Two Distinct Mantle Sources Beneath the Garibaldi Volcanic Belt: Insight from Olivine-Hosted Melt Inclusions T

Chemical Geology 532 (2020) 119346

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Chemical Geology

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Two distinct mantle sources beneath the Garibaldi Volcanic Belt: Insight from olivine-hosted melt inclusions T

Swetha Venugopala,b,*, Séverine Mouneb,c,d, Glyn Williams-Jonesa, Timothy Druittb, Nathalie Vigourouxa,e, Alexander Wilsonf, James K. Russellf a Centre for Natural Hazards Research, Department of Earth Sciences, Simon Fraser University, BC, V5A 1S6, Canada b Laboratoire Magmas et Volcans, Université Clermont Auvergne, Campus Universitaire des Cézeaux, 63178, Aubiere Cedex, France c Observatoire Volcanologique et Sismologique de Guadeloupe (OVSG), Le Houëlmont, 97113, Gourbeyre, Guadeloupe d Université de Paris, Institut de Physique du Globe de Paris, CNRS, F-75005, Paris, France e Department of Earth and Environmental Sciences, Douglas College, BC, V3M 5Z5, Canada f Department of Earth, Ocean and Atmospheric Sciences, The University of British Columbia, BC, V6T 1Z4, Canada

ARTICLE INFO ABSTRACT

Editor: Donald Dingwell The nature of the magmatic source beneath the Garibaldi Volcanic Belt (GVB) in NW Washington (USA) and SW fi Keywords: British Columbia (Canada) has been debated both due to its classi cation as the northern equivalent of the High Melt inclusions Cascades and the alkaline nature of northern basalts. Whole rock studies have shown that the GVB does not share Garibaldi Volcanic Belt the same magmatic source as the High Cascades (Mullen and Weis, 2013, 2015). Nonetheless, the presence of Cascades alkaline basalts in this arc raises questions about the exact source of mantle enrichment and whether it is related Trace elements to the young age of the downgoing Juan de Fuca Plate (< 10 Ma) or the presence of a slab tear at the northern Two mantle sources end of the arc. To gain insight into the source that underlies the GVB, we sampled olivine-hosted melt inclusions from each volcanic centre along the arc. Major, volatile and trace element data reveal a northward compositional trend from arc-typical calc-alkaline magma in the south to OIB-like melts in the north near the slab tear. Fur- thermore, contributions from the subducting slab are relatively high beneath the southern end of the arc (Cl/ Nb > 80) but rapidly decreases to the north (Cl/Nb < 50). Finally, the significant differences in Zr/Nb from south to north (80 and 9, respectively) suggest two distinct mantle sources since one source cannot produce melts with such different ratios. As such, we propose the GVB should be segmented into the Northern and Southern groups, each having its own mantle source. Based on the geographic proximity, the enriched nature of the Northern group melt inclusions is likely controlled by the slab tear at the northern termination of the subducting Juan de Fuca Plate. Melt modelling results show that 3–7 % partial melting of the primitive mantle with a garnet lherzolite residue can reproduce the composition of the Northern group. Melt inclusions from the Southern group, on the other hand, imply a depleted MORB mantle that has been modified by fluids derived from the downgoing slab. Variability within the Southern group itself reflects the amount of hydrous fluids supplied beneath each centre and is correlated with slab age and subsequent degree of dehydration. This study addresses the compositional diversity along the arc and provides evidence that the age of the downgoing plate and the presence of a slab tear exert a strong compositional control over eruptive products within one arc.

1. Introduction Stern, 2018). The presence of faults and fractures can facilitate the rise of more primitive melt by acting as favourable pathways (e.g., Connor The main compositional controls of eruptive products in subduction and Conway, 2000; Gaffney et al., 2007; Venugopal et al., 2016) zone settings are arguably the age and tectonic context of the down- whereas slab tears and slab windows can contribute melts derived from going slab (Cooper et al., 2012; Saginor et al., 2013). Dictated by the the deeper mantle that do not carry the arc signature (e.g., Gorring and age, factors such as slab temperature, subduction angle and the amount Kay, 2001). of slab-derived fluids can cause significant variations in the composi- Modelling of slab thermal structure has shown that older slabs, such tion of the wedge-generated melt (Saginor et al., 2013; Weller and as the Pacific Plate beneath the Aleutian Islands, generally have steeper

⁎ Corresponding author at: Centre for Natural Hazards Research, Department of Earth Sciences, Simon Fraser University, BC, V5A 1S6, Canada. E-mail address: [email protected] (S. Venugopal). https://doi.org/10.1016/j.chemgeo.2019.119346 Received 5 February 2019; Received in revised form 11 June 2019; Accepted 19 October 2019 Available online 21 October 2019 0009-2541/ © 2019 Elsevier B.V. All rights reserved. S. Venugopal, et al. Chemical Geology 532 (2020) 119346

Fig. 1. Regional map of the Cascade Volcanic Arc along western North America. This arc is separated into the High Cascades from northern California to northern Washington and the Garibaldi Volcanic Belt (GVB) from northwest Washington to southwest Canada. Beneath the GVB, the subducting Juan de Fuca Plate youngs in age from 10 Ma below Glacier Peak to 5 Ma beneath Mount Meager where it terminates against the Nootka Fault beneath Salal Glacier and Bridge River. subduction angles, have lower slab temperatures at a given depth, and lower dip angle (Candie et al., 1989; Wilson, 2002). This difference of supply a higher flux of fluids to the sub-arc mantle (Syracuse et al., dip produces a gap or slab tear between the two plates (Thorkelson 2010; Ohtani et al., 2004; Stern, 2002; Ulmer, 2001). Younger slabs, on et al., 2011). the other hand, achieve higher temperatures at a given depth and lower The relatively older Juan de Fuca Plate beneath the southern slab dips (Syracuse et al., 2010). Though poorly investigated, the re- Cascades is capable of supplying more subduction-derived fluids into lease of fluids (due to dehydration reactions) from young slabs has been the mantle wedge, thereby initiating partial melting and producing arc- shown to be at shallower depths than in older slab settings (Ruscitto typical calc-alkaline magma (Green and Sinha, 2005). In terms of the et al., 2010; Walowski et al., 2016). This suggests that ascending pri- GVB, however, little is known of primitive melt compositions and vo- mary derived from young, dehydrated slabs should melts should be latile contents. What is known is the coeval presence of calc-alkaline depleted in volatiles and slab-derived elements. rocks in the southern GVB and alkaline rocks in the northern GVB An excellent case study for studying the effect of slab age and tec- (Green, 2006; Mullen and Weis, 2013, 2015). The GVB therefore pro- tonic framework on the resulting eruptive compositions is the Juan de vides an opportunity to examine both the effects of slab age and slab Fuca Plate, which is considered to be the young and hot end member in tearing on the magmatic source composition and volatile contents global subduction zone systems (Syracuse et al., 2010). The north- within a single subduction zone system. easterly subduction of the Juan de Fuca Plate beneath the North Here we present olivine-hosted melt inclusion data for the volcanic American Plate produces the Cascades Volcanic Arc along the western centres within the Canadian segment of the GVB alongside melt inclu- margin of North America (Fig. 1; Moore and DeBari, 2012). This arc is sion data from Glacier Peak and Mount Baker compiled by Shaw subdivided into the High Cascades in the south and the Garibaldi Vol- (2011). Major element, volatile and trace element contents of the melt canic Belt (GVB) to the north (Fig. 1). The age of the Juan de Fuca Plate inclusions are used to decipher the dynamics of the GVB sub-arc mantle at the trench decreases northward from 10 Ma beneath the northern and to better understand the effect of slab age and slab tear on the end of the High Cascades to approximately 5 Ma beneath the GVB compositions of mantle melts. where it terminates against the Explorer Plate (Fig. 1), which has a

