Mineralogy and Petrology (2001) 73: 121±143

Major, trace element and isotope geochemistry(Sr-Nd-Pb) of interplinian magmasfrom Mt. Somma-Vesuvius (Southern Italy)

R. Somma1;2, R. A. Ayuso2, B. De Vivo1, and G. Rolandi1

1 Dipartimento di Geo®sica e Vulcanologia, UniversitaÁ di Napoli Federico II, Napoli, Italy2 U.S. Geological Survey, National Center, Reston, VA, USA

With 9 Figures

Received May 5, 2000;revised version accepted June 19, 2001

Summary

Major, trace element and isotopic (Sr, Nd, Pb) data are reported for representativesamples of interplinian (Protohistoric, Ancient Historic and Medieval Formations)activity of Mt. Somma-Vesuvius volcano during the last 3500 years. Tephra and lavasexhibit signi®cant major, trace element and isotopic variations. Integration of these datawith those obtained by previous studies on the older Somma suites and on the latestactivity, allows to better trace a complete petrological and geochemical evolution of theMt. Somma-Vesuvius magmatism. Three main groups of rocks are recognized. A ®rstgroup is older than 12.000 yrs, and includes effusive-explosive activity of Mt. Somma.The second group (8000±2700 yrs B.P.) includes the products emitted by the Ottaviano(8000 yrs. B.P.) and Avellino (3550 yrs B.P.) plinian eruptions and the interplinianactivity associated with the Protohistoric Formation. Ancient Historic Formation(79±472 A.D.), Medieval Formation (472±1139 A.D.) and Recent interplinian activity(1631±1944 A.D.) belong to the third group of activity (79±1944 A.D.). The threegroups of rocks display distinct positive trends of alkalis vs. silica, which becomeincreasingly steeper with age. In the ®rst group there is an increase in silica and alkaliswith time, whereas an opposite tendency is observed in the two younger groups.

Systematic variations are also evident among the incompatible (Pb, Zr, Hf, Ta, Th,U, Nb, Rb, Cs, Ba) and compatible elements (Sr, Co, Cr). REE document variabledegrees of fractionation, with recent activity displaying higher La/Yb ratios thanMedieval and Ancient Historic products with the same degree of evolution. N-MORBnormalized multi-element diagrams for interplinian rocks show enrichment in Rb, Th,Nb, Zr and Sm (> �10 N-MORB).

Sr isotope ratios are variable, with Protohistoric rocks displaying 87Sr/86Sr�0.70711±0.70810, Ancient Historic 87Sr/86Sr� 0.70665±0.70729, and Medieval

87Sr/86Sr� 0.70685±0.70803. Neodymium isotopic compositions in the interplinianrocks show a tendency to become slightly more radiogenic with age, from the Protohis-toric (143Nd/144Nd� 0.51240±0.51247) to Ancient Historic (143Nd/144Nd� 0.51245±0.51251). Medieval interplinian activity (143Nd/144Nd: 0.51250±0.51241) lacksmeaningful internal trends. All the interplinian rocks have virtually homogeneouscompositions of 207Pb/204Pb and 208Pb/204Pb in acid-leached residues (207Pb/204Pb�15.633 to 15.687, 208Pb/204Pb �38.947 to 39.181). Values of 206Pb/204Pb are very dis-tinctive, however, and discriminate among the three interplinian cycles of activity (Proto-historic: 18.929±18.971, Ancient Historic: 19.018±19.088, Medieval: 18.964±19.053).

Compositional trends of major, trace element and isotopic compositions clearlydemonstrate strong temporal variations of the magma types feeding the Somma-Vesuvius activity. These different trends are unlikely to be related only to low pressureevolutionary processes, and reveal variations of parental melt composition.

Geochemical data suggest a three component mixing scheme for the interplinianactivity. These involve HIMU-type and DMM-type mantle and Calabrian-type lowercrust. Interaction between these components has taken place in the source; however,additional quantitative constraints must be acquired in order to better discriminatebetween magma characteristics inherited from the sources and those acquired duringshallow level evolution.

Introduction

The eruptive activity at Mt. Somma-Vesuvius during the last 3.5 kyr has beenstudied by several authors (Arn�o et al., 1987; Rolandi et al., 1993 a, b, c, 1998;Ayuso et al., 1998). Three main eruptive periods have been distinguished, on thebasis of volcanological and geochemical criteria. The ®rst period (> 25 to 14 kyrB.P.) is characterized by lava ¯ows with small-scale explosive activity and isassociated with the formation of the Mt. Somma stratovolcano. This periodincluded the explosive plinian eruptions of Codola (25000 yr B.P.), Sarno (17000yr. B.P.), Novelle and Seggiari (16±14 kyr B.P.), and inter-plinian small-scaleeffusive products. The second period (8000±2700 yrs B.P.) is characterized by twoplinian eruptions, Ottaviano (8000 yrs B.P.) and Avellino (3550 yrs B.P.) andProtohistoric interplinian activity. The third period (79±1944 A.D.) of eruptiveactivity is marked by a major plinian (Pompei: 79 A.D.) and two sub-plinianeruptions (`̀ Pollena'' 472 A.D.; 1631 A.D.), accompanied by small-scale effusive-explosive interplinian activity (Ancient Historic, Medieval and Recent). Geo-chemical and isotopic data in the literature have primarily been focused on tephra,lavas, and ejecta erupted during the plinian phases of activity from the Mt.Somma-Vesuvius volcano (e.g., Civetta et al., 1991; D'Antonio et al., 1995;Cioni et al., 1995; Ayuso et al., 1998) and the subplinian phases (e.g. Civetta et al.,1987). Sparse data only are available on the interplinian activities (Hurley et al.,1966; Hoefs and Wedepohl, 1968; Vollmer, 1976; Hawkesworth and Vollmer,1979; Vollmer and Hawkesworth, 1980; Cortini and Hermes, 1981; Cortiniand van Calsteren, 1985; Arn�o et al., 1987; Belkin et al., 1993 a, b; Santacroceet al., 1993; Caprarelli et al., 1993; Ayuso et al., 1998), which are still poorlyknown.

The present study is aimed at ®lling this gap by reporting petrological,geochemical and isotopic data of the interplinian activity at Mt. Somma-Vesuvius,

122 R. Somma et al.122 R. Somma et al.

after the Avellino eruption (3550 yrs B.P.), as recognized by the tephrastratigraphicand tephrachronologic studies of Rolandi et al. (1998). The basic aim will be therecognition of the geochemical and isotopic characteristics and variations of thetephra and lavas, as a ®rst step toward a better understanding of the evolutionaryprocesses at Mt. Somma-Vesuvius during the last 3500 years. Magma sourcecompositions will also be considered.

