CHANGING CHEMICAL SIGNATURES IN 700 YEARS OF MAGMATISM AT MT. ETNA, ITALY: DOES A MANTLE SUBDUCTION OR A CRUSTAL ASSIMILATION SIGNATURE DOMINATE?

For centuries, Mt. Etna has been erupting trachybasaltic magmas but since 1971 AD, compositions have changed (e.g., higher K, Rb, ⁸⁷Sr/⁸⁶Sr) and eruption frequency, volume and explosivity have increased. Hypotheses that explain these changes range from increased subduction zone input to contribution from crustal melt or fluid. Magma recharge has been significant, with post 1971 recharge being more primitive and volatile-rich (Schiano et al. 2001; Ferlito et al. 2010; Viccaro et al. 2010). We have conducted in situ studies to isolate the timing and source of the post 1971 compositional and eruptive changes. (1) In situ textural, elemental and Sr isotope analyses of plagioclase from lavas erupted between 1329 and 2004 reveal dissolution textures, and higher An, Mg, K, Rb and Sr but lower Ba in post 1971 plagioclase. Most plagioclase have ⁸⁷Sr/⁸⁶Sr ≤ whole-rock (WR) and show core to rim isotope disequilibrium; the overall range is similar from 1329 to 2004. Sr diffusion calculations suggest that post 1971 plagioclase have shorter residence times than pre 1971 crystals. (2) In situ clinopyroxene (cpx) analyses combined with WR data use the geobarometer of Putirka et al. (2008) to illustrate that cpx crystallized from the Moho to shallow levels (Nelson et al., this meeting). (3) Sr isotope TIMS analyses of WR and groundmass (GM) reveal that GM has higher ⁸⁷Sr/⁸⁶Sr, and the difference between GM and WR increases from 1329 to 2004. These observations support the interpretation that the last ~700 years of magmatism at Etna has been characterized by a crustal scale magma system that has been recharged, with post 1971 recharge being more primitive, volatile rich and voluminous. In post 1971 magmas higher volatile content suppressed plagioclase crystallization to shallower levels where volatile exsolution potentially caused a burst of crystallization that led to late-stage crustal assimilation. Higher frequency/explosivity of post 1971 eruptions are tied to increased volatiles and intrusion volumes, which explain shorter crystal residence times. Increased subduction zone signature may be revealed in post 1971 plagioclase trace element characteristics, but additional analysis is needed to document whether these signatures are characteristic of intruding magma or are acquired as magma evolves in the crustal scale magma system.