The proximal marine record of the Marsili Seamount in the last 7 ka (Southern Tyrrhenian Sea, Italy): Implications for the active processes in the Tyrrhenian Sea back-arc

Global and Planetary Change 133 (2015) 2–16

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The proximal marine record of the Marsili Seamount in the last 7 ka (Southern Tyrrhenian Sea, Italy): Implications for the active processes in the Tyrrhenian Sea back-arc S. Tamburrino a,c, M. Vallefuoco a, G. Ventura a,b,⁎, D.D. Insinga a, M. Sprovieri c, M. Tiepolo d, S. Passaro a a

Istituto per l'Ambiente Marino Costiero di Napoli, Consiglio Nazionale delle Ricerche, Napoli, Italy Istituto Nazionale Geofisica e Vulcanologia, Roma, Italy c Istituto per l'Ambiente Marino Costiero U.O.S. di Capo Granitola, Consiglio Nazionale delle Ricerche, Campobello di Mazara, Italy d Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Milano, Italy b

a r t i c l e

i n f o

Article history: Received 25 March 2015 Received in revised form 3 July 2015 Accepted 14 July 2015 Available online 18 July 2015 Keywords: Holocene tephra Back-arc spreading Extensional arc Submarine active volcanism Plumbing system Marsili Seamount Etna volcano

a b s t r a c t The volcanism of back-arc basins resembles that of oceanic spreading centers, rifts, and, in vanishing stages, extensional arcs, depending on the amount and rate of the dynamic processes associated to the subduction. Marsili Seamount (MS) represents the axial ridge of the Southern Tyrrhenian Sea back-arc basin, which is connected to the slab roll-backing processes affecting the Calabrian Arc (Italy). The Southern Tyrrhenian Sea back-arc is characterized by a significant decline in the spreading rate with time (2.8–3.1 mm/a to less than 1.8 mm/a in the last 0.78–1 Ma). MS develops between about 1 Ma and 3 ka and mainly consists of lava flows erupted from central and fissural vents. The MS products belong to the calcalkaline association and range in composition from basalts to trachytes. We present new stratigraphic, geochronological, and geochemical data (glass shards and minerals) of tephra from a 2.35 m long gravity core (Marsili1 core) recovered on MS at 943 m b.s.l. We recognize five tephras [M1 (top of the core) to M5 (bottom)] represented by poorly to highly vesiculated ashes. The lowermost tephra M5 emplaced between ca. 7 and 26 ka B.P.; it represents the less evolved distal counterpart of the Unit D related to the Biancavilla–Montalto products of Mount Etna (Sicily). The M1 to M4 tephras emplaced between 2.1 and 7.2 ka B.P. and are related to strombolian-like submarine eruptions of NNE–SSW aligned MS vents. The composition of the M1–M4 glasses ranges from basaltic trachyandesites to andesites and trachytes. The M1 to M4 magmas mainly originated by crystal fractionation from a heterogeneous mantle source with varying LILE enrichments by subduction-related fluids. The degree of evolution of the MS magmas increases with decreasing time. The formation of vertically stacked magma storage zones at the crust/mantle interface and within MS is related to the vanishing Southern Tyrrhenian Sea opening, which implies the rapid (b1 Ma) evolution from a slow spreading back-arc setting to an arc system. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The volcanism at plate margins is controlled by the strain rate and by the conditions of magma storage and ascent, which, in turn, depend on the temperature field, the rheology and thickness of the lithosphere. Contractional arcs and immature rifts are characterized by null to slow opening rates, polygenic volcanism, and andesitic to silicic magmatism (Acocella, 2014), whereas extensional arcs, mature rifts and oceanic spreading centers have monogenetic, mainly basaltic volcanism and higher values of the opening and magma production rates (Crisp, 1984; Abelson and Agnon, 2001). The above described geodynamic settings represents a ‘continuum’ among different end member ⁎ Corresponding author at: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy. E-mail address: [email protected] (G. Ventura).

http://dx.doi.org/10.1016/j.gloplacha.2015.07.005 0921-8181/© 2015 Elsevier B.V. All rights reserved.

scenarios. As an example, back-arcs are associated to subduction zones and their magmatic/volcanic setting spans from that of contractional arc/immature rifts (e.g., the Puna back-arc in Argentina; Risse et al., 2013) to mature rifts and oceanic spreading centers (e.g., Eastern Lau Spreading Center, Tonga; Martinez et al., 2006). Also, back-arc spreading centers are controlled by the spatial proximity to the subduction zones, the inclination and velocity of the slab, the occurrence of possible rollback processes, and the temperature and composition of the mantle wedge. Because of this complexity, the study of the volcanism in backarc spreading centers is of primary importance to analyze the eruptive style and geometry of the plumbing systems, whose time variations may be signs of important geodynamic changes. Marsili Seamount (MS, about 1.07 Ma to 3 ka) is a NNE-SSW elongated volcanic complex located in the Southern Tyrrhenian Sea oceanic basin (Fig. 1a, b; Selli et al., 1977; Marani and Trua, 2002; Cocchi et al., 2009; Ventura et al., 2013; Iezzi et al., 2014). MS, which covers an area

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Fig. 1. a) Simplified geodynamic framework of Italy. Thick and dashed blue lines indicate the hypocentral depth of earthquakes (data from Chiarabba et al., 2008); thin and dashed blue lines indicate the depth of the Moho (data from Rosenbaum and Lister, 2004); the area with high Vp/Vs is from Chiarabba et al. (2008) and Pontevivo and Panza (2006). b) Structural sketch of the Southern Tyrrhenian Sea and location of the volcanic seamounts (red triangles) and of the Aeolian Arc volcanoes (modified from De Astis et al., 2003). c) Bathymetry of the Marsili Seamount (modified from Ventura et al., 2013) with location of the Marsili1 gravity core. d) Digital bathymetry model of the central sector of the Marsili seamount with location of the NNE–SSW aligned pyroclastic cones and Marsili1 gravity core (bathymetry data from Ventura et al., 2013). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of about 2100 km2 and rises from about 3200 m to 508 m b.s.l. (Fig. 1c) occupies the central sector of the Marsili basin, a 2 Ma old oceanic-type back-arc basin (Moho depth = 10–12 km; Pontevivo and Panza, 2006) related to the rollback of the sub-vertical, NE dipping Ionian slab below the Calabrian Arc (Malinverno and Ryan, 1986; Savelli and Schreider, 1991; Gvirtzman and Nur, 1999; Rosenbaum and Lister, 2004; Nicolosi et al., 2006). This subduction system also includes the Aeolian Islands and the associated seamounts, which bound the southern sector of the back-arc forming a ring-shaped volcanic structure emplaced on the Sicily and Calabria continental crust (Fig. 1b; De Astis et al., 2003). The geodynamic significance of MS is still debated: Marani and Trua (2002) propose that MS represents the super-inflated ridge of the Marsili oceanic basin, whereas Ventura et al. (2013) suggest that (a) MS is a volcanic arc edifice formed within the now inactive, relict Marsili back-arc, and (b) its growth is due to passive magma ascent along pre-existing fractures inherited by the early spreading activity. In fact, the spreading rate in the Southern Tyrrhenian Sea back-arc abruptly decreased from 2.8–3.1 mm/a to less than 1.8 mm/a in the last 0.78–1 Ma (Cocchi et al., 2009).

