Tracking Volatile Behaviour in Sub-volcanic Plumbing Systems Using Apatite and Glass: Insights into Pre-eruptive Processes at Campi Flegrei, Italy

Tracking Volatile Behaviour in Sub-volcanic Plumbing Systems Using Apatite and Glass: Insights... Abstract Volatile elements play an important role in many aspects of the physicochemical architecture of sub-volcanic plumbing systems, from the liquid line of descent to the dynamics of magma storage and eruption. However, it remains difficult to constrain the behaviour of magmatic volatiles on short timescales before eruption using established petrological techniques (e.g. melt inclusions); specifically, in the final days to months of magma storage. This study presents a detailed model of pre-eruptive volatile behaviour in the Campi Flegrei system (Italy), through combined analyses of apatite crystals and glass. The deposits of eight eruptions were examined, covering the full spectrum of melt compositions, eruptive styles and periods of activity at Campi Flegrei in the past 15 kyr. Measured apatite compositions are compared with thermodynamic models that predict the evolution of the crystal compositions during different fractional crystallization scenarios, including (1) volatile-undersaturated conditions, (2) H2O-saturated conditions and (3) varying P–T conditions. The compositions of clinopyroxene-hosted and biotite-hosted apatite inclusions are consistent with crystallization under volatile-undersaturated conditions that persisted until late in magmatic evolution. Apatite microphenocrysts show significantly more compositional diversity, interpreted to reflect a mixed cargo of crystals derived from volatile-undersaturated melts at depth and melts that have undergone cooling and degassing in discrete shallow-crustal magma bodies. Apatite microphenocrysts from lavas show some re-equilibration during cooling at the surface. Clinopyroxene-hosted melt inclusions within the samples typically contain 2–4 wt % H2O, indicating that they have been reset during temporary magma storage at 1–3 km depth, similar to the depth of sill emplacement during recent seismic crises at Campi Flegrei. Comparable apatite compositional trends are identified in each explosive eruption analysed, regardless of volume, composition or eruption timing. However, apatites from the different epochs of activity appear to indicate subtle changes in the H2O content of the parental melt feeding the Campi Flegrei system over time. This study demonstrates the potential utility of integrated apatite and glass analysis for investigating pre-eruptive volatile behaviour in apatite-bearing magmas. INTRODUCTION Magmatic volatiles (H2O, CO2, halogens and SO2) are minor but important constituents of most silicate melts, and affect almost every aspect of magmatic evolution and eruption. Volatiles influence mineral phase stability and the liquid line of descent (Grove et al., 2003; Zimmer et al., 2010), as well as melt density (Lange & Carmichael, 1990) and viscosity (Giordano et al., 2008), thus exerting a major control on the depths of magma storage and the crustal-scale structure of sub-volcanic systems (Annen et al., 2006). Volatile exsolution and expansion drives volcanic eruptions, and the pre-eruptive behaviour of volatiles plays an important role in controlling the style and tempo of volcanism at the Earth’s surface (Roggensack et al., 1997; Huppert & Woods, 2002; Cashman, 2004; Edmonds, 2008). Given the fundamental role of volatiles in controlling volcanic processes, quantification of their pre-eruptive concentrations remains a high priority for any investigation. A variety of petrological methods have been used to decipher the volatile histories of past eruptions, giving access to distinct snapshots of melt volatile contents over variable pre-eruptive timescales. For example, melt inclusions capture a record of magmatic volatiles at the time of entrapment, and have been used widely to constrain volatile concentrations in