Mineralogy and Petrology (2001) 73: 177±200

The behavior of chlorine and sulfur duringdifferentiation of the Mt. Somma-Vesuviusmagmatic system

J. D. Webster1, F. Raia2, B. De Vivo3, and G. Rolandi3

1 Department of Earth and Planetary Sciences, American Museum of Science,New York, USA2 Department of Earth and Atmospheric Sciences, City College of New York, USA3 Dipartimento di Geo®sica e Vulcanologia, Universita di Napoli Federico II,Napoli, Italy

With 6 Figures

Received January 10, 2000;revised version accepted July 17, 2000

Summary

Reheated silicate melt inclusions in volcanic rock samples from Mt. Somma-Vesuvius,Italy, have been analyzed for 29 constituents including H2O, S, Cl, F, B, and P2O5. Thiscomposite volcano consists of the older Mt. Somma caldera, formed between 14 and3.55 ka before present, and the younger Vesuvius cone. The melt inclusion compositionsprovide important constraints on pre-eruptive magma geochemistry, identify relation-ships that relate to eruption behavior and magma evolution, and provide extensiveevidence for magmatic ¯uid exsolution well before eruption.

The melt inclusion data have been categorized by groups that re¯ect magmacompositions, age, and style of eruptions. The data show distinct differences incomposition for eruptive products older than 14.0 ka (pre-caldera rocks) versus eruptiveproducts younger than 3.55 ka. Moreover, pre-caldera eruptions were associated withmagmas relatively enriched in SiO2, whereas eruptions younger than 3.55 ka (i.e., thesyn- and post-caldera magmas which generated the Somma caldera and the Vesuviuscone) were derived from magmas comparatively enriched in S, Cl, CaO, MgO, P2O5, F,and many lithophile trace elements.

Melt inclusion data indicate that eruptive behavior at Vesuvius correlates with pre-eruptive volatile enrichments. Most magmas associated with explosive plinian andsubplinian events younger than 3.55 ka contained more H2O, contained signi®cantlymore S, and exhibited higher (S/Cl) ratios than syn- and post-caldera magmas whicherupted during relatively passive interplinian volcanic phenomena.

Introduction

During the past 35 ka, Mt. Somma-Vesuvius has erupted violently at least eighttimes. Accordingly, magmas associated with this historic volcano have beensubjected to extensive study. Most investigations of the physical, petrologic, andgeochemical aspects of the complex magmatic system underlying Mt. Somma-Vesuvius have involved whole-rock samples and/or pumice separates (Barberi et al.,1981; Belkin et al., 1985; Civetta et al., 1991; Belkin et al., 1993a, b; Santacroceet al., 1993; Trigila and De Benedetti, 1993; Villemant et al., 1993; Ayuso et al.,1998; others). More recently, however, investigations have been broadened toinclude data from silicate melt inclusions (MI) in magmatic phenocrysts, becausesome MI provide knowledge on pre-eruptive magma chemistry (Anderson, 1974;see review in Lowenstern, 1995), and in particular, crucial information on theabundances of volatiles and mobile ¯uxing components in magma. Volatiles mayalso serve, in general, as geochemical probes into magma processes. Knowledge ofmagmatic volatile concentrations is particularly important for interpreting explosiveeruptive behavior because the exsolution and expansion of volatiles provide muchof the driving force during violent eruptions. The use of MI as a means ofunderstanding pre-eruptive magma chemistry is particularly appropriate to samplesfrom Mt. Somma-Vesuvius because recent MI investigations have reported extremeenrichments of volatiles and associated mobile components, e.g., H2O, Cl, S, F,P2O5, and CO2, in some Mt. Somma-Vesuvius magmas (Vaggelli et al., 1993;Marianelli et al., 1995, 1999; Belkin et al., 1998; Lima et al., 1999; Signorelli et al.,1999; Raia et al., 2000). It is noteworthy that our study is the ®rst to integrate thesevarious MI data sets and to attempt to interpret eruptive processes from theperspective of this large body of data.

A number of outstanding problems for this well-studied system requiringresolution include: (1) determination of pre-eruptive abundances of all magmaticvolatiles for the entire eruptive history, (2) whether the concentrations of magmaticvolatiles show systematic relationships with the age of eruption, and (3) whether theconcentrations of magmatic volatiles show systematic relationships with the variouseruptive styles. To resolve these issues we have analyzed heated MI in clino-pyroxene phenocrysts from pumice samples ejected explosively during plinian andsubplinian eruptions and MI from lavas and scorias erupted during comparativelypassive interplinian volcanic events at Mt. Somma-Vesuvius. Our samples span theinterval from > 25 ka up to the most recent A.D. 1944 eruption. The MI wereanalyzed by electron microprobe for major and minor elements, including F, Cl, S,and P, and by secondary ion mass spectrometry (SIMS) for H2O, B, and otherselected trace elements. One of our major new ®ndings is that the volatiles, ¯uxingcomponents, and incompatible trace elements in the MI exhibit a number of uniquerelationships that allow us to distinguish, chemically, the magmas associated withpre-caldera activity older than 14.0 ka from magmas associated with eruptionsyounger than 3.55 ka. Eruptions of the younger magmas generated the Mt. Sommacaldera and the Vesuvius cone. A second ®nding is that most magmas younger than3.55 ka which erupted in plinian and/or subplinian fashion were characterized bygeochemical features that were uniquely different from passively erupted, inter-plinian magmas younger than 3.55 ka.

178 J. D. Webster et al.

Geologic background

Mt. Somma-Vesuvius is a composite volcano that is located roughly 14 km east-southeast of Naples, Italy. Most eruptive products of this system were derived frombasic, silica-undersaturated, potassic magmas that ranged in composition fromtephrite to phonolite and included less-alkaline, basaltic trachyandesite magmas(Sigurdsson et al., 1985; Joron et al., 1987; Belkin et al., 1993a, b; De Vivo et al.,1993). The younger cone of Vesuvius rises from the center of the older caldera rimof Mt. Somma. Mt. Somma began its eruptive activity at least 35 ka before present(BP) (Principe et al., 1982; Scandone et al., 1993; Ayuso et al., 1998), and someconsider Vesuvius volcanism to have begun roughly 17 ka BP when Mt. Sommamagmatism and eruptions ceased (Delibrias et al., 1979; Sigurdsson et al., 1985).More recently, however, this view has been challenged by observations that aremore consistent with a younger initial age for Vesuvius, i.e., it formed after the A.D.472 plinian eruption (Rolandi et al., 1998).

The eruptive styles and cyclicity of the Mt. Somma-Vesuvius system have variedsubstantially during the past 3.5 ka. The effusive lava ¯ows and strombolian-style,scoria eruptions were the most common volcanic activity here, but they have beenpunctuated by comparatively rare but highly explosive plinian to subplinianeruptions of pumice and ash that also involved pyroclastic ¯ows and surges. Fieldstudies (Dobran, 1993) of the various eruptive units demonstrate that the morepassive strombolian and lava ¯ow phenomena involved small quantities of magma(� 0.01 km3), whereas magmas erupted in plinian and subplinian fashion were ofcomparatively greater volume (several km3 to 0.1 km3). The eruptive styles alsocorrelate with differences in major-element composition of associated whole-rocksamples. The passively erupted trachybasaltic to phonotephritic magmas containedcomparatively less SiO2, Na2O, and K2O, and the plinian and subplinian magmaswere generally more evolved toward phonolitic compositions (Civetta andSantacroce, 1992). These chemical differences in conjunction with the signi®cantlylonger magmatic repose time periods preceding the plinian and subplinian events(Macedonio et al., 1990) indicate that eruption of plinian and subplinian magmafollowed extensive chemical differentiation of magma.

