210Pb–226Ra radioactive disequilibria in recent lavas and radon degassing: inferences on the magma chamber dynamics at Stromboli and Merapi volcanoes

ELSEVIER Earth and Planetary Science Letters 172 (1999) 111–126www.elsevier.com/locate/epsl

210Pb–226Ra radioactive disequilibria in recent lavas and radondegassing: inferences on the magma chamber dynamics

at Stromboli and Merapi volcanoes

Pierre-J. Gauthier Ł,1, Michel Condomines 2

Universite Blaise Pascal, CNRS–UMR 6524, OPGC et Centre de Recherches Volcanologiques, 5 rue Kessler,63038 Clermont-Ferrand, France

Received 11 December 1998; revised version received 12 July 1999; accepted 29 July 1999

Abstract

U-series and especially 210Pb–226Ra disequilibria have been measured in recently erupted lavas from the Stromboli(Aeolian Islands, Italy) and Merapi (Java, Indonesia) volcanoes. Both volcanoes display 230Th–238U disequilibria (with aTh enrichment for Stromboli and a U enrichment for Merapi, respectively), strong 226Ra enrichments over 230Th (whereas228Ra–232Th equilibrium is ubiquitous), and a contrasted evolution of their (210Pb=226Ra) activity ratios calculated at thetime of eruption. While these ratios are always close to the equilibrium value of 1 at Stromboli, they show significantvariations at Merapi (from 0.75 to 1). It is suggested that the low (210Pb=226Ra) activity ratios result from continuousdegassing of 222Rn, the precursor of 210Pb in the decay series, from a shallow magma chamber. A model assuminga physical steady-state of the shallow magma reservoir is proposed, linking the (210Pb=226Ra) ratio of the magma tothe fraction of 222Rn lost and to the renewal rate (or residence time) of the magma in the shallow reservoir. The nearequilibrium (210Pb=226Ra) ratios at Stromboli imply a high renewal rate of the shallow reservoir (in the range 0.7–3 yr�1) inagreement with its persistent explosive activity. The volume of the reservoir is estimated at 1:3š 1:2ð 107 m3. In contrast,the variability of (210Pb=226Ra) ratios at Merapi is interpreted by successive phases of nearly closed-system evolution of theshallow magma chamber (with continuous Rn degassing) and episodes of reinjections and mixing of a deep undegassedmagma in this reservoir. Its volume is estimated at 1:6ð 107 m3. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Stromboli; Merapi; radioactivity; equilibrium; magmas; degassing; dynamics; volcanic processes

1. Introduction

During the last decade, numerous data have beenpublished on short-lived radioactive disequilibria inmany volcanoes from various geodynamic settings

Ł Corresponding author. Fax: C1 505 665 3285; E-mail: [email protected] Now at: Los Alamos National Laboratory, EES-1 Group (MS D462), Los Alamos, NM 87545, USA; E-mail: [email protected] Now at: Universite Montpellier 2, ISTEEM (cc 058), Place Eugene Bataillon, 34095 Montpellier Cedex 5, France.

(see the reviews in [1–5] and references therein).These studies mainly deal with 226Ra–230Th disequi-libria but a few additional data on the pair 210Pb–226Ra have also been reported. While 226Ra is in ex-cess over 230Th in almost all cases, the few available

0012-821X/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 1 9 5 - 8

112 P.-J. Gauthier, M. Condomines / Earth and Planetary Science Letters 172 (1999) 111–126

(210Pb=226Ra) activity ratios are usually close to unitywithin analytical uncertainties, which means that, inmost cases, radioactive equilibrium prevails betweenboth nuclides [2,6–9]. However, noticeable excep-tions have been found in some young volcanics. Ex-cept at the Batur volcano where 210Pb enrichmentsover 226Ra have been measured [10] and are proba-bly related to the accumulation of 210Pb-rich plagio-clase phenocrysts, the other cases are characterizedby (210Pb=226Ra) activity ratios lower than unity.Such disequilibria have been evidenced at OldoinyoLengai (with (210Pb=226Ra) of about 0.3, which isthe lowest activity ratio ever measured [11]), at theMerapi volcano [2] and in recent lavas from theVestmannaeyjar volcanic system in Iceland [12], andhave been interpreted as the result of radium en-richment processes, during either magma genesis ordifferentiation, only a few years or decades beforeeruption. Finally, the hypothesis of sustained radondegassing has been evoked by Rubin and Macdougall[13] in order to explain (210Pb=226Ra) activity ratiosclose to 0.85 at the Macdonald seamount.

The absence of 210Pb–226Ra disequilibria in sev-eral persistently active volcanoes exhibiting strong226Ra enrichments over 230Th, unambiguously indi-cates that Ra–Pb fractionation (if instantaneous) isusually older than 130 years, which corresponds tothe time required to reach radioactive equilibrium(TPb D 22:3 yr or ½Pb ³ 3:11 yr�1 where T and½ represent the half-life and the decay constant of210Pb, respectively). It also shows that there is verylittle lead volatilization before and during eruption,which is in agreement with a previous determinationof the lead emanation coefficient lower than 1.5%[14,15]. More importantly, it suggests that the de-gassing of radon, namely its longest-lived isotope222Rn (T D 3:8 d), has little effect on 210Pb–226Radisequilibria. Radon however should be degassedalong with other major volatiles, as shown by mea-surements on freshly erupted basic and intermediatelavas [16,17] or by recent experimental work [18].The absence of disequilibrium between 226Ra and210Pb might result from the constant renewal ofthe degassed magma by a deeper and undegassedmagma in radioactive equilibria. We have thus devel-oped a model of an open-system degassing magmachamber to discuss the evolution of (210Pb=226Ra)ratios through time. This model has been applied

to explain 210Pb–226Ra data obtained at two activevolcanoes, Stromboli (Italy) and Merapi (Indonesia),which exhibit strongly contrasted eruptive behav-iors.