different magmatic settings (e.g. Dunbar et al., 1989; Saal et al., 2002; Wallace, 2005; Stefano et al., 2011). However, recent studies have revealed the rapidity of H+ diffusion in common igneous phenocrysts under magmatic conditions, relative to the timescales of pre-eruptive magma storage (e.g. Woods et al., 2000; Ingrin & Blanchard, 2006; Reubi et al., 2013). Melt inclusion H2O contents may diffusively re-equilibrate through their host crystals within hours to weeks, limiting this volatile record to the very final stages of magma storage and/or ascent (e.g. Danyushevsky et al., 2002; Portnyagin et al., 2008; Gaetani et al., 2012; Bucholz et al., 2013; Lloyd et al., 2013; Reubi et al., 2013; Preece et al., 2014). It has also been shown that significant amounts of CO2 may migrate from the inclusion melt into shrinkage bubbles, leading to underestimates of pressure (Moore et al., 2015). Understanding volatile systematics during late-stage magma storage and the onset of magma ascent is essential for identifying eruption-triggering processes and understanding the ‘warning’ signs that would be observed at the Earth’s surface in the build-up to an eruption. Apatite [Ca4(PO4)3F, Cl, OH] is a common accessory mineral in volcanic, plutonic and ore-forming environments (Piccoli & Candela, 2002) and has received increasing attention as a potential magmatic volatile ‘probe’, owing to its ability to incorporate all major magmatic volatiles into its crystal structure. Halogens and OH are essential structural constituents in apatite and are incorporated as part of a series of exchange equilibria (Candela, 1986; McCubbin et al., 2015). Sulphate and CO32– may also substitute into apatite as trace components (e.g. Pan & Fleet, 2002; Dietterich & de Silva, 2010; Riker et al., 2018). Recent work has focused on deciphering the relationship between apatite F–Cl–OH compositions and their host melt volatile contents (e.g. Candela, 1986; Piccoli & Candela, 1994; Patiño Douce & Roden, 2006; Boyce & Hervig, 2009; McCubbin et al., 2011; Boyce et al., 2014; Stock et al., 2016). One advantage of apatite analysis is that phenocryst-hosted apatite inclusions can preserve a record of melt volatile compositions under conditions in which melt inclusions may have re-equilibrated (Stock et al., 2016). Because volatile re-equilibration in mineral-hosted apatite inclusions requires simultaneous diffusion of F, Cl and/or OH, this process will be rate-limited by halogen diffusivity in the host phenocrysts, which is significantly slower than that of H+ (Bucholz et al., 2013; Lloyd et al., 2013). In contrast, F–Cl–OH diffusion within apatite crystals is relatively rapid and microphenocrysts are therefore able to exchange volatiles with their host liquids on geologically short timescales [i.e. microphenocryst rims may re-equilibrate in weeks to years at magmatic temperatures (T); Brenan, 1993]. Because these timescales of apatite microphenocryst re-equilibration are longer than timescales of magma ascent (i.e. hours to days), apatite microphenocrysts may preserve a record of pre-eruptive conditions, even when matrix glasses degas at low pressure [(P); Stock et al., 2016]. In this study, we investigate apatite and glass compositions in juvenile samples from eight eruptions of the Campi Flegrei volcano (Italy), to determine magmatic volatile systematics and processes in the build-up to eruptions. Campi Flegrei was selected as the focus of this study because its melts are known to be apatite-bearing and volatile-rich (D’Antonio et al., 1999; Cannatelli et al., 2007; Arienzo et al., 2010, 2016). It has also recently shown signs of unrest (Chiodini et al., 2012; Moretti et al., 2017). Building on the work of Candela (1986) and Piccoli & Candela (1994) we develop thermodynamic models that predict the theoretical compositional evolution of apatite as a function of changing magma compositions during fractional crystallization in the presence or absence of different fluid