Parental Melts and Magma Storage of a Large-volume Dacite Eruption at Vetrovoy Isthmus (Iturup Island, Southern Kuril Islands): Insights into the Genesis of Subduction-zone Dacites

Abstract

Detailed mineralogical and melt and fluid inclusion constraints on magma storage, and the evolution of melts, are presented for the large-volume caldera eruption on the Vetrovoy Isthmus on Itutrup Island (Kuril Islands, Russia). The shallow magma reservoir beneath the Vetrovoy Isthmus is composed of a mush of plagio-rhyolitic melt, phenocrysts and the products of peritectic reaction(s). The melt appears to have formed as a result of partial melting of previously erupted rocks, which probably had andesitic to basaltic compositions and were metamorphosed into amphibole-bearing assemblages. The breakdown of amphibole in the partially melted precursor rocks led to the formation of early Mg-rich clino- and orthopyroxene, along with plagioclase and Fe–Ti oxides, and the release of aqueous fluids. Variations in fluid pressure are recorded by a strong increase of An contents in plagioclase. Crystallization took place at around 850°C with pressure ranging from 0·9 to 3 kbar. This study demonstrates that dacitic magmas erupted during the course of a 20 kyr voluminous eruption were the result of mixing between plagio-rhyolitic partial melts and the breakdown reaction minerals (i.e. pyroxenes, plagioclase and Fe–Ti oxides). Plagioclase and quartz were the last minerals to crystallize from these melts prior to eruption.

Quantification of Water Content and Speciation in Synthetic Rhyolitic Glasses: Optimising the Analytical Method of Confocal Raman Spectrometry

“…Subsequently, studies have been carried out to perfect this technology and to apply it to experimental and natural samples of only a few micrometres in size (e.g., Thomas 2000, Thomas et al 2002, 2008a, Chabiron et al 2004, Zajacz et al 2005, Behrens et al 2006, Di Muro et al 2006a, 2006b, Severs et al 2007, Thomas 2008b, Mercier et al 2009, 2010, Le Losq et al 2012, Di Genova et al 2017, Schiavi et al 2018, Helo et al 2020, Gonz ález-Garc´ıa et al 2020, Bonechi et al 2022. Since the establishment of the method it has been successfully applied to quantitatively measure melts and melt inclusions in host minerals (e.g., Holtz et al 2005, Müller et al 2006, Cesare et al 2009, Bachmann et al 2010, Bartoli et al 2013, Ferrero et al 2015, Kotov et al 2017, Smirnov et al 2019, Angelopoulos et al 2020, Bonechi et al 2022 Water quantification in silicate glasses by Raman spectroscopy is based on the measurement of the absolute or normalised peak height of the water Raman bands following maturing external and internal calibration. Both calibration methods require a set of silicate glass reference materials spanning a wide range of compositions to calibrate the total water content dissolved in silicate glasses.…”