Earth and Planetary Science Letters 371–372 (2013) 177–190

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Earth and Planetary Science Letters

0012-82http://d

n CorrE-m

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The composition of the foundered complement to the continental crustand a re-evaluation of fluxes in arcs

O. Jagoutz a,n, M.W. Schmidt b

a Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, USAb Institute of Geochemistry and Petrology, ETH, 8092 Zurich, Switzerland

a r t i c l e i n f o

Article history:Received 18 June 2012Received in revised form26 March 2013Accepted 31 March 2013

Editor: T. Elliot

opportunity to constrain the composition and volume of material fluxes involved in this process. In a

Available online 20 May 2013

Keywords:primitive arc meltsarc crustcontinental crustdelaminatearc fluxes

1X/$ - see front matter Published by Elsevierx.doi.org/10.1016/j.epsl.2013.03.051

esponding author.ail address: jagoutz@mit.edu (O. Jagoutz).

a b s t r a c t

Most primitive arc melts are basaltic in composition, yet the bulk continental crust, thought to begenerated in arcs, is andesitic. In order to produce an andesitic crust from primitive arc basalts, rockscomplementary to the andesitic crust have to be fractionated and subsequently removed, most likelythrough density sorting in the lower arc crust. The Kohistan Arc in northern Pakistan offers a unique

lower crustal section 410 km cumulates (dunites, wehrlites, websterites, clinopyroxene-bearinggarnetites and hornblendites, and garnet-gabbros) are exposed that are 0.1–0.3 g/cm3 denser than theunderlying mantle. The cumulates combine with the andesitic bulk Kohistan Arc crust to reproduce themajor and trace element composition of primitive basaltic arc melts. Our petrochemical analysis suggeststhat fractionation and subsequent foundering of wehrlites+ultramafic hornblende–garnet–clinopyroxenecumulates+garnet-gabbros is a viable mechanism for producing andesitic crust from a calc-alkaline/tholeiitic primitive high-Mg basalt. The mass of the foundered material is approximately twice that of thearc crust generated. For an overall andesitic arc composition, we estimate a magma flux into the arc (11–15 km3/yr) about three times the rate of arc crust production itself. Foundering fluxes of cumulates (6.4–8.1 km3/yr) are one third to half those of the globally subducted oceanic crust (�19 km3/yr). Hence, thedelaminate forms a volumetrically significant, albeit refractory and depleted geochemical reservoir in themantle. Owing to its low U/Pb and high Lu/Hf the foundered material evolves with time to a reservoircharacterized by unradiogenic Pb and highly radiogenic Hf isotopes, unlike any of the common mantleendmembers defined by OIB chemistry. The unradiogenic Pb of the foundered arc cumulates couldcounterbalance the radiogenic Pb composition of the depleted mantle. The predicted highly radiogenicHf (at rather unradiogenic Nd) of the foundered material can explain the εHf–εNd systematics observedin some abyssal peridotites and mantle xenoliths.

Published by Elsevier B.V.

1. Introduction

Even though most primitive parental melts are of basalticcomposition, in mature arcs, the average composition of theintruded and erupted magmas is close to the andesitic averagecontinental crust (Arculus, 1981; Taylor and McLennan, 1995, Jagoutzand Schmidt, 2012). This is best illustrated by SiO2 concentrations,XMg values, and total alkali contents, which are 47–52 wt%, 0.68–0.72, and 2–3 wt%, respectively, for most primitive arc basalts, and55–58 wt%, 0.50–0.53, and 4–5 wt% for average arc or continentalcrust (Rudnick and Gao, 2003; Jagoutz and Schmidt, 2012). Mostlikely, this difference can be explained by removal of dense,gravitationally unstable cumulates at the base of the arc crust. Suchcumulates may form during arc magma differentiation and possiblydensify during subsequent metamorphic re-equilibration at high

B.V.

pressures (Kay and Kay, 1993; Jull and Kelemen, 2001). Other meansof fractionation such as foundering of a granulitic or eclogiticresiduum of partial melting in the lower crust have also beenproposed (Kay and Kay, 1993; Tatsumi et al., 2008).

An unfortunate but natural consequence of this density sortingprocess is that the rocks that would complement the andesiticcrust to a primitive arc melt are rarely preserved. Accordingly,neither the chemical composition nor the magnitude of the returnflux has been constrained. As a consequence, the magnitude of thetotal arc magma flux – that is, the sum of the mostly preserved arccrust and the foundered material disappearing into the uppermantle – remains unknown. This lack of quantitative constraintson composition and mass fluxes severely limits our understandingof the role of subduction zone vs. ridge magmatism in planetarydifferentiation processes.

Seismic reflection studies typically locate the base of the arccrust at 25–35 km in island arcs and 40–70 km in continental arcs(e.g. Kodaira et al., 2007; Yuan et al., 2002), but wave speedestimates do not provide unique constraints on the composition of

Fig. 1. (a) Section of the complete Kohistan arc with (b) calculated average densities of the units. The white field illustrates calculated density of the upper mantle at ~ 60 kmdepth. (c) SiO2 vs. XMg of the lower Kohistan arc crust cumulates and gabbros. Note that the suite of hornblende–garnet–clinopyroxene dominated cumulates (including thosegarnet-gabbros which are strongly cumulative) have a negative SiO2–XMg correlation which excludes their derivation through partial melting of a precursor with basalticcomposition. (d) (Dy/Yb)N vs. (Eu/Eu*)N (Eu*N¼0.5� (SmN+GdN)) of the lower Kohistan arc crust cumulates and gabbros illustrating the effect of accumulation of hornblende,garnet and plagioclase. Green rim-less diamonds: dunites and wehrlites, green diamonds with black rims: websterites, olive-green diamonds with rim: hornblende–garnet–clinopyroxene cumulates, pink dots: garnet-gabbros, blue dots: the density stable (Sarangar-) gabbro overlying the density unstable lithologies, stars: primitive arc volcanicsrocks. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

O. Jagoutz, M.W. Schmidt / Earth and Planetary Science Letters 371–372 (2013) 177–190178

the lower crust. Information about deep arc cumulates can beinferred from xenoliths in arc magmas (Arculus and Wills, 1980;Ducea, 2002; Saleeby et al., 2003), but these do not provide acomplete sampling of the crust. Lower arc crust processes can bestudied directly in deep arc sections, but exposures of suchsections are rare. The Kohistan Arc (N Pakistan) is the only(known) section of preserved island arc crust that is complete,(from harzburgite at the base to volcanic ash layers at the top),fully exposed and accessible (Tahirkheli, 1979; Jan and Howie,1981; Bard, 1983, for review see Burg, 2011). It includes a thicksection of density-unstable ultramafic/mafic rocks at its base(Jagoutz et al., 2011).

The main objectives of this study are (i) to establish whether ornot these ultramafic/mafic cumulates in Kohistan represent materialthat is extracted from primitive melts (so as to form andesitic crust),(ii) to evaluate whether this material would be gravitationallyunstable and, thus, be likely to founder, and (iii) to quantify magmafluxes in subduction zones (that is, magma generation and founder-ing into the mantle upon crust formation in island arcs). For thispurpose, we compiled primitive arc magma compositions of islandarcs using the GEOROC database (http://georoc.mpch-mainz.gwdg.de/georoc/) and mass balanced these compositions with differentcombinations of density unstable ultramafic/mafic cumulates andeither the Kohistan bulk arc crust or the average continental crustcomposition (Rudnick and Gao, 2003; Jagoutz and Schmidt, 2012).The excellent model fits for calc-alkaline/tholeiitic primitive basaltsdemonstrate that removal of dense cumulates can produce average

continental crust and suggest that the magma and foundering fluxesare much larger than assumed before.

An almost dialectic complexity arises from the following: isolat-ing the effects of arc magma evolution and arc crust differentiationfrom interactions of arc magmas with preexisting continental crustis only possible in island arcs. Yet, many modern island arcs are (still)in a juvenile state, which is characterized by erupting dominantlybasaltic material and by a thin arc crust with relatively smallvolumes of intermediate (andesitic) composition (Calvert, 2011).Andesitic crust formation is only incipient in such arcs, while thefossil Kohistan island arc constitutes a mature island arc with a fullydeveloped 420 km thick batholith resulting in a bulk crust compo-sition that is andesitic (Jagoutz and Schmidt, 2012). Large volumes offelsic upper crust are observed in e.g., Izu Bonin (Kodaira et al., 2007)but are absent in other arcs (e.g., Aleutians, Calvert, 2011) suggestingthat the processes that we describe here may not (yet) be fullyapplicable to juvenile island arcs.

