Tracking magma dynamics by laser ablation (LA)-ICPMS trace element analysis of glass in volcanic ash: The 1995 activity of Mt. Etna

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Tracking magma dynamics by laser ablation (LA)-ICPMS trace

element analysis of glass in volcanic ash: The 1995 activity of Mt. Etna

F. Schiavi,1M. Tiepolo,2 M. Pompilio,3 and R. Vannucci1,2

Received 28 September 2005; revised 22 December 2005; accepted 4 January 2006; published 8 March 2006.

[1] Glass fragments in tephra erupted at Mt. Etna from

May to December 1995 have been analyzed by laser ablation ICPMS. The trace element compositional variability of ashes deposited during this interval reveals the presence of discrete magma batches with different crystallization degrees in the shallow plumbing system. From May to October a highly crystalline magma is predominant within the conduit with only minor sporadic input of fresh and more primitive magma batches. After October new and less evolved magma batches become more prevalent and become progressively homogenized within more evolved resident magma. In December ashes closely match the chemistry of the volcanics subsequently erupted till February 1996. This study demonstrates that the trace element characterization of ashes has important implications for volcanic monitoring and is a useful tool for the forecasting of paroxysmal events at Mt. Etna. Citation: Schiavi, F., M. Tiepolo, M. Pompilio, and R. Vannucci (2006), Tracking magma dynamics by laser ablation (LA)-ICPMS trace element analysis of glass in volcanic ash: The 1995 activity of Mt. Etna, Geophys. Res. Lett., 33, L05304, doi:10.1029/2005GL024789.

1. Introduction

[2] The monitoring of geochemical tracers (trace

elements and isotopes) in solid volcanic products during eruptions provides a means to track magma dynamics and, in open-conduit volcanoes, may aid in forecasting of paroxysmal events. Geochemical tracers are usually determined by bulk chemical methods on large samples, mainly consisting of lavas or pyroclastic ejecta. However, the availability of fresh and relatively large samples is subordinated to ease and safe access of vent area. Such impediments often preclude regular sampling of effusive products, whereas ash particles can provide suitable material for petrologic monitoring. Production of ash is often associated to the opening stages of the eruption and in the case of reactivation, apart from gas emission, identifi-cation of a juvenile component in ash produced by precur-sory eruptions is the key to establishing the presence of new magma [Cashman and Hoblitt, 2004; Watanabe et al., 1999]. Ash-fall deposits can provide information on the composition of magma within the feeder conduit and, coupled with other monitoring (e.g., gas emission, ground

deformation and seismicity) allows tracking dynamics of magmas within the shallow plumbing system.

[3] Morphological characterization and component

analysis on airborne ashes have already been employed for monitoring explosive activity [Taddeucci et al., 2002] and provided constraints on magma degassing, fragmenta-tion and related physical parameters [Taddeucci et al., 2004a, 2004b]. However, bulk tephra may not be representative of the magma composition because mineral-melt relative abundances are biased by fragmentation and transport processes. In addition, tephra often contains accidental lithics and altered clasts. In-situ analyses of glass in juvenile ash fragments may provide more appropriate indications of magma composition. Major element composition can be determined by electron microprobe (EMP) but more subtle and distinctive variations of trace elements concentrations require a sensitive technique capable of low detection limits over few microns of spot areas.

[4] In this work we have used laser ablation - inductively

coupled plasma mass spectrometry (LA-ICPMS) technique to analyze trace elements in glassy juvenile fraction of ashes erupted during 1995 from North-East subterminal Crater (NEC) of Mt. Etna. In this case relations between temporal compositional variations and magmatic processes are constrained by well-monitored changes in eruptive behavior and style.

2. The Reactivation of Mt. Etna in 1995

[5] Mt. Etna is located on the suture between the

converging European and African plates in a complex tectonic environment [Doglioni et al., 2001] and at present erupts magmas with potassic trachybasalt composition. Etna activity is quasi-continuous at summit craters. Since 1961 AD, frequency of both central and flank eruptions has increased and a large number of high magnitude and dangerous explosive events (6 Sub-Plinian eruptions and more than 150 fire fountains episodes) occurred in the summit zone [Branca and Del Carlo, 2005]. The major control of the eruptive style occurs in the shallow (up to 1 km b.s.l.) part of the plumbing system due to various processes, such as degassing-induced crystallization, mix-ing, contamination and crystal segregation [Corsaro and Pompilio, 2004a].

