JOURNAL OF PETROLOGY
VOLUME 39
NUMBER 8
PAGES 1453–1491
1998
The Geochemistry of Volcanic Rocks from Pantelleria Island, Sicily Channel: Petrogenesis and Characteristics of the Mantle Source Region LUCIA CIVETTA1∗, MASSIMO D’ANTONIO2, GIOVANNI ORSI2 AND GEORGE R. TILTON3 1
OSSERVATORIO VESUVIANO, V. MANZONI 249, NAPOLI, I-80123, ITALY ` ‘FEDERICO II’ DI NAPOLI, L.GO S. MARCELLINO 10, NAPOLI, I-80138, DIP. GEOFISICA E VULCANOLOGIA, UNIVERSITA
2
ITALY 3
DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CALIFORNIA, SANTA BARBARA, CA 93106, USA
RECEIVED APRIL 24, 1996; REVISED TYPESCRIPT ACCEPTED FEBRUARY 27, 1998
Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for mafic and felsic volcanic rocks from the island of Pantelleria. The mafic rocks, mostly basalts, range from hynormative transitional basalts, through alkali basalts, to basanites. Clinopyroxene in the mafic rocks varies in composition from Al, Tipoor diopside to Al, Ti-rich augite. These two populations can be present simultaneously in the same sample and even in the same crystal, suggesting polybaric fractionation in the pressure range 0–4 kbar, or mixing between basaltic magmas with different degrees of alkalinity. On the basis of their major and trace element and Sr–Nd–Pb isotope composition and age of eruption, two groups of basalts are distinguished: a high TiO2–P2O5 group, erupted before 50 ka BP, and a low TiO2–P2O5 group, erupted after 50 ka BP, separated by a caldera collapse. The felsic volcanic rocks have compositions ranging from comenditic trachyte to comendite and pantelleritic trachyte to pantellerite, with progressively increasing peralkalinity. The Sr–Nd isotope compositions of most of the felsic volcanic rocks are similar to those of the mafic volcanic rocks, except for some very Sr-poor pantellerites, which show post-depositional exchange with seawater strontium. On the basis of their petrographic and geochemical characteristics, Sr–Nd–Pb isotope data, computer modelling and geological observations, it is suggested that the mafic volcanic rocks represent a number of different alkaline parental magmas from which the felsic volcanic rocks were derived via prolonged, closed-system fractional crystallization. The source region for the parental magmas was heterogeneous, and may have involved
∗Corresponding author. Present address: Dip. Geofisica e Vulcanologia, Universita` Federico II di Napoli, L.go S.Marcellino 10, Napoli, I80138, Italy. Telephone: +39 81 5803110. Fax: +39 81 5527631. email:
[email protected]
at least two distinct geochemical components: a mid-ocean ridge basalt (MORB) source, relatively depleted component, and a HIMU-like enriched component. A further enriched component, similar to the Enriched Mantle 1 (EM 1) component, could also have been involved. According to geophysical data, the lithosphere is thinned beneath the island, and the asthenospheric mantle rises to a depth of 60 km. Rare earth element data require residual garnet in the source and constrain the melting process to a depth of 70–80 km. The petrological and geochemical data suggest that the mafic magmas are generated within the asthenospheric mantle, from a deep plume bringing the HIMU–EM 1 isotopic and trace element signatures. Interaction of these OIB-like magmas with the shallower asthenospheric mantle, providing a depleted MORB signature, gives rise to magmas with the observed isotopic and geochemical characteristics.