2. Radon degassing model

Because of its extremely low concentration inmagmas, 222Rn needs the presence of another ex-solved gas phase acting as a radon carrier, to beextracted and degassed from a magma batch [18].Accordingly, radon exsolution is closely related tothe exsolution of other major volatiles (CO2, H2O,SO2 : : : ). Due to its much lower solubility in mag-mas [19], CO2 degassing initiates at much greaterdepth than that of both H2O and SO2, and radondegassing should also begin at depth. However, be-cause of its short half-life (T D 3:8 d), 222Rn atomsextracted at such depth are likely to decay in situbefore being flushed out from the magma, so thatthis first exsolution event cannot change significantlythe (210Pb=226Ra) activity ratio of the deep magma.Efficient radon degassing is thus supposed to occuronly at shallower levels, mainly through H2O andSO2 exsolution. Although radon degassing is prob-ably a process occurring over a large depth range,its effects can be modeled through a simple modelof a degassing cell in a magma batch stored eitherin the upper part of the volcano plumbing systemor in a shallow magma chamber. In agreement withprevious physical degassing models [20,21], we as-sume that the deep magma, termed hereafter ‘theundegassed magma’ though already CO2-depleted,is brought by convection with a constant influx �0

into the shallow reservoir, where it exsolves its gaseswith a flux �G (Fig. 1). Assuming that this reservoircontains a constant mass M of liquid magma (phys-ical steady-state), an output �L of degassed lava hastherefore to be either erupted (�E) or intruded (�I)or recycled (�R) into depth, with �0 D �L C �G and�L D �E C �I C �R. If Þ represents the gas fractionultimately released from the magma (usually a fewpercent at most), then �G D Þ�0 and �L D .1�Þ/�0.Degassing is supposed to occur freely, i.e. we assumethat the gases evolve and escape quickly out of themagma either directly into the atmosphere or throughthe permeable conduit walls, which makes the model

P.-J. Gauthier, M. Condomines / Earth and Planetary Science Letters 172 (1999) 111–126 113

Fig. 1. Schematic model of a degassing magma chamber of mass M (constant). � and [Ik] denote fluxes and radionuclide Ik

concentrations, respectively. Indices 0, L, G, E, I, R refer to deep undegassed magma, degassed magma (lava), gas and erupted, intrudedor recycled degassed magma, respectively.

applicable to the cases of open conduit volcanoes aswell as dome-forming volcanoes [21,22].

In order to simplify the model, several assump-tions can be made. We suppose that magma in theshallow reservoir is quickly and thoroughly mixedand homogenized, such that the degassed magmainside the chamber has the same composition aserupted lavas (and intruded or recycled degassedmagmas). Second, although it has been reportedthat some plagioclase phenocrysts may grow withindecades or even days of an eruption [23], thus sug-gesting that fractional crystallization could occur insuch shallow reservoirs, we will neglect this pro-cess in order to decipher only the effect of radondegassing on 226Ra–210Pb disequilibria.

The variation over time of any radionuclide Ik

in the reservoir is described by the following massbalance equation including a term of radioactiveingrowth from its parent Ik�1, a term of radioactivedecay and mass transfer terms:

dIk

dtD ½k�1 Ik�1 � ½k Ik

C �0[Ik]0 � .1� Þ/�0[Ik]L � Þ�0[Ik]G (1)

where ½ are the decay constants of nuclides Ik andIk�1, brackets represent the concentrations in thenon-degassed magma, the degassed lava and the gas(indices 0, L and G, respectively). This equation canbe written in term of activities, denoted by parenthe-ses and expressed in dpm g�1, by multiplying bothterms by ½k=M :

d.Ik/L

dtD ½k.IK /L � ½k.Ik/L

C �0

M.Ik/0 � .1� Þ/

�0

M.Ik/L � Þ

�0

M.Ik/G (2)

In agreement with the assumption of a thoroughmixing, the activities in the reservoir have been noted.Ik�1/L or .Ik/L. Note that �0=M can be defined asthe renewal rate of the shallow magma reservoir,


which is also equal to 1=− , where − is the residencetime of the magma inside the shallow reservoir. Inorder to make the presentation easier, Ra, Rn and Pbwill refer in the following to 226Ra, 222Rn and 210Pb,respectively. Applied to 222Rn, Eq. 2 becomes:

d.Rn/L

dtD ½Rn.Ra/L � ½Rn.Rn/L

C �0

M.Rn/0 � .1� Þ/

�0

M.Rn/L � Þ

�0

M.Rn/G (3)

Because the half-life of 226Ra (TRa D 1600 yr)is considerably longer than the timescale of magmadegassing processes [14,24], we may assume that itsactivity remains constant during magma degassing.Further, since radium is not engaged in volatilecompounds [14], it will be concentrated in the liquiddegassed magma according to: (Ra)L D (Ra)0=(1 �Þ). Finally, assuming that the very short-lived 222Rnis always in radioactive equilibrium with its parentin the deep undegassed magma, we have (Rn)0 D(Ra)0 D (Ra)L (1� Þ). The last term of Eq. 3 can beexpressed as a function of f , the fraction of radonthat ultimately escapes from the magma; f is definedas:

f D Þ�0.Rn/G

[M½Rn C .1C Þ/�0] .Ra/L(4)

where M½Rn.Ra/L represents the 222Rn producedby decay of 226Ra in the shallow magma reser-voir, and �0.1 � Þ/.Ra/L D �0.Ra/0 representsthe 222Rn brought into this reservoir by the in-flux of undegassed magma. Thus, Þ�0=M.Rn/G Df [½Rn C .1� Þ/�0=M].Ra/L.

Therefore, the variation through time of the(Rn=Ra)L ratio can be written:

d�

Rn

Ra

�L

dtD .1� f /

�½Rn C .1� Þ/

�0

M

½

��½Rn C .1� Þ/

�0

M

½ �Rn

Ra

�L

(5)

After integration, and taking into account the factthat (Rn=Ra)0 D 1 before degassing, it yields:�

RnRa

�L

D .1� f /

C f exp²�

�½Rn C .1� Þ/

�0

M

½t

¦(6)

Note that the (Rn=Ra)L ratio will reach a steady-state value in a few days to one month at most(depending on the value of �0=M/, which is equal to(1� f ).

