Spherulites As In-Situ Recorders of Thermal History in Lava Flows

Downloaded from geology.gsapubs.org on June 25, 2015 Spherulites as in-situ recorders of thermal history in lava flows

Kenneth S. Befus1, James Watkins2, James E. Gardner1, Dominique Richard3, Kevin M. Befus1, Nathan R. Miller1, and Donald B. Dingwell3 1Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA 2Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA 3Department of Earth and Environmental Sciences, Ludwig Maximilian University of Munich, Theresienstrasse 41/III, 80333 Munich, Germany

ABSTRACT flow, a 70 km3 high-silica rhyolitic obsidian lava Spherulites in rhyolitic obsidian provide a record of the thermal history of their host lava that traveled up to 16 km away from its vent, during the interval of spherulite growth. We use trace element concentration profiles across which erupted effusively at 79 ± 10 ka within spherulites and into the obsidian host from Yellowstone National Park (USA) to reconstruct Yellowstone caldera (western United States; see the conditions that existed during spherulite formation. The measured transects reveal three the GSA Data Repository1; Christiansen et al., behaviors: expulsion of the most diffusively mobile elements from spherulites with no con- 2007). The sample was collected as a single, centration gradients in the surrounding glass (type 1); enrichment of slower-diffusing ele- 2 kg block from an in-situ outcrop of relatively ments around spherulites, with concentration gradients extending outward into the glass non-vesicular obsidian on the erosional surface (type 2); and complete entrapment of the slowest-diffusing elements by the spherulite (type of the flow near the eastern flow front, where 3). We compare the concentration profiles, measured by laser ablation–inductively coupled the flow remains at least 180 m thick (sampling plasma–mass spectrometry and Fourier transform infrared spectroscopy, to the output of locality: 44.247°N, 110.680°W). a spherulite growth model that incorporates known diffusion parameters, the temperature Spherulites compose a few percent of the interval of spherulite growth, the cooling rate of the lava, and data on the temporal evolution obsidian sample by volume, and occur as 1–10 of spherulite radius. Our results constrain spherulite nucleation to the temperature interval mm, near-spherical ellipsoids (Fig. 1A). Individ- 700–550 °C and spherulite growth to 700–400 °C in a portion of lava that cooled at 10–5.2 ± 0.3 ual spherulites are distributed randomly through- °C s–1, which matches an independent experimental estimate of 10–5.3 °C s–1 measured using out the sample, separated by tens of millimeters, differential scanning calorimetry. Maximum spherulite growth rates at nucleation are on the with some clusters of two to four spherulites that order of 1 mm hr–1 and are inferred to decrease exponentially with time. Hence, spherulites impinge upon one another. All spherulites are may serve as valuable in-situ recorders of the thermal history of lava flows. composed of elongate crystals of alkali feldspar

