The genesis of silicic arc magmas in shallow crustal cold zones

    The genesis of silicic arc magmas in shallow crustal cold zones John Adam, Simon Turner, Tracy Rushmer PII: DOI: Reference:

S0024-4937(16)30210-9 doi: 10.1016/j.lithos.2016.07.036 LITHOS 4013

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Received date: Revised date: Accepted date:

17 April 2016 25 July 2016 29 July 2016

Please cite this article as: Adam, John, Turner, Simon, Rushmer, Tracy, The genesis of silicic arc magmas in shallow crustal cold zones, LITHOS (2016), doi: 10.1016/j.lithos.2016.07.036

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The genesis of silicic arc magmas in shallow crustal cold zones

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JOHN ADAM*, SIMON TURNER, TRACY RUSHMER

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Department of Earth and Planetary Sciences, Macquarie University, Sydney 2109, Australia

* E-mail address: [email protected]

Lithos Review paper, 2016

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ACCEPTED MANUSCRIPT ABSTRACT A number of currently popular models for the genesis of evolved arc-magmas (from basaltic andesite to dacite) invoke repeated intrusion, partial-melting and differentiation at the base of the

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crust. However, several observations suggest that this may be the exception rather than the norm:

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(1) geobarometry often indicates shallow pressure (0.1-0.3 GPa) evolution; (2) incongruent melting of amphibolite at elevated pressures should yield magmas in equilibrium with high pressure phases like garnet, but rare earth element patterns almost ubiquiously preclude this; (3) compositionally-

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zoned caldera forming eruptions suggest differentiation at near surface depths; (4) U-series data most commonly indicate differentiation over millennia time-scales. This requires rapid cooling that,

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in turn, is most easily explained by relatively small magma volumes undergoing crystal fractionation within the shallow (i.e. cool) crust. To further test these ideas, we combined published

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experimental-data for liquidus equilibria with appropriate silicic arc-magma compositions. On projections of the ternary liquidus system nepheline-silica-olivine, recent data for Tongan silicic

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lavas plot either on or close to low-pressure (1 atm) cotectics for the rocks’ phenocryst phases,

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suggesting low-pressure differentiation. Using our own and published data from arc volcanoes around the world we find that the majority are consistent with differentiation at shallow depths,

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regardless of total crustal thickness. Combined with the typical timescales of differentation, we estimate that the volumes of magma stored during differentiation in shallow crustal zones are usually on the order of only a few km3. There is also a clear role for mixing and recharge that involves magmas that are more deeply-sourced and primitive in character (typically evolved basalts and basaltic andesites). Whether the latter differentiated in the lower-crust or at the crust/mantle boundary has important implications for the constitution and average composition of arc crust. At present, we can only conclude that evidence for more silicic arc-magma generation at these depths is generally lacking.

Keywords: Arcs; Silicic magmas; Phase equilibria; Low pressure; Magma mixing; Volumes

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ACCEPTED MANUSCRIPT 1. Introduction Most arc magmas are of basaltic-andesite or andesite composition, with less common dacites. They are therefore more SiO2-rich and less magnesian than the primary (basaltic) parent melts

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derived by partial-melting of mantle peridotite (see results of Green, 1973; 1976; Baker and Eggler, Falloon et al., 2001). The path by

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1987; Draper and Johnston, 1992;

which such siliceous magmas evolve has been topical since the early controversies between Bowen (1928) and Fenner (1929). In particular, arguments have centred around the relative importance of

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fractional crystallisation versus partial-melting and magma-mixing. Recent models have also invoked a role for the crust/mantle boundary where a density barrier provides a natural opportunity

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for the stalling of ascending basaltic magmas (e.g. Green, 1982; Bergantz, 1999; Annen and Sparks, 2002). As found in early numerical models by Huppert and Sparks (1988) this will result in

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localised heating that could lead to partial melting of the crust and production of silicic magmas. Bergantz (1999) and Annen and Sparks (2002) observed that the process will be most effective if

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basalt intrusion and stalling is repetitive.

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Arc magmas are hydrous and so the pressure of vapour saturation provides another explanation for stalling during ascent although this is likely to occur in the mid- to upper crust (Burnham, 1979;

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Witham et al., 2012; Plank et al., 2013). Nevertheless, earlier intruded basalts will solidify within the lower crust to form amphibolites. Petford and Gallagher (2001) used a simple paramaterization of available experimental data on amphibolite melting to conclude that this process may not be very efficient at producing large volumes of silicic magma. However, these authors did not take into account the supply of volatiles (in addition to heat) from intruding and crystallising basalts that could help to flux-melt amphibolites formed from earlier intruded basalts. Annen and Sparks (2002) and Annen et al. (2006) demonstrated the potential importance of this latter process and concluded that the genesis of silicic magmas in these so-called “deep crustal hot zones” could be very efficient so long as thermal incubation (i.e. the repetitive instrusion and partial solidification of hydrous basalts) extends over 100’s of kyr to several Myr. Over such timescales, hybrid magmas produced

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ACCEPTED MANUSCRIPT by a combination of basalt fractionation and partial melting of amphibolites are predicted to evolve from the earliest most silicic magmas to increasingly mafic varieties as the ambient temperature of the hot zone increases (Annen et al., 2006). Whilst the numerical veracity of these models is not in

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doubt (and to which the reader is directed for details) they remain surprisingly difficult to test. In

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order to expand the discussion further we review here the available evidence for differentiation depths from arc lavas themselves and articulate the view that the majority of silicic arc magmas are

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probably formed within the upper crust rather than at the crust/mantle interface.

2. Independent evidence for the depth of magma differentiation

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Although a number of experimentally and empirically determined mineral-mineral and mineralliquid geothermometers have been successfully applied to a wide range of arc volcanic rocks,

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reliable geobarometric constraints have, in general, been far harder to obtain (Putirka and Tepley, 2008). On Fig. 1a we show the pressure-temperature conditions inferred for a broad selection of arc

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lavas around the world whose bulk rock compositions range from 51-75 wt. % SiO2. As can be

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seen, the maximum density of the data corresponds to a depth of around 4-8 km. Interestingly, this is the depth of vapor-saturation at which the maximum dissolved H2O content of basaltic magma is

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~ 4 wt. % (Witham et al., 2012) which appears to be the average water content actually observed in arc magmas (Plank et al., 2013). Thus, this may represent a common stalling point (defined by the depth of H2O saturation in magmas) where rising basaltic magma degasses and undergoes fractionation and it is notable that this depth also corresponds to the greatest range in SiO2 on Fig. 1b. It should also be recalled that it is not uncommon that the results from the different available barometers can yield conflicting results (e.g. Blundy and Cashman, 2008; Ohba et al., 2009) and these do not always coincide with independent estimates based, for example, on melt inclusions (see Putirka and Tepley, 2008 for a review). Thus, constraints on the depth of differentiation based on geobarometry from individual rocks still remain subject to some uncertainty (see also section 5 below).

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ACCEPTED MANUSCRIPT Some geophysical studies have found evidence for the presence of sizeable bodies of magma in the upper mantle (50-90 km depth) beneath large continental arc volcanoes such as Klyuchevskoy and Katmai (Utnasin et al., 1975; Matumoto, 1971) that provide support for deep level

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differentiation. However, numerous studies in the Cascades, Tonga and Vanuatu have consistently

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failed to find any evidence for the existence of magma bodies (see summaries by Iyer, 1984; Calvert, 2011). Moreover, when crustal magma chambers have been observed beneath arc volcanoes, they tend to be small (100’s to 1000’s m3) and shallow (typically 1-3 km depth) and, by

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implication, inferred to be largely transitory features (Iyer 1984; Marsh 1989; Dvorak & Dzurisin, 1997). What may be more significant is that 1-D seismic velocity profiles indicate that abundant

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silicic material (e.g. tonalite with P wave velocities < 6.5 km s-1) is only present at depths shallower than 9 km in intra-oceanic arcs (Calvert, 2011). Strikingly, comparison of Figs. 1a, b and c shows

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that there is a significant decrease in P wave velocities for the depth at which the thermobarometry suggests that the majority of silicic arc lavas have been produced (i.e. ≤ 10 km). This may relate

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both to compositional changes (with silicic compositions predominating near the surface) and to

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density contrasts that cause ascending magmas to stall and fractionate. Rare earth element patterns can also provide information on the pressure of magmatic evolution

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because the lanthanide-contraction favours incorporation of the heavy rare earths as the size of available crystallographic sites decrease with increasing pressure (e.g., Blundy and Wood, 1994). Furthermore, many studies have investigated the partial melting of amphibolites with the important result that amphibole melts incongruently at high pressures (~ 1 GPa) to produce clinopyroxene and garnet as residual phases (e.g., Rushmer, 1991; Wolf and Wyllie, 1994; Rapp and Watson, 1995; Muntener et al., 2001). Experimental studies have also shown that the heavy rare earth elements partition strongly into these phases in basaltic to andesitic liquids (e.g. Nicholls and Harris, 1980) and yet evidence for heavy rare earth element depletion is largely lacking in arc lavas (Davidson et al., 2006, 2010) with the possible exception of some “adakitic” rocks (e.g., Drummond and Defant, 1990). The implication is that, if partial melting of amphibolite is important in the genesis of silicic

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ACCEPTED MANUSCRIPT arc magmas, this probably needs to occur in the mid- to upper crust where garnet will not be formed as a residual phase and retain the heavy rare earth elements during dehydration melting (cf. Brophy,

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2008).