The Marsili rocks have a medium to HK calc-alkaline affinity and range in composition from basalts to trachytes with the less evolved basalts related to IAB and OIB-like mantle sources (Beccaluva et al., 1982; Trua et al., 2002, 2010; Iezzi et al., 2014). Most of the MS products consist of 1.07 to 0.1 Ma old lava flows (Selli et al., 1977; Cocchi et al., 2009). Geochemical data indicate that the MS evolution is dominated by fractional crystallization processes from a heterogeneous, enriched mantle source variously modified, according to geophysical data (Pontevivo and Panza, 2006; Chiarabba et al., 2014; Zhu et al., 2012), by the addition of slab-derived fluids (Trua et al., 2002, 2010). At present, MS is affected by intra-seamount seismicity due to volcanotectonic and hydrothermal-induced activity (D'Alessandro et al., 2009). The occurrence of such hydrothermal activity and active degassing is supported by the occurrence of primary sulfide-poor Feoxyhydroxides and by geochemical data on the water column, which detect He of mantle origin (Dekov and Savelli, 2004; Lupton et al., 2011). Results of a coupled magnetic and gravity survey (Caratori Tontini et al., 2010) suggest that MS is characterized by a shallow, magmatic reservoir whose top is located at about 2500 m b.s.l., i.e., 2000 m

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from the MS summit, and by a deeper reservoir located at about 10– 12 km depth (Pontevivo and Panza, 2006). Both Caratori Tontini et al. (2010) and Ventura et al. (2013) also suggest that MS has a tsunamirelated hazard related to possible sector collapses and/or seafloor sliding associated to a renewal of explosive activity and/or gravity instability related to pore pressure variations. On the basis of the above considerations, the knowledge of the MS volcanic complex is of primary importance for (a) the reconstruction of its geological history and elucidation of its geodynamic significance, (b) the definition of its recent eruptive activity and plumbing system, (c) the evaluation of the volcanic hazard, and (c) the study of back-arc basins and, in particular, those characterized by slow spreading rates. In addition, the MS edifice may be the site of deposition of tephras related to the recent (b1 Ma) activity of the Mediterranean volcanoes. The knowledge of the dispersion of tephras is relevant for the reconstruction of the past subaerial and submarine eruptive activity and for studies on the dynamics of highly explosive eruptions. Here, we report original stratigraphic, geochemical and geochronological data on five tephras found in a 2.35 m long gravity core (Marsili1 core) recovered in the MS central sector at 943 m b.s.l. (Figs. 1 and 2).

The stratigraphic data include the granulometric characterization and the morphological and mineralogical features of the ash. The geochemical data focus on the major and trace element composition of the glasses. The AMS 14C dating on planktonic foraminifera of the muddy levels separating the tephras allows us to reconstruct the time range of tephra deposition. We discuss the provenance of the Marsili1 tephras, define the type and age of the recent MS volcanic activity and put new constraints on the recent plumbing system of the volcano. The results shed new light on the evolution of plumbing systems of submarine volcanoes located at slow spreading ridges in back-arc settings. Finally, we provide new data on the dispersion of an already known tephra of the central Mediterranean Sea related to the Etna activity. 2. The Marsili1 gravity core and tephra characterization The Marsili1 gravity core (39°15.393′N, 14°24.152′E; 943 m below sea level; total length of 235 cm) has been recovered during the marine cruise “AEOLIAN_2010” operated by CNR-IAMC of Napoli (Italy) on board of the R/V Urania in May 2010. The core is located in the MS central sector, at about 1.8 km south of the summit area (Fig. 1c, d), and lies on a flat area (slope b3°) bounded, to the west, by the presence of several pyroclastic cones forming a NNE–SSW striking volcanic ridge, and, to the east, by the eastern, up to 20° dipping flanks of the seamount. Based on macroscopic features and observations on the binocular microscope, five tephra layers have been found along the Marsili1 gravity core. The contacts between these tephras and the sediments are well defined, lacking evidence of significant erosional surfaces (Fig. 2). These tephras are numbered according to their stratigraphic position from the youngest (M1, uppermost tephra) to the oldest (M5, lowermost tephra). M1, M2, and M3 are represented by one sample due to the reduced thickness (3 cm, 2 cm and 4 cm); multiple samples were picked from M4 and M5, which have thickness of 50 cm and 8 cm, respectively (Table 1). The chemical stratigraphy of the 50 cm thick M4 tephra (Fig. 4), and the occurrence of an equivalent layer on a morphological high located at a distance of about 600 m (see Fig. 9; Iezzi et al., 2014) exclude that this deposit represents a sin-eruptive turbidite bed. The M1, M2, M3, and M4 tephras are represented by 98 to 100 vol.% of juvenile fragments and by a strongly subordinate or absent biogenous and/ or terrigenous non-volcanic fraction. By adopting the protocol recently proposed by Mahony et al. (2014) for describing the tephra occurrence in cores, M1 to M4 represent V1 and V2 tephras (samples with total or dominant glassy fraction, respectively) with Type 1 contacts (distinct top and bottom contacts) (Table 1). The M5 consists of 60 vol.% of ash and 40 vol.% of sediments. Therefore, M5 can be classified as V2 glass with Type 1 contacts (Table 1). The sedimentary layers separating the tephra contain planktonic foraminifera. Following Shepard (1954) and Schlee (1973), these layers consist of sands and gravelly sediments with −0.36 b Mdϕ b 1.73 and 0.58 b σϕ b 1.57. 3. Analytical methods 3.1. AMS 14C dating

Fig. 2. Stratigraphic reconstruction of the Marsili1 record with the position of the studied tephras and the ASM14C dating (yellow stars). The thickness (in cm) of M1, M2, M3 and M5 tephras is reported on the relative lines. The thickness of M4 is 50 cm. The numbers are cm b.s.f. and represent the base of the collected samples for chemical analyses. On the right, selected microphotographs and back-scattered SEM images of the glass shards in the analyzed M1–M5 tephras are reported. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

AMS 14C dating have been carried out on planktonic foraminifera accurately extracted from three muddy samples of the Marsili1 core. Samples were selected at 10, 116 and 227 cm b.s.f. from the top of the core (samples M1/C/8-10, M1/B/79-81 and M1/A/90-92 in Table 2). Each level was disaggregated in distilled water, wet sieved and oven dried at 50 °C. Under a binocular microscope, more than 20 mg of wellpreserved polyspecific planktonic foraminifera shells were collected in the fraction 150 μm. The 14C AMS measurements were performed at the AMS system installed at CIRCE (Centre for Isotopic Research for Cultural and Environmental heritage) laboratory in Caserta (Italy) (Terrasi et al., 2007). The system is based on a tandem accelerator 9SDH-2 (National Electrostatics Corporation, WI, USA) with a maximum terminal voltage of 3 MV. The δ13C of each sample was also measured

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Table 1 Stratigraphic position (cm b.s.f.), lithology (visual description and grain size), total number of number of analyzed points (SEM, EDS, LA_ICP-MS) of the Marsili1 tephras. Geochemical classification of the glassy fraction, magmatic source and proximal and distal equivalents. Md Ф

Tephra Depth Visual core Grain cm description size b.s.f. (VCS)

σФ

Images Points of SEM analyses (SEM)

Points of analyses (EDS)

Points of Classification (TAS) analyses (LA_ICP-MS)

Source

Marsili TEPH01 Marsili TEPH02

M1 M2

18 26

Type 1, V1 Type 1, V2

Coarse Coarse

3.44 1.86 10 2.54 1.24 5

14 4

21 16

10 7

M3

42

Type 1, V2

Coarse

2.54 1.34 8

27

16

8

M4

63

Type 1, V1

Sand

1.26 0.96 7

–

11

71

Sand

1.33 1.11 –

–

12

83

Gravel

−0.50 0.75 7

–

10

8

92

Sand

1.46 1.11 –

–

13

10

102

Sand

0.36 0.59 12

39

20

111

Gravel

13

17

12

4 –

13 9

6

M5

138 143

Type 1, V2

−0.20 0.94 8

– –

4.26 1.98 10 4.82 1.94 –

using an elemental analyzer (ThermoFinnigan EA 1112) coupled with an IRMS (Thermo Finnigan Delta plus) at the Department of Environmental Science of the Second University of Naples (Caserta, Italy). Radiocarbon ages were calibrated by using calibration software OxCal v4.2.4 Bronk Ramsey 2013 (InCal13 atmospheric curve; Reimer et al., 2013). The reservoir correction ΔR (reservoir age) used for calibration is 400 a (Siani et al., 2000, 2001). The calibrated age ranges are reported in years BP and refer to 2σ (Table 2).

