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Topic

Constraining Northern Cordilleran lithosphere thickness and xenolith residence times using mantle xenolith geochemistry

School of Earth and Ocean Sciences

Date & location

• Wednesday, December 18, 2024
• 10:00 A.M.
• Clearihue Building, Room B007

Examining Committee

Supervisory Committee

• Dr. Dante Canil, School of Earth and Ocean Sciences, University of Victoria (Supervisor)
• Dr. Ruohong Jiao, School of Earth and Ocean Sciences, UVic (Member)
• Dr. Lucinda Leonard, School of Earth and Ocean Sciences, UVic (Member)

External Examiner

• Dr. William Minarik, Department of Earth and Planetary Sciences, McGill University

Chair of Oral Examination

• Dr. Randy Scharien, Department of Geography, UVic

Abstract

This study re-examines 28 mantle peridotite xenoliths and their Quaternary host lavas from near Llangorse, northwest British Columbia to determine the thickness, reconstruct the thermal history of the Cordilleran mantle lithosphere, as well as to discover the residence time of mantle xenoliths within the host magma. Based on the equilibrium textures, the Llangorse mantle xenoliths can be separate to three distinct groups: non-sieved, weakly sieved, and strongly sieved. The sieved samples are defined by partially melted pyroxenes, with weakly sieved samples exhibiting melted rims under 50 μm wide and strongly sieved samples exhibiting broader rims. Both weakly and strongly sieved samples exhibit strong Ca zoning in olivine, with Ca concentrations increasing from core to rim, indicative of substantial heating. Closure temperatures for the xenoliths were calculated using T_Al, T_BKN, and T_REE, varied between 829 to 941 °C, 811 to 1004 °C, and 847 to 1084 °C respectively. Depth estimates for the xenoliths obtained through Ca-in-olivine barometry (P_𝑆𝐶) show that only non-sieved samples and a few sieved samples yield reasonable depths between 34 and 65 (± 6) km. These depths align with both adjacent seismic measurements and the depth of equilibration of the host lava derived from SiO₂ barometry. Based on the most robust xenolith samples, I construct a model geotherm consistent with a surface heat flow of 76 (± 3) mW/m² and heat production of 1.1 (± 0.2) mW/m³. This model geotherm intersects the peridotite solidus with 300 ppm H₂O, corresponding to a lithosphere-asthenosphere boundary at 1280 (± 15) °C and 74 (± 6) km depth. Diffusion chronometry is applied to Ca zoning profiles in olivine. The results show that all the sieved samples have been heated over timescales ranging from several years to hundreds of years depending on the assumed heat source temperatures and uncertainties in the diffusion coefficient for Ca in olivine. This timescale is consistent with typical magma storage time of several years to thousands of years observed from modern volcanoes in Iceland and Hawaii. The sieved xenolith samples either have been entrained by ascending magma at an early stage, with the magma subsequently stored in the upper mantle or crustal level, or underwent heating prior to being sampled by the ascending magma.