A chemostratigraphic investigation of the prehistoric Vavalaci lava sequence on Mount Etna: simulating borehole drilling

Scientific drilling of volcanic successions has been suggested as a way of establishing stratigraphic sequences of unexposed lava flows on large complex volcanoes, with the aim of in-depth study of magmatic processes and source geochemistry of otherwise inaccessible lava sequences. To simulate the core drilling of such sequences, lava flows from the prehistoric Vavalaci Centre exposed in the south wall of the Valle del Bove, Mount Etna, Sicily, were sampled in four stratigraphic sections. The fresh, generally strongly porphyritic Na-alkaline trachybasalts and trachyandesites show diverging sub-parallel trends of high- and low-alkali concentrations in total alkali versus silica (TAS) diagrams, whilst variations of other major and trace elements reveal two further distinct chemical groups enriched in K, REE and Ti which follow separate fractionation paths. A set of control samples was used to establish geochemical variations within a single lava flow. Primitive mantle normalised incompatible element patterns demonstrate that the lavas have highly enriched OIB signatures with a clear division in LREE, Ba, Th, Nb and Zr concentrations between the four different chemical groups. Comparison of data for the Vavalaci lavas with the compositions of other prehistoric, historic and recent eruptions of Mt. Etna indicates a temporal trend towards more basic magma compositions. The chemostratigraphy of the lavas was statistically analysed to give correlations between flows from different sections. Whilst a good number of geologically meaningful correlations were revealed, we can demonstrate that only one set of lavas was actually sampled in all four sections, whilst a number of unique lavas remain uncorrelated. Thus no individual section, or simulated borehole core, provided samples of the complete lava flow sequence without significant gaps in the stratigraphy. The trends in lava compositions are also defined in the stratigraphy, showing their evolution from low- to high-alkali lavas through the series, which may be related to temporal decrease in degree of partial melting of the mantle rather than through simple fractionation processes or mixing of magmas