Understanding Magma Evolution at Campi Flegrei (Campania, Italy) Volcanic

Transcription

1 1 2 Understanding Magma Evolution at Campi Flegrei (Campania, Italy) Volcanic Complex Using Melt Inclusions and Phase Equilibria 3 4 Cannatelli C. a, *, Spera F.J. a, Fedele L. b, De Vivo B. c a Department of Earth Science and Institute for Crustal Studies, University of California, Santa Barbara, CA USA b Department of Geosciences Virginia Tech, 4044 Derring Hall, Blacksburg, VA USA c Dipartimento di Scienze della Terra, Università di Napoli Federico II, Napoli, Italy * Corresponding author: Tel , Fax: addresses: claudia@crustal.ucsb.edu (C. Cannatelli), spera@geol.ucsb.edu (F.J. Spera), lfedele@vt.edu (L. Fedele), bdevivo@unina.it (B. De Vivo)

2 25 Abstract The magmatic evolution of Campi Flegrei (Italy) has been investigated using thermodynamic modeling (MELTS) and data from melt inclusions in phenocrysts from the Fondo Riccio (9.5 ka) and Minopoli 1 (11.1 ka) eruptions. Thermodynamic modeling enables one to test possible petrogenetic scenarios by providing constraints on eruptive mechanisms. Assuming isobaric fractional crystallization is the dominant process, major element evolution and corresponding changes in the physical and thermodynamic properties of the magma bodies from which Fondo Riccio and Minopoli 1 magmas were erupted can be tracked. Using olivine hosted melt inclusions as representative of parental melt from which the eruptive products of Fondo Riccio and Minopoli 1 were derived, the physical conditions (pressure, temperature, oxygen buffer, dissolved water content of melt, melt density, compressibility and viscosity) and crystallization path have been modeled. Results are compared to observed crystal, whole rock and homogenized melt inclusion (hosted in olivine and clinopyroxene) compositions, to evaluate the extent phase equilibria modeling can reproduce observations under the imposed conditions. The simulations show that Fondo Riccio parental magma was likely trachyandesitic, approximated by the composition of MI s in olivine (SiO 2 = 46.8%, MgO = 9.45 %), which evolved mainly through fractional crystallization at low pressure (P 0.2 GPa, 6 km depth), along the QFM±1 oxygen buffer with an initial dissolved H 2 O content of circa 3 wt%. Minopoli 1 parental magma was also trachyandesitic and it is approximated by the chemistry of MIs in olivine (SiO 2 = 47.8%, MgO = 9.37%) as well. The estimated mean pressure of crystallization of P 0.3 GPa ( 9 km depth) and oxygen fugacity (along QFM+1) is similar to that of FR although its initial H 2 O content of ~ 2 wt% is slightly less than that of Fondo Riccio. Phase equilibria modeling also suggest that mafic parental magma crystallized by about 50% to generate the more evolved (erupted) compositions. Melt inclusions in olivine phenocrysts, the first phenocryst predicted to crystallize, evidently represent fossil remnants of the parental magma. Melt inclusions within later formed clinopyroxene phenocrysts do not

3 appear to represent equilibrium liquids trapped along the liquid line of descent suggesting that reaction between trapped melt and clinopyroxene may be important. The relationship between fraction melt and temperature reveals the presence of a pseudo-invariant temperature, T inv = 880 for FR. The fraction of melt decreases abruptly at T inv due to simultaneous crystallization of alkali feldspar and plagioclase. The melt density, viscosity and dissolved water content change abruptly in a very small temperature interval around T inv. At this temperature, the volume fraction of exsolved H 2 O present within magma changes from less than 10% to more than 60 vol % and exceeds the fragmentation limit of circa 60 vol% for Fondo Riccio differentiated parent melt. In the case of Minopoli 1, simulations do not point to abrupt invariant temperature behavior but instead melt fraction (f m ) varies from 0.5 to 0.2 in a temperature span of 90 C (around 990 C), due to the crystallization of alkali feldspars, plagioclase and biotite. The different eruptive style of Fondo Riccio and Minopoli 1 may be related to their different volatile contents (especially water) in agreement with H 2 O contents measured by EMPA and SIMS for both eruptions. Fondo Riccio s explosive eruption occurred more centrally in the CF region, whereas Minopoli 1 eruption occurred along a fissure influenced by the regional fault system in the northern portion of the CF complex graben-caldera. A simple thermal model based on variation of enthalpy of the system along the liquid line of descent allowed us to estimate the duration of the differentiation event, suggesting a timescale for Fondo Riccio of 6.5 ± 3.5 kyr and for Minopoli of 2.5±1.5 kyr from the beginning of fractionation until eruption

4 Introduction Campi Flegrei (Italy) is the most active magmatic system in the Mediterranean region and has exhibited predominantly explosive volcanic activity for more than 300,000 years (Pappalardo et al., 2002). The area is well known for its intense hydrothermal activity, frequent earthquakes and long history of bradyseism including the recent episodes in and The city of Naples and surroundings, with ~4 million inhabitants, represents one of the most densely populated and volcanically active areas on Earth. The origins of Campi Flegrei s explosive volcanism have been the focus of intense research for hundreds of years and is still debated today (Di Girolamo et al, 1984; Rosi and Sbrana, 1987; Barberi et al., 1991; Pappalardo et al., 1999; De Vivo et al., 2001; Rolandi et al., 2003; De Astis et al., 2004; Marianelli et al. 2006; Bodnar at el., 2007; Di Vito et al., 2008; Lima et al., 2009). Explosive volcanic eruptions constitute a challenge for volcanologists because of their unpredictability; identification of the parameters determining the style of an eruption is of fundamental importance in efforts to understand how explosive volcanoes work. Development of models for volcanic eruption forecasting require information on the pre-eruptive chemical and physical characteristics of the magmatic system (Anderson et al., 2000; Webster et al., 2001; Roggensack et al., 2001; De Vivo et al., 2005; Metrich and Wallace 2008; Moore 2008). In particular the pre-eruptive composition of the magma before the eruption, including its dissolved volatile content, is of critical importance because composition exerts a fundamental control of magma properties and hence the style of eruptive events (Anderson, 1976; Burnham, 1979). The exsolution and expansion of volatiles (especially H 2 O) provides the mechanical energy that drives explosive volcanic eruptions. The physical properties of magmas, such as density and viscosity, (Lange 1994; Ochs and Lange, 1999; Spera et al, 2000) along with the pre-eruptive phase equilibria (Moore and Carmichael, 1998) are strongly influenced by the dissolution of volatiles in magma and affect the volcanic style of a magmatic system (Sparks et al., 1994).

