Palaeomagnetism of welded, pyroclastic-fall scoriae at Vulcano, Aeolian Archipelago

Journal of Volcanology and Geothermal Research 107 (2001) 71±86

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Palaeomagnetism of welded, pyroclastic-fall scoriae at Vulcano, Aeolian Archipelago E. Zanella a,*, G. De Astis b, R. Lanza a a

Dipartimento di Scienze della Terra, UniversitaÁ di Torino, Via Valperga Caluso 35, 10125 Torino, Italy b Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy Received 23 May 2000; revised 4 December 2000; accepted 4 December 2000

Abstract This paper deals with the magnetic fabric and remanent magnetization of welded scoriae fall deposits. Four scoriae blankets were emplaced on Vulcano (Aeolian Islands) from about 50 to 8 ka. Their detailed chronostratigraphic position has been debated in the literature and they have therefore been given a different interpretation in the reconstruction of the Vulcano's eruptive history. The aims of the study were a better understanding of the magnetic features of the scoriae fall deposits, which have so far been given little attention, and to set the chronostratigraphic relationships of the sampled deposits on the ground of their palaeomagnetic directions. Overall, the magnetic properties of the four deposits are alike. Investigation of isothermal remanence and susceptibility versus temperature measurements point to low-Ti titanomagnetite as the main ferromagnetic mineral. Magnetic foliation is well developed, moderately dispersed and in most cases close to horizontal. Lineation is either clustered or dispersed within the foliation plane. Stepwise thermal and alternating ®eld demagnetization shows that secondary components are negligible in two units. In the other two, they are always completely removed in the ®rst steps, below 20 mT or 4008C. Thereafter, the characteristic component (ChRM) is clearly isolated and the within-site dispersion of its direction is small …a95 , 48 at 16 out of 23 sites). The within-site consistency of the directional features of both magnetic fabric and remanence shows that no en masse movement of welded scoria occurred nor did individual spatter move with respect to one another after their emplacement. The scoriae therefore welded and cooled in situ. The between-site dispersion of the ChRM directions also is very small …a95 , 3:58† for three units, and their mean palaeomagnetic direction is consistent with the palaeosecular variation curve for Aeolian Islands. Statistical analysis of the results from two units whose isotopic ages are indistinguishable within the error range (Quadrara: 21.3 ^ 2.4 ka; Spiaggia Lunga: 24.0 ^ 5.0 ka) shows that they were emplaced at different times, because their ChRM directions are different at the 95% con®dence level. Moreover, the Spiaggia Lunga scoriae were emplaced by a single eruption, since their mean-site directions are statistically indistinguishable at ®ve out of six sites. The palaeomagnetic results corroborate the hypothesis that the investigated scoriae deposits correspond to different volcanic phases, related to recurrent arrival of ma®c magma, and show that those of Spiaggia Lunga and Quadrara were produced during two distinct periods of volcanic activity with little time separation between them. q 2001 Elsevier Science B.V. All rights reserved. Keywords: magnetic fabric; scoriae; thermal remanence; Vulcano

* Corresponding author. Tel.: 139-011-670-7191; fax: 139-011-670-71-55. E-mail address: [email protected] (E. Zanella). 0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0377-027 3(00)00298-5

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1. Introduction Systematic studies of rock-magnetism of pyroclastic rocks began in the 1980s (Geissman, 1980; Ellwood 1982; Incoronato et al., 1983; Knight et al., 1986) and mainly focused on tuffs and ignimbrites. Numerous papers have since investigated the reliability of their palaeomagnetic records and the relations between magnetic fabric and emplacement processes (Rosenbaum, 1986; Hillhouse and Wells, 1991; Seaman et al., 1991; MacDonald et al., 1992; Fisher et al., 1993; Palmer et al., 1996; Baer et al., 1997; Zanella et al., 1999). Coarse pyroclastic deposits such as breccia and agglomerate received little attention, probably because their deposition dynamics and the possibility of post-emplacement remobilization made researchers suspicious of their ability to faithfully record the direction of the ambient magnetic ®eld. Pioneering studies (Aramaki and Akimoto, 1957; Chadwick, 1971) stressed the basic role of emplacement temperature in determining the characteristics of the natural remanent magnetization (NRM) of these rocks. When ®nal emplacement occurs at high temperature, above the Curie point of the rock's ferromagnetic minerals, a thermal remanent magnetization (TRM) is acquired during in situ cooling. Its direction is parallel to the ambient ®eld and is the same in all individual blocks. During low temperature emplacement, on the other hand, most blocks come to rest in random positions after acquiring their TRM, which is therefore randomly oriented. Several authors have investigated the emplacement temperature of pyroclastic ¯ows with reference to the TRM characteristics of their incorporated lithic fragments (Mandeville et al., 1994; Bardot, 2000 and references therein). On the other hand, characterization of breccia and agglomerate depositional units and determination of their mode of emplacement on the grounds of rock-magnetism, as suggested by Chadwick (1971), has rarely been discussed (Grubensky et al., 1998; Smith et al., 1999). Determination of the stratigraphy of these deposits is often hampered by their irregular shape and lack of lateral continuity. Moreover, geochemical analyses and isotopic dating are of little assistance in distinguishing temporally closely spaced depositional units, whose rock chemistry may be similar and whose age differences may fall within the error range of the available

