Formation of Low-Δ18o Rhyolites After Caldera Collapse at Yellowstone, Wyoming, USA

Formation of low-δ18O rhyolites after caldera collapse at Yellowstone, Wyoming, USA

Ilya N. Bindeman John W. Valley Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA

ABSTRACT We present a new model for the genesis of low-δ18O rhyolites of the Yellowstone caldera based on analyses of zircons and individual quartz phenocrysts. Low-δ18O rhyolites were erupted soon after the massive caldera-forming Lava Creek Tuff eruption (602 ka, ~1000 km3) and contain xenocrysts of quartz and zircon inherited from precaldera rhyolites. These zircons are isotopically zoned and out of equilibrium with their host low-δ18O melts and quartz. Diffusion modeling predicts that magmatic disequilibria of oxygen isotopes persists for as much as tens of thousands of years following nearly total remelting of the hydrothermally altered igneous roots of the depressed cauldron, in which the alteration-resistant quartz and zircon initially retained their δ18O values. These results link melting to caldera collapse, rule out rapid or catastrophic magma–meteoric water interaction, and indicate wholesale melting rather than assimilation or partial melting.

Keywords: Yellowstone, zircon, oxygen isotopes, caldera, low δ18O.

INTRODUCTION rock major and trace element composition is simi- (Spicuzza et al., 1998b). We measured four to Meteoric water plays an important role in the lar to that of isotopically normal high-silica rhyo- seven aliquots of UWG-2 garnet standard on each genesis of ore deposits, explosive volcanism, and lites of precaldera lavas or lavas erupted simul- day of analysis. Nine analyses of NBS-28 quartz hydrothermal activity. Low values of δ18O un- taneously outside the caldera. yielded an average value of 9.55‰. Air abrasion of ambiguously provide evidence of this process. We report analyses of more than 200 indi- zircon fractions was done in a corundum mortar Although the alteration of rocks by meteoric water vidual quartz phenocrysts and zircon separates and normally took from 1 to 5 days. Selected ion is well understood, the incorporation of meteoric from the Lava Creek Tuff and younger intra- microprobe analyses were performed on 20-µm- water into magma remains controversial. Studies caldera lavas (602–70 ka). The ability to analyze diameter spots, 3 µm into prismatic crystal growth of low-δ18O volcanic and shallow plutonic mag- samples smaller than an individual quartz pheno- faces of zircon using a Cameca ims-4f at the Uni- mas in which tens of percent of their total oxygen cryst allows us to measure the phenocryst-scale versity of Edinburgh, Scotland, with a Cs+ primary must have been ultimately derived from meteoric (<1 mm) variability, which was not possible in beam and energy filtering (Valley et al., 1998). or surface water provide important insight into earlier studies. mechanisms of fluid-magma interaction (Fried- ZONING AND VARIABILITY OF δ18O IN man et al., 1974; Muehlenbachs et al., 1974; Criss SAMPLE PREPARATION AND INDIVIDUAL QUARTZ PHENOCRYSTS and Taylor, 1986; Taylor, 1987). The genesis of ANALYTICAL TECHNIQUE AND ZIRCONS low-δ18O magmas has been variously proposed to Individual phenocrysts of quartz and sanidine, Values of δ18O in individual obsidian spheres require: (1) direct, sometimes catastrophic mete- and obsidian spheres, were handpicked from (Table 11) within several lava flows were measured oric water addition into magma (Friedman et al., hand specimens or from crushed rock. Quartz was and are homogeneous (δ18O = ±0.3‰). These 1974; Hildreth et al., 1984); (2) more gradual, purified in fluoroboric acid to dissolve feldspars values are similar to those reported by Hildreth multistage assimilation of hydrothermally altered and glass. Special attention was paid to glass et al. (1984), demonstrating a remarkable homo- rocks (Taylor and Sheppard, 1986; Balsley and inclusions and phenocryst morphology to assure geneity of δ18O in magmas parental to low-δ18O Gregory, 1998); or (3) partial melting of the walls that all analyzed phenocrysts are primary and not lavas. In contrast, all low-δ18O rhyolites erupted of the magma chamber (Bacon et al., 1989). the result of precipitation from a secondary hydro- immediately after the Lava Creek Tuff are dis- Classic examples of low-δ18O lavas are found in thermal fluid. Zircons were separated from 20 kg tinctly variable in the δ18O values of individual the Yellowstone Plateau volcanic field, Wyoming, samples of rock using standard techniques of quartz phenocrysts (Fig. 1). Variations of 1.0‰– USA, where abrupt 18O depletions (accompanied density and magnetic separations. The extracted 1.5‰ are common among quartz phenocrysts by changes in Sr and Pb isotopes) in magmas 50–250 mg of zircons were purified from admixed from a single hand specimen, and variation is in occurred after caldera-forming eruptions at 2.0 and minerals and glass by cold HF. excess of 2‰ for the Canyon flow and the South δ18 δ18 0.6 Ma. The most dramatic drop in O values The University of Wisconsin CO2-laser mass- Biscuit Basin flow. The highest values of O (by ~5.5‰) of quartz phenocrysts occurred after spectrometer system (Valley et al., 1995) allows among quartz phenocrysts in the South Biscuit the last caldera-forming eruption, which deposited precise analysis of 1–2 mg samples with the Basin flow and the Canyon flow approach +6‰, the Lava Creek Tuff and formed the Yellowstone average reproducibility of better than ±0.10‰ 1 caldera (Hildreth et al., 1984, 1991). The >40 km3 (1 standard deviation, st.d.). Quartz was analyzed GSA Data Repository item 200079, Oxygen iso- δ18 δ18 tope ratios,Yellowstone caldera, is available on request of postcaldera, low- O lavas with O quartz by the rapid heating technique (Spicuzza et al., from Documents Secretary, GSA, P.O. Box 9140, (Qz) <3‰ appear exclusively within the Yellow- 1998a). For analyses of more reactive sanidine and Boulder, CO 80301-9140, [email protected], or stone caldera. It is remarkable that their whole- obsidian, we used an airlock sample chamber at www.geosociety.org/pubs/ft2000.htm.

