238U–230Th equilibrium in arc magmas and implications for the time scales of mantle metasomatism

Earth and Planetary Science Letters 391 (2014) 146–158

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238

U–230 Th equilibrium in arc magmas and implications for the time scales of mantle metasomatism

Olivier Reubi a,∗,1 , Kenneth W.W. Sims b , Bernard Bourdon c a b c

ETH Zurich, Institute of Geochemistry and Petrology, Switzerland Department of Geology and Geophysics, University of Wyoming, USA Laboratoire de Géologie de Lyon, Ecole Normale Supérieure de Lyon, CNRS and Université Claude Bernard Lyon 1, France

a r t i c l e

i n f o

Article history: Received 30 November 2012 Received in revised form 15 January 2014 Accepted 31 January 2014 Available online 15 February 2014 Editor: T. Elliott Keywords: U-series arc magmatism mantle melting metasomatism Mexico

a b s t r a c t Large excesses of 238 U and 226 Ra relative to 230 Th characterize many arc magmas and are commonly interpreted to represent recent addition of slab-derived fluid to the mantle wedge beneath the arc. A significant proportion of arc magmas are, however, in 238 U–230 Th radioactive equilibrium. This is generally thought to result from “buffering” of the young slab fluid U-series signal by a sediment component in secular equilibrium. Here we present new 238 U–230 Th–226 Ra and 235 U–231 Pa measurements for historic andesites from Volcán de Colima, Mexico. In all lavas (230 Th/238 U) are in equilibrium, whereas (231 Pa/235 U) and (226 Ra/230 Th) are significantly greater than one. These data demonstrate that arc magmas with (230 Th/238 U) equilibrium can have significant 231 Pa and 226 Ra excesses, precluding ageing of the magmas in the crust as the cause of 230 Th/238 U equilibrium. Quantitative modeling of metasomatic and melting processes further indicates that addition of sediment melts to a depleted mantle wedge produces significant 230 Th excesses and that 238 U excesses induced by recent addition of fluids derived from the altered oceanic crust are not sufficient to compensate these 230 Th excesses. U-series activity ratios in Colima magmas are best explained by models in which the metasomatised mantle returns to secular equilibrium before melting, implying a time lag 350 kyr, with subsequent production of 231 Pa and 226 Ra excesses by in-growth during melting rather than by addition of slab fluids. Investigation of a global compilation of U-series data in arc magma indicates that our model proposed for Colima applies to most arc magmas in or near (230 Th–238 U) equilibrium. The time lag between mantle metasomatism and melting appears to vary between hundreds years to more than 350 kyr in subduction zones. We posit that the absence of U/Th elemental fractionation during melting of arc sources in (230 Th/238 U) equilibrium reflects a higher f O2 compared to MORB sources that yield magmas with 230 Th excesses. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Arc magmatism plays a key role in the evolution of the Earth by producing new continental crust and recycling subducted oceanic lithosphere back into the mantle. Understanding the physical processes involved in the generation of arc magmas and knowledge about material fluxes through subduction zones provide, therefore, critical constraints on the chemical evolution of the continental crust and mantle. The distinctive incompatible trace element compositions of arc magmas generally require contributions from three components: (1) a variably depleted mantle wedge, (2) a sediment component derived from the subducted slab, and (3) aqueous fluid

*

Corresponding author. E-mail address: [email protected] (O. Reubi). 1 Present address: Institut des sciences de la Terre, Université de Lausanne, Switzerland. http://dx.doi.org/10.1016/j.epsl.2014.01.054 0012-821X/© 2014 Elsevier B.V. All rights reserved.

produced by dehydration of the subducted oceanic crust (Elliott et al., 1997; Gill, 1981; Hawkesworth et al., 1997; Plank and Langmuir, 1998). The relative proportions of these components are estimated from the trace element and isotopic compositions of arc magmas. The physical characteristics of these components (aqueous fluid, hydrous melt or super critical fluid) being added to the mantle wedge as well as the timing and location of their addition are still a matter of debate. The addition of aqueous fluids shortly before mantle melting is inferred from U-series disequilibria in arc magmas, suggesting a direct temporal, spatial and causal relationship between slab fluid production and melting of the mantle wedge beneath the arcs (Condomines et al., 1988; Elliott et al., 1997; Gill and Williams, 1990; Hawkesworth et al., 1997; McDermott and Hawkesworth, 1991; Sigmarsson et al., 2002; Turner et al., 2000, 2001). On the other hand, experimental data indicate that most of the water budget of the down-going slab should be transferred to the mantle

O. Reubi et al. / Earth and Planetary Science Letters 391 (2014) 146–158

Fig. 1. Histogram of (230 Th/238 U) activity ratios for arc magmas with SiO2 content <63 wt%. Data from the GEOROC database and references herein (Electronic Annex A3). The bin size corresponds to conservative analytical uncertainties reported for secular equilibrium standards [±1.5%; Sims et al. (2008a)]. Twelve percent of the samples are within ±1.5% of 230 Th–238 U secular equilibrium.

wedge at pressures lower than those of the slab directly beneath the volcanic arc (Schmidt and Poli, 1998). Given the rates of subduction one would expect a significant time lag between metasomatism of the mantle and melting corresponding to the downward transport of hydrated mantle from the dewatering to the loci of melting beneath the arc where temperatures exceed the wet solidus of peridotite. In addition, it is uncertain whether the sediment component is added synchronously to the aqueous fluid by flux melting of the down-going sediments (Plank et al., 2009) or beforehand by high-temperature melting of sediment diapirs detached from the slab and ascending through the mantle wedge (Behn et al., 2011; Gerya and Yuen, 2003). Disequilibria between the short-lived U-series isotopes in arc magmas provides important information about the time scales of fluid transfer, magma generation, and material transport beneath arcs (e.g. Turner et al., 2003 and references herein). In contrast to MORB and OIB magmas that have excesses of 230 Th over 238 U [i.e. (230 Th/238 U) > 1; where the parentheses denote activity ratios], the majority of arc lavas have (230 Th/238 U) < 1 (Allegre and Condomines, 1982; Condomines et al., 1988; Gill and Williams, 1990; McDermott and Hawkesworth, 1991; Turner et al., 2003, see Fig. 1). These 238 U excesses characteristic of arc magmas are generally attributed to addition of U to the mantle wedge by slabderived fluids and implies that the time between fluid addition and subsequent eruption of these magmas is within ∼ five halflives of 230 Th (t 1/2 = 75 kyr) (Allegre and Condomines, 1982; Gill and Williams, 1990; Elliott et al., 1997; Hawkesworth et al., 1997; McDermott and Hawkesworth, 1991; Turner and Hawkesworth, 1997). Large excesses of 226 Ra relative to 230 Th in arc magmas, as well as broad correlations between 226 Ra and 238 U excesses and elemental ratios indicative of slab-fluid addition (e.g. Ba/Th) provide additional evidence for recent addition of U and Ra to the mantle wedge by a slab component (Hoogewerff et al., 1997; Gill and Williams, 1990; Turner et al., 2000, 2001; Sigmarsson et al., 2002). Preservation of these correlations in erupted magmas would require the time between fluid addition and eruption to be significantly shorter than the half-life of 226 Ra (t 1/2 = 1.6 kyr). Alternatively, 226 Ra excess may be produced by in-growth of 226 Ra during mantle melting (Spiegelman and Elliott, 1993; Williams and Gill, 1989), and correlations with elemental ratios such as Ba/Th could indicate crustal assimilation (Huang et al., 2008; Reubi et

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al., 2011). 226 Ra excesses may, consequently, not be unequivocal proof for recent (<8 kyr) slab-derived fluid addition. In most cases, large excesses of 231 Pa over 235 U are observed in arc magmas and are interpreted as melting signals overprinting those of fluid addition for this parent-daughter pair (Pickett and Murrell, 1997; Bourdon et al., 1999; Thomas et al., 2002; Turner et al., 2006; Huang and Lundstrom, 2007). Recently, Avanzinelli et al. (2012) suggested that 231 Pa excesses may be a slab-derived signal rather than a melting signal. Although a few MORB samples have (230 Th/238 U) < 1 (Bourdon et al., 1996; Tepley et al., 2004; Elkins et al., 2011), 238 U excesses are more typical of arc settings. A large proportion of arc magmas are, nevertheless, in or close to 238 U–230 Th secular equilibrium (Condomines and Sigmarsson, 1993; McDermott and Hawkesworth, 1991) and 230 Th excesses are relatively common (e.g. Bourdon et al., 2000; George et al., 2003; Jicha et al., 2007; Reagan et al., 1994) (Fig. 1). Excesses of 230 Th have been attributed either to: (1) 230 Th in-growth during dynamic melting of lherzolitic mantle (George et al., 2003), (2) contamination by lower crustal melts extracted from lithologies with residual phases that preferentially retain U over Th (e.g. garnet) (Bourdon et al., 2000; Garrison et al., 2006; Jicha et al., 2007), or (3) melting of the subducted slab in the presence of garnet (Sigmarsson et al., 1998). These models all assume that 230 Th excesses are superimposed upon metasomatised mantle sources with 238 U excesses indicative of recent addition of slab-fluids. In this context, the significance of arc lavas in 238 U–230 Th equilibrium is less clear. It may be related to the processes described above, although it would be highly fortuitous that the resulting compositions are in 238 U–230 Th equilibrium. Other alternatives include: (1) long residence times of the magmas in the crust, (2) predominant contribution of U and Th from sediment or crustal melts in secular equilibrium, or (3) the absence of U–Th fractionation during partial melting of mantle sources in secular equilibrium. Condomines and Sigmarsson (1993) suggested that 238 U–230 Th equilibrium reflect swamping of the young slab fluid U-series signal by addition of a sediment component in secular equilibrium, and several studies have subsequently identified the 238 U–230 Th equilibrium component to be a sediment melt (Elliott et al., 1997; Johnson and Plank, 1999; Turner and Foden, 2001). In this paper, we suggest that 238 U–230 Th equilibrium at Volcán de Colima, Mexico, and other arc volcanoes, can result from ageing of the metasomatised source in the mantle wedge prior to melting (duration 350 kyr). Here, we present 238 U–230 Th–226 Ra and 235 U–231 Pa isotope data for historic lavas from Volcán de Colima that are characterized by 238 U–230 Th equilibrium but also have large excesses of 226 Ra and 231 Pa. We discuss possible mechanisms that could create the patterns of disequilibria observed for Colima lavas. We then discuss the broader implications for the interpretation of U-series disequilibria in arc magmas and for the processes and time scales of mantle metasomatism, with a particular emphasis on interpreting lavas in 230 Th–238 U equilibrium. 2. Volcán de Colima One of the most historically active volcanoes in North America, Volcán de Colima (VDC) is located in the western part of the Trans-Mexican Volcanic Belt within the N–S trending Colima graben. Quaternary volcanism within the Colima graben is associated with the subduction of the young Rivera (∼10 Ma) and Cocos (∼11 Ma) plates along the Middle-American Trench (Pardo and Suarez, 1995). The subduction rate increases toward the southeast from 2–3 (Rivera plate) to 5–6 cm/yr (Cocos plate) (Fig. 2). Pardo and Suarez (1995) estimated that the top of the subducting slab is at a depth of ∼100 km beneath VDC, and the crust thickness is estimated to be 25–30 km on the basis of gravimetric data

