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

Swetha Venugopal, Séverine Moune, Glyn Williams-Jones, Timothy H. Druitt, Nathalie Vigouroux, Alexander Wilson, James Russell

► To cite this version:

Swetha Venugopal, Séverine Moune, Glyn Williams-Jones, Timothy H. Druitt, Nathalie Vigouroux, et al.. Two distinct mantle sources beneath the Garibaldi Volcanic Belt: Insight from olivine-hosted melt inclusions. Chemical Geology, 2020, 532, pp.119346. 10.1016/j.chemgeo.2019.119346. hal-02530627

HAL Id: hal-02530627

https://uca.hal.science/hal-02530627v1

Submitted on 5 Aug 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
1

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

2

inclusions

3

Swetha Venugopal*<sup>a,b</sup>, Séverine Moune<sup>b,c,d</sup>, Glyn Williams-Jones<sup>a</sup>, Timothy Druitt<sup>b</sup>, Nathalie

4

Vigouroux<sup>a,e</sup>, Alexander Wilson<sup>f</sup> and James K. Russell<sup>f</sup>

5

6

<sup>a</sup>Centre for Natural Hazards Research, Department of Earth Sciences, Simon Fraser University, BC, V5A 1S6, Canada

7

8

<sup>b</sup>Laboratoire Magmas et Volcans, Université Clermont Auvergne, Campus Universitaire des Cézeaux, 63178 Aubiere Cedex, France

9

10

<sup>c</sup>Observatoire Volcanologique et Sismologique de Guadeloupe (OVSG), Le Houëlmont, 97113 Gourbeyre, Guadeloupe, French West Indies

11

<sup>d</sup>Université de Paris, Institut de Physique du Globe de Paris, CNRS, F-75005, Paris, France

12

<sup>e</sup>Department of Earth and Environmental Sciences, Douglas College, BC, V3M 5Z5

13

14

<sup>f</sup>Department of Earth, Ocean and Atmospheric Sciences, The University of British Columbia, BC, V6T 1Z4, Canada

15

*Corresponding author. Email address: swethav@sfu.ca

16

Abstract

17

The nature of the magmatic source beneath the Garibaldi Volcanic Belt (GVB) in NW

18

Washington (USA) and SW British Columbia (Canada) has been debated both due to its classification

19

as the northern equivalent of the High Cascades and the alkaline nature of northern basalts. Whole

20

rock studies have shown that the GVB does not share the same magmatic source as the High Cascades

21

(Mullen and Weis, 2015; 2013). Nonetheless, the presence of alkaline basalts in this arc raises

22

questions about the exact source of mantle enrichment and whether it is related to the young age of

23

the downgoing Juan de Fuca Plate (< 10 Ma) or the presence of a slab tear at the northern end of the

24

arc. To gain insight into the source that underlies the GVB, we sampled olivine-hosted melt inclusions

25

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. Furthermore, 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.

Keywords: melt inclusions, Garibaldi Volcanic Belt, Cascades, trace elements, two mantle sources
43 1. Introduction

44 The main compositional controls of eruptive products in subduction zone settings are arguably
45 the age and tectonic context of the downgoing slab (Cooper et al., 2012; Saginor et al., 2013). Dictated
46 by the age, factors such as slab temperature, subduction angle and the amount of slab-derived fluids
47 can cause significant variations in the composition of the wedge-generated melt (Saginor et al., 2013;
48 Weller et al., 2018). The presence of faults and fractures can facilitate the rise of more primitive melt
49 by acting as favourable pathways (e.g., Connor and Conway, 2000; Gaffney et al., 2007; Venugopal et
50 al., 2016) whereas slab tears and slab windows can contribute melts derived from the deeper mantle
51 that do not carry the arc signature (e.g., Gorring and Kay, 2001).

52 Modelling of slab thermal structure has shown that older slabs, such as the Pacific Plate beneath
53 the Aleutian Islands, generally have steeper subduction angles, have lower slab temperatures at a
54 given depth, and supply a higher flux of fluids to the sub-arc mantle (Syracuse et al., 2010; Ohtani et
55 al., 2004; ; Stern, 2002; Ulmer, 2001). Younger slabs, on the other hand, achieve higher temperatures
56 at a given depth and lower slab dips (Syracuse et al., 2010). Though poorly investigated, the release of
57 fluids (due to dehydration reactions) from young slabs has been shown to be at shallower depths than
58 in older slab settings (Ruscitto et al., 2010; Walowski et al., 2016). This suggests that ascending primary
59 derived from young, dehydrated slabs should melts should be depleted in volatiles and slab-derived
60 elements.

61 An excellent case study for studying the effect of slab age and tectonic framework on the resulting
62 eruptive compositions is the Juan de Fuca Plate, which is considered to be the young and hot end
63 member in global subduction zone systems (Syracuse et al., 2010). The north-easterly subduction of

64 the Juan de Fuca Plate beneath the North American Plate produces the Cascades Volcanic Arc along

65 the western margin of North America (Fig. 1). This arc is subdivided into the High Cascades in the south

66 and the Garibaldi Volcanic Belt (GVB) to the north (Fig. 1). The age of the Juan de Fuca Plate at the

67 trench decreases northward from 10 Ma beneath the northern end of the High Cascades to

68 approximately 5 Ma beneath the GVB where it terminates against the Explorer Plate (Fig. 1), which

69 has a lower dip angle (Candie et al., 1989; Wilson, 2002). This difference of dip produces a gap or slab

70 tear between the two plates (Thorkelson et al., 2011).
<figure><img src="image_2.png" /></figure>

71

72
73
74
75
76
77
78
79
80
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.

The relatively older Juan de Fuca Plate beneath the southern Cascades is capable of supplying
more subduction-derived fluids into the mantle wedge, thereby initiating partial melting and
producing arc-typical calc-alkaline magma (Green, 2005). In terms of the GVB, however, little is known
of primitive melt compositions and volatile contents. What is known is the coeval presence of calc-
81 alkaline rocks in the southern GVB and alkaline rocks in the northern GVB (Green, 2006; Mullen and
82 Weis, 2013; 2015). The GVB therefore provides an opportunity to examine both the effects of slab age
83 and slab tearing on the magmatic source composition and volatile contents within a single subduction
84 zone system.

85 Here we present olivine-hosted melt inclusion data for the volcanic centres within the Canadian
86 segment of the GVB alongside melt inclusion data from Glacier Peak and Mount Baker compiled by
87 Shaw (2011). Major element, volatile and trace element contents of the melt inclusions are used to
88 decipher the dynamics of the GVB sub-arc mantle and to better understand the effect of slab age and
89 slab tear on the compositions of mantle melts.

90 2. Geological Context of the GVB

91 The subdivision of the Cascades Arc is based on the change in strike of the arc axis and a bend in
92 the alignment of volcanic centres; the change in strike is evident between Mount Rainier and Glacier
93 Peak (Fig. 1; Mullen et al., 2017). The High Cascades, which strikes north-south, stretches between
94 northern California and northern Washington. The GVB, on the other hand, is a Quaternary-aged
95 glaciovolcanic arc extending between northern Washington and southwestern British Columbia. This
96 belt strikes northwest-southeast and terminates in the north against the Nootka Fault (Fig. 1).
97 Volcanism within the GVB has resulted in more than 2300 distinct vents and 22 major volcanic
98 structures, including Glacier Peak, Mount Baker and the volcanic fields of Mount Garibaldi, Garibaldi
99 Lake, Mount Cayley, Mount Meager Volcanic Complex, Salal Glacier, Bridge River and Silverthrone
100 (Wilson and Russell, 2018; Hildreth, 2007). Recently, the study by Mullen et al. (2017) classifies Glacier
Peak as the northern termination of the High Cascades segment. The dominant eruptive products of the GVB are intermediate calc-alkaline suites with basaltic eruptions comprising a small volume fraction in the south and a large volume fraction in the north (Bridge River to Salal Glacier; Wilson and Russell, 2018). The current geochemical literature consists of high-precision Sr, Pb and Hf isotopic data of basaltic whole-rocks, generally one sample per volcanic centre (Mullen et al., 2017; Mullen and Weis, 2013; 2015). To gain further chemical insight into the primary mantle melt beneath the arc, samples of primitive magma preserved as melt inclusions in an early-crystallizing phase such as olivine, are considered excellent proxies. Melt inclusions are tiny pockets of melt trapped within growing crystals. Upon entrapment, the melts are considered to be sealed in a closed-system environment. In ideal situations, these inclusions are protected by the host crystal during magmatic ascent and eruption, at which time they quench to glass and preserve their initial composition. However, slow ascent rates and/or slow cooling following eruption can cause post-entrapment processes such as nucleation of daughter minerals, which modify the initial inclusion composition and should be taken into account (Section 4.2). Nonetheless, analyses of melt inclusions yield valuable information about the magmatic conditions at the time of entrapment as well as the unique nature of the source at depth.

