Earth and Planetary Science Letters 255 (2007) 229–242 www.elsevier.com/locate/epsl

Rapid magma ascent and generation of 230Th excesses in the lower crust at Puyehue–Cordón Caulle, Southern Volcanic Zone, ⁎ Brian R. Jicha a, , Brad S. Singer a, Brian L. Beard a, Clark M. Johnson a, Hugo Moreno-Roa b,c, José Antonio Naranjo b

a Department of Geology and Geophysics, University of Wisconsin—Madison, 1215 West Dayton Street, Madison WI 53706, USA b Servicio Nacional de Geología y Minería (SERNAGEOMIN), Avenida Santa María, 0104 , Chile c Observatorio Volcanologico de los del Sur (OVDAS), Cerro Ñielol-Sector Antenas, Temuco, Chile Received 28 July 2006; received in revised form 7 December 2006; accepted 8 December 2006 Available online 30 January 2007 Editor: R.W. Carlson

Abstract

Basaltic to rhyolitic and erupted over the last 70 kyr at the Puyehue–Cordón Caulle volcanic complex in the Andean Southern Volcanic Zone (SVZ) were analyzed for major and trace element, Sr isotope, and U–Th isotope compositions to constrain the timescales of magmatic processes and identify the subducted and crustal components involved in magma genesis. Internal U–Th mineral isochrons from five lavas and three fall deposits are indistinguishable from their eruption ages, indicating a short period (b1000 yr) of crystal residence in the magma prior to eruption. The (230Th/232Th) ratios define a narrow range (0.80–0.83) compared to that of all SVZ lavas (0.72–0.97), suggesting that Puyehue was derived from a relatively uniform mantle source. and rhyolites have the largest U excesses and likely evolved via fractional crystallization of a plagioclase-dominated mineral assemblage. In contrast, have 1 to 6% 230Th excesses, a characteristic not previously observed in frontal arc stratovolcanoes of the Andean SVZ. The 230Th excesses are interpreted to reflect a relatively small degree of fluid flux melting coupled with assimilation and melting of the lower crust. Lower crustal processes, therefore, have dampened the 238U excesses that were generated during fluid addition to the mantle wedge. Although prior 238U–230Th–226Ra studies of lavas from other southern SVZ stratovolcanoes (36 to 41° S) have inferred that slab additions and the extent of mantle melting were nearly constant along strike of the arc, our results suggest that MASH processes envisioned by Hildreth and Moorbath [W. Hildreth, S. Moorbath, Crustal contributions to arc magmatism in the Andes of central Chile, Contrib. Mineral. Petrol. 98 (1988) 455-489] in the northern SVZ also occur in the southern SVZ, where the crust is relatively thin. © 2006 Elsevier B.V. All rights reserved.

Keywords: U–Th isotopes; Puyehue–Cordón Caulle; Southern Volcanic Zone; lower crust

1. Introduction between volcanic output and magma dynamics in the mantle and crust. Disequilibria among decay products of Determining the time involved for magma transport, U-series nuclides have been used to constrain these storage, and crystallization provides an important link processes (see reviews by [2,3]). In particular, along-arc surveys of historical lavas U–Th isotope compositions ⁎ Corresponding author. Tel.: +1 608 262 8960; fax: +1 608 262 0693. have been used to fingerprint the various subducted E-mail address: [email protected] (B.R. Jicha). components involved in magma genesis and to estimate

