He, Ar, O, Sr and Nd isotope constraints on the origin and evolution of Mount Etna magmatism

EPSL ELSEVIER

Earth and Planetary Science Letters 126 (1994) 23-39

He, Ar, O, Sr and Nd isotope constraints on the origin and evolution of Mount Etna magmatism Bernard Marty a,b, Thomas Trull c,1, Patricia Lussiez d,2, Isabelle Basile b, Jean-Claude Tanguy e a Centre de Recherches Pdtrographiques et G~ochimiques, B.P. 20, 54501 Vandoeuvre Cedex, France b Ecole Nationale Sup~rieure de G~ologie, 93 avenue De Lattre de Tassigny, 54001 Nancy Cedex, France c Laboratoire de G~ochimie des Isotopes Stables, Universit~ Paris 7, 2 place Jussieu, 75251 Paris Cedex 05, France a Laboratoire MAGIE, Universit~ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France e Universit~ Pierre et Marie Curie and CNRS, 94107 Saint-Maur des Foss~s Cedex, France

Received 16 August 1993; revision accepted 3 June 1994

Abstract The 0.5 Ma history of the origin and evolution of Mount Etna, Sicily has been investigated by analysing the isotopic composition of He, At, O, Sr and Nd in 21 selected lava samples. The near constancy of the isotopic compositions of oxygen (3180 = 5.4 + 0.3%o) and of helium trapped in olivine phenocrysts (6.7+ 0.4 R a) is interpreted as evidence of a single mantle source, despite drastic petrological changes during the volcano's history. He analyses performed by crushing cogenetic pyroxene and olivine phenocrysts show a tendency to lower 3He/SHe ratios in pyroxenes. This is best explained by crystallization of pyroxenes at a depth shallower than that of olivines a n d / o r by exchange of helium trapped in pyroxenes with atmospheric or radiogenic He before eruption. 878r/86Sr ratios of recent lavas tend to increase with time and to correlate with R b / T h ratios, and, for historical lavas, these variations are tentatively attributed to shallow selective contamination from underlying sediments. Based on the similarity of the 3He/4He ratios at Etna to those of European mantle xenoliths [1], we propose that the 'baseline' geochemical signature of isotopic tracers at Etna reflects the composition of the subcontinental mantle. Comparison to other southern Italian active volcanoes (Etna, Vulcano Ischia, Campi Flegrei, Vesuvius) shows gradual dilution of the predominantly mantle Etnean end member by more radiogenic Sr and He and material with higher 3180 and C/3He, which is reasonably explained by the progressively important influence of subducted continental crust.

1. Introduction M o u n t Etna, the largest active volcano in Europe, is a large stratovolcano about 40 km in

diameter and 3300 m in height. It is situated at the eastern end of Sicily at the intersection of several major fault systems (Fig. 1). While the occurrence of subducted lithosphere is supported

[UC] 1 Now at: Antarctic CRC, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia 2 Now at: Universit6 de Versailles, 23 rue du Refuge, 78000 Versailles, France 0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-821X(94)00128-L

24

B. Marty et al. / Earth and Planetary Science Letters 126 (1994) 23-39

by geophysical and petrological data further north and east beneath mainland Italy, and by a calc-alkaline suite in the adjacent Aeolian Islands [2], there is no clear evidence that subducted lithosphere occurs beneath Etna itself. Most of Etna's volcanic characteristics are more suggestive of intraplate or hotspot rather than subduction-re-

N I

50 km

lated volcanism. For example, the volcano has no calc-alkaline lavas, and produces predominantly alkali trachybasalts (known as etnaites [e.g., 3-7]) similar in composition to the hawaiite-mugearite lavas found in the Hawaiian volcanoes. The volcanic products also display a temporal trend from early emission of tholeiitic lavas to later emission

A e o l i a n Islands ~

I

Mt ETNA

Montilblei ~--- T e r t i a r y O t QuaetV rnary J

[-~

~

Faults

~

....

.° Fig. 1. Mount Etna. Locations of sampling sites for rocks are given in [4]. The inset shows the location of the sampling sites for fluids and the fluid 3He/4He ratios. The 6.6 R a value is for Mofette Palici [32].

B. Marty et al. / Earth and Planetary Science Letters 126 (1994) 23-39

25

Table 1 Rare gas data.

Sample

Type

Phase

mass g

4He 10 -13 moi/g

R/Ra

X

Rc/Ra

+

40Ar 10-12 mol/g

40At / 36A r

+

3$Ar / 36A r

+

Oliv. Thol. Pig. Thol. Pig. Thol. All<. Basalt Oliv. Thol.

O1 Ol Ol O1 Ol

0.4808 0,3031 0.8054 0.4172 0.4608

23.7 0.75 1.78 I0.0 1.02

7.0 n,m. 6.1 6.8 n.m.

821 50 392 75 77

7,0

0.1 0.5 0.2

341.1 360.0 444.0 307.1 426.8

0.3 2.0 5.0 1.7 9.0

0.1867 0.1879 n.m. 0.1871

0.0010 0.0060

6.1 6.9

3.90 2.10 0.45 13.9 1.68

TP2258

PAB

TP1847

PAB

PLI612

Trans, Thol. PAB Etnailc Etnaite Etnaite

1.0050 {k5905 0,4428 0.6209 0.4258 0.6742 0,5052 0.5825 0.5437 0.5122 0.6553

0.81 2.30 2.37 1,55 3.39 1.01 4.55 1,61 6.95 9.03 1.61

5.8 5.3 6.7 2.4 4.9 4.1 6.3 6.1 6.3 6.7 6.8

8.35 46 _>336 96 107 10 >_646 (,46 721 768 144

6.5 5.4 6.7 2.4 4.9 4.4 6.3 6.1 6.3 6.7 6.8

1.1 1.2 1.1 1.1 0.7 0.9 0.6 0.7 0.2 0.2 0.7

0.55 9.70 1.64 5.34 1.78 n.m. 2.96 0.60 1,22 0.89 0.78

292,4 305.9 298.0 303.7 318.2 n.m. 339.1 288.2 297.3 299.3 293.7

1.8 4.4 0.6 1.7 0.3

TF'1872 TP1887 PLI630 PL1047

OI Px Ol Px Ot Px O1 Ol Ol Ol Px

n.m. 0.1861 0.1850 0.1861 0.1883 n.m. 0.1781 0.1918 0.1894 0.1892 0.1882

Ol 0.1013 Px 0.4097 Ol 0.3672 Px 0,6434 Ol 0.4143 O1## 0.2882 Px 0.2573 O1 0.5058 F'x 0.6711 O1 0.0613 Ol 0.4798 Px 0.1734 Ol 0.3907 O1 0.3266 O1 0.3740

6.67 0.80 44.29 25.35 13.93 11.23 2.98 7.45 0.30 6.63 9.11 1.05 5.92 3.59 3.34

8.0 5.1 6.9 7.0 6.5 6.3 5.2 7,2 n.m. 6.0 6.7 2.6 6.1 6.5 6.4

34 8.2 9.42 5.6 1375 6.9 1995 7.0 53 6.6 _>1595 6.3 22 5.4 645 7.2 17 93 6.1 223 6.7 14 2.7 841 6.1 510 6,5 175 6.5

0.9 2.1 0.1 0.2 0.2 0.3 0.6 0.3

6.04 4.76 24.88 15.60 24.79 26.00 n.m. 1.85 2.75 5.84 0.14 3.05 0.27 1.45 3.76

n.m. 310.1 388.2 347,3 305.0 309.3 n.m. 319,2 300.9 294,7 n.m. 303,8 n.m. 342,6 303,4

