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Bull Volcanol (2014) 76:808 DOI 10.1007/s00445-014-0808-8

RESEARCH ARTICLE

Pre-eruptive storage conditions and eruption dynamics of a small dome: Douglas Knob, Yellowstone , USA

Kenneth S. Befus & Robert W.Zinke & Jacob S. Jordan & Michael Manga & James E. Gardner

Received: 14 May 2013 /Accepted: 5 February 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract The properties and processes that control the size, ranging from 0.03 to 0.11 MPa h−1 (∼0.4–1.3 mm s−1 ascent duration, and style of eruption of rhyolite are poorly rates). Such slow ascent would allow time for passive constrained because of a paucity of direct observations. Here, degassing at depth in the conduit, thus resulting in an effusive we investigate the small-volume, nonexplosive end-member. eruption. Using calculated melt viscosity, we infer that the In particular, we determine the pre-eruptive storage conditions that fed the eruption was 4–8 m in width. Magma flux and eruption dynamics of Douglas Knob, a 0.011-km3 obsid- through this dike, assuming fissure dimensions at the surface ian dome that erupted from a 500-m-long fissure in the represent its geometry at depth, implies an eruption duration Yellowstone volcanic system. To determine pre-eruptive stor- of 17–210 days. That duration is also consistent with the shape age conditions, we analyzed compositions of , of the dome if produced by gravitational spreading, as well as matrix glass, and -hosted glass inclusions by electron the ascent time of magma from its storage depth. microprobe and Fourier-transform infrared analyses. The pre- eruptive melt is a high-silica rhyolite (∼75 wt.% SiO )and 2 Keywords dome . Microlite . . Rhyolite . was stored at 760±30 °C and 50±25 MPa prior to eruption, Ascent rate . Strain assuming vapor saturation at depth. To investigate emplace- ment dynamics and kinematics, we measured number densi- ties and orientations of at various locations across the . Microlites in samples closest to the inferred Introduction are the most aligned. Alignment does not increase with distance traveled away from the vent, suggesting Rhyolite domes and have a wide range of sizes, from microlites record conduit processes. Strains of <5 accumulat- ≤0.01 to ≥70 km3 (Walker et al. 1973; Christiansen et al. ed in the conduit during ascent after microlite formation, 2007). The effusion of rhyolite lava remained scientifically imparted by a combination of pure and simple shear. unmonitored until the eruptions of Chaitén and Cordón Caulle Average microlite number density in samples varies from volcanoes in during 2008–2009 and 2011–2012, respec- − 104.9 to 105.7 mm 3. Using the magma ascent model of tively (Castro and Dingwell 2009;Alfanoetal.2011; Toramaru et al. (J Volcanol Geotherm Res 175:156–157, Bernstein et al. 2013;Castroetal.2013; Pallister et al. 2013; 2008), microlite number densities imply decompression rates Tuffen et al. 2013). The rarity of such eruptions means that there is little quantitative data on the ascent dynamics and formation of rhyolite lavas, despite their prevalence in major Editorial responsibility: S. A. Fagents volcanic systems and in the volcanic eruption history of the western USA. There is an opportunity to assess K. S. Befus (*) : R. W. Zinke : J. S. Jordan : J. E. Gardner Jackson School of Geosciences, The University of Texas at Austin, the controls on eruption size and duration and better constrain Austin, TX 78712, USA the hazards associated with the effusion of high-silica lavas, e-mail: [email protected] by studying the hundreds of well-exposed, but still unstudied, rhyolitic lava domes and flows around the world. Although M. Manga Department of Earth and Planetary Science, University of California, direct monitoring of those prehistoric eruptions is obviously Berkeley, CA 94720, USA not possible, petrologic and textural studies can provide 808, Page 2 of 12 Bull Volcanol (2014) 76:808 quantitative constraints on the pre-eruptive magmatic storage to assess ascent rates and microlite orientations to identify conditions, the magma ascent rate, and eruption duration. when and where strain accumulated in the erupted rhyolite. Much has been learned from the eruptions of Chaitén and Those measurements, when combined, allow us to infer ascent Cordón Caulle volcanoes. Each eruption was preceded by rate, dimensions of the pathway through which the magma relatively brief, small-volume, sub-Plinian to rose, and the duration of the eruption. (Castro and Dingwell 2009;Alfanoetal.2011;Castroetal. 2013;Schipperetal.2013; Tuffen et al. 2013). They were sourced from stored at 50–200 MPa and 800±100 °C Methods (Vogel et al. 1989; Gibson and Naney 1992;Castroand Gardner 2008; Castro and Dingwell 2009;Alfanoetal. Samples 2011; Castro et al. 2013). In addition, photogrammetry and LIDAR datasets have shown that the of Samples of obsidian were collected at 10 locations across the Chaitén constructed a 0.8-km3 dome over 2 years by exoge- dome (Fig. 1a). Each was collected at approximately the same nous and endogenous growth. The average effusion rate feed- height on the edifice, 50±20 m above its base. Six of the ing the eruption during the first 4 months was ∼45 m3 s−1 and samples were oriented prior to collecting by recording the up- gradually waned until the eruption ceased (Pallister et al. direction and the strike and dip of a planar surface from an 2013). A similar sequence of eruptive activity was observed outcrop of lava thought to be in place (i.e., not rotated by at Cordón Caulle. Initially the effusive eruptive flux was autobrecciation or ). Polished thin sections were pre- estimated to be 20–60 m3 s−1, which decreased to 1– pared from all samples, with special care taken to record the 10 m3 s−1 as the eruption progressed (Schipper et al. 2013; orientation of the thin section relative to the field-oriented Tuffen et al. 2013) The magmatic ascent rate was estimated to sample. separates of quartz, , magnetite, have been <10 mm s−1 (Castro et al. 2013). fayalite, and clinopyroxene were handpicked from three Much has also been learned from drilling Obsidian Dome crushed obsidian samples. in eastern California. The prehistoric eruption of the 0.17 km3 Quartz , 1–2 mm in size, were examined while Obsidian Dome lava dome is inferred to have been fed by submerged in mineral oil to identify glass inclusions that were pulsatory ascent through a conduit ∼7 m in diameter over isolated from the edge of the and not intersected by ∼250 days. Emplacement took place by radial spreading away cracks. All inclusions contain vapor bubbles and crystallites. from an elliptical vent with near-vent and distal portions of the A population of crystals that contained isolated polyhedral flow flattening via pure shear, and the base of the flow glass inclusions ≥40minsizewereloadedintoaAg80Pd20 deforming by simple shear (Miller 1985; Swanson et al. capsule and placed inside a TZM pressure vessel at 850 °C 1989; Fink and Griffiths 1998; Castro et al. 2002; Castro and 150 MPa for 24 h in order to rehomogenize the inclusions and Mercer 2004). to bubble- and crystallite-free glass (e.g., Skirius et al. 1990). Here, we investigate the textural and petrographic features of Douglas Knob, a small-volume, high-silica rhyolite dome Geochemical analyses erupted at ∼120 ka from the western portion of Yellowstone (Fig. 1) (Christiansen et al. 2007). Douglas Knob Mineral and glass compositions were analyzed using the forms an ellipsoidal lava dome approximately 700 m long JEOL JXA-8200 electron microprobe at the University of and 500 m wide, which rises ∼70 m above the surrounding Texas at Austin. were analyzed using a 10-nA beam terrain. Exposure is poor (∼5 %), with much of the dome current, 15 keV accelerating voltage, and a focused beam. covered by vegetation and thin soil. Outcrops up to 20 m2 Glasses were analyzed using a 10-nA beam current, 15 keV are composed of dense obsidian glass with rare small zones of accelerating voltage, and a defocused beam (10 μmdiameter) vesicular obsidian. Devitrified rhyolite occurs rarely. Surface to minimize Na and volatile migration (Nielson and features common to well-exposed silicic domes such as crease Sigurdsson Nielsen and Sigurdsson 1981). During glass and structures and pressure ridges are not observed. The dome analyses, Na migration was corrected using the Na- experienced little erosion during Pinedale glaciations, as evi- migration capability of Probe for Windows™. Working stan- denced by sparse remaining veneers of flow and dards for each analyzed phase were analyzed repeatedly to pumiceous carapace. Assuming much of the original form of monitor for analytical quality and instrument drift. the dome remains preserved, the dimensions indicate a sub- Dissolved water and carbon dioxide contents of 16 doubly aerial eruptive volume of 0.011±0.002 km3. Douglas Knob is exposed glass inclusions and matrix glass were determined by the smallest documented Central Member obsidian Fourier-transform infrared (FTIR) spectroscopy, using a lava from Yellowstone. We use compositions and ThermoElectron Nicolet 6700 spectrometer and Continuμm volatile contents in glass inclusions to constrain magma stor- IR microscope. FTIR spectra consist of 60 scans at a resolu- age depth and temperature. We use microlite number densities tion of 4 cm−1. Spectra from all samples were collected using a Bull Volcanol (2014) 76:808 Page 3 of 12, 808

