doi:10.1016/S0016-7037(03)00236-9

226Ra-excess during the 1631-1944 activity period of Vesuvius (Italy):A model of alpha-recoil enrichment in a metasomatized mantle and implications on the current state of the

magmatic system

MARIO VOLTAGGIO,1,* M ARIL I BRANCA,1 DARIO TEDESCO,1,2 PAOLA TUCCIMEI,3 and LAURA DI PIETRO4

1Istituto di Geologia Ambientale e Geoingegneria, CNR, Rome, Italy2Dipartimento di Scienze Ambientali, Universita` Napoli 2, Caserta, Italy3Dipartimento di Scienze Geologiche, Universita` Roma Tre, Rome, Italy

4Dipartimento di Scienze della Terra, Universita` “La Sapienza,” Rome, Italy

(Received October28, 2002;accepted in revised form March12, 2003)

Abstract—The origin of the226Ra-excess during the last cycle of Vesuvius activity was investigated byhigh-resolution�-spectrometry, TIMS and EDXRF. Lavas display high initial226Ra-excess (500–1000%),similar (230Th/232Th) activity ratios (0.87–0.91) and most samples show significant238U-excess. During theperiod 1631–1944 the initial absolute226Ra-excess reached the highest values (19–44 dpm g�1) recorded forearth volcanoes. Crystal fractionation and particularly leucite floating did not cause the226Ra-excess in spiteof the high226Ra activity (21–85 dpm g�1) in leucite. The presence of phlogopite in the mantle source,documented by field and petrological evidences on local mantle-derived xenoliths, rules out that equilibriumpartial melting can be responsible for the226Ra-excess. This primary feature may be explained by a multistageprocess involving metasomatic mantle fluids (MMFs) flowing through a mantle wedge where U is concen-trated as U-accessory minerals deposited along microfractures. Fluids, passing through the mantle wedge, aresupplied of226Ra,230Th and234U by �-decay recoil of parent nuclides from U-enriched microfractures. Thismodel calculates that the ascent time of fluids through the mantle wedge was� 12 ka. Successively MMFsmixed with mantle-derived melts, giving rise to226Ra-enriched magmas, which entered the Vesuviusplumbing system less than 7 ka. Crystal fractionation did not affect extensively the initial226Ra/Ba ratio,which varied in the 1631–1944 period according to a pattern reflecting periodic inputs of226Ra-enrichedmagma, variable reservoir volumes and residence times in magmatic chamber(s). The temporal trend of thereservoir volumes, extrapolated to the present time, indicates a volume of magma of� 0.021 km3, stored mostprobably in a shallow chamber.Copyright © 2004 Elsevier Ltd

1. INTRODUCTION

The purpose of this study is to investigate the origin of large226Ra-excesses in Vesuvius magma during the ‘recent histori-cal activity’, RHA (1631–1944). The Somma-Vesuvius, ayoung cone (Vesuvius) inside an older structure (Mt. SommaCaldera), began its activity 25 ka ago. The cone, built up afterthe A.D. 79 Pompeii eruption, grew during the RHA after a 500yr-long quiescence period (Rolandi et al., 1993). Since thestudy area is one of the most dangerous of the world, with�1,000,000 people at risk, this work was oriented to find a linkbetween temporal226Ra variations in magmas and Vesuviusdynamics.

The 226Ra-excess is defined as the226Ra activity (�226 �4.332 10�4a�1) in percent units, exceeding the activity of itsparent,230Th (�230 � 9.195 10�6 a�1). In a closed system,initial activity ratio (226Ra/230Th)o, relaxes towards unity on atime that depends on�226

226Raexcess� ��226Ra/230Th�t � 1� � 100� ��226Ra/230Th)o�1]

� exp� � �226t� � 100 (1)

wheret stands for present time.226Raexcesshas been observedin MORBs, OIBs and subduction-related IABs, rarely exceed-

ing a value of 300% (Gill and Williams, 1990; Gill and Con-domines, 1992; Gill et al., 1992). During the RHA, Vesuviusdisplayed a226Raexcessup to 1030%, second only to the Oldo-inyo Lengai carbonatites (8200%), although Vesuvius cumu-lates exhibit a226Raexcess 160,000% ( Capaldi et al., 1982;Williams et al., 1986).

To correlate226Ra with Ra-proxy elements (Ba, K), we usethe initial absolute226Ra-excess, i.e., the fraction of226Ra notsupported initially by230Th, given by the difference betweeninitial 226Ra and230Th activities, expressed in dpm g�1:

�226Raexcess�o � �226Ra)o � �230Th�o (2)

Physical-chemical processes fractionate (226Ra) from (230Th)causing variations of their ratio in magma. For example, whensanidine or phlogopite dominate fractional crystallization, re-sidual melts become226Ra and Ba-depleted, Ra and Ba substi-tuting for K in K-minerals. At Vesuvius, before the Pompeiieruption, this process gave rise to two stratified melts differingin Ba content by factor of 10 (Cioni et al., 1995). Additionally,Ra-enriched hydrothermal fluids (e.g., Sturchio et al., 1993) caninduce secondary226Ra enrichment in volcanic host-rocks(Volpe and Goldstein, 1993), although others noted difficultieswith this interpretation for oceanic tholeites (Pietruszka et al.,2001). In both cases, defect or excess of226Ra is revealed onlywhen the measured (226Ra/230Th) plus the lab’s 2� error differsfrom unity. Analytical uncertainity and extent of initial excess

* Author to whom correspondence should be addressed: (Istituto diGeologia Ambientale e Geoingegneria, CNR, Rome, Italy).

Pergamon

Geochimica et Cosmochimica Acta, Vol. 68, No. 1, pp. 167–181, 2004Copyright © 2004 Elsevier Ltd

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167

define the measurable temporal range, which in our study is �7 ka.

On the basis of the obtained data and previous work we willevaluate if geochemical models suggested for 226Raexcess arevalid for Vesuvius. Finally, we will propose an alternativemodel for 226Raexcess in Vesuvius magmas, exploring its im-plications on the current state of Vesuvius dynamics.

1.1. Previous Studies

Vesuvius belongs to the Roman Volcanic Province (RVP),characterized by perpotassic magmas erupted 1200–0.058 kaago. Although the geodynamic interpretation of the RVP isdebated, there is a consensus that the mantle sources weremetasomatically modified by fluids/melts released by subduct-ing slabs (Peccerillo, 2001). Isotope and minor elements geo-chemistry (see e.g., Serri, 1990; Tedesco, 1997) and regionalteleseismic-tomography inferred the presence of subductingslabs in this area (De Natale et al., 2001). A lithosphericdiscontinuity separates the Central Campania (Campi Flegrei,Ischia, Procida, Somma-Vesuvius) from the Roman subprov-ince (RoccaMonfina, Ernici, Alban Hills, Sabatini, Vico,Vulsini), but strong K and other LILE enrichment affected bothsubprovinces (Beccaluva et al., 1991; Peccerillo, 2001). The226Ra-excess does not seem a common feature of the RVP,although, among the RVP volcanoes, only Vesuvius, Ischia,Campi Flegrei and probably Alban Hills were active during thelast 7 ka. Almost coeval lavas of Ischia (Arso eruption-AD.1301) and Campi Flegrei (Mt. Nuovo eruption-AD.1538)have (226Ra/230Th)o close to unity (Gasparini, 1963; Voltaggio,unpub. data) whereas Aeolian Islands arc (Stromboli, Vul-cano), geochemically akin to Vesuvius (Peccerillo et al., 1984)albeit peripheral to the RVP, displays 226Ra-excess (Capaldi etal., 1983). Thus the study of the RHA is important to under-stand which process(es) cause 226Ra-enrichment in the RVP.

In 1909 John Joly carried out the first determinations of226Ra activity in Vesuvius lavas with electrometric methods(Joly, 1909). Several authors (e.g., Imbo, 1975; Capaldi et al.,1982; Black et al., 1998) investigated the origin of this 226Ra-excess but results were not clear-cut in interpretations. The226Ra-excess was explained as a primary character inheritedfrom a mantle source-region at radioactive disequilibrium (Ca-paldi et al., 1982) or as the result of recent fluid addition to amelt involved in a process of continuous crystallization (Blacket al., 1998).