(Or34 ± 6Ab64 ± 6An2 ± 1) and quartz that radiate out- INTRODUCTION perature-dependent rates. The process of expul- ward from a centralized nucleation site. Minor The striking appearance of millimeter- to sion and diffusion of the incompatible elements phases include equidimensional magnetite and centimeter-scale light-colored spherulites in generates element-specific concentration pro- randomly oriented, irregularly shaped voids that dark obsidian outcrops became a subject of sci- files, the shapes of which depend on spherulite most likely represent vesicles. Quartz and alkali entific inquiry more than a century ago (Cross, growth rate, element diffusivity, and the cooling feldspar crystals are both longer and wider near 1891). Studies in the intervening years have rate of the host lava (Gardner et al., 2012). the center of the spherulites. The ratio of alkali documented occurrences of spherulites, their If two of the above quantities can be estimated feldspar to quartz increases gradually outwards field distributions, and their textural charac- independently, then it should be possible to from the spherulite core, from near equal pro- teristics (Ewart, 1971; Manley and Fink, 1987; determine the unknown third quantity by direct portions near the center to ~60% alkali feldspar Tuffen and Castro, 2009). Beyond their histori- inference from measurements on spherulite- approaching the margin. The outermost ~100 cal role as a curiosity to petrologists as well as bearing glasses. Here, we measure concentra- mm is more porous and composed solely of rock collectors, spherulites have recently been tion gradients around 19 individual spherulites radiating alkali feldspar crystals and void space. recognized for their scientific value as record- and use published diffusion parameters to con- Pockets of glass trapped within the spherulites, ers of past magmatic conditions. The distri- struct a numerical model for spherulite growth documented in other studies (e.g., Castro et al., butions of trace elements within and around accompanied by outward diffusion of incom- 2008), were not observed. spherulites appear to contain information on patible elements. We use differential scanning The spherulites are hosted within a high-sil- crystallization rates, redox reactions, and the calorimetry (DSC) to independently determine ica rhyolitic glass (76 wt% SiO2) that contains thermal history of the host lava (Castro et al., the cooling rate of the sample investigated. 5–10 vol% phenocrysts, 95% of which are sani- 2008, 2009; Watkins et al., 2009; Gardner et With this information we construct model fits to dine and quartz, with the other 5% being mag- al., 2012; von Aulock et al., 2013). measured diffusion profiles around spherulites netite, clinopyroxene, and fayalite. The glassy Spherulites in rhyolitic lavas are spherical to of varying sizes to constrain spherulite growth groundmass contains magnetite and clinopyrox- ellipsoidal bodies of radiating, intergrown crys- through time. The result is a set of self-consis- ene microlites, which define diffuse flow bands. tals, typically feldspar and quartz, that form by tent physical parameters for spherulite growth Spherulites overprint those flow-induced tex- rapid crystallization of lava in response to sig- and lava cooling. Insofar as the inferred spheru- tures, as evidenced by undeflected flow bands nificant cooling (Lofgren, 1971a, 1971b; Fenn, lite growth law is generally applicable, we pro- that can be traced from the matrix glass through 1977; Swanson, 1977). As spherulites grow, pose that spherulites act as in-situ recorders of the spherulites. incompatible constituents are expelled into the thermal history of spherulite-bearing glasses the surrounding melt or glass, creating enrich- and welcome future tests of this proposal. ments of those elements along the spherulite- 1GSA Data Repository item 2015227, descrip- host boundary (Castro et al., 2008; Watkins et GEOLOGIC SETTING AND SAMPLE tions of analytical techniques, modeling parameters, and uncertainties, is available online at www al., 2009; Gardner et al., 2012). With time, the SELECTION .geosociety.org/pubs/ft2015.htm, or on request from expelled constituents diffuse away from the A hand sample of spherulite-bearing obsidian [email protected] or Documents Secretary, spherulite and into the surrounding host at tem- was collected from the Pitchstone Plateau lava GSA, P.O. Box 9140, Boulder, CO 80301, USA.

GEOLOGY, July 2015; v. 43; no. 7; p. 647–650 | Data Repository item 2015227 | doi:10.1130/G36639.1 | Published online 5 June 2015 ©GEOLOGY 2015 Geological | Volume Society 43 | ofNumber America. 7 For| www.gsapubs.org permission to copy, contact [email protected]. 647 Downloaded from geology.gsapubs.org on June 25, 2015

CONCENTRATION PROFILES AROUND lite boundary, and (2) the propagation distance A Obsidian SPHERULITES (Table DR1 in the Data Repository). Enrichment

Trace element and H2O concentration pro- (e) is defined as the concentration of an element files were measured across the spherulite-glass in the glass at the spherulite-glass boundary, nor- interface by laser ablation–inductively coupled malized to its concentration far from the spheru- Spherulite plasma–mass spectrometry and Fourier Trans- lite, and then converted to a percentage. In the form infrared spectroscopy, respectively (Fig. model, spherulite growth rates strongly control 1B; see the Data Repository). Overall, three e, with faster growth leading to greater e. For

types of behavior have been recognized (cf. the propagation distance (P∆), we use a different Gardner et al., 2012) (Fig. 1C). The fastest- metric than the commonly used e-fold distance diffusing elements (Li and K) follow “type 1” for describing the shape of diffusion profiles. behavior, exhibiting uniformly low concentra- The reason for doing so is to be able to plot “dif-

tions in the spherulite and uniformly higher con- fusion profile shape” ase / P∆ on the y-axis versus 500 μm centrations in the glass. Such profiles are inter- spherulite radius on the x-axis in order to com- preted to result from rapid diffusion away from pare model to data from all spherulites simulta- B Crack Contact the spherulite during spherulite growth. A group neously. If one were to use the e-fold distance of slower-diffusing components (Rb and H2O) (distance at which concentration decreases by follow “type 2” behavior, exhibiting enrich- a factor of e from the spherulite boundary), the ments in the glass surrounding spherulites, with ratio of enrichment to diffusion distance would Laser track concentration decreasing with distance from the be a non-unique description of diffusion profile 100 μm boundary. Such profiles are thought to result shape. For the measured profiles, we calculate

from trace elements diffusing at rates compara- P∆ as the distance from the spherulite margin C Type 1: Li ble to that of spherulite growth. The slowest-dif- that the elemental concentration exceeds by two 100 ) fusing incompatible elements (B, Cs, Mn, Mg, standard deviations that measured in the glass

