Modelling and physico-chemical constraints to the 4.5 ka Agnano-Monte Spina Plinian eruption (Campi Flegrei, Italy)

doi:10.1016/j.chemgeo.2019.119301

Chemical Geology

Volume 532, 20 January 2020, 119301

https://doi.org/10.1016/j.chemgeo.2019.119301 Get rights and content

Abstract

The 4.5 ka trachytic Plinian eruption of Agnano-Monte Spina is the largest magnitude event of the past 5 ka at Campi Flegrei caldera. The complete eruptive sequence consists of six members, three of which, named A, B and D, are characterized by the association of fallout and pyroclastic density current (PDC) deposits well preserved at proximal locations.

In this study, we analyze the textural characteristics of the pumice clasts of the three major fallout deposits (A1, B1, D1) and of their associated PDC deposits (A2, B2, D2), and link them to the physical properties of magma in order to investigate conduit fluid dynamics. A combination of data (field work, grain-size and density measurements, vesicle number densities and size distributions, crystal content, water content) is used to set up the source term conditions for numerical simulations.

Each fall/PDC transition is accompanied by distinctive changes in textural properties of the juveniles, recognized by a lowering in vesicle number densities of about one order of magnitude (from 10⁸ to 10⁷ cm⁻³), indicating a significant decrease in the magma ascent rate. Melt inclusions show a marked decrease in volatile content recurrent at each fall/PDC transition and indicate that the three main pulses of the eruption were fed by distinct and progressively deeper magma batches. Numerical simulations, taking into account magma properties derived from the textural analyses, and variations in initial water content, show decreases in the exit velocities and Mass Discharge Rate (MDR) consistent with such fall/PDC transitions. Different initial water contents together with changes in conduit diameters allow us to simulate the different column heights reconstructed for the three Plinian phases. The reconstructed scenario for the Agnano-Monte Spina eruption involves a stop-start behavior and a top-down trigger for the most voluminous and intense eruptive episode D.

Introduction

Plinian eruptions commonly show transitions from phases of sustained convective columns to phases characterized by partial or total collapse generating pyroclastic density currents (PDCs) (e.g., Orsi et al., 1992; Wohletz et al., 1995; Rosi et al., 2001; Cioni et al., 2015; Pensa et al., 2015). Furthermore, while many Plinian sequences show the “classic” transition from a lower (basal) fall deposit into Pyroclastic Density Current (PDC) deposits (Cas and Wright, 1987), many others have been described as related to more uncommon stop-start behavior (e.g. Christiansen and Peterson, 1981; Giordano, 1998; Adams et al., 2001; Yasui and Koyaguchi, 2004; Miyaji et al., 2011).

Factors that determine sustained column to PDC transitions include variations in mass discharge rate as a consequence of changing volatile contents, magma composition and/or crystallinity, which in turn affect the rheology of magma, changes in conduit geometry and magma-water interaction (e.g. Sparks, 1978; Wilson et al., 1980; Cashman and Mangan, 1994; Klug and Cashman, 1994; Lyakhovsky et al., 1996; Papale, 1999; Rust et al., 2004; Gurioli et al., 2005; Houghton et al., 2010; Shea et al., 2011, 2012; Vinkler et al., 2012; Pardo et al., 2014).

The link between eruption column and PDCs (i.e., if the PDC is related to a total or partial column collapse) is of great interest for modeling conduit dynamics aimed at volcanic hazard assessment and risk mitigation. Considerable advancements in such studies have been achieved in recent years (e.g. Esposti Ongaro et al., 2006; Dellino et al., 2007; de’ Michieli Vitturi et al., 2010d; Suzuki and Koyaguchi, 2012; Carazzo et al., 2015; Trolese et al., 2019). Despite that, the critical conditions that drive the collapse of a Plinian column still need to be thoroughly explored. Textural analysis of pyroclasts has been proved to provide reliable proxies for magma ascent dynamics (e.g. Toramaru, 2006; Gurioli et al., 2008; Piochi et al., 2008; Costantini et al., 2010; Shea et al., 2010), and, coupled with experiments or parameterization of magma rheology (e.g., Costa, 2005; Llewellin and Manga, 2005; Mader et al., 2013; Vona et al., 2016), may help in understanding and constraining the transitions in eruptive styles (Adams et al., 2006; Sable et al., 2006; Moitra et al., 2013; La Spina et al., 2016). The 4.5 ka trachytic Plinian eruption of Agnano-Monte Spina (A-MS; de Vita et al., 1999; Blockley et al., 2008; Smith et al., 2011) provides an excellent opportunity to study an uncommon explosive sequence related to at least three different eruptive pulses, each initiated by a buoyant phase later evolving into a collapsing phase. This eruption has been taken as reference eruption scenario for the next high scale eruption at Campi Flegrei (e.g. Orsi et al., 2004, 2009; Bevilacqua et al., 2015, 2017) by the Italian Civil Protection Agency and therefore it represents a very important event for the hazard studies of one of the most populated active volcanic areas on Earth.

