Geophysical Detection of Relict Metasomatism from an Archean (~3.5 Ga) Subduction Zone

Geophysical Detection of Relict Metasomatism from an Archean (~3.5 Ga) Subduction Zone Teleseismic receiver-function (RF) and magnetotelluric (MT) profiles across the Slave craton. (a) Color-coded seismic profile along the line A-A' (Fig. 1). Red and blue colors effectively represent positive (downward slow-to-fast) and negative (downward fast to slow) seismic velocity contrasts, respectively. M, Moho; R1 and R2, free-surface reverberations. (b) MT-derived resistivity profile [Jones et al., 2003] across the line B-B' shows the central Slave conductive anomaly (red to yellow) at 80-120 depth. The solid black line highlights the base of the conductive anomaly within the region of interest (dashed black box), repeated to scale in RF profiles for comparison. (c) Close-up RF traces for bins in the region of interest.
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The origin of Archean cratonic lithosphere is the subject of much debate. Geological and geochemical data are consistent with a plate tectonic origin involving assembly of predominantly oceanic terranes and micro continents into large stable cratons. Deep probing geophysical surveys of Archean cratons provide a means of understanding the processes responsible for the formation of these earliest continents, although the signals are complex and present-day cratons are only the surviving remnants of once larger entities. Here we present a unique geophysical view of structure within the Archean Slave craton, in northwestern Canada, that shows clear evidence for subduction processes frozen into Archean lithosphere. New seismic imaging results from the central Slave are synthesized with a coincident magnetotelluric model, and interpreted using petrological and geochemical constraints from mantle xenoliths. We find the most striking correlation between seismic and electrical structures ever observed in a continental setting in the form of a coincident seismic discontinuity and electrical conductor at ~100 km depth. The magnitude of both anomalies, in conjunction with the occurrence of phlogopite rich xenoliths originating at the same depth, point to a metasomatic origin. We believe that fluids were released from a subducting slab and altered the mantle directly below the base of a pre-cratonic lithosphere. Our model suggests that cratons are formed by subduction underplating and accretion of preexisting fragments, and that these processes were active as early as 3.5 billion years ago.
</p><p>References
</p><p>Chen, C.-W., S. Rondenay, R. L. Evans, and D. B. Snyder, Geophysical detection of relict metasomatism from an Archean (~3.5 Ga) subduction zone, Science, 326, 1089-1091, 2009.
</p><p>Jones, A.G., P. Lezaeta, I.J. Ferguson, A.D. Chave, R. L. Evans, X. Garcia and J. Spratt, The electrical structure of the Slave craton. Lithos, 71, 505-527, 2003.
</p><p>Acknowledgements: This work is funded by the POLARIS consortium and NSF grant EAR-0409509 (S. R.).</p>

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