Petrology‐based modeling of mantle melt electrical conductivity and joint interpretation of electromagnetic and seismic results

Abstract

The presence of melt in the Earth's interior depends on the thermal state, bulk chemistry, and dynamics. Therefore, the investigation of the physical and chemical properties of melt is a probe of the planet's structure, dynamics, and potentially evolution. Here we explore melt properties by interpreting geophysical data sets sensitive to the presence of melt (electromagnetic and seismic) with considerations of petrology and, in particular, peridotite partial melting. We present a petrology-based model of the electrical conductivity of fertile and depleted peridotites during partial melting. Seismic and magnetotelluric (MT) studies do not necessarily agree on melt fraction estimates, a possible explanation being the assumptions made about melt chemistry as part of MT data interpretation. Melt fraction estimates from electrical anomalies usually assume a basaltic melt phase, whereas petrological knowledge suggests that the first liquids produced have a different chemistry, and thus a different conductivity. Our results show that melts produced by low-degree peridotite melting (< 15 vol %) are up to 5 times more conductive than basaltic liquids. Such conductive melts significantly affect bulk rock conductivity. Application of our electrical model to magnetotelluric results suggests melt fractions that are in good agreement with seismic estimates. With the aim of a simultaneous interpretation of electrical and seismic data, we combine our electrical results with seismic velocity considerations in a joint model of partial melting. Field electrical and seismic anomalies can be explained by~1 vol % melt beneath Hawaii and~1-8 vol % melt beneath the Afar Ridge.

Why is Africa rifting?

“…There appears to be large volumes of melt beneath parts of Afar that extend well into the mantle (Desissa et al 2013). There is some discrepancy between the estimated melt volumes from magnetotelluric and seismic methods, but the effect of melt composition may explain some of this (Pommier & Garnero 2014). Linking seismic and electrical images of melt and their anisotropy is an ongoing challenge.…”

Heterogeneity and anisotropy in the lithospheric mantle

“…It depends weakly on the mineralogical composition, but it might (or not) be highly sensitive to hydrogen incorporated as a trace element in olivine (there is no consensus in the experimental data; cf., Poe et al, 2010;Yoshino and Katsura, 2013;Dai and Karato, 2014). Electrical conductivity also depends on oxygen fugacity (Duba et al, 1974) and on the presence and interconnectivity of minor, but highly conductive phases, like graphite, sulfides, or melts along grain boundaries (Ducea and Park, 2000;Jones et al, 2001;Wang et al, 2013a;Pommier and Garnero, 2014;Sifré et al, 2014). Joint seismic and magnetotelluric surveys may help discriminating between these effects, since the sensitivities of seismic and electrical properties to each parameter differ.…”

Geophysical and petrological constraints on ocean plate dynamics

“…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]. This means that MT is in fact able to detect very small silicate melt fractions (typically on the order of 0.1-0.2 vol%) at the onset of melting where CO2 is concentrated in the melt.…”

“…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].…”