How an embryonic magma feeding system evolves: Insights from the primordial pulses of Mt. Etna volcano

Volcanism at Mt. Etna (Italy) started with an early tholeiitic stage dating back to 542 ka during which subaqueous to subaerial magmas were emitted chiefly through fissure-type eruptions on widespread areas located on the southern flank of the modern volcano edifice. Volcanic products belonging to the earlier Aci Trezza Synthem (542–496 ka) and those of the later Adrano Synthem (332–320 ka) are basalts within a narrow range of variation. Despite the rather homogeneous geochemical characteristics, zoning patterns and FeMg diffusion chronometry on olivine crystals from lavas of both the Synthemic Units have evidenced different dynamics and kinetics of storage and transfer before eruptions. Specifically, one dominant, normally-zoned, Fo_83–86 olivine population makes peculiar lavas of the Aci Trezza Synthem, whose patterns can be interpreted as due to simple upward migration from deep storage reservoirs directly to the surface with timescales of 109–200 days. Volcanic rocks of the Adrano Synthem have at least three additional olivine populations (i.e., Fo_78-81, Fo_73-74, Fo_64-70) bearing more complex normal and reverse zoning patterns, features revealing that magmas ascended from the deeper storage zones and then intruded and stalled in shallower reservoirs before being erupted. Transfers throughout these magma reservoirs record both short (<46 days) and long timescales (>106 days), suggesting that tectonics could have accelerated or inhibited magma supply during this later stage of volcanic activity. This new dataset points out that the embryonic plumbing system of Mt. Etna developed a more complex architecture throughout the first ~200 ka of volcanism as a consequence of a declining effect of transtensional tectonics over time.