2. SAMPLING AND ANALYTICAL METHODS

Locations of the analyzed samples are reported in Figure 1. Wesampled the 1944 lava flow in four points (V1944a-d), spaced along theflow, to appreciate elemental variations during a typical RHA eruption.Samples S(�1700)a-b, S(�1000) and S79 were collected in the Ter-zigno Quarry. Samples S1911, V1631a and V1631b belong to a privatecollection. V-labelled lavas are phonotephrites with a strong porphy-ritic character (Table 1). During the field work we collected preferen-tially the less porphyritic varieties, therefore the Phenocrysts/Gdmratios are lower than previously reported (e.g., Marianelli et al., 1999).Groundmass ranges between 39–78% whereas phenocrysts consistmainly of leucite 8–47% and clinopyroxene 5–36%. Apatite, olivineand magnetite (�3%) are accessory phases. Plagioclase (up to 6% asphenocryst) is present mainly in groundmass with leucite and clinopy-roxene. Only one sample, (V1822), has a high biotite content (8%).

Two samples (V1737, V1944a), exhibit dicktytaxitic textures, indicat-ing high content of volatiles.

238U, 232Th, 226Ra, K were determined in twenty-one lava samples,in one tephra sample (S1911), in exhalative-hydrothermal minerals(avogadrite(K, Cs)BF4, chlornatrokaliteK6NaCl7, aphtitalite(K, Na)3

Na(SO4)) deposited onto lavas, in four samples of pumices eruptedbetween 1700 B.C.–A.D. 79 and in gypsum and calcite crystals in-cluded in RHA lavas. Coaxial and planar HPGe detectors were used todetermine Th-Ra-K-U by high resolution �-spectrometry (Simpson andGrun, 1998). 232Th, 226Ra, K, 238U were estimated from 583 keV(208Tl), 609 keV (214Bi), 1461 keV (40K) and 63.3 keV(234Th) �-rays

Fig. 1. Sketch of the Mt. Somma-Vesuvius apparatus. Location ofthe analyzed samples.

Table 1. Modal composition (%) of RHA lavas

Sample Clpx Leu Gdm Others

V-1631a 8 12 78 �2 (Mt,Ap,Ol)V-1697 13 31 54 �2 (Mt,Ap,Ol)V-1737 36 10 52 2 (Mt,Ap,Ol)V-1754 18 19 61 �2 (Mt,Ap,Ol)V-1767 15 20 63 �2 (Mt,Ap,Ol)V-1794 9 18 68 5 (Plag,Mt)V-1822 18 8 66 8 (Biot)V-1834 12 15 71 �2 (Mt,Ap,Ol)V-1858 8 22 68 �2 (Mt,Ap)V-1871 5 20 72 3 (Mt,Ap)V-1872 11 16 70 3 (Mt,Ap)V-1882 10 25 63 �2 (Mt,Ap)V-1894 10 38 47 �2 (Mt,Ap)V-1895 8 44 46 �2 (Mt,Ap)V-1906 15 26 56 3 (Plag)V-1929 12 47 39 �2 (Mt,Ap)V-1944a 14 37 43 6 (Plag)

168 M. Voltaggio et al.

respectively, using as standard the Capo di Bove leucitite (Alban Hills),a reference material (Adams and Gasparini, 1970) having U-Th-K-Racontents comparable to Vesuvius lavas. 226Ra-214Bi radioactive equi-librium was assured by sealing samples in Rn-impermeable bags andleaving them to equilibrate for 22 d. Leucite samples were analyzed for226Ra, 232Th, K by high resolution �-spectrometry; six subsamples ofLeu-crystals of different size (�0.125–1.44 mm) were separated man-ually from sample V1944a and two samples of Leu-crystals wereseparated by densitometric techniques from the 1794 and the 1881samples.

Five lava samples (see Table 2), one tephra sample (S1911), Avellino(1700 B.C.) and Pompeii(A.D.79) pumices, were measured by TIMSusing a Finnigan Mat 262 mass spectrometer equipped with a RPQ-IIsystem to obtain precise 230Th/232Th atomic ratios (Schwieters andBach, 1994). Analytical procedures concerning sample preparation forTh mass spectrometry are described in Voltaggio et al. (1995). Thionization efficiencies up to 0.1% enabled measurements of (230Th/232Th) in static mode with an accuracy of 2–3% at 2�-level. Sampleswere not blank-corrected because the total procedural blank for massspectrometric analyses was � 200 pg 232Th, a value �1000 timessmaller than that of any loaded sample. Measurements of (230Th/232Th)on the ‘Rome-Standard’ (Voltaggio et al., 1997), carried out duringsample analyses, yielded a reproducibility of 0.3%, showing that error-estimates for each individual mass spectrometer run are reasonable.

Ba was analyzed in powdered samples by Dispersive Energy X-RayFluorescence (EDXRF) using a sealed source of 226Ra (0.1 mCi) and aplanar HPGE detector with a thin Be-window. Measurements werecalibrated with standards, prepared by adding weighted amounts ofBa(OH)2 to a Ba-free powdered mixture of appropriate amounts ofSiO2 and Al2O3. Standards, with size and bulk density close to those ofthe samples, ranged between 250–5000 ppm of Ba. The low radioac-tivity of the source required counting times of � 2 h to obtain an error� 10% at 2�-level.

To assess how U and Th are distributed in groundmass, 50 g ofpowdered phenocryst-depleted fractions of samples V1944 and V1794were leached with strong HCl during 32 and 24 h, respectively. At fixedintervals, powder (1–2 g) was sampled from the leaching bath and afterwashing was dried at 105 °C, spiked with 232U-228Th tracer and

analyzed by �-spectrometry for U and Th. The results of leachingexperiments are discussed in section 3.4.

3. RESULTS AND DISCUSSION

The results of the analyses are reported in Tables 2, 3, 4. The(230Th/232Th) activity ratios, measured by TIMS on 1697, 1767,1834, 1906, 1944 lavas and 1911 tephra (mean � 0.89 � 0.02)match the �-spectrometry results of Capaldi et al. (1982) onRHA lavas (mean � 0.90 � 0.02) (Fig. 2). Thus, consideringa constant Th isotopic composition (0.90) during the RHA, wecalculated the 230Th activity of the lavas from their 232Th

Table 2. Analytical data: U, Th, Ba, K2O, (226Ra)t,(226Ra)o,(230Th/232Th),(226Ra/230Th)o,(230Th/23U).