Pre-Etnean PLAC TP643 PL734 Paterno TP591

0.0009

Ancient lavas

4.9 3.0 2.2 4.3 2.3

0.0024 0.0027 0.0040 0.0010 0.0228 0.0020 0.1894 0.0008 0.0008

Modern Lavas 1669

Etnaite

1780

Etnaite

1892

Emaite

1971

Etnaite

1983

Etnaite

1985 1989 Bomb, Voragine

Emaite Etrtaite Etnaite

AIR

1

1

0.7 0.3 1.9 0.4 0.7 0.7

295.5

3.6 4.5 2.6 6.9 3.2 0.6 0.2 4.5 4.7 2.6 0.8

n.rn. 0.1878 n.m. 0.1872 0.1890 0.1857 n.m. 0.1970 0.1875 n.m. n.m. 0.1872 n.rn. n.m. 0.1978

0.0026 0.0031 0.0206 0.0072 0.0013 0.0007

0.0057

0.0069

0.1869

Samples are listed according to their ages as estimated by Condomines et al. [5,23]. Gases were extracted by crushing, with the exception of # # for which fusion was used. PAB = porphyritic alkali basalt, the first in the alkaline series in the nomenclature of Tanguy [4]. Errors on rare gas isotopic ratios include reproducibility of air standard (of the order of 1%), 2or/~n- errors for individual measurements and, in the case of Rc/Ra, errors on He and Ne contents (_+ 5% for all rare gases), n.m. = not measured. R a is the 3He/4He ratio of air (1.39 × 10-6), X is the ( H e / N e ) s / ( H e / N e ) a ratio, where subscripts 's' and 'a' refer to sample and atmosphere, and R c / R a is the He isotopic ratios corrected for air contamination [e.g., 15].

26

B. Marty et al. / Earth and Planetary Science Letters 126 (1994) 23-39

of alkali basalts, reminiscent of the well-known tholeiitic-alkali transition in Hawaii. However, Etna differs from Hawaii and other large intraoceanic volcanoes such as REunion in that it occurs above approximately 30 km of continental crust and sedimentary rock. Previous studies of the Sr and Nd isotopes of a limited selection of samples have shown that Etnean lavas are predominantly of a mantle origin and are the least radiogenic lavas found in southern Italy [8-10]. This is in contrast to other Italian volcanoes, where more radiogenic Sr compositions occur, suggesting either involvement of subducted lithosphere, or continental crust, or a metasomatised subcontinental mantle [11-14]. Rare gases and particularly He isotopes offer the possibility of allowing us to distinguish between recycled, MORB-type (e.g., upper mantle) and hotspot sources [e.g., 15]. Oxygen isotopes are sensitive to crustal contributions, and particularly to hydrothermal alteration processes [e.g., 16]. Based on these isotopic tracers examined in rocks tracing Etna's history, we evaluate the nature of the magma and volatile sources and constrain the role of geochemical heterogeneities by examining the temporal evolution of the erupted volcanic products. In addition, the helium isotopic composition analysed in several geothermal gases from around the volcano are used to delineate the areal extent of active magmatism beneath Mount Etna (Fig. 1).

2. Geology and samples Models of the origin and evolution of Mount Etna lavas, in particular for the tholeiitic-alkaline transition, generally fall into two main categories: Either petrological changes as well as R E E and Sr isotope variations (see section 4) resulted from the contribution of two or more distinct mantle sources [3,5], or the whole magmatic system evolved from a single mantle source with variable partial melting and fractional crystallization rates [4,7]. In order to explain the predominance and long-lasting production of trachybasalts, the occurrence of a large reservoir at a depth of 20-30 km where a primary magma

would fractionate, has been suggested [3-5]. Geophysical investigations have shown a low-velocity zone in the same depth interval, which has been interpreted as a framework of magma-filled fissures [17]. Microthermometry of CO2-rich fluid inclusions showed that inclusions in magnesian olivine ( = Fos0) formed in the depth interval 11-24 km [18,19] and that Fe-Ti oxides, augitic pyroxenes, and less magnesian olivines (Fo74_75) probably formed at shallower depths, perhaps after interaction between the magma and the crustal rocks [19]. Recent alkali enrichments (K, Rb, Cs) have been observed in historical lavas and appear uncorrelated with other incompatible element variations [4,6]. These enrichments have been attributed to selective contamination of ascending magmas by sedimentary sequences underlying the volcano (such as the siliceous Capo d'Orlando Flysh and the Piedimonte Formation) on the basis of the occurrence of sedimentary xenoliths, R b / T h variations in lavas [20], and U-Th systematics [21]. It must be noted that Etna degasses at present considerably more sulphur dioxide and CO 2 than Hawaiian or other subaerial volcanoes, and may contribute more CO 2 to the atmosphere than any other individual volcano measured to date [22]. It is of fundamental importance to evaluate whether a significant portion of this CO 2 is derived from degassing of calcareous sediments present in the sub-Etnean crust. We have selected 21 lavas covering the major episodes of Etnean volcanism (Table 1), from the pre-Etnean tholeiitic pillow lavas found at Aci Castello on the coast to the alkali trachybasalts erupted in the past few years. Some of these samples have been studied previously for major and trace element chemistry, U,Th disequilibria and R E E abundances [4-7,23], while others were collected specifically for this study (see Table 1). It is of interest that one of the arguments advanced by Chester et al. [3] for the occurrence of several mantle sources feeding Mount Etna relies on the variation in the 87Sr/S6Sr ratios between basal tholeiites and more evolved lavas, and particular attention is given in the present work to Sr isotope analysis on samples with good geochronological control.

B. Marry et al. / Earth and Planetary Science Letters 126 (1994) 23-39

3. Analytical procedures Rare gases were extracted from olivine and pyroxene phenocrysts by crushing rather than by fusion in order to minimise the contribution of isotopes produced within the minerals by natural radioactivity or spallation. The samples were first crushed to a grain size of 0.5-2 mm, and then ~ 1 mm phenocrysts were separated using a Frantz-Isodynamic magnetic separator, followed by hand-picking under a binocular microscope. Mineral separates were cleaned in distilled water and high-grade acetone and loaded in on-line solenoid-driven crushers. The extraction, purification and analysis procedures are described in [24]. During the course of these experiments, the sensitivity for He was 2.6 X 10 -4 A / T o r r . Typical blanks were 2.5-5 x 10 -15 mol 4He and 2-4 x 10 -14 mol 4°Ar. The rare gas data are given in Table 1, Sr, Nd and O isotopic ratios are displayed in Table 2, and Table 3 presents He data obtained in fluids and rocks for various active volcanic areas of southern Italy.

27

Table 2 Sr, Nd and O isotopic ratios. ~ample PLAC FP643 PL734 ?aterno I'P2258 FP1847 PL1612 I'P1872 PL1630 PL1047 122BC 1381 1669 1892 1971 1983 1985 1989 1992

87Sr/S6S r

+/- 143Nd/144Nd +/-

~18O 9.5

0.703149 0.703058 0.703035 0.703167 0.703087 0.703110

13 17 13 13 11 10

0.703107 0.703149 0.703303 0.703293 (I.703298 0.703375 0.7(13404 0.703486 0.703551 0.703480 0.703473

13 12 11 11 12 14 15 17 I4 11 14

0.512982 0.512905

9 16

0.512934 0.512929

6 14

0.512924

1

0.512898 0.512886 0.512906 0.512875

9 9 11 1

5.3 5.0 5.4 5.1 5.4 5.2 5.8

55 5.4 5.4 5.9

4.1. Helium isotopic ratios

Sr and Nd isotope ratios were determined at the C R P G , Nancy following standard techniques developped in that laboratory. For Sr, repeated analyses of standard NBS987 gave a value of 0.710168+0.000014 for ten runs during the course of the sample analyses. Sr and Nd (ltr) uncertainties refer to the last two digits. Oxygen isotope compositions were determined on whole-rock powders by BrF 5 extraction and mass spectrometry of molecular 0 2 in dynamic mode, following the procedure described in [52]. Replicate sample analyses (n = 2 or 3) for three different samples were reproducible to within +0.2%~, and are reported in relation to the V - S M O W standard.

