# Magnesium isotopic composition of metasomatized upper sub-arc mantle and its implications to Mg cycling in subduction zones

Yan Hu, Fang-Zhen Teng, Dmitri Ionov

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Yan Hu, Fang-Zhen Teng, Dmitri Ionov. Magnesium isotopic composition of metasomatized upper sub-arc mantle and its implications to Mg cycling in subduction zones. Geochimica et Cosmochimica Acta, 2020, 278, pp.219-234. 10.1016/j.gca.2019.09.030. hal-03120904

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Magnesium isotopic composition of metasomatized upper sub - arc mantle and its
implications to Mg cycling in subduction zones

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4 Yan Huª, Fang-Zhen Tengª, Dmitri A. Ionovᵇ.c

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a. Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195-1310, USA

b. Géosciences Montpellier, Université de Montpellier, 34095 Montpellier, France

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c. State Key Laboratory of Isotope Geosciences, Guangzhou Institute of Geochemistry, Guangzhou 510640, China

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Abstract: 289 words

Text: 5580 words

Tables: 2

Supplementary tables: 2

Figures: 9

Submitted to **GCA "SI: The Continents"**
September, 2019

9 Emails: yanhu@uw.edu; fteng@uw.edu

<footnote>*Corresponding authors:</footnote>
10 Abstract

11 Dehydration of the subducting oceanic crust and underlying serpentinized mantle is
12 potentially the most important source of fluids for element cycling between surface-processed
13 materials and the interior of the Earth. Magnesium (Mg) isotopes are a promising tracer of
14 dehydration of serpentinites because of their high Mg contents and distinctive Mg isotopic
15 signatures relative to the mantle, which are difficult to identify using other isotopes. To further
16 constrain the primary controls on Mg isotope cycling at subduction zones, here we analyze a
17 suite of well-characterized sub-arc mantle xenoliths from the Avacha volcano in southern
18 Kamchatka arc, Russia. They are similar in modal and/or chemical composition to serpentinized
19 fore-arc harzburgites and sub-arc harzburgite xenoliths from the western Pacific. Despite
20 evidence for ubiquitous slab-related fluid metasomatism, the 23 spinel harzburgite xenoliths and
21 four pyroxenite metasomatic veins display mantle-like δ²⁶Mg values from -0.30 to -0.21‰.
22 Mineral separates have comparable δ²⁶Mg values, with an average of -0.26 ± 0.04‰ (2SD, n =
23 17) for olivine, -0.23 ± 0.04‰ (2SD, n = 17) for orthopyroxene, and -0.24 ± 0.10‰ (2SD, n = 5)
24 for late-stage interstitial clinopyroxene. The lack of a clear slab-derived δ²⁶Mg signature in the
25 Avacha sub-arc peridotites is probably due to their high MgO contents and low fluid/rock mass
26 ratios during flux melting and metasomatism. We also compare published δ²⁶Mg data for four
27 island arcs distributed worldwide. This inter-arc comparison reveals that the thermal structure of
28 a subduction zone appears to exert a strong control on the efficiency of Mg isotope cycling from
29 the downgoing slab to the mantle wedge and subsequently to the arc magmas; those arcs that
30 permit large-scale dehydration of Mg-rich serpentinized slab mantle, coupled with available
31 channelized fluid pathways, are more likely to produce isotopically diverse arc magmas.
32

33

33 **Keywords**: Magnesium isotopes, sub-arc xenoliths, fluid-rock interaction, slab dehydration, subduction zone thermal structure

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# 1. Introduction

Subduction of oceanic plates at convergent margins provides the primary pathway for transporting water to the mantle. Prior to subduction, the oceanic crust can be extensively hydrated via hydrothermal circulation, which is estimated to contain ~2% of global seawater by volume (Johnson and Pruis, 2003). Approaching the trench, bending of the incoming oceanic plates generates deep normal faults that allow further hydration of the slab mantle via serpentinization (Ranero et al., 2003; Faccenda et al., 2009). As oceanic plates dive into the mantle, fluids are gradually released to the overlying mantle wedge, triggering arc magmatism and modulating the composition of arc magmas. Tracking the nature of slab-released fluids and their interaction with the mantle is central to our understanding of global element cycling, arc magmatism and mantle heterogeneity. This has motivated development of various geochemical tracers to identify and quantify the contribution of slab-derived fluids to mantle magma sources.

Magnesium isotope studies of subduction zones have recently shown potential to be a tracer of slab input, particularly sensitive for tracking subducting carbonates and serpentinites (Teng, 2017 and references therein). In contrast to the uniform $\delta^{26}$Mg values in global peridotites ($-0.25 \pm 0.04\%$, Teng et al., 2010), subducting lithologies have highly heterogeneous Mg isotopic compositions, with carbonate phases having characteristically low $\delta^{26}$Mg values and marine sediments, altered oceanic crust and ultramafic rocks typically displaying variably high $\delta^{26}$Mg values (Beinlich et al., 2014; Teng et al., 2016; Hu et al., 2017; Liu et al., 2017; Huang et al., 2018). Consistent with these heterogeneous subducting inputs, some arc lavas are found to be isotopically heavier than the normal mantle, which are taken as evidence of fluid-mediated transfer of Mg from subducted slab to mantle wedge ($-0.35$ to $+0.06\%$, Teng et al., 2016; Li et al., 2017). By contrast, other studies argued that crust assimilation and/or fractional crystallization can alternatively drive the evolving magmas to higher $\delta^{26}$Mg values (Su et al., 2017a; Brewer et al., 2018; Pang et al., 2019), which compromises the signature of slab fluids recorded in arc magmas.

The highly variable Mg isotopic compositions of the subducting oceanic plates and arc magmas suggest that slab-derived Mg isotopic signatures could be detected in the mantle wedge, which is the primary sink for various slab fluids and the main source of arc magmas. The top of the mantle wedge is sampled most directly by peridotite xenoliths, which are fragments of sub-arc mantle lithosphere accidentally entrained in arc magmas and brought to the surface by
explosive eruptions. A preliminary study by Pogge von Strandmann et al. (2011) found negatively correlated δ<sup>7</sup>Li and δ<sup>26</sup>Mg values in six sub-arc peridotites, which was attributed to diffusion-driven interaction with slab-derived fluids. To establish a more robust link between slab input and arc lava output, more systematic studies on sub-arc peridotite are required. This, however, is hampered by the scarcity of representative xenolith samples, making the mantle wedge one of the most important, but least documented, mantle reservoirs in global Mg cycling.

The Kamchatka peninsula in far eastern Russia (Fig. 1) hosts one of the few arcs where mantle xenoliths are locally available. This arc provides an ideal place to assess the interaction between slab fluids and mantle wedge where contributions from local sediments and continental crust are estimated to be minimal based on radiogenic isotopes (Kersting and Arculus, 1995; Kepezhinskas et al., 1997). In addition, the thermal structure of Kamchatka arc is representative of many subduction zones where a major fluid pulse is liberated at sub-arc depths, in contrast to hot subduction zones where fluids are lost mainly to the fore-arc mantle or cold subduction zones where most of the slab fluids were carried to the deep mantle without a significant loss to arc magma production.

