Amphiboles as indicators of mantle source contamination: Combined evaluation of stable H and O...

Lithos 152 (2012) 141–156

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Amphiboles as indicators of mantle source contamination: Combined evaluation ofstable H and O isotope compositions and trace element ratios

A. Demény a,⁎, Sz. Harangi b, T.W. Vennemann c, R. Casillas d, P. Horváth a, A.J. Milton e,P.R.D. Mason f, A. Ulianov c

a Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Budaörsi út 45., H-1112, Budapest, Hungaryb Department of Petrology and Geochemistry, Volcanology Group, Eötvös University, Pázmány Péter sétány 1/C, H-1117 Budapest, Hungaryc Institute of Mineralogy and Geochemistry, University of Lausanne, Anthropole, CH-1015, Lausanne, Switzerlandd Departamento de Edafología y Geología, Universidad de La Laguna, 38206, La Laguna, Santa Cruz de Tenerife, Canary Islands, Spaine School of Ocean and Earth Science, Southampton Oceanography Centre, European Way, Empress Dock, Southampton SO14 3ZH, UKf Vening Meinesz Research School of Geodynamics, Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands

⁎ Corresponding author. Tel./fax: +36 1 319 3137.E-mail address: demeny@geochem.hu (A. Demény).

a b s t r a c t

Article history:Received 17 October 2011Accepted 2 July 2012Available online 11 July 2012

Keywords:AmphiboleStable isotopes of hydrogen and oxygenTrace elementsFluidMantle metasomatism

Stable isotope and trace element compositions of igneous amphiboles from different tectonic settings (oceanisland basalts, intraplate alkaline basalts, subduction-related andesitic complexes) were compiled to help un-derstand the role of fluids and melts in subduction-related mantle metasomatism and to evaluate the use ofselected trace element ratios (Pb/Pb*(N)=Pb/(√(Ce·Pr)) and Ba/Nb(N), normalized to primitive mantle) tohelp detect possible metasomatism. Comparisons of stable H and O isotope compositions and trace elementratios of amphiboles from ocean island basalts (Canary Islands), intraplate basalts, and subduction-relatedcalc-alkaline andesitic series (Carpathian–Pannonian Region, CPR) indicate systematic distributions inδD–δ18O–Pb/Pb*(N)–Ba/Nb(N) diagrams that are related to metasomatic processes in the mantle and themigration of fluids and melts derived from subducted crustal slabs. In order to interpret these data for theamphiboles from the CPR, ophiolites of the Penninic and the Meliata–Vardar complexes as potential sourcesof subducted crustal melts and fluids in the mantle of the Carpathian–Pannonian Region were also analyzed.On the basis of published fluid/rock partition coefficients the compositions of fluids emanating fromsubducted ophiolites were calculated. The calculated fluid compositions—especially for blueschists of theMeliata complex—fit the amphibole trends, indicating that such fluids could have been responsible for themantle metasomatism beneath the CPR.

1. Introduction

In subduction-related settings mantle metasomatism by slab-derived fluids and melts has been routinely invoked as a cause ofvariations in trace element contents, as well as stable and radiogenicisotope compositions in magmatic rocks. Mantle metasomatism mayalso be induced by fluids released from volatile-rich upwelling mantleplumes. Fluids related to mantle plume activity are likely to have iso-topic compositions typical of the mantle, whereas crustal materialswould be characterized by distinctly different isotope compositions(Faure, 1986; Hoefs, 2010). In addition, the subducted slab may con-tain oceanic or continental crust with varying amounts of sedimenta-ry material with different radiogenic and stable isotope compositionsand trace element contents compared to the oceanic crust and mantle.Continental crust and especially sedimentary rocks have, for example,higher 87Sr/86Sr and 18O/16O ratios amongst the reservoirs mentioned

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above. Thus, crustal contamination to mantle melt source regions isdetectable by such analyses.

On the basis of the mobility of elements in fluids and melts cer-tain trace elements are regarded as good indicators of fluid ormelt-induced metasomatism, as exemplified by experimental studies(e.g., Brenan et al., 1995a; Keppler, 1996; Stalder et al., 1998) and sys-tematic investigations of natural samples (e.g., Eiler et al., 2007; Ersoyet al., 2010; Halama et al., 2009; Sun et al., 2004, 2008). Elevatedconcentrations of fluid-mobile elements (e.g., Ba, Pb, Sr), along withcompositions typical of crustal rocks in terms of their isotopic compo-sitions, would indicate infiltration of slab-derived fluids into the man-tle region where magmas of subduction-related volcanic edificies aregenerated, whereas enrichments in fluid-immobile elements (Th,REE, HFSE) would suggest direct addition of slab-derived melts. Thispicture is complicated by magmatic processes, such as partial meltingand fractional crystallization that can also modify the trace elementcomposition. A good example is the Ce/Pb ratio (Hofmann et al.,1986), which is a frequently used indicator for crustal contamination,but which may also be affected by fractional crystallization so that theuse of Nd/Pb ratio is recommended instead (Hofmann, 2003). As a

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result, evaluating the fluid vs. melt-induced metasomatism requiresadditional constraints, including measurements of isotope composi-tions of the magmatic rocks. The effects of H2O-bearing metasomaticfluids and the origin of the fluids can be determined using hydrogenand oxygen isotopes combined with trace element measurements ofrocks and minerals. The scientific literature for stable isotope geo-chemistry of magmatic systems is rather extensive but the compre-hensive work of Hoefs (2010) gives a review for such systems.Oxygen isotope compositions of pristine mantle-derived rocks havea range of about 5 to 6‰, whereas crustal rocks generally have highervalues of up to 20–30‰ (relative to the V-SMOW internationalstandard). As a consequence, direct addition of crustal material tomantle-derived melts is readily determined on the basis of oxygenisotope compositions of magmatic rocks. In contrast, the degree of Oisotope exchange with a metasomatic fluid not only depends on thecomposition of the fluid that may not be in equilibrium with themantle-type lithologies, but also on the amount of such an infiltratingfluid. At large fluid/rock ratios the metasomatized rock can change itsO isotope composition even within the mantle (e.g., Halama et al.,2011). However, fluid/rock ratios decrease with distance from thesubducted slab, and at low fluid/rock ratios the metasomatic effecton the O isotope composition of the rock would be much smaller(Halama et al., 2011). In contrast, the hydrogen isotope compositioncan serve as a more sensitive indicator of fluid-induced metasoma-tism, as hydrogen is only a minor element in rocks but abundant inan aqueous fluid. The range of hydrogen isotope compositions of pris-tine, mantle-derived melt or fluid is also restricted (−70±10‰, rel-ative to the V-SMOW international standard), whereas subductedcrustal complexes would—in general—have a larger range of data.These considerations suggest that stable H and O isotope analysesare valuable for deciphering the influence of metasomatic fluids andcrustal contamination processes within mantle‐derived rocks, espe-cially if combined with analyses of trace element content.

Amphibole contains both hydrogen and oxygen as well as a rangeof trace elements in readily detectable amounts (e.g., Pb; Hofmann,2003), and is well suited for studies on the influence of metasoma-tism. A further advantage is that amphibole is ubiquitous in rocksthat have been affected by fluid–rock exchange (e.g., Gregoire et al.,2001; Ionov and Hofmann, 1995; Ionov et al., 1997, 2002; Powell etal., 2004) and it is rather resistant to late-stage alteration. This maybe different for phlogopite, another hydrous silicate formed frequentlyin mantle rocks, that has rather low trace element concentrations andmay be altered to minerals stable at lower temperatures such as chlo-rite. Amphibole, however, also has a disadvantage: the trace elementcontents depends on its chemical composition(Tiepolo et al., 2000)and degassing can affect the H isotope composition and oxidationstates (Demény et al., 2006), resulting in compositions that may bedifficult to interpret. In spite of these effects, Coltorti et al. (2007)presented an example of how the composition of amphibole can besuccessfully evaluated to determine melting processes in the mantle.