4.1.1. Isotopic disequilibria Comparing eight pairs of cogenetic olivines and pyroxenes suggests a tendency to lower ratios in the pyroxenes (Fig. 2), although the departure from equilibrium is only resolvable in two samples (ancient lava TP1847 and the 1983 flow). Both of these samples have olivine 3 H e / a H e ratios of 6.7 Ra, but they have pyroxene values of only 2.4 and 2.7 R a respectively. There are several possible explanations for the origin of these disequilibria, some of which can be easily eliminated while others are more difficult to evaluate. Post-eruptive addition of radiogenic helium appears unlikely given the disequilibrium in modern lava. Pre-eruptive addition of radiogenic He would preferentially affect the pyroxenes, because they have lower He concentrations than the cogenetic

olivines (about 40% lower for the ancient lava and nearly a factor of 10 lower for the modern lava). However, the accumulation times appear too long. Etnean U and Th contents average 2.5 and 9 ppm in historical lavas [5], yielding a radiogenic 4He* production rate of 2.5 x 10-17 m o l / g / y r for the magma. Alpha-particle injection into a 1 mm grain would produce accumulation at 5% of this rate [25], so that 20-40 kyr of accumulation would be required to produce the observed pyroxene 4He* contents (in excess of magmatic He) of 1.1 and 1.6 x 10 -13 mol/g. In-situ production from U in melt inclusions ( ~ 1 vol%) would require 100-200 kyr. These times are considerably longer than the < 10 kyr magma chamber refilling interval determined for Etna from U-Th disequilibrium studies [5,23]. Another reason to discount pre-eruptive 4He* accumulation is that

4. Results

28

B. Marry et al. /Earth and Planetary Science Letters 126 (1994) 23-39

Table 3 Helium and CO 2 in volcanic regions of southern Italy.

Sampling Date

He ppm vol

R/Ra

X

Rc/Ra

CO2/3He 109

Apr-91 Apr-91 Jun-81 Apr-91 Apr-91 Apr-91

7 34

3.33 5.25 6.05 5.91 5.89 6.32

3.34 18.0 4236 1298 1346 235

4.33 5.50 6.05 5.91 5.89 6.34

21.8 3.4

ETNA Zafferana Aquarossa Patemo-Sallinella Paterno-Sallinella Patemo-Sallinella Patemo-Mud volcano Northeast Crater (1) Crater area (1)

P L A C (2)

well gas hot spring bubbles n m d pool gas mud pool gas mud pool gas n m d pool gas

115 116 63

volcanic gas soil gas soil gas

1.0 0.9 1.6 5.8 5.8 4.9

fluid inclusion

2.37

7.0

1004

7.0

> 1.7

16

1198 49 226 174 125

5.02 5.09 5.18 5.66 4.52

8.1

5.7 3.7

VULCANO F5 area F5 area F5 area FA area Beach

fumarole fumarole fumarole l'umarole bubbles m the sea

Sep-82 Apt-89 Apr-89 Apr-89 Apr-89

20 40

5.02 5.01 5.16 5.63 4.49

ISCHIA

l'umarole

Apt-89

29

4.49

28

4.62

4.9

fumarole bubbles in the sea bubbles in the sea bubbles in the sea

Apr-89 Apt-89 Apr-89 Apr-89

17 13 7

2.59 2.62 2.39 1.90

42 39 27 25

2.63 2.66 2.44 1.94

14.5 3.0 20.5 47.9

bubbles in tile sca fluid inclusion fluid inclusion

Apt-89

24 0.40 1.56

2.26 2.5 1.6

5 40 2.75

2.57 2.6 2.3

10.5

CAMPI PHLEGREI Bocca Grande Porto Pozzuoli Secca Caruso Mare Morto VESUVIUS Torre del Greco 1944 Lava, px 1 * 1944 Lava, px2*

Errors on He isotopic ratios are 1-2% for fluid analyses and 27.6 and 7.0% respectively for the two Vesuvius lavas. For rock samples, the He content should be multiplied by l0 I2 mol/g. (1) Gas data are from [22]. Only soil gas data for which CO 2 in the dry gas is > 10% are considered. (2) The CO 2 content of an aliquot (0.5 g) of the separated olivines analysed for rare gases (see below) was determined. Cleaned olivine grains were crushed under vacuum by a solenoid-operated piston to a grain size of 20/xm. The evolved CO 2 content was measured with a precision of only 40% because of its low quantity. The possibility of adsorption of CO 2 on newly crushed mineral surfaces using this technique makes the C / 3 He estimate for Aci Castello a lower limit. Pyroxene separates of Vesuvius lavas consisted of 0.5-1 cm megacrysts. Measured 31H e / 4 He ratios are consistent with those very recently reported for olivine separates from the same volcano [52]. The use of fluid data where rock data where not available may introduce some bias because of possible leaching of radiogenic helium from the country rocks, but we believe that this problem is not serious because Etna and Vesuvius fluid data overlap rock results and, in the case of Vulcano, the fluid results are from high-temperature volcanic gases (up to 500°C during sampling). Furthermore, He isotope data from the different volcanic centres of Campi Flegrei [32,48,this work] and of Vesuvius are very similar, suggesting a broad-scale regional imprint for helium and supporting, as in the case of Mount Etna, deep homogeneity of the magma sources.

B. Marty et aL / Earth and Planetary Science Letters 126 (1994) 23-39

crushing releases He from fluid inclusions rather than from within the lattice where radiogenic 4He* accumulates [25]. Evaluating the possibility that the lower 3He/ 4He ratios in the pyroxenes result from 4He* accumulation within the magma over time and sequential crystallization ('magma ageing' [26]) hinges on He partition and diffusion coefficients. The small crystal-magma partition coefficients of ~ 0.01 estimated for MORB [27,28] yield a magma He concentration at the moment of pyroxene formation of ~ 10 -11 mol/g, again requiring a long time for sufficient accumulation (~ 1 x l0 s yr). The complete diffusive He loss over hours to days measured for 0.5 mm mantle xenocryst olivine and pyroxene grains [29,30] suggests that diffusive re-equilibration with the magma would prevent successively forming phenocrysts from recording such slow changes in melt 3He/aHe ratios. Assimilation, after olivine crystallization, of crustal materials containing radiogenic 4He* could explain the lower pyroxene 3He/aHe ratios, provided it was soon followed by eruption to avoid diffusive re-equilibration. The amount of 4He* required ( ~ 10 -11 mol/g as estimated for the magma above) could be obtained from a few percent assimilation of material with a He concentration of 10 - 9 mol/g, equivalent to accumu-

o= ,o

j

O

¢~

86-

.r

/

( -----t

e~

N

2

m

0

0

l

U

n

I

2

4

6

8

10

3He/4He (Rc/Ra), olivine Fig. 2. Comparison of helium isotopic ratios recorded in olivine and in pyroxene coexisting in the same lavas (data from Table 1). Each point represents an individual rock. [] = Ancient lavas; o = historical lavas.

10-12

eat)

~

n

IIllll

I

I

29 n

nPnlnn

10-13

10 -14

e-e::;-?~[ °D

n

ulnlll

I

I

I

IIIII

• +'°

zx

/

G

<~D ~

u

oo A

O ~

i

10.15

. . . . . . . . . .

. . . . . . . . .

