ИСТИНА

Diffusion modelling of zoned crystals is an important tool to estimate crystal residence times in magma reservoirs, between magma mixing, ascent, and eruption. At Piton de la Fournaise (La Réunion, Indian Ocean) eruptions are frequently fed from a shallow magma reservoir and a complex sill and dyke system that is periodically refilled with mafic magmas from depth. A new eruptive cycle initiated in June 2014 after 41 months of quiescence. Four small eruptions in June 2014, February 2015, May, and July lasted only few days and produced mostly evolved basalts, whereas during the large August-November 2015 eruption magmas became increasingly more mafic. Diffusion profiles were analyzed in olivines from ten quenched lava and tephra samples covering the eruptive sequence in 2014-2015 to constrain the residence times of crystal populations in the plumbing system. The timescales obtained by Mg-Fe diffusion modelling were linked to monitored geophysical data such as seismicity, ground deformation, and soil CO2 flux. Olivines show three types of zoning (normal, reverse, and complex) and various core compositions (Fo73-85). Olivine crystals were periodically reactivated from different, resident magma batches (in calculated equilibrium with Fo77.7-84.6) during three main episodes of days to seven months prior to the eruptions in (1) June 2014 and (2) February 2015 and during the eruptions in (3) July and August-November 2015. Short residence times of olivines erupted in more evolved magmas of June 2014 and May 2015 eruptions (in equilibrium with Fo77-80) correlate with shallow seismicity and ground deformation at that time. This indicates that the small eruptions were fed from magmas residing directly at the top of the shallow reservoir. Other olivines started to equilibrate to more mafic compositions (Fo81-84.6) at the same time, but these crystals remained unerupted until later in July and August-November 2015. The absence of deep seismicity prior to May 2015 suggests that these olivines were reactivated probably from crystal mush in the central reservoir (~ 6-8 km depth) during basaltic recharge and magma mixing. These hybrids remained in the plumbing system for > 237 days until their eruption in September-November 2015. However, residence time of olivines erupted during July 2015 show strong similarities to those of the large August-November eruption, which marks the transition between earlier shallow fed eruptions and the large, deeply fed August-November eruption.