Modelling magma flow within dyke-fed sill geometries: A coupled thermal and fluid dynamics approach

Sill geometry and magma flow indicators preserved within fossil sills are used to determine magma source locations, understand economic potential of magmatic ore deposits, and forecast potential volcanic eruption sites. However, existing models struggle to incorporate complex flow dynamics and quantify flow variability, thus inhibiting their potential to explain spatially variable magma flow within sills spanning up to hundreds of kilometres. We present results of new 2D finite element numerical simulations coupling fluid and thermal dynamics within a dyke-fed sill using a multiphysics approach. As magma enters the model sill from below via one (or several) feeding dyke(s), magma jets of variable height develop within the sill depending on dyke thickness and inlet velocity. Low-velocity zones occur near the feeding dyke(s), with recirculation present between multiple feeding dyke(s) when they are present. These findings demonstrate the significant impact that intrusion geometry has on the magma flow dynamics within the sill, and we postulate that the presence of magma jets at the dyke-to-sill transition may be one source of sill lobes. They suggest random crystal orientations could be expected close to feeder dykes (low-velocity recirculating flow and rapid solidification), but where strain rates are high crystal alignment may still occur. These results potentially explain complex magma flow as interpreted from field observations and petrographic analysis of sill. Our results show that incorporating intrusion geometries, flow dynamics and thermal processes into models is crucial for bridging the gap between field observations and the underlying processes that govern natural systems. • We model magma flow dynamics within a dyke-fed sill using Finite Element models. • Intrusion geometry influences flow dynamics and thermal processes within sills. • Magma jet height in the sill depends on inlet velocity and dyke thickness. • Low-velocity recirculation zones develop between multiple feeding dykes. • Temperatures and melt fraction are reduced in the low-velocity recirculation zones.