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3. Stratigraphic and timescale constraints

A natural consequence of the time-progressive heating in the deep crustal hot zone model, is that the composition of erupted magmas is predicted to evolve from felsic to mafic over time (Annen et

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al., 2006). Although sequential eruptions often obscure stratigraphy, many arc volcanoes such as Fonualei in the Tonga arc, Aniakchak in the Aleutian arc and St Lucia in the Lesser Antilles show

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the reverse sense of evolution with voluminous dacites overlying andesite and basaltic andesite (George et al., 2004; Turner et al., 2012; Bezard et al., 2014). Nonetheless, there are also examples

Costa Rica (Reagan et al., 2006).

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of volcanoes whose eruptive products have become more mafic with time such as Turrialba in

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The thermal conditions under which magmas evolve strongly control the time scales over which

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evolution to more differentiated compositions occurs. If crystallization occurs due to decompression and degassing, differentiation might be very rapid (potentially months to decades) assuming that

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there is an efficient mechanism for crystal – liquid separation. However, if differentiation is driven by crystallization due to cooling and perhaps mixing with partial-melts of country rocks, then the rate of differentiation will reflect the rate of magma input and the local geotherm. In principle, therefore, time scale information can also be used to constrain the depth at which arc magmas differentiate with fast differentiation rates favoured by emplacement of small to moderate sized magma batches into shallow and cool crust (e.g. Hawkesworth et al., 2003; Blake and Rogers, 2005; Dosseto et al., 2008; Turner et al., 2010). U-series isotope disequilibria can be used to place constraint upon the timescales of magma differentiation beneath arc volcanoes (see Turner et al., 2003a; Zellmer et al., 2005; Cooper and Reid, 2008; Dosseto and Turner, 2010, for recent reviews). The most common nuclide pairs

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ACCEPTED MANUSCRIPT employed are 230Th-238U and 226Ra-230Th within which the half lives of the daughter products are 75 kyr and 1600 yr, respectively and any disequilibria inherited from the parental magmas will be erased after ~ 5 half lives. Almost all arc lavas preserve 230Th-238U disequilibria and, assuming that

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this was created duing magma generation in the mantle wedge (see review by Turner et al., 2003a),

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this requires that any differentiation occurred in < 350 kyr. Studies of crystals futher suggests that most arc lava phenocrysts can often a few kyr older than the eruption age (e.g., Cooper and Reid, 2008). On Fig. 2 we plot 226Ra-230Th disequilibria that have been aquired for over 300 historic arc

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lavas ranging in composition from basalt to rhyolite (note that there have been few rhyolite eruptions in historic times). The striking observation is that the vast majority of arc lavas have some Ra excess regardeless of composition and this agrues strongly that their differentiation took less

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than a few millenia. As noted by Turner et al. (2003b) and Cooper and Kent (2014), the differing

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time scales recorded by different chronometers in crystals and whole rocks likely refect the aging of cumulates and remobilization of liquids and crystals from crystal-mush zones beneath arc

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volcanoes.

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The time scales inferred from Fig. 2 are clearly at odds with models of silicic magma production in deep crustal hot zones because these require 100’s kyr to Myr to reach the thermal maturity

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required to produce silicic magmas by partial melting of country rocks (Annen et al., 2006). Additionally, as deep crustal zones heat up with the repeated injection of basaltic sills, the rate of cooling and crystallization will become increasingly protracted and so the rate of differentiation of incoming basalts will slow significantly. It is possible that the 226Ra excesses might be buffered or imparted to silicic magmas by repeated injecting and mixing with primitive magmas from the mantle (Annen et al., 2006). However, numerical calculations indicate that the frequency of these injections needs to be shorter than the 1600 year half life of 226Ra (Hughes and Hawkesworth 1999) and that the mass fraction of 226Ra from zero age basalt in a typical erupted dacite would have to be ~ 60%.

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ACCEPTED MANUSCRIPT It is also important to consider the possible effects of crustal melting on the 226Ra signal and how these might impact upon the time scales inferred from such data. Berlo et al. (2004) modelled the effects of partial melting of amphibolites and concluded that only very modest (< 20%) 226Ra

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excesses would result from this process. Dufek and Cooper (2006) suggested that higher 226Ra

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excesses may be produced in the presence of large amounts of residual garnet. However, their model is only applicable to rare high Sr/Y adakites rather than the bulk of arc lavas that we are concerned with here (the reader is referred to the comment by Berlo et al., 2006 for further details).

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Thus, it would appear that, in the majority of situations, amphibolite melting is unlikely to produce significant 226Ra excesses and the corollary is that assimilation of mafic crust is only likely to

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reduce mantle-derived 226Ra excesses (e.g. Reubi et al., 2011). Finally, Reagan et al. (2003) and Zellmer et al. (2005) have shown that the overall extent of U-230Th disequilibria in arc lavas also decreases with increasing SiO2 in a manner analagous to

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the 226Ra excesses on Fig. 2. Whilst the significance of this must be established on a volcano by

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volcano basis, the similarity in behaviour of 230Th and 226Ra versus SiO2 despite the differences in

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their half lives points to a significant role for mixing (e.g. Reubi et al., 2011) that is supported by

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our phase equilibria analysis below.

4. A phase equilibrium approach Many of the observations just summarised have been discussed before, though not in a combined form. Below we try to provide a complementary approach through use of phase equilibria.

4.1. Background Most arc magmas are saturated with two or more phenocryst phases (usually plagioclase + pyroxene(s) ± olivine ± amphibole ± oxides) prior to their eruption. On this basis it can be argued that the magmas probably evolved by crystal fractionation along cotectics for the same crystal phases. Because the positions of these cotectics in multi-component space are influenced by

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ACCEPTED MANUSCRIPT pressure, temperature, dissolved volatiles and other compositional factors (see Kushiro, 1969; 1972; Kushiro et a., 1968; Warner 1973; Presnall et al., 1978; Longhi and Pan, 1988) their coincidence with the compositional trends of particular arc-magma suites can be used, together with appropriate

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experimental data, as a guide to the conditions of magma evolution (e.g. Walker et al., 1979; Baker

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and Eggler, 1987; Crawford et al., 1987; Grove et al., 2003). We have adopted this approach here because it allows us to conveniently overview a number of volcanic arcs and arc-volcanoes, and to use these as case studies for conditions of magma evolution.

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In order to plot major and minor element compositions (that include 11 components) on ternary liquidus projections, we used the formulations of O’Hara (1968) to recalculate all oxides: first as

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CMAS components (CaO-MgO-Al2O3-SiO2), and then as end-members in the system Qtz-DiCaAl2O4-Fo (equivalent to quartz-clinopyroxene-nepheline-olivine). Cotectics for natural

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(complex) arc magmas were bracketed using experimental data from: Eggler (1972), Fujii and Bougault (1983), Baker and Eggler (1987), Draper and Johnson (1992), Sisson and Grove (1993),

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Gaetani et al. (1994), Gaetani and Grove (1998), Moore and Carmichael (1998), Martel et al.

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(1999), Blatter and Carmichael (2001), Prouteau and Scaillet (2003), Rutherford and Devine

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(2003), Holz et al. (2005), Adam et al. (2012) and Melekhova et al. (2015).