Sample Ba ppm U ppm Th ppm(230Th/232Th)activity ratio

(226Ra)t

dpm/g(226Ra)o

dpm/g K2O %(226Ra/230Th)o

activity ratio(230Th/238U)activity ratio

S(�1700)a 1130 � 160 7.4 � 0.4 19.8 � 0.4 0.93 � 0.04 6.1 � 0.1 13.8 � 0.3 1.95 � 0.08 3.16 � 0.12 0.80 � 0.05S(�1700)b 1340 � 160 8.0 � 0.4 24.0 � 0.4 n.d. 7.3 � 0.1 16.1 � 0.3 3.34 � 0.12 3.03 � 0.12 0.90 � 0.05S(�1000) 820 � 140 9.4 � 0.6 32.4 � 0.6 n.d. 12.3 � 0.3 26.3 � 0.6 6.01 � 0.24 3.67 � 0.14 1.03 � 0.07S79 600 � 120 8.2 � 0.4 22.1 � 0.4 0.93 � 0.04 11.1 � 0.1 19.1 � 0.3 8.59 � 0.31 3.91 � 0.14 0.81 � 0.04V1631a 2350 � 240 9.0 � 0.4 29.9 � 0.6 n.d. 41.9 � 0.9 48.0 � 1.0 8.92 � 0.33 7.50 � 0.28 0.96 � 0.05V1631b 2110 � 200 8.1 � 0.4 24.4 � 0.4 n.d. 35.6 � 0.7 40.8 � 0.8 8.77 � 0.33 7.81 � 0.26 0.87 � 0.04V1697 1850 � 200 5.3 � 0.2 11.0 � 0.2 0.90 � 0.03 18.7 � 0.3 20.9 � 0.3 4.16 � 0.16 8.88 � 0.34 0.60 � 0.02V1737 1840 � 200 7.0 � 0.4 15.6 � 0.2 n.d. 28.9 � 0.6 31.8 � 0.7 4.72 � 0.18 9.52 � 0.34 0.65 � 0.04V1754 2170 � 220 9.2 � 0.4 22.8 � 0.4 n.d. 44.8 � 0.9 49.3 � 1.0 7.17 � 0.27 10.10 � 0.38 0.72 � 0.03V1767 2110 � 200 7.0 � 0.4 23.1 � 0.4 0.87 � 0.03 45.0 � 0.9 49.1 � 1.0 7.48 � 0.29 9.93 � 0.38 0.92 � 0.05V1794 1760 � 180 7.8 � 0.4 19.9 � 0.4 n.d. 24.7 � 04 26.6 � 0.5 6.11 � 0.24 6.24 � 0.24 0.74 � 0.04V1822 1920 � 200 6.9 � 0.4 22.1 � 0.4 n.d. 33.2 � 0.6 35.4 � 0.6 7.25 � 0.28 7.47 � 0.28 0.93 � 0.06V1834 1840 � 200 7.3 � 0.4 16.4 � 0.4 0.89 � 0.02 32.8 � 0.6 34.9 � 0.6 5.37 � 0.20 9.93 � 0.30 0.61 � 0.04V1858 1950 � 200 6.8 � 0.4 25.9 � 0.6 n.d. 40.0 � 0.7 42.2 � 0.7 7.65 � 0.28 7.60 � 0.28 1.10 � 0.07V1871 2050 � 200 7.7 � 0.4 23.0 � 0.4 n.d. 36.6 � 0.7 38.4 � 0.7 7.43 � 0.30 7.80 � 0.30 0.84 � 0.05V1872 1890 � 200 6.7 � 0.4 16.6 � 0.4 n.d. 26.7 � 0.6 27.7 � 0.6 5.36 � 0.20 7.80 � 0.30 0.72 � 0.05V1882 1950 � 200 9.6 � 0.6 22.3 � 0.4 n.d. 40.9 � 0.7 42.8 � 0.7 7.08 � 0.28 8.95 � 0.34 0.67 � 0.04V1894 1830 � 200 6.5 � 0.4 23.5 � 0.4 n.d. 42.9 � 0.8 44.7 � 0.8 6.63 � 0.24 8.88 � 0.34 1.05 � 0.07V1895 2170 � 200 8.2 � 0.4 23.9 � 0.4 n.d. 40.7 � 0.7 42.2 � 0.7 8.87 � 0.34 8.25 � 0.32 0.84 � 0.04V1906 1760 � 180 5.4 � 0.2 18.9 � 0.4 0.91 � 0.02 33.4 � 0.6 34.4 � 0.6 5.92 � 0.24 8.52 � 0.24 1.02 � 0.04S1911 756 � 120 5.2 � 0.4 11.2 � 0.2 0.90 � 0.02 21.8 � 0.7 22.7 � 0.7 3.33 � 0.12 9.50 � 0.38 0.62 � 0.05V1929 1980 � 200 9.0 � 0.4 19.1 � 0.4 n.d. 44.3 � 0.9 45.3 � 0.9 6.50 � 0.24 11.09 � 0.42 0.61 � 0.03V1944a 2410 � 240 7.1 � 0.4 21.8 � 0.4 0.89 � 0.02 46.3 � 0.9 47.3 � 0.9 8.07 � 0.32 10.13 � 0.38 0.88 � 0.05V1944b 2370 � 240 7.5 � 0.4 19.7 � 0.4 n.d. 46.8 � 0.9 47.6 � 0.9 7.91 � 0.32 11.29 � 0.42 0.76 � 0.04V1944c 2300 � 240 7.0 � 0.4 19.6 � 0.4 n.d. 43.7 � 0.8 44.5 � 0.8 7.63 � 0.30 10.59 � 0.40 0.81 � 0.05V1944d 2220 � 220 8.0 � 0.4 20.0 � 0.4 n.d. 43.9 � 0.9 44.8 � 0.9 7.44 � 0.30 10.46 � 0.38 0.72 � 0.04

Errors are reported at 2-� level. Subscripts t and o stay for present and initial value, respectively.

Table 3. (226Ra)t,232Th and K results in leucite samples.

Leucite sample(226Ra)t

dpm/g

232Thppm K2O %

60 selected crystalsD � 1.44 � 0.15 mm

1944a 22 � 4 1.2 � 0.3 18.9 � 1.2

102 selected crystalsD � 1.10 � 0.10 mm

1944a 23 � 4 1.3 � 0.3 20.1 � 1.2

270 selected crystalsD � 0.85 � 0.10 mm

1944a 36 � 6 1.7 � 0.5 19.9 � 1.1

Granulometric fractionD � 0.84 � 0.50 mm

1944a 49 � 6 1.8 � 0.5 18.9 � 1.0

Granulometric fractionD � 0.50 � 0.125 mm

1944a 70 � 6 2.1 � .5 21.7 � 1.0

Granulometric fraction�0.125 mm

1944a 85 � 8 1.8 � 0.5 22.1 � 1.0

Groundmass 1944a 47 � 4 34.4 � 1.3 2.2 � 0.1Leucite (in toto) 1794 38 � 6 �1.0 20.6 � 1.0Leucite (in toto) 1881 72 � 8 �1.0 20.8 � 1.0

Errors are reported at 2-� level. Subscript t � present value.

169226Ra-excess during the historical activity of Mt. Vesuvius

activity (Voltaggio et al., 1995). Most samples show 238U-excess, the (238U/230Th) ranging between 0.91–1.66. All sam-ples show high initial (226Ra/230Th) ranging from 3.03 (S-1700,226Raexcess � 203%) to 11.29 (V1944b, 226Raexcess � 1029%),with a clear difference between the 1700 B.C.–A.D.79 and theRHA. Also [226Raexcess]o during the RHA (18.6–44.4 dpmg�1) is higher than during the period 1700 B.C.–A.D. 79 (9.2�18.5 dpm g�1). Figure 3, where 230Th activities of volcanoesfrom various geodynamic environments are plotted vs.[226Raexcess]o, illustrates clearly that Vesuvius magmas displaythe highest [226Raexcess]o.

Joron et al. (1987) and Villemant et al. (1993) reportedtrace-element geochemistry of the RHA, highlighting the ho-mogeneity of the magmatic system. A single mantle-source wasinferred by the clustering of ratios between highly incompatibleelements (Th/Ta, Th/Tb, Th/Hf) (Joron et al., 1987). Our dataconfirm and extend their results as pumices deposited by plin-ian eruptions between 1700 BC and A.D.79 display (230Th/232Th)o in the same range of the RHA lavas (Fig. 2). Thissuggests a unique source with same (238U/232Th) for the entireperiod between 1700 B.C. and 1944, despite of the variable(238U/232Th). A constant (230Th/232Th)o close to 0.90 shouldrepresent the (238U/232Th) of this unique source, provided that:

i) the source were at radioactive equilibrium (238Usource �230Thsource), ii ) only partial melting caused 238U-230Th disequi-librium, iii ) the timing of magma transfer from source tosurface were negligible in comparison with 230Th half-life.Among accessory minerals, only apatite, having a high Dsolid/melt

for these elements with DUAp/melt � DTh

Ap/melt (Bourdon et al.,1994), could fractionate sensitively U from Th if it crystallizedin much larger amounts than observed in RHA lavas (�1%).Considering the mineralogical assemblage of the lavas as rep-resentative of the phases that fractionated at depth (Black et al.,1998), appears unlikely that crystal fractionation had substan-tially modified the magmatic U/Th ratio. Previous studies,however, evidenced significant positive correlations betweenincompatible elements (IE) such as Th, Ba, Ta, La, interpretedas a mingling between IE-enriched primary melts and variousamounts of IE-depleted magmatic cumulates (Villemant et al.,1993). Crystal fractionation was therefore active during thecrustal residence time of the magma and its role in producing ormodifying 226Ra-excess in RHA magmas must be discussed.

3.1. Crystal Fractionation

In the data set composed by RHA lavas, Th and K arecorrelated positively (Table 5). This correlation, already foundby Villemant et al. (1993) in tephra and RHA lavas, depends onCpxcrystal fractionation, a phase K-free with low DTh

Cpx/melt.Tephras (see sample S1911) are less Th and K-enriched, con-taining a higher percentage of Cpx-phenocrysts. Al content ofclinopyroxene strongly influences the DCpx/melt of HFSE(Lundstrom et al., 1998; Brenan et al., 1995). Because of thehigh Al content of Cpx-phenocrysts, Wood and Trigila (2001)measured a DTh

Cpx/melt of 0.12 in RHA lavas, a value muchhigher than those given in literature (0.001–0.036, see e.g.,Beattie, 1993; LaTourrette and Burnett, 1992; Salters andLonghi, 1999). However the bulkDTh

solid/melt should have beenlow enough to cause Th increase in residual magma, on con-dition that extensive differential Cpx-segregation took place.

