Recent eruptive history and magma reservoir dynamics on the southern East Pacific Rise at 17°30′S
Pre-eruptive geochemistry of the ignimbrite-forming magmas of the Campanian Volcanic Zone, Southern...
27
Mineralogy and Petrology (2003) 79: 99–125 DOI 10.1007/s00710-003-0004-6 Pre-eruptive geochemistry of the ignimbrite-forming magmas of the Campanian Volcanic Zone, Southern Italy, determined from silicate melt inclusions J. D. Webster 1 , F. Raia 2 , C. Tappen 1 , and B. De Vivo 3 1 Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY, USA 2 Department of Earth and Atmospheric Sciences, City College of New York, NY, USA 3 Dipartimento di Geofisica e Vulcanologia, Universita di Napoli Federico II, Naples, Italy Received July 15, 2002; revised version accepted March 27, 2003 Editorial handling: R. Scandone Summary The Campanian Ignimbrite rock samples include two compositionally distinct populations of clinopyroxene phenocrysts, and the entrapped MI (melt inclusions) are also different in composition. The cores of the more MgO-enriched phenocrysts carry basaltic trachyandesite MI that contain >6 wt.% MgO, whereas other phenocrysts contain MI with <4 wt.% MgO. The MgO-enriched MI also contain comparatively greater abundances of F, CaO, TiO 2 ,P 2 O 5 , SO 2 , and Sr and show marginally higher ratios of (CaO=Al 2 O 3 ) than the low-MgO MI. Most of the high-MgO MI also contain comparatively more H 2 O. The MgO-enriched MI are restricted to diopsidic clinopy- roxenes and show minimal compositional variability, demonstrating that they were derived from a common magmatic source or sources. We interpret these MI to represent primary, mafic magma. In contrast, the more evolved, low-MgO melt inclusions, which are restricted to salitic clinopyroxenes, span the compositional range of trachyandesite to trachyte. The low-MgO fractions of Campanian Ignimbrite magma evolved via fractional crystallization with or without mingling or mixing with more primitive, high-MgO magma. Interestingly, the MI from the Giugliano sample also cluster into low-MgO and high-MgO fractions, and the evolutionary trends for major, minor, and trace elements mirror those exhibited by the Campanian Ignimbrite MI, suggesting that both magmas
Transcript of Pre-eruptive geochemistry of the ignimbrite-forming magmas of the Campanian Volcanic Zone, Southern...
Mineralogy and Petrology (2003) 79: 99–125DOI 10.1007/s00710-003-0004-6
Pre-eruptive geochemistryof the ignimbrite-forming magmasof the Campanian Volcanic Zone,Southern Italy, determined fromsilicate melt inclusions
J. D. Webster1, F. Raia2, C. Tappen1, and B. De Vivo3
1 Department of Earth and Planetary Sciences, American Museum of Natural History,New York, NY, USA2 Department of Earth and Atmospheric Sciences, City College of New York, NY, USA3 Dipartimento di Geofisica e Vulcanologia, Universita di Napoli Federico II,Naples, Italy
Received July 15, 2002; revised version accepted March 27, 2003Editorial handling: R. Scandone
Summary
The Campanian Ignimbrite rock samples include two compositionally distinctpopulations of clinopyroxene phenocrysts, and the entrapped MI (melt inclusions)are also different in composition. The cores of the more MgO-enriched phenocrystscarry basaltic trachyandesite MI that contain >6 wt.% MgO, whereas other phenocrystscontain MI with <4 wt.% MgO. The MgO-enriched MI also contain comparativelygreater abundances of F, CaO, TiO2, P2O5, SO2, and Sr and show marginally higherratios of (CaO=Al2O3) than the low-MgO MI. Most of the high-MgO MI also containcomparatively more H2O. The MgO-enriched MI are restricted to diopsidic clinopy-roxenes and show minimal compositional variability, demonstrating that they werederived from a common magmatic source or sources. We interpret these MI to representprimary, mafic magma. In contrast, the more evolved, low-MgO melt inclusions, whichare restricted to salitic clinopyroxenes, span the compositional range of trachyandesiteto trachyte. The low-MgO fractions of Campanian Ignimbrite magma evolved viafractional crystallization with or without mingling or mixing with more primitive,high-MgO magma.
Interestingly, the MI from the Giugliano sample also cluster into low-MgO andhigh-MgO fractions, and the evolutionary trends for major, minor, and trace elementsmirror those exhibited by the Campanian Ignimbrite MI, suggesting that both magmas
were derived from similar or the same source(s) and that the processes of magmaevolution were equivalent for both magmas.
The MI also indicate that the Campanian Ignimbrite and Giugliano magmas did notform by evolution of Taurano magma, because the geochemical trends expressing meltevolution of the former and latter magmas are too dissimilar. Most Taurano MI showhigher (CaO=Al2O3) and contain less SiO2, (Na2OþK2O), Cl, Li, Rb, Cs, Sr, Nb, Th,and U than the high-MgO and low-MgO Campanian Ignimbrite and Giugliano MI,indicating that the Taurano MI represent magmas which were much more primitive.
Introduction
The Campanian Ignimbrite (CI) crops out in and around Naples, Italy, and itssurficial deposits are located mainly to the north and northwest of Naples in theCampanian Plain. The plain is a region of central-southern Italy that is bordered byMesozoic carbonate platforms and has experienced extensive potassic volcanismduring the past 400 ka (Scandone et al., 1991; Rolandi et al., this volume). Erup-tions, for example, at the well-known calderas of Mt. Somma-Vesuvius and atCampi Flegrei, which straddle Naples, have also contributed significantly to theregional volcanic activity (Alberico et al., 2002). The CI eruptions were the mostvoluminous of the Neapolitan region; Barberi et al. (1978) characterized it as themajor explosive event in and around the Mediterranean during the late Quaternary.
Until recently, the CI was generally considered to represent the products of alarge and particularly violent eruption dated at about 37 ka (Deino et al., 1994; Rosiet al., 1996). Civetta et al. (1997) estimated that CI eruptive material covered30,000 km2 of the Neapolitan region immediately after eruption. The CI deposits,however, actually comprise at least two separate eruptive units emplaced approxi-mately 39 ka ago (Fig. 1). The units of the CI (Fig. 1), which include the Gray Tuff(unit 1) and the overlying Incoherent Tuff and Lithic Breccia (unit 2) have beendated with a bulk age (sanidine) of 39.28þ 0.11 ka (De Vivo et al., 2001). Each ofthe two CI units is subject to variations giving rise to a Yellow Tuff facies, and wehave analyzed MI from the Yellow Tuff of the younger unit.
Recent studies indicate that other ignimbrite-forming events in the CampanianPlain preceded and followed the CI eruptions; at least three ignimbrites areolder (289–245, 184–157, 116–104 ka) than the 39 ka CI and one is younger(23–18 ka) (De Vivo et al., 2001; Rolandi et al., this volume). These ignimbriteeruptions were actually preceded by events that represent the onset of volcanicactivity in the Campanian Plain (specifically, in the area now occupied by the Mt.Somma-Vesuvius complex) at about 400 ka (Brocchini et al., 2001). Well-datedxenocryst contamination of the Taurano eruptive units shows ample evidence ofexplosive activity at >315 ka, which is consistent with volcanic activity beginningin the Mt. Somma-Vesuvius area at about 400 to 300 ka. These activities werepostdated by other ignimbrite eruptions in the Campanian Plain at approximately289–245, 184–157 and 116–104 ka (De Vivo et al., 2001; Rolandi et al., thisvolume). The CI eruptions followed at about 39 ka (De Vivo et al., 2001), and theearliest eruptions of Mt. Somma-Vesuvius followed (which began approximately35 ka ago). The latter activities were followed by the Giugliano Tuff-formingevents at about 23–18 ka (De Vivo et al., 2001), and finally the Neapolitan Yellow
100 J. D. Webster et al.
Tuff was deposited at approximately 12 ka (Principe et al., 1982; Rosi and Sbrana,1987; Scandone et al., 1991; Belkin et al., 1985; Orsi et al., 1992). Eruptions at Mt.Somma-Vesuvius and Campi Flegrei continued into recent times and overlap theeruption of some CI and Giugliano magmas.
Although the CI deposits are well studied, important issues remain unresolved.The location of the source vent(s) of the CI, for example, is of particular interest.The source is poorly understood and is currently under considerable debate (see thesummary in De Vivo et al., 2001). The whole-rock chemistry of the CI units hasbeen studied extensively. The rocks include phonolites, trachytes, trachyandesites,and alkali-trachytes (Barberi et al., 1978). Little is known, however, about thecompositions of the CI-forming magmas and whether these magmas are relatedto those that erupted earlier to form the Taurano (and other ignimbrite events) andCampi Flegrei units and later at Mt. Somma-Vesuvius and Campi Flegrei. In this
Fig. 1. Chronologic stratigraphy of potassic volcanic units in the Neapolitan region ofcentral Italy; age dates from De Vivo et al. (2001). Symbols: Giugliano Incoherent Ignim-brite (asterisk), Campanian Ignimbrite (CI) Incoherent Tuff (square filled in lower right),CI Yellow Tuff (square filled in upper left), CI Lithic Breccia (square filled in lower left),CI Gray Tuff (square filled in upper right), Taurano Yellow Tuff (diamond), 183.8 kaTaurano Ancient Stratified Pyroclastics (triangle), and 205.6 ka Taurano Ancient StratifiedPyroclastics (circle)
Pre-eruptive geochemistry 101
regard, Signorelli et al. (1999, 2001) investigated silicate melt inclusions (MI) andmatrix glasses from a basal fall unit, known as the Lower Fall Unit, that wasdeposited as plinian tephra, and they determined that the eruptions involved com-paratively evolved compositions ranging from trachyte to phono-trachyte.
