UC Riverside

The origin of intermediate depth earthquakes has been debated for 90 years yet is still under active discussion. These earthquakes are localized in double seismic zones in descending lithosphere; both zones originate very close to oceanic trenches. A leading proposed initiation mechanism for these earthquakes since 1968 has been dehydration embrittlement of serpentine under stress. Despite the considerable evidence favoring this mechanism, a major argument against it has been that the lower seismic zone initiates at ~40 km depth almost immediately below trenches and there does not appear to be a vehicle to carry water sufficiently deep to hydrate otherwise dry lithosphere. To directly address this problem, an experimental study has been carried out to investigate the minimum amount of serpentine that is required to trigger the dehydration embrittlement instability in serpentinized peridotite at high pressure (1-3 GPa) and temperature (720-750˚C). The results show that embrittlement occurs during dehydration of antigorite (the phase of serpentine stable at elevated pressure) in a wide range of compositions but both nearly dry peridotite and extensively altered peridotite are ductile. Fresh, unaltered, synthetic harzburgite and harzburgite with 4 vol% distributed antigorite are ductile, as are specimens with greater than 65% antigorite. Only compositions between 8 vol% and 65 vol% antigorite develop the instability. We suggest that very small degrees of serpentinization do not release sufficient H2O to trigger the instability and that extensive serpentinization avoids the instability because soft, ductile, antigorite becomes the interconnected matrix with olivine and pyroxene existing only as isolated crystals. In that case, dehydration simply facilitates flow.

These systematics suggest that small amounts of H2O transported down deep normal (bending) faults at trenches are sufficient to enable the instability in the lower seismic zones, thus providing additional support for dehydration embrittlement as the mechanism of intermediate-depth earthquakes. At the other end of the spectrum of serpentinization, these results are consistent with previous suggestions that extensive dehydration of altered subducting crust and mantle release copious amounts of H2O that rise to the surface of the descending slab and react with the cool mantle of the overlying plate to lead to extensive serpentinization, thereby explaining serpentine diapers in the forearc and lack of seismicity along the plate interface deeper than about 35 km.

Another long-lived controversy in mantle geophysics involves the pressure dependence of creep in olivine, the most abundant and softest phase in unaltered mantle rocks. The pressure dependence of any thermodynamically-controlled phenomenon is commonly expressed as the activation volume, ∆V*. Previous experimental investigations on the effect of pressure on creep in olivine have produced bimodal results. ΔV* obtained from solid-medium (Griggs) and gas-medium (Paterson) deformation apparatus at relatively lower pressures is ~15 cm3/mol or higher. In contrast, higher-pressure studies using multianvil apparatus at sites of synchrotron X-radiation (D-DIA apparatus) report a ΔV* near zero. To decipher this enigma and to provide a much-needed calibration of stress in the D-DIA apparatus, I have conducted systematic experiments on a synthetic, iron-free, forsterite at 1200 ˚C and pressure between 1 and 2.5 GPa using the UCR 5 GPa modified Griggs apparatus, the only apparatus capable of performing these experiments. Our results show a robust ΔV* value of 12 ± 2 cm3/mol, indicating a fairly significant pressure dependence of creep in olivine to pressures of ~3 GPa (approximately 100 km). In collaboration with other experimentalists, we plan to measure the ΔV* for creep of this material over a pressure range of 2-8 GPa in the D-DIA apparatus to both calibrate stress measurement in the D-DIA and resolve the question of change in ΔV* at higher pressures.