Tracing magma evolution at Vesuvius volcano using melt inclusions

6 Tracing magma evolution at Vesuvius volcano using melt inclusions: a review Claudia Cannatelli1, 2 1

Department of Geology, FCFM, University of Chile, Santiago, Chile; 2Andean Geothermal Center of Excellence (CEGA), University of Chile, Santiago, Chile

Geological background Mt. SommaeVesuvius (SV) is a stratovolcano consisting of an ancient truncated edifice, Monte Somma, and the recent cone of Vesuvius that developed in the caldera after the 472 AD eruption (Santacroce, 1987; Rolandi et al., 1998) (Fig. 6.1A). The volcanic complex has formed on thick Mesozoic and Cenozoic carbonates that are overlain by Miocene sediments of the Campanian Plain (D’Argenio et al., 1973; Ippolito et al., 1975). SV is located in a tectonically complex area that developed in response to movement of the Apennine chain toward the Adriatic-Ionian foreland (Fig. 6.1B and C) following the opening of the Tyrrhenian basin in the Pleistocene (Milia et al., 2003; Turco et al., 2006). Volcanic activity in the SV area began at about 400 ka based on 40 Ar/39Ar dating of lavas (Brocchini et al., 2001; Di Renzo et al., 2007). The volcanic activity of SV can be subdivided into three groups or megacycles based on the systematic variations of major and trace element contents of bulk rocks as a function of age (Ayuso et al., 1998 and references therein) (Fig. 6.2). The first megacycle (25e14ka) corresponding to the formation of the Somma volcano includes lava flows, small-scale (StrombolianeVulcanian) explosive eruptions, plinian eruptions, and various interplinian eruptions. Eruptions belonging to this period are the following: Codola (25 ka), Sarno (18 ka, also named Pomici di Base by Santacroce, 1987), and Novelle/SeggiarieBosco (16e14 ka, also named Pomici Verdoline by Santacroce, 1987) (Fig. 6.2).

Vesuvius, Campi Flegrei, and Campanian Volcanism. https://doi.org/10.1016/B978-0-12-816454-9.00006-7 Copyright © 2020 Elsevier Inc. All rights reserved.

121

(A)

(C)

(B) Figure 6.1 (A) View of Mt. SommaeVesuvius from Seiano Archeological Site, (B) map showing main structural features of the Tyrrhenian Sea, and (C) crustal thickness of the Tyrrhenian Basin. (B and C) Modified from Rosenbaum, G., Lister, S., 2004. Neogene and Quaternary rollback evolution of the Tyrrhenian Sea, the Apennines, and the Sicilian Maghrebides. Tectonics 23, TC1013.

Plinian Activity

Interplinian Activity Repose Time ? RECENT

1631 AD Repose Time 472 AD Pollena 79 AD Pompeii

3.5 ka Avellino 8.0 ka Ottaviano (Mercato) 16-14 ka Novelle (Verdoline) 18.6 ka Sarno (Pomici di Base) 25.0 ka Codola

MEDIEVAL

Repose Time ANCIENT HISTORIC

Repose Time 800 years PROTOHISTORIC

1944 AD From 1638 to 1944 a series of 18 interplinian eruptions

1139 AD From >512 AD to 1139 AD 4 interplinian eruptions

303 AD No chronologic distribution

700 BC From 1758 BC to 832 BC 3 interplinian eruptions

Repose Time 6000 years

OLDER SOMMA ACTIVITY

Figure 6.2 Plinian and interplinian volcanic activity of Mt. SommaeVesuvius, subdivided in megacycles as defined by Ayuso et al. (1998). Blue, first megacycle; green, third megacycle; pink, second megacycle. Eruptive events show nomenclature by Rolandi et al. (1998, 2004) and that in parentheses by Santacroce (1987). Ages of eruptions and repose time are as reported in Rolandi et al. (1998) and Santacroce et al. (2008).

