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

New whole-rock B, Sr, Nd isotope ratios and Sr-87/Sr-86 on clinopyroxenes h ave 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, includi ng some key prehistoric lavas, in order to interpret the secular shift of i ts 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 delta B-11 (-3.5 to -8.0 parts per thousand), Sr-87/Sr-86 ( 0.70323-0.70370), and Nd-143/Nd-144 (0.51293-0.51287). Moreover, temporal e volution to higher delta B-11 and Sr-87/Sr-86, and to lower Nd-143/Nd-144, is observed in the current activity, defining a regular trend. Sr isotopic equilibrium between whole-rock and clinopyroxene pairs indicates the succes sive introduction of three distinct magma types into the Etna plumbing syst em over time; these are characterized by differing degrees of FME enrichmen t. In addition, certain lavas exhibit evidence for country rock assimilatio n, magma-fluid interaction, or magma mixing in the shallow feeding system; at times these processes apparently lowered magmatic delta B-11 and/or indu ced Sr isotopic disequilibrium between whole rock and clinopyroxene. The re gular increase of delta B-11 values is correlated with Nb/FME and Sr-87/Sr- 86 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 delta B-11 of about -2 parts per thousand and Sr-87/Sr-8 6 of 0.708. We argue that the slab window generated by differential roll-ba ck of subducting Ionian lithosphere (with respect to Sicily) allows the upw elling of asthenosphere from below the subduction system and provides a sui table path of rise for subduction-related fluids. The increasing geochemica l signature of fluid mobile elements enrichment to Mt. Etna lavas is consid ered a consequence of the progressive opening of slab window through time. (C) 2001 Elsevier Science B.V. All rights reserved.