511900 Shoshone a Lake b

Summit Lake flow

Y88 Heart Lake

4907000 Y122 Caldera margin 8300 Y119 8400

8500 4900000 Pitchstone Plateau flow Y120 Y118 Y121 500000 5 km N Y90 Douglas Knob dome c Y92 Y117 Y116

N 200 m Fig. 1 Location and form of Douglas Knob lava dome. a Geologic map the Yellowstone volcanic field surrounding Douglas Knob. Central Pla- of Douglas Knob. Locations of oriented and unoriented samples are teau Member are shown in pink, the margin of Yellowstone shown by black and white circles, respectively. Trend of proposed fissure caldera is shown by a dashed black line. c Panoramic field photograph of vent is marked by a black dashed line. Topographic contours are shown in Douglas Knob, view looking southwest gray (20 ft. interval). b Simplified geologic map of the southern portion of

40×40 μm aperture in both the mid-IR range, using a KBr Microlite measurements beamsplitter and globular IR source, and near-IR range, using a CaF2 beamsplitter and white light. For glass inclusions, The number density of microlite crystals (MND) was deter- concentrations of dissolved molecular and hydroxyl water mined in each sample by counting the total number of were determined from absorbances at ∼5,200 and microlites within set volumes of the thin section. Counts ∼4,500 cm−1, respectively, using the model of Zhang et al. include Fe–Ti oxide, pyroxene, and rare feldspar microlites.