phases. Different populations of apatite inclusions (hosted in biotite and clinopyroxene) and microphenocrysts are identified based on their volatile compositions and, through comparison with our thermodynamic models, we use these to constrain the pattern of magmatic volatile behaviour in the sub-volcanic plumbing system at Campi Flegrei. Although melt inclusions have re-equilibrated during magma ascent, coupled interpretation of apatite and glass compositions provides additional constraints on the structure of the Campi Flegrei plumbing system and the composition of the magmatic fluid phase prior to eruption. Finally, we discuss apparent variations in apatite volatile contents that are linked to different periods of eruptive activity at Campi Flegrei, and suggest that the volatile contents of the incoming parental magmas vary with time. GEOLOGICAL SETTING Campi Flegrei comprises a nested caldera system, defined by collapse scarps that formed during the Campanian Ignimbrite (∼40 ka; Giaccio et al., 2017) and Neapolitan Yellow Tuff (NYT, ∼15 ka; Deino et al., 2004) eruptions (Fig. 1). It is one of the most active volcanoes in Europe, having produced >60 eruptions in the past 15 kyr, from vents located within the NYT caldera (Smith et al., 2011). These are divided into three ‘epochs’ that represent periods of eruptive activity, separated by prolonged quiescence (Di Vito et al., 1999). Vents for these eruptions are located within the NYT caldera (Fig. 1; Di Vito et al., 1999; Isaia et al., 2009). Epoch 1 occurred from ∼15 to 10·6 ka and produced ∼30 explosive eruptions, with a typical inter-eruptive interval of ∼70 years (Di Vito et al., 1999; Smith et al., 2011). Six low-magnitude explosive eruptions occurred in Epoch 2, between ∼9·6 and 9·1 ka, at an average interval of ∼65 years (Di Vito et al., 1999; Smith et al., 2011), followed by a long period (∼4 kyr) of quiescence. Twenty-seven eruptions occurred within the short Epoch 3 time-period, between ∼5·5 and 3·5 ka (Smith et al., 2011), with an average eruptive interval of ∼75 years (Di Vito et al., 1999). Most Epoch 3 eruptions were small, explosive events [typically producing 0·02–0·10 km3 of material, dense rock equivalent (DRE); Smith et al., 2011]. However, uniquely within the past 15 kyr, Epoch 3 also includes four effusive lava domes (Melluso et al., 1995; Di Vito et al., 1999). The most recent Campi Flegrei eruption was at Monte Nuovo in 1538 CE. This occurred after a >3 kyr dormant period (Piochi et al., 2005), substantially greater than the typical inter-eruption time interval, and is therefore not considered part of Epoch 3 (Smith et al., 2011). Table 1: Summary of eruption deposits analysed, including age estimate, deposit characteristics (i.e. eruption style), erupted volume (i.e. magnitude estimate), average matrix glass composition, vent location and sampling location Epoch Eruption Sample number Age (cal. years BP) Deposit type Erupted volume (km3) Average matrix glass composition Vent location* Sampling location UTM X UTM Y UTM X UTM Y Monte Nuovo CF195 1538 CE* Pyroclastic density current deposit ∼0·03* Phonolite 423095 4520870 423277 4520614 Epoch 3 Astroni 1 CF69 4155–4357* Ash fall with some lapilli layers ∼0·06* Phonolite 427999 4522122 428859* 4523584* Accademia CF163 4205–4507*1 Lava dome <0·01* Trachyte§ 427551 4519282 427597 4519562 Santa Maria delle Grazie CF200 4383–4507* Shallow feeder dyke <0·01* Latite‖ 427625 4519806 427421 4518909 Epoch 2 Baia–Fondi di Baia CF88 9525–9705* Pumice fall 0·02–0·04† Trachyte 421855 4517930 421844* 4518253* Epoch 1 Pisani 1 CF25 10516–12107*2 Scoria fall 0·1–0·3* Tephri-phonolite‒trachy-andesite 428260 4523970 430237* 4525994* Pomici Principali CF6 11907–12107* Plinian fall (B3 unit) 0·43–1·28‡ Tephri-phonolite‒phonolite 428698 4522284 433890* 4522346* Minopoli 1 CF13 11907–12725*3 Scoria fall <0·1* Phono-tephrite‒tephri-phonolite 432400 4522670 432805* 4522632* Epoch Eruption Sample number Age (cal. years BP) Deposit type