In the same way as in Eq. 3, we have for lead:

d.Pb/L

dtD ½Pb.Rn/L � ½Pb.Pb/L

C �0

M.Pb/0 � .1� Þ/

�0

M.Pb/L � Þ

�0

M.Pb/G (7)

However, because the volatility of lead is rathersmall [14,15], we will assume that lead does notescape from the magma chamber, and neglect thelast term of Eq. 7, which becomes:

d�

PbRa

�L

dtD ½Pb

�Rn

Ra

�L

C .1� Þ/�0

M

�Pb

Ra

�0

��½Pb C .1� Þ/

�0

M

½ �PbRa

�L

(8)

As the steady-state value is quickly reached for(Rn=Ra)L, this ratio will be assumed constant andequal to (1� f ). The variation of (210Pb=226Ra)L ac-tivity ratios in the reservoir is obtained by integrationof Eq. 8:

�Pb

Ra

�L

D.1� Þ/

�0

M

�Pb

Ra

�0

C ½Pb.1� f /

.1� Þ/�0

MC ½Pb

C½Pb

�PbRa

�0

� ½Pb.1� f /

.1� Þ/�0

MC ½Pb

ð exp²�

�½Pb C .1� Þ/

�0

M

½t

¦(9)

If we further assume that 210Pb–226Ra radioactiveequilibrium prevails in the deep undegassed magma((Pb=Ra)0 D 1), equation (9) simply becomes:�

Pb

Ra

�L

D 1� ½Pb f

.1� Þ/�0

MC ½Pb

ð�1� exp

��

�½Pb C .1� Þ/

�0

M

½t

�½(10)


Thus, the (210Pb=226Ra) activity ratios in lavasdepend on several parameters: �0=M the renewalrate of the shallow reservoir (or − D M=�0, theresidence time of the magma inside the chamber);f , the fraction of radon which is expelled from theshallow reservoir; Þ, the proportion of gas releasedfrom this magma batch; and t , the duration of thecontinuous degassing process.

Of particular interest is the simplest case of anon-renewed magma chamber (�0=M D 0). Thisrepresents the case of a magma batch brought closeto the surface and stored in a reservoir which be-haves as a closed system, except for the gas fractionthat continuously escapes from it. The (210Pb=226Ra)activity ratio in the degassed magma thus decreaseswith time according to the following equation:�

PbRa

�L

D 1� f C f exp.�½Pbt/ (11)

This corresponds, for a given fraction of radonlost ( f ), to the maximum 210Pb depletion relativeto 226Ra. The variations of (210Pb=226Ra)L activityratios as a function of the duration of degassing areshown in Fig. 2 and a set of curves is drawn fordifferent values of f . As one should expect, thehigher the loss of radon, the larger the 210Pb–226Radisequilibria. Nevertheless, it is worth noting that,even for low values of f , significant disequilibriamay be created in a few years. Radon degassingappears thus to be a very efficient process to produce210Pb–226Ra disequilibria. For instance, assuming inagreement with Gill et al. [17] that radon completelyescapes from the vesiculating magma, (210Pb=226Ra)activity ratios as low as 0.75 are reached after only 9years of closed-system degassing.

However, the model of a continuously renewedmagma chamber is probably more likely in the caseof active volcanoes characterized by a persistentdegassing [20,21,24]. The evolution with time of(210Pb=226Ra)L ratios for various values of the re-newal rate �0=M is plotted in Fig. 3. Þ has beenfixed at 0.03, but its exact value has little influenceon the curves. Note that curves are drawn on the ba-sis of a complete radon loss ( f D 1) but incompletedegassing would simply shift these curves upwardsand, hence, would yield activity ratios closer to unity.Compared to the closed system model, the effect ofradon degassing on Ra–Pb fractionation is lowered

for a given t , and if the renewal rate is high enough,(210Pb=226Ra)L activity ratios remain close to unity.Practically, this is achieved when the renewal rateof the reservoir becomes considerably higher than½Pb. For values of �0=M higher than 3 yr�1 (i.e., forresidence times shorter than 4 months), 226Ra–210Pbequilibrium will be preserved in erupted productsdespite sustained radon degassing.

If the degassing magma chamber has beenfunctioning for a sufficiently long time, the(210Pb=226Ra)L ratio will reach a steady-state value.In a closed system, this steady-state value is simply1 � f (Eq. 11), and in the open system model, it isgiven by Eq. 10:

�Pb

Ra

�L

D.1� Þ/

�0

MC ½Pb.1� f /

.1� Þ/�0

MC ½Pb

(12)

Thus, the steady-state (210Pb=226Ra)L activity ra-tios in the degassed magma only depend on themagma renewal rate (�0=M), which can be ac-curately determined, for a given radon loss ( f )and a given fraction of volatiles released from themagma (Þ, here again fixed at 0.03), when significantdisequilibria ((210Pb=226Ra)L <0.95) are measured(Fig. 4). The time tŁ necessary to reach a steady-state value can be inferred from Eqs. 10 and 11 foropen and closed systems, respectively: it is the timefor which the exponential term becomes negligiblecompared to 1. Thus tŁ should be much larger than−=.1� Þ C ½Pb−/ or 1=½Pb for open and closed sys-tems, respectively, where − is the residence time ofthe magma in the reservoir. As shown by Fig. 3, theshorter the residence time (or the higher the renewalrate), the smaller the value of tŁ.

3. Geological setting and sampling

Stromboli is the northernmost and one of thetwo presently active volcanoes of the AeolianArchipelago, an active volcanic arc lying on thefloor of the Tyrrhenian Sea, to the north of Sicily.Stromboli is well known to have been continuouslyerupting over the past two millennia. Its volcanicactivity is mainly characterized by permanent de-gassing at its three summit craters, associated with


Fig. 2. Evolution of (210Pb=226Ra) activity ratios in a non-renewed magma chamber (�0=M D 0) as a function of the duration of the de-gassing process t . Curves are drawn from Eq. 11 for different values of f , the fraction of radon lost from the magma (figures on curves).

frequent (6–8 events per hour on average) mildexplosions (strombolian activity). During historicaltimes, such a quiet activity has been interrupted atintervals of years or decades by more violent parox-ysmic events and lava flows that have been canalizedin a deep depression on the northwestern flank ofthe volcano, the Sciara del Fuoco [25]. Present-daylavas have a rather uniform chemical composition ofshoshonitic basalts with a phenocryst (plagioclase–clinopyroxene–olivine) content of about 35–40%[26]. Samples of the main effusive events occur-ring during this century (1955, 1967, 1975, 1985 anda lava flow of unknown age which could be that of1939, according to the description by Abbruzzese

[27]) were collected in 1996 at the foot of the Sciaradel Fuoco. In addition, a fresh lava bomb emittedin October 1996 was gathered on the rim of thenortheast summit crater, just after its fallout.