Trachyte Basaltic andesite–basaltic trachyandesite Basaltic andesite–basaltic trachyandesite Basaltic andesite–basaltic trachyandesite Basaltic andesite–basaltic trachyandesite Basaltic andesite–basaltic trachyandesite Basaltic andesite–basaltic trachyandesite–trachyandesite Basaltic andesite–basaltic trachyandesite–trachyandesite Basaltic andesite–basaltic trachyandesite–trachyandesite Benmoreite–trachyte Mugearite–benmoreite

Equivalent Time interval (a B.P.) 2073 b time b 7192

Marsili Marsili

Etna

Y-1

7192 b time b 26350

Pavia (CNR, Italy) laboratory (Table 5). The adopted instrument combines a laser ablation microprobe based on a Nd:YAG laser source (Brilliant, Quantel) operating at 266 nm (see Tiepolo et al., 2003 for details), and a quadrupole ICP-MS (Drc-e, Perkin Elmer). Data reduction was carried out with the software package GLITTER (van Achterbergh et al, 2001) and using NIST SRM 610 and 29Si as external and internal standards, respectively. The analytical error is less than 10%. 4. Results

3.2. Physical and chemical analyses 4.1. Chronology The grain size analysis of the Marsili1 ash layers has been conducted using a laser Horiba Partica LA-950V2. Samples were divided in grainsize classes adopting the Udden–Wentworth scale (Wentworth, 1922) and the classification was obtained following Shepard (1954). The sorting σϕ and median diameter Mdϕ are expressed according to the scale of Krumbein (1934), i.e., ϕ = log2 diameter of the particle (in mm) (Table 1). Tephra samples were observed under binocular microscope for lithological description and the whole juvenile fraction was separated by handpicking for petrographic observation, modal analysis and chemical analyses (Table 3). Modal analysis has been carried out on SEM images with the ImageJ software (http://imagej.nih.gov/ij/index.html). The chemical composition of the selected materials was determined at CISAG (University of Napoli, Italy) with a SEM–EDS system (Scanning Electronic Microscope–Energy Dispersive Spectrometry). The chemical analyses were recalculated to 100% on an anhydrous basis (Table 4). Individual analysis of glass shards with total oxide sums lower than 95% was excluded. Instrument calibration was based on international mineral and glass standards. The analytical error, measured on mineral and glass standards is within 5–10%. The trace element content of glass shards was determined by laser ablation LA-ICP-MS at the Istituto di Geoscienze e Georisorse, UOS

AMS 14C dating are representative of the top, middle and bottom of the Marsili1 core and in agreement with the stratigraphic position of the dated samples (Fig. 2, Table 2). The obtained calibrated ages are: 2073 ± 79 a B.P. for M1/C/8-10, 7192 ± 221 a B.P., for M1/B/79-81 and 26,350 ± 323 a B.P. for M1/A/90-92. These data suggest the emplacement of M1 to M4 tephras between ca. 2.1 and 7.2 ka B.P., while the M5 tephra emplaced between ca. 7.2 and 26.3 ka B.P. 4.2. Granulometry, morphology and lithology of tephras The M1, M2 and M3 tephras are well sorted (1.24 b σϕ b 1.86) with Mdϕ between 2.54 and 3.44 (coarse ash) (Table 1). The M4 tephra is well to very well sorted (0.58 b σϕ b 1.57) and show normal grading with Mdϕ in the ranges − 0.19–1.45 (between 111 and 92 cm), and − 0.08–1.26 (from 90 to 63 cm) (Table 1). With respect to the M1, M2, M3 and M4 tephras, M5 is less sorted (1.94 b σϕ b 1.98) and shows the higher Mdϕ values (Mdϕ = 4.26–4.82) (Table 1). M1 consists of dense to poorly vesiculated (0 to 30 vol.%) scorias with subcircular vesicles and tube-like scorias with elongated, partly unconnected vesicles (Fig. 2; Table 3). Crystal content varies between 0 and 32 vol.%. They are mainly plagioclase (5–20 vol.%) and

Table 2 AMS 14C dating of the analyzed samples. CalCurve: OxCal v4.2.4 (Bronk Ramsey and Lee, 2013); InCal13 atmospheric curve (Reimer et al., 2013). ΔR 400 a. Sample

Depth cm b.s.f.

Material

Stratigraphic relation to tephra

Radiocarbon age B.P. (a)

δ13Cnat

Calendar age cal. B.P. (a)

M1/C/8-10 M1/B/79-81

10 116 227

Planktonic foraminifera Planktonic foraminifera Planktonic foraminifera

8 cm above M1 6 cm below M4 84 cm below M5

2507 ± 31 6668 ± 79 22,520 ± 120

1.0 ± 0.3 0.8 ± 0.3 −1.3 ± 0.3

2073 ± 79 7192 ± 221 26,350 ± 323

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S. Tamburrino et al. / Global and Planetary Change 133 (2015) 2–16

Table 3 Volcanological features and mineralogical assemblage of the Marsili1 tephras. ID Tephra

Glass shards description

Vesicle content %

Plg

Px

Ol

Ox

Ap

Tot micropheno %

Max. dimension

M1 M2 M3 M4

Tube pumice to dense glass shards Tube scoria and dense glass shards Dense, poorly vesiculated glass shards Dense, poorly vesiculated glass shards Dense, poorly vesiculated glass shards Dense poorly vesiculated glass shards to midly vesiculated scorias (some elongated) Dense poorly vesiculated glass shards Dense, poorly vesiculated glass shards Dense, poorly vesiculated scoria Highly vesiculated scorias, zeolites in some vesicles

0–30 10–30 2–15 2–5 2–15 5–15

5–20 0–10 10–15 8 10 12

5 – b5 3 3 3

b1 – b1 – b1 –

b1 – – – – –

b1 – b1 – – –

5–20 0–5 5–20 5–11 5–15 10–15

500 micr (pheno 0–10%) 50 micr (pheno 0%) 400 micr (pheno 0%) 300 micr (pheno 0%) 800 micr (pheno b1%) 150 micr (pheno 0%)

5–10 5–10 5–10 10–50

10 15 10 1

5 5 3 1

b1 b1 – 1

– b1 b1 b1

– – – –

10–15 20 10–15 0–5

150 micr (pheno 0%) 200 micr (pheno 0%) 300 micr (pheno 0%) 120 micr (pheno 0%)

M5

clinopyroxene (about 5 vol.%) with acicular and skeletal shape and maximum size of 510 μm. Fe–Ti oxides, orthopyroxene, olivine, amphibole, and apatite are sporadic with abundance ≪ 1 vol.% (Table 3). Crystal fragments are lacking. M2 consists of poorly vesiculated (9–20 vol.%), dense clasts with subrounded to slightly elongated vesicles (Fig. 2; Table 3). The amount of crystals does not exceed 13 vol.%. The main phase is represented by acicular plagioclases with a maximum size of 500 μm; subordinate clinopyroxene, olivine, and amphibole also occur. Apatite has been found in few clasts (Table 3). M3 and M4 are characterized by poorly vesiculated clasts and shards with stretched vesicles (Fig. 2; Table 3). The vesicle content is in the range 5–28 vol.% and the crystal amount between 12 and 25 vol.%. Plagioclase and clinopyroxene crystals are acicular in shape and their size is less than 400 μm. Olivine and sporadic apatite are also present with abundance ≪1 vol.% (Table 3). The M1 to M4 tephra area characterized by fresh glass. Evidence of alteration (e.g., palagonite) or occurrence of secondary minerals (e.g., zeolites) are lacking. M5 consists of vesiculated (30 to 56 vol.%) scorias with zeolites in some vesicles (Fig. 2; Table 3). The crystal content does not exceed 5 vol.% and the main phases are plagioclase, clinopyroxene, olivine and Fe–Ti oxides, all with size b120 μm (Table 3). 4.3. Glass geochemistry According to Total Alkali/Silica diagram (TAS, Le Bas et al., 1986; Fig. 3a), the glass M1 shows a trachytic composition, whereas M2 and M3 fall in the field of basaltic trachyandesites. The glasses M4 cover a wide compositional trend ranging from basaltic trachyandesites (bottom) to trachyandesites (top) (Figs. 3a and 4).