Erupted volume (km3) Average matrix glass composition Vent location* Sampling location UTM X UTM Y UTM X UTM Y Monte Nuovo CF195 1538 CE* Pyroclastic density current deposit ∼0·03* Phonolite 423095 4520870 423277 4520614 Epoch 3 Astroni 1 CF69 4155–4357* Ash fall with some lapilli layers ∼0·06* Phonolite 427999 4522122 428859* 4523584* Accademia CF163 4205–4507*1 Lava dome <0·01* Trachyte§ 427551 4519282 427597 4519562 Santa Maria delle Grazie CF200 4383–4507* Shallow feeder dyke <0·01* Latite‖ 427625 4519806 427421 4518909 Epoch 2 Baia–Fondi di Baia CF88 9525–9705* Pumice fall 0·02–0·04† Trachyte 421855 4517930 421844* 4518253* Epoch 1 Pisani 1 CF25 10516–12107*2 Scoria fall 0·1–0·3* Tephri-phonolite‒trachy-andesite 428260 4523970 430237* 4525994* Pomici Principali CF6 11907–12107* Plinian fall (B3 unit) 0·43–1·28‡ Tephri-phonolite‒phonolite 428698 4522284 433890* 4522346* Minopoli 1 CF13 11907–12725*3 Scoria fall <0·1* Phono-tephrite‒tephri-phonolite 432400 4522670 432805* 4522632* Erupted volumes are given as dense rock equivalent. Where exposure permitted, samples were collected vertically through volcaniclastic units and included both lapilli- and ash-sized material. The Pomici Principali sample represents only the B3 Plinian phase of the eruption and the Baia–Fondi di Baia sample comprises only the basal Baia fallout units. Samples from the deposits of these eruptions show only limited diversity in glass compositions (Smith et al., 2011) and the analysed samples are therefore assumed representative of the bulk erupted material. * Data from Smith et al. (2011). All ages of Smith et al. (2011) have been recalibrated using IntCal13 (Reimer et al., 2013). † Data from Pistolesi et al. (2017). ‡ Data from Bevilacqua et al. (2016). §Whole-rock composition from Melluso et al. (2012). ‖Whole-rock composition from Isaia et al. (2009). 1Eruption not directly dated but stratigraphically older than the Solfatara eruption (4205–4427 years BP) and younger than Santa Maria delle Grazie. 2Eruption not directly dated but stratigraphically older than Pisani 3 (10 517–10 760 years BP) and younger than Pomici Principali. 3Eruption not directly dated but stratigraphically older than Pomici Principali and younger than Archiaverno (12 579–12 725 years BP). Table 1: Summary of eruption deposits analysed, including age estimate, deposit characteristics (i.e. eruption style), erupted volume (i.e. magnitude estimate), average matrix glass composition, vent location and sampling location Epoch Eruption Sample number Age (cal. years BP) Deposit type Erupted volume (km3) Average matrix glass composition Vent location* Sampling location UTM X UTM Y UTM X UTM Y Monte Nuovo CF195 1538 CE* Pyroclastic density current deposit ∼0·03* Phonolite 423095 4520870 423277 4520614 Epoch 3 Astroni 1 CF69 4155–4357* Ash fall with some lapilli layers ∼0·06* Phonolite 427999 4522122 428859* 4523584* Accademia CF163 4205–4507*1 Lava dome <0·01* Trachyte§ 427551 4519282 427597 4519562 Santa Maria delle Grazie CF200 4383–4507* Shallow feeder dyke <0·01* Latite‖ 427625 4519806 427421 4518909 Epoch 2 Baia–Fondi di Baia CF88 9525–9705* Pumice fall 0·02–0·04† Trachyte 421855 4517930 421844* 4518253* Epoch 1 Pisani 1 CF25 10516–12107*2 Scoria fall 0·1–0·3* Tephri-phonolite‒trachy-andesite 428260 4523970 430237* 4525994* Pomici Principali CF6 11907–12107* Plinian fall (B3 unit) 0·43–1·28‡ Tephri-phonolite‒phonolite 428698 4522284 433890* 4522346* Minopoli 1 CF13 11907–12725*3 Scoria fall <0·1* Phono-tephrite‒tephri-phonolite 432400 4522670 432805* 4522632* Epoch Eruption Sample number Age (cal. years BP) Deposit type Erupted volume (km3) Average matrix glass composition Vent location* Sampling location UTM X UTM Y UTM X UTM Y Monte Nuovo CF195 1538 CE* Pyroclastic density current deposit ∼0·03* Phonolite 423095 4520870 423277 4520614 Epoch 3 Astroni 1 CF69 4155–4357* Ash fall with some lapilli layers ∼0·06* Phonolite 427999 4522122 428859* 4523584* Accademia CF163 4205–4507*1 