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Topic

Thickness and thermal history of the lithosphere of southern Canadian Cordillera constrained using mantle xenoliths from West Kettle River, British Columbia

School of Earth and Ocean Sciences

Date & location

• Wednesday, December 10, 2025
• 10:00 A.M.
• Clearihue Building, Room B007

Examining Committee

Supervisory Committee

• Dr. Dante Canil, School of Earth and Ocean Sciences, ӣ��Ӱ�� (Supervisor)
• Dr. Laurence Coogan, School of Earth and Ocean Sciences, UVic (Member)
• Dr. Ruohong Jiao, School of Earth and Ocean Sciences, UVic (Member)

External Examiner

• Dr. Dejan Milidagrovic, Research Scientist, Geological Survey of Canada

Chair of Oral Examination

• Dr. Julie Zhou, Department of Mathematics and Statistics, UVic

Abstract

Approximately 1.2 Ma, mantle-derived alkaline lava sampled, erupted, and emplaced mantle xenoliths ESE of Kelowna, British Columbia, near the West Kettle River. These mantle pieces are direct samples of ambient mantle lithosphere at depth and can be used to probe thermal and mechanical lithosphere conditions. Geophysical studies show that the lithosphere of the Canadian Cordilleran is weak, hot, and thin, yet its thickness and thermal history are not fully constrained. In this thesis, I apply two approaches to help quantify the depth and thermal conditions of the lithosphere. In the first approach, I apply mantle xenolith geochemistry to six geothermometers (T_BKN, T_Al^ol, T_Ca^opx, T_Ca^ol, T_Al-Cr^opx-sp, T_Fe-Mg^ol-sp), and test two calibrations of the Ca-in-olivine geobarometer (PKB and PSC). The findings across four thermometers (T_BKN, T_Al^ol, T_Ca^opx, T_Ca^ol) show temperatures of 907–991 °C and are interpreted as mantle lithosphere temperatures at the time of sampling. Two other thermometers (T_Al-Cr^opx-sp, T_Fe-Mg^ol-sp) show temperatures from 720–1386 °C and are interpreted as showing temperatures from reheating during transit. Barometry results from P_SC are a better fit to phase equilibria and observed lithologies than results from P_KB. Combining T_Al^ol thermometry and P_SC barometry, the data show a hot maximum Moho temperature of 916±9 °C and estimates the depth to the lithosphere-asthenosphere boundary (LAB) at 68±4 km. However, scrutiny of the barometry results exposes a problem—a large range in pressures/depths for a small range in temperatures results in a questionable temperature-depth array of ~4–5 °C/km. As a comparison, a set of conductive geotherms are constructed using the minimum rim TBKN temperature of 930 °C as a proxy for the Moho temperature, the seismic Moho depth for the Cordillera of 33±3 km, a surface heat flow range of 77–94 mW/m2, and a radiogenic heat production of 1–1.6 𝜇W/m3. This approach results in a conductive geothermal gradient of 27 °C/km, a LAB temperature of 1350±25 °C, and a LAB depth of 50±5 km—estimating a thinner lithosphere than previous studies. The two approaches are compared, resulting in the geotherm-generated approach as the preferred model, despite this model displaying a curious 12-km sampling gap. The methods of geotherm modeling are applied to other mantle xenolith suites in the southern Canadian Cordillera and suggest an equivalent thin lithosphere across the orogen with LAB depths ranging from 44–54 km. Finally, the well-equilibrated thermometric results are discussed and interpreted as showing slow cooling over long timescales. This study exposes the paucity for an improved barometer for application for spinel peridotites. Future seismic studies and additional mantle xenolith and lava barometry studies for other locations in the southern Canadian Cordillera could substantiate the ~50-km thin lithosphere estimated by geothermal modeling of this study and verify ancient, slow-cooling timescales for the mantle lithosphere.