Pb, Ba, Sr, and Sc) follow “type 3” behavior, host matrix. For the model profiles, P∆ is the whereby the elements diffuse far slower than distance from the spherulite margin where the spherulites grow, such that concentrations are concentration first exceeds the background level 10 Type 3: Cs unchanged across the spherulite-glass bound- by a set percentage. To establish model uncer- ary and the elements are trapped in the growing tainties in P∆, we use 12% ± 6% and 5% ± 3%

spherulite. Here it is important to clarify that all for Rb and H2O, respectively, as those values

H2O profiles are modified near the spherulite- encompass the full range of standard devia- matrix boundary by a steep increase in concen- tions observed in the natural data. In the model, Li and Cs conc. (ppm 1 tration caused by secondary hydration, and thus P∆ increases mainly with slower cooling, faster

) 600 the steep increases are disregarded in our treat- growth, and growth at higher temperatures. Type 2: Rb ment (see the Data Repository). The first step in modeling spherulite growth 400 is to specify how growth rate varies with tem- SPHERULITE GROWTH DURING LAVA perature. In the absence of a general theory, we 200 COOLING have investigated models that invoke constant, We employ a numerical crystal growth and linearly decreasing, and exponentially decreas-

Rb conc. (ppm Model fit diffusion model that uses known temperature- ing radial growth laws (see the Data Reposi- 0 -200 0 200 400 600 800 dependent, element-specific diffusion coef- tory). Both the constant radial and the linearly Distance along transect (µm) ficients to generate model concentration pro- decreasing radial growth laws can be ruled out files (see the Data Repository). At each time on the basis that they generate profiles with 0.3 step in the model, the spherulite boundary e values much greater than those observed 0.2 moves according to the specified temperature- (Gardner et al., 2012). Our preference for the Type 2: H2O dependent growth rate and a constant frac- exponentially decreasing radial growth model 0.1 tion of each trace element is expelled into the stems from several factors: (1) it produces rea- Model fit glass. Whereas type 1 and type 3 profiles pro- sonable values of e; (2) it reproduces the posi- O conc. (wt%) 2 0 vide constraints on the temperature window tive correlations between e, P∆, and spherulite H -400 0 400 800 1200 1600 of spherulite growth, we focus the discussion size for both Rb and H2O that are observed; and Distance along transect (µm) on the type 2 profiles of Rb and H2O, because (3) it ultimately yields reasonable cooling rates these offer the most restrictive constraints on for the host lava. Furthermore, this growth law Figure 1. A: Photomicrograph (crossed po- the conditions of spherulite formation. is qualitatively consistent with the observation lars) of spherulite composed of radiating In the spherulite growth model, the shape of that crystal sizes decrease from core to rim. We crystals of quartz and feldspar hosted in the type 2 concentration profiles depends on therefore adopt the exponentially decreasing dense obsidian glass. B: Backscattered im- (1) the temperature window over which spheru- radial growth law as the best available descrip- age of spherulite-matrix contact crossed by laser ablation track. C: Examples of type 1 lites nucleate and grow, (2) the temperature-time tion of the temperature dependence of spheru- path of the spherulite, i.e., the cooling rate of lite growth. (Li, in red), type 2 (Rb and H2O, in green), and type 3 (Cs, in blue) profiles (conc.—concen- the lava flow, and (3) the temperature-dependent With a specified growth law and measured tration). Spherulite-matrix boundary occurs growth rate (Gardner et al., 2012). The shapes diffusivities, the type 2 model profiles depend at 0 m, and is marked with dashed gray line. m of the measured and modeled diffusion profiles on the nucleation temperature, the temperature- Data are from spherulite Yell-24-13 (2050 mm diameter). Error bars shown by gray bars can be quantified in terms of two parameters: dependent growth rate (note that growth rates where larger than symbol size. (1) the concentration enrichment at the spheru- are set to specific values at nucleation and