Stratigraphic features

The Tyrrhenian border of the Italian peninsula is the site of active potassic alkalinevolcanism, which represents the well-known Roman Comagmatic Province(Washington, 1906). Mt. Somma-Vesuvius volcano is the main topographic featurein the Campanian Plain. The latter is a large graben that originated during theUpper Pliocene-Lower Pleistocene, and was affected by considerable gradualsubsidence during the Quaternary. The general structure is well delineated on theborder of the plain, where NE-SW and NW-SE oriented faults acted as the principallineament for the uplift of the Appenine Chain (inland), and Mt. Massico and Mts.Lattari (carbonate massif), which sharply interrupt the plain to the North and to theSouth, respectively (Turco and Zuppetta, 1998).

A detailed description of the stratigraphic correlation of the tephra deposits andlava ¯ows during the last 3550 years has recently been the object of several studies(i.e.: Arn�o et al., 1987; Lirer et al., 1993; Rolandi et al., 1993 a, b, c, d, 1998; Rosiet al., 1993; Mastrolorenzo et al., 1993). We brie¯y summarize here the record ofmagmatic activity at Mt. Somma-Vesuvius after the huge volume of magmaerupted at the 3550 years old `̀ Avellino'' plinian eruption. This covered all of theperivolcanic area reaching the NE side of the Campanian region; the basal fallhas different dispersal axes for the white and gray pumice oriented towards Eand E-NE, respectively; the style of the eruption changed from magmatic tohydromagmatic indicating increasing magma-water interaction during theprogression of volcanic activity (Rolandi et al., 1993 d).

Three strombolian-vulcanian deposits, separated by paleosoils rich in charcoalfragments, are interbedded between the plinian eruptions of `̀ Avellino'' (3550 yrsB.P.) and `̀ Pompei'' (79 A.D.) (Rolandi et al., 1998), which constitute goodstratigraphic markers. Tephra deposits erupted during this interplinian activity, arehere de®ned as Protohistoric; they show variable grain-size and componentdistribution allowing the identi®cation of different units for each eruption.

The Pompei eruption (79 A.D.) is one of the best documented (e.g., Lireret al., 1993; Civetta et al., 1991) plinian eruptions world wide. Deposits consistof a zoned basal fall followed by and interbedded with pyroclastic ¯ows andsurges. Interplinian activity between 79 A.D. eruption and 472 A.D. (`̀ Pollena''subplinian eruption) is here termed Ancient Historic. The deposits arecharacterized by a dark, thick, sandy layer containing lithic and crystal fragmentsand outcropping regularly in the eastern sector of the volcano. The 472 A.D.Pollena eruption (Arn�o et al., 1987; Rosi et al., 1993; Civetta et al., 1987) is thelast highly destructive subplinian activity at Mt. Vesuvius. It is represented by athick basal fall layer interspersed with pyroclastic products related to the laststages of this eruption.

Trace element and isotope geochemistry 123

Eruptive events which occurred in the time span between the explosiveeruptions of 472 A.D. and 1631 A.D. have been termed Medieval interplinianactivity. Its products have been emitted by four eruptive events, characterizedby rhythmic succession of dark scoriaceous layers interbedded with darksandy layers, separated by paleosoils. The eruption of 1631 A.D. (Rosi et al.,1993; Rolandi et al., 1993c) was characterized by surtseyan, subplinian, andvulcanian activity, which gave rise to three layers with different grain size andcompositional volcanic products for the basal tephra fallout. The ®nal stage ofthe eruption was characterized by volatile ± poor magma, which generated theeruptive column collapse and production of pyroclastic ¯ows and surges. Theeruption ended with the emplacement of lava ¯ows along a line of small craterson the southern slope of the Vesuvius cone (Rolandi et al., 1993 c; Rosi et al.,1993).

Products of the Recent activity (1631±1944 A.D.) have been extensivelystudied by several authors (Arn�o et al., 1987; De Vivo et al., 1993; Spera et al.,1998). It is characterized by 18 cycles of semi-persistent activity (lava fountains,gases and vapor emissions from the crater), interrupted by short quiet periods thatnever exceeded seven years.

Analytical methods

New data for 37 samples of the Protohistoric, Ancient Historic, and Medievalinterplinian activity (3550 yrs B.P.±1139 A.D.) are presented in this paper. Resultson whole rock chemistry and radiogenic and stable isotope compositions onsamples spanning the full range of Mt. Somma-Vesuvius activity (> 25000 yrsB.P.±1944 A.D.) have been presented previously (Ayuso et al., 1998).

Table 1 and Fig. 1 give details of locations and the stratigraphic occurrences ofeach formation: Protohistoric, Ancient Historic and Medieval, using the plinian(`̀ Avellino'': 3.550 yrs B.P.; `̀ Pompei'': A.D. 79) and subplinian (`̀ Pollena'': 472A.D.; `̀ 1631 A.D.'') deposits as stratigraphic markers. Whole rock analysesinclude major-, minor-, and trace-element compositions (analyzed at the ACTLaboratories Ontario, Canada and at the U.S.G.S.). Different methodologies havebeen applied as WD/EDX-ray ¯uorescence spectrometry (Al, Ba, Ca, Ce, Cr, Cu,FeTot, K, La, Mg, Mn, Na, Nb, Ni, P, Rb, Si, Sn, Sr, Ti, Y, Zr), wet chemicaltechniques (Cl, CO2, F, FeO, H2O� , H2O±, S), IES/ICP-MS (Be, Ni, Li, Nb, V, Y),INAA (As, Au, Ba, Ca, Ce, Co, Cr, Cs, Eu, Fe, Hf, K, La, Lu, Mo, Na, Nd, Ni, Rb,Sb, Sc, Se, Sm, Sr, Ta, Tb, Ti, Th, U, Yb, W, Zn, Zr), AAS (Pb). For a completediscussion of the various analytical methods used in this paper the reader is referredto Baedecker (1987).

Sr, Nd and Pb-isotope compositions on whole rocks were measured with anautomated, 9-collector Finnigan-MAT 262 mass spectrometer at the U.S.Geological Survey, Reston, VA (USA). Thirty-seven samples were powdered inan agate mortar to minimize metal contamination. Lead isotopes were separatedusing standard anion-exchange resins (Bio-rad 1 AG1x8, 100±200 mesh inchloride form) methods and the sample was reprocessed using the same column; allthe reagents were sub-boiled in a two-bottle of Te¯on 1 still at a temperature< 80 �C to reduce the Pb blanks to < 1 ng during this study. The samples were

124 R. Somma et al.124 R. Somma et al.

loaded on single Re degassed ®laments with the classic method of the silica geland phosphoric acid; isotopic data were collected in static running mode. TheNBS-981 standard value during repeated runs was the following: 206Pb/204Pb�16.920, 207Pb/204Pb� 15.468, 208Pb/204Pb� 36.630. Analytical uncertainty was� 0.1% (2 sigma).

