Lithosphere formation in the central Slave Craton (Canada): plume subcretion or lithosphere accretion? |
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| Authors: | Sonja Aulbach William L Griffin Norman J Pearson Suzanne Y O’Reilly Buddy J Doyle |
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| Institution: | (1) GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW, 2109 North Ryde, Australia;(2) Present address: Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada, T6G 2E3;(3) CSIRO Exploration and Mining, N. Ryde, NSW, 2113, Australia;(4) Lithosphere Services, 4009 Edinburgh Street, Burnaby, BC, Canada, V5C 1R4 |
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| Abstract: | Major-element compositions of minerals in peridotite xenoliths from the Lac de Gras kimberlites provide constraints on the
mode of lithosphere formation beneath the central Slave Craton, Canada. Magnesia contents of reconstructed whole rocks correlate
positively with NiO and negatively with CaO contents, consistent with variable partial melt extraction. Alumina and Cr2O3 contents are broadly positively correlated, suggestive of melt depletion in the absence of a Cr–Al phase. Garnet modes are
high at a given Al2O3 content (a proxy for melt depletion), falling about a 7 GPa melt depletion model. These observations, combined with high
olivine Mg# and major-element relationships of FeO-poor peridotites (<7.5 wt%) indicative of melt loss at pressures >3 GPa
(residual FeO content being a sensitive indicator of melt extraction pressure), and similar high pressures of last equilibration
(∼4.2 to 5.8 GPa), provide multiple lines of evidence that the mantle beneath the central Slave Craton has originated as a
residue from high-pressure melting, possibly during plume subcretion. Apparent low melt depletion pressures for high-FeO peridotites
(>7.5 wt%) could suggest formation in an oceanic setting, followed by subduction to their depth of entrainment. However, these
rocks, which are characterised by low SiO2 contents (<43 wt%), are more likely to be the result of post-melting FeO-addition, leading to spuriously low estimates of
melt extraction pressures. They may have reacted with a silica-undersaturated melt that dissolved orthopyroxene, or experienced
olivine injection by crystallising melts. A secular FeO-enrichment of parts of the deep mantle lithosphere is supported by
lower average Mg# in xenolithic olivine (91.7) compared to olivine inclusions in diamond (92.6). |
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| Keywords: | Mantle peridotite origins Mantle melting Plume mantle Subcontinental lithosphere formation Lac de Gras lithospheric mantle |
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