COMPARATIVE ANALYSIS OF LAVA TUBE AND LAVA FLOW BASALTS, MOUNT ETNA, SICILY, ITALY

Basalts formed during the 1614-1624 northern flank eruption of Mount Etna (Sicily, Italy) exhibit an uncommon texture, containing both abundant phenocrysts and vesicles within a lava flow field dominated by pahoehoe morphology. This study analyzed the texture, mineralogy, and geochemistry of basalts from the Grotta dei Lamponi, a lava tube produced during this eruption, and compared them with aboveground pahoehoe basalts formed during the same eruption. Mineralogy and textural characteristics were observed in thin sections, and elemental weight percentages were obtained using X-ray fluorescence analysis. Textural observations included size ranges and volume percentages for vesicles and phenocrysts, as well as various characteristics of phenocrysts. Texturally, the two basalts were very similar, with a total average phenocryst content of ~ 45 vol.%, which is unusually high for a low viscosity pahoehoe lava. Geochemical analyses also showed negligible variations between the lava flow and lava tube basalts; the eruptive products are classified as sodic trachybasalt (hawaiite) using the TAS system, containing an average of 48.4 wt.% SiO_2, 9.0 wt.% FeO, 4.1 wt.% Na₂O, and 1.8 wt.% K₂O. Previous studies of the 1614-1624 eruption explained the large phenocryst volume percent in these basalts; rather than forming at depth, the bulk of nucleation and crystal growth was driven by abundant volatile loss and took place in syn- to post-eruptive conditions. Notably, this study found that microphenocryst content differed between the basalts by an average of 3.7 vol.%, with the lava tube samples displaying more microphenocrysts (7.86 vol.%). Since the formation of new crystals and the growth of existing crystals is thought to be driven by the same process, it is unclear why one would be preferred at the expense of the other in the lava tube or flow. This difference is likely the result of other variables affecting the lava’s solidification path, though an exact mechanism has not yet been identified.