P-T-H2O conditions during multistage fractional crystallization, magma replenishment and mixing through the nephelinitic–phonolitic series and their role on the eruptive styles on the Trindade Island (South Atlantic Ocean, Brazil)

Abstract

Magmas are affected by several differentiation processes as they migrate through the lithospheric plate, which are recorded by mineralogy, textures and geochemistry in igneous rocks. These processes and their related physico-chemical conditions can be reconstructed by petrography and mineral geochemistry, providing a detailed magmatic history from magma source to crystallization. We present a detailed petrography coupled with single-spot and multiple-profile mineral chemistry and whole-rock geochemistry that allows the reconstruction of P-T–H2O–depth conditions that affected the nephelinite-phonolite magmatic series and influenced eruptive styles. Results on nephelinites and phonolites suggest their cogeneticity and led to the recognition of multistage fractional crystallization, magma replenishment and mixing during their storage and eruptive events. Our data allow us to reconstruct nephelinite magma storage within the oceanic crust (20-5 km depth) with higher H2O content, and chambers lodged below the volcanic centers (<5 km depth) with lower H2O contents, which represent pre-eruptive magma storage subjected to progressive boiling. On the other hand, the main phonolite magma storage, crystallization and mixing with nephelinitic pulses occurred within the lithospheric mantle at depths of 57–37 km, indicating that the main magmatic differentiation of this series occurred at this depth. This process is accompanied by H2O increase, which may be related to magma ascension from the mantellic depths to the volcanic environment and eruption controlled by volatile contents. These interpretations are consistent with Strombolian to Hawaiian eruptive styles reported previously for nephelinite episodes and Vulcanian eruptive styles for phonolite ones.