[6] The largest lateral eruption of the last three centuries

occurred from 1991 to 1993 and it almost totally emptied the shallow plumbing system. Reactivation occurred in December 1994 at NEC and first explosions started in May 1995 with ash emissions and continued with variable intensity until October. Strombolian activity in July-August and in October was separated by a month of quiet degassing

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L05304, doi:10.1029/2005GL024789, 2006

1_{Dipartimento di Scienze della Terra, Universita` degli Studi di Pavia,}

Pavia, Italy.

2_{Istituto di Geoscienze e Georisorse, Sede di Pavia, CNR, Pavia, Italy.} 3

Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Pisa, Italy.

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and sporadic ash emission [Coltelli et al., 1998]. The climax of activity was between November and December 1995 with 5 fire fountains paroxysmal eruptions, that provided the first material available for monitoring magma with bulk methods.

3. Sampling and Analysis

[7] Six different ash-emission episodes from May to

December 1995 were selected for this work. Ashes were sampled downwind in the summit area during surveys performed throughout this interval by INGV-CT staff. Three samples are from early phreato-magmatic explosions in May – June (Sample 09/05/95) and from Strombolian activity in July – August (Samples 11/07/95 and 28/07/95). The other three samples were collected in early October during Strombolian activity (Samples 12/10/95 and 17/10/95). Ash from the largest explosive episode of the whole eruptive sequence (23 December - sample 23/12/95) was investigated for comparison.

[8] Ash samples consist of transparent, light brown,

vesicular glassy clasts (sideromelane), black crypto or micro-crystalline, poorly vesicular, blocky grains (tachylite) and disaggregated plagioclase, clinopyroxene and olivine crystals, that form the common phenocryst assemblage of Etna rocks [Coltelli et al., 1998]. For purposes of this study, only transparent and glassy sideromelano grains represent-ing fresh, not-recycled, juvenile material are considered (Figures S1a and S1b)1.

[9] Major element composition (Table S1) of the glass

portion of ashes was determined by EMP at the CNR-IGG-Firenze with a JEOL JXA-8600. See Vaggelli et al. [1999] for analytical details. Trace element concentrations (Table S2) were determined by LA-ICPMS at the CNR-IGG-Pavia using a 266 nm laser probe (Brilliant, Quantel; for further details, see Tiepolo et al. [2002]) coupled with ICPMS (DRCe, Perkin Elmer). The laser was operated at 10Hz of repetition rate, the power on the sample was 1.5 mW and spot size was set at 40mm. Masses reported in Table S2 are the best compromise between maximum isotopic abundance and minimum presence of interference. They were acquired in peak hopping mode with a dwell time of 10 ms. Nist SRM 610 and 43Ca were adopted as

external and internal standards, respectively. Precision and accuracy were evaluated on the USGS-BCR-2 reference material and are estimated to be better than 5 and 10% relative, respectively.

4. Composition of Glass Fragments in Ash

and Variation With Time

[10] According to the total-alkali-silica (TAS)

classifica-tion scheme, 1995 glass fragments extend from mugearite to phonolitic-tephrite field showing a significant enrichment in Na2O and K2O and slightly lower contents in Al2O3, CaO

and mg# ([Mg/(Mg + Fe2+)]) than the bulk rock composi-tion of 1971 – 1996 Etna volcanics [Tonarini et al., 1995; La Delfa et al., 2001; Corsaro and Pompilio, 2004b]. The chondrite-normalized incompatible trace element patterns of 1995 tephra glasses show a marked enrichment in Ba, U, Th and LREE over Zr, Hf and HREE and relative to that in bulk 1971 – 1996 volcanics, including late 1995 samples (Figure 1). Cs, Rb and Pb are depleted relative to other LIL elements and Sr shows a marked negative anomaly relative to neighboring elements. Although characterized by rela-tively higher trace element contents, this overall pattern closely resembles that of the bulk rock from December 1995 and other recent volcanics [Tonarini et al., 1995; Armienti et al., 1996].

[11] Major element compositions of glass fragments in

ash show minor variation with time and only a slight increase of the mean mg# and CaO/Al2O3 ratios after

October is noteworthy. Trace element variations with time are more distinct. The incompatible trace elements mean concentrations (Figure 2) show an almost constant behavior between May and October, whereas a significant decrease coupled with an increase in Sr concentration is observed after October. The same trend is observed for petrologically significant trace element ratios such as La/Yb. However, if the standard deviation associated to the mean value is considered, data variability (up to 62% for Li, 32% for Cs, 22% for Nb, 18% for La) is well above the analytical uncertainty of the LA-ICPMS technique, thus suggesting the occurrence of more than one statistical population. It is worth noting that the maximum variability is observed between May and October for most of the trace elements.

