[S-IT02_29PM1] Aqueous fluids and melts in subduction zones: Experiment, modeling, and geophysical observations

[SIT02-02] Imaging mantle melting processes and the effect of water beneath island arcs and backarc spreading centers

*Douglas WIENS¹, S. shawn WEI¹ (1.Washington University, St Louis, MO, USA)

Keywords:seismic tomography, island arcs, backarc spreading centers, seismic attenuation, mantle fluids, melt generation

We use arrays of land and ocean bottom seismographs to image melting processes in the Mariana and Tonga-Lau mantle wedges. Both regions show arc volcanism, active backarc spreading, and a gradient in mantle water content going from slab to backarc spreading center. The Lau backarc in particular shows a gradient in inferred mantle water content as the spreading centers approach the arc and slab in the south. Water contents range from near-MORB conditions in the Central Lau Spreading Center (CLSC) to high water content for the Eastern Lau Spreading Center (ELSC) and nearly arc-like for the Valu Fa Ridge (VFR). For both Mariana and Lau we find significant slow velocity and high attenuation anomalies in the upper 100 km of the mantle beneath the volcanic arc and the spreading center. In the Mariana region, the anomalies are separated by a high velocity, low attenuation region at shallow depths (< 80 km), implying distinct arc and backarc melting regions, with the anomalies coalescing and possibly allowing material interchange at greater depths. The maximum anomaly in the backarc is shallower (~ 30 km) than in the arc (~ 65 km), consistent with geochemical indications on the depth of melt production in these regions. The strongest anomaly beneath the backarc spreading center is narrow (~ 70 km) and extends from close to the moho to 80 km depth. Data analyses for the Tonga-Lau project are preliminary, but show similarities to the Mariana images. Extremely low seismic velocity and high attenuation are found in a 100 km wide region beneath the spreading center in the upper 80 km. At deeper depths the anomaly is displaced westward in both velocity and attenuation images, suggesting that partial melting occurs along an upwelling limb of mantle flow originating west of the backarc. 3-D images from Rayleigh wave tomography show a much stronger anomaly along the CLSC when compared to the southern ELSC and VFR. The backarc velocity and attenuation anomalies are stronger in the Lau basin than in the Mariana backarc, perhaps due to higher mantle temperatures inferred from petrology. Both Q and velocity anomalies are larger than expected for temperature effects based on laboratory-derived relationships, and their configuration is inconsistent with the expected temperature field. In addition, the observed anomalies are roughly inversely proportional to inferred mantle water content, suggesting that water content does not cause the observed large seismic anomalies. However, experimental results suggest that seismic attenuation and velocity are highly sensitive to the presence of even very small amounts of partial melt. Therefore we suggest the high attenuation and low velocity anomalies delineate the melt production regions beneath the ridge axis and volcanic arc, but that only small melt fractions (< 1 %) are required to explain the seismic data. Smaller amplitude anomalies beneath the VFR, where large amounts of subduction-derived water are incorporated into the melt, may indicate lower mantle melt porosity due to low melt viscosity and more efficient transport of the water-rich melt, or a different topology of melt in the matrix. A lower melt porosity for aqueous melts is also consistent with the smaller seismic anomaly seen for the water-rich volcanic arc melting regions compared to the backarc melt production zone for both regions.