Geochemical and Sr-isotope data on historic lavas of Mount Etna

Jourpml q[ bblcanology and Geothermal Research, 56 ( 1993 ) 5 7 - 6 9

57

Elsevier Science Publishers B.V., A m s t e r d a m

Geochemical and Sr-isotope data on historic lavas of Mount Etna M. BarbierP, R. Cristofolini b, M.C. Delitala a, M. Fornaseri a, R. Romano c, A. Taddeucci~ and L.Tolomeoa aDepartment q/Earth Sciences, University of Rome, "La Sapienza ", P. le Aldo Moro 5, 00185 Rome, lla/t" blnstitute of Earth Sciences, Catania University, Calania, Italy ~lnternational lnstitute q[ I'olcanologr, CNR, Piazza Roman. 2, 95123, Camnia, Italy (Received September 1, 1992; revised version accepted December 3. 19921

ABSTRACT The concentration of trace elements (La, Nb, Rb, Zr, Sr, Ni, Co, Cr and Cu ) have been determined in 46 samples from 14 historical eruptions of Mt. Etna ( 1444 to 1983 ) as well as their St-isotope compositions. The measured values show significant variations that are explained in terms of magma generation, differentiation, and mixing processes. The contamination by diffusion of volatile-enriched fluids, suggested by earlier authors, might be confirmed here with reference to Rb behaviour, and the same process is also invoked in order to explain variation of Sr-isotope compositions. Nevertheless, the hypothesis of source heterogeneity, stated by other authors, cannot be ruled out.

Introduction The extensive sampling of historic lavas of Mount Etna (as from 1444) obtained during and partly after the geo-volcanological survey for producing the Geological Map of Mount Etna (Romano et al., 1979), allows us to work on materials with well established characteristics. that are difficult to be clearly identified in older rocks. In fact, the following information is known for each sample: - t h e eruption year (including the day and month as from 1971 ); - the eruption "history" (the most recent ones being more and more detailed); - t h e flow topography (location of eruptive bocca, lava flow path, sections and front); -stratigraphic relationships among various lava flows. Samples, whose position with respect to the ('orrespondence 1o R. R o m a n o .

eruptive bocca and lava front were known, were selected for the oldest lavas. Samples of lavas after 1971 were studied and collected whilst still molten at the bocca at different stages of various eruptions. In this way the concentrations of elements in rocks originating from a lava flow could be checked with reference to any spatial (different areas of the lava flow) and temporal variations (subsequent phases of the eruption). This was performed to evaluate the significance of the geochemical data and reveal whether, and to what extent, the evolutionary processes occurring in the Etnean magmas are reflected in the lavas emitted over the last 500 years.

Sampling Figure 1 shows a sketch map of the 12 flows from which the 46 samples reported in this

037"7-0273/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

58

M. BARBIERI ET A I

SAMPLED

ERUPTIONS

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Fig. I. Sketch map of the sampled eruptions study were collected. In the following list the eruption years and the corresponding sample numbers are indicated. -1444:1,2,3 (Basic Mugearite - Hawaiite). Samples obtained at progressively increasing distances from the feeding area. - 1669:1,2,3,4,5,6,7 (Hawaiites). As above. - 1766 (Hawaiite). - 1865:1,2,3,4 (Hawaiites). As above. - 1892 (Hawaiite). - 1911:1,2,3,4 (Hawaiites). As above. - 1923:1,2,3,4 (Hawaiites). As above.

1928:1,2,3,4.1,4.2 (Hawaiites). Samples 4.1 and 4.2 from the same locality. 1971:1,2,3 ( 1 Tephrite, at the very bocca on 6 April; 2 and 3 Hawaiites, at the bocca on 12 May and 12 June respectively). 1974:1,2,3,4 (Basic Hawaiites). 1 from the lava front during initial stages of the Mount De Fiore eruption (Bottari et al., 1974) on 1 February; 2 also from the lava front but during the final stage of the same eruption ( 10 February); 3 scoria collected at the start of a subsequent eruption ( Mount De Fiore II)

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on 11 March; 4 from the effusive bocca during the final stage of the same eruption on 23 March. 1978:1,2,3,4 (Hawaiites). 1 and 2 from the eruptive bocca on the 1 May (initial stage) and 27 May (intermediate stage) respectively of a SE Crater eruption; 3 from a different bocca of the SE Crater during the final stage of 30 August eruption; 4 from a new bocca of the same eruptive center during the final stage of a later eruption on 30 November. 1979 (Hawaiite). From the bocca during an intermediate stage of a SE Crater eruption, on 5 August. 1981:1,2 (Hawaiites). 1 from the flow at start of eruption on 17 March; 2 at the end of the eruption from a lower bocca which opened on 23 March. 1983:1,2,3 (Hawaiites). 1 from the flow at onset of eruption on 28 March; 2 and 3 at the main effusive bocca during intermediate eruptive stage on 22 May and 29 June, respectively.

