Phenocryst/matrix trace-element partition coefficients for hawaiite-trachyte lavas from the Ellittico volcanic sequence (Mt. Etna, Sicily, Italy

Mineralogy and Petrology (1998) 64 :65-88 Mineralogy an(1

Petrology © Springer-Verlag 1998 Printed in Austria

Phenocryst/matrix trace-element partition coefficients for hawaiite-trachyte lavas from the Ellittico volcanic sequence (Mt. Etna, Sicily, Italy)

M. D'Orazio 1, P. Armienti 2, and S. Cerretini 2

Centro di Studio Geologia Strutturale e Dinamica dell'Appennino, C.N.R., Pisa, Italy 2 Dipartimento di Scienze della Terra, Universit~ di Pisa, Italy

With 6 Figures

Received February 20, 1998; revised version accepted July 21, 1998

Summary

A set of phenocryst/matrix partition coefficients was obtained for up to 29 trace elements (ICP-MS analyses) in hawaiite to trachyte lavas from the Ellittico volcanic sequence (Mr. Etna system, Sicily). Partition coefficients were determined for plagioclase, clinopyroxene, olivine, kaersutite and Ti-magnetite. These phases, along with apatite (not analysed in this work), constitute the common fractionating solid assemblage of alkaline magmas feeding Mt. Etna volcanic system. The obtained data set forms the first attempt to characterise the solid/melt trace-element partitioning for Etnean magmas, and can be usefully applied in other sites of alkaline volcanism. The partition coefficients are here used to define the scale of incompatibility of 29 trace elements and to asses the extent of differentiation processes and the prevailing oxygen fugacity of Ellittico magmas.

Zusammenfassung

Verteilungs-Koeffizienten von Spurenelementen zwischen Phiinokristallen und Matrix in Hawaiit-Trachyt Lavender vulkanischen Abfolge von Ellittico (Atna, Sizilien, Italien)

Ein Satz von Ph~inokristall/Matrix Verteilungs-Koeffizienten ftir his zu 29 Spurenele- mente wurde mittels ICP-MS Analytik in hawaiitischen bis trachytischen Lavender Ellittico Abfolge (Atna-System, Sizilien) erhalten. Die Verteilungs-Koeffizienten wurden fiir Plagioklas, Klinopyroxen, Olivin, Kaersutit und Ti-Magnetit bestimmt. Diese Phasen, zusammen mit Apatit (in dieser Arbeit nicht analysiert), stellen die fraktionierende Assoziation yon festen Bestandteilen in alkalischen Magmen, die das

66 M. D'Orazio et al.

vulkanische System des J~tna versorgen, dar. Der Datensatz ist ein erster Versuch die Verteilung yon Spurenelementen zwischen Festphasen und Schmelze fiir Magmen des Atna zu charakterisieren und kann ebenso nutzbringend auf andere Gebiete mit alkalischem Vulkanismus angewandt werden. Die Verteilungs-Koeffizienten werden hier benfitzt, um das Ausmal3 der Inkompatibilit~it von 29 Spurenelementen zu erfassen, und das Ausmal3 der Differentiationsprozesse und der vorherrschenden Sauerstoff- Fugazit~it der Ellittico-Magmen zu bestimmen.

Introduction

Geochemical investigations on magmas feeding Mt. Etna volcanic complex frequently use trace-element distribution as a powerful too1 for unravelling differentiation processes undergone by magmas before their eruption and for constraining the nature of mantle sources. Indeed, many published works report a wealth of whole-rock trace-element data for lavas, dikes and pyroclastic rocks emplaced during the 500 ka to Present magmatic activity in the Etnean area (e.g., Joron, 1984; Cristofolini et al., 1991; D'Orazio, 1993; Tonarini et al., 1995; Corsaro et al., 1996; Tanguy et al., 1997). Notwithstanding the past and current interest in this topic, studies on trace-element partitioning between phenocrysts and their coexisting melts for typical Etnean lavas are totally lacking. Thus, quantitative modelling of evolutionary processes involving crystal-liquid fractionation has been conducted at Mt. Etna by using mineral/melt partition coefficients taken from the literature and obtained for compositions often quite different from those characterising the Etnean system (Armienti et al., 1994; Treuil and Joron, 1994; Corsaro et al., 1996; D'Orazio et al., 1997).

As a first attempt to partially fill this gap of knowledge, this paper reports trace- element data for Sc, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb Cs, Ba, REE (Rare Earth Elements), Hf, Ta, Pb, Th, U determined by ICP-MS in phenocrysts (olivine, plagioclase, clinopyroxene, amphibole and Ti-magnetite) and groundmasses separated from six lavas belonging to the Ellittico volcanic sequence. This volcanic succession was chosen on account of its relatively wide and continuous compositional spectrum (from basic hawaiite up to trachyte), moreover its rock types are widespread over the whole Mr. Etna volcanic system and include lavas with intermediate to very low content of phenocrysts (from 2 to 18% by volume for the studied samples), suitable for phenocryst/matrix studies.

The obtained set of partition coefficients is potentially applicable to other sites of Na- (and/or slightly K-) alkaline volcanism, for which thorough trace-element partitioning studies are relatively scarce.