Chapter 6 Tracing magma evolution at Vesuvius volcano

The second megacycle (8e2.7 ka) includes plinian eruptions known as Ottaviano (8 ka, also labeled as Mercato by Santacroce, 1987) and Avellino (3.5 ka), as well as Protohistoric interplinian events (between 3.5 and 2.7 ka) (Fig. 6.2). The third megacycle (79e1944 AD) includes Pompeii (79 AD) and Pollena (472 AD) plinian eruptions, subplinian eruptions, such as the 1631 AD event, and small-scale effusive and explosive events belonging to the Ancient Historic (79e472 AD), Medieval interplinian (472e1631 AD), and Recent interplinian activity (1631e1944 AD) (Fig. 6.2). Each magmatic megacycle is followed by a long repose time (Fig. 6.2), and a new magmatic megacycle (composing of several smaller cycles) always starts with a plinian eruption. The third megacycle started with the 79 AD plinian eruption after w800 years of repose, and within this megacycle, plinian and subplinian eruptions occurred in 472 and 1631 AD, respectively. These events occurring at the beginning and within the third megacycle were followed by weakly to moderately explosive or explosive-effusive interplinian eruptions that ended with a repose period (De Vivo et al., 2010 and references therein). SV is now in a quiescent state, with activity expressed by only fumaroles and low-magnitude seismic activity. Some authors have associated this state of repose with progressive cooling of a residual magma body at shallow depths (Scandone et al., 2008) or within the volcanic conduit (De Natale et al., 2003). The nature of the magmatic plumbing system of SV is still a matter of intense debate. Some authors support the existence of a shallow magma body (w1.5e2.0 km depth) based on seismic tomography (Zollo et al., 1996, 1998; De Natale et al., 1998, 2003) and aeromagnetic data (Fedi et al., 1998). Other authors (Belkin et al., 1985; Belkin and De Vivo, 1993) have used fluid inclusions (FIs) data from xenoliths to constrain the depths of magma ponding at 3.5e10 km and >12 km depth (Fig. 6.3), with no evidence of shallower magma levels. Intermediate magma storage is confirmed by seismic (Auger et al., 2001; Zollo et al., 1996) and magnetotelluric evidence (Di Maio et al., 1998) that shows there is an active large magma chamber at depths of 8e10 km (Fig. 6.3). Scaillet et al. (2008) proved by phase equilibria calculations that the SV reservoir migrated from 9e11 km to 3e4 km in the last 18.5 ka. A magma chamber deeper than 12 km and perhaps extending to 30 km has been proposed by De Natale et al. (2001, 2006), who interpreted a high velocity body dipping westward from 65 km down to w 300 km as a subducted plate within the mantle. Data from deep (300 km) seismicity studies (Anderson and Jackson,

123

Chapter 6 Tracing magma evolution at Vesuvius volcano

Age < 79 AD <1631 AD

1944 AD

Km

0.3 CO2 density g/cm

124

–4

0.5

0.7

–8

–12

Figure 6.3 Trapping depths of CO2 primary fluid inclusions of Vesuvius nodules (Belkin et al., 1985; Belkin and De Vivo, 1993), suggesting at least three levels of magma ponding at depths of 3e5 km, 8e10 km, and possibly at >12 km. Modified from De Vivo, B., Petrosino, P., Lima, A., Rolandi, G., Belkin, H.E., 2010. Research progress in volcanology in Neapolitan area, Southern Italy: a review and alternative views. Mineral. Petrol. 99, 1e28.

1987; Milano et al., 1994; Faccenna et al., 2001; Billi et al., 2007) suggest there is an actively subducting slab in the central Tyrrhenian Sea. Danyushevsky and Lima (2001) postulate that the source of the Breccia Museo mafic xenoliths at Campi Flegrei could be genetically related to the pre-14 ka SV volcanic system, with the link most likely established before the emplacement of the Neapolitan Yellow Tuff (15 ka, Deino et al., 2004). Possible reactivity of SV might be triggered by the arrival of rapidly ascending magma batches forming a new shallow magma level (Aulinas et al., 2008; Principe and Marini, 2008; Scandone et al., 2008). The arrival of this new cooler magma would increase fumarole activity, produce hydrothermal explosions, and trigger earthquakes, which would provide forecasting information for the future reawakening of SV. The link between tectonic activity and volcanic eruptions at SV was suggested by Morgan et al. (2006), who modeled trace element concentration changes in sanidine from the Pompeii eruption (79 AD). These authors related Ba diffusion profiles measured in sanidine to episodic

Chapter 6 Tracing magma evolution at Vesuvius volcano

recharge events in the magmatic reservoir using a year-to-decade scale. Morgan et al. (2006) suggest that the Ba diffusion profiles can be coupled to two distinct recharge events, 22 and 15 years before the 79 AD eruption, and associate the events with the magnitude 5.7e5.9 earthquake that occurred in 62 AD.

