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

The radiogenic heat production within the lower crust is poorly constrained despite it being a major component of the Earth's lithosphere. Here we report the results of the modal abundance and in-situ geochemical analysis on minerals in 15 lower crustal xenoliths erupted by the Diavik A154 kimberlite of the Slave Craton, Northwest Territories, Canada. The primary aim of this study is to construct a heat-producing element (HPE) budget for the lower crust of the Slave craton utilizing a reconstructed bulk rock method to filter out host kimberlite infiltration and alteration. Secondarily, Fe²⁺-Mg geothermometry is utilized to independently estimate the maximum temperature of the lower crust at the time of kimberlite eruption. These data can provide important constraints on lithosphere thermal structure (e.g. Moho temperature) and potentially the conditions for diamond stability.

The Diavik lower crustal xenolith suite comprises mafic and metasedimentary granulites in an approximately 80:20 ratio. Application of mineral-pair Fe²⁺-Mg exchange geothermometers to these xenoliths indicates that the lower crust was at a maximum temperature of roughly 500 °C at the time of kimberlite eruption. The actual temperature of the lower crust was likely lower than 500 °C because the thermometers record the closure temperature of Fe²⁺-Mg exchange between touching mineral pairs rather than the ambient temperature of the rocks prior to their entrainment in the kimberlite.

HPE concentration measurements in various lower crustal minerals are compiled in an extensive database. This database allows for comparative estimates of heat producing element concentrations on various minerals, resulting in the highest heat producing element contributors being monazite (U and Th) and alkali feldspar + plagioclase (K). The heat production values of typical mafic granulite (0.08 ± 0.01 μW/m³) and metasedimentary granulite (0.35 ± 0.06 μW/m³) were reconstructed from mineral analyses and modal abundance determinations from xenoliths in our sample set. Combining these two rock types in the observed 80:20 proportions results in an average lower crustal heat production for the Slave craton of 0.13 ± 0.06 μW/m³, which is lower than the majority of published heat production estimates for the lower crust.