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2. Geological context of the GVB entrapped with the silicate melt. These melt inclusions also contained daughter crystals, which typically form due to volatile loss, slow ascent The subdivision of the Cascades Arc is based on the change in strike rates and/or slow cooling following eruption (Schiano, 2003; Wallace, of the arc axis and a bend in the alignment of volcanic centres; the 2005; Supplementary Figure S1). As such, these inclusions are not re- change in strike is evident between Mount Rainier and Glacier Peak presentative of the original magma. To reverse the daughter crystal- (Fig. 1; Mullen et al., 2017). The High Cascades, which strikes north- lisation process and retrieve the original melt compositions, inclusions south, stretches between northern California and northern Washington. from Bridge River, Salal Glacier, Mount Cayley and Garibaldi Lake were The GVB, on the other hand, is a Quaternary-aged glaciovolcanic arc reheated at the Laboratoire Magmas et Volcans (LMV) in Clermont- extending between northern Washington and southwestern British Co- Ferrand, France. Re-heating experiments for each inclusion were lumbia. This belt strikes northwest-southeast and terminates in the 15 min in duration to reduce volatile loss (Chen et al., 2011; Portnyagin north against the Nootka Fault (Fig. 1). Volcanism within the GVB has et al., 2008; Gaetani et al., 2012; Bucholz et al., 2013) and stopped resulted in more than 2300 distinct vents and 22 major volcanic when the final daughter crystal disappeared (see Supplementary structures, including Glacier Peak, Mount Baker and the volcanic fields Methods). The heating stage was not able to accommodate the high of Mount Garibaldi, Garibaldi Lake, Mount Cayley, Mount Meager temperatures required for complete melt inclusion homogenisation. Volcanic Complex, Salal Glacier, Bridge River and Silverthrone (Wilson Melt inclusions, olivine hosts and matrix glasses were analysed for and Russell, 2018; Hildreth, 2007). Recently, the study by Mullen et al. major elements (plus S and Cl for all glasses) at LMV using a SX-100 (2017) classifies Glacier Peak as the northern termination of the High CAMECA electron microprobe. The precision of electron microprobe Cascades segment. The dominant eruptive products of the GVB are in- analyses (2σ) are better than 5 % for major elements, except for MnO, termediate calc-alkaline suites with basaltic eruptions comprising a Na2O and K2O, which had an EMP precision < 10 %. The approximate small volume fraction in the south and a large volume fraction in the 2σ precision for S and Cl is 4 % and 7 %, respectively. In-situ trace north (Bridge River to Salal Glacier; Wilson and Russell, 2018). The element analyses of melt inclusions were also performed at the LMV current geochemical literature consists of high-precision Sr, Pb and Hf with a Resonetics M50 EXCIMER Laser and a 193 nm wavelength isotopic data of basaltic whole-rocks, generally one sample per volcanic coupled to an Agilent 7500 cs ICP-MS (LA-ICP-MS). Analytical precision centre (Mullen and Weis, 2013, 2015, 2017). To gain further chemical and accuracy of measurements were stable, and most elements were insight into the primary mantle melt beneath the arc, samples of pri- better than 20 % at a 95 % confidence level. Sulphur speciation (S6+/ mitive magma preserved as melt inclusions in an early-crystallizing Stotal) was obtained from the S Kα peak shifts relative to the peak shift phase such as olivine, are considered excellent proxies. Melt inclusions of barite and sphalerite, temperature and linear regression coefficients are tiny pockets of melt trapped within growing crystals. Upon en- (Wallace and Carmichael, 1994) in order to estimate the oxygen fuga- trapment, the melts are considered to be sealed in a closed-system en- city of the melt inclusions following the method of Jugo et al. (2005). vironment. In ideal situations, these inclusions are protected by the host H2O, CO2 and D/H of select melt inclusions were analysed using the crystal during magmatic ascent and eruption, at which time they CAMECA IMS 270 Ion Probe (SIMS) at the Centre de Recherches Pét- quench to glass and preserve their initial composition. However, slow rographiques et Géochimiques (CRPG) in Nancy, France. The D/H ascent rates and/or slow cooling following eruption can cause post- contents of melt inclusions provide insight into the degree of H + dif- entrapment processes such as nucleation of daughter minerals, which fusion (i.e., water loss) following entrapment. Values are commonly modify the initial inclusion composition and should be taken into ac- reported as δD, which is the permil deviation of the D/H isotope ratio count (Section 4.2). Nonetheless, analyses of melt inclusions yield va- from the D/H ratio of Standard Mean Ocean Water (δDSMOW =0‰; luable information about the magmatic conditions at the time of en- Hauri, 2002; Shaw et al., 2008). Errors per mil were calculated using trapment as well as the unique nature of the source at depth. the instrumental drift and are between 2–9 ‰ with one value of 19‰ for Mount Meager. Finally, based on current literature estimates 3. Sample description and analytical methods (Anderson and Brown, 1993; Hartley et al., 2015; Moore et al., 2015; Wallace et al., 2015; Neave et al., 2015; Aster et al., 2016 and refer-

Detailed sample descriptions and methods can be found as supple- ences therein), the quantity of CO2 within the shrinkage bubble is mentary material. The reader is referred to Shaw (2011) for descrip- roughly 70–90% of the total amount of CO2 in the melt inclusion. In tions of Glacier Peak and Mount Baker samples. Olivine-bearing basalts order to avoid the underestimation of CO2 contents, Raman analyses and basaltic-andesites were sampled from each centre between Gar- were done within the bubble for most melt inclusions; analyses for ibaldi Lake and Bridge River. Tephra samples were collected for Gar- Bridge River, Salal Glacier, Mount Cayley and Garibaldi Lake were post ibaldi Lake and Mount Meager while lava ﬂows were sampled from re-heating. The size and volume of bubbles and melt inclusions were Mount Cayley. Hyaloclastite and pyroclastic breccia were sampled from estimated using a microscope under transmitted light and Leica imaging Salal Glacier and Bridge River, respectively. Samples were crushed, software. Melt inclusion volumes were assumed to be ellipsoidal and sieved and hand-picked for olivine crystals, which ranged in size from the two observable axes were measured. The best estimate for the third 300 to 600 μm. In all samples, melt inclusions were 15–40 μm in dia- axis was approximated using the smaller ellipsoidal axis measured in meter, elliptical to spherical in shape and contained a vapour bubble the microscope. Bubble volumes were assumed to be spherical and that occupied less than 10 vol% of the inclusion (Supplementary Figure calculated using measured diameters that were accurate to 2 μm. The S1). Bubbles that occupied a larger volumetric proportion were avoided associated errors for size and volume estimates is less than 10 %. as they most likely do not represent primary vapour bubbles (Moore The analytical procedure for Mount Baker and Glacier Peak can be et al., 2015). Melt inclusions that showed evidence of decrepitation found in Shaw (2011). The main instrumental diﬀerence between Shaw

(i.e., loss of volatiles through fractures extending from the melt inclu- (2011) and this study is their use of FTIR for H2O analyses of melt sion into the olivine host) were also avoided (Maclennan, 2017). Glassy, inclusions. Further details about analytical techniques can be found as crystal-free melt inclusions were only present in the Mount Meager supplementary material. sample as well as Glacier Peak and Mount Baker (Shaw., 2011). Some Mount Meager melt inclusions contained immiscible sulphide globules 4. Results that ranged in size between 5–10 μm. Olivines from Garibaldi Lake, Mount Cayley, Salal Glacier and Bridge River hosted melt inclusions 4.1. Whole rock and groundmass glass containing pre-existing spinel and occasional sulphides. Based on their irregular shape, rounded embayments and diverse volumetric propor- Whole rock compositions for Bridge River, Salal Glacier, Mount tions relative to their olivine hosts, the sulphides were likely co- Meager, Mount Cayley and Garibaldi Lake are presented in

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Fig. 2. Post entrapment crystallisation-corrected major elements of melt inclusions in this

study. a) Total alkalis versus SiO2; b) MgO vs

SiO2;c)K2O vs SiO2 and d) FeO* vs SiO2. There is a narrow range in olivine host Fo content along the arc, meaning the melt inclusion compositions should be comparable. In each diagram, Bridge River and Salal Glacier are consistently the most primitive along the arc. This is followed by a gradual southward in-

crease in K2O and decrease in MgO and FeO* wt%. Errors are 2σ. Data from Glacier Peak and Mount Baker are from Shaw (2011).