The Ellittico volcanic sequence

The Ellittico volcano, so named after the elliptical shape of the border of its terminal caldera (Waltershausen, 1880), was one of the major central volcanic edifices of the Etnean area. It is also known as Ancient Mongibello to distinguish it from the following, the nearly co-axial, Recent Mongibello, the historically active volcanic edifice. Due to sparse radiometric dates, the onset of the Ellittico volcanic activity is very poorly constrained; however, some lavas near Barcavecchia (south-

Phenocryst/matrix trace-element partition coefficients 67

12

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VBE-22 N

VBE-14

VBE-72

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Ellittico lithostratigraphic Units [] Portella Giumenta Unit

+ Pizzi Deneri Formation

® Serra delle Concazze Formation

[] M.te Scorsone Unit

• M.te Cerasa Unit . .

40 45 50 55 60 65 70

SiO 2 wt%

Fig. 1. Total alkali vs. silica classification diagram. Arrows point to the samples selected for trace-element partitioning study. Greyed area encloses lavas from the current volcanic activity of Mt. Etna, reported for reference purposes. HW hawaiite; MU mugearite; BE benmoreite; TR trachyte. Data from Tonarini et al. (1995), D'Orazio (1993) and D'Orazio et al. (1997)

western base of the volcanic complex), ascribed to the Ellittico volcano, were dated to about 34 ka using the K-Ar method (Gillot et al., 1994). Eruptions of Ellittico terminated with an intense explosive activity culminating between 14000 and 15000 years ago with the formation of a caldera (Kieffer, 1979; Condomines et al., 1982; Gillot et al., 1994). Volcanic products of Ellittico extensively crop out in the whole Etnean area, but they are particularly well exposed along the northern walls of Valle de Bove, a huge horse shoe-shaped morpho-structural depression of the eastern flank of the volcano opening towards the Ionian Sea. Here, by recognition of some erosive or discordant surfaces, Coltelli et al. (1994) distinguished five lithostratigraphic units belonging to the Ellittico volcano and a unit (Piano delle Concazze) of lava flows filling the caldera soon after its collapse.

The Ellittico volcanics belong to a mildly Na-alkaline series ranging from basic hawaiites to trachytes with a predominance of intermediate compositions (Fig. 1). Most evolved types are restricted to the highest lithostratigraphic unit (Portella Giumenta), whereas hawaiites and mugearites predominate in the lower units.

Figure 2 resumes the model features of the phenocryst assemblage observed within Ellittico lavas: they include plagioclase, diopsidic-augitic clinopyroxene and Ti-magnetite as ubiquitous phases, while kaersutite is lacking within hawaiites, olivine is very scarce or absent in trachytes, and phenocrystic apatite is found only

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[] Hawaiites I [] Mugearites [ ] Benmoreites [] Trachytes

I I

I I I I

Olivine Amphibole Ti-Magnetite Apatite

II

I

P.I.

Fig. 2. Distribution of phenocrysts volume % by point-counting) for different products from Ellittico volcanic succession. Boxes represent the average values, whereas vertical segments join the extreme values. P.L Porphyritic Index. Data from D'Orazio et al. (1997)

in trachytes. In the figure, the most frequent abundances are marked as boxes, whereas vertical segments show the range of values. Porphyritic indexes (RI.) are relatively high (10-57%) in the less evolved lavas and are always lower (< 10%) in benmoreites and trachytes, which exhibit sub-aphyric varieties (RI. <5%). Plagioclase phenocrysts vary in composition from bytownite to andesine in hawaiites and mugearites (where complex zoning and sieve textures are widespread) to andesine in benmoreites and trachytes, where oligoclase may appear in the groundmass together with alkali feldspar. The ratio of mafic phases to plagioclase decreases quite regularly from hawaiites to mugearites and further drops in the most evolved rocks. Some mugearites may lack clinopyroxene phenocrysts. Groundmasses are typically microcrystalline and largely dominated by feldspar microlites. Olivine is absent in the evolved groundmasses, amphibole is always lacking, while apatite is an ubiquitous accessory phase. In the groundmasses, the mafic phases-plagioclase ratios are typically lower than in the corresponding phenocrystic assemblage.


Methods

Sample selection and mineral separation

Samples selected for this study represent the whole compositional spectrum of Ellittico lavas (Fig. 1). Whenever possible, only samples with low porphyricity and small (< 2ram) phenocrysts were used. Care was also taken to avoid samples containing xenocrysts, xenoliths, or textural disequilibrium (as evident through petrographic analysis of phenocryst-groundmass textures). Of the six samples selected for trace-element study, five belong to the Ellittico volcanic sequence proper. The other (VBE-71) is a sample of a sub-horizontal lava flow poured directly onto the bottom of Ellittico caldera. Table 1 summarises the main petrographic features of the studied samples, Table 2 reports major element content, whereas Tables 3-7 report mineral chemistry data for phenocrysts.

