2D Thermo-mechanical Modelling Sheds New Light on the Geochemical Evolution of Melt within Crustal Magma Chambers

Abstract:

An axisymmetric, two-dimensional numerical model has been developed that describes heat and mass transport within crustal magma chambers during repeated intrusion of mafic sills. The model includes mass transport (melt segregation) via buoyancy driven flow of melt along grain boundaries and compaction of the crystalline matrix. Results demonstrate that the majority of the magma chamber is in a mush state at low to moderate intrusion rates, consistent with previous thermal models. However, the melt segregation processes included here lead to periodic formation of high melt fraction (magma) layers within the mush which could be tapped from the chamber to erupt or ascend to a shallower reservoir. Moreover, the magma composition is chemically evolved because melt reacts with the crystal mush as it migrates upwards into cooler regions of the chamber. Eruptible magma is therefore present in the chamber at lower mafic intrusion rates than has been predicted by purely thermal models because (i) segregation causes melt to accumulate in discrete layers and (ii) the evolved magma composition remains liquid at lower temperatures. Moreover, differentiation of the magma is driven primarily by mush processes rather than by fractional crystallisation, as is assumed to dominate in traditional models of liquid-filled magma chambers. Such magma chambers can only be maintained by very high intrusion rates of new mafic material. We will use our new 2D thermo-mechanical code to model the growth of the magma chamber beneath the Soufrière Hills Volcano, Montserrat, which has previously been investigated using a thermal-only model and constraints from seismic refraction data. The model results show that melt segregation leads to decoupling of the maximum temperature and melt fraction. Its addition will therefore provide improved estimates of the dimensions and frequency of successive magma injections into the chamber, as well as shed light on the evolution of potentially eruptible magma and magmatic fluid with time.