Compared to clinopyroxene, leucite played a minor role incrystal fractionation during the RHA. The K2O/Th ratio ofRHA lavas (Fig. 4) varies within a small range (0.3–0.4 � 104)compared to the previous products and to the high K2O/Thvalues of RHA Leu-phenocrysts (from 10 to 20 � 104); awider variability would appear if floating or lateral segregationof leucite were effective.

Despite of the large content of phenocrysts, Ba, Ra, K and Thare not correlated at 95% confidence level with modal percent-ages of the main phases (Table 5), with exception of thenegative correlation between K2O and Cpx.226Ra/Ba, 226Ra/Kand (226Ra/230Th)o ratios are not correlated significantly with

Table 4. U, Th, 226Ra and K in hydrothermal minerals at Vesuvius.

Sample U ppm Th ppm 226Ra dpm/g K2O %

Avogadrite(1926) 13.6 � 1.2 26.7 � 1.6 37.1 � 2.2 8.0 � 0.9Clornatrokalite (1906) 1.7 � 0.4 6.2 � 0.8 1.6 � 0.2 16.1 � 1.2Aphtitalite (1906) 0.7 � 0.4 1.2 � 0.4 15.5 � 0.6 3.2 � 0.4Gypsumincluded in 1894 lava flow 0.4 � 0.2 0.8 � 0.4 3.1 � 0.2 �0.3Calcite in muscovite-calcite bearing ejecta (1631) 3.1 � 0.5 1.4 � 0.4 4.6 � 0.2 �0.3

Errors are reported at 2-� level.

Fig. 2. Activity ratio (230Th/232Th)of Vesuvius magmas during thelast 3700 yr. Full circles: mass spectrometry data, this work; fullsquares: �-spectrometry data by Capaldi et al. (1982). Error barsrepresent � 2�. Solid lines: 95% confidence limits. Measured data arenot initial values: however the oldest samples, if reported as (230Th/232Th)o, are � 0.91, very close to the RHA values.

170 M. Voltaggio et al.

the modal percentage of leucite, suggesting that Leu-crystalfractionation did not modify substantially the (226Ra/230Th) ofthe magma. The use of (226Ra/Ba)o or (226Ra/K)o to describetemporal 226Ra variations in magmas is, however, more appro-priate since K or Ba may be surrogates for the ‘ lacking’ stableRa isotope (Reinitz and Turekian, 1989; Rubin and Macdou-gall, 1990).

The role of leucite floating should be considered definitely asthe low density of leucite might have caused its floating inmagma chamber. As in RHA lavas K2O/Th is not correlatedwith K2O (r � 0.03) (Fig. 4), a relevant leucite floating should

be ruled out. RHA leucites are characterized by large amountsof 226Ra, showing the highest [226Raexcess]o values (up to 85dpm g�1) presently recorded for common minerals in volcanicrocks (Table 3). We calculate a DRa

Leu/melt of 1.8 using thefinest Leu-granulometric fraction and the coexisting ground-mass of sample V1944a (Table 3). This value matches theDBa

Leu/melt in RVP leucites (0.9–1.3, Francalanci et al., 1987)as well as the DBa

Leu/melt (0.8–1.9) values for 1944-Leu, cal-culated from the data of Black et al. (1998). The initial 226Raactivity of 1944-Leu (22–85 dpm g�1) matches the (226Ra)o

given by Black et al.(1998) but is notably higher than that (6.7dpm g�1) found by Capaldi et al. (1982). The different dimen-sions of the crystals could account for the disparity, as (226Ra)o

depends exponentially on leucite size (Fig. 5). If leucite dom-inated the fractional crystallization, (226Ra)o of Leu-phenoc-rysts would increase with the size as DRa

leu/melt 1, and thesmallest phenocrysts, presumably formed in a late stage, shouldbe the most Ra-depleted. This does not happen probably be-cause extensive Cpx-crystallization, during which DBa

Cpx/melt

DRaCpx/melt � 0 (Wood et al., 1999), shifted the bulkDRa

solid/

melt to � 1, causing progressive increase of (226Ra) in the melt.Thus the cumulates consisted mainly of clinopyroxene, a well-known feature of Vesuvius cumulates (Hermes and Cornell,1981; Cioni et al., 1995).

Fig. 3. 230Th activity vs. initial absolute 226Ra-excess in lava samples of volcanoes from different geodynamicenvironments. Vesuvius (full triangles � RHA activity, empty triangles � 3000 B.C.–A.D. 79). MORBs: Rubin andMacdougall (1988, 1990). OIBs: Sigmarsson et al., 1998; Sims et al., 1999; Pietruszka et al., 2001. IABs: Gill and Williams,1990; Hoogewerff et al., 1997. Aeolian Arc: Capaldi et al., 1983. Continental convergent margins: Gill and Williams, 1990;Turekian et al., 1996. Continental rifts: Turekian et al., 1996. Carbonatites: Williams et al., 1986. Samples with [226Raexcess]o

� 0 are not shown.

Table 5. Correlation coefficients in RHA lavas.

Ba Th Ra gm cplx leu

K 0.75 0.85 0.76 0.06 �0.46 0.17Ba 0.44 0.72 �0.20 �0.18 0.26

Th 0.69 0.14 �0.38 0.02Ra �0.24 �0.12 0.30

0.37 Ra/Th0.26 Ra/Ba0.18 Ra/K

Underlined values have a level of significance �0.05.

171226Ra-excess during the historical activity of Mt. Vesuvius

Since (226Ra/230Th)o of 1944-Leu (85–220) is (226Ra/230Th)o of the coexisting melt (5.9), (226Ra/230Th)o of themagma should increase as a function of floated leucite. Oppo-sitely, (226Ra/K2O)o should decrease with increasing floated

leucite as 226Ra/K2O in 1944-Leu (1.5–3.7 dpm g �1 %�1) isnotably lower than in coexisting melt (21.3 dpm g �1 %�1).Figure 6 shows the positive correlation between (226Ra/K2O)o

and (226Ra/230Th)o, which is incompatible with Leu-accumula-

Fig. 4. K2O/Th vs. K2O diagram showing the field of RHA lava flows (re-plotted using a linear scale to show internalstrucure of the data) in comparison with 1944-leucites (1794 and 1881 leucites fall out of the diagram), 1944-groundmassand previous products (1700 BC–79 AD).

Fig. 5. 226Ra activity (dpm/g) of leucite of different average diameterin sample V1944.

Fig. 6. (226Ra/230Th)o-(226Ra/K2O)o covariation diagram for RHAlava flows.

172 M. Voltaggio et al.

tion; therefore a different process played an important role inthe origin of 226Ra-excess.

3.2. 226Ra-excess by ingrowth of 226Ra duringequilibrium partial melting

Since226Ra-excess seems a primary feature of the magma, itcould originate by equilibrium partial melting of a mantlesource. Following the approach of Sims et al. (1999) andPietruszka et al. (2001), we explored two models of ingrowth ofshort-lived daughter nuclides of 238U series during equilibriumpartial melting representing two extreme cases: dynamic melt-ing (McKenzie, 1985) and equilibrium percolation melting(Spiegelman and Elliot, 1993). They differ by two main fea-tures: 1) melt extraction time, instantaneous in dynamic melt-ing, 2) transport of solid matrix in melt column, neglected indynamic melting. Both of these variables can be modelled inequilibrium percolation melting. In applying these models toVesuvius data we assume a priori that bulkDRa � bulkDTh �bulkDU. This agrees with a spinel lherzolite mantle region(Landwehr et al., 2001) set at a p 1.2 GPa, i.e., at a depthmajor than 30 km, indicated by data on mantle xenoliths of theRVP as the shallower limit for the depth of magma formation(Trigila et al.,1992).