2. The lower arc crust

2.1. Gravitational instability of lower arc crust cumulates

A prerequisite for gravitational foundering is a mass density inexcess of that of the sub-arc mantle (harzburgite: �3.2–3.3 g/cm3).In the Kohistan Arc, dense lower crustal ultramafic and maficcumulates are exposed in the Jijal section (Jan and Howie, 1981;

Table 1Averaged cumulate and bulk arc and continental crust compositions.

Kohistan cumulates and garnet-gabbro Bulk crust

dunite wehrlite webster. hbl-gar-cum

gar-gabbro

Koh.arcc

Cont.d

n 6 4 7 7 45SiO2 (wt%)a

41.4 49.5 52.9 42.6 48.6 59.3 60.6

TiO2 0.03 0.04 0.11 1.05 0.84 0.68 0.72Al2O3 0.59 1.22 2.32 16.6 19.0 17.0 15.9FeOT 11.3 7.61 6.20 13.8 11.3 6.26 6.71MnO 0.15 0.14 0.13 0.24 0.22 0.13 0.10MgO 45.7 31.7 21.1 11.7 6.66 4.32 4.66CaO 0.67 9.66 17.1 12.4 11.3 7.21 6.41Na2O 0.10 0.16 0.08 1.36 1.88 3.42 3.07K2O 0.13 0.12 1.52 1.81P2O5 0.02 0.08 0.19 0.13

XMgb 0.878 0.881 0.859 0.602 0.512 0.552 0.553

Cr (ppm) 4370 3280 3270 131 52.0 115 135Ni 1530 997 242 61.2 24.8 50.8 59.0V 38.8 58.0 173 494 329 146 138

Rb 0.039 0.069 0.101 0.434 0.571 41.2 49Sr 0.591 6.04 8.86 104 235 399 320Y 0.043 0.494 2.54 19.6 15.4 17.4 19Zr 0.026 0.116 0.514 6.49 11.1 113 132Nb 0.028 0.030 0.02 0.183 0.424 5.41 8Ba 0.602 1.56 3.09 16.5 49.3 288 456La 0.001 0.029 0.05 0.341 1.75 15.2 20Ce 0.008 0.069 0.16 1.51 4.32 30.2 43Pr 0.001 0.014 0.05 0.363 0.659 3.64 4.9Nd 0.008 0.085 0.33 2.69 3.86 14.0 20Sm 0.008 0.042 0.20 1.27 1.36 3.14 3.9Eu 0.003 0.020 0.10 0.672 0.718 0.930 1.1Gd 0.009 0.074 0.37 2.29 1.99 3.24 3.7Tb 0.001 0.014 0.07 0.446 0.377 0.432 0.6Dy 0.011 0.108 0.55 3.51 2.52 3.13 3.6Ho 0.003 0.024 0.12 0.795 0.581 0.671 0.77Er 0.009 0.068 0.33 2.26 1.61 1.83 2.1Tm 0.002 0.011 0.05 0.210 0.250 0.272 0.28Yb 0.012 0.068 0.29 2.10 1.62 1.80 1.9Lu 0.003 0.012 0.05 0.346 0.252 0.278 0.3Hf 0.004 0.008 0.035 0.308 0.303 2.49 3.7Ta 0.001 0.002 0.010 0.021 0.037 0.342 0.7Pb 0.071 0.245 0.125 0.168 0.814 7.13 11Th 0.002 0.005 0.004 0.004 0.022 4.00 5.6U 0.002 0.005 0.003 0.003 0.017 1.11 1.3

Rb/Sr 0.066 0.011 0.011 0.004 0.002 0.103 0.153Sm/Nd 1.00 0.49 0.59 0.47 0.35 0.22 0.20U/Pb 0.028 0.020 0.024 0.018 0.021 0.156 0.118Th/Pb 0.028 0.020 0.032 0.024 0.027 0.561 0.509Lu/Hf 3.00 6.00 4.60 16.5 6.81 0.81 0.43Mineral modeolivin (wt%)

490 30-60 5-10 – –

opx 0-5 5-15 5-20 o3 o3cpx 0-5 30-50 60-85 5-20 5-30garnet – – – 10-80 15-55hbl – – – 10-80 0-30plag – – – o5 5-50epidote – – – o5 o10rutile – – – 1-3 1-3quartz – – – – o10

a Major elements are normalized to 100% total.b Molar Mg/(Mg+Fetot).c Koh.arc: bulk Kohistan arc crust composition model m#1 from Jagoutz and

Schmidt (2012).d Cont.: average continental crust composition from Rudnick and Gao (2003).

O. Jagoutz, M.W. Schmidt / Earth and Planetary Science Letters 371–372 (2013) 177–190 179

Miller et al., 1991; Jagoutz et al., 2011). Calculated and measureddensities range from 3.32 g/cm3 for wehrlites to 3.50 g/cm3 forhornblende–garnet–clinopyroxene cumulates and garnet-gabbros(Fig. 1b, Miller and Christensen, 1994; Kono et al., 2009; Jagoutzet al., 2011). The Kohistan garnet-gabbro is −0.1 to −0.3 g/cm3 lessdense than the average mantle when (re-)crystallizing at o1 GPaoutside the garnet stability field, and +0.1 to +0.3 g/cm3 more densewhen (re-)crystallizing garnet at higher pressures (Jagoutz et al.,2011). Subsequently, we describe the field relationships and geo-chemistry of the density unstable rocks. For a more detaileddescription we refer to Burg et al. (2005) and Jagoutz et al. (2011).

2.2. The Kohistan lower crust

Structurally, above minor serpentinite associated with the Indussuture, a 3.5 km thick series of lithologically variable ultramaficcumulates comprising dunites, wehrlites, olivine–clinopyroxenites,and websterites (Fig. 1a, Table 1), locally with tiny amounts ofgarnet and Cr-diopside-rich dykes are preserved. Amphibolebecomes increasingly abundant up section and pyroxene-richultramafics grade into modally highly variable ultramafic hornble-nde–garnet–clinopyroxene cumulates which contain almost purehornblendites and garnetites. The uppermost �1–2 km of theultramafic cumulates are mostly these hornblende- and garnet-dominated cumulates. Due to the compositional heterogeneity, wehave used the average of these ultramafic hornblende–garnet–clinopyroxene cumulates for our mass balance calculations. Theseultramafic magmatic cumulates have a sharp originally intrusivecontact with the 4 km thick granulite-facies garnet-gabbros. Garnetaverages 20–30 vol% in this partly cumulative gabbros (Fig. 1, seealso Jagoutz, 2010), but modal variations range from garnet-freegabbro to garnetite and lenses of garnet-hornblendite up to 100 mthick (Burg et al., 2005) that become less abundant up section. Thegarnet-gabbro also contains plagioclase, clinopyroxene, amphibole,zoisite, and oxides. The granulitic lower arc crust is overlain by the(density stable) Sarangar gabbro, which contains o1% garnet, andby various complex amphibolite units composed of many mostlygabbroic to dioritic intrusions and of amphibolites (Fig. 1a, see alsoBurg et al., 2005 and Jagoutz et al., 2011).

Of relevance is the position of the Moho, the presumed crust–mantle boundary. Miller and Christensen (1994) originally placedthe sub-arc Moho below the ultramafic hornblende and garnetdominated cumulates, but Burg et al. (2005) and Kono et al. (2009)consider the Moho to be defined by the first massive appearanceof plagioclase in the garnet-gabbros. This contact would alsocorrespond to a pronounced seismic discontinuity, with VP

increasing from �7.4 to 7.9 km/s across the transition from gabbroto ultramafic cumulates (at 1 GPa, Miller and Christensen, 1994;Kono et al., 2009).

2.3. Chemistry and origin of the density unstable lower arc crust

The dense lower arc crust cumulates are characterized by highto moderate XMg (0.881−0.512, Fig. 1c, Table 1), moderate to verylow SiO2 (55−39 wt%) and high Al2O3 for the hornblende- andgarnet-rich cumulates and the garnet-gabbros (up to 22 wt%)(Garrido et al., 2006; Dhuime et al., 2009; Jagoutz et al., 2011).The olivine–pyroxene dominated cumulates (dunites, wehrlites,websterites and pyroxenites) owe their high density to a high Fecontent compared to upper mantle peridotite, while the hornble-nde–garnet–clinopyroxene cumulates and the garnet-gabbros aredense due to abundant garnet.