The bottom of the M4 tephra (from 111 cm to 92 cm) is characterized by a wide silica content (SiO2 content ranges from ca. 52 wt.% to ca. 59 wt.%), whereas on the top (from 83 cm to 63 cm) prevails a basaltic trachyandesite composition with SiO2 values clustered between 52 and 54 wt.%. The glass M5 displays a trachyandesitic composition (Fig. 3a). Almost all the glasses of the M1, M2, M3, and M4 tephras have a sub-alkaline affinity and, in particular, a HK calc-alkaline affinity (Fig. 3b), whereas M5 falls in the alkaline field (Fig. 3a). The degree of evolution of the M1–M4 tephra decreases from the bottom to the top of the Marsili1 core (Fig. 4a, top left). In the Harker's diagrams, the M1 to M4 tephras depict decreasing trends of MgO and CaO with increasing SiO2, whereas Al 2O3 is nearly constant in M2, M3, and M4, and decreases with increasing SiO 2 in M1. P 2O 5 and TiO 2 show significant variation in the basaltic trachyandesites of M2, M3 and M4 (Fig. 4). The FeOtot, MgO and CaO contents of M5 tephra decrease with increasing SiO2. Selected trace element plots of the M1 to M4 tephras are reported in Fig. 5a. La/Ce, Ba/Rb, Nb/Zr are nearly constant in M1 to M4. Ni decreases with increasing Zr. Zr and Y are linearly correlated with higher Y/Zr for Zr b 120 ppm, and a lower ratio for Zr N 120 ppm, suggesting pyroxene fractionation (Ewart and Griffin, 1994). The trace element contents normalized to primitive mantle (McDonough et al., 1992) of M1, M2, M3 and M4 tephras show a decreasing pattern from Rb to Lu with Nb, Ta, and Ti troughs and positive Pb peak (Fig. 6). In particular, the pattern of M1 shows the greater enrichment, with, in addition, Sr and Sm troughs. The chondrite-normalized (Wood et al., 1979) pattern of M1 to M4 tephras show a strong fractionation of LREE compared with HREE ([La/Lu]N = 5.2–28.7), with an almost flat patterns of HREE ([Gd/Yb]N = 0.5–2.7) (Fig. 6). The trace elements pattern normalized to the primordial mantle of M5 tephra shows enrichments from Rb to K, and strong depletion in Sr, Sm and Ti (Fig. 7). The

Table 4 Average values and standard deviation (in italic) of the major-element composition of glasses M1 to M5 from the Marsili1 core. M1

M2

M3

M4

M5

63 cm

71 cm

83 cm

92 cm

102 cm

111 cm

138 cm

143 cm

Analyses

No. 21

σ

No. 16

σ

No. 16

σ

No. 11

σ

No. 12

σ

No. 10

σ

No. 13

σ

No. 20

σ

No. 17

σ

No. 13

σ

No. 9

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 F Cl S Total Alk

64.45 0.91 17.25 4.08 0.14 0.99 3.62 4.41 3.48 0.25 0.06 0.33 0.04 97.78 7.93

1.51 0.21 1.94 0.92 0.12 0.32 1.00 0.71 0.75 0.15 0.11 0.14 0.06 2.07 0.38

55.77 1.34 16.56 8.07 0.18 3.74 7.65 3.99 1.90 0.49 0.08 0.20 0.04 98.53 5.91

0.60 0.18 0.21 0.47 0.13 0.20 0.22 0.24 0.08 0.12 0.15 0.04 0.07 1.27 0.28

53.88 1.37 16.02 8.63 0.14 4.68 9.20 3.44 2.01 0.40 0.02 0.18 0.04 98.80 5.46

0.82 0.16 0.31 0.43 0.15 0.48 0.71 0.29 0.20 0.25 0.06 0.05 0.07 1.56 0.39

53.05 1.15 16.16 8.49 0.28 4.90 10.00 3.24 2.04 0.40 0.02 0.19 0.09 98.30 5.30

0.69 0.22 0.44 0.53 0.18 0.32 0.68 0.15 0.20 0.14 0.06 0.07 0.14 0.95 0.27

53.74 1.11 16.33 8.07 0.32 5.02 9.44 3.32 2.05 0.31 0.06 0.17 0.07 98.82 5.38

1.18 0.23 0.41 0.37 0.21 0.65 0.85 0.27 0.29 0.16 0.12 0.06 0.12 1.22 0.54

54.06 1.23 16.12 8.44 0.17 4.63 9.41 3.35 2.05 0.22 0.02 0.21 0.10 98.49 5.41

0.65 0.15 0.34 0.33 0.14 0.33 0.61 0.28 0.19 0.16 0.05 0.07 0.12 1.75 0.41

54.24 1.23 16.07 8.53 0.17 4.37 8.80 3.48 2.28 0.46 0.07 0.21 0.10 98.26 5.78

1.12 0.24 0.53 0.38 0.14 0.66 0.83 0.28 0.35 0.24 0.10 0.09 0.14 1.20 0.56

55.24 1.18 16.35 8.09 0.24 3.97 8.00 3.53 2.65 0.39 0.06 0.26 0.03 97.63 6.21

1.97 0.21 0.60 0.62 0.15 1.08 1.77 0.42 0.56 0.15 0.12 0.11 0.12 1.25 0.96

54.30 1.12 16.22 8.48 0.25 4.17 8.83 3.28 2.62 0.37 0.05 0.06 0.25 96.09 5.92

1.28 0.22 0.33 0.36 0.19 0.83 1.21 0.29 0.43 0.12 0.11 0.09 0.06 1.40 0.69

59.55 1.43 17.06 5.91 0.17 1.92 4.33 5.32 3.22 0.51 0.12 0.37 0.10 97.09 8.59

1.49 0.28 0.32 0.73 0.17 0.25 0.69 0.37 0.46 0.19 0.14 0.11 0.16 1.27 0.60

59.43 1.57 17.19 5.86 0.25 2.01 4.33 4.88 3.28 0.64 0.14 0.36 0.04 98.50 8.21

1.09 0.10 0.12 0.47 0.11 0.20 0.51 0.41 0.55 0.11 0.08 0.06 0.09 1.12 0.49

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Table 5 Average and standard deviation (in italic) of trace-elements of the M1 to M5 glasses from the Marsili1 core. M1

M2

M3

M4

M5

83 cm

92 cm

111 cm

138 cm

Analyses

No. 10

σ

No. 7

σ

No. 8

σ

No. 8

σ

No. 10

σ

No. 12

σ

No. 6

σ

Li Be B Sc V Cr Co Ni Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U Eu/Eu* (La/Lu)N (La/Sm)N (Gd/Yb)N

11.6 – 29.1 11.1 86.4 21.4 7.29 3.70 91.1 106 246 29.2 224 30.6 4.94 1114 51.3 89.8 8.98 33.7 5.24 1.60 5.19 0.92 4.98 1.05 4.10 0.55 4.09 0.55 4.84 1.73 18.3 17.4 5.11 1.1 12.5 7.1 1.3

4.7 – 22.3 3.8 8.5 9.5 1.53 2.03 13.3 13 38 5.6 34 4.5 0.84 147 9.2 10.3 1.51 6.3 2.46 0.28 1.64 0.27 1.46 0.33 0.90 0.21 1.91 0.29 1.66 0.46 2.4 2.4 0.97 0.5 7.4 3.0 0.8

9.7 5.38 17.0 21.5 247.1 4.7 20.86 8.21 86.1 60 465 23.2 137 23.6 2.44 666 41.4 72.3 7.75 29.1 5.45 1.72 4.88 0.80 3.84 0.92 2.60 0.35 2.52 0.43 3.38 1.02 9.0 9.6 2.69 1.0 10.5 4.8 1.6

1.5 – 4.8 5.0 48.0 1.2 3.21 3.69 30.7 16 55 4.1 29 4.7 0.68 181 9.0 14.7 1.39 4.5 0.64 0.40 1.11 0.17 0.75 0.15 0.68 0.12 0.62 0.11 0.72 0.26 3.0 2.8 0.68 0.2 3.3 0.8 0.3

