Matthews, Stephen John; (1995) Volcanology, petrology and geochemistry of Lascar volcano, northern Chile. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This is the first comprehensive petrographic and geochemical study of Lascar Volcano, an active calc-alkaline stratovolcano located in the Atacama Desert, Northern Chile. The volcano probably initially became active during the last glacial maximum, about 20,000 to 27,000 years ago. It has successively built up three eruptive centres which form a slightly elongated structure trending ENE-WSW. The lavas and pumices produced by this volcano are predominantly 2-pyroxene andesites and dacites. The chemical and petrographic features of the eruption products are explained here in terms of a fractionating magma chamber at shallow depth which receives periodic influxes of basaltic andesite magma. This magma, which has already crystallised in middle to lower crustal magma chambers, mixes with the resident more evolved magmas to create a variety of disequilibrium textures including mafic inclusions and reaction coronae on olivine phenocrysts. These influxes of hotter magma lead to convective overturn of the magma chamber. The "primitive" magma which rises from deep levels is relatively rich in dissolved sulphur and chlorine as well a large amounts of water. A model is proposed in which these volatile phases are degassed from the magma on quenching, depressurisation and oxidation of the mafic endmember to form a separate mixed fluid phase in the magma chamber. This release of volatiles is an important contributor to the violence of the eruptions which frequently occur in such volcanoes. The discovery of anhydrite in a prehistoric dacitic pyroclastic flow, the Soncor Flow, and also in the April 1993 eruption products, indicates that sulphur is an important volatile phase and that the magma chamber is relatively oxidised, stabilising sulphate rather than sulphide in the magmas. This is confirmed by calculations of oxygen fugacity using the compositions of coexisting magnetite-ilmenite pairs. A trend of increasing fO₂ with decreasing temperature has been attributed to buffering by the SO₂-H₂S equilibrium in a coexisting H₂O-rich fluid phase. This model explains the common association of oxidised, anhydrite-bearing magmas with excessively high SO₂ emissions in some volcanoes, notably the 1991 eruption of Mount Pinatubo, Phillipines. The source of this sulphur is believed to be either the subducted oceanic crust below the volcanic front or the overlying mantle wedge. Important changes in the plumbing system are believed to have occurred in the past, producing magmas which appear to have bypassed the magma chamber on their way to the surface. This is based on whole rock geochemical trends and petrographic analysis, which shows a lack of evidence for magma mixing in such eruption products. One of these, the andesitic Chaile Flow, lies stratigraphically between dacitic flows with very similar geochemical and petrographic features indicative of magma mixing and typical of magma chamber derived products. A similar change is believed to have occurred in the present eruption cycle, as the 1986 and 1990 lavas are interpreted as having bypassed the volatile-rich magma chamber which gave rise to the 1993 eruption. This change is thought to be responsible for the switch in eruption style from shallow short-lived vulcanian explosions to a sustained sub-plinian eruptive style producing anhydrite bearing pumices. This petrological monitoring of an active volcano provides a new way of studying the magmatic system and predicting the future eruption style.

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