Explosive plinian and sub-plinian phenomena have been recognized frompumiceous pyroclastic ¯ows that were deposited during eruptions dated roughly17 ka, 16 ka, 14 ka, 8 ka, and 3.55 ka BP and at A.D. 79, A.D. 472, and A.D. 1631(Joron et al., 1987; Ayuso et al., 1998; Rolandi et al., 1998). Based on eruptiveactivity and concomitant cyclicity, Arno et al. (1987) recognize 3 general groupingsfor Mt. Somma-Vesuvius eruptive products: 25±11.5 ka BP, 7.90 ka BP to A.D. 79,and A.D. 79 to A.D. 1944. Ayuso et al. (1998) have con®rmed and detailed thegroupings of Arno et al. (1987) indicating 3 chemically gradational groups dis-tinguished by age: 1) 25 to 14 ka BP, 2) 8 to 2.7 ka BP, and 3) A.D. 79 to 1944.Further details on eruptive behavior and rock geochemistry are available inSheridan et al. (1981), Joron et al. (1987), Civetta et al. (1991), and references citedtherein.

We analyzed MI in clinopyroxene phenocrysts from 6 new samples of lava,scoria, and pumice from Mt. Somma-Vesuvius. These analytical results have beencombined with analyses of MI from 10 samples in prior studies (Belkin et al., 1998;

The behavior of chlorine and sulfur 179

Table 1. Average compositions of reheated silicate melt inclusions in clinopyroxene phenocrysts from Mt. Somma-Vesuvius rock samples

SCL-5 S2(2)D S4(4) S6 S11 S19 S20 S21

Rock type lava scoria-lava lava pumice pumice-scoria pumice scoria pumiceYears sinceor age of > 25 ka ago 25 ka to 17 ka 17 ka to 14 ka 15 ka to 14 ka 3.55 ka A.D. 79 A.D. 79 to 472 A.D. 472eruption (Avellino)(n,m) (20,6) (21,8) (5,4) (6,4) (7,4) (10,7) (4,3) (5,4)SiO2 (wt.%) 52.09�1.8 53.85�2.0 53.16�0.50 51.48�0.7 49.04�1.7 49.58�1.25 50.44�0.4 47.72�0.9TiO2 1.33�0.16 0.98�0.15 0.94�0.13 1.22�0.03 1.34�.36 1.20�0.17 0.78�0.06 1.07�0.19Al2O3 16.57�1.24 16.8�0.99 16.81�0.66 16.62�0.3 14.35�1.19 14.79�0.82 17.44�0.2 15.66�1.1FeO 7.35�2.6 5.77�1.14 5.39�0.52 6.41�0.4 5.44�0.46 6.03�0.39 5.25�0.17 5.97�0.54MgO 4.1�0.51 4.04�0.50 4.14�0.30 4.62�1.0 6.52�1.61 5.01�0.48 3.57�0.33 5.13�0.93MnO 0.14�0.05 0.12�0.05 0.11�0.03 0.12�0.01 0.06�0.06 0.13�0.03 0.12�0.03 0.16�0.04CaO 9.17�1.14 7.53�0.75 7.74�0.56 8.71�0.76 10.36�0.76 9.34�1.44 7.27�0.16 10.16�0.94K2O 4.87�0.83 6.3�0.86 6.92�0.33 5.67�0.34 5.1�1.0 6.23�0.86 7.93�0.20 6.19�1.31Na2O 2.61�0.57 2.79�0.55 2.41�0.33 2.75�0.62 2.53�0.50 2.29�0.20 3.58�0.24 2.68�0.28P2O5 1.16�0.20 0.79�0.18 0.75�0.16 0.95�0.23 1.38�0.50 1.04�0.38 0.52�0.08 0.75�0.83S 0.03�0.01 0.065�0.05 0.045�0.02 0.035�0.01 0.24�0.05 0.23�0.07 0.085�0.02 0.235�0.02Cl 0.21�0.07 0.29�0.15 0.28�0.01 0.26�0.1 0.42�0.07 0.62�0.12 0.82�0.07 0.64�0.16F 0.35�0.13 0.31�0.18 0.33�0.07 0.25�0.06 0.26�0.11 0.34�0.09 0.40�0.02 0.40�0.11TOTAL 100.01 99.7 99.07 99.13 97.28 97.06 98.29 97.00H2O (wt.%) 1.2�0.2 1.4�0.9 0.4�0.1 0.5�0.2 1.5�0.6 3.5�1.5 0.9�0.2 2.3�1.5Li (ppm) 32�9 41�27 20�6 17�3 93�12 36�35 29�3 21�8Be 14�1 10�2 7�1 10�1 4�5 7�1 13�1 9�2B 63�5 54�13 41�7 51�4 45�30 50�36 71�1 57�10Rb 78�22 175�62 186�59 126�22 182�70 154�17 194�17 185�47Sr 543�133 628�92 595�55 794�73 832�148 985�160 1184�50 1255�197Y 18�6 21�5 19�3 34�1 18�3 29�3 27�4 36�9Zr 159�45 190�41 136�31 227�16 134�3 188�28 186�25 247�100Nb 64�112 27�8 21�6 30�4 36�3 25�3 35�2 32�5Cs 2�1 12�6 18�9 9�3 18�9 16�2 15�2 21�6Ce 46�20 69�13 55�9 84�9 86�12 106�11 93�6 130�25Sm 7.0�3.2 9.0�4.9 11.7�2.2 15.0�8.5 15.0�6 19.1�6.1 17.7�1.5 26.0�1.7Dy 4.1�2.6 4.6�1.5 4.9�1.0 7.6�0.7 4.6�0.9 5.9�0.8 5.6�0.2 7.8�1.4Yb 1.6�0.6 3.8�2.0 4.3�1.4 7.5�1.0 3.4�1.7 5.8�2.1 7.2�0.6 7.8�0.6Th 5�3 10�4 6�1 8�1.8 8�1 9�1 11�1 10�2U (ppm) 1�1 2�1 2�1 3�1 3�1 2�1 4�1 4�2

R3 R4 RL-1 RL-5 V6 V16 V105 V144a

Rock type scoria scoria lava lava lava lava lava lavaYears sinceor age of A.D. 893 A.D. 1139 A.D. 1139 A.D. 1139 A.D. 1631 A.D. 1714 A.D. 1906 A.D. 1944eruption(n,m) (10,8) (6,5) (5,5) (10,6) (7,4) (10,4) (6,4) (4,3)SiO2 (wt.%) 49.89�0.80 47.17�1.6 52.00�1.1 51.92�0.69 50.55�1.12 50.68�0.81 49.70�1.2 48.97�0.57TiO2 1.08�0.12 1.05�0.07 0.69�0.11 0.80�0.09 1.04�0.11 0.95�0.95 0.96�0.11 1.08�0.23Al2O3 16.34�0.62 16.36�0.62 16.88�0.92 17.13�0.46 15.37�0.83 15.37�1.0 16.04�0.59 14.94�0.37FeO 5.70�0.46 7.59�0.74 5.04�0.34 4.76�0.29 6.07�0.64 5.80�0.8 6.45�0.43 6.86�1.1MgO 4.41�0.51 4.29�0.71 3.56�0.96 3.77�0.24 5.08�0.57 5.02�0.7 4.67�0.34 5.59�0.5MnO 0.15�0.02 0.15�0.03 0.10�0.02 0.13�0.05 0.15�0.04 0.12�0.06 0.14�0.04 0.16�0.04CaO 8.35�0.8 9.78�1.4 6.73�0.90 6.85�0.51 9.12�1.2 8.78�1.0 9.22�0.6 10.04�1.05K2O 7.43�0.60 6.62�0.83 9.19�0.30 7.68�0.85 6.48�1.46 6.62�1.0 6.61�0.23 5.78�1.22Na2O 3.05�0.33 2.83�0.35 3.18�0.54 3.49�0.62 2.75�0.46 2.38�0.36 2.97�0.32 2.40�0.13P2O5 1.04�0.17 0.80�0.45 0.60�0.50 1.05�0.12 1.02�0.15 1.04�0.15 0.95�0.32 0.86�0.04S 0.12�0.02 0.10�0.02 0.095�0.04 0.11�0.05 0.105�0.04 0.11�0.06 0.12�0.04 0.10�0.07Cl 0.69�0.05 0.69�0.03 0.63�0.10 0.57�0.15 0.64�0.26 0.56�0.15 0.57�0.06 0.49�0.19