10-18

10-17

,

10-16

,

10-15

[3He] mol/g Fig. 3. 36Ar vs. 3He contents in olivines (open symbols) and pyroxenes (closed symbols). Cogenetic minerals are connected by dashed lines. Triangles = pre-Etnean lavas; squares = ancient lavas; circles = historical lavas. For samples TP643 (ol), TP591 (ol) and 1971 (px), 3 H e / a H e ratios of 6.7 R a were assumed. For sample 1669, a 4°Ar/36Ar ratio of 310 (the value measured in the pyroxene) was assumed for olivine.

lation for ~ 108 yr for sedimentary rocks with U contents of ~ 1 ppm (such as the sedimentary xenoliths and flysh at Mount Etna). Accompanying 4°Ar* would produce only small increases (< 10%) in the magmatic 4 ° m r / 3 6 A r ratio, because of the higher Ar content of the magma ( ~ 10 X higher judging from the concentration ratios in the crystals and assuming similar He and Ar partition coefficients). In addition to their lower 3He/nile ratios the pyroxenes tend to have lower 3 H e / 36Ar ratios than the olivines (Fig. 3). All the cogenetic pairs show this relationship, with ratios from 20% to an order of magnitude lower in the pyroxenes. This is consistent with an addition of atmospheric gases, although if this process produced the lower pyroxene 3He/4He ratios it must have preferentially affected He since the pyroxene H e / N e ratios are much higher than that of air (Table 1). Diffusive atmospheric He could produce such a preferential effect. Because of the low air He content this would require extensive He exchange (>/99%, [30]), which sample grain sizes and measured He mobilities [30] suggest could have occurred if the lavas remained red hot for hours to days. Without precise knowledge of the effective

B. Marty et aL /Earth and Planetary Science Letters 126 (1994) 23-39

30

diffusive length scale in the minerals it is not possible to evaluate these possibilities further. Whatever the exact processes responsible for the He isotopic disequilibrium, its occurrence

emphasizes the value of examining cogenetic minerals, and obtaining estimates of their crystallization depths by microthermometry or other means. We consider that the tendency towards lower

0,7036 L

(OCt) ¢0

0,7034

0

L

i(/~

0

O0

0

0,7032

O0

0,7030

'0

Z

O 0.51296

,p

% 0

0.51291

Z

0

O

t')

0

0.51286

!

n"

7

0

6

¢

|

!

© .C3

T

MEAN

.

: 6.7 + 0.4

.

L

5 4 L

CPo

400

L

o<

(~ 3 0 0

O -

..... . o . ~

Q ..... o . . . . . . . . . . . . . ? . . . . . . . . . . . . . . . . . . . . .

0

OQ o

........

AIR 7

I

6

%

I

I

I

10 3

10 2

101

MEAN : 5.4 + 0.3

6

O0

I

5

4

10 6

10 5

10 4

10 0

Time, yr Fig. 4. Temporal variation in Sr, Nd and He (olivine), Ar (olivine) and O isotopic compositions (Table 1) from 0.5 Ma to the present.

B. Marty et al. / Earth and Planetary Science Letters 126 (1994) 23-39

3He/4He and 3He/36Ar ratios in pyroxenes represents shallow assimilation of atmospheric a n d / or radiogenic rare gases after the crystallization of the olivines at greater depth. For this reason, when discussing deep volatile sources for Mount Etna we focus on the olivine values.

4.1.2. Temporal and geographical variations He isotopic ratios recorded in olivines appear constant throughout the life of the volcano (Fig. 4). In particular the two samples which show the highest helium contents and therefore the highest precision on 3He/4He determinations, the primitive tholeiite at Aci Castello (sample PLAC) and the 1780 lava flow, have identical 3He/4He ratios of 7.0 +_0.1 R a and 6.9 _+0.1 R a respectively (Table 1). The Paterno Dome lava, whose connection with Etnean magmatism sensu stricto is questionable [3,4], displays an identical 3He/4He ratio of 6.9 _+0.2 R a. The only possible exceptions are the transitional tholeiite PL1612 (4.9 _+0.7 R a) and the 1669 lava sample (8.2 _+0.9 Ra), although for both samples the He ratios are only marginally different from the mean isotopic composition. The constancy of Etna's 3He/4He ratios despite significant changes in the volcano structure, petrological variations of the Etnean lavas, Sr isotope evolution (Fig. 4) and recent alkali enrichments argues against models which invoke the contribution of geodynamically diverse mantle sources [3,5] but is in agreement with evolution from a single source [4,7]. These helium isotopic ratios (weighted mean of 6.7 +_0.4 R a) are among the highest ratios reported so far in Europe [e.g., 31]. This confirms the predominantly mantle-derived character of the Etnean magmas, a point reiterated by 3He/4 He analysis of volcanic and geothermal gases from hot and cold springs on and around Mount Etna (Table 3). A ratio of 6.6 R a in Mofette Palici [32], 50 km southwest of Mount Etna, possibly on the same fracture zone as the Paterno Springs and near the Quaternary volcanic region of Monti Iblei (Fig. 1), suggests a near-constant He isotopic signature on a regional scale. This situation is in stark contrast with that of some arc volcanoes for which 3He/4He ratios typically display a sharp decrease away from the central

31

volcanic cones over distances of 5-10 km [33-35] and is compatible with the presence of a laterally extensive magma reservoir beneath Mount Etna.

4.1.3. C /3He ratios and the origin of carbon A compilation of C and He data in Etnean emanations shows that C/3He ratios vary between 0.9 x 109 and 5.8 × 109 (Table 3). (Note that the Zafferana well gas exhibits a higher C/3He ratio of 21.8 X 109 but also shows the lowest R c / R a ratio of all Mount Etna data and is therefore suspected of secondary alteration.) The analysis of volatiles in the Aci Castello olivine separate indicates a C/3He ratio of >~ 1.7 × 109. All these ratios are within the range of MORB and hotspot values [36,37] and at the lower end of typical values recorded at subduction zones where sedimentary contribution has been proposed [34,38]. In addition the ~ 1 3 C ratio of Etnean volcanic gas is -3.5%0 to ca. - 4 % o PDB [22], similar to values recorded in gas-rich MORB [39], volcanic gases from basaltic volcanoes [e.g., 40], and above the t513C range of carbonate rocks. Based on these observations, we tentatively favour a deep, volatile-rich source rather than a contribution from the sedimentary sequences underlying the volcanic edifice. Fig. 5 compares the Et1E+05 1E+04 1E+03 1E+02 1E+01 1E+00 IE-01 1E-02

BaTh U PbNbLaCe SrNdZrSmEuGd*TiDy - Er - Yb Fig. 5. Trace element concentrations (relative to C1) of Etnean lavas, compared to N - M O R B ( o ) ([54] and references therein) and to continental crust (,x [55]) concentrations. Etnean samples (solid lines) include historical and prehistorical samples with 875r/86Sr covering the whole range of Sr isotope variations. The field of carbonatites (shaded area) is from [56].

