Geothermobarometry of spinel peridotites from southern British Columbia: implications for the thermal conditions in the upper mantle

Spinel lherzolite xenoliths within alkali basalts exposed at Rayfield River and Big Timothy Mountain, south-central British Columbia, represent samples of the underlying lithospheric mantle. Electron microprobe analysis shows that most xenoliths comprise compositionally homogeneous grains of olivine, orthopyroxene, clinopyroxene, and spinel. We applied the following mineral-pair geothermometers to these rocks: orthopyroxene–clinopyroxene, spinel–orthopyroxene, and spinel–olivine. Temperatures calculated using the Brey and Köhler calibration of two-pyroxene thermometry were constrained in pressure by being required to lie on a model geotherm we develop for this region of B.C. The model geotherm is constrained to produce a temperature at the Moho (33 km) of 825 ± 25 °C to match the lowest temperature peridotite xenoliths recovered in this study. Although the overall effect of pressure on the temperature calculations is negligible (∼2 °C for 0.1 GPa), the simultaneous solution of the model geotherm and the pressure-dependent Brey–Köhler two-pyroxene thermometry removes the need for adopting an arbitrary pressure. We take these temperatures to represent peak mantle lithosphere temperatures. Fourteen Rayfield River xenoliths return two-pyroxene temperatures between 841 and 962 °C corresponding to depths of 34–42 km. Orthopyroxene–spinel and olivine–spinel results are 889 ± 60 and 825 ± 88 °C, respectively. Five Big Timothy xenoliths have two-pyroxene temperatures spanning 840–1058 °C and corresponding to depths of 34–48 km. Mean orthopyroxene–spinel and olivine–spinel temperatures are 844 ± 63 and 896 ± 232 °C, respectively. We argue that the differences in ranges of temperature do not represent closure temperatures imposed during cooling either in the mantle or during transport by the magma. Rather, these differences reflect differences in the original calibrations of the geothermometers or different degrees of equilibration in exchange reactions in dry rocks. Isochemical phase diagrams (pseudosections) constrain the pressure–temperature (P–T) field in which spinel is stable. These diagrams suggest that the spinel-bearing peridotites equilibrated at pressures ranging from ∼9.6 to 14 kbar (10 kbar = 1 GPa).