Table 1. Sample localities for pre-A.D. 1631 interplinian Protohistoric, Ancient Historicand Medieval Formations. The eruption chronology is from Rolandi et al. (1998)

3550 yr B.P. to A.D. 79: PROTOHISTORIC activity

Sample Lithotype Locality Eruption (afterRolandi et al., 1998)

S-11(1) Pumice S.Pietro a Terzigno (300 m a.s.l.) IS-12(1) Pumice S.Pietro a Terzigno (300 m a.s.l.) IIS-13(1)a Pumice S.Pietro a Terzigno (300 m a.s.l.) III Unit AS-13(1)b Pumice/scoria S.Pietro a Terzigno (300 m a.s.l.) III Unit AS-14(1) Pumice/scoria S.Pietro a Terzigno (300 m a.s.l.) III Unit BS-15(1)b Pumice S.Pietro a Terzigno (300 m a.s.l.) III Unit CS-16(1) Pumice Pozzelle Boscoreale III Unit DS17(1)a Pumice Lag. Zabatta, S.Gius. Ves III Unit ES17(1)b Pumice Lag. Zabatta, S.Gius. Ves III Unit ES-18(1) Pumice Lag. Zabatta, S.Gius. Ves IS-11(2) Pumice/scoria Lag. Fossa dei Leoni S.G. IIS-12(2) Pumice/scoria Pozzelle Boscoreale III Unit AS-13(2) Pumice/scoria Pozzelle Boscoreale III Unit BS-14(2) Pumice Pozzelle Boscoreale III Unit CS-15(2)a Pumice Pozzelle Boscoreale IIS-18bis Pumice Camaldoli della Torre >A.D.79

A.D. 79 to A.D. 472: ANCIENT HISTORIC activity

S20(1) Dark Sand Terzigno (300 m a.s.l.) Unit AS20(2) Scoria Terzigno (300 m a.s.l.) Unit AS20(3) Dark Sand Terzigno (300 m a.s.l.) Unit BS20(4) Dark Sand Terzigno (300 m a.s.l.) Unit CS20(5) Scoria Terzigno (300 m a.s.l.) Unit DS20(6) Dark Sand Terzigno (300 m a.s.l.) Unit ES20(7) Dark Sand Terzigno (300 m a.s.l.) Unit E

A.D. 472 to A.D. 1139: MEDIEVAL activity

R-1(1) Dark scoria Terzigno (300 m a.s.l.) IR-2(1) Dark scoria Terzigno (300 m a.s.l.) IIR-3(1)A Dark scoria Terzigno (300 m a.s.l.) IIIR-3(1)B Dark scoria Terzigno (300 m a.s.l.) IIIR-3(1)C Dark scoria Terzigno (300 m a.s.l.) IIIR-3(1)D Dark Sand Terzigno (300 m a.s.l.) IIIR-3(1)E Dark scoria Terzigno (300 m a.s.l.) IIIR-3(1)F Dark scoria Terzigno (300 m a.s.l.) IIIR-4(1)A Dark scoria Terzigno (300 m a.s.l.) IVR-4(1)B Dark scoria Terzigno (300 m a.s.l.) IV

Trace element and isotope geochemistry 125

The Sr-isotopes were separated using standard anion-exchange resins (Bio-Rad1 AG 50Wx4, 100±200 mesh, in hydrogen form). The 87Sr/86Sr ratio was nor-malized to 86Sr/88Sr� 0.1194. Analyses (n� 25) of NBS-987 standard gave87Sr/86Sr� 0.710246� 9 (2 �).

Nd-isotopes were separated using standard anion-exchange resins (Bio-Rad 1AG 50Wx4, 200±400 mesh, in methylactic acid form) after collecting the REE.The 143Nd/144Nd ratios were normalized to 146Nd/144Nd� 0.7219. Analyses of LaJolla standard (n� 40) gave an average for 143Nd/144Nd� 0.512845� 7 (2�) andthe Nd blank was less 0.2 ng.

Fig. 1. Generalized geologic map of Mt. Somma-Vesuvius volcano (after Ayuso et al.,1998) and location of samples used in this study

126 R. Somma et al.126 R. Somma et al.

The Sr and Nd samples treated were loaded on double Re degassed ®lamentsand the data were acquired using dynamic collection procedures. Additional detailsused during analysis of Sr, Nd and Pb isotopes are given in Arth and Ayuso (1997)and Ayuso et al. (1998).

Analyses of Sr isotopic compositions for the deposits encompassing the entireeruptive history of Mt. Somma-Vesuvius are in progress and will be the subject of aseparate paper.

Results

Major elements

Whole rock major- and trace-element analyses (Table 2) of interplinianProtohistoric, Ancient Historic and Medieval products (3550 yrs B.P.±1139A.D.) complement the geochemical data set available (Joron et al., 1987; Ayusoet al., 1998) for the interplinian Mt. Somma-Vesuvius volcanic activity. Specialattention is devoted here to a comparison of the Recent interplinian activity (Arn�oet al., 1987; Belkin et al., 1993 a, b; Civetta et al., 1987, 1991) with the olderProtohistoric, Ancient Historic and Medieval interplinian products.

Total alkali vs. SiO2 classi®cation diagram (Le Bas et al., 1986) (Fig. 2) showsthat the 17 rocks of the Protohistoric activity vary from phonolite (1st and 2nd

eruption) to tephri-phonolite (3rd eruption). Pumices and scorias show SiO2

contents varying from 50.9 wt.% to 53.7 wt.%, Na2O from 3.16 wt.% to 4.53 wt.%,and K2O between 6.47 wt.% and 8.76 wt.%. Ten samples of Ancient Historictephra have SiO2 contents between 47.9 wt.% and 49.3 wt.%, Na2O from 2.34 to3.20 wt.%, and K2O between 6.26 and 7.16 wt.%. The ten samples from Medievallavas show only a narrow variation of silica contents (47.2±48.4 wt.%) and a widertotal alkali (Na2O wt.%� 2.16±3.59; K2O wt.%� 6.05±8.25) content, overlappingthe ®eld of the Recent interplinian lavas and tephra (Belkin et al., 1993a, b). Theinset to Fig. 2 shows three generalized (i.e. both plinian and interplinian) evolutiontrends occurring during the Mt. Somma-Vesuvius eruptive history; arrows indicatedirection of compositional variations with time. These emphasize increasingsteepening of alkali vs. silica trends with time. The ®rst Plinian (I P) eruptive activityoccurring during > 25±14 kyr B.P. (Codola, Sarno, Novelle and Seggiari) displaysa positive trend of alkalis vs. silica and an increasing degree of chemical evolutionwith time. The second Plinian (II P) activity (8000±2700 yrs B.P.) shows adecreasing alkali content with respect to age. The third Plinian and Sub-plinianrocks (III P) (79±472±1631 A.D.) are aligned along an almost vertical trendoverlapping the Ancient Historic, Medieval, and Recent interplinian activity.

Figure 3(a±d) shows variation diagrams of major elements with respect to MgO(wt. %) for the Protohistoric, Ancient Historic and Medieval interplinian activity,younger than Avellino plinian eruption. The decreasing contents in Fe2O3 Tot., CaO,P2O5, TiO2 and the associated increase in Na2O and Al2O3 contents (and to a lesserextent K2O) during the evolution of the magma in each cycle suggests crystalfractionation of clinopyroxene, plagioclase, and only subordinately olivine andFe-Ti oxide, as for the Recent interplinian activity (Belkin et al., 1993a, b). There isa time-related variation of major element contents in rocks with the same MgO

Trace element and isotope geochemistry 127

Tab

le2.

