Downloaded from geology.gsapubs.org on October 1, 2010 Volcanic biotite-sanidine 40Ar/ 39Ar age discordances refl ect Ar partitioning and pre-eruption closure in biotite

John M. Hora1,2, Brad S. Singer1, Brian R. Jicha1, Brian L. Beard1, Clark M. Johnson1, Shan de Silva3, and Morgan Salisbury3 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA 2Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Geochemie, Goldschmidtstraße 1, 37077 Göttingen, Germany 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA

ABSTRACT nic biotite (148 single-crystal total fusions [TF] The 40Ar/39Ar radioisotope system is widely used to date eruption and cooling of volcanic and 25 incremental heating experiments [IH] tephra–marker horizons that commonly provide the only means of correlating and assigning using both laser and furnace) and sanidine (251 numerical ages to stratigraphy in which they are contained. This chronometer bridges the TF and 53 laser IH) (Table DR1 in the GSA Data gap between 14C and longer-lived isotopic systems that are too imprecise for dating young Repository1). All samples are from rhyolitic and samples. However, 40Ar/ 39Ar ages obtained from coevally erupted biotite and sanidine do not dacitic magmas erupted from the Andean Cen- always match. Here, we use an independent chronometer, 238U-230Th disequilibrium, to dem- tral Volcanic Zone, range in age from 5.6 Ma to onstrate that 40Ar/ 39Ar age disparity is not caused by differences in pre-eruption crystallization 40 ka, and represent a variety of eruptive styles times. Our fi ndings indicate that the presence of extraneous 40Ar in biotite, and its absence in and volumes (~800 km3 ignimbrites to 4 km3 sanidine, may result from violations of two assumptions implicit in 40Ar/ 39Ar geochronology domes): fi ve units from the Altiplano-Puna vol- on volcanic samples: (1) Prior to eruption, minerals are devoid of 40Ar due to rapid loss to an canic complex (APVC) (de Silva, 1989), three “infi nite reservoir” such as the atmosphere, and (2) closure to volume diffusion is geologically units from Cerro Galan, and lava domes associ- instantaneous and coincident with eruption. We propose a mechanism whereby the presence of ated with Parinacota volcano (Hora et al., 2007). extraneous Ar in certain minerals is explained by the relative sequence of four events in a mag- Incremental heating data indicate that sanidine matic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic typically yields younger and more precise ages equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. than biotite (Fig. 1A), and biotite age determi- These data have potentially far-reaching implications for studies that depend on geochrono- nations are not in agreement with one another logical data, necessitating re-evaluation of interpretations based solely on biotite with no inde- (Fig. 1B). The biotite grains used in our study pendent age control, particularly in young samples where the effects are most pronounced. were large (typically >1 mm diameter, >200 µm thick; Fig. DR1 in the Data Repository), pre- INTRODUCTION ing. Sanidine is a proven chronometer, whereas cluding 39Ar recoil-loss as a mechanism (Paine et In the 40K-40Ar system, radiogenic daughter biotite, despite continued use, is frequently al., 2006) for anomalously old biotite ages. The nuclide accumulation begins (i.e., diffusive loss complicated by extraneous 40Ar that manifests crystals are sourced from apparently nonaltered of 40Ar ceases) at temperatures that are rela- as anomalously old ages. Extraneous Ar is pres- rocks in one of the driest environments on Earth, tively low compared to other isotopic systems ent in numerous phases in varied geologic set- making parent isotope redistribution by weath- (e.g., Rb-Sr, Sm-Nd, U-series), thereby record- tings (e.g., metamorphic rocks)—and indeed ering (Roberts et al., 2001) or the presence of ing the cooling of volcanic units. 