Here a comprehensive data set of trace element and stable iso-tope compositions of igneous andmantle-derived amphiboles formedin different tectonic settings is presented. The data is from ocean is-land basaltic rocks (Canary Islands; Demény et al., 2004a, 2008), intra-plate basalts, and subduction-related calc-alkaline volcanic edifices(Carpathian–Pannonian Region, CPR; Demény et al., 2005, 2006). Theamphiboles are from clinopyroxenite xenoliths and occur asmegacrystsand phenocrysts, all of them representing amphibole crystallizationfrom fluid-rich melts of different magmatic environments. On thebasis of the radiogenic isotope data, Hoernle et al. (1995) suggestedthat Cenozoic volcanic rocks originating from the Eastern Atlantic(Canary and Madeira Islands), Western Mediterranean and CentralEuropean (including the Pannonian Basin) mantle regions have a com-mon component, called the Low Velocity Component, all related tomajor mantle upwelling. Thus, the amphibole compositions and thoseof the host rocks studied in this paper may also indicate a common

component that connects these geographically separated complexes.In addition, the upper mantle region of the CPR has been affected byCretaeceous to Miocene subduction during the orogenesis of theCarpathians (Harangi and Lenkey, 2007; Szabó et al., 1992). Earlierstudies have already suggested that crustal contamination was impor-tant in these systems (Demény et al., 2004b). It was proposed thatmetasomatism has affected the mantle-derived xenoliths of theCarpathian Pannonian Region and that this is related to the migrationof fluids and melts released from subducted ocean crust. The aim ofthis paper is to distinguish betweenfluid- andmelt-inducedmetasoma-tism and crustal contamination, given the oceanic crust as a contami-nant source in the CPR. The influence of the subducted componentscan be estimated by determining the trace element and stable isotopecompositions of ophiolitic rocks of the CPR that may have served assources for the fluids and melts that infiltrated into the Carpathian–Pannonian mantle. The data will be used to evaluate the usefulness ofthe Pb/Pb*(N) ratio (Pb/√(Ce·Pr), normalized to primitive mantle(Hofmann, 1988) as an indicator value for crustal contamination and as-similation. The Pb/Pb*(N) ratio was introduced by Marks et al. (2004)based on similar normalized Ce and Pr concentrations and a correlationof Nd isotope ratios measured for augite, aegirine and Ca- and Na-amphiboles. The utility of the Pb/Pb*(N) ratio will be further exploredin this manuscript.

2. Geological background and samples

The amphibole compositions discussed here from the CanaryIslands and the Carpathian–Pannonian Region have been published(Demény et al., 2004a, 2005, 2006, 2007, 2008), while those fromthe calc-alkaline rocks of the Carpathian–Pannonian Region representnew measurements.

2.1. Canary Islands

Amphibole from the Canary Islands was sampled from gabbroicintrusions, basaltic dikes, and lavas of La Palma and Fuerteventura.The submarine edifice of La Palma is composed of pillow lavas, pillowbreccias, and hyaloclastites, intruded by gabbros and a dense dikeswarm (Carracedo et al., 2001; De la Nuez, 1984; Hernández-Pacheco,1971; Staudigel and Schmincke, 1984; Staudigel et al., 1986), andforms the so-called Basal Complex. The subaerial volcanism is repre-sented by two volcanic edifices, the Northern Shield and the CumbreVieja ridge. Amphiboles were collected from pillow basalts of thesubmarine volcanic rocks, olivine-bearing gabbros and amphibole-bearing alkaline leucogabbros from the Basal Complex, as well asfrom amphibole-rich clinopyroxenite and wehrlite–clinopyroxenitexenoliths sampled from the basaltic rocks of the Cumbre Vieja Ridgeand other historical eruptions. More detailed descriptions are given byDemény et al. (2008). The amphiboles of Fuerteventura were sampledfrom Miocene basanitic rocks for the Transitional Volcanic Group thatconsists of pillow lavas, pillow breccias, and a related basaltic hypabys-sal complex (Gutiérrez, 2000). Megacrysts (kaersutites and pargasites)were collected from basanitic pillow lavas (Demény et al., 2004a).

2.2. Alkaline basalts of the Carpathian–Pannonian Region

Intraplate alkaline basalts were erupted in the Carpathian–Pannonian Region during the Neogene (Pécskay et al., 1995, 2006)due to updoming of the asthenosphere, heating and thinning of thelithosphere(e.g., Embey-Isztin et al., 1990; Horváth, 1993; Szabó etal., 1992). The alkaline basalts are remarkably fresh, moderately por-phyritic and holocrystalline (Embey-Isztin et al., 1993a,b). The basaltscontain phenocryst olivine (forsterite (Fo) content of 78–86) that isin some cases accompanied by clinopyroxene. The matrix is com-posed of plagioclase, Ti-rich clinopyroxene, olivine, titanomagnetiteoccasionally coexisting with ilmenite, and apatite (Embey-Isztin et al.,

143A. Demény et al. / Lithos 152 (2012) 141–156

1993a). The amphibole megacrysts of the alkaline basalts have beeninterpreted as fragments of igneous cumulates precipited from anearly magma intrusion and brought to the surface by later basalticmagmatism (Dobosi et al., 2003; Downes et al., 1995; Huraiová et al.,1996; Szabó and Taylor, 1994). The sampling localities range fromEastern Austria (Burgenland) to Western Romania (Apuseni Mts.),spanning the Carpathian–Pannonian Region. Detailed sample descrip-tions are given by Demény et al. (2005).

2.3. Calc-alkaline series of the Carpathian–Pannonian Region

Selected amphiboles from Miocene calc-alkaline volcanic rocks ofthe Western Carpathians (Harangi et al., 2001, 2007) were analyzedfor trace element compositions. The list of the studied occurrencesis given in Table 1. The calc-alkaline volcanism at this segment ofthe Carpathian arc was coeval with the major extensional phase ofthe Pannonian basin and thus Harangi and Lenkey (2007) suggestedthat formation of these rocks could be closely linked to lithosphericthinning rather than active subduction. Garnet-bearing andesitesand dacites can be linked to the transition from a compressional toan extensional tectonic stage (Harangi et al., 2001). Trace elementand isotopic compositions of the volcanic rocks formed from 16.5 to10 Ma are different, suggesting an evolution towards the alkaline ba-salts (Harangi et al., 2007). Amphiboles of different andesite volcanicstages were examined (Table 1). They are classified as megacrysts, cu-mulates, and phenocrysts occurring in basaltic andesites and andesites.

2.4. Ophiolite series of the Carpathian–Pannonian Region

There are two major ophiolitic series in the Carpathian–PannonianRegion: the Penninic system (the Eastern end of the Alpine nappe sys-tem), and the Meliata–Vardar system (from SE-Slovakia to N-Hungary).The Penninic system contains the Bündnerschiefer series, consistingof phyllites, calcareous phyllite, quartzites, and metaconglomerates,and a greenschist-dominated ophiolite complex with ultramafic rocksand gabbros (Koller and Pahr, 1980). Plagiogranites, rodingites andophicarbonates occur sporadically within this complex. The ultramaficrocks are strongly serpentinized and some magmatic relics are pre-served in the gabbroic rocks only (Koller, 1985). The metamorphic evo-lution can be divided into three major events: 1) oceanic hydrothermalactivity, 2) subduction-related HP/LT metamorphism, 3) late Alpinegreenschist facies metamorphism (see Koller, 1985). The ophiolite se-quence contains N-MORB-type basalts (Koller, 1985) and harzburgitesthat originated from suboceanic mantle (Meisel et al., 1997; Melcheret al., 2002). Gabbros with different chemical compositions (Fe-, Ti-,and Mg-rich) have also been distinguished (Koller, 1985), indicatingthat the original magmatic geochemical compositions have been pre-served despite the subsequent metamorphism. Massive serpentinites,gabbroic dikes, serpentinized harzburgites and ophicarbonates weresampled. Stable isotope compositions of the Penninic rocks have beenreported by Demény et al. (2007) and a full description of geologicalbackground and samples is given there. Trace element analyses weremade on the same samples studied by Demény et al. (2007).

Coupled stable isotope and trace element analyses of the ophioliticseries of the Meliata–Vardar system (the Meliata, the Bódva Valley

Table 1List of the studied samples of the Miocene calc-alkaline rocks of the Western Carpathians (

Sample name Location

VER-A Verőce, Börzsöny Mts., northern HungarySATj Siatoros, southern SlovakiaHV95, Hv12, Hv22, ZARV4 Holdvilág creek, Visegrád Mts., northern HungarySAL Salabsina creek, Visegrád Mts., northern HungarySP4x southern Pol'ana, central SlovakiaFA1 Pieniny klippen belt, MoraviaVV-E Vlci vrch, central Slovakia

and the Darnó–Szarvaskő complexes) are given for the first time inthis paper. Metabasalts, metagabbros and serpentinites representingremnants of a dismembered Triassic–Jurassic ophiolite suite areembedded in an accretionary mélange complex of the innermostWestern Carpathians. Samples were collected from various localitiesrepresenting: 1. blueschist facies metabasites of the Meliata Unitin Slovakia, 2. polymetamorphic Bódva Valley Ophiolite Complexrocks (blueschist–greenschist facies), and 3. Triassic-Jurassic Darnó-Szarvaskő Unit rocks. The blueschist facies metabasalts of the MeliataUnit outcrop in close association with metasediments and othermetavolcanic rocks of different metamorphic grade. According to thecomprehensive studies of Faryad (1995a,b), the unit contains verylow- to low-grade metamorphic rocks of magmatic and sedimentaryorigin. Árkai et al. (2003) described the petrological andmetamorphicfeatures of the metasedimentary rocks, focusing on the characteristicsof the phyllosilicates. They suggested that the low-temperature pro-grade metamorphism of the metasediments was synchronous withthe retrograde metamorphism of the blueschists (between 150 and120 Ma).