Data Repository item 200079 contains additional material related to this article.

Geology; August 2000; v. 28; no. 8; p. 719–722; 4 figures. 719 LCT values more than 1‰ higher than either the cal- a SBB 6 Precaldera Quartz culated rim composition based on zircon abra- WY AC CF NBB sion, the ion microprobe analyses of zircon rims, SCL ML or zircon in equilibrium with their host obsidian. 4 Precaldera Zircons This finding suggests that even the smallest MBB grains of zircon are isotopically zoned and con- 2 DR tain higher δ18O cores. Such zoning would suggest that much of the total mass of zircons was in-

0 Quartz herited and that diffusive exchange with δ18 O (‰, VSMOW) O (‰, low- O magma was an important process, Zircons: 18

δ cores affecting a larger mass percentage of smaller -2 >105, >149 µm crystals because of their larger surface area/mass. bulk <53 µm It is remarkable that all studied zircons from -4 rim, calculated low-δ18O rhyolites younger than the Lava Creek Tuff show evidence of δ18O zoning, whereas all b abraded zircon fractions from precaldera and Equilibrium ∆ (Qz-Zrc) extracaldera rhyolite rocks are homogeneous.

O (Qz-Zrc), ‰ 2 The Scaup Lake flow and other voluminous 18

∆ rhyolites erupted after an ~200 k.y. quiescence 0 in eruptive activity inside the caldera show no significant heterogeneity in quartz phenocrysts, no isotopic zoning in zircon, and no ∆(Qz-Zrc) 600 550 500 450 200150 100 Age, k.y. disequilibria (Fig. 1). Two processes operated to reequilibrate Figure 1. Oxygen isotope ratios of larger and smaller zircons (Zrc), measured compositions of zircon, quartz, and magma: (1) diffusive oxygen abraded cores, and calculated compositions of rims in low-δ18O rhyolites from Yellowstone caldera,Wyoming. Boxes show ranges of quartz (Qz) δ18O values.These data show (1) variability exchange, and (2) solution reprecipitation on of δ18O values among individual quartz phenocrysts, especially for low-δ18O lavas erupted after inherited cores and new overgrowth in equilib- Lava Creek Tuff (LCT) at 550–450 ka; (2) variability of different size zircons and air-abraded rium with the melt. Oxygen diffusion coefficients cores; (3) disequilibria between average quartz and zircon in most low-δ18O lavas. CF—Canyon in zircon in Yellowstone water-bearing magmas flow, DR—Dunraven Road flow, SBB, MBB, and NBB—South, Middle, and North Biscuit Basin are on the order of 10–17–10–18 cm2/s (water- flows, SCL—Scaup Lake flow, ML—Mallard Lake flow, WY—West Yellowstone flow, AC—Aster Creek flow. Ages are from Gansecki et al. (1996, 1998) and Obradovich (1992). Plotted ages of saturated experiments at 750–850 °C, Watson post-Lava Creek Tuff lavas are not exact to prevent overlap of data points. VSMOW is Vienna and Cherniak, 1997), making it possible to equili- standard mean ocean water. Note that oxygen isotopes permit making of stratigraphic distinc- brate a typical 50-µm-radius zircon grain after a tions. Our results show that three separate outcrops (hills), southern, middle, and northern, few tens of thousands of years (Fig. 2). Sluggish along Firehole River of Upper Geyser Basin, defined earlier as single unit of Biscuit Basin flow, represent three independent lava flows, which have distinctly different isotope compositions of rates of zircon growth and dissolution in magmas –15 –17 phenocrysts. We call them South, Middle, and North Biscuit Basin flows. Only Middle Biscuit (10 –10 cm/s, at the same temperatures and Basin was sampled by Hildreth