There is substantial debate, whether the garnet in the garnet-gabbros is generally magmatic (Ringuette et al., 1999) or formedduring granulitization of the lower arc crust (Yamamoto andYoshino, 1998; Garrido et al., 2006, 2007) or both (Jagoutz et al.,

2009, 2011). The origin of garnet in the garnet-gabbros is of littleconsequence to their mass density and the delamination process,but the amount of magmatically accumulated garnet in the garnet-

O. Jagoutz, M.W. Schmidt / Earth and Planetary Science Letters 371–372 (2013) 177–190180

gabbro/granulite unit determines the amplitude of the geochem-ical garnet-signature. The modal variability from o20% to 490%garnet in the ultramafic hornblende–garnet–clinopyroxene cumu-lates together with their trace element signature demonstrates amagmatic cumulative origin of garnet in these cumulates (Figs. 1dand 2b, available analyses are compiled by Jagoutz and Schmidt,2012). Along with a high Al2O3 content (up to 22 wt%), the low(Dy/Yb)N of 0.7–1.0 of the garnet-gabbro compared to the highaverage (Dy/Yb)N of primitive arc melts (�1.2) and the positiveEu/Eu* (up to �2) strongly indicate a magmatic accumulation ofplagioclase and garnet. Hence magmatically accumulated garnetand garnet formed during granulite facies reactions are bothcommon in the Kohistan lower arc crust.

2.4. Seismic evidence for the presence of similar rocks in active islandarc roots

Many seismic studies have revealed that the crust–mantletransition in island arcs is more complex than a simple step-wise change from typical crust- to mantle-like velocities (Calvert,2011). Jacob and Hameda (1972) detected a 20–40 km thickgradual crust–mantle transition in the Aleutian arc (with P-wavevelocities of �7.6 km/s), which was confirmed by later refraction(Shillington et al., 2004) and reflection surveys (Calvert andMcGeary, 2013). Transitional P-wave speeds of 7.6–8.0 km/s at 20to �35 km have also been identified in the Mariana Arc (Takahashiet al., 2008) and a �5 km thick zone between the base of the arccrust and the mantle has been ascribed to ultramafic cumulates inthe Izu-Bonin arc (Kodaira et al., 2007).

At the appropriate depths, P-wave speeds of 7.4–8.0 km/scharacterize garnet-gabbros, (garnet-) pyroxenites, and hornblen-dites (Miller and Christensen, 1994; Behn and Kelemen, 2006;Kono et al., 2009; Calvert, 2011). As garnet-rich cumulates reach P-wave speeds of 8.3 km/s at lower crustal conditions (Kono et al.,2009), mixtures of garnet-gabbros and ultramafic cumulates mayreach ≥8.0 km/s, making them difficult to distinguish from typicalmantle velocities. Sub-arc velocities of 7.5–8.0 km/s could repre-sent garnet-gabbros and ultramafic cumulates, or a mixture ofmantle and mafic to ultramafic cumulates. Velocities just below8.0 km/s could also represent partially serpentinized or melt-bearing mantle, but serpentinization is inconsistent with thetemperatures expected for a magma-rendering sub-arc mantle(see also Calvert (2011)). Similarly, a continuous melt-bearingmantle layer along and across the arc is implausible. In summary,there is ample seismic evidence for the presence of lower crustalcumulates similar to those observed in Kohistan also in active arcs.

3. The Kohistan average arc crust

Jagoutz and Schmidt (2012) calculated the average Kohistan arccomposition from 560 bulk analyses. The bulk composition of thepresent day bulk Kohistan arc section was obtained from (i) amostly mafic lower arc crust exposed in the Southern PlutonicComplex (17%) which still contains several kilometrs of garnet-gabbro, (ii) the lower- to mid-crust intrusive Chilas Complex (20%)which is dominated by a gabbro–norite, (iii) the mid- to uppercrust Kohistan batholith composed of gabbroic to granitic intru-sions but dominated by quartz-, monzo-, and granodiorites and(iv) the overlying 4 km of volcanics (together with (iii) 63%).Depending on the model of integration, the average Kohistan arccomposition has 56.5–59.6 wt% SiO2, 1.2–1.7 wt% K2O, 3.1–3.5 wt%Na2O, and XMg¼0.51–0.545 (Table 1, eTable). The major and traceelements match the average bulk continental crust (Fig. 2c) ofRudnick and Gao (2003) suggesting that the mature Kohistan Arc

is representative for crust formation processes in modern subduc-tion settings.

4. Primitive arc melts

The composition of the dominant primitive melt(s) from whichthe arc crust and, by implication, the continental crust is derived isdebated: picritic, high-Mg, or high-Al basalts, and high-Mg ande-sites have been championed (Arculus and Johnson, 1978; Kay et al.,1982; Kushiro, 1987; Gust and Perfit, 1987; Grove et al., 2002;Kelemen et al., 2003). We compiled primitive melt compositionsfor active island arcs with relative simple tectonic settings usingthe GEOROC database. We supplemented this compilation withprimitive basalts from the incipient Palau arc (Hawkins andIshizuka, 2009) and from the Kohistan paleo arc itself (Jagoutz,2010). We consider volcanic bulk rock compositions withXMg¼0.65–0.74, Ni¼150–500 ppm and Cr¼300–1100 ppm as(near) primitive and used Si, Ti, P and alkalis to distinguish calc-alkaline/tholeiitic basalts from andesites (Table 1) and from alkalibasalts (eTable). We then averaged primitive volcanic rocks of agiven type for each arc.

The dominant primitive magma compositions are calc-alkaline/tholeiitic basalts, which were compiled for the Aleutian, Bismarck,Izu-Bonin, Kurile, Marianas, Tonga–Kermadec, Vanuatu, and Yaparcs (Fig. 2a, Table 2). These average primitive melts have SiO2

contents of 49.3–51.5 wt% (on a volatile free basis), TiO2 of 0.6–1.0 wt%, XMg

's of 0.684–0.718, and total alkali contents of 1.9–4.2 wt%. These averages fall into the calc-alkaline field of Miyashiro(1974) in FeO/MgO vs. SiO2 space, but some individual analysesplot into the tholeiitic field. The composition of the averageprimitive melt from the Lesser Antilles (dominated by samplesfrom Grenada) has a distinctly lower SiO2 content (47.6 wt%).

Albeit significantly less abundant, alkaline basaltic and high-Mgandesitic primitive magmas are also present in the Aleutians, Izu-Bonin, Marianas, and Tonga. Primitive high-Mg andesites haveaverage SiO2 contents of 54.7–57.1 wt%, a lower TiO2 of 0.2–0.6 wt%, a variable Na2O+K2O of 1.8–4.2 wt% and distinctly lower FeOand MgO concentrations than the calc-alkaline/tholeiitic primitivemelts of the same arc (Table 2). Nevertheless, the high-Mgandesite are inappropriate as parental melts to the cumulatesobserved in Kohistan. Primitive high-Mg andesites would crystal-lize only olivine and orthopyroxene but not clinopyroxene nearthe liquidus (Tatsumi, 1981, 1982; Umino and Kushiro, 1989).Secondly, clinopyroxene, hornblende and garnet-rich cumulatesas observed in the Kohistan would not be in equilibrium withmelts similar to or evolved from high-Mg andesites, as thesecrystallize orthopyroxene-rich cumulates such as websterites andgabbronorites (Muntener et al., 2001). Therefore we do notconsider high-Mg andesite in the subsequent discussion.

The few primitive alkaline basalts present in the Aleutian andTonga arcs do not have the characteristic trace element signaturesof arcs – negative Nb–Ta, positive Pb and Sr anomalies – and arenot further discussed in this study.

