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The magma source at Mount Etna volcano: Perspectives from the Hf isotopecomposition of historic and recent lavas

Marco Viccaro a,⁎, Eugenio Nicotra a, Ian L. Millar b, Renato Cristofolini a

a Università di Catania, Dipartimento di Scienze Geologiche, Corso Italia 57, I-95129, Catania, Italyb British Geological Survey, NERC Isotope Geosciences Labs, Kingsley Dunham Centre, Keyworth, NG12 5GG, Nottingham, England, United Kingdom

a b s t r a c ta r t i c l e i n f o

Article history:Received 9 September 2010Received in revised form 6 December 2010Accepted 21 December 2010Available online 25 December 2010

Editor: D.B. Dingwell

Keywords:VolcanismSouthern ItalyMantle heterogeneityMetasomatismFOZOEM1

High-precision Hf isotopic data for selected historic (pre-1971 eruption) and recent (post-1971) Etnean lavasare discussed in light of their covariation with previously reported Sr–Nd–Pb isotope ratios. 176Hf/177Hf rangesbetween 0.282961 and 0.282989 in the historic lavas, and between 0.282943 and 0.282990 in the recent ones,displaying, especially among the most recently erupted products, an overall decrease. In Hf–Nd and, to aminor extent, Hf–Sr–Pb isotope spaces, Etnean lavas plot slightly below the linear array defined by MORB andOIB, between the enriched end of the FOZO and HIMU fields. This may suggest that, prior to the formation ofEtnean melts, the garnet-bearing source experienced an episode of low-degree melting; this source thenevolved for enough time to produce the 176Hf/177Hf and 143Nd/144Nd decoupling observed in the historic andrecent lavas. In addition, the distinct εHf values of the historic and recent lavas may be accounted for byportions of the mantle with distinct Lu/Hf ratios, likely related to decreasing modal proportions of garnet infavour of metasomatic phases involved in partial melting. Integration of all the available data suggests thatFOZO is the dominant component in the source of Etnean magmas, although this is not enough to solve theobserved 176Hf/177Hf change coupled with the Sr–Nd–Pb variation through time. Binary mixing betweenFOZO and EM-type components demonstrates that addition of about 10% of an EM1-type component to FOZOcan account for the isotopic variability of Etneanmagmas, a feature more emphasized in themost recent lavas.Divergences from a dominant FOZO signature of erupted magmas at Mt. Etna may therefore be attributed tosmall-scale heterogeneity produced by access into the source of enriched material with an EM1-typesignature. The coupled analysis of 176Hf/177Hf and trace element ratios, such as Sm/Hf and Th/Hf, providesevidence that this enriched infiltrating component could be metasomatizing silicate melts rather thanaqueous, low-T fluids.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Variability of radiogenic isotope ratios in basalts is a key factor forascertaining mantle heterogeneity, defined in terms of distinctmantle components (e.g., Zindler and Hart, 1986). However, thereasons for such isotopic variation still remain partly enigmatic evenat a global scale, although the increased quality of investigations andthe amounts of available data in the last decades have allowed moredetailed information about geochemistry of the mantle and itsevolution to be obtained. The development of mantle heterogeneityand unusual isotopic signatures, particularly of ocean island basalts,has been chiefly ascribed to subduction processes, which introduceoceanic crust, sediments and fluids into the mantle (e.g., Hofmann,1997 and references therein). Chemical and physical disequilibriumconditions of the subducted material can in turn trigger other

processes in the mantle including hydration, metasomatism andhydrothermal alteration, which finally lead to different paths ofgeochemical and isotopic evolution of the mantle at any possiblescale (e.g., Hofmann, 1997; Meibom and Anderson, 2003 andreferences therein).

Several previous authors have published isotopic data for Etneanmagmas (Carter and Civetta, 1977; Condomines et al., 1982, 1995;Marty et al., 1994; Tonarini et al., 2001; Corsaro and Cristofolini, 1996;Tanguy et al., 1997; Gasperini et al., 2002). The source characteristicsand their role at the melting level have been recently discussed byViccaro and Cristofolini (2008). However, the question of whichmantle components are involved in the origin of Etnean magmas stillremains a matter of debate. Several authors have suggested that aHIMU-type component superimposed on a depleted one (MORB orDM) is involved in the production of Etnean magmas (Carter andCivetta, 1977; Condomines et al., 1982; Gasperini et al., 2002).Recently, other authors (Bell et al., 2004; Cadoux et al., 2007)associated these features to the C (common component; cf. Hanan andGraham, 1996) or FOZO (FOcus ZOne; cf. Stracke et al., 2005)

Chemical Geology 281 (2011) 343–351

⁎ Corresponding author. Tel.: +39 095 7195755; fax: +39 095 7195760.E-mail address: m.viccaro@unict.it (M. Viccaro).

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component. Finally, Viccaro and Cristofolini (2008), on the basis of theintegrated analysis of trace elements and Sr–Nd–Pb–O isotopecompositions, attributed features of the Etnean source to a depletedcomponent plus a HIMU-type one (forming FOZO by mixing) uponwhich EM1 contributions may be superimposed.