dating techniques. Attribution of a unit to an eruption is important in the case of active volcanoes, since it is relevant to deeper understanding and better modeling of their eruptive processes. Various welded to weakly welded scoriae blankets were emplaced from about 50 to 8 ka at Vulcano, Aeolian Islands. The detailed stratigraphic position of some of these deposits is still uncertain and their relationships with the eruptive and tectonic history of Vulcano are debated. Keller (1980) observed that three scoriae fall deposits (Punta Luccia, Spiaggia Lunga and Quadrara, Fig. 1) had very similar composition to each other and were deposited in the same position within his stratigraphical scheme. This author tentatively ascribed their emplacement to a trachybasaltic eruptive phase related to the early stages of the La Fossa caldera formation (around 25±10 ka). Gioncada and Sbrana (1991) agreed with Keller's reconstruction and outlined further links between the volcanic phases and the caldera formation. On the basis of new stratigraphical and geochemical data as well as new K/Ar and 230Th/ 232Th ages, other authors (Frazzetta et al., 1985; De Rosa et al., 1988; De Astis, 1995) referred the three deposits to distinct arrivals of ma®c magma. In particular, the Punta Luccia unit was shown to be older than Spiaggia Lunga and Quadrara, whose distinction could only be based on small difference in composition. Preliminary palaeomagnetic results (Zanella, 1995) have shown that both remanence and magnetic fabric are usually well de®ned in the welded scoriae at Vulcano. A systematic sampling was therefore planned, to test the extent to which rock magnetic data can be used to correlate the individual units and understand their chronostratigraphical setting.

2. Volcanological setting 2.1. Volcanic history The island of Vulcano is located in the southern sector of the Aeolian Arc (southern Tyrrhenian Sea) and is formed entirely of Quaternary volcanic rocks ranging in composition from basalt to rhyolite. It is the upper part of a large, submerged volcanic edi®ce whose subaerial activity started between 135 and

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73

Fig. 1. Geological sketch map of Vulcano island (simpli®ed after De Astis, 1995) and sampling sites.

120 ka (De Astis, 1995; De Astis et al., 1997; Laj et al., 1997; Soligo et al., 1999) and has continued up to the present. The last eruption occurred in 1888±1890 from the still active La Fossa cone. The volcanological, petrological and structural evolution of this volcanic system

has been extensively investigated (Keller, 1980; Frazzetta et al., 1983; De Rosa et al., 1987; De Astis, 1995; De Astis et al., 1989, 1997; Gioncada and Sbrana, 1991; Barberi et al., 1994; Ventura, 1994; Del Moro et al., 1998; Gioncada et al., 1998; Ventura et al.,

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E. Zanella et al. / Journal of Volcanology and Geothermal Research 107 (2001) 71±86

1999). The oldest part of the island is Vulcano Primordiale (Fig. 1), a strombolian cone dissected by the Piano Caldera at about 100 ka. Intense volcanic activity eventually developed inside the caldera depression and ®lled it in with, ®rst, lava ¯ows (99.5±78.5 ka Ð Frazzetta et al., 1985) and then pyroclastic products erupted from different vents (77.8±48.5 ka Ð De Astis et al., 1989). Minor activity occurred to the north of Piano Caldera from small vents external to the depression, as suggested by K/Ar isotopic ages of the Punta Luccia scoriae blanket (48.5 ^ 6.5 ka Ð Frazzetta et al., 1985). Approximately at the end of this period, a series of volcano-tectonic faults oriented NNE±SSW affected the Piano area and the south-eastern sectors of La Fossa Caldera depression were formed. After a gap of about 20 kyr, volcanic activity resumed toward the northern part of Vulcano, as shown by 230Th/ 232Th isotopic ages from the rocks of the Monte Minico± Lentia Complex (Fig. 1) sampled in different stratigraphic positions, and lasted from 28 to 8 ka (Soligo et al., 1999). The new volcanic edi®ce was mainly formed of lava ¯ows and domes of rhyolitic and minor intermediate composition. Volcaniclastic and pyroclastic deposits of undetermined age were also locally emplaced on its slopes (Valentine et al., 1998). Volcanological evidence (De Astis, 1995) and 230 Th/ 232Th isotopic data (Delitala et al., 1997) show that at the beginning of this time interval substantial pyroclastic eruptions occurred in other parts of the island and deposited the Spiaggia Lunga (24 ^ 5 ka) and Quadrara (21.3 ^ 3.4 ka) units that blanket the western and southwestern ¯anks of Vulcano Primordiale, respectively (Fig. 1). The collapse of the eastern part of the Monte Minico±Lentia edi®ce probably occurred between 13 and 8 ka and produced the western and northern sectors of La Fossa Caldera. Intense volcanism within and along the border of this depression resulted in several lava ¯ows and pyroclastic deposits that form the youngest volcanic structures of the island: the Monte Saraceno center, La Fossa cone and Vulcanello. 2.2. Stratigraphy and lithology During some periods of Vulcano's eruptive activity, pyroclastic deposits with similar lithological features were deposited. On the basis of volcanological and compositional evidence, Keller (1980)