Rep

rese

nta

tive

majo

roxi

de

and

min

or-

and

trace

-ele

men

tabundance

of

Pro

tohis

tori

c,A

nci

ent

His

tori

cand

Med

ieva

lin

terp

linia

nF

orm

ati

ons

Sam

ple

S1

1(1

)S

12

(1)

S1

3(1

)bS

14(1

)S

15(1

)bS

16(1

)S

17(1

)bS

18(1

)aS

20(1

)S

20(3

)S

20(4

)S

20(6

)R

1(1

)R

3(1

)AR

3(1

)BR

3(1

)DR

3(1

)ER

4(1

)B

Lit

hoty

pe

Pu

mic

eP

um

ice

Sco

ria

Sco

ria

Pum

ice

Pum

ice

Pum

ice

Pum

ice

Dar

k

sand

Dar

k

sand

Dar

k

sand

Dar

k

sand

Dar

k

sand

Dar

k

sand

Dar

k

sand

Dar

k

sand

Dar

k

sand

Dar

k

sand

Fo

rmat

ion

Pro

toh

ist.

Pro

toh

ist.

Pro

toh

ist.

Pro

toh

ist.

Pro

tohis

t.P

roto

his

t.P

roto

his

t.P

roto

his

t.A

n.

His

t.A

n.

His

t.A

n.

His

t.A

n.

His

t.M

edie

val

Med

ieval

Med

ieval

Med

ieval

Med

ieval

Med

ieval

Maj

or

ox

ides

(wt.

%)

SiO

25

2.3

05

1.9

05

2.3

05

1.8

052.6

051.3

052.6

050.9

047.9

049.0

049.2

048.6

047.9

047.5

047.5

047.3

048.2

048.2

0

Al 2

O3

18

.40

18

.10

19

.00

19

.20

19.7

018.8

020.0

018.4

018.3

015.5

016.4

016.0

018.3

016.1

016.5

016.8

017.1

017.0

0

Fe 2

O3to

t4

.86

5.0

45

.13

5.7

35.4

56.0

95.0

56.4

27.1

66.7

56.7

37.1

57.1

68.1

78.2

08.2

78.1

97.7

4

FeO

tot

4.3

74

.53

4.6

25

.16

4.9

05.4

84.5

45.7

8±

±±

±±

±±

±±

±

FeO

wet

2.4

82

.45

2.4

22

.94

2.8

92.5

62.3

52.6

1±

±±

±±

±±

±±

±

Mg

O2

.76

2.9

81

.76

2.0

31.4

02.3

10.9

02.7

93.4

45.5

14.6

94.8

03.4

45.1

64.4

94.1

74.1

94.5

5

CaO

6.5

27

.46

5.6

16

.43

5.4

66.8

04.7

67.6

68.4

710.4

09.3

810.1

08.7

29.8

89.4

28.7

88.7

610.1

0

Na 2

O4

.10

4.0

34

.37

3.7

84.0

83.6

14.1

93.3

93.2

02.3

42.8

72.7

33.2

02.3

72.3

22.5

02.5

12.2

4

K2O

7.1

46

.47

6.7

17

.43

8.1

17.1

28.4

96.5

76.8

36.2

66.8

46.9

56.8

36.9

07.1

27.2

77.5

06.4

0

TiO

20

.56

0.5

70

.57

0.6

60.5

90.7

10.5

10.7

00.8

00.7

40.7

30.7

70.8

00.9

60.9

30.9

50.9

30.8

7

P2O

50

.36

0.3

50

.29

0.3

80.2

80.4

30.1

70.3

90.6

10.6

20.5

70.6

40.6

10.8

30.7

80.7

90.7

80.6

3

Mn

O0

.14

0.1

40

.13

0.1

40.1

40.1

40.1

40.1

40.1

50.1

40.1

40.1

40.1

50.1

50.1

50.1

50.1

50.1

4

CO

20

.10

0.1

60

.61

0.0

30.0

90.0

20.0

2<

0.0

1±

±±

±±

±±

±±

±

S±

±±

±±

<0.0

5±

<0.0

5±

±±

±±

±±

±±

±

Cl

0.5

20

.44

0.4

70

.71

0.7

90.5

60.8

70.3

9±

±±

±±

±±

±±

±

F0

.28

0.2

30

.24

0.2

20.2

30.2

30.2

50.1

9±

±±

±±

±±

±±

±

H2O�

0.9

20

.95

1.4

70

.73

0.8

70.9

40.8

81.1

1±

±±

±±

±±

±±

±

H2O

±0

.50

0.5

00

.53

0.2

80.3

20.2

40.3

90.2

9±

±±

±±

±±

±±

±

To

tal

99

.36

99

.16

98

.58

99

.52

100.0

299.2

899.2

099.3

496.8

697.2

697.5

597.8

897.1

198.0

297.4

196.9

898.3

197.8

7

LO

I1

.69

1.8

22

.69

1.3

21.6

81.3

31.7

81.4

91.6

71.5

01.3

40.7

91.6

70.3

51.2

01.1

10.9

41.1

1

Tra

ceel

emen

ts(p

pm

)