The Merapi volcano, a 60,000 years old stratovol-cano located on Java Island (Indonesia), is one of themost active volcanoes of the Sunda–Banda arc [28].Over the last two — and probably six — centuries,its volcanic activity has been mainly characterized bya cyclic succession of lava dome extrusion episodesinterrupted by brief destructive events generating py-roclastic flows [28]. The Merapi activity and themorphological evolution of its lava dome during thetwo past decades have been especially well docu-


Fig. 3. Evolution of (210Pb=226Ra) activity ratios in a renewed magma chamber as a function of the duration of the degassing process t .Curves are drawn from Eq. 10 for different values of the renewal rate �0=M (figures on curves), with Þ D 0:03 (Þ is the weight fractionof volatiles released from the magma) and f D 1 (complete radon degassing).

mented ([28–30] and references therein). Accordingto these authors, several lava domes have been em-placed within the summit crater that formed in 1961,and distinct major growth episodes, ending by par-tial collapse of the newly formed lava dome, havebeen recognized in 1973–79, 1979–84, 1984–92 andsince 1992. All recent erupted products display arather uniform chemical composition of basaltic an-desites with a crystallinity in the range 30–60%(plagioclase, clinopyroxene and magnetite with rareorthopyroxene, olivine and amphibole [28,31]). Oursamples were collected on the active (i.e., growing)lava dome in 1987, 1992, 1993, 1994 and 1995

and two blocks were also sampled in the 1984’sblock-and-ash flow canalized in the Putih river bed(MP 84 Þ and þ).

4. Analytical techniques

Whole-rock samples were first crushed in a steelchewing-crusher and then in a tungsten-carbide ring-crusher until the powder size was less than 100μm. Several aliquots of this powder are used forthe measurement of U- and Th-series radioactivedisequilibria and trace element contents.


Fig. 4. Evolution of steady-state (210Pb=226Ra) activity ratios as a function of the renewal rate �0=M. Curves are drawn from Eq. 12 fordifferent values of f (figures on curves), with Þ D 0:03.

230Th–238U disequilibria have been measuredthrough a two-step procedure: (1) the measurementof U and Th contents, after separation and purifi-cation of both elements on AG1-X8 anionic resin,through isotope dilution mass spectrometry, and (2)the measurement of (230Th=232Th) activity ratio, afterTh purification and electrodeposition on a stainless-steel disc, through α-spectrometry. Further detailson both chemical procedures may be found in [32].Estimated errors (2σ confidence level) are of about0.5% on U and Th contents and 2% on (230Th=232Th)activity ratios (derived from counting statistics).

Activities of other radionuclides (226Ra, 228Raand 210Pb) are measured by non-destructive gamma-

ray spectrometry using the double detector gamma-ray spectrometer described by Condomines et al.[9]. The determination of radionuclide activities isachieved by comparison with a standard volcanicrock (sample 1536 in [9]) according to the proceduredetailed by these authors. Analytical uncertaintiesderived from counting statistics are usually betterthan 2% at the 2¦ confidence level.

Finally, Ba and Pb are measured from the samesplit of powder by isotope dilution ICP–MS. Afteracid digestion of the sample in a HF-HNO3 mixture,the solution is evaporated and dissolved twice in 6N HCl. An aliquot of this solution is spiked with135Ba- and 206Pb-enriched tracers. After isotopic ho-


mogenization, this solution is evaporated to dryness,dissolved in 3 N HNO3 and then passed through aSr-SPEC resin which retains both barium and lead.After washing with 3 N HNO3, Ba is first eluted with0.05 N HNO3, then Pb with 6 N HCl. Measurementsthemselves are performed on a Fisons VG PQ2CICP–MS and 2σ errors are estimated to be lowerthan 1%. Further details on this chemical procedurewill be published elsewhere (Pin et al., in prep.).

5. Results and preliminary discussion

The analytical results for the whole suite of sam-ples from both the Stromboli and Merapi volcanoesare reported in Table 1.

5.1. Stromboli

All our samples display a uniform compositionof shoshonitic basalts. U and Th contents show verysmall variations (less than 5%), respectively from3.90 and 14.94 ppm in the 1996 lava bomb to 4.08and 15.59 ppm in the 1967 lava flow. Th=U ratios inlavas and scoriae erupted during this century are re-markably constant (3:83š0:01), as are (230Th=232Th)and (230Th=238U) activity ratios with mean values of0:900š 0:008 and 1:135 š 0:010, respectively.

226Ra is always enriched over 230Th, and activityratios for all samples, except the 1939 lava flow,are constant with a mean value of 2:45 š 0:05,which is consistent with data previously reportedby Capaldi et al. [33,34]. The higher (226Ra=230Th)ratio (2.64) in the 1939 lava is not the result ofcrystal fractionation or accumulation, because the(226Ra=Ba) ratio is also higher .8:55ð10�3 dpm=μg)than the constant value of the other samples (7:96 š0:11 ð 10�3 dpm=μg). This suggests an anomalousradium enrichment for this sample by some kind ofcontamination process, as observed in some recentlavas from Mt. Etna [8,9]. It is worth noting that228Ra is always found in equilibrium with 232Th,even in the most recent samples (1985 and 1996),as in all other volcanoes except the Oldonyo Lengai[11]. This constrains the Ra–Th fractionation, ifinstantaneous, to be between 30 and 8000 years.

