APPLICATION OF OLIVINE-MELT THERMOMETRY AND HYGROMETRY TO TENERIFE BASALTS: IMPLICATIONS FOR ORIGINS OF LOW MG# ALKALINE MELTS

Tenerife, the largest of the Canary Islands, represents a complex eruptive history that has been attributed to a mantle plume. Mafic volcanism in this study includes basanites, alkaline basalts, hawaiites, mugearites, and trachytes. Six islands in the Canarian archipelago, including Tenerife, contain mantle peridotite xenoliths, which precludes stalling in a crustal reservoir. Mafic lavas are low Mg# (<53), low SiO₂ (44-49 wt%) and alkaline, which precludes direct partial melting of peridotite. A possible hypothesis for the origin of low Mg# melts is that they originated as partial melts of an asthenospheric mantle source that included pyroxenite ± carbonate (e.g., recycled oceanic crust). The objective of this study is to evaluate temperature and dissolved water content of Tenerife basalts using olivine-melt thermometry/hygrometry. Entrainment of mantle xenoliths suggests phenocrysts in the host basalts must have grown rapidly during ascent. Basalts also display diffusion-limited rapid growth textures in olivine (e.g., skeletal/hopper) and clinopyroxene (e.g., sector zoned). Preliminary results for a basanite show that when the most Mg-rich olivine composition is paired with the whole-rock composition, the resulting ^Fe2+-MgK_D(oliv-melt) value (0.32) indicates equilibrium. Mg- and Ni-based olivine-melt thermometers are applied to the most Mg-rich olivine and the whole-rock to determine temperature at the onset of crystallization. The Ni-thermometer (independent of dissolved H₂O) gives the actual temperature of 1140°C, whereas the Mg-thermometer gives the anhydrous temperature of 1203°C. The ∆T (=T_Mg – T_Ni) represents the depression of the olivine liquidus due to H₂O, which is 63°C. This is consistent with the range of water contents (0.4 – 3.0 wt%) measured in olivine-hosted melt inclusions (Longpré et al., 2017). Thermometry and hygrometry could not be applied to an alkaline basalt, as it has an olivine population that is too forsteritic to be in equilibrium with the whole-rock composition (K_D = 0.21). Linear mixing trends in whole-rock major- and trace-elements paired with diffusion-limited growth textures in olivine and clinopyroxene suggest that this sample may have an inherited high-MgO olivine population due to magma-mixing during rapid ascent to the surface along intersecting fractures.