Rb

325

33

03

00

33

5345

345

385

350

328

289

281

257

283

268

351

285

303

293

Cs

25

.10

22

.30

25

.60

21

.60

23.7

021.9

026.8

021.7

020.0

017.0

017.0

013.0

016.0

014.0

019.0

015.0

016.0

016.0

0

Sr

600

69

08

00

86

0840

860

850

890

1080

987

956

989

1160

1050

1260

1040

1030

1060

Ba

11

00

12

00

13

50

14

50

1450

1450

1450

1450

2010

1840

1770

1840

1910

2360

2780

2220

2203

2130

Pb

57

50

59

56

64

52

69

53

46

35

31

25

24

30

41

31

31

33

La

93

.20

87

.70

81

.10

75

.80

77.5

073.3

075.6

067.5

070.6

064.2

058.3

055.9

069.6

050.5

064.0

051.5

051.0

051.0

0

Ce

15

71

47

13

91

30

13

2129

128

119

129

120

107

103

131

99

127

104

100

103

Nd

50

.60

46

.70

44

.70

44

.60

44.7

045.4

044.6

044.9

051.9

050.8

043.7

045.5

054.5

047.9

059.9

047.0

045.9

047.1

0

Sm

8.3

58

.22

8.6

08

.84

8.5

79.2

17.5

39.0

99.6

210.2

08.1

88.8

510.1

09.7

012.4

09.5

69.2

29.8

6

Eu

1.7

21

.69

1.8

31

.91

1.8

21.9

81.6

62.0

12.0

52.5

12.1

42.2

12.4

72.5

03.1

82.5

42.4

12.5

2

Gd

±±

±±

±±

±±

7.8

27.5

96.5

96.7

77.7

97.5

29.5

67.5

57.1

77.4

9

Tb

0.7

50

.77

0.8

20

.82

0.7

90.8

60.7

50.8

71.0

51.1

10.9

40.9

71.0

81.0

81.3

81.0

61.0

31.1

0

Tm

±±

±±

±±

±±

0.3

50.3

60.3

20.3

20.3

50.3

40.4

30.3

50.3

30.3

5

Yb

2.0

61

.87

1.9

52

.06

2.2

12.3

12.1

22.1

82.1

42.1

71.9

01.9

42.1

01.9

62.6

92.0

42.0

02.1

7

Lu

0.2

50

.27

0.2

90

.29

0.2

80.3

00.2

90.3

00.2

80.2

70.2

50.2

50.2

80.2

60.3

40.2

90.2

70.2

7

Y3

02

93

03

227

33

30

33

29

28

25

25

29

28

35

28

27

29

Zr

31

52

90

31

52

90

30

5285

310

270

268

245

235

224

262

233

288

240

240

221

Hf

5.8

05

.36

5.6

55

.33

5.5

65.4

25.2

65.4

01.8

02.3

01.6

01.7

02.0

01.5

01.9

01.6

01.6

01.5

0

Nb

KV

X5

34

64

94

749

42

54

36

46

39

39

36

46

32

39

34

34

32

Ta

2.2

62

.12

2.3

52

.27

2.4

02.2

42.4

62.0

71.8

02.3

01.6

01.7

02.0

01.5

01.9

01.6

01.6

01.5

0

Th

37

.60

32

.90

33

.70

31

.00

33.6

029.4

034.9

027.4

027.6

024.7

023.9

022.1

025.9

019.9

024.9

021.7

021.0

019.9

0

U1

1.5

09

.66

10

.09

9.2

710.3

48.9

210.9

08.3

09.4

08.2

18.3

57.7

38.9

86.7

98.3

87.4

57.5

46.5

2

Sc

12

.88

12

.10

7.0

77

.25

4.3

88.6

92.2

011.3

0±

±±

±±

±±

±±

±

Cr

65

65

27

20

20

18

<20

51

122

195

130

123

53

107

79

53

71

81

Co

13

.42

13

.40

11

.00

13

.38

11.1

215.8

07.6

317.2

025.0

026.0

022.0

022.0

021.0

029.0

034.0

027.0

027.0

026.0

0

Ni

22

22

12

10

10

9<

10

11

28

37

19

15

15

17

15

15

15

15

Cu

19

25

19

30

18

24

10

26

138

92

75

73

54

103

135

117

122

58

Zn

87

88

88

80

84

74

86

72

130

79

81

56

70

73

98

76

73

72

Li

36

29

30

28

41

24

34

24

±±

±±

±±

±±

±±

Be

9.2

07

.70

8.9

07

.80

9.0

07.7

010.0

08.4

0±

±±

±±

±±

±±

±

Sn

3.6

03

.30

3.8

02

.80

3.8

02.8

02.3

02.6

04.0

03.0

03.0

03.0

03.0

03.0

04.0

03.0

03.0

03.0

0

W6

.20

4.4

05

.30

5.6

06.6

06.4

07.5

05.2

06.0

05.1

05.3

05.3

06.2

05.3

07.0

05.9

05.9

05.4

0

Mo

3.8

03

.40

4.6

04

.70

5.2

04.3

05.9

03.7

05.0

04.0

04.0

04.0

05.0

05.0

07.0

05.0

05.0

04.0

0

Sb

0.7

80

.76

0.7

20

.64

0.7

40.6

00.7

60.5

90.9

00.8

00.7

00.6

00.6

00.4

00.6

00.5

00.4

00.4

0

Fe

3.6

63

.69

3.7

44

.17

4.0

34.4

93.6

44.7

6±

±±

±±

±±

±±

±

As

18

.90

16

.80

16

.50

16

.10

16.6

014.2

018.0

012.1

014.0

010.0

011.0

011.0

06.0

09.0

010.0

010.0

010.0

09.0

0

Aco

mple

tese

tof

the

anal

yti

cal

dat

aca

nbe

obta

ined

from

the

®rs

tau

thor

upon

reques

t

content; at MgO� 4±6 wt. %, Fe2O3Tot, P2O5, and TiO2 increase from AncientHistoric to Medieval and Recent activity, whereas SiO2 and Na2O display a reversetrend. The Protohistoric activity seems to follow this trend, although the lack ofma®c rocks in this group casts some doubts on this conclusion.

Trace elements

Figure 4(a±d) shows selected trace elements plotted against MgO wt. %. TheProtohistoric interplinian rocks have a well de®ned negative trend for the Rb, Cs,Pb, Zr, Th and U and ¯at trends for Sr and Ba. Overall, there are negative trendsbetween incompatible trace elements and MgO for the investigated rocks, al-though a higher content in Th, U and Zr can be observed for the Ancient Historicand, possibly, for Protohistoric rocks. Recent interplinian activity (Belkin et al.,1993 a, b) has more homogeneous composition for trace elements. Ayuso et al.(1998) used selected trace elements ratios as tools to discriminate the geochemicalsignatures of the magmas erupted at various stages of Mt. Somma-Vesuvius his-tory. In particular, they used incompatible elements like Hf and Th, and their ratios,to distinguish between the different cycles of activity. Variation diagrams of Nb/Ta,La/Yb and Hf/Th vs. MgO, which allow us to identify signi®cant differences

Fig. 2. Classi®cation diagram using the sum of alkalis (Na2O�K2O wt. %) vs. SiO2

(wt. %) for the Somma-Vesuvius rocks (see Ayuso et al., 1998). Protohistoric, AncientHistoric and Medieval (this work). The inset in the ®gure shows the plinian and subplinianeruptive activity as from Ayuso et al. (1998); arrows show within-group silica and alkalisvariation with the time

130 R. Somma et al.130 R. Somma et al.

among the rocks investigated, show a systematic variation of these ratios as afunction of age. The Protohistoric interplinian activity shows variable but overallhigher La/Yb (�23.45±17.37) than the Ancient Historic (�25.55±21.17) andMedieval (�23±20) interplinian activity. Nb/Ta ®rst increases from Protohistoric(�23.45±17.37) to Ancient Historic (�25.56±26.96) and then decreases for theMedieval (�20.00±23.00) and Recent interplinian activity. Hf/Th decreases fromProtohistoric (�0.20±0.14) to Ancient Historic (�0.09±0.07) and Medieval(�0.08±0.07) interplinian activity. Recent interplinian activity (Belkin et al.,1993 a, b) included here for comparison, has values of Nb/Ta� 18.63±13.90;La/Yb� 18.63±13.90; and Hf/Th� 0.39±0.19. Although some variation of elementratios (e.g. decrease of Hf/Th with decreasing MgO in the Recent interplinianactivity) can result from evolutionary processes, the distinct values of elementalratios for rocks with similar MgO content suggest that these differences result fromgeochemically distinct parental magmas.