et al. (1984). NBB and SBB flow ages were not determined. rate of cooling or heating of 0.01 °C/yr, Watson, 1996) suggest that it may take at least tens of thousands of years to form a 50-µm-thick zircon similar to normal-δ18O quartz in precaldera rocks cons have exchanged oxygen as a function of sur- overgrowth. U-Th disequilibria and U-Pb dating of and in Lava Creek Tuff. Despite similar whole- face area/mass. To test these hypotheses, larger zircon cores show that zircon may reside for >200 rock chemical compositions, low-δ18O lavas con- zircons (>105 µm, or >149 µm) were air abraded, k.y. in similar rhyolitic systems (e.g., Reid et al., tain fewer quartz phenocrysts. The external mor- and the residual cores were analyzed for δ18O. 1997; Brown and Fletcher, 1999). Several lines phology and internal structure of quartz (as shown Cores of zircons from low-δ18O rocks have δ18O of evidence support zircon recycling as the main by cathodoluminescence imaging) reveal resorp- values that are as much as 2.5‰ higher than mechanism of Zr mass balance: relatively short tion and solution reprecipitation. those of the smallest zircons (Fig. 1; Table 1). time scales of volcanic melting-crystallization Another characteristic feature of low-δ18O The highest δ18O values in abraded cores range phenomena, similar compositions of most Yellow- lavas is oxygen isotope disequilibrium between from 1.8‰ to 3.7‰, approaching the δ18O of zir- stone rhyolites (low-δ18O or normal) including quartz and zircon (Zrc). Values of ∆(Qz-Zrc) cons in precaldera rhyolites (+4‰) and suggest- relatively constant Zr concentrations (Hildreth between individual quartz phenocrysts and bulk ing that large zircons are inherited from the pre- et al., 1991, 1984), and thus similar zircon satura- separates of zircon are as much as a few per mil caldera magma or country rock. tion temperatures. smaller than the equilibrium fractionation of 2‰ Successive air abrasions were made of abundant for magmatic temperatures (Fig. 1). Most pre- zircons in the Middle Biscuit Basin flow to moni- OXYGEN ISOTOPE REEQUILIBRATION caldera, extracaldera, and intracaldera rhyolites tor δ18O after each step of rim removal (Fig. 2). Diffusive exchange of oxygen isotopes in erupted more than 150 k.y. after the Lava Creek Optical measurements and weight loss were used quartz and zircon was modeled using equation Tuff do not show such disequilibrium and have to determine the decrease in the average radius of 6.18 of Crank (1975) for diffusion in a normal ∆(Qz-Zrc) ~2‰. zircons that became ellipsoidal to nearly spherical. δ18O sphere surrounded by an infinite reservoir of Differences of as much as 0.7‰ in δ18O exist Values of δ18O are plotted as a function of average low-δ18O magma. Calculations were made at 750 between the smallest (<53 µm diameter) and radii of residual zircons. The resulting profile and 850 °C, the latter consistent with Fe-Ti oxide largest (>149 µm) zircons from low-δ18O lava shows a steep gradient of ~1‰ per 2–3 µm at grain temperature estimates for Yellowstone’s high- flows erupted after the Lava Creek Tuff. These boundaries while ∆(core-rim) approaches 5‰. silica rhyolites (Hildreth et al., 1984) and with our differences suggest that smaller zircons grew It is important to note that the bulk analyses of zircon saturation-temperature estimates. Measured later in a magma of changing δ18O, or that zir- the smallest zircons (<53 µm) yielded δ18O zircon zoning profiles (Fig. 2) are between pro-

