Magma mixing processes in volcanic contexts, a thermodynamic approach with the examples of St Vincent Soufrière volcano, West lndies and Cerro Chiquito, Guatemala

Different modalities and thermodynamic conditions of magma mixing are compared: heterogeneity and chemistry, temperature and water pressure, volatile (= water) content and water solubility, density and viscosity. Such data are somewhat scarce in the literature.
Two case studies have been studied in detail to highlight this review. (1) St Vincent Soufrière (West Indies ). Magma mixing may have been a major process in triggering the 1979 explosive phase. All products (whole rock analyses) are basaltic andesite (52.3–56.2 wt% SiO₂) but dacitic (60–64 wt% SiO₂) glass occurs as micropumice or inclusions in crystals. (2) Cerro Chiquito (Guatemala). A Quaternary extinct dome consisting of dacite (62.5–67.1 wt% SiO₂) enclosing 20–30% in volume of basaltic andesitic (54.9–55.3 wt% SiO₂) enclaves.
The nearly thorough mixing process of case 1 strongly differs from the commingling process of case 2. The main phases may be mafic (case 1) or acidic (case 2). Thermodynamic parameters are calculated using classical geothermometers and formulae. The range of values and errors are estimated (60–80°C in case of temperature). In addition, other processes, such as crystallization of mineralo-gical phases and fluid vesiculation, may modify thermodynamical conditions. Difference of magma temperature ranges from about 60°C (case 1) up to about 130°C (case 2). Water concentrations of 2.5 wt% in mafic magmas and 4 wt% in acidic magmas, are always lower than the upper limit of water solubility at depths where contrasting magmas were mixed. The difference between the viscosities of the two magmas varies from one order of magnitude up to five.
Various mixing scenarios may occur, each of them being linked to different petrological processes including effusive or explosive volcanism as well as plutonism.