4.2. Liquidus equilibria for mafic systems: the effects of pressure, H2O and other variables Before describing our results, it is worth reviewing relevant liquidus phase-equilibria and their more significant features. In most respects the natural systems behave like their simple-system (CMAS) analogue (which represents > 80 % of the components present in most arc volcanics). Thus at low pressure (1 atm) within the CaAl2O4-silica-forsterite plane (Fig. 3) there are cotectics for plagioclase-olivine, plagioclase-orthopyroxene, and orthopyroxene-olivine that come together at a piercing-point P1 where all three crystal phases and liquid simultaneously coexist. As pressure increases this point progressively shifts to lower values of SiO2 as the liquidus fields of plagioclase and olivine also shrink. At a pressure close to 1.0 GPa plagioclase is replaced by spinel and there is

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ACCEPTED MANUSCRIPT a change in direction as P1 moves sub-parallel to a direct olivine-addition trend (with melts becoming increasing less SiO2-saturated as they become more MgO-rich). In natural igneous systems these relationships are retained but systematically displaced from their CMAS locations by

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the influence of the additional (non-CMAS) components present in natural magmas. In particular,

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the replacement of CaO (CaAl2O4) by Na2O and K2O (NaAlSiO4 and KAlSiO4) expands the liquidus fields of olivine and pyroxenes relative to plagioclase (Fig. 4). Conversely, the addition of FeO reduces the stability fields of pyroxenes and olivine relative to plagioclase. Dissolved H2O

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expands the liquidus field of olivine relative to both plagioclase and pyroxenes (Fig. 4), and has also been shown to destabilize plagioclase relative to orthopyroxene (e.g. Yoder, 1965; Eggler and

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Burnham, 1973; Feig et al., 2006). However, the latter effect appears to be small relative to the former and is difficult to distinguish from those produced by minor variations in other components.

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When present together, alkalis and H2O can also stabilize amphibole relative to both plagioclase and clinopyroxene (Fig. 3), although this requires enough pressure (≥ 0.15 GPa) for significant

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concentrations of H2O (typically ≥ 4 wt. %) to be dissolved in the melt phase (Eggler, 1972;

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Rutherford et al., 1985; Rutherford and Devine, 2003; Berndt et al., 2005; Holz et al., 2005; Geschwind and Rutherford, 2013).

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Of the remaining non-CMAS components none appear to be of significant influence within the concentration ranges expected of normal arc magmas. The same applies to natural variations in ƒO2 (Eggler and Burnham, 1973; Martel et al., 1999; Berndt et al., 2005). Consequently we have focussed our attention on matching data for systems of comparable alkali and FeO concentrations. To do this, individual arcs and arc provinces were characterized according to whether they have low (2-4 wt. % Na2O + K2O), medium (4-6 wt. % Na2O + K2O) or high (6-8 wt. % Na2O + K2O) alkali concentrations (Fig. 5). In general, these differences are accompanied by relatively modest variations in FeO, with strong FeO enrichments associated only with the tholeiitic trends of low alkali series. Fortunately the same is also true of the various experimental starting-compositions used in published studies of liquidus relations in natural arc magmas.

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ACCEPTED MANUSCRIPT Liquidus relations from the experimental studies are compared with the distributions of natural volcanic-rock compositions for arc provinces with matching concentrations of alkalis and FeO. This is shown graphically in Figs. 6-13. A final note: most of the projections (Figs. 6-13) show liquidus

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boundaries for the CaAl2O4-silica-forsterite plane that are also saturated with diopside (± opaque

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oxides) for which no separate liquidus field is shown. We have concentrated on this projection for two reasons. One is that high-Ca clinopyroxene is almost ubiquitously present in both natural rocks (as phenocrysts) and experimental run-products. Thus it can be consistently used as a projection-

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point for other components. The CaAl2O4-silica-forsterite plane also displays some of the most evident and consistent shifts in liquidus relations arising from changes in pressure, dissolved H2O,

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and alkali concentrations in melts.

A variety of starting compositions were used in the compilation of each set of cotectics. This

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factor, combined with the tendency of alkalis to concentrate in melts with increasing degree of crystallization, inevitably causes a degree of uncertainty and systematic bias. However, natural

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magmas will tend to be similar and with few exceptions we found consistency between the

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5. Case studies

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experimental data from various sources.

Although we investigated many volcanic systems we selected those to present on the basis that they encompass the range of general observations. We begin with the simplest, intra-oceanic arcs then more complex and longer-lived, transitional oceanic arcs and finally arcs developed on thick continental crust. It must be noted that our coverage is limited to volcanic arc rocks and does not include plutonic examples (except where these are sometimes sampled as cognate inclusions in volcanic rocks). The latter will be the subject of a separate review.

5.1. Tonga

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ACCEPTED MANUSCRIPT The Tonga-Kermadec arc is a purely intra-oceanic arc, although its southern extension passes through the continental crust of New Zealand (Ewart et al., 1973; Gamble et al., 1996; Smith and Price, 2006). We consider data for three volcanic islands (Tofua, Late and Fonualei) that are part of

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the northern Tonga Arc. The islands have previously been studied by a number of researchers (e.g.

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Baker et al., 1971; Ewart et al., 1973; Caulfield et al., 2008; 2012; Turner et al., 2012) and are particularly suited to the investigative approach that has been outlined. This is because the volcanics are relatively aphyric (with typically < 10 % of phenocrysts) but contain several different

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phenocryst phases (plagioclase + clinopyroxene + orthopyroxene). Thus they represent a close approach to the ideal of magmatic liquids equilibrated under cotectic conditions. Pre-eruptive H2O

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concentrations have been determined for one volcano (Tofua). All have low total-alkali whole rock compositions with between 2 and 4 wt. % of Na2O + K2O. The Tonga samples are also distinctive

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in following a well-defined tholeiitic fractionation trend (Ewart et al., 1973). This appears to be a function of their low alkali concentrations rather than low H2O, since the original magmas were

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relatively hydrous. As noted earlier, the position of the plagioclase-pyroxene cotectic is sensitive to

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alkali concentrations but not especially to H2O. All of the other case studies cited in this review have higher alkali concentrations and follow calc-alkaline trends. There is also a broad positive

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correlation between alkali concentrations for individual case studies and the degree to which the calc-alkaline trend is developed. Exposed lavas on Tofua are mostly basaltic andesites with a single dacite lava. Crystallization temperatures inferred from coexisting phenocryst compositions are 1200-950 °C at depths of 1.52.5 km (Ewart et al., 1973; Caulfield et al., 2012). Analyses of glassy melt inclusions in phenocrysts indicate pre-eruptive water concentrations of up to 4 wt. %, but there is an anti-correlation between H2O concentrations in glasses and indices of differentiation (Caulfield et al., 2012). The volcanics from Late are mostly basaltic andesites and andesite, whereas on Fonualei they are mostly dacites with one underlying basaltic andesite/andesite (Ewart et al., 1973; Turner et al., 2012). Inferred crystallization temperatures for Fonualei (from phenocryst compositions) are 1100-1000 °C. On all

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ACCEPTED MANUSCRIPT three islands phenocryst abundances are variable (3-40 %) but typically low (< 10 %). Plagioclase is the most abundant phenocryst and coexists with clinopyroxene, orthopyroxene and (more rarely) pigeonite. Amphibole (as a phenocryst phase) has yet to be reported from any of the Tonga-

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Kermadec islands.

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On the CaAl2O4-silica-forsterite projection (Fig. 6) the Tongan whole-rock data plot either on or to the left of the 1 atm low-alkali cotectic for plagioclase, orthopyroxene (low-Ca pyroxene) and diopside. There is a small tail at the low SiO2 end which projects in the direction of plagioclase.

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This is probably the result of plagioclase accumulation. When combined with the anti-correlation between H2O concentrations and indices of fractionation in melt inclusions, the coincidence of

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bulk-rock trends with low-pressure cotectics is consistent with crystal-fractionation driven by H2O loss from magmas as they ascended to depths ≤ 7 km. This is further supported by the consistency

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of barometry for phenocrysts (0.15 to 0.25 GPa) in near-aphyric lavas of basaltic andesite to dacite composition (Ewart et al., 1973; Caulfield et al., 2012). Thus all of the bulk-rock spectrum is

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represented by melts equilibrated on the same low-pressure cotectic. This negates the suggested

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need for cryptic fractionation of arc magmas at significantly higher pressures (e.g. Davidson et al.,

5.2. Vanuatu

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2007; Melekhova et al., 2015).