Pantelleria; asthenosphere; isotope geochemistry; mantle components; HIMU KEY WORDS:
INTRODUCTION The island of Pantelleria (Fig. 1) is the type locality for pantellerite. This is a peralkaline rhyolite which, at
Oxford University Press 1998
JOURNAL OF PETROLOGY
VOLUME 39
Pantelleria, is extremely enriched in Na, Fe, Cl, and incompatible trace elements. Pre-eruptive H2O contents are moderate to high [2–4 wt %, according to Kovalenko et al. (1988); 1·4–2·1 wt %, according to Lowenstern & Mahood (1991)]. Peralkaline silicic rocks, specifically trachytes and rhyolites with molar (Na2O + K2O)/Al2O3 ratio (agpaitic index, AI) greater than unity, occur as both plutonic (peralkaline granites) and volcanic (peralkaline trachytes, comendites and pantellerites) types. They are found in many different tectonic environments. Typically they occur in non-orogenic continental regions which have been subjected to crustal doming and rifting, e.g. Tibesti, Kenya, Ethiopia, Cameroon and Niger–Nigeria in Africa; East Greenland; British Columbia in North America; China–North Korea in Asia; Pantelleria island in Europe. They are also found commonly in oceanic islands located on actively spreading ridge crests, e.g. Socorro and Easter Islands on the East Pacific Rise; Iceland, the Azores and Ascension on the Mid-Atlantic Ridge; St Paul Island on the Mid-Indian Rise. The Canary Islands, where comendites and pantellerites also occur, constitute an exception in that they are located on a continental slope region. Less commonly, peralkaline silicic rocks occur in island arcs, e.g. Mayor Island in New Zealand, New Guinea and Hokkaido in NE Japan. Still more rarely they occur on continental margins, such as at Nandewar volcano in Australia, the Basin and Range Province in western North America, Volca`n Las Navajas in Mexico, and SW Sardinia in Italy. General studies of peralkaline rocks, with many additional cited references, have been reported by Bailey et al. (1974), Sørensen (1974) and Fitton & Upton (1987). The petrogenesis of peralkaline silicic magmas, and their genetic relationships with the associated mafic magmas, are strongly debated topics in petrology. Hypotheses include: (1) protracted fractional crystallization from alkali basaltic magmas—this hypothesis is supported by the ubiquitous presence of mildly alkali basalts associated with peralkaline rocks (Ewart et al., 1968; Barberi et al., 1975; Parker, 1983; Civetta et al., 1984; Nelson & Hegre, 1990; Mungall & Martin, 1995); (2) partial melting of alkali-gabbroic cumulates, as proposed for Pantelleria (Mahood et al., 1990; Lowenstern & Mahood, 1991); (3) partial melting of different portions of upper lithospheric mantle, lower and upper crust, as proposed for trachytes, pantellerites and comendites, respectively, from Lake Naivasha in Kenya (Bailey & Macdonald, 1987). The close association, at Pantelleria, of mildly alkali basalts, peralkaline trachytes and pantellerites provides an important opportunity to address the problem of the petrogenesis of peralkaline silicic magmas. Major and trace element and Sr–Nd–Pb isotope data have been obtained for a variety of mafic and felsic volcanic rocks from Pantelleria, which, when combined with previously
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AUGUST 1998
published data, allow us to reconstruct the evolution of the magmatic system of the island and to constrain the genetic relationships between the mafic and felsic magmas. Furthermore, these data provide constraints on the characteristics of the mantle source region beneath the Sicily Channel Rift Zone. Basic volcanic rocks from the islands of Pantelleria and Linosa (Fig. 1) have the least radiogenic Sr isotope compositions of all the Quaternary alkaline volcanic rocks of Italy, and are inferred to represent magmas coming from an ‘uncontaminated’ or minimally contaminated mantle domain, thought to be representative of the mantle of the whole Italian– Tyrrhenian region before overprinting by subductionrelated fluid transfer processes (Hoernle et al., 1995; D’Antonio et al., 1996).