### 3. Sample description and analytical methods

Detailed sample descriptions and methods can be found as supplementary material. The reader is referred to Shaw (2011) for descriptions of Glacier Peak and Mount Baker samples. Olivine-bearing basalts and basaltic-andesites were sampled from each centre between Garibaldi Lake and Bridge
River. Tephra samples were collected for Garibaldi Lake and Mount Meager while lava flows were sampled from Mount Cayley. Hyaloclastite and pyroclastic breccia were sampled from Salal Glacier and Bridge River, respectively. Samples were crushed, sieved and hand-picked for olivine crystals, which ranged in size from 300 to 600 µm. In all samples, melt inclusions were 15 – 40 µm in diameter, elliptical to spherical in shape and contained a vapour bubble that occupied less than 10 vol% of the inclusion (Supplementary Figure S1). Bubbles that occupied a larger volumetric proportion were avoided as they most likely do not represent primary vapour bubbles (Moore et al., 2015). Melt inclusions that showed evidence of decrepitation (i.e., loss of volatiles through fractures extending from the melt inclusion into the olivine host) were also avoided (Maclennan, 2017). Glassy, crystal-free melt inclusions were only present in the Mount Meager sample as well as Glacier Peak and Mount Baker (Shaw., 2011). Some Mount Meager melt inclusions contained immiscible sulphide globules that ranged in size between 5 - 10 µm. Olivines from Garibaldi Lake, Mount Cayley, Salal Glacier and Bridge River hosted melt inclusions containing pre-existing spinel and occasional sulphides. Based on their irregular shape, rounded embayments and diverse volumetric proportions relative to their olivine hosts, the sulphides were likely co-entrapped with the silicate melt. These melt inclusions also contained daughter crystals, which typically form due to volatile loss, slow ascent rates and/or slow cooling following eruption (Schiano, 2003; Wallace, 2005; Supplementary Figure S1). As such, these inclusions are not representative of the original magma. To reverse the daughter crystallisation process and retrieve the original melt compositions, inclusions from Bridge River, Salal Glacier, Mount Cayley and Garibaldi Lake were reheated at the Laboratoire Magmas et Volcans (LMV) in Clermont-Ferrand, France. Re-heating experiments for each inclusion were 15 minutes in duration to reduce
volatile loss (Chen et al., 2011; Portnyagin et al., 2008; Gaetani et al., 2012; Bucholz et al., 2013) and
stopped when the final daughter crystal disappeared (see Supplementary Methods). The heating stage
was not able to accommodate the high temperatures required for complete melt inclusion
homogenisation. Melt inclusions, olivine hosts and matrix glasses were analysed for major elements
(plus S and Cl for all glasses) at LMV using a SX-100 CAMECA electron microprobe. The precision of
electron microprobe analyses (2σ) are better than 5 % for major elements, except for MnO, Na₂O and
K₂O, which had an EMP precision < 10 %. The approximate 2σ precision for S and Cl is 4 % and 7 %,
respectively. In-situ trace element analyses of melt inclusions were also performed at the LMV with a
Resonetics M50 EXCIMER Laser and a 193 nm wavelength coupled to an Agilent 7500cs ICP-MS (LA-
ICP-MS). Analytical precision and accuracy of measurements were stable, and most elements were
better than 20 % at a 95 % confidence level. Sulphur speciation (S⁶⁺/Sₘα) was obtained from the S Kα
peak shifts relative to the peak shift of barite and sphalerite, temperature and linear regression
coefficients (Wallace and Carmichael, 1994) in order to estimate the oxygen fugacity of the melt
inclusions following the method of Jugo et al. (2005). H₂O, CO₂ and D/H of select melt inclusions were
analysed using the CAMECA IMS 270 Ion Probe (SIMS) at the Centre de Recherches Pétrographiques
et Géochimiques (CRPG) in Nancy, France. The D/H contents of melt inclusions provide insight into the
degree of H⁺ diffusion (i.e., water loss) following entrapment. Values are commonly reported as δD,
which is the permil deviation of the D/H isotope ratio from the D/H ratio of Standard Mean Ocean
Water (δD<sub>S<sub>MOW</sub></sub> = 0 ‰; Hauri, 2002; Shaw et al., 2008). Errors per mil were calculated using the
instrumental drift and are between 2-9 ‰ with one value of 19 ‰ for Mount Meager. Finally, based
on current literature estimates (Anderson & Brown, 1993; Hartley et al., 2014; Moore et al., 2015;
Wallace et al., 2015; Neave et al., 2015; Aster et al., 2016 and references therein), the quantity of CO₂ within the shrinkage bubble is roughly 70-90% of the total amount of CO₂ in the melt inclusion. In order to avoid the underestimation of CO₂ contents, Raman analyses were done within the bubble for most melt inclusions; analyses for Bridge River, Salal Glacier, Mount Cayley and Garibaldi Lake were post re-heating. The size and volume of bubbles and melt inclusions were estimated using a microscope under transmitted light and Leica imaging software. Melt inclusion volumes were assumed to be ellipsoidal and the two observable axes were measured. The best estimate for the third axis was approximated using the smaller ellipsoidal axis measured in the microscope. Bubble volumes were assumed to be spherical and calculated using measured diameters that were accurate to 2 µm. The associated errors for size and volume estimates is less than 10%.

The analytical procedure for Mount Baker and Glacier Peak can be found in Shaw (2011). The main instrumental difference between Shaw (2011) and this study is their use of FTIR for H₂O analyses of melt inclusions. Further details about analytical techniques can be found as supplementary material.

## 4. Results

### 4.1 Whole rock and groundmass glass

Whole rock compositions for Bridge River, Salal Glacier, Mount Meager, Mount Cayley and Garibaldi Lake are presented in Supplementary Table S1. Major oxide compositions for these centres are broad with 47 – 55 wt% SiO₂, 8 – 14 wt% Fe₂O₃, 5 – 11 wt% MgO and 4 – 5 wt% total alkalis (Na₂O + K₂O; Fig. 2). While most samples are calc-alkaline arc basalts, the samples from Bridge River and Salal Glacier are the most alkaline (Fig. 2). Mount Baker and Glacier Peak whole rock and groundmass glass compositions were not provided by Shaw (2011).
184 4.2 Melt Inclusions

185 All melt inclusions experienced some degree of post-entrapment modification. Glassy melt
186 inclusions from Glacier Peak, Mount Baker and Mount Meager were corrected for olivine-rim
187 crystallisation considering the Fe-Mg equilibrium K<sub>D</sub> [= (FeO/MgO)<sub>olivine</sub>/(FeO/MgO)<sub>melt inclusion</sub>].