Measured (210Pb=226Ra) ratios are always veryclose to 1, within analytical uncertainties, with the

exception of the 1975 lava flow which has a ra-tio of 0.80. Initial (210Pb=226Ra)0 activity ratios atthe time of eruption are thus difficult to ascertainfor the oldest samples, but the equilibrium valuesmeasured in the most recent ones (1985 and 1996)suggest that the initial ratios of older samples werealso close to 1. The initial ratio of 0.60 for the 1975sample is the result of a lead loss of about 40%,as emphasized by its much lower Pb content (11.5ppm) compared to that of the other samples (17.3to 19.9 ppm), and its (210Pb)0=Pb ratio, which hasthe same value as in the other samples (note thatthe inferred high (210Pb)0=Pb ratio of the 1939 sam-ple is due to its anomalous Ra enrichment). Suchan important loss cannot be easily reconciled withprimary magmatic processes and it is likely that itreflects an additional process. A possible explanationis that lead removal occurred when the lava flowedon the Sciara del Fuoco and reached the sea, throughsome kind of hydrothermal alteration by seawater ofthe still hot lava flow, as previously observed duringsea-floor eruptions ([35] and references therein). Pet-rographic evidence for this process might be attestedby the transformation of olivine into iddingsite, aphenomenon which is not observed in the othersamples. Alternatively, it is also possible that somekind of interaction between the magma and seawateroccurred inside the volcanic edifice, leading to anenrichment in Cl of the magma and to an anoma-lously high degassing of lead chlorides. Nonetheless,the overall absence of radioactive disequilibrium be-tween Pb and Ra in the other samples indicatesthat no recent (younger than 130 years) fraction-ation occurred between both nuclides. Especially,although Stromboli magmas have undergone crystal-lization less than a century before an eruption, whichis attested by significant Ra–Pb disequilibria in allmineral phases [36], crystal fractionation in the up-per plumbing system appears to have little effect onthe (210Pb=226Ra) ratio of the magma. This also con-firms that 210Pb loss during degassing is negligible,which is in good agreement with the low emanationcoefficients measured at Stromboli [24].

5.2. Merapi

High-K basaltic andesites that formed the succes-sive lava domes emplaced within the summit crater

120P.-J.G

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/Earth

andP

lanetaryScience

Letters172

(1999)111–126

Table 1U- and Th-series disequilibria and trace element content in recent lavas from Stromboli and Merapi volcanoes

Sample Date of Date of SiO2 K2O U Th Ba Pb Th=U (230Th=232Th) (230Th=238U) (226Ra) (226Ra)=Ba (226Ra=230Th) (228Ra=232Th) (210Pb=226Ra) (210Pb=226Ra)0 (210Pb)0=Pberuption analysis (%) (%) (ppm) (ppm) (ppm) (ppm) (dpm=g) (dpm=μg) (dpm=μg)

ð 103

StromboliSTR X jan-39 sep-96 51.1 2.10 3.95 15.12 1027 18.3 3.83 0.900 (14) 1.136 (17) 8.78 (14) 8.55 2.64 (6) 0.99 (3) 1.01 (4) – 0.485STR 55 mrt-55 sep-96 51.1 2.06 4.04 15.44 1033 18.3 3.83 0.889 (13) 1.121 (16) 8.28 (13) 8.02 2.47 (6) 1.00 (3) 0.97 (4) – 0.439STR 67 apr-67 mei-97 50.5 2.24 4.08 15.59 1073 19.9 3.82 0.907 (13) 1.141 (16) 8.71 (14) 8.12 2.52 (6) 0.97 (3) 1.02 (4) – 0.446STR 75 nov-75 mrt-97 50.6 2.21 4.03 15.48 1063 11.5 3.84 0.910 (13) 1.151 (17) 8.40 (13) 7.90 2.45 (6) 0.98 (3) 0.80 (3) 0.60 (6) 0.438STR 85 dec-85 okt-96 51.0 2.13 4.02 15.36 1032 18.0 3.82 0.898 (11) 1.130 (15) 8.06 (12) 7.81 2.40 (5) 0.99 (3) 0.97 (4) 0.96 (6) 0.430STR 96 okt-96 feb-97 50.6 2.21 3.90 14.94 987 17.3 3.83 0.896 (12) 1.130 (15) 7.85 (12) 7.95 2.40 (6) 0.97 (3) 0.97 (4) 0.97 (4) 0.440

MerapiJPM2 a 1981 1981 55.6 2.18 1.69 7.46 534 13 4.41 0.654 (10) 0.952 (45) 3.58 (29) 6.70 2.98 (12) – 0.81 (8) 0.81 (8) 0.223MP 84Þ jun-84 sep-97 56.0 2.38 – – 522 265 – – – 3.49 (6) 6.69 – – 9.47 (35) 13.69 (58) 0.180MP 84þ jun-84 sep-97 55.7 2.37 1.57 7.02 522 17.5 4.46 0.661 (10) 0.971 (14) 3.47 (6) 6.65 3.05 (7) 0.99 (4) 1.00 (4) 1.00 (6) 0.198MP 87 jul-87 apr-97 56.4 2.14 1.54 6.87 518 16.5 4.45 0.659 (10) 0.966 (15) 3.44 (6) 6.64 3.10 (8) 0.99 (4) 0.88 (4) 0.84 (5) 0.175MP 92 sep-92 dec-92 56.0 2.23 1.56 6.95 535 16.7 4.44 0.652 (10) 0.954 (14) 3.42 (7) 6.39 3.10 (9) 0.97 (4) 0.75 (5) 0.75 (5) 0.154MP 93 apr-93 feb-97 56.4 2.14 1.58 7.05 526 18.5 4.45 0.655 (12) 0.961 (15) 3.53 (6) 6.71 3.14 (8) 1.00 (4) 0.90 (4) 0.89 (4) 0.170MP 94 okt-94 jun-97 56.5 2.10 1.58 7.02 525 18.8 4.45 0.661 (11) 0.969 (14) 3.50 (6) 6.67 3.09 (8) 0.98 (4) 0.92 (4) 0.91 (4) 0.169MP 95 sep-95 dec-95 56.6 2.10 1.59 7.05 525 19.9 4.44 0.666 (11) 0.975 (14) 3.51 (6) 6.69 3.06 (7) 0.98 (4) 1.00 (4) 1.00 (4) 0.176