(continued)

180 J. D. Webster et al.

Raia et al., 2000), and the full data set which involves 16 samples is reported asaverage values in Table 1. The lava and scoria samples represent eruptive productsof interplinian events, and the pumice and pumice-scoria samples represent theproducts of explosive plinian and subplinian eruptions. Our rock samples containrelatively abundant phenocrysts of clinopyroxene with minor olivine, leucite, andplagioclase and accessory ¯uorapatite. This phase assemblage is consistent with thatreported previously; Joron et al. (1987) observed phenocrysts of clinopyroxene,plagioclase, leucite, alkali feldspar, biotite and minor opaques in phonolitic tephritesand tephritic phonolites. The abundance of MI varies widely from grain to grainwithin a sample, and most rock samples contain MI that are generally < 60±70mmin diameter.

Sample preparation and analytical procedures

The rock samples were crushed and clinopyroxene phenocrysts were picked byhand from the powdered material. Most of the MI in clinopyroxene are partiallyto completely crystallized, so all MI-bearing phenocrysts were reheated totemperatures of 1170� to 1200 �C at 1 atm for 2 hours and quenched rapidly toglass prior to analysis. After heating and quenching, the glass of most MI containshrinkage- or vapor-bubbles, and some MI also contain small residual apatite and/oriron oxide crystals. The clinopyroxenes were mounted on glass slides with epoxyand polished to expose the reheated MI at the grain surface. It should be noted thatthe sample preparation and analytical procedures used in this study are eitheridentical or suf®ciently equivalent to those of Belkin et al. (1998) and Raia et al.

Table 1 (continued)

R3 R4 RL-1 RL-5 V6 V16 V105 V144a

F 0.33�0.09 0.31�0.05 0.26�0.10 0.40�0.10 0.40�0.2 0.35�0.13 0.39�0.09 0.34�0.11TOTAL 98.7 97.84 99.05 98.77 98.88 97.89 98.88 97.71H2O (wt.%) 1.1�0.6 2.0�1.1 0.5�0.2 1.0�0.4 0.9�0.1 0.6�0.2 1.1�0.6 0.8�0.3Li (ppm) 13�2 21�2 46�8 135�71 48�31 24�10 12�6 31�13Be 10�1 11�2 11�2 17�1 9�1 9�5 8�2 10�3B 56�6 62�17 68�3 143�42 51�3 42�23 44�16 53�11Rb 174�54 186�103 223�13 158�44 273�25 231�60 203�71 138�72Sr 1070�188 1188�492 904�211 810�168 1069�34 1124�152 1179�179 1137�187Y 26�6 47�24 17�4 11�3 23�4 26�2 23�10 32�10Zr 232�115 304�148 133�26 116�17 205�19 237�3 210�73 302�102Nb 28�5 26�15 28�3 25�15 32�2 33�7 44�7 43�9Cs 12�7 8�4 19�3 8�3 20�4 14�4 14�7 9�9Ce 88�27 161�114 62�20 47�10 101�5 116�9 101�36 128�26Sm 8.5�1.5 22.5�15 6.8�2 5.0�1 8.8�1.7 9.9�0.9 8.1�1 10.2�2.5Dy 6�1.6 11.1�7 4.6�2 2.0�1 4.9�0.9 5.3�0.4 4.9�2.0 5.8�1.7Yb 2.6�1.1 4.2�2 3.6�1 1.0�0.6 2.3�0.8 2.4�0.5 1.8�0.7 2.5�0.7Th 10�3 14�4 12�3 11�6 13�1 17�2 16�2 18�5U (ppm) 2�1 5�2 4�1 1�1 4�1 5�1 5�1 5�1

Data for SCL-5, S2(2)D, RL-5 from Belkin et al. (1998); data for R3, R4, RL-1, V6, V16, V105, V144a from Raiaet al. (2000). S19 is average composition of MI from white and gray pumice. Number of MI analyzed by electronmicroprobe (n); number of MI analyzed by ion microprobe (m)

The behavior of chlorine and sulfur 181

(2000) in their investigations of MI from Mt. Somma-Vesuvius. Thus, the combinedresults of all three studies represent a comprehensive and internally consistentdata set.

Electron microprobe (EMPA)

The glasses of the polished MI mounts were analyzed at 15 KeV and 10 nA for Na,K, Si, Al, Mg, Fe, Mn, Ca, Ti, P, and F and at 15 KeV and 30 nA for S and Cl with aCameca SX-100 electron microprobe at the American Museum of Natural Historyusing a large (10±20mm diameter) defocused electron beam. The peak counting timesfor Cl and F were 60 s and 40 s, respectively. The MI were moved under the beam tominimize volatile loss. Based on counting statistics, the relative 1� precisions foranalysis of MI glasses are less than 1% for Si, Al, Ca, and K; 1% to 2% for Fe, Mg,Na, and Ti; and from 2% to 6% for Mn, P, F, and Cl.

Replicate analyses of Cl in glass, conducted on a single spot, demonstrate thatthe Cl counts are stable at these conditions. Obsidians of known composition werealso studied to determine analytical accuracy and precision for Cl. The microprobedetermined 0.19 to 0.21 wt.% Cl in an obsidian containing 0.20 wt.% Cl as deter-mined by wet chemical analysis. The measured analytical precision for Cl in anobsidian containing 0.2 wt.% Cl is 5% relative.

Sulfur analyses on silicate glasses were conducted with count times of 60 to120 s on the S K-� peak and 30±60 s on background. Because of concerns aboutS peak shifts varying as a function of the oxidation state, S K-� wavelength scansof unknown glasses were performed ®rst to best determine the appropriate Sreference standard (anhydrite or troilite). Precision and accuracy of S analyseswere monitored with NIST reference glasses SRM 620 (1121 ppm S), SRM 621(520 ppm S), and Juan De Fuca basaltic glass JDF (1420� 40 ppm; Wallace andCarmichael, 1992). Thirty electron probe analyses of S in the JDF glass yielded anaverage S concentration of 1463 ppm (1�� 43 ppm) using the above conditions.Results on the NIST glasses are similar and routinely within 5% of the acceptedvalues.

Secondary ion mass spectrometry (SIMS)

The same MI glasses were analyzed for H (reported as H2O), Li, Be, B, Rb, Sr, Y,Zr, Nb, Cs, Ce, Sm, Dy, Yb, Th, and U by SIMS at the Woods Hole OceanographicInstitution using techniques detailed by Webster et al. (1996) and Shimizu and Hart(1982). The trace elements and hydrogen were analyzed as positively chargedsecondary ions utilizing a negatively charged oxygen primary ion beam. Thesamples were analyzed at 10 KeV accelerating potential and 1 to 2 nA beam current.The inclusions were analyzed in one spot, ®ve times each in depth pro®le mode.Based on SIMS analyses of synthetic and natural glasses, H2O concentrations arereproducible to � 0.3 to 0.4 wt.% and the trace elements to 5 to 15% relativeprecision. The silicate glasses that were used as analytical standards includemetaluminous and moderately alkaline obsidians and the NBS glasses which arehighly calcic and alkaline. The results of analyzing a single MI during differentanalytical sessions show close reproducibility (see Raia et al., 2000).

