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Authors: D'Oriano, Claudia* 
Landi, Patrizia* 
Pimentel, Adriano* 
Zanon, Vittorio* 
Title: Magmatic processes revealed by anorthoclase textures and trace element modeling: The case of the Lajes Ignimbrite eruption (Terceira Island, Azores)
Issue Date: Sep-2017
Series/Report no.: /347 (2017)
DOI: 10.1016/j.jvolgeores.2017.08.012
Keywords: Anorthoclase
Zoning textures
Trace elements
Cumulate re-melting
Reservoir dynamics
Subject Classification04.08. Volcanology
Abstract: The Lajes Ignimbrite on Terceira Island (Azores) records the last major pyroclastic density current-forming eruption of Pico Alto Volcano that occurred ca. 21 kyrs ago. This comenditic trachyte ignimbrite contains up to 30 vol% of crystals, mostly anorthoclase. Geochemical investigation of the products collected throughout two key outcrops reveals that major element compositions are poorly variable, whereas trace elements show significant variability, pointing to the presence of a zoned magma reservoir. Thermometry and oxygen fugacity estimations yielded pre-eruptive temperatures of 850-900°C and ∆NNO -2.4 to -1.8. Meltalkali-feldspar hygrometer indicates magmatic H2O contents ranging from 5.8 wt% in the upper part of the reservoir to 3.6 wt% at the bottom, indicating that the magma reservoir (confined at ~4 km depth) was mainly water-undersaturated before the eruption, except for the topmost portion. Two types of anorthoclase crystals were identified. Type 1 crystals show reverse to oscillatory zoning with An contents of 0.4-2.1 mol% and Ba of 200-2000 ppm. They formed in the middle/upper portion of the reservoir, where fractional crystallization processes dominated. Type 2 crystals, mainly present in the less evolved products, are characterized by patchy-zoned cores with large dissolution pockets surrounded by thick oscillatory-zoned rims and show a wide compositional range (An of 0.5-4.7 mol% and Ba of 142-4824 ppm). Their zoning patterns, together with whole-rock and glass compositions of the juvenile clasts, are consistent with the involvement of an anorthoclase-bearing cumulate from the bottom of the reservoir that underwent partial melting. Crystal dissolution was likely induced by the presence of a heat source at depth, without any mass transfer to the eruptible magma as suggested by the lack of petrographic and chemical evidences of mixing between the resident comenditic trachyte and a mafic/intermediate magma. The thermal instability generated convective plumes that were responsible for the admittance of crystals from the cumulate level into the intermediate portions of the magma reservoir and possibly acted as trigger of the explosive eruption.
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