Lava dome <0·01* Trachyte§ 427551 4519282 427597 4519562 Santa Maria delle Grazie CF200 4383–4507* Shallow feeder dyke <0·01* Latite‖ 427625 4519806 427421 4518909 Epoch 2 Baia–Fondi di Baia CF88 9525–9705* Pumice fall 0·02–0·04† Trachyte 421855 4517930 421844* 4518253* Epoch 1 Pisani 1 CF25 10516–12107*2 Scoria fall 0·1–0·3* Tephri-phonolite‒trachy-andesite 428260 4523970 430237* 4525994* Pomici Principali CF6 11907–12107* Plinian fall (B3 unit) 0·43–1·28‡ Tephri-phonolite‒phonolite 428698 4522284 433890* 4522346* Minopoli 1 CF13 11907–12725*3 Scoria fall <0·1* Phono-tephrite‒tephri-phonolite 432400 4522670 432805* 4522632* Erupted volumes are given as dense rock equivalent. Where exposure permitted, samples were collected vertically through volcaniclastic units and included both lapilli- and ash-sized material. The Pomici Principali sample represents only the B3 Plinian phase of the eruption and the Baia–Fondi di Baia sample comprises only the basal Baia fallout units. Samples from the deposits of these eruptions show only limited diversity in glass compositions (Smith et al., 2011) and the analysed samples are therefore assumed representative of the bulk erupted material. * Data from Smith et al. (2011). All ages of Smith et al. (2011) have been recalibrated using IntCal13 (Reimer et al., 2013). † Data from Pistolesi et al. (2017). ‡ Data from Bevilacqua et al. (2016). §Whole-rock composition from Melluso et al. (2012). ‖Whole-rock composition from Isaia et al. (2009). 1Eruption not directly dated but stratigraphically older than the Solfatara eruption (4205–4427 years BP) and younger than Santa Maria delle Grazie. 2Eruption not directly dated but stratigraphically older than Pisani 3 (10 517–10 760 years BP) and younger than Pomici Principali. 3Eruption not directly dated but stratigraphically older than Pomici Principali and younger than Archiaverno (12 579–12 725 years BP). Fig. 1. View largeDownload slide Simplified geological map of the Campi Flegrei caldera showing major structural features, adapted from Vitale & Isaia (2014). The locations of sampling sites (black squares) and vents for each eruption analysed in this study (Table 1) are indicated. Fig. 1. View largeDownload slide Simplified geological map of the Campi Flegrei caldera showing major structural features, adapted from Vitale & Isaia (2014). The locations of sampling sites (black squares) and vents for each eruption analysed in this study (Table 1) are indicated. Chemical diversity of eruptive products in the past 15 kyr The most mafic (shoshonitic) melt inclusions identified in Campi Flegrei typify the mantle melts feeding the system (Mangiacapra et al., 2008; Vetere et al., 2011). Major and trace element studies of Campi Flegrei whole-rocks and glasses show an evolutionary trend from these mafic melts to evolved trachytes or phonolites, with the entire suite formed by fractional crystallization of a single parental magma composition, punctuated by periodic recharge events (Villemant, 1988; Civetta et al., 1991; D’Antonio et al., 1999; Pappalardo et al., 1999; Di Renzo et al., 2011; Di Vito et al., 2011; Smith et al., 2011; Fourmentraux et al., 2012). Pre-NYT (older than 15 ka) melts encompass only the most evolved end of the compositional spectrum (Pappalardo et al., 1999). Post-NYT (younger than 15 ka) magmas do not follow a continual compositional trend between eruptions (i.e. from primitive to evolved), but the most primitive matrix glasses and whole-rocks are derived from Epoch 1, with Epochs 2 and 3 largely indistinguishable based on their major element compositions (D’Antonio et al., 1999; Smith et al., 2011). Matrix glasses from Monte Nuovo are unlike previous eruptions, with notably higher Na2O concentrations (Smith et al., 2011). In contrast, isotopic heterogeneity in Campi Flegrei products suggests that the magmas cannot have formed through fractional crystallization alone (Pappalardo et al., 1999, 2002; D’Antonio et al., 2007; Di Renzo et al., 