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Fig. 2. General tectonic setting of Volcán de Colima and the Trans-Mexican Volcanic Belt (TMVB). Tectonic features are from Pardo and Suarez (1995). The numbers along the trench (Middle America Trench) indicate the convergence rate in cm/yr. The black contours represent the depth of the subducted slab in km. EPR indicates the East Pacific Rise. White triangles indicate Quaternary volcanoes.

(Urrutia-Fucugauchi and Flores-Ruiz, 1996). VDC is the youngest cone within a large volcanic complex, which also includes Nevado de Colima (0.53–0.14 Ma) and Volcán Cantaro (1.52–0.95 Ma) (Cortés et al., 2005). VDC historical magmas are andesitic in composition with 57.9–61.5 wt% SiO2 (H2 O-free) (Luhr and Carmichael, 1980; Luhr, 2002; Reubi and Blundy, 2008; Reubi et al., 2013; Savov et al., 2008). All historical andesites show nearly identical primitivemantle normalized trace element patterns typical of subductionrelated magmas with negative Nb, Ta, and Ti anomalies (Luhr, 2002; Savov et al., 2008). Sr, Nd, Pb, and O isotope data for VDC historical magmas, including samples of the 1998-present activity, are reported in Valdez-Moreno et al. (2006) and Verma and Luhr (2010). VDC historical magmas show a small range of 87 Sr/86 Sr values from 0.70354 to 0.70361. 143 Nd/144 Nd range from 0.512911 to 0.512993, 206 Pb/204 Pb from 18.57 to 18.61, 207 Pb/204 Pb from 15.56 to 15.60, and 208 Pb/204 Pb from 38.28 to 38.41. δ 18 O values range between +5.7h and +6.4h (Valdez-Moreno et al., 2006). A basaltic andesite from Volcán Cantaro analysed by Verma and Luhr (2010) is isotopically similar to MORB and has the least radiogenic Sr and the most radiogenic Nd (0.702818 and 0.513050 respectively) yet measured for any locality in the Trans-Mexican Volcanic Belt. The sample suite we have analyzed for this study cover the 1998–2010 period of activity. These andesites contain 30–40% phenocrysts with plagioclase being the dominant phase, followed by orthopyroxene, clinopyroxene, titanomagnetite, and rare resorbed hornblende (Luhr, 2002; Reubi et al., 2013; Reubi and Blundy, 2008). 3. Analytical techniques Major elements were determined by X-ray fluorescence (XRF) on fused glass discs using a wave-length dispersive spectrometer at ETHZ. Trace element concentrations were determined using a PerkinElmer ELAN quadrupole ICP-MS at ETHZ. The samples (50 mg) were processed using the method described in Kelley et al. (2003). 115 In and 187 Re were used for internal standard normalization and the following rocks were used for calibration curves: BHVO-2, BCR-2, AGV-2, and BIR-1. The rock standard W2 was used as an external standard. Duplicate analyses of W2 indicate that

trace element data obtained by ICP-MS are accurate to within ±10% (<±5% for most elements) (Table 1). U-series analyses were done at ETHZ and WHOI. Full details of the chemical separation and analytical techniques used at ETH are given in Koornneef et al. (2010), and briefly described below. U, Pa, Th, and Ra concentrations and isotope ratios were determined on single aliquots (about 900 mg) spiked with 236 U, 229 Th, 233 Pa, and 228 Ra tracers before dissolution in a HF-HNO3 mixture. U, Pa, Th, and Ra purification was achieved using a combination of EiChrom® TRU-spec and Sr-spec, and Bio-Rad® AG 1-X4, AG 1-X8 and AG50W-X8 resin columns. Concentrations and isotope ratios were measured on a Nu Instrument® MC-ICP-MS fitted with a high abundance sensitivity filter. Rock standard BCR-2 and W2 which are in secular equilibrium were analysed concurrently with the samples for quality control. Results are listed in Table 1. U, Th, and Ra concentrations and isotope ratios were determined on a subset of samples at WHOI following the procedure described in detail in Sims et al. (2008a and 2008b) and Ball et al. (2008). About 1000 mg of rock powder were dissolved by a series of digestions using HF and HNO3 . Th and U were separated and purified using two anion columns filled with AG1-X8 resin. Samples were loaded in the first column in HNO3 to separate Th and U from the silicate rock matrix. In the second column, samples were loaded in HCl to separate Th from U. Th and U isotopes were measured using the WHOI ThermoFisher NEPTUNE. U and Th concentrations on separate liquid aliquots from the same rock dissolution were determined by isotope dilution using the ThermoFisher Element 2 high resolution sector-field ICPMS. Rock sample aliquot sizes contained ∼10 ng of 238 U. Each aliquot was spiked with individual 229 Th and 233 U spikes. A nitric anion column was used to separate Th and U from most of the silicate matrix. 226 Ra concentrations on separate liquid aliquots from the same rock dissolution were determined by isotope dilution using the ThermoFisher Neptune at WHOI. Rock sample aliquot sizes contained ∼30–100 fg of 226 Ra. Each aliquot is spiked with 228 Ra ‘milked’ from NIST SRM 3159 Th standard. 231 Pa concentrations were subsequently determined from different dissolution of the same samples at ETHZ using the procedure described in the preceding paragraph. 4. Results The 1998–2010 magmas are andesitic and relatively homogeneous in composition (60.2 ± 1.2 wt% SiO2 ) (Fig. 3(a)) (Electronic Annex A1). Broad linear correlations are observed between most major elements. SiO2 correlates negatively with CaO, MgO, FeO, and positively with K2 O. Incompatible trace elements also show broad positive correlations with SiO2 . VDC magmas have typical subduction-related trace element signatures showing Nb, Ta, and Ti negative anomalies (Fig. 3(b)). Large ion lithophile elements (LILE) are enriched relative to light rare earth elements (LREE) (e.g. Ba/La 39.6–43.6) and both are enriched relative to high field strength elements (HFSE) (e.g. Ba/Nb 129–145; La/Nb 3.2–3.5). The U-series data for whole rocks are reported in Table 1 and Electronic Annex A1. VDC 1998–2010 andesites have (238 U/230 Th) values within analytical error (∼1%) of secular equilibrium, while the (230 Th/232 Th) ratios range from 1.22 to 1.27 (Figs. 5 and 6). All magmas have significant excess 231 Pa relative to 235 U, with activity ratios between 1.7 and 2.0 (Fig. 6(a)). The (226 Ra/230 Th) ratios range between 1.47 to 1.61 (Fig. 6(b)). Despite the small ranges in major element contents and U-series disequilibria, broad correlations between 226 Ra and some major elements (MgO, FeO, K2 O) are observed.

