Basaltic dyke eruptions at Piton de La Fournaise: characterization of the eruptive products with implications for reservoir conditions, conduit processes and eruptive dynamics

Small-volume proximal-summit eruptions, sometimes with only little precursors, are common at Piton de la Fournaise (La Reunion Island, France). Their eruptive style ranges from Hawaiian to Strombolian, but their eruptive mechanisms are still not completely understood. To gain insight, we combined syn-eruptive field work, textural and geochemical measurements on the short-lived eruption of July 2015 and we compared it with the 2014 eruption, which opened the ongoing eruptive cycle at Piton de la Fournaise. Crystal-poor and moderately vesicular components were predominant at the beginning of the eruptions, during the lava flux peak and the Hawaiian-style activity. Their abundance decreased with time in favour of less vesiculated and crystal-rich components during the decrease of the lava flux and the Strombolian-style activity. Physical, textural and chemical characterization of the pyroclasts, as well as thermometry calculations, permit relating the different texture and chemical features of the eruptive products with the pre- and syn-eruptive processes. Geophysical precursors detected several weeks before the July 2015 eruption, as well as some evidences of magmatic reheating recorded in bulk and crystal compositions highlight that the July 2015 magma results from a pre-eruptive mixing between more and less differentiated magmas. We finally deduced that the observed syn-eruptive textural variations are related to the same batch of magma undergoing different cooling and degassing dynamics.