Element mobility during serpentinization of the Tulameen Alaskan-type intrusion, British Columbia

The serpentinization of ultramafic rocks occurs under some of the Earth's most extreme geochemical conditions, with strongly reducing fluids, extreme pH, and low silica activity, which contribute to unique element mobility, including the mobilization of aluminum in H₂O-rich fluids. This study presents a petrographic and geochemical analysis of partially serpentinized ultramafic rocks from the dunite core of the Tulameen Alaskan-type mafic-ultramafic intrusion in British Columbia, Canada. Mineral composition and textural relationships are used to establish alteration conditions during serpentinization of the intrusion, identify evidence of fluid-mediated element mobility, and reconstruct element transport mechanisms during alteration. Fluid mobile components typically exhibit anisotropic length scales of equilibrium, with fluid-mobile components equilibrating on far greater length scales parallel to permeable pathways than perpendicular to them. Electron microprobe analyses of samples from the Tulameen Intrusion reveal high aluminum content in antigorite and lizardite after olivine (0.14- 2.01 wt% Al₂O₃). Thermobarometry, mineral composition, and textural analysis indicate that most serpentinization of the Tulameen intrusion occurred at 300-450°C in the antigorite+brucite stability field, and continued as the intrusion cooled. Fluids were H₂O-dominated with high pH (>8), low oxygen fugacity (FMQ-4), low silica activity (less than 10^−2.5 at the serpentinization front), and low salinity during serpentinization. Correlation between the occurrence of Fe-rich serpentine (2.12-5.45 wt% FeO) and relatively high chlorine levels (0.02-0.05 wt% Cl) implicates salinity in fluid-based iron mobility. Comparative analysis of the alteration conditions identified in the Tulameen and known mechanisms of aluminum mobility suggests that aluminum becomes mobile in H₂O-dominated fluids at high pH via the formation of AlO₂^- anions. These discoveries have implications for ongoing research on serpentinite reactivity in carbon sequestration and on the remobilization of mineral resources during hydrothermal alteration.