5 Melt inclusions (MI) are a powerful tool to investigate the pre-eruptive magma composition since they potentially retain the pristine composition of the magma at the time of trapping (Roedder 1984). The original volatile content of magma can be estimated by analyzing melt inclusions (MI) contained in phenocrysts (Anderson, 1974; Clocchiatti, 1975; Roedder, 1979; Belkin et al., 1985; Sobolev, 1990; Lowenstern, 1994; Anderson, 2003; De Vivo and Bodnar, 2003; Wallace, 2005). Moreover, MI provide information concerning crystallization and mixing histories of magmas and also the conditions of primary melt generation and extraction (Roedder, 1984; Carroll and Holloway, 1994; Lowenstern, 1994; Sobolev, 1996; Danyushevsky et al., 2000; Frezzotti, 2001). In the present work we examine the origin of magma erupted during the Fondo Riccio, FR (9.5 ka) and Minopoli 1, M1 (10.3 ka) events by deriving constraints imposed from phase equilibria embodied in the MELTS thermodynamic model (Ghiorso and Sack, 1995), from phenocryst and glass compositions and from an analysis of MI s. Using olivine hosted melt inclusions as representative of parental melt that generated the eruptive products of Fondo Riccio and Minopoli 1, estimates of the pressure, temperature, oxygen buffer, density and viscosity can be made assuming fractional crystallization was the dominant process of geochemical evolution. Similarly to the Campanian Ignimbrite study (Fowler et al., 2007), an important aspect of our result is the identification of a pseudo-invariant temperature (T inv ) along the liquid line of descent in the case of the FR system. At this temperature, the melt fraction dramatically increases isothermally and the volume fraction of the coexisting fluid phase rises accordingly. Additional melt properties such as density and melt viscosity also undergo rapid variations. The net effect of these changes is to drive the system towards dynamic instability, the culmination of which leads to eruption (Fowler and Spera, 2008). While for Fondo Riccio, T inv = 880 C, the temperature interval for significant change for the Minopoli 1 eruption is much broader, around 70 K and may correlate with the less explosive nature of the Minopoli 1 eruption. A simple thermal model based on variation of enthalpy of the system along the liquid line of descent that affords an estimate the timescale between the start of

6 significant crystallization and the time of eruption is also presented. We apply the model to explain the timescale and the mechanisms of the magmatic systems for Fondo Riccio and Minopoli Volcanological background Campi Flegrei Volcanic District (CFVD) is a large volcanic complex (~ 200 km 2 ) located west of the city of Naples, Italy (Fig.1). Multiple eruptions have occurred in this area in the last 300 ka (Pappalardo et al., 2002), as well as intense hydrothermal activity, bradyseismic events and frequent earthquakes. Two major eruptions in the CFVD include the 39 ka Campanian Ignimbrite (CI) ((Rosi and Sbrana, 1987; Orsi et al., 1996) and the 15 ka Neapolitan Yellow Tuff (NYT) (Deino et al., 2004). De Vivo et al. (2001) and Rolandi et al. (2003) suggested that most eruptive centers align along fractures activated along the neotectonic Apennine fault system parallel to the Tyrrhenian coastline. They argue that eruptions from >300 ka to 19 ka are not confined to a unique volcanic center or isolated vent system in Campi Flegrei as suggested by Rosi and Sbrana, 1987 and Orsi et al., Rolandi et al., (2003) argued that only the Neapolitan Yellow Tuff (NYT) (15 ka, Deino et al., 2004) erupted from vents within Campi Flegrei, whereas the CI (39 ka, DeVivo et al., 2001) has a much wider source and dispersal area. According to Pappalardo et al. (2002), the interval between the CI and NYT eruptions is characterized by a large number of significantly smaller magnitude volcanic events. Since the NYT eruption, margins of the region have been the site of at least 65 eruptions, divided in three periods of activity. Eruptions were separated by quiescent periods marked by two widespread paleosols (Di Vito et al., 1999). The last eruption in 1538 A.D. formed the Monte Nuovo cone (Di Vito et al., 1987) after 3.4 ka of dormancy. In this paper we analyze the Fondo Riccio (FR) and Minopoli 1 (Mi1) eruptive products in an effort to deduce their petrogenesis. The Fondo Riccio eruption was explosive with a strombolian

7 character and occurred at kyr (D Antonio et al., 1999) from an eruptive centre on the western side of the Gauro volcano, near the centre of the Phlegrean caldera (Fig 1). The eruptive deposits are limited to the vent area and lie above the Paleosol A and below the Montagna Spaccata Tephra. The eruptive products consist of fallout deposits composed of very coarse scoria beds with subordinate coarse ash beds (Di Vito et al., 1999). According to Di Vito et al. (1999) the earlier Minopoli 1 eruption, occurred ka, and was strombolian with subordinate phreatomagmatic phases, while Di Girolamo et al. (1984), based on the degree of dispersal of Minopoli 1 s products, define this eruption as sub-plinian. The deposits are limited to the vent area formed by scoriae horizons with a composition varying from latitic to alkali-trachytic. The eruptive products are composed of alternating pumice lapilli fallout and mainly massive ash fallout beds and subordinately cross laminated ash surge beds, rich in accretionary lapilli (Di Vito et al., 1999). The Minopoli 1 eruption has a stronger phreatomagmatic component than the closely related FR eruption based on the observed volcanic stratigraphy Sample description and analytical techniques The locations of the samples utilized in this study are indicated in Figure 1. Here we give petrographic and mineralogical descriptions of the samples and describe the methods used to perform the analysis Petrography and chemical composition of Fondo Riccio For Fondo Riccio, CF-FR-C1 was collected at the top of the stratigraphic column and is a well-vesciculated scoriae containing less than 20% by volume of phenocrysts. The phenocrysts include olivine, clinopyroxene, spinel (magnetite), biotite, alkali feldspar and plagioclase. Biotite occurs as large crystals (typical size ~ 2-3 mm), while apatite phenocrysts occur as small ( ~ 0.1

8 mm) acicular needles. Clinopyroxenes and feldspars commonly exhibit intergrowth textures, suggesting cotectic crystallization. Olivine, clinopyroxene and plagioclase contain recrystallized melt inclusions (MI), while alkali feldspar phenocrysts contain apatite inclusions. Sample, CF-FR- C2, is a bomb, relatively unvesciculated, containing olivine, clinopyroxene, apatite, spinel, biotite, alkali feldspar and plagioclase. Olivine, clinopyroxene and alkali feldspar phenocrysts contain recrystallized MI s. Petrochemically, both samples are porphyritic latite with less than 20% phenocrysts, with clinopyroxene and plagioclase often found in glomeroporphyritic clots; clinopyroxene and plagioclase also occur as microlites in the groundmass. In the FR samples, olivine phenocrysts range between Fo 84 and 87, and pyroxene lies in the diopside-salite field on the pyroxene quadrilateral, with Wo and Fs Based on microprobe analyses, alkali feldspars in Fondo Riccio present a unimodal distribution with Or component of ~ 79 to 88. Plagioclase crystals are zoned with An component ranging from ~ 72 to Petrography and chemical composition of Minopoli 1 For Minopoli 1, CF-MI1-C1 was collected in the Casalesio area (Fig 1), and corresponds to the base of the deposit. The sample is greyish-black scoriae, of trachybasalt composition containing ~ 20% phenocrysts of olivine, clinopyroxene, plagioclase, alkali feldspar, spinel (magnetite), apatite and biotite. Olivine phenocrysts are weakly to unzoned with average Fo content ~ 78, while pyroxenes present Wo values between 47 and 49 and Fs between 8 and 19. Based on microprobe analyses, alkali feldspars in Minopoli 1 present a bimodal distribution of Or values which ranges from ~ 50 to 80. Alkali feldspars exhibit zonation, with higher Or cores. Plagioclase crystals are highly zoned presenting a bimodal distribution with a range from ~ 46 to 87 and peaks at 53 and Melt Inclusions description