To determine the pre-eruptive compositions of the CI magmas and those mag-mas associated with the Giugliano and Taurano eruptions and to better understandthe processes of magma evolution that ultimately led to these eruptions, we haveanalyzed 54 reheated MI in clinopyroxene phenocrysts as well as the host pheno-crysts from 15 Taurano, CI, and Giugliano rock samples. Our data are comparedwith compositions of the MI and matrix glasses from the Lower Fall Unit (ofSignorelli et al., 2001) and with whole-rock data of the Taurano, CI, and Giuglianosamples to systematically describe the compositions of these magmas prior toeruption. We have focussed this study on geochemical relationships involvingthe magmas that deposited the �39 ka old CI units and the Giugliano and Tauranoseries.
Geologic background
Detailed descriptions of CI petrography are available in Barberi et al. (1978),Di Girolamo et al. (1984), and Rosi et al. (1999). Analysis of the CI basal pumicefall units shows that they are aphyric to phenocryst poor. However, the Lower FallUnit was studied by Castelmenzano et al. (1995), Rosi et al. (1999), and Signorelliet al. (2001), and it contains minor sanidine, clinopyroxene, plagioclase, biotite,apatite, and iron oxides. We consider the Lower Fall Unit as equivalent to the CIbasal pumice (Fig. 1). The 39 ka ignimbrite phenocryst assemblage is dominatedby sanidine with lesser clinopyroxene, plagioclase, biotite, and magnetite (Barberiet al., 1978), and we have observed phenocrysts of sanidine and clinopyroxene inour CI, Taurano, and Giugliano samples. De Vivo et al. (2001) report that theTaurano samples contain mixed sanidine populations of widely varying ages.Signorelli et al. (1999) also describe pyrrhotite inclusions in Lower Fall Unitclinopyroxene crystals. We searched for inclusions of magmatic sulfides in the CIclinopyroxene phenocrysts of this study, but they were not observed. If sulfideminerals were present in the phenocrysts, then elevated SO2 contents of reheatedMI could possibly reflect S contamination through melting of accidentallyentrapped sulfides in the crystallized MI. The dearth of sulfides in the phenocrystsindicates that the abundances of SO2 in the reheated MI represent magmatic values.We, however, did observe inclusions of apatite in the rims of some zoned CIclinopyroxene phenocrysts.
Sample preparation and analytical procedures
We located, reheated, and analyzed silicate melt inclusions in clinopyroxene phe-nocrysts of eleven CI rock samples, one Giugliano sample, and three Tauranosamples (Table 1). Clinopyroxenes in samples AFYI-7, AFYI-9, AFYI-14, andICB-9c are extremely rare, so phenocrysts were separated from glass and otherminerals in these samples using sodium polytungstate solutions and a float-sink
102 J. D. Webster et al.
Tab
le1
.A
vera
ge
com
posi
tions
of
rehea
ted
sili
cate
mel
tin
clusi
ons
incl
inopyr
oxe
ne
phen
ocr
ysts
from
Cam
pania
nIg
nim
bri
te(C
I),
Taura
no,
and
Giu
gli
ano
rock
sam
ple
s
Volc
anic
Giu
gli
ano
CI
Inco
her
ent
CI
Yel
low
CI
Lit
hic
CI
Gra
yT
uff
Tau
rano
Tau
rano
Tau
rano
unit
Inco
her
ent
Tu
ffT
uff
Bre
ccia
Yel
low
Tuff
Str
atifi
edS
trat
ified
Ignim
bri
teP
yro
clas
tics
Pyro
clas
tics
Sam
ple
sIC
B-9
cS
FC
-2,
MO
-2,
SF
C-1
,A
LT
-A
R-1
ICB
-12E
,A
FY
I-14
AF
YI-
9A
FY
I-7
SP
-24
,IC
B-1
3A
AL
T-1
,V
E-1
,S
A-1
Ag
eo
f1
8to
23
39
.6�
1.9
39
.33�
0.8
33
9.0
5�
0.5
13
8.7
0to
39
.19
15
7.4�
1.0
18
3.8�
15
.22
05
.6�
3.0
eru
pti
on
ka
ka
ka
ka
ka
ka
ka
ka
(n,m
)(5
,4)
(6,6
)(1
1,9
)(4
,3)
(11
,8)
(7,4
)(5
,4)
(6,4
)S
iO2
(wt.
%)
54
.07�
4.3
55
5.6
2�
3.4
55
.17�
3.3
50
.70�
0.5
75
5.5
6�
4.1
04
8.6
1�
3.5
55
0.0
5�
3.4
34
8.7
8�
0.9
1T
iO2
0.6
9�
0.3
30
.51�
0.2
20
.71�
0.2
90
.95�
0.0
70
.67�
0.2
41
.14�
0.3
31
.13�
0.4
1.1
3�
0.1
Al 2
O3
15
.88�
1.0
71
5.4
5�
0.7
91
5.2
2�
1.0
41
3.8
0�
1.5
51
4.7
9�
1.3
71
3.0
0�
1.2
71
3.4
9�
1.3
71
3.6
2�
0.3
4F
eO5
.12�
0.9
34
.42�
0.6
64
.70�
0.6
55
.56�
0.4
14
.69�
0.8
87
.54�
2.2
36
.85�
0.5
06
.26�
0.4
9M
gO
4.4
6�
2.3
94
.64�
2.3
4.5
5�
2.2
97
.63�
0.9
24
.47�
2.7
17
.29�
0.9
66
.28�
1.5
38
.11�
0.6
6M
nO
0.1
5�
0.0
50
.13�
0.0
40
.13�
0.1
00
.09�
0.0
30
.21�
0.1
70
.17�
0.0
60
.23�
0.1
70
.13�
0.0
4C
aO8
.03�
3.0
66
.96�
0.9
46
.99�
1.9
71
0.2
4�
2.2
87
.41�
2.0
81
3.3
1�
1.5
21
2.6
5�
2.3
61
2.7
3�
1.7
0K
2O
6.3
2�
2.1
16
.82�
1.2
06
.68�
1.5
35
.36�
0.6
85
.90�
1.3
93
.33�
0.8
13
.41�
1.1
53
.74�
0.8
6N
a 2O
2.2
8�
0.4
42
.42�
0.4
02
.54�
0.7
61
.81�
0.2
23
.29�
1.1
61
.86�
0.1
82
.17�
0.8
92
.00�
0.2
3P
2O
50
.27�
0.1
70
.42�
0.2
60
.28�
0.2
50
.68�
0.1
70
.54�
0.6
60
.52�
0.1
60
.32�
0.2
10
.40�
0.1
3S
O2
0.2
5�
0.2
00
.11�
0.1
10
.09�
0.0
80
.28�
0.0
60
.14�
0.1
70
.26�
0.1
40
.31�
0.2
20
.43�
0.1
6C
l0
.23�
0.1
20
.34�
0.1
00
.33�
0.1
10
.41�
0.0
80
.39�
0.1
70
.18�
0.0
40
.23�
0.0
70
.15�
0.0
3F
0.2
3�
0.0
40
.27�
0.1
50
.27�
0.1
50
.24�
0.0
50
.23�
0.1
30
.24�
0.1
20
.35�
0.1
80
.25�
0.0
8
TO
TA
L9
7.9
99
8.1
19
7.6
69
7.7
59
8.2
99
7.4
69
7.4
79
7.7
4
H2O
(wt.
%)
2.3�
0.5
2.7�
1.0
2.6�
1.0
3.5�
1.4
1.8�
0.9
1.9�
0.5
2.7�
1.3
1.8�
0.4
Li
(pp
m)
20�
45
4�
32
51�
37
34�
19
3�
86
38�
40
20�
81
5�
10
Be
4�
13
.8�
0.6
7�
86�
11
3�
10
3.5�
1.0
6�
33
.8�
1
(co
nti
nued)
Pre-eruptive geochemistry 103
Tab
le1
(conti
nued
)
Volc
anic
Giu
gli
ano
CI
Inco
her
ent
CI
Yel
low
CI
Lit
hic
CI
Gra
yT
uff
Tau
rano
Tau
rano
Tau
rano
unit
Inco
her
ent
Tu
ffT
uff
Bre
ccia
Yel
low
Tuff
Str
atifi
edS
trat
ified
Ignim
bri
teP
yro
clas
tics
Pyro
clas
tics
B2
7�
42
8�
33
4�
24
27�
34
9�
31
20�
16
22�
13
25�
4R
b7
7�
21
11
0�
30
13
0�
33
01
50�
26
17
0�
40
56�
17
30�
15
42�
8S
r3
40�
17
04
90�
29
05
00�
24
08
70�
57
41
0�
38
05
10�
47
22
0�
14
02
20�
54
Y1
8�
42
3�
12
23�
14
18�
13
7�
38
28�
42
8�
11
12�
1Z
r8
5�
21
11
0�
18
15
0�
13
01
10�
32
30�
17
01
30�
25
12
0�
58
50�
15
Nb
9�
31
6�
72
3�
16
20�
23
4�
23
13�
11
0�
58�
2C
s5�
18�
31
1�
71
0�
21
6�
85�
22�
13�
1C
e4
1�
12
62�
21
06
8�
44
63�
31
49�
17
06
2�
13
61�
23
29�
8N
d1
0.1�
2.4
ND
ND
ND
ND
15
.9�
51
7.4�
66
.5�
2S
m8
.0�
1.8
13
.2�
61
1�
79
.1�
12
2�
29
13
.1�
41
4.2�
3.8
5.1�
1D
y2
.9�
15
.2�
34
.8�
33
.7�
18
.9�
11
4.7�
15
.6�
1.9
1.7�
1Y
b3
.5�
15
.2�
35
.2�
43
.3�
19
.1�
10
4.9�
15
.6�
2.6
1.9�
1T
h3�
15�
27�
57�
11
1�
83�
12
.6�
1.9
1.8�
1U
(ppm
)0
.5�
0.2
1�
12�
22�
14�
30
.3�
0.1
0.3�
0.2
0.2�
0.1
Num
ber
of
MI
anal
yze
dby
elec
tron
mic
ropro
be
(n);
num
ber
of
MI
anal
yze
dby
ion
mic
ropro
be
(m).