Yasur Volcano on Tanna Island, with its resurgent caldera and currently active daughter-cone (with lava-lake) is part of the southern section of the Vanuatu arc. The latter is an intra-oceanic arc formed where oceanic lithosphere of the Australian Plate is subducting beneath the Pacific Plate (Pelletier et al., 1998; Calmant et al., 2003). Yasur Volcano has consistently produced magmas of basaltic trachyandesite to trachyandesite composition for the last 630-850 years (Métrich et al., 2011; Firth et al., 2014). As with the Tongan case study, the within-caldera magmas are relatively aphyric (with only a few % of phenocrysts) although multiply-saturated with several phenocryst phases (plagioclase, clinopyroxene and olivine) prior to eruption (Firth et al., 2014). However, the

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ACCEPTED MANUSCRIPT more distal eruptives are both more evolved and phenocryst-rich. In contrast to Tonga, the Yasur magmas are alkali-rich with 6-8 wt. % of Na2O + K2O (Fig. 5). Estimated conditions of phenocryst growth (by conventional thermobarometry) are 1066-1095 °C for plagioclase, and 1049-1070 °C

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and 0.37-0.67 GPa for clinopyroxene (Firth et al., 2014) although the latter are based on only two

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estimates. Melt-inclusion homogenization temperatures from phenocrysts in the caldera basaltic andesites give relatively uniform temperatures of 1123-1093 °C (Métrich et al., 2011). Concentrations of pre-eruptive H2O (from melt inclusion data) are close to 1.0 wt. % irrespective of

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degree of melt evolution and consistent with melt evolution at constant pressure [equivalent to ~1112 km] (Métrich et al., 2011). Allied with these observations are data for short-lived U-isotope

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decay series and volatile exhalation rates (Métrich et al., 2011; Firth et al., 2014). These indicate a steady-state magma system that is sustained by continuous additions of fresh basaltic parent-melts

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together with the latent heat of crystallization resulting from ~ 50 % of crystal fractionation (as needed to produce basaltic trachyandesite and trachyandesite derivatives from basaltic parent

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magmas).

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Consistent with their level of alkali enrichment and melt-inclusion barometry, bulk-rocks and glasses plot close to the 1 atm plagioclase-olivine cotectic for high-alkali melts (Fig. 7). Thus like

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the Tongan examples, the Yasur magmas appear to have evolved at relatively low pressures (≤ 0.3 GPa), although the barometry for two clinopyroxene phenocrysts (Firth et al., 2014) suggests inputs of fractionated melts from greater (mid-crustal) depths. The latter are consistent with the dynamic and open-system environment required to maintain small and shallowly-located magma bodies, such as Yasur, over long periods of time.

5.3. The Sunda Arc The Sunda arc of Indonesia, which includes the main islands of Java and Sumatra, has been formed by subduction of the oceanic India Plate beneath the continental Sunda Plate (Katili, 1975; Hall, 2002). Although one of the most volcanically active regions of the world it is also densely

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varied phenocryst populations (Camus et al., 1987; Purbawinata, 1990; Gerbe et al., 1992; Harmon

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and Gerbe, 1992; Mandeville et al., 1996; Chesner et al., 1998; Sisson and Bonto, 1998; Andreastuti et al., 2000; Handley et al., 2008; 2010; Belousov et al., 2012; Dahren et al., 2012; Erdmann et al., 2014). Plagioclase is the most frequent with clinopyroxene, orthopyroxene and opaque oxides also

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common. Olivine is common only in magnesian basalts, but still present as a minor phenocryst in the most mafic andesites and basaltic andesites. In the latter it is often rimmed by orthopyroxene.

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Amphibole is only occasionally present (Merapi, Salak and Guntur) and most typical of evolved and/or phenocryst-rich samples. Total phenocrysts abundances are variable with dacites from

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Krakatau having ~15 % phenocrysts and andesites from Salak ~50-60 % phenocrysts. In some cases (e.g. Merapi, Salak and Gede) there are mafic enclaves, cumulate xenoliths, and complex patterns

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of compositional zoning, partial-resorption and overgrowth in phenocrysts that are indicative of

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magma mixing and mingling (e.g. Handley et al., 2008; 2010; Chadwick et al., 2013). Bulk-rock data for eight Sunda-Arc volcanoes are plotted in Fig. 8. Most of the examples come

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from western Java, but with one from central Java (Merapi) and two from the Sunda Strait (Krakatau and Anak Krakatau). Most are of intermediate alkali content (4-6 wt. % H2O) although the Merapi samples tend to higher alkali concentrations and the basaltic samples have lower alkali concentrations. Within the CaAl2O4-silica-forsterite plane (Fig. 8) samples plot on relatively narrow, near-vertical trends that diverge at low SiO2 toward olivine. The vertical part of the trend is largely encompassed by the basaltic-andesite to dacite spectrum. It is also coincident with plagioclase-pyroxene cotectics for intermediate- and high-alkali melts under low-pressure (1 atm) conditions. For basaltic andesites and andesites at the low-SiO2 end of the range, there is a further coincidence with the low-pressure cotectics for olivine-plagioclase and olivine-orthopyroxene. Although olivine phenocrysts in these rocks are not abundant and frequently overgrown by

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ACCEPTED MANUSCRIPT orthopyroxene and/or partially-resorbed, this is the down-temperature peritectic relationship normally expected for olivine and orthopyroxene in SiO2-rich arc magmas (see Zellmer et al., 2016). Thus, the relative frequency of olivine phenocrysts, and their consistency with experimental

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liquidus-equilibria, is evidence of olivine’s presence as an original low-pressure liquidus phase. In

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this case, the primary mechanism of intermediate magma evolution in the Sunda Arc is crystalfractionation along cotectics for low-pressure (≤ 0.2 GPa) phenocryst phases (plagioclase + pyroxenes ± olivine ± amphibole).

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In contrast to this assessment, previously-estimated depths of magma evolution and storage for Merapi in central Java have varied widely, from shallow crust to uppermost mantle (e.g. Beauducel

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and Cornet, 1999; Muller and Haak, 2004; Chadwick et al., 2013; Costa et al., 2013; Erdmann et al., 2014; 2016). Much of this variation can be related to the various methodologies used and their

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interpretation (see discussion of Erdmann et al., 2016). However, a careful experimental study by Erdmann et al. (2016) of a basaltic-andesite from the 2010 Merapi eruption separately replicated

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both the: (i) melt phase (rhyolite) [Fig. 8] and phenocryst rims, and (ii) phenocryst cores and relicts.

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By this means it mimicked both shallow storage conditions (at 0.1-0.2 GPa and 925-950 °C with 34 wt. % of dissolved H2O) and conditions of recharge by a basaltic-andesite magma (at 950-1000

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°C with 4-5 wt. % of dissolved H2O). These conditions are consistent with a recent re-evaluation of amphibole thermobarometry by Putirka (2016) indicating cooling of Merapi parent magmas by 190270 ºC at mostly shallow depths (≤ 0.22 GPa). In contrast to the basaltic andesites and more felsic eruptives, the most mafic of the Sunda Arc magmas (magnesian basalts) have compositions that are appropriate to high-pressure (1.5 GPa) partial melting of peridotite (Fig. 8). This factor, combined with the evidence of open-system processes (magma mixing and mingling), reveals a general coincidence of mixing trends and cotectic controls that extend the depth interval of magmas evolution from the upper crust to uppermantle. In spite of this, there is no evidence to suggest that the primary site of intermediate magma

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ACCEPTED MANUSCRIPT generation is the crust/mantle interface. Instead, the evidence favours a predominantly upper-crustal origin.

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5.4. Lesser Antilles

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The Lesser Antilles arc is formed along the eastern boundary of the Caribbean Plate where it is underthrust by subducted ocean-crust of the western mid-Atlantic (Bouysse et al. 1990). It is built on the foundations of an older (Mesozoic) arc (the Caribbean Arc) and has 17 currently active

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volcanoes, including the Soufrière Hills of Montserrat. As with Merapi, this volcano has been intensively studied due to its activity and proximity to human habitation (e.g. Barclay et al., 1998;

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Murphy et al., 2000; Druitt and Kokelaar, 2002; Zellmer, 2003; Christopher et al., 2010). It is also petrographically complex with an abundant phenocryst assemblage, including amphibole,

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plagioclase, quartz, pyroxenes and olivine. Cumulate xenoliths and mafic enclaves are also common. Although the olivine in lavas is described as resorbed, this is not necessarily an indication

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of a xenocrystic origin (as previously explained for the Sunda Arc). The compositional range of the

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Soufrière rocks is extensive (from rhyolite to basalt) and characterized by low to intermediate alkalis (3-5 wt. % Na2O + K2O).