GEOLOGICAL BACKGROUND The island of Pantelleria (Fig. 1) represents the emergent portion of a volcanic edifice that rises ~1000 m above the adjacent sea floor. It is composed dominantly of volcanic rocks which include lavas and pyroclastic deposits, varying in composition from pantellerite, through pantelleritic trachyte and comenditic trachyte, to mildly alkali basalt, in order of decreasing abundance. Exposed mafic volcanic rocks are restricted to the northwestern lobe of the island, although drilling for geothermal research in the northern sector of the island has revealed a basaltic sequence several hundreds of metres thick (Fulignati et al., 1997). K–Ar determinations on different basaltic units give ages of 118 ± 9, 83 ± 5, and ~29 ka BP (Civetta et al., 1984). Felsic volcanic rocks range in age from 324 ka BP to 4 ka BP (Civetta et al., 1984, 1988; Mahood & Hildreth, 1986; this study). The island is located in the NW–SE trending Sicily Channel Rift Zone (SCRZ) (Illies, 1981; Finetti, 1984), which results from transtensional tectonics along the northern margin of the African plate, related to the opening of the Tyrrhenian Sea (Boccaletti et al., 1987). The SCRZ transects the Pelagian Block, which is a sector of the foreland of the northern part of the African plate preserved after collision with the European plate (Burollet et al., 1978). The Pelagian Block is composed of continental crust ~40 km thick, which in the SCRZ thins from 25 km at the periphery to 18 km in the axial part (Colombi et al., 1973; Cassinis, 1981). Rifting began in the Late Miocene and was intense in the Pliocene to Late Quaternary. It is at present concentrated along the axial ridge area, where the asthenosphere rises up to ~60 km (Della Vedova et al., 1989). Intense volcanism in the SCRZ has generated two emergent volcanoes which form the islands of Pantelleria and Linosa. Pantelleria is the largest volcano, located on the axial ridge, whereas Linosa occurs at the periphery of the rift zone.
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THE GEOCHEMISTRY OF VOLCANIC ROCKS FROM PANTELLERIA
Fig. 1. Geological sketch map of the island of Pantelleria [redrawn from Orsi et al. (1991b)]. 1, Alluvium; 2, Mursia basalts: lava flows and cinder cones younger than 10 ka BP; 3, volcanics of the VI silicic cycle; 4, volcanics of the V silicic cycle; 5, volcanics of the IV silicic cycle; 6, volcanics of the III silicic cycle; 7, Punta St. Leonardo basalts (29 ka BP): lava flows and cinder cones; 8, volcanics of the II silicic cycle; 9, volcanics of the I silicic cycle (50 ka BP): Green Tuff; 10, basalts older than 50 ka BP: lava flows and cinder cones; 11, silicic activity older than 50 ka BP; 12, eruptive vents older than 50 ka BP; 13, eruptive vents younger than 50 ka BP; 14, top of escarpment of volcano-tectonic origin; 15, volcano-tectonic fault; 16, Monastero caldera rim; 17, La Vecchia caldera rim. Legend for the inset: A, normal fault; B, transcurrent fault; C, external limit of the Apennine thrust-belt between African and Eurasian continental blocks.
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VOLUME 39
Many submerged volcanoes are located either in the rift zone or on the Pelagian Block. Some of these have been active in the last few centuries. Accounts of this very young volcanism have been given by Imbo` (1965) and Zarudzki (1972). Volcanological and petrological studies on Pantelleria have been carried out since the last century (Foerstner, 1881; Washington, 1913–1914; Zies, 1960, 1962, 1966; Carmichael, 1962, 1967; Rittmann, 1967; Noble & Haffty, 1969; Romano, 1969; Villari, 1970, 1974; Korringa & Noble, 1972). More recent studies (Wright, 1980; Wolff & Wright, 1981; Cornette et al., 1982, 1983; Civetta et al., 1984, 1988; Orsi & Sheridan, 1984; Mahood & Baker, 1986; Mahood & Hildreth, 1986; Orsi et al., 1989, 1991a, 1991b; Mahood & Stimac, 1990; Lowenstern & Mahood, 1991; Lowenstern, 1994) have highlighted the structural and volcanological