The Electrical Structure of the 70Ma Pacific LAB Constrained by the NoMelt Experiment

Abstract:

Previous investigations on the electrical structure of the oceanic lithosphere-asthenosphere boundary (LAB) have found a highly resistive lithosphere underlain by a highly conductive and typically anisotropic upper asthenosphere [Evans et al., 2005; Baba et al., 2010; Naif et al., 2014]. These studies have provided very influential interpretations of the oceanic LAB structure, which generally depend on either the presence of partial melt or dissolved hydrogen to act as the weakening mechanism in the upper asthenosphere. While these earlier observations have resulted in an unprecedented view of the oceanic upper mantle, they are limited in scope focusing primarily on mid-ocean ridges or subduction zones. Thus, the observed electrical structure and resultant preferred weakening mechanism may not be descriptive of the broad ocean basin. The NOMELT experiment focused on understanding the electrical structure of 70Ma oceanic lithosphere and underlying asthenosphere in order to compare the LAB structure of a stable system to those previously observed. The preferred electrical resistivity model for the NOMELT region is isotropic and does not contain a highly conductive layer under the 70-80 km thick resistive lithosphere. This lack of a conductive layer suggests that partial melt is not present in a well-connected network within the lithosphere-asthenosphere boundary of 70Ma oceanic plate, in contrast to other regions [Evans et al., 2005; Baba et al., 2010; Naif et al., 2014]. The lack of anisotropy within the upper asthenosphere is also in contrast to previous electromagnetic studies of oceanic settings that invoked a more hydrous asthenosphere [Evans et al., 2005]. The observed conductivity profiles seem to suggest a thermal or hydrous mechanism at work in the stable oceanic upper mantle with water contents ranging between 0-400 ppm dependent on the choice of thermal gradient, laboratory dataset 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.