The metamafic rocks in the Bódva Valley Ophiolite Complex donot form a single tectonic unit, as they are imbricated with theunmetamorphosed, mostly Mesozoic sedimentary sequences of theSilica Unit. The metabasalts and metagabbros have been influencedby ocean floor hydrothermal as well as blueschist facies regionalmetamorphism with a pervasive (mostly) greenschist facies over-print (Horváth, 2000; Horváth and Árkai, 2005). The metabasaltsand metagabbros of the Darnó-Szarvaskő Unit do not contain high-pressure minerals. The Triassic metavolcanites (mostly pillow lavas)occur as olistoliths in a Jurassic clastic sedimentary matrix (mostlyshaleswithminor sandstones and radiolarites). The Jurassicmetabasicrocks are part of a volcano-sedimentary complex with intrusive rela-tionships to the sedimentary rocks (Józsa and Kovács, 2004). Theblueschist facies metamorphism occurred during the Middle Jurassic(150–165 Ma, K–Ar and 40Ar–39Ar ages obtained on phengites,Faryad and Henjes-Kunst, 1997). For a detailed study on the geochem-istry of the (meta)igneous rocks and implications for the genesis andtectonic setting see Harangi et al. (1996) and Faryad et al. (2002).

Samples were collected from various localities representing as-semblages with different metamorphic histories. Samples from theSlovakian part of theMeliata Unit are classic blueschists with a miner-alogy similar to the rocks described in Faryad (1995a,b), while theMORB-type Bódva Valley metabasites have a polymetamorphic origin(ocean floor hydrothermal metamorphism overprinted by regionalmetamorphism ranging from blueschist to greenschist facies, Horváth,2000; Horváth and Árkai, 2005). The Triassic MORB-type metabasitesof the Darnó-Szarvaskő Unit were subjected to ocean floor hydro-thermal alteration only, while metabasalts and metagabbros fromthe back-arc related to the Jurassic Szarvaskő Unit were subjected toocean floor hydrothermal alteration overprinted by low-grade meta-morphism (Sadek Ghabrial et al., 1996).

3. Analytical methods

TheO isotope compositions of silicatemineralswere determined atthe University of Lausanne, Switzerland, using a laser-based method

Northern part of the Pannonian basin).

Rock type Age (Ma) Amphibole type

Garnet-bearing andesite 16 Megacryst and phenocrystGarnet-bearing andesite 15 Megacryst and phenocrystAndesite 15 Megacryst and phenocrystsAndesite 15 PhenocrystAndesite 14 Cumulate (amphibole-rich inclusion)Basaltic andesite 13 PhenocrystBasaltic andesite 10 Megacryst

adapted after Rumble and Hoering (1994) and described in moredetail in Kasemann et al. (2001). Water content and H isotope com-positions were determined using a TC/EA method adapted after thatof Sharp et al. (2001). The raw data were corrected using the NBS-30biotite standard and two laboratory standards (Kaol#17 kaolinite andG1 biotite). The isotope compositions are expressed in the δ-notation(δ=(R1/R2−1)·1000 where R1 and R2 are the D/H and 18O/16O ratiosin the sample and the standard, respectively) in permil (‰) relativeto V-SMOW. The following δ18O values were obtained for internationalstandards in the course of the study: NBS-28 quartz 9.66±0.05‰ (n=10; accepted value: 9.58‰; Hut, 1987), NBS-30 biotite 5.12±0.08‰(n=4; accepted value: 5.1‰; Hut, 1987), UWG-2 garnet 5.88±0.13‰ (n=20; accepted value: 5.8‰, Valley et al., 1995). Given the re-producibilities of duplicates of samples and standards, the δ18O valuesare precise to within 0.15‰ (1σ). A H2O content of 3.53±0.15 wt.%and a δD value of −65.4±1.9‰ (n=9; accepted values: 3.5 wt.% and−65.7‰) were obtained for the NBS-30 biotite standard in the courseof this study. The H isotope compositions of the laboratory standardsobtained in this study were always to within ±2‰ from the expectedvalues, the reproducibilities were better than ±2‰. The precision ofH2O content is better than 0.15 wt.%.

Trace element analyses of amphiboles were conducted usingLA-ICP-MS technique at the laboratories of the University of Utrechtand the Southampton Oceanography Centre. Bulk rock samples ofophiolites were analysed at the University of Lausanne.

3.1. University of Utrecht

Trace elements of amphiboles were measured by LA-ICP-MS usinga 193 nm ArF excimer laser ablation system (MicroLas GeoLas 200Q)in combination with a quadrupoleMicromass Platform ICP-MS instru-ment (Mason and Kraan, 2002) following the methodology describedby Harangi et al. (2005). Ablation was performed at a fixed point onthe sample with an irradiance of 0.2 GW cm−2, a laser pulse repeti-tion rate of 10 Hz and an ablation crater diameter of 40–80 μm. Thesignal recorded by the ICP-MS during ablation was carefully checkedfor compositional boundaries to ensure that only data for the amphi-boles were integrated. Quantitative concentrations were calculatedusing NIST SRM 612 as a calibration standard (Pearce et al., 1997)with Ca (previously determined by electron microprobe analysis) asan internal standard element. The USGS reference glass BCR-2G wascontinuously measured throughout the analysis of the amphiboles(see Supplementary Material 2) and the results were within 5–10%of recommended values. Detection limits were typically in the range0.01–1 μg g−1 and internal precision was b5% RSD (1σ) for concen-trations above 1 μg g−1 and b15% RSD (1 σ) below 1 μg g−1.

3.2. Southampton Oceanography Centre

Laser ablation ICP-MS trace element analyses were carried out ona VG Elemental PQ2+ICPMS coupled to a 4D Engineering (Hannover,Germany) excimer laser system. Details of the analytical procedureare given in Demény et al. (2004b).

3.3. University of Lausanne

Bulk rock trace element abundances in ophiolites were analysedby LA-ICP-MS on fused lithium tetraborate glass discs at the Instituteof Mineralogy and Geochemistry using a GeoLas 200M 193 nm ArFexcimer laser ablation system (Lambda Physik, Germany) interfacedto an ELAN 6100 DRC quadrupole ICP-MS (Perkin Elmer, Canada).Operating conditions of the laser included an on-sample energy densityof ca. 15 J/cm2, a 10 Hz repetition rate and a 120 μm pit size. Heliumwas used as a cell gas. The acquisition times for the background andthe ablation interval corresponded to about 60–70 and 35 s, respective-ly. Dwell times per isotope ranged from 10 to 20 ms, peak hopping

mode was employed. The ThO+/Th+ and Ba2+/Ba+ ratios were opti-mized to c. 0.5–0.6 and 2.7%, respectively. The NIST 612 glass standardwas used for external standardization, using the average element con-tent after Pearce et al. (1997). 42Ca served as an internal standard.Intensity vs. time data were reduced in LAMTRACE (Jackson, 2008).All spectra were checked for the presence of surface contaminationand intensity ‘spikes’ and corrected when necessary. The analysis of ablank glass corresponding to 100% lithium tetraborate had trace ele-ment abundances well below the sample values.