Table 1 U-series data of Volcán de Colima magmas. Ua (ppm)

Ub (ppm)

226 Ra (fg/g)

231

Pab (fg/g)

(238 U/232 Th) 2 SE

(230 Th/232 Th) 2 SE

(230 Th/238 U) 2 SE

(234 U/238 U) 2 SE

(231 Pa/235 U)b 2 SE

(226 Ra/230 Th) 2 SE

(a) (a) (a) (a) (a) (a) (a) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b) (b)

1.497 1.561 1.531 1.553 1.570 1.582 1.605 1.603 1.630 1.651 1.601 1.552 1.597 1.545 1.682 1.750 1.762 1.680

0.621 0.650 0.635 0.652 0.656 0.658 0.665

0.627 0.657 0.642 0.647 0.659 0.669 0.658 0.659 0.675 0.678 0.662 0.639 0.657 0.635 0.694 0.720 0.707 0.690

313.6 347.7 345.8 334.7 337.0 346.5 321.7 335.5 352.8 345.8 340.3 330.9 336.7 328.0 350.6 358.5 360.5 349.9

395.1 434.1 413.2 412.3 415.5 420.8 423.3 415.8 388.3 423.1 419.4 406.5 423.2 400.9 429.7 444.9 442.3 431.5

1.259 1.264 1.259 1.273 1.268 1.262 1.256 1.247 1.257 1.246 1.254 1.249 1.248 1.248 1.252 1.248 1.218 1.247

0.022 0.022 0.017 0.021 0.017 0.018 0.021 0.028 0.028 0.028 0.028 0.029 0.028 0.028 0.029 0.027 0.030 0.032

1.264 1.262 1.259 1.261 1.265 1.264 1.258 1.247 1.248 1.250 1.268 1.247 1.259 1.252 1.260 1.250 1.227 1.245

0.003 0.005 0.002 0.003 0.002 0.002 0.002 0.012 0.008 0.017 0.015 0.018 0.016 0.015 0.020 0.013 0.021 0.025

1.004 0.998 1.000 0.991 0.998 1.002 1.001 0.999 0.993 1.003 1.011 0.999 1.009 1.003 1.007 1.002 1.007 0.999

0.035 0.035 0.027 0.033 0.027 0.028 0.028 0.028 0.028 0.028 0.028 0.029 0.028 0.028 0.029 0.027 0.030 0.032

1.002 1.002 1.002 1.002 1.002 1.002 1.001 1.008 1.000 1.007 1.008 1.003 1.005 1.006 1.007 1.008 1.005 1.007

0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.001 0.002 0.003 0.002 0.002 0.002 0.003 0.002 0.002 0.002

1.913 2.006 1.955 1.935 1.914 1.908 1.952 1.916 1.746 1.894 1.924 1.932 1.956 1.915 1.880 1.877 1.898 1.898

0.047 0.049 0.048 0.048 0.047 0.047 0.048 0.048 0.047 0.048 0.048 0.049 0.048 0.047 0.047 0.047 0.048 0.048

1.489 1.586 1.612 1.535 1.524 1.558 1.431 1.516 1.558 1.506 1.506 1.536 1.505 1.523 1.486 1.473 1.498 1.503

0.022 0.024 0.024 0.023 0.023 0.023 0.021 0.022 0.014 0.022 0.019 0.023 0.021 0.019 0.024 0.017 0.027 0.031

Quality assurance standards BCR-2 (b)

5.905

1.690

579.3

560.2

0.869

2σ (#) 0.007 (7)

0.878

2σ (#) 0.016 (11)

1.007

2σ (#) 0.009 (7)

0.999

2σ (#) 0.009 (8)

1.007

2σ (#) 0.019 (4)

1.008

2σ (#) 0.013 (4)

Sample

2003 2004 2002 2005 1999 2003 2005 2005 1998 2007 1999 2004 2004 2003 1998 1998 1998 2010

211004 31004 180202 130305 MX54 1.2003 MX61 MX61 Mx60 111407 Mx55 Mx66 Mx65 Mx53 MX69 MX71 Mx70 Mx64

Notes: Activity ratios (denoted by parentheses) calculated using decay constants: λ238 = 1.5513 × 10−10 yr−1 , λ234 = 2.8263 × 10−6 yr−1 , λ235 = 9.8485 × 10−10 yr−1 , λ232 = 4.9475 × 10−11 yr−1 , λ231 = 2.1158 × 10−5 yr−1 . λ230 = 9.1577 × 10−6 yr−1 , λ226 = 4.331 × 10−4 yr−1 . Measurements done at: (a) WHOI and (b) ETHZ. (231 Pa/235 U) are calculated using the U concentrations measured at ETH on the same aliquot used for 231 Pa measurements. Errors (2SE) are calculated using standard error propagation methods and include uncertainties in 236 U, 229 Th, 228 Ra and 233 Pa spikes used for isotope dilution, the weighing errors, and measurement precision for the samples and bracketing standards. Values for the International Standards are mean values over the period of analysis. Reproducibility is expressed as 2σ ; (#) = number of analyses.

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Th (ppm)

Eruption year

149

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Fig. 3. (a) Anhydrous K2 O versus SiO2 diagram for magma erupted between 1998 and 2010 at Volcán de Colima and (b) Primitive mantle normalized trace element patterns for Volcán de Colima 1998–2010 andesites. Normalizing and MORB values are from Sun and McDonough (1989).

5. Discussion U-series disequilibria in Colima andesites indicate that arc magmas in 238 U–230 Th equilibrium can have significant 231 Pa and 226 Ra excesses (relative to 235 U and 230 Th, respectively). Because 231 Pa and 226 Ra have shorter half-lives than 230 Th, 238 U–230 Th equilibrium cannot simply result from ageing of the magmas in the crust after production of 238 U–230 Th disequilibria in the mantle (via slab-derived fluid addition). A process or combination of processes that can produce 231 Pa and 226 Ra excesses while either preserving or yielding 238 U–230 Th equilibrium is required. Here we present quantitative geochemical models to explore plausible scenarios. A large body of evidence indicates that production of Useries disequilibria is related to mantle processes (e.g. Allegre and Condomines, 1982; Condomines et al., 1988; Bourdon et al., 2003; Gill and Williams, 1990; Hawkesworth et al., 1997; Turner et al., 2001, 2003; Williams and Gill, 1989), and consequently, we focus on mantle metasomatism and melting. Crustal processes can, nevertheless, have significant leverage on U-series disequilibria (Handley et al., 2008; Price et al., 2007; Huang et al., 2008; Reubi et al., 2011) and we will first consider this possibility before moving on to examine mantle processes. 5.1. Differentiation and crustal contamination Volcán de Colima andesites have ∼60 wt% SiO2 and ∼4 wt% MgO and are not primary magmas in equilibrium with mantle peridotite, implying that they underwent significant amount of fractional crystallization. U, Th, and Pa are all strongly incompatible with respect to the mineral assemblage olivine-pyroxenesplagioclase-amphibole (Blundy and Wood, 2003), and fractional crystallization cannot change elemental ratios sufficiently to create or alter 238 U–230 Th and 235 U–231 Pa disequilibria. On the other hand, Ra is somewhat less incompatible than Th in plagioclase (Fabbrizio et al., 2009) and fractionation or accumulation of plagioclase may modify (226 Ra/230 Th) activity ratios. Experimental data indicate that differentiation of hydrous MgO-rich basaltic melts to produce intermediate arc magmas may involve up to 20% plagioclase fractionation (Melekhova et al., 2013). Assuming an initial (226 Ra/230 Th) of 1.6 in the basaltic melt, 20% fractionation of plagioclase results in a 0.5% diminution of the 226 Ra excess [calculated using plagioclase An80 and Fabbrizio et al. (2009) partition

coefficients]. If the effect of fractional crystallization is small, timedependent differentiation may have a more pronounced leverage on U-series disequilibria (Huang et al., 2008). Preservation of 226 Ra excesses in Colima magmas implies that the residence times of the magmas in the crust did not exceed several half-lives of 226 Ra (1.6 kyr) and are, therefore, short compared to the half-lives of 231 Pa (32 kyr) and 230 Th (72 kyr). Mixing or mingling between basaltic and silicic magmas is often called upon in andesite petrogenesis (e.g. Reubi and Blundy, 2009). If the differentiated magmas that have high U and Th contents relative to the basaltic magmas have aged in the crust prior to mixing, this process may have a pronounced leverage on Useries disequilibria with little effects on Sr, Nd or Pb isotopes (Huang et al., 2008; Reubi et al., 2011). The Colima andesites result from mingling between silicic melts and 15% of older gabbroic fragments with low U and Th contents (0.2 and 0.4 ppm, respectively) (Reubi and Blundy, 2008). Assuming that the gabbroic fragments are in secular equilibrium, 15% mingling results in <2% decrease in U-series disequilibria in the silicic magmas. Assimilation of crustal rocks in secular equilibrium could also diminish mantle-derived U-series signal (Handley et al., 2008; Price et al., 2007; Reubi et al., 2011). Nd–Sr–Pb isotope ratios of VDC magmas are essentially invariant across the range of magma compositions from mafic to silicic (Luhr, 1997; Valdez-Moreno et al., 2006; Verma and Luhr, 2010). This isotopic uniformity indicates that assimilation of old radiogenic crustal material did not modify significantly the geochemistry of the magmas. Oxygen isotope data suggest, nonetheless, assimilation of Cretaceous batholiths forming the VDC basement in the historical andesites (Valdez-Moreno et al., 2006). The 1998 to 2010 andesites have δ 18 O values between 5.81 and 6.09, when the Cretaceous batholiths have an average δ 18 O of 8.5 (Valdez-Moreno et al., 2006). Starting from a basaltic melt with mantle-like δ 18 O of 5.7 (Bindeman, 2008), the Colima andesites δ 18 O may imply 4 to 15% crustal assimilation. These values represent, however, maximal amounts as crystal fractionation is expected to produce a ∼0.2h increase in δ 18 O from basalt to andesite (Bindeman, 2008). Variations in incompatible trace element and Sr–Pb isotope ratios in VDC 1998–2010 magmas can be reproduced by less than 5% assimilation of basement granodiorites (Fig. 4) and this also reproduces the range of U-series disequilibria (Figs. 5 and 6). Crustal contamination most likely causes the small range of U-series disequilibria and the broad correlations between 231 Pa and 226 Ra and some major elements observed.

