Days to weeks of syn-eruptive magma interaction: High-resolution geochemistry of the 2002-03 branched eruption at Mount Etna

Eruption onset, style and duration are governed, in part, by the movement and interaction of distinct magma bodies at depth. High-resolution investigation of erupted products can retrospectively inform our understanding of such processes and improve future interpretation of real-time monitoring signals. In 2002-03, Mount Etna (Italy) erupted two elementally and isotopically distinct magmas simultaneously, providing a unique opportunity to explore magma transport and evolution through complex, well-documented and monitored volcanic activity. In this study, melt chemistry (groundmass fraction as opposed to whole-rock) and mineral zoning (X-ray fluorescence mapping and electron microprobe) are treated as separate recorders of magma history tracking syn- and pre-eruptive processes, respectively. Elemental mapping of entire thin sections revealed a largely antecrystic, hybrid crystal cargo hosting reverse-zoned clinopyroxene and olivine, supporting extensive magma mixing in southern flank conduits. Antecryst-free groundmass chemo-stratigraphies reveal melt compositional variations on timescales of days to weeks. In agreement with previous whole-rock studies, we find that during the early-middle stages of activity, melt erupted on the southern flank was rich in MgO, CaO, Cr, Ni, and radiogenic-Sr, and depleted in Al2O3, Na2O, La, Zr, relative to its north-eastern counterpart. In addition, high resolution tracking of melt composition reveals a progressive shift to increasingly evolved compositions over the course of the S-rift eruption. A corresponding shift in 87Sr/86Sr isotope ratios (0.70366 to 0.70358) across the same period implies progressive mixing of isotopically distinct magmas as the main driver of the observed compositional change. In contrast, NE-rift products lack evidence of mixing in both melt and mineral records, in agreement with previous work suggesting geodynamically controlled draining of shallow, isolated magma pockets. Comparing recharge-to-eruption timescales derived from mineral zonation with real-time monitoring data and groundmass compositional data, we show that the onset of magma mixing in the S-rift coincided with a deep seismic event (8-18 km) and was followed by the ascent of undegassed magma approximately 2 months before eruption onset. Finally, we demonstrate that high-resolution temporal changes in melt composition, approached through melt geochemistry, are closely linked to eruption intensity and SO2 emission. We suggest that composition and explosivity on the southern rift fluctuated in response to multiple phases of recharge-mixing-hybridisation between undegassed ‘eccentric’ magma and a degassed resident magma at 3-5 km depth. Our results highlight that high-resolution geochemistry can improve our understanding of deep magmatic processes driving eruption onset, duration and intensity. Rapid analysis of melt composition integrated with traditional monitoring approaches could improve future hazard response at Mount Etna and active volcanoes globally.