Textural and geochemical window into the IDDP-1 rhyolitic melt, Krafla, Iceland, and its reaction to drilling

Abstract

The unexpected intersection of rhyolitic magma and retrieval of quenched glass particles at the Iceland Deep Drilling Project-1 geothermal well in 2009 at Krafla, Iceland, provide unprecedented opportunities to characterize the genesis, storage, and behavior of subsurface silicic magma. In this study, we analyzed the complete time series of glass particles retrieved after magma was intersected, in terms of distribution, chemistry, and vesicle textures. Detailed analysis of the particles revealed them to represent bimodal rhyolitic magma compositions and textures. Early-retrieved clear vesicular glass has higher SiO2, crystal, and vesicle contents than later-retrieved dense brown glass. The vesicle size and distribution of the brown glass also reveal several vesicle populations. The glass particles vary in δD from −120‰ to −80‰ and have dissolved water contents spanning 1.3−2 wt%, although the majority of glass particles exhibit a narrower range. Vesicular textures indicate that volatile overpressure release predominantly occurred prior to late-stage magma ascent, and we infer that vesiculation occurred in response to drilling-induced decompression. The textures and chemistry of the rhyolitic glasses are consistent with variable partial melting of host felsite. The drilling recovery sequence indicates that the clear magma (lower degree partial melt) overlays the brown magma (higher degree partial melt). The isotopes and water species support high temperature hydration of these partial melts by a mixed meteoric and magmatic composition fluid. The textural evidence for partial melting and lack of crystallization imply that magma production is ongoing, and the growing magma body thus has a high potential for geothermal energy extraction. In summary, transfer of heat and fluids into felsite triggered variable degrees of felsite partial melting and produced a hydrated rhyolite magma with chemical and textural heterogeneities that were then enhanced by drilling perturbations. Such partial melting could occur extensively in the crust above magma chambers, where complex intrusive systems can form and supply the heat and fluids required to re-melt the host rock. Our findings emphasize the need for higher resolution geophysical monitoring of restless calderas both for hazard assessment and geothermal prospecting. We also provide insight into how shallow silicic magma reacts to drilling, which could be key to future exploration of the use of magma bodies in geothermal energy.

Eruption risks from covert silicic magma bodies

“…Based on similarities between the rhyolite and experimental melts of IDDP-1 felsite, Masotta et al (2018) proposed that the IDDP-1 rhyolite was produced by high-degree (>70%) felsite remelting, and they conceptualized the crust around IDDP-1 as hosting a plexus of small felsite bodies that may be remobilized by new intrusions. Textural evidence for partial melting and felsite assimilation at the roof of the IDDP-1 magma body is consistent with such a scenario (Zierenberg et al, 2013;Saubin et al, 2021). Features of the Víti crystals also suggest some degree of felsite recycling; macrocrysts commonly host internal resorption surfaces with only slight compositional shifts across them (e.g., Figs.…”

Geothermal potential of small sub-volcanic intrusions in a typical Icelandic caldera setting

“…The active geothermal fields at Krafla are primarily hosted within basaltic lavas and hyaloclastites, some rhyolitic ignimbrites and, at depth, gabbroic and doleritic intrusions [Ármnannsson et al 1987;Eggertsson et al 2020]. Of critical relevance to this project is that, despite the recent basaltic volcanism, there is now ample evidence for shallow intrusions of rhyolite that were still molten in 2009 [Elders et al 2011], and derive from partial melting of older basaltic formations [Jónasson 1994;Árnason 2020;Saubin et al 2021].…”

“…Our conclusions support suggestions that small intrusions may already be fuelling successful geothermal extraction sites [cf. Saubin et al 2021].…”

“…Still, this timescale remains highly relevant to Krafla (where the last magmatic intrusions spanned 1975-1984 with still active magma bodies in 2009) and other frequently-active volcanic-geothermal systems. In fact, the high temperatures encountered by boreholes in Krafla indicate a potential proximity to recent sheet intrusions [Saubin et al 2021].…”

Rapid Assembly and Eruption of a Shallow Silicic Magma Reservoir, Reyðarártindur Pluton, Southeast Iceland

“…In Iceland, silicic rocks are generated by both (a) fractional crystallization, and (b) the partial melting of hydrothermally altered basaltic crust and older silicic volcanic rocks by the intrusion of hot basaltic magma (Banik et al., 2018; Jónasson, 2007; Padilla et al., 2016; Schattel et al., 2014; Sigurdsson & Sparks, 1981). Subsequently, the silicic magma may be transported via dikes and stored at shallow depths prior to eruption (e.g., Eyjafjallajökull: Keiding & Sigmarsson, 2012; and Krafla: Saubin et al., 2021). Such shallow‐level silicic magma reservoirs may be several kilometers in diameter, such as evident from Krafla (Árnadóttir et al., 1998; Friðleifsson et al., 2014; Gasperikova et al., 2015; Kennedy et al., 2018) and exposed plutons in the eroded parts of Iceland (Blake, 1966; Burchardt et al., 2012; Furman et al., 1992; Walker, 1966).…”