ABSTRACT In the recent years, continuous ash emission activity, related to mid-low intensity, lon... more ABSTRACT In the recent years, continuous ash emission activity, related to mid-low intensity, long lasting eruptions, has been increasingly described to occur at different volcanoes worldwide. Focusing on this type of deposits, a retrospective analysis of the stratigraphic successions at Vesuvius revealed that such type of eruptions have occurred repeatedly in the last 4000 years of activity. This type of activity has been overlooked in the past and the mechanism of ash production in these eruptions is not yet clear. The detailed study of the deposits suggests that these eruptions are dominated by discrete phases of repeated emission of a highly fragmented mixture, alternated with violent strombolian episodes. In this study we present morphological, textural and compositional data on the products of two ash eruptions representative of the whole variability of this activity at Vesuvius (Italy), occurred in the periods between the “Avellino” and “Pompeii” Pumice eruptions (AP3, 2,710±60 years B.P.) and after the 512 A.D. eruption, (AS1a). Juvenile fragments from different ash layers throughout the studied stratigraphic sections were fully characterized in terms of external morphology, particle outline parameterization, groundmass texture (in terms of Crystal and Vesicle Size Distributions) and glass composition. Volcanic ash holds information about magma dynamics within the volcanic conduit, where fragmentation occurs and eruption style is decided. Results of our investigations have been interpreted in terms of fragmentation processes, transport and dispositional mechanisms, dynamics of magma ascent ant timing of the eruption. The methodology of ash analysis used for this study has also important implications for tephrochronological studies, adding a complete parameterization of physical and textural properties of the ash to the routinely used compositional data.
ABSTRACT 1] Textures, petrography and geochemical compositions of products emitted during the ons... more ABSTRACT 1] Textures, petrography and geochemical compositions of products emitted during the onset of the 2011–2012 sub-marine eruption (15 October, 2011) off the coast of El Hierro have been investigated to get information on interaction mechanism between the first rising magma and the crust during the onset of the eruption as well as to get information on magma storage and plumbing systems beneath El Hierro volcano. Studied products consist of 5–50 cm bombs with an outer black to greenish, vesicular crust with bulk basanite composition containing pumiceous xenoliths (xenopumices). Our results show that xenopumices are much more hetero-geneous that previously observed, since consist of a macro-scale mingling of a gray trachyte and white rhyolite. We interpreted xenopumices as resulting from the interaction (heating) between the basanitic magma feeding the eruption, a stagnant trachytic magma pocket/s and an associated hydro-thermally altered halo with rhyolitic composition. Our find-ings confirm the importance of the study of the early products of an eruption since they can contain crucial information on the plumbing system geometry and the mechanism of magma ascent. Citation: Meletlidis, S.
Recent stratigraphic studies at Vesuvius have
revealed that, during the past 4,000 years, long la... more Recent stratigraphic studies at Vesuvius have revealed that, during the past 4,000 years, long lasting, moderate to low-intensity eruptions, associated with continuous or pulsating ash emission, have repeatedly occurred. The present work focuses on the AS1a eruption, the first of a series of ash-dominated explosive episodes which characterized the period between the two Subplinian eruptions of 472 AD and 1631 AD. The deposits of this eruption consist of an alternation of massive and thinly laminated ash layers and minor well sorted lapilli beds, reflecting the pulsatory injection into the atmosphere of variably concentrated ash-plumes alternating with Violent Strombolian stages. Despite its nearly constant chemical composition, the juvenile material shows variable external clast morphologies and groundmass textures, reflecting the fragmentation of a magma body with lateral and/or vertical gradients in both vesicularity and crystal content. Glass compositions and mineralogical assemblages indicate that the eruption was fed by rather homogeneous phonotephritic magma batches rising from a reservoir located at ~ 4 km (100 MPa) depth, with fluctuations between magma delivery and magma discharge. Using crystal size distribution (CSD) analyses of plagioclase and leucite microlites, we estimate that the transit time of the magma in the conduit was on the order of ~ 2 days, corresponding to an ascent rate of around 2× 10−2 ms−1. Accordingly, assuming a typical conduit diameter for this type of eruption, the minimum duration of the AS1a event is between about 1.5 and 6 years. Magma fragmentation occurred in an inertially driven regime that, in a magma with low viscosity and surface tension, can act also under conditions of slow ascent.