[12] In order to address the origin of this significant

compositional variability during single eruptive events, probability density plots have been drawn for each ash sample and trace element. In Figure 3, the curves for the La/Yb ratio are shown as example of trends for ratios of Figure 1. C1-normalized trace element composition of

1995 ashes (black lines), compared with bombs and lapilli ejected at the end of 1995 (dashed line) [Armienti et al., 1996] and recent historic lavas (grey field) [Tonarini et al., 1995].

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Auxiliary material is available at ftp://ftp.agu.org/apend/gl/ 2005GL024789.

Figure 2. Temporal variations of mean absolute concen-trations for representative trace elements (La and Nb) in 1995 ashes (error bars are 2s).

L05304 SCHIAVI ET AL.: THE 1995 ACTIVITY OF MT. ETNA L05304

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high to moderately incompatible elements. Noticeably, from May to October two different peaks roughly at La/Yb = 38 and La/Yb = 28 are distinguishable, suggesting the occur-rence of two compositionally different magmatic compo-nents. In contrast, from 17 October to December only ashes with La/Yb = 28 are observed, similar to La/Yb = 26 reported for the bulk rock of volcanics erupted in December 1995 [Armienti et al., 1996].

5. Inferences on Magma Dynamics in the

Shallow Plumbing System

[13] Major and trace elements provide contrasting

infor-mation when the chemical differences between the two magmatic components outlined by the statistical analysis are modeled through a simple fractional crystallization process. The observed differences in mg# and CaO/Al2O3

values of glass fragments can be reconciled only with limited (10%) fractional crystallization of a mineral assemblage made of Ol, Cpx and prevailing plagioclase, i.e., the phenocryst paragenesis of Etna lava [Corsaro and Pompilio, 2004b]. On the contrary, trace elements require (based on fluid-immobile elements, for example, HREE, and solid/liquid partition coefficients for alkali-enriched evolved melts [Vannucci et al., 1998; Morra et al., 2003]) up to 30% crystallization of an overall Ol (7%) + Cpx (28%) + Plg (65%) assemblage to account for the differ-ences between the endmember glass compositions. Excep-tion is made for some highly incompatible elements and, particularly, for Li, B and Pb, which are enriched in some glass fragments from the more evolved ashes, thus exceed-ing the maximum allowed by the above fractional crystal-lization model. Thus, quite apart from the possible role of Fe-oxides in affecting trace element changes, it seems plausible that the more evolved magma is not directly derived from the less evolved one via one stage of crystal fractionation, but possibly formed from a common parent through a different evolutionary path. Trace element modeling indicates that both the less and more evolved magmas underwent crystallization of Ol + Cpx at greater

depth or en route toward the surface, whereas only the more evolved magma crystallized prevailing plagioclase and additional clinopyroxene in the shallow plumbing system thus acquiring only slightly lower mg# and CaO/Al2O3

values and extending its total alkali content relative to Ca. [14] The proposed two-step fractional crystallization for

the most evolved magma is also consistent with the increase of about 1.4 times of its La/Yb value (La/Yb = 38) relative to that (La/Yb = 28) of the less evolved one. It can be easily observed that such a variation is not accounted for by the slight difference between bulk S/LDLa and S/LDYb

(both0.20) for the Ol + Cpx + Pl assemblage and requires the early crystallization of olivine and clinopyroxene (bulkS/LDLaS/LDYb), only lately followed by plagioclase

(S/LDLa S/LDYb) in order to explain the negative Sr

anomaly. The finding within recent lavas of mafic cumu-lates rich in olivine and clinopyroxene [Andronico et al., 2005] provides further support to this interpretation. Fur-thermore the abundance of clinopyroxene phenocrysts and phenocryst fragments in tephra erupted throughout this interval demonstrates the extensive crystallization of clino-pyroxene. Despite the uncertainty onS/LD values and on the fractionating assemblage, an exotic component has to be invoked to explain the observed compositional difference in Li, B and Pb. Due to the high compatibility of these elements in fluids [Brenan et al., 1998; Kessel et al., 2005], contributions from the hydrothermal system at the crater vent level or from a vapor phase concentrating volatiles in the upper portion of the magma chamber, as suggested for the Mt. St. Helens [Berlo et al., 2004], are the most plausible exotic components.