Samples were crushed using a steel jaw-crusher, and then reduced to sand-size with a disk-mill. Depending on phenocryst size, two sieved fractions were used for the separations: 100-160gin and 75-100gin. The phenocrysts of olivine, plagioclase, clinopyroxene and amphibole and the matrix were separated by repeated passages through the Frantz magnetic separator and heavy liquid (aqueous solution of sodium polytungstate, 6 ~ 2.9 g cm-3). Ti-magnetite microphenocrysts were concentrated by means of a hand magnet; glass and groundmass adhering to Ti-magnetite crystals were then removed by leaching with 10% HF for 30 rain. at room temperature. The final purification was performed by hand-picking under the stereoscopic microscope. Before being analysed, the separated phases were repeatedly washed with ultrapure H20 in an ultrasonic bath. The degree of purity of the separated materials, checked under the microscope, is generally better than

Table 1. Petrographic outlines of the studied samples

sample rock type texture phenocryst assemblage (% by volume)

VBE-72 hawaiite

VBE-14 hawaiite

VBE-22 mugearite

VBE-71 benmoreite

VBE-66 benmoreite

VBE-32 trachyte

porphyritic, intergranular

porphyritic, intersertal

porphyritic, fluidal

sub-aphyric, trachytic



groundmass assemblage

Cpx (8.3), Plag (6.8), O1 (3.0), Ti-Mag (0.3)

Plag (11.1), O1 (2.2), Cpx (0.2), Ti-Mag (0.4)

Plag (5.3), Cpx (2.5), Ti-Mag (0.8), O1 (0.3)

Plag (1.6), Ti-Mag (0.3), Cpx (0.2), O1 (0.1), Ap (0.1)

Plag (2.2), Cpx (0.2), Krs (0.2), Ap (<0.1), Ti-Mag (<0.1)

Plag (2.4), Krs (1.4), Ti-Mag (0.2), O1 (<0.1), Cpx (<0.1), Ap (<0.1)

Plag, Cpx, O1, Ti-Mag, Ap



Plag, Cpx, Ti-Mag, Kfs, Ap

Plag, Cpx, Ti-Mag, Kfs, Ap

Plag, Kfs, Cpx, Bt, Ap, Ti-Mag

Data from D'Orazio et al. (1997). Phenocryst contents measured by point-counting (2000 points counted on a 0.2x0.3 mm grid). Crystals with maximum length > 0.3 mm were considered as phenocrysts. Ap apatite, Bt biotite, Cpx clinopyroxene, Kfs K-feldspar, Krs kaersutite, O1 olivine, Plag plagioclase, Ti- Mag titano-magnetite


Table 2. _Major element dam Jbr the studied samples

Sample VBE-72 VBE-14 VBE-22 VBE-71 VBE-66 VBE-32

SiO2 (wt %) 49.01 49.82 51.13 54.40 58.07 60.21 TiO2 1.58 1.47 1.39 1.58 1.25 0.95

A1203 17.61 19.09 19.03 17.89 18.22 18.42 Fe203 3.30 2.52 3.58 1.30 1.80 0.65 FeO 5.81 6.53 4.67 6.45 3.87 3.45 MnO 0.15 0.17 0.16 0.18 0.16 0.15 MgO 6.57 4.67 • 4.00 2.84 2.08 1.44 CaO 9.20 7.94 7.56 5.54 4.02 2.82 Na20 3.80 4.46 5.04 5.73 6.38 7.19 K20 1.48 1.91 2.03 2.69 3.12 3.56

P205 0.53 0.61 0.68 0.81 0.45 0.31 LOI 0.95 0.81 0.76 0.58 0.58 0.86

Mg# 60.2 51.8 50.6 43.0 43.4 41.9

Mg#=Mg/(Mg+Fe 2+) • 100, calculated assuming Fe203/FeO (wt%) = 0.15. Data from D'Orazio et al. (1997)

99% and decreases in the order: amphibole (the most pure), clinopyroxene, groundmass, plagioclase, Ti-magnetite and olivine. The most frequent impurities are represented by very tiny glass and spinel inclusions in olivine and crystallised melt inclusions in plagioclase.

Electron miclvprobe analyses

Major element data for the phenocryst phases were obtained by means of a JEOL JXA-8600 electron microprobe (Centro di Studio per la Minerogenesi e la Geochimica Applicata, C.N.R., Florence) operating at 15 kV and 10 nA and with a matrix correction procedure after Bence and Albee (1968).

Trace element analyses

The concentrations of Sc, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, REE, Hf, Ta, Pb, Th and U were determined by ICP-MS (Fisons PQII+ STE). Rock and mineral powders were dissolved by conventional HNO3+HF acid digestion (for sample dissolution details see D'Orazio and Tonarini, 1997) and then spiked with Rh, Re and Bi (internal standards). Sample solutions, with a final total dissolved solid <1 mg m1-1 in a 2% (v/v) HNO3 matrix, were measured with external calibration carried out using the well-certified international reference rock BE-N. ICP-MS measurements were subjected to a correction procedure including blank sub- traction, drift-monitoring and oxide-hydroxide isobaric interferences (for the REE). Detection limits (/ag g-i in the solid sample), estimated by calculating the concentration corresponding to 3 times the standard deviation of blank solution counts,


range from 0.001 to 0.05 gg g-i for Co, Y, Nb, Cs, REE, Ta, Th and U, and from 0.05 to 0.5 gg g-1 for Sc, V, Ni, Rb, Sr, Zr, Ba and Pb. The detection limit for Cr is higher (1 idem), mainly due to heavy interference problems. Analytical precision, estimated by repeated analyses of geochemical reference samples, typically ranges from ~ 10% at 10 x detection limit to 3-5% at 100 × detection limit; conse- quently, different numbers of significant digits are used to report results. The accuracy of data, evaluated by regularly analysing well-certified geochemical reference samples, typically ranges from 0 to 7% for elements at 50-100 ×detection limit.