A set of experiments have been performed on
volcanic materials from Etna, Stromboli and Vesuvius ... more A set of experiments have been performed on volcanic materials from Etna, Stromboli and Vesuvius in order to evaluate how the exposure to thermal and redox conditions close to that of active craters affects the texture and composition of juvenile pyroclasts. Selected samples were placed within a quartz tube, in presence of air or under vacuum, and kept at T between 700 and 1,130 C, for variable time (40 min to 12 h). Results show that reheating reactivates the melt, which, through processes of chemical and thermal diffusion, reaches new equilibrium conditions. In all the experiments performed at T = 700–750 C, a large number of crystal nuclei and spherulites grows in the groundmass, suggesting conditions of high undercooling. This process creates textural heterogeneities at the scale of few microns but only limited changes of groundmass composition, which remains clustered around that of the natural glasses. Reheating at T = 1,000–1,050 C promotes massive groundmass crystallization, with a different mineral assemblage as a function of the redox conditions. Morphological modifications of clasts, from softening to sintering as temperature increases, occur under these conditions, accompanied by progressive smoothing of external surfaces, and a reduction in size and abundance of vesicles, until the complete obliteration of the pre-existing vesicularity. The transition from sintering to welding, characteristic of high temperature, is influenced by redox conditions. Experiments at T = 1,100–1,130 C and under vacuum produce groundmass textures and glass compositions similar to that of the respective starting material. Collapse and welding of the clasts cause significant densification of the whole charge. At the same temperature, but in presence of air, experimental products at least result sintered and show holocrystalline groundmass. In all experiments, sublimates grow on the external surfaces of the clasts or form a lining on the bubble walls. Their shape and composition is a function of temperature and fO2 and the abundance of sublimates shows a peak at 1,000 C. The identification of the features recorded by pyroclasts during complex heating– cooling cycles allows reconstructing the complete clasts history before their final emplacement, during weakly explosive volcanic activity. This has a strong implication on the characterization of primary juvenile material and on the interpretation of eruption dynamics.
Deposits of mid-intensity basaltic explosive eruptions are characterized by the coexistence of di... more Deposits of mid-intensity basaltic explosive eruptions are characterized by the coexistence of different types of juvenile clasts, which show a large variability of external properties and texture, reflecting alternatively the effects of primary processes related to magma storage or ascent, or of syn-eruptive modifications occurred during or immediately after their ejection. If fragments fall back within the crater area before being re-ejected during the ensuing activity, they are subject to thermally- and chemically-induced alterations. These ‘recycled’ clasts can be considered as cognate lithic for the eruption/explosion they derive. Their exact identification has consequences for a correct interpretation of eruption dynamics, with important implications for hazard assessment. On ash erupted during selected basaltic eruptions (at Stromboli, Etna, Vesuvius, Gaua-Vanuatu), we have identified a set of characteristics that can be associated with the occurrence of intra-crater recycling processes, based also on the comparison with results of reheating experiments performed on primary juvenile material, at variable temperature and under different redox conditions.