[15] The dominance of the more differentiated magma in

May (i.e., at the beginning of the eruption) followed by the more primitive component from October is not consistent with the evolution of the shallow plumbing system through a single magma batch. A more complex scenario with multiple injections of new magma is likely. We interpret glass compositions as representing distinct magma batches that fractionated to different extents from similar parental magmas. The more evolved compositions resulted from longer residence time in the shallow zone of the plumbing system and may represent magma from the walls or roof of the conduit; in contrast, the more primitive component had shorter residence times in the shallow zone, representing the hotter and less degassed portions of the magma which subsequently rose to refill the magma chamber and conduit close to the surface.

[16] Based on the observed trace element variations with

time the following scenario for the shallow plumbing system during the 1995 activity may be drawn (Figure 4). From May to early October a relatively crystalline and ‘‘cold’’ magma dominates in the conduit. The generally high La/Yb ratios observed in glasses of May and July ashes suggest that most likely this magma was stagnant at deeper levels where it formed olivine and clinopyroxene-rich mafic cumulates before reaching the shallow plumbing system and conduit where degassing and extensive crystallization of plagioclase occurred. During this stage, the magma was enriched in Li, B and Pb, either through interaction with hydrothermal fluids or a magmatic vapor phase. From October to December the arrival in the shallow plumbing system of new batches of less differentiated magma is Figure 3. Probability density plot for La/Yb values of

1995 ashes erupted during the investigated period.

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progressively revealed by increased abundance of glass fragments with relatively depleted incompatible elements and low La/Yb ratios. The degassing of the new primitive magma batches is the most likely cause of explosive eruptions. After December volcanics closely matching the chemistry of the less evolved ashes are poured out.

6. Concluding Remarks

[17] Trace element characterization of glass fragments in

airborne ashes is a useful petrologic tool enabling the recognition of magmatic processes that are not readily apparent in subtle differences of major element composi-tions. The application of this method to the 1995 Mt. Etna products enables detailed petrologic modeling of shallow magmatism during the May – October time span, and for which no chemical information was previously available due to the absence of large samples. During this eruptive interval, multiple pulses of primitive and hot magma were injected into a more crystallized and evolved magma close to the top of the feeder conduit. Noticeably, large batches of primitive magma dominated from October to December just before the large paroxysmal event of 23 December.

[18] This study has important implications for volcanic

monitoring since it allows to recognize the arrival within the conduit of primitive and poorly degassed magma batches. The easy sampling of ash together with fast sample prep-aration and analysis enables regular and reliable chemical monitoring of the trace element signatures of magma and efficient assessment of possible eruptive scenarios. As demonstrated in this study of trace elements in glasses of 1995 Mt. Etna ashes, the degassing of the new primitive magma batches causes explosive eruptions. In the future, systematic monitoring of glass compositions in ash pro-duced during earliest eruptive activity should be a viable tool for forecasting paroxysmal events.

[19] Acknowledgments. R. A. Corsaro and L. Miraglia (I. N. G. V. Catania) are gratefully acknowledged for their advice in sample selection. This manuscript has improved significantly through constructive criticism and suggestions from C. Thornber and an anonymous reviewer.

References

Andronico, D., et al. (2005), A multi-disciplinary study of the 2002 – 03 Etna eruption: Insights into a complex plumbing system, Bull. Volcanol., 67, 314 – 330.

Armienti, P., M. D’Orazio, F. Innocenti, S. Tonarini, and L. Villari (1996), October 1995 – February 1996 Mt. Etna explosive activity: Trace element and isotopic constraints on the feeding system, Acta Vulcanol., 8, 1 – 6.

Berlo, K., J. Blundy, S. Turner, K. Cashman, C. Hawkesworth, and S. Black (2004), Geochemical precursors to volcanic activity at Mount St. Helens, USA, Science, 306, 1167 – 1169.

Branca, S., and P. Del Carlo (2005), Types of eruptions of Etna Volcano AD 1670 – 2003: Implications for short-term eruptive behaviour, Bull. Volcanol., 67, 732 – 742.

Brenan, J. M., F. J. Ryerson, and H. F. Shaw (1998), The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium and lithium dur-ing subduction: Experiments and models, Geochim. Cosmochim. Acta, 62, 3337 – 3347.

