EVOLUTION OF THE MOUNT ETNA MAGMA - CONSTRAINTS ON THE PRESENT FEEDING SYSTEM AND ERUPTIVE MECHANISM

Volcanism in the Mount Etna area began some 500,000 years ago with spa rse effusions of subaphyric olivine tholeiites showing primary charact eristics (0.3-0.4% K2O, 12-10% MgO, 500 to 400 ppm Cr and 350-200 ppm Ni). At 300 ky BP, pigeonite tholeiites were emitted, soon followed by increasingly porphyric transitional tholeiites (0.4-0.7% K2O), slight ly evolved porphyritic alkali basalts (0.6-1.2% K2O), and trachybasalt s (1.3-2.2% K2O) close to hawaiites, though rich in calcic plagioclase phenocrysts. All these ancient lavas, either tholeiitic or alkaline, cover the same range of Sr-87/Sr-86 ratios (0.7030-0.7032). Since 200 ky BP, porphyritic trachybasalts have composed most of the various par ts of Mt. Etna proper. They were accompanied from time to time by more differentiated products (porphyritic or aphanitic trachyandesites and trachytes) whose eruptions eventually culminated in caldera collapse. For the last 14 ky, Etna has continued to erupt porphyritic trachybas alts and rarely aphyric basalts, some of which are strongly enriched i n K, Rb, Ca, and have higher Sr-87/Sr-86 (0.7033 to 0.7037). The gradu al shift in chemical and mineralogical composition from tholeiites to alkaline types is consistent either with a change in the melting degre e of an initially homogeneous mantle source, or more likely with melti ng of upper mantle levels metasomatized by previous infiltrations of K -rich, small-degree melts from the same source. The primary magma even tually evolved to alkali olivine basalt from which the porphyritic alk ali basalts and trachybasalts are shown to be derived by high-pressure (8-10 kbar) fractional crystallization, involving clinopyroxene and o livine as dominant Liquidus phases. The younger trachyandesites and tr achytes are products of low-pressure fractionation of minerals, mainly plagioclase, present as phenocrysts in porphyric types. Sudden increa ses in K, Rb, Cs, and Sr-87/Sr-86 ratios, like those in the post-1971 period, may be explained by selective assimilation, through a fluid ph ase, of particular crustal levels beneath the volcanic pile. It is sug gested that upwelling of the asthenosphere first caused extensive melt ing of a mantle diapir, allowing tholeiitic magma to accumulate near t he mantle-crust interface. Then, increasingly alkaline basalt was gene rated and fed the entire volcanism of Mt. Etna by undergoing continuou s but Limited differentiation (trachybasalts) in a subcrustal reservoi r, possibly at the top of the mantle diapir. Superimposed on this basi c mechanism, more pronounced differentiation (trachyandesites and trac hytes) occurred in temporary, superficial crustal chambers, for which there is geophysical and morphological evidence (calderas). At present , the 20-30 km deep subcrustal reservoir appears of critical importanc e in controlling volcanic activity: Variations of magmatic pressure wi thin it (input/output of magma) should trigger opening of fractures in the crust, exchange with phreatic fluids and selective assimilation, and finally fissure eruptions. A 'volcano-tectonic' model is presented that accounts for the various eruptive styles.