Experimental determination of melt interconnectivity and electrical conductivity in the upper mantle

Abstract

International audienceThe presence of a small fraction of basaltic melt is a potential explanation for mantle electrical conductivity anomalies detected near the top of the oceanic asthenosphere. The interpretation of magnetotelluric profiles in terms of the nature and proportion of melt, however, relies on mathematical models that have not been experimentally tested at realistically low melt fractions (<0.01). In order to address this, we have performed in situ electrical conductivity measurements on partially molten olivine aggregates. The obtained data suggest that the bulk conductivity follows the conventional Archie's law with the melt fraction exponents of 0.75 and 1.37 at melt fractions greater and smaller than 0.5 vol.% respectively at 1350 °C. Our results imply multiple conducting phases in melt-bearing olivine aggregate and a connectedness threshold at ∼0.5 vol.% of melt. The model predicts that the conductive oceanic upper asthenosphere contains 0.5 to 1 vol.% of melt, which is consistent with the durable presence of melt at depths over millions years while the oceanic plates spread apart at the mid-ocean ridge. Beneath ridges a minimum permeability may allow mid-ocean ridge basalts to rise out of the mantle, where our model indicates that melt is present in proportions of up to 4 vol.%

Influence of High Conductive Magnetite Impurity on the Electrical Conductivity of Dry Olivine Aggregates at High Temperature and High Pressure

“…To explain the anomalous high conductivity in geophysical field observations of magnetotelluric and geomagnetic deep sounding data, many in situ laboratory-based high-pressure conductivity studies have been reported for relevant minerals and rocks in recent decades. Some potential explanations for high conductivity anomalies in the deep interior of the Earth and other planets have been proposed such as the hydrogen-bearing of nominally anhydrous minerals [1,2], dehydration of minerals [3,4], partial melting [5,6], dehydrogenation of minerals [7], anisotropy [8], and the presence of an interconnected high conductivity impurity phase [9].…”

Metallized ε-FeOOH and the heterogeneous electrical conductivity structure in the lower mantle

“…6). In addition, when hydrous ε-FeOOH meets hotter regions in the mantle, it causes dehydration melting that would further boost the overall transport properties 63 . We therefore suggest that those localized domains with topped 10 2 S/m EC may have incorporated higher concentration of ε-FeOOH, or the enrichment of other superior EC components.…”

A földköpeny reológiai kutatása: mennyiségi Fourier transzformációs infravörös spektrometria alkalmazása egy Persány hegységi xenolit példáján

“…JONES et al , 2011FULLEA 2017), továbbá a kis mennyiségben jelen lévő mafikus (pl. NI et al 2011, MILLER et al 2015, LAUMONIER et al 2017 vagy karbonátos olvadék jelenlétét is (SIFRÉ et al 2014).…”