The 12.4 ka Upper Apoyeque Tephra, Nicaragua: stratigraphy, dispersal, composition, magma reservoir conditions and trigger of the plinian eruption

Abstract

Upper Apoyeque Tephra (UAq) was formed by a rhyodacitic plinian eruption in west-central Nicaragua at 12.4 ka BP. The fallout tephra was dispersed from a progressively rising plinian eruption column that became exposed to different wind speeds and directions at different heights in the stratosphere, leading to an asymmetric tephra fan with different facies in the western and southern sector. Tephra dispersal data integrated with geochemical compositions of lava flows in the area facilitate delimitation of the source vent to the south of Chiltepe Peninsula. UAq, Lower Apoyeque Tephra, Apoyeque Ignimbrite, and two lithic clasts in San Isidro Tephra together form a differentiation trend distinct from that of the younger tephras and lavas at Chiltepe Volcanic Complex in a TiO2 versus K2O diagram, compositionally precluding a genetic relationship of UAq with the present-day Apoyeque stratovolcano. Apoyeque Volcano in its present shape did not exist at the time of the UAq eruption. The surface expression of the UAq vent is now obscured by younger eruption products and lake water. Pressure-temperature constraints based on mineral-melt equilibria and fluid inclusions in plagioclase indicate at least two magma storage levels. Clinopyroxenes crystallised in a deep crustal reservoir at ~24 km depth as inferred from clinopyroxene-melt inclusion pairs. Chemical disequilibrium between clinopyroxenes and matrix glasses indicates rapid magma ascent to the shallower reservoir at ~5.4 km depth, where magnesiohornblendes and plagioclase fractionated at a temperature of ~830°C. Water concentrations were ~5.5 wt. % as derived from congruent results of amphibole and plagioclase-melt hygrometry. The eruption was triggered by injection of a hotter, more primitive melt into a water-supersaturated reservoir.