(1997). Water contents (H2O) are reported as the averaged Microlites were continuously counted at changing depths sums of dissolved molecular water and hydroxyl contents. For through the thin section by focusing through the transparent matrix glasses water concentrations were estimated using the host obsidian glass using an optical microscope. The shape of absorbance at ∼3,500 cm−1, using the Beer–Lambert Law and volumes of glass used for the microlite counts were rectangu- an absorption coefficient of 71±2 (L cm−1 mol−1)(Befusetal. lar prisms, whose volumes were calculated from the dimen-

2012). Dissolved molecular carbon dioxide (CO2) contents sions of the selected surface area (i.e., 60×60 μm) and thick- were determined from absorbance at ∼2,350 cm−1, using the ness of the thin section, as measured by the optical method Beer–Lambert Law and an absorption coefficient of 1,214±16 described above. (L cm−1 mol−1)(Behrensetal.2004). Linear backgrounds Orientations of microlites were measured in all samples. were assumed for absorbances at 2,350, 3,500, and Given that the development of a preferred orientation of 5,200 cm−1. Background for absorbances at the 4,500 cm−1 microlites depends on their aspect ratios (Manga, 1998), we peak was estimated using a flexicurve set tangential to the IR measured orientations of acicular microlites with aspect ratios spectrum on either side of the peak. The thickness of each between 8 and 15, taking care to avoid microlites with mineral inclusion at the spot of the FTIR analysis was measured overgrowths. For all samples the average width of measured optically by focusing on the top and bottom of the sample microlites was 1 μm. First, the trend (ϕ)ofanacicular using a petrographic microscope equipped with a Heidenhain microlite was measured using the goniometer on the micro- focus drive linear encoder. The standard error on those mea- scope stage (Fig. 2a). Individual ϕ measurements are accurate surements is ±0.6 μm, as determined by repeatedly focusing to ±0.5°. Next, the width and apparent length of the microlite through a sample of known thickness. was measured using the reticle in the binocular eyepiece. The 808, Page 4 of 12 Bull Volcanol (2014) 76:808

a 0° b

apparent length φ

D1 D2 270° 90° θ true length

180° Fig. 2 Schematic diagram showing how the trend (φ) and plunge (θ)of Microlite length and θ are calculated geometrically using the apparent microlites are measured in obsidian glass relative to the plane of the thin length and difference in depths (D2–D1) to the tips of each end of the section. a φ is measured using the microscope stage goniometer. b microlite. Modified after Castro et al. (2002) depth of each endpoint of the microlite was then measured by MND and orientations focusing through the transparent obsidian glass and recording the depth measured by the Heidenhain focus drive linear MND across Douglas Knob varies between 104.6 and encoder (Fig. 2b). With those measurements, the true length 106.2 mm−3 (Table 2). Petrographically, individual samples and plunge (θ) of the microlite were then determined. appear texturally homogenous, occasionally with diffuse flow Individual θ measurements are precise to ±2°, as determined bands and patchy microlite domains that cause MND to vary by repeatedly measuring the orientation of a microlite. After within individual samples by 0.4 to 1.2 orders of magnitude orientations for more than 120 microlites were determined in (Fig. 4). Average MND varies within individual samples each thin section, their orientations were rotated to reflect their from 104.9 to 105.7 mm−3. Microlites are preferentially true field orientation using Stereonet (Cardozo and oriented in all samples (Fig. 5). Standard deviations of Allmendinger 2012). For the four unoriented samples, each ϕ and θ (σφ and σθ) in field-oriented samples range was re-oriented such that the mean trend of the microlites is from 17° to 46° and 16° to 36°, respectively (Table 2). σφ and north and the mean plunge is horizontal, to show their relative σθ in unoriented samples range from 14° to 36° and 9° to 24°, degree of microlite alignment. respectively (Table 2).

Results Discussion

Phenocryst and glass composition Douglas Knob is a small-volume, 0.011±0.002 km3,high- silica rhyolite dome which was emplaced at ∼120 ka near the Douglas Knob obsidian is comprised of phenocrysts of quartz, western margin of Yellowstone Caldera (Christiansen et al. sanidine, magnetite, clinopyroxene, and , all set in a 2007). Christiansen (2001) mapped the vent of Douglas Knob glassy matrix of high-silica rhyolite, which comprises ∼95 % as a point source at the topographic high near the center of the of the rock by volume (Table 1). Matrix glass contains 0.14± dome (Fig. 1a). We note, however, that Douglas Knob is an

0.05 wt.% H2O. Quartz occurs as subhedral to anhedral crys- elongate, NW-SE-trending ridge and its morphology is sym- tals, 800–2,000 μmindiameter,andisoccasionallyembayed metric about the trend of the topographic high. Nearby Central with reentrants. Quartz-hosted glass inclusions contain 1.2 to Plateau Member rhyolite lavas, such as Summit Lake, Grants

2.5 wt.% H2O and 56 to 254 ppm CO2 (Fig. 3). Euhedral to Pass, and Bechler River, are all interpreted to have erupted subhedral crystals and crystal fragments of sanidine can reach through fissures (Christiansen et al. 2007). Vents for those