REE-chondrite normalized patterns (Pearce, 1996) for the Protohistoric, AncientHistoric and Medieval rocks (not shown) exhibit smooth and parallel curves andresemble patterns reported by Ayuso et al. (1998) for magmas erupted during theentire Mt. Somma-Vesuvius activity. We note that the LREE concentrations broadlyincrease with increasing silica within each of the three interplinian cycles (e.g.La� 67.5±93.2 ppm for Protohistoric, La� 55.9±70.6 ppm for Ancient Historic,

Fig. 3. Variation diagrams for major oxides as function of MgO (wt. %) for the investigatedrocks. Data on Recent activity are from Belkin et al. (1993 a, b)

Trace element and isotope geochemistry 131

and La� 48.3±69.6 ppm for Medieval samples); contents of the HREE have theopposite trend. N-MORBs-normalized multi-element diagrams (Pearce, 1996) ofthe three interplinian groups (not shown) indicate an enrichment of Rb, Th, Nb, Zrand Sm compared to N-MORBs (> 10 �N-MORB).

Isotope data (Sr, Nd and Pb)

Isotopic analyses of Sr, Nd and Pb (Table 3) for whole rocks from the threeformations (Protohistoric, Ancient Historic and Medieval) are compared with datafrom the literature representing the entire Mt. Somma-Vesuvius activity (Vollmer,1976; Vollmer and Hawkesworth, 1980; Cortini and Hermes, 1981; Cortini and vanCalsteren, 1985; Civetta et al., 1987, 1991; Santacroce et al., 1993; Caprarelliet al., 1993; Cioni et al., 1995; D'Antonio et al., 1995; Ayuso et al., 1998).143Nd/144Nd show an overall variation from 0.51232� 5 to 0.51251� 5. Valuesplot well within the trend de®ned by the magmas erupted along the Tyrrhenianborder of the Italian peninsula (Fig. 8); Nd isotopic compositions for lavas andtephras measured by different authors for the same eruption are slightly different.This may be due to different values of the same standard utilized to normalize theNd isotope ratios in the different laboratories, but may also reveal actual variationsof 143Nd/144Nd ratios for the same eruption unit (Caprarelli et al., 1993).

Fig. 4. Variation diagrams for selected trace elements as a function of MgO (wt. %) for theinvestigated rocks. Data on Recent activity are from Belkin et al. (1993 a, b)

132 R. Somma et al.132 R. Somma et al.

Table 3. Representative Sr, Nd and Pb isotopic compositions of Protohistoric, AncientHistoric and Medieval interplinian Formations

Sample 206Pb/204Pb wr

207Pb/204Pb wr

208Pb/204Pb wr

143Nd/144Nd wr 87Sr/86Sr

R-1(1) res 19.034 15.679 39.141 0.512504 0.706846R-1(1) L1 19.053 15.709 39.225 na naR-1(1) L2 19.024 15.672 39.111 na naR-3(1)A res 18.977 15.647 39.005 0.512453 0.706851R-3(1)A L1 18.973 15.641 38.994 na naR-3(1)A L2 18.965 15.630 38.960 na naR-3(1)B res 19.014 15.670 39.106 0.512440 naR-3(1)B L1 18.976 15.632 38.987 na naR-3(1)B L2 19.000 15.656 39.070 na naR-3(1)D res 19.041 15.687 39.181 0.512436 naR-3(1)D L1 19.032 15.675 39.142 na naR-3(1)D L2 19.020 15.662 39.102 na naR-3(1)E res 19.000 15.635 39.012 0.512471 naR-3(1)E L1 18.990 15.621 38.964 na naR-3(1)E L2 19.031 15.672 39.135 na naR-4(1)B res 19.005 15.663 39.070 0.512406 0.707308R-4(1)B L1 18.981 15.647 39.013 na naR-4(1)B L2 19.043 15.640 39.017 na naS20(1) res 19.049 15.655 39.089 0.512449 0.707286S20(1) L1 19.033 15.634 39.019 na naS20(1) L2 19.043 15.649 39.066 na naS20(3) res 19.061 15.660 39.103 0.512462 naS20(3) L1 19.050 15.646 39.058 na naS20(3) L2 19.044 15.640 39.038 na naS20(4) res 19.041 15.647 39.050 0.512477 0.706979S20(4) L1 19.044 15.643 39.044 na naS20(4) L2 19.062 15.668 39.124 na naS20(6) res 19.019 15.644 39.019 0.512467 naS20(6) L1 19.032 15.638 39.019 na naS20(6) L2 19.045 15.656 39.074 na naS11(1) res 18.978 15.673 39.104 0.512405 0.707449S11(1) L1 18.942 15.622 38.928 na naS11(1) L2 18.967 15.652 39.029 na naS12(1) res 18.971 15.652 39.030 0.512443 naS12(1) L1 18.966 15.645 39.005 na naS12(1) L2 18.961 15.641 38.993 na naS13(1)b res 18.950 15.639 38.988 0.512453 naS13(1)b L1 18.937 15.630 38.954 na naS13(1)b L2 18.943 15.637 38.978 na naS14(1) res 18.942 15.634 38.970 0.512470 0.707611S14(1) L1 18.957 15.653 39.030 na naS14(1) L2 18.949 15.633 38.960 na naS15(1)b res 18.950 15.642 38.995 na naS15(1)b L1 18.948 15.637 38.975 na naS15(1)b L2 18.933 15.619 38.917 na na

(continued)

Trace element and isotope geochemistry 133

Figure 5 plots Nd isotopic compositions of rocks from the three interplinianactivities as a function of stratigraphic position as established by Rolandi et al.(1998). It is important to note that Nd isotopic compositions change broadlyaccording to the stratigraphic position of the sample, with an increase from theProtohistoric formation (143Nd/144Nd: 0.51240±0.51247) to the Ancient Historicformation (143Nd/144Nd: 0.51245±0.51251); medieval rocks display less radiogenicNd higher in the sequence (143Nd/144Nd: 0.51250±0.51241). The Nd isotopic

Table 3 (continued)

Sample 206Pb/204Pb wr

207Pb/204Pb wr

208Pb/204Pb wr

143Nd/144Nd wr 87Sr/86Sr

S16(1) res 18.963 15.662 39.058 0.512456 naS16(1) L1 18.998 15.692 39.157 na naS16(1) L2 18.977 15.672 39.089 na naS18(1)a res 18.955 15.639 38.980 0.512469 0.707522S18(1)a L1 18.953 15.638 38.972 na naS18(1)a L2 18.954 15.640 38.981 na na

A complete set of the analytical data can be obtained from the ®rst author upon request

Fig. 5. Nd isotopic composition as a function of the stratigraphic position for the volcanicserupted by the Mt. Somma-Vesuvius after the Avellino plinian eruption (3550 yrs B.P.).Source of data: Recent activity (Caprarelli et al., 1993; Ayuso et al., 1998); `̀ Avellino'',`̀ Pompei'' plinian and `̀ A.D. 1631'' subplinian eruptions (Ayuso et al., 1998); `̀ Pollena''subplinian eruption (Civetta et al., 1987)

134 R. Somma et al.134 R. Somma et al.

compositions of the Recent activity products are highly variable (Caprarelli et al.,1993; Ayuso et al., 1998).