9 The MI s present in both FR and M1 are generally devitrified and partially recrystallized, present a bubble (shrinkage ± exsolution of volatiles) and daughter minerals (generally apatite and oxides). MI s generally have elongated ellipsoidal shapes and range from 30 to 80 µm (most between 20 and 50 µm). In order to be analyzed, MI s needed to be re-heated to a homogenous glass. Detailed descriptions of melt inclusion reheating procedures, sample preparation and analytical methods are elsewhere (Cannatelli et al., 2007 and reference therein) Analytical methods Major and minor elements analyses of phenocrysts were performed in the Department of Earth Science at University of California, Santa Barbara using a Cameca SX-50 electron microprobe equipped with five wavelength dispersive spectrometers. Phenocrysts analyses were performed using a 1µm focused beam at 15 kev accelerating voltage and a beam current of 15nA. Uncertainty of analyses was in the order of 1% for most elements. Quantitative electron microprobe analyses (EMPA) on phenocrysts and MIs were performed at Virginia Tech and at University of Rome La Sapienza (IGAG-CNR, Rome, Italy) on a Cameca SX-50 equipped with four wavelength dispersive spectrometers. The analytical scheme for MIs was chosen for major/minor oxide analyses. Analysis of SiO 2, TiO 2, Al 2 O 3, FeO, MnO 2, MgO, CaO, Na 2 O, K 2 O, NiO, Cr 2 O 3, P 2 O 5, and Cl, S and F and standardization were preformed using silicate, oxide, phosphate and glass standards, and the data were corrected with the PAP method, developed by Pichou and Pouchoir (1985), using vendor supplied software. Analyses were performed at 15 kv, using a current of 20 na with a defocused beam diameter of 10 µm and counting time 10 seconds, as recommended by Morgan and London (1996). Relative one-sigma precision is estimated to be 1 to 2 % for major elements and 5 to 10 % for minor elements. In each analytical run, alkalis were counted first, and no correction has been made for Na loss. Test runs made prior to the beginning of the analysis on

10 synthetic and natural glass standards of known composition showed no significant alkali migration under the specified analytical conditions. Selected MI s were analyzed for H (reported as H 2 O), light and rare earth elements by Secondary Ion mass Spectrometry (SIMS) at the Woods Hole Oceanographic Institution, using techniques detailed by Shimizu and Hart (1982) and Webster et al. (1996). Accelerating potential was 10 kv and beam current was 1-2 na. Precision and accuracy were monitored with NBS (National Bureau of Standards) reference glasses NBS 610. Results on the NBS glasses are similar and within 5% of the accepted values; H 2 O concentrations are reproducible to to 0.4 wt% and trace elements to 5 to 15% (for more details see Webster et al., 2001) Phase equilibria modeling Procedures to select the parental melt composition Phase equilibria modeling has been carried out using the software MELTS, a thermodynamic model of crystal-liquid equilibria. The MELTS algorithm is based on classical equilibrium thermodynamics and has been object of extensive reviews in the past years (Ghiorso and Sack, 1995, Asimow and Ghiorso, 1998). The use of MELTS to reconstruct the crystallization path of a magma requires specification of initial conditions, including 1) the initial state of the system (parental melt composition including H 2 O content, starting temperature and pressure, and oxygen fugacity) and 2) constraints under which the magmatic evolution proceeds (open or closed system, fractional or equilibrium crystallization, minimization of appropriate thermodynamic potential based on imposed constraints). In this work we investigate isobaric crystallization scenarios and explore both equilibrium and fractional crystallization scenarios. By a long shot, fractional crystallization generates results closer to observations (see below).

11 The search of parental melt composition starts with the assumption that MI s within phenocryst phases can be related to a unique parental melt during cotectic (olivine +clinopyroxene) crystallization. The graphical method developed by Watson (1976) is used to test the hypothesis that MI s are primary or nearly so. MI s composition(s) of interest are further culled by selecting ones that exhibit the lowest concentrations of incompatible trace elements and highest MgO contents as input for the phase equilibria calculations. In the case of Fondo Riccio, 7 MI s were selected, hosted in olivine and pyroxene and have been plotted on a CaO-MgO-Al 2 O 3 coordinates, as described by Watson (1976). The intersection I (Figure 2a) of olivine and clinopyroxene fractionation lines is in the field occupied by FR-C1-o6 M1, a melt inclusions hosted in olivine O6. This melt inclusion represents the predicted composition of the melt at the cotectic point, where olivine and clinopyroxene crystallize simultaneously, so it is reasonable to hypothesize that the Parental Melt (PM) composition should be more primitive than FR-C1-o6 M1. The MIs FR-C1-o2 M1 (9.45 wt% MgO), and FR-C1-o1 M1 (8.05 wt % MgO) possess high MgO contents and the lowest concentration of incompatible trace elements and are consequently considered the best candidates to represent PM. We carried out phase equilibria calculations using FR-C1-o1 M1 (not shown) and FR-C1-o2 M1 and differences were small; based on this we decided to select the one with the highest MgO content. In the case of Minopoli 1, by applying the Watson graphical method we found that Mi1-C1-P8 M1, a MI hosted in the clinopyroxene P8 (fig 2b) represents the composition of the melt at the cotectic point. We selected the parental melt composition choosing the MI with the highest MgO content and lowest incompatible trace element concentrations as an approximation to the PM. The MI that best fit the criteria and was closest to Mi1-C1-P8 M1 in Fig. 2b was hosted in olivine o5 with a MgO content of 9.37 wt%, and values of Ce, and Nd of 69 and 61ppm. It is probable that MI s in olivine can undergo some re-equilibration with the host (Danyushevsky and co-workers, find reference; Kress and Ghiorso, 2004). However in our case the MELTS results agree very well with the compositions for the MI s in olivine for both FR amd Mi1 samples. Our interpretation of