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Fig. 4. (a) U/Th vs. Nb/Th, (b) Ba/Th vs. Th/La and (c) 87 Sr/86 Sr vs. 206 Pb/204 Pb diagrams for Volcán de Colima 1998–2010 andesites and metasomatism models. The thick solid curves show the effect of addition of sediment melts to a depleted mantle with the composition given by Salters and Stracke (2004). The sediment melts are calculated using melt/eclogite bulk partition coefficients from Martindale et al. (2013) and the Cocos plate sediments average composition from Plank (2014). The grids in (a) and (b) show the composition of the magmas as a function of the proportion of sediment melts [Martindale et al. (2013) 850 ◦ C Ds] and slab-fluids added to the mantle wedge and taking into account 15% partial melting of the metasomatised sources and 50% crystallization. The slab fluids are assumed to be produced in the altered oceanic crust (AOC) and were calculated using the AOC global average trace element composition from Kelley et al. (2003), Sr and Pb isotopic composition of the Cocos altered oceanic crust from Verma (2002), and fluid/eclogite bulk partition coefficients from Kessel et al. (2005) 800 ◦ C experiments. The crustal contamination trends show the effect of assimilation of Cretaceous granodiorites forming the basement beneath Colima [data from Valdez-Moreno et al. (2006)]. Sr and Pb isotope data for VDC magmas are from Valdez-Moreno et al. (2006). In (c) the black curves show mixing between sediment melts [Martindale et al. (2013) 850 ◦ C Ds], slab-fluids and the mantle wedge for a fixed proportion 2/3 slab-fluid and 1/3 sediment melt in the slab-derived metasomatic agent. The orange curve shows mixing between sediment melt [Martindale et al. (2013) 900 ◦ C Ds] and the mantle wedge which corresponds to the orange mixing trends in (a) and (b). The compositions of the components and the partition coefficients used in the models are provided in Electronic Annex A2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

We conclude that intracrustal magmatic processes certainly modified mantle-derived U-series disequilibria in Colima magmas and that crustal contamination is likely to be the most important process involved. The amount of crustal contamination in our least-contaminated samples is <5% but could, nonetheless, have diminished mantle-derived U-series excesses. However, the significant 231 Pa excesses clearly indicate that intracrustal magmatic processes did not modify drastically the U-series disequilibria in Colima magmas and are not responsible for 238 U–230 Th equilibrium.

5.2. Trace element constraints on the slab-derived components Primitive mantle normalized trace element patterns of VDC whole rocks show typical enrichment and depletion patterns for arc magmas, indicating addition of slab-derived agents to the mantle wedge (Fig. 3(b)). Sr and Nd isotopic ratios [0.702818 and 0.513050, respectively (Verma and Luhr, 2010)] for a basaltic andesite from Cantaro Volcano (20 km north from Colima and part of the Colima graben) are similar to those estimated for the depleted MORB mantle (DM) isotopic composition [0.7027 and 0.5131, respectively (Salters and Stracke, 2004)]. This indicates that

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Fig. 5. U–Th activity ratios for Volcán de Colima 1998–2010 magmas and metasomatism and assimilation models. Metasomatism by slab-fluids and sediment melts models are calculated as in Fig. 4 using Kessel et al. (2005) 800 ◦ C and Martindale et al. (2013) 850 and 900 ◦ C partition coefficients, respectively (Electronic Annex A2). Several distinct scenarios are shown: In (a) the DM source is metasomatised by 2% AOC fluid and the sediment melt proportions shown on the black mixing curve. In (b) metasomatism is due to a sediment melt only. (c) Shows ageing of a source metasomatised by 1% sediment melt (850 ◦ C partition coefficients) and 2% AOC fluid. In (d) metasomatism occurs in two successive steps; first a time lag 350 kyr allows the sediment melt to return to secular equilibrium before addition of the AOC fluid and metasomatism of the DM. The crustal contamination trend is calculated as in Fig. 4. The grey boxes on the metasomatism trends show the mixing proportions permissible by the trace elements and Sr, Nd, Pb isotopes models shown in Fig. 4.

the mantle wedge beneath VDC is similar to a MORB source. Starting from a DM source, the incompatible trace element ratios of VDC magmas appear to require contributions from one or several components enriched in LILE and collectively characterized by high Ba/Th, U/Th, Th/La and Th/Ce, and low Nb/Th and Sm/La. A number of studies of arc magmas have identified separate contributions from subducted sediments and fluids originating from altered oceanic crust (AOC) (e.g. Elliott et al., 1997; Hawkesworth et al., 1997; Turner et al., 1998; Plank and Langmuir, 1998). VDC magmas can be reproduced by addition of AOC fluids or hydrous melts (high Ba/Th and U/Th component) and/or sediment melts (high Th/La and Th/Ce, low Nb/Th and Sm/La component) to the DM source (Fig. 4). Estimates of the relative contribution from each component were calculated by modeling the effect of metasomatism. The DM composition from Salters and Stracke (2004) was used as the initial composition of the mantle wedge. The AOC-derived fluid (or hydrous melt) was calculated using fluid/eclogite bulk partition coefficient from Kessel et al. (2005) 800 ◦ C experiments. The trace element composition of the AOC comes from a global average of Kelley et al. (2003), and the Sr and Pb isotopic compositions of the altered Cocos plate oceanic crust were taken from Verma (2002). The trace element and isotopic compositions of the sediments represent an average of sediments overlying the Cocos plate from Plank (2014). As the U and Th budget of the magmas are predominantly controlled by the sediment component (Fig. 7), to simplify the models, the composition of the AOC fluid was kept constant while the sediment melt compositions were calculated using hydrous melt/eclogite bulk partition coefficient from Johnson and Plank (1999), Kessel et al. (2005), Klimm et al. (2008), Hermann and Rubatto (2009), Skora and Blundy (2010) and Martindale et al. (2013), respectively. It is worthwhile mentioning that amongst

these experiments only those from Johnson and Plank (1999) and Martindale et al. (2013) used natural sediment as starting material. The other studies used material doped with trace elements which extends the stability field and amount of accessory phases that incorporate certain trace elements (e.g. LREE and Th in allanite/monazite, HFSE and Ti in rutile), thus leading to artificially low fluid-melt/solids bulk partition coefficient (Martindale et al., 2013). The compositions of the components used in the models are provided in Electronic Annex A2. The U/Th ratio of the combined (sediment ± AOC) slab component can be constrained by the observed mixing relationship between the DM and the VDC magmas (Fig. 7). Considering that the AOC fluid has higher U/Th, the sediment component must have U/Th equal to (absence of AOC fluid) or lower than the mixture. The average Mexican sediment has a higher U/Th than the mixture and only sediment melts calculated using partition coefficient from Martindale et al. (2013) experiments satisfy the mixing requirement (Fig. 7). The best fit between mixing model and observations were obtained using two component (AOC fluid + sediment melt) models and the Martindale et al. (2013) partition coefficients at 850 ◦ C (Fig. 4). Alternatively, models where the slab component consists only of sediment melt with negligible influence from the AOC fluid are possible using Martindale et al. (2013) 900 ◦ C bulk partition coefficient (Fig. 4), although these models do not fit to the Sr and Pb isotope data as well as the AOC fluid + sediment melt models. Both models calculated using Martindale et al. (2013) partition coefficient are used in the following section to assess the role of the slab-derived agents, particularly the sediment melts, on U-series disequilibria in VDC magmas. 5.3. Modeling U-series disequilibria To explore the conditions that can produce 231 Pa and 226 Ra excesses together with 238 U–230 Th equilibrium, we employ a twostep model: (1) metasomatism of the mantle wedge by the slabderived agents and (2) subsequent dynamic partial melting of the metasomatised mantle. The formulation of Williams and Gill (1989) and partition coefficients from Blundy and Wood (2003) were used in our dynamic melting models. D Th , D Pa and D Ra were calculated assuming a spinel lherzolite comprising 57% olivine, 28% orthopyroxene, 13% clinopyroxene and 2% spinel. Partitioning of U between melt and clinopyroxene depends on the oxygen fugacity and it has been shown that D U / D Th increase with decreasing oxygen fugacity (Lundstrom et al., 1994). To account for this effect, D U is arbitrarily chosen to produce a range of D U / D Th ratios. In all our melting models, the residual porosity of the source is assumed to be constant at 0.2%. The sediment melts were calculated using the parameters described in Section 5.2. There are no experimental constraints on the hydrous melt/eclogite bulk partition coefficient for Ra and Pa. Consequently our quantitative metasomatism models focus on U and Th. To provide first-order estimates of 226 Ra and 231 Pa disequilibria in the sediment melt and AOC fluid we used Ba and Nb partition coefficients as chemical proxies for Ra and Pa, respectively. In the case of Pa, Fe–Ti oxides were assumed to be the controlling phases and the relationship D Pa = 0.01 × D Nb (Blundy and Wood, 2003) was used. D Ra was calculated for the residual phase proportion obtained by Martindale et al. (2013) at 850 ◦ C and 3 GPa and D Ra / D Ba relationships from Blundy and Wood (2003). Several important observations can be drawn from the mantle metasomatism models (first step only) (Fig. 5): (i) The (230 Th/232 Th) activity ratios of the sediment melts are too high to account for the activity ratios of Colima magmas independent of the partition coefficients and amount of AOC fluid used. (ii) The sediment melts have sizeable 230 Th excess. (iii) When considering the proportion of sediment melt and AOC fluid permissible by the trace element

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Fig. 6. (231 Pa/235 U) vs. (230 Th/238 U) and (226 Ra/230 Th) vs. (230 Th/238 U) diagram for Volcán de Colima 1998–2010 magmas. Metasomatism and dynamic melting models are also shown. Two distinct scenario are shown; in (#) the metasomatised mantle source is assumed to start melting instantaneously after addition of the slab-derived agents: in (∗) a time lag 350 kyr allows the metasomatised mantle to return to secular equilibrium before melting. The source is assumed to be initially in secular equilibrium, to have the depleted mantle composition given by Salters and Stracke (2004), and to be composed of 57% olivine +28% orthopyroxene +13% clinopyroxene +2% spinel. Metasomatism by slab-fluids and sediment melts models are calculated as in Fig. 4. Dynamic melting calculations according to the formulation of Williams and Gill (1989) and partition coefficients from Blundy and Wood (2003), except for D U that is arbitrarily chosen to produce a range of D U / D Th ratios. The source residual porosity is constant at 0.2%. The grids in (a) show the dynamic melting results for ranges of melting rates (in kg/m3 /yr) and bulk D U / D Th . In (b) dynamic melting models are calculated using a melting rate of 10−4 kg/m3 /yr. The hydrous melt/eclogite bulk partition coefficient for Ra and Pa are poorly constrained. 226 Ra and 231 Pa disequilibria in the metasomatised mantle are, therefore, only indicative (see text for detail). Crustal assimilation trends are as in Figs. 4 and 5.