40Ar/39Ar ages some aspects of the present discussion (i.e., intergrown alteration phases (Smith et al., 2008) have been used in this capacity to calibrate por- partitioning) may be more broadly applicable— unlikely causes of the excess age. tions of the geologic time scale (Gradstein et however, here we focus on behavior of Ar in al., 2004), correlate stratigraphy (Smith et al., magmatic systems. Biotite and sanidine coexist Interpretation 2006), delineate geomagnetic polarity reversals in dacitic-rhyolitic magmas where this chro- Whereas Singer et al. (1998) resolved radio- (Singer et al., 2005), and constrain the pace of nometer is commonly used and age discordance genic, inherited, and trapped 40Ar components hominid evolution and migration (Leakey et al., is prevalent (e.g., Bachmann et al., 2007; Spell in a heterogeneous mixture of plagioclase phe- 1998; Swisher et al., 1994) by providing numer- and Harrison, 1993). Whereas previous studies nocrysts and xenocrysts of contrasting age, ical ages that bracket the geological events of have established that age discordance occurs, biotites in general rarely exhibit saddle-shaped interest. These ages are fundamental to our the number of independent analyses for a given spectra (Lanphere and Dalrymple, 1976) 40 understanding of the recent geological and cli- phase within any single sample has been limited. associated with excess Ar ( Arxs). More com- 40 matological evolution of Earth and life on it. Ar Consequently, it has been diffi cult to gauge the monly, high-T Arxs is substantially homog- geochronology’s expanding role in dating Neo- magnitude and reproducibility of apparent age enized with radiogenic 40Ar* upon analysis gene events has largely been driven by analyti- differences, or establish whether differences in (Renne, 1995). This situation is undetectable cal advances that allowed increasingly precise pre-eruption crystal residence times can account using the inverse isochron method, explaining 40Ar/39Ar measurements to be made on young for the discordance. Our goals here are to more why most of our biotite IH yield atmospheric samples (Chen et al., 1996; Hu et al., 1994; thoroughly document biotite 40Ar/39Ar age anom- 40Ar/ 36Ar intercepts. Furthermore, biotite, like Renne et al., 1997). To avoid fl awed interpreta- alies, independently determine pre-eruption resi- other hydrous sheet silicates, is unstable during tions, evaluations of geological reasons behind dence times, and offer a conceptual explanation. apparently precise yet inaccurate data are needed, because systematic errors of constant 40Ar/ 39Ar GEOCHRONOLOGY 1GSA Data Repository item 2010256, compris- absolute magnitude have increasing relative ing Figures DR1−DR4, Tables DR1−DR5, and Text File DR1, is available online at www.geosociety contribution when applied to younger samples. Results .org/pubs/ft2010.htm, or on request from editing@ Biotite and sanidine are commonly used We present the largest currently available geosociety.org or Documents Secretary, GSA, P.O. K-rich mineral phases in K-Ar and 40Ar/39Ar dat- database of 40Ar/ 39Ar analyses on coeval volca- Box 9140, Boulder, CO 80301, USA.

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600 1.2 A Parinacota rhyodacite dome in the context of these problems, and may mask Parinacota rhyodacite domes sample PAR-03-26 true intracrystal Ar distributions. In this study, (PAR-03-18 and PAR-03-26) 500 Eruption ages 46 ± 2 ka we use integrated and total fusion ages, as these Biotite 363.7 ± 18.8 ka 1.0 400 are indicative of bulk Ar isotope composition. 168 ka Biotite Equiline (>350 ka) The magnitude of the age difference between Hbl 300 coeval biotite and sanidine (Δage) is limited 0.8 Biotite 278.7 ± 13.7 ka Th) Sanidine Plag 47.7 ka

200 (<600 k.y.; Fig. 1C), implying an upper limit for 232 Hbl Glass the process responsible. WR Th/ Apparent age (ka) 100 0.6 230