Cashman, K. V., and R. P. Hoblitt (2004), Magmatic precursors to the 18 May 1980 eruption of Mount St. Helens, USA, Geology, 32, 141 – 144. Coltelli, M., M. Pompilio, P. Del Carlo, S. Calvari, S. Pannucci, and

V. Scribano (1998), Mt. Etna: The eruptive activity, Acta Vulcanol., 10, 141 – 148.

Corsaro, R. A., and M. Pompilio (2004a), Dynamics of magmas at Mount Etna, in Mt. Etna: Volcano Laboratory, Geophys. Monogr. Ser., vol. 143, edited by A. Bonaccorso et al., pp. 91 – 110, AGU, Washington, D. C. Corsaro, R. A., and M. Pompilio (2004b), Magma dynamics in the shallow

plumbing system of Mt. Etna as recorded by compositional variations in volcanics of recent summit activity (1995 – 1999), J. Volcanol. Geotherm. Res., 137, 55 – 71.

Doglioni, C., F. Innocenti, and G. Mariotti (2001), Why Mt Etna?, Terra Nova, 13, 25 – 31.

Kessel, R., M. W. Schmidt, P. Ulmer, and T. Pettke (2005), Trace element signature of subduction-zone fluids, melts and supercritical liquids at 120 – 180 km depth, Nature, 437, 724 – 727.

La Delfa, S., G. Patane`, R. Clocchiatti, J. L. Joron, and J. C. Tanguy (2001), Activity of Mount Etna preceding the February 1999 fissure eruption: Inferred mechanism from seismological and geochemical data, J. Volcanol. Geotherm. Res., 105, 121 – 139.

Morra, V., M. Lustrino, L. Melluso, G. Ricci, R. Vannucci, A. Zanetti, and F. d’Amelio (2003), Trace element partition coefficients between feldspar, clinopyroxene, biotite, Ti-magnetite, apatite and felsic potassic glass from Campi Flegrei (S. Italy), Geophys. Res. Abs., 5, Abstract 05966. Taddeucci, J., M. Pompilio, and P. Scarlato (2002), Monitoring the

explosive activity of the July – August 2001 eruption of Mt. Etna (Italy) by ash characterization, Geophys. Res. Lett., 29(8), 1230, doi:10.1029/ 2001GL014372.

Taddeucci, J., M. Pompilio, and P. Scarlato (2004a), Conduit processes during the July – August 2001 explosive activity of Mt. Etna (Italy): Inferences from glass chemistry and crystal size distribution of ash particles, J. Volcanol. Geotherm. Res., 137, 33 – 54.

Taddeucci, J., O. Spieler, B. Kennedy, M. Pompilio, D. B. Dingwell, and P. Scarlato (2004b), Experimental and analytical modeling of basaltic ash explosions at Mount Etna, Italy, 2001, J. Geophys. Res., 109, B08203, doi:10.1029/2003JB002952.

Tiepolo, M., P. Bottazzi, M. Palenzona, and R. Vannucci (2002), A laser probe coupled with ICP-double-focusing sector-field mass spectrometer for in situ analysis of geological samples and U-Pb dating of zircon, Can. Mineral., 41, 259 – 272.

Tonarini, S., P. Armienti, M. D’Orazio, F. Innocenti, M. Pompilio, and R. Petrini (1995), Geochemical and isotopic monitoring of Mt. Etna 1989 – 1993 eruptive activity: Bearing on the shallow feeding system, J. Volcanol. Geotherm. Res., 64, 95 – 115.

Vaggelli, G., F. Olmi, and S. Conticelli (1999), Evaluation of analytical errors during microprobe analyses of silicate minerals international reference samples, Acta Volcanol., 11, 297 – 303.

Vannucci, R., P. Bottazzi, E. Wulff-Pedersen, and E. R. Neumann (1998), Partitioning of REE, Y, Sr, Zr and Ti between clinopyroxene and silicate melts in the mantle under La Palma (Canary Islands): Implications for the nature of the metasomatic agents, Earth Planet. Sci. Lett., 158, 39 – 51. Watanabe, K., T. Danhara, K. Terai, and T. Yamashita (1999), Juvenile

vol-canic glass erupted before the appearance of the 1991 lava dome, Unzen volcano, Kyushu, Japan, J. Volcanol. Geotherm. Res., 89, 113 – 121.

M. Pompilio, Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, I-56126 Pisa, Italy.

F. Schiavi and R. Vannucci, Dipartimento di Scienze della Terra, Universita` degli Studi di Pavia, I-27100 Pavia, Italy. (schiavi@crystal. unipv.it)

M. Tiepolo, Istituto di Geoscienze e Georisorse, Sede di Pavia, CNR, I-27100 Pavia, Italy.

Figure 4. Schematic representation of magma dynamics in the shallow plumbing system of Mt. Etna during the 1995 eruptive period. Before May a crystalline (plg-rich) and cold magma dominate in the conduit; after May a more primitive, less degassed and hotter magma reaches the shallow zone, causing explosive eruption.