Trace-element partitioning 13phenocryst/matrix Apparent partition coefficients, ~element , are here defined as the mass ratio

of a given trace element in a solid phase (phenocryst) and the coexisting silicate liquid (groundmass = matrix). Due to crystal zoning, late magmatic or deuteric modification of the system, matrix inhomogeneity, etc., this ratio only describes trace-element distribution between solid and liquid and could deviate to various extent from the partition coefficient expected for thermodynamic equilibrium.

Table 8 reports measured trace-element concentrations in minerals and in their corresponding groundmasses; Table 9 lists the calculated partition coefficients, that are also graphically shown in Figs. 3-5.

Plagioclase

In the studied rocks, plagioclase is the dominant phenocryst phase (except for VBE-72, which has more clinopyroxene than plagioclase). The most evolved lavas contain clear and poorly zoned euhedral crystals, whereas hawaiites and mugearites are frequently characterised by plagioclase with glassy inclusions and complex zoning patterns. Electron microprobe data indicate bytownite to labradorite plagioclase compositions for the hawaiites, bytownite-andesine for the mugearites and andesine for the benmoreites and trachytes (Table 3).

The 3d transition elements (3dTE) were not determined for this phase as their contents are known to be very close to the detection limits of our analytical technique.

REE

The partition coefficients for the REE increase from 0.014-0.025 for Lu to 0.10-0.21 for La. As expected, Eu is characterised by higher partition coefficients, that, however, are always < 1 (0.29-0.65). The partitioning of Eu into plagioclase with respect to other REE, increases with the differentiation degree as revealed

I ) P l a g / h P l a g and Th content of the groundmass by positive correlation between ~Eu /~Sm 1-3Plag (Tables 8, 9). Our estimated ~'I~EE are in good agreement with those reported by

Lemarchand et al. (1987) for hawaiite-mugearite lavas; whereas, for benmoreites - . - r,~Plag and trachytes, uu, ~kEE are generally lower. The REE partition coefficients

estimated in this work for the hawaiite-mugearite lavas are also in agreement with the "average" values for alkaline series of Villemant et al. (1981), and with the values by Dostal et al. (1983), obtained for basalts and basaltic andesites.


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Sc V Cr Co Ni Sc V Cr Co Ni

Fig. 3. Transition element (3d) partition coefficients determined for clinopyroxene and kaersutite (a), olivine and Ti-magnetite (dashed) (b) from Ellittico volcanics

Other hygromagmaphile elements (HYGE) / r . , P l a g With the exception of S~ ~Osr = 1.8-4.2), all the hygromagmaphile elements have

partition coefficients < 1. The highest D-values were found for Ba (0.40-0.78) and Pb (0.15-0.37), the lowest were found for Th (0.008-0.018) and U (0.011-0.030).

Our D-values for the hygromagmaphile elements are in close agreement with those reported for hawaiite-benmoreite lavas by Lemarchand et al. (1987), but, again, some minor differences exist for more differentiated rocks. With respect to values published by Villemant et al. (1981), our data exhibit a good agreement with somewhat lower D-values for U, Th and Zr. Ewart and Griffin (1994) reported D- values for Pb (0.19-0.38), Ba (0.44-1.6), Rb (0.008-0.16) and Zr (0.022-0.21) in plagioclases from latitic and andesitic rocks which are in agreement with our data.

F~Plag found by the latter authors are significantly higher (5.3-12.0). However, ~Sr

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O.Ol t (a) t I ~ t t t F t I I t t ~ L t I 0 . 0 0 1

La Ce Pr N d P m S m E u Gd T b Dy Ho Y Er T m Y b Lu

Kaersutite

-~ - Clinop~roxene

Ti-magnetite

(b)

ka Ce Pr Nd P m S m E u G Ho Y Er T m Y ku

Fig. 4. REE partition coefficients determined for plagioclase (a), clinopyroxene, kaersutite and Ti-magnetite (dashed) (b) from Ellittico volcanics


Clinopyroxene

Clinopyroxene phenocrysts were separated from four samples (hawaiites VBE-72, VBE-14; mugearite VBE-22 and benmoreite VBE-71). Even if clinopyroxene phenocrysts are also present in the more differentiated rocks, their abundance is too low to obtain a separated fraction sufficiently pure for trace-element analyses. Clinopyroxene crystals analysed by electron microprobe (Table 4) tend to be zoned and diopsidic and augitic in composition. It is worth noting that in terms of quadrilateral components, clinopyroxenes from rocks with quite different degree of differentiation have compositions varying within narrow limits ( W o 4 4 - 4 7 E n 3 7 - 4 4 F s 1 o - 1 6 ) .

3dTE 13CP x The partition coefficients for the 3dTE are all > 1 and increase in the order ~v <-

~-,Cpx _ r- ,Cpx _ ~-,Cpx < ~-,Cpx . ~ . tJco < t)Ni < t)Sc tJCr . 1he most primitive rock (VBE-72) has the lowest D- value (except for Cr). Our data are in close agreement with phenocryst/matrix D- values for basalts-hawaiites-mugearites published by Lemarchand et al. (1987). Villemant et al. (1981) reported D-values for the 3dTE somewhat lower than ours. Francalanci (1989) found D-values for Co, Sc and Ni in close accordance with our

Cpx • - • data whereas her Dcr are slgmficantly higher.

