EVIDENCE OF RAPID MAGMA MIXING DURING ASCENT ALONG FRACTURES FROM PERIDOTITE-BEARING HAWAIITES OF THE CIMA VOLCANIC FIELD, CA

The Cima volcanic field, located in the south-central Basin and Range province, includes >70 small-volume cones and flows erupted over the last 8 Myr. This study focuses on the youngest units (≤ 1 Ma), which are low Mg# hawaiites (≥ 56) derived from an asthenospheric mantle source (Farmer et al., 1995). The Cima hawaiites, and adjacent Dish Hill basanites, are well known for carrying large mantle peridotite xenoliths, which precludes stalling in a crustal reservoir and points to rapid transit through the crust. This raises the question of how low Mg# hawaiites, which cannot be in equilibrium with peridotite mantle, formed and differentiated while successfully carrying dense, mantle xenoliths. Several hypotheses are tested and the only one shown to be viable is magma mixing between high-MgO basanites (with entrained mantle xenoliths) and low-MgO mugearites, which formed by partial melting of mafic lower crust under relatively dry and reducing conditions. Multiple lines of evidence, including the presence of dense mantle xenoliths in erupted hawaiites, diffusion-limited growth textures in olivine and clinopyroxene phenocrysts, and notably thin Fe-rich rims on high-MgO olivine crystals (inherited xenocrysts from basanite), indicate magma mixing must have occurred rapidly (days or less) during ascent to the surface along intersecting fractures, and not in a stalled crustal magma chamber. Olivine-melt thermometry and hygrometry (Pu et al., 2017) applied to a high-MgO (9.8 wt%) basanite from Dish Hill gives a temperature of ~1196°C and a melt water content of ~1.4 wt% H₂O at the onset of olivine crystallization during ascent, which is consistent with olivine-hosted melt inclusions (Plank and Forsyth, 2016). Calculated ascent velocities for the Dish Hill basanite carrying 5-20 cm diameter mantle xenoliths are ≥ 0.3-4.9 km/h, enabling ascent through the 36-km thick crust in ≤ 7-119 h.