Sanidine 47.4 ± 2.1 ka ( 238U-230Th DISEQUILIBRIA 0.8 1.0 0 40 39 10 20 30 40 50 60 70 80 10090 Previously, Ar/ Ar age differences were Titanite & Cumulative 39Ar released (%) 0.4 magnetiteMagnetite attributed to differing ages of the crystals them- Equiline Old xtls B All Parinacota selves, i.e., differing pre-eruption residence oung xtls Sanidine t = 0 Y rhyodacite domes times (Bachmann et al., 2007) that have been 0.2 documented in large-volume silicic systems 0.2 0.4 0.6 Young xtls Eruption age 238 232 (Charlier and Zellmer, 2000; Reid et al., 1997). ( U/ Th) Old xtls There is a paucity of crystal residence time data for small-volume (<10 km3) silicic systems Figure 2. Parinacota dome 238U-230Th crys- like those that fed the Parinacota rhyodacite tallization ages. On an equiline diagram, a Biotite short-lived burst of crystallization produces domes, which show similar biotite-sanidine Relative probability a linear isochron. Protracted crystallization age discrepancies previously observed in large (cf. Charlier and Zellmer, 2000) produces systems. Because of their youth, the Parinacota wedges bounded by isochrons indicating onset and cessation of crystallization (inset). 0 0.1 0.20.3 0.40.5 0.6 0.7 lava domes present a unique opportunity to both Age (Ma) Parinacota dome minerals span ~120 k.y. measure the magnitude of biotite age excess and bounded by isochrons of 168 and 47.7 ka; the C Partially degassed Pastos Grande ig. independently determine crystallization ages of younger isochron is in close agreement with biotite phenocrysts Tara ignimbrite the phases involved via 238U-230Th disequilibrium sanidine-determined 40Ar/ 39Ar eruption ages and antecrysts Juvina high-SiO2 ig. Chuhuilla ignimbrite (applicable to samples younger than 300 ka). (46 ± 2 ka). Ellipses indicate 2σ external re- Guacha ignimbrite Eruption age (sanidine) producibility. WR—whole rock; Plag—plagio- Cerro Galan north 238U-230Th data (Table DR2) indicate that both Cerro Galan east clase; Hbl—hornblende. The relative position C. Galan intracaldera biotite and sanidine predate eruption by up to of any given phase within the wedge gives an Parinacota domes ~120 k.y. (Fig. 2), but sanidine shows no evi- approximation of its (bulk) age—long-lived dence of this prior history in its 40Ar/39Ar age. crystals have cores that are older and rim This implies substantial differences in partition- ages that are younger than this “integrated” ‘True’ xenocrysts age. With one exception, both sanidine and Relative probability incorporated during ing and/or degassing behavior for Ar among eruption biotite are ~120 k.y. older than eruption, as sanidine and biotite prior to eruption; conse- they plot near the older bounding isochron. -0.25 0 0.25 0.5 0.75 1.0 1.25 1.5 1.75 quently a valid model of excess age in biotite ∆age (m.y.) (Bt-San) must also explain why equally old sanidine

40 39 40 Figure 1. Ar/ Ar age discordance. A: Rep- crystals are devoid of extraneous Ar. of which are proportional to pAr (Table DR3). resentative biotite and sanidine 40Ar/ 39Ar age Solubility in melt increases from basalt to spectra from the same Parinacota rhyodacite PARTITIONING rhyolite by approximately an order of magni- dome sample. Despite extreme discordance Because most biotite 40Ar/39Ar ages are older tude (Carroll and Stolper, 1993), from <0.1 to between analyzed aliquots, all increments of any individual step-heating experiment are than the sum of independently known eruption 0.8 ppm/barAr. Therefore, the capacity of melts concordant. B: Relative probability plot of age and pre-eruption crystal residence time con- to act as Ar transfer media between mineral sanidine (black symbols) and biotite (gray) strained by 238U-230Th disequilibria, in situ–pro- sources and the atmospheric sink decreases with 40Ar/ 39Ar apparent ages from Parinacota duced inherited Ar (40Ar ) cannot be the sole increasing SiO content and pressure. dome samples. Each dot represents a single K 2 reason for discordance; externally sourced 40Ar Because Ar solubility (S ) is proportional to incremental heating or total fusion; hori- xs Ar 40 zontal lines indicate ±2σ uncertainty; >90% must also