attributed the same stratigraphic position to three major deposits of welded scoriae cropping out along the ¯anks of Vulcano Primordiale, and reported as Punta Luccia, Quadrara and Saraceno±Spiaggia Lunga (Spiaggia Lunga in the more recent literature). This stratigraphic scheme is no longer tenable for the Punta Luccia scoriae, which have been shown by K/ Ar dating (48.5 ^ 6.5 ka) to be older than the other two units, whereas the 230Th/ 232Th ages of Spiaggia Lunga (24 ^ 5 ka) and Quadrara (21.3 ^ 3.4 ka) scoriae are indistinguishable within the error range. Both are covered by the surge deposits called Tu® di Grotte dei Rossi Inferiori (TGR Inf. Ð De Astis, 1995) and are never in contact with each other. The Spiaggia Lunga Unit (SL) consists of two different pyroclastic deposits on the western ¯anks of Vulcano Primordiale (Fig. 1). Their thickness is maximum (about 60 m) at Scoglio di Capo Secco, where the most complete sequence crops out, and decreases toward the south. About 4 m of ashy hydromagmatic tuffs with plane-parallel to sandwave lamination (De Rosa et al., 1987, 1988) form the lowest exposed part of the unit. The base of these surge beds is not exposed; at the top, they are covered by a fall layer of partially welded scoriae and intrusive xenoliths, which produce impact sags on the underlying deposit. This layer gradually passes upward to a spatter agglutinate and a completely welded scoriae blanket characterized by quenching fractures and/or weak columnar jointing. A thin, massive bed of ashes lies at some places on the top of the blanket and is in turn overlain by a fallout scoriae blanket with a lower degree of welding and a maximum thickness of 7±8 m. Scoriae are porphyritic, with phenocrysts of plagioclase, augitic pyroxenes, and subordinate olivine and microphenocrysts of Fe±Ti oxides, set in a generally glassy groundmass. All the samples are classi®ed as shoshonitic basalts (De Astis, 1995) according to the K2O vs. SiO2 classi®cation diagram (Peccerillo and Taylor, 1976). The Quadrara Unit (Q) extends over the southern ¯anks of Vulcano Primordiale (Fig. 1). It reaches its maximum thickness of about 10 m at Quadrara and grows thinner toward both south and north. The unit shows an erosive contact with the underlying deposits and consists of two distinct layers. The basal layer is composed of whitish pumices, one to ten cm in size and with a trend to inverse grading. It is followed by a

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and Taylor (1976) classi®cation diagram. Scoriae samples display compositions between shoshonite and latite (De Astis, 1995). In addition to Spiaggia Lunga and Quadrara units, palaeomagnetic investigation was extended to the Punta Luccia and Monte Saraceno units to get a larger data set on the magnetic properties of welded scoriae. The Punta Luccia unit (PL) blankets the slopes of Monte Luccia (Fig. 1) with a variable thickness (maximum of about 30 m). It has a non-erosive contact with the underlying deposits and at the base is so strongly welded as to appear lava-like. The thickness of this basal part is laterally variable and nowhere exceeds 2 m. The degree of welding progressively decreases toward the upper part of the blanket,

blanket of bombs, welded scoriae and spatter, mainly reddish. Lava lithic fragments occur in both layers and their amount is higher (7±10% of volume) in the basal layer. The degree of welding is maximum in the central part of the scoriae blanket, which is characterized by roughly columnar jointing. The pumices contain microphenocrysts of biotite and K-feldspar, with microlites of plagioclase and minor pyroxene set in a glassy groundmass. The scoriae are weakly porphyritic and contain microphenocrysts of feldspar, pyroxene, biotite, and few iddingsitized olivines set in a glassy groundmass containing abundant opaque and feldspar microlites. The usual composition of the pumices is trachytic; for some samples it falls between the latite and trachyte ®elds of the Peccerillo