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In the CaAl2O4-silica-forsterite plane (Fig. 9) the trend of the Soufriere samples is similar to that for the Sunda Arc, except that there is a slight curvature towards CaAl2O4 at low SiO2. The felsic and intermediate compositions (rhyolites to basaltic andesites) follow plagioclase-pyroxene cotectics for low pressure (1 atm) melts of intermediate-alkali content (Fig. 9). At the SiO2-poor end of this range (basaltic andesites) they are coincident with the plagioclase-pyroxene-olivine piercing point for the same system (Fig. 9). Under these circumstances the occasional presence of relict olivine phenocrysts is significant and consistent with low-pressure fractionation. However, the compositions of the most mafic enclaves and lavas approach those of high-pressure (≥ 1.5 GPa) peridotite melts (Fig. 9). A role for low-pressure fractionation and shallow storage is well demonstrated by the experimental works of Barclay et al. (1998) and Rutherford and Devine (2003)

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ACCEPTED MANUSCRIPT who reproduced the matrix and phenocryst compositions of a Soufrière andesite at ≤ 0.2 GPa (Fig. 9). But this work also demonstrated a significant role for magma-mixing and open-system processes, a conclusion that is consistent not only with the abundance of mafic enclaves, anticrysts,

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and complex phenocryst zoning (Murphy et al., 1998), but with the mass-balance requirements of

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volcanic degassing (Christopher et al., 2010). The latter require ongoing re-charge of the andesitic magma chamber by fresh mafic melts from depth. Thus as with Yasur and the Sunda Arc, the data suggest a coincidence between low-pressure cotectic controls and mixing trends that involve

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contributions from more deeply-sourced and primitive magmas. There is no evidence suggesting that the intermediate and more felsic magmas of Soufrière were primarily the result of high-

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pressure fractionation at the crust/mantle interface 5.5. Cascades

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The Cascade arc is formed where the continental crust of North American is under-thrust by subducted oceanic plates of the adjacent Pacific Ocean (Priest, 1990; Hildreth, 2007). The volcanics

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are generally of medium to high alkali content and compositionally diverse (from MgO-rich basalt

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to rhyodacite). Phenocrysts include plagioclase, orthopyroxene, clinopyroxene, olivine, Fe-Tioxides and amphibole (Hildreth et al., 2003; Grove et al., 2005; Green, 2006; Jicha et al., 2009;

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Koleszar et al., 2012). Olivine phenocrysts are most abundant in the basalts, but also present in some basaltic-andesites, andesites and dacites. Bulk-rock compositions for six Cascade volcanoes are plotted in Fig. 10.

As for the Soufrière Hills and Sunda Arc, most of the Cascade data fall on relatively narrow and linear trends that span a wide SiO2 interval. These trends are coincident with plagioclase-pyroxene cotectics for intermediate- and high-alkali melts at low pressure (1 atm). However, there is a significant subset with low SiO2 that diverges in the direction of enstatite (see Fig. 10). The origin of this group (with specific reference to Mount Shasta samples) has been variously attributed to: (1) the fractionation of primitive basaltic-melts produced by hydrous peridotite-melting at 1 GPa (Grove et al. 2003; 2005; 2007); and/or (2) wall-rock contamination by crustal serpentinites (Streck

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ACCEPTED MANUSCRIPT et al., 2007). An initial comment is that the Mount Shasta samples are less alkali-rich than the other Cascade volcanoes considered in this study (Fig. 5). This shifts the plagioclase-pyroxene cotectic to the right of the main trend in Fig. 10 (see earlier discussion). It can also be noted that either of the

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explanations previously cited is consistent with the relations shown in Fig. 10. However, option (2)

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requires special circumstances and similar divergences from the main vertical trend at low-SiO2 are common to other Cascade volcanoes. The simplest explanation is that the divergent magmas are relatively unevolved and therefore not yet established on the cotectics of common phenocryst

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phases (plagioclase and pyroxenes).

As with the Sunda and Soufrière examples there are well-defined conditions of shallow magma

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storage established for some volcanoes (e.g. Rutherford and Devine, 2008). Most intermediate magmas were also multiply-saturated with low-pressure phenocryst phases prior to their eruption,

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whereas the most mafic examples have the compositional characteristics of mantle-derived melts. Thus, magma evolution spans a range of depths (from upper-crust to mantle) although the upper-

5.6 The Andes

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crust appears to be the principle location of intermediate magma evolution and production.

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The Andes volcanic arc is formed where continental crust of the South American Plate is underthrust by subducting oceanic crust of the adjacent Pacific Ocean (James, 1971a). Current volcanism is erupted through a considerable thickness of continental crust (> 70 km, James, 1971b). Thus it provides a counterpoint to the earlier described examples of young intra-oceanic arcs (Tonga and Vanuatu) where the arc crust is relatively thin. In Fig. 11 we have plotted data for two currently-active volcanoes from the Central Andes. These are Taapaca and Parinacota volcanoes (see Hora, 2007; Wӧrner et al., 1988; Clavero et al., 2004; and Mamani et al., 2010). The eruptive products from both are alkali-rich (5-8 wt. % Na2O + K2O) [Fig. 5] and of rhyolite to mafic andesite composition. Phenocryst phases include amphibole, biotite, plagioclase, K-feldspar, clinopyroxene and olivine (Wӧrner et al., 1988). Some of the younger and less porphyritic andesites are

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ACCEPTED MANUSCRIPT distinctively rich in olivine phenocrysts but plagioclase free. This accords well with the lowpressure (1 atm) phase equilibria of alkali-rich melts (Fig. 11). Indeed, apart from what can be attributed to their generally alkali-rich character, the trends of the Taapaca and Parinacota volcanics

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(Fig. 11) are not noticeably different from those of other arc magmas that were erupted through

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comparatively thin crust (Figs. 6-10).

5.7 The Aleutians

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The Aleutian Arc is formed where both oceanic and continental crust of the North American Plate are underthrust by subducting oceanic crust of the Pacific Plate (Vallier et al., 1994). We have

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chosen three volcanoes to represent the transition from oceanic to continental segments of the arc. Thus we also represent a transition from relatively thin (oceanic) crust to thicker (continental) crust.

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The volcanoes include Akutan (ocean margin), Aniakchak (continental shelf), and Katmai (continental mainland) [Hildreth and Fierstein, 2000; George et al., 2004; Turner et al., 2010]. The

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erupted products are of intermediate to high alkali content (Fig. 5) with Akutan and Aniachak being

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more alkali-rich than Katmai. Samples include rhyolites (Katmai only), dacites and andesites. Phenocrysts in the dacites and andesites include plagioclase, clinopyroxene, orthopyroxene, olivine

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(in some andesites only) and opaque oxides. In rhyolites they include quartz, plagioclase, orthopyroxene and opaque oxides. Previous experimental work by Hammer et al. (2002) on eruptive products from Katmai indicated pre-eruptive conditions of magma storage that were quite shallow (1-5 km). Consistent with this, plotted whole rock compositions for Katmai are conformable with 1 atm cotectics for melts of intermediate alkali content (Fig. 12). The Akutan and Aniakchak samples form a discrete group (Fig. 12) which can be attributed both to their higher alkali concentrations (Fig. 5) and the inclusion of more mafic compositions. There is nothing in the plotted relationships (Fig. 12) to indicate a positive correlation between crustal thickness and depth of magma fractionation.

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ACCEPTED MANUSCRIPT 5.8. The Whangarei Volcanics and Solander Island, New Zealand Two final two case studies are used to illustrate the range of the possibilities being debated. One is the Miocene garnet-bearing Whangarei Volcanics of Northland, New Zealand (Day et al., 1992;

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Green 1992; Booden et al., 2011; Bach et al., 2012). The other is Solander Island (Harrington and

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Wood, 1958; Bishop, 1986; Mortimer et al., 2008; Foley et al. 2013) a Quaternary adakite volcano from just south of New Zealand’s South Island. Both were erupted through continental crust. The Miocene Whangarei Volcanics document some of the earliest evidence for the subduction of the

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Pacific plate beneath northeastern New Zealand (Booden et al., 2011). They are noteworthy because of their garnet “phenocrysts” and garnet-bearing cumulate xenoliths (see Day et al., 1992). The

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latter are apparently cognate (in a broad sense) and of demonstrable mantle origin (see Day et al., 1992; Green et al., 1992; Bach et al., 2012). Although the Solander volcanics do not contain garnet,

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their acknowledged adakite-like character make them (almost by definition) a popular contender for a high-pressure origin (see Drummond and Defant, 1992).