Trace elements of all calc-alkaline/tholeiitic basalts and high-Mg andesites have typical arc signatures with enriched LILE andnegative Nb–Ta, Zr–Hf, and Ti anomalies (Fig. 2a). Importantly, ourdata suggest that primitive arc melts have a considerable spread ofHREE to MREE ratios and a fractionation of (Dy/Yb)N that rangesfrom 0.86 to 1.56 (with an average of 1.2470.16 (1s), seeElectronic supplement). Dy/Yb correlates positively with Sr/Y,which ranges from 2 to 100. Such high Sr/Y and Dy/Yb areattributed to fractionation of garnet and imply that garnet couldconstitute a more common residual phase in the subarc mantlethan previously thought. This observation needs to be considered

Fig. 2. Trace element patterns normalized to primitive mantle of (a) primitive arc melt averages (Table 2). Calc-alkaline/tholeiitic island arcs include the Aleutians, Bismarck,Izu-Bonin, Kuriles, Marianas, Tonga and Vanuatu arcs. The Yap and Palau arcs are incipient arcs characterized by lower LILE concentrations and thus a lower slab contribution.For comparison we also plot the primitive high-Mg andesite averages of the Aleutians, Izu-Bonin, Marianas and Tonga arc. Note that the Aleutians high-Mg andesites havesimilar trace element concentrations to the above primitive basalts while the other primitive three high-Mg andesites are distinctly depleted with respect to calc-alkaline/tholeiitic primitive basalts. (b) Trace element characteristics of the Kohistan arc cumulate averages. Note that both the garnet-gabbro and the hornblende–garnet cumulateshave, relative to calc-alkaline/tholeiitic primitive arc melts enriched HREE. (c) Comparison of the bulk delaminate trace element patterns to depleted mantle, MOR basalts(both enriched and normal) and to the Kohistan bulk arc and continental crust averages. w–h–g denotes when the cumulates are fitted containing wehrlites, hornblende–garnet cumulates and garnet-gabbro, w–h is limited to the former two. For each cumulate/crust combination 5 lines are plotted: the best three fits to primitive arc melts(Aleutians, Bismarck and Kuriles), Kohistan, and the average fit. Note that the resulting foundering compliment to the continental crust is relative insensitive to the exactprimitive melt or crust composition. The relatively depleted nature of the delaminate is the most likely reason why this geochemical reservoir twice the size of thecontinental crust remained largely elusive.

O. Jagoutz, M.W. Schmidt / Earth and Planetary Science Letters 371–372 (2013) 177–190 181

Table 2Averaged compositions of primitive arc melts.

Primitive calc-alkaline / tholeiitic basalts Transitional High-Mg andesites

Aleutians Bismark Kuriles Izu-Bonin Marianas Tonga Vanuatu Yap Palau Kohistan L.Antilles Izu-Bonin Marianas Tonga Aleutiansc

n 12 9 14 29 20 13 31 4 13 14 67 45 18 9 4SiO2 (wt%)a 49.8 50.5 51.5 51.0 50.2 49.3 49.4 49.5 51.2 50.7 47.6 57.1 56.8 54.7 56.3TiO2 0.75 0.99 0.83 0.79 0.91 0.60 0.69 0.59 0.60 0.80 0.92 0.18 0.34 0.25 0.64Al2O3 15.74 14.88 14.69 15.64 15.70 12.05 12.95 15.21 16.51 15.67 14.99 12.07 14.08 11.05 15.48FeOtot 8.82 8.97 8.82 8.61 8.12 9.34 9.64 8.62 7.89 8.60 9.03 8.33 7.61 9.27 6.73MnO 0.16 0.17 0.16 0.16 0.13 0.18 0.19 0.17 0.16 0.14 0.18 0.16 0.15 0.17 0.13MgO 11.14 10.89 10.86 10.54 10.88 13.37 12.76 11.10 10.69 10.87 12.72 11.62 10.36 13.49 7.95CaO 10.46 11.13 9.90 10.33 11.30 10.51 11.11 12.91 9.94 9.88 11.48 8.41 7.95 9.26 8.50Na2O 2.38 2.20 2.25 2.38 2.33 2.05 2.03 1.67 2.62 2.43 2.25 1.69 2.02 1.38 2.85K2O 0.70 0.31 0.83 0.45 0.37 2.12 0.98 0.18 0.28 0.61 0.63 0.43 0.63 0.39 1.32P2O5 0.13 0.11 0.14 0.12 0.12 0.46 0.20 0.05 0.07 0.19 0.21 0.08 0.06 0.05 0.14

XMgb 0.691 0.684 0.687 0.684 0.705 0.718 0.702 0.697 0.707 0.693 0.715 0.712 0.708 0.722 0.678

Cr (ppm) 611 545 647 575 456 697 666 507 689 512 781 781 648 861 445Ni 208 213 196 186 240 300 242 153 207 167 318 197 204 214 126

a Major elements are normalized to 100% total.b Molar Mg/(Mg+Fetot).c The Aleutian primitive andesite is distinct from the other three and technically a high-XMg andesite with relatively low MgO and FeO and high Al2O3 and K2O.

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when using trace element ratios to infer the importance of garnetin the formation of felsic derivative melts.

Fig. 3. Mass proportions of the differentiates of primitive melts illustrating the relativeamounts of gravitationally unstable cumulates and arc crust. The two left hand columnsare the result of fitting the average continental crust or the present day Kohistan bulkarc together with wherlitic and hornblende–garnet–clinopyroxene cumulates to primi-tive melts (compare to Table 3). The two right hand columns include garnet-gabbros(in pink) as additional cumulate. A small fraction of garnet-gabbro remained conservedin the Kohistan arc crust but is principally gravitational unstable. If all three densityunstable cumulate are taken into account, the best fits (to the Aleutians, Bismarck,Kuriles and Kohistan primitive melts) result into cumulate/crust ratios of 2.370.4.The Moho is indicated as it would result after complete foundering of the densecumulates. P-wave velocities (VP) and densities (ρ) are given in Kilometer per second andgram per centimeter cube. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

5. Mass balancing primitive melts, arc crust and cumulates

5.1. Mass balancing major element compositions

We use least square regression to fit the major elements ofprimitive arc melts with the present day Kohistan bulk arccomposition (Jagoutz and Schmidt, 2012; their model composition♯1) and different combinations of the dense cumulates of thelower Kohistan Arc (Fig. 1b, Table 1). Best fit qualities wereobtained (i) for wehrlite+hornblende–garnet–clinopyroxenecumulates (Fig. 3), yielding an excellent sum of residual squarevalues (Σr2) of 0.4–2.7 for the Aleutians, Bismarck, Kurile, andKohistan calc-alkaline/tholeiitic primitive arc basalts (Table 3), or(ii) for a combination of wehrlite, hornblende–garnet–clinopyrox-ene cumulates, and garnet-gabbro, which yielded r2 values of 0.4–2.3. Adding dunites did not change the fit quality significantly andcalculated amounts remained within 73%. Other cumulate typessuch as websterites (Table 1) or a more cumulative garnet-gabbrotype (Jagoutz and Schmidt, 2012) degrade the fit and lead tophysically meaningless negative mass coefficients. Using theaverage continental crust of Rudnick and Gao (2003) instead ofthe average Kohistan arc crust, yields similar results and r2 values(Table 3).

We note that magmatic garnet is likely absent in arcs witho26 km thin crust (see below). There, ultramafic cumulates willbe dominated by hornblende + clinopyroxene instead of hornble-nde+garnet+clinopyroxene. This results in only slightly differentmajor element compositions and hence in slightly different massbalanced cumulate proportions. Hornblende+clinopyroxene domi-nated ultramafic cumulates at the base of the arc crust would onlydensify through metamorphic garnet formation when the arc crustmatures.