0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.12.017 230 B.R. Jicha et al. / Earth and Planetary Science Letters 255 (2007) 229–242 ascent rates [4–11]. Relatively few studies have focused Nd–Pb isotope characteristics of the erupted products on extended eruptive periods of individual volcanoes are relatively well-characterized [31], 3) its eruptive (i.e. 104 to 105 yr), and yet this approach is important for history is marked by numerous explosive sub-Plinian to understanding the tempo of magma storage, mixing, and Plinian eruptions, including the most recent eruption fractional crystallization over the lifetime of an arc which occurred on May 24, 1960, ∼38 h after the main [12–15]. shock of the Mw 9.5 Great Chilean earthquake, whose The majority of U-series data from subduction zones epicenter was 240 km to the west [32,33], and 4) it is the are from island arc lavas that are typically enriched in target of a 40Ar/39Ar-based geochronological investiga- 238U over 230Th, which have been interpreted to reflect tion aimed at characterizing the eruptive and volumetric the addition of slab-derived, U-rich fluids to the mantle growth rate history during the past several hundred kyr wedge. Interpretation of U-series isotope data from [34,35]. This study combines new U-series, trace- continental arc lavas tends to be complicated by element, and Sr isotope data to examine magma sources magma–crust interaction [9–11,16–23]. Excess 226Ra and intracrustal magmatic processes over the last 70 kyr, over 230Th is a widespread characteristic in both island a period when the Puyehue stratocone grew rapidly to its and continental arc lavas, which requires rapid magma present form. ascent within 5 half-lives of 226Ra (b8 kyr). 226Ra excesses may be generated during fluid addition to the 2. Geologic setting and volcanological overview mantle wedge, melting and assimilation of crust, or during melt transport, all of which may relax the Puyehue–Cordón Caulle is located at 40.5° S in requirement of rapid magma ascent rates [24–26]. 230Th Southern Volcanic Zone of Chile, and comprises excesses in continental arc lavas, like 226Ra excesses, ∼140 km3 of Pleistocene to Holocene lavas and tephras may be due to variable amounts of fluid and melt that crop out over ∼800 km2 [31,34–36]. The complex addition to the mantle wedge [8,10,20–22] or assimi- includes the Puyehue and the Cordón lation–fractional crystallization processes [17,23]. Caulle fissural zone, which extends 20 km to the The Andean Southern and Austral Volcanic zones northwest of Puyehue (Fig. 1). Geologic mapping, (SVZ and AVZ, respectively; Fig. 1) are adjacent stratigraphy, and more than forty 40Ar/39Ar age continental arcs where petrologic and U-series studies determinations indicate that two broad shield volcanoes have revealed dramatic differences in magma sources dominated the Puyehue–Cordón Caulle region from 240 [10–12,18,27–30]. 230Th excesses and high Sr/Y and to 70 ka [34–36]. The modern Puyehue stratocone was La/Yb ratios in AVZ lavas may reflect partial melting of built on the remains of the southernmost shield from 69 subducted oceanic crust [10]. In contrast, most SVZ to 1 ka, and consists of ∼18 km3 of and tephra that lavas have 238Uand226Ra excesses that are well span a continuum from basalt (up to 10 wt.% MgO) to 10 9 correlated with Be/ Be [11,18]; this is interpreted to rhyolite with up to 72 wt.% SiO2 (Table A.1). The reflect addition of sediment-derived fluid to the mantle, basalts which contain 7–10 wt.% MgO are among the followed by rapid magma ascent (i.e., 102 to 103 yr) [11]. most Mg-rich lavas erupted from stratovolcanoes in The aim of this paper is to explore petrologic the entire southern SVZ [31,37]. Dacitic to rhyolitic processes and their timescales over an extended period activity is primarily restricted to the last 19 kyr during of several tens of kyr at the Puyehue–Cordón Caulle which a dome complex grew atop the volcano in volcanic complex through U–Th isotope compositions several effusive and explosive stages. A sub-Plinian to determined on lavas, tephra falls and their constituent phreatoplinian rhyodacitic eruption at ∼1kaobliter- minerals. We present the first U–Th isotope data ated these domes and formed the modern 2.5 km- available for minerals from SVZ lavas. In addition, we diameter summit crater. Along the Cordón Caulle fis- have analyzed several mafic lavas from small eruptive sural zone, ∼9km3 of rhyodacitic to rhyolitic domes, centers in the Puyehue–Cordón Caulle region in order to lava flows, pumice falls, and pyroclastic cones have compare the isotopic characteristics of basalts erupted at erupted from at least 27 vents during the past a mature stratovolcano to those of the surrounding ∼16.5 ka, including rhyodacitic to rhyolitic eruptions cinder cones (e.g., [18]). Puyehue–Cordón Caulle was in 1921–22 and 1960 [33–36]. chosen because: 1) it is unique among other predom- inantly mafic centers of the southern SVZ in that it has 3. Sample selection and analytical techniques produced a wide compositional spectrum of lavas ranging from high-MgO basalt to voluminous rhyoda- We have analyzed 26 lavas and tephra fall deposits cite and rhyolite, 2) the petrologic, geochemical, Sr– from Puyehue–Cordón Caulle erupted during the last B.R. Jicha et al. / Earth and Planetary Science Letters 255 (2007) 229–242 231

70 kyr, one coarse-grained, dioritic xenolith that was Puyehue lavas are known precisely from 40Ar/39Ar found in ∼1 ka pumice on the south rim of the Puyehue geochronology, AMS 14C ages ([34,35], Table A.2), or crater, nine late Holocene to historic basalts from nearby historical observations. Three of the five Puyehue small eruptive centers (e.g., Mirador, Carrán, and basalts (PU 02 39, PU 02 03, PU 05 29) have not Rininahue centers of Carrán-Los Venados; Antillanca, been dated directly, but their ages are constrained by Pajaritos, Cerro Mirador, Anticura, an unnamed cinder stratigraphic relations relative to lavas flows that erupted cone 3 km south of Puyehue), and one basaltic lava from between 14.9±2.9 and 11.5±1.1 ka based on 40Ar/39Ar the 1835 eruption of volcano, located ∼65 km geochronology (Fig. 1). The ages of three late Holocene southwest of Puyehue. The eruptive ages of 20 of the rhyolitic domes along Cordón Caulle (PU 02 13, PU 02

Fig. 1. Geologic map of the Puyehue–Cordón Caulle volcanic complex showing the locations of the samples (○) and their eruptive ages (in ka) constrained by 40Ar/39Ar dating. Hachured lines indicate or vent margins. Contour interval is 100 m. Map modified from [36]. Inset is an enlarged view of the Southern Volcanic Zone (SVZ) showing Puyehue–Cordón Caulle and the other stratovolcanoes mentioned throughout the text. Within the inset is a map of showing the various volcanic zones (e.g., NVZ, CVZ, SVZ, AVZ). 232 B.R. Jicha et al. / Earth and Planetary Science Letters 255 (2007) 229–242