To further constrain the primary factors that control Mg cycling at subduction zones, we analyze a suite of well-characterized sub-arc mantle xenoliths from the Avacha volcano in southern Kamchatka arc (Fig. 1), including 23 peridotite xenoliths that represent *in situ* materials processed in the mantle wedge and four pyroxenite veins cross-cutting the peridotites in the lithosphere. Importantly, a detailed analysis of these peridotites by Ionov (2010) found no evidence for late-stage interaction with host magmas or extensive recrystallization reported for other xenolith suites from Avacha that was mistakenly ascribed to subduction metasomatism (e.g., Arai et al., 2003; Ishimaru et al., 2007; Ishimaru and Arai, 2008). Therefore, although these samples are not as strongly metasomatized as other xenolith suites, they are probably more representative of the normal mantle wedge and thus may provide more relevant insights into slab-mantle Mg isotope cycling applicable to many subduction zones. In addition, we explore the relationship between slab dehydration and δ<sup>26</sup>Mg variation in arc magmas by examining the thermal structure of subduction zones and possible dehydration mineral phases. Taken together, our study suggests that fluids derived from serpentinized oceanic mantle are more effective than those derived from altered oceanic crust in transporting slab δ<sup>26</sup>Mg signatures, particularly when channelized flow pathways are available for rapid fluid migration in the sub-arc mantle.
## 2. Geologic setting

The Kamchatka arc lies in the northern part of the approximate 2000 km-long Kamchatka-Kurile convergent margin, formed by subduction of the Mesozoic Pacific plate beneath the Eurasian plate in northwestern Pacific at a rate of 8 cm/yr (Gorbatov et al., 1997) (Fig. 1). Orthogonal intersection with the Aleutian transform fault-trench system, where the Pacific plate subducts beneath the North American plate, sets the northern bound of the Kamchatka arc and divides it into a volcanically inactive north segment and active central and south segments (Hochstaedter et al., 1994; Kepezhinskas et al., 1997). At the north segment, westward subduction of the young (< 25 Ma) and hot oceanic lithosphere formed by short-lived spreading in the Komandorsky basin (Bogdanov, 1988) had produced arc volcanism in the Sredinny Range (SR) until 2 Ma (Firsov, 1987). Closure of the Vetlovka basin and subsequent accretion of the Kronotsky terrane led to formation of the Eastern Volcanic Front (EVF) at the southern segment and the 200 km-wide Central Kamchatka depression (CKD) between the SR and EVF in the central segment.

The Avacha (Avachinsky) stratovolcano (53°15′N, 158°52′E) is located in the middle part of the EVF (Fig. 1). Since its formation in the late Pleistocene, it has been one of the most active volcanoes in the Kamchatka peninsula, with two recent eruptions in 1991 and 2001. The volcano is situated ~ 120-150 km above the slab surface, where the 70 km-thick Pacific slab is subducting at an angle of 55° (Gorbatov et al., 1997). Seismic data suggest a local Moho depth at ~ 37 km (Levin et al., 2002) and an asthenospheric mantle wedge between 70-130 km (Nizkous et al., 2006). The xenolith samples analyzed in this study were recovered from recent andesitic volcanic ash and scoria (< 7250 $^{14}$C years, B.P., Braitseva et al., 1998) from the western slope of the Avacha volcano (Ionov, 2010).

## 3. Samples and evidence for slab-mantle interaction

### 3.1. Petrology and origin of the Avacha xenoliths

The peridotite xenoliths investigated in this study are among the largest and freshest from active arc settings. They are medium- to coarse-grained, texturally equilibrated spinel harzburgites; composed primarily of olivine (67-82.5%) and orthopyroxene (Opx, 18-30%) as
well as accessory spinel, interstitial clinopyroxene (Cpx, 1.5-3%) formed mainly by exsolution from high-temperature Opx on cooling, and amphibole (≤ 1%) (Ionov, 2010; Bénard et al., 2017). Although the peridotites were last equilibrated in the lithospheric mantle, they recorded an evolutionary history since their initial derivation from partial melting in the asthenosphere. These peridotites are highly refractory (≥ 44 wt.% MgO) and contain no primary Cpx. They were inferred to represent residues of high-degree (28-35%) flux melting in the shallow mantle wedge, variably affected by fluid metasomatism (Ionov, 2010; Hopp and Ionov, 2011; Ionov et al., 2011; Bénard et al., 2017), pyroxenite veining (Halama et al., 2009; Bénard and Ionov, 2012; 2013; Bénard et al., 2016), mineral hydration and deformation (Soustelle et al., 2010).

## 3.2. Two stages of slab-derived metasomatism

A distinctive characteristic of the Avacha and other arc-associated peridotites is the significant enrichment in silica (hosted by Opx) relative to fertile peridotites (Fig. 2A). It was initially suggested, based on studies of xenoliths containing fine-grained Opx, that the Avacha peridotites enriched in Opx after melt extraction due to reaction of primary olivine with subduction-related fluids shortly before their capture by host magmas (e.g. Arai et al., 2003; Ishimaru et al., 2007). By contrast, Ionov (2010) reported silica enrichments in peridotites from this study that contain no fine-grained Opx and argued that the enrichments took place prior to, or during the melting in the asthenospheric mantle wedge because of input of slab-derived components and effects of fluids on modal proportions in the residues. This is consistent with the preservation of microstructural deformation features indicative of typical asthenospheric conditions in the Avacha peridotites, coupled with Opx formation from reactive porous flow of silica-rich fluids (Soustelle et al., 2010). More recently, modeling of hydrous mantle melting by Bénard et al. (2017) further confirms that the composition of residual Avacha harzburgites requires a subduction-modified protolith that is enriched in Opx prior to or during partial melting.

Slab-derived metasomatism of the Avacha harzburgites continued after their accretion to the lithospheric mantle. This is evidenced by pyroxenite veins formed from percolation of low-volume boninitic melts that were produced by slab fluid-fluxed melting of a metasomatized refractory harzburgitic source (Bénard and Ionov, 2013). These melts thus inherited a trace element signature, and possibly a δ²⁶Mg signature, of the slab fluids. The initial melts either crystallized rapidly to form Opx-rich veins (Type 1A), or fractionated to hydrous derivatives
accompanied by continuous reaction with host peridotites to form rare, thin veins and
metasomatic pockets (Type 1B) (Bénard and Ionov, 2013), as well as melt inclusions in spinels
(Ionov et al., 2011; Bénard et al., 2016). Taken together, the Avacha xenoliths show ubiquitous,
but weak effects of percolation of slab-derived fluids. This is consistent with the findings by
Ionov (2010) that nearly all Avacha harzburgites contain accessory amphibole and show U-
shaped trace element patterns with widespread, although low-degree, enrichments in fluid-
mobile elements (e.g., Rb, Ba, U, Pb, Sr) relative to fluid-immobile, melt-mobile elements (e.g.,
MREE, Nb, Y) (Fig. 2B), as well as compositional similarity to the Mariana serpentinized fore-
arc peridotites that were metasomatized by slab fluids (Savov et al. 2005) (Fig. 2C).

3.3. Potential sources of metasomatic fluids

The dominance of dehydrated fluids rather than slab-derived melt in the slab input to the mantle wedge is supported by thermal modeling for the Kamchatka arc where the old and cold Pacific plate is subducting beneath (Manea and Manea, 2007). The Sr-Nd-Pb-Os-O isotopic signatures of the southern Kamchatkan basalts and xenoliths indicate that the fluids are derived predominantly from altered oceanic crust (e.g., Kersting and Arculus, 1995; Kepezhinskas et al., 1997; Pineau et al., 1999; Dorendorf et al., 2000; Churikova et al., 2001; Widom et al., 2003; Saha et al., 2005; Duggen et al., 2007) with a possible contribution from serpentinized slab mantle, as suggested by B concentration and δ¹¹B of the lavas, volatile (S, Cl and F) and B, Li contents in olivine-hosted glass inclusions in basalts, δ³⁴S of spinel-hosted sulfate-rich glass inclusions in peridotites as well as integrated thermomechanical-thermodynamic-geochemical modeling (Ishikawa et al., 2001; Churikova et al., 2007; Konrad-Schmolke et al., 2016; Bénard et al., 2018). Serpentinite dehydration is independently supported by the presence of a double seismic zone in Kamchatka (Gobatov et al., 1994), which is commonly attributed to breakdown of antigorite (Peacock et al., 2001).

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4. Analytical methods

Magnesium isotopic analysis was performed at the Isotope Laboratory of the University
of Washington, Seattle. Detailed procedures for sample digestion, chemical separation, and
instrumental analysis are presented in Teng et al. (2007, 2010) and are summarized below. The
whole-rock powders analyzed here are the same as those used in Ionov (2010), Bénard and Ionov
(2013), and Bénard et al. (2017). Olivine and pyroxene fragments were hand-picked under a binocular. Whole-rock powders and pre-cleaned mineral fragments were dissolved in a mixture of Optima-grade HF-HNO₃-HCl on a hotplate until no insoluble residue was present in the solution. After complete dissolution, sample solutions were evaporated to dryness and subsequently fluxed with 1 N HNO₃ for column chromatography. Magnesium was quantitatively extracted (~ 100% Mg yield) and purified by two passes of the pre-cleaned Bio-Rad cation exchange resin (AG50W-X8 resin, 200-400 mesh) with 1 N HNO₃. The whole procedural blank is < 5 ng, which is negligible compared to the mass of Mg (~ 10 µg) loaded onto the columns.

