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Brown, Richard J.
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Brown, Richard J.
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- PublicationRestrictedEvidence for a large-magnitude eruption from Campi Flegrei caldera (Italy) at 29 ka(2019)
; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ; ; ; ; ; ;The 40 ka caldera-forming eruption of Campi Flegrei (Italy) is the largest known erup- tion in Europe during the last 200 k.y., but little is known about other large eruptions at the volcano prior to a more recent caldera-forming event at 15 ka. At 29 ka a widespread volcanic ash layer, termed the Y-3 tephra, covered >150,000 km2 of the Mediterranean. The glass compositions of the layer are consistent with Campi Flegrei being the source, but no prominent proximal equivalent in the appropriate chrono-stratigraphic position had been previously identified. Here we report new glass chemistry data and 40Ar/39Ar ages (29.3 ± 0.7 ka [2σ]) that reveal the near-source Y-3 eruption deposit in a sequence at Ponti Rossi and a nearby borehole (S-19) in Naples. The dispersal and thickness of the deposits associ- ated with this eruption, herein named the Masseria del Monte Tuff, were simulated using a tephra sedimentation model. The model indicates that ~16 km3 dense rock equivalent of the magma erupted was deposited as fall. This volume and the areal distribution suggest that the Masseria del Monte Tuff resulted from a magnitude (M) 6.6 eruption (corresponding to volcanic explosivity index [VEI] 6), similar to the 15 ka caldera-forming Neapolitan Yellow Tuff (M 6.8) eruption at Campi Flegrei. However, the lack of coarse, thick, traceable, near- vent deposit suggests peculiar eruption dynamics. Our reconstruction and modeling of the eruption show the fundamental role that distal tephrostratigraphy can play in constraining the scale and tempo of past activity, especially at highly productive volcanoes.117 9 - PublicationOpen AccessPetrography and Mineral Chemistry of Monte Epomeo Green Tuff, Ischia Island, South Italy: Constraints for Identification of the Y-7 Tephrostratigraphic Marker in Distal Sequences of the Central Mediterranean(2021)
; ; ; ; ; ; ; ; ; ; ; The 56 ka Monte Epomeo Green Tuff (MEGT) resulted from the largest volume explosive eruption from Ischia island (south Italy). Its tephra is one of the main stratigraphic markers of the central Mediterranean area. Despite its importance, a detailed characterisation of the petrography and mineral chemistry of MEGT is lacking. To fill this gap, we present detailed petrographic description and electron microprobe mineral chemistry data on samples collected on-land from the MEGT. Juvenile clasts include pumice, scoria, and obsidian fragments with porphyritic/glomeroporphyritic, vitrophyric, and fragmental textures. The porphyritic index is 13–40 vol.%, and phenocryst phases include alkali-feldspar, plagioclase, clinopyroxene, ferrian phlogopite, and titano-magnetite, in order of decreasing abundance; accessory phases include sphene, hydroxy-fluor-apatite, and rare edenite. Plagioclase varies from predominant andesine to subordinate oligoclase, whereas alkali-feldspar is more variable from sanidine to anorthoclase; quasi-pure sanidine commonly occurs as either rim or recrystallisation overgrowth of large phenocrysts due to hydrothermal alteration. Secondary minerals include veins and patches of carbonate minerals, Fe-Mn oxyhydroxides, clay minerals, and zeolites. Clinopyroxene is ferroan diopside (En45–29Fs7–27) and never reaches Na-rich compositions. This feature allows the discrimination of MEGT from aegirine-bearing, distal tephra layers erroneously attributed to MEGT, with implications for the areal distribution of Ischia explosive deposits356 11 - PublicationRestrictedGeochemical and isotopic insights into the assembly, evolution and disruption of a magmatic plumbing system before and after a cataclysmic caldera-collapse eruption at Ischia volcano (Italy)(2014)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; New geochemical and isotopic data on volcanic rocks spanning the period ~75–50 ka BP on Ischia volcano, Italy, shed light on the evolution of the magmatic system before and after the catastrophic, caldera-forming Monte Epomeo Green Tuff (MEGT) eruption. Volcanic ac tivity during this period was influenced by a large, composite and differentiating magmatic system, replenished several times with isotopically distinct magmas of deepprovenance. Chemical and isotopic variations highlight that the pre-MEGT eruptions were fed by trachytic/phonolitic magmas from an isotopically zoned