26, PU 02 34) are not known precisely, but on the basis 4. U–Th isotope results of their surface morphologies, degree of weathering, and inferred stratigraphic positions, they likely erupted Whole-rock (230Th/232Th) ratios for Puyehue–Cor- within the last two millennia. dón Caulle lavas have a restricted range (0.80–0.83), Approximately 20 to 270 mg of mineral separates were and are lower than most of those reported for other SVZ prepared from fresh lava samples by crushing, sieving to volcanoes with the exception of Osorno volcano 250–500 μm mesh size, magnetic and density sorting, and (Fig. 2a) [9,11,18]. In contrast, there is a 14% variation careful hand picking under a binocular microscope to in (238U/232Th) ratios. Puyehue basalts have 230Th ex- remove crystals that contain glass or melt inclusions. Air cesses ranging from 1 to 6%, whereas , dacites, abrasion was used on several of the pyroxene separates to and rhyolites have 1 to 8% U excesses (Table 1). Basalts remove glass that was adhered to the surface of the crystals. from the small eruptive centers in the Puyehue–Cordón A portion of the 180–250 μm fraction of the bulk separate Caulle region are more variable in (230Th/232Th) ratios was ground repeatedly in an agate mortar and pestle under (0.78–0.85), and they have much higher U excesses (up acetone, followed by separation of magnetite with a hand to 40%) (Fig. 2b). The dioritic xenolith is in secular magnet. Whole-rock powders and high-purity mineral equilibrium in terms of (230Th/232Th) and (238U/232Th), separates were spiked with a mixed 235U–229Th tracer; and these ratios are significantly higher than those of the silicates were dissolved using HF–HNO3–HCl in Teflon SVZ lavas (Fig. 2a). beakers, and magnetite was dissolved using 8 M HCl. U All of the mineral separates analyzed have U and Th and Th were separated using two passes through ion concentrations that are lower than those of the whole-rock exchange columns with BioRad AG 1X8 200–400 mesh samples (Table 1), which indicates that U and Th are anion exchange resin [15].Isotopicmeasurementswere enriched in the groundmass or accessory minerals. Plagio- done at the UW—Madison Radiogenic Isotope Laboratory clase tends to have the lowest (238U/232Th) ratios, whereas using a GV Instruments Isoprobe MC-ICP-MS. Additional magnetite has (238U/232Th) ratios that are considerably analytical details are in [15]. External precision, reproduc- higher than those of the whole-rocks. Internal U–Th ibility, and accuracy of Th and U isotope measurements mineral isochrons from five lavas and three tephra fall were evaluated through analyses of Table Mountain Latite deposits yielded ages that are identical to their eruption (TML), AThO, AGV-1, and BCR-1 rock standards (Table ages within analytical uncertainties (Table 1; Fig. 3). For a A.3). The results of rock standards are in agreement with few samples, the whole-rock or groundmass point lies at published values. Whole-rock standards and sample the left end of the isochron, and mass balance requires the unknowns were analyzed using a standard–sample– presence of a phase that has a low U/Th ratio (e.g., standard technique. The data show no evidence of memory plagioclase) that was not analyzed but is present in the effects because on-peak zero measurements for U and Th whole rock. The plagioclase separate for sample PU 05 remained unchanged after washout between samples. U 11A has a (230Th/232Th) ratio that is higher than those of and Th standard solutions were analyzed over a factor of the other mineral separates and the whole rock compo- five range in concentration, and this wide range in con- sition, suggesting that the plagioclase separate may centrations produced only a 1.7‰ per mass unit change contain xenocrysts or inclusions of older material. The in mass bias, which is entirely negligible. Throughout isochron age calculated using the other minerals is the analytical period, 24 measurements of a thorium consistent with the eruptive age as determined by 14C reference solution, IRMM-035, yielded a 232Th/230Th dating of charcoal within the deposit (Table A.2). ratio of 87,247±36 (2 SE), which lies within error of the certified value of 87,100±592. Total procedural blanks 5. Petrogenesis of Puyehue–Cordón Caulle magmas (n =12) were b35 pg for U and b55 pg for Th, which were insignificant because the signal to noise ratio for 5.1. Fractional crystallization and magma mixing most samples was ≫1000, with the exception of seve- ral plagioclase separates that had signal to noise ratios Gerlach et al. [31] proposed that dacitic to rhyolitic of ∼300. Decay constants used to calculate activity ratios Puyehue–Cordón Caulle magmas formed by low pressure are from [38]: λ238U=1.55125×10− 10/yr, λ230Th= (b5 kb) fractional crystallization of a plagioclase- 9.156×10− 6/yr, λ232Th=4.9475×10− 11/yr. Isochrons dominated anhydrous mineral assemblage, and basaltic were calculated using Isoplot version 3.00. Strontium andesites and andesites were produced via mixing of isotopes were measured following the procedures of Jicha basalt and or fractional crystallization of basalt. et al. (2005). Twelve measurements of NIST SRM-987 Compatible–incompatible element variations such as gave an 87Sr/86Sr ratio of 0.710272±0.000011 (2 SD). Co–Rb in Puyehue–Cordón Caulle lavas erupted over B.R. Jicha et al. / Earth and Planetary Science Letters 255 (2007) 229–242 233