188 Corrections were preformed using Petrolog3 software (Danyushevsky and Plechov, 2011) and the
189 equation calibrated by Toplis (2005). Overall, melt inclusions experienced minimal post entrapment
190 crystallization (PEC = 0 - 6.5%) and are in equilibrium with the olivine host (K<sub>D</sub> = 0.30 – 0.36; Toplis,
191 2005; Supplementary Table S2). Melt inclusions from the other centres experienced daughter
192 crystallisation. Reversing this process through re-heating also reverses the post-entrapment olivine
193 rim crystallisation, thus yielding melt inclusions in equilibrium with the olivine host (K<sub>D</sub> = 0.30 – 0.33;
194 Supplementary Table S2). The temperature at which all daughter crystals melted within the inclusion
195 range from 1150 to 1190 °C (Supplementary Table S2). It should be noted that some re-heated Salal
196 Glacier melt inclusions contained anomalously high FeO* contents (up to 29 wt %). This phenomenon,
197 described by Rowe et al. (2006), is due to the presence of pre-entrapment magnetite, chromite and
198 Fe-Ni sulphides. As the latter are present in three inclusions, we attribute Fe gain during re-heating to
199 the partial melting of Fe-Ni sulphides within Salal Glacier inclusions. These inclusions are not
200 considered representative of the initial trapped melt and will not be shown in diagrams nor discussed
201 in this study.

202 The complete dataset of corrected major, volatile and trace element data is provided in
203 Supplementary Tables S2 and S3.

204 4.1.1 Major elements
205

Olivine hosts from all volcanic centres have forsterite contents of 77 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*, 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 highest SiO₂, K₂O and lowest FeO* are from Glacier Peak, Mount Baker and Garibaldi Lake (Fig. 2). Bridge River and Salal Glacier melt inclusions are the most primitive with the lowest SiO₂ and highest MgO and FeO* (Fig. 2). These melts also contain up to 2.8 wt% TiO₂, which is higher than typical arc values (up to 1.5 wt%; Aragón et al., 2013).

206

207

208

209

210

211

<figure><img src="image_3.png" /></figure>

212
213

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

214

215

216

217

218

219

220

## 4.1.2 Volatiles

221

Volatile contents (H₂O, CO₂, S and Cl) are variable along the arc. Chlorine and sulphur contents vary significantly between 100 – 1700 ppm and 90 – 4000 ppm, respectively (Supplementary Table S2). The highest Cl contents are from Garibaldi Lake and Mount Cayley while the highest S contents are from Mount Meager, which is consistent with the presence of sulphide globules and suggests saturation of the melt with respect to S. For many samples, carbon dioxide was measured both in the melt and in the shrinkage bubble. Without the bubble, the melt inclusion CO₂ contents across the arc range between 90 – 6000 ppm (Fig. 3a; Supplementary Table S2). Both Glacier Peak and Mount Baker contain the lowest CO₂ contents (< 850 ppm) while Mount Meager contains the highest (> 1500 ppm).

222

223

224

225

226

227

228

229

Raman analyses allowed for the calculation of the CO₂ density within shrinkage bubbles from Garibaldi Lake, Mount Cayley, Mount Meager, Salal Glacier and Bridge River. Using the distance between the main CO₂ peaks and the equation by Wang et al. (2011), the CO₂ densities range from 0.15 to 0.6 g/cm³. From here, the total amount of CO₂ in GVB melt inclusions was calculated and varies between 1300 – 10,000 ppm (Fig. 3b; Supplementary Table S2); this range excludes Glacier Peak and Mount Baker (explained below). The highest total CO₂ values are from Bridge River. Consistent with current literature estimates (Anderson and Brown, 1993; Hartley et al., 2014; Moore et al., 2015; Wallace et al., 2015; Neave et al., 2015; Aster et al., 2016; Maclennan, 2017 and references therein),

230

231

232

233

234

235

236
approximately 40 – 80 % of the total CO₂ is found in the bubble (see Supplementary Table S5 for a worked example)

Shaw (2011) did not perform Raman analyses in the bubbles of Glacier Peak and Mount Baker samples, thus the total CO₂ content of the melt inclusion is unknown. The amount of CO₂ that partitions between the glass and bubble depends on several factors including the PEC, magma ascent pathway, closure temperature and the potential for decrepitation (Moore et al., 2015; Wallace et al., 2015; Aster et al., 2016; Maclennan, 2017 and references therein). Comparing with the glassy Mount Meager melt inclusions, Glacier Peak and Mount Baker have similar PEC (1-5%; Supplementary Table S2) and entrapment temperatures (Section 4.3). Since none of the melt inclusions in this study showed evidence of decrepitation, the only unknown parameter is the magma ascent pathway, which we assume to be similar between the 3 centres. Based on these factors, the amount of CO₂ that partitions into the bubble of Mount Meager samples can be extrapolated to Glacier Peak and Mount Baker. The average amount of the total CO₂ found in the bubble of Mount Meager inclusions is 60% (total range 38 – 80%). Therefore, using 60%, the total CO₂ content of Glacier Peak and Mount Baker increases to 400 – 2000 ppm and the total along arc CO₂ content becomes 400 – 10,000 ppm (Fig 3b; Supplementary Table S2). However, we would like to emphasize that the addition of 60% is relatively subjective but provides a reasonable baseline for comparison of the total melt inclusion CO₂ contents along the arc.

Water contents, measured in the melt, varies between 0.1 – 3 wt% (Fig. 3a, b, c). The lowest water contents are in melt inclusions from Garibaldi Lake and Mount Cayley, while the highest values are
from Mount Meager (Fig. 3; Supplementary Table S2). The D/H isotopic ratio was measured in a select number of melt inclusions in order to assess whether the observed H₂O trends are due to post-entrapment H⁺ loss as opposed to trapping of variably degassed melts. Values are expressed as δD, which is the permil deviation from Standard Mean Ocean Water (δD<sub>SMOW</sub>). The values of δD from Garibaldi Lake to Bridge River range from -17 to -150 ‰ (Fig. 3c; Supplementary Table S2). Values of δD between -20 and -80 ‰ are typical of slab fluids to the average upper MORB mantle (Fig. 3c; Shaw et al., 2008) while values closer to -150 ‰ are representative of the upper limit of terrestrial mantle values (-140 ± 20 ‰; Fig. 3c; Hallis et al., 2015). Positive δD values, and a negative trend with H₂O wt%, suggest loss of hydrogen, addition of seawater and/or secondary hydrothermal alteration (Kyser and O'Neil, 1984; Shaw et al., 2008; Portnyagin et al., 2008). The highest δD values (least negative) belong to melt inclusions from Garibaldi Lake and Mount Cayley while melt inclusions with δD values closest to mantle values belong to Bridge River and Salal Glacier (Fig. 3c). Nonetheless, the δD of these melt inclusions imply primary water contents that have been negligibly affected by post-entrapment H⁺ diffusion. Previous authors have shown that long duration re-heating experiments, on timescales of hours to days, can result in significant water loss from the melt inclusion (i.e., Portnyagin et al., 2008; Gaetani et al., 2012; Bucholz et al., 2013). δD of re-heated melt inclusions in this study do not provide any evidence for extensive water loss during experiments.
274

<figure><img src="image_4.png" /></figure>

275

276

277

278

279

280

281

282

283

284

285

286

287

288

289

290

291

292

293

294

295

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).

296

297

### 4.1.3 Trace elements

298

299

300

From Glacier Peak to Mount Meager, trace element contents of melt inclusions show arc-like characteristics with a depletion in immobile, high field strength elements (HFSE) as well as a relative enrichment in large ion lithophile elements (LILE) over the light rare earth elements (LREE) when
compared to normal mid-ocean ridge basalt (NMORB) and ocean island basalt (OIB) magmas (Fig. 4a, b; Supplementary Table S3). Further, there are notable enrichments in fluid-mobile, large ion lithophile elements (LILE), such as Ba, Pb and Sr, which suggest the presence of a hydrous subduction component (Fig. 4a, b; Supplementary Table S3; Sun and McDonough, 1989). These centres also show low concentrations of heavy rare earth elements (HREE) relative to primitive mantle values ((Sm/Yb)N = 1.1 – 5.7), which is indicative of garnet in the mantle source (Jennings et al., 2017; Mullen and Weis, 2015; Sun and McDonough, 1989).

