Pressure Controlled Permeability in a Conduit Filled with Fractured Hydrothermal Breccia Reconstructed from Ballistics from Whakaari (White Island), New Zealand

Abstract

Breccia-filled eruption conduits are dynamic systems where pressures frequently exceed critical thresholds, generating earthquakes and transmitting fluids. To assess the dynamics of breccia-filled conduits, we examine lava, ash tuff, and hydrothermal breccia ballistics with varying alteration, veining, fractures, and brecciation ejected during the 27 April 2016 phreatic eruption of Whakaari/White Island. We measure connected porosity, strength, and permeability with and without tensile fractures at a range of confining pressures. Many samples are progressively altered with anhydrite, alunite, and silica polymorphs. The measurements show a large range of connected porosity, permeability, and strength. In contrast, the cracked samples show a consistently high permeability. The cracked altered samples have a permeability more sensitive to confining pressure than the unaltered samples. The permeability of our altered ballistics is lower than surface rocks of equivalent porosity, illustrating that mineral precipitation locally blocked pores and cracks. We surmise that alteration within the conduit breccia allows cracks to form, open and close, in response to pore pressure and confining pressure, providing a mechanism for frequent and variable fluid advection pulses to the surface. This produces temporally and spatially variable geophysical and geochemical observations and has implications for volcano monitoring for any volcano system with significant hydrothermal activity.

Hydrothermal eruption dynamics reflecting vertical variations in host rock geology and geothermal alteration, Champagne Pool, Wai-o-tapu, New Zealand

“…Similar brecciated and recemented clasts are found in other hydrothermal breccia deposits (e.g. Nairn and Wiradiradja 1980;Hedenquist and Henley 1985;Kennedy et al 2020;Montanaro et al 2020) and drill cores from geothermal fields (White et al 1975;Krupp and Seward 1987;Dobson et al 2003). Their textures were attributed to hydraulic fracturing (Phillips 1979;Bromley and Mongillo 1994), produced by over-pressurised fluids along fault/fractures that break rocks and precipitate silica along fractures.…”

“…Continuous flashing of fluids likely fuelled shallow explosions and subsequently enlarged the crater. A similar mechanism is described for the formation of craters at Lake Okaro and Whakaari/White Island (Kennedy et al 2020;Montanaro et al 2020). Slumping of shallow unconsolidated deposits into the central vent area likely intermittently blocked it, inducing further deep explosions and sporadic debris jets transporting deeper clasts into the shallow ejection zone (cf., Sweeney and Valentine 2015).…”

“…Host rock properties of geothermal systems strongly affect cratering processes, eruption style, and related hazards during steam-driven hydrothermal eruptions (Mastin 1995;Browne and Lawless 2001;Haug et al 2013;Montanaro et al , 2020Heap et al 2020;Kennedy et al 2020). Analysis of the Champagne Pool breccia and drill core materials, along with decompression experiments, allowed us to fingerprint the eruption source depth and fluid state during all phases of the eruption.…”

“…In particular, the large abundance of OI material with low impact angle (Fig. 4a-f), and the large deposit thickness to the north, point to multiple shallow explosions at later stages of the eruption Table 2 Comparison of petrophysical properties, and median-fine grain size and componentry of naturally/experimentally fragmented material from drill cores vs. breccia blocks and matrix Drill cores that migrated/excavated northward Kennedy et al 2020;Montanaro et al 2020). Geothermal features such as hot-spring conduits, hydrothermal eruption vents, and collapsed craters, are most commonly found close to faults (Curewitz and Karson 1997;Rowland and Simmons 2012;D'Elia et al 2020).…”

“…The most deadly eruptions in recent times have been associated with vent-hosted hydrothermal systems on volcanoes, e.g. 2014 Ontake eruption, Japan (Yamamoto 2014) and 2019 Whakaari/White Island, New Zealand (Kennedy et al 2020). Sites of intermittent volcanism in humid environments are highly prone to steam-driven eruptions (Kilgour et al 2019;Jolly et al 2020).…”