2011). The isotopic compositions of <15 ka Campi Flegrei melts reflect mixing between three distinct end-members, defined by Di Renzo et al. (2011) as the ‘NYT component’ (87Sr/86Sr 0·70750–0·70753, 143Nd/144Nd ∼0·51246, 206Pb/204Pb ∼19·04, δ11B ∼ –7·9‰), the ‘Minopoli 2 component’ (87Sr/86Sr ∼0·70860, 143Nd/144Nd ∼0·51236, 206Pb/204Pb ∼18·90, δ11B –7·32‰), and the ‘Astroni 6 component’ (87Sr/86Sr ∼0·70726, 143Nd/144Nd ∼0·51250, 206Pb/204Pb ∼19·08, δ11B –9·8‰). The isotopic composition of Campi Flegrei magmas is defined in the deep crust, before significant crystal fractionation, but the major and trace element compositions of these primitive liquids must be closely similar to permit evolution along the same liquid line of descent (Pappalardo et al., 2002). The prevalence of these isotopic components correlates with the different epochs of activity: Epochs 1 and 2 show mixing between ‘NYT’ and ‘Minopoli 2’ components; Epoch 3 shows mixing between ‘NYT’, ‘Minopoli 2’ and ‘Astroni 6’ components; the Monte Nuovo eruption sampled near end-member ‘Astroni 6 component’ melts (Di Renzo et al., 2011). Enriched δ11B in Epochs 1 and 2 reflects a more metasomatized mantle source caused by a higher slab-derived fluid input (D’Antonio et al., 2007) and/or a lower subducted sediment input into the mantle wedge (Tonarini et al., 2004; Di Renzo et al., 2011). Higher 87Sr/86Sr and lower 143Nd/144Nd and 206Pb/204Pb ratios demonstrate greater crustal assimilation in Epochs 1 and 2 than in Epoch 3 and Monte Nuovo (D’Antonio et al., 2007; Di Renzo et al., 2011). However, the Campi Flegrei liquid line of descent and extent of Sr and Pb isotopic heterogeneity is compatible only with very minor assimilation (D’Antonio et al., 2007; Fowler et al., 2007). Water contents of Campi Flegrei melt inclusions typically vary from ∼1 to ∼4 wt % and show no systematic relationship with the degree of magma differentiation (Mangiacapra et al., 2008; Arienzo et al., 2016; Stock et al., 2016). Campi Flegrei melt inclusions generally have very low CO2 concentrations (<250 ppm), with a few analyses extending up to 400–500 ppm (Marianelli et al., 2006; Arienzo et al., 2010, 2016; Stock et al., 2016). SAMPLES AND METHODS Eruptions studied The eruptions investigated cover the full range of melt compositions, eruption sizes and styles of activity from Campi Flegrei in the past 15 kyr (Table 1; for full stratigraphy see Smith et al., 2011). Vent locations, average matrix glass compositions and absolute eruption ages for the samples analysed in this study are given in Fig. 1 and Table 1. The Minopoli 1 tuff cone was sampled because it is a small, phono-tephritic to tephri-phonolitic explosive eruption, typical of the early Epoch 1 eruptions that followed the NYT event (Smith et al., 2011). Scoria was also sampled from the slightly larger, slightly more evolved Pisani 1 eruption, which occurred in mid–late Epoch 1 (Smith et al., 2011). Deposits from more recent eruptions are typically more evolved. We sampled Astroni 1 from Epoch 3, which represents the first of seven small explosive eruptions from the Astroni vent between 4 and 4·4 ka (Isaia et al., 2004; Smith et al., 2011). These deposits comprise phreatomagmatic surge beds interbedded with subordinate Strombolian pumice layers (Smith et al., 2011). Pomici Principali (PP) was the largest eruption in the last 15 kyr, generating a Plinian column and pyroclastic density currents (Di Vito et al., 1999; Smith et al., 2011; Bevilacqua et al., 2016). These were sampled to investigate relationships between eruption magnitude and apatite volatile compositions. The Baia–Fondi di Baia (B–FdB) and Monte Nuovo eruptions were sampled because they produced highly evolved melts from vents in the western NYT caldera. Baia–Fondi di Baia was the first eruption of Epoch 2 and was particularly explosive, owing to magma–H2O phreatomagmatic interaction, but it expelled only a relatively small volume of material (Pistolesi et al., 2017). The most