720 GEOLOGY,August 2000 Figure 2. Zoning profile of δ18O in 105 µm zircons (52.5 µm a 7 850oC measured radius) for magma residence times in Middle Biscuit core Initial Qz 4 mm Qz, cores ranges: 6 Basin flow based on successive air abrasions (thick 0.5 k .y. Quartz 4 2.74‰ 1 mm Qz, bulk curve). Left-oriented horizontal marks on vertical lines at SBB 40–50 µm are measured core compositions; horizontal 0. 77 ‰ 5 Zircon 1.1‰ 100 µm Zrc, CF marks at bottom are rim compositions. Rim composition 5 k .y. Initial Zrc cores was calculated by mass balance using measured δ18O of 4 18 2

δ VSMOW core, O of preabrasion bulk, and percentage of abra- 10 k .y. 50 µm Zrc, bulk ‰ 3 MBB sion (percent of material removed). Thin curves indicate O Overgrowth: , VSMOW) 0.0 1 ‰ 18 50 µm Zrc, growing calculated diffusion profiles (see text). Numbers (in per δ -15 ‰ at 10 cm/s rate mil) are δ18O of whole grain, calculated (italic) and 2 0 25 k .y. measured (bold). Striped box is measured compositional O ( Equilibrium Qz, final range of host obsidian. Note steep isotopic gradients pre- -0.78-0.78‰ 1 18 DR dicted for residence times of 500–10 000 yr. Compositions δ 0 of rims, based on analyses of prismatic faces of four indi- -2 vidual zircon crystals by ion microprobe, are shown as Equilibrium Zrc, final filled circle (±1σ). δ18O values of measured and predicted -1 rims are 2‰–4‰ lower than those of bulk of zircon, b 0 20 40 rim Equilibrium ∆ (Qz-Zrc)

abraded cores and smallest measured zircons, but rims ‰ 2 are in equilibrium with host obsidian (see Fig. 1). Radius (µm) 0 ∆(Qz-Zrc) VSMOW—Vienna standard mean ocean water. bulk (Qz-Zrc)

