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Based on detailed field, petrographic, chemical, and isotopic data, this paper shows that the youngest magmas of the active Nisyros volcano (South Aegean Arc, Greece) are an example of transition from rhyolitic to less evolved magmas by multiple refilling with mafic melts, triggering complex magma interaction processes. The final magmatic activity of Nisyros was characterized by sub-Plinian caldera-forming eruption (40 ka), emplacing the Upper Pumice (UP) rhyolitic deposits, followed by the extrusion of rhyodacitic post-caldera domes (about 31-10 ka). The latter are rich in magmatic enclaves with textural and compositional (basaltic-andesite to andesite) characteristics that reveal they are quenched portions of mafic magmas included in a cooler more evolved melt. Dome-lavas have different chemical, isotopic, and mineralogical characteristics from the enclaves. The latter have lower Sr/Sr and higher Nd/Nd values than dome-lavas. Silica contents and Sr/Sr values decrease with time among dome-lavas and enclaves. Micro-scale mingling processes caused by enclave crumbling and by widespread mineral exchanges increase from the oldest to the youngest domes, together with enclave content. We demonstrate that the dome-lavas are multi-component magmas formed by progressive mingling/mixing processes between a rhyolitic component ( post-UP) and the enclave-forming mafic magmas refilling the felsic reservoir (from 15 wt.% to 40 wt.% of mafic component with time). We recognize that only the more evolved enclave magmas contribute to this process, in which recycling of cumulate plagioclase crystals is also involved. The post-UP end-member derives by fractional crystallization from the magmas leftover after the previous UP eruptions. The enclave magma differentiation develops mainly by fractional crystallization associated with multiple mixing with mafic melts changing their composition with time. A time-related picture of the relationships between dome-lavas and relative enclaves is proposed, suggesting a delay between a mafic magma input and the relative dome outpouring. We also infer that the magma viscosity reduction by re-heating allows dome extrusion without explosive activity. [ABSTRACT FROM AUTHOR]

In this paper the installation and long-term operation of a system for continuous monitoring of fumarolic gases is described. Several physicochemical and gas-geochemical parameters such as the concentration of CO2, H2S and CO in the fumarolic emissions, as well as the temperatures of the hydrothermal steam and soil in close vicinity of the fumarole and steam pressure are measured in short-time intervals (typically 15 seconds). Data are logged on-site and in parallel transferred to a remote station by digital telemetry. Specially developed software enables the real-time observation of the local conditions in the crater and full bidirectional control of the monitoring system. Fluctuations in the monitored parameters are also reported. From the data presented it can be concluded that environmental conditions (direction and strength of wind, precipitation) will interact with some of the parameters monitored. These influences can only be revealed by continuously operated monitoring systems. [ABSTRACT FROM AUTHOR]

The islet of Pyrgousa, 8 km west of Nisyros, has andesites overlain by proximal deposits of the Kos Plateau Tuff (KPT). These andesites record the evolution of the Kos-Nisyros volcanic centre prior to the large KPT eruption at 161 ka. This precursory activity had previously been recorded only in dacites and rhyolites in the Kefalos Peninsula, farther from the volcanic centre. This study investigated the age, geochemistry, mineralogy, and petrogenesis of the andesite domes, probable flows, and associated talus breccias on Pyrgousa. Analysed samples are basaltic andesite and andesite, with strong enrichment in Ba and Sr and low values of Ti and Zr. An 40Ar/39Ar date of 1.9 ± 0.1 Ma on biotite is similar to dates from dacite and rhyolite stocks and domes in the Kefalos Peninsula, and like those dates is the maximum age due to excess 40Ar. There is no evidence on Pyrgousa for a stratovolcano precursor of the KPT eruption. The andesite domes geochemically resemble Pliocene domes in Methana and Aegina that mark the onset of magmatism in the northwestern South Aegean Arc, with an important magma component derived from subcontinental lithospheric mantle. These early andesites heated the upper crust, thus facilitating the growth of upper crustal magma chambers, which were filled by felsic crystal mush derived by differentiation of hydrous intermediate magmas with a more asthenospheric signature. [ABSTRACT FROM AUTHOR]