Petrographic observations from Puyehue basaltic andesites and andesites, which have erupted several times over the last 70 ka, indicate the presence of re- sorbed and reversely zoned plagioclase phenocrysts, signaling that magma mixing has occurred repeatedly throughout the lifetime of the volcano. Furthermore, the 34 ka rhyodacitic lava flow contains about 10 vol.% quenched inclusions of basaltic , which indi- cates that mafic magma was present in the Puyehue plumbing system and in physical contact with evolved magma several thousand years before basaltic effusions at ca. 15–11 ka. Despite petrographic and chemical evidence for magma mixing, the majority of the phenocrysts in the erupted magmas largely post-date this process. Agree- ment between U–Th mineral isochron ages and the eruptive ages as constrained by 40Ar/39Ar or 14C dating for all eight samples indicates that phenocryst residence times for the various mineral phases in the magma prior to eruption must have been very brief (i.e., a few thousand years or less). Moreover, the fact that the minerals and glass lie on the same isochron suggests that the phenocrysts grew after differentiation and magma mixing, assuming that magma mixing would tend to blend isotopically distinct magmas, which would add scatter to U–Th isochrons. We envision that these pre- eruptive magmatic processes occurred in three steps: 1) mafic, mantle-derived magmas ascended into the middle to lower crust where fractionation and/or mixing may have occurred, 2) crystals were likely segregated from melt along solidification fronts at the edges of the 230 232 238 232 Fig. 2. ( Th/ Th) vs. ( U/ Th). (a) Equiline diagram showing magma chamber [39], and 3) new phenocrysts formed published data from southern SVZ lavas and data from this study. via decompression-driven crystallization as magmas (230Th/232Th) ratios have been age-corrected to the time of eruption. (230Th/232Th) ratios of Puyehue–Cordón Caulle lavas are lower than ascended through the upper crust [40,41]. most other SVZ lavas with the exception of Osorno. Data for Osorno The observed U excesses in andesitic to rhyolitic lavas are from this study and [9]. A dioritic xenolith found in the ∼1ka Puyehue–Cordón Caulle lavas may be the result of the Plinian deposits from Puyehue is in secular equilibrium with 230 232 238 232 intracrustal fractionation processes described above. For ( Th/ Th) and ( U/ Th) ratios that are significantly higher example, (238U/232Th) ratios show weak correlations with than those of the lavas. Most of the basaltic lavas from small eruptive 2 2 centers (SEC) in the Puyehue–Cordón Caulle region in have U Zr (R =0.38) and Rb (R =0.44), common indicators of excesses. Published U–Th isotope data from SVZ lavas is from differentiation. Apatite fractionation has also been pro- [9,11,18,30]. (b) Enlarged view of Puyehue–Cordón Caulle data posed as a mechanism for generating 238U excesses in showing that the lavas define a sub-horizontal array with basalts 230 evolved magmas because it retains Th over U during having Th excesses and andesites to rhyolites having U excesses. crystallization [12,23].Our(238U/232Th) data, however, do 2 not show a correlation with P2O5 (R =0.01).Wetherefore the last 70 kyr support Gerlach et al.'s [31] model of conclude that the U excesses in the evolved Puyehue– crystallization and magma mixing (Fig. 4). Based on the Cordón Caulle lavas do not reflect fractionation of apatite. enrichment of incompatible elements such as Rb, The exact origin of the U excesses remains unclear. rhyolites may be produced by ∼85% crystallization of a basaltic to basaltic andesitic parent (e.g., 52–53 wt.% 5.2. Crustal melting/assimilation SiO2)or40–55% crystallization of an andesitic parent (Fig. 4). Similar results are obtained using other The evidence of magma–crust interaction that is incompatible elements such as Ba, Zr, and Th. prevalent in the northern SVZ (33–36° S) diminishes 234 B.R. Jicha et al. / Earth and Planetary Science Letters 255 (2007) 229–242