Pumice and scoria from different eruptive layers of Mt. Vesuvius volcanic products contain mafic ... more Pumice and scoria from different eruptive layers of Mt. Vesuvius volcanic products contain mafic minerals consisting of High-Fo olivine and Diopsidic Pyroxene. These phases were crystallized in unerupted trachibasaltic to tephritic magmas, and were brought to surface by large phonolitic/tephri-phonolitic (e.g. Avellino and Pompei) and/or of tephritic and phono-tephritic (Pollena) eruptions. A large set of these mm-sized crystals was accurately separated from selected juvenile material and measured for their chemical compositions (EPMA, Laser Ablation ICP-MS) and 18O/16O ratios (conventional laser fluorination) to constrain the nature and evolution of the primary magmas at Mt. Vesuvius. Uncontaminated mantle δ18O values are hardly recovered in Italian Quaternary magmas, mostly due to the widespread occurrence of crustal contamination of the primary melts during their ascent to the surface (e.g. Alban Hills, Ernici Mts., and Aeolian Islands). At Mt. Vesuvius, measured olivine and clinopyroxene share quite homogeneous chemical compositions (Olivine Fo 85-90 ; Diopside En 45-48, respectively), and represent phases crystallized in near primary mafic magmas. Trace element composition constrains the near primary nature of the phases. Published data on volatile content of melt inclusions hosted in these crystals reveal the coexistence of dissolved water and carbon dioxide, and a minimum trapping pressure around 200-300 MPa, suggesting that crystal growth occurred in a reservoir at about 8-10 km depth. Recently, experimental data have suggested massive carbonate assimilation (up to about 20%) to derive potassic alkali magmas from trachybasaltic melts. Accordingly, the δ18O variability and the trace element content of the studied minerals suggest possible contamination of primary melts by an O-isotope enriched, REE-poor contaminant like the limestone of Vesuvius basement. Low, nearly primitive δ18O values are observed for olivine from Pompeii eruption, although still above the range of typical mantle minerals. The δ18Oolivine and δ18Ocpxof the minerals from all the studied eruptions define variable degrees of carbonate interaction and magma crystallization for the different eruptions, and possibly within the same eruption, and show evidence of oxygen isotope equilibrium at high temperature. However, energy-constrained AFC model suggest that carbonate assimilation was limited. On the basis of our data, we suggest that interaction between magma and a fluxing, decarbonation-derived CO2 fluid may be partly accounted for the measured O-isotope compositions.
Mafic phenocrysts from selected products of the last 4 ka volcanic activity at Mt. Vesuvius were ... more Mafic phenocrysts from selected products of the last 4 ka volcanic activity at Mt. Vesuvius were investigated for their chemical and O-isotope composition, as a proxy for primary magmas feeding the system. 18O/ 16O ratios of studied Mg-rich olivines suggest that near-primary shoshonitic to tephritic melts experienced a flux of sedimentary carbonate-derived CO 2, representing the early process of magma contamination in the roots of the volcanic structure. Bulk carbonate assimilation (physical digestion) mainly occurred in the shallow crust, strongly influencing magma chamber evolution. On a petrological and geochemical basis the effects of bulk sedimentary carbonate digestion on the chemical composition of the near-primary melts are resolved from those of carbonate-released CO 2 fluxed into magma. An important outcome of this process lies in the effect of external CO 2 in changing the overall volatile solubility of the magma, enhancing the ability of Vesuvius mafic magmas to rapidly rise and explosively erupt at the surface.
Direct observations of mid-intensity eruptions, in which a huge amount of ash is generated, indic... more Direct observations of mid-intensity eruptions, in which a huge amount of ash is generated, indicate that ash recycling is quite common. The recognition of juvenile vs. recycled fragments is not straightforward, and no unequivocal, widely accepted criteria exist to support this. The presence of recycled glassy fragments can hide primary magmatic information, introducing bias in the interpretations of the ongoing magmatic and volcanic activity. High temperature experiments were performed at atmospheric pressure on natural samples to investigate the effects of reheating on morphology, texture and composition of volcanic ash. Experiments simulate the transformation of juvenile glassy fragments that, falling into the crater or in the upper part of the conduit, are recycled by following explosions. Textural and compositional modifications obtained in laboratory are compared with similar features observed in natural samples in order to identify some main general criteria to be used for the discrimination of recycled material. Experiments were carried out on tephra produced during Strombolian activity, fire fountains and continuous ash emission at Etna, Stromboli and Vesuvius. Coarse glassy clasts were crushed in a nylon mortar in order to create an artificial ash, and then sieved to select the size interval of 1-0.71 mm. Ash shards were put in a sealed or open quartz tube, in order to prevent or to reproduce effects of air oxidation. The tube was suspended in a HT furnace at INGV-Pisa and kept at different temperatures (up to to 1110°C) for increasing time (0.5-12 hours). Preliminary experiments were also performed under gas flux conditions. Optical and electron microscope observations indicate that high temperature and exposure to the air induce large modifications on clast surface, ranging from change in color, to incipient plastic deformation till complete sintering. Significant change in color of clasts is strictly related to the presence of air, irrespective of temperature while sintering is favored by the high temperature and low fO2. Re-heating promotes nucleation and growth of crystals in the groundmass and associated change of glass composition, sometimes accompanied by growth and coalescence of vesicles in the size of 10-50 µm and cracking of the external surface.