Magnesium isotopic ratios were measured on a Nu Plasma II Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) using standard-sample bracketing for correction of instrumental fractionation. Sample solutions were introduced in 3% Optima-grade HNO₃ under “wet” plasma conditions and were analyzed in low-resolution mode. Sample intensity was matched to within 5% of that of the bracketing standard at 300 ppb Mg concentration. Magnesium isotopic data are expressed as per mil (‰) deviation relative to the DSM3 standard:

$$ \delta^{x}Mg (‰) = \left\{ \frac{\left( \frac{xMg}{24Mg} \right)_{\text{sample}}}{\left( \frac{xMg}{24Mg} \right)_{\text{DSM3}}} - 1 \right\} \times 1000 $$

where x = 25 or 26. The associated error of each datum is 2SD, which represents two times the standard deviation of all the analyses of bracketing standards during an analytical session.

The accuracy of the method was checked by processing and measuring three international rock standards, and two in-house standards (San Carlos olivine and Hawaiian seawater) along with samples (Table S1). The two peridotite standards (PCC-1 and DTS-1) yield average δ²⁶Mg of -0.25 ± 0.03‰ (n = 4) and -0.31 ± 0.05‰ (n = 2), and the basalt standard (JB-1) has a similar average δ²⁶Mg of -0.28 ± 0.03‰ (n = 3). The average δ²⁶Mg for 15 measurements on San Carlos olivine and 7 analyses on Hawaiian seawater are -0.25 ± 0.03‰ and -0.85 ± 0.03‰, respectively. All these standard values agree well with recommended values listed in Table S1 (Hu et al., 2016a; Teng, 2017).
## 5. Results

Magnesium isotopic compositions for bulk Avacha peridotites and associated pyroxenite veins are reported in Table 1. Mineral Mg isotopic data are presented in Table 2 together with the magnitude of inter-mineral Mg isotopic fractionation calculated as $\Delta^{26}\text{Mg}_{\text{X-Y}} = \delta^{26}\text{MgX} - \delta^{26}\text{MgY}$, where X and Y refer to two different mineral phases. All measurements during the course of this study fall on a mass-dependent fractionation line with a slope of 0.51 (R² = 0.99) (not shown) and thus only $\delta^{26}\text{Mg}$ values are discussed hereafter.

Overall, both bulk peridotites and their constituent minerals display narrow variations in $\delta^{26}\text{Mg}$, which are not correlated with their modal or major element compositions (Fig. 3). The $\delta^{26}\text{Mg}$ values of the 23 Avacha peridotites vary from -0.30 to -0.21‰ with an average of -0.27 ± 0.05‰ (2SD). Similarly, olivine separates from the Avacha harzburgites have uniform $\delta^{26}\text{Mg}$ values ranging between -0.30 and -0.23‰. Opx and Cpx have slightly higher and more heterogeneous $\delta^{26}\text{Mg}$ values of -0.27 to -0.19‰ and -0.32 to -0.20‰, respectively. The average $\delta^{26}\text{Mg}$ are -0.26 ± 0.04‰ (2SD, n = 15) for olivine, -0.23 ± 0.04‰ (2SD, n = 15) for Opx and -0.24 ± 0.10‰ (2SD, n = 5) for Cpx. The 15 olivine-Opx pairs yield $\Delta^{26}\text{Mg}_{\text{Opx-Ol}}$ from +0.01 to +0.08‰, with an average of +0.04 ± 0.04‰, while the five olivine-Cpx pairs yield $\Delta^{26}\text{Mg}_{\text{Cpx-Ol}}$ from -0.06 to +0.08‰, with an average of +0.02 ± 0.11‰ (Fig. 4). For the pyroxenite veins, the two rapidly crystalized boninitic veins (Type 1A) have $\delta^{26}\text{Mg}$ values of -0.26‰ and -0.21‰, and the two fractionated hydrous veins (Type 1B) have similar $\delta^{26}\text{Mg}$ values of -0.27‰ and -0.24‰. These values are indistinguishable from those of the harzburgite xenoliths.

## 6. Discussion

The Avacha tuff-hosted xenoliths were rapidly carried to surface by volatile-rich andesitic magmas and cooled down immediately after explosive eruption. They have not been modified by post-eruption alteration as indicated by petrographic observation and negative loss on ignition (LOI) values (Ionov, 2010). Therefore, these xenoliths provide an opportunity to examine the genuine nature and processes of Mg isotope cycling in an active subduction zone setting. Detailed petrographic and geochemical investigation on the Avacha peridotite suite suggest that their modal and chemical compositions mainly reflect melt extraction and moderate
subduction-related enrichments (Ionov, 2010). Since partial melting does not produce measurable Mg isotope fractionation (Teng et al., 2007, 2010), the discussion below focuses mainly on the effects of fluid-rock interaction on the Mg isotope systematics in subduction zones.

**6.1. Magnesium isotopic compositions of mantle wedge peridotites**

The ambient mantle has uniform δ²⁶Mg values of -0.25 ± 0.04‰ as constrained by global peridotite xenoliths (Teng et al., 2010). By contrast, the Mg isotopic compositions of the subducting slabs can be highly variable, with values both significantly lower and higher (-2.76 to +0.52‰; Beinlich et al., 2014; Teng et al., 2016; Hu et al., 2017; Liu et al., 2017; Huang et al., 2018). In addition, subduction processes are likely to further fractionate Mg isotopes in subducting components via chemical diffusion between lithologies with strong chemical contrasts (Pogge von Strandmann et al., 2015) or fluid-mediated differential isotopic exchange between ²⁶Mg-rich silicates and ²⁶Mg-depleted carbonates (Wang et al., 2014a). Therefore, slab-derived fluids are expected to have highly variable Mg isotopic compositions.

Despite a large range of δ²⁶Mg predicted for fluids released by slab dehydration, the metasomatized Avacha xenoliths reveal uniform Mg isotopic compositions that fall well within the range of global peridotite xenoliths (Fig. 3A and B), and display no correlation with common indices of fluid metasomatism, such as Ba/La and Pb/Ce (Fig. 5). Similar to peridotites, both the rapidly crystallized pyroxenite veins and the fractionated hydrous veins have mantle-like δ²⁶Mg values. These peridotite and pyroxenite xenoliths have bulk δ²⁶Mg values (-0.30 to -0.21‰) that are consistent with published data for Kamchatkan arc lavas of -0.35 to -0.24‰ (Li et al., 2017), suggesting insignificant Mg isotope fractionation during local fluid metasomatism. This is further supported by previous determination of mantle-like Li and Fe isotopic signatures in the same suite of Avacha xenoliths (Weyer and Ionov, 2007; Ionov and Seitz, 2008). Our data, however, are slightly lower than three out of the six Avacha peridotites analyzed by Pogge von Strandmann et al. (2011), who found a negative correlation between δ⁷Li and δ²⁶Mg, and attributed it to diffusion-driven interaction with slab-derived fluids. Given that their duplicate analyses on the same samples differ by 0.07‰, and the total variation in δ²⁶Mg is within their analytical reproducibility, the apparent δ⁷Li vs. δ²⁶Mg correlation and higher δ²⁶Mg values might be fortuitous.
Although percolation of slab-derived fluids has weak effects on bulk-rock Mg isotopic compositions of the Avacha peridotites, it seems to have a stronger influence on the comparatively low-Mg, fine-grained Cpx. Based on theoretical calculations (Schauble, 2011; Huang et al., 2013) and measurements on peridotite xenoliths (e.g., Huang et al., 2011; Liu et al., 2011; Xiao et al., 2013), Cpx and Opx are expected to have $\delta^{26}$Mg values higher than coexisting olivine by ~ 0.1‰ and 0.06‰, respectively. However, while Opx-olivine pairs all fall on the equilibrium fractionation line, five out of six Cpx have $\delta^{26}$Mg values lower than that would be expected at isotopic equilibrium (Fig. 4). This slight isotope disequilibrium is consistent with the late-stage origin of the Cpx with possible metasomatic contributions (Ionov, 2010).