∆ -2 ∆(Qz-Zrc) cores files calculated for 500 and 5000 yr at 850 °C. result suggests that Middle Biscuit Basin magma -4 -2-10 1 2 3 4 5 18 Figure 3 shows the measured δ O ranges of existed long enough to reequilibrate quartz, but log 10(years) zircon and quartz phenocrysts and the calculated not long enough to reequilibrate zircon. There- Figure 3. A: Calculations of isotopic exchange exchange curves at 850 °C. Crystals of observed fore, the observed heterogeneities in quartz and of normal zircon (Zrc) (δ18O = +4.0‰, thick lines) sizes were chosen and the δ18O values of both the zircon also indicate that their residence in low- and quartz (Qz) (+6.0‰, dotted lines) crystals core and the bulk crystal were computed. The δ18O magma was on the order of 102–103 yr at (sizes are diameters) immersed into low-δ18O water-saturated diffusion coefficients of Watson 850 °C or 103–104 yr at 750 °C. melt (0.0‰) predict period of quartz-zircon disequilibrium for residence times to 50 k.y. B: Up and Cherniak (1997) were used for zircons. We to 5.5‰ disequilibrium in ∆(Qz-Zrc) exists dur- also consider the case in which new overgrowth of ORIGIN OF LOW-δ18O MAGMAS ing time intervals to 50 k.y. See Figure 1 for unit a rim in equilibrium with the melt occurs simul- Given the slow rates of oxygen diffusion, high abbreviations and text for discussion. Each taneously with isotopic exchange by diffusion. closure temperatures, and slow crystal growth of core is innermost 50% of crystal’s radius. Over- growth implies constant rate of deposition of For quartz, the diffusion coefficients of Farver and zircon (Watson and Cherniak, 1997; Watson, new rim (δ18O = –1‰) in equilibrium with melt at Yund (1991), Elphick et al. (1986), Giletti and 1996), the newly discovered isotope heteroge- rate of 10–15cm/s (Watson, 1996). Yund (1984), and Dennis (1984) all yield similar neity both in zircon and quartz (Figs. 1 and 2) results for water-saturated oxygen diffusion at rules out a rapid (<100 yr) or catastrophic (a few 850 °C. A decrease in temperature to 750 °C hours to days) process such as phreatic meteoric lar argument applies to quartz. Lower abundance, increases the time of equilibration (~10 times) water-magma interaction at the time of eruption smaller sizes, and resorbed morphology of quartz without changing the relative position of the or posteruptive oxygen exchange or overgrowth. in low-δ18O as opposed to normal lavas is consist- curves (owing to the similar activation energy for The exchange calculations herein also restrict the ent with a relict or xenocrystic origin. The source oxygen diffusion in quartz and zircon). maximum period over which such heterogeneity rock may have achieved low-δ18O values of ~0‰ The curves of δ18O equilibration between might be preserved. or lower through hydrothermal alteration of vol- cores of quartz, cores of zircon, and melt show Assimilation or partial melting of wall rocks canic glass, matrix, and feldspars, but quartz and that maximum disequilibrium and core-rim hetero- as a mechanism of genesis of tens of cubic kilo- zircon phenocrysts are alteration resistant (Valley geneity would occur 500–5000 yr after normal meters of magmas with δ18O values of ~0‰ et al., 1994; Criss and Taylor, 1986). Any initial δ18O crystals became immersed in low-δ18O would require mixing nearly equal proportions δ18O variations in melt anneal several orders of melt. During this transition, the predicted of Lava Creek–type (+6‰) magma and unusu- magnitude faster (even by diffusion) than varia- ∆(Qz-Zrc) shows wide variations similar to our ally low-δ18O magma or rock (–5‰ to –15‰) tions in quartz and zircon (Zhang et al., 1991). measurements in low-δ18O lavas (Fig. 1 and (e.g., Hildreth et al., 1984, 1991). Such low-δ18O Caldera collapse created conditions favorable Fig. 3, bottom). Although the natural quartz crys- rocks (or low-δ18O xenocrysts) have not been to remelting of the hydrothermally altered roots tals are larger, the zircons need 100 times longer found at Yellowstone despite detailed searching. of the cauldron and thus for the genesis of low- to reequilibrate with the magma by diffusion. As Furthermore, it would be difficult to achieve the δ18O magmas (Fig. 4). The estimated vertical a result, intervals with the maximum difference observed bulk textural, chemical, and isotopic drop due to caldera subsidence following erup- of δ18O between cores of larger crystals of quartz homogeneity of low-δ18O lavas by mixing of tion of the Lava Creek Tuff was at least ~500 m and zircons, and the bulk of their smaller crystals, two viscous rhyolites, given the time considera- (Hildreth et al., 1984), and large blocks may have are offset in time. For example, variability of tions already discussed. been brought down nearer the magma chamber. δ18O in quartz in the South Biscuit Basin flow The critical observation for understanding of Following the views of Smith (1979) and Smith and the Canyon flow (Table 1) indicates a shorter the genesis of the low-δ18O Yellowstone magmas and Christiansen (1980) that most magma re- time of immersion in magma before eruption is the δ18O zonation and variability, which indi- mains in the batholithic-scale magma chambers (~500 yr at 850 °C) because the quartz had not cates that most of the mass of zircon (even the beneath the Yellowstone caldera, caldera subsi- achieved homogeneity, whereas the zircons are smallest crystals) in low-δ18O magmas was dence would have juxtaposed the hydrothermally already zoned. In contrast, quartz phenocrysts in inherited from a higher δ18O source, similar to altered and water-rich intracaldera roots of the the Middle Biscuit Basin flow and the Dunraven normal precaldera rocks. Dispersal of old zircons down-dropped block with significantly hotter Road flow show only limited remaining hetero- in a new magma could occur by near total fusion underlying magma. Most 18O-depleted source geneity, whereas zircons are highly zoned. This of source rocks containing these zircons. A simi- rocks were originally located closer to the