be present. Noble gasses are normally pAr, the maximum amount of Arxs, and conse- of analyzed sanidine is concordant with thought to be very incompatible in minerals and quently the predicted maximum age excess (txs), the sanidine analysis shown in A, consti- melts (Kelley, 2002). Transit of Ar through a would, at equilibrium without chemical potential tuting a well-defi ned probability maximum. magmatic system is commonly viewed as uni- gradients caused by active degassing, depend Biotites form a broad distribution skewed toward older apparent ages. Stratigraphy directional: (1) sourced in K-rich minerals, (2) on total pressure (Ptotal), proportion of Ar in the constrains dome age to <116 ka and >30 ka, followed by rapid diffusion into the surrounding ambient fl uid/vapor phase (XAr), and its (steady- indicating that sanidines provide accurate melt, which approximates a zero-concentration state) isotopic composition (40Ar/36Ar) as eruption ages. C: Relative probability distri- boundary (Baxter et al., 2002), and (3) degas- butions for Δage from all studied units, al- sing to its ultimate sink, the atmosphere. This ⎛ lowing comparison irrespective of eruptive λ PXS =+1 ⋅ total Ar Ar,Bt age; biotite ages are consistently older. model is valid at low pressures during and after txs,Bt ln ⎜1 λ ⎝ λλ+ ′ ⎡ 40 K⎤ eruption and implies that Ar concentrations in ee⎣ ⎦Bt minerals and the melt are low. Elevated partial ( 40036Ar Ar) − 295. 5⎞ vacuum heating, and gas release occurs via pressure of Ar (p ) invalidates the zero-concen- ⋅ ⎟. (1) Ar ( 40 36 ) + dehydration reactions instead of volume dif- tration boundary approximation, and equilib- Ar Ar 1 ⎠ fusion (Harrison et al., 1985; Lo et al., 2000). rium concentrations of Ar in minerals are dic- Concordance of step ages within a single tated by their relative solubilities: vapor phase Equation 1 derivation and values of decay experiment (Fig. 1A) is therefore not surprising >> melt >> biotite > sanidine (Kelley, 2002), all constants (λ) are given in the Data Repository.

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Incorporation of atmospheric 40Ar (i.e., 40Ar/ 36Ar diffusion domain size, in Fig. DR3). For faster extremely fast cooling rates (Fig. 3A). We con- = 295.5) does not result in excess age, because pre-eruption dT/dt, amphibole and biotite can sider post-eruption cooling of a lava or tephra this component is subtracted during data pro- partially close to diffusion at higher tempera- as geologically instantaneous, and only pre- cessing. At magma storage depths, the impact tures, similar to pre-eruption storage for rhyo- eruption dT/dt enters into the determination of 40 of high pAr on age may be twofold: More Ar lites, whereas sanidine remains open, even at Tc. Determination of the time and Ar concen- is partitioned into the minerals, and likeli- tration at which the transition from partial loss hood of complete atmospheric equilibration to retention occurs is constrained by boundary is decreased. Observed biotite age excesses A Time needed to cool 300 °C (k.y.) conditions on degassing rate and timing, T(t), 10,000 1000 100 10 require only modest pressures (1–3 km depth), and starting conditions in equation 1 (Fig. DR2). with X approximately two times atmospheric 1000 Andesitic Regardless of the model used, retention of extra- Ar cones concentrations (~2% of fl uid phase), and can Dacitic neous Ar in biotite, in contrast to the apparently occur in magmas that have been substantially ignimbrites Dacite and effi cient degassing of Ar in sanidine, implies that 800 (though not completely) equilibrated with the Rhyolitic rhyodacite the relative order of four events is important in 40 ignimbrites domes atmosphere (~98% atmospheric Ar, 2% Arxs; determining whether any phase (i.e., biotite and Table DR4). Hornblende potentially hornblende) yields anomalously old If preserved, partitioned Ar in minerals would 600 ages: (1) crystallization, (2) closure with respect be present in all minerals, according to relative to volume diffusion, (3) degassing and equili- Phlogopite solubilities. Predicted age excess would be bration with atmospheric Ar, and (4) quench- 400 approximately 3.8 times smaller in sanidine than Closure temperature (°C) Sanidine ing of the system by eruption (Fig. 3B). If the in biotite due to lower solubility and higher stoi- order of events changes, no age excess will be chiometric K content. Because no age excesses recorded by the mineral. 