In this study, we apply a multidisciplinary approach, using geological data (chemistry and texture of the emitted products) to set up the source term conditions for the modelling of the three major fallouts (A1, B1, D1) and of the related PDCs (A2, B2, D2) (de Vita et al., 1999; Dellino et al., 2001, 2004a). The aim of the study is to relate the textural and physical features of the deposit to intrinsic parameters of the magma during ascent along the conduit and to determine to what extent, if any, changes in magma properties may have affected the course of the A-MS eruption.

Section snippets

Campi Flegrei volcanic system and Agnano-Monte Spina eruption

The Campi Flegrei caldera is a nested structure which results from two main collapses that followed the Campanian Ignimbrite (39 ka; Fisher et al., 1993; Orsi et al., 1996; Rosi et al., 1996; Civetta et al., 1997; Fedele et al., 2008) and the Neapolitan Yellow Tuff eruptions (14 ka; Orsi et al., 1992, 1995; Wohletz et al., 1995; Deino et al., 2004). The activity during the last 15 ka was concentrated in three epochs (15-9.5, 8.6-8.2, 5.5-3.8 ka BP) separated by two quiescence periods (Di Vito

Sampling

We selected two key stratigraphic sections that well represent the volcanic stratigraphy of A-MS eruption: "Cavone degli Sbirri" (CS) and "Eremo dei Camaldoli" (EC) (Fig. 1A and Appendix A). These two sites are located at a distance of 2 and 5 km respectively from the eruptive center (Monte Spina). Both sections are located along the ENE-WSW axis of maximum dispersal of the main fallout deposits. The main fallout and PDC deposits have been sampled in the two sites. Bottom and top parts of the B1

Chemistry

The bulk rocks from the A-MS deposit ranges in composition from alkali-trachyte to trachyte (de Vita et al., 1999; Arienzo et al., 2010; Smith et al., 2011; Fig. 1c). Pumices are from near-aphyric to porphyritic (up to 13% of crystal in layer B1). Glass compositions of each A-MS layer and average analyses for all units are reported in Table 1 and Fig. 1c, respectively. The glass compositions, reported in Fig. 1c and Table 1, show a general decrease in silica content (from 62.3 to 59.8 wt% SiO₂)

Numerical simulations of conduit dynamics during AM-S eruption

Based on the measured textural and chemical properties of the erupted products, we performed some 1-D numerical simulations using Confort 15 software (Campagnola et al., 2016a), in order to investigate the eruptive dynamics during magma rise and constrain the style and conditions of fragmentation, focusing in particular on the transitions from sustained to collapsing column. Confort 15 is an open source numerical model for steady-state magma flow through vertical and cylindrical conduits open

Conclusions

The 4.5 ka Agnano-Monte Spina eruption is the highest magnitude event of the past 5 ka at Campi Flegrei caldera. Despite the small volume of erupted magma (1 km³, Di Renzo et al., 2011), this event caused significant environmental impact over a very large area. According to different studies (e.g. Orsi et al., 2004; Bevilacqua et al., 2015), a future vent most likely will open in the area between the Agnano and San Vito plains and a future explosive eruption, in spite of the magnitude of the

Acknowledgements

This work was funded by the DPC-INGV Project V1-UR RM3 (resp. C. Romano). The Grant of Excellence Departments, MIUR-Italy (ARTICOLO 1, COMMI 314 - 337 LEGGE 232/2016) is also gratefully acknowledged. We thank Sergio Lo Mastro (LIME, Roma-tre, Italy) for the valuable technical support with SEM analyses. We also thank Stefania Sicola for valuable help with Raman data acquisition. Prof. Lucia Gurioli and an anonymous reviewer are strongly thanked for their thoughtful comments.

