No abstract

[1] A recent global compilation of the thermal structure of subduction zones is used to predict the metamorphic facies and H 2 O content of downgoing slabs. Our calculations indicate that mineralogically bound water can pass efficiently through old and fast subduction zones (e.g., in the western Pacific), whereas hot subduction zones such as Cascadia see nearly complete dehydration of the subducting slab. The top of the slab is sufficiently hot in all subduction zones that the upper crust, including sediments and volcanic rocks, is predicted to dehydrate significantly. The degree and depth of dehydration in the deeper crust and uppermost mantle are highly diverse and depend strongly on composition (gabbro versus peridotite) and local pressure and temperature conditions. The upper mantle dehydrates at intermediate depths in all but the coldest subduction zones. On average, about one third of the bound H 2 O subducted globally in slabs reaches 240 km depth, carried principally and roughly equally in the gabbro and peridotite sections. The predicted global flux of H 2 O to the deep mantle is smaller than previous estimates but still amounts to about one ocean mass over the age of the Earth. At this rate, the overall mantle H 2 O content increases by 0.037 wt % (370 ppm) over the age of the Earth. This is qualitatively consistent with inferred H 2 O concentrations in the Earth's mantle assuming that secular cooling of the Earth has increased the efficiency of volatile recycling over time.

[1] The location and motion of subducting plates relative to volcanic arcs provide a first-order constraint on theories of arc magmagenesis. We compile volcano-specific subduction parameters for 33,000 km of the global arc system at 839 volcanic centers, measuring the depth to the top of the slab (H) beneath each volcano. The compilation also includes estimates of slab strike and dip, incoming plate velocity, and age, all available in accompanying auxiliary material. The slab geometry is contoured from the top surface of Wadati-Benioff zones (WBZs) for a variety of teleseismic and local seismicity catalogs, which provides a reference surface for evaluating the distribution of seismicity within subducting plates. The WBZ thickness exceeds that expected from hypocentral errors in a manner correlating with plate age, indicating that old plates have thicker regions in which earthquakes can occur. When averaged over 500-km-long arc segments, H ranges from 72 to 173 km with a global average of 105 km, increasing by 20 km when hypocentral error effects are taken into account. These depths correlate poorly with most subduction parameters, but significant correlations exist between H and slab dip (correlation coefficient is 0.54 for 45 arc segments). The dip correlation can be explained if the melting region is displaced from the WadatiBenioff zone by a constant-thickness boundary layer. For the north Pacific, H varies inversely with descent rate; this trend may reflect the manner in which wedge thermal structure affects arc location. Over short distances some arc segments exhibit abrupt variations in arc location but not slab geometry, indicating that upper-plate tectonic processes also exert control on H. These along-strike trends in H also correlate with geochemical proxies for the degree of melting, at least in one test case. Thus slab geometry and kinematics provide an important control on the melting that produces arc volcanoes.

[1] The Central American subduction zone exhibits large variations in geochemistry, downgoing plate roughness and dip, and volcano locations over a short distance along the arc. Results from joint inversions for Vp, Vp/Vs, and hypocenters from the Tomography Under Costa Rica and Nicaragua (TUCAN) experiment give insight into its geometry and structure. In both Costa Rica and Nicaragua, the intermediate-depth seismic zone is a single layer no more than 10 to 20 km thick. Tomographic images show that throughout Nicaragua and Costa Rica the slowest mantle P wave velocities appear below and behind the volcanic front, indicating likely zones of highest temperature extending 80 to 120 km depth.

[1] We have calculated slab fluid temperatures for 51 volcanoes in 10 subduction zones using the newly developed H 2 O/Ce thermometer. The slab fluid compositions were calculated from arc eruptives, using melt inclusion-based H 2 O contents, and were corrected for background mantle contributions. The temperatures, adjusted to h, the vertical depth to the slab beneath the volcanic arc, range from $730 to 900 C and agree well (within 30 C on average for each arc) with sub-arc slab surface temperatures predicted by recent thermal models. The coherence between slab model and surface observation implies predominantly vertical transport of fluids within the mantle wedge. Slab surface temperatures are well reconciled with the thermal parameter (the product of slab age and vertical descent rate) and h. Arcs with shallow h ($80 to 100 km) yield a larger range in slab surface temperature (up to $200 C between volcanoes) and more variable magma compositions than arcs with greater h ($120 to 180 km). This diversity is consistent with coupling of the subducting slab and mantle wedge, and subsequent rapid slab heating, at $80 km. Slab surface temperatures at or warmer than the H 2 O-saturated solidus suggest that melting at the slab surface is common beneath volcanic arcs. Our results imply that hydrous melts or solute-rich supercritical fluids, and not H 2 O-rich aqueous fluids, are thus the agents of mass transport to the mantle wedge.