6 mm in size, and are compositionally unzoned (Or50±1Ab50 flows are all aligned NW-SE, similar to the long axis of ±1). Magnetite (Mgt48±2 Usp52±2) forms euhedral, equant crys- Douglas Knob, an orientation which parallels the southwest- tals 50–400 μm across, and generally occurs as glomerocrysts ern margin of Yellowstone Caldera. We thus conclude that associated with clinopyroxene and rare fayalite. Douglas Knob actually extruded through a fissure instead of a Clinopyroxene occurs as euhedral, elongate crystals up to central vent. Using the petrology, volatile contents, microlite 1,000 μm long. They are compositionally unzoned ferroaugite number density and orientation, we now seek to constrain: (1)

(En13±1Fs46±1Wo41±1). Fayalite is compositionally unzoned Storage conditions, (2) ascent dynamics, and (3) emplacement (Fa90±1), and euhedral. history. Bull Volcanol (2014) 76:808 Page 5 of 12, 808

Ta b l e 1 Compositions of glass and major phenocryst phases

Matrix glass Glass inclusion Sanidine Clinopyroxene Fayalite Magnetite

SiO2 77.11 (0.63) 74.41 (1.47) 66.4 (0.78) 49.17 (0.91) 30.38 (0.82) 0.11 (0.05)

TiO2 0.17 (0.07) 0.16 (0.04) – 0.27 (0.11) 0.05 (0.02) 18.55 (0.59)

Al2O3 11.79 (0.12) 11.78 (0.35) 19.12 (0.23) 0.43 (0.14) 0.02 (0.02) 0.99 (0.06) FeOa 1.33 (0.29) 1.46 (0.23) 0.13 (0.03) 26.22 (0.74) 64.21 (0.90) 77.57 (0.81) MnO 0.03 (0.02) 0.03 (0.02) – 0.65 (0.04) 1.50 (0.33) 0.83 (0.05) MgO 0.03 (0.03) 0.02 (0.01) 0.02 (0.01) 4.17 (0.50) 3.03 (0.31) 0.19 (0.05) CaO 0.10 (0.04) 0.11 (0.06) – 18.34 (0.39) 0.27 (0.01) 0.01 (0.01)

Na2O 3.63 (0.18) 4.76 (0.34) 5.57 (0.24) 0.28 (0.06) ––

K2O 5.28 (0.27) 4.97 (0.11) 8.56 (0.31) –––

H2O 0.14 (0.04) 1.78 (0.35) –– –– b CO2 0 153 (50) –– –– Fb – 1,997 (190) –– –– Clb – 1,120 (173) –– –– Total 99.61 99.64 99.80 99.25 99.46 98.25 na 16 16 21 50 18 79

Analyses by electron microprobe and FTIR spectroscopy. Major oxides reported in weight percent, except where indicated, and are averages of n samples. Values in parentheses represent standard deviations “–” oxide was not analyzed, na number of glass analyses or number of individual phenocrysts analyzed a Total reported as FeO b Composition in parts per million

Storage conditions 0.4 log units below the quartz–fayalite–magnetite buffer, using QUILF (Andersen et al. 1993). That temperature is Compositions of coexisting, compositionally unzoned mag- similar to those of other rhyolitic obsidian domes (Manley netite, fayalite, clinopyroxene, and quartz indicate the rhyolit- and Bacon 2000; Castro and Gardner 2008). It is cooler than ic magma was stored at 760±30 °C at an oxygen fugacity of nearly all published temperatures for magmas in the Yellowstone/ system, but agrees with the

400 75 MPa general trend that younger and more easterly erupted lavas were emplaced at cooler temperatures (Vazquez et al. 2009; 40:60 (H2O:CO2)

300 50 MPa 50:50 Ta b l e 2 Microlite number density (MND), aspect ratio, and standard 60:40 deviations of trend (φ) and plunge (θ)

(ppm) 200 2 a b σφ σθ 25 MPa Sample MND (mm-3) Aspect ratio ° ° CO

100 Y88 104.6–105.2 9 (2.0) 38 9 Y90 104.6–105.4 12 (1.9) 14 24 5.3– 5.7 0 Y92 10 10 10 (2.1) 17 20 0123 Y116 105.3–105.9 12 (1.5) 36 17 4.6– 5.8 H2O (wt.%) Y117 10 10 10 (2.1) 30 31 4.4– 5.6 Fig. 3 Volatile contents dissolved in 16 quartz-hosted glass inclusions. Y118 10 10 11 (1.6) 32 23 4.6 5.3 Error bars (1σ) are shown in gray when larger than the symbol size and Y119 10 –10 11 (2.0) 17 16 are from uncertainties in thickness measurements. Equilibrium solubil- Y120 105.0–106.1 10 (1.4) 46 36 ities at 25, 50, and 75 MPa and 760 °C are shown in gray (Liu et al. 2005). Y121 105.0–105.7 10 (1.9) 30 29 Dashed black lines are fluid compositions for closed-system degassing Y122 105.1–106.2 10 (2.9) 26 18 pathways for rhyolite in equilibrium with a mixed H2O–CO2 fluid (Liu et al. 2005). Dotted light gray line represents the open system fluid a Sample locations are shown in Fig. 1a degassing pathway for rhyolite in equilibrium with the highest H2O– b CO2 concentrations found at Douglas Knob (Newman and Lowenstern Aspect ratio equals microlite length divided by width, values in paren- 2002) theses represent 1σ error 808, Page 6 of 12 Bull Volcanol (2014) 76:808

