New Constraints on the Structure and Composition of the Lithospheric Mantle Beneath the Slave Craton, Nw Canada from 3-D Magnetotelluric Data – Origin of the Central Slave Mantle Conductor and Possible Evidence for Lithospheric Scale Fluid Flow

Abstract

We use previously collected magnetotelluric (MT) data to produce a new 3-D resistivity model of the lithosphere beneath the Slave craton to improve our knowledge of its lithospheric architecture. A region of low resistivity (< 10 Ωm) at ~100 km depth, coined the Central Slave Mantle Conductor (CSMC) in past studies, was confirmed and better resolved in the central Slave craton. Earlier studies interpreted the CSMC as graphite films but further review indicates that it is unlikely that graphite films are stable/continuous at this depth. This implies that upper mantle conductors should not be used as an indicator of carbon concentration in the mantle to prospect for diamondiferous regions studies. Using recent laboratory experiments we review alternative conduction mechanisms for the CSMC including (a) metasomatic phlogopite, (b) grain boundary sulphides and (c) high density fluids (HDFs) emplaced from a Mesozoic subduction event. None of these hypotheses are without flaws. HDFs may undergo wall rock reaction and hence solidification; unreasonably large quantities of phlogopite (>20% volume) are required to reproduce the observed resistivity and grain boundary sulphides when found have not formed inter-connected films though experiments under oxidising conditions indicate they may do so. Our resistivity model indicates a lithospheric thickness of 210 ± 10 km beneath the observable Slave craton, consistent with seismology and xenoliths. A water content of 10 – 150 ppm at 100 – 170 km depth in the lithospheric mantle is inferred from resistivity, broadly agreeing with estimates from mantle xenoliths from the central Slave craton lithosphere. Below 170 km the bulk lithosphere is dry (< 10 ppm), which may prevent the cratonic root from being eroded by the underlying asthenosphere. Low resistivity fingers extend to surface in multiple locations that may represent paleo-fluid flow pathways through the lithosphere, broadly resembling those observed beneath Olympic Dam, Australia.

Keywords: Magnetotellurics, 3-D inversion, Lithospheric thickness, Slave craton, Electrical resistivity