Fig. 7. U vs. Th contents of the Cocos plate sediments and melts of these sediments. The sediment melts are calculated using experimentally determined melt/eclogite bulk partition coefficients from Johnson and Plank (1999), Hermann and Rubatto (2009), Kessel et al. (2005), Klimm et al. (2008), Skora and Blundy (2010) and Martindale et al. (2013). The continuous oblique line shows U/Th ratios equal to the Cocos plate sediments (Plank, 2014). Assuming that the sediments are initially in secular equilibrium, the melts to the left will have 238 U excesses whereas the melts to the right have 230 Th excesses. The dashed line shows U/Th ratios required in the metasomatic agent (slab-fluid + sediment melt). Although obtained using OIB-derived volcaniclastic sediments unlike the continental arc-derived Mexican sediments, Martindale et al. (2013) partition coefficients give the best fit of the Colima magma compositions.

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models, the metasomatised mantle retains 230 Th excess. (iv) For these mixing proportions, 238 U–230 Th–232 Th activity ratios in the magmas can be reproduced if the mantle source ages for >350 kyr after metasomatism. (v) A mantle source initially metasomatised by sediment melts and that subsequently returned to secular equilibrium before addition of AOC fluids during a second metasomatism event, shortly before eruption, develops 238 U excess. Thus, accordingly these models suggest that addition of sediment melts cannot account for 238 U–230 Th equilibrium in VDC magmas, in contrast with the mechanism suggested by Condomines and Sigmarsson (1993) to explain 238 U–230 Th equilibrium in arc magmas. Our modeling suggests a time lag (350 kyr) between metasomatism by the sediment melts and AOC fluids and mantle melting In the previous section we evaluated the conditions that may produce (230 Th/238 U) equilibrium by addition of slab-derived components. Colima magmas are characterized by significant 231 Pa and 226 Ra excesses together with 230 Th–238 U equilibrium. If the metasomatised mantle was in secular equilibrium at the onset of melting, which we believe to be the most likely scenario, in-growth during mantle melting has to produce 231 Pa and 226 Ra excesses. 226 Ra excesses are generally taken to indicate slab fluid addition to the mantle wedge shortly (<8 kyr) before eruption (Hoogewerff et al., 1997; Gill and Williams, 1990; Sigmarsson et al., 2002; Turner et al., 2000, 2001). In-growth during melting, nevertheless, also produces 226 Ra excesses (Spiegelman and Elliott, 1993; Williams and Gill, 1989), a process testified by (226 Ra/230 Th) measured in MORB (up to 4) (Lundstrom, 2003 and references herein). In arc magmas, the relative contributions from slab fluid and ingrowth during melting depend on the melting rate (Bourdon et al., 2003). Starting from a source in secular equilibrium, high 231 Pa excesses in Colima magmas require the melting rate to be low (<2 × 10−4 kg/m3 /yr) (Fig. 6(a)). With this low melting rate, the duration of melting greatly exceeds the half-life of 226 Ra and (226 Ra/230 Th) is controlled by in-growth during melting with no dependence on the initial disequilibria in the source (Fig. 6(b)). Dynamic melting models, using the melting conditions required by the 231 Pa excesses, yield 226 Ra excesses higher than those measured in VDC magmas even when crustal contamination is taken into account (Fig. 6(b)). This discrepancy most likely indicates that the time scales of melt residence and differentiation in the upper mantle and crust are of the same order of magnitude as the half-life of 226 Ra. In addition, 238 U–230 Th equilibrium requires D U / D Th ≈ 1 during mantle melting. D U / D Th bulk partition coefficient ratios determined experimentally for spinel lherzolite at pressure between 2.5–3 GPa and below FMQ buffer range from 1.27 to 1.33 (Landwehr et al., 2001). U and Th partitioning is predominantly controlled by clinopyroxene during melting of spinel lherzolite and D U / D Th in clinopyroxene decreases under increasingly oxidizing conditions (Lundstrom et al., 1994). Lower bulk D U / D Th than obtained by Landwehr et al. (2001) are, therefore, expected for sources with higher f O2 and values close to unity may be anticipated for metasomatised mantle in arc setting. Production 231 Pa and 226 Ra excesses while preserving 238 U–230 Th equilibrium by in-growth during melting of a metasomatised source in secular equilibrium appears to be the most likely scenario for Colima magmas. Our quantitative models support melting of an aged metasomatised mantle source in secular equilibrium. However, this conclusion depends strongly on the parameters used in the models, particularly the composition of the sediments. If the sediments have lower initial U/Th [i.e. lower (230 Th/232 Th)] than the average composition used in the models, other possible scenarios include: (1) 230 Th excess in the sediment melt balanced by 238 U excess in the AOC fluid, (2) metasomatism due only to sediment melts in 238 U–230 Th equilibrium. Alternatively, in-growth during melting may overprint the metasomatised mantle disequilibria to

yield magmas in 238 U–230 Th equilibrium. These scenarios have the common requirement that all the parameters involved (compositions, amounts and U-series disequilibria of the slab-derived agents, and melting conditions) are exactly balanced to produce (238 U–230 Th) of unity, a rather unlikely scenario. Beyond the fortuitous physiognomies of these scenarios, they raise two important issues. Scenarios 1 and 2 require the slab-derived agent to be in 238 U–230 Th equilibrium and have 231 Pa and 226 Ra excesses. Although there is good evidence that the slab-derived components have 226 Ra excesses (Gill and Williams, 1990; Turner et al., 2000, 2001; Sigmarsson et al., 2002), whether they have 231 Pa excesses or deficits is open to debate (see Avanzinelli et al., 2012). The second issue is whether 238 U in-growth during mantle melting can compensate the 230 Th excesses induced by the sediment melt in the metasomatised mantle. Above we argued that 238 U–230 Th equilibrium reflects D U / D Th ≈ 1 in mantle sources with higher f O2 than MORB sources, but experimental data indicate that this ratio is likely to range to less than 1 (Lundstrom et al., 1994). On this basis, Huang et al. (2011) and Beier et al. (2010) argued that 238 U excesses in arc magmas and in some back-arc basalts, respectively, may be melting rather than metasomatism signatures. Our dynamic melting models of metasomatised mantle sources carrying 230 Th excesses inherited from the slab-derived components (i.e. absence of time lag between metasomatism and melting) indicate that these 230 Th excesses are overcome for D U / D Th values up to 0.9 at low melting rates (1 × 10−4 kg/m3 /yr) (Fig. 6(a)). It is, therefore, possible that 238 U–230 Th equilibrium in Colima magmas represent a situation where in-growth during melting coincidently cancel out the initial 230 Th excesses of the source whilst producing or enhancing 231 Pa and 226 Ra excesses. Considering the number of parameters that need to exactly concur, this scenario is, however, highly improbable. In summary, the main outcome of our metasomatism and melting models is that addition of sediment melts ± AOC fluids from the subducting plate is unlikely to induce 238 U–230 Th equilibrium if the depleted mantle is metasomatised shortly before melting. Instead, melting of a source that has returned to secular equilibrium after addition of the AOC fluids and sediment melts (i.e. ageing 350 kyr) is the more likely explanation. In this case, 238 U–230 Th equilibrium reflects the absence of fractionation between U and Th during mantle melting due to higher mantle f O2 compared to MORB sources. Alternative models require fortuitous circumstances but cannot be ruled out and include two distinct options: (1) U-series disequilibria are buffered by a slab-derived agent characterized by 238 U–230 Th equilibrium and 231 Pa and 226 Ra excesses or (2) in-growth during melting coincidently balance the metasomatism-induced 230 Th excesses of the mantle source while producing or enhancing 231 Pa and 226 Ra excesses. 5.4. Implications for arc magmatism The large proportion of magmas in or near 238 U–230 Th equilibrium is a characteristic of subduction zone magmatism that was first noticed in the initial global studies of U-series disequilibria in volcanic arc rocks (Gill and Williams, 1990; McDermott and Hawkesworth, 1991; Condomines and Sigmarsson, 1993). Even with a significantly larger dataset and higher analytical precision of modern mass spectrometry techniques this observation persists and 32% of analyzed arc magmas with <63 wt% SiO2 are within 5% from 238 U–230 Th equilibrium (Fig. 1). Based on the observation that arc magmas close from 238 U–230 Th equilibrium often show geochemical evidence for sediment addition, this was originally interpreted to reflect swamping of the U and Th signal by addition of sediments in secular equilibrium (Condomines and Sigmarsson, 1993; Elliott et al., 1997; Hawkesworth et al., 1997; Turner and Foden, 2001). Pickett and Murrell (1997) noted that

