Magmatic volatile contents and degassing-induced crystallization at Volcán Jorullo, Mexico: Implications for melt evolution and the plumbing systems of monogenetic volcanoes

Monogenetic basaltic cinder cones are abundant on Earth and exhibit a wide range of eruptive styles, including violent explosions. However, the mechanisms driving explosive cinder cone eruptions are still poorly understood. Here we investigate relations between volatiles, degassing, and crystallization in a long-lived, historical, cinder cone eruption to better understand the plumbing systems of monogenetic volcanoes. We present volatile (H2O, CO2, S, Cl), major, and trace element data for olivine-hosted melt inclusions, estimates of olivine residence times based on Fe–Mg zoning, and measurements of groundmass crystallinity for tephra from the eruption of Volcán Jorullo, Mexico. Jorullo melt inclusions trapped some of the most volatile-rich (≤ 5.3 wt.% H2O, ≤ 1000 ppm CO2), primitive (≤ 10.5 wt.% MgO) melts yet measured in an arc setting, as well as more degassed, evolved compositions. Furthermore, the melt inclusions record temporal changes in both melt composition and crystallization. Early erupted inclusions are Mg-rich and record variable trapping pressures (10–400 MPa), whereas late inclusions were trapped only shallowly beneath the volcano (3–19 MPa) and contain increasingly evolved melts. Disparities between the compositions of the melt inclusions and the whole-rock lava samples provide evidence for a two-stage crystallization process: 1) cooling-induced fractionation of amphibole + olivine ± clinopyroxene in the lower crust, which drove the bulk melt evolution over time; and 2) degassing-induced crystallization of melts during ascent at pressures < 400 MPa. Additionally, olivine residence times calculated from diffusion profiles suggest that as the eruption progressed, olivine crystals were being stored for longer periods of time (up to 1300 days) within more evolved melts that had risen from depth. These data, taken together with temporal decreases in crystallization depths and increases in groundmass crystallinity, suggest the formation of a shallow reservoir (or simply an enlarged region of the conduit) beneath the volcano late in the eruption. Shallow storage permitted degassing and crystallization of the Jorullo melts, and facilitated assimilation of the host rock by the resident magma.