Highly Sr radiogenic tholeiitic magmas in the latest inter-Plinian acitivity of Santorini volcano, Greece.

Mineralogical, geochemical and melt inclusion analyses have been performed on scoria samples from a small scoria-fall deposit interbedded between the Cape Riva (21 ka) pyroclastic deposits and the Minoan (3.6 ka) Plinian eruptions of Santorini volcano and located near the resort of "Imerovigli" (25 25´ 14´´ / 36 26´ 01´´, WGS84), close to the town of Phira. The scoria samples are basaltic-andesites with normally zoned olivine, reversely zoned pyroxenes and variably zoned plagioclase. Olivine contains two types of melt inclusions. Type I inclusions, hosted in cumulus Fo85-90 olivine, are primitive low-K basalts with MgO 6-7 wt.%, and contain volatile element abundances of about 500 ppm F 1000 ppm Cl and 1300 ppm S. Low abundance of incompatible elements and REE patterns indicate an arc tholeiitic composition. The ?18O in host olivine is low (5.3 ?) and lower than the value for the groundmass (6.5 ?), whereas 87Sr/86Sr of the Type I inclusions is rather high (0.70579) and higher than the whole-rock and groundmass values (0.70465 and 0.70468, respectively). Type II inclusions, hosted in Fo80-83 olivine rims, are calc-alkaline andesites and have low volatile contents. Interstitial glasses (groundmass) are similar in composition to the Type II inclusions. We propose that the tholeiitic, high Sr-radiogenic, Type I primitive magma was generated by high partial melting degrees of a MORB-like mantle wedge metasomatised by subducted sediment melts and minor aqueous fluids. The input amount of sediment melts and the degree of mantle melting decreased with time, providing the parental magmas of the calc-alkaline, less Sr-radiogenic, Type II andesitic melts. The scoria-fall eruption was fed by hybrid magma with multiple generations of olivine, clinopyroxene and plagioclase growth, and occurred a short time after the mixing process. This magma was probably last stored in a small side reservoir, separated from the Cape Riva and maybe from the Minoan reservoirs, and was emplaced from a subaerial vent close to "Imerovigli" by a Strombolian-type ephemeral eruption.