Sil
ica
thro
ugh
tota
lby
elec
tron
mic
ropro
be
(wt.
%);
H2O
(wt.
%)
and
Li
(pp
m)
thro
ug
hU
(pp
m)
by
ion
mic
rop
rob
e.T
ota
ld
oes
no
tin
clu
de
H2O
or
trac
eel
emen
ts.
All
iro
nas
FeO
.N
Dn
ot
det
erm
ined
104 J. D. Webster et al.
method. A solution with a density of 2.55 g=ml was prepared by mixing distilledwater with sodium polytungstate powder, and the density of the solution wasmeasured with a NIST-calibrated hydrometer. Prior to suspension in the solution,the samples were crushed to roughly 2 mm size, cleaned ultrasonically in distilledwater, and dried at 150 �C. Fifteen to 25 g of crushed sample were added to 50 mlof sodium polytungstate reference solution, and the mixture was stirred briefly andleft undisturbed, overnight, to allow ample time for sample separation. Mineralswith densities greater than 2.55 g=ml (including clinopyroxene) settled to the bot-tom of the tube, and they were trapped there by freezing the lower half of thesuspension with liquid nitrogen. These heavy mineral concentrates were subse-quently removed, rinsed with distilled water, and dried, and clinopyroxene pheno-crysts were picked by hand from the concentrates. Rock samples containingcomparatively abundant phenocrysts (MO-2, SFC-1, SFC-2, SP-2, ALT-1, ALT-4, ICB13A, ICB12E, SA-1, VE-1, and AR-1) were crushed by hand and clinopy-roxene concentrates were separated by handpicking.
The MI (melt inclusions) are partially to completely crystallized, so all MI-bearing phenocrysts were reheated in a muffle furnace at 1 atm to temperatures of1180� to 1220 �C and quenched rapidly. This range in temperature permitted com-plete remelting of all included daughter minerals in a two-hour heating period.After heating and quenching, most MI consist of glass with a shrinkage- or vapor-bubble. Most MI in these samples range from 5 to 40 mm in diameter, and mostanalyzed MI were 20 to 40 mm in diameter. The clinopyroxenes were mounted onglass slides with epoxy and polished to expose the reheated MI at the grain surfacefor study and analysis.
Electron microprobe
The compositions of the MI glasses were determined with a Cameca SX-100electron microprobe using a large (10–20 mm diameter) defocused electron beam.Sodium oxide, K2O, SiO2, Al2O3, MgO, FeO, MnO, CaO, TiO2, P2O5, and F wereanalyzed at 15 keV and 10 nA, and SO2 and Cl at 15 keV and 40 nA. The peakcounting times for Cl and F were 60 s and 40 s, respectively. Pyroxenes wereanalyzed for SiO2, Al2O3, MgO, FeO, MnO, CaO, TiO2, K2O, and Na2O at15 keV, 10 nA, and with a 4mm spot via rim to rim traverses involving 8 to 12analyses per phenocryst (20 s on peak).
Based on counting statistics, the relative 1� precisions for analysis of MIglasses are less than 1% for SiO2, Al2O3, CaO, and K2O; 1 to 2% for FeO,MgO, Na2O, and TiO2; and 2 to 6% for MnO, P2O5, F, SO2, and Cl. The MI weremoved under the electron beam to minimize volatile loss. Replicate analyses,conducted on a single spot, demonstrate that the Cl counts are stable at theseconditions. Natural obsidians of known composition were also studied to determineanalytical accuracy and precision for Cl. The microprobe determined 0.19 to0.21 wt.% Cl in an obsidian containing 0.20 wt.% Cl as determined by wet-chemi-cal analysis. The measured analytical precision for Cl in an obsidian containing0.2 wt.% Cl is 5% relative.
Sulfur was determined with count times of 60 to 120 s on the K-� peak and30–60 s on background. Sulfur K-� wavelength scans of unknown glasses were
Pre-eruptive geochemistry 105
performed first to best determine the appropriate S reference standard (anhydrite ortroilite), because S peaks shift with oxidation state. Precision and accuracy of Sanalyses were monitored with NIST (i.e., NIST¼NBS¼National Bureau of Stan-dard) reference glasses SRM 620 (1121 ppm S), SRM 621 (520 ppm S), and JuanDe Fuca basaltic glass JDF (1420� 40 ppm; Wallace and Carmichael, 1992).Thirty electron probe analyses of S in the JDF glass yielded an average S concen-tration of 1463 ppm (1�¼ 43 ppm). Results on the NIST glasses are similar andwithin 5% of the accepted values.
Secondary ion mass spectrometry (SIMS)
The MI were analyzed for H (reported as H2O), Li, Be, B, Rb, Sr, Y, Zr, Nb, Cs,Ce, Nd, Sm, Dy, Yb, Th, and U by SIMS at the Woods Hole OceanographicInstitution using techniques detailed by Shimizu and Hart (1982) and Websteret al. (1996). The glasses were analyzed with an accelerating potential of 10 keVand beam currents of 1 to 2 nA. Trace elements and hydrogen were analyzed aspositively charged secondary ions utilizing a negatively charged oxygen primaryion beam. The inclusions were analyzed in one spot, five times each in depth profilemode. Based on SIMS analyses of synthetic and natural glasses, H2O concentra-tions are reproducible to �0.3 to 0.4 wt.% and the trace elements to 5 to 15%relative precision. The glass standards used include metaluminous and moderatelyalkaline obsidians and the NBS glasses which are highly calcic and alkaline.Replicate analyses of single MI during different analytical sessions exhibit goodreproducibility.
Results
Clinopyroxene compositions
The compositions of the clinopyroxene phenocrysts range from salite to diopside,and although some of the clinopyroxene phenocrysts are chemically homogeneous,most are zoned. Some phenocrysts have Mg-enriched rims, whereas most clino-pyroxenes have cores characterized by high Mg numbers and Fe-enriched rims(Table 2). The transition in composition from magnesian cores to Fe-enriched rims,and vice versa, is not gradational but is, instead, quite abrupt. This variety ofzoning is consistent with mixing primitive MgO-enriched magma with moreevolved MgO-poor magma, and subsequent crystallization of pyroxene rims thatare distinctly out of equilibrium with the phenocrysts cores. Interestingly, the Fe-enriched rims of several phenocrysts contain high densities of apatite inclusionswhereas the magnesian cores of these same grains contain no apatite inclusions.
Are these melt inclusions representative ofpre-eruptive melt compositions?
The H2O concentrations of the MI are, to some extent, less than those of theentrapped samples of melt they represent because some H2O was lost from theMI after entrapment via diffusion of H2O and=or H2 while the magma and rocks