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Both groups form diagonal trends within the ternary CaAl2O4-silica-forsterite system (Fig. 13)

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that parallel the slope of the garnet-pyroxene cotectic (Fig. 13) produced by amphibolite melting at 1.5 GPa (see Adam et al., 2012). Direct experimental evidence for one of the Whangarei samples

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(the Taipa Dacite) confirms that hydrous liquids of this composition (with 5 wt. % of dissolved H2O) can be equilibrated with garnet-websterite at 1.1 GPa (Green, 1992). This result is matched by data for heavy rare earth elements (HREE) that are also indicative of garnet fractionation. Lu decreases with increasing SiO2 in bulk-rocks (Fig. 14a) whereas Ho/Lu increases exponentially as Lu decreases (Fig. 14b). Thus the most silica-rich samples have pronounced HREE depletions. Although the Solander adakite-like rocks are not garnet-bearing they are HREE depleted and petrographically complex. Most are of andesite composition, but contain an array of complexlyzoned phenocrysts, anticrysts, mafic-enclaves, and cumulate xenoliths, at least some of which are of mantle character. The majority of phenocrysts are of low-pressure origin (0.1-0.4 GPa, Foley et al., 2013). On this basis, and the absence of steeply-inclined HREE patterns, Foley et al. (2013)

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ACCEPTED MANUSCRIPT proposed a relatively shallow origin for the Solander adakites by open-system crystal fractionation of more primitive, yet adakite-like parent melts. In this context, is worth noting that adakites of relatively recent origin (2.5. Ma) have been recovered from an extensional setting at the northern

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termination of the Tonga Trench (Falloon et al., 2008) where expected crustal thicknesses are

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insufficient for deep fractionation that is constrained by the crust/mantle boundary. Thus either the adakites evolved by relatively shallow fractionation, as proposed by Foley et al. (2013) for Solander, or they were produced at greater depths that were unconstrained by the crust/mantle

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boundary.

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6. Summary and implications

Most of the evidence reviewed in this paper is consistent with the pre-eruptive equilibration of

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felsic and intermediate arc magmas along cotectics that are controlled by low-pressure phenocryst phases (plagiocalse and pyroxenes ± olivine ± amphibole). Thus we found no correlation between

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depths of magma fractionation (as indicated by magma composition) and total crustal thickness, as

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would be expected if silicic arc magmas are generated at the crust/mantle interface. These conclusions have implications for the composition of primary arc crust, as well as conditions of

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magma storage, although to some degree they must be moderated to take account of the scale and complexity of arc-magma systems. The latter must transect the mantle to crust-surface transition and may be active for significant periods of time (kyr to Myr). This creates conditions that are inherently favourable to open-system, polybaric fractionation. Important aspects of open-system behaviour are magma mixing and continuous fractionation, although most of the magmas involved in mixing appear to be the result of fractionation along the same or similar cotectics. The possibility of cryptic high-pressure (> 0.6 GPa) fractionation is difficult to entirely exclude. However, it is not necessary for the explanation of felsic to intermediate arc-magma compositions and also requires the operation of an undisclosed mechanism for purging magmas of high-pressure crystal residues. In only one of the cases reviewed (Whangarei, New Zealand) are these phases unambiguously

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ACCEPTED MANUSCRIPT observed together with corroborating trace element evidence. Thus allowing for complexities and exceptions, our analysis continues to favour a shallow crustal origin for the majority of felsic and intermediate arc magmas.

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When combined with the brief timescales of arc-magma generation (a few 10 kyr) the upper-

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crustal origin we propose for most felsic and intermediate arc-magmas favours magma chambers that are both small and shallow (thereby promoting rapid heat-loss and freezing). Using the relationships illustrated on Fig. 15b, combined with the timescale inferences from Fig. 2, we

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suggest that the typical magma volumes stored in shallow crustal zones at any given time are on the order of only a few km3 (Fig. 15b). Nevertheless, magmatism is maintained by steady

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replenishments of more mafic magmas rising from depth. The dynamic aspect of this balance is inherently less stable and more amenable to change than the slow approach to thermal equilibrium

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intrinsic to models of lower-crustal heating by underplating magmas (e.g., Huppert and Sparks, 1988; Bergantz, 1999; Annen and Sparks, 2002). In this sense, it is also more consistent with the

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rapid and non-systematic variations in magma chemistry that can be observed for actual arc

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volcanoes. It is worth acknowleding, however, that reservoir volumes do vary and that this includes some very significant exceptions to the generalizations that we have made (e.g. super-volcanoes)

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although the latter are comparatively rare (see Mason et al., 2004). The most primitive arc magmas resemble high pressure partial-melts of peridotite and are of essentially mantle derivation. They are compositionally continuous with more evolved arc magmas and presumeably parental to them. However, the degree of crystal fractionation required to produce even the most primitive basaltic andesites is relatively high, approaching 40 % (Fig. 15a). Thus it remains a significant question as to what degree arc-magma evolution is either: (i) continuously distributed between the mantle and upper-crust, or (ii) concentrated at particular depths. The answer has important implication for the constitution and average composition of arc crust. As mentioned previously, most erupted arc-magmas are either basaltic andesites or andesites (with less

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ACCEPTED MANUSCRIPT common dacites). They have an average composition equivalent to a mafic andesite (with ~ 57.6 wt. % SiO2). Such a composition may be equated with the bulk arc-crust, but this can only be true if basaltic parent-magmas undergo significant fractionation prior to becoming part of the arc crust,

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as previously noted by Kushiro (1990).

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Prevailing P wave velocities > 6.5 km s-1 at depths greater than 8-10 km (> 0.2 GPa) indicate that the lower arc-crust is predominantly mafic and in parts possibly even ultramafic (Calvert, 2011). Consistent with this factor, the bulk-rock trends of arc volcanics show a linear continuity

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with the high-pressure extensions of plagioclase-pyroxene cotectics (Figs. 5-13). On this basis, the influence of the plagioclase-pyroxene cotectic on magma compositions may continue to depths of

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35-40 km (~ 1.0 GPa). There is already some suggestion of this in the greater depths of equilibration typically inferred by thermobarometry and experiments for magmas of increasingly

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mafic composition (Fig. 1b). It may be implied from this that fractionation occurs over a range of depths and produces increasingly felsic magmas as the surface is approached. In this case, much of

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the lower crust will be composed of gabbroic cumulates that are complementary to the most

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primitive of the intermediate igneous rocks that predominate within the uppermost crust. This does not presuppose that ponding of arc magmas at the crust/mantle interface, together with attendent

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processes (e.g., wall-rock assimilation and crystal fractionation), cannot occur. Additional mechanisms, such as delamination (in response to metamorphism), may also strip mafic material from beneath thickened layers of intermediate composition resulting in a generally more felsic composition for some segments of arc crust (see Calvert, 2011). But we conclude that in most cases substantive fractionation of arc-magmas (from basalt to rhyolite) commences at depths sufficiently shallow to ensure that primary arc-crust is of dominantly basaltic rather than intermediate composition. In counterpoint, the continuation of fractionation within the shallow crust ensures that most erupted arc-volcanics are of intermediate rather than mafic composition. Auxilary processes, including granite plutonism (a consequence of lower-crustal melting), sedimentation, and the tectonics of melange environments, may also selectively favour the inclusion of more felsic arc-

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ACCEPTED MANUSCRIPT components at shallow crustal depths. This may explain why the average composition of upper

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crust in the Japan Arc is dacitic rather than andesitic, for example (see Togashi et al., 2000).

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Acknowledgements

It is a pleasure to acknowledge the enjoyable and constructive conversations on this topic that have taken place over the years with many people but especially those with Jon Blundy and Steve

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Sparks and we hope they will forgive our attempt at flattery with the title! Heather Handley and

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Georg Zellmer are thanked for making available their compilations of data.

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primitive intra-oceanic Tonga Arc: petrogenesis of basaltic andesites at Tofua Volcano. Journal of Petrology 53, 1197-1230.

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Turner, S., Sandiford, M., Reagan, M., Hawkesworth, C., Hildreth, W., 2010. Origins of largevolume, compositionally zoned volcanic eruptions: new constraints from U-series isotopes and numerical thermal modelling for the 1912 Katmai-Novarupta eruption. Journal of Geophysical Research 115, B1201. Doi: 10.1029/2009JB007195 Turner, S., Caulfield, J., Rushmer, T., Turner, M., Cronin, S., Smith, I., Handley, H., 2012. Magma evolution in a primitive, intra-oceanic arc setting: rapid petrogenesis of dacites at Fonualei Volcano, Tonga. Journal of Petrology 53, 1231-1253. Turner, S., Foden, J., 2001. U, Th and Ra disequilibria, Sr, Nd and Pb isotope and trace element variations in Sunda arc lavas: predominance of a subducted sediment component. Contributions to Mineralogy and Petrology 142, 43-57. Turner, S., George, R., Jerram, D., Carpenter, N., and Hawkesworth, C., 2003b. Case studies of plagioclase growth and residence times in island arc lavas from Tonga and the Lesser Antilles, and a model to reconcile discordant age information. Earth and Planetary Science Letters, 214, 279-294. 37

ACCEPTED MANUSCRIPT Turner, S., Hawkesworth, C.J., Rogers, N., Bartlett, J., Worthington, T., Hergt, J., Pearce, J., Smith, I., 1997. 238U-230Th disequilibria, magma petrogenesis and flux rates beneath the depleted Tonga-Kermadec island arc. Geochimica et Cosmochimica acta 61, 4855-4884. Turner, S., Sandiford, M., Reagan, M., Hawkesworth, C., Hildreth, W., 2010. The origins of large

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volume, compositionally-zoned volcanic eruptions – new U-series isotope and numerical

Research 115, doi:10.1029/2009JB007195.