features of the island, as well as the mineralogical and geochemical characteristics of the volcanic rocks. The structural setting of the island of Pantelleria is defined by both tectonic and volcano-tectonic lineaments. The tectonic lineaments include faults and fractures related to regional deformation events, which have the same orientation as the rift-bounding faults. NW–SE trending fractures and strike-slip faults are the dominant lineaments, although NE–SW and N–S trending features are common. All these regional features occur outside the area of caldera collapse (see below). A NE–SW tensile fault system divides the island into two sectors and probably represents a crustal discontinuity along the axial ridge of the rift (Fig. 1). The northwestern sector includes most of the exposed basaltic rocks, whereas the southeastern sector includes silicic peralkaline rocks. The former has been affected only by NW–SE crustal fractures through which mafic magmas have reached the surface, as testified by the alignment of basaltic vents ranging in age from 80 ka BP to 29 ka BP (Cornette et al., 1983; Civetta et al., 1984), and to 1891 7 km NE of the island (Imbo`, 1965). In the southeastern sector the eruption of differentiated magmas and the occurrence of calderas suggest that crustal magma chambers were established, probably at the intersection of the main tectonic lineaments. The volcano-tectonic features of the island include caldera collapses and resurgence inside the youngest caldera. At least two caldera collapses have affected the island in recent times. The oldest caldera, the La Vecchia caldera, is dated at 114 ka BP (Mahood & Hildreth, 1986; Fig. 1). The youngest is related to the eruption of the Green Tuff (50 ka BP; Orsi & Sheridan, 1984) and has been named the Monastero caldera by Cornette et al. (1983) and the Cinque Denti caldera by Mahood & Hildreth (1983). Inside the Monastero caldera resurgence has taken place with uplifting and tilting of the Montagna Grande block (Orsi et al., 1991a).
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AUGUST 1998
The volcanic history of the island is characterized by large explosive eruptions, some of which produced caldera collapses, alternating with periods dominated by less energetic eruptions. The history before 50 ka BP cannot be reconstructed in detail because only remnants of the erupted products are exposed. This is due either to repeated collapse of the central part of the island and erosion along the coastal cliffs, or to blanketing of the whole island by the Green Tuff erupted at ~50 ka BP (Cornette et al., 1983). The history since this last large eruption has been subdivided by Civetta et al. (1984, 1988) into six silicic cycles sometimes intercalated with basaltic eruptions. The Green Tuff is considered representative of the first silicic cycle. It is the product of a complex eruption including ignimbrites, fall and surge horizons (Orsi & Sheridan, 1984). The chemical composition of the Green Tuff varies from the base upwards from pantellerite to comenditic trachyte (Civetta et al., 1984). All the other silicic cycles, dated at around 35–29, 22, 20–15, 14–12 and 10–4 ka BP, are characterized by eruptive products ranging in composition from pantellerite to pantelleritic trachyte, or to comenditic trachyte. Even though it is not always possible to arrange all the analysed samples in stratigraphic succession, for many cycles it has been demonstrated that the most differentiated magmas were erupted early in the cycle. This has been interpreted as the consequence of eruptions tapping a zoned magma chamber at progressively deeper levels during each eruptive cycle (Civetta et al., 1988).