3.2.1. Dynamic melting

Zou and Zindler (2000) provided ready-to-use equations todescribe 238U-series disequilibrium in dynamic melting as afunction of bulkDs, melting porosity, melting rate and meltingtime. Previous formulations of dynamic melting are particularsolutions of the Zou-Zindler’s equations. These equations col-lapse to the description of trace element behavior in extractedmelt setting all �s to zero and show that extremely smalldegrees of melting are not required to explain large 238U-seriesdisequilibria.

Assuming that the adjacent, nearly coeval volcanic area ofCampi Flegrei was not involved in the same melting process,we define a cylindrical volume with a Vesuvius-centred ray of�10 km and a height of 30 km, as volume of the magmasource-region, Vs, corresponding to the entire spinel-lherzolitestability field (at depth between 30–60 km). The 1631–1944mean emission rate of 106 Ta�1 (Scandone et al., 1993),divided by Vs, gives a mantle melting rate of � 2 � 10�4 kgm�3a�1. Melting porosity was set to 10�5 as a minimal valueandbulkDU, bulkDTh, bulkDRa were set to 0.024, 0.020 and 0.001respectively (Lundstrom et al., 1998; Landwehr et al., 2001).By solving the Zou-Zindler’s equations for RHA data, themeasured 226Ra-excesses appear compatible with dynamicmelting, no matter how long the melting time is; but theassumed constraint bulkDU/bulkDTh 1 prevents 238U-excessproduction.

3.2.2. Equilibrium percolation melting

Spiegelman and Elliott (1993) showed that high (226Ra/230Th) in magmas can rise by processes including the fluiddynamics of melt segregation. In their model, the residencetime of 238U-series radionuclides in a columnar melting systemcontrols radioactive disequilibria in the melt. The different

residence times of any radionuclide depend on �s, bulkDssolid/melt,surface adsorption and interaction with crystal grain bound-aries. As long as the daughter radionuclide spends less timethan the parent in the melting system, daughter-excess will beproduced. Solving two approximate solutions for the equilib-rium transport equations, whose difference depends on whetherthe 226Ra- and 230Th-extraction time is slow or fast comparedwith their half-lives (Spiegelman and Elliot, 1993), yields max-imum values of (226Ra/230Th) and (230Th/238U) close tobulkDTh/bulkDU and bulkDRa/

bulkDTh respectively, both compat-ible with a slow extraction time. In this regime, the extent ofexcess depends only on porosity , reaching maximum valueswhen �� bulkDs. As initially we assumed �� bulkDs, themeasured 226Ra-excesses are compatible with this model. Butlike in the dynamic melting model, the constraint representedby bulkDU/bulkDTh 1 prevents 238U-excess production.

Up to now we have considered bulkDs values typical of a‘standard’ spinel lherzolite source-region independently of pet-rologic and geochemical constraints of the RVP source-region,which will be examined in the next section.

3.3. The role of phlogopite

Potassic basic liquids may result from small degrees ofpartial melting of a Phlog-bearing peridotite under CO2 satu-rated conditions (Wendlandt and Eggler, 1980; Hornig andWorner, 1991) or from partial melting of a Phlog-enrichedclinopyroxenite with higher degrees of melting than from aperidotitic source (Lloyd et al., 1985). Phlog-bearing pyroxen-ite was recognized as the most probable source rock of RVPmagmas (Dolfi, 1981). The presence of phlogopite in the man-tle wedge, where is believed to form veins of metasomaticorigin (Foley, 1992), explains some geochemical characteris-tics of the RVP (e.g., negative Eu anomaly) (Peccerillo et al.,1984). In particular, the range of Sr isotopic composition atVesuvius (0.7071–0.7079), not affected by crustal contamina-tion (Civetta et al., 1991), and high K and Rb contents recordedin RHA lavas could be explained by partial melting of aPhlog-enriched mantle source (Peccerillo, 2001). Studies car-ried out on RVP xenoliths revealed the presence of phlogopitein the mantle-source (Trigila et al., 1992). The main mineral-ogical feature of these xenoliths is a high Cr-diopsidic clinopy-roxene, a phase that permits to distinguish ‘mantle-derived’xenoliths from the ‘crustal’ ones. Ultramafic mantle-derivedxenoliths in RVP volcanics are mainly Cr-wehrlites, a restiticassemblage of metasomatic Phlog-bearing spinel lherzolite,and Cr-clinopyroxenites, representing melts generated by ametasomatized spinel lherzolite, crystallized at depth (Gaeta etal., 1989). Several experimental studies determined DTh

Phlog/melt

and DBaPhlog/melt for a variety of melts (Guo and Green, 1990;

LaTourrette et al., 1995; Foley et al., 1996; Schmidt et al.,1999). DBa

Phlog/melt varies between 0.3–3.7 as a function of Tiand Al content of the phlogopites, whereas DTh

Phlog/melt isalways � 0.01. Experimental studies on chemical equilibriumpartition of Ra between phlogopite and melt are not yet avail-able, but from the lattice strain energy model of Brice (1975),the similarity of ionic radius (1.61 Å, Ba; 1.62 Å, Ra) and theoptimum size (1.64 Å) of the K site in phlogopite, one candeduce that DRa

Phlog/melt DBaPhlog/melt 1 (Blundy and

Wood, 1994; Schmidt et al., 1999). As phlogopite in RVP

173226Ra-excess during the historical activity of Mt. Vesuvius

mantle-derived xenoliths is generally 4% (Gaeta et al.,1989), the resulting bulkDRa

solid/melt should be higher than thebulkDTh

solid/melt. It is therefore impossible to produce 226Ra-excess by equilibrium partial melting in presence of relevantphlogopite in a mantle at radioactive equilibrium. If phlogopiteoccurs significantly in the mantle wedge beneath Vesuvius,equilibrium partial melting should lead to a reduced mobility of226Ra, which, like Ba and differently from Th, would be re-tained preferentially in phlogopite (Edgar, 1992). To sustain theproduction of 226Ra-excess exclusively by partial melting wemust: 1) keep hold equilibrium partial melting, ruling out thepresence of relevant phlogopite in the magma source-region, inspite of field petrological evidences or 2) consider 226Raexcess

as inherited from chemical disequilibrium melting (Bedard,1989) of a Phlog-bearing clinopyroxenite constrained at radio-activedisequilibrium by continuous fluid addition.

At p � 1.2 GPa (�30 km) there is an inversion of the Dsvalues with DU

Cpx/melt � DThCpx/melt (Wood et al., 1999;

Landwehr et al., 2001), therefore equilibrium partial melting ofa Phlog-bearing clinopyroxenite set at p � 1.2 GPa mightoriginate the 238U-excess displayed during RHA, although thislow depth contrasts with petrologic data. But other processesdifferent from equilibrium partial melting can give rise to238U-excess. 238U-excess is common in subduction-related vol-canoes while in continental rifts is confined to CO2-rich mag-mas; this restriction suggests that fluids in magmatic processescarry preferentially U (Gill and Williams, 1990).

3.4. Fluids and production of 238U-excess and 226Ra-excess in RHA magma

Capaldi et al. (1982) proposed that, at Vesuvius, CO2-richfluids produced an excess of 226Ra over 230Th and, at a smallerextent, of 238U over 230Th because of their different behaviorduring chemical complexing (Condomines et al., 1988). The226Ra-excess, however, must be explained differently inasmuchas (226Ra/230Th)o are not correlated with (238U/230Th) (Fig. 7).CO2-rich fluids able to complex, partition and transport ele-ments like REE (Wendlandt and Harrison, 1979) were presentduring metasomatic and melting processes (Joron et al., 1987)but experimental data, indicating that such fluids producedchemical Ra-enrichment in melts, are not available. Only a high226Raexcess (8000%) in carbonatitic magmas (Williams et al.,1986) is in accord with this hypothesis. The similarity of DBa

and DRa between carbonatitic and silicate melts, both 1(Jones et al., 1995; Veksler et al., 1998), should result in a highBa content coupled with 226Raexcess as occurs in carbonatiticmagmas (Williams et al., 1986). But because of the highDBa

Phlog/carbonatiticmelt (1.04 according to Sweeney et al., 1995) it isunlikely that a carbonatitic melt, after rising through a Phlog-enriched mantle, can transport high amounts of Ba and Ra. Themoderate amount of 226Ra in calcite of a 1631-muscoviteejecta, likely crystallized at the roof of the magmatic reservoir(Table 4), indicates no evidence of 226Ra transport by CO2-bearing fluids.