7.5 2.67 11.0 25.8 244.3 24.4 26.06 21.52 84.2 51 409 19.4 89 12.3 2.69 662 27.5 49.8 5.35 21.7 4.74 1.39 3.88 0.57 3.67 0.74 2.03 0.26 2.01 0.29 2.07 0.53 10.0 7.1 1.98 1.0 9.9 3.7 1.6

2.0 0.35 3.7 4.8 24.0 37.6 3.44 6.47 11.9 9 41 2.5 16 2.3 0.42 92 4.3 6.9 0.85 3.2 0.81 0.27 0.64 0.11 0.60 0.18 0.29 0.05 0.50 0.05 0.33 0.13 1.3 1.2 0.33 0.2 0.7 0.4 0.3

7.4 3.34 10.2 23.9 241.4 13.0 24.35 19.15 91.2 52 432 19.0 86 12.2 2.68 675 27.9 51.4 5.45 21.4 4.34 1.36 3.73 0.66 3.19 0.72 1.96 0.26 2.07 0.29 2.19 0.58 11.2 7.2 2.03 1.1 10.2 4.1 1.5

2.2 1.20 2.9 4.4 25.7 8.0 3.19 7.37 24.4 8 61 2.6 11 1.8 0.39 75 3.7 5.8 0.81 2.3 0.89 0.18 0.76 0.12 0.46 0.17 0.23 0.05 0.32 0.06 0.32 0.09 2.6 0.9 0.28 0.2 1.6 0.6 0.4

6.9 3.52 14.7 21.1 230.4 9.5 23.15 14.95 69.6 58 451 18.1 95 14.0 2.75 701 30.1 53.3 5.66 22.9 4.38 1.42 3.95 0.58 3.53 0.71 1.83 0.28 1.89 0.28 2.32 0.65 13.8 8.5 2.40 1.1 11.2 4.3 1.7

1.3 1.90 2.6 3.3 46.9 2.0 3.32 7.55 21.4 18 54 4.5 35 5.7 0.71 183 10.4 17.2 1.59 7.1 1.31 0.37 1.20 0.10 1.19 0.18 0.59 0.07 0.43 0.10 0.66 0.28 7.9 2.9 0.88 0.2 1.6 0.4 0.3

12.0 3.59 14.7 25.4 289.2 13.9 25.02 13.58 87.2 74 421 23.7 119 15.9 3.40 810 32.4 60.1 6.60 25.7 5.36 1.77 4.66 0.72 4.64 0.93 2.26 0.40 2.66 0.35 2.92 0.78 18.9 9.5 2.71 1.1 9.4 3.8 1.5

5.1 1.09 8.1 7.2 58.2 9.5 3.03 7.49 18.0 36 82 5.5 37 7.2 1.52 448 13.9 24.8 2.72 9.0 1.66 0.51 1.13 0.19 0.99 0.29 0.59 0.13 0.78 0.08 1.32 0.43 13.5 4.5 1.34 0.3 3.0 0.9 0.6

21.0 – 46.3 13.7 87.4 54.1 8.04 19.68 126.9 68 920 35.2 398 97.8 2.36 1365 134.3 232.4 23.73 90.3 13.09 3.53 9.80 1.39 7.64 1.29 4.28 0.59 4.50 0.49 8.98 4.89 21.9 22.3 6.39 1.0 30.2 6.5 1.9

6.1 – 33.7 4.8 13.5 16.2 2.16 19.96 33.8 17 87 3.9 48 13.6 0.93 190 13.8 29.6 2.84 13.7 0.67 1.22 3.53 0.38 2.90 0.54 1.68 0.19 1.76 0.16 3.58 0.98 10.1 4.7 1.15 0.3 6.3 0.8 0.9

chondrite-normalized pattern exhibits high HREE/LREE ([La/Lu]N = 19.6–38.6), with a flat pattern of HREE ([Gd/Yb]N = 1.1–3.4) (Fig. 7). 4.4. Petrography and mineral chemistry The studied Marsili tephras generally show a groundmass of clean glass, as previously reported. Cryptocrystalline textures occur in some clasts of the thick M4 deposit, where plagioclase and pyroxene microcrystals prevail over minor olivine crystals. Plagioclase often form glomerophyric clusters with each other or with pyroxene, olivine and the few amphibole found in the samples. The most ubiquitous phase in the MS tephras is plagioclase, which show both subhedral and anhedral features. In tephras M3 and M4, they cover a mild compositional range from An84–66 and An78–44, respectively (Fig. 8; Supplementary Table 1). Plagioclase from tephra M1 and M2 have a more restricted compositional range (An62–57 and An43–33, respectively) (Fig. 7). Sanidine also occurs in tephra M1. Clinopyroxene was analyzed in tephras M1, M3 and M4. Orthopyroxene also occurs in tephra M1. In the pyroxene classification diagram (Morimoto et al., 1988), samples from tephras M4 and M3 plot in the augite field with part of the composition straddling the diopside and augite boundary (Fig. 8; Supplementary Table 1). Pyroxenes from tephra M1 are augite and enstatite. Looking at the Mg# number (Mg# = 100 ∗ Mg / (Mg + Fet), where Fet refers to all Fe expressed as FeO, the tephra M4 displays the larger variation

of this value, which ranges from 63 to 83. Olivine crystals were analyzed in tephra M4; they show a narrow range of variation (Fo72–68). The lowest value characterizes the tephra M1 (Fo65) (Supplementary Table 1). The amphiboles in tephra M1 are Mg-hastingsites according to Leake et al. (1977). By utilizing the geothermobarometer of Ridolfi et al. (2010), the temperature and pressure of crystallization of ca. 1000 °C and 3 kbar, respectively (Supplementary Table 1). 5. Discussion 5.1. Tephrostratigraphy The M1, M2, M3, and M4 tephras represent primary volcanic deposits, as suggested by the large abundance of ash (over 98%) within the Marsili1 core. They record the deposition of ash from explosive submarine or subaerial explosive eruptions. The M5 tephra, instead, is a cryptotephra with predominant glass fraction coupled with nonjuvenile material. On the basis of our geochronological data, the M1, M2, M3 and M4 events occurred between ca. 2.1 and 7.2 ka B.P., whereas the M5 tephra emplaced between ca. 7.2 and 26.3 ka B.P. The correlation of the Marsili1 tephras with equivalent explosive events, discussed below, is based on the single-shards micro-analysis (EDS and LA-ICP-MS) compared with the available data from literature created with the same analytical approach and with bulk rock analyses.

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Fig. 3. Classification of the M1 to M5 Marsili1 glasses according to (a) the Total Alkali/Silica (TAS, Le Bas et al., 1986) diagram and (b) the SiO2 vs. K2O diagram for calcalkaline rocks (Peccerillo and Taylor, 1976). The composition of the tephras TEPH01 and TEPH02 from the COR02 is also reported (data from Iezzi et al., 2014). The compositional fields of the Campanian Plain, Etna, Marsili and Aeolian proximal and distal volcanic products are shown for comparison. The line of Irvine and Baragar, 1971, is also reported. Data from: Peccerillo (2005), Beccaluva et al. (1982), Trua et al. (2002, 2003) and Iezzi et al. (2014).

