Crustal evolution leading to successive rhyolitic supereruptions in the Jemez Mountains volcanic field, New Mexico, USA

Abstract The magmatic systems that feed supereruptions result from high magma supply from depth that sustains large regions of partial melt in the shallow crust. However, many questions remain around the processes and timescales over which super-sized magmatic systems develop. We present new zircon age, trace elemental and isotopic data from the Jemez Mountains volcanic field (JMVF) to reveal contrasting magmatic processes that led to successive rhyolitic supereruptions at 1.60 and 1.25 Ma. Before the supereruptions, zircons from lavas erupted over ~8.4 Myr have predominantly unimodal age distributions, close to their respective eruption ages. This highlights that distinct magma domains gradually formed in the JMVF crust. Zircon crystal inheritance shows that magma from the first supereruption at 1.60 Ma included at least three of these precursor plutons and basement rock that were partially molten by enhanced magma supply from ~2 Ma. In contrast, absence of inherited zircon and a change in zircon chemistry in ignimbrite from the second supereruption at 1.25 Ma indicates a thermal resetting of the remnants of the old magma reservoir and rapid construction of a new super-sized magma body. The sudden change in eruptive behaviour in the young JMVF reflects both thermal and chemical maturation of the crust, as well as elevated magma supply and partial melting of precursor plutons. Our study highlights that although crustal conditioning may occur during gradual magmatism over millions of years, the assembly of super-sized magma bodies occurs over much faster timescales and may occur in succession due to thermal adjustment following evacuation.