Magma evolution at SommaeVesuvius volcano Several studies on magma evolution (Joron et al., 1987; Trigila and De Benedetti, 1993; Marianelli et al., 1995; Cioni et al., 1998; Lima et al., 1999; Raia et al., 2000; Webster et al., 2001; Aulinas et al., 2008; Pappalardo and Mastrolorenzo, 2010; Dallai et al., 2011; Redi et al., 2017 and references therein) have suggested that the crystallization and fractionation of clinopyroxene (Cpx) and olivine (Ol) are the controlling factors in the evolution of parental melts beneath SV. These studies focused on specific SV eruptions rather than the entire volcanic history of SV. Only Redi et al. (2017) have studied the full range of eruptive products, including Cpx and Ol mineral compositions linked to volcanic products younger than 40 ka. The selected time span is considered to be the most representative of the volcanic system as all SV products are younger than the Campanian Ignimbrite deposit (w40 ka, Giaccio et al., 2008) and Mt. Somma formed as a stratovolcano only after w40 ka (Brocchini et al., 2001; Santacroce and Sbrana, 2003). The SV volcanic complex has produced rocks that range from shoshonite to trachy-phonolite and from alkali-basalt to tephrite and phonolite (Fig. 6.4). The degree of silica undersaturation increases through time, and rocks with the lowest SiO2 content belong to the third megacycle (Fig. 6.4). Volcanic products of SV have been petrographically described by many authors (Rosi and Santacroce, 1983; Joron et al., 1987; Santacroce et al., 1993; Marianelli et al., 1999). Medium silica-undersaturated rocks older than 472 AD contain phenocrysts of Cpx as well as microlite, plagioclase (Pl), FeeTi oxides (Ox), biotite (Bio), and apatite (Ap), in order of abundance. In the least evolved rocks, Ol appears as phenocrysts. In the most evolved rocks, leucite (Leu) appears as microlite and microphenocrysts, while K-feldspar (K-fsp) is only present as phenocrysts. Leucite is absent in rocks ranging from shoshonites to trachytes of the first megacycle. Nepheline (Neph) is the only feldspathoid in the Avellino volcanic products. High-silica undersaturated rocks of the third megacycle are porphyritic with abundant Cpx and Leu, an average amount of Ox and Ap and a minor or rare amount of Pl, Bio, and K-fsp.

125

Chapter 6 Tracing magma evolution at Vesuvius volcano

16 st

1 mega-cycle

Phonolite

14

2

nd

mega-cycle

rd

3 mega-cycle

Tephriphonolite

12

Trachyte

Na2O+K2O

126

10

Nephelinite or Melilitite

PhonoTephrite

Trachyandesite

8 Tephrite or Basanite Trachybasalt

6

Trachydacite Rhyolite

Basaltic trachyandesite Dacite

4

Basalti anddesite

Basalt

2 0 35

Andesite

Picrobasalt

40

45

50

55 SiO2

60

65

70

75

Figure 6.4 Total alkali (Na2O þ K2O)esilica (SiO2) diagram (Le Bas et al., 1986) showing distribution of Mt. SommaeVesuvius bulk rock samples. Fields are obtained by using data from Piochi et al. (2006). Blue field, first megacycle; green field, third megacycle; pink field, second megacycle.

Bulk rock Sr and Nd isotope compositions from Sarno (18 ka), Avellino (3.55 ka), Pompeii (79 AD), and Pollena (472 AD) range from w0.7062 to w0.7078 and 0.51225 to 0.51257, respectively (Civetta et al., 1991; Civetta and Santacroce, 1992; Cioni et al., 1995; Ayuso et al., 1998; Bertagnini et al., 1998; Somma et al., 2001; Chapter 5), suggesting there was a chemically and isotopically zoned magma chamber located within the carbonate platform (Barberi and Leoni, 1980; Del Moro et al., 2001). According to Chapter 5, several processes have affected magmas during SV evolution, such as fractional crystallization, magma mixing and recharge, and wall rock assimilation. Piochi et al. (2006) suggest that some of these processes, particularly mixing and recharge, occurred repeatedly throughout the volcanic history of SV and were more prevalent during interplinian activity (250, 1000, 2500, and 15,000 years BP), as shown by the variation of 87 Sr/86Sr of erupted magmas with age (Fig. 6.5).