648 www.gsapubs.org | Volume 43 | Number 7 | GEOLOGY Downloaded from geology.gsapubs.org on June 25, 2015 allowed to decrease following exponentially The results yield a cooling rate of 10–5.3 °C s–1 than smaller spherulites. Spherulite growth and decreasing radial growth from that tempera- across the glass transition temperature interval, diffusion modeling of type 2 compositional ture), and the cooling rate of the host lava. We with an uncertainty of +0.2 log units and -0.5 profiles suggests a time-integrated cooling consider a broad parameter space of temperature log units. Although this cooling rate is only rate of 10–5.2 ± 0.3 °C s–1 for the host lava during (800–400 °C), growth rate (0.01–100 mm hr–1), representative of the sample’s thermal history spherulite growth between 700 and 400 °C. That and cooling rate (10-3 to 10-7 °C s–1). In order to at the crossing of the glass transition tempera- estimate, based only on the temperature range simultaneously match both Rb and H2O, we find ture, the value should closely approximate the and style of spherulite growth, agrees with the that nucleation temperatures must be roughly slope of the temperature-time curve for a con- cooling rate of 10–5.3 °C s–1 measured by relax- 625 ± 75 °C and that the cooling rate must be on ductively cooled lava over the majority of its ation geospeedometry at the glass transition. the order of 10-5 °C s–1. We show the influence cooling (Manley, 1992). Consequently, we propose that spherulites act of varying the cooling rate and growth rate in Adopting the cooling rate of 10–5.3 °C s–1 from as in-situ recorders of thermal history and can Figures 2A and 2B, which are constructed by relaxation geospeedometry as the true cooling therefore be used to explore both the cooling simulating spherulite growth and expulsion of rate, we perform a similar analysis as above, and crystallization of lava.

Rb or H2O at a fixed nucleation temperature of treating instead growth rate and nucleation tem- 600 °C. Figure 2A also shows the influence of perature as unknowns (Fig. 2C). Model results SPHERULITE GEOSPEEDOMETRY varying the nucleation temperature. The cool- replicate Rb data using growth rates of ~0.4 To assess the viability of the modeling of ing rate and growth rate curves that bracket the mm hr–1 (at 600 °C), whereas the model curves concentration gradients around spherulites data provide a first-order estimate of the range replicate H2O data using growth rates of ~0.3 in relation to lava cooling and crystallization of conditions over which the spherulites might mm hr–1 (at 600 °C). Note that these growth kinetics, we compare our results with experi- have grown, provided that the assumptions built rate estimates are slightly slower than would be mental constraints on spherulite formation. Past into the model are reasonably valid. Rb concen- inferred from the earlier analysis (Figs. 2A and experimental studies have examined how felsic tration data indicate that spherulites grew at 1.2 2B), because the nucleation temperatures here minerals crystallize from melt in response to ± 0.6 mm hr–1 while the lava cooled at 10–5.1 ± 0.2 are generally hotter than 600 °C. The two sets variable degrees of undercooling (Fenn, 1977; –1 °C s (Fig. 2A). H2O data indicate that spheru- of data yield a nucleation temperature window Swanson, 1977; Baker and Freda, 2001; Castro –1 lites grew at 0.6 ± 0.3 mm hr while the lava of ~75 °C, with the H2O data suggesting some- et al., 2009). Spherulites were the stable form cooled at 10–5.4 ± 0.2 °C s–1 (Fig. 2B). The uncer- what hotter nucleation temperatures than the in experiments that experienced large under- tainties are estimates based on a detailed assess- Rb data. Although the two data sets should ide- coolings ranging from 100 to 400 °C. The liq- ment of Figure 2; see the Data Repository for ally yield the same growth rates and nucleation uidus temperature of the Pitchstone Plateau lava further treatment of model uncertainties. temperatures, we consider these values to be with 0.1 wt% H2O is estimated to be ~1000 °C As a test of the model results, and to further in reasonably good agreement given the many using Rhyolite-MELTS software (Gualda et constrain the conditions of spherulite growth, assumptions and uncertainties in the idealized al., 2012). If true, then our estimated spheru- we determined the cooling rate of the obsidian spherulite growth and diffusion models. lite nucleation temperatures of ~600–700 °C sample via relaxation geospeedometry using To summarize, the form of the measured con- are equivalent to undercoolings of 300–400 DSC measurements (see the Data Reposi- centration gradients indicates that the spheru- °C, values similar to those determined in com- tory). This method allows the cooling rate of lites grew with an exponentially decreasing parable experiments. Furthermore, quartz and a pristine glass sample to be inferred from the radial growth style over a temperature window alkali feldspar were experimentally estimated to hysteresis exhibited in the heat capacity ver- from ~700 °C to 400 °C. We interpret the range grow at rates of 0.1–6 mm hr–1, which are largely sus temperature curve obtained upon heating in spherulite sizes as a consequence of larger consistent with our estimates (Swanson, 1977; the natural glass across the glass transition. spherulites nucleating at hotter temperatures Baker and Freda, 2001; Castro et al., 2009).