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thermal modeling constraints on the 1912 Katmai-Novarupta eruption. Journal of Geophysical

Utnasin, V.K., Abdurakhimov, A.I., Anasov, G.I., Balesta, S.T., Budyanskiy, Yu. A., Markhinin, Ye. K., Fedorenko, V.I., 1975. Deep structure of Klyuchevskoy group of volcanoes and problem of magmatic hearths. International Geology Review 17, 791-806.

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Vallier, T.L., Scholl, D.W., Fisher, M.A., Burns, T.R., Wilson, F.H., 1994. Geological framework of the Aleutian Arc, Alaska, in: Plafker, G., Berg, H.C. (eds) The Geology of Alaska. The

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Geological Society of America Inc., Boulder, Colorado pp. 367-388. Vukadinovic, D., Sutawidjaja, I., 1995. Geology, mineralogy and magma evolution of Gunung Slamet Volcano, Java, Indonesia. Journal of Southeast Asian Earth Sciences 11, 135-164.

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Walker, D., Shibata, T., DeLong, S.E. 1979. Abyssal tholeiites from the Oceangrapher Fracture Zone. Contributions to Mineralogy and Petrology 70, 111-125.

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Warner, R.D. 1973. Liquidus relations in the system CaO-MgO-SiO2-H2O. American Journal of Science 273, 925-946.

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Witham, F., Blundy, J., Kohn, S., Lesne, P., Dixon, J., Churakov, S.V., Botcharnikov, R., 2012. SolEx: a model for mixed COHSCI-volatile solubilities and exsolved gas compositions in

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basalt. Computer Geoscience. 45, 87-97. Yoder, H.S. Jr., 1965. Diopside-anorthite-water at five and ten kilobars and its bearing on explosive volcanism. Carnegie Institution of Wasgington Yearbook 64, 82-89. Zellmer, G.F., Hawkesworth, C.J., Sparks, R.S.J., Thomas, L.E., Harford, C.L., Brewer, T.S., Loughlin, S.C. 2003. Geochemical evolution of the Soufrière Hills volcano, Montserrat, Lesser Antilles Volcanic Arc. Journal of Petrology 44 (8), 1349-1374. Zellmer, G.F., Annen, C., Charlier, B.L.A., George, R.M.M., Turner, S.P., Hawkesworth, C.J., 2005. Magma evolution and ascent at volcanic arcs: constraining petrogenetic processes through rates and chronologies. Journal of Volcanology and Geothermal Research 140, 171191. Zellmer, G.F., Sakamto, N., Matsuda, N., Lizuka, Y., Moebis, A., 2016. On progress and rate of the peritectics reaction Fo + SiO2 → En in natural andesitic magmas. Geochimica et Cosmochimica Acta 185, 383-393.

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ACCEPTED MANUSCRIPT Fig. 1. Plots of (a) temperature and (b) SiO2 versus depth as inferred from published mineralmineral and mineral-liquid thermobarometry for various arc volcanoes [solid black circles] (see appendix for data sources). Black open squares on (a) are the results of experimental

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studies compiled in Table 1 of Blundy and Cashman (2008). Note that the peak density of the

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data correspond to a depth of ~ 6 km at which basalt containing 4 wt. % H2O will become vapor-saturated. The grey field on (c) encompases 1D P wave velocity profiles from a broad range of island arcs indicating that voluminous felsic crust only exists at depths shallower

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than ~ 8 km (modified from Calvert, 2011). For data sources see Appendix 1. Fig. 2. Plot of (226Ra/230Th) versus rock type for a global compilation of arc lavas showing that

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measurable 226Ra excesses are common across the full compositional spectrum of arc lavas implying that most have spent less than a few millennia transiting the arc crust. For data

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sources see Appendix 2.

Fig. 3. Liquidus relations for the CMAS (CaO-MgO-Al2O3-SiO2) [black lines] and HNCMAS

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(H2O-Na2O-CaO-MgO-Al2O3-SiO2) [dashed blue lines] systems projected within the

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CaAl2O4-silica-forsterite plane. Data from Kushiro (1974), Presnall et al. (1979), and Longhi et al. (1987). The CMAS relations are for a diopside-saturated liquidus surface.

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Fig. 4. Diopside-saturated liquidus relations for the alkali-free CMAS system at 1 atm [thin black lines] and for melts of intermediate alkali concentration (3-5 wt. %). The latter include: anhydrous melts from experiments on natural volcanic rocks at 1 atm [thick red lines] and 1.5 GPa [dashed black lines]; and hydrous melts (with ~5 wt. dissolved H2O) produced during experiments on natural volcanic rocks at 0.2 GPa [dashed blue lines]. Data are from Baker and Eggler (1987), Longhi et al. (1987), Moore and Carmichael (1998), Grove et al. (2003), and Feig et al. (2006). Fig. 5. Total alkali concentrations (Na2O + K2O) versus SiO2 in erupted lavas and tuffs from the volcanic arcs and arc volcanoes used as case studies in this report. Data sources are as for

39

ACCEPTED MANUSCRIPT Figs. 6-13. The line dividing alkaline from sub-alkaline compositions is from Irvine and Baragar (1971). Fig. 6. Compositional data for Tongan volcanic rocks projected within the CaAl2O4-silica-forsterite

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system, together with diopside-saturated liquidus relations for matching low-alkali melts.

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Data sources for liquidus relations include: Baker and Eggler (1987), Moore and Carmichael (1998), Draper and Johnston (1992), Grove et al. (2003). Data sources for Tongan volcanic rocks include: Ewart et al. (1977), Caulfield et al. (2008, 2012), Turner at al. (2012).

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Fig. 7. Compositional data for whole rocks and glasses from Yasur Caldera (Vanuatu) projected within the CaAl2O4-silica-forsterite system, together with diopside-saturated liquidus relations

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for matching high-alkali melts. Liquidus data are from Baker and Eggler (1987), and Draper and Johnston (1992). Note that at 1.5 GPa the plagioclase liquidus field is co-saturated with

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spinel. Data for bulk-rocks and glasses are from Firth et al. (2014). Fig. 8. Compositional data for Sunda Arc volcanic rocks projected within the CaAl2O4-silica-

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forsterite system, together with diopside-saturated liquidus relations for both the CMAS

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system and melts of natural (complex) composition. Note that at 1.5 GPa the plagioclase liquidus field is co-saturated with spinel. The CMAS data are from Longhi (1987). Data for

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intermediate- and high-alkali liquids are from Baker and Eggler (1987), Draper and Johnston (1992), Moore and Carmichael (1998), and Feig et al. (2006). Data for Sunda Arc rocks are from Camus et al. (1987), Edwards (1990), Edwards et al. (1993), Vukandinovic and Sutawidjaja (1995), Turner and Foden (2001), Gertisser and Keller (1993), and Handley et al. (2014). Fig. 9. Compositional data for volcanic rocks from the Soufrière Hills volcano of Monserrat (Lesser Antilles Arc) projected within the CaAl2O4-silica-forsterite system, together with diopsidesaturated liquidus relations for both the CMAS system and melts of natural (complex) composition. Note that at 1.5 GPa the plagioclase liquidus field is co-saturated with spinel. Data sources for the liquidus relations are as for Fig. 8 but include an experimentally

40

ACCEPTED MANUSCRIPT reproduced Soufrière groundmass (melt) composition from Rutherford and Devine (2003). Whole-rock data are from Zellmer et al. (2003), Baker (1984), Davidson (1987), Devine et al. (1998), Murphy et al. (1998; 2000), Murphy and Sparks (1999), and Harford (2000).

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Fig. 10. Compositional data for volcanic rocks from the Cascade Arc of western North America

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projected within the CaAl2O4-silica-forsterite system, together with together with diopsidesaturated liquidus relations for both the CMAS system and melts of natural (complex) composition. Note that at 1.5 GPa the plagioclase liquidus field is co-saturated with spinel.