However, these latter products have a content in trace elements (e.g., Rb, Sr, Nb and Zr) that differs from that of the of the Marsili1 tephras (Fig. 5c). On the basis of the above considerations, then, we exclude a provenance of the M1–M4 tephras from the Aeolian volcanoes. As a result, the M1 to M4 tephra are not related to subaerial eruptions of the Quaternary continental volcanoes of the Italian peninsula or volcanic islands of the Tyrrhenian Sea. The overall mineral and geochemical composition of the M1–M4 tephras resembles those of the Marsili Seamount. The MS products mainly consist in medium-K calc-alkaline basalts, with evolved high-K andesites (Trua et al., 2002, 2014) produced in late Pliocene through effusions and lava flows (Figs. 5a, 6). The magmatic activity is characterized by a progression in time from IAB-type basalts and andesites with calcalkaline affinity, to OIB-like basalts to form several small cones to the summit. Recently, Iezzi et al. (2014) found on the summit of the MS two tephras ascribable to the previously unknown, recent explosive activity. In this context, the M1–M4 tephras represent four explosive submarine events that have produced the deposition of ash fall in historical times. Concerning the M5 tephra, we exclude a provenance from the Campanian and Aeolian volcanoes because these latter volcanic provinces do not have a Na-alkaline geochemical affinity. In the Tyrrhenian area and surroundings, the only edifices with such affinity are the Mt. Etna and Pantelleria Island (e.g., Peccerillo, 2005). A provenance of M5 from Pantelleria can be rejected because of the iron-rich peralkaline- and silica-oversaturated products of the Pantelleria magmas (Esperança and Crisci, 1995; Civetta et al., 1998; Avanzinelli et al., 2004). The major, trace element composition and the REE pattern of M5 show a consistent similarity with that of the Biancavilla–Montalto (BMI) products from the Etna volcano (17.030 ± 0.180 ka B.P.; Siani et al., 2001). The BMI and the Unit D deposits represent co-genetic eruptions with complex temporal and geochemical relationships and overlaps both at the proximal and distal sites (Coltelli et al, 2000). In particular, more evolved proximal products towards SE are related to the BMI-D2b (which represent the youngest event) and to an unknown oldest eruption D1b (Fig. 7; ca. 19.8 ka B.P.; Albert et al., 2013), respectively. Less evolved products, instead, characterize the eruptions dispersed to the E (D2a and D1a), which occurred in the intermediate period (between ca. 18.3 and 18.8 ka B.P.; Albert et al., 2013). In this context, the M5 tephra seems to be, both in major and trace element term, the distal counterpart of a less evolved event (Figs. 3 and 7). 5.2. Proximal–distal correlations of the Marsili1 tephras

Despite the correlation between grain-discrete and bulk rock data suffers of several problems, the single-shards method represents, to date, the most appropriate in geochemical characterization of distal deposits made up by fine-grained materials. The HK-calcalkaline affinity of the M1 to M4 tephras exclude that the source area of these products is the Campanian Plain volcanic area (Vesuvius, Campi Flegrei, Ischia) because of these products have a potassic undersaturated to ultrapotassic affinity (Fig. 3a; Peccerillo, 2005). We also exclude a provenance of these tephras from the Etna or Pantelleria volcanoes, because these latter rocks have a Na-alkaline affinity (Lustrino and Wilson, 2007 and references therein). The composition of M1 to M4 tephra overlaps that of the Aeolian rocks, and, in particular, of Salina, Lipari, Vulcano and Stromboli islands (Fig. 3a, b). The rocks from these islands are characterized by calcalkaline, HK-calcalkaline and, at Vulcano and Stromboli, also by undersaturated shoshonitic products (De Astis et al., 1997; Francalanci et al., 2013). However, the Salina activity ceased at about 13 ka B.P., whereas the Lipari products erupted in the last 8 ka have a rhyolitic composition (Crisci et al., 1991; De Rosa et al., 2003). The Vulcano products emitted in the last 8 ka consist of rhyolites, undersaturated potassic rocks and shoshonites (De Astis et al., 1997). At Stromboli, magmas with a HK-calcalkaline affinity were not emitted in the last 26 ka B.P. with the exception of the Pizzo and San Bartolo lavas, which have ages between 380 BC and 557 AD (Francalanci et al., 2013).

With respect to the marine and terrestrial tephras recognized in the Tyrrhenian area, whose source areas are the Campanian, Etna, Aeolian and Marsili volcanoes (e.g., Paterne et al., 1988; 1988; Wulf et al., 2004, 2012; Albert et al., 2012; Iezzi et al., 2014), the composition of the M1 and M4 tephras is similar to that of TEPH01 and TEPH02 tephras (Figs. 3 and 5) found in the CORE02 gravity core on MS (Fig. 5; Iezzi et al., 2014). In particular, the M1 tephra has the same composition of TEPH01 (trachyte), while the compositional range of the M2, M3, and M4 tephras partly overlap that of the TEPH02 (trachyandesite). According to the pattern of a number of incompatible trace elements ratios (e.g., Th/Yb vs Th/Nb) it is possible to indicate the M2-TEPH02 correlation as the most reliable (Fig. 5b). In addition, these two tephra are characterized by the same paragenesis (Table 3; Supplementary Table 1). Therefore, the M1 and M2 tephras testify already known explosive events of MS, whereas the M3 and M4 tephras suggest the occurrence of up to now unknown eruptive events (Fig. 9). This conclusion is also supported by our chronological data. The M1 and M2 tephras erupted between ca. 2 and 7 ka B.P., a time span overlapping the ages of the TEPH01 and TEPH02 emplacement (recovered between 3 and about 5 ka B.P., Iezzi et al., 2014). On the base of the age constraints for the Marsili1 record and of the M1-TEPH01 and M2-TEPH02 correlations here proposed, we conclude that the M1 to M4 tephras emplaced between 3

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9

Fig. 4. Variation range (bars) and average value (dots) of SiO2 content of the glasses M1 to M4 vs. stratigraphic height in the Marsili1 core (top left) and major elements variation vs. SiO2 content of the Marsili1 glasses M1 to M4. The compositional field of the Marsili basaltic lavas is shown for comparison. Data from Trua et al. (2002) and Iezzi et al. (2014) and references therein.

and 7 ka B.P. (Fig. 9). The thickness of the correlated M1 (thickness = 3 cm) and TEPH01 (thickness = 15 cm) tephras decreases from the location of CORE02 to that of Marsili1, i.e., from east to west (Fig. 9). The thickness of the correlated M2 (thickness = 4 cm) and TEPH02 (thickness = 60 cm) tephras also decreases from west to east (Fig. 9). This suggests that the source vents of the M1-TEPH01 and M2-TEPH02 tephras were located west of the Marsili1 and CORE2 gravity cores, whose distance is about 600 m. Such vents may be one or more of the NNE–SSW aligned pyroclastic cones forming the Marsili ridge in the central sector of the volcano (Figs. 1d and 9), but further data need to be collected to better constrain this hypothesis. As concerns the source vents of the M3 and M4 tephras, the available data do not allow us to locate the position of the possible emission center(s) due to the lack of proximal–distal correlations. The M5 tephra testifies the deposition in the northern direction respect to the Etna volcano, of less evolved products respect to the BMI (Fig. 7). According to the event stratigraphy proposed by Albert et al. (2013), the M5 tephra appears similar to the “Y-1” (previous to the BMI) found at Lago di Mezzano, Central Adriatic, Lago Grande di Monticchio (TM-11) and Bannock basin. No definitive attribution of

M5-LMZ to the proximal counterpart is possible to establish due the lack of information of the basal flow deposits in the Biancavilla region (Fig. 7). 5.3. Volcanological significance of the M1–M4 tephras The low vesicle content (≤ 30 vol.%) of the M1 to M4 tephras, the concomitant occurrence of scorias with undeformed vesicles and stretched and tube-like scorias, as well as the lack of erosional surfaces at the interface between the volcanic levels and the sedimentary layers indicate that, according to the criteria proposed by Head and Wilson (2003), the M1 to M4 tephra represent ash fall deposits related to submarine strombolian-like eruptions. 5.4. Inferences on the source of the MS magmas The M1, M2, M3 and M4 tephras show an HK-calcalkaline affinity and variable degree of evolution (Figs. 3b). None of collected samples represents a primitive basaltic magma however the less evolved glasses M2, M3 and M4 are characterized by enrichments of Rb, Ba, Th and K relative to Nb. These enrichments also characterize the MS basaltic