12 these relations is that that post entrapment changes for these MI s are small to negligible. We conclude that the method espoused 35 years ago by Watson is indeed useful and that by careful use of MI s one can in this circumstances estimate the parental melt composition reasonably well Phase equilibria constraints To reconstruct the magmatic evolution the initial state of the system, devolatilized PM composition, dissolved H 2 O content of PM, initial temperature, pressure, and oxygen buffer are specified. Here we present results of closed system isobaric fractional crystallization where the Gibbs energy is the appropriate thermodynamic potential to be minimized. These runs clearly show the effects of varying pressure, f O2 and the initial water content of the parental melt on the liquid line of descent and on the composition and abundance of all crystalline phases and the temperature at which melt becomes water saturated. After setting P, f O2 and dissolved H 2 O content, we compare predicted phase and melt compositions to those observed in order to determine the range of physical conditions leading up to eruption for FR and M1. We selected the best case based on correspondence between mineralogical and geochemical data and the phase equilibria calculations. Calculations were rejected when the deviation between observation and model was deemed too large. This is a judgment that will naturally vary from petrologist to another; there is no ab initio method to judge closeness, although an experienced petrologist will be able to spot a poor solution, one that provides no new insight into the petrogenesis of the system. One must keep in mind the assumptions of the method and the realities of Nature. For example, the computation assumes perfect fractional crystallization. However, in situations where crystals are removed from liquid by some physical process such as crystal settling or liquid filter pressing, there will always be some reaction between earlier formed crystals and ambient liquid. Similarly, the calculation assumes there is a single parental composition from which all differentiated liquids develop. It is easy to imagine that compositional heterogeneities would be present a priori even if convective mixing was reasonably efficient. Additionally, the calculation assumes that crystallization is

13 isobaric, exactly. The approximate nature of this assumption should be clear to anyone who ever mapped a pluton in rugged terrain. The point of performing phase equilibria calculations using an imperfect thermodynamic model (no thermodynamic model is perfect) with constraints that are obviously approximate is to evaluate the overall reasonability of the proposed scenario. If, for example, crystallization is grossly polybaric, then no isobaric model will come close to reproducing observed phase compositions, abundances and glass (melt) compositions. One could then perform a 306 constrained polybaric simulation and ask if that procedure produces better agreement. If assimilation plays an important part of the petrogenesis, then no closed system phase equilibria model will produce satisfactory correspondence to observations and one would rightfully seek to explore petrogenetic models involving significant assimilation and reaction. In this study (see below) we find that isobaric closed system fractional crystallization at low pressure produces results that bear a close (but not perfect) correspondence to observed relations and that the implications of the calculation suggest a causative link between crystallization and magma eruption (see below) Fondo Riccio The initial water content in the parental melt has been estimated starting from the values obtained for MI s by SIMS analyses. Fondo Riccio s MI s belong to two different populations of inclusions, one with water contents ranging between 1 and 4 wt% and the other with water values around 6 wt%. As starting water content we tested values ranging between 1 and 5 wt%, but from petrographic observations values of H 2 O >3wt% were discarded because of the high water saturation temperature. For example, in the case of H 2 O = 4wt% the temperature of water saturation was 1070 C. At this temperature the system is saturated in water and crystallizing mineral phases such as clinopyroxene, plagioclase and alkali feldspar should trap fluid inclusions during the cooling process. There is no petrographic evidence of fluid inclusions hosted in these phases in the samples studied here. In the cases of H 2 O < 2 wt%, each run generated a rhombohedral oxide phase (ilmenite) at low melt fractions, inconsistent with the phase assemblage observed. Although not

14 shown, calculated runs with initial water content in the PM less than 2 wt% and greater than 4wt% did not predict the phase assemblage observed in the Fondo Riccio. We therefore conclude that initial water content in the PM around 3 wt% is the most realistic case for the Fondo Riccio eruptive model. Although we acknowledge that this is a judgment, we believe it to be the best estimate based on the congruence between calculation and observation. The majority of the runs were made isobarically and for Fondo Riccio at P < 0.3 GPa; at greater P the presence of predicted minerals such as garnet or muscovite is not compatible with the FR phenocryst assemblage. To understand better the effect of a changing pressure on our system, we compared MELTS generated TAS diagrams calculated at a fixed f O2 = QFM+1, QFM and P = 0.1, 0.15, 0.2 and 0.3 GPa. For the case of f O2 = QFM and QFM+1 we observe good agreement between phase equilibria (MELTS) predictions with the Fondo Riccio s data (see Fig. A in Supplementary Material). The best case scenario of oxygen fugacity for Fondo Riccio was chosen for P 0.2 GPa, corresponding to ~6 km depth, and compatible with recent studies by Zollo et al., 2003 suggesting that a hypothetical magma body at Campi Flegrei is about 6 km deep. From petrographic investigation we found the presence of spinel (in the form of magnetite solid solution) in olivine and clinopyroxene, but not in plagioclase and feldspars. We also noticed an abundant presence of biotite. We compared several MELTS generated mineral distribution diagram with our petrographic observations and we found that the best agreement is reached when f O2 varies between QFM-1 and QFM+1. We also noticed, as expected, the strong dependence of the ironbearing phases on the variation of oxygen fugacity. For example, when we consider the case of Fondo Riccio with initial water content of 2wt%, an increase in the oxygen fugacity from QFM-2 to QFM+2, stabilizes spinel at higher temperature, while not affecting the crystallization temperature of clinopyroxenes and feldspars (see Fig. B in Supplementary Material). The stabilization of spinel at higher temperatures corresponds to a decrease of FeO tot and increase of SiO 2 content in the melt. Our choice of best case has been mostly influenced by the spinel stabilization temperature; the

15 inconsistency between observed mineral assemblage and MELTS generated mineral distribution has lead us to discard oxygen fugacity values of QFM-2, QFM-1 and QFM+2. In summary, the physical conditions that produce the closest correspondence between the model and observation is fractional crystallization of a parental melt of (anhydrous) composition (given in Table 1 Supplementary Material) with 3wt % H 2 O added at pressure of 0.15 GPa and oxygen fugacity around the QFM buffer Minopoli 1 Water contents of MIs from the Minopoli 1 eruptive products were measured by SIMS and range from 1 to 4wt% (Cannatelli et al., 2007). The effect of varying the initial water concentration in the parental melt was examined in the Minopoli case through isobaric fractional crystallization, similarly to FR. Petrographic studies of Minopoli 1 s thin sections reveal the presence of large (1-2 mm) biotite crystals. The presence of such crystals implies initial water contents greater than 2 wt%. Therefore simulations obtained by setting the water content less than 2wt% were discarded, regardless of oxygen fugacity and pressure values. Furthermore, in the case of H 2 O > 2wt% we observed a lack of intersection between the MELTS generated oxides trends and the real data field of Minopoli 1. In particular, values of water content greater of 3wt% were discarded for f O2 = QFM QFM+2 and pressure greater than 0.3 GPa, because of the predicted presence of garnet and leucite, inconsistent with the observed assemblage. Values of water greater than 4wt% were discarded because of the high water saturation temperature (T ~ 1080 C) which would result in the presence of fluid inclusions in the phenocrysts of Minopoli 1 sample, not observed in Minopoli 1. The initial water content of the parental melt for Minopoli 1 is around 2 wt% at bit lower than for FR. Several simulations were carried out using a fixed value of initial water content of 2-3wt%, and varying the pressure and the oxygen fugacity. Many runs were discarded because of mismatch between observed and predicted phases, such in the cases of f O2 > QFM or P 0.1GPa. A small

