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Eruption Risks from Covert Silicic Magma Bodies Shane M

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Eruption Risks from Covert Silicic Magma Bodies Shane M https://doi.org/10.1130/G48697.1 Manuscript received 7 December 2020 Revised manuscript received 18 February 2021 Manuscript accepted 23 February 2021 © 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 15 April 2021 Eruption risks from covert silicic magma bodies Shane M. Rooyakkers1*, John Stix1, Kim Berlo1, Maurizio Petrelli2 and Freysteinn Sigmundsson3 1 Department of Earth & Planetary Sciences, McGill University, Montreal, Quebec H3A 0E8, Canada 2 Dipartimento di Fisica e Geologia, Università degli Studi di Perugia, Perugia 06123, Italy 3 Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, 101 Reykjavík, Iceland ABSTRACT was first encountered during drilling in 2008 in Unintentional encounters with silicic magma at ∼2–2.5 km depth have recently occurred well KJ-39 (Fig. 1; drilled by Landsvirkjun), during drilling at three volcanoes: Kilauea (Hawaii), Menengai (Kenya), and Krafla (Iceland). where quenched glass (Fig. S1 in the Supple- Geophysical surveys had failed to warn about shallow magma before each encounter, and mental Material) was found to comprise up to subsequent surveys at Krafla have been unable to resolve the size or architecture of its silicic 30% of cuttings from the well base (2571 m magma body. This presents a conundrum for volcano monitoring: Do such shallow “covert” depth; Mortensen et al., 2010). Pre-drilling magma bodies pose an eruption risk? Here, we show that Krafla’s most recent explosive seismic and geodetic observations had revealed eruption, a mixed hydrothermal-magmatic event in 1724 C.E. that formed the Víti maar, in- magma interpreted to lie in the 3–7 km depth volved rhyolite essentially indistinguishable in composition from magma encountered during range (Einarsson, 1978; Tryggvason, 1986), but drilling in 2009. Streaks of quenched basalt in some Víti pumices provide direct evidence for these findings were interpreted in the context interaction between co-erupted rhyolitic and basaltic magmas, but crystals in these pumices of the 1975–1984 Krafla Fires basaltic erup- show no evidence for late-stage heating or re-equilibration with more mafic melt, implying tions (e.g., Wright et al., 2012). In 2009, silicic mixing time scales of at most several hours. Covert silicic magma thus presents an eruption magma was again intercepted in well IDDP-1, risk at Krafla and may be mobilized with little warning. Difficulties in resolving magma bod- the first well of the Iceland Deep Drilling Proj- ies smaller than ∼1 km3 with geophysical surveys mean that covert silicic magma may exist ect (Fig. 1). The well was intended to reach the at many other volcanoes and should be considered in hazard and risk assessments. margin of the large basaltic storage zone, in- ferred from MT observations to reside at depths INTRODUCTION may not be mapped with resolution better than of ≥4–4.5 km (Friðleifsson et al., 2014), but Eruptions from mature magmatic systems 1–2 km (e.g., Schuler et al., 2015). Similarly, drilling ceased when magma was encountered have traditionally been thought to tap large bod- sensitivity testing suggests that a 1 km3 magma at 2096 m. Cuttings from ∼30–80 m above the ies of melt-dominant magma in the middle to body could go undetected by magnetotelluric magma consisted of fresh fine-grained mafic upper crust (“magma chambers”; Marsh, 1989). (MT) observations (Lee et al., 2020). Recent (“diabase”) and felsic (“felsite”) intrusive ma- However, it is increasingly recognized that erupt- unintentional encounters with shallow (≤2.5 km terial (Zierenberg et al., 2013). Circulation loss ible magma can be much more distributed than depth) silicic magmas in boreholes at Kilauea, prevented sampling of the 30 m interval above previously thought, and recent petrologic evi- Hawaii (dacite; Teplow et al., 2009); Menengai, the magma, but quenched rhyolitic glass host- dence suggests that some eruptions tap small and Kenya (trachyte; Mbia et al., 2015); and twice ing <3 modal% crystals and occasional intru- discrete magma bodies from a range of depths at Krafla, Iceland (dacite to rhyolite; Mortensen sive fragments were sampled from the magma. rather than a single chamber (e.g., Cashman et al., 2010; Elders et al., 2011) underline the Further details of these drilling encounters and and Giordano, 2014; Sparks et al., 2019). Such problem. In each case, pre-drilling geophysical the geology of Krafla are given in the Supple- distributed magmatic systems present a chal- surveys failed to detect or warn about shallow mental Material. lenge for volcano monitoring because magma silicic magma. bodies up to ∼1 km3 are difficult to detect with To better understand the covert silicic mag- VÍTI–IDDP-1 LINK geophysical surveys. Seismic waves used to im- ma at Krafla, we studied juvenile products from Our petrologic data reveal a link between the age