On the Development of the Calc-Alkaline and Tholeiitic Magma Series: a Deep Crustal Cumulate Perspective ∗ Emily J

Earth and Planetary Science Letters 482 (2018) 277–287

Contents lists available at ScienceDirect

Earth and Planetary Science Letters

www.elsevier.com/locate/epsl

On the development of the calc-alkaline and tholeiitic magma series: A deep crustal cumulate perspective ∗ Emily J. Chin a, , Kei Shimizu b, GrantM.Bybeec, Monica E. Erdman d a Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, United States b Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC, United States c School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa d Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, TX, United States a r t i c l e i n f o a b s t r a c t

Article history: Two distinct igneous differentiation trends – the tholeiitic and calc-alkaline – give rise to Earth’s oceanic Received 31 July 2017 and continental crust, respectively. Mantle melting at mid-ocean ridges produces dry magmas that Received in revised form 3 November 2017 differentiate at low-pressure conditions, resulting in early plagioclase saturation, late oxide precipitation, Accepted 9 November 2017 and Fe-enrichment in mid-ocean ridge basalts (MORBs). In contrast, magmas formed above subduction Available online xxxx zones are Fe-depleted, have elevated water contents and are more oxidized relative to MORBs. It is widely Editor: A. Yin thought that subduction of hydrothermally altered, oxidized oceanic crust at convergent margins oxidizes Keywords: the mantle source of arc magmas, resulting in erupted lavas that inherit this oxidized signature. Yet, cumulates because our understanding of the calc-alkaline and tholeiitic trends largely comes from studies of erupted calc-alkaline trend melts, the signals from shallow crustal contamination by potentially oxidized, Si-rich, Fe-poor materials, tholeiitic trend which may also generate calc-alkaline rocks, are obscured. Here, we use deep crustal cumulates to “see mid-ocean ridge through” the effects of shallow crustal processes. We find that the tholeiitic and calc-alkaline trends are arc indeed reflected in Fe-poor mid-ocean ridge cumulates and Fe-rich arc cumulates, respectively. A key iron finding is that with increasing crustal thickness, arc cumulates become more Fe-enriched. We propose that the thickness of the overlying crustal column modulates the melting degree of the mantle wedge + (lower F beneath thick arcs and vice versa) and thus water and Fe3 contents in primary melts, which subsequently controls the onset and extent of oxide fractionation. Deep crustal cumulates beneath thick, mature continental arcs are the most Fe-enriched, and therefore may be the “missing” Fe-rich reservoir required to balance the Fe-depleted upper continental crust. © 2017 Elsevier B.V. All rights reserved.

1. Introduction plot (Fig. 1). Magmas evolving along the calc-alkaline trend evolve towards Fe-depletion, whereas magmas evolving along the tholei- The calc-alkaline and tholeiitic magma series are the two most itic trend become Fe-enriched. It has also long been recognized important igneous differentiation trends on Earth and are broadly that calc-alkaline lavas, when they erupt, tend to be more oxidized expressed by the crustal dichotomy of andesitic continental crust than MORB (Christie et al., 1986; Richards, 2015), but the reasons and basaltic oceanic crust (MORB), respectively. Whereas tholeiitic for this are unclear. magmas may occur in all tectonic settings, calc-alkaline magmas Controversy surrounds a fundamental issue: whether the calc- appear to be uniquely associated with the convergent margin set- alkaline signature originates from the mantle source (Carmichael, ting (Miyashiro, 1974; Zimmer et al., 2010). Convergent margin 1991)or whether magmatic fractionation (Bowen, 1928)or con- (arc) magmatism is thought to be a key process by which conti- tamination and mixing with Si-rich, Fe-poor crustal materials nental crust is formed (Taylor, 1977). Thus, understanding the pro- (Grove et al., 1982) could also generate Fe-depleted calc-alkaline cesses that govern the calc-alkaline magma series is of importance magmas. Current prevailing hypotheses favor the source as the to understanding the origin and evolution of continental crust. culprit. One popular view is that, unlike the dry and low f O The behavior of Fe deﬁnes the calc-alkaline and tholeiitic 2 MORB mantle, calc-alkaline magmas result from elevated water trends. This is illustrated in the classic AFM (alkali–FeOT–MgO) content and f O2 inherited from a mantle wedge hydrated and oxidized by slab-derived material (Brounce et al., 2014; Eggins, 1993; * Corresponding author. Evans et al., 2012; Kelley and Cottrell, 2009; Parkinson and Ar- E-mail address: [email protected] (E.J. Chin). culus, 1999). Other hypotheses implicating a source origin for https://doi.org/10.1016/j.epsl.2017.11.016 0012-821X/© 2017 Elsevier B.V. All rights reserved. 278 E.J. Chin et al. / Earth and Planetary Science Letters 482 (2018) 277–287