200 are observed in sanidine, any mechanism that 0.01 0.1 1 10 100 Thermometry from several ignimbrites from Δ 40 explains age must lock in Arxs in biotite, Cooling rate (°C/k.y.) the Altiplano-Puna volcanic complex indicates ≈ while allowing it to escape from sanidine once storage and eruption at T Tc,biotite (Lindsay et al., B T T T T the system becomes open to the atmosphere and C, Bt If C,biotite > eruption > C,sanidine 2001). For Parinacota domes, Fe-Ti oxide ther- reaches the low pressures of eruption. T mometry yields T = 792 ± 30 °C (Table DR5; eruption Fig. DR4). If biotite and hornblende are closed T CLOSURE TEMPERATURE C, San or partially closed prior to eruption, some of the 40 40 As the system departs from equilibrium, in situ–produced ArK and partitioned Arxs

40 Temperature the proportion of maximum partitioned Arxs may be “locked” in those phases; in contrast, that is retained in biotite is dictated by kinetics sanidine begins accumulating 40Ar only after and depends on the shifting balance between Biotite coincident eruption and closure (Fig. 3B). net partial loss and net partial retention of Ar Sanidine Because volcanic groundmass and glass are (Fig. DR2). At high T, loss depends largely liquid at the time of eruption, these components on the chemical potential difference of Ar D / P behave similarly to sanidine in terms of rapid between crystal and melt (i.e., degree of melt diffusive equilibration with the atmosphere and degassing), because diffusion is fast relative likewise give reliable eruption ages, although to radiogenic ingrowth. Alternating episodes t Δage Time they are more susceptible to post-eruption alter- c, Bt age of volatile degassing and recharge <1 yr prior sanidine ation (hydration in particular) than sanidine. 40 Figure 3. Pre-eruption closure: a mechanism to eruption are recorded by Arxs in Mount St. for retention of partitioned 40Ar . A: Closure Helens plagioclase (Layer and Gardner, 2001) xs DISCUSSION temperature (T ) as a function of cooling rate c Δ and by hydroxyl zoning in apatite from Cerro (dT/dt); shaded bands approximate mineral The magnitude of age appears to be deter- Galan (Boyce and Hervig, 2008). Although H2O closure intervals—i.e., the range of diffusivi- mined by the degree of isotopic equilibration is approximately one order of magnitude more ties between 10 × DTc and 0.1 × DTc. Scale with the atmosphere before biotite closure, and spans the range of crustal storage condi- soluble in silicate melts relative to CO2 and Ar whether eruption temperature is lower than bio- (Carroll and Stolper, 1993), the late loss of vola- tions. Boxes indicate estimated pre-eruption tite T . Consequently, extraneous 40Ar contents T and dT/dt for volcanic samples. B: Sche- c –13 –11 tiles that is implied by apatite zoning (Boyce matic time series in a model system where of 10 to 10 mol per gram of biotite limit and Hervig, 2008) suggests that elevated p age excess to <600 k.y. (Fig. 4). Here we show Ar Tc,biotite > Teruption > Tc,sanidine, shown for the end- 40 may persist until shortly prior to eruption. member case of inherited ArK only—no that the discrepancy is not analytical; rather, 40Ar . As pre-eruption cooling at depth pro- As the magma cools, the temperature-depen- xs instrumental precision has approached the level ceeds, biotite passes through its closure in- dent rate of diffusion determines the transition terval, initiating gradual increase in daugh- where relatively small systematic differences are between loss and