Insight into monogenetic eruption processes at Pelagatos volcano, Sierra Chichinautzin, Mexico: a combined melt inclusion and physical volcanology study

Eruptions and magma evolution of monogenetic volcanoes are thought to be controlled by rapid ascent of magmas over a short period of time. Volatiles degassing from magmas control the ascent velocity and therefore eruption intensity. Complex feedbacks exist between the rate and extent of volatile exsolution at shallow levels and groundmass crystallization, affecting the magma rheology, extent of fragmentation, resulting eruptive style and related hazards. Melt inclusions record the volatile contents and compositions of melts at various stages during their evolution, providing insights into degassing–crystallization processes at shallow crustal levels. Here we present new volatile and major element data from olivine-hosted melt inclusions from Pelagatos scoria cone, Mexico. These new data, combined with recent geochemical and textural data on Pelagatos eruptive products, allow us to propose a model for shallow magmatic processes at this volcano. Discharge of volatile-poor, low-viscosity magma drives early effusive activity along the fissure. Decreasing magma fluxes lead to the clogging of the fissure and the formation of a shallow magma reservoir where degassing and fractional crystallization take place. Subsequent explosive cone-forming activity is triggered by influx of deeper (c. 5 km), less evolved, more volatile-rich magma into this shallow (c. 1 km) reservoir.

学界普遍认为，单成因火山（monogenetic volcanoes）的喷发活动与岩浆演化受控于短时间尺度内的岩浆快速上升过程。岩浆中的挥发分脱气作用可调控岩浆上升速度，进而决定喷发强度。浅部环境中挥发分出溶的速率与规模和基质结晶之间存在复杂反馈机制，该机制会影响岩浆流变特性、岩浆破碎程度，最终作用于喷发样式及相关灾害风险。熔体包裹体（melt inclusions）能够记录岩浆演化不同阶段的熔体挥发分含量与成分组成，为探究地壳浅部的脱气-结晶过程提供了关键依据。本文报道了墨西哥佩拉加托斯火山渣锥（Pelagatos scoria cone）中橄榄石寄主熔体包裹体的全新挥发分与主量元素数据。结合该火山喷发产物的最新地球化学及岩相学数据，我们可为该火山的浅部岩浆作用提出一套成因模型。早期裂隙带的喷溢活动由挥发分匮乏、低粘度的岩浆驱动；岩浆通量的下降会引发裂隙堵塞，并形成一处可发生脱气与分离结晶作用的浅部岩浆房。随后，深部（约5公里）演化程度较低、挥发分含量更高的岩浆注入该浅部（约1公里）岩浆房，触发了后续的爆炸型锥状喷发活动。