Constraining magma storage conditions at a restless volcano in the Main Ethiopian Rift using phase equilibria models

- Isobaric crystallisation can reproduce the compositional diversity seen at Aluto.
- The magma storage conditions at Aluto are: fO2 = QFM; P = 150 MPa; initial H2O = 0.5 wt%.
- Inferred magma storage depth is consistent with modelling of ground deformation.
- Magma storage conditions are comparable to those at other peralkaline volcanoes.

The Main Ethiopian Rift hosts a number of peralkaline volcanic centres, several of which show signs of recent unrest. Due to the low number of historical eruptions recorded in the region and lack of volcanic monitoring, conditions of magma storage in the Main Ethiopian Rift remain poorly constrained. Aluto is one of these restless volcanic centres and identifying magma storage conditions is vital for evaluating the significance of recent periods of unrest. Using Aluto as a case study, we explore magma storage conditions using Rhyolite-MELTS thermodynamic modelling software. We performed ~ 150 fractional crystallisation models using a primitive basalt as the starting composition, and for a range of pressures (50-300 MPa), initial H2O contents (0.5-3 wt%) and oxygen fugacities (QFM − 2-QFM + 1). Predicted liquid lines of descent from these models are compared with published whole-rock data and, together with new observations of natural phase assemblages and erupted mineral compositions, provide constraints on magma storage conditions.Using a statistical approach to compare empirical data and thermodynamic model outputs, we find that compositions of evolved peralkaline rhyolites from Aluto are best reproduced by protracted (90%) isobaric fractional crystallisation from a rift-related basaltic composition, without the need for significant crustal assimilation. The required extent of fractional crystallisation suggests that much of the magmatic system may exist as a highly crystalline mush with only a small lens of rhyolitic melt. The best agreement between models and natural samples is at low pressures (150 MPa), low initial H2O concentrations (0.5 wt%) and an oxygen fugacity near the QFM buffer. The depth of magma storage derived from these results (~ 5.6 ± 1 km) is consistent with the source depths modelled from measured ground deformation. Data from other peralkaline volcanic centres in the Main Ethiopian Rift (Boset and Gedemsa), and other locations globally (e.g. Pantelleria, Italy) suggest that these storage conditions are a common feature of many peralkaline volcanic centres.