32

B. Marty et al. / Earth and Planetary Science Letters 126 (1994) 23-39

nean trace element pattern to that of the fields of carbonatites, of continental crust and of NMORB. Etnean trace element patterns are clearly different from pure M O R B and continental crust end members, and present significant depletions in Pb, Zr and Ti, which possibly trace the influence of carbonatitic metasomatism in the source region. This leaves the possibility that the high enrichments in trace elements and the high volatile content of the Etnean magmas (as is evident from fluid and melt inclusion data [18,19] as well as from very high volcanic plume fluxes [22]) may reflect C-rich metasomatism in the source region of Etnean magmatism. 4.2. A r g o n isotopes

Etnean 38Ar/36Ar ratios are mostly atmospheric, within the uncertainties, w h i l e 4°Ar/36Ar ratios vary between the air value of 295.5 and 444, and are thus much lower than depleted mantle (MORB) values, which reach 2.8 x 10 4 [41]. Near-atmospheric 4°Ar/36Ar ratios are also often found in lavas from hotspots (e.g., Loihi, REunion), although whether this reflects a 'primitive' source or shallow air contamination is a matter for debate [41,42]. The 3He-36Ar diagram shown in Fig. 3 reveals that Etnean 3He/36Ar ratios are closer to those of air (2.32 x 10 -7) than those of Loihi Seamount (3.0 X 10 -2) and M O R B (3.6) [41], which indicates that the low Ar ratios at Mount Etna are probably due to atmospheric contamination. In addition, atmospheric addition appears to have preferentially affected the shallower crystallizing pyroxene samples, which all have lower 3He/36Ar ratios than cogenetic olivines (Fig. 3). Given the low Ar isotopic ratios measured in olivine crystals, some of which were formed at depths of 11-24 km, it follows that atmospheric contamination must have also occurred to at least these depths. Deep addition of atmospheric argon is also suggested by the weak 3He-36Ar correlation in Fig. 3 (because 3He is negligible in air and comes only from the deep source). From C / 3 H e ratios in gases, estimates of the initial S content in the magma ( ~ 1.0 x 10 -4 m o l / g [43]), C / S in volcanic gases ( C / S = ~ 1,

[22,40]) and in the plume ( C / S = 12.6, [22]) we compute a minimum He concentration in the magma of 2 X 10 -9 m o l / g , similar to, or higher, than that of undegassed M O R B (1.5 X 10 -9 m o l / g , section 5), confirming the volatile enrichment of the Etnean magmas. The initial 36Ar content (computed with 2 x 1 0 - 4 ~< 3He/36Ar ~< 2 x 10 -3 from Fig. 4) admits a lower limit of 10 -11 m o l / g . This order of magnitude is probably reasonable given the observation of 36Ar values of 10-13-10 -15 m o l / g in olivine and considering a partition coefficient of 10-2-10 -3 for argon [27]. Such a concentration is extremely high: Typical M O R B 36mr contents are 10 -15 m o l / g and those of OIB are 10-13-10 -14 m o l / g [41]. If we suppose that water in Etncan alkaline magma, estimated to total ~ 3% [19], is of surficial origin and we take an air-saturated water (ASW) concentration at 0-25°C of 5.7-4.3 x 10 -11 m o l / g , only 1 to ca. 2 X 10 -12 mol 36Ar/g could have been added to the magma through simple binary mixing. Therefore the high atmospheric Ar content in Etnean magmas requires an additional source of atmospheric argon (or a selective enrichment process such as metasomatism as suggested in section 4.1), which in this case should have taken place after addition of atmospheric gases. 4.3. Oxygen isotopes

One of the most striking results of this study is the constancy of the ~lSo signature through time at values (+5.4_+ 0.3%o) typical of mantle-derived magmas [16]. This range is consistant with a 6180 value of 5.6%o previously reported for the 1964 lava flow [44]. The only exception is the pillow basalt PLAC for which a value of + 9.5%0 is probably due to seawater alteration associated with the emplacement of these lavas into shallow sediments. No increase in 6!80 occurs concomitantly with the 875r/86Sr and K, Rb content increases in modern lavas (see next subsection), arguing against--except in small amounts (a few to several p e r c e n t ) I t h e bulk addition of flysch, cherts and carbonates (all of which typically have 6180 values of + 10%o or higher [e.g., i6]). No low ~aso lavas (e.g., in the range + 1 to +5%0)

33

B. Marty et al. / Earth and Planetary Science Letters 126 (1994) 23-39

such as those that occur in Iceland and are considered to indicate strong interaction of magma with hydrothermally altered crust were found [45]. These observations suggest that the interaction with sediments advocated (next subsection) to explain Sr isotope and alkali element enrichments are more likely to involve selective partial fusion than transport in an aqueous fluid.

0.7036

I

o r.~' 0.7034 '.,D

1985

o

1983

1971

199

1892

o 122BC

0.7032

1669

TP643

d p2258

.P1847 A

4.4. Strontium and n e o d y m i u m isotopes

The whole-rock 87Sr/86Sr ratios (Table 2) vary between 0.703035 (+0.000013) for the Paterno neck (about 200,000 yr BP) and 0.703551 ( + 0.000014) for the 1985 lava flow. They show a remarkable increase through time (Fig. 4), suggesting gradual mixing between a mantle source and a more radiogenic component, with most of the increase taking place recently, during the last century. The shift in Sr isotopic ratios is not accompanied by a correlated shift in Nd, O or He isotopic ratios (Table 2 and Fig. 4). He mobility in mantle minerals [30] appears too low to allow homogenization of He isotopes over the probable kilometre length scales of magma sources, given the lifetime of magmatism in this region. Thus, the constancy of the 3 H e / 4 H e ratio implies that the mantle source was homogeneous on the scale of melting, or that some fluid-mediated homogenization took place, which would also be expected to have homogenized the Sr isotope ratios. Sr isotope variations could also be the result of shallow assimilation, but the lack of covariation between R E E fractionation (e.g., C e / Y b ) and the 87Sr/S6Sr shift (not shown) does not support the occurrence of an AFC-type process. Another possibility would be selective contamination of strontium by sediments underlying Mount Etna, as has been proposed for alkali elements on the basis of R b / T h ratio variations [6,20]. In agreement with this view, a plot of R b / T h ratios vs. 875r/86Sr ratios reveals a fair correlation in the case of historical lavas (Fig. 6). This hypothesis must also take into account the observation that neodymium, helium and oxygen isotopes do not show correlated variations. The case of Nd is easily explained by the low abundance of neodymium in sediments. Helium analysed in olivines

i

o

[]

0.703

~

2

l==][~L 1 6 1 2

Paterno

A

I

3

PLI630 i

I

i

4

I

5

i

6

Rb/Th Fig. 6. 87Sr/86Srratios vs. Rb/Th ratios. Sr data from Table 2 and Rb/Th ratios from [5] and unpublished data obtainable at the CRPG. Same symbols as in Figs. 2 and 3.

was probably trapped at a depth greater than that of sedimentary contamination and we do not expect alteration of isotopic ratios if closed system conditions prevailed. For oxygen, the maximal contribution can be computed assuming the boundary case of binary mixing. The mass of contaminant necessary to account for a Sr isotopic shift from 0.7030 to' 0.7035 in the magma, assuming that this contaminant is of the Capo d'Orlando-type, is about 5%. The mean (~180 value of Etnean lavas is + 5.4%o and, allowing a possible shift of 0.3%o (which corresponds to the standard deviation on all measurements), the contaminant should have a maximum 6180 signature of + 13%o, well in the range of typical sedimentary values. The case of ancient and pre-Etnean lavas is left open to further investigation, which will require examination of other trace element and isotopic systems (e.g., Pb, Os) in order to better unravel the contributing sources and processes.

