CONSTRAINTS ON SOURCE AND TIMING OF ALKALI-ENRICHMENT AT MT. ETNA, SICILY: CLINOPYROXENE GEOBAROMETRY AND IN SITU SR ISOTOPE DATA

Since 1971, Mt. Etna, Europe’s largest and most active volcano, has exhibited increased eruption frequency and explosivity, emphasizing the need for hazard evaluation. In association with this increased activity, researchers have documented increased abundances of elements such as K and Rb, as well as elevated ⁸⁷Sr/⁸⁶Sr in the lavas. The source of this alkali-enrichment has been hotly debated; while some researchers favor changes in the mantle source region due to subduction, evidence for late-stage crustal assimilation is implicated by distinct Sr isotope disequilibrium between whole-rock and groundmass. To further evaluate the mantle vs. crustal debate, clinopyroxene (cpx) from ten samples erupted between 1329 and 2004 was targeted for in situ analysis. Cpx representing a broad range of textures and sizes was selected for electron microprobe analysis, and these data were combined with whole-rock analyses to determine the pressures of formation for each crystal. Cores and rims of the largest crystals, defined by a wide array of pressures, were then selected for microdrilling and Sr separation, followed by Sr isotope analysis via thermal ionization mass spectrometry. Cpx has Mg# from 36 to 43, and many crystals show evidence for reverse zoning. Geobarometry using the method of Putirka et al. (2003) reveals a pressure range extending from the Moho (~27 km) to the upper crust (~6.0-6.6 km). These data support a complex and dynamic polybaric plumbing system, in which magma interacts and differentiates at multiple levels within the crust. Selected preliminary in situ Sr isotope data range from 0.70334-0.70372 (±0.000001), are broadly similar to whole-rock, and are more radiogenic than plagioclase in situ ⁸⁷Sr/⁸⁶Sr. Cores and rims of post-1971 cpx are more radiogenic than those of pre-1971 cpx, and post-1971 cpx rims that crystallized at ~12-15 km have higher Mg# and ⁸⁷Sr/⁸⁶Sr than cores that crystallized at similar depths. Such core to rim zoning likely represents an increasingly important contribution of a subduction zone signature. Additional in situ cpx Sr isotope data and evaluation of the Sr isotope disequilibrium among cpx, plagioclase, whole-rock and groundmass will help resolve the role that crustal contamination plays in the changing character of Etna magmas since 1971 (see Bohrson et al., this meeting).