DOCUMENTING MAGMA EVOLUTION OF THE FOSSA DELLE FELCI (SALINA ISLAND, SOUTHERN TYRRHENIAN SEA) VOLCANIC SEQUENCE THROUGH INTEGRATION OF QUANTITATIVE MODELING AND IN SITU CHEMICAL ANALYSIS

Integrating in situ chemical and textural analyses of plagioclase with MELTS (Ghiorso & Sack 1995) and Spera & Bohrson’s (2001) Energy-Constrained (EC) formulation provides quantitative information about magma chamber processes such as recharge (R), assimilation (A) and fractional crystallization (FC). Plagioclase growth records melt characteristics adjacent to the crystal, while MELTS and EC results constrain pressure/temperature paths, mineral assemblages and compositions, including plagioclase An content, and the efficacy of RAFC. Previous studies on Salina Island (SI), South Tyrrhenian Sea, Italy, have thoroughly documented a basalt to dacite series with volcanism occurring from ~500-13 ka (Gertisser & Keller 2000). Central to this study is the Fossa delle Felci (FdF) basaltic andesite to dacite sequence. Stratigraphic relations within the FdF indicate an inverted eruption sequence from a compositionally stratified chamber. Whole-rock geochemical and petrologic studies indicate FdF magmas differentiated primarily through FC with assimilation having a minor role. Samples from each FdF unit were collected, and six representative samples were chosen for in situ chemical and textural analyses of plagioclase. The most SiO₂ poor sample shows the least complex plagioclase textures, as documented by Nomarski Differential Interference Contrast imaging. Textural complexity increases with increasing (whole-rock) SiO₂. Similarly, no overlap occurs in core and rims An values in the most SiO₂ poor sample, with varying degrees of core to rim An overlap as SiO₂ increases. The overall An (mol%) range is An₉₃ to An₃₉. Best-fit MELTS models suggest magmas underwent polybaric FC from 3-4 kbar to the surface, had an fO₂ between QFM+2.12 – QFM-2.96 and 1.25 – 2.5 wt% initial H₂O; the predicted An range, An₈₁-An₅₅, is narrower than the observed range. Plagioclase textural and compositional data coupled with MELTS results suggest magma mixing due to a potentially catalytic recharge event. The importance of recharge and assimilation will be further evaluated with in situ trace element and isotopic analyses and EC-RAFC modeling. The ability to quantitatively constrain RAFC processes will allow better insight into eruptive triggers and the possible eruptive products and thus assist in hazard prediction and mitigation.