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Sci Rep ; 7(1): 5344, 2017 07 13.
Artigo em Inglês | MEDLINE | ID: mdl-28706289


Nominally anhydrous minerals formed deep in the mantle and transported to the Earth's surface contain tens to hundreds of ppm wt H2O, providing evidence for the presence of dissolved water in the Earth's interior. Even at these low concentrations, H2O greatly affects the physico-chemical properties of mantle materials, governing planetary dynamics and evolution. The diffusion of hydrogen (H) controls the transport of H2O in the Earth's upper mantle, but is not fully understood for olivine ((Mg, Fe)2SiO4) the most abundant mineral in this region. Here we present new hydrogen self-diffusion coefficients in natural olivine single crystals that were determined at upper mantle conditions (2 GPa and 750-900 °C). Hydrogen self-diffusion is highly anisotropic, with values at 900 °C of 10-10.9, 10-12.8 and 10-11.9 m2/s along [100], [010] and [001] directions, respectively. Combined with the Nernst-Einstein relation, these diffusion results constrain the contribution of H to the electrical conductivity of olivine to be σH = 102.12S/m·CH2O·exp-187kJ/mol/(RT). Comparisons between the model presented in this study and magnetotelluric measurements suggest that plausible H2O concentrations in the upper mantle (≤250 ppm wt) can account for high electrical conductivity values (10-2-10-1 S/m) observed in the asthenosphere.

Sci Adv ; 2(5): e1501631, 2016 05.
Artigo em Inglês | MEDLINE | ID: mdl-27386526


Mantle wedge regions in subduction zone settings show anomalously high electrical conductivity (~1 S/m) that has often been attributed to the presence of aqueous fluids released by slab dehydration. Laboratory-based measurements of the electrical conductivity of hydrous phases and aqueous fluids are significantly lower and cannot readily explain the geophysically observed anomalously high electrical conductivity. The released aqueous fluid also rehydrates the mantle wedge and stabilizes a suite of hydrous phases, including serpentine and chlorite. In this present study, we have measured the electrical conductivity of a natural chlorite at pressures and temperatures relevant for the subduction zone setting. In our experiment, we observe two distinct conductivity enhancements when chlorite is heated to temperatures beyond its thermodynamic stability field. The initial increase in electrical conductivity to ~3 × 10(-3) S/m can be attributed to chlorite dehydration and the release of aqueous fluids. This is followed by a unique, subsequent enhancement of electrical conductivity of up to 7 × 10(-1) S/m. This is related to the growth of an interconnected network of a highly conductive and chemically impure magnetite mineral phase. Thus, the dehydration of chlorite and associated processes are likely to be crucial in explaining the anomalously high electrical conductivity observed in mantle wedges. Chlorite dehydration in the mantle wedge provides an additional source of aqueous fluid above the slab and could also be responsible for the fixed depth (120 ± 40 km) of melting at the top of the subducting slab beneath the subduction-related volcanic arc front.

Cloretos/química , Desidratação , Condutividade Elétrica , Sedimentos Geológicos , Modelos Teóricos
Sci Adv ; 2(5): e1600246, 2016 05.
Artigo em Inglês | MEDLINE | ID: mdl-27386548


The low-velocity zone (LVZ) is a persistent seismic feature in a broad range of geological contexts. It coincides in depth with the asthenosphere, a mantle region of lowered viscosity that may be essential to enabling plate motions. The LVZ has been proposed to originate from either partial melting or a change in the rheological properties of solid mantle minerals. The two scenarios imply drastically distinct physical and geochemical states, leading to fundamentally different conclusions on the dynamics of plate tectonics. We report in situ ultrasonic velocity measurements on a series of partially molten samples, composed of mixtures of olivine plus 0.1 to 4.0 volume % of basalt, under conditions relevant to the LVZ. Our measurements provide direct compressional (V P) and shear (V S) wave velocities and constrain attenuation as a function of melt fraction. Mantle partial melting appears to be a viable origin for the LVZ, for melt fractions as low as ~0.2%. In contrast, the presence of volatile elements appears necessary to explaining the extremely high V P/V S values observed in some local areas. The presence of melt in LVZ could play a major role in the dynamics of plate tectonics, favoring the decoupling of the plate relative to the asthenosphere.

Sedimentos Geológicos , Geologia , Modelos Teóricos