Chemical and textural analysis by EMPA and synchrotron X-ray micro-CT: insights into lava tube formation at Vesuvius (Italy)

Lava flow emplacement depends on the effusion rate, the viscosity of the lava and the topography. The transport of lava within lava flows usually occurs through the formation of lava channels or by the development of lava tubes. Lava flows that form tubes are able to feed their front with molten lava, due to the insulation of the erupted lava by the roof of the tubes, preventing atmospheric cooling. These tube-fed lava flows can thus emplace over longer distances, increasing the hazard caused by lava flows. Lava tube formation within a lava flow can be influenced by different factors such as: eruption rate, pre-eruptive topography, geochemical composition, and lava viscosity. Understanding how these mechanisms influence the formation of a lava tube is of great importance to better assess the hazard caused by lava flows and better react in case of eruptive crisis. In this study, the 1858 lava flow field formed during the 1858-1861 eruption at Vesuvius was examined as it formed several lava tubes, including one which can be accessed. The 1858 lava flow field and the largest lava tubes were sampled from the vent to the front and from the inside to the outside respectively. We performed Electron Microprobe Analysis (EMPA) to measure crystal and glass compositions, and used phase-contrast synchrotron X-ray computed microtomography (µ-CT) to retrieve the 3D structure of our samples and characterize the crystal phases. Our results show a variation between the microlitic matrix of the samples in terms of size and number of crystals, suggesting different cooling conditions during lava flow emplacement and lava tube development. Moreover, the volume fractions of the different phases composing the lava, leucites and plagioclase, clinopyroxenes, oxides, also show variations. These variations in volume fraction, number and size of the microlitic matrix can be used to decipher the local conditions of emplacement within the lava flow and lava tube. Understanding the petrology and geochemistry of the lavas forming the flow field and the lava tube help us to further comprehend lava flow emplacement and lava tube formation at Vesuvius.