Supplementary Table S1. Major oxide compositions for these centres melt and will not be shown in diagrams nor discussed in this study. are broad with 47–55 wt% SiO2,8–14 wt% Fe2O3,5–11 wt% MgO and The complete dataset of corrected major, volatile and trace element 4–5 wt% total alkalis (Na2O+K2O; Fig. 2). While most samples are data is provided in Supplementary Tables S2 and S3. calc-alkaline arc basalts, the samples from Bridge River and Salal Gla- cier are the most alkaline (Fig. 2). Mount Baker and Glacier Peak whole 4.2.1. Major elements rock and groundmass glass compositions were not provided by Shaw Olivine hosts from all volcanic centres have forsterite contents of 77 (2011). to 89 mol % (Supplementary Table S2). Corrected melt inclusions along

the arc are rather variable and have 43–58 wt% SiO2,4–15 wt% FeO*, 4.2. Melt inclusions 2.5–10 wt% MgO, 0.4–2 wt% K2O and 2–8 wt% total alkalis (Fig. 2; Supplementary Table S2). The most evolved compositions, with the

All melt inclusions experienced some degree of post-entrapment highest SiO2,K2O and lowest FeO* are from Glacier Peak, Mount Baker modification. Glassy melt inclusions from Glacier Peak, Mount Baker and Garibaldi Lake (Fig. 2). Bridge River and Salal Glacier melt inclu- and Mount Meager were corrected for olivine-rim crystallisation con- sions are the most primitive with the lowest SiO2 and highest MgO and sidering the Fe-Mg equilibrium KD [= (FeO/MgO)olivine/(FeO/MgO)melt FeO* (Fig. 2). These melts also contain up to 2.8 wt% TiO2, which is inclusion]. Corrections were preformed using Petrolog3 software higher than typical arc values (up to 1.5 wt%; Aragón et al., 2013). (Danyushevsky and Plechov, 2011) and the equation calibrated by Toplis (2005). Overall, melt inclusions experienced minimal post en- 4.2.2. Volatiles trapment crystallization (PEC = 0 - 6.5%) and are in equilibrium with Volatile contents (H2O, CO2, S and Cl) are variable along the arc. the olivine host (KD=0.30 – 0.36; Toplis, 2005; Supplementary Table Chlorine and sulphur contents vary significantly between S2). Melt inclusions from the other centres experienced daughter 100–1700 ppm and 90–4000 ppm, respectively (Supplementary Table crystallisation. Reversing this process through re-heating also reverses S2). The highest Cl contents are from Garibaldi Lake and Mount Cayley the post-entrapment olivine rim crystallisation, thus yielding melt in- while the highest S contents are from Mount Meager, which is conclusions in equilibrium with the olivine host (KD = 0.30 – 0.33; Sup- sistent with the presence of sulphide globules and suggests saturation of plementary Table S2). The temperature at which all daughter crystals the melt with respect to sulfide. For many samples, carbon dioxide was melted within the inclusion range from 1150 to 1190 °C (Supplemen- measured both in the melt and in the shrinkage bubble. Without the tary Table S2). It should be noted that some re-heated Salal Glacier melt bubble, the melt inclusion CO2 contents across the arc range between inclusions contained anomalously high FeO* contents (up to 29 wt %). 90–6000 ppm (Fig. 3a; Supplementary Table S2). Both Glacier Peak and

This phenomenon, described by Rowe (2006), is due to the presence of Mount Baker contain the lowest CO2 contents (< 850 ppm) while pre-entrapment magnetite, chromite and Fe-Ni sulphides. As the latter Mount Meager contains the highest (> 1500 ppm). Raman analyses are present in three inclusions, we attribute Fe gain during re-heating to allowed for the calculation of the CO2 density within shrinkage bubbles the partial melting of Fe-Ni sulphides within Salal Glacier inclusions. from Garibaldi Lake, Mount Cayley, Mount Meager, Salal Glacier and

These inclusions are not considered representative of the initial trapped Bridge River. Using the distance between the main CO2 peaks and the

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Fig. 3. Comparing the water contents of all melt inclusion against a) CO2 in the glass b) total melt inclusion CO2 (glass + bubble) and c) δD. Isobars calculated using VolatileCalc (Newman and Lowenstern, 2002) are shown in a and b. a) Using only the glass measurements of CO2, the pressure of entrapment varies between 1–4 kbar, with the exception of a few melt inclusions from Bridge River and Salal Glacier. b) Pressure estimates rapidly increase when considering the amount of CO2 that partitions into the vapor bubble. Raman analyses were performed for all samples except for Glacier Peak and Mount Baker. Using Mount Meager as a reference, the average amount of CO2 that entered the bubble is 60%. Therefore, we use this amount to correct the CO2 contents of Glacier Peak and Mount Baker. See text for details. Bridge River and Salal Glacier consistently have the highest pressure of entrapment along the arc, while Glacier Peak and Mount Baker have the lowest. c) δD of melt inclusions, normalized to SMOW, excluding Glacier Peak and Mount Baker. Most inclusions lie within the range of MORB (grey box), but with elevated H2O contents. MORB values from Shaw et al. (2008) and Haui (2002). More negative values (up to -150) are typical of the upper limit of the MORB mantle (Hallis et al., 2015). Positive δD of melt inclusions suggest diﬀusive H + loss either during magmatic ascent or during re-heating experiments. Typically, melt inclusions suﬀer rapid water loss during re-heating experiments, on timescales of hours to days (i.e., Portnyagin et al., 2008; Gaetani et al., 2012; Bucholz et al., 2013). However, our experiments were 15 minutes long and melt inclusions were hosted in large olivine hosts (e.g. Chen et al., 2011). As such, the δD of re-heated melt inclusions (Garibaldi Lake, Mount Cayley, Salal Glacier and Bridge River) are primary and not indicative of extensive water loss. Error bars are 2σ and, unless otherwise shown, are the same size as data markers. Data for Glacier Peak and Mount Baker are from Shaw (2011).

equation by Wang et al. (2011), the CO2 densities range from 0.15 to as opposed to trapping of variably degassed melts. Values are expressed 3 0.6 g/cm . From here, the total amount of CO2 in GVB melt inclusions as δD, which is the permil deviation from Standard Mean Ocean Water was calculated and varies between 1300 – 10,000 ppm (Fig. 3b; Steele- (δDSMOW). The values of δD from Garibaldi Lake to Bridge River range MacInnis et al., 2011; Supplementary Table S2); this range excludes from -17 to -150 ‰ (Fig. 3c; Supplementary Table S2). Values of δD

Glacier Peak and Mount Baker (explained below). The highest total CO2 between -20 and -80 ‰ are typical of slab ﬂuids to the average upper values are from Bridge River. Consistent with current literature esti- MORB mantle (Fig. 3c; Shaw et al., 2008) while values closer to -150 ‰ mates (Anderson and Brown, 1993; Hartley et al., 2015; Moore et al., are representative of the upper limit of terrestrial mantle values 2015; Wallace et al., 2015; Neave et al., 2015; Aster et al., 2016; (-140 ± 20 ‰; Fig. 3c; Hallis et al., 2015). Positive δD values, and a