REE

The REE partition coefficients show the typical pattern for the clinopyroxenes grown in basic to intermediate liquid compositions (e.g. Green, 1994), they have relatively constant values fiom Sm to Lu and steeply decrease from Nd to La. The elements from Sm to Lu have D-values very close or greater than one.

hCpx slightly lower than Clinopyroxenes of samples VBE-22 and VBE-72 exhibit ~Eu neighbouring elements. Francalanci (1989), Villemant et al. (1981) and Lemarc- hand (1987) reported D-values for the REE in Ca-pyroxene which are in agreement with our data.

Other HYGE

With the exception of Hf (for the sample VBE-22) all the partition coefficients for the remaining hygromagmaphile elements are <1. The lowest D-values were observed for U (0.016-0.037), Th (0.022-0.051), Rb (0.011-0.058), Ba (0.022- 0.065) and Cs (0.027-0.063), the highest were found for Hf (0.64-1.2), Zr (0.38- 0.70) and Sr (0.10-0.19). Ewart and Griffin (1994) reported comparable partition coefficients for Zr, whereas the range of D-values for Sr in plagioclase determined for mafic to intermediate compositions varies between 0.06 and 0.28 (e.g. Francalanci, 1989; Green et al., 1989; Kuehner et al., 1989; Shearer et al., 1991; Ewart and Griffin, 1994). Clinopyroxene of sample VBE-72 has an unusually high

1-,~Cpx content of Pb which results in an exceedingly high ~eb • We think that the high Pb concentration value could be ascribed to contamination during the sample preparation and analysis.


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Fig. 5. Hygromagmaphile element partition coefficients determined for plagioclase (a), clinopyroxene and kaersutite (b) and Ti-magnetite (e) from Ellittico volcanics

Olivine

Olivine phenocrysts were separated from two hawaiites and one mugearite. Observed ranges of major element composition as determined by electron microprobe, are: F078_86 (VBE-72), F073_77 (VBE-14) and FO71-75 (VBE-22) (Table 5).


Hygromagmaphile element contents appear to be unusually high, indeed, by using the experimental olivine/liquid partition coefficients proposed by Beattie (1994) they result to be up to three orders of magnitude more abundant than expected. Thus we do not propose any partition coefficient for these elements in olivine. Small amounts of glass included in olivine and/or matrix material adhering to the crystal surfaces may account for the observed enrichment. Assuming the groundmasses as contaminant material, we calculated that their maximum fraction can not exceed 3% for samples VBE-72 and VBE-14, whereas it is higher for sample VBE-22. Nevertheless, considering uncertainties due to analytical errors, the partition coefficients of Sc, Cr, Co and Ni for the three samples are not significantly affected by such levels of contamination. On the contrary, the concentrations of V, due to the strongly different content in olivine and groundmass, were significantly modified by glass and/or matrix contamination. Thus, the reported D °1 have to be considered with caution.

3dTE

Olivine strongly incorporates Ni and Co (DN°I = 12.2-17.2, Dc °l = 6.1-7.6). Ni, Co and Sc partition coefficients for the most primitive sample (VBE-72), are slightly lower than those of the more evolved rocks. This is in accordance with the thermodynamic analysis of olivine-melt partitioning (Beattie et al., 1991) that shows how the D-values of these elements correlate positively with the FeO/MgO ratio of a coexisting liquid. Ranges of phenocryst/matrix D-values for olivine published by Lemarchand et al. (1987) and Villemant et al. (1981) for basaltic- hawaiitic compositions encompass our values. Dostal et al. (1983) determined V, Cr and Co in olivines and coexisting matrix from basaltic andesites and found D- values very close to ours.

Amphibole

Amphibole is commonly found as a phenocryst phase in the most evolved alkaline lavas from Mt. Etna volcanic system. Occasionally, it was also observed in basaltic or hawaiitic lavas (Tanguy et al., 1997). Indeed, some authors invoke amphibole fractionation to explain some peculiar geochemical features, such as the low TiO2 content of some lavas from Trifoglietto volcanic centre (Chester et al., 1985; Tanguy et al., 1997). Within the Ellittico volcanic sequence kaersutite phenocrysts were found in some mugearite, benmoreite and trachyte lavas and pumices. Cumulate rocks, with prevailing kaersutite and subordinate plagioclase and clinopyroxene, were found as xenoliths, both in lavas and pyroclastites, within the volcanic successions of some eruptive centres pre-dating the Ellittico volcano.

Subhedral kaersutite phenocrysts (Table 6), with a poorly developed Fe-Ti oxide reaction rim, were separated for trace-element analysis from the benmoreite lava VBE-66.

3dTE {hAmph With the exception of Cr (DcAr mph =0.33), the 3dTE have D-values > 1 ~'sc

~Amph ~Amph ~Amph > IACo > IA V > lANi ). Wi th respect to analysed clinopyroxenes, kaersut i te can


Table 6. Average composition for the kaersutite phenocrysts of sample VBE- 66

cores n=5 rims n=5

Average St. Dev. Average St. Dev.

SiO2 41.20 0.14 41.35 0.32

TiO2 4.72 0.14 4.68 0.O4

A1203 11.46 0.21 11.55 0.23

FeO 11.61 0.35 11.64 o.3o MnO 0.29 0.O7 0.27 O.O4 MgO 13.72 O.2O 13.67 0.17 CaO 11.74 0.14 11.70 0.16 NazO 2.77 0.O4 2.77 0.10 KzO 0.89 O.O3 0.92 o.o3

Tot. 98.40 98.55

incorporate significantly less Cr and Ni. Our partition coefficients for Sc and V are quite similar to those reported by Sisson (1994) for hornblendes found in basaltic andesite, andesite and dacite from Guatemala, while we found a significantly lower value for Cr. D-values published by Lemarchand et al. (1987) are similar to our values for Sc and Co, but are higher for Ni.