Table 1 U–Th data from Puyehue–Cordón Caulle region 238 232 230 232 238 230 230 232 Sample SiO2 Age/location Material ( U/ Th) ( Th/ Th) ( U/ Th) Th U ( Th/ Th)0 (wt.%) (ppm) (ppm) Puyehue–Cordón Caulle volcanic complex PU 02 16 69.6 A.D. 1960 (lava flow) wr 0.827±0.002 0.823±0.003 1.005±0.006 8.969 2.445 0.823±0.003 my 0.863±0.001 0.825±0.002 1.046±0.004 1.714 0.487 plag 0.889±0.005 0.824±0.012 1.079±0.022 0.061 0.018 gm 0.831±0.002 0.822±0.003 1.011±0.006 8.472 2.322 PU 02 25 70.3 A.D. 1960 (pumice) wr 0.830±0.002 0.821±0.002 1.011±0.005 8.348 2.284 0.821±0.002 gm 0.842±0.002 0.820±0.002 1.027±0.005 8.459 2.347 opx 0.937±0.005 0.821±0.006 1.141±0.015 1.046 0.323 mt 0.951±0.002 0.822±0.003 1.157±0.007 0.668 0.209 PU 02 29 69.5 A.D.1921–22 (lava flow) wr 0.843±0.002 0.819±0.003 1.029±0.006 8.248 2.291 0.819±0.003 gm 0.877±0.003 0.819±0.004 1.071±0.009 8.139 2.353 opx 0.869±0.002 0.820±0.004 1.060±0.008 1.131 0.324 plag 0.831±0.003 0.816±0.011 1.018±0.018 0.066 0.018 mt 0.933±0.003 0.820±0.005 1.138±0.011 0.747 0.230 PU 05 03 65.4 730–1530 a a (pumice) wr 0.834±0.002 0.822±0.001 1.015±0.004 7.737 2.127 0.822±0.001 mt 0.950±0.001 0.824±0.001 1.154±0.004 0.443 0.139 plag 0.805±0.003 0.820±0.003 0.982±0.007 0.512 0.136 gl 0.831±0.001 0.822±0.001 1.011±0.002 7.721 2.115 opx 0.872±0.002 0.823±0.002 1.060±0.005 0.872 0.250 PU 05 11A 66.5 730–1530 a a (pumice) wr 0.840±0.001 0.822±0.002 1.022±0.004 7.588 2.101 0.822±0.002 gl 0.838±0.001 0.821±0.002 1.021±0.004 7.819 2.159 opx 0.905±0.002 0.820±0.003 1.104±0.007 0.993 0.296 mt 1.041±0.002 0.826±0.003 1.260±0.007 0.443 0.152 plag 0.814±0.001 0.852±0.002 0.955±0.003 0.376 0.101 PU 02 39 53.1 11.5–14.9 ka b wr 0.794±0.001 0.820±0.002 0.968±0.004 1.752 0.458 0.823±0.002 gm 0.801±0.001 0.822±0.002 0.974±0.004 1.845 0.487 Olivine 1.059±0.006 0.843±0.009 1.256±0.021 0.029 0.010 mt 0.718±0.001 0.813±0.002 0.883±0.003 0.710 0.168 PU 03 27 66.5 11.5±1.1 ka b wr 0.846±0.003 0.821±0.004 1.030±0.009 7.647 2.131 0.818±0.005 cpx 0.849±0.001 0.823±0.002 1.032±0.004 0.708 0.198 mt 1.018±0.002 0.844±0.003 1.206±0.007 0.716 0.240 plag 1.011±0.003 0.840±0.006 1.204±0.012 0.122 0.041 gm 0.842±0.001 0.822±0.002 1.024±0.004 8.318 2.309 PU 03 10 64.7 31.6±5.3 ka b wr 0.847±0.002 0.815±0.002 1.039±0.005 7.933 2.215 0.804±0.003 gm 0.870±0.002 0.820±0.003 1.061±0.006 8.459 2.425 opx 0.791±0.003 0.808±0.005 0.979±0.010 0.445 0.116 mt 1.253±0.002 0.912±0.003 1.374±0.007 0.607 0.251 plag 0.761±0.003 0.794±0.007 0.958±0.012 0.103 0.026 PU 02 13 70.2 b2000 a wr 0.882±0.001 0.814±0.002 1.084±0.004 8.319 2.419 0.814±0.002 PU 02 26 71.1 b2000 a wr 0.832±0.001 0.814±0.002 1.022±0.004 8.736 2.394 0.814±0.002 PU 02 34 70.9 b2000 a wr 0.840±0.002 0.818±0.003 1.027±0.006 8.578 2.375 0.818±0.003 PU 05 12 66.4 730–1530 a a wr 0.844±0.001 0.814±0.002 1.037±0.004 7.669 2.133 0.814±0.002 PU 05 15 63.4 730–1530 a a wr 0.844±0.001 0.817±0.001 1.033±0.002 7.323 2.037 0.816±0.001 PU 02 36 70.4 5.2±2.4 ka b wr 0.822±0.001 0.819±0.002 1.004±0.004 8.817 2.389 0.819±0.002 PU 05 25 70.2 6.9±1.6 ka b wr 0.810±0.001 0.817±0.002 0.991±0.004 7.832 2.090 0.817±0.002 PU 04 04E 65.6 7.2±0.1 ka a wr 0.822±0.001 0.818±0.002 1.005±0.004 8.199 2.222 0.818±0.002 PU 02 03 48.2 11.5–14.9 ka b wr 0.805±0.001 0.811±0.002 0.993±0.003 0.859 0.228 0.812±0.002 PU 05 29 51.1 11.5–14.9 ka b wr 0.778±0.002 0.823±0.003 0.945±0.006 1.924 0.494 0.829±0.004 PU 02 40 63.9 14.9±2.9 ka b wr 0.872±0.003 0.815±0.004 1.070±0.009 7.019 2.018 0.807±0.005 PU 02 20 69.3 18.7±2.1 ka b wr 0.835±0.001 0.820±0.002 1.018±0.004 8.374 2.304 0.817±0.003 PU 02 02 68.3 34.4±0.7 ka b wr 0.856±0.003 0.818±0.004 1.046±0.009 8.357 2.359 0.804±0.007 PU 02 02i 52.7 34.4±0.7 ka b wr 0.800±0.001 0.805±0.001 0.994±0.002 1.831 0.483 0.807±0.002 PU 02 11 60.2 44.5±1.5 ka b wr 0.852±0.001 0.828±0.002 1.029±0.004 5.384 1.513 0.816±0.004 PU 03 07 53.0 51.4±14.5 ka b wr 0.819±0.002 0.814±0.003 1.006±0.006 2.374 0.641 0.811±0.006 PU 02 12 52.7 65.5±4.5 ka b wr 0.781±0.001 0.805±0.002 0.970±0.004 2.622 0.675 0.825±0.004 PU 03 15 54.1 69.3±1.6 ka b wr 0.812±0.001 0.809±0.002 1.004±0.004 3.148 0.843 0.806±0.005 PU 05 16 58.1 Xenolith wr 1.115±0.001 1.114±0.002 1.001±0.003 2.018 0.742 B.R. Jicha et al. / Earth and Planetary Science Letters 255 (2007) 229–242 235

