Subduction-like fluids in the genesis of Mt. Etna magmas: evidence from boron isotopes and fluid mobile elements

摘要

New whole-rock B, Sr, Nd isotope ratios and ~(87)Sr/~(86)Sr on clinopyroxenes have been collected to study the enrichment of fluid mobile elements (FMEs) observed in Mt. Etna volcanics. Etna volcano, one of the most active in the world, is located in an extremely complex tectonic context at the boundary between colliding African and European plates. The analytical work focuses on current (1974-1998) and historic (1851-1971) eruptive activity, including some key prehistoric lavas, in order to interpret the secular shift of its geochemical signature to more alkaline compositions. Boron is used as a tool to unravel the role of fluids in the genesis of magmas, revealing far-reaching consequences, beyond the case study of Mt. Etna. Small variations are observed in δ~(11)B (-3.5 to -8.0‰), ~(87)Sr/(86)Sr (0.70323-0.70370), and ~(143)Nd/~(144)Nd (0.51293-0.51287). Moreover, temporal evolution to higher δ~(11)B and ~(87)Sr/~(86)Sr, and to lower ~(143)Nd/~(144)Nd, is observed in the current activity defining a regular trend. Sr isotopic equilibrium between whole-rock and clinopyroxene pairs indicates the successive introduction of three distinct magma types into the Etna plumbing system over time; these are characterized by differing degrees of FME enrichment. In addition, certain lavas exhibit evidence for country rock assimilation, magma-fluid interaction, or magma mixing in the shallow feeding system; at times these processes apparently lowered magmatic δ~(11)B and/or induced Sr isotopic disequilibrium between whole rock and clinopyroxene. The regular increase of δ~(11)B values is correlated with Nb/FME and ~(87)Sr/~(86)Sr ratios; these correlations are consistent with simple mixing between the mantle source and aqueous fluids derived from nearby Ionian slab. The best fit of Mt. Etna data is obtained using an enriched-MORB mantle source and a fluid phase with δ~(11)B of about -2‰ and ~(87)Sr/~(86)Sr of 0.708. We argue that the slab window generated by differential roll-back of subducting Ionian lithosphere (with respect to Sicily) allows the upwelling of asthenosphere from below the subduction system and provides a suitable path of rise for subduction-related fluids. The increasing geochemical signature of fluid mobile elements enrichment to Mt. Etna lavas is considered a consequence of the progressive opening of slab window through time.