Table 1 Palaeomagnetic data for the Vulcano scoriae. Symbols: n ˆ number of collected and demagnetized specimens; NRM and ChRM ˆ natural and characteristic remanent magnetization; Jr ˆ NRM intensity; D, I ˆ declination, inclination; k ˆ Fisher's precision; a 95 ˆ semi-angle of con®dence Unit and age

Monte Saraceno (8.3^1.6 ka)

Site

n

Jr (A/m)

NRM

ChRM

D

I

a 95

D

I

k

a 95

48.6 51.9 52.8 53.0 53.2 43.5 50.5

296 203 75 26 127 53 377

3.2 4.3 7.8 10.3 3.2 7.7 3.5

MS1 MS2 MS3 MS4 MS5 MS6 All

9 7 6 9 17 8 6

3.6 2.8 2.1 7.1 9.5 35.6 10.1

5.6 3.3 300.4 Dispersed 8.3 4.6 351.7

43.0 49.0 25.3

4.9 7.5 14.3

51.6 13.1 39.7

3.6 14.9 28.9

6.9 8.5 10.4 9.6 8.0 3.3 7.6

Quadrara (21.3^3.4 ka)

Q1 Q2 Q3 Q4 Q5 Q6 Q7 All

19 11 25 18 9 11 13 7

8.9 11.0 6.3 9.5 3.3 3.8 9.7 7.5

353.5 7.6 4.0 1.8 356.9 28.5 359.7 3.1

49.7 43.1 52.6 56.8 50.6 67.2 46.2 52.7

3.8 3.8 1.9 3.3 4.9 8.8 12.2 7.4

353.4 4.5 3.7 359.4 1.5 7.9 10.1 3.0

51.2 46.3 52.7 58.0 52.2 52.9 48.5 52.5

86 188 222 119 216 377 451 328

3.6 3.3 1.9 3.2 3.5 2.4 2.0 3.3

Spiaggia Lunga (24.0^5.0 ka)

SL1 SL2 SL3 SL4 SL5 SL6 All

28 22 16 13 5 11 6

4.6 2.1 1.7 2.5 2.3 2.0 2.5

1.9 2.4 3.2 357.5 Dispersed 350.2 359.1

46.8 41.6 45.3 41.7

3.8 1.9 2.8 2.0

47.7 44.7

5.1 4.5

3.6 0.7 3.4 357.3 3.2 3.1 1.8

46.4 42.7 43.2 43.0 48.1 46.7 45.0

73 653 209 460 29 95 769

3.2 1.2 2.6 1.9 14.5 4.7 2.4

PL1 PL2 PL3 PL4 All

22 10 18 32 4

39.9 1.8 2.2 2.2 11.5

28.1 39.5 25.6 37.8 33.7

11.9 3.5 2.7 2.1 18.9

44.0 22.7 18.5 16.4 25.4

28.5 34.1 23.2 32.7 30.1

23 716 171 285 45

6.6 1.9 2.6 1.6 13.8

Punta Luccia (48.5^6.5 ka)

52.0 21.6 16.4 14.6 26.4

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The Monte Saraceno Unit (MS) crops out along the western rim of the Piano Caldera (Fig. 1). Keller (1980) considered this subaerial scoriae blanket as one of the two depositional facies of his ªAlighieri formationº, the other being lacustrine hyaloclastitic lapilli-tuffs. Later studies (De Astis, 1995) showed that the scoriae underlie the tuffs and the two facies form two distinct depositional units. The maximum thickness of the scoriae blanket is of about 20 m. The unit has an erosive contact with the underlying Tu® di Grotte dei Rossi Inferiori. Its very basal part is poorly to non-welded and consists of highly vesiculated, cm-sized scoriae with few spindle bombs. Otherwise, the deposit is formed of large, ¯attened bombs and spatter bombs up to 1 m in size, whose degree of welding increases upwards. The scoriae are mainly vitrophyric and contain scarce microphenocrysts of pyroxene and abundant magnetite grains, together with ma®c felsic microlites. Their composition is shoshonitic (De Astis, 1995) and their K/Ar age is 8.3 ^ 1.6 ka (De Astis et al., 1989); some small lava ¯ows and dikes with similar petrochemical af®nity are associated with the scoriae blanket and probably erupted in the ®nal stages of its emplacement. Fig. 2. (a) Isothermal remanent magnetization (IRM) acquisition curve and (b) thermal demagnetization of the IRM components. Symbols: dot ˆ low- ; square ˆ intermediate- ; triangle ˆ highcoercivity component.