higher MND lower MND pressures, assuming equilibrium conditions, indicate magmat- ic storage pressures of 50±25 MPa (Liu et al. 2005). That value corresponds to a storage depth of 2±1 km, assuming −3 Fe-Ti oxide depth-averaged crustal density of ∼2,500 kg m . Results from Rhyolite-MELTS simulations of the observed pheno- cryst assemblage are consistent with those magmatic storage conditions (Gualda et al. 2012). pyroxene

30 µm Eruption ascent rate Fig. 4 Photomicrograph of obsidian displaying MND variations in a sample with strong microlite alignment. MND is higher on the left half of the image. Black acicular microlites are Fe–Ti oxides. Clear prismatic Microlite number density varies by little more than an order of microlites are pyroxene magnitude across Douglas Knob. If the lava crystallized dur- ing emplacement, then the number density of microlites would Watts et al. 2011;Almeevetal.2012). Measured concentra- be expected to increase with increasing distance from the tions of H2OandCO2 in quartz-hosted glass inclusions are fissure. Instead, MND does not vary systematically with dis- dispersed along a closed-system degassing trend for melt in tance in the flow. Nor does MND vary systematically for Fe– equilibrium with a fluid composed of 50±10 mol.% CO2 Ti oxides, pyroxene, or feldspar when treated separately. The (Fig. 3)(Liuetal.2005). Geochemical trends of the glass lack of systematic increase suggests that all microlites nucle- inclusions indicate the range of volatile contents do not result ated at depth rather than during emplacement. Furthermore, from crystallization of the quartz host. Volatile saturation the variability displayed by MND suggests that eruptive

unoriented unoriented sample sample

5 Y88 10 5 Y122 8300 Y119 8400

Y120

Y118 5 Y121 10 5 Y90

Y92 Y117 Y116

51015 10 5 N 200 m

unoriented unoriented sample sample

Fig. 5 Stereograms and rose diagrams show the orientations of acicular contours, with colors representing orientation frequency per unit area microlites of field-oriented samples (black circles) and unoriented sam- (where blue is 2 %, green is 8 %, yellow is 14 %, orange is 20 %, red is ples (white circles). Rose diagrams display the orientation of acicular 26 %, and pink is 32 %). Trend of fissure vent is marked by a black microlites. The degree of microlite alignment is represented by Kamb dashed line Bull Volcanol (2014) 76:808 Page 7 of 12, 808

degassing, water content, and temperature varied with time its lower SiO2 and higher H2O contents, which are both and position during the eruption, but did so unsystematically important variables in the model (70–74 and 4.1±1.2 wt.%, (Fig. 6). respectively; Hervig et al. 1989; Vogel et al. 1989). The ascent

Microlites nucleate and grow in response to degassing rate of the effusive eruption at Cordón Caulle (∼70 wt.% SiO2) during decompression, because the liquidus temperature of is estimated to be 10 to 40 mm s−1 (Castro et al. 2013). the melt rises as exsolve (Swanson et al. 1989; Although ascent rate for the effusive eruption at Chaitén