a Sample JPM-2 from Gill and Williams [2].– D not determined.Major oxides (in wt.% on the basis of 100% water-free) are measured by ICP–AES (2σ error of about 5%). U and Th contents are measured by ID–TIMS (2σ error of about 0.5%). Ba and Pb are analyzedby ID–ICP–MS with a 2σ error of about 1%. The accuracy of these measurements was checked on two international standards (recommended values for AGV-1: Ba D 1226 ppm, Pb D 36 ppm; for RGM-1:Ba D 807 ppm, Pb D 24 ppm; measured values for AGV-1: BaD 1215 ppm, Pb D 37 ppm; for RGM-1: Ba D 817 ppm, Pb D 24 ppm). All reported errors (between parentheses) are 2σ uncertainties on thelast significant figures. Reported (210Pb=226Ra)0 activity ratios are calculated at the time of eruption. Precise initial ratios cannot be calculated for the oldest samples close to equilibrium. For these samples,(210Pb)0=Pb ratios have been calculated using the present-day (210Pb) activity (figures in italic).


since 1984 have a very uniform chemical composi-tion, similar to that of older products erupted duringthe modern period [28,31]. Both U and Th contentsexhibit very small variations (less than 3%), suggest-ing that recent lavas have a nearly constant degreeof differentiation. As for Stromboli, Th=U ratiosand both (230Th=232Th) and (230Th=238U) are remark-ably constant with average values of 4:45 š 0:01,0:659š0:005, and 0:966š0:008, respectively. Suchvalues agree with previous data published on historicMerapi products [2,37,38].

In all our samples, 226Ra appears to be consider-ably (more than 200%) enriched over 230Th, a patternthat was previously observed by Gill and Williams[2] in a sample from the 1981 lava dome and byCondomines et al. [39] on historical Merapi lavas.(226Ra=230Th) activity ratios remain nearly constantwith an average value of 3:09š 0:03, whereas 228Raand 232Th are always found in radioactive equilib-rium, constraining the age of Ra–Th fractionation, ifinstantaneous, between 30 and 8000 yr. On the basisof their observed (210Pb=226Ra) ratio lower than 1 inthe 1981 andesite, Gill and Williams [2] proposed amodel of instantaneous or continuous radium enrich-ment in the magma chamber, which allowed them tocalculate the age of this event (or its beginning) byassuming an initial (210Pb=226Ra) activity ratio of 1.These models cannot account for the irregular varia-tions of the (210Pb=226Ra) ratios observed here, whilethe (226Ra=230Th) ratios remain constant. Obviously,(210Pb=226Ra) variations result from another process.And 226Ra–230Th disequilibria are probably inheritedfrom fractionations older than 130 years, occurringat a deeper level in the magma, either during partialmelting or during magma evolution in the crust (seefor example [1,2]).

Whereas all recent products display (210Pb=226Ra)0 activity ratios lower than unity (ranging be-tween 0.75 and 1), the sample MP 84Þ, a block fromthe 1984 nuee ardente deposit, has an exceptionallead enrichment (Pb D 265 ppm; (210Pb) D 47.78dpm g�1). Its (210Pb=226Ra)0 activity ratio of 13.7 isthe highest ratio ever measured in a volcanic rock.It is clear that such a Pb (and 210Pb) enrichmentcannot be accounted for by any crystal fractionationor accumulation process. The most likely expla-nation is that this sample has been contaminatedby lead sublimates. The existence of such com-

pounds, either sulfides of compositions ranging be-tween galena (PbS) and galenabismuthite (PbBi2S4),or chlorides (KPbCl3), has been reported at Merapi[40,41]. Assuming that the uncontaminated rock hasthe same Pb content as the MP 84þ sample, theweight fraction of the contaminating Pb salt (assum-ing it is KPbCl3) can be calculated at 4:2 ð 10�4.Its (210Pb)=Pb ratio can then be calculated: not sur-prisingly, it is very close to the value of the bulkrock (0.179), since most Pb comes from the Pb sub-limate. The lower value compared to that of the MP84þ sample could be due to an age effect in the Pbsublimate. If this sublimate had the same ratio as theuncontaminated rock (0.198) at the time of its forma-tion, it would take 3.2 years to get the observed ratioof 0.179 by simple 210Pb decay. Note that a similarresult is obtained if we assume that the contaminantis a lead sulfide. The MP 84Þ sample might thereforecorrespond either to a fragment of an older domewith its lead sublimates, emplaced 3 years beforeand incorporated in the 1984 block-and-ash flow, orto a juvenile magma fragment that assimilated anold lead sublimate during the eruption. In any case,it is worth mentioning that there is no petrographicevidence for such an assimilation of lead sublimates,which is not surprising in view of the very lowproportion of sublimates involved in this process.This emphasizes the need for extreme caution whendiscussing 210Pb–226Ra radioactive disequilibria.

6. Implications for magma dynamics and volumebeneath Stromboli and Merapi volcanoes

The (210Pb=226Ra) variations observed at Strom-boli and Merapi are now interpreted using the radondegassing model presented in Section 2.

6.1. Stromboli

The results obtained on recent unaltered lavaflows suggest that the (210Pb=226Ra) ratios havealways remained close to 1. In view of the per-sistent activity and homogeneity of the eruptedproducts, Stromboli can be considered as a steady-state degassing magmatic system. The steady-state(210Pb=226Ra) ratio only slightly lower than 1, de-spite continuous radon degassing, is easily explained