Trace element contents of melt inclusions from Salal Glacier and Bridge River are unique along the arc and characterized by enrichment in HFSE relative to primitive mantle values. These patterns do not resemble previously discussed centres. The signatures of Bridge River and Salal Glacier are similar to enriched asthenospheric melts as opposed to arc magmas and this is mirrored in the low Ba/Ta values between 140 – 220 for Salal Glacier and 120 – 250 for Bridge River (typical arc values are > 500; Borg et al., 2002; Fig. 5). Relative to the trend between Glacier Peak and Mount Meager, there is a weaker depletion of HREE, which suggests a reduced importance of garnet in the source (Jennings et al., 2017). Between these two volcanoes, Salal Glacier melts tend to be slightly more enriched in HFSE and LILE. Overall, the trend for both Salal Glacier and Bridge River closely follows that of ocean island basalts based on data by Sun and McDonough (1989) (Fig. 4c).
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

318

<caption>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</caption>
concentrations of fluid-mobile LILE and depletions in HFSE and LREE. Conversely, 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.

328

## 4.2 Magmatic conditions

Melt inclusion temperatures were calculated from olivine - melt inclusion equilibria using equation 13 from Putirka (2008). Where applicable, calculated values are consistent with re-heating experiments, which range from 1130 – 1190 °C. Using corrected melt inclusion compositions, Garibaldi Lake melts are 1117 – 1178 °C, Mount Cayley are 1090 – 1149 °C, Mount Meager 1057 – 1142 °C, Salal Glacier 1186 – 1303 °C and Bridge River 1241 – 1312 °C (Fig. 5). Errors for temperature values are ± 50 °C. These values are also consistent with Petrolog3, which estimates the liquidus temperature of the melts (Danyushevsky and Plechov, 2011). From Shaw (2011), temperatures were originally determined using the equations from Sugawara (2000) and Medard and Grove (2008) but are comparable to the recalculated temperatures using equation 13 from Putirka (2008) and Petrolog3: Glacier Peak melts are 1113 – 1179 °C while Mount Baker melts are 1076 – 1116 °C (Fig. 5; Supplementary Table S2).

330

331

332

333

334

335

336

337

338

339

340
<figure><img src="image_8.png" /></figure>
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).

The pressure of inclusions was estimated using the volatile saturation model of Papale et al. (2006) using H₂O and CO₂ contents. Recalculated CO₂ contents for all volcanic centres range between 420 – 10,000 ppm and subsequently yield pressure estimates between 0.4 to 6.8 kbars (Fig. 5; Supplementary Table S2). These values reflect the variably degassed nature of melt inclusions (shown previously by H₂O and δD) and suggest polybaric olivine crystallization. As such, we consider these pressure values as a minimum. To avoid underestimation, the maximum recorded pressure (Pmax) for each centre is 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 (Supplementary Table S2). Oxygen fugacity cannot be calculated for Garibaldi Lake, Mount Cayley, Salal Glacier and Bridge River since the inclusions have been re-
369 heated. The sulphur speciation of these melt inclusions will only yield fO₂ values of the heating apparatus, not the magma. The Fe³⁺/ΣFe 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⁺/ΣFe 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).
370
371
372
373
374 For Bridge River and Salal Glacier, Fe³⁺/ΣFe 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).
375
376
377
378

379 5. Discussion

380

381 5.1 Chemical trends along the Garibaldi Volcanic Belt

382 Based on the above results and calculated magmatic conditions, there are clear along-arc trends.

383 Consistent with previous work (Mullen and Weis, 2013; 2015), there is a subtle transition from arc-
384 typical calc-alkaline magmas in the south beneath Glacier Peak and Mount Baker towards increasingly
385 alkalic melts in the north (Fig. 2). Not only are Bridge River and Salal Glacier the most alkaline, these
386 melts are also the least hydrous and contain the lowest chlorine contents along arc (Supplementary
387 Table S2).

388 There is also a clear northward increase in maximum melt inclusion entrapment temperature and
389 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.

## 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 (Fig. 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 (Fig. 4, 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 Figure 5, Ba/Ta is the lowest beneath the northern end of the arc (up to 250) and therefore suggests minimal or lack of subduction-derived fluids (Borg et al., 2002). Centres between Glacier Peak and Mount Meager show typical arc patterns with low Nb (5 – 12 ppm), and high Sr and Ba/Ta (up to 1920 ppm and 1900, respectively) (Fig. 5; Supplementary Table S3), all of
which are indicative of subduction-related processes (e.g., Borg et al., 2002; Portnyagin et al., 2007; Vigouroux et al., 2012 and references therein). The variability of Ba/Ta between Glacier Peak to Mount Meager could be explained by extensive plagioclase fractionation prior to melt inclusion entrapment leading to lower Ba/Ta below Glacier Peak and Mount Baker. However, this would produce a negative Eu anomaly, which is not seen on Figure 4. A more likely explanation is the variable deliverance of subduction-derived fluids between these centres. This is discussed further in Section 5.4.1. Finally, all centres have very low Th/Yb (< 3.2) and Sr/Y (< 90), which shows that for the whole arc, there is respectively little sediment and slab melt from the downgoing plate.

<figure><img src="image_9.png" /></figure>

Fig. 6. Cl/Nb vs Sr/Nd as an indicator of subduction-derived fluids beneath each centre along the GVB, with a logarithmic y-axis. Each respective elemental pair shows similar partitioning behaviour during magmatic processes, except in fluids derived from the downgoing slab for which Cl and Sr have a high affinity. Bridge River and Salal Glacier show much lower values for both ratios, supporting the lack of subduction zone fluids while each centre between Glacier Peak and Mount Meager show elevated
424
425

Cl/Nb and Sr/Nd, illustrating the importance of subduction-derived fluids. Typical 2σ error bars are shown. Data for Glacier Peak and Mount Baker are from Shaw (2011).

426

427

These along-arc variations are best shown by comparing Cl/Nb and Sr/Nd (Fig. 6). Each respective elemental pair have similar partitioning during magmatic processes except in the presence of hydrous fluids, for which Cl and Sr have high affinity (Walowski et al., 2016; Portnyagin et al., 2007). Therefore, these ratios are excellent tracers for subduction-derived fluids. There is a clear break between Bridge River and Salal Glacier and the rest of the arc. The enrichment in Cl and Sr from Glacier Peak to Mount Meager, coupled with the higher water contents, provides strong evidence for the circulation of hydrous fluids within the mantle wedge. In terms of the northern centres, the depletion in Cl and Sr suggests negligible subduction-related fluids.

428

429

430

431

432

433

434

<figure><img src="image_10.png" /></figure>

435
436

437

438

439

440

Fig. 7. Ba versus Nb for all melt inclusions. Two distinct trends clearly represent two separate mantle sources beneath the arc. One subduction modified source beneath Glacier Peak to Mount Meager (Ba/Nb = 45 - 60) and another beneath Bridge River and Salal Glacier (Ba/Nb < 13), which experiences minor subduction influence. Typical 2σ error bars are shown. Data for Glacier Peak and Mount Baker are from Shaw (2011).

441

442

443

444

445

446

447

448

Overall, the distinct nature of the northernmost volcanoes can be explained by the Ba and Nb contents of melt inclusions, which define two distinct trends (Fig. 7). This suggests the presence of two different mantle sources beneath the GVB. One source underlies Glacier Peak to Mount Meager which is capable of producing magmas with Ba/Nb > 30 and another beneath Bridge River and Salal Glacier that can produce significantly lower Ba/Nb (<13). We therefore propose that the arc 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).

449

450

451

452

453

454

455

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).