Hammer 2008). Previous studies have related MND to de- (∼75 wt.% SiO2) is not known, the effusion rate is well compression rate, with faster decompression resulting in constrained. If we assume a range of reasonable estimates higher MND (e.g., Couch et al. 2003; Castro and Gardner for the diameter of its circular conduit (10–100 m), then 2008; Martel 2012). Using experimental data and kinetic magmas ascended at 6 to 600 mm s−1 during the first 4 months theory, (Toramaru et. al. 2008) developed a model that of the effusion of the Chaitén lava dome (Pallister et al. 2013). quantitatively links pyroxene and oxide MND to the decom- Ascent rates at Obsidian Dome, Cordón Caulle and Chaitén pression rates of silicic magmas. Using compositional data of are thus one to three orders of magnitude faster than those the pre-eruptive Douglas Knob melt and the range in average estimated for Douglas Knob. The faster ascent rates may be a MND, decompression rates are estimated to have been 0.03 to consequence of their lower melt viscosities. 0.11 MPa h−1. Such rates are equivalent to ascent rates of 0.4 The ascent rate of Douglas Knob can be used to constrain to 1.3 mm s−1, assuming a lithostatic gradient and a calculated the geometry of the fissure feeding the eruption. The mean magma density of 2,330 kg m−3 (Lange and Carmichael ascent rate U (m s−1) during steady, laminar flow of an 1990). Similar ascent rates (∼1mms−1)havebeendocument- incompressible magma between two parallel walls separated ed for effusive, silicic, dome-forming eruptions at Unzen, by a distance w (m), and driven by a pressure gradient dP/dz Mount Saint Helens, Soufriére Hills, and (kg m−3), is (Devine et al. 1998; McCanta et al. 2007; Nakada and Motomura 1995; Rutherford and Devine 2003;Rutherford w2 U ¼ dP dz ð1Þ 2008). Slow ascent rates allow time for open system degassing 12μ in the conduit, which is why the magma effused instead of erupting explosively (e.g., Jaupart and Allégre 1991; where μ is viscosity (Pa s)(Turcotte and Schubert 1982). Rutherford and Gardner 2000). Except near a dike tip, the ascent of silicic magma is deter- The microlite assemblage of Fe–Ti oxides, pyroxene, and mined by a balance between local buoyancy forces and vis- at Obsidian Dome appears identical to that of cous resistance (e.g., Lister and Kerr 1991). We equate dP/dz Douglas Knob. MND are likewise similar, with those from to the product of gravitational acceleration and density differ- Obsidian Dome ranging from 105.1 to 105.7 mm−3 (Castro ence between magma and crust, typically assumed to be et al. 2002). Those MND yield decompression rates of 0.2 to 200 kg m−3 (e.g., Petford et al. 1993). The viscosity of magma 4.1 MPa h−1, which correspond to ascent rates of 3 to with the composition and average water content of the glass −1 50 mm s (Toramaru et. al. 2008). Although MND are inclusions (1.8 wt.% H2O), 5 wt.% crystallinity, and temper- similar at Douglas Knob and Obsidian Dome, the order of ature of 760 °C is 106.9 Pa s (Spera 2000;Giordanoetal. magnitude faster ascent rate for Obsidian Dome is caused by 2008). A temperature uncertainty of 30 °C translates into a viscosity uncertainty of a factor of 2. Ascent rates of 0.4 to 1.3 mm s−1 thus imply that w is in the range of 4 to 8 m with an 6.5 additional uncertainty of 50 % if we allow for the possible range of temperatures. If we take the 500-m length of the 6 fissure at the surface as representative of that at depth, the ) -3 volume flux would have been 0.6 to 7.5 m3/s. Emplacing a 5.5 0.011 km3 dome took 17 to 210 days. Interestingly, this duration is similar to the 20 to 70 days that would have been 5 required for magma to ascend from the chamber to the surface,

log MND (mm assuming the average pre-eruptive storage pressure of 50 MPa 4.5 and the estimated decompression rates of 0.03 to 0.11 MPa h−1. A volume flux of 0.6 to 7.5 m3 s−1 is similar 4 0 100 200 to those observed during dome-forming effusions at Merapi, Distance from fissure vent (m) Mount St. Helens, Unzen, and Soufriére Hills (Swanson et al. 1987;NakadaandMotomura1995;Sparksetal.1998; Fig. 6 Relationship between microlite number density (MND) and dis- tance from fissure vent. Black dots represent average MND. Gray bars Hammer et al. 2000; Pallister et al. 2013), and estimated for encompass maximum and minimum values within sample the rhyolitic dike at Hrafntinnuhryggur (Tuffen and Castro 808, Page 8 of 12 Bull Volcanol (2014) 76:808

2009) but smaller than during the historical rhyolite dome- of microlites, suggesting the importance of the inherited ori- forming eruptions at Chaitén and Cordón Calle (20–60 m3 s−1; entation from ascent in the conduit. Castro et al. 2013; Pallister et al. 2013; Tuffen et al. 2013). We The degree of alignment of acicular microlites in Douglas recognize all those duration and ascent rate estimates have Knob can be directly compared with theoretical results to significant uncertainties (many of which are difficult to re- estimate how much strain accumulated during emplacement duce) and neglect processes such as degassing on magma following microlite formation (Appendix). Values of σφ indi- viscosity and the evolution of dike geometry in space and cate Douglas Knob accumulated strains of 0.2 and 5.0, as- time. However, our calculations do provide first-order esti- suming either end-member pure and simple shear, respective- mates of time and length scales for systems in which we ly. Values of σθ indicate Douglas Knob accumulated a mini- previously had few quantitative constraints. mum strain of 0.8 strain, but the upper limit is unconstrained because the σθ in those samples are too low (i.e., well aligned) Emplacement of the lava dome to be generated solely by simple shear, implying pure shear as a partial mechanism during deformation. Microlite orientations provide insight into the deformation Strain estimates are scattered with respect to increasing history of melt and hence understanding of ascent and dome distance from vent (Fig. 7). Microlites in vent samples are as emplacement. Long axes become aligned in the direction of well aligned, or better, than those in samples farther from the local extension, which allows microlite orientations to be used vent. Importantly, the absence of increasing strain with increas- to infer the dominant directions of fluid stretching and track ing distance from vent indicates that the lava did not accumulate flow lines in a lava (Manga 1998;Castroetal.2002). measurable strain (on the flow’s upper surface) in the ≤240 m of Microlites are oriented during simple shear flow by progres- subaerial flow during emplacement. Instead, strain estimates sively aligning in the direction of extension with increasing from σφ in distal samples are generally smaller than those at strain, typically thought to be perpendicular to the flow front. the vent. Thus, strain generated by emplacement was sufficient to Microlite orientation induced by pure shear can be either rotate the preferred orientation of microlites from near vertical perpendicular or parallel to the flow margin but should have a limited vertical component in response to gravitational flat- 5 tening of the dome (Manga 1998;Merle1998; Castro et al. a 2002). Analog experiments and sparse field studies show that 4 φ