Maclennan, 2017 and references therein), approximately 40–80 % of negative trend with H2O wt%, suggest loss of hydrogen, addition of the total CO2 is found in the bubble (see Supplementary Table S5 for a seawater and/or secondary hydrothermal alteration (Kyser and O’Neil, worked example) 1984; Shaw et al., 2008; Portnyagin et al., 2008). The highest δD values Shaw (2011) did not perform Raman analyses in the bubbles of (least negative) belong to melt inclusions from Garibaldi Lake and

Glacier Peak and Mount Baker samples, thus the total CO2 content of Mount Cayley while melt inclusions with δD values closest to mantle the melt inclusion is unknown. The amount of CO2 that partitions be- values belong to Bridge River and Salal Glacier (Fig. 3c). Nonetheless, tween the glass and bubble depends on several factors including the the δD of these melt inclusions imply primary water contents that have PEC, magma ascent pathway, closure temperature and the potential for been negligibly affected by post-entrapment H + diffusion. Previous decrepitation (Moore et al., 2015; Wallace et al., 2015; Aster et al., authors have shown that long duration re-heating experiments, on 2016; Maclennan, 2017 and references therein). Comparing with the timescales of hours to days, can result in significant water loss from the glassy Mount Meager melt inclusions, Glacier Peak and Mount Baker melt inclusion (i.e., Portnyagin et al., 2008; Gaetani et al., 2012; have similar PEC (1–5 %; Supplementary Table S2) and entrapment Bucholz et al., 2013). δD of re-heated melt inclusions in this study do temperatures (Section 4.3). Since none of the melt inclusions in this not provide any evidence for extensive water loss during experiments. study showed evidence of decrepitation, the only unknown parameter is the magma ascent pathway, which we assume to be similar between the 4.2.3. Trace elements 3 centres. Based on these factors, the amount of CO2 that partitions into From Glacier Peak to Mount Meager, trace element contents of melt the bubble of Mount Meager samples can be extrapolated to Glacier inclusions show arc-like characteristics with a depletion in immobile, Peak and Mount Baker. The average amount of the total CO2 found in high field strength elements (HFSE) as well as a relative enrichment in – the bubble of Mount Meager inclusions is 60% (total range 38 80%). large ion lithophile elements (LILE) over the light rare earth elements Therefore, using 60%, the total CO2 content of Glacier Peak and Mount (LREE) when compared to normal mid-ocean ridge basalt (NMORB) and – Baker increases to 400 2000 ppm and the total along arc CO2 content ocean island basalt (OIB) magmas (Fig. 4a, b; Supplementary Table S3). – becomes 400 10,000 ppm (Fig. 3b; Supplementary Table S2). How- Furthermore, there are notable enrichments in fluid-mobile, large ion ever, we would like to emphasize that the addition of 60 % is relatively lithophile elements (LILE), such as Ba, Pb and Sr, which suggest the subjective but provides a reasonable baseline for comparison of the presence of a hydrous subduction component (Fig. 4a, b; Supplemen- total melt inclusion CO2 contents along the arc. tary Table S3; Sun and McDonough, 1989). These centres also show low – Water contents, measured in the melt, varies between 0.1 3 wt% concentrations of heavy rare earth elements (HREE) relative to primi- (Fig. 3a, b, c). The lowest water contents are in melt inclusions from tive mantle values ((Sm/Yb)N = 1.1–5.7), which is indicative of garnet Garibaldi Lake and Mount Cayley, while the highest values are from in the mantle source (Jennings et al., 2017; Mullen and Weis, 2015; Sun Mount Meager (Fig. 3; Supplementary Table S2). The D/H isotopic ratio and McDonough, 1989). was measured in a select number of melt inclusions in order to assess Trace element contents of melt inclusions from Salal Glacier and whether the observed H2O trends are due to post-entrapment H + loss Bridge River are unique along the arc and characterized by enrichment

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Fig. 4. Primitive mantle-normalised spider diagram of melt inclusions from a) Glacier Peak and Mount Baker (data from Shaw, 2011) b) Garibaldi Lake, Mount Cayley and Mount Meager and c) Salal Glacier and Bridge River. Reference compositions of EMORB, NMORB and OIB are from Sun and McDonough (1989). Centres from Glacier Peak to Mount Meager exhibit arc-like patterns with enriched concentrations of ﬂuid-mobile LILE and depletions in HFSE and LREE. Con- versely, Salal Glacier and Bridge River show depletions in LILE and elevated HFSE, particularly Nb and Ta. These enrichments suggest preservation of mantle-derived HFSE without the dilution supplied by a subduction component. The trace elements patterns of Salal Glacier and Bridge River closely follow that of OIB, further supporting a lack of subduction inﬂuences.

in HFSE relative to primitive mantle values. These patterns do not re- inclusion compositions, Garibaldi Lake melts are 1117–1178 °C, Mount semble previously discussed centres. The signatures of Bridge River and Cayley are 1090–1149 °C, Mount Meager 1057–1142 °C, Salal Glacier Salal Glacier are similar to enriched asthenospheric melts as opposed to 1186–1303 °C and Bridge River 1241–1312 °C (Fig. 5). Errors for tem- arc magmas and this is mirrored in the low Ba/Ta values between perature values are ± 50 °C. These values are also consistent with 140–220 for Salal Glacier and 120–250 for Bridge River (typical arc Petrolog3, which estimates the liquidus temperature of the melts values are > 500; Borg et al., 2002; Fig. 5). Relative to the trend be- (Danyushevsky and Plechov, 2011). From Shaw (2011), temperatures tween Glacier Peak and Mount Meager, there is a weaker depletion of were originally determined using the equations from Sugawara (2000) HREE, which suggests a reduced importance of garnet in the source and Medard and Grove (2008) but are comparable to the recalculated (Jennings et al., 2017). Between these two volcanoes, Salal Glacier temperatures using equation 13 from Putirka (2008) and Petrolog3: melts tend to be slightly more enriched in HFSE and LILE. Overall, the Glacier Peak melts are 1113–1179 °C while Mount Baker melts are trend for both Salal Glacier and Bridge River closely follows that of 1076–1116 °C (Fig. 5; Supplementary Table S2). ocean island basalts based on data by Sun and McDonough (1989) The pressure of inclusions was estimated using the volatile satura-

(Fig. 4c). tion model of Papale et al. (2006) using H2O and CO2 contents. Re- calculated CO2 contents for all volcanic centres range between 420 – 10,000 ppm and subsequently yield pressure estimates between 0.4 to 4.3. Magmatic conditions 6.8 kbars (Fig. 5; Supplementary Table S2). These values reﬂect the

variably degassed nature of melt inclusions (shown previously by H2O Melt inclusion temperatures were calculated from olivine - melt and δD) and suggest polybaric olivine crystallization. As such, we inclusion equilibria using equation 13 from Putirka (2008). Where consider these pressure values as a minimum. To avoid under- applicable, calculated values are consistent with re-heating experi- estimation, the maximum recorded pressure (Pmax) for each centre is ments, which range from 1130 to 1190 °C. Using corrected melt