456

5.3 Tectonic link
457

The segmentation of the arc strongly correlates with the presence of the Nootka Fault, which is a transform fault that defines the boundary between the Juan de Fuca and Explorer Plates and transects the Northern group (Fig. 1; Hyndman et al., 1979). Offshore, it is a left-lateral strike slip fault between the two plates. On shore, it is interpreted as the surficial manifestation of a slab tear or fracture zone between the subducting Juan de Fuca Plate and the relatively stagnant Explorer Plate (Hyndman et al., 1979). Schmidt et al. (2008) proposed that this fracture zone was a pathway for seawater to serpentinise the 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.

458

459

460

461

462

463

464

465

466

467

468

## 5.4 Modelling the magmatic source

469

### 5.4.1 Southern Group

470

471

472

473

474

475

476

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). Indeed 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 lack of a sediment input (across arc Th/La <0.15 and based on high precision whole rock isotope data; Mullen et al., 2017; Mullen and Weis, 2015 and references therein). Based on these factors, the main subduction component beneath the GVB are most likely fluids delivered by the downgoing slab.

Considering the contrasting melt models for the GVB and the High Cascades, we initially model magma generation beneath the GVB with a depleted MORB mantle (DMM; Salters and Stracke, 2004) and add a hydrous subduction component which was calculated using the method of Portnyagin et al. (2007) (Fig. 8a; Supplementary Table S6). Determining the composition of the hydrous subduction component involves first estimating the amount of partial melting of the DMM using elements with low solubility in slab-derived fluids (Nb, Zr and Yb). The degree of partial melting varies between 5 - 12 %. Using the degree of melting and the melt inclusion composition, the composition of the subduction-modified mantle wedge is calculated with the batch melting equation. Finally, the
difference, or excess, between the DMM and the subduction-modified mantle wedge reflects the addition by the hydrous subduction component. The amount of hydrous fluids added can be estimated by the H₂O content of the melt inclusions. However, factors such as magma degassing prior to melt inclusion trapping, and to a lesser degree post-entrapment H loss, would lead to an underestimation of the initial H₂O contents and the amount of hydrous subduction fluids contributed. Therefore, the melt inclusion H₂O values are corrected using the maximum H₂O/K₂O of each centre (Supplementary Table S2; following the methods of Aster et al., 2016 and Walowski et al., 2016). We perform this correction for the sole purpose of estimating the amount of hydrous fluids contributed by the downgoing slab. Reconstructed H₂O values for the GVB range between 0.7 – 4.7 wt%. Using only the maximum reconstructed values (H₂Omax) for each centre, the 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 amount of hydrous subduction fluids delivered to the mantle wedge varies between 2 – 10 wt% (Fig. 8a). The composition of the hydrous subduction component varies slightly between the centres of the Southern group. Interestingly, the modelled concentrations of Ba, Cl and Sr in the hydrous component are the lowest beneath Mount Baker, reach a maximum beneath Garibaldi Lake and Mount Cayley, and wane beneath Mount Meager (Supplementary Table S6). Not only is this trend mirrored in the along arc values of Ba/Ta on Figure 5, but is also consistent with the Ba, Sr and Cl-rich nature of Mount Cayley and Garibaldi Lake melt inclusions (up to 1700 ppm Cl). This is also seen in Fig. 6, where there is a gradual increase in Cl/Nb and Sr/Nd from Mount Meager and Mount Baker towards Mount Cayley and Garibaldi Lake.

518
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>
Fig. 8. Modelling the two distinct magmatic sources beneath the GVB. a) H₂O/Ce vs Sr/Nd. Modelling the source beneath the Southern group starts with the depleted MORB mantle (black diamond) and adding variable amounts of a hydrous subduction component, calculated using the method of Portnyagin et al. (2007). This component is enriched in Ba, Sr, Cl, H₂O and Pb. Grey area represents the modelling results that involve 5 – 12 % partial melt of DMM (black lines) mixed with 2 – 10 wt% of the hydrous subduction component (dashed lines). Inclusions that lie above the grey area are assumed to have elevated slab fluid content and those that lie below are assumed to be more degassed melts. b) Modelling the enriched mantle source beneath the Northern group in terms of Dy/Yb and Nb/Y, with the Southern group melt inclusions plotted as reference. These ratios are optimal markers for the degree of partial melting, the presence of garnet and heterogeneities in the mantle (Vigouroux et al., 2012). Elevated Dy/Yb and Nb/Y of the Northern group cannot be modelled by even low degrees of partial melting (1 %) of the DMM (black diamond). Melt inclusions 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 by tick marks along the model curve. The residue is garnet lherzolite (7 cpx: 25 opx: 60 olv: 8 grt). Typical 2σ error bars are shown. Data for Glacier Peak and Mount Baker are from Shaw (2011).

537

538
5.4.2 Northern Group

To first test the hypothesis of a single mantle source beneath the GVB, the Northern group melt inclusions are modelled using the same procedure as the Southern Group, i.e., depleted mantle with fluids delivered by the subducting slab. Even with very low degrees of partial melting (< 1 %), the Northern group melt inclusion compositions cannot be recreated using this method (Fig. 8b). As discussed earlier, these melt compositions suggest an enriched mantle source that is potentially controlled by the Nootka Fault. It is thus useful to compare the Dy/Yb and Nb/Y contents of melt inclusions as these elements are derived entirely from the mantle and not added by subduction-related components (Fig. 8b). Furthermore, the Dy/Yb ratio is considered sensitive to variations in the mantle source composition while Nb/Y is a useful proxy for the degree of partial melting and episodes of previous melt extraction (Vigouroux et al., 2012 and references therein). Typical MORB compositions have Dy/Yb between 1.3 and 2.0 and Nb/Y < 0.4 while OIB have Dy/Yb > 2 and Nb/Y >
0.8 (Fig. 8b; PetDB database; Lehnert et al., 2000). Dy/Yb can be affected by amphibole fractionation, and this would be shown by a negative correlation between Dy/Yb and SiO₂ wt%, which is not seen in the dataset. Both Bridge River and Salal Glacier have higher Dy/Yb and Nb/Y than typical MORB melts but overlap with OIB compositions. This suggests low degrees of partial melting of an enriched mantle source. Modelling of the Northern group thus begins with the mass balance equation for batch melting and with the primitive mantle as the source (Fig 8b; Sun and McDonough, 1989). Distribution coefficients are provided in Supplementary Table S4. With melt fractions below 7 % and garnet lherzolite as a residual phase, the modelled partial melt compositions bear a strong resemblance to the Northern group in terms of Dy/Yb and Nb/Y ratios (Fig. 8b). Modelling results are consistent with the elevated FeO* wt% (> 9), La/Ta (<20) and low HREE contents of Northern group melt inclusions, all of which support OIB-like melt generation below the garnet-spinel transition zone at 70 km (Gorring and Kay, 2001). This model is also consistent with the whole-rock modelling results of Mullen and Weis (2013). However, the main and rather significant difference is their incorporation of Mount Meager as having the same enriched source as the Northern group. Although Mount Meager shows transition alkaline compositions, the trace element contents are clearly subduction zone derived.

## 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 melting of two distinct mantle sources (Fig. 9). However, there are significant tectonic factors to consider. In the case of the Northern group, the derivation of deeply sourced primitive and enriched melts beneath Bridge River and Salal Glacier suggests that the Nootka
570 Fault and slab tear is persistent through to the asthenosphere and is a favourable pathway for
571 ascending enriched melts.