by a high renewal rate (or short residence time) ofthe magma in the reservoir. This can be estimated ifsome assumptions are made about the possible val-ues of f , the fraction of 222Rn lost from the magma.In basaltic magmas, Rn is very efficiently extractedby the degassing of major volatiles and f should bearound 1 [17,18]. However, during the rise of gasbubbles in the reservoir, some radon atoms could de-cay, owing to its very short half-life, to 210Pb beforebeing lost to the atmosphere and this must be takeninto account in estimating the bulk f . The escap-ing time � of gases at Stromboli, estimated throughradon daughters disequilibria in the gaseous phase[24], is usually shorter than a few hours or half a dayat most, which would enable about 10% radon decay.Thus, f could be reasonably estimated between 0.9and 1. Assuming that the steady-state (210Pb=226Ra)activity ratio in Stromboli lavas is between 0.96 and0.99, �0=M is computed from Eq. 12 (with Þ D 3%[24]) between 0.7 and 3.2 yr�1 and the residencetime − ranges between 110 and 520 days, whichis of the same order of magnitude as the residencetime derived from 210Pb–210Bi–210Po disequilibria inStromboli gases and aerosols (20–200 days [24]).This suggests that the model of a steady-state shal-low degassing reservoir closely approximates theactual subvolcanic system and gives confidence inthe calculated residence time. With such a short res-idence time, the steady-state will be attained after atime tŁ × − , i.e. in a few years, which is shorterthan the time elapsed between two lava eruptions[25]. Thus, our results do not rule out the possibilitythat Stromboli may diverge from steady-state overshort periods between effusive events. It is worthnoting that our results are significantly lower thanthose found at Stromboli as well as at other basalticvolcanoes, which are usually of the order of a fewdecades [9,42,43]. A possible explanation would bethat these authors studied geochemical variations oc-curring in a deeper and larger, and consequently,more slowly renewed magma chamber, while ourstudy only regards the upper part of the plumbingsystem where degassing takes place. The volume Vof this shallow reservoir may be estimated throughthe relation: V D −ð�0, where �0 is the flux of deepundegassed magma entering the system. Both sulfurbudgets [44] and thermal measurements [45] providean estimate of the magma supply rate at Stromboli

in the range 10,000–50,000 m3=day, which leads toa volume of about 1:3š 1:2ð 107 m3. Although thisestimate is given with a large uncertainty, mainlydue to the near equilibrium value of activity ratios,it is consistent with the volume inferred from short-lived disequilibria in the gaseous plume of Stromboli[24]. Furthermore, it is worth noting that this figuremust be taken as an upper limit for the size of thereservoir in so far as some of the small 210Pb deficitcould be also explained by lead degassing. In thatcase, it would suggest that radon degassing has nosignificant effect on Ra–Pb disequilibria and, conse-quently, that − is shorter than 110 days, strengtheningthe agreement between both estimates.

6.2. Merapi

The situation at Merapi volcano appears to bedifferent and more complex than at Stromboli, asemphasized by the large variations of (210Pb=226Ra)initial activity ratios recorded in lavas emplaced dur-ing the two past dome growth episodes. From 1984to 1992, (210Pb=226Ra) ratios decrease from unity to0.75, then they progressively increase to reach againan equilibrium value in 1995 (Fig. 5). Whereas itis obvious that a steady-state model cannot be ap-plied in the Merapi case, the lowering of activityratios between 1984 and 1992 because of continuousradon degassing remains conceivable. Indeed, thePb content of these lavas remains nearly constant,and only 210Pb is decreasing (Table 1). No data onthe radon release efficiency at Merapi are availablebut, by analogy with Arenal basaltic andesites [17],Merapi lavas should also completely degas radon.Accordingly, we assess f to be close to unity, fur-ther assuming that radon decay during gas exsolutionto the atmosphere is negligible as at Stromboli. Asshown in Fig. 5, the progressive 210Pb depletion ob-served between 1984 and 1992 could be modeledby 9 years of more or less closed-system degassing(Eq. 11, with f D 1), this processes initiating shortlybefore the beginning of the dome growth episode in1984. It is worth mentioning that the lowering ofthe (210Pb=226Ra) ratios cannot be due to continu-ous radon loss from solidified rocks of the degassedlava dome, which might have been emplaced severalyears before sampling. Recent experimental work[18] shows indeed that radon does not escape from


Fig. 5. Time-dependent variations of (210Pb=226Ra)0 activity ratios in Merapi lavas as a function of the magma dynamics (diamond forsample JPM-2 from [2]). Error bars are at the 2¦ confidence level. Curve a is drawn from Eq. 11 with f D 1 (complete radon degassing).Curve b is drawn from Eq. 13 with (210Pb=226Ra)i D 1, (210Pb=226Ra)0 D 0.75 and �0=M D 0:5 yr�1. Upward arrows indicate thebeginning of a lava dome growth episode. Years during which pyroclastic eruptions have occurred are underlined. Also shown is thecumulative volume of erupted lavas between 1981 and 1995 (thick curve; from [30,48]).

the solid degassed andesites from the Merapi lavadome, even at high temperatures (�1000ºC).

Regarding the post-1992 increase of (210Pb=226Ra) activity ratios to unity, it is clear that thereturn towards equilibrium cannot be explained by aradioactive evolution over such a short time interval.Therefore, we propose that a new undegassed magmawas injected into the shallow reservoir in 1992and progressively mixed with the resident degassedmagma. The same phenomenon could also have oc-curred between 1981 and 1984, as already stated byBerthommier et al. [46] on the basis of petrologic ar-guments, and would thus explain the sudden increaseof (210Pb=226Ra) activity ratios. However, it is worthnoting that no significant variations of U-series dise-quilibria or U and Th contents are detectable beforeand after magma injection. These observations arguefor the presence of a deeper magma chamber, wheremagmas could differentiate and crystallize, beforefeeding the upper degassing reservoir. The modelof a progressive injection and instantaneous mixingof a new magma in a reservoir of constant volume(physical steady-state) has been treated for example

by Condomines et al. [32] and recently applied toStromboli by Francalanci et al. [43]. The variation ofthe concentration of a trace element C through timeis given by:

C � Ci D .C0 � Ci /ð exp��0

Mt

�(13)

where C0 and Ci are the initial concentration of themagma in the reservoir and the concentration of thenew injected magma, respectively. �0 is the input fluxof magma and M the mass of magma in the reservoir.A plot of ln (C � Ci ) versus time should give astraight line of slope �0=M (i.e. the renewal rate,equal to 1=− ). Applying the same model to 210Pb andassuming that (226Ra)i D (226Ra)0, it can be easilydemonstrated that a plot of ln [(210Pb=226Ra)�1]should give a straight line of slope ½PbC�0=M , fromwhich the residence time can be calculated. Althoughonly three values of (210Pb=226Ra) ratios can be used(1992, 1993 and 1994), they allow a rough estimateof − at about 2 years. Assuming that the magmainflux was equal to the average production rate oflava at the surface between 1992 and 1993 (8 ð 106


m3 yr�1 [30]), the volume of the shallow reservoircan be thus estimated at around 1:6ð 107 m3.