Fig. 1. AFM (A = Na2O+K2O, F = FeOT, and M = MgO) diagrams showing oceanic vs. arc cumulates. For clarity, data are plotted based on provenance: blue triangles and circles = mid-ocean ridges and island arcs, respectively; orange circles = continental arc (full data references available in Supplementary Information). Arrows are schematic and show calc-alkaline (CA) vs. tholeiitic trends (TH). Large symbols represent median values. (a) AFM diagram showing mid-ocean ridge cumulates (blue triangles), MORBs (from Gale et al., 2013), and experimental cumulates and melts (full data references available in Supplementary Information). (b) AFM diagram showing arc cumulates (blue circles = island arcs; orange circles = continental arcs), arc volcanic and plutonic rocks from GEOROC database, and experimental cumulates and melts (see Supplementary Information for references). For simplicity, experimental cumulates and corresponding experimental melts are shown as open squares and crosses, respectively (experiments, sorted by individual studies, are plotted with unique symbols in Supplementary Figs. 1, 2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) calc-alkaline magmas involve a mantle wedge that has experienced (Kelley and Cottrell, 2012; Plank et al., 2013)aim to circum- significant melt-rock reaction to explain the genesis of high Mg# vent some of these issues, but melt inclusions may also suffer arc andesites (Kelemen, 1990). More recently, variation of mantle from diffusive re-equilibration and thus do not necessarily reflect wedge oxidation state with potential temperature (Gaetani, 2016) the f O2 or H2O conditions of their origin (Gaetani et al., 2012; or variation in the wedge melting regime itself (Plank and Lang- Hartley et al., 2017). Lastly, owing to the large variance of sili- muir, 1988; Turner and Langmuir, 2015)have also been proposed. cate liquids due to their absence of stoichiometry and high com- On the other hand, a number of studies are at odds with the pressibilities, thermobarometry on silicate/liquid systems is more popular view of a uniquely oxidized arc mantle source for calc- challenging compared to thermobarometry using subsolidus min- alkaline magmas. For example, studies of arc lavas using redox- eral equilibria (Putirka, 2008). As such, it is difficult to pin down sensitive element ratios (Lee et al., 2010) and Fe isotopes (Dauphas precisely the depth within the crustal column a magma may first et al., 2009)show that the oxidation state of the arc lavas’ source become calc-alkaline, because such magmas will have likely re- is identical to that of MORB, and thus the oxidized nature of equilibrated to shallower conditions prior to eruption. arc lavas may arise instead from differentiation and/or mixing Here, we “see through” shallow crustal processes by examin- processes that occur in the lithosphere and/or crust. Melt oxida- ing deep crustal cumulates – the crystalline solids fractionated tion has also been attributed to shallow-level degassing of H2O from differentiating magmas (Irvine, 1982). Deep crustal cumu- (Humphreys et al., 2015); however other studies argue that de- lates, while not as abundant and well-exposed as lavas or shallow gassing has no effect on melt oxidation (Crabtree and Lange, 2012; intrusive rocks, are nevertheless sampled as exhumed terranes and Waters and Lange, 2016). Tectonic controls may also be important. xenoliths or by cores and dredges in arcs and ocean ridge settings, Two recent studies of global arc magmas observed a positive cor- respectively. Despite numerous studies of cumulates from arcs and relation between crustal thickness and degree of Fe depletion in mid-ocean ridges, there is a lack of synoptic studies of the role arc magmas (Chiaradia, 2014; Farner and Lee, 2017), suggesting of cumulates in crust formation. Thus, the goals of this paper are that intracrustal processing of magmas or, arc crustal thickness it- twofold: 1) to present a comprehensive survey of global mid-ocean self, may also control primary magma water content and f O2 and ridge and arc cumulates and 2) to use these cumulates, together therefore promote the generation of calc-alkaline rocks. with published experimental data and simple petrologic models, A common thread in the vast majority of studies concern- to better understand the development of calc-alkaline and tholei- ing arc redox conditions, and more broadly, genesis of the calc- itic magma trends. Although we systematically surveyed a wide alkaline versus tholeiitic suites, is that the melts/liquid perspec- compositional range of cumulates (Mg# 40–90), we emphasize tive has been the dominant focus. However, there are several fac- high Mg# cumulates (Mg# > 75), as such cumulates are closest in tors that potentially complicate reliance on melt evolution paths equilibrium with primary mantle-derived melts and therefore less alone. The most obvious one is that all erupted lavas, includ- susceptible to shallow crustal processes. Our approach provides an ing primitive MORBs, experience some extent of crystal fraction- intuitive, yet less well-recognized, complementary “crystal” view to ation during ascent to the surface (O’Hara, 1968). Transport of the melt perspective on the long-standing debate over the origins magmas through pre-existing thick crust, as found in mature of tholeiitic and calc-alkaline magmas. continental arcs such as the Andes (Hora et al., 2009; Leeman, 1983), further increases the likelihood of crustal contamination 2. Geologic background and assimilation (Annen et al., 2006; Dufek and Bergantz, 2005; Hildreth and Moorbath, 1988). Thus, the combined processes of 2.1. Oceanic magmas and cumulates crystal fractionation and/or contamination/assimilation may ob- scure primary signals from the mantle wedge source of arc mag- Fractional crystallization at low pressures and anhydrous con- mas. Recently, studies on primitive olivine-hosted melt inclusions ditions and evolution toward Fe-enrichment characteristic of the E.J. Chin et al. / Earth and Planetary Science Letters 482 (2018) 277–287 279 tholeiitic trend (Fig. 1a) is the paradigm of MORB differentiation bly involves a mix of these two end-member processes. Regard- (Bryan and Moore, 1977; Grove et al., 1993). This paradigm has less of the generation mechanism, there is broad consensus that endured and still adequately explains much of the major element a large extent of chemical differentiation takes place in the deep variation of MORB, from the earliest samples of seafloor basalts to crust (Annen et al., 2006), because generation of voluminous Si- the current database that now includes over 10,000 MORBs from rich melts requires an even greater volume of mafic cumulates and the global mid-ocean ridge system (Gale et al., 2014; Rubin and restites which are too dense to be gravitationally stable in the up- Sinton, 2007). However, some recent studies suggest that the sim- per crust (Muntener et al., 2001; Ward et al., 2017). ple fractional crystallization model may fail to account for certain As arc melts are diverse, their cumulates are also diverse and peculiar aspects of MORB, such as an apparent over-enrichment crystallized under a range of tectonic stress states depending on in incompatible elements relative to that predicted by the model arc type (Ducea et al., 2015). Unlike the generally shallow con- (Coogan and O’Hara, 2015; Lissenberg et al., 2013; O’Neill and Jen- ditions of crystallization in mid-ocean ridges, arc cumulates form ner, 2016). Thus, more complex processes in addition to fractional both at high pressures where garnet is stable (>1GPa) in thick, crystallization are likely involved in MORB genesis and differen- compressional arcs (e.g., the Andes, Weber et al., 2002; and Sierra tiation. In addition to this recognition of complex melt evolution Nevada, Ducea and Saleeby, 1998; Lee et al., 2006) and at lower in forming MORBs, there are several long-recognized conundrums, pressures (<1GPa) in thin, extensional arcs (e.g., the Marianas, such as that MORB phenocryst assemblages do not reflect cotec- Bloomer and Hawkins, 1983). As such, pyroxenitic cumulates, not tic proportions and are often out of equilibrium with each other gabbroic cumulates, are the predominant deep crustal cumulate (Coogan, 2014), and that clinopyroxene, though required in suc- lithology beneath most arcs. A general consensus has emerged that cessful crystal fractionation models, is rarely ever observed in mafic–ultramafic arc cumulates form the igneous counterparts to MORB phenocryst assemblages (Francis, 1986). Many of the un- arc granitoids formed by fractional crystallization; this is borne resolved issues of MORB differentiation, therefore, need to also be out in numerous studies focusing on individual arcs (Greene et investigated from the perspective of the MORB cumulates. al., 2006; Jagoutz, 2010; Jagoutz et al., 2009; Kay and Kay, 1985; As with any melt-dominated system, the evolution of MORB Lee et al., 2006). Indeed, a complementary relationship between is inherently complex, involving the aggregation of mantle-derived calc-alkaline arc magmas and their cumulates has been recognized melts that, in passing through the crustal column, fractionally crys- for decades (Conrad and Kay, 1984; Gill, 1981), and is supported tallize, assimilate, mix, and potentially react with their surround- by detailed studies from individual arcs (Arculus and Wills, 1980; ings (Grove et al., 1993). In contrast, lower oceanic crust in the Greene et al., 2006; Jagoutz et al., 2009; Lee et al., 2006; Spandler form of gabbros and cumulates has been conceptualized in rel- et al., 2003). These studies, focused on specific localities and indi- atively simple end-member scenarios – either viscous-flow dom- vidual rock suites, have been instrumental in improving our under- inated (Morgan and Chen, 1993; Sleep, 1975)or sill-dominated standing of the magmatic processes involved in differentiating arcs (Kelemen et al., 1997; Natland and Dick, 1996); hybrid models and thus continental crust. have also been proposed that incorporate elements of the two end-members (Boudier et al., 1996). Even at the fastest spreading 3. Data compilation ridges, 75% of the oceanic crust formed is plutonic and composed primarily of gabbroic cumulates that can be modeled as products Locations of compiled cumulate data from individual published of simple fractional crystallization (Gillis et al., 2014). Furthermore, studies are shown on the map in Fig. 2 (mid-ocean ridge cu- extraction of interstitial melt from deep crustal cumulates is con- mulates (n = 236) and arc cumulates (n = 387; continental arcs, sidered to be highly efficient and rapid, implying that nearly all the n = 256; island arcs, n = 131)). All compiled samples were previ- incompatible elements will be concentrated in the basalts and not ously interpreted as cumulates based on textural criteria and/or by the gabbroic lower crust (Natland and Dick, 1996). Natland and petrologic relationship to observed volcanics or shallower intrusive Dick (1996), in their study of Hess Deep cumulates, also showed rocks. Only studies with 10 or more reported cumulate compo- that all the recovered cumulates were found to crystallize on co- sitions were considered; samples were additionally screened for tectics and contained clinopyroxene, supporting the dominant role alteration or hydration by selecting only whole rock oxide totals of crystal fractionation in MORB genesis and providing a reso- >95 wt.%. lution of the MORB “pyroxene paradox” (Francis, 1986). At slow Arc crustal thicknesses, where available, were taken from seis- and ultraslow ridges, melt stagnation and reaction with wallrocks mic studies or constraints from exposed arc sections and xenolith may be more common. Pyroxenites interleaved with abyssal peri- thermobarometry (Table 1). By way of comparison to natural cu- dotites have been reported from ultraslow ridges and point to melt mulate compositions, experimentally produced cumulate composi- stagnation within the oceanic lithosphere (>20 km; Dantas et al., tions (under a variety of conditions) have also been compiled from 2007; Laukert et al., 2014), suggesting that some component of the literature. References for all compiled whole-rock data, both oceanic lower crust may also begin accreting close to the base of from natural and experimental cumulates, are provided in the Sup- the thermal lithosphere (Cannat, 1996). plementary Information.

2.2. Arc magmas and cumulates 4. Results

Unlike the relatively narrow compositional range of MORB, For the purposes of this study, we focus on whole-rock ma- arc magmas are far more diverse, trending towards both Fe- jor element compositions. When properly ﬁltered, whole-rock ma- enrichment (tholeiitic) and Fe-depletion (calc-alkaline) (Fig. 1b) jor element concentrations retain a robust signature of primary (Zimmer et al., 2010). No single process has been invoked to crystallization compared to incompatible trace element composi- explain such diversity of magmatism in arcs, but there is gen- tions, which are more susceptible to small degrees of contamina- eral agreement that two distinct processes are responsible for tion and secondary alteration. Whole-rock major element composi- development of intermediate to silicic arc magmas: 1) fractional tions (weight percent oxide) for all compiled cumulates are plotted crystallization of primary magma in the uppermost mantle and against Mg# (molar Mg/(Mg+Fe); used as an index for degree of within the crust (Gill, 1981; Grove and Kinzler, 1986) and 2) par- crystallization) in Fig. 3 (SiO2, Al2O3, CaO) and Fig. 4 (FeOT and tial melting of pre-existing crust (Atherton and Petford, 1993; TiO2). For simpler data visualization, we also plotted the median Chappell et al., 1987). In reality, arc magma generation proba- and interquartile range of each major element, binned by Mg#, 280 E.J. Chin et al. / Earth and Planetary Science Letters 482 (2018) 277–287

Fig. 2. Map showing locations of compiled cumulates (full data references available in Supplementary Information).