evolved melts identified in Campi Flegrei were produced in the historical Monte Nuovo tuff cone eruption (Smith et al., 2011). The latitic Santa Maria delle Grazie (SMdG) lava, which is thought to be part of a shallow dyke that fed the SMdG scoria cone (Isaia et al., 2009), and the subsequent trachytic Accademia lava dome (Isaia et al., 2009; Melluso et al., 2012) were sampled to assess differences between apatite volatile compositions in these deposits and explosive units. Samples All samples were collected from proximal deposits (Fig. 1), either by Smith et al. (2011) or during fieldwork for this study in March 2013 and September 2014. Minopoli 1, PP, Pisani 1, B–FdB and Astroni 1 samples are CF13, CF6, CF25, CF88 and CF69 of Smith et al. (2011). The Astroni 1 sample was investigated by Stock et al. (2016). CF88 is from the initial fallout (Baia) phase of the B–FdB eruption (Pistolesi et al., 2017). The Monte Nuovo sample (CF195) is from the upper pyroclastic flow unit (Table 1; Unit II of Piochi et al., 2005). The SMdG (CF200) and Accademia (CF163) samples were collected from the centre of the NYT caldera. All samples have the major phase assemblage K-feldspar + plagioclase + clinopyroxene + biotite + apatite + magnetite. Samples also contain fluorite ± sulphides ± sodalite ± leucite, with precipitation of these accessory phases constrained to late in magmatic evolution by their absence as phenocryst-hosted inclusions in natural samples, and by experimental studies and thermodynamic models (Fowler et al., 2007; Arzilli et al., 2016). Olivine is reported in mafic samples from Campi Flegrei (Cannatelli et al., 2007) but was not observed in this study. Crystal contents are typically <5–30% (from qualitative observations and Isaia et al., 2004; Piochi et al., 2005; Mastrolorenzo & Pappalardo, 2006) but are notably lower in B–FdB (⁠ ≪1%; Mastrolorenzo & Pappalardo, 2006). In Monte Nuovo, two pyroxene populations can be identified in hand specimen: one black and one green, as in other eruptions at Campi Flegrei and Vesuvius (Cioni et al., 1998; D’Antonio et al., 1999). As apatite and melt inclusions show no systematic compositional difference between these pyroxene populations, they are not separated in the following discussion. Analytical methods Clinopyroxene and biotite phenocrysts were hand-picked from the 250–500 µm size fraction in samples from explosive eruptions and the Accademia lava dome. Heavy liquid and magnetic separation techniques were used to extract apatite microphenocrysts from the 44–250 µm size fraction. Crystals and matrix ash grains were mounted in epoxy, ground and polished for analysis. Lava samples were prepared as polished thin sections. Samples were examined using an FEI Quanta 650 FEG scanning electron microscope (SEM), operating with a 20 kV, ∼6–7 nA beam, in the Department of Earth Sciences, University of Oxford. Only apatite inclusions away from cracks and fully enclosed within host phenocrysts were analysed, to ensure that they were trapped during phenocryst growth and were unable to subsequently re-equilibrate with melt or fluids. Melt inclusions were analysed only if they did not show visual evidence for post-entrapment crystallization and were located away from cracks in their host phenocryst. Ash, lapilli fragments and lavas were also assessed by SEM to identify microlite-free regions for analysis of the matrix glass compositions. Mineral and glass major, trace and halogen element compositions were analysed using a JEOL 8600 electron microprobe at the Research Laboratory for Archaeology and the History of Art, University of Oxford. Samples were re-polished prior to electron probe microanalysis (EPMA) to remove any compositional modification induced by SEM electron-beam exposure (Stock et al., 2015), and subsequently carbon coated along with secondary standards to avoid variable light element X-ray attenuation. Apatite was analysed using a defocused (5 µm), 15 kV, 10 nA beam, with halogens