RECONCILING SEISMIC STRUCTURES AND LATE CRETACEOUS KIMBERLITE MAGMATISM IN NORTHERN ALBERTA, CANADA

Kimberlites often carry xenoliths and diamonds from great depths (>150 km) to the surface and provide vital information on the composition and dynamic processes of the subcontinental lithosphere. Their scientific and economic significance has prompted growing research interest in petrogenesis and migration of kimberlite melts. In northern Alberta (Canada), the Late Cretaceous kimberlites intruded into the Paleo-Proterozoic crust and represent a nonconventional setting for the discovery of diamonds, yet their origin remains unclear. In this study, we examined the mechanism of kimberlite magmatism using a multidisciplinary approach. A new teleseismic survey reveals a low-velocity (−1%) corridor that connects two deep-rooted (>200 km) quasi-cylindrical anomalies underneath the Birch Mountains and Mountain Lake kimberlite fields in northern Alberta. The radiometric data, including a new U-Pb perovskite age of 90.3 ± 2.6 Ma for the Mountain Lake intrusion, indicates a northeast-trending age progression in kimberlite magmatism, consistent with the (local) plate motion rate of North America constrained by global plate reconstructions. Collectively, these observations favor a deep stationary (relative to the lower mantle) source region for kimberlitic melt generation. Two competing models, mantle plume and slab subduction, can satisfy kinematic constraints and explain the exhumation of ultradeep diamonds. The plume hypothesis is less favorable due to the apparent age discrepancy between the oldest kimberlites (ca. 90 Ma) and the plume event (ca. 110 Ma). Alternatively, magma generation may have been facilitated by decompression of hydrous phases (e.g., wadsleyite and ringwoodite) within the mantle transition zone in response to thermal perturbations induced by a cold slab. The three-dimensional lithospheric structures largely controlled melt migration and intrusion processes during the Late Cretaceous kimberlite magmatism in northern Alberta.