Here, we present the first systematic study based on high-precision MC-ICP-MS Hf isotope data for Etnean lavas and discussthe covariation of previously analyzed trace elements and radiogenicisotope ratios (Sr–Nd–Pb from Viccaro and Cristofolini, 2008) in thelight of 176Hf/177Hf ratios obtained on the same samples. This worksignificantly extends our knowledge of the Etnean magma sourcefeatures, contributing particularly to understanding the processes thathave led to the physical characteristics of the mantle. This studyreinforces the idea that the small-scale mantle heterogeneity could bethe reason for short-term variations of the geochemical signature ofMt. Etna alkaline magmas.

2. Volcanological and geochemical background

Mount Etna is a composite stratovolcano located near the easterncoast of Sicily (Southern Italy) at the intersection of three structuraldomains (Fig. 1a), namely: 1) the Apennine–Maghrebian overthrustbelt to the north, related to the collision of the European and Africanplates; 2) the Hyblean Plateau to the south; 3) the Ionian oceanicdomain to the east, considered a remnant of the Mesozoic Tethys(Cristofolini et al., 1985; Bousquet et al., 1988; Scribano et al., 2009).Several authors provided data concerning the Etnean magmatism andits geodynamic setting in an attempt to explain the unusual location ofthe volcanic structure in relation to the complex geodynamicframework of the central Mediterranean area (Fig. 1a; e.g., Gvirtzmanand Nur, 1999; Doglioni et al., 2001; Schellart, 2010). Volcanic activityin the Etnean region began ~500 ka ago (Gillot et al., 1994; Branca et al.,2008) with emission of submarine tholeiitic lavas (Kamenetsky andClocchiatti, 1996 and references therein), followed by eruption ofsubaerial tholeiites (~300 ka ago). Products gradually changed incomposition ~200 ka ago to transitional and later to Na-alkaline(Tanguy et al., 1997, and references therein). The later activity involvedalkaline central-conduit edifices, forming a sequence of several distinctvolcanic centres characterized by cycles of strombolian and effusiveactivity, alternating with violent explosive episodes associated withcaldera-forming collapses. Throughout the historical record (since 1329AD) and particularly over the last few centuries, volcanic activity hasbeen dominated by effusive eruptions and strombolian ejections fromsummit craters or monogenetic vents on the volcano flanks, with asignificant short-term increase in eruption frequency, emission ratesand amounts of erupted magma after the benchmark represented bythe 1971 eruption (Viccaro and Cristofolini, 2008 and referencestherein). In addition, some authors have shown that trachybasalticmagmas erupted since the 2001 eruption are LIL- and volatile-enrichedcompared to the previous ones, and are often associated withuncommonly explosive dynamics of extrusion. This led to the 2001event being considered as themost recent turning point concerning thegeochemistry of lavas and eruptive behaviour ofMt. Etna (Clocchiatti etal., 2004; Allard et al., 2006; Viccaro et al., 2006; Ferlito et al., 2008,2009a; Collins et al., 2009).

Many hypotheses were suggested to account specifically for theprogressive short-term enrichment in K, Rb, Cs, 87Sr/86Sr anddepletion in Th, 143Nd/144Nd, 206/Pb/204Pb, 207/Pb/204Pb and 208/Pb/204Pb since the 1971 eruption. Early hypotheses pointed towardsselective contamination by crustal material underlying the volcano(Joron and Treuil, 1984; Clocchiatti et al., 1988; Marty et al., 1994;Condomines et al., 1995; Tonarini et al., 1995; Tanguy et al., 1997),although the following studies, based on more extensive geochemicaland isotopic databases, definitively dismissed a significant contribu-tion from the sedimentary basement as being responsible for thevariations. Conversely, all the available data suggest that these

changes are related to distinct partial melting mechanisms coupledwith heterogeneous compositions of the magma source region (e.g.,Armienti et al., 1989; Barbieri et al., 1993; Corsaro and Cristofolini,1996; Peccerillo, 2005; Viccaro and Cristofolini, 2008). Other authorshave associated these differences with the progressive contribution ofan island-arc component connected with the southward migration ofthe Ionian slab (Schiano et al., 2001) or to the metasomatic action ofaqueous fluids derived from the subducting Ionian slab (Tonariniet al., 2001).

3. Sample selection and analytical methods

Some of the volcanic rocks from the extensive database of traceelements and Sr–Nd–Pb–O isotope data provided by Viccaro andCristofolini (2008), have been analyzed in order to obtain high-precision Hf isotope data. Samples are lavas collected on the southernand northeastern flanks of the volcano from thirteen flow units ofselected eruptions ranging from 1763 to 2005, nine of which eruptedafter 1971 (Table 1). On the whole, these lavas are geochemically andisotopically homogeneous within the same eruption, although themost recent eruptive episodes showed the simultaneous eruption of

Fig. 1. A) Structural and geodynamic framework of Southern Italy with the principaldomains involved in active tectonics (modified after Monaco et al., 2005); B) DigitalElevation Model (DEM; from Monaco et al., 2008) of Mt. Etna volcano with location ofthe analyzed volcanic rocks.

