Variations in lava composition during the March 1981 eruption of mount etna and the implications of a compositional comparison with earlier historic eruptions

The March 1981 eruption of Mount Etna occurred on the northern slopes of the volcano in an area previously inactive for more than 400 a. A radial fissure system extending 7.5 km and producing 20±2×10⁶ m³ of basic hawaiite lava, opened at 2600 m a.s.l. on 17th March and migrated downslope to 1125 m a.s.l. where activity ceased on 23rd March. Gravity evidence points to the draining of a radial storage-dyke, located between sea-level and +1 km a.s.l. Whole-rock chemistry and petrography suggests that the magma in the dyke was a hybrid supporting crystal and compositional gradients developed as a result of progressively greater mixing of residual 1979 basic hawaiite magma with fresh basic hawaiite magma during permissive filling of the dyke around September 1980. These mixing gradients caused the eruption to display a significant contrast in the concentrations of Al₂O₃, Sr, CaO, MgO and Fe₂O₃ (total) between early, high-altitude and later, low-altitude products. The fresh magma which entered the high-level system around September 1980 ascended rapidly from a deep storage region at 16–24 km depth. On a FMA plot, the composition range of the 1981 lava lies at the end of a temporal variation trend defined by the composition of successive flank eruptions back to 1971 and appears to be repeated by successive eruptions between 1923 and 1949. This suggest that the chemistry of historic lavas may, in a general way, reflect the evolution path of magma in the deep storage region despite masking of detail by high-level processes such as mixing. These temporal trends may, if real, reflect successive batches of freshly generated magma from the mantle source region entering into and subsequently differentiating within the deep storage region. Paper presented to the Symp. on Formation and Evolution of Magma Chambers: Physical and Chemical Processes, 18th IUGGIAVCEI Gen. Assembly, Hamburg, F.R.G., August 1983.