 km Schmidt & Poli 1998

Modeling water transport and magma production in the arc Cagniocle et al., 2007

Arc lavas contain signature of subducted sediments Be10 has half life of 1.5 Ma, formed by cosmogenic processes in atmosphere Concentrated in the upper oceanic sediments High Be10 concentrations in arc magmas show that sediment melts or else Be10 is transported to the arc magmatic system in subduction fluids

Development of Arc Crust

Comparison with Basalt Geobarometry = maximum anomaly Si & Mg thermobarometry suggest a final equilibration depth of magma: 21-34 km for the backarc spreading center 34-87 km for the volcanic arc [Kelley et al., in press] Corresponds perfectly with the strongest anomalies beneath the backarc and arc Shows that velocity and Q anomalies delineate the melt production region Anomalies result from small fractions of in-situ melt

TUCAN Broadband Seismic Experiment

Nicaragua vs Costa Rica Attenuation – More Water? Rychert et al. [2008] Nicaragua shows significantly higher attenuation than Costa Rica Geochemical/petrologic constraints suggest similar mantle temperatures The greater attenuation is consistent with higher mantle water contents and extents of melting inferred from geochemistry and melt inclusions.

Nicaragua Attenuation & Vp/Vs Ratio – evidence of magma ascent? (from Rychert et al. 2008) Vp/Vs ratio (from Syracuse et al. 2008)