SAMPLE SELECTION AND ANALYTICAL TECHNIQUES Lava flows, pumice-fall deposits and ignimbrites from Pantelleria have been sampled for geochemical and isotopic analysis. In addition, some trachytic enclaves, thought to represent fragments of a crystal-rich, lower portion of a stratified magma chamber (Mahood & Hildreth, 1986), have also been collected from lavas and pyroclastic units. Mineral phases were analysed by combined WDS–EDS techniques using a CAMECA SX50 electron microprobe at the Centro di Studi per il Quaternario e l’Evoluzione Ambientale–CNR (Rome). Data reduction was made using the ZAF4/FLS software by Link Analytical. Representative analyses are reported in Tables 1 and 2. Both whole-rock lavas and pumice fragments, and glass separated from pumice fragments were ground in an agate mill, after careful washing in distilled water to remove any seawater-derived salt deposits. Major element compositions and Sc abundances were determined by inductively coupled plasma–atomic emission spectrometry (ICP-AES), and the remainder of the trace
1456
1457
85·57
1·001 0·286 0·005 1·693 0·007 0·006 2·999
71·76
1·001 0·559 0·008 1·421 0·008 0·001 2·999
59·24
1·007 0·800 0·015 1·163 0·008 0·000 2·993
36·77 34·93 0·65 28·47 0·28 b.d.l. 101·10
O-1013 Hawaiite 3-5-r p-rim
46·46
1·006 1·045 0·021 0·907 0·013 0·002 2·994
34·58 42·98 0·86 20·92 0·42 0·08 99·84
O-1013 Hawaiite 3-4 mp-core
15·06
0·991 1·592 0·118 0·282 0·026 0·000 3·009
30·68 58·94 4·31 5·86 0·74 b.d.l. 100·53
O-361i Com. Tr. 2 mp-core
0·022 2·000
Na Sum M1+M2
44·92 44·06 11·02
0·003 0·839 0·855
Mn Mg Ca
Ca at. % Mg at. % Fe∗ at. %
0·019 0·042 0·014 0·067 0·139
48·20 35·93 15·87
0·041 2·000
0·006 0·659 0·884
0·000 0·121 0·004 0·096 0·189
1·703 0·286 0·011 2·000
3·78 6·02 100·54
2·44 4·55 101·04
1·851 0·149 0·000 2·000
45·35 4·30 6·47 0·12 9·42 0·19 11·77 21·97 0·57 100·16
O-120b Basalt 3-1-r mp-rim
50·58 1·53 3·89 0·47 6·75 0·10 15·37 21·80 0·31 100·80
O-120b Basalt 5-1-c mp-core
AlVI Ti Cr Fe3+ Fe2+
SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O 1·000 Sum 1·749 0·132 Fe2O3 0·109 FeO 0·009 Sum 0·000 3·000 Formula Si 5·86 AlIV Fe3+IV Sum T
30·28 63·29 4·73 2·21 0·26 b.d.l. 100·77
O-361i Sample: Com. Tr. Rock type: 1 Spot: mp-rim
Clinopyroxene
40·73 34·04 25·23
0·037 2·000
0·033 0·662 0·792
0·004 0·012 0·003 0·045 0·412
1·963 0·037 0·000 2·000
1·56 12·99 100·39
51·71 0·43 0·91 0·10 14·39 1·04 11·69 19·46 0·50 100·24
O-120b Basalt 6-1-rm p-rim
43·12 45·65 11·23
0·023 2·000
0·002 0·787 0·829
0·020 0·025 0·010 0·054 0·159
1·898 0·102 0·000 2·000
1·98 5·19 100·73
51·85 0·92 2·82 0·36 6·97 0·08 16·08 21·13 0·32 100·53
O-1013 Hawaiite 1-1-c p-core
41·25 41·31 17·44
0·025 2·000
0·008 0·786 0·768
0·017 0·039 0·000 0·016 0·308
1·927 0·073 0·000 2·000
0·56 9·92 100·48
51·90 1·39 2·00 b.d.l. 10·42 0·25 14·19 19·71 0·56 100·42
O-1013 Hawaiite 4c-3 ml
41·46 31·20 27·34
0·040 2·000
0·036 0·606 0·805
0·001 0·016 0·002 0·039 0·456
1·968 0·032 0·000 2·000
1·35 14·16 99·64
51·11 0·56 0·72 0·07 15·37 1·09 10·55 19·50 0·54 99·51
O-361i Com. Tr. 3-r p-rim
41·29 25·03 33·67
0·040 2·000
0·050 0·488 0·805
0·000 0·010 0·000 0·054 0·552
1·965 0·018 0·017 2·000
2·42 16·89 99·90
50·25 0·35 0·38 b.d.l. 19·06 1·51 8·37 19·21 0·53 99·66
O-361i Com. Tr. 2-c mp-core
39·96 17·30 42·74
0·061 2·000
0·065 0·333 0·770
0·000 0·010 0·003 0·072 0·698
1·981 0·012 0·007 2·000
2·26 20·98 100·10
49·79 0·33 0·25 0·09 23·01 1·93 5·62 18·06 0·79 99·87
O-361i Com. Tr. 1-c mp-core
Sum
Sum A
Na K
Ca Na Sum B
Ti Mg Fe2+ Mn Sum C
Formula Si AlIV Sum T AlVI Cr Fe3+
SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K 2O F Sum
15·935
0·935
0·638 0·297
0·599 1·401 2·000
0·455 0·589 3·689 0·176 5·000
7·781 0·219 8·000 0·086 0·006 0·000
49·42 3·84 1·64 0·05 27·02 1·27 2·42 3·55 6·68 1·48 1·87 99·24
Sample: O-361i Rock type: Com. Tr. Spot: 1 ml
Amphibole
For the sample labels: O, Opl. Mineral formulae calculated on the basis of: three cations for olivine; four cations for clinopyroxene, according to the procedure proposed by Vieten & Hamm (1978) and Vieten (1980); 23 oxygens for amphibole, according to the procedures proposed by Leake (1978) and Droop (1987). p, phenocryst; mp, microphenocryst; ml, microlite; Fo, forsterite; Fe∗ = Fe2+ + Fe3+ + Mn; Com. Tr., comenditic trachyte; b.d.l., below detection limit.
