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The Cima volcanic field, in the southern Basin and Range province (California, USA), includes >70 eruptive units over the last 8 m.y. The youngest (≤1 Ma) are low Mg# (≥56) hawaiites derived from an asthenospheric mantle source. The Cima hawaiites, and adjacent Dish Hill basanites, are known for carrying large mantle xenoliths, which precludes stalling in a crustal reservoir. This raises the question of how low Mg# hawaiites, which cannot be in equilibrium with peridotite mantle, formed and differentiated while carrying dense, mantle xenoliths. Several hypotheses are evaluated and the only one shown to be viable is mixing between high-MgO basanite (with entrained mantle xenoliths and sparse olivine phenocrysts) and low-MgO mugearite liquids, which formed by partial melting of mafic lower crust under relatively dry and reducing conditions. Multiple lines of evidence, including the presence of mantle xenoliths in hawaiites, diffusion-limited growth textures in olivine and clinopyroxene, and notably thin Fe-rich rims on high-MgO olivine crystals (inherited), indicate magma mixing must have occurred rapidly (days or less) during ascent to the surface along intersecting fractures, and not in a stalled crustal reservoir. Abundant evidence points to clinopyroxene growth immediately after mixing, and application of clinopyroxene-melt barometry constrains the depth of mixing to the lower and middle crust (0.8−0.4 GPa). Results from olivine-melt thermometry/hygrometry (∼1196 °C and ∼1.4 wt% H2O) applied to a basanite from Dish Hill carrying 5−20 cm mantle xenoliths leads to calculated ascent velocities ≥0.3−4.9 km/h, enabling ascent through the 36 km thick crust in ≤7−119 h. 

Abstract The Quaternary Big Pine (BP) volcanic field in eastern California is notable for the occurrence of mantle xenoliths in several flows. This points to rapid ascent of basalt through the crust and precludes prolonged storage in a crustal reservoir. In this study, the hypothesis of phenocryst growth during ascent is tested for several basalts (13–7 wt% MgO) and shown to be viable. Phenocrysts of olivine and clinopyroxene frequently display diffusion‐limited growth textures, and clinopyroxene compositions are consistent with polybaric crystallization. When the most Mg‐rich olivine in each sample is paired with the whole‐rock composition, resulting(olivine‐melt) values (0.31–0.36) match those calculated from literature models (0.32–0.36). Application of a Mg‐ and a Ni‐based olivine‐melt thermometer from the literature, both calibrated on the same experimental data set, leads to two sets of temperatures that vary linearly with whole‐rock MgO wt%. Because the Ni thermometer is independent of water content, it provides the actual temperature at the onset of olivine crystallization (1247–1097°C), whereas the Mg thermometer gives the temperature under anhydrous conditions and thus allows ΔT(=T_Mg − T_Ni = depression of liquidus due to water) to be obtained. The average ΔTfor all samples is ~59°C, which is consistent with analyzed water contents of 1.5–3.0 wt% in olivine‐hosted melt inclusions from the literature. Because the application of olivine‐melt thermometry/hygrometry at the liquidus only requires microprobe analyses of olivine combined with whole‐rock compositions, it can be used to obtain large global data sets of the temperature and water contents of basalts from different tectonic settings.