Abstract
Ignimbrites within calderas host intrusions with hazardous and/or economically significant hydrothermal systems. The Hvítserkur ignimbrite at Breiðuvík caldera, north-eastern Iceland, is intruded by basaltic dykes. Our data show that the ignimbrite immediately adjacent to the dyke is hard, dark-coloured, recrystallised quartz, plagioclase, and alkali feldspar with a low permeability and porosity and frequent macrofractures. At 1-2 m from the dyke, the ignimbrite is hard, dominantly glassy with pervasive perlitic microfractures, has high permeability, but low porosity and frequent macrofractures. A narrow zone of pervasive unlithified clay exists 2 m from the dyke. Beyond this, the ignimbrite is soft and zeolite-rich, has low permeability, high porosity and fewer macrofractures. The dyke intrusion promoted a narrow zone of welding, fracturing and perlitisation in the ignimbrite resulting in high permeability and focussed alteration. Our study shows how intrusions and their thermal aureoles create vertical pathways for, and horizontal barriers to, geothermal fluid flow.
Highlights
IntroductionDykes and hydrothermal systems within porous volcanic rocks represent a hazard related to hydrothermal explosions [e.g. Hedenquist and Henley 1985; Bixley and Browne 1988; Barberi et al 1992; Bromley and Mongillo 1994; Browne and Lawless 2001; Montanaro et al 2020; 2021] and a geothermal resource [e.g. Bibby et al 1995; Bertrand et al 2012; Rowland and Simmons 2012; McNamara et al 2016; Heap et al 2020a; Spittler et al 2020]
It is observed in aquifers, petroleum reservoirs, geothermal fields [e.g. Quinao et al 2013; Rateau et al 2013; McNamara et al 2016; Senger et al 2017], and hydrothermal systems at volcanoes [e.g. Hurwitz 2003; Delcamp et al 2016; Ball et al 2018; Finn et al 2018]; and can be driven by mineral precipitation associated with chemical [Farquharson et al 2019], barometric, or thermal gradients or structural discontinuities such as faults and shear zones [e.g. Watson et al 2007; Árnason 2020]
Figure : [A] Fracture spacing data recorded at scanline intersection binned every cm to illustrate fracture density as a function of distance from the dyke margin. [B] Stereographic lower hemisphere, equal area projection of fracture orientations planes in the different rock types including comparison to the orientation of the dyke investigated in this study and other dykes in Hvítserkur measured in the field
Summary
Dykes and hydrothermal systems within porous volcanic rocks represent a hazard related to hydrothermal explosions [e.g. Hedenquist and Henley 1985; Bixley and Browne 1988; Barberi et al 1992; Bromley and Mongillo 1994; Browne and Lawless 2001; Montanaro et al 2020; 2021] and a geothermal resource [e.g. Bibby et al 1995; Bertrand et al 2012; Rowland and Simmons 2012; McNamara et al 2016; Heap et al 2020a; Spittler et al 2020]. Bibby et al 1995; Bertrand et al 2012; Rowland and Simmons 2012; McNamara et al 2016; Heap et al 2020a; Spittler et al 2020] In these systems, the fluid composition [Garden et al 2020], matrix permeability [Saar and Manga 1999; Heap et al 2017a; Kushnir et al 2018; Mordensky et al 2018b; Cavazos-Álvarez et al 2020], and fracture pathways [Rissmann et al 2011; Heap and Kennedy 2016; Garden et al 2017; Mordensky et al 2018a; Kennedy et al 2020] within the volcanic rock determine the type of hazard, as well as the resource potential. The composition, temperature, width, and dynamic history of the dyke determines its impact on the properties of the surrounding rocks [e.g. Kennedy et al 2010; Schauroth et al 2016; Annen 2017]
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