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geochemically distinct magmas (e.g., Viccaro et al., 2006; Ferlito et al.,2009b). This led us to analyze more volcanic rocks from the eruptionsof 2001 (three samples from three main active sites) and of 2002–03(two samples from the fracture systems active on the northeasternand southern volcano flanks respectively). Analytical procedures usedfor obtaining trace element compositions as well as Sr–Nd–Pb isotopedata used here are extensively described in Viccaro and Cristofolini(2008). However, given the rather high detection limit for Hf (1 ppm)and Lu (0.05 ppm) reported in that dataset, Hf and Lu concentrationson the selected samples have been reanalyzed at the SGS Laboratories(Toronto, Canada) by means of a Perkin Elmer ELAN 6100 inductivelycoupled plasma mass spectrometer after four acid digestions (hydro-fluoric–perchloric–hydrochloric–nitric) of the rock powders(Table 1). Four calibration runs were performed on internationalcertified referencematerials at the beginning and end of the analyticalsession. This method ensures a detection limit for Hf of 0.02 ppm andfor Lu of 0.01 ppm, with precision of the order of 5%.

Samples of knownHf content (4–5 ppm), wereweighed in Savillexdissolution vessels, and dissolved by a standard HF–HNO3 digestion.Hf was separated using a single LN-SPEC column procedure followingthe method of Münker et al. (2001). The Hf isotope composition wasthen analyzed at the NERC Isotope Geosciences Labs of the BritishGeological Survey on a Nu-plasma HR multicollector plasma ioniza-tion mass spectrometer, using static multicollection (Table 1).Correction for Lu and Yb interference on mass 176 was performedby standard reverse-mass-bias correction of empirically predeter-

mined values in all the samples. 176Lu/177Hf and 176Yb/177Hf ratioswere b10−6 for all samples, so corrections were negligible. Sixteenanalyses of the JMC475 solution standard gave a value of 0.282150±0.000008 (2σ). One analysis of the BCR-2 rock standard analyzedwiththe samples gave a 176Hf/177Hf value of 0.282878±0.000006 (2SE),after normalization to a value of 0.282160 for JMC475. This isindistinguishable from the preferred value for BCR-1 of 0.282879±0.000008 quoted by Blichert-Toft (2001).

4. Results

Hf isotope ratios measured on the selected samples are presentedin Table 1. As observed from the integrated analysis of Sr–Nd–Pbisotope compositions (cf. Viccaro and Cristofolini, 2008), complexvariations also occur for 176Hf/177Hf within the Etnean historic lavas(before the 1971 eruption, hereafter HL) and the recent ones (post-1971, hereafter RL). 176Hf/177Hf in HL ranges between 0.282961 and0.282989; it displays a negative correlation with 143Nd/144Nd and apositive one with 87Sr/86Sr (Figs. 2–3). In contrast, a lack of relevantcorrelation has been observed between 176Hf/177Hf and 208Pb*/206Pb*[calculated as (208Pb/204Pbmeasured−29.476)/(206Pb/204Pbmeasured−9.307); cf. Galer and O'Nions, 1985; Fig. 3]. Larger variations areobserved in RL, as 176Hf/177Hf ranges between 0.282943 and0.282990. The Hf isotope variation for RL is relatively similar to thatof HL only for the RL erupted before 2001. Products emitted since2001 reach the lowest 176Hf/177Hf values within the suites. On the

Table 1Hf and Lu data (ppm) and Hf isotope data for some historic (pre-1971; HL) and recent (post-1971; RL) Etnean volcanics.

Sample Latitude Longitude Eruption year Hf Lu 176Yb/177Hf 179Hf/177Hfa 178Hf/177Hfb 176Hf/177Hfc ±2σd

HL MNT3 37° 42′ 52.58″N 15° 0′ 13.73″E 1763 4.58 0.30 0.000005 0.747272 1.467268 0.282977 0.000009CAL8 37° 41′ 56.59″N 15° 0′ 39.76″E 1766 4.89 0.32 0.000003 0.747552 1.467244 0.282989 0.000009GEM5 37° 38′ 18.58″N 14° 59′ 58.01″E 1886 4.44 0.30 0.000000 0.747161 1.467202 0.282979 0.000010SIL2 37° 41′ 37.58″N 15° 1′ 0.64″E 1892 4.79 0.30 0.000003 0.747049 1.467230 0.282961 0.000009

RL ET74 37°44′37.76″N 14°55′18.26E 1974 4.74 0.32 0.000004 0.747049 1.467202 0.282971 0.0000091ET83 37° 40′ 2.77″N 14° 59′ 13.52″E 1983 5.19 0.35 0.000004 0.747328 1.467240 0.282990 0.0000092ET85 37° 42′ 5.53″N 14° 59′ 31.26″E 1985 5.38 0.35 0.000002 0.747049 1.467231 0.282981 0.000010CAL4 37° 42′ 1.16″N 15° 0′ 9.97″E 2001 — Vent 2100 m 4.66 0.32 0.000002 0.747328 1.467241 0.282953 0.000009LAG11 37° 43′ 12.90″N 15° 0′ 20.77″E 2001 — Vent 2550 m 5.15 0.33 0.000001 0.747161 1.467184 0.282954 0.0000093SE-PL 37° 43′ 55.38″N 15° 0′ 9.32″E 2001 — Summit Vent 5.07 0.34 0.000002 0.747105 1.467237 0.282976 0.0000093TDF02 37° 43′ 32.60″N 14° 59′ 41.26″E 2002–03 South 4.68 0.33 0.000003 0.747049 1.467205 0.282943 0.0000091PP02 37° 47′ 57.31″N 15° 2′ 14.27″E 2002 North 5.07 0.33 0.000003 0.747384 1.467256 0.282976 0.0000091SE04 37° 44′ 39.08″N 15° 0′ 22.71″E 2004–05 4.67 0.33 0.000003 0.747496 1.467156 0.282947 0.000010

