Temperatures and Heat Production in the Slave Craton Lower Crust: Evidence from Xenoliths in the Diavik A-154 Kimberlite

• Main Author: Gruber, Benjamin Hess
• Other Authors: Chacko, Thomas (Earth and Atmospheric Sciences), Pearson, Graham (Earth and Atmospheric Sciences)
• Format: Thesis
• Language: English
• Published: University of Alberta. Department of Earth and Atmospheric Sciences. 2019
• Subjects: geo; socio; Canada; Northwest Territories
• Online Access: https://era.library.ualberta.ca/items/37ea80ba-573f-43c8-84e6-b3073ae5cde3

Degree: Master of Science Abstract: Lower crustal heat production is poorly constrained due to the relative inaccessibility of lower crustal samples and their inherent complexity. To obtain the requisite information, the current project conducts spatially resolved geochemical analyses on minerals in 15 lower crustal xenoliths erupted via the Diavik A-154 kimberlite of the Northwest Territories, Canada. The aims are to: 1) conduct geothermometric measurements on lower crustal minerals, 2) construct a heatproducing element budget of the lower crust of the Slave craton, and 3) test the validity of these measurements in a parameter space relevant to geodynamic modeling and diamond exploration. The Diavik lower crustal xenolith suite comprises two main lithologies, mafic granulite (garnet-plagioclase-clinopyroxene ± orthopyroxene) and metasedimentary granulite (garnetplagioclase- orthopyroxene ± quartz ± K-feldspar ± kyanite), which are present in proportions of approximately 80:20, respectively. Application of mineral-pair, iron-magnesium exchange geothermometers (garnet-biotite, garnet-amphibole, and garnet-clinopyroxene) to these xenoliths indicates that the lower crust was at a maximum temperature of roughly 500 °C at the time of kimberlite eruption (~ 55 Ma). The actual temperature of the lower crust is likely lower than 500 °C as the geothermometers probably record the closure temperature of diffusional Fe2+-Mg exchange between touching mineral pairs rather than the ambient temperature of the rocks prior to their entrainment in the kimberlite magma. Heat-producing element (HPE) concentration measurements show that the lower crustal heat production of the Slave craton is likely 0.14 ± 0.02 μW/m3, which is lower than most values in the literature but broadly comparable to some geophysical estimates. This estimate is the result of (20:80) bimodal mixing of idealized lower crustal endmembers: a metasedimentary lower crust (0.37 ± 0.06 μW/m3) and a mafic lower crust (0.08 ± 0.01 μW/m3). These endmembers were iii ...