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Fig. 5. a) Selected trace elements variation diagrams of the Marsili1 glasses M1 to M4. The compositional fields of the Marsili basaltic lavas and of the TEPH01 and TEPH02 glasses from the COR02 core are reported for comparison fields are also shown for comparison (data from Trua et al. (2002) and Iezzi et al. (2014) and references therein). b) Th/Nb vs. Th/Yb plot of the Marsili1 glasses M1 to M4 and compositional fields of the TEPH01 and TEPH02 glasses from the COR02 core (data from Iezzi et al., 2014). c) Nb vs Zr plot of the Marsili1 glasses M1 to M4 glasses, and compositional fields of the TEPH01 and TEPH02 glasses from the COR02 core (data from Iezzi et al., 2014), of the Marsili lava flows (data from Trua et al., 2002), and of the HKCA pyroclastics of the Stromboli volcano erupted between 380 B.C. and 557 A.D. (data from Francalanci et al., 2013).

lava flows and the less evolved magmas of the neighbor Aeolian Islands (Trua et al., 2002, 2010; Peccerillo, 2005), suggesting the occurrence of a LILE-enriched mantle source. The LILE enrichment observed in the M4 less evolved glasses, as well as the high fractionation of LREE compared to HREE, may be explained in light of the detected geophysical

anomalies, i.e., low Vp, low Vs, high Qp and high Vp/Vs, in the Marsili basin (Pontevivo and Panza, 2006; Chiarabba et al., 2008), which are consistent with a release of fluids from the roll-backing slab beneath the Aeolian volcanoes and MS. In light of these information, we conclude that the LILE enrichment and LREE fractionation of the Marsili1

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Fig. 6. Selected trace elements normalized to the primitive mantle (Sun and McDonough, 1989) and REE chondrite (Boynton, 1984) diagrams of the M1 to M4 glasses from the Marsili1 core. The compositional range of the Marsili lava flows is also reported. Data from Trua et al. (2002).

less evolved glasses could be due to the uprising of subduction-related fluid in the overlying, lithospheric mantle. In the Nb/Yb vs Zr/Yb plot (Fig. 10a), the less evolved (Zr b 80 ppm) glasses of the M4 tephra depict a continuous trend extending from enriched MORB-related magmas of the Aeolian Islands to the OIB-like Marsili, Etna and Pantelleria sources. According to Trua et al. (2003), this feature suggests that the recent, less evolved Marsili magmas derive from a heterogeneous mantle characterized by the interaction of IAB and OIB sources. This model is consistent with the available geophysical data (Rosenbaum et al., 2008), which indicate the occurrence of a tear fault in the roll-backing slab along which the Etna-like, OIB-like melts migrate towards the Aeolian Islands and blend with the mantle wedge underlying MS and the Aeolian volcanoes. 5.5. Mechanisms of magma evolution The decrease of CaO, MgO, and FeO with increasing degree of evolution and the mineralogical association of the M1–M4 tephras are consistent with the fractionation of clinopyroxene (Fig. 4; Table 3). The lack of Eu anomaly in the normalized trace element pattern

(Fig. 6) and the Eu/Eu* values in Table 5 suggest that plagioclase does not play a major role in the evolution of the M1–M4 magmas, although it has been recognized to be relevant in the evolution from basalts to basaltic trachyandesites recorded in the MS lava flows (Trua et al., 2002). The large variation of TiO2 and P2O5 at constant SiO2 values is not peculiar of the M1–M4 glasses and it has also been recognized in the MS basaltic lava flows (Fig. 4). This large variation of TiO2 and P2O5 in the MS basalts has been interpreted to be a sign of a heterogeneous mantle source (Trua et al., 2002, 2003). Therefore, the M1–M4 basaltic trachyandesites may result from the evolution this heterogeneous source. The higher TiO2 and P2O5 content of the less evolved M2–M4 glasses with respect to the basaltic MS lavas reflects the lack of significant Fe–Ti oxides and apatite fractionation from basalts to basaltic trachyandesites (Fig. 4). On the contrary, the lower values of these oxides in the M1 trachyte indicate the involvement of these two phases in the evolution from trachyandesites to trachytes. In the Ba vs. Rb/Ba and Nb vs. Zr/Nb plots a variation of the ratio between incompatible elements is found, mainly in the M4 tephra (Fig. 10b). In particular, two main trends may be recognized: a) a first trend characterized by nearly constant ratios between trace elements

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Fig. 8. a) Composition of plagioclase in the Marsili1 tephras, b) classification of the pyroxenes representative of the M1, M3 and M4 tephras.

higher ratios of incompatible trace elements. This latter process is compatible, according to the above discussed results, with the occurrence of a heterogeneous source below MS. 5.6. Interpretation of the Marsili plumbing system in the last 7 ka and its relations with the back-arc geodynamics Fig. 7. Top: Total Alkali/Silica (TAS) diagram (Le Bas et al., 1986) showing the M5 glass composition compared with proximal and distal deposits found in the central and eastern Mediterranean setting. BMI VL = Biancavilla Montalto Ignimbrite (Vallone Licodia); BMI CM = Biancavilla Montalto Ignimbrite (Contrada Monaci); D1a, D2a = eruptive Unit D from Acireale (Ellittico volcano activity); D1b, D2b = eruptive Unit D from Giarre (Ellittico volcano activity) (data from Albert et al., 2013); “Y-1” (data from Insinga et al., 2014; Albert et al., 2013; Wulf et al., 2008; Siani et al., 2004; Calanchi et al., 1998; Paterne et al., 1988; Keller et al., 1978). Lower part: normalized primitive mantle (after Sun and McDonough, 1989) and normalized chondrite (after Boynton, 1984) diagrams for M5 tephra. The compositional range of Etna intermediate, BMI and Unit D products are shown for comparison (data from Albert et al., 2013 and Peccerillo, 2005).

of the M1, M2, M3 and M4 glasses and, b) a second one with variable ratios of the less evolved M4 glasses. The trend a) is consistent with fractional crystallization processes, whereas the trend b) testifies the blending of magmas with a similar degree of evolution but different trace element pattern. The results of the geochemical modeling of such two processes following Neuman et al. (1954) and Langmuir et al. (1978) suggest that the M1, M2, and M3 magmas and the more evolved M4 magmas originate by fractionation of a M4 basaltic trachyandesite characterized by low Rb/Ba and Zr/Nb, whereas the less evolved M4 magma results from the interaction between this latter M4 end-member and a melt with the same degree of evolution but

The SiO2 vs. depth plot shows that the degree of evolution of the glasses decreases from M1 to M3 (Fig. 4). In the M4 tephra, the SiO2 content increases with depth. Therefore, the M4 tephra reflects the emptying of a normally zoned magma chamber, while the M3, M2 and M1 tephras evidence the formation of a progressively more evolved reservoir with decreasing time. No appreciable difference in the degree of evolution exists between the last erupted M4 magma and the early erupted M3 magma. The variations of the degree of evolution of the M1–M4 glasses with the depth in the Marsili1 core and the above discussed geochemical data allow us to put some constraints of the MS plumbing system. The chemostratigraphy of the M4 tephra and the recognized mechanism of evolution deduced from the glass chemistry testify the emptying of normally zoned magma chamber in which magma mainly evolved by fractional crystallization processes. This zoned reservoir was affected by the arrival of new, fresh magma. This latter magma has the same degree of evolution of the less evolved resident melt and could be responsible for the triggering of the M4 eruption (Fig. 11), although the involvement of a nonerupted basaltic melt cannot be excluded. Successively, the reservoir was filled by new magma that evolved by fractional crystallization

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Fig. 9. Location of the Marsili1 (this study) and COR02 (Iezzi et al., 2014) records in the central sector of MS and schematic stratigraphic position of the tephras and M1-TEPH01 and M2-TEPH02 correlations proposed in this study.

and the M2 and M3 basaltic trachyandesites were erupted (Fig. 11). With decreasing time, the residual melt evolved producing the M1 trachytes and trachyandesites. The M2 and M3 trachyandesites have the same chemical signature of the less evolved resident melts of the M1 reservoir. The available geochemical data (Trua et al., 2002, 2003; Iezzi et al., 2014), and the new data presented here indicate that the Marsili reservoir is fed by melts from a heterogeneous mantle source characterized by magmas with different TiO2, P2O5, Rb, and Zr contents. Between 7.2 and 2.1 ka B.P., the Marsili reservoir was mainly filled by magmas with low Rb/Ba and Zr/Nb (Fig. 11). These magmas suffered fractional crystallization and evolved up to trachytes. The above discussed general model of the recent MS activity involves a deeper, heterogeneous basaltic source, and a shallower reservoir in which magma mixing and fractionation processes operate. Trua et al. (2002) also invoke a mantle source for the basalts and estimate a depth of about 1 km below the top of the edifice for the fractionation of the trachyandesitic magmas. This polybaric arrangement of the MS plumbing system is consistent with the available geophysical data, which evidence a deeper reservoir at the Moho depth (10 km; Pontevivo and Panza, 2006), and a shallower reservoir at about 1.5– 2 km depth below the top of MS (Caratori Tontini et al., 2010). The depth of the deeper reservoir is fully consistent with our preliminary geothermobarometric estimates (Section 4.4 and Supplementary Table 1), which indicate a pressure of 300 MPa, a value roughly corresponding to a depth of 10 km by assuming a MS average rock density of 2700 kg/m3 (Caratori Tontini et al., 2010).