Methods and results

Concentrations of trace elements were determined by X-ray fluorescence spectrometry according to the "standard addition" method. Relative errors are estimated between 5 and 10%. The results are reported in Table 1. Strontium isotopic compositions were measured by means o f a VG Micromass 54 E mass spectrometer. The results are also shown in Table 1.

Discussion

Petrologic notes The petrochemical characters of the analyzed rocks are shown on the TAS diagram (Fig. 2; Le Maitre, 1989).

M. BARBIERt ET 4L,

The lavas of the eruptions analyzed conform to those of historical dated eruptions at Mount Etna. As shown by Cristofolini et al. (1987), features like porphyricity and color index, phenocryst compositions and sizes are not clearly related to the rather restricted chemical variation (SIO2=44-52 %) among the various lavas. Even though details of the modal compositions within the same flow and among different lavas may vary to a large extent, general features - such as major phases (pl, cpx, ol, mt) among the phenocrysts, complicated zoning pattern in plagioclases, rather high porphyricity index with plagioclase predominating over mafic phases - are c o m m o n to all the analyzed products. Their generally porphyritic character shows that the chemistry of these lavas does not strictly represent melt compositions. Each of the samples was also analyzed for major elements. The overall analytical data for major elements do not constitute a very homogeneous set, as they were produced by various methods in different laboratories ( Bottari et al., 1974: chemical analyses 2, 4, I, III; Rittm a n n et al., 1971 : ch. an. a, h, n; R o m a n o and Guest, 1979: ch. an. MEN 98-1923, MEN 1441865; R o m a n o and Sturiale, 1975: ch. an. Be 41-1669, Be 54-1444, 1981: ch. an. MES 2551766; International Institute of Volcanology, unpubl, data; Institute Earth Sciences, University of Catania, unpubl, data). Nevertheless, the major-element variation shows a more coherent pattern than the petrographic characters. The former reveals a fairly large withinflow range that is mostly accounted for by differences in phenocryst proportions. The 1444 lavas appear the most differentiated, being enriched in silica and alkalies, particularly K20, and depleted in CaO and FeO tot (MgO is abnormally high in one out of the three analyses). Almost all the remaining analyses of lavas up to 1928 appear as a rather homogeneous group, with widely overlapping compositional ranges. The products from 1971 to 1983 tend to be

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characterized by a marked increase of K20 (up to 2 %), though clearly being among the most basic lavas. This is clear for the 1974 lavas which are the most potash-rich and mafic (Tanguy and Kieffer, 1976-77) in the analyzed group. They are also aphyric and thus represent actual melt compositions. This variation is not regular with time, some of the most recent lavas having potash contents quite similar to the previous ones and matching what had already been shown on the basis of independent chemical data (Cristofolini et al., 1984; Tanguy and Clocchiatti, 1984; Clocchiatti et al., 1986 ). This feature raises a problem related to the origin of the m a g m a feeding the most recent volcanic activity at Mount Etna, which may not be accounted for by crystal fractionation processes alone.

Strontium isotopes The Sr-isotopic composition ranges from 0.70346_+0.00002 for a "'1444"" sample to 0.70374 _+0.00003 for a "1971"" sample. All the Sr-isotope ratios are consistent with a time-integrated, Rb-depleted source region, compared to the bulk Earth and original mantle compositions (Carter and Civetta, 1977). In spite of the scatter of the isotopic data (t:ig. 3 ), even within the same eruption, their fluctuations do not seem to be related to petrological variations, to eruption ages. or to traceelement contents. Several processes may be assumed as responsible for the isotopic values observed: ( 1 ) mixing of an alkali magma with a magma oftholeiitic affinity (Condomines et al.. 1982 ):

62

M. BARBIERI ET AL.

0.70380

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1979

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Time

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Fig. 3. Variation of the strontium isotopic composition. © = 1444; 0 = 1669: © = 1766; ~]= 1865; i = 1 9 1 1 ; A = 1923;A= 1 9 2 8 ; * = 1971; X = 1974;+ ~- 1978;D--- 1 9 7 9 ; 0 = 1 9 8 1 ; 0 = 1983.