Melt inclusions Melt inclusions (MIs) are small portions of silicate melt that are trapped in surface irregularities or defects of host crystals during growth in a magma body (e.g., Cannatelli et al., 2016 and references therein). The MIs typically contain variable amounts of daughter crystals, glass, and/or vapor and are very common in volcanic rocks. MIs represent time capsules created during

Chapter 6 Tracing magma evolution at Vesuvius volcano

Figure 6.5 87Sr/86Sr versus age of eruption for Mt. SommaeVesuvius rocks. Circles, first megacycle; diamonds, second megacycle; squares, third megacycle; triangles, transitional period. Closed symbols represent data from plinian eruptions; open symbols represent compositions of interplinian rocks. Modified from Piochi, M., Ayuso, R.A., De Vivo, B., Somma, R., 2006. Crustal contamination and crystal entrapment during polybaric magma evolution at Mt. SommaVesuvius volcano, Italy: geochemical and Sr isotope evidence. Lithos 86, 303e329.

degassing or magma differentiation and allow researcher to trace the evolution of magma from its formation at mantle depths to its release at the surface (e.g., Cannatelli et al., 2016 and references therein). Several authors have studied MIs hosted in eruptive products of SV (Table 6.1), obtaining a wide range of chemical composition data (Fig. 6.6). The first studies of FIs and MIs of SV were undertaken by Roedder (1965), who investigated the 1858 and 1944 AD lavas for the presence of CO2 FIs in Cpx and Leu phenocrysts and observed very small shrinkage bubbles in small (<4 mm) Leu-hosted MIs, which he suggested formed due to relatively slow cooling. Sobolev et al. (1972) studied recrystallized Cpxand Leu-hosted MIs from recent lavas (1944 AD). The authors reheated the MIs and obtained extremely high (>1600 C) homogenization temperatures, suggesting that mass leakage occurred during the experiment (Bazarova and Krasnov, 1975). Sobolev et al. (1972) measured a gas content of 23.3 vol% CO2 þ 76.8 vol % N2 þ rare gases (vol%) for Leu-hosted Mis and 0.4 vol% CO2 þ 91.6 vol% N2 þ rare gases(vol%) for the Cpx-hosted MIs. One of the first studies of Vesuvius samples that used MIs to determine the geochemistry of the preeruptive magma was conducted by Barberi et al. (1981). In this work, the authors determined that the phonolitic pumice ejected during the two most recent and important plinian eruptions of Vesuvius (Avellino and Pompeii, Fig. 6.2) originated from the fractionation of a

127

Table 6.1 Studies on melt inclusions in volcanic rocks from Mt. SommaeVesuvius. Methods

Results

References

Year Host(s)

Raman/ P-T Melt Petrography Heating EMPA SIMS LAICPMS FTIR conditions evolution

Roedder Sobolev et al. Barberi et al. Belkin et al. Cortini et al.

1965 1972 1981 1985 1985

X X X X X

X X X X

Vaggelli et al. Vaggelli et al. Marianelli et al. Belkin et al. Cioni et al. Lima et al. Marianelli et al. Raia et al. Webster et al. Fulignati et al.

1992 1992 1995 1998 1998 1999 1999 2000 2001 2001

X X X X X X X X X X

X X X X X X X X X X

Fulignati et al. Schiano et al. Fulignati et al. Fulignati and Marianelli Lima et al. Balcone-Boissard et al. Balcone-Boissard et al. Klebesz et al.

2004 2004 2007 2007

X X X X

X

2007 Ol, Cpx 2008 Cpx, Fsp

X X

X

2012 Cpx, Fsp

X

2012 Cpx

X

Leu Cpx, Leu Leu, Cpx, Ol Cpx Cpx, Ol, Sp, Mica Cpx, Fsp Cpx, Fsp Ol, Cpx Cpx, Fsp Cpx, Fsp Cpx, Fsp Cpx, Ol, Leu Cpx Cpx Neph, Fsp, Leu, Cpx Ol, Fsp Ol Cpx, Fsp Cpx, Fsp

Volatiles

X X X

X X

X X X X X X X X X X X X X X X

X X

X X

X

X X X X X

X

X

X X

X X

X X

X

X X

X

X X X X X X X X X X X X

X X X X X X X

X X X

X

X X

X

X X

X

X

X

X X

Cpx, clinopyroxene; EMPA, Electron Microprobe Analysis; Fsp, feldspar; FTIR, Fourier Transform Infrared Spectroscopy; LAICPMS, Laser Ablation Inductively Coupled Plasma Mass Spectrometry; Leu, leucite; Neph, nepheline; Ol, olivine; Sal, salite; SIMS, Secondary Ion Mass Spectrometry; Sp, spinel.