182 J. D. Webster et al.

Results

Major elements

The compositions of the MI glasses from all 16 Mt. Somma-Vesusius rock samplesdemonstrate that the magmas of this system were basic, alkaline, and potassic. Theaverage compositions of the MI from scoria and lavas reported in Table 1 are similarto the compositions of most of the corresponding whole-rock samples (Joron et al.,1987; Belkin et al., 1993a, b; Ayuso et al., 1998), except that the MI containdistinctly higher abundances, in general, of H2O, Cl, F, and S. More importantly, asdetailed below, trends shown by constituents of whole-rock samples that re¯ectmagma evolution are mirrored by trends in MI compositions.

Volatiles and ¯uxing components

These magmas were strongly enriched in volatiles (H2O, Cl, and S) and ¯uxingcomponents (P, B, and F). Average pre-eruptive abundances of H2O, ascertainedfrom compositions of heated MI for each sample, range from 3 to < 0.5 wt.%.Several individual MI from plinian sample S19 (the A.D. 79 eruption) containnearly 6 wt.% H2O even after the MI were reheated in the laboratory prior toanalysis. It is noteworthy that the H2O contents of reheated MI may be somewhatlower than those in magma, prior to eruption, as described below. The con-centrations of S are also quite high; average pre-eruptive S concentrations rangefrom < 0.05 to 0.24 wt.%. Mt. Somma-Vesuvius magmas also contained some of thehighest reported Cl contents for any magmatic system on Earth (Vaggelli et al.,1993; Belkin et al., 1998; Marianelli et al., 1999; Lima et al., 1999; Signorelli et al.,1999; Raia et al., 2000). The melt phases contained from 0.2 to more than 0.8 wt.%Cl prior to eruption, on average, and several MI contain more than 1 wt.% Cl (seealso Vaggelli et al., 1993; Marianelli et al., 1999; Raia et al., 2000). These magmasalso contained from 1 to 1.5 wt.% P2O5� F. The B contents of Mt. Somma-Vesuvius melts, ranging from 30 to 160 ppm, were quite high as compared to otherprimitive subduction-related magmas, and this is consistent with the observations ofKamenetsky et al. (1995) and Metrich et al. (1998) of high B contents in MI fromthe Vulsini volcanic district of Italy.

Melt evolution

The combined MI data sets of this study, Belkin et al. (1998), and Raia et al. (2000)(Figs. 1 and 2) indicate that most residual fractions of melt evolved toward lowerMgO and CaO contents and increasing abundances of alkalies, Al2O3, and SiO2.Concomitant variations in whole-rock trace-element levels generally re¯ectincreasing U, Cs, Ta, Rb, Sr, REE's, and Th and decreasing Co, Cr, Sc, and Niwith diminishing abundances of MgO in melt. Figure 3 illustrates representativecovariations involving Cs versus Rb, Be versus B, and Sm versus Yb in MI glasses.

It is well established that the evolution of magmas which generated plinian,subplinian, and interplinian eruptions at Mt. Somma-Vesuvius was stronglygoverned by crystallization and fractionation of clinopyroxene (Joron et al., 1987;

The behavior of chlorine and sulfur 183

Trigila and De Benedetti, 1993; Belkin et al., 1993a, b; Lima et al., 1999; Raia et al.,2000). The positive covariations of MgO with (CaO/Al2O3) in our MI data indicatethat clinopyroxene crystallization was important, and this covariation is consistentwith previous observations on whole rocks and MI (Fig. 1a). Clearly however,fractionation of other crystalline phases occurred. The SiO2 contents of Mt. Somma-Vesuvius clinopyroxene phenocrysts (Cundari and Salviulo, 1987) are similar tothose of the MI, so fractionation of clinopyroxene alone should not modify the SiO2

Fig. 1. Plots of a MgO versus (CaO/Al2O3), b CaO versus SiO2, and c Sr versus Ce inindividual reheated silicate melt inclusions in clinopyroxene phenocrysts from pre-calderarocks older than 14.0 ka (open squares) and syn- and post-caldera rocks younger than3.55 ka (®lled squares) of Mt. Somma-Vesuvius. Major-element data in a re¯ect thein¯uence of fractional crystallization of clinopyroxene in melt evolution, but data in bdemonstrate that melt evolution was not in¯uenced solely by crystallization ofclinopyroxene because the SiO2 contents of clinopyroxene are similar to those of thebulk melts. Trace-element data (c) show that older melts contained less Sr and Ce prior toeruption. Data from this study, Belkin et al. (1998), and Raia et al. (2000); maximumrepresentative values for 1� precision are shown for datum in (c)

184 J. D. Webster et al.

contents of residual fractions of melt signi®cantly as it depletes residual melt inCaO. Theoretically, melt evolution resulting solely from clinopyroxene crystal-lization should be characterized by SiO2 variations that are independent of theassociated changes in CaO in melt for plots of CaO versus SiO2. The data in Fig. 1are inconsistent with this, however, because the SiO2 contents of individual MI varyfrom 47 to 57 wt.% while CaO ranges from 13 to 6 wt.%, respectively. Thus, meltevolution was in¯uenced by fractionation of feldspar and lesser olivine as well asclinopyroxene crystallization. Prior work has also demonstrated that magma com-positions and melt evolution were in¯uenced by frequent input of new batches ofmagma and through subsequent and variable magma mixing (Civetta et al., 1991;Santacroce et al., 1993).

Pre-eruptive magma chemistry as a function of the age and style of eruption

The MI of this study, Belkin et al. (1998), and Raia et al. (2000) exhibit com-positional differences that are a distinct function of the age of eruption. Forexample, MI from all rock samples that are at least 14 ka in age contain, on average,more SiO2 and TiO2 and less CaO, MgO, P2O5, F, Li, Rb, Sr, Zr, Nb, Ce, Sm, Cs,Th, and U than MI from samples that are younger than 3.55 ka (Figs. 1, 2, and 3).This time boundary is chronologically consistent with two of the groupings of Ayuso

Fig. 2. Plot of the average concentrations of (Na2O�K2O) versus average (CaO/Al2O3)for reheated silicate melt inclusions in pre-caldera rocks (open squares) and syn- and post-caldera rocks (®lled squares) from Mt. Somma-Vesuvius. The magmas of both agegroupings (or magmatic systems) evolved toward lower (CaO/Al2O3) and higher alkalicontents with differentiation, but each magmatic system evolved along separate paths. Meltinclusions from rocks erupted during plinian and subplinian activity (large symbols) appearto be more primitive, chemically, than melt inclusions from rocks erupted duringstrombolian to vulcanian (i.e., interplinian) activities (small symbols). Data from this study,Belkin et al. (1998), and Raia et al. (2000). Error bars represent maximum 1� precision

The behavior of chlorine and sulfur 185

et al. (1998). These data also indicate that the average abundances of Na2O and K2Oin residual fractions of melt increased as magmas of both age groups evolved towardlower CaO and MgO concentrations and higher Al2O3 contents, because the twotrends in Fig. 2 exhibit similar slopes. The two trends are not colinear, however. Theextent of melt evolution was distinctly different for the younger and older varietiesof magma, because MI from rocks older than 14 ka contain lower total alkalies andexhibit lower (CaO/Al2O3) than MI from rocks younger than 3.55 ka. Moreover, theaverage abundances of the alkalies, Cl, and S in the MI also exhibit distinctrelationships with the age of eruption (Fig. 4). The four oldest eruptive units exhibita relatively tight clustering of the average pre-eruptive Na2O, K2O, Cl, and S

Fig. 3. Plots of average abundances of a Cs versus Rb, b Be versus B, and c Sm versus Ybin reheated silicate melt inclusions from Mt. Somma-Vesuvius. Although melts evolved viacrystal fractionation toward higher abundances of Cs, Rb, Be, and B and lower Sm and Yb,the older pre-caldera magmas evolved along marginally different paths than youngermagmas. Data from this study, Belkin et al. (1998), and Raia et al. (2000); symbols same asin Fig. 2. Error bars represent maximum 1� precision. Dashed lines are interpretive ®ts tocompositions of melt inclusions in pre-caldera rocks; solid lines are interpretive ®ts toinclusions in syn- and post-caldera rocks

