The geochemical evolution and current state of magmatic systems of the different islands of the archipelago are of fundamental importance to understand the past activity of the volcanoes and to model their future behaviour. In this project the rock suite cropping out in the islands of Graciosa, Faial and Terceira will be studied to understand the chemical evolution of magma that generated those islands of the central group and to provide new hints for the volcanic hazard assessment on the archipelago. These islands will represent the central group, being sufficiently distant each other, and were also chosen due to the existence of previous studies of stratigraphy which allowed tracing their geological history.
All the samples (lava flows and pyroclastic ejecta) are already available and are stored in the CVARG archive. They will be studied in three single steps. A micropetrographic study of the rock structures and textures will be an important step for the successive development of the project. In this phase it will be determined which sample will be analyzed as bulk rock for major, trace elements and isotopes (second step) and which will be used to detail chemical analyses of the constituting mineral phases (third step). Besides, in this phase it will be possible to understand the existing mineral equilibrium and discriminate the possible presence of melt inclusions that will eventually be analyzed.
By combining modal and geochemical features of the samples it will be possible to define the most probable primary magmas. Once defined the composition of the primary magmas, these compositions will be used as starting material to model geochemical processes occurring in the plumbing system, and the melting processes occurring in the mantle, respectively. The combination of these data will evidence the role played by the basaltic injection into the rift systems in triggering the explosions of central volcanoes. The possible geochemical contamination and thermodynamic destabilization of the trachytic reservoir will be analysed and discussed in detail.
Furthermore, it will be discussed also the genetic mechanism of comenditic rhyolites and pantellerites of Terceira island and the role of degassing on the rheology of these evolved magmas.
This project will be developed with cooperation with the University of Perugia (Italy), which already studied similar geochemical process occurring below some volcanoes of Ethiopia.