Hirth Greg

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Subject: Magnetotellurics ×

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  • Article
    Electrical structure beneath the northern MELT line on the East Pacific Rise at 15°45′S
    (American Geophysical Union, 2006-11-16) Baba, Kiyoshi ; Tarits, Pascal ; Chave, Alan D. ; Evans, Rob L. ; Hirth, Greg ; Mackie, Randall L.
    The electrical structure of the upper mantle beneath the East Pacific Rise (EPR) at 15°45′S is imaged by inverting seafloor magnetotelluric data obtained during the Mantle ELectromagnetic and Tomography (MELT) experiment. The electrical conductivity model shows no evidence for a conductive region immediately beneath the ridge, in contrast to the model previously obtained beneath the EPR at 17°S. This observation can be explained by differences in current melt production along the ridge, consistent with other observations. The mantle to the east of the ridge at 60 –100 km depth is anisotropic, with higher conductivity in the spreading direction compared to the along-strike direction, similar to the 17°S region. The high conductivity in the spreading direction can be explained by a hydrated mantle with strain-induced lattice preferred orientation of olivine or by partial melt preferentially connected in the spreading direction.
  • Article
    The electrical structure of the central Pacific upper mantle constrained by the NoMelt experiment
    (John Wiley & Sons, 2015-04-18) Sarafian, Emily K. ; Evans, Rob L. ; Collins, John A. ; Elsenbeck, James R. ; Gaetani, Glenn A. ; Gaherty, James B. ; Hirth, Greg ; Lizarralde, Daniel
    The NoMelt experiment imaged the mantle beneath 70 Ma Pacific seafloor with the aim of understanding the transition from the lithosphere to the underlying convecting asthenosphere. Seafloor magnetotelluric data from four stations were analyzed using 2-D regularized inverse modeling. The preferred electrical model for the region contains an 80 km thick resistive (>103 Ωm) lithosphere with a less resistive (∼50 Ωm) underlying asthenosphere. The preferred model is isotropic and lacks a highly conductive (≤10 Ωm) layer under the resistive lithosphere that would be indicative of partial melt. We first examine temperature profiles that are consistent with the observed conductivity profile. Our profile is consistent with a mantle adiabat ranging from 0.3 to 0.5°C/km. A choice of the higher adiabatic gradient means that the observed conductivity can be explained solely by temperature. In contrast, a 0.3°C/km adiabat requires an additional mechanism to explain the observed conductivity profile. Of the plausible mechanisms, H2O, in the form of hydrogen dissolved in olivine, is the most likely explanation for this additional conductivity. Our profile is consistent with a mostly dry lithosphere to 80 km depth, with bulk H2O contents increasing to between 25 and 400 ppm by weight in the asthenosphere with specific values dependent on the choice of laboratory data set of hydrous olivine conductivity and the value of mantle oxygen fugacity. The estimated H2O contents support the theory that the rheological lithosphere is a result of dehydration during melting at a mid-ocean ridge with the asthenosphere remaining partially hydrated and weakened as a result.