Table 1 (continued ) 238 232 230 232 238 230 230 232 Sample SiO2 Age/location Material ( U/ Th) ( Th/ Th) ( U/ Th) Th U ( Th/ Th)0 (wt.%) (ppm) (ppm) Small eruptive centers and Osorno volcano OS 02 02 52.1 Osorno wr 0.921+0.001 0.724+0.002 1.272+0.005 0.910 0.276 0.724+0.002 PU 04 10 50.7 Antillanca wr 0.819+0.002 0.817+0.003 1.002+0.006 1.344 0.363 0.817+0.003 PU 05 05 53.5 Pajaritos lava wr 1.093+0.003 0.781+0.003 1.400+0.009 7.543 2.718 0.781+0.003 PU 05 07 49.1 Cerro Mirador wr 0.853+0.001 0.819+0.002 1.041+0.004 1.454 0.409 0.819+0.002 PU 05 31 57.0 S. of Puyehue wr 0.853+0.001 0.820+0.002 1.040+0.004 4.208 1.183 0.820+0.002 PU 03 06 51.9 1907 Rininahue wr 0.994+0.001 0.851+0.002 1.168+0.003 0.701 0.230 0.851+0.002 PU 05 32 51.8 1979 Mirador wr 0.946+0.001 0.817+0.001 1.159+0.003 0.771 0.240 0.817+0.001 PU 05 33 51.4 1955 Carrán wr 0.959+0.002 0.851+0.001 1.127+0.003 0.778 0.246 0.851+0.001 PU 05 35 51.2 Anticura wr 0.755+0.001 0.794+0.001 0.951+0.003 1.716 0.427 0.794+0.001 PU 05 37 52.0 Antillanca wr 0.854+0.001 0.795+0.002 1.074+0.004 2.432 0.685 0.795+0.002 Abbreviations: wr, whole-rock; gm, groundmass; cpx, clinopyroxene; opx, orthopyroxene; gl, glass; mt, magnetite; plag, plagioclase. All analytical U and Th isotope uncertainties are reported at 2σ precision. 230 232 40 39 14 ( Th/ Th)0 ratios were calculated using the whole-rock values and the Ar/ Ar or C age determinations. a Eruptive ages constrained by radiocarbon dating. Summary of 14C results is given in Supplementary data. b Eruptive ages constrained by 40Ar/39Ar dating. Summary of 40Ar/39Ar results is given in Supplementary data. southward where the crust thins to ∼35 km [1]. be explained by crustal interaction. In the discussion Numerous studies of northern SVZ mafic lavas have below, we provide several lines of evidence favoring suggested that the shifts in isotopic and select trace- lower crustal contamination of magmas at Puyehue– element compositions relative to those expected for Cordón Caulle and throughout much of the SVZ. We mantle-derived magmas are due to contamination in the utilize incompatible element ratios to identify the possible lower crust [1,42–45]. In the southern SVZ, McMillan crustal components that may have contaminated Puyehue et al. [46] proposed that variations in the O, Sr, Nd, and basalts, and then model lower crustal assimilation/melting Pb isotope compositions and incompatible trace-ele- processes via batch melting calculations. ment ratios of mafic lavas at Mocho-Choshuenco vol- Within the SVZ, incompatible element ratios such as cano (40° S), located 70 km to the north of Puyehue– K/Rb and Ba/Rb have been used to distinguish crustal Cordón Caulle, were likely produced in the lower crust contamination from variations in mantle source regions or mantle wedge. Further south at 41.2° S, elevated [1,42,45,46]. Large ion lithophile elements (LILE) K 87Sr/86Sr ratios and δ18O values in lavas have and Rb are not significantly fractionated from one been interpreted to reflect bulk assimilation of a another during differentiation due to their incompatible relatively small amount (∼10%) of Paleozoic metase- nature relative to the liquidus mineral assemblage (ol+ dimentary rocks in the lower crust [28]. Thus, MASH plag+cpx+opx+mt) [42]. Plagioclase may fractionate zone processes [1] are not restricted to the northern half K from Rb [50], but the resulting melt will have a K/Rb of the SVZ. These processes have likely occurred in the ratio that is only slightly lower than the starting compo- lower crust of the southern SVZ because assimilation of sition. The most primitive Puyehue basalts (b51 wt.% upper crustal Miocene granitoids [36,47–49], which are SiO2; N6 wt.% MgO) have much higher K/Rb ratios enriched in Rb, La, and Ba relative to Zr, Ta, and Th, than the more evolved lavas (Fig. 5), and fractional would result in an increase in the Rb/Zr, La/Ta, and Ba/ crystallization curves indicate that the lower K/Rb ratios Th ratios in the erupted magmas, which is not observed. in the Puyehue andesites, dacites, and rhyolites cannot Moreover, the dioritic xenolith found at Puyehue has be produced by crystal fractionation. Many of the (230Th/232Th) and (238U/232Th) ratios that are markedly evolved lavas can only be produced from basaltic different than the lavas (Fig. 2). Assimilation of upper andesites that have low K/Rb ratios (330) and elevated crust equivalent to this xenolith would move magmas Rb concentrations (18–20 ppm). The large range in K/ toward higher (230Th/232Th) ratios along the equiline, Rb ratios among Puyehue basalts and basaltic andesites away from the horizontal array observed for the must therefore reflect mantle-source heterogeneity or Puyehue rocks. interaction with lower crust that has low K/Rb ratios. Although lower crustal assimilation seems like an Possible crustal contaminants to the Puyehue basalts unlikely process to explain the U–Th isotope composi- include biotite-bearing tonalites of the late Paleozoic to tions in the Puyehue–Cordón Caulle lavas, other chemical Jurassic Panguipulli batholith, which crop out ∼25 km characteristics of the rocks including excess 230Th, may to the north of Puyehue–Cordón Caulle [35,51,52] and 236 B.R. Jicha et al. / Earth and Planetary Science Letters 255 (2007) 229–242