3. Rock magnetism

which consists of poorly sorted spatter agglutinate, ¯attened scoriae and bombs whose proportions vary throughout the deposit, together with a large amount of lithic clasts, which have been referred to the collapse of crater walls. The intermediate and upper parts of the deposit are characterized by density grading, evidenced by the increase of dimensions of weakly welded scoriae and bombs. Spatter bombs are absent and the amount of lithic clasts decreases from about 30% at the base to about 10% at the top. Scoriae are porphyritic with phenocrysts of plagioclase, clinopyroxene, olivine and Fe-oxides set in a partially microcrystalline groundmass composed of the same minerals and glass. Crystals often exhibit dimensional preferred orientation. The porphyricity of the scoriae clasts decreases towards the top of the blanket, whereas their vesicularity increases. Chemical analyses (De Astis, 1995) have shown that all samples have shoshonitic composition.

3.1. Laboratory procedures and magnetic properties

Twenty-three sites were drilled with a batterypowered tool. At each site, the cores were randomly distributed at different heights and over distances up to 10±15 m. They were oriented both with magnetic and solar compass, and magnetic variation was found to be in the range 58W±58E between sites, and less than ^38 within each site. One or two standard cylindrical specimens were then cut in the laboratory. The number of specimens per site depended on the outcrop condition, and varied from 5 to 32.

Natural remanent magnetization (NRM), and susceptibility and its anisotropy (AMS) were measured using a JR-4 or JR-5 spinner and a KLY-2 bridge, respectively. Magnetic mineralogy was investigated by isothermal remanent magnetization (IRM), given by a pulse magnet with a maximum ®eld of 1.5 T, and susceptibility versus temperature experiments run with Bartington MS2 equipment.

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3.2. Palaeomagnetism

Fig. 3. (a) Normalized susceptibility vs temperature curve and (b) normalized intensity decay curve during thermal demagnetization. Symbols: full/dashed line ˆ heating/cooling cycle; k0 ˆ room temperature value.

The NRM intensity varies between 1 and 11 A/m (Table 1), except for two sites (MS6 and ML1) with a very high intensity of 35±45 A/m. The magnetic properties are similar in the four units. The IRM saturation was always reached at an applied ®eld of 100±400 mT (Fig. 2a). Thermal demagnetization of the IRM components according to Lowrie's (1990) method showed that the high-coercivity component is negligible and the medium- and low-coercivity components are completely removed within 550± 5708C (Fig. 2b). The susceptibility versus temperature experiments yielded Curie points in the same temperature range (Fig. 3a). These results show that low-Ti titanomagnetite, with its limited coercivity and Curie point lower than 5758C, is the main carrier of magnetization in all the investigated units.

Two or more pilot-specimens per site were stepwise demagnetized, one thermally up to 6008C and one in alternating ®eld (AF) up to a peak-®eld of 100 mT. The median destructive ®eld was 20±40 mT. The blocking temperature spectrum ranged from 350±4008C to the Curie point in the specimens from Quadrara Unit and was wider in those from the other units, whose intensity began to decrease in the ®rst demagnetization steps (Fig. 3b). The two methods provided similar results. Secondary components were absent or negligible at Spiaggia Lunga (Fig. 4a) and Punta Luccia, stronger at Quadrara (Fig. 4b and c) and Monte Saraceno. They were always removed in the ®rst steps, below 20 mT or 4008C. Thereafter, the Zijderveld diagrams pointed straightforward to the origin and a stable, characteristic component (ChRM) was isolated. AF demagnetization at 20±30 mT proved enough to remove any secondary component in all pilot specimens and these values were used to demagnetize all the remaining specimens. The site mean values of the NRM and ChRM directions are listed in Table 1, together with the values of the precision parameter k and the semiangle of the cone of con®dence a 95 of Fisher's statistics. At most sites, the ChRM and NRM directions are similar (Table 1), but after demagnetization dispersion is always reduced (Fig. 5) as shown by the signi®cant improvement in the statistical parameter values. The within-site dispersion of the ChRM directions is usually small: a 95 is less than 48 at 16 sites and more than 108 at only two sites (Table 1). The between-site dispersion is also very small within each unit with the exception of Punta Luccia (Fig. 6). 3.3. Magnetic fabric The magnetic susceptibility of titanomagnetite grains depends on their shape. For non-equant grains, it is maximum when measured parallel to the longest axis, minimum when parallel to the shortest. The AMS of a titanomagnetite-bearing rock therefore mirrors the spatial arrangement and preferred orientation of its titanomagnetite grains and is a powerful tool for investigation of its fabric. AMS is de®ned by the three principal susceptibilities …k1 . k2 . k3 † and their directions. The results of the susceptibility

