Geochemical and isotopic monitoring of Mt. Etna 1989–1993 eruptive activity: bearing on the shallow feeding system

Two flank eruptions took place at Mt. Etna during 1989–1993. The former (September–October 1989) produced lavas among the most primitive of this century while the latter (December 1991–March 1993) was unusually long lasting and the largest of the last three centuries. Alkali basalts of 1989 displayed initial enrichments of Rb and K accompanied by an higher value of radiogenic-Sr and isotopic disequilibrium between host-rock and clinopyroxene (0.70364 and 0.70356, respectively). Subsequently, within one week, K, Rb and 87Sr86Sr ratio decreased to constant values (87Sr86Sr = 0.70355). Nd isotopic ratio did not show significant variations. These features can be explained by a selective contamination process involving essentially Rb, K and radiogenic-Sr. lk from the 1991–1993 eruption hawaiites, were erupted between January and May 1992. In this period, lavas (here called JML) were characterised by uniform geochemical and isotopic compositions and Sr-isotopic equilibrium between whole rock and pyroxene (0.70355). In other periods (December 1991 and June 1992) we observed lower and variable Sr isotopic compositions coupled with disequilibria between lavas and their clinopyroxenes (0.70351 in the w.r. and 0.70345 in the cpx of the sample of 15 December 1991). Starting from June 1992, the products seem to reveal fractionation effects as suggested by the continuous rise in concentrations of incompatible elements. he observed variability cannot be ascribed to fractionation effects, we are not able to discriminate mixing from selective contamination. However, lavas erupted during December 1991 evolved towards the isotope composition of JML that was never exceeded, moreover the regular behaviour of Rb, K and radiogenic-Sr of 1989 lavas was not observed during the 1991–1993 eruption. Thus, we prefer to interpret these variations as mainly due to mixing processes between an end-member of JML composition and another one having the same, or lower, isotopic composition of the clinopyroxene found in the sample of December 1991. Several petrogenetic processes are contemporaneously active at Mt. Etna; they encompass magma mixing, selective contamination and fractional crystallization. Their relative importance varies according to the original volume of each magma batch and to the status of the shallow feeding system. The complexity of these processes, acting in the short time span of a few months, claims caution about the geochemical representativity of a single lava sample for each eruption.