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The complex sequential compositional zoning of clinopyroxene crystals erupted <5 ka during the Agnano-Monte Spina (A-MS) eruption, which is considered to be the reference event for a future large-scale explosive eruption at Campi Flegrei caldera (Italy), has been characterized in detail. Concentration profiles (20–800 μm long, spacing 2.5–10 μm) of major (Si, Ti, Al, Fe, Mg and Ca) and minor (Mn, Na, Ni and Cr) elements were measured along different directions in the clinopyroxene crystals. The zoning patterns of Fe–Mg and selected elements (e.g., Al, Ti) often consist of two or more compositional plateaus with both sharp and/or slightly diffuse boundaries between the plateaus. Each compositional population results from a growth stage in a distinct Magmatic Environment (ME), which is identified as a defined set of intensive thermodynamic variables (pressure, temperature, bulk composition and fugacity of fluids including oxygen). The large range of chemical compositions of clinopyroxenes reveals the existence of at least four MEs that are characterized by different compositional populations; two of these are dominant. The variation in zoning pattern from one plateau composition to another fingerprints the transfer of the crystal from one ME to another. In combination with Sr isotopic data and thermobarometric estimates, our systematic characterization of clinopyroxene zoning patterns suggests recharge by deep mafic magmas (ME0: Mg# = 85–92) of an evolved shallow reservoir (ME2: Mg# = 70–78). Such a process also led to the formation of a compositionally intermediate environment (ME1: Mg# = 84–80), that is detected in the clinopyroxene zoning pattern.
A new method has been developed in this work to evaluate the effective diffusive modifications that affect the concentration profiles of zoned crystals. The application of different diffusion modeling methods indicates that deep and shallow reservoirs beneath the Campi Flegrei caldera were connected to each other over several tens of years until the amount of mafic recharge increased during the last 10–15 years before the A-MS eruption. This study highlights the complex relationships between events of magma recharge and the onset of eruption. Our results provide a contribution to the knowledge of timescales of magmatic processes that have recently occurred beneath the Campi Flegrei caldera, which is useful for risk assessment.
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Citation Excerpt :
The knowledge of water content in natural glasses is fundamental for understanding the numerous physical and chemical processes occurring before, during, and after the eruption (Bachmann et al., 2009; Berry et al., 2008; Blundy and Cashman, 2005; Di Genova et al., 2017; Dingwell, 2006; Hartley et al., 2014; Kennedy et al., 2005; Kotov et al., 2021; Métrich et al., 2010; Romano et al., 2020; Schiavi et al., 2018).
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Classification of ignimbrites and their eruptions
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The term “ignimbrite” probably encompasses the one of the largest ranges of deposit types on Earth, associated with the partial to total collapse of explosive eruption columns feeding pyroclastic density currents. Surprisingly, there is no quantified classification scheme for ignimbrite types, as there is for fallout deposits, and this is a remarkable deficiency of modern volcanology. This has so far prevented the identification of standardized descriptors for ignimbrites and the improvement of methods for the documentation of their characteristics, such as happened for fallout deposits, building on the classification scheme proposed by Walker in 1973. Despite some earlier attempts, ignimbrite types do not conform to eruption style nomenclature. In this paper, we explore and discuss descriptors for a classification scheme based on the correlation of runout, areal extent, aspect ratio and volume from a compiled database comprising 92 ignimbrites, which then allows current understanding of pyroclastic flow dynamics to be considered. We refer to single ignimbrite outflow units, i.e. emplaced without significant breaks in their sedimentation, in extra-caldera settings and forming individual cooling units, irrespective of internal lithofacies architecture. Our main finding is that ignimbrites show remarkable power-law relationship between dispersal area/equivalent runout and bulk volume. Runout is directly related to increasing mass flow rate feeding the pyroclastic current. Volume is related to the magnitude of the flow event. We therefore propose that by measuring first order field observables such as bulk volume and dispersal area provides the opportunity to evaluate magnitude and intensity of related pyroclastic currents and, for large eruptions dominated by ignimbrites, of the eruption. Based on the relationships identified we propose that ignimbrites that originated from the collapse of single point-source eruption columns, usually smaller than 1 km³, are named “Vulcanian ignimbrites” and “Plinian ignimbrites” depending on the style of the eruption they are associated with. Larger ignimbrites that originated from caldera-forming eruptions along ring-fault fissure vents should be regarded as related to a separate eruption style - with respect to the common Hawaiian-Plinian trend -, where the effect of increased mass flow rate due to ring-fissure vents is dominant and controls the dynamics of the resulting collapsing fountains and pyroclastic flows, irrespective of the kind of eruption style that preceded the onset of the caldera collapse. These are named “caldera-forming ignimbrites” and are further subdivided into small, intermediate, large and super, based on their increasing erupted volume.