GEOLOGY,August 2000 721 Figure 4. Model of genesis Plateau Volcanic Field: Journal of Petrology, δ18 of low-δ18O rocks by total Omelt = 0‰ v. 32, p. 63–138. fusion of hydrothermally Muehlenbachs, K., Anderson, A.T., and Sigvaldsson, altered caldera roots after Qz G.E., 1974, Low-δ18O basalts from Iceland: caldera collapse. Quartz Zrc Geochimica et Cosmochimica Acta, v. 38, (Qz) and zircon (Zrc) were p. 577–588. not altered by hydrother- Obradovich, J.D., 1992, Geochronology of the late Ceno- mal fluids and retained zoic volcanism of Yellowstone National Park and precaldera higher δ18O adjoining areas, Wyoming and Idaho: U.S. Geolog- values. Purely conductive ical Survey Open-File Report, v. 92–408, p. 1–45. heating of ∆T = 200–300 °C Reid, M.R., Coath, C.D., Harrison, M.T., and of depressed cauldron McKeegan, K.D., 1997, Prolonged residence with vertical dimension of o C times for the youngest rhyolites associated with 230 238 ~500 m, proceeding at 800 Long Valley Caldera: Th- U ion microprobe thermal diffusivity of 0.005 δ18 dating of young zircons: Earth and Planetary Sci- Omelt = +6‰ cm2/s, would yield heating ence Letters, v. 150, p. 27–39. on order of 10–2 °C/yr Smith, R.B., and Christiansen, R.L., 1980,Yellowstone (Carslaw and Jagger, 1959). At this heating rate from Tsolidus to Tmax (700–850 °C) zircon growth Park as a window on the Earth’s interior: Scien- rate is 10–16 to 10–17 cm/s (Watson, 1996). 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