186 J. D. Webster et al.

Fig. 4. Plots of average concentrations of a Cl, b S, and c H2O versus (Na2O�K2O) insilicate melt inclusions from Mt. Somma-Vesuvius rock samples. Increasing meltevolution, as expressed by increasing (Na2O�K2O), caused increasing Cl contents ofmelt. Data from this study, Belkin et al. (1998), and Raia et al. (2000), however, suggestthat average abundances of S in plinian and subplinian and in interplinian, syn- and post-caldera magmas were ®xed at roughly 0.235 and 0.11 wt.%, respectively. Symbols used fordata of this study, Belkin et al. (1998), and Raia et al. (2000) are the same as in Fig. 2.Large triangles represent average compositions of melt inclusions from plinian andsubplinian pumice and scoria of 3.55 ka, A.D. 472, and A.D. 79 eruptions (Marianelli et al.,1995); open cross represents melt inclusions in plagioclase, amphibole, and garnetphenocrysts (P�A�G) in white pumice from Avellino eruption (Signorelli et al., 1999).Rocks formed in A.D. 1944 and A.D. 1906 eruptions and analyzed by Marianelli et al.(1995, 1999) shown as small triangles representing melt inclusions trapped in olivine (O) atcomparatively high pressures and coexisting melt inclusions from degassed magma trappedat lower pressures in clinopyroxene and leucite (C&L). Error bars represent maximum 1�precision

The behavior of chlorine and sulfur 187

contents, and hence, they were apparently derived from magmas exhibiting a smallrange in melt compositions as compared to those of the younger magmas. Chlorideabundances of the 4 oldest melts and those of the 12 younger melts appear toincrease with increasing alkali contents of melt, and hence, with increasing meltevolution (Fig. 4a), so the most primitive batches of magma (i.e., those with meltcontaining the highest CaO/Al2O3 and lowest Na2O�K2O) were not the moststrongly enriched in Cl. The differences in pre-eruptive abundances of S in thesemelts are even more remarkable (Fig. 4b). All four magmas that erupted � 14 kaago contained low S contents (i.e., < 0.07 wt.% in melt), and the apparent covari-ations suggest that S concentrations increased with pre-eruptive alkali abundancesduring melt evolution. Thus, the plinian, subplinian, and interplinian melts thaterupted� 3.55 ka BP (i.e., those spanning time from the Avellino eruption to the A.D.1944 events) contained much higher Cl and S concentrations, on average (Fig. 4b).Likewise, we suggest that all explosive eruptions that took place at 17 ka, 16 ka, 14 ka,and 8 ka involved magmas characterized by lower CaO/Al2O3 and generally lowerabundances of Cl than S than all four explosive events occuring since 3.55 ka BP.

The data of this study, Belkin et al. (1998), and Raia et al. (2000) indicate thatthe magmas which fed the younger (i.e., � 3.55 ka) plinian and subplinian eruptivephenomena contained less SiO2, Al2O3, MnO, Na2O, K2O, Be, B, Rb, Sr, Nb, Th,and U and more TiO2, MgO, CaO, P2O5, Ce, and REE than the younger, inter-plinian magmas (Table 1). The average abundances of the alkalies, S, and H2O inthe MI from rocks younger than 3.55 ka also exhibit relationships that vary as afunction of the style of eruption (Fig. 4). For example, the magmas that erupted aslava ¯ows and as scoria (i.e., those produced during non-plinian eruptions) show alarger range in total alkali contents than the more explosively erupted (plinian andsubplinian) magmas. Moreover, the interplinian magmas that erupted � 3.55 ka agoshow a relatively constant range in S contents of 0.08 to 0.12 wt.% in melt, whereasthe concentrations of S in plinian and subplinian magmas that erupted � 3.55 ka agowere much higher. The latter magmas contained from 0.23 to 0.24 wt.% S in melt,on average. Remarkably, all MI from the three plinian and subplinian magmas thaterupted � 3.55 ka ago contain > 1.5 wt.% H2O, on average, whereas the averageH2O concentrations of MI from all of the other younger interplinian magmas(except for that which erupted as the A.D. 1139 scoria) are � 1.2 wt.% H2O. Insummary, the magmas that generated the younger plinian to subplinian eruptionscontained pre-eruptive abundances of Cl that were equivalent, on average, to thosein the younger interplinian magmas.

Potential problems associated with MI compositions

The concentrations of H2O in the MI are less than those of the entrapped aliquots ofmelt they represent because some H2O was undoubtedly lost from the MI afterentrapment (via diffusion of H2O and/or H2 while the magma and rocks were stillhot ± Roedder, 1984) and later when the phenocrysts were reheated in thelaboratory. We tried to minimize the loss of H2O by reheating the MI for only 2hours. Water losses notwithstanding, the relative differences in the concentrations ofH2O in the MI are useful for constraining relative differences in pre-eruptiveabundances of H2O in the magmas. Even though the reheated MI may not

188 J. D. Webster et al.

necessarily provide accurate information on pre-eruptive H2O concentrations, theywere heated under similar conditions so the relative abundances of H2O should notvary strongly. Moreover, it is noteworthy that the average abundances of H2O in theheated MI from all of our samples of interplinian magmas are equivalent, withinassociated precision, to the average abundances of H2O in the unheated MI ofMarianelli et al. (1999) (Fig. 4). This indicates that the measured H2O con-centrations in our heated MI were not strongly reduced through loss or diffusion ofH2 or H2O by heating for two hours.

Post-entrapment crystallization of clinopyroxene on the walls of MI willarti®cially reduce the CaO and MgO concentrations of the MI and increase theabundances of all other melt constituents except for SiO2. Insuf®cient heating ofcrystallized MI due to short heating durations and/or heating at low temperatureswill have similar consequences. Conversely, reheating crystallized MI at excessivelyhigh temperatures will melt too much clinopyroxene, and the MI compositions willshow arti®cially high concentrations of CaO and MgO and abundances of all othermelt constituents that are lower than those of the melt at the time of entrapment. Asnoted previously, SiO2 abundances in MI entrapped in clinopyroxenes are relativelyindependent of host phase crystallization and of excessive or insuf®cient reheatingof the host phase because the SiO2 contents of the MI are very similar to those of theclinopyroxenes. One can constrain or limit the consequences of this potentialproblem through simple computations. For example, the range in average CaOcontents for most reheated MI is only 7 to 10 wt.%. If, for the sake of argument, weassume that this difference only re¯ects excessive melting of clinopyroxene, thenless than 20 wt.% of excess clinopyroxene must be melted to generate a melt withthe maximum CaO content of 10 wt.% from a melt initially containing 7 wt.% CaO.Alternatively, if the difference is the sole result of excessive crystallization ofclinopyroxene to generate a melt with 7 wt.% CaO from a melt that initiallycontains 10 wt.% CaO, then < 20 wt.% crystallization of clinopyroxene must occur.Twenty percent melting or crystallization of clinopyroxene induces changes in allother melt constituents (save SiO2) that are < 20% relative. In contrast, thecompositional data and associated trends of this study show covariations that farexceed the relative 20% variability that could be a result of clinopyroxene meltingor crystallization. Moreover, the data in Fig. 1b demonstrate that the SiO2 contentsof the reheated MI vary by nearly 10 wt.%. Thus, the compositional trends shown bythe MI are not a result of problems associated with the reheating procedures.

The presence of small residual apatite and/or iron oxide crystals in somereheated MI may also bear on how well the reheated MI are chemically re-presentative of the pre-eruptive Mt. Somma-Vesuvius melt compositions. If theseminerals are, in fact, daughter phases that crystallized from the melt afterentrapment, then some MI are arti®cially low in CaO, P2O5, F, with or without FeO.

