Reconstructing the deposition of a 5.5 Ma lithified tephra by multi-scale and -analytical characterisations: the Messinian Maccarone section (Italy)

Fallout pyroclasts from large explosive volcanic eruptions consist of loose material that accumulates at significant distances from volcanic vents, i.e., primary tephra; they can become secondary tephra if reworked by sedimentary processes. This discrimination is challenging for old, lithified tephra due to limited outcrops and the inability to use sieving techniques. Here, a multi-analytical protocol is adopted and validated on a 5.5 Ma old, lithified (tuff-like), thick, and widely distributed tephra in Central Italy (the Maccarone section). The deposit and mesoscopic samples are mainly massive, lacking erosive surfaces and allochthonous lithics. Geochemical and mineralogical (XRPD) analyses reveal a calc-alkaline, crystal-poor (feldspars and biotite) deposit, rich in both glass and bulk SiO2. Sub-μm secondary sheet-silicates are widespread, whereas sedimentary calcite is significant in specific horizons. EPMA and SEM show rhyolitic, stubby, and poorly vesiculated glassy ashes, with 2D (by image analysis) grain-size distributions from ~200 to a few μm and highly sorted; ashes are also elongated and angular and poorly rounded. These microscopic shape attributes, coupled with the massive, structure-free mesoscopic character, indicate primary tephra deposition; conversely, the minor horizons with plane-parallel, curvilinear, or cross-layered aspects are instead ascribed to secondary and local remobilisations, which are unable to alter the textures of the ashes. The last hypothesis may represent a common intermediate depositional process between primary ashes settled in seawater and secondary pyroclast-rich turbidites. Trace elements indicate a volcanic-arc origin, possibly from a large eruption at approximately 5.5 Ma.