Direct measurement of decomposition rates of pure, polycrystalline methane hydrate reveals a thermal regime where methane hydrate metastably "preserves" in bulk by as much as 75 K above its nominal equilibrium temperature (193 K at 1 atm). Rapid release of the sample pore pressure at isothermal conditions between 242 and 271 K preserves up to 93% of the hydrate for at least 24 h, reflecting the greatly suppressed rates of dissociation that characterize this regime. Subsequent warming through the H 2 O ice point then induces rapid and complete dissociation, allowing controlled recovery of the total expected gas yield. This behavior is in marked contrast to that exhibited by methane hydrate at both colder (193-240 K) and warmer (272-290 K) test conditions, where dissociation rates increase monotonically with increasing temperature. Anomalous preservation has potential application for successful retrieval of natural gas hydrate or hydrate-bearing sediments from remote settings, as well as for temporary low-pressure transport and storage of natural gas.

996). 20. We acquired VSP data by firing a 300-cubic-inch (4.9-liter) air gun and recording the shots on a threecomponent Woods Hole Oceanographic Institution borehole seismometer clamped at 8-1-17 intervals from about 150 to 700 mbsf. Ten to 20 air-gun shots were recorded at each depth and summed to form a stacked section. 21. We perform a weighted, damped, least squares inversion of VSP traveltimes for slowness, minimizing L = llT -Zql + %1141, where T. Z, and U are, respectively, the traveltime, depth, and slowness vectors, P is the second derivative of U, and % is the Lagrange pararneter that governs trade-off between data misfit and model smoothness. We chose % to be 2.5 to 3.0 for the inversions shown here. The traveltime datacannot resolve thin (<20 m thick) high-or low-velocity layers.

Inclined zones of earthquakes are the primary expression of lithosphere subduction. A distinct deep population of subduction-zone earthquakes occurs at depths of 350 to 690 kilometers. At those depths ordinary brittle fracture and frictional sliding, the faulting processes of shallow earthquakes, are not expected. A fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics, including the discovery and characterization of transformational faulting, a shear instability connected with localized phase transformations under nonhydrostatic stress. These developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine-rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km. Predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes, the maximum depths of earthquakes in individual subductions zones, and key source characteristics of deep events.

[1] To provide a better understanding of rheological properties of mantle rocks under lithospheric conditions, we carried out a series of experiments on the creep behavior of polycrystalline olivine at high pressures (∼4-9 GPa), relatively low temperatures (673 ≤ T ≤ 1273 K), and anhydrous conditions, using a deformation-DIA. Differential stress and sample displacement were monitored in situ using synchrotron X-ray diffraction and radiography, respectively. Experimental results were fit to the low-temperature. On the basis of this analysis, the low-temperature plasticity of olivine deformed under anhydrous conditions is well constrained by our data with a Peierls stress of s P = 5.9 ± 0.2 GPa, a zero-stress activation energy of E k (0) = 320 ± 50 kJ mol, and A P = 1.4 × 10. Compared with published results for high-temperature creep of olivine, a transition from low-temperature plasticity to high-temperature creep occurs at ∼1300 K for a strain rate of ∼10 −5 s −1 . For a geological strain rate of 10 −14 s −1 , extrapolation of our low-temperature flow law to 873 K, the cutoff temperature for earthquakes in the mantle, yields a strength of ∼600 MPa. The low-temperature, high-stress flow law for olivine in this study provides a solid basis for modeling tectonic processes occurring within Earth's lithosphere.

[1] Methane clathrate hydrate (structure I) is found to be very strong, based on laboratory triaxial deformation experiments we have carried out on samples of synthetic, high-purity, polycrystalline material. Samples were deformed in compressional creep tests (i.e., constant applied stress, s), at conditions of confining pressure P = 50 and 100 MPa, strain rate 4.5 Â 10 À8 _ e 4.3 Â 10 À4 s À1 , temperature 260 T 287 K, and internal methane pressure 10 P CH4 15 MPa. At steady state, typically reached in a few percent strain, methane hydrate exhibited strength that was far higher than expected on the basis of published work. In terms of the standard high-temperature creep law, _ e = As n e À(E*+PV*)/RT the rheology is described by the constants A = 10 8.55 MPa Àn s À1 , n = 2.2, E* = 90,000 J mol À1 , and V* = 19 cm 3 mol À1 . For comparison, at temperatures just below the ice point, methane hydrate at a given strain rate is over 20 times stronger than ice, and the contrast increases at lower temperatures. The possible occurrence of syntectonic dissociation of methane hydrate to methane plus free water in these experiments suggests that the high strength measured here may be only a lower bound. On Earth, high strength in hydratebearing formations implies higher energy release upon decomposition and subsequent failure. In the outer solar system, if Titan has a 100-km-thick near-surface layer of highstrength, low-thermal conductivity methane hydrate as has been suggested, its interior is likely to be considerably warmer than previously expected.