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 forme r (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 basa lts of 1989 displayed initial enrichments of Rb and K accompanied by a n higher value of radiogenic-Sr and isotopic disequilibrium between ho st-rock and clinopyroxene (0.70364 and 0.70356, respectively). Subsequ ently, within one week, K, Rb and Sr-87/Sr-86 ratio decreased to const ant values (Sr-87/Sr-86 = 0.70355). Nd isotopic ratio did not show sig nificant variations. These features can be explained by a selective co ntamination process involving essentially Rb, K and radiogenic-Sr. The bulk from the 1991-1993 eruption hawaiites, were erupted between Janu ary and May 1992. In this period, lavas (here called JML) were charact erised by uniform geochemical and isotopic compositions and Sr-isotopi c equilibrium between whole rock and pyroxene (0.70355). In other peri ods (December 1991 and June 1992) we observed lower and variable Sr is otopic compositions coupled with disequilibria between lavas and their clinopyroxenes (0.70351 in the w.r. and 0.70345 in the cpx of the sam ple of 15 December 1991). Starting from June 1992, the products seem t o reveal fractionation effects as suggested by the continuous rise in concentrations of incompatible elements. When the observed variability cannot be ascribed to fractionation effects, we are not able to discr iminate mixing from selective contamination. However, lavas erupted du ring December 1991 evolved towards the isotope composition of JML that was never exceeded, moreover the regular behaviour of Rb, K and radio genic-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 ha ving the same, or lower, isotopic composition of the clinopyroxene fou nd in the sample of December 1991. Several petrogenetic processes are contemporaneously active at Mt. Etna; they encompass magma mixing, sel ective contamination and fractional crystallization. Their relative im portance varies according to the original volume of each magma batch a nd to the status of the shallow feeding system. The complexity of thes e processes, acting in the short time span of a few months, claims cau tion about the geochemical representativity of a single lava sample fo r each eruption.