ABSTRACT In the recent years, continuous ash emission activity, related to mid-low intensity, lon... more ABSTRACT In the recent years, continuous ash emission activity, related to mid-low intensity, long lasting eruptions, has been increasingly described to occur at different volcanoes worldwide. Focusing on this type of deposits, a retrospective analysis of the stratigraphic successions at Vesuvius revealed that such type of eruptions have occurred repeatedly in the last 4000 years of activity. This type of activity has been overlooked in the past and the mechanism of ash production in these eruptions is not yet clear. The detailed study of the deposits suggests that these eruptions are dominated by discrete phases of repeated emission of a highly fragmented mixture, alternated with violent strombolian episodes. In this study we present morphological, textural and compositional data on the products of two ash eruptions representative of the whole variability of this activity at Vesuvius (Italy), occurred in the periods between the “Avellino” and “Pompeii” Pumice eruptions (AP3, 2,710±60 years B.P.) and after the 512 A.D. eruption, (AS1a). Juvenile fragments from different ash layers throughout the studied stratigraphic sections were fully characterized in terms of external morphology, particle outline parameterization, groundmass texture (in terms of Crystal and Vesicle Size Distributions) and glass composition. Volcanic ash holds information about magma dynamics within the volcanic conduit, where fragmentation occurs and eruption style is decided. Results of our investigations have been interpreted in terms of fragmentation processes, transport and dispositional mechanisms, dynamics of magma ascent ant timing of the eruption. The methodology of ash analysis used for this study has also important implications for tephrochronological studies, adding a complete parameterization of physical and textural properties of the ash to the routinely used compositional data.
ABSTRACT 1] Textures, petrography and geochemical compositions of products emitted during the ons... more ABSTRACT 1] Textures, petrography and geochemical compositions of products emitted during the onset of the 2011–2012 sub-marine eruption (15 October, 2011) off the coast of El Hierro have been investigated to get information on interaction mechanism between the first rising magma and the crust during the onset of the eruption as well as to get information on magma storage and plumbing systems beneath El Hierro volcano. Studied products consist of 5–50 cm bombs with an outer black to greenish, vesicular crust with bulk basanite composition containing pumiceous xenoliths (xenopumices). Our results show that xenopumices are much more hetero-geneous that previously observed, since consist of a macro-scale mingling of a gray trachyte and white rhyolite. We interpreted xenopumices as resulting from the interaction (heating) between the basanitic magma feeding the eruption, a stagnant trachytic magma pocket/s and an associated hydro-thermally altered halo with rhyolitic composition. Our find-ings confirm the importance of the study of the early products of an eruption since they can contain crucial information on the plumbing system geometry and the mechanism of magma ascent. Citation: Meletlidis, S.
Recent stratigraphic studies at Vesuvius have
revealed that, during the past 4,000 years, long la... more Recent stratigraphic studies at Vesuvius have revealed that, during the past 4,000 years, long lasting, moderate to low-intensity eruptions, associated with continuous or pulsating ash emission, have repeatedly occurred. The present work focuses on the AS1a eruption, the first of a series of ash-dominated explosive episodes which characterized the period between the two Subplinian eruptions of 472 AD and 1631 AD. The deposits of this eruption consist of an alternation of massive and thinly laminated ash layers and minor well sorted lapilli beds, reflecting the pulsatory injection into the atmosphere of variably concentrated ash-plumes alternating with Violent Strombolian stages. Despite its nearly constant chemical composition, the juvenile material shows variable external clast morphologies and groundmass textures, reflecting the fragmentation of a magma body with lateral and/or vertical gradients in both vesicularity and crystal content. Glass compositions and mineralogical assemblages indicate that the eruption was fed by rather homogeneous phonotephritic magma batches rising from a reservoir located at ~ 4 km (100 MPa) depth, with fluctuations between magma delivery and magma discharge. Using crystal size distribution (CSD) analyses of plagioclase and leucite microlites, we estimate that the transit time of the magma in the conduit was on the order of ~ 2 days, corresponding to an ascent rate of around 2× 10−2 ms−1. Accordingly, assuming a typical conduit diameter for this type of eruption, the minimum duration of the AS1a event is between about 1.5 and 6 years. Magma fragmentation occurred in an inertially driven regime that, in a magma with low viscosity and surface tension, can act also under conditions of slow ascent.