Further insights into the Mg isotopic compositions of sub-arc mantle can be obtained from previous studies on other peridotite lithologies of likely mantle wedge origin. For instance, the Horoman massif peridotites from Japan are interpreted as samples of an ancient mantle wedge infiltrated by slab-derived fluids/melts, leading to enriched geochemical compositions and formation of secondary Opx (e.g., Yoshikawa and Nakamura, 2000). Nevertheless, these massif peridotites preserve mantle-like $\delta^{26}$Mg values of -0.27 to -0.19‰ (Lai et al., 2015), similar to those of the Avacha sub-arc peridotites analyzed in this study. In comparison, Su et al. (2015) reported a slightly wider $\delta^{26}$Mg variation (-0.28 to -0.14‰) coupled with a negative trend with $\delta^{56}$Fe for a suite of Tibetan ophiolitic peridotites and hinted at a role of sub-arc metasomatism. It appears that the subduction-related fluid infiltration has a varied effect on the Mg isotopic composition of sub-arc mantle wedge at different convergent margins, which is likely to be controlled by certain physical parameters of the subduction systems and fluid/rock ratios.

## 6.2. Magnesium isotope geochemistry in the slab-mantle wedge system

Our data on the Avacha sub-arc mantle peridotites suggest that infiltration of the mantle wedge by fluids expelled from the Pacific slab does not affect its Mg isotope composition (Fig. 5). This can be explained by the following reasons. Firstly, Mg behaves conservatively during metamorphic dehydration at relatively shallow depths; therefore, only limited amounts of Mg partition to the fluids (e.g., $MgO \le 0.5$ wt%, Manning, 2004). This is supported by the consistent Mg isotopic ratios measured in a series of genetically related prograde metamorphic rocks ranging from greenschist to amphibolite to eclogite in a continental subduction zone (Wang et al., 2014b). A similar conclusion has been reached for metamorphic dehydration of metapelites after
as much as 75% loss of the initial volatiles (Li et al., 2014). These studies imply that the majority of Mg in the slab inventory will be retained in silicates during metamorphic dehydration and be carried to the deeper mantle whereas the fluids released beneath the arc volcanic regions must have relatively low Mg contents.

Secondly, although Mg can be transported by fluids, it is a highly compatible element with a strong affinity for Mg-silicates relative to fluids ($D_{Mg}^{rock/fluid}$ of ~ 10 at sub-arc depth, Ayers et al., 1997). While slab-released fluids react with and are consumed by surrounding mantle along their ascending pathways, dissolved Mg in such fluids would also be effectively sequestered by mantle minerals, in a process analogous to chromatography (e.g., Ionov et al., 2002; Tomascak et al., 2002). Therefore, only limited amounts of remaining fluids and fluid-borne slab Mg could make their way to the shallower part of the mantle wedge, from where the xenoliths were originally formed, subsequently cooled, and finally entrained by andesitic magmas. This is likely the case for the moderately metasomatized Avacha harzburgites given that they originate from a depth of less than 65 km (Ionov, 2010), whereas the slab surface is located approximately 120 km deep (Gorbatov et al., 1997). The relatively low fluid/host ratios during the metasomatism of the Avacha harzburgites is also consistent with their initial formation by hydrous melting at low to moderately fluxing conditions via reactive porous flow (e.g., Soustelle et al., 2010; Bénard et al., 2017), the low abundance of amphibole (< 1 %), and the widespread but low-degree enrichments in fluid-mobile elements (Ionov, 2010).

Thirdly, migration of slab-expelled fluids in the mantle is likely accompanied by progressive re-equilibration of their Mg isotopic compositions with the ambient mantle. The equilibrium Mg isotope fractionation between olivine and Opx in the Avacha harzburgites indicates that Mg isotopes reached exchange equilibrium within and between individual samples. This is further supported by the lack of Mg concentration zoning in minerals (Ionov, 2010), the lack of correlation between $\Delta^{26}\text{Mg}_{\text{Opx-Ol}}$ with mineral chemical compositions (Fig. 3C and D), and identical Mg isotopic compositions obtained for different batches of dissolution for the same mineral phase from a single xenolith sample (Table 2). Therefore, diffusive Mg isotopic equilibration between metasomatic fluids and ambient peridotite is probably a relatively recent and rapid process in the sub-arc mantle. This inference is also supported by the links of amphibole (and other metasomatic phases) disseminated in the peridotites with quenched veins (Bénard and Ionov, 2013). The Mg isotopic composition of percolating fluids and fluid-rich
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melts can thus be effectively buffered by the mantle wedge as a result of the high Mg content and
much larger mass of the mantle wedge compared to the relatively limited amounts of fluids.

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The lack of a clear fluid-imparted δ²⁶Mg signature in the Avacha harzburgites does not rule out the possibility that fluid metasomatism may alter the mantle wedge locally where the fluid flux is unusually high with substantially enriched Mg concentrations. This has been demonstrated by significant Mg isotope variation in exhumed metamorphic rocks from subduction channels or fore-arc mélange, where fragments of different lithologies intensively interact with each other (Pogge von Strandmann et al., 2015; Chen et al., 2016, 2018; Wang et al., 2017). Figure 6 depicts the trajectories of δ²⁶Mg change in mantle peridotites that interact with a high-δ²⁶Mg fluid phase at various integrated fluid/rock ratios and Δ²⁶Mgₗock-fluid values in an open system. It shows that fluids derived from slab crust are too depleted in Mg to shift the Mg isotopic composition of the mantle because such a shift requires high fluid/rock ratios and large Δ²⁶Mgₗock-fluid. In comparison, fluids derived from serpentinized peridotites are more effective metasomatic agents for Mg. In addition, the efficiency of metasomatism could be further enhanced if the fluids travel through the mantle by channelized flow, in which case they will react less with surrounding peridotites and therefore preserve their slab δ²⁶Mg signatures for longer travel distances during fluid ascent.

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In contrast to source modification by direct addition of slab fluids, Foden et al. (2018) recently proposed a process-driven mechanism to alter the δ⁵⁷Fe values of the sub-arc mantle by ~ 0.26‰, via diffusive addition of isotopically light Fe to surrounding mantle during repeated, fast extraction of partial melts formed in the mantle wedge. This process is presumably coupled with preferential loss of light Mg isotopes to the percolating melts, leaving a corresponding isotopically heavy mantle veneer in contact with the magma conduits. However, given that MgO content in peridotite is approximately five times of FeO, the isotopic effects for Mg would be significantly diluted by a much larger proportion of isotopically unfractionated Mg, and are thus probably unresolvable with current analytical precision.

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## 6.3. Comparison with arc lavas: Implications for Mg cycling in subduction zones and beyond

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Compositions of arc lavas provide complementary constraints on the role of slab input in sub-arc mantle sources. Although Mg isotopic data on arc lavas remain scarce, they have
revealed significant variations between different arcs. These lavas display dominant
contributions from slab-derived fluids except that those from Lesser Antilles also show a strong
sediment signal (Fig. 7A and B). Arc lavas from the Kamchatka and Costa Rica volcanic fronts
have mantle-like Mg isotopic compositions whereas those from Lesser Antilles and Philippines
are characterized by variably higher δ²⁶Mg values (Fig. 7C and D) (Teng et al., 2016; Li et al.,
2017). More recently, slightly heavier δ²⁶Mg values are also found in some of the Cascadia and
Makran arc lavas, but are only limited to more evolved samples, which are attributed mainly to
assimilation and fractional crystallization within the crust (Brewer et al., 2018; Pang et al., 2019).
They are thus not included in the following discussion. Overall, there is no apparent correlation
between the δ²⁶Mg values of these lavas and their Ba/Th or Dy/Yb ratios (Fig. 7C and D),
indicating that neither the amount of fluid additions to their mantle sources nor fractional
crystallization of isotopically light, Yb-rich garnet from the magmas could be responsible for the
variably ²⁶Mg-enriched arc lava compositions. Instead, the higher δ²⁶Mg values in these arc lavas
are explained to reflect inputs from slab fluids, particularly those derived from serpentinized slab
mantle (Teng et al., 2016; Li et al., 2017).