REE

REE partition coefficients for kaersutite are quite high, ranging from 0.49 for La to 2.6 for Tb. They show a typical convex-upward pattern with the highest values from Eu to Er, the lowest values for the lightest REE (La-Nd) and intermediate values for the heaviest REE (Yb, Lu). On the whole, patterns of amphibole partition coefficients resemble those of clinopyroxene, but the latter exhibit lower D-values for all the REE. Very similar REE partition coefficient patterns were reported by Sisson (1994).

Other HYGE

The lowest partition coefficients were observed for U (0.028) and Th (0.033); highly incompatible elements in kaersutite are also Cs (0.040) and Rb (0.066). Moderately high D-values were found for Sr (0.69), Zr (0.53), Ba (0.52), Hf (0.85) and Pb (0.10). For Nb and Ta D-values are very similar and slightly larger than 1.

In comparison with the published phenocryst/matrix D-values, we can observe a general agreement for most elements with the data published by Lemarchand et al. (1987), with the exception of U, Th and Cs, that display higher D-values. The phenocryst/matrix D-values of Villemant et al. (1981) are slightly higher for U, Th, Zr, Ba, Cs and Rb, lower for Ta and quite similar for Sr and Hf.

Phenocryst/matrix trace-element partition coefficients

Table 7. Selected analyses of Ti-magnetite microphenocrysts

79

Sample VBE-72 VBE- 14 VBE-22 VBE-71 VBE-66 VBE-32

core core rim core core rim core core rim core rim core rim core rim

TiO2 14.24 14.84 14.42 15.75 9.16 11.84 13.16 13.01 14.65 11.76 12.51 12.32 13.40 21.40 21.41

A1203 4.71 4.89 4.83 3.75 7.75 5.82 4.70 4.64 4.36 3.33 3.44 4.06 3.45 1.72 1.69

FeOtot 71.86 68.64 68.62 68.92 72.60 72.74 73.50 75.28 74.70 77.22 75.51 74.27 75.39 70.06 70.43

MnO 0.43. 0.63 0.48 0.75 0.60 0.62 0.86 .0.56 0.59 0.71 0.77 0.61 0.65 1.84 1.56

MgO 5.17 5.05 5.31 5.28 4.73 3.79 3.23 3.14 2.85 4.02 3.22 4.53 2.79 1.06 1.11

Tot. 96.41 94.05 93.66 94.45 94.84 94.81 95.45 96.63 97.15 97.04 95.45 95.79 95.68 96.08 96.20

Mg/(Mg+Fe,o,; 0.11 0.12 0.12 0.12 0.10 0.08 0.07 0.07 0.06 0.08 0.07 0.10 0.06 0.03 0.03

Ti-magnetite

Ti-magnetite microphenocrysts were separated from hawaiite VBE-14, mugearite VBE-22 and benmoreite VBE-71. Hawaiite is thought to supply a fraction of magnetite grains included in olivines; at Mt. Etna these crystals are known to be distinctly enriched in A1-Mg spinel component (Chester et al., 1985); Table 7 shows that Ti-magnetite of hawaiites have higher Mg/(Mg+Fetot) ratios (0.11-0.12) with respect to Ti-magnetite of more evolved lavas (0.03-0.10). Compositional differences of Ti-magnetite through the series, are expected to cause strong changes of trace-element partitioning.

3dTE

The 3dTE all have D-values > 1 (l)Ti-Mag hTi-Mag> I~Ti-Mag I~Ti-Mag ' .~Cr > > ~Ni -- ~"V > ~Co

> L)sc~Ti-Mag.). The very high ~CrI3Ti-Mag of VBE-14 is not surprising given the elevated fraction of Mg-A1 spinel component of its Ti-magnetite. High D-values for Cr in Ti-magnetic were also reported by Dostal et al. (1983). The partition coefficients for Sc, V, Co and Ni are more usual and consistent with those determined by Francalanci et al. (1987), Francalanci (1989), Villemant et al. (1981), Lemarchand et al. (1987) and Dostal et al. (1983).

REE

REE partition coefficients for Ti-magnetite are strongly contrasting in hawaiite sample with respect to the other two. The former displays very low l)Ti-Mag ~'REE (0.04- 0.025), roughly rising from La to Sm and then almost constant from Tb to Lu. In

~'T1 Mag this sample, DEiu - is lower than the D-values of the adjacent elements, as also observed by Franealanci (1989) for the Ti magnetite of a latite lava from Stromboli volcano. A quite similar pattern of D-values for Ti-magnetites was found experimentally by Nielsen et al. (1992). VBE-71 and VBE-22 exhibit higher

Ti Mag DRE E (0.06--0.22), with a sub-horizontal or very slightly convex-upward pattern.

Other HYGE

Very low partition coefficients (0.001-0.06) were observed for Rb, Sr, Ba, Cs, Pb, Th. Moderately high D-values (0.15-0.77) were found for the high-field-strength

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elements (Zr, Nb, Hf and Ta). An even higher affinity of these elements for the spinel structure was reported by many authors (e.g. Francalanci, 1989; Ewart and Griffin, 1994; Nielsen et al., 1994).