572 Based on the glaciovolcanic overview by Wilson and Russell (2018), there has been one recorded
573 deposit of calc-alkaline composition within the Salal Glacier Volcanic Field. Though one known calc-
574 alkaline deposit does not necessarily suggest that the Northern group experiences occasional
575 subduction influence, it may however provide insight into the nature of the ascending pathway. A
576 likely case for melt generation is the ascent of asthenospheric mantle through the slab tear and the
577 subsequent decompression-induced melting in the sub-arc mantle (Fig. 9). The ascent of
578 asthenospheric melts through faults and fractures directed along the northern edge of the Juan de
579 Fuca Plate followed by the periodic mixing with the subduction-modified source beneath the Southern
580 group would account for the rare calc-alkaline signature within the Salal Glacier Volcanic Field. We
581 therefore propose that the area of slab tear is more of a densely fractured zone between the Juan de
582 Fuca and Explorer Plates with each fault acting as a potential pathway for ascending asthenospheric
583 mantle (Fig. 9).
<figure><img src="image_13.png" /></figure>

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.

The Southern group requires a hydrous subduction component that varies in composition between each centre. The amount of the hydrous subduction component contributed to the mantle wedge is at a maximum beneath Mount Cayley and Garibaldi Lake and lowest beneath Glacier Peak and Mount Baker (Fig. 9). The idea of more fluids exsolved from a younger slab is contrary to the normally accepted relationship between slab age and fluid exsolution. A younger slab typically implies higher slab temperatures and thus, smaller additions of slab-derived hydrous components to the mantle wedge (Mullen and Weis, 2015; Green and Harry, 1999). However, if we consider the slab
602 dehydration model by Walowski et al. (2016), then the slightly older plate beneath Glacier Peak and
603 Mount Baker would have already dehydrated prior to reaching sub-GVB depths thereby supplying
604 smaller amounts of fluids. As the slab becomes progressively younger to the north, the degree of
605 dehydration decreases (i.e., the slab remains hydrated) and thus the amount of fluids supplied
606 increases. Nevertheless, the Juan de Fuca Plate is one of the youngest subducting plates in the world
607 and therefore the total amount of fluids supplied would be very low in comparison to global
608 subduction zones with older slabs.

609 6. Conclusions

610 Olivine-hosted melt inclusions from the GVB provide new insight into the critical factors
611 controlling the composition of subduction zone magmas. Factors such as the age of the downgoing
612 slab, degree of fluid release into the mantle wedge as well as the tectonic framework of the GVB itself
613 have resulted in a unique and pronounced north-south geochemical transition.

614 Beneath the northern GVB, the reduced nature of melt inclusions coupled with higher
615 temperatures and pressures of entrapment and the elevated contents of HFSE suggest an enriched
616 mantle source rather than a subduction-modified source. We attribute these signatures to a slab tear,
617 or dense fracture zone, between the northern edge of the Juan de Fuca Plate and the neighbouring
618 Explorer Plate. Modelling of the enriched source beneath the Northern group suggests that the slab
619 tear is pervasive enough to reach the upper mantle. Through decompression melting, asthenospheric
620 melts ascend through the tear and erupt at the surface. In a global context, there are have been few
621 eruptions of similarly enriched melts in an arc setting. Gorring and Kay (2001) show that Patagonia
plateau lavas have a distinct OIB-like signature that can be attributed to the slab window formed by the subduction of the Chile Ridge between the Nazca and Antarctic Plates. Trace element and modelling results show that, similar to the Northern Group, melt generation began near the garnet-spinel at 70 km depth (Gorring and Kay, 2001).

Melt inclusions from the southern GVB, on the other hand, reveal arc-typical compositions produced by a mix between the depleted MORB mantle and hydrous subduction components. The main difference between the centres of the Southern group is the amount and concentrations of Sr, Ba and Cl of the hydrous subduction component. The maximum amount of fluids with the highest Sr, Ba and Cl is delivered to the mantle wedge beneath Mount Cayley and Garibaldi Lake while the lowest amounts are below Mount Baker and Glacier Peak. This pattern of fluid exsolution correlates positively with the age of the downgoing Juan de Fuca Plate. Though Walowski et al. (2016) found that the downgoing Juan de Fuca Plate beneath the southern Cascades had significantly dehydrated prior to reaching sub-arc depths, our modelling results show that slab fluids are the dominant subduction component beneath the southern GVB. These results suggest that the difference in slab age between the southern Cascades and the southern GVB (3 – 4 Ma) is sufficient to preserve a more hydrated slab.

To test the global applicability of younger slabs retaining their fluids, melt inclusion compositions from young (<10 Ma) and hot subduction zones, such as South Chile and Mexico (Syracuse et al., 2010), would provide further insight into the relationship between the slab age and the degree of dehydration.
641 Our results show that abrupt along-arc compositional changes can be used to identify

642 structures in the slab that cannot otherwise be identified by geophysical methods, either due to low-

643 resolution imaging or a lack of available information. Furthermore, this work illustrates the

644 importance of the age and degree of dehydration of the downgoing slab in subduction zone settings.

645 From the delivery of slab melts in the southern Cascades, to the exsolution of fluids in the southern

646 GVB and the termination against the Nootka Fault and slab tear beneath the northern GVB, the Juan

647 de Fuca Plate is capable of producing a diverse array of volcanic centres along the western margin of

648 North America.

649

650 Acknowledgments

651 This study is a Clervolc contribution. This work was supported by NSERC Discovery grants to Williams-

652 Jones and Russell and a ClerVolc grant to Moune. Many thanks to Jean-Luc Devidal and Etienne

653 Deloule for their expertise in electron microprobe and SIMS analysis. The work and preparation of an

654 andesitic CO₂ standard by Federica Schiavi and Didier Laporte is greatly appreciated. Estelle Rose-Koga

655 and Nico Cluzel is thanked for the use of the re-heating stage at LMV. We would also like to thank Paul

656 Wallace and an anonymous reviewer for many helpful comments that greatly improved the quality of

657 this work.

658

659
660 Bibliography

661 Anderson, A.T. & Brown, G. G. (1993). CO₂ contents and formation pressures of some Kilauean
662 melt inclusions. American Mineralogist. 78,794-803.

663

664 2. Aragón, E., Pinotti, L., Castro, A., Rabbia, O., Coniglio, J., Demartis, M., Hernando, I., Cavarozzi, C.,Aguilera, Y. E. (2013). The Farallon-Aluk ridge collision with South America: Implications for the geochemical changes of slab window magmas from fore- to back-arc. *Geoscience Frontiers*. 4. 377-388. https://doi.org/10.1016/j.gsf.2012.12.004.

668 3. Aster, E. M., Wallace, P. J., Moore, L. R., Watkins, J., Gazel, E., & Bodnar, R. J. (2016).
669 Reconstructing CO₂ concentrations in basaltic melt inclusions using Raman analysis of vapour
670 bubbles. *Journal of Volcanology and Geothermal Research*, 323, 148–162.

671 https://doi.org/10.1016/j.jvolgeores.2016.04.028

672 4. Borg, L., Blichert-Toft, J., Clynne, M. A. (2002). Ancient and Modern Subduction Zone
673 Contributions to the Mantle Sources of Lavas from the Lassen Region of California Inferred from
674 Lu-Hf Isotopic Systematics. *Journal of Petrology*, 43(6), 705–723,
675 https://doi.org/10.1093/petrology/43.4.705

676 5. Bucholz, C.E., Gaetani, G.A., Behn, M.D., Shimizu, N. (2013). Post-entrapment
677 modification of volatiles and oxygen fugacity in olivine-hosted melt inclusions. *Earth
678 Planet. Sci. Lett.* 374, 145–155.

679 6. Candie, S.C., La Brecque, J.L., Larson, R.L., Pittman III, W.C., Golovchenko, X., Haxby, W.F.
680 (1989). Magnetic Lineations of the World's Ocean Basins. *American Association of Petroleum
681 Geologists*, Tulsa, Oklahoma.

682

683 7. Connor, C.B., Conway, F.M. (2000). Basaltic volcanic fields. *Encyclopaedia of Volcanology*.
684 Academic Press, pp. 331–343.