The present-day activity at the Merapi volcano, atleast for the two past decades, could thus be con-trolled by successive injections of deep undegassedmagma of constant composition into the upper partof the edifice, separated by short periods of moreor less closed-system degassing. It is worth notingthat reinjections pointed out in 1984 and 1992 co-incide with two major pyroclastic eruptions which,in both cases, almost completely destroyed the oldlava dome and marked the beginning of a new domegrowth cycle [28,29]. The most destructive eventscould thus be triggered by an important influx ofdeep undegassed magma towards the surface. Thishypothesis is further supported by the record of bothvolcanotectonic seismic signals prior to the 1984and 1992 eruptions [29], and volcanomagnetic sig-nals prior to the 1992 eruption [47], which could berelated to the upward propagation of deep magma.On the other hand, two other pyroclastic eruptionsoccurred in 1986 and 1994, i.e. two years afterthe domes began to grow, when there was neitherevidence of changes in the magma dynamics norgeophysical precursor [29,47]. These events mightthus be unrelated to internal processes and couldmerely result from a gravitational collapse of thelava dome. Furthermore, the curve showing cumula-tive volumes of erupted lavas (Fig. 5) indicates thatthe eruption rate increased from an average value of0:8 ð 106 m3=yr during the period 1984–1992, toa much higher value of about 5 ð 106 m3=yr since1992 [30,48]. This large difference correlates withthe periods of contrasted magma dynamics, i.e. withthe difference in the renewal rate of the shallowmagma chamber, inferred from 210Pb–226Ra disequi-libria. This suggests that the shallow level magmadynamics controls the volcano eruptive (explosiveand effusive) behavior.

7. Conclusion

In order to interpret the 210Pb–226Ra disequilib-ria observed at the Stromboli and Merapi volcanoes,we have developed a simple model showing the ef-fects of the degassing of 222Rn, precursor of 210Pbin the decay chain. It shows that radon degassing,

even incomplete, may lead to significant 210Pb de-pletion relative to 226Ra in a few years only, exceptwhen the renewal rate of the chamber is consid-erably higher than the 210Pb decay constant, i.e.,higher than 3 yr�1. Our study shows that, in orderto interpret 210Pb data, accurate measurements of Pbare also necessary, because they allow one to dis-tinguish between the effects of radon degassing andother magmatic or post-eruptive processes (e.g., leadloss by volatilization or hydrothermal alteration, orcontamination by lead sublimates).

The constant near-equilibrium (210Pb=226Ra) ac-tivity ratios measured at Stromboli may be accountedfor by a steady-state model involving a high renewalrate of the magma chamber (in the range 0.7–3yr�1) or, in other words, short residence times of themagma inside the chamber (<520 days). Estimatesof both the magma residence time in the chamberand the deep undegassed magma supply suggest amagma chamber volume of 1:3š 1:2ð 107 m3.

In contrast, the 25% variability of (210Pb=226Ra)activity ratios at Merapi may be explained by a suc-cession of short periods (<10 years) of closed-sys-tem evolution and continuous degassing inducing aprogressive 210Pb depletion in erupted products, andepisodes of reinjection of deep undegassed magma inradioactive equilibrium that mixes with the residentdegassed magma in the chamber. Using a simplereinjection model, we calculate a magma residencetime of about 2 years during such a reinjection period(1992–1995), and a volume for the shallow magmareservoir of the order of 1:6 ð 107 m3. The eruptivebehavior of the volcano appears to be controlled bythe shallow-level magma dynamics. Major explosiveeruptions (1984, 1992) might indeed be triggered bythe injection of deep undegassed magma into theshallow reservoir, and the lava production rate at thesurface strongly depends on the renewal rate of theshallow magma reservoir.

Our results emphasize the interest of short-lived radioactive disequilibria measurements infreshly erupted lavas. Studying the evolution of(210Pb=226Ra) activity ratios offers a powerful meansto infer shallow level magma dynamics at activevolcanoes. It allows determination of such essen-tial parameters as the renewal rates or residencetimes of the magmas, which can be highly relevantfor predicting the eruptive behavior of the volcano.


However, further studies are required to assess thepossible influence of other magmatic processes (e.g.crystal fractionation or accumulation), besides thatof radon degassing, on the 210Pb–226Ra disequilibria.

Acknowledgements

This work was undertaken in the course of astudy on short-lived disequilibria in active volca-noes, supported by the European Community (Vol-canology Program ‘Pre-eruptive processes: model-ing and parametrization’, Contract No. ENV4-CT96-0259 coordinated by J. Marti). MC is grateful to C.J.Allegre for a fruitful and encouraging discussion onthe problem of radon degassing. We are grateful toA. Finizola and M. Dejean for their invaluable helpduring field trips at Stromboli. G. Camus, M.-F. LeCloarec, F. Lecuyer and J. Guilbert provided some ofthe Merapi samples and are warmly acknowledged.We wish to thank W.S. Tjetjep, R. Sukhyar, M.A.Purbawinata, the Merapi Volcano Observatory staffand M. Dejean for their efficient support and kindhospitality during a sampling campaign at Merapivolcano. We are indebted to Ch. Pin and C. De-niel for their help with lead chemistry and to S.Joannon for ICP–MS analyses. PJG acknowledgesthe ‘Societe de Secours des Amis des Sciences’ fora financial support during part of this research. T.Elliott, J. Gill and B. Villemant are gratefully ac-knowledged for their detailed and careful reviews ofthe manuscript. [CL]

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Documents

210Pb–226Ra radioactive disequilibria in recent lavas and radon degassing: inferences on the magma chamber dynamics at Stromboli and Merapi volcanoes