Groundmass isotope signatures reveal decadal source evolution and mixing at Mount Etna volcano (Sicily, Italy).

Abstract:

An abrupt shift in magmatic isotope composition occurred at Mount Etna in 1971, accompanied by an increase in eruptive frequency and explosivity. In this study, we revisit this isotopic (Sr-Nd-Pb-Hf) evolution in historic products through the analysis of groundmass separates as proxies for melt compositions. Igneous minerals are known to retain the isotopic signature of the melt from which they crystallised; however, magmas often inherit crystals upon ascent, through interaction with pre-existing crystal mushes. If these mushes are isotopically distinct from the entraining magma, the erupted composition represents a mixture of historic and present-day signatures. We examine a suite of carefully hand-picked groundmass separates from both historic (pre-1971) and recent (1974-2014) eruptions, avoiding the effects of crystal inheritance. The abrupt change in composition from historic to recent eruptions is accentuated in our results (⁸⁷Sr/⁸⁶Sr: 0.703403-0.703663; ¹⁷⁶Hf/¹⁷⁷Hf: 0.282985-0.292939) and two endmember eruptions are identified; 1669 and 2002-03. Most post 1970 lavas likely represent a mixture of these two endmember compositions whose signatures, according to previous studies, are generated at the mantle level. Progressive enrichment and depletion of ⁸⁷Sr/⁸⁶Sr in the groundmass is also observed, perhaps reflecting episodes of magma recharge, a key driver of eruptive activity. In whole-rock material, however, these trends are disguised by the inclusion of a less radiogenic, antecrystic cargo. Given that recent increases in eruption explosivity and frequency are correlated with the arrival of a new isotopically distinct melt, accurate tracking of this signature may signpost the direction of future activity at Mount Etna. We show that only groundmass separates succeed in fingerprinting the isotopic evolution and mixing of melts through the plumbing system, bringing new insights to decadal eruptive trends at a highly active volcano.