A set of experiments have been performed on
volcanic materials from Etna, Stromboli and Vesuvius ... more A set of experiments have been performed on volcanic materials from Etna, Stromboli and Vesuvius in order to evaluate how the exposure to thermal and redox conditions close to that of active craters affects the texture and composition of juvenile pyroclasts. Selected samples were placed within a quartz tube, in presence of air or under vacuum, and kept at T between 700 and 1,130 C, for variable time (40 min to 12 h). Results show that reheating reactivates the melt, which, through processes of chemical and thermal diffusion, reaches new equilibrium conditions. In all the experiments performed at T = 700–750 C, a large number of crystal nuclei and spherulites grows in the groundmass, suggesting conditions of high undercooling. This process creates textural heterogeneities at the scale of few microns but only limited changes of groundmass composition, which remains clustered around that of the natural glasses. Reheating at T = 1,000–1,050 C promotes massive groundmass crystallization, with a different mineral assemblage as a function of the redox conditions. Morphological modifications of clasts, from softening to sintering as temperature increases, occur under these conditions, accompanied by progressive smoothing of external surfaces, and a reduction in size and abundance of vesicles, until the complete obliteration of the pre-existing vesicularity. The transition from sintering to welding, characteristic of high temperature, is influenced by redox conditions. Experiments at T = 1,100–1,130 C and under vacuum produce groundmass textures and glass compositions similar to that of the respective starting material. Collapse and welding of the clasts cause significant densification of the whole charge. At the same temperature, but in presence of air, experimental products at least result sintered and show holocrystalline groundmass. In all experiments, sublimates grow on the external surfaces of the clasts or form a lining on the bubble walls. Their shape and composition is a function of temperature and fO2 and the abundance of sublimates shows a peak at 1,000 C. The identification of the features recorded by pyroclasts during complex heating– cooling cycles allows reconstructing the complete clasts history before their final emplacement, during weakly explosive volcanic activity. This has a strong implication on the characterization of primary juvenile material and on the interpretation of eruption dynamics.
Deposits of mid-intensity basaltic explosive eruptions are characterized by the coexistence of di... more Deposits of mid-intensity basaltic explosive eruptions are characterized by the coexistence of different types of juvenile clasts, which show a large variability of external properties and texture, reflecting alternatively the effects of primary processes related to magma storage or ascent, or of syn-eruptive modifications occurred during or immediately after their ejection. If fragments fall back within the crater area before being re-ejected during the ensuing activity, they are subject to thermally- and chemically-induced alterations. These ‘recycled’ clasts can be considered as cognate lithic for the eruption/explosion they derive. Their exact identification has consequences for a correct interpretation of eruption dynamics, with important implications for hazard assessment. On ash erupted during selected basaltic eruptions (at Stromboli, Etna, Vesuvius, Gaua-Vanuatu), we have identified a set of characteristics that can be associated with the occurrence of intra-crater recycling processes, based also on the comparison with results of reheating experiments performed on primary juvenile material, at variable temperature and under different redox conditions.
Pumice and scoria from different eruptive layers of Mt. Vesuvius volcanic products contain mafic ... more Pumice and scoria from different eruptive layers of Mt. Vesuvius volcanic products contain mafic minerals consisting of High-Fo olivine and Diopsidic Pyroxene. These phases were crystallized in unerupted trachibasaltic to tephritic magmas, and were brought to surface by large phonolitic/tephri-phonolitic (e.g. Avellino and Pompei) and/or of tephritic and phono-tephritic (Pollena) eruptions. A large set of these mm-sized crystals was accurately separated from selected juvenile material and measured for their chemical compositions (EPMA, Laser Ablation ICP-MS) and 18O/16O ratios (conventional laser fluorination) to constrain the nature and evolution of the primary magmas at Mt. Vesuvius. Uncontaminated mantle δ18O values are hardly recovered in Italian Quaternary magmas, mostly due to the widespread occurrence of crustal contamination of the primary melts during their ascent to the surface (e.g. Alban Hills, Ernici Mts., and Aeolian Islands). At Mt. Vesuvius, measured olivine and clinopyroxene share quite homogeneous chemical compositions (Olivine Fo 85-90 ; Diopside En 45-48, respectively), and represent phases crystallized in near primary mafic magmas. Trace element composition constrains the near primary nature of the phases. Published data on volatile content of melt inclusions hosted in these crystals reveal the coexistence of dissolved water and carbon dioxide, and a minimum trapping pressure around 200-300 MPa, suggesting that crystal growth occurred in a reservoir at about 8-10 km depth. Recently, experimental data have suggested massive carbonate assimilation (up to about 20%) to derive potassic alkali magmas from trachybasaltic melts. Accordingly, the δ18O variability and the trace element content of the studied minerals suggest possible contamination of primary melts by an O-isotope enriched, REE-poor contaminant like the limestone of Vesuvius basement. Low, nearly primitive δ18O values are observed for olivine from Pompeii eruption, although still above the range of typical mantle minerals. The δ18Oolivine and δ18Ocpxof the minerals from all the studied eruptions define variable degrees of carbonate interaction and magma crystallization for the different eruptions, and possibly within the same eruption, and show evidence of oxygen isotope equilibrium at high temperature. However, energy-constrained AFC model suggest that carbonate assimilation was limited. On the basis of our data, we suggest that interaction between magma and a fluxing, decarbonation-derived CO2 fluid may be partly accounted for the measured O-isotope compositions.