5. Discussion 5.1. Constraints on the source region

The mean He isotopic signature of M o u n t Etna is slightly equal to or below the lower end of

34

B. Mart), et aL / Earth and Planetary Science Letters 126 (1994) 23-39

the M O R B range (8 + 1 R a) and similar to the values r e c o r d e d in some island arc volcanic gases [e.g., 15]. It is also within the range of values r e c o r d e d in several o c e a n island basalts (e.g., Tubuaii, Saint Helena, G o u g h , Tristan da C u n h a [e.g., 28,46]). This certainly eliminates the occurrence of a ' h i g h - 3 H e ' hotspot (characterized by 3 H e / 4 H e ratios higher than M O R B and inferred to outgas m o r e primitive H e than that f o u n d in the u p p e r mantle) like Hawaii, Iceland, R6union, Afar, Yellowstone etc., but leaves the possibility that M o u n t E t n a m a g m a t i s m may be linked with a mantle region affected by an ancient source enrichment, or the contribution of a recycled c o m p o n e n t , or both. Given the specific tectonic context of Etna, we now discuss these possibilities. First, the hypothesis of a d e e p m a g m a reservoir at the base of the crust (section 2) is relevant to the M A S H m o d e l of Hildreth and M o o r b a t h [47] in which basaltic Melts Assimilate and mix with ~ 25% of lower crustal material at the m a n t l e / crust interface or within the lowermost crust, and are Stored and H o m o g e n i s e d in such zones. Hilton et al. [48] p r o p o s e d that the signature of helium in the central A n d e a n zone was lowered f r o m a typical M O R B value by stripping radiogenic helium from the lowermost crust. In the case of M o u n t E t n a however, the H e and Sr isotopic ratios are too close to typical mantle values to be compatible with the assimilation of ~ 25% of lower crustal material. In addition,

although calc-alkaline m a g m a t i s m is generally associated with the M A S H model, it is typically absent in the M o u n t E t n a region. A recent study of H e isotopes in continental xenoliths from T e r t i a r y - Q u a t e r n a r y volcanic regions of western E u r o p e by D u n a i and B a u r [1] has shown a striking uniformity o f 3 H e / 4 H e ratios when radiogenic and cosmogenic contributions are filtered. T h e m e a n 3 H e / a H e ratios of the Massif Central, Eifel, Spitsbergen and Kapfenstein are 6.5 + 0.2, 6.0 + 0.1, 6.6 + 0.2 and 6.1 _+ 0.7 R a respectively. T h e observation that the m e a n E t n e a n 3 H e / n i l e ratio falls within the same range and that m a g m a t i s m in this region originates in a mantle source located b e n e a t h a continental crust suggests that the nature of the sampled geochemical reservoir may be similar. T h e E u r o p e a n subcontinental mantle may have a lower 3 H e / 4 H e ratio ( ~ 6.5 R a) than the suboceanic M O R B mantle ( 7 - 9 R a) for a variety of reasons. D u n a i [pers. commun.] favours a 1 - 2 % average crustal c o n t a m i n a t i o n of the m a g m a source having o c c u r r e d 3 5 0 - 7 0 0 M a ago in the precollisional/collisional stage of the Hercynian orogeny. Alternatively, 'ageing' without enrichm e n t could also lead to lowering 3 H e / 4 H e ratios, provided that the subcontiental mantle is decoupied from the u p p e r mantle sampled at mid-ocean ridges. Taking a M O R B U / H e ratio of ~ 80 p p b / 1 . 5 × 10 -9 mol H e / g suggests that ~ 0.5 G a are required in o r d e r to p r o d u c e 3 H e / 4 H e ratios in the range 6 - 7 R a.

Fig. 7. He isotopic ratios ( R c / R a) vs. C/3He and Sr and O isotopic ratios, o = Etna; [] = Vulcano; zx = Ischia; • = Vesuvius; • = Campi Flegrei. C/3He and He data from Table 1 and 3, Sr data from Table 2 and [8-14], 3180 data from Table 2 and [13,16]. In the STSr/S6Sr and 3180 diagrams, bars represent isotopic variations recorded for the corresponding regions for the least differentiated lavas. Mixing lines between MORB and upper continental crust (UC), lower continental crust (LC), carbonate sediments (CARB) (10% carbonate and 90% shale of continental derivation (50% UC and 50% LC) following Varekamp et al. [38]) and oceanic crust (OC) have been computed using the following data: N-MORB UC LC OC CARB Sr (ppm) S7Sr/86Sr He (10- l0 mol/g) 3He/4He (R a) C (10- 5 mol/g) 3180 (%0)

141 0.7025 15 8 3.6 5.5

160 0.7369 145 0.01 1.7 8

104 0.7170 27 0.01 1.7 8

400 0.7041 0.036 2.0 6.7 5.7

200 0.7250 4 0.05 400 10

For this table the following data sources have been used: C and He = [36-38,58]; Sr = [54-57], O = [16]. For the continental crust components, an age of 1 Ga has been assumed in computing the He concentrations.

B. Marty et aL / Earth and Planetary Science Letters 126 (1994) 23-39

•

Can~

CARB\

•

UC /

35

3C

n

~

iqi!:!~,i~:~::~!:i~i~

oo

109

9ili!ii!ii!ii!!i!i!!i

108

CARB 0,708 t_

r~

0,706

~"~ UC /

1.1

r~

LC \~._,

0,704

0,702 9

CARB/

8

=o

7 UC

6

Bna

5 4 0

1

2

3

4

RC/Ra

5

6

7

8

9

36

B. Marty et al. / Earth and Planetary Science Letters 126 (1994) 23-39

5.2. Comparison with other southern Italian volcanoes

Magmas south of the Roman region are characterized by an abundance of potassic and ultrapotassic lavas with high Sr and 180/160 ratios and low Nd ratios, reflecting either the influence of intracrustal or enriched mantle sources. When shallow crustal contamination and assimilation/ fractional crystallization processes are filtered, oxygen and strontium isotopic ratios appear well grouped around + 5.8 to + 7.5%0 and 0.706-0.710 respectively [e.g., 16], which has lead to the general consensus that an enriched mantle source exists produced by either ancient fractionation or 'recent' addition of a slab-derived component [11,12-14], with the timing of enrichment being a matter of debate [12,14]. Considering 3He/4He and C/3He ratios in addition to O and Sr isotopes allows us to further delineate the potential enrichment processes (Fig. 6). The C/3He vs. 3He/aHe ratio variations suggest increases in the carbon content relative to mantle-derived 3He as the helium isotopic ratio decreases, which could result from the gradual contribution of a sedimentary component rich in C, or an altered oceanic crust rich in carbon veins. This possibility is in agreement with the observation that the ¢~13Cin the Vesuvius, Campi Flegrei and Vulcano fumaroles cluster around 0%o [49], close to marine carbonate values. However, 3He/4He ratios from Campi Flegrei-Vesuvius are very low [32,49,this work] and, among active volcanoes, only volcanoes of the East Sunda Arc show such low (or even lower) He isotopic ratios [50]. Such low 3He/4He ratios cannot be produced by the sole subduction of oceanic crust or carbonate sediments since these two reservoirs can hardly accumulate the required amount of radiogenic helium [50], and we must therefore appeal to the contribution of another geochemical end member which, in the case of the Sunda Arc, was identified as subducted continental crust [50]. The mixing curves of Fig. 7 show that subducted continental crust also represents an end member capable of producing the regional variations in southern Italy. Dehydration during subduction could introduce the metasomatic fluids

proposed by Taylor and Sheppard [16] in order to explain the O and Sr isotopic compositions, along with U and radiogenic 4He. If the subcontinental mantle initially had He contents similar to the MORB source (~ 10-l°mol/g), the admixture of tens of percent of 109 yr old crust (with ~ 10 -9 mol/g He for 1 ppm U and complete He retention) could explain the lowered 3He/4He ratios in the north. Much smaller amounts of continental material would be necessary if the subduction and dehydration 'flushed out' the original mantle helium. In this view, the south to north variations represent the changing extents of subduction and the introduction of crustal fluids into the subcontinental lithospheric mantle. This interpretation is similar to the explanation of Sano et al. [51] for the origin of hydrothermal fluid He isotopic variations in southern Italy, in that it calls for varying contributions from the high-3He/4 He mantle end member across the region, although it differs in that we favour a deep subducted source for the radiogenic He, Sr and high-6180 end member, whereas Sano et al. preferred a shallow, crustal, hydrothermally transported radiogenic He source.