Discussion

Order of incompatibility of trace elements and extent of fractionation

The data obtained in this study for trace-element partitioning between phenocrysts and matrix (Table 9), coupled with the observed relative abundances of phenocrysts (Table 1), allow to evaluate the order of incompatibility for trace elements during fractional crystallisation of Ellittico magmas.

As long as we consider apatite-free phenocryst assemblages, the two elements that preserve a highly incompatible behaviour throughout the compositional range from hawaiite to trachyte are Th and U. For the typical phenocryst assemblage of hawaiite and mugearite lavas (VBE-72, 14, 22), an incompatible behaviour is also displayed by Rb, Cs, Nb, La, Ce, Pr, Zr, Ta, Hf and Pb. For the same bulk-solid assemblages, the compatible elements are; Cr, Ni, Sr, Co, Sc and V (sample VBE- 14, which is clinopyroxene-poor, has quite low bulk partition coefficients for Sc and V), whereas moderately incompatible elements are represented by Eu, Ba and HREE.

The bulk-solid trace-element partitioning for the evolved compositions is much more variable due to the diverse proportion of kaersutite phenocrysts and apatite microphenocrysts. Ta, Nb and HREE could be effectively removed from the system by fractionating assemblages dominated by kaersutite, whereas the fractionation of significant amounts of apatite could potentially affect the distribution of middle REE (the elements with the highest partitioning with apatite; Watson and Green, 1981). The most evolved lavas issued by the Ellittico volcano have certainly segregated apatite microphenocrysts, as evidenced by petrographic observations and by the abrupt decrease of P205 as the MgO content falls below 2.5 wt% (D'Orazio et al., 1997). However, the chondrite-normalised REE patterns of the groundmasses share all the same shape and do not show any inflection of the middle REE. Thus, we argue that the D-values for the REE in apatite of Etnean magmas should not exceed the relatively low values experimentally determined for hawaiite-andesite liquids by Watson and Green (1981).

Sr and Nd isotope data for the Ellittico lavas allow us to state that, while some differences are inherited from the source or acquired during crustal contamination, major evolutionary trends are imposed by extensive fractional crystallisation (D'Orazio et al., 1997). Thus, even if the less evolved products cannot be considered as strictly comagmatic, their observed range of Th content can be used to assess the possible intervals of fractionation leading to the formation of the various members of the Ellittico series. For initial Th contents of basic hawaiites varying from 9.5 to 10.8 ~g g-i (whole-rock concentrations; D'Orazio et al., 1997) and DbuJk for Th varying from 0.00 to a maximum of 0.05 (as calculated with values from Table 9), the most evolved lava of Table 2 can be obtained by fractionating the initial magma to an extent ranging from 57 to 64%, while


intermediate compositions can be attained for a fractionation varying between 26 and 36% for mugearites and between 47 and 55% for benmoreites.

Whilst benmoreite and trachyte products are relatively scarce, mugearite compositions are the most abundant within Ellittico volcanic succession. Thus, the high velocity body underlying the central-eastern part of Mt. Etna volcanic system (Him et al., 1997) could likely represent the igneous counterpart of the erupted differentiated lavas. Several lines of evidence (fluid and melt inclusions in clinopyroxene phenocrysts, Frezzotti et al., 1991 and Kamenetsky et al., 1986; crystal chemistry of clinopyroxenes, Nimis et al., 1996; phase stability relation- ships, Armienti et al., 1988) indicate that Etnean ancient and historical magmas crystallised over a low-pressure interval ranging from about 500 to 100 MPa ( ~ 1 7 - 3 k m ) .

The robustness of the obtained set of partition coefficients can be assessed by calculating the trace-element distribution resulting from a given step of crystal fractionation. The test was accomplished choosing the interval from the less evolved hawaiite (VBE-72) to the benmoreite VBE-71; this is the largest possible step not involving amphibole and/or apatite. Calculations were performed by adopting Th as the most incompatible element ( D b u l k = 0.02) and recasting the Dbulk'S of remaining elements on the basis of the Rayleigh equation. These values and the obtained mineral/melt partition coefficients were fitted by means of least- squares procedure to get the best guess of the fractionating assemblage (plagioclase = 44.4, clinopyroxene = 30.0, olivine = 17.3, Ti-magnetite = 8.3, wt% basis). The obtained paragenesis is well consistent with the phenocryst assemblages prevailing among hawaiite to benmoreite lavas. Figure 6 compares the observed trace-element distribution of the benmoreite VBE-71 with that obtained from the calculated fractionating assemblage and Rayleigh equation. In the figure trace-element concentrations are normalised to that of the starting composition. Data fitting is fully satisfactory, with differences between observed and calculated values usually below 7%. The largest discrepancies are found for U (17%), Cr (15%) and Ba (12%).

The close petrographical and chemical similarity between Ellittico hawaiites and current Mt. Etna products (Fig. 1), enables application of these mineral/melt partition coefficients to modem products, obtaining the same order of incompatibility determined for Ellittico hawaiites. On the same basis, the limited variability of modal composition of Mt. Etna hawaiites permits the assessment of their bulk partition coefficients, even if a detailed treatment of the chemical variability of present-day lavas is beyond the purposes of this work.

Estimate off02 during fractionation

Several lines of evidence point to a high oxygen fugacity for the alkaline magmas of Mt. Etna: a) occurrence of Ti-magnetite as an early crystallising phase; b) high fO2 directly measured for the volcanic gases (Sato and Moore, 1973); c) extremely high production of the oxidised species CO2 and SO2 (Allard et al., 1991); d) high content of H20 (up to 2.3 wt% in the basaltic glass included in the olivines from the 1892 eruption; Metrich et al., 1993).