(2) crustal contamination of mantle-originated magma during uprising; ( 3 ) heterogeneous mantle source. Actually, the isotopic differences might be related to different degrees of mixing between two deep magmas, but no evidence for the endmembers has been found. Crustal contamination must be ruled out because no correlation was found between Sr-isotope ratios and Sr elemental contents or other trace-element trends; furthermore, on the basis of mass balance calculations, no simple binary mixing is possible (Armienti et al., 1989 ). Moreover, a sudden short-term alkali enrichment has been pointed out by Clocchiatti et al. (1988) in recent and present-day lavas, some

1892. ,5 =

of which have been studied in this paper (see next section). The authors propose an alkaliselective diffusion mechanism as a result of an interaction of magmas with sedimentary country rocks, such as the Piedimonte formation and the Capo d'Orlando Flysch. The process also seems to involve a fluid phase, connected to the phreatic system in the basement. This model agrees with the observed isotopic variation, as selective diffusion could have greatly affected the radiogenic Sr content. In fact, the diffused fluids can be enriched in Sr-87, that can be preferentially removed from the site previously occupied by Rb-87 absorbed by clay minerals in the country rock. Moreover, the process could easily explain the scattering of

GEOCI{EMICAL

AND

SR-ISOTOPE

DATA

ON HISTORIC

LAVAS OF MOUNT

the isotopic data which are not correlated to petrological variations in m o d e m lavas. On the other hand, it is not possible to rule out the presence of a heterogeneous mantle source, as postulated by Armienti et al. (1989) on the basis of Sr- and Nd-isotope data.

of Figure 4, the concentration of four incompatible (La, Rb, Nb and Sr) and three compatible elements (Ni, Co and Cr) are plotted against the distance of the sample from the effusive vent. The variability appears quite high (greater than the analytical error), but no particular trends can be observed. Zr, Cu and Zn contents have been plotted at the top (b) of the diagram. Roughly speaking this suggests that the variability of trace-element contents may depend on mineralogical control and we can assume that such control depends either on effusion rate variations or on some possible heterogeneity within the uprising magma. Figure 5 shows variations in the average

Trace-element geochemistry The first step involved studying the internal variability of the element concentrations within a single eruption. The 1669 lava flow was chosen, since it is one of the most heterogeneous and also because of the larger number of samples ( 7 ) available. In the lower part (a) 1669

63

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64

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ET M_,

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Fig. 5. Variation of the Nb and La (incompatibles) concentration as compared with that of Co and Ni (compatibles). ,Average value for each eruption.

contents of two incompatible (Nb and La) and two compatible (Co and Ni) elements from lava flows within the considered time span (the vertical bars represent the variability within the single lava flow, at 1a level). Also in this case,

no particular trends but only random oscillations can be detected. However, the patterns appear quite coherent: Nb parallels La and Co parallels Ni, whereas the incompatible Nb and La are broadly symmetrical with respect to the

GEOCHEMICAL

AND SR-ISOTOPE

DATA ON HISTORIC

LAV,XS ( I F M O U N T

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Fig. 6. "Spider" diagram for the analyzed rocks I

compatibles Co and Ni. According to the previous petrographic considerations, the 1444 lavas seem to be the most evolved, being enriched in incompatible and depleted in compatible elements. On the contrary, the 1974 lavas are depleted in incompatible and enriched in compatible elements and can be considered as the least evolved rocks of the whole set studied. In Figure 6, the contents of the elements have been normalized to the "primordial mantle" (Wood et al., 1979). Here too, the 1444 lavas appear as the most evolved, whereas the rocks which erupted in 1974 lie amongst the least differentiated ones. Their abnormal behavior with respect to Rb and K agrees with the already quoted hypothesis by Clocchiatti et al. (1986), Joron and Treuil (1986) and Clocchiatti et al. (1988) of an alkali-selective diffusion mechanism that affected some of the more recent lavas (see previous section). M a x i m u m spread in the entire set of samples occurs at Nb (factor 4) and La (factor 3 ) level, and therefore it seems worthwhile to consider

r

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Fig. 7. k a / N b vs. ka plot of the analyzed rocks. ( For symbols see Fig. 3 ). Average ~alue for each eruption.

the correlation between this pair of elements in detail. Figure 7 (Treuil and Joron, 1975) suggests that the 1974 lavas represent the cooling product of a liquid generated by a low degree of partial melting of the source rock. The 1865 and 1444 lavas may have been formed by fractional crystallization of a liquid which was generated at a higher degree of partial melting, corresponding with point A (the corresponding lavas are unknown ). The rocks dating from 1928, 1971 and 1983 could represent fractional crystallization products from a parent derived by an even higher degree of melting. The fairly good alignment of the points representing the 1669, 1911, 1923, 1978, 1979 and 1981 materials supports the hypothesis that they could be the products of mixing of two

66

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end-members ( 1974 and 1865 respectively), even if each of them could also be considered as the product of fractional crystallization of distinct liquids generated by different degrees of partial melting. La contents reported for Etnean tholeiitic to alkaline basalts by Condomines et al. ( 1982 ) are not in conflict with this model. 70

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Fig. 10. (a,b) Variation of compatible element (Ni, Co, Cr) contents; average value for each eruption. (b) Expanded version for eruptions from 1865 to 1983; in the insert is the expanded version for eruptions from 1971 to 1983. (For symbols see Fig. 3 )

G E O C H E M I C A L A N D S R - I S O T O P E D A T A O N H I S T O R I C LAVAS O F M O U N T ETN ~',. 09

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Fig. 11. Nb/La ratios in the analyzed rocks (average values and sample variation range). (For symbols see F,g. 3).

