Timescales of magmatic processes during the eruptive cycle 2014-2015 at Piton de la Fournaise, La Réunion, obtained from Mg-Fe diffusion modelling in olivine
Résumé
Piton de la Fournaise started a new eruptive cycle in June 2014 after 41 months of quiescence. The small eruptions in June, February 2015, May, and July produced evolved basalts, whereas magmas of the large August-November eruption became increasingly mafic. Compositional zoning of olivine crystals was analysed to model diffusion times representing residence times between magma mixing and eruption. These correlate to the geophysical and geochemical records. Olivine crystals of various core compositions (Fo73.2-85.1) were periodically reactivated days to 7 months prior to June 2014 and February 2015 eruptions and during July and August-November by different magmas (in equilibrium with Fo77.7-84.5). June 2014, February, and May eruptions were fed from the top of the reservoir as shown by the eruption of evolved magmas and olivine crystals with short diffusion times. At the same time, olivine crystals were reactivated probably from mush by recharging basalt during the formation of hybrid magmas (in equilibrium with Fo80.1-84.5) in the central reservoir. These hybrids remained unerupted (> 235 days) until October 2015. However, products of July 2015 eruption bear already olivine crystals with long diffusion times, which is a strong similarity to those of the August-November eruption 2015. This eruption marks the transition between earlier small eruptions fed from shallow levels to those fed from the central reservoir. Such linking of magma mixing and activation processes and geophysical signals helps for a better understanding of the process and timescale of eruption at Piton de la Fournaise and basaltic hotspot-related magma systems in general.