5.2. A comparison of trace element compositions

We also calculated trace element compositions of the Kohistanbulk arc+cumulates (eTable) using the proportions obtained fromthe major element fits. The average trace element deviation to the

calc-alkaline/tholeiitic primitive melts of the Kohistan, Aleutians,Bismarck, Kurile, Marianas, Tonga, and Vanuatu arcs amounts to20–39% (Supplementary figure). Interestingly, for some arcs, thelargest deviations are observed for LIL elements (Rb, Ba, Th, U, K)

Table 3Results from mass balance calculations

Crust composition Kohistan arc crust - m#1 Average cont. crust RG03

primitive basalt from Aleutians Bismarck Kuriles Kohistan average⁎ Aleutians Bismarck Kuriles Kohistan average⁎

wehrlite (wt%) 10.4 11.4 13.7 11.2 11.7 8.5 9.0 9.8 9.5 9.6hbl-gar-cpx cumulate 50.1 47.1 37.9 43.8 44.8 54.7 53.2 49.8 47.0 49.6crust 39.5 41.5 48.4 45.1 43.6 36.8 37.8 40.4 43.5 40.8delaminate/crust (wt-ratio) 1.53 1.41 1.07 1.22 1.30 1.71 1.64 1.47 1.30 1.45∑r2 2.6 2.7 0.4 1.8 1.4 4.6 4.6 3.4 3.7 3.1av.dev. traces (%) 24.0 48.7 18.4 27.2 17.8 29.0 68.3 24.0 48.4 30.2wehrlite (wt%) 13.0 16.0 16.8 14.0 15.4 16.4 13.6 17.8 14.7 14.9hbl-garnetite 34.2 17.8 18.7 26.5 22.2 9.2 31.3 6.9 18.1 21.2gar-gabbro 20.8 40.3 27.2 22.9 29.6 57.5 24.4 44.9 37.2 34.5crust 32.0 25.9 37.4 36.7 32.9 17.0 30.7 30.3 30.0 29.5

delaminate/crust (wt-ratio) 2.13 2.85 1.68 1.73 2.04 4.89 2.26 2.29 2.34 2.39∑r2 2.3 2.0 0.4 1.6 1.1 2.8 3.3 0.9 2.6 2.1av.dev. traces (%) 23.3 34.2 19.6 19.9 16.1 25.7 52.8 21.3 24.8 16.1ΔSiO2 (wt%) 0.31 0.0 0.14 0.32 0.30 0.51 0.06 0.46 0.49 0.43ΔTiO2 0.02 -0.28 -0.14 10.07 −0.13 −0.03 −0.23 -0.15 −0.08 10.11ΔAl2O3 −0.52 0.34 0.15 −0.52 −0.16 −0.38 −0.01 0.04 −0.66 −0.30ΔFeOtot 1.26 0.90 0.47 1.02 0.84 1.35 1.22 0.61 1.25 1.15ΔMgO −0.23 0.07 0.10 −0.22 −0.07 −0.24 0.15 0.02 −0.21 −0.05ΔCaO −0.29 −0.96 −0.19 −0.01 −0.40 −0.16 −1.21 −0.31 −0.10 −0.49ΔNa2O −0.41 −0.29 −0.18 −0.36 −0.31 −0.62 −0.35 −0.35 −0.54 −0.45ΔK2O −0.14 0.16 −0.20 0.01 −0.05 −0.31 0.32 -0.21 0.00 −0.01

⁎ Fit to average primitive melt of Aleutians-BIsmarck-Kohistan-Kuriles.

0.01

0.1

1

10

100

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

prim

itive

man

tle n

orm

aliz

ed

Rb Ba Th U K Ta Nb La Ce Pb Pr Sr Nd P Hf Zr SmEu Gd Ti Tb Dy Y Ho Er TmYb Lu V Mn Cr Ni

average calc-alkaline/tholeiitic island arc primitive meltKohistan arc primitive meltbulk cont. crust

Kohistan arc crustw-h-g delaminate

a b

Fig. 4. Comparison of the trace element content of the Kohistan arc and continental crust with the preferred delaminate (containing all density unstable lithologies) andwith the average composition of primitive calc-alkaline/tholeiitic arc basalts (Table 2, Aleutians–Bismarck–Izu-Bonin–Kuriles–Marianas–Tonga–Vanuatu) compiled in thisstudy. The delaminate and the Kohistan arc crust are complementary, with their combined compositions reproducing the compositions of calc-alkaline/tholeiitic arc basalts.

O. Jagoutz, M.W. Schmidt / Earth and Planetary Science Letters 371–372 (2013) 177–190 183

which are dominantly derived from subducted material ratherthan from the mantle wedge (Fig. 2a). This large intra-arc variationof the LILE-suite is easily interpreted as a variable subductioncomponent between individual arcs.

Our results indicate that the cumulate sequence preserved atthe base of the Kohistan Arc, balances the bulk continental crustcomposition to primitive calc-alkaline/tholeiitic basaltic melts, notonly in terms of major elements, but also with respect to traceelements (Fig. 4, Supplementary figure). If significant volumes ofcontinental crust have been derived from rare high-Mg andesiticmelts, their respective cumulates must have been compositionallydistinct from those preserved in Kohistan. While such a possibilitycannot be excluded by our study, the quantitative similarity

between primitive calc-alkaline/tholeiitic basalts and the cumu-lates+arc crust indicates that such basalts represent the dominantparental melts of the arc and continental crust.

6. The proportion and chemical composition of thedelaminate

6.1. Relative proportion of arc crust and delaminate

The calculations above reveal that the cumulates to be removedfrom the arc system are (i) wehrlites (XMg¼0.881, Table 1, Fig. 1)fractionating from primitive or close-to-primitive mantle melts,

Table 4Estimated bulk composition of delaminate.

Wehrlite+hbl-gar-cpx cumulate +garnet-gabbro

Wehrlite+hbl-gar-cpxcumulate

Koh-m#1c RG03d Koh-m#1c RG03d

SiO2 (wt%)a 46.8 47.0 44.0 43.7TiO2 0.73 0.73 0.84 0.89Al2O3 14.1 14.5 13.4 14.1FeOT 11.3 11.3 12.6 12.8MnO 0.21 0.21 0.22 0.22MgO 14.1 13.5 15.9 15.0CaO 11.3 11.3 11.9 12.0Na2O 1.31 1.36 1.11 1.16K2O 0.10 0.10 0.10 0.11P2O5 0.04 0.05 0.02 0.02

XMgb 0.689 0.617 0.693 0.647

V (ppm) 321 321 404 423Cr 818 757 782 643Ni 260 241 255 213Rb 0.411 0.424 0.359 0.375Sr 139 147 83.8 88.1Y 13.4 13.5 15.7 16.5Zr 7.06 7.40 5.17 5.45Nb 0.254 0.268 0.151 0.158Ba 27.5 29.4 13.4 14.1La 0.890 0.964 0.277 0.290Ce 2.42 2.58 1.21 1.28Pr 0.413 0.434 0.291 0.306Nd 2.61 2.71 2.15 2.27Sm 1.03 1.05 1.02 1.07Eu 0.543 0.557 0.537 0.566Gd 1.65 1.68 1.83 1.93Tb 0.317 0.321 0.357 0.376Dy 2.29 2.31 2.81 2.96Ho 0.524 0.528 0.636 0.670Er 1.47 1.48 1.81 1.90Tm 0.182 0.188 0.169 0.178Yb 1.42 1.44 1.68 1.77Lu 0.228 0.230 0.277 0.292

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(ii) hornblende–garnet–clinopyroxene cumulates (XMg¼0.602),and (iii) garnet-gabbros (XMg¼0.512), the latter two forming fromevolved, lower XMg and somewhat H2O-enriched liquids at≥1.0 GPa. Fractionation of wehrlites decreases MgO-contents ofthe evolving melt, whereas fractionation of hornblende–garnetdominated cumulates also decreases FeO-contents and increaseSiO2-concentrations due to their low SiO2 content (�42.5 wt%).The mass fractions of these cumulates relative to the parentalprimitive melt results in 10–14 wt% wehrlites and 38–50 wt%hornblende–garnet–clinopyroxene cumulates when only consider-ing ultramafic cumulates, or 13–17 wt% wehrlites, 18–34 wt%hornblende–garnet–clinopyroxene cumulates and 21–40 wt%garnet-gabbro (Table 3, Fig. 3) when considering all negativelybuoyant lithologies. The total amount of cumulates fractionatedfrom the primitive melt is thus 52–60 wt% when excluding and63–74 wt% when including garnet-gabbros (percentages relative tothe Kohistan arc bulk crust, for results relative to the continentalcrust see Table 3). The preponderance of olivine, clinopyroxene,garnet, and amphibole is also in agreement with fractional crystal-lization experiments on H2O-bearing basaltic melts (see compila-tion in Pilet et al., 2010).

The mass of cumulates amounts to 1.1–1.7 (average 1.570.2)times the mass of the andesitic arc crust (Table 3, Fig. 3) for thecombination of wehrlite+hornblende–garnet–clinopyroxenecumulates or to 1.7–2.9 (average 2.370.5) for a delaminatecomposed of wehrlite+hornblende–garnet–clinopyroxene cumu-late+garnet-gabbro for most calc-alkaline/tholeiitic primitivemelts of Table 2. The exception is the Si-poor primitive melt fromthe Lesser Antilles which cannot be fit well using the Kohistancumulate compositions. On geological time scales it is expectedthat all density unstable lithologies founder due to relatively highMoho temperatures and the resulting low viscosity of the subarcmantle (Jull and Kelemen, 2001). Therefore, we propose that thefoundering complement to the continental crust forms a geo-chemical reservoir about twice the size of the continental crust.