4. Results

The compositions of amphiboles from the Canary Islands and fromthe alkaline basalts of the Carpathian–Pannonian Region (CPR) havebeen published and only representative mean values are given inSupplementary Table 1. All of the samples are Ca-amphiboles, theamphiboles from La Palma are kaersutites (Demény et al., 2008),those of Fuerteventura and the CPR alkaline basalts are kaersutites,Mg-hastingsites, and pargasites (Demény et al., 2004a, 2005). Theamphiboles from the calc-alkaline suite (see Supplementary MaterialTable 1) are mostly magnesiohastingsites, whereas the phenocrystsare tschermakites on the basis of the IMA classification scheme(Leake et al., 1997; Fig. 1). Comparing them with the amphibolesin andesitic rocks, the calc-alkaline amphiboles have relatively highalumina contents (especially the andesite-hosted megacrysts withAl2O3 >15 wt.%). The amphiboles from the basaltic andesites havehigher TiO2 contents (2.4 to 2.6 and 3.1 to 4.1 wt.% for the amphibolesfrom Vlci Vrch and the Pieniny klippen belt, Table 1) than most of thecalc-alkaline amphiboles (Fig. 2). The high Al2O3 content (Al2O3>14 wt.%) could be indicative of crystallization at higher pressure andtemperature (Bachmann and Dungan, 2002; Harangi et al., 2001,2007), whereas the elevated TiO2 content may imply a compositionaldependence on the host magma composition. Remarkably, the am-phiboles of the basaltic andesites deviate from the other amphibolegroups and resemble the magnesiohastingsites found in the 2001eruptive products of Etna (Viccaro et al., 2007). The amphibolesfrom the basaltic andesites and the Neogene alkaline basalts havevery similar compositions (Figs. 1 and 2), whereas the Fuerteventuraand especially the La Palma amphiboles have lower Al2O3 and higherTiO2 contents compared to the Etna samples. Figs. 1 and 2 show thatthe main difference between these amphiboles is in the TiO2 contents,while in terms of the Si and mg# values the studied samples arerather similar. This suggests that trace element compositions maybe influenced by crystal chemical effects, given their variations inTiO2 contents. However, the studied amphiboles are expected topartition the trace elements in a similar fashion during crystalliza-tion (cf. Tiepolo et al., 2000, 2007).

Stable isotope compositions of ophiolitic rocks and their mineralseparates (amphiboles, excepting PEN-8/6 talc) are listed in Table 2and are shown in Fig. 3. The metagabbros of the Meliata–Vardar serieshave preserved values that can be considered to represent normalupper mantle values with δ18O of about 5.5‰ and δD of about−70‰, whereas the glaucophane schists have high δ40O and δDvalues of up to 16‰ and about −50‰, respectively. Values from thePenninicminerals and rocks practically overlapwith those of mineralsfrom Meliata–Vardar, with one exception: PEN-3/3 with a δD value of−106‰. This value was interpreted by Demény et al. (2007) as beingrelated to interaction with infiltrating meteoric water during meta-morphism. The δD values plotted as a function of H2O contents show asimilar distribution (Fig. 3B), with the Meliata–Vardar rocks containingminerals with low water content and normal mantle δD values ingeneral, although there might be the tendency to have values enrichedin D at higher water contents. The Penninic rocks overlap with the fieldfor the Meliata–Vardar rocks, with the exception of some serpentiniteswith higher water contents but similar isotopic composition.

Fig. 1. Amphibole classification plot following Leake et al. (1997). Calculation of theamphibole cation numbers and the estimation of the ferric ironwere performed followingthe procedure of Schumacher (1997). Published data: solid circles—La Palma (Deményet al., 2008), empty squares—Fuerteventura (Demény et al., 2004a), solid triangles—CPRalkaline basalts (Demény et al., 2005). This study, calc-alkaline (CA) series of the CPR:grey filled squares.

Fig. 2. TiO2 vs. Al2O3 (in wt.%) contents in the amphiboles of the Canary Islands and theCarpathian Pannonian Region. See Fig. 1. for legends. Grey field: magnesiohastingsitesfound in the 2001 eruptive products of Etna (Viccaro et al., 2007). Small empty circles:andesites worldwide gathered from the GEOROCK database (http://georoc.mpch-mainz.gwdg.de/georoc/).

Table 2Stable hydrogen and oxygen isotope compositions (δD and δ18O values in‰ relative toV-SMOW) and H2O contents (in wt.%) of mineral separates and bulk rock samples ofophiolites of the Meliata complex (S—Slovakia), the Bükk Mts. (NE-Hungary) and thePenninic rocks of the Kőszeg–Rechnitz complex (W-Hungary and E-Austria). Thedata for the Penninic rocks are from Demény et al. (2007).

Sample Rock type Sample H2Owt.%

δ18O δD

Vardar-Meliata ocean crustBukk 1/1 Low grade metamorphic

gabbroAmphibole 1.31 5.2 −79

Bukk 1/2 Low grade metamorphicgabbro

Chloritizedamphibole

1.48 5.3 −75

Bukk 3/1 Low grade metamorphicgabbro

Amphibole 3.98 5.5 −70

Bukk 4/A Unmetamorphic pillowbasalt

Bulk rock 3.98 14.2 −71

KO-11/30 Retrograde blueschist Chloritizedamphibole

2.27 6.5 −76

SZo-4/190 Low grade metamorphicgabbro

Brown amphibole 1.78 5.6 −64

TK-3/494 Low grade metamorphicgabbro

Matrix actinolite 1.99 13.5 −70

H1 Glaucophane schist Glaucophane 2.35 12.2 −64SZ04-1 Unmetamorphic pillow

basaltBulk rock 5.02 12.3 −53

SZ04-2 Glaucophane schist Bulk rock 2.87 13.2 −54SZ04-5 Glaucophane schist Glaucophane 2.81 16.5 −55SZ04-7c Glaucophane schist Glaucophane 4.9 13.2 −49

Penninic ocean crustPEN-3/3 Serpentinite Bulk rock 13.72 7.4 −106PEN-3/4 Serpentized harzburgite Bulk rock 13.36 6.3 −64PEN-8/2 Ophicalcite Blue amphibole 3.65 16.2 −71PEN-8/6 Ophicalcite Talc 3.24 16.2 −60PEN-10 Serpentinite Bulk rock 12.24 11.2 −85PEN-2 Serpentinite Bulk rock 4.53 16.2 −65PEN-5 High-Mg metagabbro Amphibole 1.3 5.1 −64PEN-6 High-Fe metagabbro Blue amphibole 4.72 7.4 −75PEN-7 Metagabbro Blue amphibole 2.47 15.5 −79

The trace element compositions of the ophiolitic rocks (seeSupplementaryMaterial Table 2) have a large range (Fig. 4), especiallyfor the Penninic rocks. The Meliata–Vardar rocks have rather uniformtrace element compositions, with the exception of two samples(SZo-4/190 and H1) that have higher Pb contents (Fig. 4A). ThePenninic rocks have either similar patterns to those of the Meliata–Vardar rocks (serpentinized gabbros, PEN-2,6,7), or very low traceelement contents with a sediment-like pattern (samples PEN-3,5,10;

Fig. 4B; see also Demény et al., 2004b) similar to the ophicarbonateswith carbonate of sedimentary origin (Demény et al., 2007).

Trace element compositions of amphiboles from the calc-alkalineseries of the CPR as well as representative compositions for theamphiboles from the Canary Islands and from alkaline basalts of theCPR are listed in Supplementary Material Table 2. The primitivemantle-normalized trace element patterns of the different amphibolegroups (Fig. 5) have distinct trace element variations. The megacrysts

Fig. 3. Stable hydrogen and oxygen isotope compositions (in ‰ relative to V-SMOW)and H2O contents (in wt.%) of mineral separates (amphiboles and talc) and bulk rocksamples of ophiolites of the Meliata–Vardar series (SE-Slovakia and NE-Hungary) andthe Penninic rocks of the Kőszeg–Rechnitz complex (W-Hungary and E-Austria, seeDemény et al., 2007). See Table 2 for sample details. Upper mantle ranges are fromKyser and O'Neil (1984), Kyser (1986), Agrinier et al. (1993), Mattey et al. (1994)and Chazot et al. (1997), δD values for the Carpathian–Pannonian mantle region arefrom Demény et al. (2005).

and the amphiboles from cognate xenoliths typically have lower in-compatible element contents, except for Rb and Ba. The phenocrystshave higher trace element contents and negative anomalies in Sr andEu, consistent with plagioclase crystallization. Amphibole megacrystsfrom the youngest calc-alkaline rocks (basaltic andesites from VlciVrch) and amphibole phenocrysts from the Late Miocene basalticandesitic dikes of the Pieniny Klippen belt (sample FA-1, Table 1, Sup-plementary Material Tables 1 and 2) differ from both groups and havesimilar compositions. They have a slightly LREE enriched pattern, aredepleted in Rb, U, and Th (but not in Ba), and have a negative anomalyin Pb, and a negative anomaly in Zr compared to the amphibolemegacrysts. A further important feature of the trace element pattern isthe relative concentration of Th and U with respect to the neighbouringelements. A depletion of these elements is evident in the amphibolesof the basaltic andesites as well as in the megacrysts and in amphi-boles from cognate xenoliths. This trace element pattern is charac-teristic of amphiboles occurring in mantle xenoliths of intraplatesettings (Coltorti et al., 2007).