**6.3.1. Importance of subduction zone thermal structure**

Thermal structure of a given subduction zone has a profound influence on the
dehydration path of a subducting plate and the nature as well as the flux of slab-mantle mass
transfer agent (fluid vs. melt), which ultimately affect the compositions of arc magmas. The
average δ²⁶Mg values of these arcs display co-variation trends with their subduction parameters,
e.g. increasingly higher δ²⁶Mg values are associated with an increase in slab surface temperatures
and slab depths (Fig. 8). These correlations suggest that the Mg isotopic signatures of the
volcanic front lavas may be controlled by dehydration reactions that occur at P-T conditions
specific to each individual subduction zone. For magmas that originate from relatively shallow
depths, the primary water suppliers in the oceanic crust are amphibole, chlorite, lawsonite,
zoisite, and paragonite (Schmidt and Poli, 2014). These minerals either do not contain much Mg
(e.g., lawsonite, zoisite), or likely have Mg isotopic compositions similar to those of mantle
olivine and pyroxene due to similar Mg-O bond strength (e.g., amphibole) (Fig. 9). By contrast,
arc magmas formed from partial melting at greater depths may bear a stronger slab δ²⁶Mg
signature because these deeper fluids have stronger dissolving capacity and are more solute-
enriched (e.g., Manning, 2004). Furthermore, the higher slab temperatures also facilitate fluid
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401

liberation from the partially serpentinized slab mantle. Compared to oceanic crust, serpentinites have much higher water contents and Mg concentrations. More importantly, serpentinized peridotites and constituent phyllosilicates often have Mg isotopic compositions that are different from those in the normal mantle (Fig. 9). For example, variably heavy Mg isotopic compositions have been reported in talc and antigorite (up to +0.30‰, Beinlich et al., 2014), as well as in altered abyssal peridotites (e.g., Liu et al., 2017) and exhumed whiteschists metasomatized by Mg-rich fluids derived from serpentinite dehydration (–0.07 to +0.72‰, Chen et al., 2017).

402

**6.3.2. Role of fluid transport mode**

403
404
405
406
407
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Island arcs having P-T conditions conducive to serpentinite dehydration, however, do not necessarily generate arc magmas with distinctive Mg isotopic compositions. First, high degree serpentinization of the slab mantle mainly occurs within plates formed at slow-spreading ridges (Schmidt and Poli, 2014), such as the Atlantic plate subducting beneath the Lesser Antilles arc, and within plates that inherited a seafloor spreading fabric oriented nearly parallel to the trench axis thus facilitates faulting during plate bending (Ranero et al., 2003). Furthermore, fluids released from serpentinites would also need to make their way up to the melting zone or all the way to the sub-arc lithosphere. The presence of channelized fluid pathways in the sub-arc mantle is thus important in preserving the slab isotopic signature of an abundant element, such as Mg, which would otherwise be buffered by the mantle. For example, although slab mantle dehydration is a suggested possibility for the Kamchatka arc (see section 3.3 for details), both the Avacha peridotites (this study) and Kamchatkan arc lavas (Li et al., 2017) display mantle-like Mg isotopic compositions. This could be reconciled with percolation of low volume slab-derived fluids as reactive porous flow rather than as focused channelized flow in the Avacha sub-arc regions, as suggested by Soustelle et al. (2010) and Bénard et al. (2017).

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An alternative mechanism to transfer the slab isotopic signature, which has been increasingly advocated, is through buoyancy-driven mélange diapirs (e.g., Marschall and Schumacher, 2012; Nielsen and Marschall, 2017). While this is a possible mechanism for distinctive Mg isotopic signatures to survive, as supported by a broad Mg isotopic variation (–1.55 to +0.72‰) displayed in exhumed metamorphic assemblages (Chen et al., 2017, 2018; Wang et al., 2017; Li et al., 2018), it is currently not possible to quantify this effect on arc magma composition given the limited δ²⁶Mg database reported for arc lavas. However, because the δ²⁶Mg values of partially melted mélange diapirs would be dominated by the most fusible
sediment component, the abnormally high δ²⁶Mg values of the Philippine arc lavas that originate from a slab-fluid dominated regime cannot be ascribed to the mélange mechanism (Figs. 7 and 8). Even for the Lesser Antilles arc that has a significant sediment input, this mechanism is also questionable because local subducting sediments do not have a sufficiently high δ²⁶Mg value (Teng et al., 2016).

### 6.3.3. Recycling of slab-borne Mg beyond sub-arc depths

Subducted slab components that have been left from arc magma production could then contribute to deep mantle heterogeneity. For example, phengite and magnesite are stable at typical melting depths beneath island arcs (Schmidt and Poli, 2014). They are hence capable of carrying their distinctively heavy (up to +0.59‰, Li et al., 2011; Liu et al., 2011) and light (< -1‰, Beinlich et al., 2014) Mg isotopic signatures further down in the mantle (Fig. 9). These may later be involved in the formation of isotopically light continental basalts and potassium-rich rocks, such as those in eastern China (Yang et al., 2012; Huang et al., 2015; Li et al., 2017; Su et al., 2017b; Sun et al., 2017). Alternatively, such melts formed at greater depths could be consumed in the subcontinental lithospheric mantle to generate a variety of metasomatic rocks such as wehrlite and pyroxenite that were found to have variable Mg isotopic compositions and disequilibrium inter-mineral Mg isotope fractionations (e.g., Xiao et al., 2013; Hu et al., 2016b).

## 7. Conclusions

The heterogeneous Mg isotopic compositions in different components of the downgoing slabs, and non-mantle Mg isotopic ratios found in a few studied arc lavas, indicate that the mantle wedge may bear distinctive Mg isotopic signatures. Our Mg isotopic data, however, show that there is no significant difference between the normal mantle and the metasomatized sub-arc lithospheric mantle sampled by the Avacha xenoliths from the Kamchatka arc. Therefore, we conclude that a typical mantle wedge where slab input is dominated by addition of fluids from the subducting crust would probably have an overwhelmingly mantle-like Mg isotopic composition. This could be due to a combination of dehydration of hydrous minerals with mantle-like Mg isotopic compositions (e.g., amphibole), chromatographic removal of Mg from slab-derived fluids and diffusive Mg isotopic re-equilibration between migrating fluids and ambient mantle rocks. Furthermore, we propose that arc lavas with distinctive Mg isotopic
456

compositions are more likely to be formed in subduction zones with deeper and hotter slab
surface that allows dehydration of serpentinized peridotites to occur, along with rapid fluid
transport by channelized flow.

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460

Acknowledgements

461

Drs. Scott Kuehner, Bruce Nelson and Bernard Evans are thanked for stimulating discussions.
462
Constructive comments from Dr. Paul Tomascak, Dr. Oliver Nebel and an anonymous reviewer
463
are greatly appreciated for improving the quality of this manuscript. Efficient and careful
464
editorial handling by the guest editor Dr. Sonja Aulbach and Editor-in-Chief Dr. Jeffrey Catalano
465
is gratefully acknowledged. This work was financially supported by the National Science
466
Foundation (EAR-1340160 and EAR-1747706) to F.-Z. Teng and Harry Wheeler Scholarship to
467
Y. Hu. D. Ionov acknowledges support from the Chinese Academy of Sciences (CAS)
468
President's International Fellowship Initiative (PIFI) for 2017-18 and from the PNP-INSU
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program in France.

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721 **Figure captions**

722 **Fig. 1.** Geological map of the Kamchatka peninsula, Russia, showing the regional tectonic
723 setting. The boxed area of Kamchatka in (A) is enlarged in (B) to show the locations of major
724 Quaternary volcanoes. Modified from Kersting and Arculus (1995) and Ionov (2010). EVF =
725 Eastern Volcanic Front, SR = Sredinny Range, CKD = Central Kamchatka Depression.

726 **Fig. 2.** Geochemical evidence for subduction-related metasomatism recorded in the Avacha
727 peridotite xenoliths. In (A), major and trace element concentrations are from Ionov (2010). In
728 (B), Ce/Pb and U/Th abundances are normalized to primitive mantle of McDonough and Sun
729 (1995). The field of slab melts trapped in the Kamchatka mantle are based on analyses of glass
730 veins and melt inclusions in Kamchatka mantle xenoliths (Kepezhinskas et al., 1996;
731 Kepezhinskas and Defant, 1996). In (C), the compositions of serpentinized forearc peridotites
732 (Savov et al., 2005) and N-MORB (Sun and McDonough, 1989) are plotted for comparison.

733 **Fig. 3.** δ²⁶Mg values of bulk Avacha peridotite xenoliths and their constituent minerals plotted
734 against modal and chemical parameters. The grey band and horizontal solid line in each panel
735 represent normal mantle range and average mantle value from Teng et al. (2010). Whole-rock
736 Al₂O₃ content, olivine mode, and Mg# of olivine, Opx and Cpx are from Ionov (2010).