volcanoes typically have wavelengths rang- seven eruptions spanning the system’s known IDDP-1 rhyolite and Krafla’s youngest rhyolitic ing from several hundred meters to kilometers, history of rhyolitic volcanism and compared products, erupted in 1724 C.E. during the Mývatn with inherent difficulties in resolving features them with magma sampled during drilling. Our Fires rifting episode. The eruption formed Víti, a with sub-wavelength dimensions. For example, methods are outlined in the Supplemental Mate- small (∼300-m-diameter) maar ∼500 m northeast seismic tomography will smear boundaries of rial1. Krafla is a predominantly basaltic system, of IDDP-1 (Fig. 1). Approximately 0.45 km3 of real velocity anomalies so that their outlines consisting of an ∼8 × 10 km caldera (65.73°N, basaltic lava effusion followed from fissures 16.78°W) transected by an ∼100-km-long fis- ∼2 km to the west in 1727–1729 (Grönvold, *E-mail: [email protected] sure swarm (Sæmundsson, 1991). Silicic melt 1984). Víti ejecta are mainly altered country 1Supplemental Material. Details of methods, an outline of the geology of Krafla, supplemental details of the drilling encounters with magma, supplemental figures, and a spreadsheet containing all compositional data collected from Víti pumice samples. Please visit https://doi.org/10.1130/GEOL.S.14346863 to access the supplemental material, and contact [email protected] with any questions. CITATION: Rooyakkers, S.M., et al., 2021, Eruption risks from covert silicic magma bodies: Geology, v. 49, p. 921–925, https://doi.org/10.1130/G48697.1 Geological Society of America | GEOLOGY | Volume 49 | Number 8 | www.gsapubs.org 921 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/8/921/5362088/g48697.1.pdf by guest on 25 September 2021 0 16°45'W 60 N 16°45'W Krafla caldera The IDDP-1 and Víti crystal populations also show similarities. Anorthite (An) contents 65°45' 500 for IDDP-1 rhyolite plagioclase cores and rims 800 W (Masotta et al., 2018) overlap with the primary modes for Víti (Fig. 3A), while trace elements La, Ce, and Eu overlap with the lower end of the N Víti range (Fig. 3B). Major-element composi- 65°45' 600 800 Legend 700 tions of Víti augites strongly overlap with those Oldest rhyolite Superficial deposits Mafic lava (historic) from the IDDP-1 rhyolite and felsite (Fig. 3C), 700 Mafic lava (pre-historic) while their trace-element contents closely match Gæsafjallarani Víti crater 700 500 600 Subglacial hyaloclastite 700 Andesite those produced in crystallization experiments Rhyolite IDDP-1 K800 J-36 on the IDDP-1 rhyolite (Masotta et al., 2018; Halarauður ignimbrite 700 600 600 Fig. S2). Similarly, IDDP-1 pigeonite composi- 500 KJ-3500 9 Jörundur Lake 600 tions overlap with the more magnesian grains 700 Hrafntinnuhryggur 600 800 Caldera topographic margin 500 from Víti (Fig. 3C) and have similar rare earth 600 400 Fissure swarm boundary 500 600 element (REE) patterns. Subtle differences and 500 700 Wellhead with well path 500 500 Hlídarfjall V/psV <1.67 (2 km depth) the narrower range of IDDP-1 crystal composi- 500 0 123 S shadows (3 km depth) 16°45'W km tions may point to a real difference between the rhyolites, but they could also be an artifact of Figure 1. Geologic map of the Krafla caldera (Iceland), modified after Sæmundsson et al. the lower number of published analyses from (2012). Inset shows location of the caldera in Iceland’s neovolcanic zone. White areas are gla- ciers. Low V /V zone is after Schuler et al. (2015); S-wave shadows are after Einarsson (1978). IDDP-1 and/or the small magma volume sam- p s pled during drilling. Based on similarities between the rhyolite rock fragments, consistent with a predominantly glasses (Fig. S1). More revealing are the trace and experimental melts of IDDP-1 felsite, Ma- hydrothermal eruption, but occasional juvenile elements (Fig. 2); Víti and IDDP-1 bulk and glass sotta et al. (2018) proposed that the IDDP-1 basaltic scoria and rhyolitic pumice clasts imply compositions almost completely overlap and are rhyolite was produced by high-degree (>70%) a magmatic trigger. IDDP-1 glass major-element distinct from other Krafla rhyolites, with similar felsite remelting, and they conceptualized the compositions resemble glasses in erupted Krafla highly incompatible element ratios implying a crust around IDDP-1 as hosting a plexus of small rhyolites and largely overlap with Víti pumice common source. felsite bodies that may be remobilized by new A C 2 0.25 Legend Víti 1.75 Other erupted rhyolites IDDP-1 rhyolite( M2018) 1.5 0.2 IDDP-1 felsite(M2018) IDDP-1 felsite( Z2013) b 1.25 Th/N . IDDP-1 Rhyolite of Z2013 Av 1 0.15 0.75 Red = IDDP-1 rhyolite all analyses (Zierenberg et al., 2013; Masotta et al., 2018) Concentration/ Blue = IDDP-1 felsite high-degree melt (FLS1 glass of Masotta et al., 2018) 0.5 0.1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Hf Ta 2 3 4 5 Ta (ppm) B D 2 0.7 1.75 1.5 /Th 0.5 Ta 1.25 1 Concentration/IDDP-1 Rhyolite Masotta et al.
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