Sample distribution in the diagram in Figure 8 agrees with the previously postulated model only as to the origin of the 1974 and 1444 rocks. With the exception of the 1978, 1979, 1981 and probably 1669 lavas, all the materials seem to be enriched in Rb, as can be seen by comparing their position with respect to the fractioning line from A to 1444 samples in the two diagrams. Figure 9 shows the variation of the average

6 7

contents for La, Nb and Rb within the time span from 1444 to 1983. These elements behave coherently up to 1971, after which the Nb pattern is substantially opposite to the La and Rb ones: at the moment, no explanation is given for such behaviour. In Figure 10a,b the quite good correlation among Ni, Co and Cr can be observed over the entire time span. The more frequent lava-rocks sampling performed of the eruptions of the last century revealed fluctuation of element correlations, whereas this was not observed for the earlier eruptions where sampling was infrequent. ] h e most probable reason for the observed fluctuations may be: (1) source heterogeneity; (2) different degrees of melting; (3) contamination processes; (4) mixing of magmas: and ( 5 ) crystal fractionation processes. As already shown by the "spider diagram" of Figure 6, analysis of Nb and La data may provide indications of the genesis and evolution of the Etnean magmas. In fact, since these elements are highly incompatible and immobile. their ratio is less affected by mineralogical control and gaseous transfer. Moreover.

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Fig. 12. (a) Nb vs. La plot for overall samples• (b) Nb vs. La plot for the rocks showing a crystal tractionation trends. In the insert the more significant trend A is fully depicted. (For symbols see Fig. 3 ).

68

M. BARBIERI E T A L

their contents are very sensitive to melting and differentiation processes (Treuil and Joron, 1975). Figure 11 shows the averaged values and the ranges of the N b / L a ratios for the analyzed eruptions, ordered in a time sequence. Although most of the values fall within a relatively narrow range, some of them are much lower ( 1974, 1981 ). However, it is clear that crystal fractionation from a single parent melt cannot account alone for the observed distributions. Further, the within-flow variation for some eruptions is fairly high and contrasts the model of differentiation by crystal fractionation, as supported by the plots in Figure 12a,b. In Figure 12a the Nb vs. La values are plotted for the whole set of analyzed samples. The overall distribution appears poorly ordered, but, as shown in detail in Figure 12b, alignments intercepting the origin can be detected for some eruptions. According to the model proposed by Treuil and Joron (1975), this suggests crystal fractionation from melts originated by different degrees of fusion, a n d / o r by magma mixing processes.

positions can be explained taking into account the contamination mechanism proposed by Clocchiatti et al. (1988), that involves an interaction between the magma and the basement sedimentary rocks, rich in Rb and in easy leachable "hot" Sr-87. The geochemical elemental behaviour of Rb and Sr are quite different, the former being more incompatible and more sensitive to volatile diffusion processes: that explains the lack of correlation between the two values.

Conclusions

References

Trace-element contents and Sr-isotope compositions of the analyzed historical lavas of Mt. Etna show significant variations both among different eruptions and within single lava flows. Mineral control and/or heterogeneities in the feeding system may be responsible for the trace-element variation within a single eruption. In the La-Nb plots (Figs. 7, 12a,b) there is evidence for several trends indicating that degrees of partial melting, crystal fractionation, and mixing processes as well, have been active in different cases and may be responsible for the observed variations. The hypothesis of mantle heterogeneity, proposed by Armienti et al. (1989) on the grounds of Sr and Nd isotopes, cannot be ruled out. However the enrichment of Rb in most recent lavas and the variation of Sr-isotope com-

Acknowledgements

The authors are very grateful to Dr. R. Clocchiatti (Laboratoire Pierre Sue, CEN Saclay. France) for his friendly comments and suggestions. Thanks are due to Dr. T. Caltabiano (Istituto Internazionale di Vulcanologia, CNR, Italy) and dr. M.F. Grasso for their precious help in the setting of the paper. This work was supported by grants from CNR-IIV (R.R.), CNR-CSGASR (M.B.) and MURST (R.C. and A.T.).

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