Composition and compositional zoning of olivine as a tracer for pre-eruptive magmatic processes: Application to Piton de la Fournaise, Laacher See, and Shiveluch volcano

Abstract

This dissertation deals with the reconstruction of pre-eruptive magmatic processes with respect to their timescales. Deep recharges of mafic magma into shallower reservoirs and subsequent magma mixing are assumed to play a key role in triggering eruptions. The timespan between mixing and eruption is therefore a crucial factor to answer the question how fast magmatic systems can reactivate and erupt. Timing of magmatic processes and chemical composition of interacting magmas are archived in the zoning of crystals. This study uses zoned olivine crystals as a tool to track pre-eruptive recharge and mixing events of magmas and characterize the interacting magmas with respect to their composition for three different volcanic settings: (1) the subduction-related basaltic volcanism at Shiveluch, Kamchatka, (2) the basaltic hot-spot volcano Piton de la Fournaise on La Réunion, and (3) the alkaline intraplate volcanic field of the Eifel in western Germany with basanitic to phonolitic magma compositions.Olivine crystals in clasts from a maar deposit of Shiveluch volcano, Kamchatka, were analyzed with line profiles at the electron microprobe to track crystal histories. High-Fo olivine cores (Fo86-91) show complex zoning with a normal zoned core, a dissolution boundary, and a rim overgrowth (Fo90). The normal zoned cores are interpreted as result from magma mixing, subsequent dissolution, and partial re-equilibration, whereas the overgrowths represent crystallization subsequent to magma mixing and during ascent. Diffusion modeling of Mg-Fe and Ni reveal times of 100-2000 days for the partial equilibration of olivine cores, but only 1-10 days since the rim overgrowth formed indicating that the ascent of mafic arc magmas can occur quite fast.At Piton de la Fournaise, La Réunion, olivine crystals were analyzed in basaltic samples from three small and one large eruption occurring during the eruptive cycle 2014-2015 after nearly four years of inactivity. Magmas erupted during the small eruptions in June 2014, February, May, and July 2015 were evolved basalts and became increasingly more mafic during the large August-November 2015 eruptions. Olivine cores show variable compositions (Fo73.2-85.1), whereas the rims have similar compositions in every sample, which are in equilibrium with the host melt in which the crystals were erupted. Olivine crystals from small eruptions in June 2014 and May 2015 have only short diffusion times of days to few months. Combined with the more evolved magma compositions and associated shallow seismicity, this indicates that the small eruptions were fed from more shallow magma batches after recharge. In contrast, olivine crystals in lavas from the large August-November 2015 eruption started re-equilibration during three distinct episodes days up to seven months prior to the eruptions in June 2014, February 2015, and in the time between the eruptions in July and August-November. The correlation to deep seismic signals and the eruption of increasingly more mafic magmas during the large August-November er...