Fo mol %
Formula Si Fe Mn Mg Ca Ni Sum
38·01 25·39 0·38 36·19 0·29 0·04 100·30
SiO2 FeO MnO MgO CaO NiO Sum
40·09 13·67 0·22 45·46 0·28 0·30 100·02
O-120b Basalt 3-5-r p-rim
Sample: O-120b Rock type: Basalt Spot: 3-6-c p-core
Olivine
Table 1: Representative analyses of olivine, clinopyroxene and amphibole in mafic and felsic volcanic rocks from Pantelleria CIVETTA et al. THE GEOCHEMISTRY OF VOLCANIC ROCKS FROM PANTELLERIA
1458
99·22
Sum
100·14
b.d.l.
0·14
2·68
15·06
0·26
0·67
32·20
0·06
49·07
0·008
7·237
0·075
0·080
3·205
0·732
0·017
0·000
20·032
81·07
18·51
0·42
Ti
Al
Fe3+
Mg
Ca
Na
K
Ba
Sum
An mol %
Ab mol %
Or mol %
0·83
24·16
75·01
20·004
0·000
0·033
1·57
33·21
65·22
20·015
0·000
0·063
1·331
2·614
0·014
0·137
6·484
0·025
9·347
5·34
53·83
40·83
20·056
0·000
0·211
2·125
1·612
0·108
0·251
5·528
0·031
10·190
99·54
b.d.l.
0·91
6·04
8·29
0·40
1·66
25·85
0·23
56·16
ml
3s-5
Basalt
S-17a
2·17
48·36
49·47
20·030
0·000
0·087
1·947
1·992
0·000
0·111
5·923
0·015
9·955
100·37
b.d.l.
0·38
5·58
10·33
b.d.l.
0·74
27·92
0·11
55·31
ml
4c-4
Hawaiite
O-1013
15·90
74·34
9·76
20·038
0·111
0·647
3·028
0·397
0·000
0·036
4·407
0·000
11·523
100·53
0·54
2·86
8·80
2·09
b.d.l.
0·24
21·07
b.d.l.
64·93
p-core
4-c
Com. Tr.
O-361i
36·48
62·43
1·09
19·868
0·000
1·398
2·393
0·042
0·000
0·084
3·930
0·009
12·011
100·67
b.d.l.
6·19
6·97
0·22
b.d.l.
0·57
18·83
0·07
67·82
mp-rim
3-r
Com. Tr.
O-361i
28·04
67·72
4·24
19·971
0·052
1·116
2·694
0·169
0·000
0·028
4·144
0·024
11·797
99·79
0·25
4·89
7·77
0·88
b.d.l.
0·19
19·66
0·18
65·97
mp-rim
2-r
Com. Tr.