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Fig. 4. Zijderveld diagrams for AF (a, b) and thermal (c) demagnetization. Symbols: diamond ˆ declination; dot ˆ apparent inclination; ®gures ˆ peak-®eld (mT) or temperature (8C) values.

measurements are reported in Table 2, which lists the mean site values of the bulk susceptibility km and some AMS parameters together with the lineation and the foliation pole, calculated according to Jelinek (1978). km varies in the range 4660±35; 360 £ 1026 SI units, and the degree of anisotropy, P ˆ k1 =k3 ; which gives the degree of the preferred orientation (Tarling and Hrouda, 1993), is low …1:005 # P # 1:029†: The

Punta Luccia, Spiaggia Lunga and Monte Saraceno scoriae show similar values, whereas at Quadrara km is usually lower and the P values are the lowest. The fabric is always well de®ned at both specimen and site level. Magnetic foliation, F ˆ k2 =k3 ; is usually better developed than lineation, L ˆ k1 =k2 (Table 2), and its poles, represented by the minimum susceptibility axes k3, cluster more or less close to the vertical. The

Fig. 5. Equal-area projection of NRM and ChRM directions for site Q7. Symbol: star ˆ site mean value with a 95 con®dence ellipse.

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Fig. 6. Equal-area projection of the site mean ChRM directions. Symbol: star ˆ unit mean value. The a 95 con®dence ellipses of Monte Saraceno, Quadrara and Spiaggia Lunga are too small to be drawn.

maximum k1 and intermediate k2 axes are usually dispersed within the foliation plane (Fig. 7). The characteristics of the fabric are similar at all sites, with the exception of SL2 and SL3. Foliation either is horizontal or dips some 10±208. Lineation, too, is mainly close to the horizontal and its direction varies little at Spiaggia Lunga and Quadrara, where it is mainly oriented WNW±ESE and SW±NE, respectively. At sites SL2 and SL3, the principal directions are more scattered than at other sites and the k1 and k3 axes are exchanged (Table 2). The foliation plane is hence nearly vertical (Fig. 8). The exchange might be

ascribable to single-domain grains whose relation between geometry and susceptibility axes is opposite to that mentioned above: the longest and shortest axes correspond to k3 and k1, respectively. Occurrence of single-domain grains may be checked by investigation of remanence anisotropy, whose maximum axis is parallel to the grain's longest dimension irrespective of the magnetic state, single- or multi-domain (Stepehnson et al., 1986; Potter and Stephenson, 1988). The IRM anisotropy (AIRM) was therefore measured for seven specimens from site SL2. First, they were tumble-demagnetized in an alternating ®eld

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Table 2 Magnetic anisotropy (AMS) data for the Vulcano scoriae (same specimens as in Table 1). Symbols: n ˆ number of specimens; km ˆ bulk susceptibility; P ˆ degree of anisotropy; L ˆ magnetic lineation; F ˆ magnetic foliation; D, I ˆ declination, inclination; a 1, a 2 ˆ angles of the 95% con®dence ellipse for k3 Unit

