Science plan for the NWT (slave) POLARIS array:
Diamonds and Ancient Continental Roots

The Archean Slave craton in northwestern Canada is an ideal natural laboratory for investigating Archean lithospheric formation and subsequent evolution, and has become an international focus of broad geoscientific investigation following the discovery of economic diamondiferous kimberlite pipes in the centre of the craton during the early 1990s.  The first diamond mine, BHP's Ekati mine, went into production in October 1998, and Diavik's kimberlites on their property are expected to be mined by 2003, closely followed by DeBeers' Snap Lake kimberlite dyke. Given such intensity of geoscientific scrutiny, the Slave craton was chosen for more detailed geophysical imaging using Polaris both to contribute to studies of Archean lithospheric formation and to obtain baseline geophysical data against which other prospective diamond-producing areas could be compared.
A reconnaisance teleseismic study by Bank et al. (Tectonophysics, 2000) shows tantalizing differences in SKS directions between the northern, central, and southern sites. Also, there are differences in slowness of the lithospheric mantle, with a fast "core" beneath the cental Slave region enveloped by a slow region. Deep-probing magnetotelluric surveys conducted since 1996 revealed an unexpected and remarkable anomaly in electrical conductivity, collocated with the Lac de Gras kimberlite field, which is modeled as a spatially-confined upper-mantle region of extremely low resistivity (<30 ?.m) at depths of 80-120 km. This anomalous upper mantle region is spatially coincident with the above fast seismic core and a geochemically-defined ultra-depleted harzburgitic layer interpreted as oceanic or arc-related lithosphere emplaced during early tectonism.
These mantle images obtained from two geophysical techniques imply a north-south division of the Slave's lithospheric mantle into three roughly NE-SW-striking domains, consist with Grutter et al.'s (1999) proposal based on G10 garnet populations. They are not consistent with the dominent east-west division of the Slave's crust into an eastern arc terrane and a western proto-Slave basement complex, and thus are likely unrelated to the ca. 2690 Ma north-south suturing of these two. The tectonic processes that emplaced this geophysical-geochemical body are more likely related to the subduction of a craton of unknown provenance to the south during 2630-2620 Ma. The Polaris array will provide high resolution geophysical images of the region beneath the Lac de Gras kimberlite field and the mantle domains to the north and south.

Bank, C.-G., Bostock, M.G., Ellis, R.M., and Cassidy, J.F., 2000. A reconnaissance teleseismic study of the upper mantle and transition zone beneath the Archean Slave craton in NW Canada: Tectonophysics, v. 319, 151-166.
Grütter, H.S., D.B. Apter and J. Kong, 1999. Crust-mantle coupling; evidence from mantle-derived xenocrystic garnets. Contributed paper at: The 7th International Kimberlite Conference Proceeding, J.B. Dawson Volume, 1, 307-31..

(Click on the thumbnail to get a larger image)

Back to Poster

Back to Map of Canada

Back to