16 decrease in oxygen fugacity leads to a decrease of spinel stabilization temperature of almost 100 C and a longer crystallization interval for feldspars with a consequent greater generated mass of feldspars in the mineral assemblage. Comparisons among feldspars plotting model results and observations on ternary diagrams (An-Ab-Or) and spinel diagrams (FeO-Fe 2 O 3 -TiO 2 ) were conducted in order to establish the best fit between observation and prediction. In general higher pressures better match observed phases (Fig. C in Supplementary Material). In particular, for f O2 = QFM+1 as starting oxygen fugacity value, we can observe a the good fit of generated spinel and feldspars data with the observed Minopoli 1 phenocryst compositions especially for P=0.3GPa and water content of 2 wt%. The best case chosen from the several Minopoli 1 simulations is represented by a parental melt of (anhydrous) composition (Table 1 in Supplementary Material) at pressure P ~ 0.3 GPa (6-9 km depth), water content of 2 wt % along the QFM+1 oxygen buffer Results Fondo Riccio We present results for Fondo Riccio for isobaric fractional crystallization of the estimated parental composition. In fact, we have used a number of possible parental compositions and although small differences in results are obtained, the salient features are robust. The parental melt composition of FR-C1-o2 M1 with an initial water content of 3 wt% is used to generate the results below. The fractional crystallization path along the QFM to QFM+1 oxygen buffer at 0.15 GPa has been computed. MELTS correctly predicts the mineral phases observed. Olivine is the liquidus (T= 1260 C) phase, followed by clinopyroxene, magnetite, H 2 O, plagioclase, Alkali feldspar and biotite at 1110 C, 1100 C, 1070 C and 880 C respectively. Mineral distribution, abundances and temperature at which water saturates are shown in Fig. 3. It is interesting to note the abrupt change

17 in melt composition around 880 C due to a simultaneous crystallization of alkali feldspar, plagioclase and biotite. As in the case of the Campanian Ignimbrite, we can define this temperature as pseudo-invariant point temperature (Fowler et al., 2007). At this temperature, a major change in melt fraction (f m ), from 0.5 to 0.1 and melt composition occurs (Fig. 3). The properties of the melt (density and shear viscosity) and of magma (density, volume fraction of bubbles, shear viscosity) change dramatically around this temperature (see below). The growth of alkali feldspars and plagioclase dominates the crystallization path at T ~ 880 C and below. In fig.4 crystallization patterns for f O2 = QFM and QFM+1 are portrayed. Concentrations of SiO 2, K 2 O, Na 2 O and Al 2 O 3 initially increase with decreasing MgO due to the crystallization of olivine and continue to increase as clinopyroxene, spinel and apatite crystallize (Fig. 4a-f). The increase of CaO concentration ends when melt becomes saturated in clinopyroxene and then decreases slowly with cooling. FeO tot concentrations slightly decrease in the early stages of crystallization and decrease abruptly when spinel joins the already fractionated phase minerals. Results for QFM and QFM+1 are quite similar for Al 2 O 3, K 2 O and Na 2 O while for SiO 2, FeO tot and CaO we can observe a more close approximation to the observed trends for f O2 =QFM+1. At T=T inv there is a rapid change in the variation diagram trajectories of circa 2 wt% for SiO 2 and Al 2 O 3, 1 wt% for K 2 O and 0.5 wt% for CaO and Na 2 O (Fig.4). For T<T inv SiO 2, CaO, Al 2 O 3 and K 2 O show a sudden decrease, while Na 2 O continues to increase as a result of feldspar fractionation. These compositional changes at T=T inv are associated to a change in the physical properties of both melt and magma with significant consequences for eruption probability and dynamics. As noted on Fig. 4, MI s hosted in olivine and clinopyroxene agree well with the predicted liquid line of descent making melt inclusions, especially hosted in olivine. There is a good agreement between observed and simulated clinopyroxene and olivine compositions (Fig. D in Supplementary Material) remembering that calculated values assume perfect fractional crystallization. Alkali feldspar trends compare favourably; predicted plagioclase becomes more sodic than observed values near the solidus presumably related to the breakdown of the assumption

18 of perfect fractional crystallization near the solidus. In addition, differences were noted for perfect fractionation of both crystals and exsolved fluid or just solid crystallization. In general, the best agreement was found for the case when both precipitated solids and exsolved H 2 O were removed in fractional crystallization. The spinel ternary diagram also show good agreement between MELTS predictions and spinel compositions from FR samples Minopoli 1 Based on the assumption that the most realistic parental melt has a composition of Mi1-C1-o5- with water content of 2-3 wt%, we present results for calculations with oxygen buffer set at QFM+1 and pressure at GPa. In Fig. 5 we can see the mineral distribution along the crystallization path for the case P= 0.3 GPa, 2wt% H 2 O and f O2 = QFM+1. The liquidus phase is olivine at T= 1300 C, followed by clinopyroxene (T= 1160 C), spinel (T=1070 C) and apatite (T=1020 C). At 990 C, 960 C and 900 C respectively plagioclase, biotite and alkali feldspar join the mineral assemblage, dominating the crystallization path. From Fig. 6a-f, concentrations of SiO 2, Al 2 O 3, K 2 O and Na 2 O increase with decreasing MgO during the crystallization of olivine and then continue to increase as clinopyroxene, apatite and spinel crystallize. The increase of CaO ends when clinopyroxene begins crystallization and then 445 decreases slowly with cooling. FeO tot concentrations slightly decrease in the early stages of crystallization, then remain constant and decrease abruptly only when spinel begins fractionation. In the case of Minopoli 1 an abrupt change in melt composition noted in Fondo Riccio is not evident; instead, in a temperature span of about 80 C (around T= 990 C) there is a change of f m from 0.5 to 0.2, due mainly to the crystallization of feldspar. At T=990 C, there are changes in the calculated oxides trends of about 3 wt% for SiO 2, CaO and K 2 O, 2wt% for Al 2 O 3 and 1 wt% for Na 2 O and FeO tot. Parenthetically, this comparative behaviour shows how sensitive phase equilibria are to small changes in melt starting composition and ambient conditions. This indicates that the approach

