Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.u52a..02f&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #U52A-02
Physics
7240 Subduction Zones (1207, 1219, 1240), 7270 Tomography (6982, 8180), 8170 Subduction Zone Processes (1031, 3060, 3613, 8413)
Scientific paper
Recent seismic imaging in the Nicaragua-Costa Rica subduction zone, in combination with geochemical data, has revealed systematic along-arc variations in volatile cycling from the subducting slab, through the mantle wedge, to the melts generated at arc volcanoes. Velocity and attenuation tomography based on P and S phases from local events recorded by the 2004-2006 TUCAN Broadband Seismic Experiment resolve a high velocity, low attenuation subducting slab, a shallow wedge corner with intermediate attenuation, and a slower, more highly attenuating mantle wedge beneath the arc and back-arc. However, velocity and attenuation structures also manifest strong along-arc variations. The subducting slab beneath Nicaragua contains a shallow layer of slow velocities and is more attenuating than the slab beneath Costa Rica, consistent with greater slab hydration (for example, 10-20% mantle serpentinization). Continuing the along-arc trend, attenuation in the Nicaraguan mantle wedge at depths of 60-100 km is significantly higher than in Costa Rica, suggesting that enhanced slab hydration beneath Nicaragua results in a more hydrated wedge. Moving to the arc, olivine melt inclusions indicate that Nicaraguan magmas are more water-rich on average than those erupted in the Costa Rican arc within the region best-imaged by the TUCAN array. Assuming that these wet melts equilibrated with olivine in the mantle wedge, then the inferred decrease in wedge hydration from Nicaragua to Costa Rica can explain the along-arc variation in wedge attenuation. In addition, although bulk magma compositions in the vicinity of the Nicaragua and Costa Rica attenuation profiles are significantly different, they imply a similar temperature of 1265 ± 25°C at a depth comparable to the middle of the wedge (75 km) when primary melt water content is taken into account in calculating olivine-liquid temperatures. A roughly vertical column of high Vp/Vs extends from the slab surface to the arc in Nicaragua, but this feature dies away along the arc to Costa Rica. This anomaly could reflect larger melt fractions in the Nicaraguan wedge, possibly produced by the apparently greater wedge hydration.
Abers Geoffrey A.
Fischer Karen M.
Plank Terry
Rychert Catherine A.
Syracuse Ellen M.
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