Preliminary data for Sr isotopic composition of the interplinian activitiesstudied show opposite trends as Nd, with signi®cant variations in the Protohistoric(87Sr/86Sr: 0.70711±0.7081), Ancient Historic (87Sr/86Sr: 0.70729±0.70665), andMedieval (0.70803±0.70685) products.

Lead isotopic analyses have been carried out following a stepwise acid leachingprocedure applied to the whole rocks. If we consider the Pb isotopic compositionsof the Protohistoric, Ancient Historic and Medieval interplinian rocks (Table 3) wenote that all values of the three leached fractions (R, L1 and L2) are within thecompositional range documented for the entire eruptive activity at Mt. Somma-Vesuvius (Ayuso et al., 1998) (206Pb/204Pb� 18.963±19.120; 207Pb/204Pb�15.617±15.717; 208Pb/204Pb� 38.984±39.285). Figure 6 illustrate the Pb isotopecompositions for whole rocks residue fraction, showing values for each interpliniancycle arranged along regression lines that are almost parallel. These trends havedistinct values of 206Pb/204Pb, which can be used to discriminate between thedifferent cycles of interplinian activity. The 206Pb/204Pb ratios systematicallyincrease from the Protohistoric (206Pb/204Pb 18.929±18.990) to Ancient Historic(206Pb/204Pb 19.018±19.088), and then decrease during the Medieval interplinianactivity (206Pb/204Pb 18.964±19.053). The Recent (1631±1944 A.D.) interplinianproducts (Ayuso et al., 1998) show a wider compositional range for 206Pb/204Pb(18.944±19.050), overlapping the values of all interplinian activities studied.

Fig. 6. Pb isotope diagrams for whole rocks residue fraction 208Pb/204Pb vs. 206Pb/204Pb.Protohistoric, Ancient Historic and Medieval (this study); Ventotene xenoliths (De Vivoet al., 1995); Somma Cumulate (Cortini and Hermes, 1981): Recent (Ayuso et al., 1998)

Trace element and isotope geochemistry 135

Figure 7 shows the 206Pb/204Pb values of residue, leach 1, and leach 2 of rocksof the Medieval interplinian activity as a function of stratigraphic position. Leach 1and Leach 2 are compositions of material extracted by two leaching steps andrepresent the composition of non-magmatic material. As stated above 143Nd/144Ndvalues and, to a greater extent the wide range of lead isotopic compositions allowus to reconstruct a geochemical stratigraphy for the interplinian formations. Ingeneral, there is a trend in isotopic values from more radiogenic residue (R) to lessradiogenic leach 1 (L1) and leach 2 (L2). In particular, the leach 2 (L2) Pb isotopiccomposition is very close to the residue (R) fraction. Considering the Pb isotopiccompositions of the Protohistoric interplinian activity in detail we note that all,except for a few samples (S14(1); S16(1) and S17(1), representing the upper partof the stratigraphic column), follow the general rule. The same is true for samplesof the Ancient Historic (S20(4); S20(5); S20(6)) and Medieval (R1(1); R2(1);R3(1)C; R3(1)E; R4(1)A; R4(1)B) interplinian formations.

Discussion and conclusions

The volcanic rock groups of various ages display different composition for severalmajor, trace elements and isotopes. Therefore, petrological and geochemical datacan be distinctive and discriminate among groups of magmas during the entireeruptive history of the Mt. Somma-Vesuvius (Joron et al., 1987; Ayuso et al., 1998).Each group displays distinct trends of internal variations. Magmas erupted duringearly stages of activity at Mt. Somma-Vesuvius (Ayuso et al., 1998) have shown arather ¯at trend of alkali vs. silica with Na2O�K2O: � 6.3±11.7 and SiO2:� 48.1±58.8. The second group, represented by the Ottaviano and Avellino plinian

Fig. 7. 206Pb/204Pb isotopic composition of leached samples as a function of thestratigraphic position for the Medieval volcanics erupted by the Somma-Vesuvius. A lineconnects residue fractions extracted

136 R. Somma et al.136 R. Somma et al.

eruptions and Protohistoric interplinian activity, shows a steeper trend,(Na2O�K2O: � 15.5±9.96; SiO2: � 54.1±50.9). Rocks of the third group displayan almost vertical trend, with strong enrichment in alkalis at almost constant silica(Na2O�K2O: � 15.3±6.46; SiO2: � 52.7±46.76) contents. These trends highlightsigni®cant variations of magma evolution with time, which are the effect ofdifferent processes but probably also re¯ect an increasing alkaline character ofprimary melts with time.

Figure 3a±d shows overall decrease in Al2O3, Na2O, K2O, and increase inFe2O3Tot., CaO, TiO2, and P2O5 from the Protohistoric to the Ancient Historicand Medieval activity. These variations result from crystal fractionation ofclinopyroxene, plagioclase, and subordinate olivine and Fe-Ti oxides (Civetta andSantacroce, 1992; Belkin et al., 1993 a, b; Villemant et al., 1993; Trigila et al.,1993). This conclusion is supported by variations in compositions of some traceelements as shown in Fig. 4a±d. However, ratios of some incompatible elements(i.e. Nb/Ta, La/Yb and Hf/Th) display variable values for different rock groups,at the same MgO level. These differences cannot be explained by simplemechanism of crystal fractionation and associated crustal assimilation, althoughthe internal variations within each group could well result from evolutionaryprocesses. Most of these elements (e.g. Ta, Nb, Th) have similar partitioncoef®cients for the main minerals in potassic rocks and their ratios should notchange signi®cantly during fractional crystallization. On the other hand,interaction with the continental crust may modify incompatible element ratios;however, crustal contamination is unable to discriminate signi®cantly amongsome incompatible elements and the large differences of e.g. Nb/Ta and Hf/Thfor rocks cannot result from this process. Therefore, the geochemical differencesobserved among the various groups are likely to re¯ect pristine compositionaldifferences of parental magmas.

The inferred variations of incompatible element ratios in the Somma-Vesuviusmagmas are unlikely to depend on various degrees of partial melting of a singlemantle source, and most probably re¯ect contributions from different mantlereservoirs tapped during the interplinian activities.

It is noteworthy that Nd isotopic compositions of the Avellino and Pompeiplinian eruptions and Pollena plinian eruption decrease during each eruptive event.We interpret this as possibly re¯ecting the progressive emptying of a zoned magmachamber and tapping of less evolved magma at the bottom of the chamber.Reactivation of the eruptive activity at Mt. Somma-Vesuvius, after plinianeruptions, with the interplinian eruptions (Protohistoric and Ancient Historic),shows an inverse geochemical trend, consistent with emptying of the magmachamber after the Avellino and Pompei plinian eruptions. For the plinian`̀ Pollena'' 472 A.D. eruption, emptying of the magma chamber is accomplishedonly at the end of the Medieval interplinian activity.

Geochemical heterogeneity in the mantle source of Mt. Somma-Vesuvius maybe due to a complex interplay of different components involved in mantleevolutionary history, such as subduction-related crustal rocks, ¯uids and sediments,and intraplate material (Hawkesworth and Vollmer, 1979; Civetta et al., 1987;Santacroce, 1987; Di Girolamo, 1987; Ellam et al., 1989; D'Antonio et al., 1995;Ayuso et al., 1998; Peccerillo, this volume).

