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

Previously collected magnetotelluric data were used to produce an improved 3-D resistivity model of the lithosphere beneath the Slave craton. A region of low resistivity (<10 Ωm) at ∼100 km depth was imaged beneath the central Slave craton, and identified as the Central Slave Mantle Conductor (CSMC) reported in previous studies. Earlier studies interpreted the CSMC as graphite films but new analysis from laboratory experiments indicates that it is unlikely that graphite films are stable/continuous at this depth. 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 including hydrous carbonatite melts or brines. None of these hypotheses are without flaws requiring either unreasonably high-volume fractions or have petrological challenges. However, brines are the preferred explanation as they (1) can explain geophysical observations, (2) are observed in fibrous diamonds coincident with the CSMC and (3) require small volume fractions (<1%). We hypothesize their emplacement was related to a Mesozoic subduction event. This implies that upper mantle conductors should not be used as an indicator of carbon concentration in the mantle to prospect for diamondiferous regions. Our resistivity model indicates a lithospheric thickness of 210 ± 10 km beneath the Slave craton, consistent with seismic and xenolith studies. 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 explain why the cratonic root has been resistant to erosion 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.