106 J. D. Webster et al.
Tab
le2
.A
vera
ge
com
posi
tions
ofin
div
idualcl
inopyr
oxe
ne
phen
ocr
ysts
host
ing
rehea
ted
sili
cate
mel
tin
clusi
ons
from
Cam
pania
nIg
nim
bri
te(C
I),
Taura
no,
and
Giu
gli
ano
rock
sam
ple
s
Sam
ple
no
.A
LT
-4-1
SA
1b
SF
C-1
aA
LT
-1-5
AR
-1A
LT
-4-4
SP
3S
FC
2-1
(Mg
-en
rich
ed(F
e-en
rich
ed(u
nzo
ned
(un
zon
ed(u
nzo
ned
(un
zon
ed(F
e-en
rich
ed(F
e-en
rich
ed
nri
m)
2ri
m)
1cp
x)
3cp
x)
3cp
x)
4cp
x)
1ri
m)
2ri
m)
1U
nit
CI
Yel
low
CI
Gra
yC
IY
ello
wC
IG
ray
CI
Lit
hic
CI
Yel
low
CI
Yel
low
CI
Inco
her
ent
Tu
ffT
uff
Tu
ffT
uff
Bre
ccia
Tu
ffT
uff
Tu
ff
SiO
2(w
t.%
)4
9.6
8�
1.0
45
3.6
3�
1.2
54
9.8
0�
1.2
55
2.2
0�
0.3
65
3.2
6�
0.6
95
0.9
1�
0.3
45
2.0
0�
1.4
45
1.5
3�
1.2
0T
iO2
0.6
8�
0.1
60
.36�
0.1
10
.58�
0.1
10
.60�
0.0
90
.42�
0.1
10
.56�
0.0
80
.37�
0.1
20
.42�
0.1
5A
l 2O
34
.24�
0.8
31
.67�
0.4
63
.67�
0.4
32
.74�
0.5
62
.52�
0.5
83
.53�
0.2
22
.10�
0.5
02
.51�
0.8
5F
eO6
.50�
1.2
74
.90�
2.7
28
.13�
0.2
58
.98�
0.5
63
.60�
0.4
48
.54�
0.1
13
.97�
1.8
53
.85�
0.9
1M
gO
14
.49�
0.6
41
6.2
1�
2.3
11
3.3
0�
0.4
01
3.0
3�
0.5
91
6.9
3�
0.4
41
3.4
7�
0.2
01
6.6
7�
1.3
51
6.2
8�
0.6
3M
nO
0.2
4�
0.1
00
.24�
0.3
00
.28�
0.0
70
.62�
0.2
20
.08�
0.0
20
.37�
0.0
60
.15�
0.2
60
.09�
0.0
4C
aO2
3.3
7�
0.6
92
4.0
3�
0.7
92
3.0
8�
0.1
52
3.2
7�
0.2
12
4.3
5�
0.3
22
3.5
4�
0.2
42
4.2
7�
0.4
02
3.8
6�
0.4
4K
2O
0.0
1�
0.0
20
.00�
0.0
00
.00�
0.0
00
.00�
0.0
00
.00�
0.0
00
.00�
0.0
00
.01�
0.0
10
.00�
0.0
0N
a 2O
0.2
7�
0.0
90
.26�
0.2
40
.29�
0.0
40
.56�
0.1
60
.13�
0.0
20
.39�
0.0
50
.21�
0.1
80
.13�
0.0
4
TO
TA
L9
9.8
61
01
.31
99
.12
10
2.0
21
01
.29
10
1.3
39
8.7
79
8.7
0
Sam
ple
no
.V
E-1
MO
-2S
P3
AL
T-1
AF
YI-
14
ICB
9c
AF
YI-
9A
FY
I-7
(Fe-
enri
ched
(Fe-
enri
ched
(Fe-
enri
ched
(un
zon
edcp
x)
(Mg
-en
rich
ed(F
e-en
rich
ed(F
e-en
rich
ed(u
nzo
ned
cpx
)
nri
m)
3ri
m)
2ri
m)
21
rim
)3
rim
)3
rim
)2
4U
nit
CI
Gra
yC
IIn
coh
eren
tC
IG
ray
CI
Lit
hic
Tau
ran
oG
iug
lian
oT
aura
no
Tau
ran
oT
uff
Tu
ffT
uff
Bre
ccia
Yel
low
Tu
ffIn
coh
eren
tS
trat
ified
Str
atifi
edIg
nim
bri
teP
yro
clas
tics
Py
rocl
asti
cs
SiO
2(w
t.%
)5
2.3
3�
0.8
05
1.4
7�
1.2
15
0.6
5�
0.5
85
0.2
4�
0.5
94
8.9
9�
2.3
34
8.0
7�
2.2
64
9.4
6�
1.2
84
9.6
6�
0.5
7T
iO2
0.3
7�
0.0
90
.40�
0.1
20
.59�
0.1
20
.53�
0.1
10
.89�
0.3
61
.16�
0.5
41
.25�
0.2
91
.09�
0.1
0A
l 2O
32
.12�
0.5
62
.46�
0.6
73
.53�
0.8
13
.10�
0.3
74
.82�
1.5
64
.92�
1.8
05
.94�
0.9
95
.48�
0.3
5F
eO3
.95�
1.6
44
.41�
1.7
98
.48�
0.6
38
.45�
0.4
36
.54�
2.0
81
0.1
3�
1.4
75
.68�
0.6
55
.87�
0.1
7M
gO
16
.43�
1.5
81
6.5
2�
1.3
81
3.3
3�
0.7
71
3.3
6�
0.4
31
4.3
7�
1.7
21
1.3
2�
1.7
41
4.4
2�
0.7
51
4.5
9�
0.2
7M
nO
0.1
2�
0.2
30
.13�
0.1
10
.39�
0.2
50
.41�
0.1
00
.14�
0.0
40
.49�
0.0
60
.10�
0.0
20
.11�
0.0
3C
aO2
4.2
1�
0.4
62
3.9
1�
0.3
22
2.6
2�
0.1
92
2.6
6�
0.2
22
3.0
7�
0.3
52
2.4
9�
0.3
22
2.8
5�
0.2
22
2.9
2�
0.2
6K
2O
0.0
0�
0.0
00
.00�
0.0
00
.00�
0.0
00
.00�
0.0
00
.00�
0.0
00
.01�
0.0
10
.00�
0.0
00
.00�
0.0
0N
a 2O
0.1
7�
0.1
40
.16�
0.0
90
.37�
0.1
80
.42�
0.0
90
.26�
0.0
60
.55�
0.1
30
.16�
0.0
30
.19�
0.0
1
TO
TA
L9
9.7
39
9.5
31
00
.00
99
.23
99
.77
99
.18
99
.91
99
.95
Anal
yse
sby
elec
tron
mic
ropro
be
(see
text
for
det
ails
).A
llir
on
asF
eO.Z
onin
go
fcl
inopyro
xen
ees
tabli
shed
by
rim
thro
ugh
core
tori
man
alyti
cal
trav
erse
s;n
nu
mb
ero
fm
elt
incl
usi
on
sh
ost
edb
ycp
x
Pre-eruptive geochemistry 107
were still hot (Roedder, 1984). Some water was also probably lost while the phe-nocrysts were reheated in the laboratory, but this contribution to the loss of H2Owas minimized by reheating the MI for only 2 hours. Water losses notwithstanding,the relative differences in the concentrations of H2O in the MI are useful forconstraining relative differences in pre-eruptive abundances of H2O in the magmas.Even though the reheated MI do not provide accurate information on pre-eruptiveH2O concentrations, they were heated under similar conditions so the relativeabundances of H2O should not vary strongly. We also note that the average abun-dance of H2O in all reheated MI of this study (i.e., 2.3� 1.4 wt.% determined bySIMS) is actually � the 1.6� 0.3 wt.% H2O determined by FTIR for unheated MIfrom the CI Lower Fall Unit (Signorelli et al., 2001).
We have analyzed MI to investigate geochemical trends that are indicative ofprocesses of magma evolution (Table 1), but the MI may also reflect processes ofpost-entrapment melt modification. Post-entrapment crystallization of clinopyrox-ene (Table 2) on the walls of the MI, for instance, will reduce the CaO, MgO, andFeO concentrations of the MI and increase the abundances of most other meltconstituents. Although, the great majority of the reheated MI show no residualdaughter crystals in the MI glass, all clinopyroxene that precipitated on thehost phase walls after entrapment must be remelted to return the MI to theirentrapment compositions. If the phenocrysts containing crystallized MI arereheated at temperatures that are too low, not all post-entrapment clinopyroxenewill melt and the MI compositions will contain lower MgO and FeO, higher SiO2,and exhibit larger values for the [CaO=Al2O3] ratio – making the MI compositionsappear comparatively evolved. Alternatively, reheating crystallized MI at exces-sively high temperatures will melt excess clinopyroxene from the host phase wallsof the MI, and the MI will contain excess CaO and MgO and appear relativelyprimitive.
This potential problem is compounded by the fact that the MI compositionsalso reflect crystallization of clinopyroxene and other phenocrysts prior to entrap-ment, so it is difficult to determine how much of the compositional variation seenin the MI reflects magmatic evolutionary versus post-entrapment change. Thetrends of Fig. 2 indicate that crystallization of clinopyroxene (pre-entrapment crys-tallization, post-entrapment crystallization, or both) has influenced the MI compo-sitions, and that other phenocrysts (e.g., magnetite and feldspar) have crystallized.Thus, the compositional variability of the MI is not the sole result of post-entrap-ment modification (including problems associated with reheating the phenocrysts),because the MI compositions also reflect magmatic processes of melt evolution.Moreover, MI from samples AFYI7 and AFYI14 have higher CaO contents than allother MI, on average, and higher MgO than most other MI, on average, but MIfrom samples AFYI7 and AFYI14 also contain higher average abundances of TiO2
as well as FeO than all other MI. If these differences were a result of rehomoge-nization problems (i.e., melting too much or too little clinopyroxene), then TiO2
and FeO in the reheated MI would be reduced by dilution.Another potential concern is that MI may form by entrapment of boundary
layer melts, that are out of equilibrium with bulk melt, along the surface of rapidlygrowing phenocrysts. It has been suggested, however, that the entrapment ofboundary layer melt as MI should not influence the compositions of MI >25 mm
108 J. D. Webster et al.
in diameter (Lowenstern, 1995; Thomas et al., 2001), and most MI of the presentstudy are greater than 20 mm in diameter. Furthermore, one means of establishingthe potential impact of entrapment of boundary layer melt in MI is to compare theconcentrations of large, highly-charged cations, that are incompatible with the hostphase, in the MI with their abundances in associated whole-rock samples. If aboundary layer develops along the margins of phenocrysts, then the concentrationsof those cations that diffuse comparatively slowly through silicate melt will becomparatively enriched and the MI will contain greater abundances of these cationsthan the associated whole rocks. The whole-rock compositions (Table 3) are muchhigher in Zr and Nb than the corresponding MI. We interpret these differences tomean that if the MI do indeed represent boundary layer melts, then these boundarylayer melts had not evolved to extreme enrichments in slowly diffusing cations. Wehave no evidence that these MI represent boundary layer melts, but if in fact theyactually do then the melt compositions are not more highly enriched in incompat-ible elements than the rocks hosting the MI.