100 10 700 A 10-3 Rb B 10-3 H2O C 100 Rb ) ) -1 -1 550 10-5 10 1 500 700 10 10-5 650 10 (% μm (% μm ε (%) -1 -1 ∆ ∆ 1 1 0.1 1 300

ε P 600 650 ε P -7 H O 10-7 10 0.1 700 2 550 0.1 600 650 0.1 0.01 100 102 103 104 105 102 103 104 105 102 103 104 Spherulite radius (µm) Spherulite radius (µm) Spherulite radius (µm)

Figure 2. A,B: Ratio of e (enrichment) to P∆ (propagation distance) for Rb (A) and H2O (B) as function of spherulite radius during exponentially decreasing growth. Solid black curves show lava cooling rates in °C s–1. Dashed curves are spherulite growth rates in mm hr–1; gray fields represent model uncertainty. Model results were generated assuming nucleation temperature of 600 °C at the specified growth and cooling rates. Gray curves in A labeled 550 and 650 show effect of changing nucleation temperature (in °C) on model results for 10-5 °C s–1. C: Model variations of e as function of spherulite radius for exponentially decreasing radial growth. Assuming cooling rate of 10–5.3 °C s–1, data are –1 –1 duplicated when growth rates are set to 0.4 mm hr and 0.3 mm hr for Rb and H2O, respectively, at 600 °C, which was then allowed to vary proportionally as function of nucleation temperature (tick marks in °C). Rb and H2O data are shown as white and black circles, respectively; black lines represent error bars where larger than symbol size.

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The cooling rate estimates determined inde- (grant EAR-1049829) and a National Park Service Lofgren, G., 1971a, Experimentally produced devit- pendently from both techniques described research permit (YELL-05678). rification textures in natural rhyolitic glass: Geological Society of America Bulletin, v. 82, herein can be interpreted to provide a first- REFERENCES CITED p. 111–124, doi:10.1130/0016-7606(1971)82 order constraint on the longevity of a large- Baker, D.R., and Freda, C., 2001, Eutectic crystal- [111:EPDTIN]2.0.CO;2. volume prehistoric lava. The duration over lization in the undercooled Orthoclase-Quartz- Lofgren, G., 1971b, Spherulitic textures in glassy and crystalline rocks: Journal of Geophysical which lava flows are mobile is, amongst other H2O system: Experiments and simulations: factors, controlled by how fast lavas cool. 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Watkins, J., Manga, M., Huber, C., and Martin, M., tion and analyzed with proper geologic context Gardner, J.E., Befus, K.S., Watkins, J., Hesse, M., 2009, Diffusion-controlled spherulite growth if applied to lava spreading. The veracity of and Miller, N., 2012, Compositional gradients in obsidian inferred from H2O concentration these estimates now awaits testing in environ- surrounding spherulites in obsidian and their profiles: Contributions to Mineralogy and relationship to spherulite growth and lava cool- Petrology, v. 157, p. 163–172, doi:10.1007 ments where active flows cease and the prod- ing: Bulletin of Volcanology, v. 74, p. 1865– /s00410-008-0327-8. ucts can be sampled for analysis. 1879, doi:10.1007/s00445-012-0642-9. Gualda, G.A.R., Ghiorso, M.S., Lemons, R.V., and ACKNOWLEDGMENTS Carley, T.L., 2012, Rhyolite-MELTS: A modi- Manuscript received 22 January 2015 Careful reviews by Eric Christiansen, Hugh Tuffen, fied calibration of MELTS optimized for silica- Revised manuscript received 12 May 2015 and anonymous reviewers greatly improved this man- rich, fluid-bearing magmatic systems: Journal Manuscript accepted 13 May 2015 uscript. This research was made possible by a grant of Petrology, v. 53, p. 875–890, doi:10.1093 from the National Science Foundation to Gardner /petrology/egr080. Printed in USA

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Geology

Spherulites as in-situ recorders of thermal history in lava flows

Kenneth S. Befus, James Watkins, James E. Gardner, Dominique Richard, Kevin M. Befus, Nathan R. Miller and Donald B. Dingwell

Geology 2015;43;647-650 doi: 10.1130/G36639.1

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Notes