Temporal Evolution of the Lithosphere-Asthenosphere Boundary and its Effect on the “Diamond-Window” in the Western Superior Craton

Karen Smit1,2, Graham Pearson1, Thomas Stachel1 and Michael Seller3, (1)University of Alberta, Earth and Atmospheric Sciences, Edmonton, AB, Canada, (2)Gemological Institute of America, Carlsbad, CA, United States, (3)De Beers Canada, Toronto, ON, Canada

Contact First Author: Karen Smit; ksmit@gia.edu

Previously Published Material: Findings in the abstract have been accepted as part of two scientific papers: in Journal of Petrology (2014: 55, 9, 1829-1863) and Contributions to Mineralogy and Petrology (2014: 167, 962).

Abstract ID#: 35294

 

English Abstract:
The Attawapiskat area of the Superior craton provides the ideal opportunity to evaluate the thermal and compositional effects of a rifting event on the cratonic lithospheric mantle. Rather than being a stable entity since Archaean craton amalgamation (2.7 Ga), the lithospheric keel records evidence for significant thinning, melt impingement and diamond destruction, all related to the impact of the Midcontinent Rift.

Kimberlites in the Attawapiskat area of the western Superior sample the lithospheric mantle at two time periods, i) during the Mesoproterozoic, concurrent to the 1.1 Ga Midcontinent Rift and ii) during the Jurassic.

Thermobarometry studies of peridotite xenoliths and xenocrysts from the two kimberlitic age groups reveal a temporal evolution of the lithospheric mantle beneath Attawapiskat. The Palaeoarchaean lithospheric keel was thinned to less than 180 km at the time of Midcontinent rifting. The rifting resulted in higher heat flow into the base of the lithosphere and appears to have caused diamonds residing in the deep lithosphere to be destroyed.

Small-degree rift-related melts infiltrated and refertilised the lithospheric mantle as evidenced by (1.) enrichment of incompatible trace elements in clinopyroxene, (2.) the generation of Mesoproterozoic Re depletion (TRD) ages, and (3.) decreased Mg# in olivine (89 – 91).

After the thermal and igneous effects of the Midcontinent Rift subsided, the lithospheric root cooled and thickened, such that xenoliths erupted in Jurassic kimberlites define P-T arrays close to ‘typical’ cratonic geotherms - that correspond to crustal heat flow of ~ 40 mW/m2. This thermal thickening of the lithospheric mantle led to the lithosphere-asthenosphere boundary being extended to depths of 200 km. This new stable, cooler and thickened cratonic root provided the optimum conditions for post-Midcontinent Rift diamond formation at Attawapiskat.

Back to: Multidisciplinary Perspectives on the Continental Mantle Lithosphere and Lithosphere-Asthenosphere Boundary I