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Age, origin and composition of the Attawapiskat lithospheric mantle and its diamonds (western Superior craton, Canada) Open Access


Other title
Type of item
Degree grantor
University of Alberta
Author or creator
Smit, Karen V
Supervisor and department
Stachel, Thomas (Earth and Atmospheric Sciences)
Examining committee member and department
Rob Creaser (Earth and Atmospheric Sciences)
Larry Heaman (Earth and Atmospheric Sciences)
Bruce Kjarsgaard (Geological Survey of Canada)
Graham Pearson (Earth and Atmospheric Sciences)
Department of Earth and Atmospheric Sciences

Date accepted
Graduation date
Doctor of Philosophy
Degree level
This study reports results from peridotitic and eclogitic xenoliths and di- amond samples from the Attawapiskat area to understand the processes of craton formation and modification in the Superior lithospheric mantle. These diamondiferous kimberlites also provide an opportunity to study the associa- tion of a primary diamond deposit (Victor) with a post-Archaean rift system - the Midcontinent Rift which impacted the southern Superior at ∼ 1.1 Ga. Existence of a depleted mantle reservoir beneath the Attawapiskat area since the Palaeoarchaean is indicated by TRD ages in sulphides and PGE’s trapped in olivine. Mg# up to 93.6 in olivine indicates high degrees of partial melting leading to harzburgitic - dunitic residues. High Cr# in garnet and positive slopes in depleted garnet HREEN , indicate that melting occurred during frac- tional polybaric melt extraction ending at low pressures. Low-Mg eclogites have shallow origins as plagioclase-bearing protoliths likely in former oceanic crust, emplaced into the SCLM by subduction during the Archaean. Partial melting of low-Mg eclogites and interaction of these melts with overlying peri- dotite in the mantle wedge resulted in the formation of the high-Mg eclogites and pyroxenites. TRD ages of ∼2.7 Ga in peridotite with residual PGEN pat- terns also indicate melting in the mantle related to subduction - accretion and hydrous melts infiltrating the overlying mantle wedge led to I-PGE alloy formation. Diamond destruction occurred in the Attawapiskat lithospheric mantle due to the thermal impact of the Midcontinent Rift - seen in an ele- vated geotherm and narrow ”diamond window” in 1.1 Ga kimberlites - with high mantle residence temperatures recorded in pre-rift diamonds. Rift basalts interacted with variably depleted peridotite, leading to P-PGE-enrichment and Mesoproterozoic TRD ages. Older depleted domains are, however preserved. After the thermal impact of the rift subsided, diamond-stable conditions were extended to shallower depths in the lithosphere and diamonds sampled by post-rift kimberlites, formed after the Midcontinent Rift. These diamonds are likely to be both lherzolitic and eclogitic - pyroxenitic, as indicated by their δ13C compositions, and favourable high pressure compositions in both lherzolitic and high-Mg eclogitic - pyroxenitic garnets at Attawapiskat.
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