Compositions of melt inclusions and whole-rock samples
The whole-rock samples of this study include basaltic trachyandesites, trachyan-desites, trachytes, and phonolites, but trachytes and phonolites dominate (Table 3;Fig. 3) as observed previously (Barberi et al., 1978). The MI of this study show a
Fig. 2. Plot of the MgO concentration versus the (CaO=Al2O3) ratio for reheated silicatemelt inclusions (MI) of the Campanian Ignimbrite (CI), Taurano Tuff, and Giugliano Tuff(symbols same as in Fig. 1). The Campanian Ignimbrite MI cluster into high-MgO and low-MgO groupings. The dispersion shown by the low-MgO MI could be a result of crystalfractionation of clinopyroxene with or without sanidine. The Taurano MI imply that Tau-rano magmas were more primitive in composition than CI and Giugliano magmas.Giugliano MI groupings mirror those of Campanian Ignimbrite MI. One � precision isequivalent to the size of the symbols
Pre-eruptive geochemistry 109
Tab
le3
.C
om
posi
tions
of
Cam
pania
nIg
nim
bri
te(C
I),
Taura
no,
and
Giu
gli
ano
whole
-rock
sam
ple
s
Unit
Giu
gli
ano
CI
Inco
her
ent
CI
Yel
low
Tuff
CI
Lit
hic
CI
Gra
yT
uff
Tau
rano
Tau
rano
Tau
rano
Inco
her
ent
Tu
ffB
recc
iaY
ello
wT
uff
Str
atifi
edS
trat
ified
Ign
imb
rite
Pyro
clas
tics
Pyro
clas
tics
Sam
ple
sIC
B-9
cM
O-2
SF
C-1
aA
R-1
VE
-1A
FY
I-1
4A
FY
I-1
0�
AF
YI-
8�
Ag
eo
f1
8–
23
39
.6�
1.9
39
.33�
0.8
33
9.0
5�
0.5
13
8.7
0–
39
.19
15
7.4�
1.0
18
3.8�
15
.22
05
.6�
3.0
eru
pti
on
ka
ka
ka
ka
ka
ka
ka
ka
SiO
2(w
t.%
)58.4
758.3
59.2
57.6
60.0
57.8
359.7
455.5
5T
iO2
0.5
20
.41
0.4
10
.46
0.4
50
.74
0.9
81
.00
Al 2
O3
18
.15
18
.01
8.3
18
.21
8.5
21
.05
23
.78
24
.09
FeO
4.1
05
.20
5.0
73
.81
3.8
45
.14
.55
7.0
6M
gO
1.1
80
.69
0.6
41
.38
0.6
11
.55
1.5
61
.67
Mn
O0
.12
0.1
60
.17
0.1
10
.20
0.1
90
.06
0.0
7C
aO3
.08
2.4
72
.33
3.8
12
.22
5.6
83
.46
4.3
6K
2O
8.0
67
.95
8.0
48
.67
7.1
15
.24
2.9
13
.19
Na 2
O3
.80
4.5
24
.68
2.9
65
.14
0.8
41
.49
1.8
4P
2O
50
.21
0.1
30
.12
0.2
50
.20
0.1
70
.37
0.3
7S
ND
0.0
80
.01
0.0
20
.01
ND
ND
ND
Cl
ND
0.5
70
.51
0.2
80
.73
ND
ND
ND
FN
D0
.05
0.0
70
.12
0.1
7N
DN
DN
D
TO
TA
L9
7.6
89
9.4
39
9.9
29
9.3
09
9.4
19
8.3
89
8.9
09
9.2
0
LO
I(w
t.%
)N
D3.0
52.5
01.5
91.4
4N
DN
DN
DL
i(p
pm
)N
D4
14
12
35
8N
DN
DN
DB
eN
D1
21
25
18
ND
ND
ND
BN
DN
DN
DN
DN
DN
DN
DN
DR
b3
05
32
53
39
24
53
80
69
91
74
14
0S
r6
04
19
81
82
59
11
50
22
56
76
39
68
Y3
44
74
42
36
05
74
44
8Z
r2
99
41
54
10
17
95
80
42
74
30
45
4
110 J. D. Webster et al.
Nb
45
.27
27
22
51
93
64
.17
1.8
77
.2C
s1
6.5
11
9.8
20
9.7
62
4.9
32
21
.01
6.7
Ce
14
11
40
15
58
7.4
21
51
84
.31
47
.71
93
.5S
m1
0.5
10
.71
0.6
7.0
13
.81
4.7
9.8
12
.2D
y6
.8N
DN
D4
.4N
D8
.55
.87
.1Y
b3
.03
.83
.82
.15
.13
.12
.43
.0T
h3
13
43
31
5.2
46
.11
92
7.9
36
.3U
(pp
m)
ND
10
.59
.84
4.5
14
.07
.87
.37
.5
Note
that
we
hav
eno
whole
-rock
anal
yse
sfo
rT
aura
no
seri
essa
mple
sA
FY
I7or
AF
YI9
,bu
tfo
rpurp
ose
sof
com
par
ison
we
report
dat
afo
rA
FY
I8an
dA
FY
I10
(whic
hhav
enot
bee
nag
edat
ed).
Sam
ple
AF
YI1
0under
lies
AF
YI9
whic
hunder
lies
AF
YI8
whic
hunder
lies
AF
YI7
.N
Dco
nst
ituen
tnot
det
erm
ined
Pre-eruptive geochemistry 111
wider range in composition than the rock samples, because the MI also includetrachybasalts (Table 1).
Chemically, MI from the 11 CI rock samples (that represent at least 2 strati-graphically distinct volcanic units) cluster into two discrete groupings (Fig. 2).Excluding the single outlier that contains 4.9 wt.% MgO, the MI from the 11 CIsamples can be distinguished as either high in MgO (i.e., those containing>6 wt.%) or low MgO (those containing <4 wt.%) (Table 4). Rock samples ofthe CI Incoherent Tuff, Yellow Tuff, and Gray Tuff rock contain both high-MgOand low-MgO MI, but only high-MgO MI were observed in rocks of the CI LithicBreccia. All high-MgO MI, on average, also contain less SiO2, Na2O, and K2O,more F, CaO, TiO2, P2O5, SO2, and Sr and show marginally higher ratios of(CaO=Al2O3). Most of the high MgO MI also contain comparatively moreH2O and less Y, Zr, Sm, Dy, and Yb. The high-MgO CI MI plot, in fact, in thecompositional fields for trachybasalt and basaltic trachyandesite whole rocks(Fig. 3).
Fig. 3. Plots of (Na2OþK2O) versus SiO2 in reheated silicate melt inclusions (MI) ofCampanian Ignimbrite (CI), Taurano Tuff (open diamonds, triangles, and circles as in Fig.1) and Giugliano Tuff (asterisks) overlain on classification scheme for igneous rock fieldsdescribed by Le Bas et al. (1986). CI whole-rock samples are shown as filled circles(Barberi et al., 1983) and as filled triangles (Bohrson et al., personal communication).The Campanian Ignimbrite MI cluster into high-MgO (open squares) and low-MgO group-ings (open crosses), and the Giugliano MI groupings are similar. The limited variability ofhigh-MgO Campanian Ignimbrite MI is shown in the clustered data of the dotted area. Thelow-MgO Campanian Ignimbrite MI display greater dispersion. The Taurano MI are moreprimitive than Campanian Ignimbrite and Giugliano MI. Most CI whole-rock samplesare more evolved than most Campanian Ignimbrite MI. One � precision is approximatelyequivalent to the size of the symbols
112 J. D. Webster et al.
Interestingly, the average composition of the low-MgO CI MI is very similar tothat of the unheated MI of the CI Lower Fall Unit investigated by Signorelli et al.(1999) (Table 4). Within quoted analytical precision, the concentrations of SiO2,
Table 4. Average compositions of reheated, high-MgO and low-MgO melt inclusions (MI) inclinopyroxene phenocrysts from 39 ka old CI rock samples of this study compared withunheated MI from CI pumiceous Lower Fall Unit of Signorelli et al. (1999)
Inclusions Low-MgO High-MgO Lower FallMI MI Unit MI
Samples SA-1, SFC-1, SA-1, SP-3, Vos1, Vos3SFC-2, AR-1,SP-2, SP-3, MO-2, VE-1,ALT-4, ALT-1, ICB-13A-95ICB-12E-95
(n,m) (18, 13) (14, 13) (8, 0)SiO2 (wt.%) 57.83� 1.52 50.97� 0.56 58.67� 1.2TiO2 0.50� 0.19 0.93� 0.10 0.38� 0.08Al2O3 15.52� 1.18 14.18� 0.88 18.16� 0.49FeO 4.39� 0.67 5.23� 0.60 3.33� 0.26MgO 2.88� 0.70 7.56� 0.79 0.57� 0.22MnO 0.21� 0.13 0.08� 0.03 0.19� 0.04CaO 6.09� 1.28 9.38� 1.36 2.4� 0.62K2O 7.11� 1.31 5.19� 0.39 9.41� 0.37Na2O 3.26� 0.92 1.96� 0.18 3.26� 0.46P2O5 0.18� 0.19 0.68� 0.14 0.14� 0.03SO2 0.06� 0.04 0.54� 0.18 0.08� 0.04Cl 0.34� 0.16 0.39� 0.07 0.38� 0.04F 0.17� 0.07 0.36� 0.12 0.21� 0.05
TOTAL 98.41 97.19 97.18
H2O (wt.%) 2.1� 0.9 2.9� 1.1 1.60� 0.32Li (ppm) 43� 25 83� 74 NDBe 11� 10 4� 2 NDB 43� 31 29� 7 NDRb 140� 43 140� 35 NDSr 250� 190 770� 140 NDY 38� 29 15� 3 NDZr 230� 160 98� 18 NDNb 30� 23 19� 3 NDCs 13� 9 10� 3 NDCe 130� 140 53� 13 NDSm 22� 21 6.7� 2 NDDy 9.3� 8 2.8� 1 NDYb 9.8� 8 2.5� 1 NDTh 10� 8 6� 1 NDU (ppm) 3� 3 2� 1 ND
Number of MI analyzed by electron microprobe (n); number of MI analyzed by ionmicroprobe (m). Water in Vos 1 and Vos 3 MI determined by FTIR (Signorelli et al.,1999). ND not determined. All iron as FeO. Total does not include H2O or trace elements
Pre-eruptive geochemistry 113
TiO2, MnO, Na2O, P2O5, SO2, Cl, F, and H2O are statistically equivalent for bothMI groups. This consistency indicates that the former represent trapped samples ofmore highly evolved magma that was nearly equivalent in composition to thatwhich erupted to form the Lower Fall Unit. Moreover, the equivalence betweenthe MI from the two different studies, albeit representing different units of the CI,confirms our interpretation that our reheated MI were heated at appropriate tem-peratures and that they were not significantly influenced by melting too much ortoo little of the host grains.