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Fig. 8. U–Th equiline diagram for arc magmas with SiO2 content <63 wt%. (a) Global compilation of U-series data in arc magmas from the GEOROC database and references herein. Arc volcanoes in or near 230 Th–238 U secular equilibrium are shown in (b), (c) and (d). The mantle metasomatism trends show the effect of addition of variable amount of sediment melts and constant AOC fluids to a depleted mantle source. Metasomatism models are calculated as in Figs. 4 and 5, except for the sediment melts that are calculated using the average sediment composition for each arc from Plank (2014). The amount of AOC fluids is constant and arbitrarily set to reproduce the highest (238 U/232 Th) ratios in arc magmas in the absence of sediment melts. Data are from: Martinique (Turner et al., 1996), Soufrière Hills Volcano (Zellmer et al., 2003), Aniakchak (George et al., 2004), Katmai (Turner et al., 2010), Kamchatka (Turner et al., 1998; Dosseto et al., 2003) and GEOROC database (Electronic Annex A3).

(231 Pa/235 U) versus (230 Th–238 U) trends for arc magmas failed to cross near the point of secular equilibrium for these two systems and suggested that magmas in 238 U–230 Th equilibrium were likely to have 231 Pa excesses. Our new VDC dataset is the first unequivocal demonstration of this case. If 238 U–230 Th equilibrium was due to swamping by a sediment signature, this implies that the sediment component should have 231 Pa and 226 Ra excesses. Thus 238 U–230 Th equilibrium cannot simply result from ageing of the sediment component. Recent studies have shown that the solubility and residual abundance of accessory phases controls the trace element signature of slab-derived melts or fluids (Hermann and Rubatto, 2009; Klimm et al., 2008; Skora and Blundy, 2010). Monazite and allanite exert a strong control on U–Th partitioning because of the high retention of Th in these minerals. Melts expelled from a garnet + rutile-bearing eclogite residue will have 230 Th excesses as a result of the strong preference for U over Th in these minerals, but residues with small amounts of monazite or allanite

will produce melts with 238 U excess (Skora and Blundy, 2010; Martindale et al., 2013). On this basis, Avanzinelli et al. (2012) proposed that U-series disequilibria in Marianas arc magmas are controlled by variable contributions of 230 Th excess bearing sediment melts produced at temperatures above the stability field of monazite or allanite coupled with near constant flux of 238 U excess bearing AOC fluids generated deeper in the subducting oceanic crust at temperatures where these minerals are stable. The high proportion of arc of magmas in or near 238 U–230 Th equilibrium is difficult to account for in this context as it would require that the residual abundances of monazite or allanite and the relative proportion of AOC fluid and sediment melts fortuitously produce slab-derived agents near 238 U–230 Th equilibrium in one third of the cases. Arc volcanoes in or near 238 U–230 Th equilibrium cover almost the complete range of (230 Th/232 Th) ratios measured in arc magmas (Fig. 8) and include magmas showing geochemical evidence for sediment addition [e.g. Martinique and Soufriere Hill, Lesser

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Antilles (Turner et al., 1996; Zellmer et al., 2003); Krakatau, Sunda Arc (Turner and Foden, 2001)], as original observed by Condomines and Sigmarsson (1993) and Hawkesworth et al. (1997), but also magmas with limited geochemical evidence for sediment addition [e.g. Kizimen, Ksudach, Plosk and Klyuchevskoy, Kamchatka arc (Turner et al., 1998; Dosseto et al., 2003)]. Systematic covariation of the parameters to yield magmas in 238 U–230 Th equilibrium is coincidental even when one volcano is considered and extremely unlikely to account for the significant proportion of arc magmas in or near 238 U–230 Th equilibrium considering the variable influence of slab sediments. Furthermore, the (230 Th/232 Th) ratio in the sediment melt often rules out this component as the cause for 238 U–230 Th equilibrium (Figs. 5 and 8(c), (d)). The mantle metasomatism trends shown in Fig. 8 illustrate the well-known buffering effect of sediment melts on the amplitude of 238 U excesses (Condomines and Sigmarsson, 1993; Elliott et al., 1997; Hawkesworth et al., 1997), but also demonstrate that metasomatism by variable amounts of sediment melts cannot account for the distribution of magmas in or near 238 U–230 Th equilibrium along the equiline (Fig. 8). Rather we conclude that aging (>350 kyr) of the source after metasomatism by slab-derived sediment melts and, importantly, the AOC fluids followed by subsequent melting is common in arc setting. In the case where melting of aged metasomatised sources is common in arc setting, the predominant peak near U–Th equilibrium for arc magmas (Fig. 1) potentially reflects the higher oxidation state of the metasomatised mantle wedge compared to MORB sources, and the resulting shift toward bulk D U / D Th values near unity. As this ratio depends on the f O2 , pressure and proportion of clinopyroxene in the source, variations in bulk D U > D Th to D U < D Th will occur and melting of metasomatised mantle sources in secular equilibrium would be expected to produce a range from 238 U to 230 Th excesses distributed around 238 U–230 Th equilibrium, in agreement with the significant peak in the global dataset and the spread observed along the equiline (Fig. 8). In our view, the global U–Th array is best explained in terms of a time lag between metasomatism and melting varying from less than a few hundred years for magmas characterized by very high 226 Ra and 238 U excesses to more than 350 kyr for the magmas on or near the equiline. This implies that in some cases mantle melting has a close temporal and spatial relationship to fluid fluxing, while in other cases significant delays occur potentially allowing downward transport of the hydrated mantle to the loci of melting beneath the arc. The subduction rates beneath the volcanoes in or near 238 U–230 Th equilibrium vary significantly (Fig. 9), suggesting that ageing between metasomatism and melting is independent of the downward velocity of the subducting plate.

Fig. 9. Subduction rate and slab surface depth beneath volcanoes in or near 238 U–230 Th equilibrium. References are as in Fig. 8.

the slab-derived components is a common situation in arc setting. Although the amplitude of 238 U excesses in arc magmas is predominantly controlled by the amount sediment melts added to the source as previously suggested (Condomines and Sigmarsson, 1993; Elliott et al., 1997; Hawkesworth et al., 1997), the U–Th arrays require the time lag between metasomatism and melting to vary from less than few hundred years to more than 350 kyr at global and regional scales. The temporal and spatial relationship between fluid fluxing and mantle melting is, therefore, variable in subduction zones. Whether this implies distinct physical conditions of melting will need further consideration, but the time interval between metasomatism and melting should be considered as an important parameter in investigations of arc magmatism. Acknowledgements This work was funded by a Swiss NSF Ambizione fellowship to OR. K.W.W.S. acknowledges support from NSF. We are grateful to J.M. Koornneef and C. Waters for help with U-series measurements, F. Oberli and C. Maden for their help to run the MC-ICPMS, and to L. Cooper for comments on an earlier version of the manuscript. Thorough reviews by Fang Huang and an anonymous reviewer and editorial handling by Tim Elliott are gratefully acknowledged. Finally, this paper is dedicated to Ian Carmichael (K.W.W.S.).

6. Concluding remarks Appendix A. Supplementary material Historic lavas from Volcán de Colima (Mexico) demonstrate that arc magmas in 238 U–230 Th radioactive equilibrium can have significant 231 Pa and 226 Ra excesses. These excesses of 231 Pa and 226 Ra rule out the possibility that equilibrium between 238 U–230 Th is simply the result of long residence times of the magmas in the crust. Our geochemical modeling indicates that melting of a source that has returned to secular equilibrium after addition of AOC fluids and sediment melts (i.e. ageing 350 kyr) is the most likely explanation. Although 238 U in-growth during mantle melting may fortuitously compensate the 230 Th excesses in the metasomatised mantle source and produce the 231 Pa and 226 Ra excesses or the slab-derived agent may coincidently be in 238 U–230 Th equilibrium whilst carrying 231 Pa and 226 Ra excesses in the Colima case, this cannot account for the significant proportion of arc magmas in or near 238 U–230 Th equilibrium. We conclude that melting of sources that have returned to secular equilibrium after metasomatism by

Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2014.01.054. References Allegre, C.J., Condomines, M., 1982. Basalt genesis and mantle structure studied through Th-iotopic geochemistry. Nature 299, 21–24. Avanzinelli, R., Prytulak, J., Skora, S., Heumann, A., Koetsier, G., Elliott, T., 2012. Combined 238 U–230 Th and 235 U–231 Pa constraints on the transport of slab-derived material beneath the Mariana Islands. Geochim. Cosmochim. Acta 92, 308–328. Ball, L., Sims, K.W.W., Schwieters, J., 2008. Measurement of 234 U–238 U and 230 Th–232 Th in volcanic rocks using the Neptune MC-ICP-MS. J. Anal. At. Spectrom. 23, 1693. Behn, M.D., Kelemen, P.B., Hirth, G., Hacker, B.R., Massonne, H., 2011. Diapirs as the source of the sediment signature in arc lavas. Nat. Geosci. 4, 641–646. Beier, C., Turner, S., Sinton, J.M., Gill, J.B., 2010. Influence of subducted components on back-arc melting dynamics in the Manus Basin. Geochem. Geophys. Geosyst. 11.