Hf 0.237 0.242 0.246 0.259Ta 0.024 0.025 0.017 0.018Pb 0.470 0.500 0.184 0.181Th 0.012 0.013 0.004 0.004U 0.010 0.010 0.003 0.003Rb/Sr 0.0029 0.0029 0.0043 0.0043Sm/Nd 0.39 0.39 0.47 0.47U/Pb 0.020 0.021 0.019 0.018Th/Pb 0.026 0.026 0.023 0.023Lu/Hf 0.96 0.95 1.13 1.13

a Major elements are normalized to 100% totalb Molar Mg/(Mg+Fetot)c Calculated from fit to the bulk Kohistan arc crust composition model m#1

from Jagoutz and Schmidt (2012)d Calculated from fit to the average continental crust composition from Rudnick

and Gao (2003)

6.2. The chemical composition of the delaminate

In the following we discuss the geochemistry of the delaminatecomposed of wehrlite+hornblende–garnet–clinopyroxene cumu-late+garnet-gabbro (Table 4), i.e. of all negatively buoyant lowerarc lithologies. In Fig. 2c we plot the delaminate compositionscalculated from the fits with either the Kohistan arc crust or theaverage continental crust to the four best matching primitive arcmagmas (Aleutians, Bismarck and Kuriles, and Kohistan). Thesmall spread demonstrates the insensitivity of the cumulatecomposition to the exact primitive melt or crust compositionemployed.

6.2.1. The major element composition of the delaminateThe bulk composition of the delaminating cumulates has 43.7–

47.9 wt% SiO2, an XMg of 0.67–0.69, 13.1–15.8 wt% MgO and, as theresult of the important role of magmatically accumulated garnetand/or plagioclase, considerable Al2O3 (13.5–14.8 wt%, Table 4).Total alkali contents are moderate (1.1–1.4 wt% Na2O+K2O). Thebulk Na2O/K2O-ratio is 11–13, resulting from a low K2O-content of�0.1 wt%, which is hosted in amphibole and plagioclase. As such,the delaminate forms a silica-undersaturated, alkali-poor reser-voir. The fertility of this reservoir mostly depends on the smallamount of H2O stored in amphibole and clinozoisite. With respectto MORB, the calculated bulk cumulate compositions have similarFeO concentrations, significantly more MgO, and lower total alkalicontents.

6.2.2. The trace element composition of the delaminateThe delaminating rocks are strongly depleted in incompatible

elements when compared to primitive arc magmas or the arc crust(Fig. 4, Table 4). Positive spikes in an incompatibility diagram areobserved for Ba, Sr, K and Pb, while Th, U, Nb, Ta, Zr, and Hfconcentrations are markedly depleted with respect to primitivemantle (McDonough and Sun, 1995). LREE concentrations aresimilar to primitive mantle while abundant cumulative garnetresults in HREE 3–4 fold those of primitive mantle. The mostimportant effect of delaminating this reservoir is a 2–3-foldenrichment (with respect to the original primitive arc magma) ofincompatible trace elements in the buoyant material forming thecontinental crust.

It has been postulated that delamination of lower crustal rockscould result in some of the enriched endmembers seen in OIB

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isotope geochemistry (e.g., Arndt and Goldstein, 1989) althoughthe chemical composition of this delaminated reservoir has neverbeen determined. Our results indicate that, on average, thedelaminate will have Rb/Sr (0.0036) and Sm/Nd (0.43) ratios(Table 4) comparable to those of depleted upper mantle (Rb/Sr0.0065; Sm/Nd 0.41 after Workman and Hart, 2005), thus a time-evolved contribution from the delaminate would not be recogniz-able in Sr–Nd isotope signature. However, the delaminates havelow U/Pb ratios due to the high concentration of Pb hosted inchalcopyrite in the ultramafic rocks and plagioclase in the gabbros,and high Lu/Hf due to the presence of magmatic garnet. The effectsof these trace element ratios for the evolution of the isotopic“mantle zoo” are discussed below.

7. Genesis of the delaminate: fractional crystallization vs.partial melting

Trace element characteristics alone cannot easily distinguishbetween direct fractional crystallization producing evolved meltsand ultramafic hornblende–garnet–clinopyroxene cumulates vs.partial re-melting of a gabbroic precursor yielding similar ultra-mafic restites and evolved partial melt. In both cases andesitic togranitic melts equilibrate with hornblende, garnet, plagioclase andclinopyroxene. Thus, “partial-melting” experiments (approachingequilibrium from the solidus side), are equal to “partial-crystal-lization” experiments (approaching equilibrium from theliquidus side).

The decisive geochemical difference between a continuousfractional crystallization process and partial remelting of a pre-viously fully crystallized basaltic arc melt is a loss of 60–80% of theinitial H2O-content from the primitive basaltic arc melt for thelatter scenario. As a consequence, partial melting can only producegranitoids with significantly lower H2O-contents than observed inarcs and requires higher temperatures for the same melt composi-tions (on a volatile free basis).

Most primitive basaltic arc magmas have 1.5–6 wt% H2O(Sobolev and Chaussidon, 1996; Grove et al., 2003; Arculus,2004; Wallace, 2005) and a relatively hydrous liquid line ofdescent is volumetrically dominant in arcs (Jagoutz et al., 2011).Complete crystallization of a basaltic melt with 2–4 wt% initial H2Ocontent leads to hornblende-gabbros that typically contain 20–50 wt% amphibole and thus 0.4–1.1 wt% H2O (Blundy and Sparks,1992; Costa et al., 2002). The amount of crystallizing amphibolelessens when basaltic melts crystallize within the garnet stabilityfield at ≥1 GPa where the modal increase of garnet and clinopyr-oxene result in o20% amphibole and thus o0.4 wt% bulk H2O(Sen and Dunn, 1994). Accordingly, in Kohistan hornblende is onlya minor constituent of the garnet-gabbros (Yoshino et al., 1998).

The bulk H2O-content of crystallized gabbros (aka amphibo-lites) is then crucial for the mass balance of partial melting. Toobtain andesitic melts with 58–60 wt% SiO2, 30–50% of partialmelting is necessary (Rapp and Watson, 1995) resulting in max-imum 0.9–2.8 wt% H2O in the partial melts. Such low H2O-contents are generally not reported for andesitic arc magmas,which are typically fluid saturated with 4–7 wt% H2O (Scaillet andPichavant, 2003). Similarly, low H2O andesites crystallize twopyroxenes (Beard and Lofgren, 1991), yet in the Kohistan batholithintermediate intrusives with pyroxenes are only reported fromtwo areas (Nawaz et al., 1987; Jagoutz et al., in press). Mostgranitoids have amphibole-only and are void of pyroxenes, sug-gesting that the batholith dominantly formed from more hydrousparent melts than can be produced by partial melting. Fractionalcrystallization of a basaltic parental melt with initial 2–4 wt% H2Oresults in andesitic melts with 4–8 wt% H2O after removal of �50%

cumulate and in dacitic melts with 6–10 wt% H2O after removal of60–70% cumulate.

We thus reject a partial melting process of gabbroic precursorsas the principal formation mechanism of evolved melts in arcs andtherefore consider ultramafic/mafic rocks in the lower arc crust asmagmatic cumulates. This is supported by the large variation inmodal hornblende, garnet, and clinopyroxene on a 10–100 m scalein the Kohistan cumulates (Burg et al., 2005) in accordance withaccumulation processes in magma chambers. In contrast, partialmelting would produce restites with similar hornblende–garnet–clinopyroxene modes that would vary as a function of degree ofmelting. Finally, the ultramafic hornblende–garnet–clinopyroxenedominated cumulates, as well as the strongly cumulative garnet-gabbros from Kohistan display a positive correlation of whole rockXMg vs. SiO2 (Fig. 1c). Partial melting processes producing silica-rich liquids would lead to the opposite, i.e. a negative correlationof XMg vs. SiO2, while crystal fractionation from primitive mantlemelts perfectly explains the observed correlation (Jagoutz et al.,2011).

