MAGMA MUSH RECHARGE EXPLAINS THE DEFORMATION AND GAS EMISSIONS OBSERVATIONS DURING ``SURGES" IN TRANS-CRUSTAL BASALTIC MAGMATIC SYSTEMS

Magma recharge into shallow crustal magma reservoirs plays a critical role in the growth and eruptibility of magma bodies. It is thus integral to understanding how magmatic systems evolve, under which conditions volcanoes erupt, and how eruptions and volcano hazards develop. Most magma transport models assume that deep magma influx flows through liquid-dominated conduits/dikes directly into melt-rich reservoirs, bypassing any magmatic mushes. However, given the paradigm shift towards transcrustal magmatic systems – composed of small, localized melt regions within an extensive magmatic mush region – we need to reevaluate this assumption and assess the potentially dominant role of mushes in magma recharge, as indicated by extensive geochemical evidence for mush-magma interaction. In this study, we focus on two volcanoes hosting basaltic calderas, Kīlauea, Hawaii and Masaya, Nicaragua, as multiple magma surges globally have been reported in these systems. We jointly analyze ground deformation (GPS and InSAR) and gas emissions (CO₂) data for time periods spanning potential magma surges (2002-2010 & 2011-2018 respectively), and model the transfer of magma to the shallow visco-elastic crustal reservoir through an intermediate non-linear poro-elastic crystalline mush. Our model broadly reproduces the observed gas emissions, seismicity, and ground deformation at each volcano using a simple input flux history into the base of the mush zone. Additionally, our theoretical calculations show that the shape of magma recharge through a mush is different from the input function at the base of the mush, and that mushes naturally lead to diffusive-like magma recharge histories into the shallow reservoirs. We find that including local pore pressure dependent non-linear permeability (due to local mush compaction-decompaction) introduces significant history dependence and system feedbacks, as the fluid pressures-stress state in the magmatic system does not fully relax in between recharge and eruption induced de-pressurization events. Overall, we suggest that magmatic mushes potentially control the rate and timescale of magma recharge into shallow reservoirs over years to decades. Hence, incorporating mushes into magma dynamics models is key to understanding magma surge events and for long-term forecasting of volcanic hazards.