In general, the MI from the three stratigraphically different Taurano samplesare compositionally equivalent to one another. The differences in major- andminor-element concentrations in MI from the Taurano Yellow Tuff and those fromthe Taurano Stratified Pyroclastics are not statistically different. The sole differ-ence, that is significant, is that the former MI do contain more Sr than the otherTaurano MI.
The MI from the three Taurano units are compositionally different, on average,from the MI from the four younger CI units (Table 1). Nearly all Taurano MIcontain abundances of MgO that are similar to but generally higher than thoseof the high-MgO CI MI. The Taurano MI represent distinctly more primitive melts,because, on average, most also contain more TiO2, FeO, and CaO, and less Al2O3,K2O, Na2O, Cl, Rb, Y, Cs, Th, and U. These distinctions are mirrored by the rocksamples as the Taurano whole rocks generally contain more MgO and Sr than CIwhole rocks.
The MI extracted from the single Giugliano sample can also be grouped byMgO content, and they are similar to the MI of CI eruptive materials. High-MgOGiugliano MI, for instance, exhibit comparatively larger (CaO=Al2O3) and containless SiO2 much like that shown by the CI MI. Moreover, the low-MgO and high-MgO MI of the Giugliano and CI samples show similar geochemical trends reflect-ing melt evolution. The groupings in plots of MgO versus (CaO=Al2O3) (Fig. 2),total alkalis versus silica (Fig. 3), Na2O versus volatiles (Fig. 4), and others forGiugliano MI are nearly identical to those of all CI MI.
Volatiles and fluxing components in the melt inclusions
Melt inclusions from the CI volcanic units contain, on average, from 1.8 to3.5 wt.% H2O and approximately 0.3 to 0.7 wt.% P2O5, 0.3 to 0.4 wt.% Cl, and0.23 to 0.27 wt.% F (Table 1). Most CI MI contain approximately 0.1 wt.% SO2,but the average SO2 content of MI from the CI Lithic Breccia is much higher(0.27 wt.%). Hence, the magmas represented by these inclusions were moderatelyenriched in volatiles (H2O, SO2, and Cl) and melt fluxing components (F andP2O5). These levels of volatile and fluxing component abundances are approxi-mately similar to those determined for MI in the pumiceous Lower Fall Unit of theCI (Signorelli et al., 2001). We have no data for CO2 in these MI, but Signorelliet al. (2001) report that the CO2 concentrations of MI in the Lower Fall Unit arebelow the detection limit of 10–20 ppm.
The MI from the associated Taurano volcanic units contain, on average, from1.8 to 2.7 wt.% H2O, 0.3 to 0.5 wt.% P2O5, 0.15 to 0.23 wt.% Cl, 0.26 to 0.43 wt.%SO2, and 0.24 to 0.35 wt.% F. These values are very similar to those determined for
114 J. D. Webster et al.
CI MI. The MI from the Giugliano sample contain, on average, 2.3 wt.% H2O,0.27 wt.% P2O5, 0.23 wt.% Cl, 0.25 wt.% SO2, and 0.23 wt.% F. These values aresimilar to those determined for CI and Taurano MI.
Discussion
Geochemical investigations of magma evolution based on compositions of MI offerdistinct advantages over studies involving volcanic whole-rock samples (Anderson,1974). Melt inclusions provide strong constraints on pre-eruptive abundances of
Fig. 4. Plots of concentrations of A SO2, B Cl, and C H2O versus Na2O in reheated silicatemelt inclusions (MI) from Campanian Ignimbrite (CI), Taurano Tuff, and Giugliano Tuffrock samples (symbols same as in Fig. 3). The limited variability of high-MgO CampanianIgnimbrite MI for some constituents is shown as the clustered data in the dotted areas.Curves are fitted to low-MgO MI. The Giugliano MI mirror behavior of low-MgO MI.High-MgO MI contain more SO2 and H2O than low-MgO MI. SO2 in low-MgO MI appearsto be buffered by some unknown phase, Cl generally appears to increase with increasingNa2O (reflecting melt evolution by fractional crystallization), and H2O appears indepen-dent of increasing melt evolution. One � precision is slightly greater than the size of thesymbols
Pre-eruptive geochemistry 115
volatiles and mobile fluxing components, for example, and these data are criticalfor interpreting processes of pre-eruptive and eruptive degassing. Volcanic whole-rock samples, in general, are subject to eruptive and post-eruptive alteration, andthis has been reported for CI rocks (Barberi et al., 1983; Giannetti and Luhr, 1983;Devine et al., 1984; Palais and Sigurdsson, 1989; Rolandi et al., this volume), butmost silicate melt inclusions represent pristine samples. Moreover, melt inclusionsfrom a single hand specimen can include a diversity of compositions representingvariably evolved melts.
Compositional evolution of Campanian Ignimbrite magmas
The Taurano and Giugliano MI show geochemical trends that reflect magmaticevolution, but we will address the MI of the Campanian Ignimbrite in greater detailbecause they show the most pronounced compositional variations. The low-MgObatches of CI magma are comparatively enriched in SiO2, Na2O, and K2O anddepleted in F, H2O, MgO, CaO, TiO2, P2O5, SO2, and Sr, so they represent morehighly evolved magma than the high-MgO magma. The tendency of CI magmas toevolve toward decreasing CaO and MgO and increasing Na2O and SiO2 wasobserved by Signorelli et al. (1999) in their study of CI matrix glasses and coex-isting MI, but their investigation was limited to major and minor elements.
The high MgO, and hence more primitive, batches of magma show far lessdispersion for numerous constituents (e.g., SiO2, Na2O, K2O, Cl, Rb, Cs, Th, Zr,Nb, REE, and others) than the evolved, lower-MgO magma (Fig. 3, 4b, and 5). Forinstance, the more primitive batches of magma cluster tightly in plots of REE andother incompatible elements, whereas the evolved magma batches cover a muchwider range (Fig. 6). The small extent of dispersion expressed by this clusteringdemonstrates that the various aliquots of magma which were captured along thesurfaces of clinopyroxene phenocrysts were very similar in composition at thestage of MI entrapment.
Some characteristics of the clinopyroxene phenocrysts also correlate with MIcompositions. The (MgO=MgOþFeO) ratios of the MI correlate positively withthe (MgO=MgOþ FeO) ratios of the corresponding host phenocrysts (Fig. 7),indicating that evolved MI were entrapped in clinopyroxenes characterized bycomparatively low (MgO=MgOþ FeO). The opposite relationship is also true;the high-MgO MI are restricted to the MgO-enriched interiors of the zoned hostgrains. Even though the rims of many clinopyroxenes containing high-MgO MI arezoned to lower (MgO=MgOþFeO), the high-MgO MI are found only in MgO-enriched interiors of the host grains. Taken together, these relationships indicatethat the high-MgO MI were entrapped in clinopyroxenes crystallizing from ahomogeneous magma (homogeneous for SiO2, Na2O, K2O, Cl, Rb, Cs, Th, Zr,Nb, REE) that was enriched in H2O, CaO, TiO2, P2O5, F, SO2, and Sr as well asMgO (Table 4). The minor compositional variability of these MI indicates that thehigh-MgO magma was not detectably influenced by fractional crystallization ormixing processes during the interval of inclusion trapping. We suggest that thehigh-MgO MI may, in fact, represent aliquots of primary magma, because ofthe: (1) primitive nature of the magma; (2) enrichments in H2O, F, P2O5, andSO2; and (3) the low dispersion. In this regard, MI representing primitive magma
116 J. D. Webster et al.
from nearby Mt. Somma-Vesuvius were also observed to show preferential enrich-ments in many of these volatiles and fluxing components (Marianelli et al.,1995).
Fig. 5. Plots of abundances of (Na2OþK2O) versus the (CaO=Al2O3) ratio (A) and Rbversus Cs (B) of reheated silicate melt (MI) inclusions from Campanian Ignimbrite (CI),Taurano Tuff, and Giugliano Tuff rock samples (symbols same as in Fig. 3) compared withreheated MI from Mt. Somma-Vesuvius (SV) samples (shown as crosses and x’s) (Vaggelliet al., 1993; Belkin et al., 1998; Raia et al., 2000; Webster et al., 2001). The range in variabilityof high-MgO CI MI is shown by the dotted area. Partial overlap of Campanian Ignimbrite MIwith SV MI in A reflects similar processes of magma evolution, but differences in B suggestthat CI and SV evolved via different magmas and processes. The Taurano MI are moreprimitive as reflected by comparatively lower alkalis (Na, K, Cs, and Rb) and higher(CaO=Al2O3) ratio. One � precision is approximately equivalent to the size of the symbols
Pre-eruptive geochemistry 117
It is unlikely that the low-MgO melt compositions were generated throughcrystal fractionation of sanidine, magnetite, or clinopyroxene from a magma ofhigh-MgO CI MI composition. In this regard, it is noteworthy that the low-MgO CIMI are clustered relative to the more mafic MI compositions, meaning that the CImelt inclusions do not exhibit a continuous spread in MgO contents, so the com-plete data set does not show a continuous range from the least to the most evolvedcompositions. This apparent clustering is consistent with the groupings of high-MgO and low-MgO CI MI for other elements described below.
