Complex core-rim zoning of Mg-Fe-Ni-Ca-Cr-Al-P in high-Mg olivine crystals from a tuff ring of Shiveluch volcano, Kamchatka, enables reconstruction of the entire olivine crystallization history from mantle conditions to eruption. Bell-shaped Fo86–92 and Ni profiles in crystal cores were formed by diffusion after mixing with evolved magma. Diffusion proceeded to the centres of crystals and completely equilibrated Fo and Ni in some crystals. Diffusion times extracted from Fo and Ni core profiles range from 100 to 2000 days. During subsequent mixing with mafic mantle-equilibrated melt, the cores were partially dissolved and overgrown by Fo90 olivine. Times extracted from Fo and Ni diffusion profiles across the resorption interface between the core and its overgrowth range within 1–10 days, which corresponds to the time of magma ascent to the surface. The overgrowth shows identical smooth Fo-Ni decreasing zoning patterns for all crystals towards the margin, indicating that all crystals shared the same growth history after last mixing event prior to eruption. At the same time, Ca, and to an even greater extent Cr, Al, and P have oscillatory growth patterns in the crystals overgrowth. Our data show that magma ascent can be extremely short during maar/tuff ring eruption.

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Diffusion profiles in olivine crystals from the final mafic eruption products of the compositionally zoned Laacher See tephra deposit were measured to identify recharge and eruption-triggering events prior to the eruption of the Laacher See volcano (12.9 kyr). These products represent the hybrids of mixing between phonolite and intruding basanite at the bottom of the reservoir, which is likely related to the eruption-triggering event. Additionally, olivine crystals from ten basanitic scoria cones and maar deposits (East Eifel) and two nephelinites (West Eifel) were analyzed to constrain histories of olivine in Quaternary basanite magmas. Olivine crystals from the Laacher See hybrids vary in core composition (Fo 83-89) and show reversely zoned mantles with high Fo 87.8-89 compared to olivine in East Eifel basanites erupted in nearby, older scoria cones. Towards the crystal margin, olivine in the hybrids develop a normally zoned overgrowth (Fo 86.5-87.5). Olivine from East Eifel basanites show similar zonation and core compositions (Fo 80-88) but have less forsteritic mantles (Fo 83-88) indicating that these basanites are less primitive than those recharging the Laacher See reservoir (> Fo 89). Olivine in the West Eifel nephelinites show mantles similar to those from Laacher See (Fo 87.5-90), but have normal zoning and high-Fo cores (Fo 88-92). This indicates that olivine in the Laacher See hybrids were entrained by a near-primary basanite from older cumulates just before hybridization of the basanite with the phonolite. Diffusion modeling indicates maximum timescales between entrainment and eruption of Laacher See of 30-400 days that are comparable to those calculated for olivine from basanitic scoria cones (10-400 days).

<p>Diffusion profiles in sanidine (Ba) and olivine (Mg-Fe, Ca, Mn, and Ni) were used to track recharge events prior to the eruption of the Laacher See volcano, East Eifel volcanic field, western Germany (12.9 ka). Sanidine crystals were analyzed in samples from cumulates and mafic to intermediate phonolites. Olivine crystals occur only in the final mafic eruption products of the compositionally zoned tephra deposit and represent the hybrids of mixing between differentiated phonolite, crystal cumulates, and intruding basanitic magma at the bottom of the magma reservoir. This mixing event is likely related to the eruption triggering event. Additionally, olivine crystals from ten basanitic scoria and maar deposits in the East Eifel and two locations in the West Eifel (Pulvermaar melilith-nephelinite, Meerfelder Maar ol-nephelinite) were analyzed to represent Quaternary parent mafic magmas in Eifel volcanism.</p><p>Olivine from the mafic component that mixed with the Laacher See phonolite are always reversely zoned from cores of variable composition (Fo<sub>83-89</sub>). Zoning of all crystals show trends to a common rim composition (Fo<sub>87.5-89</sub>). Most crystals show additionally a narrow (<10 μm) normally zoned overgrowth at the outermost grain boundary (Fo<sub>86.5-87.5</sub>). Olivine crystals from mafic cones in the East Eifel show similar zoning patterns and core compositions (Fo<sub>80-88</sub>) as those from Laacher See hybrids, but their rims are more variable and always less forsteritic (Fo<sub>83-88</sub>). The lack of olivine rims with >Fo<sub>88</sub> indicates that East Eifel basanites are less primitive than the basanite that intruded into the Laacher See reservoir with olivine rim composition >Fo<sub>89</sub>. However, olivine in samples from the West Eifel nephelinite maar deposits show rim compositions similar to the olivines from Laacher See (Fo<sub>87.5-90</sub>), but are dominantly normal zoned and have high-Fo cores (Fo<sub>88-92</sub>).</p><p>We interpret these observations to indicate that olivine crystals on Laacher See hybrids probably originate from a cumulate or crystal mush with low melt fraction that was disaggregated by the ascending basanite before hybridization. Diffusion modeling of olivine rims indicate a time scale between mixing and eruption of less than 49 days.</p><p>Diffusion times of the sanidine phenocrysts from the intermediate phonolite indicate older recharge events every 1500-3000 yrs that did not result in complete hybridization and eruption. Ba-diffusion times are much shorter for sanidines from the mafic phonolite (4-8 yrs) and the cumulates (months). The reactivation of crystals from cumulates, that can be related to the eruption-triggering recharge event, occurred therefore only months prior to the eruption of Laacher See. These timescales between recharge and eruption are remarkably shorter than the diffusion times calculated for olivine from basanite erupted from scoria cones (up to 500 days).</p>

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...