a Measured 179Hf/177Hf ratio used for fractionation correction.b 178Hf/177Hf ratio corrected for fractionation (mean value=1.46722±0.00006, 2σ, cf. mean value for standard runs=1.46722±0.00002).c 176Hf/177Hf ratio corrected for Lu and Yb interferences and fractionation, and normalized to the preferred value of 0.282160 for JMC475.d 2σ errors derived by propagating within-run precision of each analysis (0.000004–0.000006 2SE) with the external reproducibility of the standard analyses.

Fig. 2. Hf–Nd isotope correlation diagram of the historic (HL) and recent (RL) Etnean lavas within the context of all oceanic volcanics. Data for Nd isotope are from Viccaro andCristofolini (2008). Fields are defined after Salters and White (1998), except for FOZO, reconstructed from Austral Cook Islands (Raivave volcano) of Lassiter et al. (2004), as well asthe EM1 and EM2 spots that result from average values of Eisele et al. (2002) and Pfander et al. (2007) respectively. SA & SF correspond to volcanic rocks from San Ambrosio and SanFelix Islands (Chile).

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whole, Hf isotope ratios in RL are positively correlated with Nd andnegativelywith Sr isotope ratios (Figs. 2–3). Amore complex variationoccurs in 208Pb*/206Pb* vs. 176Hf/177Hf space: Hf was rather constantfor RL until the 2001 eruption, although it is generally associated withrather higher 208Pb*/206Pb* values seen in the HL (Fig. 3). For a given208Pb*/206Pb*, lavas erupted after 2001 display significantly lower176Hf/177Hf than all the other analyzed samples.

In Nd–Hf isotope space, Etnean lavas plot slightly below the lineararray defined by the mid-ocean-ridge and ocean islands basalts(Salters, 1996; cf. also Patchett, 1983 and Patchett and Tatsumoto,1980), falling between the FOZO and HIMU fields (Fig. 2; cf. Strackeet al., 2005). Similar features are also seen in the relationship between176Hf/177Hf vs. 87Sr/86Sr and 208Pb*/206Pb*, although Etnean lavas havehigher 87Sr/86Sr values, comparable 208Pb*/206Pb* and slightly lower176Hf/177Hf with respect to the FOZO component (Fig. 3; although theextent of the Hf isotope field for FOZO is not well constrained by theliterature data).

5. Discussion

5.1. Mantle components

Integration of Hf isotope data with other radiogenic isotope valuesprovides better constraints on the characteristics of the mantlecomponents involved in magma genesis at Mt. Etna. Particularly,features that are unclear when considering only the covariation of Sr–Nd–Pb isotopes become more evident when the 176Hf/177Hf ratio is

taken into account (cf. Viccaro and Cristofolini, 2008). Hf isotope datain the literature for basalts (s.l.) are limited, so that comparison of theHf signature of Etnean lavas with those that might be diagnostic oftheir origin from definite mantle components is difficult. Using 176Hf/177Hf, Etnean products appear to be in general more “Hf-enriched”compared to HIMU (Figs. 2–3) and display, for a given 176Hf/177Hf,higher 87Sr/86Sr and 208Pb*/206Pb* as well as comparable 143Nd/144Nd.On the whole these signatures are more akin to a FOZO source than toa mixture between HIMU and MORB components (cf. Carter andCivetta, 1977; Condomines et al., 1982; Gasperini et al., 2002). Theorigin of FOZO has been attributed by Stracke et al. (2005) to theenriched end of the compositional spectrum expected for recycledoceanic lithosphere that is relatively little affected by sub-arcalteration and with a different composition and age than the recycledcrust which produces the rarer HIMU sources. This implies that FOZO-and HIMU-like compositions may be the result of the evolution of tworelatively distinct recycled mantle domains (Stracke et al., 2005).Closer similarities to a FOZO-type source for Etnean magmas are alsodemonstrated by considering an evolution model for the formation ofHIMU from oceanic crust (cf. Salters and White, 1998). In Hf-Ndisotope space, the Etnean source does not fit the lines expected for theageing of 2 or 3 Ga old MORBs (recycled oceanic crust) to produceHIMU, as 176Hf/177Hf is too high compared to the related 143Nd/144Nd(Fig. 4a). This implies, also according to Stracke et al. (2005), theinvolvement of a recycled oceanic crust that evolved for a time(≤1 Ga) significantly less than that expected for HIMU. This should bean argument against an Etnean source with features related to mixing

Fig. 3.Hf–Sr–Pb isotope correlation diagrams of the historic (HL) and recent (RL) Etnean lavas within the context of all oceanic volcanics. Sr and Pb isotope data are from Viccaro andCristofolini (2008). MORB, HIMU, OIB and FOZO fields are defined after Stracke et al. (2005); EM1 from Pitcairn Islands of Eisele et al. (2002); EM2 from Society Islands of Pfander etal. (2007). Hf isotope values are slightly different in the two diagrams because coupled Sr and Pb isotope ratios are not available for all the samples.