Trends involving major elements in the low-MgO MI are consistent with deri-vation of low-MgO magma from more primitive melts of compositions somewhatsimilar to those of older Taurano magma through fractional crystallization of thephenocrysts present, but we do not mean to imply that the low-MgO MI werederived directly from Taurano magma. Most Taurano MI show higher(CaO=Al2O3) and contain less SiO2, (Na2OþK2O), Cl, Li, Rb, Cs, Sr, Nb, Th,and U than the high-MgO and low-MgO CI MI. The former MI represent magmasthat were much more primitive. The low-MgO CI MI trend projects toward theTaurano MI compositions (Fig. 2). The internal evolutionary trend shown by the
Fig. 6. Plots of Sm versus Yb (A) and Zrversus Nb (B) of reheated CampanianIgnimbrite (CI) silicate melt (MI) inclu-sions and whole-rock samples (symbolssame as in Fig. 3, but rocks shown as filledtriangles). The limited range in variabilityof high-MgO MI is shown as the dottedareas. The dispersion of low-MgO MI re-flects magma evolution via fractionalcrystallization with or without magmamixing (see text for discussion). One �precision is approximately equivalent tothe size of the symbols
118 J. D. Webster et al.
low MgO CI magma could reflect crystal fractionation of clinopyroxene, with orwithout sanidine, and evolution toward lower (CaO=Al2O3) and even lower MgO.Clinopyroxene crystallization drives residual melts to progressively lower MgOand (CaO=Al2O3), but sanidine crystallization drives residual melt compositionsalong a trajectory that is nearly opposite to that of the clinopyroxene trend. Con-sequently, crystallization of mixtures of clinopyroxene plus sanidine will movemelts along approximately the same trajectory as clinopyroxene alone, the onlydifference between the two mineral assemblages is the relative rate of change ofmelt composition (for these two parameters) with fractional crystallization. Inother words, simultaneous crystallization of sanidine and clinopyroxene will slowthe rate of decrease in MgO content and decreasing (CaO=Al2O3) ratio, as com-pared to crystallization of clinopyroxene alone. These data further suggest that thecrystallization of magnetite had little to do with the evolution of the low-MgOmelts.
The volatiles provide further constraints on melt evolution (Fig. 4). It is clearthat the more primitive CI magma was comparatively enriched in H2O, F, and SO2;whereas higher Cl concentrations appear to result from progressive evolution ofmagma because the highest Cl contents occur in the most sodic, low-MgO meltfractions. For example, the plot of Cl versus Na2O for low-MgO MI (Fig. 4b)projects away from the origin and from more primitive melt compositions (i.e.,like that of the Cl- and Na-deficient Taurano MI) suggesting that the abundances ofNa and Cl in magma increased simultaneously during fractional crystallization. Incontrast, the abundances of SO2 are independent of Na2O during progressive meltevolution and are limited to values less than 0.15 wt.%. This implies that some
Fig. 7. Plot of average (MgO=MgOþ FeO) ratio of clinopyroxene phenocrysts versusaverage (MgO=MgOþFeO) ratio of reheated high-MgO (squares) and low-MgO (crosses)silicate melt inclusions (MI) in the same phenocrysts of Campanian Ignimbrite (CI) sam-ples. Magnesium-enriched MI are restricted to diopsidic clinopyroxene; whereas, low-MgOMI occur in salite-diopside phenocrysts. Large variability of average high-MgO clinopy-roxene compositions is a result of zoned phenocrysts
Pre-eruptive geochemistry 119
S-bearing phase (or phases) was present to fix the SO2 contents of residual frac-tions of melt. Signorelli et al. (2001) report the presence of sulfide inclusions inclinopyroxene phenocrysts of the Lower Fall Unit and that the S content of the meltwas apparently buffered by sulfide, and we have noted, already, the compositionalsimilarity of the Lower Fall Unit MI and the low-MgO CI MI. However, we havenot found sulfides in these rocks. Nevertheless, it is clear that some crystalline orvolatile phase was present to limit the S enrichments of residual fractions ofmelt during compositional evolution. The abundances of H2O in low-MgO CI meltalso appear to be independent of increasing Na2O and increasing melt evolution.The H2O contents of the low-MgO MI are restricted to the 1–3 wt.% range.Remarkably however, the high-MgO CI MI show a much more dramaticrange in pre-eruptive H2O concentrations, and we suggest that this reflects initialheterogeneities in the primitive melt fractions much like those observed forSO2.
Conceivably, other processes such as volatile phase exsolution could have influ-enced magmatic evolution. Computations based on relationships involving Cl in theMI and matrix glasses of the CI Lower Fall Unit led Signorelli et al. (2001) to suggestthat the evolved Lower Fall Unit magma may have exsolved a Cl-bearing aqueousphase prior to melt entrapment. We, on the other hand, observe no evidence toindicate that this process influenced either the low-MgO or high-MgO fractions ofCI melt. In this regard, Signorelli et al. (2001) also observed two populations ofclinopyroxene phenocrysts hosting MI, based on clinopyroxene composition. Theyreported no significant differences in abundances of H2O, Cl, or F between thediopside=salite- and the salite-hosted MI of the Lower Fall Unit. This and otherrelationships led them to conclude that the eruptive styles were not significantlycontrolled by initial volatile abundances. In contrast, the high-MgO MI of this studyare distinctly enriched in SO2, F, and P2O5 relative to the low-MgO MI. This suggeststo us that the intrusion of a more primitive and volatile-enriched magma of basaltictrachyandesite into the body of more evolved trachyandesitic to andesitic composi-tion, with subsequent mingling and mixing, could have played a significant role intriggering the eruptions due to the input of heat and volatiles.
Assimilation may have also led to changes in magma composition. It is note-worthy that De Vivo et al. (2001) reported the presence of older xenocrystic sani-dine phenocrysts in the CI Gray Tuff rocks. These sanidines were possibly derivedfrom Taurano eruptive materials, and indicate that some entrainment and=or assim-ilation of older volcanic materials took place. The CI MI show no compositionalevidence of such processes, however.
The role of mingling and=or mixing inCampanian Ignimbrite magma evolution
Interpretation of the compositional evolution of the CI magmas is complicated bythe two, i.e., the high-MgO and low-MgO, magma compositions. The presence oftwo populations of MI and the sharp chemical transitions in zoned clinopyroxenephenocrysts are consistent with the mingling and=or mixing of a primitive magmaand a more-evolved magma prior to eruption. Mingling and mixing of magmas,
120 J. D. Webster et al.
prior to and during eruption of the CI, have been discussed previously by Rosi et al.(1996), Civetta et al. (1997), and Signorelli et al. (1999, 2001).
The small degree of variability for some constituents in the high-MgO MI andthe fact that those constituents characterized by greater dispersion (e.g., MgOin Fig. 2, Rb and Cs in Fig. 5, H2O in Fig. 4) do not show evidence of mixingwith magma represented by the low-MgO MI are inconsistent with mixing betweenthe low-MgO and high-MgO varieties of magma prior to MI entrapment. Thehigh-MgO MI were apparently entrapped and removed from subsequent melt(s)before mixing occurred. Some constituents of the low-MgO magma, however,show relationships that could be interpreted as evidence of magma mixing as wellas fractional crystallization. Some low-MgO MI extend toward the clustered high-MgO compositions (Figs. 3, 5, and 6), and this may reflect variable but minormixing of high-MgO and low-MgO magma prior to entrapment of low-MgO MI.The lack of clear and distinct evidence of magma mixing=mingling in our samples,however, suggests that these processes occurred after most of the MI wereentrapped. Furthermore, it appears that the magma mixing=mingling must haveoccurred shortly before eruption; otherwise, more MI would exhibit evidence ofmixed magma compositions.
Fig. 8. Plot of compositions of reheated silicate melt inclusions (MI) of Campanian Ignim-brite (CI), Taurano Tuff, and Giugliano Tuff compared with compositions of Campi Flegreiwhole-rock samples (dotted area; Rosi and Sbrana, 1987), overlain on classification schemefor igneous rock fields described by Le Bas et al. (1986). Most symbols same as in Fig. 3. Therange in compositions of CI and Giugliano MI are similar to those shown by Campi Flegreirocks. One � precision is approximately equivalent to the size of the symbols
Pre-eruptive geochemistry 121
Magmatism in the Neapolitan region
The MI-bearing clinopyroxene phenocrysts in the Taurano rocks were erupted to thesurface well before the Campi Flegrei activity, deposition of the CI, and beforeeruptions of Mt. Somma-Vesuvius. The MI-bearing clinopyroxenes in CI rocks wereerupted before the onset of eruptions at nearby Mt. Somma-Vesuvius (SV), but theSVactivities overlapped with the eruption of the Giugliano rocks. Given the overlapof some of these various eruptive activities and the fact that the eruptive centers arelocated so relatively close to one another, it is useful to compare the compositions ofmagmas that fed these different eruptive systems to better understand trends inmagmatism of the Neapolitan region. For example, not only did the Taurano erup-tions occur well before all other activities of this region, but the Taurano eruptionsinvolved materials that were much more primitive in composition. Consequently,there is no reason to relate the Taurano magmas to subsequent eruptive activities.
The compositions of the low- and high-MgO MI in all of the CI rocks studiedare very consistent to one another and to Lower Fall Unit MI (Signorelli et al.,2001), demonstrating that they were all derived from the same magmatic sources.Moreover, the CI MI are more consistent with Campi Flegrei eruptive materialsthan with those of Mt. Somma-Vesuvius. The change in (Na2OþK2O) versus SiO2
for Campi Flegrei whole rocks (Rosi and Sbrana, 1987), for instance, is similar tothe trends shown by low- and high-MgO CI MI (Fig. 8).
Fig. 9. Plot of compositions of reheated silicate melt inclusions (MI) of Campanian Ignim-brite (CI), Taurano Tuff, and Giugliano Tuff compared with compositions of Mt. Somma-Vesuvius MI (dotted area; Webster et al., 2001), overlain on classification scheme for igneousrock fields described by Le Bas et al. (1986). Most symbols are the same as in Fig. 3. The rangein compositions of Somma-Vesuvius MI overlaps only the high-MgO Campanian IgnimbriteMI compositions. One � precision is approximately equivalent to the size of the symbols
122 J. D. Webster et al.
In contrast, the MI of the oldest, precaldera SV eruptive units (meaning thoseranging from 35 to 14 ka in age) and the younger postcaldera SV eruptive units(those younger than 3.55 ka) represent tephritic, phonotephritic, and tephriphono-litic magmas (Webster et al., 2001); whereas the CI MI show a similar range inSiO2 contents but lower total alkalis (Fig. 9) and involve compositions that aremore trachytic. The latter MI show a larger range in (CaO=Al2O3) which reflects agreater extent of melt evolution (Fig. 5), than the SV MI.