19 used here is not one size fits all even when differences in the systems (FR and M1) are relatively small. For T<T inv SiO 2, CaO, FeO tot and K 2 O show a sudden decrease, while Na 2 O and Al 2 O 3 continues to increase as a result of feldspar fractionation. From Fig. 6, MIs hosted in olivine phenocrysts agree with MELTS predicted liquid line of descent, while MI hosted in clinopyroxenes do not. A possible scenario in this case could be a post-entrapment diffusive re-equilibration, with the presence of trapped melt and the host crystal not in equilibrium during cooling (Qin et al., 1992; Danyushevsky et al., 2000; Cottrell et al., 2002; Michael et al., 2002). If the cooling rate is slow, the diffusive gradient in the crystal may extend to the host magma resulting in re-equilibration between the melt inclusion and the magma surrounded the phenocrysts (Gaetani and Watson, 2000). If the cooling rate is fast, such as in the case of scoriae or pumices, post-entrapment re-crystallization could take place as well and the crystallization of the host mineral on the walls of the inclusion modifies the composition of the melt inclusion between the time of entrapment and quenching. We do not have any independent evidence of post entrapment re-equilibration in melt inclusions so the cause of the divergence remains open. Good agreement is found between observed and computed clinopyroxene, plagioclase and alkali feldspar compositions (Fig. E in Supplementary Material), considering that simulated data are calculated assuming a perfect fractional crystallization at a single pressure. The closed-system model reproduces the range of observed phenocrysts compositions reasonably well. Although we cannot rule out any involvement of assimilation, there is no indication that this process played a critical petrogenetic role for either the FR or Minopoli 1 system Changes in properties at T=T inv Significant changes in properties with temperature of melt and magma can be observed in Fig. 7 and 8 respectively for Fondo Riccio and Minopoli 1. All variations in properties become more

20 significant near the invariant temperature T inv, especially for Fondo Riccio. Fig. 7a and 8a shows the variation of melt density with temperature along the liquid line of descent, where the most dramatic change of physical properties for FR and Mi1 occurs at T T inv, because the melt density decreases as a result of a temperature decrease and mass fraction of the fluid phase (! fluid ) significant increase The variation of dissolved water in the melt along the liquid line of descent can be observed in Fig. 7b and 8b. For FR, melt saturates with respect to H 2 O at 1108 C at about 4 wt % H 2 O and increases as crystallization occur and heat is extracted. At T inv the H 2 O content jumps from about 4.5 wt% to 5 wt% H 2 O and has a rate of increase of 1 wt% H 2 O per 30 C. For Minopoli 1 the saturation of melt with respect to H 2 O occurs at 800 C at about 8 wt%. Around the invariant interval the value of dissolved water jumps from 3.55wt% to 4.33 wt% and increases with a rate of 1.0 wt% per 30 C. The viscosity of melt as a function of temperature along the crystallization path is shown for both eruptions in Fig. 7c and 8c respectively. For Fondo Riccio the variation of viscosity is similar of what has been observed for the Campanian Ignimbrite (Fowler et al., 2007). Melt viscosity for Fondo Riccio system present a cusped path; a rapid increases with falling of temperature between T liquidus and T inv (due to cooling and the silica enrichment of evolved melt) and then a dramatic drop for T<T inv (due to the increasing concentration of water dissolved in melt). As we can see from fig. 8c, for Minopoli 1 the behaviour of the system is different from the FR trend: during cooling the increase of viscosity and dissolved water content with temperature is more gradual. In Fig. 7d, the volume fraction of water in the magma along the crystallization path for the Fondo Riccio is depicted, where magma has been defined as a homogeneous mixture of oversaturated melt plus bubbles of supercritical fluid (Fowler et al., 2007). The magma density was calculated according to: 503 " magma = " fluid " melt " melt # fluid + " fluid (1$ # fluid ) (1)

21 where! fluid is the mass fraction of the fluid phase in the mixture, ρ fluid is the density of exsolved H 2 O and ρ melt is the density of volatile-saturated melt. At T=T inv there is a dramatic increase in volume fraction of water, from about 15% vol to 60% vol just below T inv. The exsolution and expansion of H 2 O provides the mechanical energy that drives explosive volcanic eruptions. According to Cashman et al., (2000), a pyroclastic eruption can occur when the fluid volume fraction exceeds roughly 70% by volume at which magma fragmentation occurs. Our phase equilibria calculations are consistent with the following picture for Fondo Riccio. Isobaric crystal fractionation of parental basaltic trachyandesitic melt initially containing about 3wt% H 2 O generates a liquid line of descent consistent with melt inclusion and phenocryst compositional data. In the absence of magma decompression, the crystallization of almost 60% of the original melt and the drastic increase in the volume fraction of supercritical fluid just below T inv = 880 C, leads to an abrupt increase of the volume of the system and consequent fluid expulsion. This occurs at the same time that the liquid fraction of the system is rapidly decreasing. At this point, a fluid cap develops at the top of the magma body producing roof hydrofracture and the propagation of volatile-saturated magma filled cracks. The resultant release of pressure during decompression causes further exsolution of fluids from the melt since the solubility of volatiles decreases as pressure is reduced. As the volume fraction of fluid in the magma increases, the magma viscosity also decreases which in turn allows for even more rapid ascent. Via this mechanism of positive feedback the system becomes unconditionally unstable and an eruption ensues. For Minopoli 1 s magma, the phase equilibria calculations suggest that the system was deeper (~ 9 km depth) and drier (2wt% H 2 O) than Fondo Riccio. Unlike the case of FR, for Minopoli 1 simultaneous saturation of plagioclase, alkali feldspar and biotite crystallization took place in a temperature span of ~90 C and not isothermally at T inv as for FR. The smaller rate of change of fraction crystallized with temperature naturally leads to less abrupt changes in the melt composition, properties and physical state of the magma. A decrease in melt viscosity (from 10 5 to 10 4 Pa s),

22 coupled with a smaller change in the volume fraction of water in magma (from 0.05 to 0.2) and a decrease in melt density nevertheless drove the system towards dynamical instability and acted as a destabilizing eruption trigger. A prediction of this model is that the Minopoli 1 eruption was less explosive than that of FR. This prediction may be tested by analysis of the volcanic stratigraphy and by granulometric studies on available samples. Poor exposures make this test a difficult one to carry out although one worth trying Timescale for Fondo Riccio and Minopoli 1 magma evolution While phase equilibria modelling can constrain the thermodynamic and transport properties of magmas, the evolutionary timescale cannot be determined without additional considerations. Here we apply a simple thermal model in order to estimate the time interval between the start of fractionation and the eruption in the context of the phase equilibria model. This model can be tested using isotopic data on the various phenocryst phases, although these data do not presently exist. The timescale is estimated by determining the time it takes for sufficient heat to be removed from the magma in order to drive the geochemical evolution from liquidus to eruption temperature. That is, it is assumed that parental melt of volume V (V FR or V MI for Fondo Riccio and Minopoli 1, respectively) and density ρ loses heat at flux rate q& and that the total amount of heat that needs to be removed is the difference in enthalpy (ΔH) between the initial and final states. The fraction of parental melt volume (f m ) that differentiates to form the FR and Mi1 melt compositions and the fraction (α) of that volume that erupts to form Fondo Riccio and Minopoli 1 (respectively V EFR and V EMI ) are linked by the following: V EFR = α f m V FR V EMI = α f m V MI (3a) (3b)