retention regimes. Closure ter nuclide retention (increase in daughter/ detected among phases that compose magma. temperature (Tc) is a function of phase-specifi c parent ratio [D/P] analogous to the case of a These data underscore the reliability of sanidine parameters that describe diffusivity, the size and cooling pluton) before eruption. Its effective and glass analyses, but they should not be taken time of closure (t ) refl ects early retention. shape of the diffusion domain, and cooling rate c as a blanket indictment of 40Ar/39Ar geochronol- Eruption abruptly decreases temperature. (dT/dt) (Dodson, 1973). Due to large differences Sanidine passes through its closure interval ogy on biotite; adverse effects may be undetect- in dT/dt found in nature, widely reported “plu- equally abruptly, and is fully retentive of Ar able in samples older than Cretaceous age, or shortly after eruption (the usual assump- tonic” Tc are inappropriate for pre-eruption con- where Teruption > Tc,biotite. Rather, we urge caution ditions of most volcanic systems, which accu- tion in 40Ar/39Ar geochronology on volcanic and point to an instance where advances in ana- 40 rocks). With additional partitioned Arxs, mulate, cool, and erupt on shorter time scales. biotite has initial D/P > 0, and Δage will be lytical precision are outpacing our understand- In Figure 3A, we show the dependence of Tc larger relative to the inheritance-only case ing of processes that are recorded in biotite and on dT/dt (full results, including dependence on (see also Fig. DR2 [see footnote 1]). sanidine respectively. As techniques improve,

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10,000 raphy: Geology, v. 36, p. 63–66, doi: 10.1130/ Lo, C.H., Lee, J.K.W., and Onstott, T.C., 2000, Ar- G24184A.1. gon release mechanisms of biotite in vacuo and Carroll, M.R., and Stolper, E.M., 1993, Noble gas the role of short-circuit diffusion and recoil: 1000 solubilities in melts and glasses: New experi- Chemical Geology, v. 165, p. 135–166, doi: 10 mental results for argon and the relationship 10.1016/S0009-2541(99)00167-9. -11 between solubility and ionic porosity: Geochi- Paine, J.H., Nomade, S., and Renne, P.R., 2006, 100 39 5 × 10 mica et Cosmochimica Acta, v. 57, p. 5039– Quantifi cation of Ar recoil ejection from 5051, doi: 10.1016/0016-7037(93)90606-W. GA1550 biotite during neutron irradiation as -1 2 Charlier, B., and Zellmer, G., 2000, Some remarks a function of grain dimensions: Geochimica et 10 10 - -12 10 13 mol on U-Th mineral ages from igneous rocks with Cosmochimica Acta, v. 70, p. 1507–1517, doi: prolonged crystallisation histories: Earth and 10.1016/j.gca.2005.11.012. 40 Ar Planetary Science Letters, v. 183, p. 457–469, Reid, M.R., Coath, C.D., Harrison, T.M., and McK- xs / 1 g B doi: 10.1016/S0012-821X(00)00298-3. eegan, K.D., 1997, Prolonged residence times Limit of t Chen, Y.S., Smith, P.E., Evensen, N.M., York, D., and for the youngest rhyolites associated with Long analytical 230 238 uncertainty Lajoie, K.R., 1996, The edge of time: Dating Valley Caldera: Th- U ion microprobe dat- Difference in age (%) Difference 0.1 young volcanic ash layers with the 40Ar-39Ar la- ing of young zircons: Earth and Planetary Sci- ser probe: Science, v. 274, p. 1176–1178, doi: ence Letters, v. 150, p. 27–39, doi: 10.1016/ 0.01 0.1 1 10 100 Sample age (Ma) 10.1126/science.274.5290.1176. S0012-821X(97)00077-0. de Silva, S., 1989, Geochronology and stratigraphy Renne, P.R., 1995, Excess 40Ar in biotite and horn- Figure 4. Age discrepancy (%) as a function of the ignimbrites from the 21°30′S to 23°30′S blende from the Noril’sk 1 intrusion, Siberia: Im- of sample age. Dots indicate variability of portion of the Central Andes of northern Chile: plications for the age of the Siberian Traps: Earth biotite ages for studied units (vertical axis), Journal of Volcanology and Geothermal Re- and Planetary Science Letters, v. 131, p. 165–176, connected by vertical lines representing cor- search, v. 