It is also interesting that the Giugliano MI show numerous trends that aresimilar to those of the CI MI. Melt inclusions from both magmatic systems arecharacterized by high-MgO and low-MgO varieties, and the abundances and rela-tionships of major, minor, and trace elements (Figs. 2, 3, 4, and 5) for Giuglianomagma mirror those of the CI magmas. These similarities suggest that the magmasource(s) and processes of magma evolution for the 39 ka Campanian Ignimbritewere equivalent to those of the 18 ka Giugliano tuff and that they may have eruptedfrom a common source.
Acknowledgements
We appreciate reviews of the manuscript by an anonymous referee, H. Belkin, and C.Szab�oo. We thank C. Mandeville for invaluable help during EMPA analysis and usefuldiscussion. N. Shimizu and G. Layne provided assistance during SIMS analyses. This workwas partially supported by INGV (GNV) to B. De Vivo for year 2001.
References
Alberico I, Lirer L, Petrosino P, Scandone R (2002) A methodology for the evaluation oflong-term risk from pyroclastic flows in Campi Flegrei (Italy). J Volcanol Geotherm Res116: 63–78
Anderson AT (1974) Chlorine, sulfur, and water in magmas and oceans. Geol Soc Am Bull85: 1485–1492
Barberi F, Innocenti F, Lirer L, Munno R, Pescatore T, Santacroce R (1978) The CampanianIgnimbrite: a major prehistoric eruption in the Neapolian area (Italy). Bull Volcanol41–1: 10–22
Barberi F, Innocenti F, Lirer L, Munno R, Pescatore T, Santacroce R (1983) Compositionalvariations in the Campanian Ignimbrite, Italy: hydrothermal modification or pre-eruptivefractionation? – A reply. Bull Volcanol 46: 433–434
Belkin HE, De Vivo B, Roedder E, Cortini M (1985) Fluid inclusion geobarometry fromejected Mt. Somma-Vesuvius nodules. Am Mineral 70: 288–303
Belkin HE, De Vivo B, T€oor€ook K, Webster JD (1998) Pre-eruptive volatile content, meltinclusion chemistry, and microthermometry of interplinian Vesuvius lavas (pre-1631A.D). J Volcanol Geotherm Res 82: 79–95
Brocchini D, Principe C, Castradori D, Laurenzi MA, Gorla L (2001) Quaternary evolutionof the southern sector of the Campania Plain and early Somma-Vesuvius activity: insightsfrom Trecase 1 well. Mineral Petrol 73: 67–91
Castelmenzano A, Grieco G, Vezzoli L, Rosi M (1995) The fall deposit of the CampanianIgnimbrite eruption, Phlegrean Fields, Italy (abs). AGU 76: F676
Civetta L, Orsi G, Pappalardo L, Fisher RV, Heiken G, Ort M (1997) Geochemical zoning,mingling, eruptive dynamics and depositional processes – the Campanian Ignimbrite.Campi Flegrei caldera, Italy. J Volcanol Geotherm Res 75: 183–219
Pre-eruptive geochemistry 123
Deino AL, Southon I, Terrasi F, Campajola L, Orsi G (1994) 14C and 40Ar=39Ar dating of theCampanian Ignimbrite, Phlegrean Fields, Italy. Abstract ICOG, Berkely, CA, USA
Devine JD, Sigurdsson H, Davis AN, Self A (1984) Estimates of sulfur and chlorine yield tothe atmosphere from volcanic eruptions and potential climatic effects. J Geophys Res 89:6309–6325
De Vivo B, Rolandi G, Gans PB, Calvert A, Bohrson WA, Spera JF, Belkin HE (2001) Newconstraints on the pyroclastic eruptive history of the Campanian volcanic Plain (Italy).Mineral Petrol 73: 47–65
Di Girolamo P, Ghiara MR, Lirer L, Munno R, Rolandi G (1984) Vulcanologia e petrologiadei Campi Flegrei. Boll Soc Geol It 103: 349–413
Giannetti B, Luhr JF (1983) Compositional variation in the Grey Campanian Ignimbrite,Italy: hydrothermal modification or pre-eruptive fractionation? Bull Volcanol 46:429–431
Le Bas MJ, Le Maitre RW, Streckeisen A, Zanettin B (1986) A chemical classification ofvolcanic rocks based on the total alkali-silica diagram. J Petrol 27: 745–750
Lowenstern JB (1995) Applications of silicate melt inclusions to the study of magmaticvolatiles. In: Thompson JFH (ed) Magmas, fluids, and ore deposits. Mineral Assoc Can23: 71–99
Marianelli P, Metrich N, Santacroce R, Sbrana A (1995) Mafic magma batches at Vesuvius: aglass inclusion approach to the modalities of feeding stratovolcanoes. Contrib MineralPetrol 120: 159–169
Orsi G, D’Antonio M, de Vita S, Gallo G (1992) The Neapolitan Yellow Tuff, a largemagnitude trachytic phreato-plinian eruption: eruptive dynamics, magma withdrawaland caldera collapse. J Volcanol Geotherm Res 53: 275–287
Palais JM, Sigurdsson H (1989) Petrologic evidence of volatile emissions from majorhistoric and pre-historic volcanic eruptions. In: Kidson JW (ed) Understanding climatechange. Am Geophys Union Monogr 52: 31–53
Principe C, Rosi M, Santacroce R, Sbrana A (1982) Workshop on explosive volcanism:Guidebook on the field excursion to Phlegrean Fields and Vesuvius. Con Naz delle RichItal, Rome, 121 p
Raia F, Webster JD, De Vivo B (2000) Pre-eruptive volatile contents of Vesuvius magmas:constraints on eruptive history and behavior. I. The medieval and modern interplinianactivites. Eur J Mineral 12: 179–193
Roedder E (1984) Fluid inclusions. Mineral Soc Am Rev Mineral 12: 644 pRolandi G, Bellucci F, Heizler MT, Belkin HE, De Vivo B (2003) Tectonic controls on the
genesis of ignimbrites from the Campanian Volcanic Zone, southern Italy. Mineral Petrol79: 3–31
Rosi M, Sbrana A (1987) The Phlegrean Fields. Quad Ric Sci, 175 pRosi M, Vezzoli L, Aleotti P, De Censi M (1996) Interaction between caldera collapse and
eruptive dynamics during the Campanian Ignimbrite eruption. Phlegrean Fields, Italy.Bull Volcanol 57: 541–554
Rosi M, Vezzoli L, Castelmenzano A, Grieco G (1999) Plinian pumice fall deposit of theCampanian Ignimbrite eruption (Phlegraean Fields, Italy). J Volcanol Geotherm Res 91:179–198
Scandone R, Belluci F, Lirer L, Rolandi G (1991) The structure of the Campanian Plainand the activity of the Neapolitan volcanoes (Italy). J Volcanol Geotherm Res 48: 1–31
Shimizu N, Hart SR (1982) Applications of the ion microprobe to geochemistry and cos-mochemistry. Ann Rev Earth Planet Sci 10: 483–526
Signorelli S, Vaggelli G, Francalanci L, Rosi M (1999) Origin of magmas feeding the Plinianphase of the Campanian Ignimbrite eruption, Phlegraean Fields (Italy): constraints
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based on matrix-glass and glass-inclusion compositions. J Volcanol Geotherm Res 91:199–220
Signorelli S, Vaggelli G, Romano C, Carroll MC (2001) Volatile element zonation inCampanian Ignimbrite magmas (Phlegraean Fields, Italy): evidence from the study ofglass inclusions and matrix glasses. Contrib Mineral Petrol 140: 543–553
Thomas J, Bodnar R, Shimizu N (2001) Melt inclusions in zircon: microautoclaves fordetermination of trace element partition coefficients. Eleventh V. M. Goldschmidt ConfAbs 3480, Lunar Planet Inst, Houston (CD-ROM)
Vaggelli G, De Vivo B, Trigila R (1993) Silicate melt inclusions in recent Vesuvius lavas(1631–1944). II. Analytical chemistry. J Volcanol Geotherm Res 58: 367–376
Wallace P, Carmichael ISE (1992) Sulfur in basaltic magmas. Geochim Cosmochim Acta56: 1863–1874
Webster JD, Burt DM, Aguillon RA (1996) Volatile and lithophile trace-element geochem-istry of heterogeneous Mexican tin rhyolite magmas deduced from compositions of meltinclusions. Geochim Cosmochim Acta 60: 3267–3283
Webster JD, Raia F, De Vivo B, Rolandi G (2001) The behavior of chlorine and sulfurduring differentiation of the Mt. Somma-Vesuvius magmatic system. Mineral Petrol 73:177–201
Authors’ addresses: J. D. Webster (e-mail: [email protected]) and C. Tappen (e-mail: [email protected]), Department of Earth and Planetary Sciences, American Museum of NaturalHistory, Central Park West at 79th Street, New York, NY 10024-5192, USA; F. Raia (e-mail:[email protected]), Department of Earth and Atmospheric Sciences, City College ofNew York, New York, NY 10031, USA; B. De Vivo (e-mail: [email protected]), Diparti-mento di Geofisica e Vulcanologia, Universita’ di Napoli Federico II, Via Mezzocannone 8,I-80134 Napoli, Italy
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