23 The volume of the magma body that crystallizes can be expressed in function of surface area A and a dimensionless constant K that depends on the shape of the magma reservoir, such that A = KV 2/3. The shape of the magma reservoir can be approximated with a cubical, disk-like or spherical volume, for which 7 < K < 5. With these assumptions the timescale can be calculated as: 559 * ( FR) = th )' H & V $ Kq& % ( f EFR m #! " 1/ 3 (4a) 560 * ( MI) = th )' H & V $ Kq& % ( f EMI m #! " 1/ 3 (4b) The time t since the start of fractionation for each mineral phase is t =! H th (5) Where H is the dimensionless enthalpy and it is function of melt fraction or temperature and is defined: H = H liquidus " H (T ) #H (6) Some parameters such as!,! H and f m near the solidus are fairly constant and we choose values of 2200 kg/m 3, 1MJ/kg and 0.05 (for Minopoli 1) and 0.1 (for Fondo Riccio). At Campi Flegrei the present day heat flow ( q& ) range between 1 and 2.5 W/m 2 (AGIP, 1987; Wohletz et al, 1999; De Lorenzo et al., 2001), as measured at geothermal boreholes in Mofete and San Vito. The fraction (α) of differentiated magma that erupted to form Fondo Riccio and Minopoli 1 eruptive fields can be estimated between 0.5 and 1 (Crisp et al., 1984; White et al., 2006), which we have chosen as the maximum and minimal values. The estimated DRE eruptive volume of Fondo Riccio and Minopoli 1 is 0.16 km 3 and 0.1 km 3 respectively (Di Girolamo et al., 1984) which leads to a timescale τ of 6.5 ± 3.5 ka for Fondo Riccio and 2.5 ± 1.5 ka for Minopoli 1 (Fig. 9a-b). The values obtained for τ using the simple thermal model allow us to approximate the timescale for the fractionation process and to give an estimate of the age of each mineral phase. The more evolved compositions of Fondo

24 Riccio melt inclusions and eruptive products can be explained by the longer stationing of the batch magma in the chamber before the eruption, allowing the melt to fractionate up to 60 vol % Conclusions The present study has been conducted with the goal to reconstruct the history of Fondo Riccio and Minopoli 1 eruptions using a combination of melt inclusion data, thermodynamic and thermal modelling. The simulations were carried out using MELTS and varying the initial water content, the oxygen fugacity and the pressure. Both systems were assumed to evolve by fractional crystallization in a closed system. Melt inclusions in olivine phenocrysts, the first phenocryst to crystallize, evidently represent fossil remnants of the parental magma and were used to represent the starting composition. Parental melt for Fondo Riccio has about 3 wt% H 2 O and evolved by isobaric fractional crystallization at pressure near 0.15 GPa (equivalent to 5-6 km of depth) among QFM - QFM+1 oxygen buffer. Calculated phase equilibria along the liquid line of descent show that for P = 0.15 GPa, olivine is the liquidus phase (T liq = 1260 C), followed by clinopyroxene (1110 C), magnetite (1100 C), saturation of water (1070 C) plagioclase, alkali feldspar and biotite (880 C). The calculated oxides trend and composition of phase mineral well agree with observed melt inclusions and mineral assemblage suggesting that Fondo Riccio s system has most likely evolved by closed-system fractional crystallization. At a temperature of 880 C, the magmatic system is subject to a dramatic variation in its physical properties (viscosity, density and water dissolution) as biotite, plagioclase and alkali feldspars start to crystallize. At this temperature, an abrupt decrease in the fraction of melt from 0.5 to 0.1 occurs. The sudden decrease of viscosity and density at this pseudo invariant point temperature and the dramatic change in volume fraction of water from 0.1 to 0.6 is, we speculate, the trigger mechanism for the eruption of Fondo Riccio magma. Minopoli 1 s petrological evolution has been simulated by isobaric fractional crystallization. The starting parental composition based on MI s in olivine suggests a more primitive parent. The system, containing 2 wt% H 2 O, has evolved from pressure of 0.3 GPa and oxygen fugacity values around QFM+1. The crystallization sequence is represented by olivine (T liq = 1300 C), followed by

25 clinopyroxene (T= 1160 C), spinel (T=1070 C), apatite (T=1020 C), plagioclase (T=990 C), biotite (T=960 C) and alkali feldspar (T = 900 C). In the case of Minopoli 1, simulations have not shown invariant temperature behaviour but only a variation of melt fraction (fm) from 0.5 to 0.1 in a temperature span of 90 C (around 990 C), due to the crystallization of alkali feldspars, plagioclase and biotite. A good agreement between observed and calculated mineral compositions suggests that also Minopoli 1 has undergone to a fractional crystallization process even though melt inclusions within later formed clinopyroxene phenocrysts do not appear to represent equilibrium liquids trapped along the liquid line of descent. This suggests that reaction between trapped melt and clinopyroxene may be important. The different eruptive style of Fondo Riccio and Minopoli 1 may be related to their different volatile contents in agreement with H 2 O contents measured by EMPA and SIMS for both eruptions, different melt fraction vs T relationship and eruptive vent location. Fondo Riccio s explosive eruption occurred at centre of the CF caldera, while the more effusive-like Minopoli 1 eruption occurred along a fissure fracture influenced by the regional fault system in the northern portion of the CF caldera. The timescale of evolution of Fondo Riccio magmatic system can be constrained based on rates of heat loss from the nearby geothermal system of La Mofete, the volume of the system and the difference between the enthalpy at the liquidus and the enthalpy at the lowest melt fraction (f m = 0.05). The results show that Fondo Riccio s system has evolved over a time interval of 6.5 ± 3.5 ka, meaning that the magma probably evolved from a basaltic trachy andesitic melt to a trachytic composition over about 6000 years. Thermal timescale calculation for Minopoli 1 gives estimate of potential evolving of the system from a basaltic to a trachy andesitic composition in a time span of 2.5 ± 1.5 ka References - Agip, Modello geotermico de1 sistema flegreo (Sintesi). Servizi Centrali per 1 Esplorazione. SERG-MESG, San Donato, 23 pp.