Fig. 3. U–Th mineral isochrons for eight lavas and tephra fall deposits from the Puyehue–Cordón Caulle volcanic complex. All eight internal isochron ages are statistically indistinguishable from the eruptive ages. The plagioclase separate for sample PU 05 11A is denoted with an open diamond because it likely contains xenocrysts of older material, and therefore is not considered when calculating an isochron age. Data from Table 1. Abbreviations: plag, plagioclase; wr, whole rock; cpx, clinopyroxene; opx, orthopyroxene; mt, magnetite; gm, groundmass; gl, glass; ol, olivine. B.R. Jicha et al. / Earth and Planetary Science Letters 255 (2007) 229–242 237

oxene, 10%; orthopyroxene, 15%; magnetite, 10%) produces up to 14% 230Th excesses. Assuming a mantle-derived basalt has ∼15% 238U excess, which it acquired following a modest degree of fluid-flux melting, assimilation of ∼3–10% of a lower crustal melt would produce the 1–5% 230Th excesses observed in Puyehue basalts and basaltic andesites, which seems a reasonable scenario. Little assimilant is required be- cause of the large difference in concentration between the basaltic magma (∼0.8 ppm Th) and a partial melt of either hornblende gabbro or tonalite (N11 ppm Th). Similar degrees of 230Th excesses have been identified in basalts from other continental arc settings (e.g., Alaska [8],Cascades[21,55],Kamchatka[6,20],Nicaragua[22], Costa Rica [22], and Andean CVZ [17] and NVZ [23]), Fig. 4. Plot of Co vs. Rb (ppm) for Puyehue–Cordón Caulle lavas and and have been attributed to dynamic partial melting, lower tephras erupted over the last 70 kyr. The fractional crystallization crustal melting of a garnet-rich protolith, or partial melting model curve assumes crystallization of a basaltic parent with a of the subducted oceanic crust under eclogite facies composition of 33 ppm Co and 10 ppm Rb. Bulk partition coefficients (D =0.1, D =2.5) used in Rayleigh fractionation calculations conditions. Bourdon et al. [17] and Garrison et al. [23] Rb Co 230 reflect the strong incompatibility and compatibility of Rb and Co, proposed that Th excesses in magmas from respectively in the observed anhydrous phenocryst assemblages. and volcanoes in the Andean CVZ and NVZ, Approximately 85% crystallization can produce the most evolved respectively, are consistent with lower crustal melting and – rhyolites. Dacites and rhyolites can also be produced by 40 55% assimilation of eclogite or garnet-bearing pelitic schist. crystallization of an andesitic parent. Tick marks indicate degree of ∼ crystallization in 10% increments. Most of the basaltic andesitic to The crust beneath these two volcanoes is 70 km thick, dacitic lavas define a linear trend which may be consistent with mixing and it has been suggested that crustal thickness may rather than fractional crystallization. influence the degree of 230Th excess in continental arc magmas [23].Puyehue–Cordón Caulle, however, is built hornblende-bearing gabbros similar to those found as xenoliths in Calbuco andesites [53](Fig. 5). Because the crust is relatively thin in this region, it should be possible to preserve these lithologies in the lower crust. If mantle-derived basalt assimilated a small percentage of a partial melt of one or both of the proposed contaminants, the hybrid magma would have a compo- sition similar to Puyehue basaltic andesite, and thus would be a suitable parent to the andesites to rhyolites. We propose that the 230Th excesses observed in Puyehue basalts are likely attributed to melting and assimilation of the lower crustal contaminants men- tioned above. Formation of a 230Th-enriched partial melt requires the presence of a refractory phase that pre- ferentially retains U over Th. Both of these lower crustal ⁎ contaminants have abundant magnetite (DU =10 DTh) ⁎ and trace amounts of zircon (DU =6 DTh). Modal batch Fig. 5. K/Rb vs Rb for Puyehue–Cordón Caulle lavas. Curves labeled melting calculations based on the partition coefficients FC are Rayleigh fractional crystallization paths for the mineral in Table 6 of [54] and the modal proportions of the assemblage plagioclase (65%)+olivine (15%)+clinopyroxene (10%)+ Panguipulli tonalite (plagioclase, 60%; quartz, 20%; magnetite (10%). Compositional range of proposed lower crustal – alkali feldspar, 5%; biotite, 3%; magnetite, 7%) [39] contaminants (tonalite of the late Paleozoic Jurrassic Panguipulli 230 batholith and hornblende gabbro) are shown in the shaded regions. indicate that 5% melting produces 12% Th excesses Dashed curves represent potential mixing curves between parental (Table A.4). Five percent melting of hornblende-bearing basalt and a partial melt of gabbro and tonalite. See text for further gabbro (plagioclase, 50%; amphibole, 15%; clinopyr- discussion. 238 B.R. Jicha et al. / Earth and Planetary Science Letters 255 (2007) 229–242 on crust only half as thick as that beneath Parinacota and Cotopaxi, and yet the 230Th excesses are the same at all three volcanoes. 230Th excesses may, therefore, be produced via assimilation of a garnet-free lower crust, and may be a far more sensitive indicator of contamina- tion in thin, immature arc crust than, for example, Sr isotopes (e.g., [1,50]).

5.3. Contributions from the subducted slab and partial melting

The isotopic compositions of the Puyehue–Cordón Caulle magmas, may be due, in part, to variable con- tributions from the subducting Nazca plate [9,56]. The Sr, Be, and U–Th–Ra isotope compositions of young mafic SVZ lavas have been ascribed to fluid addition to the magma source region a few thousand years ago [9,11,18,57]. Because the primary source of 10Be is subducted sediment, slab-derived fluids will likely leach 10Be from the sediment during dehydration [11]. Subducted sediments, whether terrigenous or pelagic, 87 86 have significantly higher Sr/ Sr ratios and Sr contents Fig. 6. 87Sr/86Sr and La/Yb vs. latitude (°S) for SVZ (36–42°S) basalts. than the SVZ mantle; therefore, sediment-derived fluids 87Sr/86Sr ratios increase from 36 to 41° S, which may be due to 87 86 would also be expected to have elevated Sr/ Sr ratios. increased lower crustal contamination southward. From 36 to 39° S, A simple fluid-fluxing model for magma genesis is La/Yb ratios increase northward, which is either due to lower degrees problematic in the southern SVZ because the greatest of mantle melting or increasing degrees of lower crustal contamination 10 87 86 with garnet as a residual phase. Basaltic lavas from 39 to 41° S have enrichment in Be and Sr/ Sr in uncontaminated, similar La/Yb ratios, however, Puyehue basalts are slightly higher. mantle-derived magmas should occur where sediment Data from: this study; [9,18,27–31,42,43,45–47,56,58–61]. flux is highest, such as is the case in the Aleutian Island Arc [8]. The highest 10Be/9Be ratios, however, are basaltic lavas from Osorno, San Jorge (a SEC near observed in lavas at 39° S, but 87Sr/86Sr ratios Villarrica), and several of the monogenetic centers in the increase southward until they reach a maximum value of Puyehue–Cordón Caulle region have large U-excesses 0.7044 at ∼41° S (Fig. 6a). There is no strong isotopic and low La/Yb ratios (Fig. 7), thereby supporting an or geophysical evidence to suggest that sediment flux origin involving a greater amount of fluid addition to the increases southward in the arc. In fact, the rate of plate mantle wedge and a larger percent partial melting. convergence decreases to the south [58]. Therefore, the Another factor that could govern ascent rates and lower processes governing the Sr and Be isotope signatures in crustal signatures in the SVZ is the tectonic regime of the SVZ lavas appear to be decoupled. continental crust, which can favor rapid magma ascent or Based on our new U–Th isotope data and consider- partially arrest magma and facilitate interaction with the ation of existing data from the SVZ, we propose that the crust [28,62]. Hence, it appears that each southern SVZ subtle geochemical and isotopic differences among stratovolcano may be tapping a discrete mantle source, volcanic centers cannot be explained by a common and derivative basalts interact to different degrees with petrogenetic process. For example, the La/Yb ratios of the lower crust. This obviates simple petrogenetic basalts from Puyehue and SEC near Villarrica and models linking all SVZ volcanism to a single source or Calbuco are higher than those of other stratovolcanoes set of processes, and suggests that MASH processes may south of 38° S, which may reflect a smaller degree of be occurring beneath volcanoes located atop much partial melting of the mantle (Fig. 6b). Because slab thinner crust than previously envisioned. input commonly correlates with extent of melting, a relatively minor amount of U-rich fluid addition to the 6. Timescales of magmatic processes mantle beneath these volcanoes may not generate suf- ficient U-excesses to offset the 230Th excesses that are Estimates of crustal residence time for continental arc produced during lower crustal melting. Conversely, magmas are generally thought to range from 103 to B.R. Jicha et al. / Earth and Planetary Science Letters 255 (2007) 229–242 239

