New work by Lawrence Livermore National Laboratory (LLNL) scientists shows the dispersal of water (incorporated as hydrogen in olivine, the most abundant mineral in the upper mantle), could account for high electrical conductivity seen in the asthenosphere (part of the upper mantle just below the lithosphere that is involved in plate tectonic movement).
Minerals formed deep in the mantle and transported to the Earth’s surface contain tens to hundreds of parts per million in weight (ppm wt) of water, providing evidence for the presence of dissolved water in the Earth’s interior. Even at these low concentrations, water greatly affects the physico-chemical properties of mantle materials. The diffusion of hydrogen controls the transport of water in the Earth’s upper mantle, but until now was not fully understood for olivine.
* new analysis determines there is five times more water in the mantle than the previous model and looks at a previously poorly understood rock that has water in its structure
A mantle nodule collected from San Carlos, Arizona, brought to the surface during a deep volcanic eruption about 1 million years ago. Olivine, which is the focus of the LLNL study, is the predominant light green-colored mineral that is present in this rock. Photo by Wyatt Du Frane/LLNL
Earth’s hydrosphere is a distinctive feature of our planet where massive oceans affect its climate and support its ecosystem. The distribution of water on Earth is not limited to its outermost shell (hydrosphere and hydrated minerals), but extends to great depths within the planet. Downwelling oceanic lithosphere (at subduction zones) and upwelling magmas (at mid ocean ridges, volcanoes and hotspots) are vehicles for transport of H2O between the surface and the Earth’s deep interior.
Olivine electrical conductivity as a function of inverse temperature (in Kelvin) and H2O content. Calculations are shown for dry olivine36 as well as hydrous (this study), with 10, 80 and 250 ppm wt H2O. Yellow bands display electrical conductivity anomalies measured in the asthenosphere16,17,18, 48 (horizontal) and adiabatic temperatures calculated at such depths (~1350–1450 °C49, vertical). Temperatures in degrees °C are also shown at the top of the diagram.
“The amount of hydrogen required to match geophysical measurements of electrical conductivity inside Earth are in line with the concentrations that are observed in oceanic basalts. This demonstrates that geophysical measurements of electrical conductivity are a promising tool for mapping out water distributions deep inside the Earth,” Du Frane said.
Abstract
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 (less than 250 ppm wt) can account for high electrical conductivity values (10−2–10−1 S/m) observed in the asthenosphere.
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