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Fig. 5. Trace element and magmatic conditions of melt inclusions with distance along the arc. Pressure values are calculated using the volatile saturation model

of Papale et al. (2006) using H2O and CO2 contents. CO2 contents include the bubble and include the addition of 60% for Glacier Peak and Mount Baker (see text for details). Temperatures are calculated using olivine - melt equilibria using equation 13 from Putirka (2008). There is a marked increase in Nb and decrease in Ba/Ta between Bridge River, Salal Glacier and the other centres. As Nb is mainly derived from the mantle, its high concentration in the northern magmas suggest the mantle signature is preserved and not overprinted by addition of any subduction component. Bridge River and Salal Glacier also report the highest temperatures and the deepest pressures of melt inclusion entrapment along the arc. Conversely, the remaining centres, from Glacier Peak to Mount Meager, show an arc-typical trend: low Nb contents that become overprinted by the addition of subduction-derived components, which is shown by high values of Ba/Ta. The addition of subduction-derived components also lowers the temperature of entrapment. Typical 2σ error bars are shown. Data for Glacier Peak and Mount Baker are from Shaw (2011).

considered representative of the deepest storage conditions. The lowest Pmax values are from Glacier Peak and Mount Baker (2 kbars) while the highest are from Salal Glacier (6.8 kbars). The sulphur speciation was measured following the method of Jugo et al. (2005) and can be used to estimate the magmatic oxygen fugacity beneath Glacier Peak, Mount Baker and Mount Meager. Glacier Peak magmas are +0.3 < NNO < +0.82 and Mount Baker magmas are

NNO + 0.29. Mount Meager magmas are fO2 NNO + 0.74 (Supple- mentary Table S2). Oxygen fugacity cannot be calculated for Garibaldi Lake, Mount Cayley, Salal Glacier and Bridge River since the inclusions have been re-heated. The sulphur speciation of these melt inclusions

will only yield fO2 values of the heating apparatus, not the magma. The Fe3+/∑Fe ratio, another useful proxy for the oxidation state of magma, was estimated using the major oxide glass composition as well as

temperature and fO2 (Kress and Carmichael, 1991). Glacier Peak and Mount Baker have Fe3+/∑Fe values between 0.14 to 0.26 while Mount Meager melts are more oxidised with values between 0.26 and 0.28 (Supplementary Table S2). For Bridge River and Salal Glacier, Fe3+/ ∑Fe was measured using wet chemistry of whole rocks by Mathews (1958) and Green (2006) and yield values of 0.16. From this value, the

fO2 conditions can be back-calculated as the required parameters are known (T, P and major elements of glasses). The magma beneath both Bridge River and Salal Glacier is much less oxidized than the other

centres with fO2 values of -0.37 < NNO < -1.27 and -0.64 < NNO < -1.36, respectively (Supplementary Table S2).

5. Discussion

5.1. Chemical trends along the Garibaldi Volcanic Belt

Based on the above results and calculated magmatic conditions, there are clear along-arc trends. Consistent with previous work (Mullen and Weis, 2013, 2015), there is a subtle transition from arc-typical calc- alkaline magmas in the south beneath Glacier Peak and Mount Baker towards increasingly alkalic melts in the north (Fig. 2). Not only are Bridge River and Salal Glacier the most alkaline, these melts are also the least hydrous and contain the lowest chlorine contents along arc (Supplementary Table S2). There is also a clear northward increase in maximum melt inclusion entrapment temperature and pressure and a decrease in the degree of mantle oxidation (Fig. 5; Supplementary Table S2). Over the narrow range in Fo contents of olivine hosts along the entire arc, Glacier Peak and Mount Baker yield the lowest temperatures, shallowest pressures (average 1120 °C and up to 2 kbars) and the most oxidized magmas along the arc; Bridge River and Salal Glacier melts are signiﬁcantly hotter, trapped much deeper (average 1260 °C and up to 6.8 kbars), and are much more reduced than the other centres within the GVB.

7 S. Venugopal, et al. Chemical Geology 532 (2020) 119346

5.2. Magmatic sources: northern and southern groups

Trace element patterns along the arc can provide insight into the nature of the magmatic source beneath the arc (Figs. 4 & 5). The northernmost volcanoes show trace element variations that are more typical of OIB magmas, which is consistent with the higher melt inclusion temperatures (Figs. 4 and 5; Sun and McDonough, 1989). They also show the greatest enrichment in Nb (> 20; Fig. 5) as well as all other HFS elements (Supplementary Table S3). As these HFS elements are relatively fluid-immobile, and derived from the mantle, this suggests preservation of initial mantle values and thus minimal addition (i.e. dilution) of these elements by the subducting slab. This is further shown in the depletion in fluid-mobile, slab-sourced elements such as Sr relative to typical arc magmas (maximum 836 ppm; Supplementary Table S3). Based on LILE/HFSE ratios, the northernmost volcanoes lack a subduction or arc signature. For example, arc magmas typically have elevated Ba/Ta values due to fluids delivered by the subducting slab. Referring to Fig. 5, Ba/Ta is the lowest beneath the northern end of the arc (up to 250) and therefore suggests minimal or lack of subduction- Fig. 7. Ba versus Nb for all melt inclusions. Two distinct trends clearly re- derived fluids (Borg et al., 2002). Centres between Glacier Peak and present two separate mantle sources beneath the arc. One subduction modified – Mount Meager show typical arc patterns with low Nb (5 12 ppm), and source beneath Glacier Peak to Mount Meager (Ba/Nb = 45–60) and another high Sr and Ba/Ta (up to 1920 ppm and 1900, respectively) (Fig. 5; beneath Bridge River and Salal Glacier (Ba/Nb < 13), which experiences minor Supplementary Table S3), all of which are indicative of subduction- subduction influence. Typical 2σ error bars are shown. Data for Glacier Peak related processes (e.g., Borg et al., 2002; Portnyagin et al., 2007; and Mount Baker are from Shaw (2011). Vigouroux et al., 2012 and references therein). The variability of Ba/Ta between Glacier Peak to Mount Meager could be explained by extensive subduction-derived fluids. There is a clear break between Bridge River plagioclase fractionation prior to melt inclusion entrapment leading to and Salal Glacier and the rest of the arc. The enrichment in Cl and Sr lower Ba/Ta below Glacier Peak and Mount Baker. However, this would from Glacier Peak to Mount Meager, coupled with the higher water produce a negative Eu anomaly, which is not seen on Fig. 4. A more contents, provides strong evidence for the circulation of hydrous fluids likely explanation is the variable deliverance of subduction-derived within the mantle wedge. In terms of the northern centres, the deple- fl uids between these centres. This is discussed further in Section 5.4.1. tion in Cl and Sr suggests negligible subduction-related fluids. Finally, all centres have very low Th/Yb (< 3.2) and Sr/Y (< 90), Overall, the distinct nature of the northernmost volcanoes can be which shows that for the whole arc, there is respectively little sediment explained by the Ba and Nb contents of melt inclusions, which define and slab melt from the downgoing plate. two distinct trends (Fig. 7). This suggests the presence of two different These along-arc variations are best shown by comparing Cl/Nb and mantle sources beneath the GVB. One source underlies Glacier Peak to Sr/Nd (Fig. 6). Each respective elemental pair have similar partitioning Mount Meager which is capable of producing magmas with Ba/Nb > 30 fl during magmatic processes except in the presence of hydrous uids, for and another beneath Bridge River and Salal Glacier that can produce ffi which Cl and Sr have high a nity (Walowski et al., 2016; Portnyagin significantly lower Ba/Nb (< 13). We therefore propose that the arc et al., 2007). Therefore, these ratios are excellent tracers for can be segmented into two groups: the Northern group (Bridge River and Salal Glacier; Ba/Nb = 13; Fig. 7) and the Southern group (Glacier Peak, Mount Baker, Garibaldi Lake, Mount Cayley and Mount Meager; Ba/Nb = 45–60; Fig. 7). The Zr/Nb ratio can be used to further distinguish these different mantle sources, as these values are not affected by subduction input or fractional crystallization (Mullen and Weis, 2015; Pearce and Norry, 1979). Beneath the Southern group, the average Zr/Nb is 27, which is typical of N-MORB values (∼30; Sun and McDonough, 1989) and indicative of partial melting of a depleted mantle plus a subduction component. Conversely, beneath the Northern group, the average Zr/ Nb is 7, which is more typical of an enriched mantle source (Vigouroux et al., 2012; Sun and McDonough, 1989). A single source cannot produce magmas which such different Zr/Nb ratios (Mullen and Weis, 2015).