737 **Fig. 4.** Inter-mineral fractionation between pyroxene (Px) and olivine (Ol) in Avacha peridotites.
738 The solid red and black lines in (A) correspond to theoretical equilibrium fractionation lines for
739 Cpx-Ol and Opx-Ol pairs, respectively, across a relevant temperature range for the Avacha
740 peridotites, whereas these lines in (B) are calculated equilibrium fractionation lines at an average
741 temperature of 950°C for the peridotites. These theoretical fractionation lines are calculated
742 using the parameters provided in Schauble (2011). The estimated temperatures for Avacha
743 peridotites are from Ionov (2010).

744 **Fig. 5.** Lack of correlation between δ²⁶Mg in Avacha peridotites with common indices for slab
745 fluid-peridotite interaction. The star symbol labeled as ‘PM’ denotes the trace element and Mg
746 isotopic composition of the primitive mantle. The trace element concentrations of the PM are
747 from Sun and McDonough (1989) and its δ²⁶Mg is assumed to be identical to the normal mantle
748 average from Teng et al. (2010).
**Fig. 6** Change of the bulk peridotite δ²⁶Mg value during open-system fluid-rock interaction calculated for a range of integrated fluid/rock ratios (indicated by the x axis), MgO concentrations of slab-derived fluids (curves coded by different colors), and rock-fluid fractionation factors (Δ²⁶Mgrock-fluid values, specified by the number on each colored curve). The sub-arc mantle is assumed to have an average MgO content identical to that of Avacha harzburgites (45.4 wt%, Ionov, 2010) with a δ²⁶Mg of normal mantle average (–0.25‰, Teng et al., 2010). Representative MgO concentrations of slab fluids derived from oceanic basalt and peridotite are taken from experimental studies of Kessel et al. (2005) and Dvir et al. (2011), respectively. The These fluids are assumed to have a δ²⁶Mg value of +1.1‰, which is at the high-δ²⁶Mg end of biotite from the Ivrea Zone metapelites, NW Italy (Wang et al., 2015). See Table S2 for calculation details.

**Fig. 7.** (A) and (B) Trace element and ⁸⁷Sr/⁸⁶Sr characteristics of arc lava samples from four island arcs that have been studied for Mg isotopes in the literature. These characteristics indicate a dominant role of slab-derived fluids in Philippines, Kamchatka, and Costa Rica, whereas both slab-fluids and sediments have contributions to the composition of Lesser Antilles arc lavas. (C) and (D) Published δ²⁶Mg values for these arc lavas plotted against indices for fluid addition (Ba/Th ratio) and garnet crystallization (Dy/Yb ratio), respectively. Data are from Teng et al. (2016) and Li et al. (2017).

**Fig. 8.** Published δ²⁶Mg values for the four island arcs plotted as a function of the slab surface temperature (°C) and slab depth (km) beneath each corresponding arc, which are summarized in Syracuse et al. (2010). The arc parameter values used for Philippines and Lesser Antilles are the average of the North and South Philippines and North and South Lesser Antilles, respectively. The black horizontal bar represents the average lava δ²⁶Mg value for each arc. Magnesium isotopic data are from Teng et al. (2016) and Li et al. (2017).

**Fig. 9.** Dehydration diagram of key Mg-bearing hydrous minerals with increasing depth, associated with possible δ²⁶Mg values of released fluids. Modified from Schmidt and Poli (2014).
<table><caption>775 Table 1. Magnesium isotopic compositions of bulk Avacha harzburgite xenoliths and pyroxenite veins.</caption><thead><tr><th>Sample</th><th>δ<sup>26</sup>Mg (‰)</th><th>2SD (‰)</th><th>δ<sup>25</sup>Mg (‰)</th><th>2SD (‰)</th></tr></thead><tbody><tr><td>Spinel harzburgite</td><td></td><td></td><td></td><td></td></tr><tr><td>Av-1</td><td>-0.29</td><td>0.07</td><td>-0.13</td><td>0.04</td></tr><tr><td>replicate</td><td>-0.29</td><td>0.05</td><td>-0.15</td><td>0.05</td></tr><tr><td>average</td><td>-0.29</td><td>0.04</td><td>-0.14</td><td>0.03</td></tr><tr><td>Av-2</td><td>-0.30</td><td>0.07</td><td>-0.17</td><td>0.04</td></tr><tr><td>Av-3</td><td>-0.30</td><td>0.07</td><td>-0.14</td><td>0.04</td></tr><tr><td>Av-4</td><td>-0.28</td><td>0.07</td><td>-0.13</td><td>0.03</td></tr><tr><td>duplicate</td><td>-0.30</td><td>0.07</td><td>-0.14</td><td>0.04</td></tr><tr><td>replicate</td><td>-0.30</td><td>0.05</td><td>-0.16</td><td>0.05</td></tr><tr><td>average</td><td>-0.29</td><td>0.04</td><td>-0.14</td><td>0.02</td></tr><tr><td>Av-5</td><td>-0.26</td><td>0.06</td><td>-0.13</td><td>0.04</td></tr><tr><td>Av-6</td><td>-0.27</td><td>0.06</td><td>-0.15</td><td>0.04</td></tr><tr><td>duplicate</td><td>-0.27</td><td>0.07</td><td>-0.14</td><td>0.05</td></tr><tr><td>replicate</td><td>-0.25</td><td>0.05</td><td>-0.14</td><td>0.05</td></tr><tr><td>average</td><td>-0.26</td><td>0.04</td><td>-0.14</td><td>0.03</td></tr><tr><td>Av-7</td><td>-0.27</td><td>0.08</td><td>-0.13</td><td>0.05</td></tr><tr><td>Av-8</td><td>-0.30</td><td>0.07</td><td>-0.15</td><td>0.05</td></tr><tr><td>replicate</td><td>-0.29</td><td>0.05</td><td>-0.14</td><td>0.05</td></tr><tr><td>average</td><td>-0.29</td><td>0.04</td><td>-0.14</td><td>0.04</td></tr><tr><td>Av-9</td><td>-0.27</td><td>0.05</td><td>-0.14</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.27</td><td>0.07</td><td>-0.13</td><td>0.04</td></tr><tr><td>average</td><td>-0.27</td><td>0.04</td><td>-0.13</td><td>0.03</td></tr><tr><td>Av-10</td><td>-0.28</td><td>0.05</td><td>-0.15</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.21</td><td>0.07</td><td>-0.10</td><td>0.04</td></tr><tr><td>average</td><td>-0.25</td><td>0.04</td><td>-0.12</td><td>0.03</td></tr><tr><td>Av-11</td><td>-0.30</td><td>0.05</td><td>-0.15</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.30</td><td>0.07</td><td>-0.16</td><td>0.04</td></tr><tr><td>average</td><td>-0.30</td><td>0.04</td><td>-0.16</td><td>0.03</td></tr><tr><td>Av-12</td><td>-0.24</td><td>0.05</td><td>-0.12</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.25</td><td>0.07</td><td>-0.12</td><td>0.04</td></tr><tr><td>average</td><td>-0.24</td><td>0.04</td><td>-0.12</td><td>0.03</td></tr><tr><td>Av-13</td><td>-0.29</td><td>0.05</td><td>-0.16</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.24</td><td>0.07</td><td>-0.11</td><td>0.04</td></tr><tr><td>average</td><td>-0.27</td><td>0.04</td><td>-0.13</td><td>0.03</td></tr><tr><td>Av-14</td><td>-0.27</td><td>0.05</td><td>-0.14</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.28</td><td>0.07</td><td>-0.12</td><td>0.04</td></tr><tr><td>average</td><td>-0.27</td><td>0.04</td><td>-0.13</td><td>0.03</td></tr><tr><td>Av-15</td><td>-0.27</td><td>0.05</td><td>-0.12</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.22</td><td>0.07</td><td>-0.11</td><td>0.04</td></tr><tr><td>average</td><td>-0.25</td><td>0.04</td><td>-0.12</td><td>0.03</td></tr><tr><td>Av-16</td><td>-0.29</td><td>0.05</td><td>-0.14</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.29</td><td>0.07</td><td>-0.13</td><td>0.04</td></tr><tr><td>average</td><td>-0.29</td><td>0.04</td><td>-0.13</td><td>0.03</td></tr><tr><td>Av-17</td><td>-0.27</td><td>0.05</td><td>-0.13</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.23</td><td>0.07</td><td>-0.12</td><td>0.04</td></tr><tr><td>average</td><td>-0.25</td><td>0.04</td><td>-0.12</td><td>0.03</td></tr><tr><td>Av-33</td><td>-0.27</td><td>0.07</td><td>-0.15</td><td>0.04</td></tr><tr><td>Av-40</td><td>-0.30</td><td>0.07</td><td>-0.15</td><td>0.04</td></tr><tr><td>Av-41</td><td>-0.28</td><td>0.06</td><td>-0.14</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.27</td><td>0.05</td><td>-0.13</td><td>0.04</td></tr><tr><td>average</td><td>-0.28</td><td>0.04</td><td>-0.14</td><td>0.03</td></tr><tr><td>Av-42</td><td>-0.31</td><td>0.07</td><td>-0.16</td><td>0.04</td></tr><tr><td>duplicate</td><td>-0.28</td><td>0.05</td><td>-0.14</td><td>0.04</td></tr></tbody></table>
<table><tr><td>average</td><td>-0.29</td><td>0.04</td><td>-0.15</td><td>0.03</td></tr><tr><td>Av-G1</td><td>-0.20</td><td>0.07</td><td>-0.11</td><td>0.04</td></tr><tr><td>duplicate</td><td>-0.22</td><td>0.07</td><td>-0.11</td><td>0.05</td></tr><tr><td>average</td><td>-0.21</td><td>0.05</td><td>-0.11</td><td>0.03</td></tr><tr><td>Av-G2</td><td>-0.23</td><td>0.06</td><td>-0.12</td><td>0.04</td></tr><tr><td>duplicate</td><td>-0.22</td><td>0.07</td><td>-0.11</td><td>0.05</td></tr><tr><td>average</td><td>-0.23</td><td>0.05</td><td>-0.12</td><td>0.03</td></tr></table>