Mafic phenocrysts from selected products of the last 4 ka volcanic activity at Mt. Vesuvius were ... more Mafic phenocrysts from selected products of the last 4 ka volcanic activity at Mt. Vesuvius were investigated for their chemical and O-isotope composition, as a proxy for primary magmas feeding the system. 18O/ 16O ratios of studied Mg-rich olivines suggest that near-primary shoshonitic to tephritic melts experienced a flux of sedimentary carbonate-derived CO 2, representing the early process of magma contamination in the roots of the volcanic structure. Bulk carbonate assimilation (physical digestion) mainly occurred in the shallow crust, strongly influencing magma chamber evolution. On a petrological and geochemical basis the effects of bulk sedimentary carbonate digestion on the chemical composition of the near-primary melts are resolved from those of carbonate-released CO 2 fluxed into magma. An important outcome of this process lies in the effect of external CO 2 in changing the overall volatile solubility of the magma, enhancing the ability of Vesuvius mafic magmas to rapidly rise and explosively erupt at the surface.
Direct observations of mid-intensity eruptions, in which a huge amount of ash is generated, indic... more Direct observations of mid-intensity eruptions, in which a huge amount of ash is generated, indicate that ash recycling is quite common. The recognition of juvenile vs. recycled fragments is not straightforward, and no unequivocal, widely accepted criteria exist to support this. The presence of recycled glassy fragments can hide primary magmatic information, introducing bias in the interpretations of the ongoing magmatic and volcanic activity. High temperature experiments were performed at atmospheric pressure on natural samples to investigate the effects of reheating on morphology, texture and composition of volcanic ash. Experiments simulate the transformation of juvenile glassy fragments that, falling into the crater or in the upper part of the conduit, are recycled by following explosions. Textural and compositional modifications obtained in laboratory are compared with similar features observed in natural samples in order to identify some main general criteria to be used for the discrimination of recycled material. Experiments were carried out on tephra produced during Strombolian activity, fire fountains and continuous ash emission at Etna, Stromboli and Vesuvius. Coarse glassy clasts were crushed in a nylon mortar in order to create an artificial ash, and then sieved to select the size interval of 1-0.71 mm. Ash shards were put in a sealed or open quartz tube, in order to prevent or to reproduce effects of air oxidation. The tube was suspended in a HT furnace at INGV-Pisa and kept at different temperatures (up to to 1110°C) for increasing time (0.5-12 hours). Preliminary experiments were also performed under gas flux conditions. Optical and electron microscope observations indicate that high temperature and exposure to the air induce large modifications on clast surface, ranging from change in color, to incipient plastic deformation till complete sintering. Significant change in color of clasts is strictly related to the presence of air, irrespective of temperature while sintering is favored by the high temperature and low fO2. Re-heating promotes nucleation and growth of crystals in the groundmass and associated change of glass composition, sometimes accompanied by growth and coalescence of vesicles in the size of 10-50 µm and cracking of the external surface.
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revealed that, during the past 4,000 years, long lasting,
moderate to low-intensity eruptions, associated with continuous
or pulsating ash emission, have repeatedly occurred.