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of a depleted component and HIMU, and instead favours thehypothesis that FOZO is the main component in the source of Etneanmagmas.

However, if one considers the Hf isotope variation of Etnean lavasintegrated with their Sr–Nd–Pb isotope signature, the FOZO compo-nent alone does not seem to be enough in order to account for theobserved differences between the HL and RL: specifically, changes tolower Hf–Nd–Pb (but slightly higher 208Pb*/206Pb*) and higher Srisotope ratios which occurred in some products erupted after 2001(Figs. 2–3). This feature could be ascribed to the addition of enrichedmaterial (EM-type), a characteristic observed in most ocean islandbasalts having a FOZO signature (cf. Stracke et al., 2005; Elliott et al.,2007; Beier et al., 2007). The origin of the EM1 component in ocean-island basalts (e.g. Pitcairn, Walvis Ridge) has been attributed toinvolvement in the partial melting process of: 1) significant amountsof ancient pelagic sediments (Richardson et al., 1982; Eisele et al.,2002); 2) recycled lower continental crust (Willbold and Stracke,2006, 2010); 3) sub-continental lithosphere metasomatized by fluidsand/or melts (Gibson et al., 2005). The EM2 component has beengenerally ascribed to recycling of terrigenous sediments (Zindler andHart, 1986) or, more generally, to recycled upper continental crust(Willbold and Stracke, 2010). Even though several authors discrim-inated between these two types of enriched sources through traceelement concentrations (cf. Weaver, 1991), a significant result can beonly obtained by investigating integrated isotopic systematics, as EM1and EM2 do not show regular changes of incompatible elements ortheir ratios (Willbold and Stracke, 2006). Involvement of amounts ofan EM2-type component should lead to rather constant 176Hf/177Hf,slightly lower 143Nd/144Nd, 206Pb/204Pb and 208Pb/204Pb (but moder-ately increasing 208Pb*/206Pb*) and, at the same time, to significantlyhigher 87Sr/86Sr and lower 207Pb/204Pb. On the other hand, addition ofEM1 material should more significantly affect 176Hf/177Hf, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, producing their netdecrease and leading to higher 87Sr/86Sr and 208Pb*/206Pb*. In thisregard, modelling involving simple binary mixing between FOZO andEM1 as well as FOZO and EM2 gives evidence that addition of about10% of an EM1-type component to a dominant FOZO source canexplain the observed variability of Etnean magmas. This feature tendsto be more emphasized in products emitted after 2001 (Fig. 4b). Thus,we may reasonably state that the complex changes through time ofthe Hf–Sr–Nd–Pb isotope signature of the HL and RLmay be due to the

involvement of an EM1-type component rather than of an EM2 one.The signature of Etnean magmas may be therefore consistent withmixing of only two components, dominant FOZO and a subordinateEM1. The reasons for divergences from the FOZO signature of eruptedmagmas may be explained by invoking small-scale heterogeneityproduced by metasomatic inputs of enriched materials with an EM1signature, a feature consistent with worldwide observations for oceanisland basalts.

5.2. Evolution of the Etnean mantle

Previous studies, chiefly based on Sr–Nd isotope data, havehighlighted the depleted character of the Etnean magma sourcerelative to the Bulk Earth (e.g., Armienti et al., 1989; Corsaro andCristofolini, 1996; Peccerillo, 2005; Viccaro and Cristofolini, 2008 andreferences therein). Indeed, Sr and Nd isotope values for both the HLand RL plot near the field of Pacific and Atlantic MORBs (not shown),being characterized by lower Sr andhigherNd isotope ratios relative tothe Bulk Earth, a feature also consistent with Pb isotope compositions(See Figs. 5–7 in Viccaro and Cristofolini, 2008). Concerning the Hfisotope signature, the Etnean lavas fall at the lower boundary of thearray formed by non-HIMU ocean island basalts, with 176Hf/177Hfvalues slightly lower for a given 143Nd/144Nd compared to this array(Fig. 2; cf. Salters, 1996; Blichert-Toft and Albarède, 1997; Salters andWhite, 1998). This indicates that these Etnean samples bear asignature of a source that experienced a period of time when the Lu/Hf ratio was low compared to the Sm/Nd ratio. Reasons for such adisplacement may be found in a different fractionation for Lu/Hfrelative to Sm/Nd, which led to decoupling of the 176Hf/177Hf and143Nd/144Nd trends. This is possibly related to melting in the presenceof garnet that results in a larger fractionation for Lu/Hf than for Sm/Nd,since Lu is more compatible than Hf in garnet compared to thebehaviour of Sm relative to Nd (Salters, 1996; Salters and Hart, 1989;Blichert-Toft and Albarède, 1997; Salters and White, 1998). This maycorrespond to a magma source that experienced a low degree ofmelting in the presence of garnetwhich then evolved for a long enoughtime to develop decoupling of the 176Hf/177Hf and 143Nd/144Nd ratios,and a consequent shift away from the ocean island basalt array. Thedepleted character of the Etnean magma source is even moreemphasized looking at the εHf values (initial 176Hf/177Hf=0.28286defined by Patchett and Tatsumoto, 1980). The positive εHf values,