6. Summary The evolution of Mount Etna has been investigated using several isotopic tracers (He, Ar, O, Sr, Nd). The following has been shown: (1) The results of rare gas extractions by crushing of cogenetic olivine and pyroxene suggest a tendency to lower 3He/aHe and 3He/36Ar ratios in the pyroxene. This effect is attributed to a higher 'sampling' capability of olivine, which traps helium at greater depth than pyroxene and preserves the original magmatic signature, while pyroxene is subject to shallower contributions of radiogenic or atmospheric helium. (2) Whole-rock ~180 values (5.4 +0.3%o) and olivine 3He/aHe ratios (6.7+0.4 R a) are constant throughout the volcano's history, supporting the occurrence of a single mantle source feeding Etnean magmas. The 3He/4 He ratios are very similar to those recorded

B. Marry et al. /Earth and Planetary Science Letters 126 (1994) 23-39

in continental xenoliths from various European volcanic areas, with the implication that the mantle source supplying Etnean magmas is the subcontinental mantle. (3) Argon isotopic ratios indicate the presence of atmospheric argon. Argon addition is not solely a shallow process and probably took place before or during olivine crystallization at depths close to the mantle-crust boundary. The mechanism of its introduction remains to be elucidated. (4) 87Sr/86Sr ratios show temporal variations between 0.7030 and 0.7035 which, for recent historical lavas, are tentatively attributed to selective shallow contamination by sedimentary rocks underlying the volcano. (5) T h e 3 H e / 4 H e - C / 3 H e - s 7 S r / 8 6 S r - t ~ 1 8 0 systematics of Etna, Vulcano, Ischia, Campi Flegrei and Vesuvius are consistent with a progressive influence of a continental crust-type component, perhaps attributable to a progressive change in the nature of subducted material from oceanic to continental crust.

Acknowledgements This research was in part supported by the Commission of the European Communities (DG XII, Environment Programme, Climatology and Natural Hazards Unit) as part of contract EV5VCT92-0177. We are indebted to A. Criaud, D.L. Pinti, R. Romano, D. Tedesco, J.P. Toutain, M. Navratilova, F. Tonani and the late S. Matsuo for their assistance and good humour during various field trips. R. Clocchiatti kindly supplied sample 1892. N. Vassard, M. Lenoble (Laboratoire MAGIE), A. Michel (Laboratoire de G~ochimie des Isotopes Stables) and C. Alibert and D. Dole (CRPG) provided help and expertise. The manuscript benefited from critical reading by S. Barth, M. Chaussidon and T. Dunai. We are also indebted to an anonymous reviewer and to D. Hilton for very helpful comments. This is CRPG contribution 1042.

37

References [1] T.J. Dunai and H. Baur, He and Ar systematics of the European subcontinental mantle, Terra Abstr. 5, 425, 1993. [2] F. Barberi, L. Civetta, P. Gasparini, F. Innocenti, R. Scandone, G. Ferrara and L. Villari, Evolution of a section of the African-Europe plate boundary: Paleomagnetic and volcanological evidence from Sicily, Earth Planet. Sci. Lett. 21,269-276, 1974. [3] D.K. Chester, A.M. Duncan, J.E. Guest and C.R.J. Kilburn, Mount Etna: The Anatomy of a Volcano, Chapman and Hall, London, 1985. [4] J.C. Tanguy, L'Etna: &ude p6trologique et pal~omagn~tique, implications volcanologiques, Doct. Etat, Thesis, Univ. Paris VI, 1980 (Unpubl.). [5] M. Condomines, J.C. Tanguy, G. Kieffer and C.J. All~gre, Magmatic evolution of a volcano studied by 23°Th-23Su disequilibrium and trace element systematics: The Etna case, Geochim. Cosmochim. Acta 46, 1397-1416, 1982. [6] J.L. Joron and M. Treuil, Etude g6ochimique et p6trogen~se des laves de l'Etna, Sicile, Italic, Bull. Volcanol. 47, 1126-1144, 1984. [7] J.C. Tanguy, Tholeiitic basalt magmatism of Mount Etna and its relations with alkaline series, Contrib. Mineral. Petrol. 66, 51-67, 1978. [8] S.R. Carter and L. Civetta, Genetic implications of the isotope and trace element variations in the Eastern Sicilian volcanics, Earth Planet. Sci. Lett. 36, 168-180, 1977. [9] P. Afmienti, F. Innocenti, R. Petrini, M. Pompilio and L. Villari, Petrology and Sr-Nd isotope geochemistry of recent lavas from Mr. Etna: Bearing on the volcano feeding system, J. Volcanol. Geotherm. Res. 39, 315-327, 1989. [10] D.W. Graham, A. Giacobbe, F. Spera and G. Tilton, Chemical and isotopic variations in historical lavas from Mount Etna, Eos 73(43), 611, 1992. [11] C.J. Hawkesworth and R. Vollmer, Crustal contamination versus enriched mantle: 143Nd/144Nd and 87Sr/86Sr evidence from the Italian volcanics, Contrib. Mineral. Petrol. 69, 151-165, 1979. [12] R.M. Ellam, C.J. Hawkesworth, M.A. Menzies and N.W. Rodgers, The volcanism of Southern Italy: role of subduction and the relationship between potassic and sodic alkaline magmatism, J. Geophys. Res. 94, 4589-4601, 1989. [13] G. Ferrara, M.A. Laurenzi, H.P. Taylor, Jr., S. Tonarini and B. Turi, Oxygen and strontium isotope studies of K-rich volcanic rocks from the Alban Hills, Italy, Earth Planet. Sci. Lett. 75, 13-28, 1985. [14] B. Luais, Mantle mixing and crustal contamination as the origin of the high-Sr radiogenic magmatism of Stromboli (Aeolian arc), Earth Planet. Sci. Lett. 88, 93-106, 1988. [15] J. Lupton, Terrestrial inert gases: Isotope tracer studies and clues to primordial components in the Earth. Annu. Rev. Earth Planet. Sci. 11,371-414, 1983.

38

B. Marty et al. / Earth and Planetary Science Letters 126 (1994) 23-39

[16] S.R. Taylor and S.M.F. Sheppard, Igneous rocks: I Processes of isotopic fractionation and isotope systematics, in: Stable Isotopes in High Temperature Geological Processes, J.W. Valley, H.P. Taylor, Jr. and J.R. O'Neil, eds., Rev. Mineral. 16, 227-269, 1986. [17] A.D.L. Sharp, P.M. Davis and F. Gray, A low velocity zone beneath Mt. Etna and magma storage, Nature 287, 587-591, 1980. [18] R. Clocchiatti, J, Weisz, M. Mosbah and J.C. Tanguy, Coexistence de 'verres' alcalins et thol~iitiques satur~s en CO 2 dans les olivines des hyaloclastites d'Aci Castello (Etna, Sicile, Italie). Arguments en faveur d'un manteau anormal et d'un r~servoir profond, Acta Vulcanol. 2, 161-173, 1992. [19] N. Metrich and R. Clocchiatti, Melt inclusion investigation of the volatile behaviour in historic alkali basaltic magmas of Etna, Bull. Volcanol. 51, 185-198, 1989. [20] R. Cloechiatti, J.L. Joron and M. Treuil, The role of selective alkali contamination in the evolution of recent historic lavas of Mr. Etna, J. Volcanol. Geotherm. Res. 34, 241-249, 1988. [21] B. Villemant, V. Michaud and N. Metrich, Wall rockmagma interactions in Etna, Italy, studied by U-Th disequilibrium and rare earth element systematics, Geochim. Cosmochim. Acta 57, 1169-1180, 1993. [22] P. Allard, J. Carbonelle, D. Dajlevic, J. Le Bronec, P. Morel, M.C. Robe, J.M. Maurenas, R. Faivre-Pierret, D. Martin, J.C. Sabroux and P. Zettwoog, Eruptive and diffuse emissions of CO 2 from Mount Etna, Nature 351, 387-391, 1991. [23] M. Condomines, R. Bouchez, J.L. Ma, J.C. Tanguy, J. Amosse and M. Piboule, Short-lived radioactive disequilibria and magma dynamics in Etna volcano, Nature 325, 607-609, 1987. [24] B. Marty, I. Appora, J.A. Barrat, C. Deniel, P. Vellutini and Ph. Vidal, He, Ar, Sr, Nd and Pb isotopes in volcanic rocks from Afar: Evidence for a primitive mantle component and constraints on magmatic sources, Oeochem. J. 27, 219-228, 1993. [25] T.W. Trull, M.R. Perfit and M.D. Kurz, Helium and strontium isotopic constraints on subduction contributions to Woodlark Basin volcanism, Solomon islands, Geochim. Cosmochim. Acta 54, 441-453, 1990. [26] A. Zindler and S.R. Hart, Helium: problematic primordial signals, Earth Planet. Sci. Lett. 79, 1-8, 1986. [27] B. Marty and P. Lussiez, Constraints on rare gas partition coefficients from analysis of olivine-glass from a picritic mid-ocean ridge basalt, Chem. Geol. 106, 1-7, 1993. [28] M.D. Kurz, Mantle heterogeneity beneath oceanic islands: Some inferences from isotopes, Proc. R. Soc. London A342, 91-103, 1993. [29] S.R. Hart, He diffusion in olivine, Earth Planet. Sci. Lett. 70, 297-302, 1984. [30] T.W. Trull and M.D. Kurz, Experimental measurements of 3He and 4He mobility in olivine and clinopyroxene at