The present work focuses on the AS1a eruption, the
first of a series of ash-dominated explosive episodes which
characterized the period between the two Subplinian
eruptions of 472 AD and 1631 AD. The deposits of this
eruption consist of an alternation of massive and thinly laminated ash layers and minor well sorted lapilli beds,
reflecting the pulsatory injection into the atmosphere of
variably concentrated ash-plumes alternating with Violent
Strombolian stages. Despite its nearly constant chemical
composition, the juvenile material shows variable external
clast morphologies and groundmass textures, reflecting the
fragmentation of a magma body with lateral and/or vertical
gradients in both vesicularity and crystal content. Glass
compositions and mineralogical assemblages indicate that the
eruption was fed by rather homogeneous phonotephritic
magma batches rising from a reservoir located at ~ 4 km
(100 MPa) depth, with fluctuations between magma delivery
and magma discharge. Using crystal size distribution (CSD)
analyses of plagioclase and leucite microlites, we estimate that
the transit time of the magma in the conduit was on the order
of ~ 2 days, corresponding to an ascent rate of around 2×
10−2 ms−1. Accordingly, assuming a typical conduit diameter
for this type of eruption, the minimum duration of the AS1a
event is between about 1.5 and 6 years. Magma fragmentation
occurred in an inertially driven regime that, in a magma
with low viscosity and surface tension, can act also under
conditions of slow ascent.
volcanic materials from Etna, Stromboli and Vesuvius in
order to evaluate how the exposure to thermal and redox
conditions close to that of active craters affects the texture
and composition of juvenile pyroclasts. Selected samples
were placed within a quartz tube, in presence of air or
under vacuum, and kept at T between 700 and 1,130 C, for
variable time (40 min to 12 h). Results show that reheating
reactivates the melt, which, through processes of chemical
and thermal diffusion, reaches new equilibrium conditions.
In all the experiments performed at T = 700–750 C, a
large number of crystal nuclei and spherulites grows in the
groundmass, suggesting conditions of high undercooling.
This process creates textural heterogeneities at the scale of
few microns but only limited changes of groundmass
composition, which remains clustered around that of the
natural glasses. Reheating at T = 1,000–1,050 C promotes
massive groundmass crystallization, with a different
mineral assemblage as a function of the redox conditions. Morphological modifications of clasts, from softening to
sintering as temperature increases, occur under these conditions,
accompanied by progressive smoothing of external
surfaces, and a reduction in size and abundance of vesicles,
until the complete obliteration of the pre-existing vesicularity.
The transition from sintering to welding, characteristic
of high temperature, is influenced by redox conditions.
Experiments at T = 1,100–1,130 C and under vacuum
produce groundmass textures and glass compositions similar
to that of the respective starting material. Collapse and
welding of the clasts cause significant densification of the
whole charge. At the same temperature, but in presence of
air, experimental products at least result sintered and show
holocrystalline groundmass. In all experiments, sublimates
grow on the external surfaces of the clasts or form a lining
on the bubble walls. Their shape and composition is a
function of temperature and fO2 and the abundance of
sublimates shows a peak at 1,000 C. The identification of
the features recorded by pyroclasts during complex heating–
cooling cycles allows reconstructing the complete
clasts history before their final emplacement, during
weakly explosive volcanic activity. This has a strong
implication on the characterization of primary juvenile
material and on the interpretation of eruption dynamics.
of juvenile clasts, which show a large variability of external properties and texture, reflecting alternatively the
effects of primary processes related to magma storage or ascent, or of syn-eruptive modifications occurred
during or immediately after their ejection. If fragments fall back within the crater area before being
re-ejected during the ensuing activity, they are subject to thermally- and chemically-induced alterations.
These ‘recycled’ clasts can be considered as cognate lithic for the eruption/explosion they derive. Their exact
identification has consequences for a correct interpretation of eruption dynamics, with important
implications for hazard assessment. On ash erupted during selected basaltic eruptions (at Stromboli, Etna,
Vesuvius, Gaua-Vanuatu), we have identified a set of characteristics that can be associated with the
occurrence of intra-crater recycling processes, based also on the comparison with results of reheating
experiments performed on primary juvenile material, at variable temperature and under different redox
conditions.
revealed that, during the past 4,000 years, long lasting,
moderate to low-intensity eruptions, associated with continuous
or pulsating ash emission, have repeatedly occurred.