Table 1 Crustal thickness data for selected arcs. Locality Crustal thickness Method(s) Reference(s) (km)a Continental arcs Sierra Nevada, California, USA 60 Xenolith thermobarometry Ducea and Saleeby (1996) North Pacific 37 Seismic reflection Klemperer et al. (2002) Colombian Andes 45 Xenolith thermobarometry1 1Weber et al. (2002) Seismic refraction2 2Meissnar et al. (1976) Gravity model3 3Case and MacDonald (1973) Famatinian arc, Andes 26 Exposed arc section Otamendi et al. (2009) New Zealand (Fiordland) 50 Exposed arc section1 1Daczko et al. (2002) Seismic refraction2 2Bourguignon et al. (2007) Arizona, USA 70 Xenolith thermobarometry1; 1Erdman et al. (2016) Seismic data using receiver 2Levander et al. (2011) functions and Rayleigh wave tomography1,2 Panama 28 Seismic reflection Briceno-Guarupe (1978)

Island arcs Kohistan 40 Thermobarometry & laboratory Miller and Christensen (1994) measurement of seismic velocities and densities of exposed arc section rocks Kermadec 18 Seismic refraction Karig (1970) Talkeetna–Bonanza 30 Exposed arc section Greene et al. (2006) Marianas 20 Seismic reflection Takahashi et al. (1998) Sunda 25 Seismic refraction Curray et al. (1977) a Uncertainties are ∼10% for seismic reflection and refraction constraints on Moho depths (considered a maximum bound; Grad and Tiira, 2009); these uncertainties are also comparable to those based on mineral thermobarometry (Hacker et al., 2008). in the righthand panels of Figs. 3 and 4. These values and other is also borne out by the factor-of-two increase in Na2O between statistics are reported in Table 2. ridges and arcs (Table 2). In contrast to plagioclase-rich ridge cumulates, high Mg# arc cumulates are dominated by pyroxenites 4.1. Major element systematics in cumulates (Lee et al., 2006) and therefore their bulk Al2O3 contents are much lower. Al2O3-depletion is most pronounced in high Mg# continen- We now discuss major element trends observed in the com- tal arc cumulates (Fig. 3f), which is complemented by elevated piled data within the context of cumulate petrology and miner- CaO (Fig. 3i), reflecting the important role of early clinopyroxene alogy. SiO2 contents are relatively uniform, varying by at most in continental arc cumulates. ∼10%, amongst mid-ocean ridge, island arc, and continental arcs Another major difference between arc and ridge cumulates is (Fig. 3a–c). In contrast, there is much greater variation in Al2O3, observed in FeOT and TiO2 (Fig. 4). Median compositions of arc CaO, FeOT, and TiO2 between mid-ocean ridges and arcs. At high cumulates are substantially enriched in FeOT (∼10 wt.%) and TiO2 Mg#s (>80) mid-ocean ridge cumulates show much higher Al2O3 (∼0.8 wt.%) compared to mid-ocean ridges (FeOT ∼6wt.%, TiO2 contents – more than a 2-fold increase between Mg# 80–85 – ∼0.4 wt.%) (Table 2). If we consider only early-formed cumulates compared to arcs (Fig. 3d–f). This is because plagioclase satu- with Mg# > 75, a pronounced enrichment in FeOT is already ob- rates early in mid-ocean ridge basalt crystallization (Grove et served in arc cumulates compared to ridge cumulates (Fig. 4c). al., 1993), leaving behind early-formed olivine gabbros. Further- Ridge cumulates eventually achieve higher FeOT than arcs, but this more, the plagioclase crystallizing in ridge cumulates is, in gen- does not occur until low Mg#s (<50) (Fig. 4c). A parallel late en- eral, more sodic than plagioclase crystallizing in arc cumulates richment in TiO2 is observed in low Mg# ridge cumulates – for (Beard, 1986). The importance of early and continuous (sodic) pla- instance, at Mg# 45–50, there is a nearly 3-fold increase in TiO2 gioclase fractionation in mid-ocean ridge cumulates, but not arcs, in ridges versus arcs (Fig. 4f). As we will discuss further below, the E.J. Chin et al. / Earth and Planetary Science Letters 482 (2018) 277–287 281

Fig. 3. Whole-rock SiO2, Al2O3 and CaO (wt. %) versus Mg# (molar Mg/(Mg+Fe)) systematics. Left and middle panels (a, b, d, e, g, h) show all compiled natural cumulates (mid-ocean ridge cumulates = blue triangles, island arc cumulates = blue circles, continental arc cumulates = orange circles), experimental cumulates of Grove et al. (1993) and Yang et al. (1996) (white triangles), experimental cumulates of Muntener et al. (2001) and Alonso-Perez et al. (2009) (white circles) (these experiments are plotted for simplicity because they bracket the compositional variability of the full experimental dataset), and modeled cumulates using alphaMELTS (dotted colored lines). Regions outlined by thin black lines representing all mid-ocean ridge cumulate (“MORC”) compositions from the left panels are also shown for comparison with the arc data. Right panels (c, f, i) show the median and interquartile ranges of mid-ocean ridge (gray), island arc (blue) and continental arc (orange) cumulates. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 4. Whole-rock FeOT and TiO2 versus Mg#. Symbols and trends same as in Fig. 3. major element systematics, particularly Fe and Ti, observed in arc fractionation of instantaneous cumulates from a primary basalt and ridge cumulates reflect the contrasting behavior of the calc- (Run #19 in Table 4 (hereafter “HK19”) from Hirose and Kushiro, alkaline versus tholeiitic magma series. 1993). We considered two model scenarios. The first simulates fractional crystallization at mid-ocean ridge conditions: pressure 5. Analysis: cumulate reference models is set to 600 bars, which is based on the depth of seismically observed magma chambers at the East Pacific Rise (Carbotte and To aid in the interpretation of compositional differences be- ◦ tween mid-ocean ridge versus arc cumulates, we establish base- Scheirer, 2004); initial temperature is set to 1320 C (the approx- line reference models using alphaMELTS (Ghiorso and Sack, 1995; imate liquidus temperature of normal MORB picrite at 1bar; ◦ Smith and Asimow, 2005). We consider the simple case of isobaric cf. Green et al., 2001) and decreases at 5 C increments; oxygen 282 E.J. Chin et al. / Earth and Planetary Science Letters 482 (2018) 277–287

Table 2 Summary of compiled cumulate compositions from mid-ocean ridges, island arcs, and continental arcs. The median (Q2) and interquartile range (Q1 or ﬁrst quartile – 75% of the data have a value larger than Q1 and 25% lower than Q1; Q3 or third quartile – 25% of data have value larger than Q3 and 75% lower than Q3) and the mean and standard deviation are reported.