Microscale textural heterogeneity and tip-streaming instability in alkaline magmas: Evidence in tube pumices from Campi Flegrei, Italy
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Citation Excerpt :
For magma quenched in AMS fallout pumices, pre-eruptive temperature, pressure and dissolved water content were ca. 900 °C, 100 MPa and 2–2.5 wt%, respectively [Piochi et al., 2008; Arienzo et al., 2010; Romano et al., 2020]. On the basis of temperature, water content and texturally determined crystal abundance (mostly 10% in both microvesicular and tube pumices and generally not more than 20% on average; see also Supplement), magma viscosity was estimated at ca. 104.5–105 Pa s [Piochi et al., 2008; Romano et al., 2020]. Melt viscosity referred to AMS products (0.7 wt% of water content) is at ca. 105.5 Pa s [Piochi et al., 2008].
We present a textural investigation of compositionally homogeneous alkaline tube pumices from the plinian fallouts of the Agnano Monte Spina eruption (ca. 4.1 ka), Campi Flegrei, Italy. The pumice clasts present strong textural complexity from hand specimen to microscopic scale of inspection. The clasts are characterized by large vesicle tubes that extend up to several millimetres along the clast main axis. They also exhibit small and elongated vesicles arranged in stretched bands around textural domains of larger, sub-spherical vesicles associated with pyroxene, feldspar, and biotite phenocrysts. Clasts, vesicle tubes, vesicle deformation, major axes of pyroxene phenocrysts and crystals with size ≤100 μm align in the same direction. In addition, we observe chains of gradually smaller stretched vesicles likely generated from break-up of a larger parent vesicle (i.e., tip-streaming phenomenon). We use image analysis to quantify the heterogeneity of vesicle sizes and shapes. Then, we deduce the Capillary number (Ca) of the each single vesicle, which represents the ratio between the deforming viscous stress on a bubble and the restoring stress supplied by surface tension, and obtain the spatial distribution of Ca values within pumice clasts. As Ca depends on strain-rate and viscosity, the spatial distribution of Ca values provides us with a snapshot of the sub-millimetre scale variation in magma strain-rate and rheology before fragmentation. Ca in highly stretched vesicles is close to values (0.45 to 0.6) required for bubble-breakup induced by tip-streaming. Instead, low Ca values (0.3 to 0.45) are calculated for large sub-spherical vesicles mostly distributed around phenocrysts. We suggest that sub-millimetre heterogeneities in vesicle sizes and shapes recorded in tube pumices are caused by a combination of tip-streaming, heterogeneous bubble nucleation and second-boiling processes prior to fragmentation. The heterogeneous spatial distribution of Ca values may be the result of local micrometric shear localisation and consequent deformation upon break-up experienced by magma while ascending in the conduit. This deformation suggests that a variation of magma strain-rate and viscosity occurred before fragmentation at the clast microscale and this is likely to affect magma dynamics and fragmentation. Our results claim for future experimental and numerical modelling studies that investigate and consider the effect of such micrometric heterogeneities on magma rheology and flow condition variations inside volcanic conduits.
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The study of medium-large magnitude and intermediate-high intensity eruptions (VEI = 4/5; Column Height 15–20 km) fed by poorly evolved magmas is one of the main topics in volcanology. In this framework, the 472 AD (Pollena) eruption from Somma-Vesuvius (Italy) represents a key case study, as it is one of the subplinian eruptions used to constrain the reference scenario adopted by the Italian Department of Civil Protection in case of renewal activity at Somma-Vesuvius. The Pollena eruption experienced a complex dynamics, with abrupt shifts in eruptive style. This study focused on the fall products (L1-L8) of the magmatic eruptive phases (Phases I and II), which preceded the onset of the final phreatomagmatic phase (Phase III). Phase I was characterized by unsteady magma discharge resulting in an oscillating convective column, whereas Phase II involved pulsating activity with alternation of sustained and collapsing columns. To evaluate the role of textural variability in controlling magma rheology (and therefore variations in magma discharge), a detailed textural analysis of the juvenile products has been performed.
Pyroclast textures record a variable degree of outgassing efficiency and lateral textural stratification of magma in the conduit, related to differential magma ascent rates and resulting in variable eruption intensity. Repetitive patterns of magma densification, achieved by progressive outgassing and crystallization, led to stages of decreasing ascent velocity resulting in the end of the eruptive pulse (during the oscillatory Phase I) or anticipating PDC emplacement (at the end of Phase I and during the pulsatory Phase II). Further decompression of the outgassed magma induced intense clinopyroxene microlite crystallization, that markedly increased magma viscosity and promoted the restoration of sustained columns during the eruption climaxes (L3 and L8). Magma densification patterns and consequent unsteadiness in magma discharge at the surface may derive from small heterogeneities in the initial volatile budget and/or represent a natural evolution of low viscosity magmas, as those feeding Pollena eruption, where efficient crystallization in the conduit can deeply and abruptly modify magma rheology, outgassing ability, conduit flow and, ultimately, eruptive style.
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Modelling and physico-chemical constraints to the 4.5 ka Agnano-Monte Spina Plinian eruption (Campi Flegrei, Italy)