Trace element and isotope geochemistry 137

Additional information on the identity of possible magma sources involvedduring the formation of the volcanic rocks erupted during the interplinian activitycan be obtained from the Sr, Nd and Pb isotopic compositions. If we take intoaccount the compositional range of the lead isotopes (208Pb/204Pb) of theinterplinian episodes we note that 206Pb/204Pb values can be used to discriminateamong interplinian cycles (Fig. 6). The lead isotopic compositions of the sampleslie along regression lines that vary systematically with age, increasing fromProtohistoric to Ancient Historic, and then becoming less radiogenic for206Pb/204Pb in the Medieval and Recent interplinian episodes.

Variations of the 206Pb/204Pb isotope composition with time are shown in Fig. 7for the Medieval magmas and these allow to reconstruct a geochemical strati-graphy. 206Pb/204Pb values in the acid-leached rock residues of the Protohistoricmagmas decrease for rocks of the ®rst to the second eruptive cycle and thenincrease again. The same trend is noted for the 206Pb/204Pb isotopic compositionsof the residue fractions of the Ancient Historic. 206Pb/204Pb increases initiallywithin unit A, decreasing in units B, C and D, and ®nally increasing at the end ofthe interplinian activity (unit E). Less distinctive is the evolutionary trend for206Pb/204Pb in the Medieval residues fraction, which changes rapidly. In summary,the Nd and Pb isotopic compositions of the Protohistoric, Ancient Historic andMedieval interplinian activity show consistent changes within each cycle that canbe explained by the presence of a zoned magma chamber and between cycles dueto different magma contributions.

Magma batches originating from oceanic or continental lithosphere subduction,or intraplate mantle upwelling could have been involved at Mt. Somma-Vesuvius(see Peccerillo, this issue). Zindler and Hart (1986) classi®ed mantle magmasources as Depleted Mantle MORB (DMM) and Enriched Mantle (EM). The lattercan be divided into mantle with high U/Pb ratio (HIMU); mantle with low U/Pbvalues and low to intermediate Rb/Sr ratios and low 143Nd/144Nd (EMI); mantlewith low to intermediate values of 143Nd/144Nd, and high values of Rb/Sr and206Pb/204Pb (EMII). Figure 8 shows the 87Sr/86Sr and 143Nd/144Nd values ofmagmas erupted along the Tyrrhenian Sea, consistent with a regional variationfrom NE to SW showing decreasing 87Sr/86Sr and increasing 143Nd/144Nd values(e.g. Vollmer, 1976, 1977; Hawkesworth and Vollmer, 1979; Rogers et al., 1985;Civetta et al., 1987, 1991; Santacroce, 1987; Caprarelli et al., 1993; De Vivo et al.,1995; Cioni et al., 1995; Ayuso et al., 1998). The Vulsini district, Ventotene island,Mt. Sabatini and Albani Hills have high 87Sr/86Sr and low 143Nd/144Nd ratios.Roccamon®na volcano has a wider compositional range for Sr and Nd, overlappingthe Central Italy volcanic districts to the north, and Mt. Somma-Vesuvius, CampiFlegrei, Ischia and Procida islands to the south. All values are above the NorthernHemispere Reference Line (Hart, 1984). Considering the main petrologicalcharacteristics of magmas (e.g. the strong degree of silica undersaturation and thema®c composition of many rocks), trace element concentrations and the isotopiccomposition of oxygen (Turi and Taylor, 1976; Ferrara et al., 1986; Ayuso et al.,1998), a simple process of AFC cannot explain the regional isotopic variation ofmagmas (e.g., Peccerillo, 1999). Taking into account magmas erupted in Centraland Southern Italy (Fig. 8) it is likely that they illustrate a mixing trend betweenHIMU and EMII-type mantle sources.

138 R. Somma et al.138 R. Somma et al.

Figure 9 shows 208Pb/204Pb vs. 206Pb/204Pb for the magmas erupted in theRoman and Campanian Region along the Tyrrhenian border. It is evident that206Pb/204Pb decrease from the south to the north, but the various provinces plotbetween the intraplate volcanoes of Etna and Mt. Iblei (eastern Sicily), and uppercrust composition, such as those of the Calabrian basement. 206Pb/204Pb temp-oral variations for the interplinian activity can be related to the involvement ofdifferent amount of crustal material of Calabrian type during the magmasformation.

In summary, the temporal evolution of major, trace elements and isotopecompositions shown by the various volcanic groups and within the different groupsresult from the combined effect of the differences in the magma sources and ofcomplex evolutionary processes (e.g. AFC, magma mixing) occurring in themagma chamber. The negative correlation indicated by Cr and MgO vs. 87Sr/86Sr(®gure not shown) within each group can be attributed to the different role playedby crustal assimilation with time. This process becomes even more complicated forthe presence of zoned magma chambers as shown by the evolution of 143Nd/144Ndand other parameters. Moreover, the geochemical differences among the differentgroups cannot be explained as due exclusively to different degrees of magmaevolution. These processes cannot explain the variation of incompatible elementsratios (e.g. Na/Ta), or other geochemical differences exhibited by the different

Fig. 8. 143Nd/144Nd vs. 87Sr/86Sr diagram for the volcanic products erupted along theThyrrhenian border of the Italian peninsula. Somma-Vesuvius: D'Antonio et al. (1995);Caprarelli et al. (1993); Ayuso et al. (1998) and this study. Ischia and Campi Flegrei:Hurley et al. (1966). Roccamon®na: Vollmer (1976): Cortini and Hermes (1981). RomanProvince: Hurley (1966); Turi and Taylor (1976); Vollmer (1977); Holm and Munksgaard(1982); Rogers et al. (1985); Ferrara et al. (1986). Etna: Carter and Civetta (1977)

Trace element and isotope geochemistry 139

groups at a similar degrees of evolution. They are caused by different parentalmagmas as indicated by the Pb isotope compositions.

Acknowledgements

This work was supported initially with funds to B. De Vivo from Consiglio Nazionale delleRicerche-Gruppo Nazionale di Vulcanologia and by the United States Geological Surveyfor 1997 as part of a proposed three-year project. Funds, however, were not available fromCNR-GNV for the conclusive third year of the project. The work continued mostly withsupport from the USGS to R. Somma during his stay at the USGS, Reston VA as a visitingscientist and as part of a collaborative program between R. A. Ayuso and B. De Vivo. The con-structive criticism of two Mineralogy and Petrology reviewers is gratefully acknowledged.

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Authors' addresses: R. Somma, B. De Vivo and G. Rolandi, Dipartimento di Geo®sica eVulcanologia, Universita' di Napoli Federico II, Via Mezzocannone 8, I-80134 Napoli,Italy, e-mail: rsomma@biol.dgbm.unina.it; R. A. Ayuso, US Geological Survey, NationalCenter, Mail Stop 954, Reston, VA 20192, USA

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