Cumulate locality Major element SiO2 TiO2 Al2O3 FeOT MgO CaO Na2OMg# (wt.%) Mid-ocean ridge Median 50.14 0.44 16.29 6.28 9.21 12.20 2.54 73.40 n = 236 Q1 48.27 0.30 15.04 5.21 8.00 10.83 2.20 65.61 Q3 51.20 0.82 17.48 7.99 10.55 13.21 3.00 77.62 Mean 49.48 0.90 15.95 7.22 10.39 11.86 2.53 70.66 Standard deviation 2.50 1.43 3.80 3.15 5.43 2.15 0.84 11.42

Island arcs Median 46.74 0.62 17.96 9.20 7.88 12.46 1.2 60.82 n = 131 Q1 44.11 0.24 16.10 7.28 6.20 9.97 0.81 52.02 Q3 49.38 0.94 19.30 11.27 10.73 14.96 2.40 73.62 Mean 46.79 0.64 17.02 9.36 9.96 12.19 1.63 62.56 Standard deviation 3.64 0.43 5.16 3.21 6.51 3.52 1.07 12.39

Continental arcs Median 46.69 0.89 16.47 10.90 8.97 12.13 1.24 60.88 n = 256 Q1 44.05 0.46 12.45 8.52 7.24 10.03 0.76 52.18 Q3 49.50 1.27 17.99 13.38 13.12 13.63 2.06 72.07 Mean 46.77 0.99 14.94 10.98 11.15 11.84 1.52 61.62 Standard deviation 3.66 0.73 5.06 3.41 6.30 3.37 1.07 12.77 fugacity is set to the fayalite–magnetite–quartz (FMQ) buffer. The towards Fe-enrichment, thus their complementary cumulates must second scenario simulates fractional crystallization at arcs. Here, be Fe-depleted (Fig. 1, Fig. 4). Such cumulates, at least in the early we added 4wt.% H2O to the melt (the melt composition was then stages of differentiation, do not contain abundant oxide phases. re-normalized to 100%, yielding an effective H2O of 3.86 wt.%). Only until substantial fractionation is achieved (Mg# < 55) do Fe– 4wt.% H2O was chosen as a maximum bound scenario since Ti oxides (magnetite, ilmenite, spinel) begin forming as cumulus 4wt.% is the global average constrained from melt inclusions in phases. We observe that these oxide-bearing cumulates fall on mafic arc magmas (Plank et al., 2013). The initial temperature was modeled instantaneous cumulate trends at Mg#’s lower than 55 in ◦ ◦ 1320 C, and decreased at 10 C intervals to smooth out the instan- Fig. 4d. Because they occur after significant crystallization (>80%) taneous cumulate compositions. The pressure was set to 10,000 of the starting melt, such oxide cumulates should be minor. In fact, bars to approximate the conditions of a deep crustal “hot zone” they account for only 6% of our compiled mid-ocean ridge dataset. (Annen et al., 2006; Hildreth and Moorbath, 1988) and the oxy- It is probably for this reason that the mean mid-ocean ridge FeOT gen fugacity was set to FMQ+1, which is a lower estimate for (7.2 wt.%) is higher than the median (6.3 wt.%) (Table 2). arc environments (Ballhaus, 1993). In contrast, arc magmas evolve towards Fe-depletion along the Broadly speaking, there is overall agreement between modeled, calc-alkaline trend, because their complementary cumulates are experimental, and natural cumulates as shown in the left and mid- Fe-enriched (Fig. 1, Fig. 4). By looking collectively at as many arc dle panels in Figs. 3 and 4. Mid-ocean ridge cumulates show the cumulates as possible, we find that, when the data are normalized best agreement between MELTS and experimental cumulate trends. to a primitive arc basalt (Baker et al., 1994), Fe- and Ti-enrichment For arcs, experimental cumulates show better overlap with natural in arc cumulates appears to be a global feature (Fig. 5a). Intrigu- data compared to MELTS models (note, also, that natural arc cu- ingly, we observe that Fe- and Ti-enrichment is highest in conti- mulates are considerably more scattered than mid-ocean ridge cu- nental arc cumulates. Garnet is a common phase in continental arc mulates). MELTS predicts arc cumulates with lower SiO2 than ob- cumulates (Erdman et al., 2016; Lee, 2014) and is relatively high in served in both experiments and natural samples, especially at low FeOT (∼20 wt.%), making it a possible candidate for Fe-enrichment Mg#s (<50) (Fig. 3b). In addition, MELTS may over-stabilize early in cumulates. However, garnet crystallization cannot explain the clinopyroxene (Villiger et al., 2004); experiments predict lower CaO Fe-enrichment trend in island arc cumulates, because island arcs, in early cumulates (Fig. 3h). Villiger et al. (2004) and Zimmer et al. which generally lack the thick crust and high pressures required (2010) noted that MELTS may also under-stabilize spinel and other for garnet stability (> ∼30 km; O’Hara et al., 1971), do not usually Fe–Ti oxides, in contrast to experiments on hydrous arc basalts contain abundant garnet-bearing cumulates. that contain spinel on the liquidus (Sisson and Grove, 1993). We A more plausible candidate for causing Fe- and Ti-enrichment also recognize that accurate modeling of hornblende, a common in island arc cumulates is the fractionation of magnetite and/or mineral in arc assemblages, is currently difficult to conduct us- spinel. Fractionation of magnetite and/or spinel can also explain ing MELTS. Considering all of these caveats, the modeled MELTS Fe- and Ti-enrichment in continental arc cumulates that form at trends for hydrous arc compositions should serve only as qualita- low pressures (< ∼30 km). We point out that only a small amount tive predictions, and a more accurate comparison can be had using of Fe–Ti oxide is capable of making the bulk cumulates’ Fe con- experimental cumulates. tent high enough to result in Fe-depletion in corresponding melts, efficiently driving the calc-alkaline trend (Fig. 1b). Using the frac- 6. Discussion tional crystallization experiments of Nandedkar et al. (2014), we calculated that just 2% oxide mode in the bulk cumulate is suffi- 6.1. Fe and Ti in deep cumulates and the tholeiitic vs. calc-alkaline cient to complement a low-Fe magma with composition similar to trends average upper continental crust. Thus, while Fe–Ti oxides may not be volumetrically dominant in arc cumulates (but are nonetheless The contrasts in Fe and Ti contents between mid-ocean ridge present throughout a wide crystallization interval), their 1) high Fe and arc cumulates is essentially a manifestation of the tholeiitic (and Ti) concentrations and 2) early and continuous precipitation versus calc-alkaline trends, but viewed from the perspective of is important for the Fe- and Ti-evolution of the arc magmas, es- crystal cumulates complementary to MORB and arc magmas. Dur- pecially in low pressure conditions. Increasing differentiation and ing early differentiation along the tholeiitic trend, MORBs evolve fractionation of Fe–Ti oxides should therefore result in Ti-depletion E.J. Chin et al. / Earth and Planetary Science Letters 482 (2018) 277–287 283

Fig. 5. (a) Median compositions of arc cumulates normalized to primitive arc basalt Mg# 85–44 from Baker et al. (1994). Symbols represent individual arcs and are scaled by crustal thickness (where available) as shown in (b) and reported in Table 1. Average upper continental crust (UCC) composition is taken from Rudnick and Gao (2003). Blue circles = island arcs; orange circles = continental arcs. Error bars on crustal thickness are 10%, generally considered a maximum bound for seismic reflection and refraction constraints on Moho depth (Grad and Tiira, 2009)and are also comparable to uncertainties√ based on thermobarometry (Hacker et al., 2008). Uncertainties at the 95% confidence interval around the median FeOT values were calculated using (1.58 × IQR)/ N where IQR = interquartile range (difference between third and first quartiles) and N = number of individual cumulate compositions per arc. Abbreviations: PA = Panama, AZ = Arizona, CA = Colombian Andes, NZ = New Zealand, NP = North Pacific, FA = Famatinian arc, Andes, SN = Sierra Nevada, California, KO = Kohistan, TB = Talkeetna–Bonanza, SU = Sunda–Andaman, MA = Marianas, KR = Kermadec. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) in the melt, a feature that is observed in arc lavas, but not MORBs, worldwide (Thirlwall et al., 1994). Finally, we note that Fe–Ti oxide fractionation occurs early during magmatic differentiation, because primitive arc cumulates (Mg# > 75) are already Fe-enriched relative to similarly primitive ridge cumulates (Fig. 4).

