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

  • Author / Creator
    Smit, Karen V
  • 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.

  • Subjects / Keywords
  • Graduation date
    Fall 2013
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.