THERMODYNAMIC EVALUATION OF THE ROLE OF MAGMA MIXING IN THE 1968-2010 ERUPTION OF ARENAL VOLCANO, COSTA RICA USING THE MAGMA CHAMBER SIMULATOR

Magma mixing is one process responsible for the compositional diversity of erupted magmas and can potentially catalyze eruptions, making documentation of the size and relative timing of mixing events important for improving eruption prediction. Thousands of cases of magma mixing have been documented in the literature, but quantitative thermodynamic analyses of magma mixing are relatively rare. Arenal Volcano, Costa Rica has experienced a multi-decadal eruption (1968-2010) in which whole rock and mineral compositions of erupted products suggest magma mixing produced lavas that are homogenous at the whole-rock scale but highly heterogeneous at the crystal scale. The hypothesis of Streck et al. (2005) that four distinct magmatic components mixed to produce Arenal’s lavas was analyzed by computational modeling utilizing the Magma Chamber Simulator (MCS), an open-system mass and energy constrained thermodynamic model. About 100 models were run to constrain the pressure, oxygen fugacity and H₂O content of each parental magma component prior to mixing. Best-fit results reproduce distinct mineral assemblages representing the four magma mixing components. A mafic component consisting of olivine+high Cr-cpx+plagioclase+spinel likely evolved at shallow crustal levels prior to mixing (<300MPa), had an initial H2O content of ~3-4 wt.% and evolved under fO₂ conditions of QFM-1. The other three components are likely andesitic and evolved from a similar parent but at different pressure-temperature conditions and volatile contents. The mineral assemblages include low Cr-cpx+titanomagnetite+plagioclase±opx±olivine. Modeling thus far has shown medium to high H₂O contents play an important role in defining the crystal assemblages of these three mixing components, and a range of crustal pressures is also implicated, with one component crystallizing at pressures above 300 MPa. Preliminary MCS mixing simulations suggest that the four components mixed in a shallow chamber in relatively constant proportions to maintain the homogenous whole rock composition. Thermodynamic analysis at Arenal permits a quantitative understanding of the masses and relative timing of recharge events, thus providing potential ties to observable geophysical changes leading to an improved understanding of eruption precursors.