37, p. 93–191, doi: 10.1016/0377- doi: 10.1016/0012-821X(95)00015-5. responding sanidine age (in Ma, horizontal 0273(89)90065-6. Renne, P.R., Sharp, W.D., Deino, A.L., Orsi, G., axis). Labeled dashed lines represent equal Dodson, M.H., 1973, Closure temperature in cool- and Civetta, L., 1997, 40Ar/39Ar dating into the concentrations of excess Ar, and indicate ing geochronological and petrological systems: historical realm: Calibration against Pliny the 40 Contributions to Mineralogy and Petrology, Younger: Science, v. 277, p. 1279–1280, doi: limits on amount of retained Arxs detected in these samples. Gray shaded region repre- v. 40, p. 259–274, doi: 10.1007/BF00373790. 10.1126/science.277.5330.1279. sents current limits on analytical precision. Gradstein, F.M., Ogg, J.G., and Smith, A.G., 2004, Roberts, H.J., Kelley, S.P., and Dahl, P.S., 2001, A geologic time scale 2004: Cambridge, UK, Obtaining geologically meaningful 40Ar-39Ar New York, Cambridge University Press, 589 p. ages from altered biotite: Chemical Geology, Harrison, T.M., Duncan, I., and Mcdougall, I., 1985, v. 172, p. 277–290, doi: 10.1016/S0009-2541( the region beyond analytical limits in Figure 4 Diffusion of 40Ar in biotite: Temperature, pres- 00)00255-2. will contract, and evaluation of inherent geologi- sure and compositional effects: Geochimica et Singer, B.S., Wijbrans, J.R., Nelson, S.T., Pringle, M.S., cal limitations on accuracy will be increasingly Cosmochimica Acta, v. 49, p. 2461–2468, doi: Feeley, T.C., and Dungan, M.A., 1998, Inherited 40 39 important. Our fi ndings indicate that sanidine, 10.1016/0016-7037(85)90246-7. argon in a Pleistocene andesite lava: Ar/ Ar Hora, J.M., Singer, B.S., and Wörner, G., 2007, Vol- incremental-heating and laser-fusion analyses in general, appears to behave as the common cano evolution and eruptive fl ux on the thick of plagioclase: Geology, v. 26, p. 427–430, doi: assumptions suggest. Conversely, biotites may crust of the Andean Central Volcanic Zone: 10.1130/0091-7613(1998)026<0427:IAIAPA> retain evidence of pre-eruption conditions that 40Ar/39Ar constraints from Volcán Parinacota, 2.3.CO;2. complicate interpretations of their apparent ages. Chile: Geological Society of America Bulletin, Singer, B.S., Hoffman, K.A., Coe, R.S., Brown, v. 119, p. 343–362, doi: 10.1130/B25954.1. L.L., Jicha, B.R., Pringle, M.S., and Chauvin, Hu, Q., Smith, P.E., Evensen, N.M., and York, D., A., 2005, Structural and temporal requirements ACKNOWLEDGMENTS 1994, Lasing in the Holocene: Extending for geomagnetic fi eld reversal deduced from We thank the Oregon State University Radiation the 40Ar-39Ar laser probe method into the 14C lava fl ows: Nature, v. 434, p. 633–636, doi: Center for sample irradiation, Sue Kay for supply- age range: Earth and Planetary Science Let- 10.1038/nature03431. ing Cerro Galan samples, and John Fournelle for ters, v. 123, p. 331–336, doi: 10.1016/0012 Smith, M.E., Singer, B.S., Carroll, A.R., and Four- assistance with Fe-Ti oxide thermometry. This work -821X(94)90277-1. nelle, J.H., 2006, High-resolution calibration was supported by National Science Foundation grant Kelley, S., 2002, Excess argon in K-Ar and Ar-Ar geo- of Eocene strata: 40Ar/39Ar geochronology of EAR-0538159 to Singer; grants EAR-0538206 and chronology: Chemical Geology, v. 188, p. 1–22, biotite in the Green River Formation: Geology, EAR-0710545 to de Silva; and Geological Society doi: 10.1016/S0009-2541(02)00064-5. v. 34, p. 393–396, doi: 10.1130/G22265.1. of America, Sigma Xi, and Weeks grants to Hora. Lanphere, M.A., and Dalrymple, G.B., 1976, Iden- Smith, M.E., Singer, B.S., Carroll, A.R., and Four- We thank editor William Collins, Paul Renne, and tifi cation of excess 40Ar by 40Ar/39Ar age spec- nelle, J.H., 2008, Precise dating of biotite in an anonymous reviewer for comments that improved trum technique: Earth and Planetary Science distal volcanic ash: Isolating subtle alteration this manuscript. 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