O. Reubi et al. / Earth and Planetary Science Letters 391 (2014) 146–158

Bindeman, L., 2008. Oxygen isotopes in mantle and crustal magmas as revealed by single crystal analysis. Minerals, inclusionsions and volcanic processes. Rev. Mineral. Geochem. 69, 445–478. Blundy, J., Wood, B., 2003. Mineral-melt partitioning of uranium, thorium and their daughters. In: Uranium-Series Geochemistry. In: Rev. Mineral. Geochem., vol. 52, pp. 59–123. Bourdon, B., Turner, S., Allegre, C., 1999. Melting dynamics beneath the TongaKermadec Island are inferred from 231 Pa–235 U systematics. Science 286, 2491–2493. Bourdon, B., Turner, S., Dosseto, A., 2003. Dehydration and partial melting in subduction zones: Constraints from U-series disequilibria. J. Geophys. Res. 108, 1–19. Bourdon, B., Worner, G., Zindler, A., 2000. U-series evidence for crustal involvement and magma residence times in the petrogenesis of Parinacota volcano, Chile. Contrib. Mineral. Petrol. 139, 458–469. Bourdon, B., Zindler, A., Elliott, T., Langmuir, C.H., 1996. Constraints on mantle melting at mid-ocean ridges from global 238 U–230 Th disequilibrium data. Nature 384, 231–235. Condomines, M., Hemond, Ch., Allegre, C.J., 1988. U–Th–Ra radioactive disequilibria and magmatic processes. Earth Planet. Sci. Lett. 90, 243–262. Condomines, M., Sigmarsson, O., 1993. Why are so many arc magmas close to 238 U–230 Th radioactive equilibrium. Geochim. Cosmochim. Acta 57, 4491–4497. Cortés, A., Garduno-Monry, V.H., Navarro-Ochoa, C., Komorowski, J.C., Saucedo, R., Macias, J.L., Gavilanes, J.C., 2005. Carta Geologica del Complejo Volcanico de Colima. In: Mexico, U.N.A.d. (Ed.), Cartas Geologicas y Mineras, vol. 10. Dosseto, A., Bourdon, B., Joron, J.L., Dupre, B., 2003. U–Th–Pa–Ra study of the Kamchatka arc: New constraints on the genesis of are lavas. Geochim. Cosmochim. Acta 67, 2857–2877. Elkins, L.J., Sims, K.W.W., Prytulak, J., Elliott, T., Mattielli, N., Blichert-Toft, J., Blusztajn, J., Dunbar, N., Devey, C., Mertz, D.F., Schilling, J.G., Murrell, M., 2011. Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using 238 U, 230 Th, and 231 Pa excesses. Geochim. Cosmochim. Acta 75, 6300–6329. Elliott, T., Plank, T., Zindler, A., White, W., Bourdon, B., 1997. Element transport from slab to volcanic front at the Mariana arc. J. Geophys. Res. 102, 14991–15019. Fabbrizio, A., Schmidt, M.W., Guenther, D., Eikenberg, J., 2009. Experimental determination of Ra mineral/melt partitioning for feldspars and 226 Ra-disequilibrium crystallization ages of plagioclase and alkali-feldspar. Earth Planet. Sci. Lett. 280, 137–148. Garrison, J., Davidson, J., Reid, M., Turner, S., 2006. Source versus differentiation controls on U-series disequilibria: Insights from Cotopaxi volcano, Ecuador. Earth Planet. Sci. Lett. 244, 548–565. George, R., Turner, S., Hawkesworth, C., Bacon, C.R., Nye, C., Stelling, P., Dreher, S., 2004. Chemical versus temporal controls on the evolution of tholeiitic and calc-alkaline magmas at two volcanoes in the Alaska-Aleutian arc. J. Petrol. 45, 203–219. George, R., Turner, S., Hawkesworth, C., Morris, J., Nye, C., Ryan, J., Shu-Hui, Z., 2003. Melting processes and fluid and sediment transport rates along the AlaskaAleutian arc from an integrated U–Th–Ra–Be isotope study. J. Geophys. Res. 108, ECV6-1-25. Gerya, T.V., Yuen, D.A., 2003. Rayleigh–Taylor instabilities from hydration and melting propel ‘cold plumes’ at subduction zones. Earth Planet. Sci. Lett. 212, 47–62. Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Springer, Berlin. Gill, J.B., Williams, R.W., 1990. Th-isotope and U-series studies of subduction-related volcanic-rocks. Geochim. Cosmochim. Acta 54, 1427–1442. Handley, H.K., Turner, S.P., Smith, I.E.M., Stewart, R.B., Cronin, S.J., 2008. Rapid timescales of differentiation and evidence for crustal contamination at intraoceanic arcs: Geochemical and U–Th–Ra–Sr–Nd isotopic constraints from Lopevi Volcano, Vanuatu, SW Pacific. Earth Planet. Sci. Lett. 273, 184–194. Hawkesworth, C.J., Turner, S.P., McDermott, F., Peate, D.W., vanCalsteren, P., 1997. U–Th isotopes in arc magmas: Implications for element transfer from the subducted crust. Science 276, 551–555. Hermann, J., Rubatto, D., 2009. Accessory phase control on the trace element signature of sediment melts in subduction zones. Chem. Geol. 265, 512–526. Hoogewerff, J.A., van Bergen, M.J., Vroon, P.Z., Hertogen, J., Wordel, R., Sneyers, A., Nasution, A., Varekamp, J.C., Moens, L.E., Mouchel, D., 1997. U-series, Sr–Nd– Pb isotope and trace-element systematics across an active island arc-continent collision zone: Implications for element transfer at the slab-wedge interface. Geochim. Cosmochim. Acta 53, 1057–1072. Huang, F., Lundstrom, C.C., 2007. 231 Pa excesses in arc volcanic rocks: Constraint on melting rates at convergent margins. Geology 35, 1007–1010. Huang, F., Gao, L., Lundstrom, C.C., 2008. The effect of assimilation, fractional crystallization, and ageing on U-series disequilibria in subduction zone lavas. Geochim. Cosmochim. Acta 72, 4136–4145. Huang, F., Lundstrom, C.C., Sigurdsson, H., Zhang, Z., 2011. U-series disequilibria in Kick’em Jenny submarine volcano lavas: A new view of time-scales of magmatism in convergent margins. Geochim. Cosmochim. Acta 75, 195–212. Jicha, B.R., Singer, B.S., Beard, B.L., Johnson, C.M., Roa, H.M., Naranjo, J.A., 2007. Rapid magma ascent and generation of 230 Th excesses in the lower crust at Puyehue-Cordon Caulle, Southern Volcanic Zone, Chile. Earth Planet. Sci. Lett. 255, 229–242. Johnson, M.C., Plank, T., 1999. Dehydration and melting experiments constrain the fate of subducted sediments. Geochem. Geophys. Geosyst. 1, 1–26.