Fig. 10. a) Nb/Yb vs. Zr/Yb diagram for M4 tephra with Zr b 80 ppm showing the Marsili IAB and OIB-like basalt as end-members according to Trua et al. (2002, 2010) and with the compositional fields of Aeolian, Etna and Ustica volcanic products (data from Peccerillo, 2005). b) Ba vs. Rb/Ba and Nb vs. Zr/Nb diagrams. The calculated fractional crystallization trend (Neuman et al., 1954) between the M1 and M4 magmas is reported as green curve; the used partition coefficients (Kd) values are also reported. The calculated mixing trend (Langmuir et al., 1978) among the M4 less evolved magmas is reported as light blue curve. Symbols as in Figs. 3–5.

The available geochronological, volcanological and petrological data on MS indicate that the seamount mainly emitted lava flows of prevailing basaltic and minor trachyandesitic composition between 1.07 Ma (Trua et al., 2002) and, on the basis of our data, about 7 ka. In the last 7 ka, ashes of evolved composition (trachytes) associated to explosive, submarine strombolian-like eruptions were erupted. These products are responsible for the formation of the NNE–SSE aligned pyroclastic cones located along the main axis of the seamount (Sections 5.2 and 5.3; Iezzi et al., 2014). The younger MS lava flows, which mantle the top of the seamount between 550 and 800 m b.s.l. have also a trachytic composition (Trua et al., 2002). Therefore, a change in the MS plumbing system occurred with decreasing time. This change may be due to different processes including (a) a decrease in the spreading rate, (b) a variation in the stress field, and (c) a decrease in the output rate. Both (a) and (b) processes affected the Marsili back-arc basin and MS. The spreading rate decreased from 2.8–3.4 mm/a to 0–1.8 mm/a in the last 0.78–1 Ma (Cocchi et al., 2009), and, at about 0.8–0.5 Ma, the Southern Tyrrhenian Sea opening virtually stopped (Goes et al., 2004). These processes may have favored, according to models of magma

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Fig. 11. Conceptual model of the processes occurring in the MS plumbing system during the last 7.2 ka.

storage in slow spreading ridges (Wright et al., 2012 and references therein), a decrease in the magma supply rate and the formation of vertically stacked reservoirs at the crust/mantle interface and within MS. The occurrence of vertically stacked reservoirs at MS is supported by the available geophysical and geothermobarometric data, as previously reported. In addition, a change in the stress field occurred in the Southern Tyrrhenian Sea; this change was marked by a transition from a purely extensional regime associated to the opening of the Marsili back-arc (2–1 Ma) to a present-day strike-slip and compressive regime affecting the southern and western boundaries of the Marsili basin (De Astis et al., 2003). This major change occurred in the last 0.5–0.8 Ma, when the convergence of the African plate in Sicily produced thrusting in the Southern Tyrrhenian Sea (Goes et al., 2004). Accordingly, the average output rate of MS is 3.9 ± 0.2 10− 3 km3/a, a value consistent with that of arc volcanism (10− 5 to 10− 3 km3/a) and not with spreading ridges (10− 2 to 10− 1 km3/a) (Ventura et al., 2013). The above summarized data and results suggest, following the classification of magmatic systems along plate boundaries by Acocella (2014), that the MS volcanism records the evolution from a slow spreading ridge to an arc system. This may be due to the decline of back-arc spreading processes in the Southern Tyrrhenian Sea.

b)

c)

d)

6. Conclusions e) The results of this study may be summarized in the following main points: a) The Marsili1 sequence contains five tephras related to the eruptive activity of the Marsili Seamount (Central Tyrrhenian) and of the Etna volcano (Sicily). The Marsili eruptions occurred between 2.1 and 7.2 ka B.P. Two Marsili tephra (M1 and M2) are well correlated to already known tephra (TEPH01 and TEPH02), whereas the M3 and M4 tephra record previously unknown eruptive events. We conclude that at least four explosive eruptions occurred at Marsili seamount in the last 7 ka. The Etna-related M5 tephra represents the less evolved distal counterpart of the Unit D. In particular, M5 tephra is ascribable to an unknown explosive event occurred during the Ellittico-caldera formation, the distal

equivalents of which were recognized in the northern direction respect to the volcano. The M1–M4 tephras record the emplacement of fall deposits. The coarse ashes characterizing the tephras consist of poorly vesiculated (vesicles b30 vol.%) scorias indicative of strombolian-like submarine eruptions. The source vent(s) of the M1 and M2 tephras are probably represented by the NNE–SSW aligned pyroclastic cones located at a distance between 0.8 and 1.5 km west of Marsili1 core, in the central sector of the seamount. The source vents of the M3 and M4 tephra are, at the present, unknown. The composition of M1–M4 tephras, with HK-calcalkaline affinity, depicts a nearly continuous evolution trend from basaltic trachyandesites to trachytes. The M1 tephra is trachytic in composition, whereas the M2 and M3 are basaltic trachyandesites. The M4 tephra composition range from basaltic trachyandesites to trachytes. Two basaltic trachyandesites characterized by different ratio of incompatible elements are consistent, according to geochemical data on lava flows, with an origin from a heterogeneous mantle source affected by different degree of LILE enrichment by subduction-related fluids. The compositions of the predominant phases in the M1–M4 tephras also confirm the calcalkaline affinity of the tephras characterized by fractionated crystallization with M1 tephra being the most evolved term. In the last 7.2 ka, the early eruption (M4) was triggered by the arrival of a basaltic trachyandesite within a normally zoned magma reservoir in which fractional crystallization processes operate. The successive activity resulted from the filling and successive emptying of a reservoir in which magma with a basaltic trachyandesite composition evolves with a dominant fractionation processes. In this reservoir, the degree of evolution increases with decreasing time. The more evolved magma derives from the fractionation of un-erupted residual melts stored in the reservoir. This mechanism is compatible, according to the available geophysical data, with the occurrence of two reservoirs located at different depth. The formation of reservoirs at the Moho depth and within the MS edifice is related to a decrease in the back-arc spreading rate and evolution of the Southern Tyrrhenian Sea towards an arc system.

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Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.gloplacha.2015.07.005.

Acknowledgments We thank the GPC reviewer and S. Cloetingh for the comments, suggestions, and editorial handling. We would like to thank Giorgio Tranchida for the grain size analyses and Roberto dè Gennaro for his skilled assistance during the SEM–EDS acquisition. We also acknowledge Filippo Terrasi for the AMS14C dating. This research is supported by CNR funds to IAMC and by IYPE-UNESCO accredited ‘Creep’ project awarded to G. Ventura. We thank the colleagues of INGV and IAMC, and G. Iezzi of Chieti University for the numerous discussions on the Marsili volcanic complex and the geodynamics of the Southern Tyrrhenian Sea. We also thank G. Milano of OV-INGV for the perceptive observations on the topics developed in this study, and the technical team of RV Urania for the help and support. References Abelson, M., Agnon, A., 2001. Hotspot activity and plume pulses recorded by geometry of spreading axes. Earth Planet. Sci. 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