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31 Sobolev, A.V., Melt inclusions in minerals as a source of principle petrological information. Petrology 4, Sobolev, A.V., Kamenetsky, V.S., Metrich, N., Clocchiatti, R., Koronova, N.N., Devirts, A.L., Ustinov, V.I., Volatile regime and crystallization conditions in Etna hawaiite lavas. Geochem. Int Sparks R.S.J., Barclay J., Jaupart C., Mader H.M., Phillips J.C Physical aspects of magmatic degassing I. Experimental and theoretical constraints on vesciculation. Rev. Mineral 30, pp Spera, F. J., Stein, D. J., Lejeune, A. M., Bottinga, Y., Trull, T. W. & Richet, P. (2000). Rheology of bubble-bearing magmas; discussion and reply [modified]. Earth and Planetary Science Letters 175, Wallace P.J., 2005 Volatiles in subduction zone magmas; concentrations and fluxes based on melt inclusion and volcanic gas data. J. Volcanol. Geotherm. Res. 140, 1-3, Watson, E. B. (1976). Glass inclusions as samples of early magmatic liquid; determinative method and application to South Atlantic basalt. Journal of Volcanology and Geothermal Research 1, Webster, J.D., Burt, D.M., Aguillon, R.A., Volatile and lithophile trace-element geochemistry of heterogeneous Mexican tin rhyolite magmas deduced from compositions of melt inclusions. Geochim. Cosmochim. Acta 60, Webster, J.D., Raia, F., De Vivo, B., Rolandi, G., The behavior of chlorine and sulfur during differentiation of the Mt. Somma Vesuvius magmatic system. Mineral. Petrol. 73, White, S.M., J.A. Crisp, and F.J. Spera, Long-term volumetric eruption rates and magma budgets, Geochemistry, Geophysics, Geosystems, 7, Q03010, doi: /2005gc Wohletz, K., Civetta, L. & Orsi, G. (1999). Thermal evolution of the Phlegraean magmatic system. Journal of Volcanology and Geothermal Research 91, Zollo, A., Judenherc, S., Auger, E., D Auria, L., Virieux, J., Capuano, P., Chiarabba, C., DeFranco, R., Makris, J., Michelini, A., and Musacchio, G., 2003, Evidence for a buried rim of

32 Campi Flegrei caldera from 3-D active seismic imaging: Geophysical Research Letters, v. 30, 2002, doi: /2003GL Figure Captions Fig. 1 Schematic geological map of Campi Flegrei Volcanic District (CFVD), FR = Fondo Riccio, Mi1= Minopoli 1. Modified after Orsi et al. (1996) and Peccerillo (2005) Fig. 2 CaO-MgO-Al 2 O 3 triangular diagrams (Watson, 1976) for melt inclusions hosted in olivine (circles) and pyroxene (squares). Point I is the intersection of olivine and pyroxene fractionation lines and represent the composition of the magmatic liquid at the time of melt inclusion formation. (a) Fondo Riccio, (b) Minopoli Fig. 3 Fondo Riccio s phase proportion diagrams as a function of temperature for MELTS simulation at variable oxygen fugacity, P = 0.15 GPa and H 2 O = 3wt%. Ap = apatite, Bio = biotite, Cpx =clinopyroxene, Ksp = alkali feldspar, Ol = olivine, Plag = plagioclase feldspar, Rh-ox = rhombohedral oxide, Sp = spinel. (a) QFM+1, (b) QFM Fig.4 Oxides diagram for Fondo Riccio in the best cases of 0.15 GPa, 3wt% H 2 O and varying f O2 between QFM and QFM+1. (a) FeOtot, (b) Al 2 O 3, (c) Na 2 O, (d) K 2 O, (e) SiO 2 and (f) CaO Fig. 5 Minopoli 1 phase proportion as a function of temperature for MELTS simulation at variable oxygen fugacity, P = 0.3 GPa and H 2 O = 2wt%. Ap = apatite, Bio = biotite, Cor = corundum, Cpx

33 =clinopyroxene, Ksp = alkali feldspar, Leu = leucite, Ol = olivine, Plag = plagioclase feldspar, Rut = rutile, Sp = spinel. (a) QFM, (b) QFM Fig. 6 Oxides diagram for Minopoli 1 in the best case produced by MELTS, P = 0.3 GPa, 2wt% H 2 O and f O2 = QFM. (a) SiO 2, (b) CaO, (c) FeO tot, (d) Al 2 O 3, (e) Na 2 O and (f) K 2 O Fig. 7 Variation of melt physical properties for Fondo Riccio along the liquid line of descent for the case P= 0.15 GPa, 3wt% H 2 O and QFM+1. (a) Density of melt versus T, (b) Dissolved water content versus T, (c) melt viscosity versus T, (d) volume fraction of water versus T. H 2 O saturates at 1070 C Fig. 8 Variation of melt physical properties for Minopoli 1 along the liquid line of descent for the case P= 0.3 GPa, 2wt% H 2 O and QFM. (a) Density of melt versus T, (b) Dissolved water content versus T, (c) melt viscosity versus T, (d) volume fraction of water versus T. there is no saturation of water along the liquid line of descent for this case Fig. 9 Timescale evolution and temporal crystallization history showed by phase proportion in function of magma temperature. (a) Fondo Riccio, (b) Minopoli 1. According to the best fit model, ages represent time before each eruption when specific phase mineral begin crystallizing

34 Supplementary material Figure captions Fig. A Total Alkali Silica diagram (Le Bas et al., 1986) showing the MELTS results simulations for variable f O2. Symbols are shown in the legend. Shaded area represents field for all Fondo Riccio melt inclusions data (Cannatelli et al., 2007) Fig. B Phase proportion as a function of temperature for MELTS simulation at variable oxygen fugacity, P = 0.2 GPa and H 2 O = 2wt% for Fondo Riccio. Ap = apatite, Bio = biotite, Cor = corundum, Cpx =clinopyroxene, Ksp = alkali feldspar, Leu = leucite, Ol = olivine, Plag = plagioclase feldspar, Rut = rutile, Sp = spinel. (a) QFM+2, (b) QFM+1, (c) QFM, (d) QFM-1, (e) QFM Fig. C Minopoli 1 calculated compositions for f O2 = QFM+1, H 2 O = 2wt% and P = 0.2 GPa and 0.3 GPa. (a) Feldspar, (b) Spinel Fig. D Fondo Riccio s calculated mineral compositions for the best case of MELTS simulations. (a) Circles = olivine, diamonds = clinopyroxene; (b) triangles = feldspar; (c) triangles = spinel. Grey symbols are MELTS generated data, open symbols are mineral data collected by EMPA. 857

35 Fig. E Minopoli 1 s calculated mineral compositions for the best case of MELTS simulations. (a) Circles = olivine, diamonds = cpx; (b) triangles = feldspar. Grey symbols are MELTS generated data, open symbols are mineral data collected by EMPA.

36 0 4 km Recent Sediments Volcanics younger than 15 ky Neapolitan Yellow Tuff (15 ky) Campanian Ignimbrite (39ky) and volcanics between 39 and 15 ky Volcanics older than 15 ky Caldera Crater Fault