analysed first. Where possible, apatite crystals were analysed with the c-axis parallel to the plane of the mount. This routine limits the potential for time-dependent variability in halogen X-ray counts during analysis (Stormer et al., 1993; Goldoff et al., 2012; Stock et al., 2015), while maintaining reasonable precision for low-concentration elements (i.e. Cl). In glass, most elements were measured using a defocused (10 µm), 15 kV, 6 nA beam to minimize Na2O and SiO2 migration (e.g. Humphreys et al., 2006); when in low abundance, SO2, P2O5 and Cl were measured in a second analysis using a 30 nA current. In both crystals and glass, count times were 20–30 s for major elements and 30–90 s for minor elements (120 s for Cl and SO2 in apatite). Backgrounds were determined by counting for half of the on-peak count time on either side of the peak. Glass and apatite analytical totals were typically 95–99% and 96–102%, respectively. Data were filtered to remove analyses with totals <92%, and >101·5% in glass. Apatite totals >100% probably reflect minor electron-beam induced compositional modification (Stock et al., 2015) and totals significantly <100% probably result from the absence of trace elements [e.g. rare earth elements (REEs)] in the analytical routine. Apatite OH contents in EPMA data were calculated ‘by difference’, assuming stoichiometry. Typical analytical uncertainties are reported in Tables 2 and 3. Table 2: Major and volatile element analyses of representative Campi Flegrei glasses Eruption: Monte Nuovo Monte Nuovo Monte Nuovo Monte Nuovo Monte Nuovo Monte Nuovo Monte Nuovo Monte Nuovo Monte Nuovo Monte Nuovo Analysis #: CF195_cp x113_m6 CF195_cp x64_m1 CF195_cp x86_m2 CF195_cp x86_m3 CF195_cp x110_m1 CF195_cp x113_m7 CF195_cp x122_m1 CF195_cp x122_m2 CF195_cp x123_m3 CF195_cp x137_m5 Type: mi mi mi mi mi mi mi mi mi mi SiO2 58·77 57·35 57·81 56·19 58·05 57·63 58·35 58·26 59·68 58·71 Na2O 4·45 4·04 4·11 3·94 5·04 4·65 4·20 4·08 5·03 4·42 MgO 0·55 0·77 0·75 0·74 0·76 0·59 0·87 0·81 0·51 0·70 Al2O3 18·17 17·70 18·27 17·26 16·54 18·02 18·03 18·12 16·97 17·71 K2O 8·40 8·57 8·49 7·89 6·86 8·37 8·48 8·94 7·43 8·42 CaO 2·22 2·59 2·55 2·48 2·72 2·26 2·81 2·83 2·33 2·24 TiO2 0·43 0·48 0·48 0·42 0·55 0·46 0·50 0·44 0·53 0·49 MnO 0·09 0·09 0·11 0·15 0·19 0·08 0·10 0·12 0·12 0·11 FeOt 3·31 3·82 3·46 3·39 3·96 3·38 3·83 3·66 3·85 3·37 P2O5 0·08 0·15 0·13 0·14 0·08 0·12 0·13 0·13 0·10 0·11 Cl 0·95 0·66 0·73 0·64 0·85 0·84 0·67 0·66 0·94 0·77 F 0·25 0·22 0·23 0·22 0·22 0·28 0·25 0·22 0·23 0·28 H2O 2·82 3·48 2·84 5·56 3·47 2·45 2·44 1·04 2·28 2·54 Total 100·47 99·93 99·96 99·01 99·29 99·12 100·67 99·31 100·01 99·87 Eruption: Monte Nuovo Monte Nuovo Monte Nuovo Monte Nuovo Monte Nuovo Monte Nuovo Monte Nuovo Astroni 1 Astroni 1 Astroni 1 Analysis #: CF195_cp x159_m4 CF195_cp x188_m1 VF55_cf19 5_1_1 VF55_cf19 5_2_1 VF55_cf19 5_3_1 VF55_cf19 5_4_1 VF55_cf19 5_4_2 CF69_cp x22_m1 CF69_cp x23_m2 CF69_cp x55_m1 Type: mi mi mg mg mg mg mg mi mi mi SiO2 57·83 58·59 56·50 57·19 58·62 57·66 57·48 57·70 59·17 56·74 Na2O 4·85 4·77 6·88 7·88 7·56 7·69 7·29 4·39 4·25 4·26 MgO 0·66 0·68 0·21 0·25 0·28 0·17 0·18 0·65 0·42 0·54 Al2O3 17·10 17·52 18·28 18·79 19·04 18·69 18·87 17·90 17·84 18·07 K2O 7·18 8·25 6·88 7·23 7·25 7·20 7·30 8·08 8·36 8·89 CaO 2·59 2·52 1·81 1·51 1·66 1·69 1·75 2·47 2·19 2·17 TiO2 0·50 0·47 0·49 0·50 0·39 0·45 0·43 0·45 0·49 0·41 MnO 0·20 0·13 0·24 0·37 0·20 0·20 0·28 0·10 0·19 0·04 FeOt 3·91 3·38 3·08 3·30 2·74 2·89 3·19 3·36 3·65 3·15 P2O5 0·15 0·11 0·01 0·00 0·00 0·05 0·07 0·12 0·10 0·08 Cl 0·89 0·78 0·90 0·97 0·97 1·05 1·17 0·96 1·00 0·87 F 0·24 0·32 1·21 0·65 0·73 0·82 0·68 0·28 0·28 0·26 H2O 3·71 2·38 0·88 0·76 0·97 1·06 1·08 2·37 2·22 2·12 Total 99·80 99·88 97·36 99·41 100·41 99·60 99·77 98·82 100·15 97·60 Eruption: Astroni 1 Astroni 1 Astroni 1 Astroni 1 Astroni 1 Astroni 1 Astroni 1 Astroni 1 Astroni 1 Astroni 1 Analysis #: CF69_cpx 55_m2 CF69_cpx 66_m1 CF69_cpx 66_m3 CF69_cpx 82_m1 CF69_cpx 91_m1 CF69_cpx 92_m1 CF69_cpx 97_