Electrical conductivity during incipient melting in the oceanic low-velocity zone

Abstract

A low viscosity layer in the upper mantle, the Asthenosphere, is a requirement for plate tectonics1. The seismic low velocities and the high electrical conductivities of the Asthenosphere are attributed either to sub-solidus water-related defects in olivine minerals2-4 or to a few volume percents of partial melt5-8 but these two interpretations have shortcomings: (1) The amount of H2O stored in olivine is not expected to be higher than 50 ppm due to partitioning with other mantle phases9, including pargasite amphibole at moderate temperatures10, and partial melting at high temperatures9; (2) elevated melt volume fractions are impeded by the too cold temperatures prevailing in the Asthenosphere and by the high melt mobility that can lead to gravitational segregation11,12. Here we determined the electrical conductivity of CO2-H2O-rich melts, typically produced at the onset of mantle melting. Electrical conductivity modestly increases with moderate amounts of H2O and CO2 but it dramatically increases as CO2 content exceeds 6 wt% in the melt. Incipient melts, long-expected to prevail in the asthenosphere10,13-15, can therefore trigger its high electrical conductivities. Considering depleted and enriched mantle abundances in H2O and CO2 and their effect on the petrology of incipient melting, we calculated conductivity profiles across the Asthenosphere for various plate ages. Several electrical discontinuities are predicted and match geophysical observations in a consistent petrological and geochemical framework. In moderately aged plates (>5Ma), incipient melts most likely trigger both the seismic low velocities and the high electrical conductivities in the upper part of the asthenosphere, whereas for young plates4, where seamount volcanism occurs6, higher degree of melting is expected.

Geophysical and petrological constraints on ocean plate dynamics

“…Other effects on melt conductivity are Na content, which can increase conductivity at the incipient stages of melting [Pommier and Garnero, 2014] and CO2 content, which at the very early stages of melting can dramatically increase conductivity [Sifre et al, 2014]. The above studies indicate the effect of melt on conductivity is greater than previously thought when interpreting field results [Pommier and Garnero, 2014;Sifre et al, 2014].…”

“…In addition, dissolved H2O can enhance the electrical conductivity of mantle minerals [Wang et al, 2006;Yoshino et al, 2009;Poe et al, 2010;Dai and Karato, 2014], yet there is still considerable disagreement between the various laboratories reporting conductivity measurements on hydrous olivine (see discussion in Evans [2012] and Jones et al [2012]). A recent analysis has attempted to reconcile the published data with a reanalysis of all data sets resulting in a single "unified hydrous olivine" model (UHO) [Gardes et al, 2014]. Nonetheless, subsequent studies have reopened the controversy, with results apparently suggesting that two conduction mechanisms are operative, one at low temperatures expected in the lithosphere and one at asthenospheric temperatures [Dai and Karato, 2014].…”

“…Shankland and Waff [1977] showed that silicate melt is a semiconductor with conductivity dependent on temperature and weakly on pressure. Partial melts of mantle materials have conductivities in the range of 1-10 S/m, substantially higher than anhydrous subsolidus olivine, although the conductivity of a particular melt depends on composition (H2O and CO2 in particular) [Yoshino et al, 2012;Sifre et al, 2014], temperature, and pressure [Roberts and Tyburczy, 1999;Toffelmier and Tyburczy, 2007;Yoshino et al, 2012;Sifre et al, 2014]. For example, at constant pressure the addition of H2O to melt promotes an increase in conductivity [e.g., Ni et al, 2011;Pommier et al, 2008].…”

Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere

“…Receiver-function studies and underside reflections of SS precursors detect a sharp velocity discontinuity (the Gutenberg discontinuity) at a depth of 40 to 150 km (4, 6, 10-12). The sharpness of this discontinuity may indicate the presence of a small amount of melt (4), and the lack of a strong dependence of the discontinuity depth on plate age (6, 10, 13) is roughly consistent with a carbonated peridotite fluid (7,14). Alternatively, a transition from water-poor to water-rich conditions could establish a sharp reduction in seismic velocity.…”

Ultralow viscosity of carbonate melts at high pressures

“…Kimberlite magma is characterized by low silica with high magnesium and a C-O-H volatile-rich composition [5][6][7][8] , and is thought to be formed in the presence of H 2 O and CO 2 under conditions close to the carbonate-peridotite solidus in the Earth's mantle [9][10][11] . Small amounts of carbonate-rich melt may also be present in the asthenosphere beneath mid-ocean ridges, according to recent electrical conductivity studies 12,13 and high-pressure and hightemperature experiments 10,[14][15][16] . In addition, strongly carbonated silicate melt is thought to be partly responsible for the presence of oceanic low-velocity zones 10,17 .…”