5.3. Tectonic link

The segmentation of the arc strongly correlates with the presence of Fig. 6. Cl/Nb vs Sr/Nd as an indicator of subduction-derived fluids beneath fi each centre along the GVB, with a logarithmic y-axis. Each respective elemental the Nootka Fault, which is a transform fault that de nes the boundary pair shows similar partitioning behaviour during magmatic processes, except in between the Juan de Fuca and Explorer Plates and transects the fluids derived from the downgoing slab for which Cl and Sr have a high affinity. Northern group (Fig. 1; Hyndman et al., 1979). Offshore, it is a left- Bridge River and Salal Glacier show much lower values for both ratios, sup- lateral strike slip fault between the two plates. On shore, it is inter- porting the lack of subduction zone fluids while each centre between Glacier preted as the surficial manifestation of a slab tear or fracture zone be- Peak and Mount Meager show elevated Cl/Nb and Sr/Nd, illustrating the im- tween the subducting Juan de Fuca Plate and the relatively stagnant portance of subduction-derived fluids. Typical 2σ error bars are shown. Data for Explorer Plate (Hyndman et al., 1979). Schmidt et al. (2008) proposed Glacier Peak and Mount Baker are from Shaw (2011). that this fracture zone was a pathway for seawater to serpentinise the

8 S. Venugopal, et al. Chemical Geology 532 (2020) 119346 subducting oceanic crust. However, this implies that the Northern group magmas would experience some degree of subduction inﬂuence. Conversely, Green (2006) and Mullen and Weis (2015) suggest this fracture zone is deep enough to allow asthenospheric upwelling to occur and exert a compositional control on the eruptive suites, which would explain the enriched nature of the Northern group magmas.

5.4. Modelling the magmatic source

5.4.1. Southern group The current whole-rock literature states that one enriched mantle source, similar to that of intraplate basalts, underlies the GVB, and that the along-arc variability can be explained through variable mixing proportions of depleted mantle with slab-derived melts (Mullen et al., 2017; Mullen and Weis, 2015 and references therein). Conversely, Walowski et al. (2016) modelled the melt inclusion compositions from the Lassen region in the southern High Cascades through two-component mixing between enriched MORB mantle and oceanic crustal melts (i.e., sediment and slab components). This model begins with fluid-flux melting of the upper basaltic layer of the downgoing oceanic crust beneath the arc and the subsequent migration of these hydrous melts into the overlying mantle wedge to induce melting and generate hydrous calc-alkaline basalts (Walowski et al., 2016). This model is based on findings that show the downgoing slab is already dehydrated once it reaches sub-arc depths. Based on the MORB-like boron contents of Lassen melt inclusions, the subducting slab is considered dry and therefore contributes no fluids to the mantle wedge (Walowski et al., 2016). However, melt inclusion compositions across the GVB do not show evidence for significant slab melt contribution to the mantle wedge (La/Yb < 25; Defant and Drummond, 1990; Moyen, 2009). In- deed the Juan de Fuca Plate subducts beneath both the High Cascades and the GVB, there is a distinct compositional difference between the two arcs, namely the transitional alkaline nature of GVB basalts and the Fig. 8. Modelling the two distinct magmatic sources beneath the GVB. a) H2O/ lack of a sediment input (across arc Th/La < 0.15 and based on high Ce vs Sr/Nd. Modelling the source beneath the Southern group starts with the precision whole rock isotope data; Mullen et al., 2017; Mullen and depleted MORB mantle (black diamond) and adding variable amounts of a Weis, 2015 and references therein). Based on these factors, the main hydrous subduction component, calculated using the method of Portnyagin et al (2007). This component is enriched in Ba, Sr, Cl, H O and Pb. Grey area re- subduction component beneath the GVB are most likely fluids delivered 2 presents the modelling results that involve 5–12 % partial melt of DMM (black by the downgoing slab. lines) mixed with 2–10 wt% of the hydrous subduction component (dashed Considering the contrasting melt models for the GVB and the High lines). Inclusions that lie above the grey area are assumed to have elevated slab Cascades, we initially model magma generation beneath the GVB with a fluid content and those that lie below are assumed to be more degassed melts. depleted MORB mantle (DMM; Salters and Stracke, 2004) and add a b) Modelling the enriched mantle source beneath the Northern group in terms hydrous subduction component which was calculated using the method of Dy/Yb and Nb/Y, with the Southern group melt inclusions plotted as re- of Portnyagin et al. (2007) (Fig. 8a; Supplementary Table S6). De- ference. These ratios are optimal markers for the degree of partial melting, the termining the composition of the hydrous subduction component in- presence of garnet and heterogeneities in the mantle (Vigouroux et al., 2012). volves first estimating the amount of partial melting of the DMM using Elevated Dy/Yb and Nb/Y of the Northern group cannot be modelled by even elements with low solubility in slab-derived fluids (Nb, Zr and Yb). The low degrees of partial melting (1 %) of the DMM (black diamond). Melt in- degree of partial melting varies between 5–12 %. Using the degree of clusions from the Northern group can be best modelled by low degrees of partial melting (3–7 %) of the primitive mantle (Sun and McDonough, 1989) indicated melting and the melt inclusion composition, the composition of the by tick marks along the model curve. The residue is garnet lherzolite (7 cpx: 25 subduction-modified mantle wedge is calculated with the batch melting opx: 60 olv: 8 grt). Typical 2σ error bars are shown. Data for Glacier Peak and ff equation. Finally, the di erence, or excess, between the DMM and the Mount Baker are from Shaw (2011). subduction-modifi ed mantle wedge reflects the addition by the hydrous subduction component. The amount of hydrous fluids added can be amount of hydrous subduction fluids delivered to the mantle wedge estimated by the H2O content of the melt inclusions. However, factors varies between 2–10 wt% (Fig. 8a). The composition of the hydrous such as magma degassing prior to melt inclusion trapping, and to a subduction component varies slightly between the centres of the lesser degree post-entrapment H loss, would lead to an underestimation Southern group. Interestingly, the modelled concentrations of Ba, Cl of the initial H2O contents and the amount of hydrous subduction fluids and Sr in the hydrous component are the lowest beneath Mount Baker, contributed. Therefore, the melt inclusion H2O values are corrected reach a maximum beneath Garibaldi Lake and Mount Cayley, and wane using the maximum H2O/K2O of each centre (Supplementary Table S2; beneath Mount Meager (Supplementary Table S6). Not only is this trend following the methods of Aster et al., 2016 and Walowski et al., 2016). mirrored in the along arc values of Ba/Ta on Fig. 5, but is also con- We perform this correction for the sole purpose of estimating the sistent with the Ba, Sr and Cl-rich nature of Mount Cayley and Garibaldi amount of hydrous fluids contributed by the downgoing slab. Re- Lake melt inclusions (up to 1700 ppm Cl). This is also seen in Fig. 6, constructed H2O values for the GVB range between 0.7–4.7 wt%. Using where there is a gradual increase in Cl/Nb and Sr/Nd from Mount only the maximum reconstructed values (H2Omax) for each centre, the Meager and Mount Baker towards Mount Cayley and Garibaldi Lake. values vary between 1.5–4.7 wt%, which is consistent with maximum reconstructed estimates for the Lassen region (1.3–3.4 wt%; Walowski et al., 2016). Using the reconstructed H2O contents, the modelled