157

Kelley, K.A., Plank, T., Ludden, J., Staudigel, H., 2003. Composition of altered oceanic crust at ODP Sites 801 and 1149. Geochem. Geophys. Geosyst. 4. Kessel, R., Schmidt, M.W., Ulmer, P., Pettke, T., 2005. Trace element signature of subduction-zone fluids, melts and supercritical liquids at 120–180 km depth. Nature 437, 724–727. Klimm, K., Blundy, J.D., Green, T.H., 2008. Trace element partitioning and accessory phase saturation during H2 O-saturated melting of basalt with implications for subduction zone chemical fluxes. J. Petrol. 49, 523–553. Koornneef, J.M., Stracke, A., Aciego, S., Reubi, O., Bourdon, B., 2010. A new method for U–Th–Pa–Ra separation and accurate measurement of 234 U–230 Th–231 Pa–226 Ra disequilibria in volcanic rocks by MC-ICPMS. Chem. Geol. 277, 30–41. Landwehr, D., Blundy, J., Chamorro-Perez, E.M., Hill, E., Wood, B., 2001. U-series disequilibria generated by partial melting of spinel lherzolite. Earth Planet. Sci. Lett. 188, 329–348. Luhr, J.F., 1997. Extensional tectonics and the diverse primitive volcanic rocks in the western Mexican Volcanic Belt. Can. Mineral. 35, 473–500. Luhr, J.F., 2002. Petrology and geochemistry of the 1991 and 1998–1999 lava flows from Volcan de Colima, Mexico: implications for the end of the current eruptive cycle. J. Volcanol. Geotherm. Res. 117, 169–194. Luhr, J.F., Carmichael, I.S.E., 1980. Colima Volcanic Complex, Mexico. 1. Post-Caldera Andesites from Volcan Colima. Contrib. Mineral. Petrol. 71, 343–372. Lundstrom, C.C., 2003. Uranium-series disequilibria in mid-ocean ridge basalts: Observations and models of basalt genesis. In: Uranium-Series Geochemistry. In: Rev. Mineral. Geochem., vol. 52, pp. 175–214. Lundstrom, C.C., Shaw, H.F., Ryerson, F.J., Phinney, D.L., Gill, J.B., Williams, Q., 1994. Compositional controls on the partitioning of U, Th, Ba, Pb, Sr and Zr between clinopyroxene and haplobasaltic melts – implications for uranium series disequilibria in basalts. Earth Planet. Sci. Lett. 128, 407–423. Martindale, M., Skora, S., Pickles, J., Elliot, T., Blundy, J., Avanzinelli, R., 2013. High pressure phase relations of subducted volcaniclastic sediments from west pacific and their implications for the geochemistry of Mariana arc magma. Chem. Geol. 342, 94–109. McDermott, F., Hawkesworth, C., 1991. Th, Pb, and Sr isotope variations in young island-arc volcanics and oceanic sediments. Earth Planet. Sci. Lett. 104, 1–15. Melekhova, E., Annen, C., Blundy, J., 2013. Compositional gaps in igneous rock suites controlled by magma system heat and water content. Nat. Geosci. 6, 385–390. Pardo, M., Suarez, G., 1995. Shape of the subducted Rivera and Cocos plates in southern Mexico: seismic and tectonic implications. J. Geophys. Res. 100, 12357–12373. Pickett, D.A., Murrell, M.T., 1997. Observations of 231 Pa–235 U disequilibrium in volcanic rocks. Earth Planet. Sci. Lett. 148, 259–271. Plank, T., 2014. The Chemical Composition of Subducting Sediments, 2nd ed. Treatise Geochem., pp. 607–629. Plank, T., Cooper, L.B., Manning, C.E., 2009. Emerging geothermometers for estimating slab surface temperatures. Nat. Geosci. 2, 611–615. Plank, T., Langmuir, C.H., 1998. The chemical composition of subducting sediment and its consequences for the crust and mantle. Chem. Geol. 145, 325–394. Price, R.C., George, R., Gamble, J.A., Turner, S., Smith, I.E.M., Cook, C., Hobden, B., Dosseto, A., 2007. U–Th–Ra fractionation during crustal-level andesite formation at Ruapehu volcano, New Zealand. Chem. Geol. 244, 437–451. Reagan, M.K., Morris, J.D., Herrstrom, E.A., Murrell, M.T., 1994. Uranium series and beryllium isotope evidence for an extended history of subduction modification of the mantle below Nicaragua. Geochim. Cosmochim. Acta 58, 4199–4212. Reubi, O., Blundy, J., Varley, N., 2013. Volatiles contents, degassing, and crystallisation of intermediate magmas at Volcan de Colima, Mexico, inferred from melt inclusions. Contrib. Mineral. Petrol. 165, 1087–1106. Reubi, O., Blundy, J., 2008. Assimilation of plutonic roots, formation of high-K exotic melt inclusions and genesis of andesitic magmas at Volcán de Colima, Mexico. J. Petrol. 49, 2221–2243. Reubi, O., Blundy, J., 2009. A dearth of intermediate melts at subduction zone volcanoes and the petrogenesis of arc andesites. Nature 461, 1269–1273. Reubi, O., Bourdon, B., Dungan, M.A., Koornneef, J.M., Selles, D., Langmuir, C.H., Aciego, S., 2011. Assimilation of the plutonic roots of the Andean arc controls variations in U-series disequilibria at Volcan Llaima, Chile. Earth Planet. Sci. Lett. 303, 37–47. Salters, V.J.M., Stracke, A., 2004. Composition of the depleted mantle. Geochem. Geophys. Geosyst. 5. Savov, I.P., Luhr, J.F., Navarro-Ochoa, C., 2008. Petrology and geochemistry of lava and ash erupted from Volcan Colima, Mexico, during 1998–2005. J. Volcanol. Geotherm. Res. 174, 241–256. Schmidt, M., Poli, S., 1998. Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation. Earth Planet. Sci. Lett., 361–379. Sigmarsson, O., Chmeleff, J., Morris, J., Lopez-Escobar, L., 2002. Origin of 226 Ra–230 Th disequilibria in arc lavas from southern Chile and implications for magma transfer time. Earth Planet. Sci. Lett. 196, 189–196. Sigmarsson, O., Martin, H., Knowles, J., 1998. Melting of a subducting oceanic crust from U–Th disequilibria in Austral Andean lavas. Nature 394, 566–569. Sims, K.W.W., Gill, J.B., Dosseto, A., Hoffmann, D.L., Lundstrom, C.C., Williams, R.W., Ball, L., Tollstrup, D., Turner, S., Prytulak, J., Glessner, J.J.G., Standish, J.J., Elliott,

158

O. Reubi et al. / Earth and Planetary Science Letters 391 (2014) 146–158

T., 2008a. An inter-laboratory assessment of the thorium isotopic composition of synthetic and rock reference materials. Geostand. Geoanal. Res. 32, 65–91. Sims, K.W.W., Hart, S.R., Reagan, M.K., Blusztajn, J., Staudigel, H., Sohn, R.A., Layne, G.D., Ball, L.A., 2008b. 238 U–230 Th–226 Ra–210 Pb–210 Po, 232 Th–228 Ra, and 235 U–231 Pa constraints on the ages and petrogenesis of Vailulu’u and Malumalu lavas, Samoa. Geochem. Geophys. Geosyst. 9. Skora, S., Blundy, J., 2010. High-pressure hydrous phase relations of radiolarian clay and implications for the involvement of subducted sediment in arc magmatism. J. Petrol. 11, 2211–2243. Spiegelman, M., Elliott, T., 1993. Consequences of melt transport for Uranium series disequilibrium in young lavas. Earth Planet. Sci. Lett. 118, 1–20. Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in the Ocean Basins, pp. 313–345. Tepley, F.J., Lundstrom, C.C., Sims, K.W.W., Hekinian, R., 2004. U-series disequilibria in MORB from the Garrett transform and implications for mantle melting. Earth Planet. Sci. Lett. 223, 79–97. Thomas, R.B., Hirschmann, M.M., Cheng, H., Reagan, M.K., Edwards, R.L., 2002. (231 Pa/235 U)–(230 Th/238 U) of young mafic volcanic rocks from Nicaragua and Costa Rica and the influence of flux melting on U-series systematics of arc lavas. Geochim. Cosmochim. Acta 66, 4287–4309. Turner, S., Bourdon, B., Gill, J., 2003. Insights into magma genesis at convergent margins from U-series isotopes. In: Uranium-Series Geochemistry. In: Rev. Mineral. Geochem., vol. 52, pp. 255–315. Turner, S., Bourdon, B., Hawkesworth, C., Evans, P., 2000. 226 Ra–230 Th evidence for multiple dehydration events, rapid melt ascent and the time scales of differentiation beneath the Tonga-Kermadec island arc. Earth Planet. Sci. Lett. 179, 581–593. Turner, S., Evans, P., Hawkesworth, C., 2001. Ultrafast source-to-surface movement of melt at island arcs from 226 Ra–230 Th systematics. Science 292, 1363–1366. Turner, S., Foden, J.D., 2001. U–Th–Ra disequilibria, Sr–Nd–Pb isotope and trace element variations in Sunda arc lavas: predominance of a subducted sediment component. Contrib. Mineral. Petrol. 142, 43–57.

Turner, S., Hawkesworth, C., 1997. Constraints on flux rates and mantle dynamics beneath island arcs from Tonga-Kermadec lava geochemistry. Nature 389, 568–573. Turner, S., Hawkesworth, C., vanCalsteren, P., Heath, E., Macdonald, R., Black, S., 1996. U-series isotopes and destructive plate margin magma genesis in the Lesser Antilles. Earth Planet. Sci. Lett. 142, 191–207. Turner, S., McDermott, F., Hawkesworth, C., Kepezhinskas, P., 1998. A U-series study of lavas from Kamchatka and the Aleutians: constraints on source composition and melting processes. Contrib. Mineral. Petrol. 133, 217–234. Turner, S., Regelous, M., Hawkesworth, C., Rostami, K., 2006. Partial melting processes above subducting plates: Constraints from 231 Pa–235 U disequilibria. Geochim. Cosmochim. Acta 70, 480–503. Turner, S., Sandiford, M., Reagan, M., Hawkesworth, C., Hildreth, W., 2010. Origins of large-volume, compositionally zoned volcanic eruptions: New constraints from U-series isotopes and numerical thermal modeling for the 1912 KatmaiNovarupta eruption. J. Geophys. Res. 115, B12201. Urrutia-Fucugauchi, J., Flores-Ruiz, J.H., 1996. Bouguer gravity anomalies and regional crustal structure in Central Mexico. Int. Geol. Rev. 38, 176–194. Valdez-Moreno, G., Schaaf, P., Schaaf, P., Macias, J.L., Kusakabe, M., 2006. New Sr– Nd–Pb–O isotope data for Colima volcano and evidence for the nature of the local basement, neogene-quarternary continental margin volcanism: A perspective from Mexico. Geol. Soc. Amer. Spec. Publ. 402, 45–63. Verma, S.P., 2002. Absence of Cocos plate subduction-related basic volcanism in southern Mexico: A unique case on Earth? Geology 30, 1095–1098. Verma, S.P., Luhr, J.F., 2010. Sr, Nd, and Pb isotopic evidence for the origin and evolution of the Cantaro-Colima volcanic chain, Western Mexican Volcanic Belt. J. Volcanol. Geotherm. Res. 197, 33–51. Williams, R.W., Gill, J.B., 1989. Effects of partial melting on the Uranium decay series. Geochim. Cosmochim. Acta 53, 1607–1619. Zellmer, G.F., Hawkesworth, C.J., Sparks, R.S.J., Thomas, L.E., Harford, C.L., Brewer, T.S., Loughlin, S.C., 2003. Geochemical evolution of the Soufrière Hills Volcano, Montserrat, Lesser Antilles Volcanic Arc. J. Petrol. 44, 1349–1374.