6.2. Source vs. process?

A key new finding that emerged from our study is that with increasing crustal thickness, deep arc cumulates become more Fe-rich (Fig. 5b). This is a complimentary and consistent observation that echoes recent studies showing a positive correlation between degree of Fe depletion in arc magmas and crustal thickness (Chiaradia, 2014; Farner and Lee, 2017). The complementary behavior of Fe in arc cumulates and melts with variation in arc crustal thickness is consistent with prediction from mass balance, and with the major role of fractional crystallization, over crustal assimilation, in the development of the calc-alkaline trend. How- ever, questions still stand as to what causes the positive correlation between Fe content in cumulates and crustal thickness? Is this due to a source effect or igneous differentiation? The elevated Fe and Ti in continental arc (thicker crust) cumulates compared to island arcs (thinner crust) suggests earlier saturation of an Fe-rich phase (oxide/garnet) in the former. Ear- Fig. 6. alphaMELTS models of instantaneous cumulates formed from a primitive lier oxide saturation is generally ascribed to elevated f O2 and/or MORB with 0 to 4 wt.% H2O undergoing fractional crystallization at 1 GPa and FMQ+1. Top panel shows FeO of instantaneous cumulates and bottom panel H2O in the melt (Osborn, 1959; Zimmer et al., 2010). To inves- T shows spinel mode in instantaneous cumulates. tigate the effect of H2O on oxide precipitation, we return to the reference MELTS model of primary basalt HK19 (see Section 5) but varied H2O from 0 to 4wt.% (we chose a maximum of 4wt.% H2O since this is the global average from arc mafic melt inclusions silicate melt becomes more depolymerized (Mysen et al., 1985). based on Plank et al., 2013). Fig. 6 shows that, under arc-like con- In fact, natural melt composition data show a positive correlation 3+ ditions (1 GPa, FMQ+1) all hydrous melts produced early, high- between melt H2O and Fe /Fe (Kelley and Cottrell, 2009). How- Mg# cumulates with substantially higher FeOT compared to the ever, H2O alone is a poor oxidizer (Frost and Ballhaus, 1998), so anhydrous case and b) increasing melt H2O stabilized more spinel, variations in melt f O2 must instead arise from other processes, even amongst the earliest and most primitive (Mg# 85) cumulates. which we discuss below. Thus, melt H2O plays an important role in promoting early oxides The question is why melt H2O and f O2 would positively cor- and Fe-enrichment (Sisson and Grove, 1993; Zimmer et al., 2010). relate with arc crustal thickness. Our observation that Fe and Ti Furthermore, an increase in melt H2O could also change the rela- in primitive cumulates (Mg# > 75) increases with crustal thick- + + tive melt/residue partition coefficient of Fe2 and Fe3 (Evans et ness (Fig. 5) suggests a first-order relationship with the overriding al., 2012). This is because 1) addition of H2O depolymerizes sili- plate, rather than the downgoing slab. One possibility is that the + + cate melts (Zotov et al., 1996) and 2) Fe2 /Fe3 decreases as the degree of melting of the mantle wedge is modulated by the over- 284 E.J. Chin et al. / Earth and Planetary Science Letters 482 (2018) 277–287