9 S. Venugopal, et al. Chemical Geology 532 (2020) 119346

5.4.2. Northern group melting of two distinct mantle sources (Fig. 9). However, there are To first test the hypothesis of a single mantle source beneath the significant tectonic factors to consider. In the case of the Northern GVB, the Northern group melt inclusions are modelled using the same group, the derivation of deeply sourced primitive and enriched melts procedure as the Southern Group, i.e., depleted mantle with fluids de- beneath Bridge River and Salal Glacier suggests that the Nootka Fault livered by the subducting slab. Even with very low degrees of partial and slab tear is persistent through to the asthenosphere and is a fa- melting (< 1%), the Northern group melt inclusion compositions vourable pathway for ascending enriched melts. cannot be recreated using this method (Fig. 8b). As discussed earlier, Based on the glaciovolcanic overview by Wilson and Russell (2018), these melt compositions suggest an enriched mantle source that is po- there has been one recorded deposit of calc-alkaline composition within tentially controlled by the Nootka Fault. It is thus useful to compare the the Salal Glacier Volcanic Field. Though one known calc-alkaline de- Dy/Yb and Nb/Y contents of melt inclusions as these elements are deposit does not necessarily suggest that the Northern group experiences rived entirely from the mantle and not added by subduction-related occasional subduction influence, it may however provide insight into components (Fig. 8b). Furthermore, the Dy/Yb ratio is considered the nature of the ascending pathway. A likely case for melt generation is sensitive to variations in the mantle source composition while Nb/Y is a the ascent of asthenospheric mantle through the slab tear and the useful proxy for the degree of partial melting and episodes of previous subsequent decompression-induced melting in the sub-arc mantle melt extraction (Vigouroux et al., 2012 and references therein). Typical (Fig. 9). The ascent of asthenospheric melts through faults and fractures MORB compositions have Dy/Yb between 1.3 and 2.0 and Nb/Y < 0.4 directed along the northern edge of the Juan de Fuca Plate followed by while OIB have Dy/Yb > 2 and Nb/Y > 0.8 (Fig. 8b; PetDB database; the periodic mixing with the subduction-modified source beneath the Lehnert et al., 2000). Dy/Yb can be affected by amphibole fractiona- Southern group would account for the rare calc-alkaline signature tion, and this would be shown by a negative correlation between Dy/Yb within the Salal Glacier Volcanic Field. We therefore propose that the and SiO2 wt%, which is not seen in the dataset. Both Bridge River and area of slab tear is more of a densely fractured zone between the Juan Salal Glacier have higher Dy/Yb and Nb/Y than typical MORB melts but de Fuca and Explorer Plates with each fault acting as a potential overlap with OIB compositions. This suggests low degrees of partial pathway for ascending asthenospheric mantle (Fig. 9). melting of an enriched mantle source. Modelling of the Northern group The Southern group requires a hydrous subduction component that thus begins with the mass balance equation for batch melting and with varies in composition between each centre. The amount of the hydrous the primitive mantle as the source (Fig. 8b; Sun and McDonough, subduction component contributed to the mantle wedge is at a max- 1989). Distribution coefficients are provided in Supplementary Table imum beneath Mount Cayley and Garibaldi Lake and lowest beneath S4. With melt fractions below 7 % and garnet lherzolite as a residual Glacier Peak and Mount Baker (Fig. 9). The idea of more fluids exsolved phase, the modelled partial melt compositions bear a strong resem- from a younger slab is contrary to the normally accepted relationship blance to the Northern group in terms of Dy/Yb and Nb/Y ratios between slab age and fluid exsolution. A younger slab typically implies (Fig. 8b). Modelling results are consistent with the elevated FeO* wt% higher slab temperatures and thus, smaller additions of slab-derived (> 9), La/Ta (< 20) and low HREE contents of Northern group melt hydrous components to the mantle wedge (Mullen and Weis, 2015; inclusions, all of which support OIB-like melt generation below the Green and Harry, 1999). However, if we consider the slab dehydration garnet-spinel transition zone at 70 km (Gorring and Kay, 2001). This model by Walowski et al. (2016), then the slightly older plate beneath model is also consistent with the whole-rock modelling results of Glacier Peak and Mount Baker would have already dehydrated prior to Mullen and Weis (2013). However, the main and rather significant reaching sub-GVB depths thereby supplying smaller amounts of fluids. difference is their incorporation of Mount Meager as having the same As the slab becomes progressively younger to the north, the degree of enriched source as the Northern group. Although Mount Meager shows dehydration decreases (i.e., the slab remains hydrated) and thus the transition alkaline compositions, the trace element contents are clearly amount of fluids supplied increases. Nevertheless, the Juan de Fuca subduction zone derived. Plate is one of the youngest subducting plates in the world and therefore the total amount of fluids supplied would be very low in comparison to global subduction zones with older slabs. 5.5. Magma generation beneath the GVB

The along-arc geochemical variations expressed by melt inclusions collected from the majority of GVB volcanic centres requires partial

Fig. 9. Schematic diagram of magma generation beneath the Northern and Southern groups of the GVB. Starting at the slab tear in the north, the age of the downgoing Juan de Fuca Plate decreases southward from 5 Ma to 10 Ma. This subsequently leads to southward increase in slab dip and the degree of slab dehydration, according to the model proposed by Walowski et al. (2016). The amount of ﬂuids delivered (blue arrows) to the depleted sub-arc mantle is highest beneath Mount Cayley and Mount Garibaldi and lowest beneath Mount Baker and Glacier Peak. Beneath the Northern group, the main compositional control is the slab tear. This gap between the Explorer and Juan de Fuca Plates allows for the upwelling of enriched asthenospheric mantle (orange arrow), thus yielding the OIB-like signature of the Northern group melts. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article).