Chapter 6 Tracing magma evolution at Vesuvius volcano

16 14

Na2O + K2O

12 10

Nephelinite Trachydacite

Rhyolite

8 Tephrite

6

Dacite

4 Basalt

Andesite

Picrobasalt

2 0

Basaltic Andesite

35

40

45

50

55

60

65

70

75

Si2O

Figure 6.6 Color coding as in Fig. 6.5. Circles, pre-Avellino; diamonds, Avellino; squares, 79 AD; stars, Medieval Age; tilted crosses, Modern Age; vertical crosses, 472 AD. Data from studies in Table 6.1.

parental (tephritic) magma in a shallow (w2e4 km) cylindrical chamber located within the Mesozoic limestone that represents the sedimentary basement of the volcano (Fig. 6.7). According to Barberi et al. (1981), the parental magma underwent w70% fractionation and interacted at 800 C (Avellino) and 850 C (Pompeii) with the calcareous country rock, producing skarns whose solids were partly incorporated into the magma. Belkin et al. (1985) and Cortini et al. (1985) suggest that nodules from the skarns underwent a multistage crystallization history based on FIs and MIs data. Belkin et al. (1985) concluded that the skarn nodules formed by crystallization of magma from a peripheral assimilation zone contaminated with the carbonate country rock (Fig. 6.7). Two types of MIs were observed in the skarns, suggesting that at least locally, different melt compositions were present at the same time but most likely at different depths (between 3.5 and 13 km) based on the bimodal CO2 density distribution of FIs (Fig. 6.3). In a later study, Fulignati et al. (2001) concluded that nephelineand K-feldsparehosted MIs represent the NaeKeCa carbonatee chloride melt formed as a result of the interaction between early high-temperature hypersaline fluids (Gilg et al., 2001) and carbonate country rocks, which create an immiscibility process that leads besz et al. to the formation of intrusion-related skarn systems. Kle (2012) shed new light on the nature of the skarns, indicating that Cpx-hosted MIs in the skarn nodules represent samples from the mush zone of the active plumbing system of SV.

129

130

Chapter 6 Tracing magma evolution at Vesuvius volcano

SW Trecase 0

2.5 Km NE SV Pyroclastic deposits

Sandstones and Siltstones

Moderate Salinity

Hydrostatic Pf

Brine + Steam

id d Ol

Ma

Pf

gm

tic

at

ic

ta

os

Flu

th

Limestones and Dolostones

Depth (Km)

Li

s

–4

Magma Body

–8

Thrust Fault

NW

SE Trecase

Clastic and volcanoclastic Quaternary deposits

–12

Low Velocity Zone

Meso-Cenozoic fold thrust belt Low velocity Zone

Figure 6.7 Stratigraphic successions below Mt. SommaeVesuvius and zone of transition from lithostatic to hydrostatic conditions. The area of transition between brittle to plastic is a self-sealed zone with an average P w 1 kbars and depths between 3.6 and 4.5 km (assuming T ¼ 720 C). T and fluid pressure (Pf) gradients are very steep across the interface. Modified from Lima, A., De Vivo, B., Fedele, L., Sintoni, F., Milia, A., 2007. Geochemical variations between the 79 AD and the 1944 AD Somma-Vesuvius volcanic products: constrains on the evolution of the hydrotermal system based on fluid and melt inclusions. Chem. Geol. 237, 401e417.