Fig. 7. Cartoon showing cumulates versus tectonic setting. H2O and f O2 increase from mid-ocean ridges to island arcs to continental arcs, along with a concomitant increase in overlying crustal thickness, which may control the melting degree and thus initial H2O and f O2 in primary melt. lying crustal thickness (Chin et al., 2015; Karlstrom et al., 2014; arc magmas (Lee et al., 2006). However, unless the primary melt Plank and Langmuir, 1988; Turner et al., 2016). If this is the case, is substantially more evolved (andesitic) than picrobasalt, oxides 3+ incompatible elements such as H2O and Fe in primary arc mag- will still crystallize first before garnet (Alonso-Perez et al., 2009; mas should increase with increasing crustal thickness. Muntener et al., 2001). Thus, some combination of both oxide and We illustrate this using a simple model that can explain the garnet fractionation could explain the positive trend in Fig. 5b. variation of FeOT in deep crustal cumulates across island to con- The process of thickening will also transport already Fe-rich is- tinental arcs (Fig. 7). In this model, the main parameter is the land arc cumulates to higher pressures, where they may transform overlying arc crustal thickness which modulates the melting de- to dense garnet-bearing assemblages (however, this would be an 3+ gree and therefore the primary melt H2O content and Fe /Fe isochemical process sensu stricto) (Hacker et al., 2008). The impli- ratio. In a juvenile island arc, the overlying crust is relatively thin, cations of stabilizing garnet (either isochemically, by new igneous which may allow production of relatively high-degree melts with crystallization, or a combination of both) are important for arc 3+ low H2O and Fe /Fe, producing a spectrum of arc magmas that lithosphere stability because the high density of garnet cumulates, include both tholeiitic and calc-alkaline affinities (Zimmer et al., relative to underlying mantle, renders the former gravitationally 2010). In comparison, in a mature continental arc, the thicker over- unstable; and so mafic root removal drives arc crust to the inter- lying crustal/lithospheric column may reduce the accommodation mediate composition similar to continental crust. Bearing this in space for melt generation in the convecting wedge (Chin et al., mind, present-day seismic constraints on crustal thickness in some 2015; Turner et al., 2016), resulting in lower-degree melts with continental arcs, particularly those that are very old, are likely to 3+ elevated H2O and Fe /Fe. This would promote earlier accumu- be minimum bounds if crustal foundering occurred at some time(s) lation of oxides, generation of higher Fe cumulates, and hence in the past. For instance, the cumulates of the Paleozoic Famatinian greater Fe-depletion in magmas and thus stronger calc-alkaline arc, despite being Fe-rich, record relatively shallow depths com- trends in arcs with thicker as opposed to thinner crust (Fig. 5b). Di- pared to other mature, but younger, arcs such as the Sierra Nevada rect comparison of cumulate and magma compositions from mid- (Cretaceous) and Arizona (Late Cretaceous–Paleogene) (Fig. 5b). Re- ocean ridges and arcs, solely based on crustal thickness (i.e. degree cent work on the Famatinian arc suggests that convective root of mantle melting), is less straightforward as the latter involves removal must have occurred at least once, if not multiple times, subduction while the former doesn’t. However, arc cumulates (this since the inception of the arc in order to satisfy mass balance study) and magmas (Chiaradia, 2014)do become more similar and radiometric age constraints (Ducea et al., 2017). In fact, if to those in mid-ocean ridges as arc crustal thickness decreases the Famatinian arc is excluded in Fig. 5b, the correlation improves 2 2 (Fig. 5b), suggesting that the oxygen fugacity and H2O content in (R = 0.6 compared to R = 0.4). We note that for the cases of arc magmas decrease as arc crustal thickness decreases. Hence, the the Sierra Nevada and Arizona arcs, xenoliths were used to con- cause of the calc-alkaline trend is more complicated than being strain maximum equilibration pressures; yet today, the thick roots solely due to subduction of the oxidized oceanic crust and oxida- indicated by xenolith thermobarometry are thought to be absent tion of the wedge, and other causes such as difference in degree of beneath these arcs owing to recent convective removal (Erdman et mantle melting and mantle potential temperature (Gaetani, 2016) al., 2016; Lee et al., 2001). should also be explored in more detail, but are beyond the scope of the present paper. Our proposed hypothesis, based on the ob- 7. Summary and implications served relationship between crustal thickness and cumulate FeOT, is that to a first-order, variations in overlying crustal thickness may Using a “crystal” perspective provided by 500+ global cumu- 3+ regulate the melting degree and thus primary water and Fe con- lates, we show that primitive arc cumulates are Fe-rich whereas tent of arc magmas. mid-ocean ridge cumulates are Fe-poor, reflecting the dichotomy of the calc-alkaline and tholeiitic magma series. Our results provide 6.3. The role of lithospheric thickening clear evidence that fractional crystallization, rather than crustal assimilation/mixing, is the main driver of the calc-alkaline trend and Lithospheric accretion and thickening are important steps in therefore of andesite (and continental crust) formation. An Fe-rich the maturation of island arcs into continental arcs (Chin et al., reservoir, in the form of deep crustal cumulates, is required to bal- 2012; Lee et al., 2007). Higher pressures/thicker lithosphere pro- ance the Fe-depleted upper continental crust (Fig. 5a). Analysis of mote fractionation of garnet pyroxenites from primary, hydrous arc cumulates shows that with increasing crustal thickness, deep E.J. Chin et al. / Earth and Planetary Science Letters 482 (2018) 277–287 285 arc cumulates increase in Fe and Ti, suggesting earlier oxide frac- Chappell, B.W., White, A.J.R., Wyborn, D., 1987. The importance of residual source tionation beneath thicker arcs. We speculate that arc crustal thick- material (restite) in granite petrogenesis. J. Petrol. 28, 1111–1138. ness modulates melting degree, which in turn governs the initial Chiaradia, M., 2014. Copper enrichment in arc magmas controlled by overriding 3+ plate thickness. Nat. Geosci. 7, 43. H2O content and Fe /Fe ratio of primitive arc melts, and which Chin, E.J., Lee, C.T.A., Blichert-Toft, J., 2015. Growth of upper plate lithosphere con- ultimately controls the degree of Fe enrichment in arc cumulates trols tempo of arc magmatism: constraints from Al-diffusion kinetics and cou- and the complementary degree of Fe depletion in arc magmas. In pled Lu–Hf and Sm–Nd chronology. Geochem. Perspect. Lett. 1, 20–32. 3+ Chin, E.J., Lee, C.-T.A., Luffi, P., Tice, M., 2012. Deep lithospheric thickening and refer- contrast, the low H2O and Fe /Fe in MORBs generate low Fe tilization beneath continental arcs: case study of the P, T and compositional ridge cumulates and the complementary Fe enrichment in MORBs. evolution of peridotite xenoliths from the Sierra Nevada, California. J. Petrol. 53, Our observation shows that, compared to cumulates and magmas 477–511. from thick-crust arcs, those from thin-crust arcs are more similar Christie, D.M., Carmichael, I.S., Langmuir, C.H., 1986. Oxidation states of mid-ocean to cumulates and magmas from mid-ocean ridges, suggesting that ridge basalt glasses. Earth Planet. Sci. Lett. 79, 397–411. the cause of greater oxidation state in arc magmas is more com- Conrad, W.K., Kay, R.W., 1984. Ultramafic and mafic inclusions from Adak Island: crystallization history, and implications for the nature of primary magmas and plicated than being solely due to subduction of oxidized oceanic crustal evolution in the Aleutian Arc. J. Petrol. 25, 88–125. crust. Coogan, L., O’Hara, M., 2015. MORB differentiation: in situ crystallization in replenished-tapped magma chambers. Geochim. Cosmochim. Acta 158, 147–161. Acknowledgements Coogan, L.A., 2014. The lower oceanic crust. In: Holland, Heinrich D., Turekian, K.K. (Eds.), Treatise on Geochemistry, second edition. Elsevier, Oxford, pp. 497–541. Crabtree, S.M., Lange, R.A., 2012. An evaluation of the effect of degassing on the Ideas for this paper were initiated between EJC, GMB and KS oxidation state of hydrous andesite and dacite magmas: a comparison of pre- + during the 2015 Interior of the Earth Gordon Research Conference. and post-eruptive Fe2 concentrations. Contrib. Mineral. Petrol. 163, 209–224. We gratefully acknowledge Cin-Ty A. Lee for his guidance in hon- Curray, J.R., Shor, G.G., Raitt, R.W., Henry, M., 1977. Seismic refraction and reflec- ing our ideas and providing informal reviews of the manuscript. tion studies of crustal structure of the eastern Sunda and western Banda arcs. J. Geophys. Res. 82, 2479–2489. We also thank Jay Chapman, Oliver Jagoutz, Massimo Chiaradia, Daczko, N., Clarke, G., Klepeis, K., 2002. Kyanite–paragonite-bearing assemblages, and four anonymous referees for their constructive and insightful northern Fiordland, New Zealand: rapid cooling of the lower crustal root to a reviews of this version and earlier versions of the manuscript. Cretaceous magmatic arc. J. Metamorph. Geol. 20, 887–902. Dantas, C., Ceuleneer, G., Gregoire, M., Python, M., Freydier, R., Warren, J., Dick, H., 2007. Pyroxenites from the Southwest Indian Ridge, 9–16 E: cumulates from Appendix A. Supplementary material incremental melt fractions produced at the top of a cold melting regime. J. Petrol. 48, 647–660. Supplementary material related to this article can be found on- Dauphas, N., Craddock, P.R., Asimow, P.D., Bennett, V.C., Nutman, A.P., Ohnenstetter, line at https://doi.org/10.1016/j.epsl.2017.11.016. D., 2009. Iron isotopes may reveal the redox conditions of mantle melting from Archean to present. Earth Planet. Sci. Lett. 288, 255–267. Ducea, M.N., Bergantz, G.W., Crowley, J.L., Otamendi, J., 2017. Ultrafast magmatic References buildup and diversification to produce continental crust during subduction. Ge- ology 45, 235–238. Alonso-Perez, R., Muntener, O., Ulmer, P., 2009. Igneous garnet and amphibole frac- Ducea, M.N., Saleeby, J.B., 1996. Buoyancy sources for a large, unrooted mountain tionation in the roots of island arcs: experimental constraints on andesitic liq- range, the Sierra Nevada, California: evidence from xenolith thermobarometry. uids. Contrib. Mineral. Petrol. 157, 541–558. J. Geophys. Res. 101, 8229–8244. Annen, C., Blundy, J.D., Sparks, R.S.J., 2006. The genesis of intermediate and silicic Ducea, M.N., Saleeby, J.B., 1998. The age and origin of a thick mafic–ultramafic magmas in deep crustal hot zones. J. Petrol. 47, 505–539. keel from beneath the Sierra Nevada batholith. Contrib. Mineral. Petrol. 133, Arculus, R.J., Wills, K.J., 1980. The petrology of plutonic blocks and inclusions from 169–185. the Lesser Antilles island arc. J. Petrol. 21, 743–799. Ducea, M.N., Saleeby, J.B., Bergantz, G., 2015. The architecture, chemistry, and evo- Atherton, M.P., Petford, N., 1993. Generation of sodium-rich magmas from newly lution of continental magmatic arcs. Annu. Rev. Earth Planet. Sci. 43, 299–331. underplated basaltic crust. Nature 362, 144–146. Dufek, J., Bergantz, G.W., 2005. Lower crustal magma genesis and preservation: a Baker, M.B., Grove, T.L., Price, R., 1994. Primitive basalts and andesites from the Mt. stochastic framework for the evaluation of basalt–crust interaction. J. Petrol. 46, Shasta region, N. California: products of varying melt fraction and water content. 2167–2195. Contrib. Mineral. Petrol. 118, 111–129. Eggins, S.M., 1993. Origin and differentiation of picritic arc magmas, Ambae (Aoba), Ballhaus, C., 1993. Redox states of lithospheric and asthenospheric upper mantle. Vanuatu. Contrib. Mineral. Petrol. 114, 79–100. Contrib. Mineral. Petrol. 114, 331–348. Erdman, M.E., Lee, C.-T.A., Levander, A., Jiang, H., 2016. Role of arc magmatism and Beard, J.S., 1986. Characteristic mineralogy of arc-related cumulate gabbros: impli- lower crustal foundering in controlling elevation history of the Nevadaplano and cations for the tectonic setting of gabbroic plutons and for andesite genesis. Colorado Plateau: a case study of pyroxenitic lower crust from central Arizona, Geology 14, 848–851. USA. Earth Planet. Sci. Lett. 439, 48–57. Bloomer, S., Hawkins, J., 1983. Gabbroic and ultramafic rocks from the Mariana Evans, K.A., Elburg, M.A., Kamenetsky, V.S., 2012. Oxidation state of subarc mantle. Trench: an island arc ophiolite. In: The Tectonic and Geologic Evolution of Geology 40, 783–786. Southeast Asian Seas and Islands: Part 2, pp. 294–317. Farner, M.J., Lee, C.-T.A., 2017. Effects of crustal thickness on magmatic differentia- Boudier, F., Nicolas, A., Ildefonse, B., 1996. Magma chambers in the Oman ophiolite: tion in subduction zone volcanism: a global study. Earth Planet. Sci. Lett. 470, fed from the top and the bottom. Earth Planet. Sci. Lett. 144, 239–250. 96–107. Bourguignon, S., Stern, T.A., Savage, M.K., 2007. Crust and mantle thickening beneath Francis, D., 1986. The pyroxene paradox in MORB glasses – a signature of picritic the southern portion of the Southern Alps, New Zealand. Geophys. J. Int. 168, parental magmas? Nature 319, 586–589. 681–690. Frost, B.R., Ballhaus, C., 1998. Comment on “Constraints on the origin of the ox- Bowen, N.L., 1928. The Evolution of the Igneous Rocks. Dover Publications, Inc., New idation state of mantle overlying subduction zones: an example from Sim- York. coe, Washington, USA” by AD Brandon and DS Draper. Geochim. Cosmochim. Briceno-Guarupe, L.A., 1978. The Crustal Structure and Tectonic Framework of the Acta 62, 329–332. + Gulf of Panama. Master’s Thesis. Oregon State University, p. 79. Gaetani, G.A., 2016. The behavior of Fe3 /Fe during partial melting of spinel lher- + Brounce, M.N., Kelley, K.A., Cottrell, E., 2014. Variations in Fe3 /Fe of Mariana arc zolite. Geochim. Cosmochim. Acta 185, 64–77. basalts and mantle wedge f O2. J. Petrol. 55, 2513–2536. Gaetani, G.A., O’Leary, J.A., Shimizu, N., Bucholz, C.E., Newville, M., 2012. Rapid Bryan, W., Moore, J.G., 1977. Compositional variations of young basalts in the Mid- reequilibration of H2O and oxygen fugacity in olivine-hosted melt inclusions. Atlantic Ridge rift valley near lat 36 49 N. Geol. Soc. Am. Bull. 88, 556–570. Geology 40, 915–918. Cannat, M., 1996. How thick is the magmatic crust at slow spreading oceanic ridges? Gale, A., Dalton, C.A., Langmuir, C.H., Su, Y., Schilling, J.G., 2013. The mean composi- J. Geophys. Res., Solid Earth 101, 2847–2857. tion of ocean ridge basalts. Geochem. Geophys. Geosyst. 14, 489–518. Carbotte, S.M., Scheirer, D.S., 2004. The global mid-ocean ridge. In: Hydrogeology of Gale, A., Langmuir, C.H., Dalton, C.A., 2014. The global systematics of ocean ridge the Oceanic Lithosphere with CD-ROM, p. 59. basalts and their origin. J. Petrol. 55, 1051–1082. Carmichael, I.S.E., 1991. The redox states of basic and silicic magmas: a reflection of Ghiorso, M., Sack, R., 1995. Chemical mass transfer in magmatic processes IV. A re- their source regions? Contrib. Mineral. Petrol. 106, 129–141. vised and internally consistent thermodynamic model for the interpolation and Case, I., MacDonald, W., 1973. Regional gravity anomalies and crustal structure in extrapolation of liquid–solid equilibria in magmatic systems at elevated temper- northern Colombia. Geol. Soc. Am. Bull. 84, 2905–2916. atures and pressures. Contrib. Mineral. Petrol. 119, 197–212. 286 E.J. Chin et al. / Earth and Planetary Science Letters 482 (2018) 277–287

Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Springer-Verlag, Berlin. Lee, C.-T.A., Morton, D.M., Kistler, R.W., Baird, A.K., 2007. Petrology and tectonics of Gillis, K.M., Snow, J.E., Klaus, A., Abe, N., Adrião, Á.B., Akizawa, N., Ceuleneer, G., Phanerozoic continent formation: from island arcs to accretion and continental Cheadle, M.J., Faak, K., Falloon, T.J., 2014. Primitive layered gabbros from fast- arc magmatism. Earth Planet. Sci. Lett. 263, 370–387. spreading lower oceanic crust. Nature 505, 204–207. Leeman, W.P., 1983. The influence of crustal structure on compositions of Grad, M., Tiira, T., 2009. The Moho depth map of the European Plate. Geophys. J. subduction-related magmas. J. Volcanol. Geotherm. Res. 18, 561–588. Int. 176, 279–292. Levander, A., Schmandt, B., Miller, M., Liu, K., Karlstrom, K., Crow, R., Lee, C.A., Green, D.H., Falloon, T.J., Eggins, S.M., Yaxley, G.M., 2001. Primary magmas and man- Humphreys, E., 2011. Continuing Colorado plateau uplift by delamination-style tle temperatures. Eur. J. Mineral. 13, 437–452. convective lithospheric downwelling. Nature 472, 461. Greene, A.R., DeBari, S.M., Kelemen, P.B., Blusztajn, J., Clift, P.D., 2006. A detailed Lissenberg, C.J., MacLeod, C.J., Howard, K.A., Godard, M., 2013. Pervasive reactive geochemical study of island arc crust: the Talkeetna arc section, south-central melt migration through fast-spreading lower oceanic crust (Hess Deep, equato- Alaska. J. Petrol. 47, 1051–1093. rial Pacific Ocean). Earth Planet. Sci. Lett. 361, 436–447. Grove, T.L., Gerlach, D.C., Sando, T.W., 1982. Origin of calc-alkaline series lavas at Meissnar, R.O., Flueh, E.R., Stibane, F., Berg, E., 1976. Dynamics of the active plate Medicine Lake volcano by fractionation, assimilation and mixing. Contrib. Min- boundary in southwest Colombia according to recent geophysical measure- eral. Petrol. 80, 160–182. ments. Tectonophysics 35, 115–136. Grove, T.L., Kinzler, R.J., 1986. Petrogenesis of andesites. Annu. Rev. Earth Planet. Miller, D.J., Christensen, N.L., 1994. Seismic signature and geochemistry of an island Sci. 14, 417–454. arc: a multidisciplinary study of the Kohistan accreted terrane, northern Pak- Grove, T.L., Kinzler, R.J., Bryan, W.B., 1993. Fractionation of mid-ocean ridge istan. J. Geophys. Res., Solid Earth 99, 11623–11642. basalt (MORB). In: Mantle Flow and Melt Generation at Mid-Ocean Ridges, Miyashiro, A., 1974. Volcanic rock series in island arcs and active continental mar- pp. 281–310. gins. Am. J. Sci. 274, 321–355. Hacker, B.R., Mehl, L., Kelemen, P.B., Rioux, M., Behn, M.D., Luffi, P., 2008. Recon- Morgan, J.P., Chen, Y.J., 1993. The genesis of oceanic crust: magma injection, struction of the Talkeetna intraoceanic arc of Alaska through thermobarometry. hydrothermal circulation, and crustal flow. J. Geophys. Res., Solid Earth 98, J. Geophys. Res., Solid Earth 113. 6283–6297. Hartley, M.E., Shorttle, O., Maclennan, J., Moussallam, Y., Edmonds, M., 2017. Olivine- Muntener, O., Kelemen, P.B., Grove, T.L., 2001. The role of H2O during crystalliza- hosted melt inclusions as an archive of redox heterogeneity in magmatic sys- tion of primitive arc magmas under uppermost mantle conditions and genesis tems. Earth Planet. Sci. Lett. 479, 192–205. of igneous pyroxenites: an experimental study. Contrib. Mineral. Petrol. 141, Hildreth, W., Moorbath, S., 1988. Crustal contributions to arc magmatism in the An- 643–658. des of Central Chile. Contrib. Mineral. Petrol. 98, 455–489. Mysen, B.O., Virgo, D., Neumann, E.-R., Seifert, F.A., 1985. Redox equilibria Hirose, K., Kushiro, I., 1993. Partial melting of dry peridotites at high pressures: and the structural states of ferric and ferrous iron in melts in the sys- determination of compositions of melts segregated from peridotite using aggre- tem CaO–MgO–Al2O3–SiO2–Fe–O; relationships between redox equilibria, melt gates of diamond. Earth Planet. Sci. Lett. 114, 477–489. structure and liquidus phase equilibria. Am. Mineral. 70, 317–331. Hora, J.M., Singer, B.S., Wörner, G., Beard, B.L., Jicha, B.R., Johnson, C.M., 2009. Shal- Nandedkar, R.H., Ulmer, P., Müntener, O., 2014. Fractional crystallization of primi- low and deep crustal control on differentiation of calc-alkaline and tholeiitic tive, hydrous arc magmas: an experimental study at 0.7 GPa. Contrib. Mineral. magma. Earth Planet. Sci. Lett. 285, 75–86. Petrol. 167, 1–27. Humphreys, M.C.S., Brooker, R.A., Fraser, D.G., Burgisser, A., Mangan, M.T., McCam- Natland, J.H., Dick, H.J., 1996. Melt migration through high-level gabbroic cumulates mon, C., 2015. Coupled interactions between volatile activity and Fe oxidation of the East Pacific Rise at Hess Deep: the origin of magma lenses and the deep state during arc crustal processes. J. Petrol. 56, 795–814. crustal structure of fast-spreading ridges. In: Proceedings – Ocean Drilling Pro- Irvine, T., 1982. Terminology for layered intrusions. J. Petrol. 23, 127–162. gram Scientific Results. National Science Foundation, pp. 21–58. Jagoutz, O., 2010. Construction of the granitoid crust of an island arc. Part II: a O’Hara, M., 1968. Are ocean floor basalts primary magma? Nature 220, 683–686. quantitative petrogenetic model. Contrib. Mineral. Petrol. 160, 359–381. O’Hara, M., Richardson, S., Wilson, G., 1971. Garnet-peridotite stability and occur- Jagoutz, O., Burg, J.P., Hussain, S., Dawood, H., Pettke, T., Iizuka, T., Maruyama, S., rence in crust and mantle. Contrib. Mineral. Petrol. 32, 48–68. 2009. Construction of the granitoid crust of an island arc. Part I: geochrono- O’Neill, H.S.C., Jenner, F.E., 2016. Causes of the compositional variability among logical and geochemical constraints from the plutonic Kohistan (NW Pakistan). ocean floor basalts. J. Petrol. 57, 2163–2194. Contrib. Mineral. Petrol. 158, 739–755. Osborn, E.F., 1959. Role of oxygen pressure in the crystallization and differentiation Karig, D.E., 1970. Ridges and basins of the Tonga–Kermadec island arc system. of basaltic magma. Am. J. Sci. 257, 609–647. J. Geophys. Res. 75, 239–254. Otamendi, J., Vujovich, G., de La Rosa, J., Tibaldi, A., Castro, A., Martino, R., Pinotti, Karlstrom, L., Lee, C.T.A., Manga, M., 2014. The role of magmatically driven litho- L., 2009. Geology and petrology of a deep crustal zone from the Famatinian spheric thickening on arc front migration. Geochem. Geophys. Geosyst. 15, paleo-arc, Sierras de Valle Fértil and La Huerta, San Juan, Argentina. J. South 2655–2675. Am. Earth Sci. 27, 258–279. Kay, S.M., Kay, R., 1985. Role of crystal cumulates and the oceanic crust in the for- Parkinson, I.J., Arculus, R.J., 1999. The redox state of subduction zones: insights from mation of the lower crust of the Aleutian arc. Geology 13, 461–464. arc-peridotites. Chem. Geol. 160, 409–423. Kelemen, P.B., 1990. Reaction between ultramafic rock and fractionating basaltic Plank, T., Kelley, K.A., Zimmer, M.M., Hauri, E.H., Wallace, P.J., 2013. Why do mafic magma I. Phase relations, the origin of calc-alkaline magma series, and the for- arc magmas contain ∼4 wt.% water on average? Earth Planet. Sci. Lett. 364, mation of discordant dunite. J. Petrol. 31, 51–98. 168–179. Kelemen, P.B., Koga, K., Shimizu, N., 1997. Geochemistry of gabbro sills in the crust– mantle transition zone of the Oman ophiolite: implications for the origin of the Plank, T., Langmuir, C.H., 1988. An evaluation of the global variations in the major oceanic lower crust. Earth Planet. Sci. Lett. 146, 475–488. element chemistry of arc basalts. Earth Planet. Sci. Lett. 90, 349–370. Kelley, K.A., Cottrell, E., 2009. Water and the oxidation state of subduction zone Putirka, K.D., 2008. Thermometers and barometers for volcanic systems. Rev. Min- magmas. Science 325, 605–607. eral. Geochem. 69, 61–120. Kelley, K.A., Cottrell, E., 2012. The influence of magmatic differentiation on the ox- Richards, J.P., 2015. The oxidation state, and sulfur and Cu contents of arc magmas: idation state of Fe in a basaltic arc magma. Earth Planet. Sci. Lett. 329–330, implications for metallogeny. Lithos 233, 27–45. 109–121. Rubin, K.H., Sinton, J.M., 2007. Inferences on mid-ocean ridge thermal and magmatic Klemperer, S.L., Miller, E.L., Grantz, A., Scholl, D.W., Group, B.-C.W., 2002. Crustal structure from MORB compositions. Earth Planet. Sci. Lett. 260, 257–276. structure of the Bering and Chukchi shelves: deep seismic reflection profiles Rudnick, R.L., Gao, S., 2003. Composition of the continental crust. Treatise across the North American continent between Alaska and Russia. Geol. Soc. Am. Geochem. 3, 1–64. Spec. Pap. 360, 1–24. Sisson, T., Grove, T., 1993. Experimental investigations of the role of H2O in Laukert, G., von der Handt, A., Hellebrand, E., Snow, J.E., Hoppe, P., Klügel, A., 2014. calc-alkaline differentiation and subduction zone magmatism. Contrib. Mineral. High-pressure reactive melt stagnation recorded in abyssal pyroxenites from the Petrol. 113, 143–166. ultraslow-spreading Lena trough, Arctic Ocean. J. Petrol. 55, 427–458. Sleep, N.H., 1975. Formation of oceanic crust: some thermal constraints. J. Geophys. Lee, C.-T., Rudnick, R.L., Brimhall Jr., G.H., 2001. Deep lithospheric dynamics beneath Res. 80, 4037–4042. the Sierra Nevada during the Mesozoic and Cenozoic as inferred from xenolith Smith, P.M., Asimow, P.D., 2005. Adiabat_1ph: a new public front-end to the MELTS, petrology. Geochem. Geophys. Geosyst. 2. pMELTS, and pHMELTS models. Geochem. Geophys. Geosyst. 6. Lee, C.T.A., 2014. Physics and chemistry of deep continental crust recycling. In: Hol- Spandler, C.J., Arculus, R.J., Eggins, S.M., Mavrogenes, J.A., Price, R.C., Reay, A.J., 2003. land, Heinrich D., Turekian, K.K. (Eds.), Treatise on Geochemistry, second edition. Petrogenesis of the Greenhills complex, Southland, New Zealand: magmatic dif- Elsevier, Oxford, pp. 423–456. ferentiation and cumulate formation at the roots of a Permian island-arc vol- Lee, C.-T.A., Cheng, X., Horodyskyj, U., 2006. The development and refinement of cano. Contrib. Mineral. Petrol. 144, 703–721. continental arcs by primary basaltic magmatism, garnet pyroxenite accumula- Takahashi, N., Suyehiro, K., Shinohara, M., 1998. Implications from the seismic tion, basaltic recharge and delamination: insights from the Sierra Nevada. Con- crustal structure of the northern Izu-Bonin arc. Isl. Arc 7, 383–394. trib. Mineral. Petrol. 151, 222–242. Taylor, S.R., 1977. Island arc models and the composition of the continental crust. Lee, C.-T.A., Luffi, P., Le Roux, V., Dasgupta, R., Albaréde, F., Leeman, W.P., 2010. The In: Island Arcs, Deep Sea Trenches and Back-Arc Basins. AGU, Washington, DC, redox state of arc mantle using Zn/Fe systematics. Nature 468, 681–685. pp. 325–335. E.J. Chin et al. / Earth and Planetary Science Letters 482 (2018) 277–287 287

