Two distinct eruptive events characterize the volcanic activity at Mount Etna during the 2002 to 2005 period. We identified signals of magma ascent preceding these eruptions by geochemical monitoring of both chemical composition and He‐isotope ratio of gas emissions from five locations in the peripheral area of the volcano. The geochemical signals are interpreted using the models proposed by Caracausi et al. (2003a, 2003b) and allow identification of episodes of magma ascent and estimation of the pressures of degassing magma. As observed for the 2001 eruption (Caracausi et al., 2003b), magma ascent probably triggered the onset of the 2002–2003 eruption, and minor events of magma ascent were observed between May and December 2003. In contrast to the previous two eruptions, the 2004–2005 eruption was not preceded by significant geochemical signals of volcanic unrest, suggesting that this eruption was mainly triggered by the failure of the upper portion of the volcanic edifice under the magmatic hydrostatic pressure in the conduits. High 3He/4He ratio revealed new volatile‐rich magma accumulation. The 2002–2003 eruption was preceded by a much shorter period of new magma accumulation from deep levels of the feeding system. Few minor signals of magma migration were detected at some of the sites during the months preceding the 2004–2005 eruption, suggesting that the degassed 3He‐depleted magma resident in the volcanic conduits was not replaced by new volatile‐rich magma. This is in agreement with the lack of explosive activity during the 2004–2005 eruption and with petrologic observations that the parent magma probably erupted in 2000 and 2001. New geochemical signals of magma ascent from the deep reservoir have been identified since June 2005, indicating that the volcanic activity of Mount Etna is evolving toward new pre‐eruptive conditions.

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between Type-1 and Type-2 end-members. Type-2/Type-3 MIs testify to en route processes (plagioclase assimilation and volatile fluxing) peculiar for ''deep dike fed'' eruptions. The latter are strongly controlled by tectonics or flank instability that occasionally promote upraise of undegassed, more radiogenic primitive magma, which may interact with plagioclase-rich crystal mush/cumulates before erupting. Type-2/Type-3 MIs approach the less radiogenic Pb isotopic composition of plagioclase from prehistoric lavas, thus suggesting geochemical overprinting of present-day melts by older products released from distinct mantle sources. Our study emphasizes that MIs microanalysis offers new insights on both source characteristics and en route processes, allowing to a link between melt composition and magma dynamics.

[1] We report on the first geochemical investigation of the Monticchio maar lakes (Mt. Vulture volcano, southern Italy) covering an annual cycle that aimed at understanding the characteristic features of the physical structures and dynamics of the two lakes. We provide the first detailed description of the lakes based on high-resolution conductivity-temperature-depth (CTD) profiles, chemical and isotopic (H and O) compositions of the water, and the amounts of dissolved gases (e.g., He, Ar, CH 4 , and CO 2 ). The combined data set reveals that the two lakes, which are separated by less than 200 m, exhibit different dynamics: one is a meromictic lake, where the waters are rich in biogenic and mantle-derived gases, while the other is a monomictic lake, which exhibits complete turnover of the water in winter and the release of dissolved gases. Our data strongly suggest that the differences in the dynamics of the two lakes are due to different density profiles affected by dissolved solutes, mainly Fe, which is strongly enriched in the deep water of the meromictic lake. A conceptual model of water balance was constructed based on the correlation between the chemical composition of the water and the hydrogen isotopic signature. Gas-rich groundwaters that feed both of the lakes and evaporation processes subsequently modify the water chemistry of the lakes. Our data highlight that no further potential hazardous accumulation of lethal gases is expected at the Monticchio lakes. Nevertheless, geochemical monitoring is needed to prevent the possibility of vigorous gas releases that have previously occurred in historical time.