Electrical conductivity of continental lithospheric mantle from integrated geophysical and petrological modeling: Application to the Kaapvaal Craton and Rehoboth Terrane, southern Africa

Abstract

[1] The electrical conductivity of mantle minerals is highly sensitive to parameters that characterize the structure and state of the lithosphere and sublithospheric mantle, and mapping its lateral and vertical variations gives insights into formation and deformation processes. We review state-of-the-art conductivity models based on laboratory studies for the most relevant upper mantle minerals and define a bulk conductivity model for the upper mantle that accounts for temperature, pressure, and compositional variations. The bulk electrical conductivity model has been integrated into the software package LitMod, which allows for petrological and geophysical modeling of the lithosphere and sublithospheric upper mantle within an internally consistent thermodynamic-geophysical framework. We apply our methodology to model the upper mantle thermal structure and hydrous state of the western block of the Archean Kaapvaal Craton and the Proterozoic Rehoboth Terrane, in southern Africa, integrating different geophysical and petrological observables: namely, elevation, surface heat flow, and magnetotelluric and xenolith data. We find that to fit the measured magnetotelluric responses in both the Kaapvaal and Rehoboth terranes, the uppermost depleted part of the lithosphere has to be wetter than the lowermost melt-metasomatized and refertilized lithospheric mantle. We estimate present-day thermal lithosphere-asthenosphere boundary (LAB) depths of 230-260 and 150 ± 10 km for the western block of the Kaapvaal and Rehoboth terranes, respectively. For the Kaapvaal, the depth of the present-day thermal LAB differs significantly from the chemical LAB, as defined by the base of a depleted mantle, which might represent an upper level of melt percolation and accumulation within the lower lithosphere.

Deformation and hydration of the lithospheric mantle beneath the Kaapvaal craton, South Africa

“…the Fig. 8 in Fullea et al, 2011) suggests however that the vertical variation of water contents in olivine observed in the present study and in Peslier et al (2010) may be representative of the present-day hydration state of the Kaapvaal mantle, favoring the last scenario and implying that metasomatism resulted in extensive re-hydration of the cratonic mantle at intermediate depths.…”

The electrical resistivity of Canada’s lithosphere and correlation with other parameters: contributions from Lithoprobe and other programmes

“…13. Log(resistivity)-temperature (T) relationships defined by pure olivine that is dry or contains 15, 30, 100, or 200 ppm water, including small polaron, magnesium vacancy, and proton conduction, using the equations given in Fullea et al (2011) with the water model of Jones et al (2012). The dots are the resistivity-temperature pairs for 100 km depth (red) and 200 km depth (black).…”

“…We can determine ambient temperature from electrical resistivity, given the exponential effect of temperature on resistivity, if we know the mineral composition of the mantle (a second-order effect) and water content of these minerals (Fullea et al 2011;Jones et al 2013), and there is no contribution to conduction from exotic minor phases, such as carbon in graphite form (Duba and Shankland 1982;Jones et al 2003) or sulphides (Ducea and Park 2000). These are reasonable assumptions for cratonic mantle lithosphere , and have been used by Ledo and Jones (2005) to put bounds on the temperature of the upper mantle beneath part of the Yukon, NW Canada (discussed later in the text).…”

“…Olivine-dominated dry cratonic lithosphere would have a value of the order of 12 S resistivity-temperature relation for small polaron and magnesium vacancy conduction, derived using the formulae inFullea et al (2011), is shown inFig. 13(solid black line).…”

An audio-magnetotelluric investigation of the Otjiwarongo and Katima Mulilo regions, Namibia

“…More recently, lithospheric 2D conductivity models have been produced for parts of South Africa, Namibia, and Botswana that indicate variations in conductivity between the older cratonic regions and younger surrounding mobile belts (Muller et al, 2009;Evans et al, 2011;Miensopust et al, 2011;Khoza et al, 2013). Furthermore, the conductivity models were used to make inferences about the thermal, mechanical, and chemical properties of the geologic terranes (Fullea et al, 2011) and to provide control on laboratory measurements of water in olivine . The correlation between diamond prospectivity and regional resistive/conductive boundaries in Namibia and South Africa has also been examined with SAMTEX results (Jones et al, 2009b;Muller et al, 2009).…”