Compared to calc-alkaline series, alkaline magmas are characterized by an enrichment in alkali oxides (Na2O+K2O) relative to SiO2, which is strongly impacting their rheological (ie, viscosity) properties and their chemical differentiation. Although it constitutes a minor volumetric component of the Earth's current magmatism, the study of alkaline magmas and associated volcanism has today major economic and social impacts. First, some alkaline felsic rocks are recognized strategic deposits of REE and other critical metals; yet, explaining the formation of such anomalous deposits in the chain of processes involved in alkaline magmatism still constitute a major scientific challenge. Secondly, whereas alkaline magmatism represents a major component in the long-term construction of volcanic islands, their recurrent reactivation represents, at short term, a major threat to the population. This is evidenced by recent eruptions at Canary and Cabo Verde Islands. To better understand future eruptions and predict the formation of critical metal deposits, it is crucial to precisely determine the storage and differentiation conditions of alkaline systems.

The conditions under which parental basanites, extracted from the convective mantle, fractionate to produce phonolite/trachyte derivatives are not clear. Mineral and fluid barometers support a multiple storage+extensive fractionation occurring over a large crustal-mantle section (~30 to 3 km or 1 GPa-100 MPa). This contrasts with findings from newly experimental works, which constrain that felsic magma production mostly operates at shallow crustal levels (100 to 300 MPa) over a limited range of temperatures and fO2. The mismatch between these two methodologies highlights the poor resolution of currently used petrological-barometers for predicting the storage and magma evolution in alkaline systems. As illustrated here, systematic crystallization experiments performed at variable P-T-fO2 conditions allow to develop new and specific petrological tools (e.g. FeO* in melt and Fo content in olivine as a function of P) for understanding these uncommon magmas