O-361i
100·14
99·65
38·58
7·78
98·87
0·27
2·76
0·66
45·58
0·11
48·97
0·40
ml
c-3
Hawaiite
1·566
Fe2+
X’Ilm
X’Usp
0·73
3·000
0·000
Ca Sum
0·135
Mg
0·024
0·515
Fe3+
Mn
0·036
0·716
0·008
Al
Ti
Si
0·92
2·000
0·007
0·102
0·014
0·801
0·145
0·003
0·915
0·010
(magnetite) and 2 (ilmenite) cations
Formula on the basis of 3
Sum
18·69 51·15
FeO
98·27
b.d.l.
2·47
0·76
67·97
0·83
26·01
0·23
ml
c-2
Basalt
O-1001
Magnetite Ilmenite S-17a
Fe2O3
Sum
CaO
MgO
MnO
FeO
Al2O3
TiO2
SiO2
Spot:
Rock type:
Sample:
Sum
Cl
F
P 2O5
Cr2O3
K 2O
Na2O
CaO
MgO
MnO
FeO
Al2O3
TiO2
SiO2
Spot:
Rock type:
Sample:
99·14
b.d.l.
b.d.l.
b.d.l.
0·06
0·41
6·99
0·33
0·35
1·26
38·95
1·45
7·25
42·09
mp
ae1
Pantellerite
99·86
0·21
5·00
39·99
b.d.l.
b.d.l.
0·24
53·09
0·10
b.d.l.
0·71
0·09
b.d.l.
0·43
ml
7
Com. Tr.
O-361i
Aenigmatite Apatite O-330
NUMBER 8
0·949
2·947
0·071
0·103
6·931
0·008
8·962
99·84
b.d.l.
0·27
3·76
13·36
0·05
0·90
30·13
0·18
51·19
mp-rim
3-1-r
Basalt
O-120b
Anorthoclase
VOLUME 39
For the sample labels: O, Opl; S, Sic. p, phenocryst; mp, microphenocryst; ml, microlite; Com. Tr., comenditic trachyte; b.d.l., below detection limit; X’Usp, molar fraction of ulvo¨spinel; X’Ilm, molar fraction of ilmenite. Opaque formulae calculated according to the procedure proposed by Stormer (1983).
8·678
Si
Formula on the basis of 32 oxygens
0·07
b.d.l.
BaO
2·04
Na2O
K 2O
0·29
0·49
FeO
16·17
33·19
Al2O3
CaO
0·06
MgO
46·91
TiO2
p-rim
p-core
SiO2
1c-1-r
1c-3-c
Spot:
Basalt
Basalt
Rock type:
S-17a
S-17a
Sample:
Plagioclase
Table 2: Representative microprobe analyses of feldspars, opaques, aenigmatite and apatite in mafic and felsic volcanic rocks from Pantelleria
JOURNAL OF PETROLOGY AUGUST 1998
CIVETTA et al.
THE GEOCHEMISTRY OF VOLCANIC ROCKS FROM PANTELLERIA
Table 3: Major (wt %) and trace element (ppm) compositions of mafic volcanic rocks from Pantelleria Sample:
Opl 120b
Sic 16
Opl 1013
Sic 5
Sic 3
Opl 104b
Sic 17a
Sic 48
Opl 1001
Location:
Cuddie
Punta San
Punta
Cuddia del
Cuddia del
Punta San
Punta San
Punta
Punta della
Rosse
Leonardo
Sidere
Cat
Cat
Leonardo
Leonardo
Karuscia
Guardia
Age (ka):
120
29
50–80
>50
120
29
29
50 ka(WR)
18·3†
465†
0·70313±1
11·35
48·44
0·51298±1
0·7
3·6
1·5
Opl 120b
B>50 ka(WR)
27·5†
702†
0·70320±1
11·56
59·88
0·51294±1
1·7
7·2
3·1
19·937
Sic 3
B>50 ka(WR)
25·2†
528†
0·70315±1
11·25
53·58
0·51296±1
n.a.
n.a.
1·9
19·910
Sic 5
B>50 ka(WR)
16·5†
532†
0·70310±1
10·78
52·82
0·51297±1
1·1
4·7
n.a.
18·302
leached
19·664
19·591
Sic 16
B6·9% MgO, and Ni and Cr contents are always