Site

n

km £ 10 26 SI

P

L

F

Lineation k1

Foliation pole k3

D

I

D

I

a1

a2

Monte Saraceno

MS1 MS2 MS3 MS4 MS5 MS6

9 7 6 9 17 8

7880 22,850 31,500 16,640 11,810 10,160

1.013 1.019 1.020 1.008 1.008 1.021

1.007 1.007 1.007 1.004 1.004 1.010

1.006 1.012 1.013 1.004 1.005 1.010

68 341 282 340 185 49

14 1 4 10 0 0

293 122 105 ± 275 ±

71 89 86 ± 88 ±

25 13 12 ± 23 ±

12 8 6 ± 16 ±

Quadrara

Q1 Q2 Q3 Q4 Q5 Q6 Q7

19 11 25 18 9 11 13

14,340 9560 4660 9415 9680 10,440 10,800

1.010 1.005 1.006 1.007 1.007 1.007 1.005

1.002 1.002 1.002 1.004 1.003 1.003 1.002

1.007 1.002 1.004 1.004 1.004 1.004 1.002

265 274 217 214 ± 353 242

18 7 11 59 ± 38 17

72 126 22 ± 47 141 120

72 81 78 ± 83 48 75

27 21 24 ± 28 19 45

9 16 18 ± 11 10 24

Spiaggia Lunga

SL1 SL2 SL3 SL4 SL5 SL6

28 22 16 13 5 11

31,620 32,480 11,710 15,820 7100 35,360

1.026 1.023 1.014 1.013 1.018 1.021

1.014 1.008 1.005 1.005 1.009 1.008

1.012 1.015 1.008 1.007 1.009 1.013

122 151 215 289 106 108

6 72 78 13 1 0

287 255 109 60 8 18

84 5 4 70 81 65

46 57 53 28 25 30

27 32 42 18 13 16

Punta Luccia

PL1 PL2 PL3 PL4

22 10 18 32

22,120 34,300 15,840 19,480

1.013 1.029 1.014 1.016

1.006 1.016 1.005 1.007

1.007 1.012 1.009 1.009

22 254 216 246

11 4 6 1

188 3 82 354

79 78 81 87

38 19 17 26

21 8 11 17

of 80 mT, then given a 15 mT direct ®eld. The acquired IRM was measured and the procedure repeated in six different positions to compute the AIRM tensor. The directions of the AMS and AIRM maximum and minimum axes were actually exchanged (Fig. 9). The rock fabric of SL2 and SL3 is thus similar to that of other sites, and the AMS difference solely results from single-domain grains. 4. Discussion The within-site consistency and low dispersion of the AMS and ChRM directions of the scoriae of Vulcano are their most interesting magnetic features and a substantial mark for their interpretation. Although the degree of anisotropy is low, at 18 out

of the 23 investigated sites the magnetic fabric is characterized by a well-developed foliation, which is close to the horizontal at 13 sites. Lineation is weak and often rather dispersed, even if its mean values are consistent within each unit with the exception of Mt. Saraceno. The mean site lineation is directed NW±SE at all four sites of Spiaggia Lunga (leaving aside SL2 and SL3, as mentioned above), NE±SW at three out of four sites of Punta Luccia and NE±SW to E±W at ®ve out of seven sites of Quadrara. The fabric features are related to the emplacement processes, and an attractive hypothesis is that they are acquired at the very moment the spatter ¯attens on the ground. Foliation is consistent with the ¯attened shape and the lineation direction could be related to the direction the spatter comes from. This tentative explanation needs to be substantiated by a physical model and investigation of other

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Fig. 7. Equal-area, lower hemisphere projection of the principal susceptibility axes and 95% con®dence ellipses. Symbols: square ˆ k1, maximum; triangle ˆ k2, intermediate; dot ˆ k3, minimum axis; great circle ˆ magnetic foliation.

pyroclastic-fall scoriae from other volcanoes. As far as the scoriae of Vulcano are concerned, the main conclusion drawn from the AMS measurements is that no relative movement at the outcrop scale occurred next to the emplacement. This result is further substantiated by the low dispersion of the ChRM directions, which were acquired below the Curie point (550±5708C) and

hence well below solidi®cation. As no evidence has been found for widespread occurrence of secondary ferromagnetic minerals, the ChRM is a primary, thermal remanence. It was acquired throughout cooling of a hard rock and should have recorded the direction of the Earth's ®eld at the time of emplacement. Within each unit, with the exception of Punta Luccia, the values of the statistical parameters show a very

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Fig. 8. Magnetic fabric orientation in the Vulcano scoriae. The foliation plane is shown as a great circle, the lineation azimuth as a black triangle and its 95% con®dence level as a black band.

good between-site consistency of the ChRM directions (Table 1 and Fig. 6). This suggests that they represent the ambient ®eld. This hypothesis can be checked against the palaeosecular variation (PSV) curve of the Aeolian Islands (Zanella, 1995). The part of the curve younger than 60 ka is shown in Fig. 10, together with the mean direction and age uncertainty of the four scoriae units. The Monte Saraceno, Quadrara and Spiaggia Lunga directions are consistent with those of coeval lava ¯ow units, and their deviation from the local geocentric axial dipole …D ˆ 08; I ˆ 578† are within the PSV range. The

results as a whole show beyond any doubt that these pyroclastic-fall scoriae are a reliable source of palaeomagnetic information. The situation with Punta Luccia is different. Its within-site consistency is similar to that of the other units and therefore points to a primary origin for the ChRM. On the other hand, its between-site dispersion is higher (Table 1; Fig. 6) and the palaeomagnetic directions depart from the PSV curve (Fig. 10). Their inclination is 25±308 lower than that of other volcanic units of similar age Two hypotheses can be advanced to explain the anomalously low inclination: short-lived ¯uctuations of the

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83

the 95% con®dence level at ®ve out of six sites. Most of these scoriae actually recorded the same direction and could have been emplaced by a single eruption. The directions of the Quadrara scoriae are statistically different at ®ve out of seven sites. This suggests that they either were emplaced by a series of temporally closely spaced eruptions, and their age is thus slightly different from site to site, or underwent a very small, random tilting after the emplacement. 5. Conclusions

Fig. 9. Equal-area projection of the susceptibility (AMS) and isothermal remanence (AIRM) principal directions at site SL2. Symbols as in Fig. 7.