In a magma chamber environment, gaseous transfer and fluidtransport may be relevant for U and Th fractionation. Acidleaching experiments on samples V1794 and V1944 show thatmore than 70% of U and Th is leachable (Fig. 8). As theobserved losses cannot be justified by dissolution of apatite and

magnetite because of their low modal percentages, U and Thmust occur in U- and Th-enriched leachable accessory mi-crophases, like perrierite or zirconolite, dispersed in ground-mass. Perrierite was found in porphyritic potassic lavas atVulcano (Voltaggio et al.,1995); de Hhog and van Bergen(2000) found zirconolite in vesicle-fillings in porphyritichigh-K lavas of Lewotolo volcano explaining this occurrenceas a mobilization of HFSE, REE and Actinides during fluid-driven processes in a shallow magmatic environment. Similar238U-excesses characterize other lavas of the district of Le-wotolo (East Sunda Arc) and Vulcano (Aeolian Islands)(Capaldi et al., 1983; Hoogewerff et al., 1997). Different valuesof the formation constants of U and Th-complexes with fluid

Fig. 7. (226Ra/230Th)o vs. (238U/230Th)o for samples of the 1700B.C.–A.D.1944 period. Dashed lines represent radioactive equilibriumbetween the considered pairs. Full triangles �1631–1944 activity,empty triangles � 3000 B.C.– A.D. 79 period

Fig. 8. Percent cumulative losses of U and Th during acid leachingof two lava samples (V1794 and V1944).

174 M. Voltaggio et al.

species, favouring U over Th transport, might account for the238U-excess. If such a process were active at Vesuvius, itshould have modified the magmatic (226Ra/230Th), causing adecrease of the original 226Ra-excess. This possibility suggeststo use (226Ra/Ba)o or (226Ra/K)o ratios for tracing 226Ra tem-poral variations in Vesuvius magmas.

Vesuvius eruptions younger than 3.55 ka derived from mag-mas enriched in volatiles (H2O, Cl, S) and fluxing components(B, F). In RHA lavas F and Cl range between 0.35–0.65 wt %(Webster et al., 2001). During degassing in magma chamber,volatile fluoride and chloride species might transport out U(Voltaggio et al., 1998) and Th (ThCl4 volatilizes at T 770°C). Indeed, first studies considered U and Th losses by exha-lative processes as the main cause of 226Ra-excess (Imbo,1975). We tested this hypothesis by analyzing fumarolic min-erals deposited onto RHA lavas. Because avogadriteas well aschloronatrokalite(Table 4) are not rich enough in these ele-ments and aphtitalitedisplays (226Ra/230Th) even higher (54.1)than the coeval lava (V1906), this hypothesis should be rejecteddefinitely. Finally, the low 226Ra content (Table 4) in an ag-glomerate of gypsumcrystals in the 1894 lava indicates that thehydrothermal system connected to the magma chamber (Fulig-nati et al., 1995) provided no 226Ra contribution to the magma.

3.5. Direct �-recoil Model of 226Ra-excess

In RHA lavas, Ba correlates apparently (Fig. 9) with[226Raexcess]o in accordance with the similar geochemical andcristallochemical behavior of Ba, a natural analog for a stableRa isotope (Rubin and Macdougall, 1990). But in theBa�[226Raexcess]o diagram the samples describe a break linerevealing two different trends: at right of the knee of the breakline the two elements are strictly correlated; at left, [226Raexcess]o

is variable and Ba keeps a constant value of � 1800 ppm. Thisindicates that Ra and Ba were decoupled during a first stage ofthe magmatic history and that a physical factor involving

exclusively Ra, probably direct �-recoil, produced the most226Ra-excess.

To explain 226Ra-excesses in MORBs, Osmond (1998) sug-gested a model based on daughter-recoil occurring in �-decay,a process able to fractionate U-series radionuclides in exoge-nous environments. Recent data on U-distribution in the man-tle, reported by Grachev and Komarov (1994), support thisidea. Their extensive study of mantle xenoliths (mainly spinellherzolites) showed that 90% of U is located in thin films ofintergranular material along grain boundaries of crystals, cracksand fractures indicating that MMFs can transport and deposit Uin mantle as microdispersed and stable U-high accessory min-erals (Grachev and Komarov, 1994). The evidence that inlherzolite mantle U can be sequestered preferentially in U-highaccessory minerals rather than in common ones, applies spe-cifically for the metasomatized mantle related to the RVP. Wepresent in next sections our version of the direct �-recoil modelof the 226Ra-excess at Vesuvius, consisting of two stages sep-arated spatially and temporally.

3.5.1. Stage I: MMFs and226Ra recoil-induced excess

Pressurized MMFs, interpreted in this geodynamic environ-ment as slab-derived fluids, are made mostly of H2O withdissolved solids and CO2. At p � 20 kb and T � 1200°C,MMFs should have a density close to 1, and a viscosity (�0.005 poise) enough low to permit their migration throughmicrofractures (Spera, 1981). As microfractures are rich inU-accessory minerals, daughter-recoil of respective parent nu-clides from microfracture walls supplies 226Ra, 230Th and 234Uto flowing fluids. The temporal variation of (226Ra/230Th) influids, due to recoil effects is given by (Cherdyntsev, 1971):

�226Ra/230Th�f � A(t) � �1 � e��226tat�/�1 � e��230tat� (3)

In Eqn. 3 the activities of recoiled226Ra and 230Th into the fluidare:

�226Ra�f � K � �R/f� � �2rRa/w� � �230Th�F � �1 � e��226tat�

(4)

�230Th�f � K � �R/f� � �2rTh/w� � �234U�F � �1 � e��230tat�

(5)

where K is an efficiency factor (equal for all daughters) for�-recoil from U-enriched fracture walls into the fluid, R/f isthe ratio between rock and fluid density, rRa and rTh are the226Ra and 230Th recoil range in fluid (rRa rTh) w is theaverage width of microfractures, (234U)F and (230Th)F are theaverage 234U and 230Th activity of U-enriched microfracturewalls, and tat is the ascent time of MMFs through the mantlezone where this process occurs. In Eqns. (4–5) 238U-seriesradionuclides in fracture walls are assumed at secular radioac-tive equilibrium (i.e., 230ThF � 234UF � 238UF). In Eqn. 4,R/f and rRa are known and the others variables can be mod-elled. The average width of fractures along which MMFs move,w, determines the absolute value of (226Ra)f at constant tat,because the activity of recoiled radionuclides increases withdecreasing w. If, on the average, w may be considered constantin the RVP mantle, the critical factors in determining large226Ra-excess in fluids of the mantle beneath the Vesuvius are

Fig. 9. Ba content vs.[226Raexcess]o (absolute initial 226Ra-excess) forRHA lava flows.

175226Ra-excess during the historical activity of Mt. Vesuvius

(238U)F and tat. (238U)F does not must be confused with thelocal 238U mantle activity, which might be similar to that ofother zones. (238U)F is the activity in microfracture walls, theunique sources of the recoiled 226Ra and 230Th atoms, anddepends on intensity and duration of the action of MMFs.Figure 10 shows how (226Ra)f and (230Th)f activities and theirratio vary in the pure recoil component of MMFs. In agreementwith Eqn. 3, (226Ra/230Th)f ranges between 47 (i.e.,� �226/�230) and 1. A short ascent time implies high (226Ra/230Th)f andlow values of (226Ra)f and (230Th)f, whereas a long ascent timeimplies (226Ra/230Th)f � 1 as well as high (226Ra)f and (230Th)f.Note as the maximum [226Raexcess]f is reached at tat � 10 ka.

3.5.2. Stage II: mixing of MMFs with partial melting-derivedmelts

When 226Ra-enriched fluids enter the zone of partial meltingthey mix with partial melting-derived melts. Since the presenceof phlogopite prevents production of 226Ra-excess by equilib-rium partial melting also disequilibrium mantle melting (Be-dard, 1989) could be compatible with the proposed model.