Discussion

Distinguishing magmas of Mt. Somma-Vesuvius

Understanding the eruptive history of the Mt. Somma-Vesuvius magmatic systemfor the purpose of searching for cyclic behavior and forecasting future volcanic

The behavior of chlorine and sulfur 189

activity depends, fundamentally, on the capacity to recognize differences in thenumerous batches of magma and to distinguish the volcanic products derived frommagmas of differing sources. The MI data of this and prior studies (Belkin et al.,1998; Raia et al., 2000) provide an effective means of distinguishing andcategorizing Mt. Somma-Vesuvius magmas. For instance, prior to formation of theMt. Somma caldera volcanism was fed by magmas that were geochemically distinctfrom those that generated the Mt. Somma caldera and drove subsequent eruptions ofVesuvius. Differences in the MI compositions ®x the time boundary as occurringbetween 14 and 3.55 ka BP.

The average compositions of MI from the four pre-caldera rock samples that areolder than 14 ka (i.e., those comparatively enriched in SiO2 and TiO2 and depletedin H2O, S, Cl, CaO, MgO, P2O5, F, Li, Rb, Sr, Zr, Nb, Ce, Sm, Cs, Th, and U) showtrends re¯ecting magmatic evolution distinct from those indicated by MI of thetwelve syn- and post-caldera rock samples (i.e., those rocks younger than 3.55 kaand formed during or after the Avellino plinian eruption). Some trends areconsistent with the work of Ayuso et al. (1998) on whole-rock samples. Theyreported that rocks from the older cycle (25 to 14 ka in age) exhibit internallyconsistent relationships involving trace elements and that some of these trace-element variations differ from those shown by rocks of the younger cycle (A.D. 79to 1944). More importantly, the MI indicate that although the magmas controllingeruptive events at Mt. Somma-Vesuvius show similar (i.e., parallel) trends ofevolution, the trends are not colinear (Figs. 2 and 3). Fractional crystallization led torelatively lower abundances of CaO and concomitantly higher concentrations of Na,K, and Al in residual batches of Mt. Somma-Vesuvius magmas. The pre-calderamagmas, however, contained comparatively lower (CaO/Al2O3) ratios than mostyounger magmas. Moreover, S and Cl in the older or pre-caldera magmas showcovariations that are dramatically unlike those in the younger magmas (Fig. 4).Whereas the abundances of Cl, and arguably S, in melt appear to increase weaklywith increasing alkali contents during progressive evolution of the pre-calderamagmas, the S abundances of the melts younger than 3.55 ka appear comparatively®xed at values near either 0.11 wt.% or 0.24 wt.%.

To understand how dissimilar the two magmatic systems were, it is important torecognize that the younger magmas did not evolve from the older, pre-calderamagmas via simple crystal fractionation even though the greater abundances ofH2O, S, and Cl in the younger magmas and the apparent trends involving S and Clin melt (Fig. 5) could be interpreted this way. This is highly unlikely, because theranges in abundance for most of the major, minor, and trace elements in the pre-caldera magmas are similar to those in the younger ones (Table 1 and Figs. 1, 2, 3,and 4). If the younger magmas did, in fact, evolve from the older magmas by simplecrystal fractionation, then the data in Fig. 2 should not exhibit two distinct trendsand the pre-caldera magmas should contain lower abundances of incompatibleelements (e.g., Cs, Rb, Be, B, and REE) than the younger magmas.

Differentiation of pre-caldera magma

The trends shown by the major, minor, and trace elements in heated MI from fourMt. Somma rock samples of this study and Belkin et al. (1998) are generally

190 J. D. Webster et al.

consistent with the geochemical relationships expressed in whole-rock data. Priorinvestigations of whole rocks at least 14 ka in age have reported that residualfractions of melts were characterized by increasing abundances of SiO2, Al2O3, andalkalis and decreasing CaO, MgO, P2O5, FeO, and TiO2 contents with progressiveevolution (Joron et al., 1987; Ayuso et al., 1998 and references cited therein). Theresponse of SiO2 to fractional crystallization with or without magma mixing,however, varied with the degree of silica undersaturation of the pre-caldera parentmagmas. Whole-rock investigations show strong SiO2 enrichments in somefractions of melt (i.e, that represented by the average compositions of somewhole-rock samples) during evolution, whereas the variation in SiO2 contents ofother melt fractions was negligible. Concomitant variations in whole-rock trace-element levels generally re¯ect increasing U, Cs, Ta, Rb, Sr, REE's, and Th anddecreasing Co, Cr, Sc, and Ni with decreasing abundances of MgO in melt. Many ofthe trace elements show a response to melt evolution that varies depending on thedegree of silica saturation of the parent melt, much like that described for SiO2

(Joron et al., 1987; Ayuso et al., 1998).

Differentiation of syn- and post-caldera magmas

The geochemical trends expressed by the compositions of the heated MI from thesixteen rock samples of this and associated studies (Belkin et al., 1998; Raia et al.,

Fig. 5. Plots of average abundances of S versus Cl in Mt. Somma-Vesuvius magmas priorto eruption. Explosively erupted, syn- and post-caldera plinian and subplinian melts fromVesuvius clearly contained higher abundances of S than passively erupted, interplinianmagmas before eruption. Data and symbols are the same as in Fig. 4. Error bars representmaximum 1� precision. Arrows signify apparent changes in pre-eruptive volatileabundances of magma for the Avellino (long arrow) and the A.D. 1944 (short arrow)eruptions (from comparatively early S-rich to subsequently degassed S-de®cient magma)

The behavior of chlorine and sulfur 191

2000) generally conform with those expressed in whole-rock and MI data. Priorinvestigations of whole rocks from all eruptive events and activities younger than3.55 ka in age suggest fractionation trends evolving toward increasing SiO2, Al2O3,and alkalis and decreasing CaO, MgO, P2O5, FeO, and TiO2 (Joron et al., 1987;Belkin et al., 1993a, b; Ayuso et al., 1998). This is consistent with the observationthat MI in phenocrysts erupted in 1944 with lava ¯ows, ®re fountain products, andvulcanian eruptive products show distinct changes in CaO and K2O which re¯ectthe evolution of this single batch of magma (Marianelli et al., 1999).

The syn- and post-caldera magmas trapped in clinopyroxene phenocrysts thatwere erupted during plinian and subplinian events were compositionally distinctfrom magmas trapped in clinopyroxenes from intervening, interplinian events; theformer were generally more primitive in composition. Heated MI from the formereruptive units contain less SiO2, Al2O3, MnO, Na2O, K2O, Be, B, Rb, Sr, Nb, Th,and U and more S, H2O, TiO2, MgO, CaO, P2O5, Ce, Sm, and Dy (Table 1). Thesyn- and post-caldera magmas which supplied explosive plinian and subplinianevents were also characterized by higher (CaO/Al2O3) and lower total alkalis(Fig. 2).

The chemical evolution of these magmas also re¯ects a gain of Cl and loss ofS from individual batches of magma over time. The large and small arrows inFig. 5 clearly show decreasing abundances of S and increasing Cl in MI entrappedduring the Avellino and 1944 eruptions, respectively. Melt inclusions trappedcomparatively early in olivine and clinopyroxene phenocrysts contain much more Sand less Cl than MI entrapped later in plagioclase, leucite, amphibole, garnet andapparently late-forming clinopyroxene crystals. These differences apparently re¯ectchanges in pre-eruptive volatile abundances in melt during progressive meltevolution.