N105 yr [19,21,23]. Puyehue–Cordón Caulle lavas define a sub-horizontal array on an equiline diagram, which indicates that U–Th fractionation in the source, differentiation, mixing, and eruption occurred within a few thousand years (Fig. 2). Concordance between U– Th mineral isochron and eruptive ages provides further evidence that the phenocryst crystallization and subse- quent storage in crustal reservoirs was b103 yr prior to eruption. These processes apparently took place after MASH processes and therefore at shallow crustal levels. Our results demonstrate the powerful combination of U- series and 40Ar/39Ar geochronometers to place con- straints on ascent and residence times over a protracted period of a volcano's eruptive history (e.g., [15]). To further explore magmatic evolution and distinguish between open- and closed-system processes over the 230 232 lifetime of the volcano, we have plotted ( Th/ Th)0 vs. time (eλt)inFig. 8. If magmas are extracted from an evolving reservoir that is an isotopically closed system, 230 232 their ( Th/ Th)0 ratios would vary linearly with time in such a diagram [15,63,64]. Our data show no 230 232 Fig. 8. (a) Initial ( Th/ Th)0 as a function of time for Puyehue– 230 232 230 232 linear correlation of ( Th/ Th)0 ratios over the last Cordón Caulle lavas. The subtle variations in ( Th/ Th)0 ratios 70 kyr, suggesting that the erupted magmas were suggest that not all magmas were extracted from the 23 same reservoir. derived from several discrete sources or evolved by Yet, these magmas were likely derived from a fairly homogeneous 87 86 – distinct open-system pathways (Fig. 8a). The range in source. (b) Sr/ Sr vs. age (ka) for Puyehue Cordón Caulle lavas. Basaltic to dacitic lavas erupted between 15 and 11 ka are the most 230 232 – ( Th/ Th)0 ratios at Puyehue (0.80 0.83) is very isotopically diverse. Conversely, b6 ka rhyolites display similar narrow relative to the entire range encompassed by the 87Sr/86Sr ratios. SVZ (0.72–0.97), indicating that basalt at Puyehue has tappedamantlesourceregion that has been essentially uniform with respect to U–Th isotope composition over Temporal changes in 87Sr/86Sr ratios of Puyehue– 230 232 the last 70 kyr. Cordón Caulle lavas mimic those of the ( Th/ Th)0 ratios in the sense that there are no systematic va- riations with time or rock type (Fig. 8b). Six basaltic to rhyolitic lavas erupted between 19 and 11 ka show the largest range in Th and Sr isotope ratios. The diversity in isotope compositions for the 19–11 ka lavas may reflect addition of a new subducted component to the magma source, or incorporation of a heterogeneous lower crustal component into the magma that over- whelmed the mantle signature. In contrast, rhyolitic lavas erupted over the last 6 ka have a very restricted 87 86 230 232 range in Sr/ Sr and ( Th/ Th)0 ratios, which may reflect fractionation from parental magmas that were extensively homogenized during the Holocene [31]. 238 230 Fig. 7. ( U/ Th) vs. La/Yb for basaltic (b53 wt.% SiO2) SVZ lavas. Villarrica, Osorno, San Jorge, and Carrán-Los Venados lavas have the 7. Conclusions largest U-excesses (i.e., (238U/230Th)N1) and smallest La/Yb ratios, which suggests the addition of U-rich slab fluids during magma U–Th isotope data and trace-element modeling generation. The higher La/Yb ratios for SEC near Villarrica and Puyehue lavas suggest limited slab fluid input to the mantle wedge provide constraints on the timescales of magmatic beneath these centers and a lower degree of melting. U-series data processes over the past 70 kyr at the Puyehue–Cordón sources same as Fig. 4. Caulle volcanic complex. Puyehue rhyolites may be 240 B.R. Jicha et al. / Earth and Planetary Science Letters 255 (2007) 229–242 produced by ∼85% fractional crystallization of an References anhydrous mineral assemblage from basaltic andesite, but cannot be directly evolved from high-MgO, low- SiO basalt. Geochemical and petrographic evidence [1] W. Hildreth, S. Moorbath, Crustal contributions to arc magma- 2 tism in the Andes of central Chile, Contrib. Mineral. Petrol. 98 suggests that minor volumes of basaltic andesite and (1988) 455–489. andesite likely formed throughout the lifetime of the [2] M. Condomines, P.-J. Gauthier, O. Sigmarsson, Timescales of volcano as a result of mixing between basaltic and magma chamber processes and dating of young volcanic rocks, dacitic to rhyolitic magmas. U–Th mineral isochrons in: B. Bourdon, G.M. Henderson, C.C. Lundstrom, S.P. 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