Thirlwall, M., Smith, T., Graham, A., Theodorou, N., Hollings, P., Davidson, J., Arculus, Waters, L.E., Lange, R.A., 2016. No effect of H2O degassing on the oxidation state of R., 1994. High ﬁeld strength element anomalies in arc lavas: source or process? magmatic liquids. Earth Planet. Sci. Lett. 447, 48–59. J. Petrol. 35, 819–838. Weber, M.B., Tarney, J., Kempton, P.D., Kent, R.W., 2002. Crustal make-up of the Turner, S.J., Langmuir, C.H., 2015. The global chemical systematics of arc front stra- northern Andes: evidence based on deep crustal xenolith suites, Mercaderes, tovolcanoes: evaluating the role of crustal processes. Earth Planet. Sci. Lett. 422, SW Colombia. Tectonophysics 345, 49–82. 182–193. Yang, H.-J., Kinzler, R.J., Grove, T., 1996. Experiments and models of anhydrous, Turner, S.J., Langmuir, C.H., Katz, R.F., Dungan, M.A., Escrig, S., 2016. Parental arc basaltic olivine–plagioclase–augite saturated melts from 0.001 to 10 kbar. Con- magma compositions dominantly controlled by mantle-wedge thermal struc- trib. Mineral. Petrol. 124, 1–18. ture. Nat. Geosci. 9, 772–776. Zimmer, M.M., Plank, T., Hauri, E.H., Yogodzinski, G.M., Stelling, P., Larsen, J., Singer, B., Jicha, B., Mandeville, C., Nye, C.J., 2010. The role of water in generating the Villiger, S., Ulmer, P., Muntener, O., Thompson, A.B., 2004. The liquid line of descent calc-alkaline trend: new volatile data for Aleutian magmas and a new tholeiitic of anhydrous, mantle-derived, tholeiitic liquids by fractional and equilibrium index. J. Petrol. 51, 2411–2444. crystallization – an experimental study at 1.0 GPa. J. Petrol. 45, 2369–2388. Zotov, N., Keppler, H., Hannon, A.C., Soper, A.K., 1996. The effect of water on Ward, K.M., Delph, J.R., Zandt, G., Beck, S.L., Ducea, M.N., 2017. Magmatic evolution the structure of silicate glasses – a neutron diffraction study. J. Non-Cryst. of a Cordilleran ﬂare-up and its role in the creation of silicic crust. Sci. Rep. 7, Solids 202, 153–163. 9047.