<table><tr><td colspan="5">Pyroxenite veins</td></tr><tr><td colspan="5">Type 1A (rapid crystallized veins)</td></tr><tr><td>Av-20</td><td>-0.26</td><td>0.04</td><td>-0.13</td><td>0.03</td></tr><tr><td>Av-21</td><td>-0.21</td><td>0.07</td><td>-0.11</td><td>0.05</td></tr></table>

<table><tr><td colspan="5">Type 1B (fractionated hydrous veins)</td></tr><tr><td>Av-18</td><td>-0.27</td><td>0.04</td><td>-0.14</td><td>0.03</td></tr><tr><td>Av-19</td><td>-0.24</td><td>0.04</td><td>-0.12</td><td>0.03</td></tr></table>

776 In this and the following table, duplicate refers to repeat instrumental analysis on the same purified Mg

777 solutions during different analytical sessions, and replicate refers to repeat sample dissolution, column

778 chemistry, and instrumental analysis.

779 Italics indicate error-weighted average values calculated using Isoplot 3.75-4.15.
<table><caption>Table 2. Magnesium isotopic compositions of mineral separates and calculated magnitudes of inter-mineral fractionation.</caption><thead><tr><th rowspan="2">Samples</th><th colspan="4">Olivine (Ol)</th><th colspan="4">Orthopyroxene (Opx)</th><th colspan="4">Clinopyroxene (Cpx)</th><th rowspan="2">Δ<sup>26</sup>Mg<sub>Opx-Ol</sub> (‰)</th><th rowspan="2">2SD (‰)</th><th rowspan="2">Δ<sup>26</sup>Mg<sub>Cpx-Ol</sub> (‰)</th><th rowspan="2">2SD (‰)</th></tr><tr><th>δ<sup>26</sup>Mg (‰)</th><th>2SD (‰)</th><th>δ<sup>25</sup>Mg (‰)</th><th>2SD (‰)</th><th>δ<sup>26</sup>Mg (‰)</th><th>2SD (‰)</th><th>δ<sup>25</sup>Mg (‰)</th><th>2SD (‰)</th><th>δ<sup>26</sup>Mg (‰)</th><th>2SD (‰)</th><th>δ<sup>25</sup>Mg (‰)</th><th>2SD (‰)</th></tr></thead><tbody><tr><td>Av-1</td><td>-0.27</td><td>0.07</td><td>-0.13</td><td>0.06</td><td>-0.18</td><td>0.07</td><td>-0.10</td><td>0.05</td><td>-0.24</td><td>0.05</td><td>-0.11</td><td>0.06</td><td>0.06</td><td>0.04</td><td>0.01</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.23</td><td>0.06</td><td>-0.10</td><td>0.05</td><td>-0.18</td><td>0.06</td><td>-0.09</td><td>0.04</td><td>-0.24</td><td>0.07</td><td>-0.12</td><td>0.04</td><td></td><td></td><td></td><td></td></tr><tr><td>Fine grain</td><td>-0.23</td><td>0.06</td><td>-0.12</td><td>0.52</td><td>-0.18</td><td>0.05</td><td>-0.09</td><td>0.04</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>duplicate</td><td>-0.26</td><td>0.06</td><td>-0.14</td><td>0.04</td><td>-0.23</td><td>0.07</td><td>-0.11</td><td>0.06</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td>-0.25</td><td>0.03</td><td>-0.12</td><td>0.03</td><td>-0.19</td><td>0.03</td><td>-0.10</td><td>0.02</td><td>-0.24</td><td>0.04</td><td>-0.11</td><td>0.03</td><td></td><td></td><td></td><td></td></tr><tr><td>Av-2</td><td>-0.26</td><td>0.07</td><td>-0.14</td><td>0.06</td><td>-0.21</td><td>0.05</td><td>-0.10</td><td>0.04</td><td></td><td></td><td></td><td></td><td>0.04</td><td>0.08</td><td></td><td></td></tr><tr><td>duplicate</td><td></td><td></td><td></td><td></td><td>-0.23</td><td>0.07</td><td>-0.12</td><td>0.06</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td>-0.22</td><td>0.04</td><td>-0.11</td><td>0.03</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Av-3</td><td>-0.26</td><td>0.06</td><td>-0.13</td><td>0.04</td><td>-0.24</td><td>0.05</td><td>-0.12</td><td>0.04</td><td>-0.32</td><td>0.05</td><td>-0.16</td><td>0.06</td><td>0.02</td><td>0.07</td><td>-0.06</td><td>0.07</td></tr><tr><td>duplicate</td><td></td><td></td><td></td><td></td><td>-0.24</td><td>0.07</td><td>-0.12</td><td>0.06</td><td>-0.33</td><td>0.06</td><td>-0.15</td><td>0.04</td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td>-0.24</td><td>0.04</td><td>-0.12</td><td>0.03</td><td>-0.32</td><td>0.04</td><td>-0.16</td><td>0.03</td><td></td><td></td><td></td><td></td></tr><tr><td>Av-4</td><td>-0.28</td><td>0.07</td><td>-0.15</td><td>0.06</td><td>-0.22</td><td>0.05</td><td>-0.11</td><td>0.04</td><td></td><td></td><td></td><td></td><td>0.08</td><td>0.06</td><td></td><td></td></tr><tr><td>duplicate</td><td>-0.32</td><td>0.06</td><td>-0.17</td><td>0.04</td><td>-0.24</td><td>0.07</td><td>-0.13</td><td>0.06</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td>-0.30</td><td>0.04</td><td>-0.16</td><td>0.04</td><td>-0.22</td><td>0.04</td><td>-0.12</td><td>0.03</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Av-7</td><td>-0.25</td><td>0.06</td><td>-0.14</td><td>0.05</td><td>-0.24</td><td>0.05</td><td>-0.12</td><td>0.04</td><td></td><td></td><td></td><td></td><td>0.01</td><td>0.07</td><td></td><td></td></tr><tr><td>duplicate</td><td></td><td></td><td></td><td></td><td>-0.21</td><td>0.07</td><td>-0.11</td><td>0.06</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td>-0.23</td><td>0.04</td><td>-0.12</td><td>0.03</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Av-8</td><td>-0.28</td><td>0.07</td><td>-0.14</td><td>0.06</td><td>-0.25</td><td>0.05</td><td>-0.14</td><td>0.04</td><td></td><td></td><td></td><td></td><td>0.02</td><td>0.05</td><td></td><td></td></tr><tr><td>duplicate</td><td>-0.26</td><td>0.06</td><td>-0.12</td><td>0.05</td><td>-0.28</td><td>0.07</td><td>-0.15</td><td>0.06</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>duplicate</td><td>-0.29</td><td>0.06</td><td>-0.15</td><td>0.04</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td>-0.28</td><td>0.04</td><td>-0.14</td><td>0.03</td><td>-0.26</td><td>0.04</td><td>-0.14</td><td>0.03</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Av-9</td><td>-0.24</td><td>0.06</td><td>-0.11</td><td>0.05</td><td>-0.17</td><td>0.06</td><td>-0.09</td><td>0.04</td><td>-0.21</td><td>0.05</td><td>-0.11</td><td>0.06</td><td>0.06</td><td>0.05</td><td>0.03</td><td>0.05</td></tr><tr><td>Fine