Chapter 6 Tracing magma evolution at Vesuvius volcano

In the 1980s, researchers primarily concentrated on analyzing major elements in FIs and MIs, while in the 1990s, the majority of studies focused on geochemical composition, including volatiles and trace elements. Vaggelli et al. (1993) analyzed MIs from recent Vesuvius lavas (1631e1944 AD) and pointed out that fractional crystallization was the dominant process during differentiation. However, magma evolution was also affected by convective processes involving Cpx that resulted in normal and/or reverse zoning likely due to inputs of a new primitive unfractionated magma. Marianelli et al. (1995) determined major elements, S, Cl, and H2O content of mafic magma batches that supplied the Vesuvius plumbing system over the last 4000 years. The authors selected Ol- and Cpx-hosted MIs from plinian (Avellino, Pompeii, Pollena) and subplinian (1906e1944 AD) eruptions and determined that K-tephritic H2OeClePeF-rich MIs hosted in the 1906 olivine (Fig. 6.6) likely represent primary magmas of Vesuvius, consistent with a metasomatized mantle origin. Cpx- and Ol-hosted MIs from the other plinan and subplinian eruptive products range from K-basalt to K-tephrite in composition (Fig. 6.6) and represent magma batches that supplied the Vesuvius plinianesubplinian chamber from the Avellino (3.5 ka) and Pollena (472 AD) eruptions (Fig. 6.6). Belkin et al. (1998) studied Cpx-, Ol-, Leu-, and Pl-hosted MIs from the pre-1631 AD products of SV interplinian activity and determined H2O (0.6e2.7 wt%), Cl (up to 1 wt%), F (up to 0.63 wt%), and SO3 (up to 0.5 wt%) content. The authors suggested that the moderately low volatile content of MIs indicate that the nonplinian eruptions at SV were relatively low-energy events. Marianelli et al. (1999) and Cioni et al. (1998) proposed a different evolution for the SV plumbing system during periods of open or closed conduit conditions. When the conduit is open (1906 and 1944 AD eruptions, third cycle, Fig. 6.2), a small magma chamber forms in the upper portions of the volcano (1e2 km depth, Fig. 6.7) and the resident K-tephritic magma forms as a result of several processes (new magma batches, magma mixing, and extractions). When the conduit is closed, larger and deeper (2e5 km) plinian- or subplinian-layered magma chambers form and grow by a periodic arrival of deep magma batches with a deep origin (Fig. 6.7). Cioni et al. (1998) suggested that the increasing volume of the magma chamber is related to changes in the aspect ratio, with the chamber classified as an initial stage or moderate volume chamber (0.01e0.1 km3, 1906 open conduit style), a young stage or medium volume chamber (0.1e0.5 km3, Pollena-type chamber), or a mature stage or large volume chamber (0.5e5 km3, Pompeii-type chamber).

131

132

Chapter 6 Tracing magma evolution at Vesuvius volcano

Marianelli et al. (1999), using MI compositions (major and volatile elements), determined that the magmas feeding the 1944 AD eruption of SV underwent differentiation at different pressures. The authors also suggest that K-tephritic volatile-rich melts (up to 3 wt% H2O, 3000 ppm CO2, and 0.55 wt% Cl) evolved to reach K-phonotephritic compositions (Fig. 6.6) by crystallization of Cpx and Ol at pressures higher than 300 MPa, which involved mixing, open-system degassing, and crystallization of Leu, Cpx, and Pl. According to Marianelli et al. (1999), the eruption was triggered by the input of a volatile-rich magma batch from a depth of 11e22 km into the shallow magma chamber (Fig. 6.7). Lima et al. (1999) studied MIs in scoriae from an SV eruption of Medieval times (Formazione di Terzigno, A.D. 893, Rolandi et al., 1998) and concluded that the MIs have a less evolved magma composition that those reported by Belkin et al. (1998) and Marianelli et al. (1995). Lima et al. (1999) assumed that Cpx-hosted MIs in the Terzigno scoria represented primitive melts similar to those that supply plinian and subplinian magma chambers. They suggest that the polygenetic source of Cpx may indicate that the SV magmatic system retains “records” of the most recent plinian event. The first comprehensive studies (29 chemical components including H2O, S, Cl, F, B, and P2O5) of reheated MIs in products from SV spanning from >14ka to younger than 3.5 ka (Webster et al., 2001) and Medieval products (Raia et al., 2000) provided important constraints on the preeruptive magma geochemistry and eruption behavior relative to magma evolution and evidence for magmatic fluid exsolution occurring well before eruption. Webster et al. (2001) showed there are distinct differences in composition between precaldera rocks (older than 14 ka with magmas slightly enriched in SiO2) and products younger than 3.5 ka (magmas moderately enriched in S, Cl, CaO, MgO, P2O5, F, and many LILE). Furthermore, Webster et al. (2001) determined for the first time that the eruptive behavior at Vesuvius correlates with preeruptive volatile enrichments. These authors indicated that most magmas from explosive plinian and subplinian events <3.5 ka had higher volatile contents (H2O, S, and S/Cl) than magmas from interplinian volcanic phenomena. Webster and De Vivo (2002) and Webster et al. (2003) constrained a Cl solubility model for SV magma evolution from phonotephrite to phonolitic compositions by fractional crystallization. These authors showed that Cl solubility was dramatically lowered by decreasing Ca, Mg, and Fe in the residual melt and simultaneously but gradually had increased volatiles in the melt due to crystallization of volatile-free minerals. The increase in volatiles and