Abstract

Introduction

Section snippets

Campi Flegrei volcanic system and Agnano-Monte Spina eruption

Sampling

Chemistry

Numerical simulations of conduit dynamics during AM-S eruption

Conclusions

Acknowledgements

Chem. Geol.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Chem. Geol.

Earth Planet. Sci. Lett.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Geophys. Res. Lett.

J. Volcanol. Geotherm. Res.

Earth Planet. Sci. Lett.

J. Volcanol. Geotherm. Res.

Earth Planet. Sci. Lett.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Chem. Geol.

Chem. Geol.

J. Volcanol. Geotherm. Res.

Chem. Geol.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Italy. J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Earth Planet. Sci. Lett.

J. Volcanol. Geotherm. Res.

Chem. Geol.

The physical volcanology of the 1600 eruption of Huaynaputina, southern Peru

Bull. Volcanol.

Abrupt transitions during sustained explosive eruptions: examples from the 1912 eruption of Novarupta

Alaska. Bull. Volcanol.

Geologia e Geofisica del sistema geotermico dei Campi Flegrei

Insights into eruption dynamics from textural analysis: the case of the May, 2008, Chaitén eruption

Bull. Volcanol.

Quantifying volcanic hazard at Campi Flegrei caldera (Italy) with uncertainty assessment: 1. Vent opening maps

J. Geophys. Res. Solid Earth

The effects of vent location, event scale, and time forecasts on pyroclastic density current hazard maps at Campi Flegrei Caldera (Italy)

Front. Earth Sci.

Petrologic reconstruction of magmatic systems variables and processes

Experimental constraints on degassing and permeability in volcanic conduit flow

Bull. Volcanol.

Large-Scale Plinian Eruptions of the Colli Albani and the Campi Flegrei Volcanoes: Insights from Textural and Rheological Studies

Confort 15 model of conduit dynamics: applications to Pantelleria Green Tuff and Etna 122 BC eruptions

Contrib. Mineral. Petrol.

Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns

Bull. Volcanol.

Volcanic Successions: Modern and Ancient