Contents volume 71, 1996

Abstract

The base-surge products of the Monte Guardia sequence on Lipari contain evidence suggesting that magma mixing accompanied the main eruption. Principal data include at least two essential glass populations with discrete compositions, dominance of crystals that are incompatible with the rhyolitic glass, and contrasting quench and superheating textures in the various types of glass pryoclasts. Four distinct types of non-phenocrystic essential pyroclasts are present: vesicular white glass, non-vesicular white glass, brown devitrified clasts, and black microcrystalline clasts. The first three clast types have a uniform rhyolitic composition that is quite distinct from the alkali trachytic composition of the fourth type. These latter black microcrystaline grains have a wide range of compositions that fall in the trachyte field. Discrete crystal grains also occur as essential pyroclasts. The dominant crystal phases are augite and bytownite, with minor amounts of orthopyroxene, olivine pseudomorphed by iddingsite, magnetite, ilmenite, and sanidine. Biotite and hornblende are trace constituents. The diverse mineral assemblage and composition of the most abundant crystals suggest that their origin is different from that of the rhyolitic clasts. Pyroclast textures indicate that these magmas of different composition have been intimately mixed during eruption. The cryptocrystalline textures of the trachyte pyroclasts is due to their chilling within the rhyolitic magma. Strongly vesiculated rhyolite and glass in contact with augite and bytownite crystals suggests that these crystals were able to locally cause exsolution of volatiles. These relationships suggest a separate source for the mafic crystals, and discrete chambers for the trachytic and rhyolitic glass. The eruption model thus involves the mixing of two magmas during the interaction of the erupting materials with near-surface (sea) water.