Chapter 6 Tracing magma evolution at Vesuvius volcano

simultaneous decrease in Cl solubility may force exsolution of a hydrous chloride melt directly from the Cl-enriched mafic magmas (Webster and De Vivo, 2002; Webster et al., 2006) as also indicated by the chemistry of MIs from Pollena’s products (Fulignati and Marianelli, 2007). Schiano et al. (2004) determine that MIs from 1906 AD eruptive products of SV represent subduction-related magma, product of mantle metasomatic processes. Furthermore, incompatible elements in MIs are consistent with a phlogopitebearing mantle source, likely related to reactions of interaction and hybridization with K2O- and H2O-rich fluids released from the slab at greater depths. Based on MI chemistry, Lima et al. (2007) proposed that the interplinian activity (particularly the one that occurred in 1944 AD) was triggered by magmas that continuingly ascended through the system, undergoing simultaneous degassing and fractionation. The combination of ongoing magma supply and frequent eruptions resulted in near steady-state conditions under the volcano, which explains the small geochemical variation in the composition of the 1944 AD magma (Lima et al., 2007). According to De Vivo et al. (2010), magma cooling and precipitation of new minerals can seal the active magmatice hydrothermal system in the last stages of interplinian eruptions. This process can create a “closed system” condition and favor the onset of plinian eruptions. Lima et al. (2003, 2007) and De Vivo et al. (2010) indicated that there is a carapace at depths between 3.6 and 4.5 km (Fig. 6.7), which represents the limit between brittle and plastic behavior of the surrounding rocks. Tectonic earthquakes can cause a breach of the carapace, which can potentially activate the local fault system that could eventually lead to an eruption. In the last 10 years, fewer studies have carried out analyses of MIs geochemical compositions and have instead focused primarily on the determination of the physicalechemical conditions of SV magma chamber(s). Balcone-Boissard et al. (2008) determined the preeruptive conditions and degassing processes of the 79 AD plinian eruption using F and Cl volatile elements measured in MIs from white pumice (WP) and gray pumice (GP). In their study, the authors conclude that magmas that generated WP and partly GP were fluid saturated (a Cl-rich H2O vapor phase and a brine), based on Cl content in the melt of w5300 ppm and H2O of w5 wt%. From these results, they estimated a depth of w7.5 km for the WP magma chamber, and a magma reservoir for the GP was just beneath it (Fig. 6.7). The same authors (BalconeBoissard et al., 2012) also investigated the preeruptive conditions

133

134

Chapter 6 Tracing magma evolution at Vesuvius volcano

and sin-eruptive degassing processes of the Avellino eruption using the volatile contents (F, Cl, H2O) of Cpx- and K-fspehosted MIs. The authors concluded that based on the preeruptive Cl content of MIs, the Plinian phase of the Avellino eruption was fed by an H2O-saturated magma in equilibrium with a free fluid phase at the top of the reservoir. Based on MIs volatile content, the authors suggest that the geochemistry of magmas from the Avellino and Pompeii eruptions, along with similar magma chamber pressure conditions (180e210 MPa), was capable to form and feed sustained plinian column.

Conclusions This work has been focused on providing a comprehensive review of the numerous studies carried out on the SV magmatic system using MIs. In the last two decades, several authors have determined that the trigger mechanisms for the varying eruptive styles of SV can be attributed to different rates of magma supply, upward migration of the shallower magmatic reservoir, and pristine differences in parental melts feeding distinct cycles. Over 50 years, MIs geochemical data have provided fundamental information on the SV magma evolution, source of SV magmas, and effects of volatiles on the style and frequency of SV eruptions and magma chamber(s) location(s). Furthermore, MI data have sparked intense scientific debate and produced several alternative physical and geochemical models for the magma feeding system of SV, motivating scientists to continue this research to rigorously reconstruct the SV eruptive history and determine its eruption style, size, recurrence intervals, and long-term forecasts of future activity.

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