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

magmatic temperatures, Geochim. Cosmochim. Acta 57, 1313-1324, 1993. B. Marty, R.K. O'Nions, E.R. Oxburgh, D. Martel and S. Lombardi, Helium isotopes in Alpine regions, Tectonophysics 206, 71-78, 1992. B. Polyak, I.V. Konovov, I.N. Tolstikhin, B.A. Mamyrin and L.V. Khabarin, The helium isotopes in thermal fluids, in: Thermal and Chemical Problems of Thermal Waters, A.I. Johnson, ed., Int. Assoc. Hydrol. Sci. PuN. 119, 17-33, 1976. Y. Sano, Y. Nakamura and H. Wakita, Helium-3 emission related to volcanic activity, Science 224, 150-151, 1984. B. Marty, A. Jambon and Y. Sano, Helium isotopes and CO a in volcanic gases of Japan, Chem. Geol. 76, 25-40, 1989. S.N. Williams, Y. Sano and H. Wakita, Helium-3 emission from Nevado del Ruiz, Columbia, Geophys. Res. Lett. 14, 1035-1038, 1987. B. Marty and A. Jambon, C / 3 H e in volatile fluxes from the solid Earth: Implications for carbon geodynamics, Earth Planet. Sci. Lett. 83, 16-26, 1987. T.W. Trull, S. Nadeau, F. Pineau, M. Polv~ and M. Javoy, C-He systematics in hotspot xenoliths: Implications for mantle carbon contents and carbon recycling, Earth Planet. Sci. Lett. 118, 43-64, 1993. J.C. Varekamp, R. Kreulen, R.P.E. Poorter and M.J. Van Bergen, Carbon sources in arc volcanism, with implications for the carbon cycle, Terra Nova 4, 363-373, 1992. M. Javoy and F. Pineau, The volatile record of a 'popping' rock from the Mid-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles, Earth Planet. Sci. Lett. 107, 598-611, 1991. B.E. Taylor, Magmatic volatiles: isotopic variation of C, H and S, in: Stable Isotopes in High Temperature Geological Processes, J.W. Valley, H.P. Taylor, Jr. and J.R. O'Neil, eds., Rev. Mineral. 16, 185-226, 1986. C.J. All~gre, T. Staudacher and Ph. Sarda, Rare gas systematics: formation of the atmosphere, evolution and structure of the Earth's mantle, Earth Planet. Sci. Lett. 81, 127-150, 1986/1987. D.B. Patterson, M. Honda and I. McDougall, Atmospheric contamination: a possible source for heavy noble gases in basalts from Loihi Seamount, Hawaii, Geophys. Res. Lett. 17, 705-708, 1990. N. Metrich, R. Clocchiatti, M. Mosbah and M. Chaussidon, The 1989-1990 activity of Etna. Magma mingling and ascent of H20-CI-S rich basaltic magma. Evidence from melt inclusions, J. Volcanol. Geotherm. Res. 59, 131-144, 1993. M. Javoy, G~ochimie isotopique de l'oxyg~ne dans les roches ign6es, Doct. Etat Thesis, Univ. Paris 7, 1974 (Unpubl.). C. Hemond, N.T. Arndt, U. Lichtenstein and A.W. Hoffman, The heterogeneous Iceland Plume: Nd-Sr-O iso-

B. Marty et a l . / Earth and Planetary Science Letters 126 (1994) 23-39

[46]

[47]

[48]

[49]

[50]

[51]

[52]

topes and trace element constraints, J. Geophys. Res. 98(9), 15833-15850, 1993. D.W. Graham, S.E. Humphris, W.J. Jenkins and M.D. Kurz, Helium isotope geochemistry of some volcanic rocks from Saint Helena, Earth Planet. Sci. Lett. 110, 121-131, 1992. W. Hildreth and S. Moorbath, Crustal contributions to arc magmatism in the Andes of Central Chile, Contrib. Mineral. Petrol. 98, 455-489, 1988. D.R. Hilton, K. Hammerschmidt, S. Teufel and H. Friedrichsen, Helium isotope characteristics of Andean geothermal fluids and lavas, Earth Planet. Sci. Lett. 120, 265-282, 1993. D. Tedesco, P. Allard, Y. Sano, H. Wakita and R. Pece, Helium-3 in subaerial and submarine fumaroles of Campi Flegrei Caldera, Italy, Geochim. Cosmochim. Acta 54, 1105-1116, 1990. D.R. Hilton, J.A. Hoogewerff, M.J. Van Bergen and K. Hammerschmidt, Mapping magma sources in the east Sunda-Banda arcs, Indonesia: Constraints from helium isotopes, Geochim. Cosmochim. Acta 56, 851-859. Y. Sano, H. Wakita, F. Italiano and M. Nuccio, Helium isotopes and tectonic interactions in Italy, Geophys. Res. Lett. 16(6), 511-514, 1989. J. Halbout, F. Robert and M. Javoy, Oxygen and hydrogen isotope relations in water and acid residues of car-

[53]

[54]

[55]

[56]

[57]

[58]

39

bonaceous chondrites, Geochim. Cosmochim. Acta 50, 1599-1609, 1986. D.W. Graham, P. Allard, C.R.J. Kilburn, F.J. Spera and J.E. Lupton, Helium isotopes in some historical lavas from Mount Vesuvius, J. Volcanol. Geotherm. Res. 58, 359-366, 1993. Le Roex, Source regions of mid-ocean ridge basalts: Evidence for enrichment processes, in: Mantle Metasomatism, M.A. Menziez and C.J. Hawkesworth, eds., pp. 389-422, Academic Press, 1987. S.R. Taylor and S.M. McLennan, The Continental Crust: Its Composition and Evolution. (An Examination of the Geological Record Preserved in Sedimentary Rocks.), Blackwell, 1985. E.H. Hauri, N. Shimizu, J.J. Dieu and S.R. Hart, Evidence for hotspot-related carbonatite metasomatism in the oceanic upper mantle, Nature 365, 221-226, 1993. D.J. DePaolo, W.I. Manton, E.S. Grew and M. Halpern, Sm-Nd, Rb-Sr and U-Th-Pb systematics of granulite facies rocks from Fyfe Hills, Enderby Land, Antarctica, Nature 298, 614-618, 1982. C. France-Lanord, A. Michard and A.M. Karpoff, Major element and Sr isotope composition of interstitial waters in sediments from Leg 129: The role of diagenetic reactions, Proc. ODP, Sci. Results 129, 267-281, 1992.