Evolution of the Mount Etna magma: Constraints on the present feeding system and eruptive mechanism

Abstract

Volcanism in the Mount Etna area began some 500,000 years ago with sparse effusions of subaphyric olivine tholeiites showing primary characteristics (0.3–0.4% K 2O, 12–10% MgO, 500 to 400 ppm Cr and 200–350 ppm Ni). At 300 ky BP, pigeonite tholeiites were emitted, soon followed by increasingly porphyric transitional tholeiites (0.4–0.7% K 2O), slightly evolved porphyritic alkali basalts (0.6–1.2% K 2O), and trachybasalts (1.3–2.2% K 2O) close to hawaiites, though rich in calcic plagioclase phenocrysts. All these ancient lavas, either tholeiitic or alkaline, cover the same range of 87Sr 86Sr ratios (0.7030–0.7032). Since 200 ky BP, porphyritic trachybasalts have composed most of the various parts 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 trachybasalts and rarely aphyric basalts, some of which are strongly enriched in K, Rb, Cs, and have higher 87Sr 86Sr (0.7033 to 0.7037). The gradual shift in chemical and mineralogical composition from tholeiites to alkaline types is consistent either with a change in the melting degree of an initially homogeneous mantle source, or more likely with melting of upper mantle levels metasomatized by previous infiltrations of K-rich, small-degree melts from the same source. The primary magma eventually evolved to alkali olivine basalt from which the porphyritic alkali basalts and trachybasalts are shown to be derived by high-pressure (8–10 kbar) fractional crystallization, involving clinopyroxene and olivine as dominant liquidus phases. The younger trachyandesites and trachytes are products of low-pressure fractionation of minerals, mainly plagioclase, present as phenocrysts in porphyric types. Sudden increases in K, Rb, Cs, and 87Sr 86Sr ratios, like those in the post-1971 period, may be explained by selective assimilation, through a fluid phase, of particular crustal levels beneath the volcanic pile. It is suggested that upwelling of the asthenosphere first caused extensive melting of a mantle diapir, allowing tholeiitic magma to accumulate near the mantle-crust interface. Then, increasingly alkaline basalt was generated and fed the entire volcanism of Mt. Etna by undergoing continuous but limited differentiation (trachybasalts) in a subcrustal reservoir, possibly at the top of the mantle diapir. Superimposed on this basic mechanism, more pronounced differentiation (trachyandesites and trachytes) 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 importance in controlling volcanic activity: Variations of magmatic pressure within 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.