emplacement of lava domes is primarily controlled by pure σ shear mechanisms, especially in the upper portions of lavas 3 where we collected samples, but natural lavas may experience simple shear simple shear, pure shear, or a combination of both especially 2 in different parts of the flow (Merle 1998;BuissonandMerle Strain from 2002; Ventura 2004; Schipper et al. 2013). 1 Assuming Douglas Knob erupted from a 500-m long fis- pure shear sure that runs the length of the dome, two samples were collected where the rhyolite emerged at the surface b (Fig. 1a). Microlites in those samples are strongly 8 θ aligned, indicating significant strain (Fig. 5). Microlites σ in the field-oriented sample are preferentially oriented 6 close to vertical, which suggests alignment was caused 4 by deformation during vertical flow, as expected from from Strain ascent in the conduit. 2 Microlites in samples collected away from the fissure vent simple shear pure shear also have preferred orientations, roughly perpendicular to the 0 flow front. We would expect some scatter in the preferred 0 50 100 150 orientations, because some microlite orientations may record Distance from fissure vent (m) local flow fields, caused by flow folds or brittle deformation, Fig. 7 Strain estimates recorded by the standard deviation of φ a and θ b which are relatively common in rhyolitic lavas (Merle 1998; of microlites plotted against distance from fissure vent. The lower bound Castro and Cashman 1999; Buisson and Merle 2002; Tuffen estimates represent strain induced solely by pure shear, whereas upper et al. 2003; Gonnermann and Manga 2005). It is unlikely, strain estimates assume strain induced by only simple shear. The range however, that strains induced from such local features could between those end-members displays strain accumulated by combina- tions of simple and pure shear. Microlites are so well aligned with respect overprint dominant orientation trends. In each sample, there is to θ in some samples that they leave the upper bound for those strain a significant vertical component to the preferred orientations estimates extending to infinity Bull Volcanol (2014) 76:808 Page 9 of 12, 808 to roughly perpendicular to the flow front, but was unable to equal to 4 to 8 m, as estimated previously from our ascent rate improve the microlite orientation distributions. It is plausible calculation, τ is about 0.5 to 2 years, long enough that the that the rotation of preferred orientations worsened the σφ in magma can reach the surface without solidifying (given the microlite orientation distributions. Alternatively, all of the var- inferred ascent times), yet short enough that some cooling will iation in microlite orientation distributions record strain accu- occur. In sum, the eruption duration implied by the dome mulated during conduit ascent. Thus, variations in strain reflect shape is consistent with that based on ascent rates and conduit variations in conduit characteristics, such as diameter or ascent dimensions. rate. If so, then the variations indicate that changes in conduit The eruption duration also allows flow front advance rates conditions were pulsatory to chaotic rather than systematic to be estimated. Viscous flow is assumed to have been viable (e.g., Geschwind and Rutherford 1995). during the duration of the eruptive event, and likely continued Microlites indicate the sampled domains of Douglas Knob for up to a few years following the end of the eruption (e.g., experienced negligible strain during subaerial emplacement, Tuffen et al. 2013). Assuming that, the flow front of Douglas whereas strain was found to accumulate during emplacement Knob lava dome advanced at tens of cm to a few m per day. of Obsidian Dome (Castro et al. 2002). We attribute this Those values are on par with velocity estimates from Cordón disparity to the vastly different sizes of the two flows. Caulle lava (1.5–3 m day−1) (Tuffen et al. 2013). Such esti- Obsidian Dome traveled hundreds to thousands of m more mates provide first-order constraints important for hazard than did Douglas Knob. It is thus possible that the degree of assessment associated with the effusive eruptions from the microlite alignment in very small domes are vestiges of con- Yellowstone volcanic system. duit flow, and thus preserve a record of conduit processes. In larger domes and flows, however, significant strain may ac- cumulate during emplacement that erases and overprints Summary microlite textures derived in the conduit (Castro et al. 2002). Finally, the morphology of the dome also provides a con- The eruption of Douglas Knob tapped a high-silica rhyolitic straint on eruption duration. The distance R (m) that magma magma that was shallowly stored at 50±25 MPa and 760± spreads away from the vent depends on eruption rate and 30 °C. Microlite number densities indicate ascent rates of 0.4 magma viscosity following: to 1.3 mm s−1. Using the measured glass composition and