Earth's ®eld or tectonic tilting related to the formation of La Fossa Caldera, whose eastern wall is close to Punta Luccia (Fig. 1). Present data, however, are not enough to choose between the two possibilities. Our work was also aimed at stratigraphic correlation between Quadrara and Spiaggia Lunga. As mentioned above, Keller (1980) correlated these two units, whereas other workers suggested that they represent volcanic episodes fed by different magma reservoirs because of their different geochemical characteristics (De Astis, 1995). Isotopic dating provided ages of 21.3 ^ 2.4 for Quadrara and 24.0 ^ 5.0 ka for Spiaggia Lunga, and was therefore inconclusive, since the errors overlap. Closer inspection of palaeomagnetic data helps to better de®ne the time relations of the two units. Their mean directions are 7.68 apart and differ at the 95% con®dence level, because the cones of con®dence do not intersect (a95 ˆ 3:38 and 2.48, Table 1). Hence, palaeomagnetic data strongly support the hypothesis that the Quadrara and Spiaggia Lunga scoriae were emplaced at different times, since they recorded different Earth ®eld directions. At the site level, the con®dence cones, though very small, often intersect and a statistical test must be used to check the hypothesis of a common direction. Application of the Mc Fadden and Lowes (1981) test within each unit gives different results. The directions of Spiaggia Lunga are statistically indistinguishable at

The ability of rock-magnetism investigations to characterize volcanic breccias and scoriae has not been extensively exploited. Grubensky et al. (1998) and Smith et al. (1999) have shown the usefulness of the magnetic approach to distinguish breccias of different origin (pyroclastic, autoclastic, levee, debris-¯ow) and hence characterize the constructional units of composite volcanoes. The present study focussed on welded, scoria fall at Vulcano, and reached two main conclusions about their mode and time of emplacement: 1. The within-site consistency of the magnetic fabric showed that tephra welded immediately after the fall-out. The agglomerates formed in their present position and no en-masse, ¯ow-like movement occurred nor did individual spatter move relative to one another. 2. The overall consistency of the ChRM directions con®rmed that the emplacement temperature was higher than the Curie point (550±5708C) of the rocks' ferromagnetic minerals. The small withinand between-site dispersion showed that welded scoriae are a reliable indicator of palaeosecular variation of the Earth's ®eld, in good agreement with the results from historically dated welded scoriae from Etna (Tanguy et al., 1999). Moreover, they gave statistical signi®cance to small differences in palaeomagnetic directions and yielded a very ®ne time resolution. Emplacement of Spiaggia Lunga and Quadrara scoriae occurred at two distinct times in Vulcano history. Moreover, the Spiaggia Lunga scoriae were emplaced by a single eruption.

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D (˚)

I (˚)

-30 -20 -10 0 10 20 30 40 50

10

20

30

40

50

60

70

0

10

20

30

40

50

60

Age (ka) Fig. 10. Palaeosecular variation (PSV) curve for Aeolian Islands. Symbols: D, I ˆ declination, inclination (vertical axes correspond to the local values of the geocentric axial dipole, D ˆ 08; I ˆ 578†; crosses ˆ data from Zanella (1995); square ˆ Monte Saraceno; diamond ˆ Quadrara; triangle ˆ Spiaggia Lunga; dot ˆ Punta Luccia; vertical bar ˆ age error; horizontal bar ˆ con®dence limit on declination, DD, and inclination, DI.

These results substantiated those from volcanological (De Astis, 1995) and isotopic (Delitala et al., 1997) studies and helped in understanding the Vulcano's eruptive history: 1. The Punta Luccia, Spiaggia Lunga and Quadrara units correspond to distinct volcanic episodes and can not be referred to a single trachybasaltic phase related to the early stages of the La Fossa Caldera formation. 2. The arrival of ma®c magma can be considered a

recurrent event in the volcanic activity, as shown by the different palaeomagnetic directions and ages of the Punta Luccia ( < 48 ka), Spiaggia Lunga ( < 24 ka), Quadrara ( < 21 ka) and Monte Saraceno ( < 8 ka) deposits. 3. The small time separation between the eruptions of Spiaggia Lunga and Quadrara corroborates the existence of a rather complex plumbing system, as suggested by De Astis et al. (1997). According to their hypothesis, a deep, less evolved reservoir fed both the Spiaggia Lunga eruption and a

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shallower reservoir, which eventually became zoned and fed the Quadrara eruption.

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