Pre-eruptive exsolution of volatile phases from syn- and post-caldera magmas

There now exists a large body of evidence indicating that many, if not most, of thesyn- and post-caldera magmas at Mt. Somma-Vesuvius exsolved one or morevolatile phases (supercritical ¯uid, aqueous vapor, or saline brine) prior to MIentrapment and prior to eruption. Belkin et al. (1985) and Belkin and De Vivo (1993)observed H2O- and CO2-bearing ¯uid inclusions in nodules erupted during plinianeruptions and CO2-bearing ¯uid inclusions in nodules ejected during non-plinian,eruptive events. These observations are consistent with the suggestion of Marianelliet al. (1999) that CO2 should have exsolved from these magmas early on (i.e., at acomparatively great depth). The latter study further suggested that up to 50% of theCl in magma involved in the 1944 eruption may have been lost during degassingbecause of open-conduit conditions. Signorelli and Capaccioni (1999) andSignorelli et al. (1999) provide clear evidence indicative of and detailed discussionson pre-eruptive degassing of Cl-enriched volatile phases for the A.D. 79 and 3.55 kaAvellino eruptions, respectively. More recently, Raia et al. (2000) called upon theexsolution of magmatic ¯uids, well before eruption, for magmas involved medievalinterplinian eruptive events (i.e., for Vesuvius magmas dated between A.D. 512 andA.D. 1139). These ¯uids apparently in¯uenced the pre-eruptive abundances of thecomponents H2O, S, Cl, B, Be, and Li in melt, and the principal evidence that was

192 J. D. Webster et al.

cited for volatile exsolution was the apparently ®xed (i.e., volatile phase-buffered)Cl abundances in the medieval magmas. It is noteworthy that the investigations ofSignorelli and Capaccioni (1999), Signorelli et al. (1999), and Raia et al. (2000)speci®cally consider the simultaneous exsolution of two volatile phases (e.g., vaporand coexisting saline liquid) from the syn- and post-caldera magmas at Vesuvius.Degassing of melt is also the apparent cause for the correlation of decreasingabundances of S with increasing melt evolution for the Avellino and 1944 eruptions(Fig. 5).

We note, in passing, that the compositions of MI entrapped in plagioclase,amphibole, and garnet phenocrysts in white pumice fragments of the Avellinoeruption (Signorelli et al., 1999) are quite different from those of MI entrapped inclinopyroxene, olivine, and leucite phenocrysts, because the former MI most likelyrepresent late-stage, highly evolved, and strongly degassed fractions of melt.Consequently, although we have included these data in Fig. 4 for comparativepurposes, we will not consider them further because they appear to bear moststrongly on late-stage magma chemistry and late magmatic processes at Mt.Somma-Vesuvius.

The present study and associated studies of Belkin et al. (1998) and Raia et al.(2000) show relatively ®xed, average S abundances in MI from the pre- and syn-caldera magmas, and we suggest that the restricted S contents are a result of and anindication of buffering by the presence of aqueous-carbonic ¯uid phases. The rocksof Mt. Somma-Vesuvius are not reported to contain sul®de minerals of any signif-icance, magmatic sulfate minerals have not been oberved in these rocks, and it isunlikely that repeated episodes of magma input and mixing would result in suchreproducible average S contents. An aqueous, aqueous-saline, or aqueous-carbonicvapor, liquid, or ¯uid is the most likely phase responsible for sequestering SO2 andH2S and ®xing the residual S concentrations of melts at relatively constant values,on average, because experiments demonstrate that S partitions strongly in favor ofaqueous ¯uids relative to coexisting silicate melts (Scaillet et al., 1998). Further-more, we suggest that multiple episodes of degassing apparently took place and thatdegassing of S occurred in a closed magmatic system under generally similarconditions of pressure and temperature, because it is unlikely that open-systemdegassing would result in such elevated and similar S contents in melt.

The behavior of S, Cl, and H2O in syn- and post-caldera magmas

This study and the associated investigations of Belkin et al. (1998) and Raia et al.(2000) indicate that the concentrations of some of the volatiles in the MI entrappedin clinopyroxene phenocrysts from younger (syn- and post-caldera) magmas variedas a function of the mode of eruption. The average abundances of S in the plinianand subplinian melts were ®xed or buffered at approximately 0.24 wt.%; whereasthe average abundances of S in the interplinian melts were buffered at approx-imately 0.11 wt.%. In contrast, the average Cl contents of the latter range fromroughly 0.5 to 0.7 wt.% whereas Cl in the former varied through a similar range of0.4 to 0.6 wt.%. The average pre-eruptive abundances of H2O in the plinian andsubplinian melts were also apparently free to vary strongly from 1.5 to 3.5 wt.%,whereas most MI from interplinian magmas contained � 1 wt.% H2O.

The behavior of chlorine and sulfur 193

It is useful to include the results of other studies. All MI investigations to date(Marianelli et al., 1995, 1999; Belkin et al., 1998; Signorelli et al., 1999; Lima et al.,1999; Raia et al., 2000; this study) indicate high, pre-eruptive Cl contents for thesyn- and post-caldera magmas (Figs. 4 and 5), and most studies suggest that syn- andpost-caldera, plinian and sub-plinian Vesuvius magmas involved melts containingroughly 1.5 wt.% H2O or more, on average. Some of the observations from Belkinet al. (1998), Raia et al. (2000), and this study appear to differ from thosedetermined in other investigations (Marianelli et al., 1995, 1999) on MI from thesame eruptive units. For example, the data sets demonstrate that most interplinianphenomena were generated by magmas containing < 1.5 wt.% H2O, on average, butthe mean H2O content of the melt erupted during the 1139 event (Raia et al., 2000)was apparently greater and the average, pre-eruptive H2O contents of magmaerupted during the 1906 and 1944 lava ¯ow and lapilli-forming events were also> 1.5 wt.%. In summary, 15 of the 18 syn- and post-caldera MI data sets (whichre¯ect 11 different eruptive events) indicate that the more explosive behavior ofVesuvius was associated with H2O-enriched melts.

The combined MI data sets (Marianelli et al., 1995, 1999; Belkin et al., 1998;Raia et al., 2000; this study) also indicate that the average pre-eruptive abundancesof S in interplinian, sub-plinian, and plinian magmas were high (i.e., � 1000 ppm).But here again, some of the results for S from Belkin et al. (1998), Raia et al.(2000), and this investigation appear to differ from those of other studies. Speci-®cally, the average, pre-eruptive S contents of the Avellino, A.D. 79, 472, 1906, and1944 magmas (Marianelli et al., 1995, 1999) are clustered near 0.17 wt.% and showno relationship with the style of eruption.

It is not clear why the S abundances of these studies appear to differ. Thisdiscrepancy could be a result of systematic differences in the analytical methodo-logies, but the accuracy and precision for the various techniques used are welldescribed and there is no reason for assigning the differences to methodologicalerror(s). It should also be noted that the speci®c values determined for S differdepending on the type of phenocryst involved. One cannot necessarily compare thecompositions of MI in pyroxene or leucite phenocrysts with the composition of MIin olivine from the same or different eruptions to constrain differences in pre-eruptive magma compositions for different eruptions. Marianelli et al. (1999)investigation of the 1944 eruption clearly demonstrates that minerals whichcrystallize and grow comparatively late in the evolutionary history of a magma willtrap and sample melt that is more evolved and may have degassed volatiles to agreater extent. Conversely, early forming clinopyroxene and olivine phenocrystshave a greater tendency of sampling aliquots of more primitive melt that have notyet exsolved a volatile phase or have lost less H2O, S, and Cl to a volatile phase. Inthis regard, the combined MI data sets of Belkin et al. (1998), Raia et al. (2000),and this study involve clinopyroxene phenocrysts that presumably grew at similarstages of melt evolution and were reheated under similar conditions. Closeexamination of Fig. 4b suggests that Marianelli et al. (1995, 1999) averaged Sconcentrations may overlap with the S contents determined by Belkin et al. (1998),Raia et al. (2000), and this study if the 1� precision for each of the studies isaccounted for. Thus, the apparent differences in pre-eruptive S contents may not bestatistically signi®cant.