1=8 volatile contents, the required fissure width is 4 to 8 m, similar ρgV 3 R ¼ 0:7 t1=8 ð2Þ to rhyolite dikes elsewhere (e.g., Tuffen and Castro, 2009). 3μ Such slow ascents are reasonable when compared with other silicic effusive, dome-forming eruptions, and slow enough to −3 μ where p is density (kg m ), is viscosity (Pa s), V is eruptive allow degassing to occur (e.g., Eichelberger et al. 1986; 3 −2 volume (m ), g is 9.8 m s ,andt is time (s) (Huppert 1982). Gonnermann and Manga 2005; Castro and Gardner 2008; This model assumes a constant viscosity and neglects the Rutherford 2008; Castro and Dingwell 2009). The emplace- possible feedbacks between strain localization and viscous ment time based on those ascent rates, as well as the morphol- dissipation. As viscosity during emplacement is unknown, ogy of the dome, is 17–210 days, equivalent to eruption rates we adopt the approach of testing whether the eruption rate of 0.6 to 7.5 m3 s−1. Microlites in all samples from Douglas inferred in the previous section is consistent with a reasonable Knob are well aligned, especially in near-vent locations, and viscosity. Assuming an eruption duration of 17 to 210 days, do not become better aligned with distance. Microlite align- −3 radial spreading of 240 m, and density of 2,330 kg m ,the ment also demonstrates the lava accumulated strains <5, 11 12 viscosity of the lava must have been 10 to 10 Pa s. Such which were imparted by components of both simple and pure 6.9 viscosities are higher than those estimated at depth (10 Pa s) shear in the conduit following microlite nucleation during ∼ owing to the loss of water (from 1.8 to 0.14 wt.% H2O) and ascent. cooling. Using the composition of matrix glass those viscos- ities imply surface emplacement temperatures between 695 Acknowledgments The authors thank Hugh Tuffen and Jonathan Cas- and 735 °C, and thus cooling of 25 to 65 °C. As water content tro for thorough critical reviews which significantly improved this man- decreases from 1.8 to 0.14 %, temperature will decrease about uscript. This work was funded by a grant from the National Science 30 °C mainly as a result of decompression of the vapor phase Foundation (EAR-0738664) to J.E.G. and M.M. (Proussevitch and Sahagian 1998; Mastin and Ghiorso 2001; Gonnermann and Manga 2007). There should be additional cooling owing to thermal conduction to the surrounding coun- Appendix: modeling microlite orientation try rocks. The time scale for thermal diffusion (τ)isw2/κ, where κ=5×10-7 m2 s−1 is the thermal diffusivity and w is The degree of alignment of a microlite population, irrespective fissure width (m) (Bagdassarov and Dingwell 1994). Using w of preferred orientation, can be used to understand how lavas 808, Page 10 of 12 Bull Volcanol (2014) 76:808

60 dθ R2−1 ¼ G sinθcosθsinϕcosϕ ð4Þ 50 dt R2 þ 1 40 and the equations governing particle motion in pure shear 30 σ flow are: θ 20 dϕ R2−1 ¼ G sinð2ϕÞð5Þ σ 2 þ 10 σ φ dt R 1 σφ θ Standard Deviation (degrees) Standard Deviation 0 0 2 4 6 8 10 12 14 16 dθ 1 R2−1 ¼ − G cosð2ϕÞsinð2θÞð6Þ Strain dt 2 R2 þ 1 Fig. 8 Model standard deviations of the φ and θ of a population of rod- −1 shaped microlites with an aspect ratio of 10 plotted as a function of strain where G is strain rate (s ), R is microlite aspect ratio accumulated during simple (solid curves) and pure shear (dashed curves). (dimensionless), and t is time (s). During flow, the lava is Standard deviations for sample Y118 are plotted in gray domains as an assumed to deform solely with plane strain and the melt is example for obtaining strain estimates from the degree of microlite alignment assumed to be a Newtonian fluid. Microlite concentrations are assumed to be sufficiently dilute so that the microlites do not interact with one another. Original orientations are assumed to be random. Microlite concentration is considered dilute when are emplaced. Microlite alignment is measured by the stan- MND*R2*d is less than 1, where d is microlite diameter dard deviation of ϕ and θ (σφ and σθ), with smaller standard (Manga 1998). All samples from Douglas Knob meet that deviations indicating better alignment. Those deviations can criterion for being a dilute suspension. be compared with theoretical σφ and σθ of a population of The model predicts that in simple shear flow initially 100,000 microlites that experience variable degrees of strain randomly oriented microlites progressively align in the direc- in either simple and pure shear flows. We use a discrete, finite- tion of extension with increasing strain, but that they contin- difference model with an explicit timestep to numerically uously rotate, thus never becoming perfectly aligned. integrate the differential equations that predict the motion of Conversely, microlites in pure shear flow are expected to rod-shaped particles in either simple shear flow and pure shear eventually become perfectly aligned in the direction of exten- flow (Jeffery 1922;Gay1968; Manga 1998). Following sion with increasing strain (i.e., σφ and σθ go to zero). The Manga (1998), equations governing particle motion in simple degree of microlite alignment in natural lavas can thus be used shear flow are: to constrain the type of shear and the amount of strain imposed on microlites during flow (Fig. 8). Samples with small vari- dϕ G ÀÁ ¼ R2cos2ϕ þ sin2ϕ ð3Þ ances in trend or plunge cannot be explained solely by simple 2 þ dt R 1 shear. Because equations of motion for pure and simple shear

Fig. 9 Modeled effects of a θ alignment b φ alignment combining relative proportions of 16 50 strain induced by end member 4:1 4:1 2:1 1:1 2:1 1:1 simple shear and pure shear on the 14 45 alignment of microlite θ (a)andφ 40 (b). Proportions of simple shear to 12 pure shear are shown by white 35 10 dashed lines. Any degree of 30 microlite alignment can be 8 25 explained by variable proportions 1:2 1:2 of simple shear and pure shear 6 20 15 Simple Shear Strain 4 1:4 1:4 10 2

5 Std. Dev. of alignment (degrees)

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