Sigmarsson et al. (1998) sustained that a 226Ra-excess byfluid addition can be tested assuming that fluids add only 226Raand no 230Th to a melt. In this case the data pattern in a(226Ra/230Th)o vs.1/230Th diagram should define a straight lineintersecting the y-axisat a (226Ra/230Th) lower than unity. Thisshould be true if crystal fractionation or assimilation of Leu-cumulates did not affect the (226Ra/230Th) ratio, a possibilitythat cannot be ruled out for Vesuvius magma. Moreover, in ourmodel, fluids add also 230Th. Thus an alternative approach totest the hypothesis of MMF addition is proposed. (226Ra) and(230Th) in magma form partly by 1) direct �-recoil from 238U-accessory minerals into fluid-filled cracks and intergranularfilms and by 2) partial melting of the mantle source. Direct�-recoil and partial melting occur in two different zones of

mantle whereas mixing of the two components of (226Ra) and(230Th) takes place close to the zone of partial melting. Pro-vided that the fluid-filled porosity due to fractures/cracks isinterconnected, the mass of fluid involved in the recoil, Mf, is:

Mf � Vrec � v � f (6)

where �v is the volume porosity and Vrec is the involvedmantle volume.

The mass of melt, Mm, is

Mm � Vm � m (7)

and the mass of magma M after mixing is:

M � Mf � Mm (8)

After mixing, 226Ra and 230Th activities in the magma are equalto:

�226Ra�M � ���Mm/M� � �226Ra�m�� � ��1 � �Mm/M��

� �226Ra�f� (9)

�230Th�M � ���Mm/M� � �230Th�m�� � ��1 � �Mm/M��

� �230Th�f� (10)

where (226Ra)f & (230Th)f and (226Ra)m & (230Th)m are activi-ties in the fluid (f) and in the melt (m) before mixing and(226Ra)M & (230Th)M are activities in the magma (M) aftermixing. Assuming that Ba derives entirely from partial melting,then:

BaM/Bam � Mm/M (11)

Bam and BaM being the Ba concentrations in the melt beforemixing and in the magma after mixing, respectively.

Substituting Eqn. 11 in Eqns. (9–10) it follows that:

�226Ra�f � ��226Ra�M � ��BaM/Bam� � �226Ra�m��/�1

� �BaM/Bam�� (12)

�230Th�f � ��230Th�M � ��BaM/Bam� � �230Th�m��/�1

� �BaM/Bam�� (13)

Dividing Eqn. 12 by Eqn. 13 and remembering Eqn. 3:

A(t) � ��226Ra/Ba�M � �226Ra/Ba�m�/��230Th/Ba�M

� �230Th/Ba�m� (14)

Eqn. 14 may be rewritten as:

�230Th/Ba�M � ���226Ra/Ba�M � �226Ra/Ba�m�/A(t)�

� �230Th/Ba�m (15)

In disequilibrium mantle melting the behavior of trace elementsdoes not differ from that of major elements (i.e., all Dssolid/melt

are equal) (Shaw, 2000), thus:

�226Ra�m/�226Ra�mantle� �230Th�m/�230Th�mantle (16)

If mantle is at secular equilibrium, then (226Ra)mantle �(230Th)mantleand consequently:

Fig. 10. Variation of (226Ra/230Th)f vs. (226Ra)f and (230Th)f overincreasing ascent times (up to 30 ka) for a fluid having initially (226Ra)� (230Th) � 0 and that accumulates these radionuclides only by direct�-recoil from U-rich fracture walls with (238U)F � (234U)F � (230Th)F

�1600 dpm g�1, R/f � 2.5, 2r/w �5*10�2, K � 0.5

176 M. Voltaggio et al.

�226Ra�m � �230Th�m. (17)

Substituting Eqn. 17 in Eqn. 15 it follows that:

�230Th/Ba�M � ���226Ra/Ba�M� � �1/A(t)�� � �230Th/Ba�m

� �1 � �1/A(t)�� (18)

Assuming equilibrium partial melting in presence of relevantphlogopite, then (226Ra)m � (230Th)m and Eqn. 17 does notapply; therefore Eqn. 15 becomes:

�230Th/Ba�M � ���226Ra/Ba�M� � �1/A(t)�� � �230Th/Ba�m

� ��226Ra/Ba�m � �1/A(t)�� (19)

Eqn. 18 (for disequilibrium partial melting) and 19 (for equi-librium partial melting) can be indifferently used to calculatethe ascent time tat. In fact, since A(t), (230Th/Ba)m and (226Ra/Ba)m may be considered constant, plotting (230Th/Ba)M vs.(226Ra/Ba)M one obtains, in case of mixing, a straight line withslope corresponding to 1/A(t) i.e, (230Th/226Ra)f, a function oftat (Eqn. 3), and whose intercept on y-axis is a constant. Fittinga straight line through RHA data, plotted in a (230Th/Ba) vs.(226Ra/Ba)o diagram (Fig. 11), yields a best-fit line whoseequation is:

�230Th/Ba�M � ��226Ra/Ba�M � �0.10 � 0.03�� � 0.0007.

(20)

The slope value close to 0.10 � 0.03 corresponds to (226Ra/230Th)f � 10 � 3. Solving Eqn. 3 for this value yields 12 � 4ka as timing of formation of the 226Ra recoil component, tat.This is the ascent time of MMFs through the local mantlewedge, whereas the age of fluid addition to the melt derived bypartial melting must be � 7 ka, otherwise the 226Raexcess wouldnot be measurable. This model assumes that the 226Ra recoilcomponent of the magma formed during a constant time lag tat

and that, after mixing of the components, the magma rose

toward surface within a time �� 226Ra half-life without sig-nificant fractionation between 230Th and Ba.

Since MMFs add 230Th to the melt, it remains to understandthe observed constancy of (230Th/232Th)M, considering that(226Ra/230Th)M varied by a factor of 2 during the RHA. In the(230Th/232Th)M vs.(226Ra/230Th)M mixing system, mixing linesbetween two end-members are straight lines therefore (230Th/232Th)f must be similar to (230Th/232Th)m. Probably MMFscontain sufficient fluorine to form stable fluoride Th-com-plexes, being able to transport not only a pure 230Th recoil-component but also a primary content of 230Th and 232Th(Keppler and Wyllie, 1990). A difference less than 10% be-tween Th isotopic composition of fluids and melts gives avariability of (230Th/232Th)M and (226Ra/230Th)M similar to thatmeasured at Vesuvius. Under these conditions, small fractionsof fluid component (0.5–2%) should give variable (226Ra/230Th)M ratios coupled with an almost constant (230Th/232Th)M.The consequence of the presence of primary 230Th in MMFs isthat the calculated (226Ra/230Th)f. represents a minimum valueand the calculated tat is actually a maximum time. Thus the real(226Ra/230Th)f is 10 and the real tat must be � 12 ka.

4. 226RA-ENRICHED MAGMAS AND VESUVIUSPLUMBING SYSTEM

The TOMOVES and BROADVES seismic projects (De Na-tale et al., 1998, 2001) recently investigated the crustal zonebeneath the volcano. Within the first 5 km there were noevidences of large magmatic reservoirs, whereas between 6 and10 km the seismic resolution did not permit a good discrimi-nation. Between 12–15 km, more probably at around 12 km, areflection/conversion surface was observed, interpreted as asill-like intrusion. The relation of this magma body with theRHA can be discussed on the basis of the geochemical con-straints. Although major elements apparently did not vary sis-tematically between 1631 and 1944 (Belkin et al., 1993), Ca-prarelli et al. (1993) reported 87Sr/86Sr variations during theRHA, that might be interpreted as a temporal trend. Cortini andHermes (1981) and Cortini and Scandone (1982) reportedchanges of the Sr isotopic composition of the magma inferringthat two magmatic reservoirs, set at different depth, alternatedduring the RHA, being active contemporaneously in the 1861–1881. Variable Sr isotopic composition was considered as theresult of mixing of magmas with different isotopic signaturewithin the magma chamber(s) (Civetta et al., 1991). Ne and Heisotopic compositions of 1906 samples show a very high vari-ability between the same eruptive event; this appears anoma-lous because the entire RHA trend is characterized by a mo-notonous decreasing 3He/4He and suggests the existence of twomagmatic reservoirs (Tedesco et al., 1997). Figure 12 illustratesthat (226Ra/Ba)o varied by a factor of 2 during the RHA. If thedimensions of the sill-like reservoir were very large, the massof resident magma should have buffered and re-homogenizedany isotopic variation. Therefore either the sill-shaped reservoirwas not involved during the RHA or it has a limited extension.

