Combined magnetotelluric and petrologic constrains for the nature of the magma storage system beneath the Late Pleistocene Ciomadul volcano (SE Carpathians)

The Ciomadul is the youngest volcano of the Carpathian–Pannonian region, which erupted last time at 32 ka. It produced high-K dacitic lava domes and pumiceous pyroclastic rocks. The dacite is crystal-rich and contains plagioclase, amphibole in addition to biotite, titanite, apatite, zircon and occasionally quartz, K-feldspar as well as olivine, clinopyroxene and orthopyroxene. There are two groups of amphiboles, characterized by low-Al and high-Al, respectively. They occur in the same samples and also as different zones of the same crystals. Thermobarometric calculations suggest that the low-Al amphiboles were formed from a low temperature (< 800 °C) silicic magma, whereas the high-Al amphiboles crystallized at about 950 °C from a more mafic melt. A near-solidus silicic crystal mush body was stored at 7–14 km depth, where an eruptible magma batch was produced by major reheating (about 200 °C temperature increase) due to the intrusion of hot mafic magma into the silicic magma reservoir. A magnetotelluric survey was performed to reveal whether any melt-bearing magma body could presently reside beneath the volcano. Both the 2D and 3D inversion modeling calculations indicate low electric resistivity values in the depth interval of 5–25 km, just beneath the volcanic centers. This can be interpreted as implying a partially melted zone, i.e. a crystal mush body containing about 5–15% melt fraction. In addition, the 2D modeling calculation indicates also a deeper low resistivity anomaly at 30–40 km depth. The consistent petrologic and magnetotelluric constrains on the magma storage beneath Ciomadul are corroborated by the recent seismic tomography result, which pointed out a low-velocity anomaly at 8–20 km depth zone. Thus, results of independent models suggest the presence of a melt-bearing crystal mush body beneath the seemingly inactive volcano. Since there are implications for long repose periods during the lifetime of the volcano as well as for effective and rapid remobilization of the low-temperature silicic crystal mush body prior to volcanic eruptions, the present existence of a low melt fraction silicic crystal mush beneath Ciomadul could mean that there is still a potential for a rejuvenation in the future. We suggest for long-dormant or seemingly inactive volcanoes, such as Ciomadul, having melt-bearing magmatic body at depths to term as ‘volcano with potentially active magma storage’ or PAMS volcano.