Electrical properties of methane hydrate + sediment mixtures

Knowledge of the electrical properties of multicomponent systems with gas hydrate, sediments, and pore water is needed to help relate electromagnetic (EM) measurements to specific gas hydrate concentration and distribution patterns in nature. Toward this goal, we built a pressure cell capable of measuring in situ electrical properties of multicomponent systems such that the effects of individual components and mixing relations can be assessed. We first established the temperature-dependent electrical conductivity (?) of pure, single-phase methane hydrate to be ~5 orders of magnitude lower than seawater, a substantial contrast that can help differentiate hydrate deposits from significantly more conductive water-saturated sediments in EM field surveys. Here we report ? measurements of two-component systems in which methane hydrate is mixed with variable amounts of quartz sand or glass beads. Sand by itself has low ? but is found to increase the overall ? of mixtures with well-connected methane hydrate. Alternatively, the overall ? decreases when sand concentrations are high enough to cause gas hydrate to be poorly connected, indicating that hydrate grains provide the primary conduction path. Our measurements suggest that impurities from sand induce chemical interactions and/or doping effects that result in higher electrical conductivity with lower temperature dependence. These results can be used in the modeling of massive or two-phase gas-hydrate-bearing systems devoid of conductive pore water. Further experiments that include a free water phase are the necessary next steps toward developing complex models relevant to most natural systems

Transformation kinetics for olivine with ~75 ppm H_2O into ringwoodite

The existence of metastable olivine (MO) has been debated as a possible trigger of deep focus earthquakes, but of equal importance its existence may constrain the amount of hydrogen being brought to Earth’s mantle transition zone (MTZ) via subduction. The rate which olivine (ol) transforms into wadsleyite and ringwoodite (rw) is dependent on hydrogen content, and determines the likelihood that metastable olivine could persist into the MTZ. Previous results indicate that 300 ppm of H_2O is too much to allow for MO [1,2]. How much is too much? We present rw rim growth rates for olivine containing as little as 75 ppm of H_2O

Ringwoodite growth rates from olivine with ~ 75 ppmw H_2O: Metastable olivine must be nearly anhydrous to exist in the mantle transition zone

It has been previously demonstrated that as little as 300 ppmw H_2O increases wadsleyite and ringwoodite growth rates to magnitudes that are inconsistent with the metastable olivine hypothesis. To further test this hypothesis, we present new ringwoodite growth rate measurements from olivine with ∼75 ppmw H_2O at 18 GPa and 700, 900, and 1100 °C. These growth rates are nearly identical to those from olivine with ∼300 ppmw H_2O, and significantly higher than those from nominally anhydrous olivine. We infer that transformation of olivine with 75–300 ppmw H_2O is primarily enhanced by hydrolytic weakening of reaction rims, which reduces the elastic strain-energy barrier to growth. We present a new method for fitting non-linear nominally anhydrous data, to demonstrate that reduction of growth rates by elastic strain energy is an additional requirement for metastable olivine. Based on previous thermokinetic modeling, these enhanced growth rates are inconsistent with the persistence of metastable olivine wedges into the mantle transition zone. Metastable persistence of olivine into the mantle transition-zone would therefore require <75 ppmw H_2O

Electrical properties of polycrystalline methane hydrate

Electromagnetic (EM) remote-sensing techniques are demonstrated to be sensitive to gas hydrate concentration and distribution and complement other resource assessment techniques, particularly seismic methods. To fully utilize EM results requires knowledge of the electrical properties of individual phases and mixing relations, yet little is known about the electrical properties of gas hydrates. We developed a pressure cell to synthesize gas hydrate while simultaneously measuring in situ frequency-dependent electrical conductivity (?). Synthesis of methane (CH4) hydrate was verified by thermal monitoring and by post run cryogenic scanning electron microscope imaging. Impedance spectra (20 Hz to 2 MHz) were collected before and after synthesis of polycrystalline CH4 hydrate from polycrystalline ice and used to calculate ?. We determined the ? of CH4 hydrate to be 5 × 10?5 S/m at 0°C with activation energy (Ea) of 30.6 kJ/mol (?15 to 15°C). After dissociation back into ice, ? measurements of samples increased by a factor of ?4 and Ea increased by ?50%, similar to the starting ice samples