In a previous paragraph we showed that the partition coefficient for Eu between plagioclase and melt is somewhat lower than expected, with Dsr/DEu varying from


p..

] . - -

0 . 1 - -

0.01 t

U Ba ©

- ° VBE-71 Observed

© VBE-71 Calculated

i i i i 1 1 1 1 1 [ 1 1 [ 1 1 1 1 1 1 1 1 [ i i ~ 1 1

Fig. 6. Comparison between observed and calculated trace-element concentrations of benmoreite VBE-71. Values are normalised to the starting composition (hawaiite VBE-72). See text for details of calculations

5 to 9. The partition coefficients between feldspar and melt for E u 2+ a r e very close to those for Sr (Philpotts, 1970; Drake and Weill, 1975), whereas the partition coefficient for Eu3+is almost two orders of magnitude lower; thus the low values of DEu with respect to Dsr are reasonably due to a high value of the Eu3+/Eu2+ratio. The speciation of Eu and its partitioning between mineral phases and melt primarily depend on the oxygen fugacity even if some control by melt structure and composition has been invoked. M6eller and Muecke (1984) however emphasised that the compositional and structural control on Eu3+]Eu 2+ ratio is more pronounced for highly silicic, strongly polymerised melts. Hence, we think that for magmas of this study the speciation of Eu primarily depends on oxygen fugacity. Drake (1975) obtained an approximate experimental relation between fO2 and Eu3+/Eu 2+ partitioning between plagioclase and melt:

logfO 2 = -4.6108(([EU]Plag - - DEu3+ [EU]L )/([Eu]LD~u2+ - [Eu]plag)) - 10.9

where: [Eu]plag = total concentration of Eu in the plagioclase; [EU]L = total concentration of Eu in the liquid, i.e. in the groundmass; Dgu2+ = Dsr DEu3+, calculated by interpolation between D s m and DTb.

By using the above equation, our data would indicate fO2 values ranging from 10 7,8 (VBE-72) to 10 .6.5 (VBE-71) bars, without any systematic relationship with the degree of evolution, suggesting an open system behaviour with respect to oxygen exchanges. The higher fO2 values reach the range of the magnetite-


hematite buffer at 900-1000 °C even if we have not observed the latter mineral in any lava. However hematite is reported as tiny intergrowths in magnetite crystals within some Etnean trachytes (Tanguy, 1980).

Similar fO2 values (10 -vs bars at 1080 °C, AFMQ+2.1) were found by applying the experimentally calibrated equation of Kilinc et al. (1983) to the residual glasses of hawaiite lavas from the 1991-93 Mt. Etna eruption (Armienti et al., 1994). High pristine oxygen fugacities (AFMQ from ÷ 1 to +2.7) were recently calculated by Kamenetsky and Clocchiatti (1997), for primitive alkaline Etnean magmas, on the basis of the composition of spinel crystals hosted in forsteritic olivines.

The low partition coefficients found in the present work for V in Ti-magnetite, with respect to that of Cr, could be due to high vS+/v 3+ ratios, as the partition coefficient for V 3+ should be similar to that of Cr 3+ and much higher than that of V 5+ (Horn et al., 1994). This is again indicative of high oxygen fugacities.

Conclusions

The set of phenocryst/matrix partition coefficients presented in this paper for the Etnean volcanic complex represents the first attempt to evaluate the partitioning between phenocrysts and liquids for a large number (up to 29) of trace elements; it is also applicable to other occurrences of alkaline volcanism compositionally similar to that characterising Mt. Etna (e.g., Hawaii).

Determined D-values ensure that Th and U behave as highly incompatible elements in the whole compositional range of Etnean volcanics, thus their use as differentiation indexes (e.g., Joron and Treuil, 1984; Condomines et al., 1995) is largely justified. Rb, Cs, Zr, Hf, Ta, Nb, LREE also exhibit a marked incompatibility, however their adoption as differentiation indexes must be subject to some controls. In fact, Rb and Cs contents should not be modified by other enrichment processes like selective contamination and source effects that have been invoked for recent lavas of Mt. Etna (Clocchiatti et al., 1998; Condomines et al., 1995). Moreover an accurate petrographical control must ensure that Nb-Ta are not removed by the fractionation of amphibole.

During the differentiation of Ellittico magmas, fO2 conditions, estimated by means of the partitioning of Eu between plagioclase and melt, were quite high and comparable to those observed for the preceding and following alkaline phases of activity of Mt. Etna (+1 < A f m q < + 3 ) .

Acknowledgements

E Innocenti is acknowledged for helpful discussion. Constructive reviews by J. Bryce and an anonymous reviewer helped to improve the manuscript. This work was financially supported by C.N.R-GNV.

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Authors' addresses: M. D'Orazio, Centro di Studio Geologia Strutturale e Dinamica dell' Appennino, Via S. Maria, 53, 1-56126 Pisa, Italy; P. Armienti and S. Cerretini, Dipartimento di Scienze della Terra, Universit~ di Pisa, Via S. Maira, 53, 1-56126 Pisa, Italy.

Documents

Phenocryst/matrix trace-element partition coefficients for hawaiite-trachyte lavas from the Ellittico volcanic sequence (Mt. Etna, Sicily, Italy