5. Discussions

Given the plate tectonic environment of the studied series, the sta-ble isotope and trace element compositions may reflect a combinationof pristine mantle compositions, crystallization-related fractionation,contamination by crustal material and late stage effects includingdegassing and alteration. The latter is not expected to play an impor-tant role in our case as the amphiboles are rather resistant to low-

temperature alteration and generally well-preserved crystals wereselected for the analyses.

5.1. Effects on hydrogen isotope compositions of amphiboles

Hydrogen isotope compositions ofminerals and rocks can be used toevaluate fluid–rock interactions and the influence of metasomatism,but the effects that canmodify the original D/H ratio should be clarified.Degassing maymodify the H2O content and the δD value of the magmaand also the amphibole. Dehydrogenation (H2 release) can cause oxida-tion in the magma and the mineral, leading to elevated Fe3+/Fetotal ra-tios. However, further magma evolution (mixing of different magmabatches, fractional crystallization, etc.) can modify the composition ofthemagma, obscuring the degassing-related H2O-δD-Fe3+ correlations.Degassing of amphiboles can result in a change towards higher positiveor negative δD values depending on the fluid species released. Dehydro-genation (H2 release) or dehydration (H2O release) would result in arelative increase or decrease in D-content, respectively (see Deményet al., 2006). On the basis of a combined evaluation of H2O-δD-Fe3+ cor-relations, Demény et al. (2005, 2006) determined the potential fraction-ation effects of dehydrogenation and dehydration of amphiboles andestablished a primary H isotope composition of about −40‰ for thePannonianmantle for the volcanic rocks analyzed. The primary compo-sition of−40‰ is higher than the range of values considered to repre-sent pristine mantle (−70±10‰, Boettcher and O'Neil, 1980; Kyserand O'Neil, 1984). These values of −40‰ were interpreted as a signof mantle metasomatism by subducted crustal material by Deményet al. (2005, 2006). Similarly, Demény et al. (2008) determined possibledegassing and alteration effects for the Canary Island amphiboles, andselected amphiboles whose compositions can be related to mantleprocesses.

5.2. Evaluation of trace element ratios as indicators of crustalcontamination and crystal chemistry effects

As described above, trace elements may be used not only to detectthe presence of crustal components in mantle-derived magmas, butalso to discriminate between fluid-dominated metasomatism or meltcontamination (e.g., Ersoy et al., 2010; Hofmann et al., 1986; Sun etal., 2004, 2008). Beside the Ce/Pb ratio that is traditionally used asan indicator of slab-derived material (Hofmann et al., 1986), thesestudies have demonstrated that combinations of trace element ratiosof Sr, Zr, Nb, Ba, Ce, Yb, Pb, Th and U can be used to decipher contam-ination and metasomatic processes. Ba/Th, Ba/Nb, and Nb/U showcorrelations with Sr and Nd isotope compositions indicating that con-tamination by crustal material, either through assimilation at crustallevels or metasomatism bymelts and fluids released from a subductedslab, can be detected by studies of trace element compositions (e.g.,Jung, 1999; Rehkämper and Hofmann, 1997; Thirlwall et al., 1997).This approach was also used by Demény et al. (2008), who describeda coupled increase in Ba/Nb (N) ratios and 87Sr/86Sr ratio in amphi-boles from the ocean island basalts of the Canary Islands and—as acomparision—from the rocks of the Carpathian–Pannonian Region. Inaddition, themobility of Ba and Sr in fluidsmake these elements effec-tive indicators of metasomatism, hence elevated Ba/Nb ratios coupledwith decreased Ce/Pb and 143Nd/144Nd ratios and higher 87Sr/86Srratios would not only reflect contamination by crustal material, butalso indicate that the contamination was induced by fluids releasedfrom the crust or subducted slab. Fig. 6A, B, and C show Th/Nb, Ba/Nb, Th/Yb, Ba/Yb, Ce/Zr, and Sr/Zr ratios, following Ersoy et al.(2010). Elevated Ba and Sr contents relative to Nb, Yb and Zrwould in-dicate fluid-induced metasomatism, whereas a relative increase of Thwould suggest melt addition (Ersoy et al., 2010). This scheme is validat relatively low pressure and temperature conditions (below about4 GPa and 1000 °C), as at pressures of 6 GPa (about 180 km depth)and 1200 °C Nb, Ta, Zr, and Hf become incompatible and enter the

Fig. 4. Trace element compositions (normalized to primitive mantle, Hofmann, 1988) of ophiolites of the A) Meliata–Vardar series (SE-Slovakia and NE-Hungary) and theB) Penninic rocks of the Kőszeg–Rechnitz complex (W-Hungary and E-Austria). Solid squares in Fig. 4A mark glaucophane schist (sample H1). Penninic system: thick grey lines—ophicarbonates; thick black lines—gabbros, serpentinized gabbros; thin black lines—serpentinites.

supercritical fluid or melt released from the subducted slab (Kesselet al., 2005). This change of incompatibility would result in a decreaseof the Ba/Nb or Sr/Zr ratios of the fluid or melt released at 6 GPa and1200 °C. Under these conditions the generated supercritical fluid istransitional between fluid and melt, though, with less than 50% fluidcontent (Kessel et al., 2005), thus the boundary between fluid andmelt is diminished both in terms of physical properties (fluid/solutecontents) and trace element concentrations.

The Th/Nb–Ba/Nb plot is consistent with the order of La Palma–Fuerteventura–CPR alkaline basalts–CPR andesites as an increasingcrustal contamination trend (see Demény et al., 2008), whereas Th/Yb–Ba/Yb and Ce/Zr–Sr/Zr plots indicate more complex processes.The andesite-hosted amphiboles show a continuous trend in the Ce/Zr–Sr/Zr plot (Fig. 6C). Those from which oxygen isotope composi-tions are available prove that the Ce/Zr–Sr/Zr trend is not related to

the amount of crustal material as it has no relationship with the δ18Ovalues (Fig. 6D). The most straightforward explanation of the Ce/Zr–Sr/Zr trend is fractionation during plagioclase and Fe–Ti mineral crys-tallization prior to amphibole formation.

Following Sun et al. (2008), the Nb/U ratios were plotted againstthe Ce/Pb ratios (Fig. 6E), giving positive trends with the La Palmaamphiboles having the highest Ce/Pb and Nb/U ratios and theandesite-hosted amphiboles at the lower end of the trend, which isin agreement with a crustal contamination trend (see above). Thegood positive correlation between Nb content and Ce/Pb ratios(Fig. 6F) would suggest that Nb concentration is related to the amountof crustal component in the melt.

However, from the selected elements, Nb is most affected by am-phibole and melt chemistry, hence crystal chemical effects shouldalso be investigated. With increasing TiO2 content in amphibole, the

Fig. 5. Trace element compositions (normalized to primitive mantle, PM, Hofmann, 1988) of amphiboles hosted by the calc-alkaline volcanic series of the Carpathian–PannonianRegion.

mineral/melt Nb partition changes (Tiepolo et al., 2000). The amphi-boles of the present study have a large range of TiO2 contents from0.2 to 7.2 wt.%. Given the amphibole/liquid distribution coefficientsfor Nb and Ti (D(Nb) and D(Ti)) after Tiepolo et al. (2000), this in-crease in TiO2 content would induce an over tenfold increase in theamphibole/melt D value for Nb and a concomitant increase in the Nbcontent of the amphibole just due to crystal chemical effects. The

positive relationship between TiO2 and Nb contents observed for thestudied amphiboles (Fig. 7A) may support the presumption of a crys-tal chemical effect, although the Nb content has a range from 1 to240 ppm, far exceeding the variation related to crystal chemistry. Incontrast, no relationship can be detected between Nb contents andMg-concentration (Fig. 7B). The effect of crystal chemistry may betracked in amphiboles if the Nb/Ta ratios behave as predicted for the

Fig. 6. Trace element ratios (following Sun et al., 2008, and Ersoy et al., 2010) of amphiboles from the Canary Islands and the Carpathian–Pannonian Region (CPR). La Palma: filledcircles, Fuerteventura: empty squares, alkaline basalts of the CPR: solid triangles, calc-alkaline rocks of the CPR: grey filled squares.