The present work focuses on the AS1a eruption, the
first of a series of ash-dominated explosive episodes which
characterized the period between the two Subplinian
eruptions of 472 AD and 1631 AD. The deposits of this
eruption consist of an alternation of massive and thinly laminated ash layers and minor well sorted lapilli beds,
reflecting the pulsatory injection into the atmosphere of
variably concentrated ash-plumes alternating with Violent
Strombolian stages. Despite its nearly constant chemical
composition, the juvenile material shows variable external
clast morphologies and groundmass textures, reflecting the
fragmentation of a magma body with lateral and/or vertical
gradients in both vesicularity and crystal content. Glass
compositions and mineralogical assemblages indicate that the
eruption was fed by rather homogeneous phonotephritic
magma batches rising from a reservoir located at ~ 4 km
(100 MPa) depth, with fluctuations between magma delivery
and magma discharge. Using crystal size distribution (CSD)
analyses of plagioclase and leucite microlites, we estimate that
the transit time of the magma in the conduit was on the order
of ~ 2 days, corresponding to an ascent rate of around 2×
10−2 ms−1. Accordingly, assuming a typical conduit diameter
for this type of eruption, the minimum duration of the AS1a
event is between about 1.5 and 6 years. Magma fragmentation
occurred in an inertially driven regime that, in a magma
with low viscosity and surface tension, can act also under
conditions of slow ascent.
volcanic materials from Etna, Stromboli and Vesuvius in
order to evaluate how the exposure to thermal and redox
conditions close to that of active craters affects the texture
and composition of juvenile pyroclasts. Selected samples
were placed within a quartz tube, in presence of air or
under vacuum, and kept at T between 700 and 1,130 C, for
variable time (40 min to 12 h). Results show that reheating
reactivates the melt, which, through processes of chemical
and thermal diffusion, reaches new equilibrium conditions.
In all the experiments performed at T = 700–750 C, a
large number of crystal nuclei and spherulites grows in the
groundmass, suggesting conditions of high undercooling.
This process creates textural heterogeneities at the scale of
few microns but only limited changes of groundmass
composition, which remains clustered around that of the
natural glasses. Reheating at T = 1,000–1,050 C promotes
massive groundmass crystallization, with a different
mineral assemblage as a function of the redox conditions. Morphological modifications of clasts, from softening to
sintering as temperature increases, occur under these conditions,
accompanied by progressive smoothing of external
surfaces, and a reduction in size and abundance of vesicles,
until the complete obliteration of the pre-existing vesicularity.
The transition from sintering to welding, characteristic
of high temperature, is influenced by redox conditions.
Experiments at T = 1,100–1,130 C and under vacuum
produce groundmass textures and glass compositions similar
to that of the respective starting material. Collapse and
welding of the clasts cause significant densification of the
whole charge. At the same temperature, but in presence of
air, experimental products at least result sintered and show
holocrystalline groundmass. In all experiments, sublimates
grow on the external surfaces of the clasts or form a lining
on the bubble walls. Their shape and composition is a
function of temperature and fO2 and the abundance of
sublimates shows a peak at 1,000 C. The identification of
the features recorded by pyroclasts during complex heating–
cooling cycles allows reconstructing the complete
clasts history before their final emplacement, during
weakly explosive volcanic activity. This has a strong
implication on the characterization of primary juvenile
material and on the interpretation of eruption dynamics.
of juvenile clasts, which show a large variability of external properties and texture, reflecting alternatively the
effects of primary processes related to magma storage or ascent, or of syn-eruptive modifications occurred
during or immediately after their ejection. If fragments fall back within the crater area before being
re-ejected during the ensuing activity, they are subject to thermally- and chemically-induced alterations.
These ‘recycled’ clasts can be considered as cognate lithic for the eruption/explosion they derive. Their exact
identification has consequences for a correct interpretation of eruption dynamics, with important
implications for hazard assessment. On ash erupted during selected basaltic eruptions (at Stromboli, Etna,
Vesuvius, Gaua-Vanuatu), we have identified a set of characteristics that can be associated with the
occurrence of intra-crater recycling processes, based also on the comparison with results of reheating
experiments performed on primary juvenile material, at variable temperature and under different redox
conditions.