Fig. 4. A) 176Hf/177Hf vs. 143Nd/144Nd diagram displaying the location of Etnean lavas within the context of MORB and OIB volcanics. Two possible patterns of isotopic evolution ofoceanic crust are reported on the diagram (N-MORBs; starting from data of Hofmann, 1988), which derives from 2 (filled line) or 3 (dotted line) Ga oldMORB sources (modified fromcalculation of Salters andWhite, 1998). Numbers on the two lines refer to the age of eruption for MORB volcanics. See text for discussion; B) Results frommodelling of binary FOZO-EM1 and FOZO-EM2mixing in Hf–Nd space. 143Nd/144Nd ratios and Nd ppm data for historic and recent Etnean lavas are taken from Viccaro and Cristofolini (2008), whereas Hf ppmdata are from this paper. FOZO end-member is derived from the Austral-Cook Islands data average of Lassiter et al. (2004), namely: Nd=43.5 ppm (average of 35 data),Hf=4.88 ppm (average of 33 data), 143Nd/144Nd=0.512912 (average of 39 data), 176Hf/177Hf=0.282981; EM1 end-member is derived from the Pitcairn Islands and Walvis Ridgedata average of Salters and Hart (1991), Richardson et al. (1982), Dietrich et al. (1984), Richardson et al. (1984), Liu and Schmitt (1984), Eisele et al. (2002) and Salters and Sachi-Kocher (2010), namely: Nd=37.9 ppm (88 data), Hf=8.38 ppm (average of 57 data), 143Nd/144Nd=0.512466 (average of 59 data), 176Hf/177Hf=0.282713 (average of 46 data);EM2 end-member is derived from the Society Islands data average of Pfander et al. (2007), namely: Nd=43.4 ppm (2 data), Hf=8.47 ppm (average of 5 data), 143Nd/144Nd=0.512805 (average of 3 data), 176Hf/177Hf=0.282994 (average of 6 data).

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which range between +2.9 and +4.6, indicate that Etnean magmasare enriched in radiogenic 176Hf compared with the chondriticreservoir, and that they presumably originated from a source thathad higher Lu/Hf than the present-day chondritic value. Provided thatresidual garnet was responsible for the inferred significant Lu/Hffractionation, the short-term evolution of the εHf values of Etneanlavas provides information about the possible role of garnet in themelting process. Distinct εHf values may be related to portions of themantle with distinct Lu/Hf ratios, which may in turn be connected todifferent modal proportions of garnet participating in partial melting.Specifically, a contribution to themelt of a sourcewith higher amountsof residual garnet could be responsible for magmas with lower Lu/Hfratios, and therefore higher 176Hf/177Hf. The lower εHf values revealedfor magmas of some eruptions of Mt. Etna (1892, 2001, 2004–2005)might then be related to lower garnet proportions in the partialmelting process. However, looking at some trace element ratios suchas Gd/Yb or Sm/Yb, which should give information on the role ofresidual garnet in the source, there is a lack of correlation with 176Hf/177Hf, as these ratios are fairly constant among the analyzed products(Fig. 5). This contradicts the previous statement about the involve-ment of sources with different garnet proportions. Nonetheless,Viccaro and Cristofolini (2008) demonstrated that the degree ofpartial melting slightly increased especially in products from the

recent activity (RL) with respect to the historic one (HL). This mayresult in a “buffer effect” of the Gd/Yb or Sm/Yb ratios, since higherfractionations of these ratios can be compensated by the increaseddegree of partial melting. Furthermore, these authors showed thatpartial melting of a source characterized by increased amounts ofmetasomatic hydrous phases (i.e., phlogopite±amphibole) appearsconsistent with the peculiar geochemical features of the post-1971magmas. This apparent decoupling between garnet and metasomaticphases could be an indication that the heterogeneous mantle domains

Fig. 5. 176Hf/177Hf vs. Gd/Yb and Sm/Yb diagrams for the historic (HL) and recent (RL)lavas of Mt. Etna (data for Sm, Gd and Yb from Viccaro and Cristofolini, 2008). Noevident correlation with time or between Hf isotope and trace element ratios areobserved.

Fig. 6. Ba/Th vs. Lu/Hf ratios resulting from modal fractional melting modelling of twodifferent mantle sources. Numbers on the curves indicate the degree of partial melting.White diamonds are calculations for a peridotite source with a FOZO isotopic signature(data source from Stracke and Bourdon, 2009). Modal proportions have been hereassumed as ol:cpx:opx:grt:amph:phl=66:12:13:7:1:1. Black diamonds are calcula-tions for a source resulting by mixing the previous peridotite composition (Stracke andBourdon, 2009) with 10% of an EM1 source (data source from Willbold and Stracke,2006). Modal proportions have been here assumed as ol:cpx:opx:grt:amph:phl=64:12:12:5:4:4. Distribution coefficients: olivine, clinopyroxene, orthopyroxeneand garnet from Beattie (1993), Hart and Dunn (1993), Kelemen et al. (1993);amphibole and phlogopite from Adam et al. (1993), Irving and Frey (1984), LaTourretteet al. (1995). Compositional fields for the historic (HL) and recent (RL) Etnean lavas andMt. Maletto have been plotted on the diagram after their re-equilibration to mantlevalues by adding 15–20% of an ultramafic mineral assemblage, constituted by olivine,augite, Ti-magnetite and amphibole, until crystallizing olivine within the magma wasequilibrated to that in the mantle with Fo88. Data from Mt. Maletto (Etnean eruption of7 ka ago) are from Armienti et al. (1988). Mineral phase compositions for thisultramafic assemblage are from Aurisicchio and Scribano (1987); distributioncoefficients are from D'Orazio et al. (1998) for olivine, augite and Ti-magnetite andfrom Viccaro et al. (2007) for amphibole.