Ti–Mg#–Nb/Ta relationships established by Tiepolo et al. (2000).Predicted Nb/Ta D(amphibole/liquid) values were calculated usingEq. (2) of Tiepolo et al. (2000):

D(amphibole/liquid)=2.45–1.26 mg#–0.84 Ti(Tot). Although theymentioned that their equations are valid for titanian pargasites andkaersutites and hence andesite-hosted amphibole data should be treatedwith caution, about half of the studied amphiboles fulfil this criterium. Ifcrystal chemical effects are important during amphibole crystallization,Nb/Ta ratios would change with TiO2 variation. The degree of Nb/Ta in-crease relative to the lowest ratio found (Nb/Ta increase=Nb/Tai/Nb/Tamin) is shown in Fig. 7C. The expected positive correlation betweenmeasured Nb/Ta ratios and predicted D(Ba/Nb) values does not appear.However, amphiboles from basalts and basaltic andesites are separatedfrom those of the andesites. Tiepolo et al. (2001) reported higher HFSEcompatibility in amphibole with increasing SiO2 content in the melt.This effect would cause a decrease in the Ba/Nb ratio of the amphibole(at constant Ba concentration), acting against its use as a tracer for crustalcontamination. However, in the Ba/Nb vs. Ce/Pb plot (Fig. 7D) a negativecorrelation emerges, indicating that the Ba/Nb ratio has indeed a rela-tionship with the amount of crustal component in the melts fromwhich the amphiboles crystallized. The andesite-hosted amphiboledata are slightly shifted to lower Ba/Nb and Ce/Pb ratios from the trenddetermined by the amphiboles from basalts (Canary Islands and CPR)

and basaltic andesites (CPR), in accordance with the expected Nb com-patibility increase and concomitant Ba/Nb decrease in the amphibolesdue to higher SiO2 contents of the andesites compared to basalts. This ob-servation indicates that crystal chemistry does affect the amphiboles’compositions, but it is exceeded by the effect of crustal contamination.

In addition, the amphiboles of the present study follow the Zr–Nbclassification scheme of Coltorti et al. (2007) with samples from theocean island basalt series having the highest Zr and Nb contents inthe intraplate amphibole (I-amph) field and the andesite-hostedones in the suprasubduction amphibole (S-amph) field at the lowZr–Nb end (Fig. 8). Coltorti et al. (2007) excluded the effects of crystalchemistry as their amphiboles had restricted compositions (Mg#=80–94, SiO2=40–48 wt.%). Although the amphiboles of the presentstudy have wider compositional characteristics, their Zr–Nb distribu-tion is rather similar to that Coltorti et al. (2007). This data distributionindicates that the Nb concentration is mainly related to genetic differ-ences with crystal chemical effects being subordinate in importance.Ba partitioning between amphibole and melts is rather insensitiveto amphibole and melt chemistry (Tiepolo et al., 2007), hence theBa/Nb ratio may be effectively used as a tracer of crustal contaminationprocesses. D values for Pb and REEs would change with the SiO2 contentof melts, but large changes are expected only in the high-SiO2 rocks(Tiepolo et al., 2007). Thus, crystal chemical effectmay not exert a strong

Fig. 7. Nb content of amphiboles as a function of their TiO2 content (A) and Mg values (B). Legends as in Fig. 6. C). Amphibole/melt Nb/Ta partition coefficients (calculated usingEq. (2) of Tiepolo et al., 2000) and the degree of Nb/Ta increase in the studied amphiboles (relative to the lowest Nb/Ta ratio detected, see text). Grey filled squares: amphibolesfrom basalts (Canary Islands and the CPR) and basaltic andesites (CPR), grey filled circles: amphiboles from andesites (CPR). D). Ce/Pb vs. Ba/Nb ratios in the studied amphiboles.Legends as in Fig. 6. “ba”: basaltic andesites (see Table 1).

influence on the Ce/Pb nor the Pb/Pb* ratios. Ba/Nb(N) ratios were,therefore, selected as an indicator of fluid-induced metasomatismin order to evaluate the application potentials of the Pb/Pb*(N) valueestablished by Marks et al. (2004).

5.3. Mantle compositions and detection of crustal contamination

As Hoernle et al. (1995) have pointed out, Cenozoic basaltic rocksof the European continent carry varying amounts of a common com-ponent (referred to as the Low Velocity Component by them) whoseorigin can be found in the Canary Island plume. Consequently, theamphiboles from the Canary Island rocks can best reflect this plumecomponent. Coupled stable isotope and trace element analyses areavailable for La Palma and Fuerteventura, so these data will bediscussed first. Pb/Pb*(N) and Ba/Nb(N) ratios were calculated andnormalized to primitive mantle (Hofmann, 1988). The lowest ratiosmeasured in these rocks indicate a starting mantle composition(Demény et al., 2008). Comparing these trace element ratios with

Fig. 8. Zr vs. Nb discrimination diagram after Coltorti et al. (2007). See Fig. 6 for legends.I-amph: intraplate amphiboles, S-amph: suprasubduction amphiboles.

stable isotope compositions helped to detect that the plume-relatedmagma had a low-δ18O value that was modified by contaminationeffects, which is noted even in the amphiboles of La Palma andFuerteventura (Demény et al., 2008). The exact cause of the contami-nation is unknown at present. However, seismic studies have revealedthick (~10 km) perhaps subducted sedimentary complexes belowsome of the islands of the Canary Archipelago (Rad et al., 1982; Yeet al., 1999). Thus, the Canary Islands have a transitional position be-tween a typical ocean island system developed purely within oceaniccrust and continental basalts. The crustal contamination may derivefrom assimilation at crustal levels, source contamination by deepsubduction, or incorporation of mantle material metasomatized byslab-derived melts and fluids related to an earlier subduction. TheFuerteventura amphiboles have low δD and δ18O values and elevatedPb/Pb*(N) and Ba/Nb(N) ratios relative to the plume-related com-positions defined by the La Palma amphiboles (lowest Pb/Pb*(N)and Ba/Nb(N) ratios, δD of about −90‰, δ18O of about 5‰; Figs. 9and 10). Coexisting phlogopites from the same rocks in Fuerteventurahave δD values closer to the plume composition of−90‰ inferred forLa Palma (Demény et al., 2004a). This difference was interpreted as aresult of fractional crystallization in the sequence of phlogopite toamphibole and parallel assimilation of crustal material to affectmagma compositions (Demény et al., 2004a, 2008). The La Palma am-phiboles also generally have higher δD for higher values of Pb/Pb*(N)for differentmagmatic rocks (gabbro intrusions, lava-hosted xenolithsand megacrysts, see Demény et al., 2008), trending toward the com-positions of amphiboles hosted by alkaline basalts and calc-alkalineandesitic rocks of the CPR.

Crustal contamination could have taken place during two princi-pal processes in the CPR magmas. The mantle may have beenmetasomatized by slab-derived fluids and melts, followed by directassimilation of crustal rocks during the rise of the magma. The amphi-bole megacrysts of alkaline basalts crystallized deep enough (Dobosiet al., 2003) to exclude the latter possibility, although it cannot be ex-cluded a priori for the andesite-hosted amphiboles. The combined

Fig. 9. Pb/Pb*(N) ratios normalized to primitive mantle (Hofmann, 1988) vs. (A) oxygenand (B) hydrogen isotope compositions (in ‰ relative to V-SMOW) of amphiboles fromthe ocean island complexes of La Palma (LP, solid circles) and Fuerteventura (FV, emptysquares), from the alkaline basalts (CPR-A, filled triangles) and calc-alkaline rocks ofthe CPR (CPR-CA, grey filled squares), and bulk rocks of ophiolites of the Meliata–Vardarseries (+) and the Kőszeg-Rechnitz Penninic system (x). Data sources for basaltic amphi-boles: Demény et al. (2004a, 2005, 2006, 2008); for Penninic rocks: Demény et al. (2007).Gl. sch.: glaucophane schists of the Meliata–Vardar series. Ophic.: ophicarbonates.