194 J. D. Webster et al.

Most, if not all, of the melts genetically associated with the plinian and sub-plinian eruptive events contained distinctly more S, distinctly to marginally moreH2O, and similar Cl abundances as did melts in the interplinian magmas, onaverage. Thus, the eruption dynamics were correlated to pre-eruptive, magmaticvolatile contents, and some of the volcanic activity may have been controlled by thestrong and variable enrichments in volatiles. This is reasonable given that much ofthe driving force for explosive volcanic eruptions is provided by the rapid exsolutionand expansion of magmatic volatiles.

The explosive, plinian to subplinian, syn- and post-caldera eruptions of Mt.Somma-Vesuvius magma should have released S-rich gases as well as gasescharacterized by comparatively high (S/Cl) ratios. Most of the plinian to subplinianmagmas were characterized by higher (S/Cl) ratios than the interplinian magmas(Fig. 6). Moreover, experimental investigations and modeling show that S partitionsstrongly in favor of aqueous ¯uids whereas Cl partitions much less strongly in favorof a ¯uid for ¯uid-saturated basaltic melts (Sisson and Layne, 1993; Scaillet et al.,1998; Webster et al., 1999). Thus, the experimental constraints and the observeddifferences in pre-eruptive magma compositions indicate that the syn- and post-caldera explosive eruptions should have released volcanic gases that were

Fig. 6. Plot of average values for (S/Cl) ratios of melt inclusions from Mt. Somma-Vesuvius rock samples versus the relative sequence of ages of eruption beginning with aMt. Somma eruption dated at > 25 ka BP and ending with the A.D. 1944 eruption.Explosively erupted, syn- and post-caldera plinian and subplinian melts from Vesuviusclearly contained higher ratios of (S/Cl) than nearly all other melts prior to eruption; theexceptions to this include melt inclusions formed during the 1906 and 1944 eruptions andinvestigated by Marianelli et al. (1995, 1999). As described in text, high (S/Cl) ratios ofvolcanic gases from Vesuvius may be excellent indicators of plinian and subplinianeruptive behavior. Data sources, symbols, and error bars same as in Figs. 2 and 4

The behavior of chlorine and sulfur 195

comparatively enriched in S. In addition, most investigations of this volcano haveeither implied or concluded that pre-eruptive degassing was common. Conse-quently, we suggest that strong increases in the (S/Cl) ratio and/or the S contents ofvolcanic gases may be an important indicator of impending explosive activity of Mt.Somma-Vesuvius, if the pre-eruptive degassing events occur at suf®ciently shallowdepths which facilitate rapid transfer of magmatic vapors to the crater surface. Thisargument also assumes that kinetic constraints (e.g., the relative rates of diffusion ofS and Cl through silicate melts) do not strongly enhance the partitioning of Cl intoan aqueous ¯uid relative to the partitioning of S during disequilibrium degassingand, hence, reduce the S/Cl ratio.

These observations raise the question of how the batches of volatile-enriched butgeochemically primitive magma that supplied the younger plinian and subplinianeruptions were derived. Theoretically, primitive, S- and H2O-enriched magmascould have been fed directly to a shallow magma chamber from depth. It has beensuggested, for instance, based on isotopic evidence (Cortini and Hermes, 1981) and¯uid inclusion geobarometry (Belkin et al., 1985; Cortini et al., 1985) that eruptionsof Mt. Somma-Vesuvius were fed from shallow (4 to 5 km depth) and deeper(roughly 9 to 13 km depth) magma chambers. Tomographic data (Zollo et al., 1998)indicate the occurrence of a sharp transition into a low-velocity zone (i.e., a magmachamber) at about 10 km depth, but they do not record evidence of a shallowermagma body. Thus, the ®ndings of Cortini and Hermes (1981), Belkin et al. (1985),and Cortini et al. (1985) are consistent with those of Marianelli et al. (1995, 1999)that shallow and deep magma chambers underlie this volcano. This scenario isconsistent with our observations of comparatively higher (CaO/Al2O3), higher MgOcontents, and lower abundances of alkalies in the S- and H2O-enriched magmas andwith data of Marianelli et al. (1999) that indicate the primitive Vesuvius magmaswere relatively enriched in volatiles. A second means of generating melts with thegeochemical characteristics of the plinian and subplinian magmas is by the input ofhot and primitive, volatile-poor magma into a shallow chamber containing evolved,volatile-enriched magma. Mixing of both magma types could generate S- and H2O-enriched magmas characterized by relatively high (CaO/Al2O3), high MgOcontents, and low abundances of alkalies. A third conceivable scenario involvesthe exsolution of H2O- and S-enriched aqueous ¯uid from magma at depth, ascentof this ¯uid to shallow levels, and resorption of the bulk of the ¯uid by relativelyprimitive magma. Aqueous ¯uids exsolved from subduction-related magmas likethese, are typically enriched in H2O and S (�alkali chlorides) (Wallace andGerlach, 1994; Gerlach et al., 1996; Mandeville et al., 1996; Webster et al., 1999),and as a consequence, they are a potential source of H2O and S. Vapor phasetransport of S has been described previously for the 1991 eruption of Mt. Pinatubo(Hattori, 1993). Resolving the origin of the high-S batches of magma at Mt.Somma-Vesuvius requires further investigation.

Concluding remarks

Chemical analysis of reheated silicate melt inclusions in clinopyroxene phenocrystsfrom volcanic rock samples at Mt. Somma-Vesuvius, Italy, provides importantconstraints on pre-eruptive magma geochemistry, magma evolution, and eruptive

196 J. D. Webster et al.

behavior. The melt inclusion compositions also provide extensive evidence formagmatic ¯uid exsolution well before eruption.

These data show distinct differences in composition for pre-caldera eruptiveproducts older than 14.0 ka versus eruptive products younger than 3.55 ka. The pre-caldera eruptive magmas were relatively enriched in SiO2, whereas the syn- andpost-caldera magmas were comparatively enriched in S, Cl, CaO, MgO, P2O5, F,and many lithophile trace elements.

Melt inclusion data indicate that eruptive behavior at Vesuvius correlates withpre-eruptive volatile abundances. Most magmas associated with explosive plinianand subplinian events younger than 3.55 ka contained more H2O, contained signif-icantly more S, and exhibited higher (S/Cl) ratios than syn- and post-calderamagmas which erupted during relatively passive interplinian volcanic phenomena.Thus, strong increases in the (S/Cl) ratio and/or the S contents of volcanic gasesmay be an important indicator of impending explosive activity of Mt. Somma-Vesuvius.

Acknowledgements

We appreciate reviews of the manuscript by C. Mandeville and M. Carroll. We thank C.Mandeville for invaluable help during EMPA analysis and useful discussion. N. Shimizuand G. Layne provided assistance during SIMS analyses. This work was partially supportedby CNR (23/15/00) grant, CNR-GNV grant (Legge Spec. 74/96) for the ®scal years 1996and 1997 to B. De Vivo, and an American Museum of Natural History Study Grant (1997)to F.R.

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Authors' addresses: Dr. J. D. Webster (jdw@amnh.org), Department of Earth and PlanetarySciences, American Museum of Natural History, Central Park West at 79th Street, NewYork, NY 10024-5192, USA; F. Raia (raia@sci.ccny.cuny.edu), Department of Earth andAtmospheric Sciences, City College of New York, New York, NY 10031, USA; B. De Vivo(devivo@biol.dgbm.unina.it), and G. Rolandi (rolandi@biol.dgbm.unina.it), Universita' diNapoli Federico II, Dipartimento di Geo®sica e Vulcanologia, Via Mezzocannone 8,I-80134 Napoli, Italy

200 J. D. Webster et al.: The behavior of chlorine and sulfur