Further support for the dominance of fractionation over crustalre-melting comes from thermal modeling (Dufek and Bergantz,2005): the ratio of crustal to mantle derived melts is typicallyo0.2 in both incipient arcs with a thin crust and low fluxes and inmature arcs with a thicker crust and higher fluxes (their Fig. 12). Inother words, for most arcs less than 15–30% of the arc melt stemsfrom crustal remelting.

8. Foundering and mass fluxes

8.1. Depth of foundering

The depth range corresponding to pressures of �0.8–1 GPawhere garnet becomes stable varies depending on the densitystructure of the arc crust. In juvenile arcs composed of basalticmaterial with densities of �2.9–3.1 g/cm3, 0.8–1 GPa correspondto depths of �26–34 km. In contrast, in mature arcs with asignificant granitic upper crust (e.g., Izu Bonin) with densities of2.6–2.8 g/cm3, 0.8–1 GPa could correspond to depths of 32–40 km.Accordingly, delamination due to garnet formation may occur426 to 440 km depth.

8.2. Mass fluxes in arcs

The magnitude of the modern magma flux at divergent plateboundaries (�19 km3/yr) is reliably estimated from the averagecrustal thickness of the oceanic crust (�6 km), the average oceanridge spreading rate of 46.6 mm/yr and the total spreading ridgelength of 67,338 km (Bird, 2003) (Fig. 5). Similar efforts todetermine the magnitude of magma fluxes in arcs have remainedpoorly constrained as the magnitude and composition of thedelaminated material were unknown. Arculus (2004) alreadysuggested that the arc magma flux could be twice as large as thearc crust production rate. Total arc magma flux (Jarc) is the sum ofthe flux leading to arc crust formation (Jarc_crust) and the delamina-tion flux (Jdelam):

Jarc ¼ JarccrustþJdelam ð1Þ

As shown above, our mass balance calculations constrain thedelamination flux to approximately twice that of the arc crust flux:

Jdelam � 2:3 Jarccrust ð2Þ

The arc crust flux (Jarc_crust) calculated here is the sum of the arccrust growth rate and the subduction erosion rate (typically

O. Jagoutz, M.W. Schmidt / Earth and Planetary Science Letters 371–372 (2013) 177–190186

derived on a time scale of tens of Ma):

Jarc crust ¼ dVarccrust=dtþ dVsubderos=dt ð3ÞEstimates for arc crust production rates vary significantly, but

on average most island arcs have a long term arc crust productionrate of 50–90 km3/km arc length/Myr with an average of 67 km3/km/Myr (Dimalanta et al., 2002; Jicha et al., 2006; Jagoutz andSchmidt, 2012). These growth rates are derived from crustal crosssections in active arcs combined with the time period of arcbuilding (25 to 450 Ma for the arcs included in this average). Forisland arcs, 23,800 km in length (Bird, 2003), a crust productionrate of 1.8–2.8 km3/yr results. For continental arcs, totaling27,500 km in length (Bird, 2003), we adopt the arc growth ratefrom Haschke and Gunther (2003) of 35 km3/km/Myr derived forthe Andes. Hence, we approximate global arc crust growth ratesdVarc crust/dt of 2.2–3.1 km3/yr, with a preferred average of 2.6 km3/yr (Fig. 5).

Subduction erosion rates for erosive margins vary between 15and 120 km3/km/Myr and have been integrated globally to adVsubd_eros/dt of 1.3–1.7 km3/yr (Clift et al., 2009; Scholl and vonHuene, 2009). Hence the arc crust formation flux averages 3.5–4.8 km3/yr with a preferred average of 4.0 km3/yr.

A complete arc crust flux Jarc crust would also need to accountfor surface erosion. To our knowledge, global estimates for arccrust surface erosion rates or the contribution of arc terranes toglobal sedimentation do not exist. For the Aleutian arc, the surfaceerosion rate has been estimated from the intrusion pressure of anow exposed pluton to 34–51 km3/km/Myr for the past 35 Ma(Jicha et al., 2006), which is an extrapolation from a single point tothe entire arc. This rate is driven by glacial erosion and cannot besimply extrapolated to lower latitude arcs. We thus do not includeany surface erosion in arcs and hence our arc crust production flux(Jarc_crust) represents a lower bound.

From the average arc crust formation flux Jarc_crust we approx-imate the modern delamination flux Jdelam (with an average

Fig. 5. Summary of global magmatic fluxes (km3/yr) represented in a cartoon depictingleads to a triplication of the magma flux into arcs with respect to the arc production ratesubaerial erosion. The latter is unfortunately unconstrained and cannot be included intocorner flow through the average degree of melting in the subarc mantle. Similar, the propthe fact that 2/3 of the arc magma flow returns to the mantle as cumulates. The fluxes givcontinental arcs in terms of volume/km_arc_length/time (see text).

delaminate density of 3.47 g/cm3) as �6.4–8.1 km3/yr with aaverage of 7.2 km3/yr. Thus, the arc magma flux Jarc is �11–15 km3/yr (with a melt density of 2.8 g/cm3) with a best averageof 12.9 km3/yr (Fig. 5), which corresponds to 220–290 km3/km/Myr. We estimate that this flux might increase by 10% if the effectof surface erosion is included.

The volume of the preserved continental crust corresponds to�0.59 wt% of the bulk silicate Earth (BSE). Assuming that surfaceand subduction erosion had the same efficiency as today for atleast 2 Gyr, continental crust of at least 1.1 wt% of the BSE wasproduced in arcs over the Earth's history. Scholl and von Huene(2009) estimate that over the past 3 Gyr about the same mass astoday's continental crust has already been destroyed. Our massbalance, yielding a mass of the foundering material �2.3 timesthat of the andesitic crust produced, indicates that about 2.5 wt%of foundered cumulate with respect to BSE should exist in themantle.

The magmatic arc flux also constrains the minimum influx ofnew mantle material into the subarc melting region. Usingestimates of the average degree of subarc mantle melting(F�0.15; Plank and Langmuir, 1988; Davies and Bickle, 1991;Hawkesworth et al., 1994), we approximate this (minimum) influxas �77 km3/yr. This value for the sub-arc corner flow represents aminimum rate, as we account only for the inflowing mantleundergoing melt extraction. In a mature arc with ≥30 km arccrust, the dense cumulates will almost be entirely removed and, asthey amount to �2/3 of the degree of mantle melting, the hotoutflowing mantle will contain about 10 wt% of founderingcumulates.

In summary, these results suggest a return flux of material intothe upper mantle due to cumulate foundering in the sub-arcregion equal to about half the return flux contributed by subduc-tion of basaltic oceanic crust (19 km3/yr at present, Bird, 2003).Thus, the foundering return flux has to be evaluated and incorpo-rated into global mass balances.

a mature island arc with an arc crust of ≥30 km. The foundering of the delaminate. The arc production rate is the sum of the arc growth rate, subduction erosion andthe arc mass fluxes. Note that the magma flux into arcs requires a minimum mantleortion of foundered material in the subarc mantle is constrained for mature arcs byen are globally integrated annual rates but note that fluxes differ for island arcs and

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9. Implications of the delamination flux for the isotopic“mantle zoo”

Our calculations document that foundering of density unstablecumulates at the crust–mantle transition in arcs is a volumetricallyimportant return flux and a natural consequence of plate tectonics.Yet, because of its ‘hidden’ character the delaminate has so farbeen largely underappreciated even though it is volumetricallycomparable to the flux of subducted oceanic crust. In the subse-quent paragraphs we discuss the role of the arc return flux on theisotopic evolution of the upper mantle.

9.1. An unradiogenic Pb reservoir counterbalancing the depletedmantle?

Important chemical characteristics of the delaminating cumu-lates are elevated Pb concentrations (0.13−0.8 ppm) compared tothe depleted upper mantle (Pb�0.02 ppm, Salters and Stracke,2004; Workman and Hart, 2005) and low U concentrations(≤0.01 ppm U for both cumulates and depleted mantle). Thisresults in a low U/Pb (�0.02) compared to the depleted mantle(U/Pb: 0.172) and thus low 238U/204Pb ratios (μ�0.3–0.5, Fig. 6a).The Pb concentrations of the main constituent minerals of thedelaminate (olivine, clinopyroxene, garnet and amphibole) aregenerally too low to explain the high Pb concentration of thebulk. We thus speculate that a significant portion of the Pb presentin the cumulates is concentrated in the observed sulfide minerals(e.g., chalcopyrite).