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References for experimental liquidus data are as for Fig. 8. Whole rock data are from Green (1981), Condie and Swenson (1973), Mullineaux (1974), Venezky and Rutherford (1997),

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Leeman et al. (1990), Halliday et al. (1983), Criswell (1987), Pallister et al. (1992), Smith and Leeman (1993), Gardner et al. (1995), Leeman et al. (1990), Bacon et al. (1997), Hildreth and

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Fierstein (1997), Wise (1969), Cribb and Barton (997), and Baker et al. (1994). Fig. 11. Compositional data for Taapaca and Parinacota Volcanoes of the central Andes (South

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America) projected within the CaAl2O4-silica-forsterite system, together with diopside-

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saturated liquidus relations for the CMAS system and melts of natural (complex) composition. References for the liquidus data are as for Fig. 8. Whole rock data for Taapaca

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and Parinacota are from Hora (2007), Wӧrner et al. (1988), Clavero et al. (2004), and Mamani et al. (2010).

Fig. 12. Compositional data for Akutan, Aniakchak and Katmai volcanoes, together with diopsidesaturated liquidus relations for melts of intermediate alkali content. Note that at 1.5 GPa the plagioclase liquidus field is co-saturated with spinel. The liquidus relations are based on data from Longhi (1987), and Baker and Eggler (1987). Whole rock data are from George et al. (2004) and Turner et al. (2010). Fig. 13. Compositional data for the Whangarei Volcanics and Solander Island Volcano of New Zealand projected within the CaAl2O4-silica-forsterite system, together with both low and high pressure liquidus relations for melts of high-alkali content. Data for 1 atm liquidus

41

ACCEPTED MANUSCRIPT relations are from Baker and Eggler (1987); those for melts in equilibrium with plagioclase (+spinel), orthopyroxene and olivine at 1.5 GPa are from Draper and Johnson (1992). The 1.5 GPa garnet-clinopyroxene cotectic is from Adam et al. (2012). Bulk-rock data are from

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Booden et al. (2011) and Foley et al. (2013).

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Fig. 14. Plots of Lu versus SiO2 (a) and Ho/Lu versus Lu (b) in volcanic rocks from Whangarei (Northland, New Zealand) and Solander Island (New Zealand). Data are from Booden et al. (2011) and Foley et al. (2013).

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Fig. 15. (a) Plot of SiO2 versus extent of fractionation obtained using MELTS (Ghiorso and Sack, 1995) for a typical arc basalt. Grey field shows that the average silica content of the lavas

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investigated in this study requires ~ 50 % fractional crystallisation of this basaltic starting composition. (b) Plot of wall rock temperature versus volume of magma contoured for the

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amount of time (in kyr) required to crystallise 50% due to cooling. Grey field shows the intersection of the wall rock temperatures anticipated to occur at 6 km depth (cf. Fig. 1) and a

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timescale for crystallisation that would permit residual 226Ra excesses to be preserved (cf. Fig.

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2). The implication is that the typical magma volumes stored in shallow crustal zones at any

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given time is on the order of a few km3. See Turner et al. (2010) for details of the modelling.

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ACCEPTED MANUSCRIPT Appendix 1. References for Fig. 1.

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Alvarado, G.E., Carr, M.J., Turrin, B.D., Swisher, C.C., Schmincke, H.U. and , K.W., 2006. Recent volcanic history of Irazú volcano, Costa Rica: Alternation and mixing of two magma batches, and pervasive mixing. Geological Society of America Special Papers, 412, 259-276.

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Wise, W.S., 1969. Geology and petrology of the Mt. Hood area: a study of high Cascade volcanism. Bulletin of the Geological Society of America 80 (6), 969-1006. Witter, J.B., Kress, V.C. and Newhall, C.G., 2005. Volcán Popocatépetl, Mexico.

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Petrology, magma mixing, and immediate sources of volatiles for the 1994–present

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eruption. Journal of Petrology, 46(11), 2337-2366.

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Appendix 2. References for Fig. 2.

Caulfield, J.T., Turner, S.P., Smith, I.E.M., Cooper, L.B. and Jenner, G.A., 2012. Magma evolution in the primitive, intra-oceanic Tonga arc: petrogenesis of basaltic andesites at Tofua volcano. Journal of Petrology, 53,1197-1230. Cunningham, H.S., Turner, S.P., Dosseto, A., Patia, H., Eggins, S.M., and Arculus, R.J., 2009. Temoral variations in U-series disequilibria in an active caldera, Rabaul, Papua New Guinea. Journal of Petrology 50, 507-529. Cunningham, H., Gill, J. Turner, S., Edwards, L., and Day, S., 2012. Rapid magmatic processes accompany arc-continent collision: the westerns Bismarck arc, Papua New Guinea. Contributions to Mineralogy and Petrology, 164, 789-804. Garrison, J., Davidson, J., Reid, M., and Turner, S., 2006. Source versus differentiation controls on U-series disequilibria: insights from Cotopaxi volcano, Ecuador. Earth and Planetary Science Letters 244, 548-565. George, R., Turner, S., Hawkesworth, C., Morris, J., Nye, C., Ryan, J., and Zheng, 48

ACCEPTED MANUSCRIPT S.-H., 2003. Melting processes and fluid and sediment transport rates along the AlaskaAleutian arc from an integrated U-Th-Ra-Be isotope study. Journal of Geophysical Research, 108 (B5), 2252, doi:10.1029/2002JB001916. George, R., Turner, S., Hawkesworth, C., Nye, C., Bacon, C., Stelling, P., Dreher, S.,

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2004. Chemical versus temporal controls on the evolution of tholeiitic and calc-alkaline magmas at two volcanoes in the Aleutian arc. J. Petrol. 45, 203-219.

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Price, R.C., George, R., Gamble, J.A., Turner, S., Smith, I.E.M., Cook, C., Hobden, B., and Dosseto, A., 2007. U-Th-Ra fractionation during crustal-level andesite formation at Ruapehu volcano, New Zealand. Chemical Geology 244, 437-451. Price, R.C., Turner, S., Cook, C., Hobden, B., Smith, I.E.M., Gamble, J.A., Handley,

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H., Mass, R., and Mobis, A., 2010. Crustal influences on U-Th disequilibrium at Ngauruhoe and Ruapehu volcanoes, New Zealand. Chemical Geology, 277, 355-373.

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Reagan, M.K., Morris, J.D., Herrstrom, E.A., and Murrell, M.T., 1994. Uranium series and beryllium isotope evidence for an extended history of subduction modification of the mantle below Nicaragua. Geochimica et Cosmochimica Acta, 58, 4199-4212. 226

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Sigmarsson, O., Chmeleff, J., Morris, J., and Lopez-Escobar, L., 2002. Origin of Ra-230Th disequilibria in arc lavas from southern Chile and magma transfer time. Earth

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and Planetary Science Letters, 196, 189-196. Turner, S., Evans, P., and Hawkesworth, C., 2001. Ultra-fast source-to-surface

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movement of melt at island arcs from 226Ra-230Th systematics. Science 292, 1363-1366. Turner, S., Sims, K., Reagan, M., and Cook, C., 2007. A 210Pb-226Ra-230Th-238U study

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of Klyuchevskoy and Bezymianny volcanoes, Kamchatka. Geochimica et Cosmochimica Acta 71, 4771-4785. Turner, S., Sandiford, M., Reagan, M., Hawkesworth, C., Hildreth, W., 2010. The origins of large volume, compositionally-zoned volcanic eruptions – new U-series isotope and numerical thermal modeling constraints on the 1912 Katmai-Novarupta eruption. J. Geophys. Res. 115, doi:10.1029/2009JB007195. Turner, S., Caulfield, J., Rushmer, T., Turner, M., Cronin, S., Smith, I. and Handley, H., 2012. Magma evolution in the primitive, intra-oceanic Tonga arc: rapid petrogenesis of dacites at Fonualei volcano. Journal of Petrology 53, 1231-1253.

Vezezky, D.Y., Rutherford, M.J., 1997. Pre-eruptive conditions and timing of daciteandesite magma mixing in the 2.2 ka eruption at Mount Rainier. Journal of Geophysical Research 102 B9, 20069-20086. Doi: 10.1029/97JB1590 Volpe, A.M., and Hammond, P.E., 1991. 238U-230Th-226Ra disequilibrium in young Mt. 49

ACCEPTED MANUSCRIPT St. Helens rocks: time constraint for magma formation and crystallization. Earth and Planetary Science Letters,107. 475-486. Volpe, A.M., 1992. 238U-230Th-226Ra disequilibrium in young Mt. Shasta andesites and dacites. Journal of Volcanology and Geothermal Research, 53, 227-238.

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Yokoyama, T., Kobayashi, K., Kuritani, T., Nakamura, E., 2002. Mantle

metasomatism and rapid ascent of slab components beneath island arcs: evidence from 238UTh-226Ra disequilibria of Miyakejima volcano, Izu arc, Japan. Journal of Geophysical

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Most silicic arc magmas form in the shallow crust

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The average composition of arc crust is not silicic

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Sub volcanic magma bodies do not exceed a few km cubed in volume

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