grain</td><td>-0.24</td><td>0.07</td><td>-0.12</td><td>0.06</td><td>-0.18</td><td>0.05</td><td>-0.10</td><td>0.04</td><td>-0.24</td><td>0.07</td><td>-0.12</td><td>0.04</td><td></td><td></td><td></td><td></td></tr><tr><td>duplicate</td><td>-0.26</td><td>0.06</td><td>-0.12</td><td>0.04</td><td>-0.20</td><td>0.06</td><td>-0.11</td><td>0.04</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>duplicate</td><td></td><td></td><td></td><td></td><td>-0.21</td><td>0.07</td><td>-0.10</td><td>0.06</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td>-0.25</td><td>0.04</td><td>-0.12</td><td>0.03</td><td>-0.19</td><td>0.03</td><td>-0.10</td><td>0.02</td><td>-0.22</td><td>0.04</td><td>-0.12</td><td>0.03</td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><tbody><tr><td>Av-10</td><td>-0.22</td><td>0.07</td><td>-0.11</td><td>0.06</td><td>-0.22</td><td>0.05</td><td>-0.11</td><td>0.04</td><td></td><td></td><td>0.01</td><td>0.05</td></tr><tr><td>duplicate</td><td>-0.25</td><td>0.06</td><td>-0.13</td><td>0.05</td><td>-0.22</td><td>0.07</td><td>-0.10</td><td>0.06</td><td></td><td></td><td></td><td></td></tr><tr><td>duplicate</td><td>-0.24</td><td>0.06</td><td>-0.12</td><td>0.04</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td>-0.23</td><td>0.04</td><td>-0.12</td><td>0.03</td><td>-0.22</td><td>0.04</td><td>-0.11</td><td>0.03</td><td></td><td></td><td></td><td></td></tr><tr><td>Av-11</td><td>-0.29</td><td>0.07</td><td>-0.16</td><td>0.06</td><td>-0.27</td><td>0.05</td><td>-0.15</td><td>0.04</td><td></td><td></td><td>0.02</td><td>0.06</td></tr><tr><td>duplicate</td><td>-0.29</td><td>0.06</td><td>-0.14</td><td>0.05</td><td>-0.25</td><td>0.07</td><td>-0.13</td><td>0.06</td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td>-0.29</td><td>0.05</td><td>-0.14</td><td>0.04</td><td>-0.27</td><td>0.04</td><td>-0.14</td><td>0.03</td><td></td><td></td><td></td><td></td></tr><tr><td>Av-12</td><td>-0.22</td><td>0.07</td><td>-0.12</td><td>0.06</td><td>-0.25</td><td>0.05</td><td>-0.14</td><td>0.04</td><td>-0.22</td><td>0.07</td><td>-0.12</td><td>0.04</td></tr><tr><td>duplicate</td><td>-0.26</td><td>0.06</td><td>-0.14</td><td>0.04</td><td>-0.23</td><td>0.07</td><td>-0.11</td><td>0.06</td><td>-0.22</td><td>0.07</td><td>-0.11</td><td>0.03</td></tr><tr><td>duplicate</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.24</td><td>0.06</td><td>-0.12</td><td>0.04</td></tr><tr><td>Fine grain</td><td>-0.29</td><td>0.07</td><td>-0.14</td><td>0.06</td><td>-0.22</td><td>0.05</td><td>-0.11</td><td>0.04</td><td></td><td></td><td></td><td></td></tr><tr><td>duplicate</td><td>-0.29</td><td>0.06</td><td>-0.14</td><td>0.05</td><td>-0.25</td><td>0.06</td><td>-0.13</td><td>0.04</td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td>-0.27</td><td>0.03</td><td>-0.13</td><td>0.03</td><td>-0.24</td><td>0.03</td><td>-0.12</td><td>0.02</td><td>-0.23</td><td>0.04</td><td>-0.11</td><td>0.02</td></tr><tr><td>Av-13</td><td>-0.27</td><td>0.07</td><td>-0.14</td><td>0.06</td><td>-0.25</td><td>0.05</td><td>-0.13</td><td>0.04</td><td></td><td></td><td></td><td>0.03</td><td>0.06</td></tr><tr><td>duplicate</td><td>-0.27</td><td>0.06</td><td>-0.15</td><td>0.04</td><td>-0.23</td><td>0.07</td><td>-0.11</td><td>0.06</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td>-0.27</td><td>0.04</td><td>-0.14</td><td>0.04</td><td>-0.24</td><td>0.04</td><td>-0.12</td><td>0.03</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Av-14</td><td>-0.25</td><td>0.07</td><td>-0.13</td><td>0.05</td><td>-0.23</td><td>0.05</td><td>-0.10</td><td>0.04</td><td></td><td></td><td></td><td>0.03</td><td>0.08</td></tr><tr><td>duplicate</td><td></td><td></td><td></td><td></td><td>-0.23</td><td>0.07</td><td>-0.13</td><td>0.06</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td>-0.23</td><td>0.04</td><td>-0.11</td><td>0.03</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Av-15</td><td>-0.28</td><td>0.07</td><td>-0.15</td><td>0.05</td><td>-0.22</td><td>0.05</td><td>-0.11</td><td>0.04</td><td>-0.21</td><td>0.07</td><td>-0.12</td><td>0.04</td><td>0.05</td><td>0.06</td></tr><tr><td>duplicate</td><td>-0.28</td><td>0.06</td><td>-0.14</td><td>0.04</td><td>-0.26</td><td>0.07</td><td>-0.13</td><td>0.06</td><td>-0.19</td><td>0.06</td><td>-0.09</td><td>0.04</td><td></td><td></td></tr><tr><td>average</td><td>-0.28</td><td>0.04</td><td>-0.14</td><td>0.03</td><td>-0.23</td><td>0.04</td><td>-0.12</td><td>0.03</td><td>-0.20</td><td>0.04</td><td>-0.11</td><td>0.03</td><td></td><td></td></tr><tr><td>Av-16</td><td>-0.27</td><td>0.06</td><td>-0.13</td><td>0.05</td><td>-0.21</td><td>0.05</td><td>-0.10</td><td>0.04</td><td></td><td></td><td></td><td>0.06</td><td>0.07</td></tr><tr><td>duplicate</td><td></td><td></td><td></td><td></td><td>-0.22</td><td>0.07</td><td>-0.11</td><td>0.06</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td>-0.21</td><td>0.04</td><td>-0.10</td><td>0.03</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Av-17</td><td>-0.27</td><td>0.07</td><td>-0.13</td><td>0.06</td><td>-0.21</td><td>0.05</td><td>-0.10</td><td>0.04</td><td></td><td></td><td></td><td>0.05</td><td>0.06</td></tr><tr><td>duplicate</td><td>-0.25</td><td>0.06</td><td>-0.14</td><td>0.05</td><td>-0.22</td><td>0.07</td><td>-0.12</td><td>0.06</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>average</td><td>-0.26</td><td>0.05</td><td>-0.14</td><td>0.04</td><td>-0.21</td><td>0.04</td><td>-0.11</td><td>0.03</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_3.png" /></figure>

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

<figure><img src="image_5.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_6.png" /></figure>

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

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

<figure><img src="image_9.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 4</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Figure 5</figcaption></figure>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>Figure 6</figcaption></figure>
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>Figure 7</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>Figure 8</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>Figure 9</figcaption></figure>