The cumulates are initially more radiogenic than depleted MORmantle (DMM) because of the addition of a radiogenic slab-derived component to the mantle wedge and hence to theprimitive arc magma (Fig. 2a). Nevertheless, with time, the low μ

Fig. 6. Isotopic characteristics of the delaminate. 206/204Pb vs. 207/204Pb and εNd vs. εHf difor three different crustal growth scenarios since 3.8 Ga ago. Green triangle: crustal grlinear crustal growth. Orange diamond: crustal growth occurred mainly in the Phanerozthe isotopic composition of the average lower continental crust assuming the same crustthrough time, and thereby of the foundered material, is calculated using the Staceycomposition is compiled from PetDB (www.petdb.org). E-DMM is from Workman andmixing lines. Half ticks indicate 1% mixing intervals (for 1–10%) and full ticks 10% mixingmaterial in the melt extracted depleted mantle is sufficient to counterbalance even thecircles) are volcanic rocks compiled from georoc (http://georoc.mpch-mainz.gwdg.de/geoare initial isotopic compositions from mineral and whole rock compositions of ultramaficlinopyroxene separates from abyssal peridotites (Stracke et al., 2011) and black diamxenoliths from the Panglai and Shanwang basalts (Chu et al., 2009). Lines connecting thetriangle) are mixing lines. Lines subparallel to the mantle array are isopleths of mixing,variation observed in some peridotite xenolith suites can be explained by a contributionfigure legend, the reader is referred to the web version of this article.)

cumulates evolve little in Pb isotope space while the relativelyhigher μ depleted mantle becomes more radiogenic through in-growth of Pb. Through time, the delaminate will thus evolve to lessradiogenic compositions than bulk silicate earth or DMM. Such anunradiogenic Pb reservoir may counterbalance the radiogenicMORB and OIB compositions (the so-called terrestrial lead para-dox, Allegre, 1969). To elucidate the global role of the delaminatedreservoir for Pb isotope systematics, we modeled the isotopicevolution of the delaminate assuming that the preserved conti-nental crust formed through the last 3.8 Ga solely by subductionzone mechanisms. The question we address is whether thevolumes of delaminate and their isotopic composition are sig-nificant for the terrestrial lead paradox. As the influence of such anisotopic reservoir depends on the timing of crust formation, weintegrate the isotopic evolution of the delaminate over threedifferent “endmember” crustal growth curves (Fig. 6a): (i) crustalgrowth occurred dominantly in the Archean (Dewey and Windley,1981) (ii) crustal growth has been constant and linear over timeand (iii) crustal growth occurred dominantly in the Phanerozoic/Proterozoic (Hurley and Rand, 1969).

As a result, dominantly Archean crustal growth (Dewey andWindley, 1981) yields the least radiogenic composition whereasdominantly Phanerozoic/Proterozoic growth (Hurley and Rand,1969) predicts the most evolved one (Fig. 6a). Importantly, anycrustal growth model yields a delaminate reservoir that evolves torelatively unradiogenic Pb compositions and plots to the left of thepresent day (4.57 Gyr) geochron. Mixing trends between depletedMOR mantle (Workman and Hart, 2005) and the unradiogenicdelaminate are roughly sub-parallel to the trend of the MORB fieldfor both the linear and the Hurley and Rand (1969) crustal growthmodels but slightly oblique for the Dewey and Windley (1981)crustal growth model.

agrams showing the time integrated isotopic composition of the foundered materialowth occurred mainly during the Archean (Dewey and Windley, 1981). Red circle:oic/Proterozoic (Hurley and Rand, 1969). The grey triangle, circle and diamond areal growth cures, respectively: (a) The initial Pb isotopic composition of the arc meltsand Kramers (1975) Pb isotope evolution model (black line). The MORB isotopicHart (2005). Lines connecting E-DMM and time integrated composition representintervals. The modeling indicates that generally an amount of ≤10% of the founderedenriched DMM composition to the chondritic geochron. (b) The mantle array (greyroc/) and PetDB. Three examples of extreme εHf values are shown: Open diamondsc xenoliths from the Williams kimberlite (Carlson et al., 2004); grey diamonds areonds are initial isotopic compositions of clinopyroxene separates from ultramaficvalues of the mantle array with the time integrated delaminate composition (greenlabeled with the amount of delaminate component (in %). The extreme Hf isotopicof �10–20% of the delaminate. (For interpretation of the references to color in this

O. Jagoutz, M.W. Schmidt / Earth and Planetary Science Letters 371–372 (2013) 177–190188

The delaminated cumulate reservoir, complementary to thevolume of the modern continental crust, could counterbalance theradiogenic Pb of a DMM reservoir if the foundered material is�10 mass% of the DMM. This results implies that if the sub-arcdegree of melting is F�0.15, and if about 2/3 of the mass ofprimitive melts are returned to the mantle as dense cumulates, thedelaminate would always counterbalance the continental crust aslong as this crust is dominantly formed in arcs through fractiona-tion and foundering of dense cumulates. This also agrees well withsimilar approximations of the Pb-isotopic evolution of xenolithsfrom the Sierra Nevada (Lee et al., 2007).

9.2. Extremely radiogenic Hf

Lu/Hf ratios of the delaminate (�1.0, Table 4) differ stronglyfrom the depleted upper mantle (Lu/Hf: 0.053). The extreme Lu/Hfof the delaminate results from the accumulation of magmaticgarnet in some of these rocks. Similar and even more extreme Lu/Hf ratios (Fig. 6b) are documented for garnet-cumulative garnet-clinopyroxenite dikes in Beni-Bousera and Ronda (Blichert-Toftet al., 1999; Garrido and Bodinier, 1999; Gysi et al., 2011) some ofwhich have been shown to be delaminated lower crust (Gysi et al.,2011). The high Lu/Hf of the delaminate coupled with its averageSm/Nd ratio will evolve through time to very radiogenic Hf atmoderately radiogenic Nd isotopic compositions (Fig. 6b). Suchdecoupling of the Hf–Nd systematic is indeed documented(Stracke et al., 2011; Chu et al., 2009; Carlson et al., 2004). Highlyradiogenic Hf isotopic compositions are additionally recorded fromrocks that sample the sub-continental lithosphere (Choukrounet al., 2005).

10. Concluding remarks

The delaminating hornblende-bearing cumulates would not bestable when sinking into the mantle to 470–80 km depth. TheH2O-bearing amphibole would decompose either when reaching41050 1C or 42.5–3.0 GPa (Lambert and Wyllie, 1974; Vielzeufand Schmidt, 2001). Thus, we expect low melt fractions of Al-rich,possibly nepheline-normative, slightly hydrous basalts (e.g.Holloway, 1973) to form from hornblende+garnet dominatedcumulates at �75 km depth.

Our model calculations document that the delaminated cumu-late reservoir in arcs is compositionally very different from thepreserved lower continental crust. Over the Earth history at least2 wt% (with respect to silicate Earth) of such cumulates have beenformed. Because of their high density, these rocks sink into thelower mantle and with time constitute an important unradiogenicPb reservoir, which needs consideration in global “plumbo-tec-tonic” models (Zartman and Doe, 1981). Similarly, the extreme Lu/Hf ratio of the foundered delaminate will evolve with time tohighly radiogenic Hf-compositions.

Nevertheless, the delaminate may remain elusive within themantle. After minor low pressure melting, the even more depletedcumulates become highly refractory and hence geochemicallyinconspicuous. The depleted delaminate may not contribute sig-nificantly to melting within the convecting heterogeneous mantle.

Last but not least, the role of garnet and amphibole forcontinental crust genesis is once more emphasized. The differ-entiation of these minerals and their removal from the crust leadsto the production of the modern continental crust. Last and least,we note that the time integrated delaminate is volumetricallycomparable to the D′′ layer; a coincidence that may or may not befortuitous.

Acknowledgments

OJ's work was supported by NSF EAR 0910644. MWS thanksETH for generous funding supporting the Kohistan projects. Wethank JP Burg for inviting us to join the Kohistan project, for hiscompanionship during field work and also for his invaluablecomments. The manuscript was improved by C.E. Bucholz, we alsothank two anonymous reviewers for their criticism andsuggestions.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.epsl.2013.03.051.

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