10 S. Venugopal, et al. Chemical Geology 532 (2020) 119346

6. Conclusions Estelle Rose-Koga and Nico Cluzel is thanked for the use of the re- heating stage at LMV. We would also like to thank Paul Wallace and an Olivine-hosted melt inclusions from the GVB provide new insight anonymous reviewer for many helpful comments that greatly improved into the critical factors controlling the composition of subduction zone the quality of this work. magmas. Factors such as the age of the downgoing slab, degree of fluid release into the mantle wedge as well as the tectonic framework of the Appendix A. Supplementary data GVB itself have resulted in a unique and pronounced north-south geochemical transition. Supplementary material related to this article can be found, in the Beneath the northern GVB, the reduced nature of melt inclusions online version, at doi:https://doi.org/10.1016/j.chemgeo.2019. coupled with higher temperatures and pressures of entrapment and the 119346. elevated contents of HFSE suggest an enriched mantle source rather than a subduction-modified source. We attribute these signatures to a References slab tear, or dense fracture zone, between the northern edge of the Juan de Fuca Plate and the neighbouring Explorer Plate. Modelling of the Anderson, A.T., Brown, G.G., 1993. CO2 contents and formation pressures of some enriched source beneath the Northern group suggests that the slab tear Kilauean melt inclusions. Am. Mineral. 78, 794–803. Aragón, E., Pinotti, L., Castro, A., Rabbia, O., Coniglio, J., Demartis, M., Hernando, I., is pervasive enough to reach the upper mantle. Through decompression Cavarozzi, C., Aguilera, Y.E., 2013. The Farallon-Aluk ridge collision with South melting, asthenospheric melts ascend through the tear and erupt at the America: implications for the geochemical changes of slab window magmas from surface. In a global context, there are have been few eruptions of si- fore- to back-arc. Geosci. Front. 4, 377–388. https://doi.org/10.1016/j.gsf.2012.12. 004. milarly enriched melts in an arc setting. Gorring and Kay (2001) show Aster, E.M., Wallace, P.J., Moore, L.R., Watkins, J., Gazel, E., Bodnar, R.J., 2016. that Patagonia plateau lavas have a distinct OIB-like signature that can Reconstructing CO2 concentrations in basaltic melt inclusions using Raman analysis be attributed to the slab window formed by the subduction of the Chile of vapour bubbles. J. Volcanol. Geotherm. Res. 323, 148–162. https://doi.org/10. Ridge between the Nazca and Antarctic Plates. Trace element and 1016/j.jvolgeores.2016.04.028. Borg, L., Blichert-Toft, J., Clynne, M.A., 2002. Ancient and modern subduction zone modelling results show that, similar to the Northern Group, melt gen- contributions to the mantle sources of lavas from the lassen region of California in- eration began near the garnet-spinel at 70 km depth (Gorring and Kay, ferred from Lu–Hf isotopic systematics. J. Petrol. 43 (6), 705–723. https://doi.org/ 2001). 10.1093/petrology/43.4.705. Bucholz, C.E., Gaetani, G.A., Behn, M.D., Shimizu, N., 2013. Post-entrapment modifica- Melt inclusions from the southern GVB, on the other hand, reveal tion of volatiles and oxygen fugacity in olivine-hosted melt inclusions. Earth Planet. arc-typical compositions produced by a mix between the depleted Sci. Lett. 374, 145–155. MORB mantle and hydrous subduction components. The main differ- Candie, S.C., La Brecque, J.L., Larson, R.L., Pittman III, W.C., Golovchenko, X., Haxby, W.F., 1989. Magnetic Lineations of the World’s Ocean Basins. American Association ence between the centres of the Southern group is the amount and of Petroleum Geologists, Tulsa, Oklahoma. concentrations of Sr, Ba and Cl of the hydrous subduction component. Connor, C.B., Conway, F.M., 2000. Basaltic volcanic fields. Encyclopaedia of Volcanology. The maximum amount of fluids with the highest Sr, Ba and Cl is de- Academic Press, pp. 331–343. Cooper, L.B., Ruscitto, D.M., Plank, T., Wallace, P.J., Syracuse, E.M., Manning, C.E., 2012. livered to the mantle wedge beneath Mount Cayley and Garibaldi Lake Global variations in H2O/Ce: 1. Slab surface temperatures beneath volcanic arcs. while the lowest amounts are below Mount Baker and Glacier Peak. Geochem. Geophys. Geosystems 13 (3). https://doi.org/10.1029/2011GC003902. This pattern of fluid exsolution correlates positively with the age of the Danyushevsky, L.V., Plechov, P., 2011. Petrolog3: integrated software for modelling crystallization processes. Geochem. Geophys. Geosystems 12 (7). downgoing Juan de Fuca Plate. Though Walowski et al. (2016) found Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arc magmas by melting that the downgoing Juan de Fuca Plate beneath the southern Cascades of young subducted lithosphere. Nature 347, 662–665. had significantly dehydrated prior to reaching sub-arc depths, our Gaffney, E.S., Damjanac, B., Valentine, G.A., 2007. Localization of volcanic activity; 2. ff – modelling results show that slab fluids are the dominant subduction E ects of pre-existing structure. Earth Planet. Sci. Lett. 263, 323 338. Gorring, M.L., Kay, S.M., 2001. Mantle Processes and Sources of Neogene Slab Window component beneath the southern GVB. These results suggest that the Magmas from Southern Patagonia, Argentina. J. Petrol. 42 (6), 1067–1094. https:// difference in slab age between the southern Cascades and the southern doi.org/10.1093/petrology/42.6.1067. fl GVB (3–4 Ma) is sufficient to preserve a more hydrated slab. To test the Green, N.L., 2006. In uence of slab thermal structure on basalt source regions and fl melting conditions: REE and HFSE constraints from the Garibaldi Volcanic Belt, global applicability of younger slabs retaining their uids, melt inclu- northern Cascadia subduction system. Lithos 87 (1–2), 23–49. https://doi.org/10. sion compositions from young (< 10 Ma) and hot subduction zones, 1016/j.lithos.2005.05.003. such as South Chile and Mexico (Syracuse et al., 2010), would provide Green, N.L., Harry, D.L., 1999. On the relationship between subducted slab age and arc basalt petrogenesis, Cascadia subduction system, North America. Earth Planet. Sci. further insight into the relationship between the slab age and the degree Lett. 171 (3), 367–381. https://doi.org/10.1016/S0012-821X(99)00159-4. of dehydration. Green, N.L., Sinha, A.K., 2005. Consequences of varied slab age and thermal structure on Our results show that abrupt along-arc compositional changes can enrichment processes in the sub-arc mantle of the northern Cascadia subduction system. J. Volcanol. Geotherm. Res. 140 (1–3), 107–132. https://doi.org/10.1016/j. be used to identify structures in the slab that cannot otherwise be jvolgeores.2004.07.017. identified by geophysical methods, either due to low-resolution ima- Hallis, L.J., Huss, G.R., Nagashima, K., Taylor, G.J., Halldórsson, S.A., Hilton, D.R., ging or a lack of available information. Furthermore, this work illus- Meech, K.J., 2015. Evidence for primordial water in Earth’s deep mantle. Science 350 (6262), 795–797. https://doi.org/10.1126/science.aac4834. trates the importance of the age and degree of dehydration of the Hartley, M.E., Neave, D.A., Maclennan, J., Edmonds, M., Thordarson, T., 2015. Diffusive downgoing slab in subduction zone settings. From the delivery of slab over-hydration of olivine- hosted melt inclusions. Earth Planet. Sci. Lett. 425, melts in the southern Cascades, to the exsolution of fluids in the 168–178. https://doi.org/10.1016/j.epsl.2015.06.008. southern GVB and the termination against the Nootka Fault and slab Hauri, E., 2002. SIMS analysis of volatiles in silicate glasses, 2: isotopes and abundances in Hawaiian melt inclusions. Chem. Geol. 183, 115–141. tear beneath the northern GVB, the Juan de Fuca Plate is capable of Hildreth, W., 2007. Quaternary magmatism in the Cascades; geologic perspectives. U.S. producing a diverse array of volcanic centres along the western margin Geological Survey Professional Paper 1744. pp. 125. — of North America. Hyndman, R., Riddihough, R., Herzer, R., 1979. The Nootka Fault Zone a new plate boundary off western Canada. Geophys. J. R. Astron. Soc. 58 (3), 667–683. Jennings, E.S., Gibson, S.A., Maclennan, J., Heinonen, J.S., 2017. Deep mixing of mantle Acknowledgments melts beneath continental flood basalt provinces: constraints from olivine- hosted melt inclusions in primitive magmas. Geochim. Cosmochim. Acta 196, 36–57. Jugo, P., Luth, R., Richards, J., 2005b. Experimental data on the speciation of sulfur as a This work is a Clervolc contribution number 380. This work was function of oxygen fugacity in basaltic melts. Geochim. Cosmochim. Acta 69, supported by NSERC Discovery grants to Williams-Jones and Russell 497–503. https://doi.org/10.1016/j.gca.2004.07.011. (R03953) and a ClerVolc grant to Moune. Many thanks to Jean-Luc Kress, V.C., Carmichael, I.S.E., 1991. The compressibility of silicate liquids containing Fe2O3 and the effect of composition, temperature, oxygen fugacity and pressure on Devidal and Etienne Deloule for their expertise in electron microprobe their redox states. Contrib. Mineral. Petrol. 108 (1–2), 82–92. https://doi.org/10. and SIMS analysis. 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