The Role of Tuffs in Sealing Volcanic Conduits

Abstract

Hydrothermal alteration by2 SO E -rich fluids governs the physicochemical properties of stratovolcanoes worldwide (Mayer et al., 2016;Zimbelman et al., 2005). As these fluids alter the conduit rocks, inevitable changes in rock porosity and permeability can limit outgassing and promote explosive volcanic behavior. This was observed at Poás volcano (Costa Rica), where the formation of a pressurized hydrothermal seal by secondary mineral precipitation and volatile accumulation limited 2 SO E -emissions for two years and triggered the 2017 phreatomagmatic eruption (de Moor et al., 2019). Similar hydrothermal sealing driven by acid-sulphate alteration also played a role in triggering phreatic and phreatomagmatic eruptions at Soufrière Hills (Montserrat) (Edmonds et al., 2003), and Ontake (Japan) (Stix & de Moor, 2018). At Whakaari-White Island volcano (New Zealand), a partially sealed hydrothermal system is implied to have played a role in the recent 2019, and past eruptions (Burton et al., 2021;Christenson et al., 2017). However, mineralogical and microimaging evidence of such sealing and its evolution for different lithologies within the conduit is lacking. Moreover, the effect of such hydrothermal processes on the fluid flow and elastic properties of conduit-filling rocks remains unconstrained, despite being of paramount importance for accurately inverting geophysical data to understand volcano pressurization.The development of an effective seal requires conduit rocks with low porosity and low permeability that limit outgassing and favor fluid accumulation and pressure build-up (Stix & de Moor, 2018). So far, experimental studies on the effects of alteration on porosity and permeability of volcanic rocks have been mainly based on surface-collected rocks and show conflicting observations. For example, pore and fracture filling secondary minerals in lavas (

High Remanent Magnetization Measured in Hydrothermally Altered Lavas

“…Based on the secondary mineral assemblage, it is inferred that the samples were altered by hot acidic (pH $$< $$ 2–6) fluids at temperatures less than 400°C (Hedenquist et al., 1993; Pirajno, 2009; Zimbelman et al., 2005). The samples were classified based on the proportion of secondary phases as fresh to slightly altered (0%–5%), moderately altered (5%–40%), and highly altered (>40%) (Kanakiya et al., 2021). A list of reference IDs for the measured samples and sample photos are provided in Table and Figure , respectively.…”

“…Sample collection, as well as characterization of hydrothermal alteration mineralogy and intensity using X-ray diffraction, petrography, backscattered-electron imaging (BSE), and energy dispersive spectroscopy (EDS), is reported by Kanakiya et al (2021). Overall, these samples underwent acid-sulfate alteration in which their primary minerals (plagioclase, pyroxene, volcanic glass, and Fe-Ti oxides) were consumed to different extents and replaced by a secondary mineral assemblage of alunite ± cristobalite ± tridymite ± anhydrite ± clays ± pyrite ± amorphous phases (silica/aluminosilicates) (Kanakiya et al, 2021). Based on the secondary mineral assemblage, it is inferred that the samples were altered by hot acidic (pH 𝐴𝐴 𝐴 2-6) fluids at temperatures less than 400°C (Hedenquist et al, 1993;Pirajno, 2009;Zimbelman et al, 2005).…”

“…The sample set consists of 42 variably altered rocks encompassing conduit-filling lithologies: lavas, tuffs, and breccias ejected from the subsurface as ballistics during the 2016 eruption, as well as vesicular lavas and sulfur flows collected from in-situ surface outcrops. Sample collection, as well as characterization of hydrothermal alteration mineralogy and intensity using X-ray diffraction, petrography, backscattered-electron imaging (BSE), and energy dispersive spectroscopy (EDS), is reported by Kanakiya et al (2021). Overall, these samples underwent acid-sulfate alteration in which their primary minerals (plagioclase, pyroxene, volcanic glass, and Fe-Ti oxides) were consumed to different extents and replaced by a secondary mineral assemblage of alunite ± cristobalite ± tridymite ± anhydrite ± clays ± pyrite ± amorphous phases (silica/aluminosilicates) (Kanakiya et al, 2021).…”

“…The composition of magnetic minerals was identified by analyzing elemental X-ray maps, BSE, and EDS data. The data were acquired using the JEOL Field Emission Electron Probe Microanalyser System 8530F at the University of Auckland on eight thin sections of variably altered lavas, tuffs, and breccias from Kanakiya et al (2021). X-ray maps of elements Si, Al, Fe, S, Ti, Na, Mg, P, O, K, Ca, and Mn, were acquired for a representative 1.5 × 1.5 mm area of the thin section.…”

Data report: in situ elastic properties of hydrothermally altered volcanic rocks, IODP Expedition 376, Brothers volcano, Kermadec arc

“…Brothers volcano is an active submarine caldera and one of 34 large volcanic complexes of the Kermadec arc (de Ronde et al, 2001(de Ronde et al, , 2003. Volcanic rocks span a wide range in physical and hydraulic properties, the result of their varied lithologies and associated microstructures, vesicularity, fracturing (primary and subsequent faulting), alteration mineral assemblages, alteration intensity, and compaction as a consequence of sequential deposition of volcanic products (McPhie and Allen, 1992;Bartetzko et al, 2005;Wyering et al, 2014;Heap et al, 2017;Clarke et al, 2020;Durán et al, 2019;Kanakiya et al, 2021).…”