Fig. 10. A) Pb/Pb*(N) ratios vs Ba/Nb(N) ratios (normalized to primitive mantle,Hofmann, 1988) of the studied amphiboles and rocks (see Fig. 9 for sample types andlegends). Data sources for amphiboles from basalts: Demény et al. (2004a, 2005,2008). Fields denoted as “calc.” mark fluid compositions in equilibrium with ophioliticrocks (“ophiolites calc.”) studied in this paper calculated on the basis of bulk rock dataand published fluid/rock partition coefficients (see text). The fields of “gl. sch. calc.”and “ophic. calc.” denote the compositions of fluids in equilibrium with glaucophaneschists of the Meliata–Vardar series and ophicarbonates of the Penninic complex, re-spectively. B) Pb/Pb*(N) ratios vs Ba/Nb(N) ratios of amphiboles of calc-alkalinerocks. Phenocrysts: circles; megacrysts and amphiboles from cognate xenoliths: solidsquares; amphiboles from basaltic andesites: triangles.

evaluation of stable isotope and trace element compositions may helpto further constrain these possibilities. Taking the relative H and Ocontents of the mantle rocks and the metasomatizing fluid into ac-count, the H isotope composition of the product will be largely deter-mined by the composition of the fluid, while the oxygen isotopecomposition of the metasomatized rock would not change much, un-less large amounts of extraneous fluids are involved. This is in contra-diction with metamorphic complexes, where shifts in δ18O valuesin metasomatized rocks can be detected, but where much largeramounts of fluids may migrate along fracture zones, leading to veryhigh fluid/rock ratios and hence to measurable changes in stable iso-tope compositions. Since stable isotope fractionations are generallylow at magmatic temperatures (O'Neil, 1986), temperature variationswithin the magmatic system do not cause large changes in the H andO isotope compositions of the metasomatized material. The main rea-son of changes in δD and δ18O values observed in metasomatizedmantle-derived rocks is the addition of crustal material in the formof fluids and/or melts released from a subducted slab with generallyelevated δD and δ18O values (>−70‰, and >7‰, respectively,Hoefs, 2010). Fluid/rock partition coefficients are higher for Ba thanfor Pb (Brenan et al., 1995b), thus the fluid released from the crustal

slab can have higher values of Ba/Nb(N) relative to Pb/Pb*(N) andthese would also be reflected by the metasomatized rocks. Continen-tal crustal rocks and especially sediments are usually more 18O-richthan magmatic rocks of the oceanic crust (see Hoefs, 2010 for a com-prehensive review), thus release of fluids and melts from the sedi-mentary part of an inhomogeneous slab would cause larger changesin δ18O values than those from magmatic oceanic crust. These as-sumptions suggest that fluid metasomatism can result in strong δDand Ba/Nb(N) shifts, while slab-derived melt infiltration carryingsedimentary components would also change the δ18O value and Pb/Pb*(N) ratio.

These effects are all shown by the δD-δ18O-Pb/Pb*(N)-Ba/Nb(N)distributions of the amphiboles from the Canary Islands and alkalinebasalts and andesitic rocks of the CPR. The δ18O-Pb/Pb*(N) plot(Fig. 9A) shows a coherent trend for the amphiboles from the CanaryIslands and the CPR alkaline basalts, whereas the andesite-hostedamphiboles have a larger scatter and higher δ18O values. This may in-dicate a higher amount of crust-derived melt or sedimentary compo-nent in the source of the calc-alkaline rocks. The δD-Pb/Pb*(N) plot(Fig. 9B) illustrates a more complex scenario as the H isotope

compositions are also affected by degassing effects. This is demon-strated by the CPR basalt-hosted amphiboles (Demény et al., 2006)that have a wide range of δD values at constant a Pb/Pb*(N) ratio.However, if degassing effects are taken into account, a H isotopecomposition of about −40‰ can be inferred for the CPR mantle(Demény et al., 2005, 2006). This composition together with theclose-to-mantle δ18O values and slightly elevated Pb/Pb*(N) ratios in-dicate that the crustal contamination was induced by slab-derivedfluids. The Canary Island and CPR basalt-hosted amphiboles formagain a coherent trend in the Pb/Pb*(N)-Ba/Nb(N) plot (Fig. 10),whereas those from calc-alkaline andesites are displaced from thistrend toward higher Pb/Pb*(N) values. These considerations raisequestions that are discussed in the following sections:

– What could the sources for the inferred crustal contamination be,and what was (were) the mechanisms influencing these rocks?

– How are the assumed processes reflected in other studies onsubduction-related tectonic settings?

5.4. Sources and mechanisms

On the basis of chemical compositions of glass veins and meltpockets in mantle-derived xenoliths of the Carpathian–Pannonian Re-gion Demény et al. (2004b) proposed that these carbonate-bearing si-licious melts derived from melting of subducted oceanic crust. In theCarpathian–Pannonian Region two major ocean crust complexes maybe taken into account: the Penninic and the Meliata–Vardar systems.Demény et al. (2007) have shown that subduction of the Penninicrocks may have released D-enriched fluids that metasomatizedthe upper mantle and that is reflected by the H isotopic compositionsof alkaline basalt-hosted amphiboles. The ophiolitic rocks of theMeliata–Vardar system have large scatters in the H2O-δD-δ18O plots(Fig. 3), originating from the diverse nature of rock types studied.The metagabbros of the Darnó-Szarvaskő Unit fall close to the CPRbasalt-hosted amphiboles in the δ18O-Pb/Pb*(N) plot (Fig. 9A),suggesting that their magmatic compositions are partially preserved.The serpentinites, serpentinized gabbros and harzburgites have ele-vated Pb/Pb*(N) values but small range in δ18O values that fits thetrend of the basalt-hosted amphiboles, and may suggest that thesetypes of rocks could have served as a source of fluids and melts. Thecalc-alkaline andesite-hosted amphiboles trend towards the Meliata–Vardar series, and especially toward the glaucophane schists. The am-phibole data extend towards the ophiolite data in the δD-Pb/Pb*(N)plot (Fig. 9B). The large scatter in this case may be explained by addi-tional processes (degassing, late-stage alteration, and isotope exchange)as outlined above.

The amphiboles from the calc-alkaline series form a rather coher-ent trend in the Pb/Pb*(N) vs. Ba/Nb(N) diagram, although somesystematic differences between amphibole types can be detected(Fig. 10B). The phenocrysts and megacrysts have a linear trend withthe latter extending towards higher values. The amphiboles fromthe Late Miocene basaltic andesites partly overlap the field denotedby the amphiboles of the Pliocene to Quaternary alkaline basalts ofthe Pannonian basin. This is consistent with the observation providedby Harangi et al. (2007) based on the bulk rock compositional varia-tion and suggests a major change in the source region of the magmasfrom a metasomatized lithospheric mantle to an enriched astheno-spheric mantle region.

Interestingly, the ophiolites do not plot on the basalt- or andesite-hosted amphibole trends in the Pb/Pb*(N) vs. Ba/Nb(N) plot (Fig. 10A),hence the ophiolites as possible sources of metasomatizing fluids andmelts for the trends should be evaluated in more detail. As we haveseen, the stable isotope data of amphiboles fromalkaline basalts (higherδD values with δ18O values close to those typical for the mantle) wouldindicate fluid-inducedmetasomatism in themantle. Thus, possible fluidcompositions that would be produced by deep degassing of the oceanic

crust slabs should be investigated. Fluid/rock partition coefficients forPb, Ba, Nb, Ce ,and Pr were taken from published data for eclogite as asource rock type (Ayers, 1998; Brenan et al., 1995b; Kelemen et al.,2003) and fluid compositions were calculated from bulk rock data ofthe ophiolite rocks studied in this paper. Plotting the calculated fluidcompositions in the Pb/Pb*(N)-Ba/Nb(N) plot (Fig. 10), the fluids frombasalt- and andesite-hosted amphiboles trend towards fluids releasedfrom serpentinites and glaucophane schists of the Meliata ophiolitesand Penninic ophicarbonates. These blueschists and ophicarbonatesmay thus represent fluid sources as they have elevated δD values, thusthe fluid emanating from the subducting slab would be D-enriched,similar to that required to account for the amphiboles of the CPR alka-line basalts (Demény et al, 2007).