685 8. Cooper, L. B., Ruscitto, D. M., Plank, T., Wallace, P. J., Syracuse, E. M., & Manning, C. E. (2012).
686 Global variations in H₂O/Ce: 1. Slab surface temperatures beneath volcanic arcs. *Geochemistry,
687 Geophysics, Geosystems*, 13(3). https://doi.org/10.1029/2011GC003902

688 9. Danyushevsky, L.V., Plechov, P. (2011). Petrolog3: integrated software for modelling
689 crystallization processes. *Geochemistry, Geophysics, Geosystems*, 12 (7).

690

691 10. Defant, M. J. & Drummond, M. S. Derivation of some modern arc magmas by melting of young
692 subducted lithosphere. *Nature* 347, 662–665 (1990)

693

694 11. Gaffney, E.S., Damjanac, B., Valentine, G.A. (2007). Localization of volcanic activity; 2. Effects of
695 pre-existing structure. *Earth Planet. Sci. Lett.* 263, 323–338.

696 12. Gorring, M. L., & Kay, S. M. (2001). Mantle Processes and Sources of Neogene Slab Window
697 Magmas from Southern Patagonia, Argentina. *Journal of Petrology*, 42(6), 1067–1094.

698 https://doi.org/10.1093/petrology/42.6.1067

699 13. Green, N. L. (2006). Influence of slab thermal structure on basalt source regions and melting
700 conditions : REE and HFSE constraints from the Garibaldi Volcanic Belt, northern Cascadia
701 subduction system. *Lithos* 87(1-2):23-49. https://doi.org/10.1016/j.lithos.2005.05.003
702

703

704

14. Green, N. L., & Harry, D. L. (1999). On the relationship between subducted slab age and arc basalt petrogenesis, Cascadia subduction system, North America. *Earth and Planetary Science Letters*, 171(3), 367-381. https://doi.org/10.1016/S0012-821X(99)00159-4

705

706

707

708

15. Green, N. L., & Sinha, A. K. (2005). Consequences of varied slab age and thermal structure on enrichment processes in the sub-arc mantle of the northern Cascadia subduction system. *Journal of Volcanology and Geothermal Research*, 140(1-3), 107-132. https://doi.org/10.1016/j.jvolgeores.2004.07.017

709

710

711

16. Hallis, L. J., Huss, G. R., Nagashima, K., Taylor, G. J., Halldórsson, S. A., Hilton, D. R., 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

712

713

714

17. Hartley M. E., Neave D. A., Maclennan J., Edmonds M. and Thordarson T. (2015). Diffusive overhydration of olivine-hosted melt inclusions. *Earth Planet. Sci. Lett.* 425, 168-178. http://dx.doi.org/10.1016/j.epsl.2015.06.008.

715

716

18. Hauri, E. (2002) SIMS analysis of volatiles in silicate glasses, 2: isotopes and abundances in Hawaiian melt inclusions. *Chem Geol.* 183:115-141.

717

718

19. Hildreth, W. (2007). Quaternary magmatism in the Cascades; geologic perspectives. *U.S. Geological Survey Professional Paper 1744*, 125.

719

720

721

20. Hirschmann, M. M., A. C. Withers, and C. Aubaud. (2006), Petrologic structure of a hydrous 410 km discontinuity. *Earth's Deep Water Cycle, Geophys. Monogr. Ser.*, vol. 168, edited by S. D. Jacobsen and S. van der Lee pp. 277-287, AGU, Washington, D. C., doi:10.1029/168GM21.

722

723

724

21. Hyndman, R., Riddihough, R., & Herzer, R. (1979). The Nootka Fault Zone — a new plate boundary off western Canada. *Geophysical Journal of the Royal Astronomical Society*, 58(3), 667-683.

725

726

727

22. Jennings, E.S., Gibson, S.A., Maclennan, J., Heinonen, J.S. (2017). Deep mixing of mantle melts beneath continental flood basalt provinces: constraints from olivine- hosted melt inclusions in primitive magmas. *Geochim. Cosmochim. Acta*. 196, 36-57.

728

729

23. Jugo, P. J. (2009). Sulfur content at sulfide saturation in oxidized magmas. *Geology*, 37(5), 415-418. https://doi.org/10.1130/G25527A.1

730

731

732

24. Jugo, P., Luth, R., and Richards, J. (2005b). Experimental data on the speciation of sulfur as a function of oxygen fugacity in basaltic melts: *Geochimica et Cosmochimica Acta*, v. 69, p. 497-503, doi: 10.1016/j.gca.2004.07.011.

733

734

735

736

25. Kress, V. C., & Carmichael, I. S. E. (1991). The compressibility of silicate liquids containing $Fe_2O_3$ and the effect of composition, temperature, oxygen fugacity and pressure on their redox states. *Contributions to Mineralogy and Petrology*, 108(1-2), 82-92. https://doi.org/10.1007/BF00307328

737

738

739

26. Kyser, T.K., O'Neil, J.R. (1984). Hydrogen isotope systematics of submarine basalts. *Geochimica et Cosmochimica Acta*. 48, 2123-2133.

740

741

742

743

27. Lehnert, K., Y. Su, C. H. Langmuir, B. Sarbas, and U. Nohl. (2000). A global geochemical database structure for rocks. *Geochemistry, Geophysics, Geosystems*, 1(5), 1012, doi:10.1029/2000GC000026.
744
745
746

747
748
749
750

751
752

753
754
755

756
757

758
759
760

761
762
763

764
765

766
767
768

769
770
771

772
773

774
775

776
777
778
779

780
781
782
783

28. Maclennan, J. (2017). Bubble formation and decrepitation control on the CO2 content of olivine-hosted melt inclusions. *Geochemistry, Geophysics, Geosystems*, 2(18), 597-616.

29. Mathews, W.H. (1958). Geology of the Mount Garibaldi map-area, southwestern British Columbia, Canada: Part II. Geomorphology and Quaternary volcanic rocks. *Geological Society of America Bulletin* 69, 179-198

30. Medard, E. and Grove, T.L. (2008). The effect of H₂O on the olivine liquidus of basaltic melts: experiments and thermodynamic models. *Contrib Mineral Petrol* 155:417-432

31. Moore, N. E., & DeBari, S. M. (2012). Mafic magmas from Mount Baker in the northern Cascade arc, Washington: Probes into mantle and crustal processes. *Contributions to Mineralogy and Petrology*, 163(3), 521-546. https://doi.org/10.1007/s00410-011-0686-4

32. Moyen, Jean-François. (2009). High Sr/Y and La/Yb ratios: The meaning of the “adakitic signature”. *Lithos*, 112(3-4), 556-574. https://doi.org/10.1016/j.lithos.2009.04.001

33. Mullen, E. K., & Weis, D. (2015). Evidence for trench-parallel mantle flow in the northern Cascade Arc from basalt geochemistry. *Earth and Planetary Science Letters*, 414, 100-107. https://doi.org/10.1016/j.epsl.2015.01.010

34. Mullen, E. K., Weis, D., Marsh, N. B., & Martindale, M. (2017). Primitive arc magma diversity: New geochemical insights in the Cascade Arc. *Chemical Geology*, 448, 43-70. https://doi.org/10.1016/j.chemgeo.2016.11.006

35. Newman, S., Lowenstern, J.B., 2002. VolatileCalc: a silicate melt-H₂O-CO₂ solution model written in Visual Basic for excel. *Comput. Geosci.* 28, 597-604

36. Ohtani, E., K. D. Litasov, T. Hosoya, T. Kubo, and T. Kondo. (2004). Water transport into the deep mantle and formation of a hydrous transition zone. *Physics of the Earth and Planetary Interiors*, 143-144, 255-269, doi:10.1016/j.pepi.2003.09.015.

37. Papale, P., Moretti, R., & Barbato, D. (2006). The compositional dependence of the saturation surface of H₂O + CO₂fluids in silicate melts. *Chemical Geology*, 229(1-3), 78-95. https://doi.org/10.1016/j.chemgeo.2006.01.013

38. Pearce, J. A. and Norry, M. J. (1979). Petrogenetic implications of Ti, Zr, Y, and Nb variations in volcanic rocks. *Contrib Mineral Petrol* 69:33-47

39. Porter, K. A., W. M. White (2009), Deep mantle subduction flux. *Geochem. Geophys. Geosyst.*, 10, Q12016, doi:10.1029/2009GC002656.

