Unveiling the occurrence of transient, multi-contaminated mafic magmas inside a rhyolitic reservoir feeding an explosive eruption (Nisyros, Greece)

The investigation of heterogeneous magma systems enhances the understanding of magma differentiation and
transfer processes in active volcanoes, thus constraining the dynamics driving the eruptions and the related
hazard. Magma heterogeneity is generally preserved in the coeval juvenile products of explosive eruptions, as it
occurs in the Upper Pumice sequence, emplaced by the last sub-Plinian explosive eruption at Nisyros volcano
(Greece). The deposit comprises a basal fallout, overlaid by pyroclastic density current units, followed by a lagbreccia level. White-yellow, porphyritic, rhyolitic pumices constitute the main juvenile component. Grey, crystalrich juvenile clasts (CRCs) are less abundant (up to 10-15%), and are characterised by three different texture
types (Type-A, -B and -C), with specific recurrence in the different depositional units and well correlated to the
magma evolution. In the basal unit CRCs occur as andesitic to dacitic lapilli with Type-A and -B vesicular textures
associated with highly variable trace element and isotopic compositions. In the lag-breccia deposit, the juvenile
clasts occur as bombs with crenulated or bread-crust surfaces, displaying diktytaxitic Type-C textures and less
evolved andesitic compositions, covering a larger Nd-isotope range at lower Sr-isotopes compared to the others.
The CRCs are interpreted as the result of the rapid cooling of more mafic magma blobs sequentially intruded in
the cooler rhyolitic host magma, in which they attained variable textures by different undercooling conditions,
due to their variable compositions. We suggest that a two-stage AFC (Assimilation plus Fractional Crystallisation)
process occurred at different pressures, before intrusion in the host magma, accounting for their heterogeneous
chemical and isotopic characteristics. Firstly, the most primitive melts variably assimilated gneissic wallrock at
depth, acquiring a variable Nd-isotope signature. On the way to the surface, they later experienced shallow AFC
processes within different small magma reservoirs, involving heterogeneous carbonate-rocks such as pure
limestone, metasomatised marble and skarn. Sequential dynamics of ascent and intrusion into the rhyolitic
magma chamber lead the more evolved and skarn-contaminated Type-A and -B melts to firstly move in the upper
part of the reservoir to be erupted in the early fallout deposits. Type-C more mafic melts later intruded the
rhyolitic reservoir and were erupted in the lag-breccia deposit. The lowest Nd-isotopes recorded by CRCs, with
respect to all the volcanic products of the Kos-Nisyros volcanic field, reveal the peculiar transient history for
these magmas at relatively shallow levels in the crust. The CO2 release from the carbonate-rock assimilation has
also possibly contributed to trigger the explosive eruption, discharging a large amount of CO2 into the
atmosphere.