Short-term variability of (226Ra/Ba)o, which depends firstlyon different proportions of the two magma-forming compo-nents (sections 3.5.1 and 3.5.2.), can help to understand thepresent Vesuvius dynamics. Studies of silicate-melt inclusionsemphasized the presence of episodes of magmatic refilling

Fig. 11. (230Th/Ba)M vs. (226Ra/Ba)M. Linear fitting of the pointsgives a slope of 0.1 � 0.03 corresponding to (226Ra/230Th)f � 10 � 3.The method used to fit the data is a version of York’s algorithm byLudwig (1994).

177226Ra-excess during the historical activity of Mt. Vesuvius

during the RHA, involving a shallow and a deep reservoir(Vaggelli et al.,1993; Marianelli et al., 1999). Such a refillingshould occur through frequent and small magma pulses(Civetta et al., 1991); thus magmas with different (226Ra/Ba)o

should have refilled periodically the magma chambers and(226Ra/Ba)o variations permit to estimate the magma residencetime in these reservoirs.

Magma chambers can be regarded as chemical reactors (Al-barede, 1993) in a steady or quasi-steady state (succession oftransient steady states) where input (feeding magma) and out-put (erupted magma or injected as dikes along deep fractures)mass rates are equal. In these conditions, is the systemresponse time, i.e., the time required to reach a new steady statewhen the input mass rate is perturbated or jumps to a differentconstant value. Concentration changes of an inert tracer inmagma, injected within the reservoir as a step feed input, arerecorded at the exit (erupted magmas) as a curve of variation ofthe output tracer concentration vs. time, the form of the curvedepending on the transport mechanism within the reservoir.Such a system may be regarded as a combination of two idealreactors: the plug flow reactor (PFR) and the continuouslystirred tank reactor (CSTR) (Smith, 1981). In a PFR magmachamber, the flow is unidirectional without mixing. Magma,having an input tracer concentration equal to Cin, travels thechamber like a plug flow; the output concentration, Cout, isequal to the concentration of resident old material, Ca, at time0 � t � and becomes equal to Cin at t � . In a CSTR magmachamber the entering magma mixes completely and instanta-neously with all the resident magma and Cout varies accordingto:

Cout � Ca � e�t/ � Cin � �1 � e�t/ �. (21a)

Albarede (1993) and Francalanci et al. (1999) adopted CSTR

models of magma chamber with mixing time � to estimate in volcanic reservoirs of Vesuvius, Piton de la Fournaise, Mt.Etna, Stromboli.

A more realistic reactor is a PFR reactor with progressivemixing (PFR-PM) where a limited volume of new enteringmaterial displaces like a plug a comparable volume of adjacentold material, mixing instantaneously only with it. The processof displacement and mixing between equal volumes propagatesthrough the reactor where at the exit:

Cout � Ca � �t/ � �Cin � Ca�� at 0 � t � (21a) and Cout

� Cin at t � (21b)

In case of ratio of two tracers, C1 and C2, if

�C2in/C

2a� � 1 (22)

from Eqns. 21a–21b one obtains:

�C1/C2�out � �C1/C2�a � �t/ � ��C1/C2� in � �C1/C2�a�� at 0 � t

� (23a)

and

�C1/C2�out � �C1/C2� in at t � (23b)

Thus from the output function (C1/C2)out, it is possible toestimate if C2

in /C2a �1. The residence time is the �t required

to (C1/C2)out for varying linearly from a point of discontinuityto another (Fig. 12). It coincides with the solution given byAlbarede (1993) for a magma chamber with a constant varia-tion of isotopic composition. Using isotopic or proxy elementsratios is advantageous because they are much less sensitivethan single elements to effects of crystal fractionation as theterms of the ratios change at a same extent (Albarede, 1993).

Fig. 12. (226Ra/Ba)o magmatic variations during the RHA. The grey line describes the effect of periodical inputs of amagma with 226Ra/Ba close to 0.026 dpm/ppm into a magma chamber continuously fed by a magma whith 226Ra/Baincreasing from 0.08 to 0.016 dpm/ppm during the RHA. Estimated residence times are shown in the lowest side of thefigure. Upper diagram: temporal evolution of the magmatic chamber volume, calculated by: rate of magma feeding � .

178 M. Voltaggio et al.

Ba in sequential samples of RHA varies by a factor � 1.2 (butsample S-1911), then one can assume Bain/Baa � 1 (Eqn. 22)and estimate for magma during the RHA, by interpreting the(226Ra/Ba)o variations in terms of (C1/C2)out of a PFR-PMmagma chamber (Fig. 12). Changes in (226Ra/Ba)o result fromperiodic inputs of 226Ra-enriched magma with constant (226Ra/Ba)o (upper solid line, Fig. 12) into a magma chamber mainlyfed with a less 226Ra-enriched magma with (226Ra/Ba)o in-creasing linearly over time (lower solid line, Fig. 12). Feedingrates were constant during three periods: 1631–1860 (1.0 � 106

m3a�1), 1860–1911 (2.7 � 106 m3a�1) and after 1911 (0.75 �106 m3a�1) (Cortini and Scandone, 1982). In steady stateconditions the product of volumetric flux by equals themagma volume stored into the magma reservoir(s), thus theRHA reservoir volumes can be found from the estimated s andthe known RHA volumetric fluxes. After plotting the calculatedvolumes vs.time, the volume of the magma chamber appears todecrease exponentially from an initial value of 0.1 km3 to apresent magma volume of � 0.021 km3 (upper diagram, Fig.12). Recent assessment of flow hazard at Vesuvius has assumeda steady magma supply of the shallow reservoir evaluated at2–4 106 m3a�1 (Todesco et al., 2002). The authors, neglectingvariations in the feeding rate, inferred that the magma beingstored in the chamber since 1944 corresponds to � 0.2 km3, aresult diverging by an order of magnitude from the presentstudy. By contrast, our estimation of 0.021 km3 is comparableto that calculated (0.056–0.042 km3) independently by Rosi etal.(1987) and is in accordance with the monotonous decreasingtrend of the 3He/4He ratio during the RHA, indicating a con-tinuous volume-decrease of the surficial magma chambers (Te-desco et al., 1997).

The current size of the available magma estimated by the(226Ra/Ba)o variations is too small to be detected by a 3-Dtomography but does not decrease the hazard of this area. Ifsuch a volume of magma is stored within Vesuvius structurenear the surface, a future eruption might start from a muchmore surficial depth than currently thought, increasing thepresent hazard. In conclusion, the variations of (226Ra/Ba)o,which at Vesuvius can be determined with a high precision dueto the unusual [226Raexcess]o of the RHA, contribute to estimatethe volumetric size of the shallow magmatic reservoir(s)present beneath the volcano, providing a model on the currentstate of Vesuvius feeding system.

5. CONCLUSIONS

The 226Ra-excess occurring in magmas erupted during therecent historical eruptive cycle (1631–1944) at Vesuvius couldbe regarded as a multistage process involving daughter-recoilinto slab-derived metasomatic mantle fluids (MMFs). The pres-ence of phlogopite in the mantle-source is not compatible withan origin of the 226Ra-excess exclusively by partial meltingprocesses. MMFs, flowing through a metasomatised mantlewedge, enriched in 226Ra recoiled by �-decay of U-accessoryminerals concentrated in microfractures. The ascent time of thefluids, before their mixing with partial melting-derived melts,was � 12 ka. Mixing of different proportions of MMFs withmantle-derived melts resulted in magmas characterized by vari-able 226Ra-excess, which entered the Vesuvius plumbing sys-tem in the last 7 ka. Processes of crystal fractionation did not

modify (226Ra/Ba)o of these magmas. This ratio varied over the1631–1944 period as function of pulses of feeding magmaswith different (226Ra/Ba)o ratios.

Short-term variability of (226Ra/Ba)o was also influenced byvariable residence times of the magma within shallow cham-ber(s) decreasing in size over time. Assuming steady stateconditions, the estimated volume of the present magmatic res-ervoir is � 0.021 km3. This result implies that, though small,some magma is still located and stored close to the surface andmight be remobilized if physical parameters or thermodynamicconditions inside the reservoir change.

Acknowledgments—This work is dedicated to the memory of ClaudioPetrucciani, a fine friend and gifted colleague of ours. We will bealways grateful for the years of scientific work and friendship we haveshared with him.

Associate editor:Y. Amelin

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181226Ra-excess during the historical activity of Mt. Vesuvius