Fig. 7. Sm/Hf vs. 176Hf/177Hf diagram for the historic (HL) and recent (RL) lavas of Mt.Etna. Sm data are from Viccaro and Cristofolini (2008). Data from Mt. Maletto (Etneaneruption of 7 ka ago) are from Gasperini et al. (2002) for 176Hf/177Hf ratio, whereas Smand Hf ppm are from Armienti et al. (1988). Samples from Pico and Sao Miguel (AzoresIslands) have been plotted for comparison (data from Elliott et al., 2007).

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involved in melting have different bulk Lu/Hf ratios, inherited bydifferent proportions of these phases.

In order to test in a semi-quantitative way the previous inferenceon the possible role of the garnet and metasomatic phases, two modalfractional melting simulations have been performed (Fig. 6). In thefirst instance, the starting material is a peridotite composition fromStracke and Bourdon (2009) with an isotopic signature correspondingto FOZO. In this case the assumed modal content of garnet exceedsthat of metasomatic phases. In the second fractional meltingmodelling, the starting material is a mixture of the previouslyreported peridotite composition (Stracke and Bourdon, 2009)enriched with 10% of an EM1 source (data from Willbold and Stracke,2006), which is in accordancewith earlier discussions (Section 5.1). Inthis case the assumed modal content of metasomatic phases exceedsthat of garnet. Given the behaviour of the chosen elements, the Ba/Thand Lu/Hf ratios could represent respectively variable degrees ofmodal metasomatism or different amounts of garnet in the source. Inaddition to HL and RL, the sample ET75 from Mt. Maletto (prehistoriceruption dated at ~7 ka ago on the north-western flank of Mt. Etna)has also been considered in this diagram, for which coupled traceelements and Hf isotope data are available [data from Armienti et al.(1988) and Gasperini et al. (2002); Fig. 6]. This diagram shows thatthe mantle-equilibrated HL and RL can be produced by 3.5 to 4% ofpartial melting with an increasing degree from HL to RL, coupled withan increase in the Ba/Th and Lu/Hf ratios (Fig. 6). Given themeaning ofthe two curves, the changes observed in HL and RL can be interpretedas due to an increasing involvement of a metasomatizing agentcoupled with decreasing of garnet participating to melting. In thispicture, the Mt. Maletto lava can be explained by an even moreincreased degree of partial melting from a source in which thecontribution of the enriched component is higher than that of RL.

Further support for small-scale mantle heterogeneity derives froma consideration of εHf values of the lavas simultaneously eruptedduring the 2001 and 2002–03 events from fractures located on thesouthern and northeastern flanks or from the summit of the volcano.During the 2001 and 2002–03 eruptions, products with geochemicaland isotopic signature similar to the Etnean HL (with higher εHf:+4.1) were erupted from the summit and by some vents on thenortheastern flank, whilst lavas with features peculiar of the RL (withlower εHf: +2.9) were emitted from the southern flank (cf. Viccaroet al., 2006; Viccaro and Cristofolini, 2008; Ferlito et al., 2009b). Thisstrongly supports the idea that melts with distinct signatures mayhave originated in the source region beneath the volcano at the sametime. These magmas can then rise and reside at various levels withinthe feeding system and finally reach the surface at different times(scale of ka) or almost simultaneously (decades, few years, the sameeruption) along different tectonically activated pathways.

5.3. Nature of the metasomatizing enriched component

An alternative perspective to the models proposed by Schiano etal. (2001) and Tonarini et al. (2001) has been recently suggested byViccaro and Cristofolini (2008) in order to explain repeated and rapidchanges of the Etnean magma geochemistry. According to thishypothesis, fluids or melts inherited from recycled altered oceaniccrust (that is, fluids not linked to the subducting Ionian slab) areassumed to have fluxed a depleted mantle causing its re-fertilizationand formation of a heterogeneously veined source. Available datasupporting the model, however, were not able to give a preciseindication of the actual nature of this metasomatizing agent (low-T orhigh-T fluids s.l.) and also its characteristics in terms of components.These are the reasons that led us to obtain Hf isotope data. One of thebest ways for interpreting the meaning of the Hf isotope variations isto compare them with elemental ratios such as M/Hf, where M is atrace element selected for its particular mobility or incompatibilitywith respect to Hf. With this regard, Sm and Hf are fairly equally