5.5. Fluids vs. melts in earlier studies and comparisons with the presentdata

In order to understand the processes that can produce theobserved compositional variations, literature data on subduction-related magmatic complexes have been compiled. There are numer-ous subduction-related systems (calc-alkaline volcanic edifices, ex-posed mantle wedge complexes, metamorphic systems, etc.), butfew have been studied with a full spectrum of trace element composi-tions including Pb and Pr, and even less for the composition of amphi-boles in such complexes. To the comparative data base the amphibolecompositions reported in this paper should also be compared to thepublished bulk rock data. Thus, besides collecting published amphi-bole data, theoretical amphibole compositions were also calculatedfrom bulk rock data for rocks that do not contain amphibole orwhere amphibole compositions are not reported, using publishedbulk rock compositions and amphibole/melt partition coefficientscompiled in the GERM database. The coefficients are different for dif-ferent studies and rock types, so estimates were made on the basisof averages listed for andesites and dacites in the GERM database:Ba=0.17, Nb=0.20, Pb=0.33, Ce, Pr=0.53. The literature data areshown in Fig. 11. The field for amphiboles from the Canary Islandsand the CPR alkaline basalts is shown for comparison. The data aredivided into four groups: adakitic rocks, as these may reflect directslab melting effects (Fig. 11A), settings where mantle sources weresuggested to have been metasomatized by slab-derived fluids(Fig. 11B and C; data from Vannucci et al, 2007 are shown separatelyfor reasons of clarity) and the rocks from the Carpathian–PannonianRegion (Fig. 11D). A special case where melts derived from sedimen-tary components of a slab are thought to alter mantle compositionsin Tibet (Zhu et al., 2009) is also shown in Fig. 11D. The overall fieldfor adakitic complexes does not differ from that of the Canary Islandto CPR amphiboles, although some of the series (in Japan and Tibet)are displaced toward higher Pb/Pb*(N) ratios. The rocks originatingfrom fluid metasomatized sources may have higher Pb/Pb*(N) ratios(Fig. 11B and C), but are generally similar to the adakites, indicatingthat melt- or fluid-induced metasomatism may not be easily distin-guished. The amphibole data presented by Scambelluri et al. (2006),spanning a subduction zone from the slab to the metasomatized man-tlewedge in the Italian Alps, provides further constraints on these pro-cesses. Amphiboles formed in the metasomatized mantle wedge plotclose to the assumed fluid influenced field (lower Pb/Pb*(N) at higherBa/Nb(N) ratios), whereas those formed close to the subducted slabhave higher Pb/Pb*(N) and lower Ba/Nb(N) ratios (the trend frommantle wedge to slab is shown by the arrow in Fig. 11C). A similarshift appears in some of the adakites and in the rocks from Tibet thatare thought to contain a component from sediment melting (Zhu etal., 2009).

These distributions can be interpreted as a result ofmixing of severalprocesses: metasomatism induced by infiltration of slab-derived fluids(high Ba/Nb(N)) or melt (small increase in Ba/Nb(N)) and addition ofsedimentary slab material (increase in Pb/Pb*(N)). Comparing these

Fig. 11. Pb/Pb*(N) ratios vs Ba/Nb(N) ratios (normalized to primitive mantle, Hofmann, 1988) for various subduction-related settings. A) Adakitic rocks. Tibet: Gao et al. (2010);Ecuador: Chiaradia et al. (2009); Japan: Yamamoto and Hoang (2009); Central America: Gazel et al. (2011). B) and C) Magmatic systems where metasomatic influences inducedby slab-derived fluids are assumed; Grenada: Vannucci et al. (2007); Italian Alps: Scambelluri et al. (2006); Izu-Bonin arc: Hochstaedter et al. (2001); Kurile islands: Hoanget al. (2011). D) Carpathian–Pannonian Region: Seghedi et al. (2003); Tibet: Zhu et al. (2009). Shaded field: amphiboles from the Canary Islands and the CPR alkaline basalts.All data are calculated amphibole compositions using published bulk rock data and amphibole/rock partition coefficients, excepting those of Scambelluri et al. (2006) that areamphiboles formed in a metasomatized mantle wedge complex and the amphibole data obtained in this study. The thick arrow shows a trend from the metasomatized mantlewedge to the subduction zone.

distributions with those obtained for amphiboles presented in thispaper suggests that the amphiboles from the Canary Island and CPR al-kaline basalts indicate mantle sources altered by infiltration of slab-derived fluids. Since a thick sedimentary complex has been detectedunder some of the Canary Islands (Rad et al., 1982; Ye et al., 1999)and Fuerteventura is less than 200 km from the African continentalcrust, contamination by crustal material can be assumed in Canary is-landmagmas. The data distribution of amphiboles shown here suggeststhat the contamination of basaltic magmas may have been induced byfluid infiltration released from crustal complexes. The metasomatismmay have taken place either in the mantle related to earlier subductionevents, or at crustal levels due to heating of sedimentary rocks that re-leased volatiles.

The above scenario is in agreement with the H isotope changesobserved in the amphiboles of the Canary Island rocks and the CPR ba-salts and andesites. The changes in δD values in the La Palma andFuerteventura samples have been described by Demény et al. (2008)to be related to an interaction with external fluids. The high δD values(up to −40‰ excluding those samples that were subjected todegassing) of the amphiboles of the alkaline basalts of the CPR serveas a sign of mantle metasomatism related to fluids released fromsubducted ocean crust. These fluid effects are also given by the Pb/Pb*(N)–Ba/Nb(N) data distribution with the amphiboles of CPR alka-line basalts plotting at the high Ba/Nb(N) end. In contrast to thebasalts, the calc-alkaline andesitic series of the CPR contain a sedimen-tary component added to the melt by direct assimilation at crustallevels or by infiltration of melts derived from sedimentary precursors

as indicated by their higher Pb/Pb*(N) ratios and δ18O values. Thatthese melts had exchanged with a fluid is suggested by the high δDvalues of the andesite-hosted amphiboles as well as their high δ18Ovalues.

6. Conclusions

The question of fluid vs. melt induced metasomatism in the man-tle source of volcanic systems has been addressed for a number oflocations worldwide. Combined trace element and radiogenic isotopecompositions are frequently used to distinguish between these twoprocesses. However, stable isotope compositions are rarely deter-mined along with trace element or radiogenic isotope compositions,even though stable H and O isotope compositions are more directlyrelated to fluid–rock or fluid–melt interactions. Amphiboles can beespecially useful in this case as they are characterized by high traceelement concentrations and contain both hydrogen and oxygen. TheH and O isotope compositions and a full spectrum of trace elementcompositions including Pb and the indicator ratio, Pb/Pb*(N) (definedas Pb/√(Ce·Pr) normalized to primitive mantle, as proposed byMarkset al., 2004) can be used to constrain fluid–rock or fluid–melt interac-tions. This combined evaluation of stable isotope compositions andtrace element ratios (Pb/Pb*(N) and Ba/Nb(N)) has allowed distinc-tions to be made between fluid-induced metasomatism and siliciousmelt addition, the latter most probably derived from melting of sedi-mentary slab components. Some of the rocks of La Palma (CanaryIslands) define plume-related mantle compositions with low Pb/

Pb*(N) and Ba/Nb(N) ratios and δD and δ18O values of about −90‰and 5‰, respectively. Changes in composition from those typical ofa mantle plume for amphiboles from Fuerteventura (Canary Islands)can be partially attributed to interaction with external fluids releasedfrom crustal complexes, although the exact source is not known atpresent. Fluids and melts derived from subducted ocean crust havebeen inferred for the mantle metasomatism in the Carpathian–Pannonian Region (CPR). New trace element and stable isotope com-positions are presented for ophiolites of the Carpathian–PannonianRegion (CPR) in order to investigate the possibility of a contributionfrom the Penninic subducted oceanic crust and/or the Meliata–Vardarsystems. It is suggested that oceanic crustal blocks similar to theblueschists of the Meliata series could have served as a source offluids in the mantle region of the CPR. The high δD values and Ba/Nb(N) ratios with modestly high δ18O values and Pb/Pb*(N) ratiosfor amphiboles of the alkaline basalts of the CPR support fluid-related metasomatism, whereas higher amounts of slab-derivedmelt is compatible with the andesite-hosted amphibole compositions(high δ18O values and Pb/Pb*(N) ratios).

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2012.07.001.

Acknowledgments

The work presented in this study was financially supported bythe Hungarian National Research Fund (OTKA T 43098 and OTKA T029078). Financial support to A.D. was kindly provided by theFondation Herbette, allowing for a three-month stay at the Universityof Lausanne where analyses of silicate minerals were conducted.Further support was received from projects CGL 2006-00970/BTEand CGL2009-07775 of the Spanish Ministry of Education and Scienceand the Spanish Ministry of Science and Innovation, 2008/0250(Agencia Canaria de Investigación, Innovación y Sociedad de laInformación del Gobierno de Canarias), from an Integrated Action be-tween Hungary and Spain (HH2004-0027 and MEC E-38/01, E-22/04)and the International Complementary Action PCI2006-A7-0520 ofthe Spanish Ministry of Education and Science. The help of ProfessorFriedrich Koller was essential during sampling of the Penninic rocks.A.D. thanks the Alexander von Humboldt Stiftung for providing traveland accommodation costs to attend the PERALK-CARB 2011 meetingin Tübingen. Official reviews provided by Ralf Halama and MassimoTiepolo greatly helped to clarify our ideas and are gratefully acknowl-edged. The authors are deeply indebted to Michael Marks for his edi-torial help.

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Amphiboles as indicators of mantle source contamination: Combined evaluation of stable H and O...

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