40. Portnyagin, M., Hoernle, K., Plechov, P., Mironov, N., Khubunaya, S. (2007). Constraints on mantle melting and composition and nature of slab components in volcanic arcs from volatiles (H₂O, S, Cl, F) and trace elements in melt inclusions from the Kamchatka Arc. *Earth and Planetary Science Letters*, 255(1-2), 53-69. https://doi.org/10.1016/j.epsl.2006.12.005

41. Portnyagin M., Almeev R. R., Matveev S., Holtz F. (2008). Experimental evidence for rapid water exchange between melt inclusions in olivine and host magma. *Earth Planet. Sci. Lett.* 272, 541-552. http://dx.doi.org/10.1016/j.epsl.2008.05.020.
784 42. Putirka, K. (2008). Thermometers and barometers for volcanic systems. In: Putirka K, Tepley F
785 (eds) Minerals, inclusions and volcanic processes, vol 69. *Reviews in Mineralogy and
786 Geochemistry, Mineralogical Soc. Am.*, pp. 61–120

787

788 43. Rowe, M. (2006). Anomalously high Fe contents in rehomogenized olivine-hosted melt inclusions from oxidized magmas. *American Mineralogist*. 91(1). 82-91.
789 https://doi.org/10.2138/am.2006.1818
790

791 44. Ruscitto, D. M., Wallace, P. J., Johnson, E. R., Kent, A. J. R., & Bindeman, I. N. (2010). Volatile contents of mafic magmas from cinder cones in the Central Oregon High Cascades: Implications for magma formation and mantle conditions in a hot arc. *Earth and Planetary Science Letters*, 298(1-2), 153-161. https://doi.org/10.1016/j.epsl.2010.07.037

792
793
794

795 45. Saginor, I., Gazel, E., Condie, C., Carr, M. J. (2013), Evolution of geochemical variations along the Central American volcanic front. *Geochem. Geophys. Geosyst.*, 14, doi:10.1002/ggge.20259.

796

797 46. Salters, V. J. M., & Stracke, A. (2004). Composition of the depleted mantle. *Geochemistry, Geophysics, Geosystems*, 5(5). https://doi.org/10.1029/2003GC000597

798

799 47. Schiano, P. (2003). Primitive mantle magmas recorded as silicate melt inclusions in igneous minerals. *Earth-Science Reviews*. 63. 121-144. https://doi.org/10.1016/S0012-8252(03)00034-5.

800

801 48. Schmidt, M. E., Grunder, A. L., & Rowe, M. C. (2008). Segmentation of the Cascade Arc as indicated by Sr and Nd isotopic variation among diverse primitive basalts. *Earth and Planetary Science Letters*, 266(1-2), 166-181. https://doi.org/10.1016/j.epsl.2007.11.013

802
803

804 49. Shaw, S. D. (2011). H₂O Contents in Olivine-Hosted Melt Inclusions From Primitive Magmas in the Northern Cascade. MSc thesis. University of Western Washington.

805

806 50. Shaw, A. M., Hauri, E. H., Fischer, T. P., Hilton, D. R., & Kelley, K. A. (2008). Hydrogen isotopes in Mariana arc melt inclusions: Implications for subduction dehydration and the deep-Earth water cycle. *Earth and Planetary Science Letters*, 275(1-2), 138-145.
809 https://doi.org/10.1016/j.epsl.2008.08.015

807
808
809

810 51. Steele-MacInnis, M., Esposito, R., Bodnar, R.J. (2011). Thermodynamic model for the effect of post-entrapment crystallization on the H₂O-CO₂ systematics of vapor-saturated, silicate melt inclusions. *Journal of Petrology*, 52, 2461–2482.

811
812

813 52. Stern, B. (2002). Subduction zones. *Rev. Geophys.*, 40(4), 1012, doi:10.1029/2001RG000108,

814

815 53. Sugawara, T. (2000). Thermodynamic analysis of Fe and Mg partitioning between plagioclase and silicate liquid. *Contrib. Mineral. Petrol.* 138, 101–113.

816

817 54. Sun, S., & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. *Geological Society, London, Special Publications*, 42(1), 313–345. https://doi.org/10.1144/GSL.SP.1989.042.01.19

818
819

820 55. Syracuse, E. M., van Keken, P. E., Abers, G. A., Suetsugu, D., Bina, C., Inoue, T., Jellinek, M. (2010). The global range of subduction zone thermal models. *Physics of the Earth and Planetary Interiors*, 183(1-2), 73–90. https://doi.org/10.1016/j.pepi.2010.02.004

821
822
823
824
825

56. Thorkelson, D. J., Madsen, J. K., & Sluggett, C. L. (2011). Mantle flow through the Northern Cordilleran slab window revealed by volcanic geochemistry. *Geology*, *39*(3), 267–270.
https://doi.org/10.1130/G31522.1

826
827
828
829

57. Toplis, M. J. (2005). The thermodynamics of iron and magnesium partitioning between olivine and liquid: Criteria for assessing and predicting equilibrium in natural and experimental systems. *Contributions to Mineralogy and Petrology*, *149*(1), 22–39.
https://doi.org/10.1007/s00410-004-0629-4

830
831

58. Ulmer, P., 2001. Partial melting in the mantle wedge—the role of H₂O in the genesis of mantle-derived “arc-related” magmas. *Phys. Earth Planet. Inter.* 127, 215–232.

832
833
834

59. Venugopal, S., Moune, S., & Williams-Jones, G. (2016). Investigating the subsurface connection beneath Cerro Negro volcano and the El Hoyo Complex, Nicaragua. *Journal of Volcanology and Geothermal Research*, 325, 211–224.

835
836
837
838

60. Vigouroux, N., Wallace, P. J., Williams-Jones, G., Kelley, K., Kent, A. J. R., & Williams-Jones, A. E. (2012). The sources of volatile and fluid-mobile elements in the Sunda arc: A melt inclusion study from Kawah Ijen and Tambora volcanoes, Indonesia. *Geochemistry, Geophysics, Geosystems*, 13(9), 1–22. https://doi.org/10.1029/2012GC004192

839
840
841

61. Wallace, P. J. (2005), Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data, *J. Volcanol. Geophys. Res.*, 140, 217–240,
doi:10.1016/j.jvolgeores.2004.07.023.

842
843

62. Wallace, P.J., Kamenetsky, V.S., Cervantes, P. (2015). Melt inclusion CO₂ contents, pressures of olivine crystallization, and the problem of shrinkage bubbles. *Am. Mineral.* 100 (4), 787–794.

844
845
846

63. Walowski, K. J., Wallace, P. J., Clynne, M. A., Rasmussen, D. J., & Weis, D. (2016). Slab melting and magma formation beneath the southern Cascade arc. *Earth and Planetary Science Letters*, 446, 100–112. https://doi.org/10.1016/j.epsl.2016.03.044

847
848
849

64. Weller, D.J., Stern, C.R., (2018). Along-strike variability in primitive magmas (major and volatile elements) inferred from olivine-hosted melt inclusions from volcanoes of the southernmost Andean Southern Volcanic Zone, Chile. *Lithos*. 296–299(2018 233–244).

850
851
852

65. Wilson, A., and Russell, J. K. (2018). Quaternary glaciovolcanism in the Canadian Cascade volcanic arc - paleoenvironmental implications. *Special Paper of the Geological Society of America*. 538(06). 1-26. DOI: 10.1130/2018.2538(06).

853
854

66. Wilson, D.S. (2002). The Juan de Fuca Plate and slab: isochron structure and Cenozoic plate motions. US Geological Survey Open-File Report 02-328.