incompatible under mantle melting conditions, so that effects ofsource depletion on Sm/Hf ratios should be low. However, Sm isthought to be more strongly enriched in slab-derived fluids than Hf(e.g., Mysen, 1979; Spera et al., 2007; Elliott et al., 2007 andreferences therein). High Sm/Hf ratios may then be attributed toselective addition of Sm by fluids. Similar indications may be alsoobtained, for example, by the Nd/Hf or Ba/Hf ratios (cf. Spera et al.,2007). Looking at the Sm/Hf vs. 176Hf/177Hf diagram (Fig. 7), datafrom Mt. Etna plot in an enriched sector characterized by high Sm/Hfratios, suggesting therefore Sm enrichment due to addition of fluids.Samples from Pico Island and Sao Miguel (Azores) have been plottedin the same diagram. The former are considered to be derived fromrecycling of an old oceanic crust, whereas the latter can be explainedwith involvement of an enriched component very similar to EM1,interpreted as underplated basalts intruded into oceanic mantlelithosphere (Elliott et al., 2007) or as the simple recycling of oceaniccrust (Beier et al., 2007). Interestingly, the Mt. Maletto lava and oursamples define a different trend with respect to those displayed bythe Azores lavas, suggesting the involvement of a different type ofenriched component than that expected for Pico Island and SaoMiguel (Beier et al., 2007; Elliott et al., 2007). At Mt. Etna, the natureof this enriched component may be fluids derived from recycledoceanic crust. In order to further examine the relative role of low-Tfluids (i.e., aqueous fluids) or high-T fluids (i.e., silicate melts)constituting such enriched component, the diagram Sm/Hf vs. Th/Hfcan be used (Fig. 8). Indeed, aqueous fluids carry Sm but not Th andHf, whereas silicate melts carry Sm and to a lesser extent Th, but notHf (e.g., Mysen, 1979; McDade et al., 2003; John et al., 2004; Spera etal., 2007; Elliott et al., 2007 and references therein). Even in thisdiagram Mt. Etna data define different trends with respect to theAzores and, more interestingly, HL and RL have diverse distributions.In particular, HL shows variations of Sm/Hf and Th/Hf within thesame range without a significant correlation, whereas RL exhibits acoupled variation of the two ratios. From this we conclude that theincrease in Sm/Hf together with Th/Hf ratios can be related to theinvolvement of metasomatizing silicate melts in the partial meltingprocess of Etnean magmas. At constant Sm/Hf, the differences in Th/Hf ratios exhibited by the Mt. Maletto lava, the HL and RL may beaccounted for by different degrees of partial melting. In accordance towhat was demonstrated in Fig. 6, the changes of Th/Hf ratio observedin Mt. Maletto lava, HL and RL can be explained by variable degrees ofpartial melting through time. This reinforces the idea that geochem-ical changes of Mt. Etna lavas are not progressive through time, but

Fig. 8. Sm/Hf vs. Th/Hf diagram for historic (HL) and recent (RL) lavas of Mt. Etna. Smand Th data for Mt. Etna lavas are from Viccaro and Cristofolini (2008). Data for Mt.Maletto for Sm, Hf and Th ppm are from Armienti et al. (1988). Samples from Pico andSao Miguel (Azores Islands) plotted for comparison (data from Elliott et al., 2007).

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are instead random and related to melting of heterogeneous mantledomains that have been variably metasomatized.

6. Conclusions

Selected Etnean lavas, whose trace elements and Sr–Nd–Pb–Oisotope compositions have been previously obtained (cf. Viccaro andCristofolini, 2008), were analyzed in order to obtain information ontheir Hf isotope signature, and to define the characteristics of themagma source. The new data presented here expand the coupled Sr–Nd–Pb–Hf database for worldwide volcanic rocks, as Hf isotope datareported in the literature are scarce.

The main results of this study are the following:

1) The integrated analysis of Hf–Sr–Nd–Pb isotope systematicshighlights strong similarities with a FOZO-type source for Etneanmagmas, on which subordinate contributions of enriched material(EM1-type) are superimposed.

2) The Etnean magma source shows evidence of a previous low-degree melting event in the presence of garnet, which produceddecoupling of the 176Hf/177Hf and 143Nd/144Nd ratios as well as ashift off the ocean island basalts array.

3) The decrease of the εHf values (and 176Hf/177Hf) especially withinsome of the most recent Etnean magmas may be related to lowermodal proportions of garnet participating in partial melting.

4) The εHf values also suggest that different Etnean magmas mayhave been simultaneously produced from mantle portions withdifferent Lu/Hf ratios, probably inherited by distinct proportionsbetween garnet and metasomatic components within the meltingmantle domain, and that magmas can ascend towards the surfacewith different timescales (from ka to few days during the sameeruption) along distinct tectonically activated pathways.

5) The integrated study of 176Hf/177Hf and trace element ratios, usingincompatible elements with distinct affinity for fluids (such as Sm,Th and Hf), suggests that the enriched component may be silicatemelts rather than aqueous, low-T fluids, which infiltrated andmetasomatized the Etnean mantle source.

6) This evidence could imply a physical scenario where the ambientEtnean mantle derives from recycling of an ancient, altered oceaniclithosphere (FOZO) later metasomatized by infiltrating melts thatcarry the enriched signature (EM1-type). This supports the idea thatsmall-scale heterogeneity is responsible for changes in the geo-chemical signature of Etnean magmas over any possible timescale.

Acknowledgements

We would like to thank Elouise Haywood for technical assistancein the Hf isotope analyses. Wendy Bohrson and Hilary Downes aregreatly acknowledged for having critically revised the early submittedversion of the manuscript and for their constructive comments andsuggestions. This work has been supported by grant to Marco Viccarofrom the University of Catania and by INGV-DPC 2007-09 V3_ProjectLAVA grant to Stefano Gresta.

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