reservoir that were poorly enriched in radiogenic Sr and became progressively less radiogenic with time. Just prior to the MEGT eruption, the magmatic system was recharged by an isotopically distinct magma, relatively more enriched in radiogenic Sr with respect to the previously erupted magmas. This second magma initially fed several SubPlinian explosive eruptions and later supplied the climactic, phonolitic-to-trachytic MEGT eruption(s). Isotopic data, together with erupted volume estimations obtained for MEGT eruption(s), indicate that >5–10 km3of this relatively enriched magma had accumulated in the Ischia plumbing system. Geochemical modelling indicates that it accumulated at shallow depths (4–6 km), over a period of ca. 20 ka. After the MEGT eruption, volcanic activity was fed by a new batch of less differentiated (trachyte-latite) magma that was slightly less enriched in radiogenic Sr. The geochemical and Sr–Nd-isotopic variations through time reflect the upward flux of isotopically distinct magma batches, variably contaminated byHercynian crust at 8–12 km depth. The deep-sourced latitic to trachytic magmas stalled at shallow depths (4–6 km depth), differentiated to phonolite through crystal fractionation and assimilation of a feldspar-rich mush, or ascended directly to the surface and erupted.74 1 - PublicationOpen AccessA multi-methodological approach to record dynamics and timescales of the plumbing system of Zaro (Ischia Island, Italy)(2024-05-04)
; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ;; ;; ; Determining the time spans of processes related to the assembly of eruptible magma at active volcanoes is fundamental to understand magma chamber dynamics and assess volcanic hazard. This information can be recorded in the chemical zoning of crystals. Nevertheless, this kind of study is still poorly employed for the active volcanoes of the Neapolitan area (Southern Italy), in particular, for Ischia island where the risk is extremely high and this information can provide the basis for probabilistic volcanic hazard assessment. For these reasons, we acquired chemical composition on clinopyroxene crystals erupted at Ischia during the Zaro eruption (6.6 ± 2.2 ka) and performed numerical simulations of the input of mafic magma into a trachytic reservoir, in order to investigate various aspects of pre-eruptive dynamics occurring at different timescales. This event emplaced a ~ 0.1 km3 lava complex, in which the main trachytic lava flows host abundant mafic to felsic enclaves. Previous petrological investigation suggested that mafic magma(s) mixed/mingled with a trachytic one, before the eruption. In this work, the clinopyroxene zoning patterns depict the growth of crystals in different magmatic environments, recording sequential changes occurred in the plumbing system before the eruption. The evolution of the plumbing system involved a hierarchy of timescales: a few hours for magma mingling caused by mafic recharge(s) and likely occurred multiple times over a decade during which a dominant magmatic environment was sustained before the eruption. Such timescales must be considered in volcanic hazard assessment at Ischia and similar active volcanoes in densely populated areas.225 4 - PublicationRestrictedNew insights into Late Pleistocene explosive volcanic activity and caldera formation on Ischia (southern Italy)(2008-03)
; ; ; ;Brown, R.; Department of Earth Sciences, University of Bristol, ;Orsi, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;De Vita, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia; ; A new pyroclastic stratigraphy is presented for the island of Ischia, Italy, for the period ∼75–50 ka BP. The data indicate that this period bore witness to the largest eruptions recorded on the island and that it was considerably more volcanically active than previously thought. Numerous vents were probably active during this period. The deposits of at least 10 explosive phonolite to basaltictrachyandesite eruptions are described and interpreted. They record a diverse range of explosive volcanic activity including voluminous fountain-fed ignimbrite eruptions, fallout from sustained eruption columns, block-and-ash flows, and phreatomagmatic eruptions. Previously unknown eruptions have been recognised for the first time on the island. Several of the eruptions produced pyroclastic density currents that covered the whole island as well as the neighbouring island of Procida and parts of the mainland. The morphology of Ischia was significantly different to that seen today, with edifices to the south and west and a submerged depression in the centre. The largest volcanic event, the Monte Epomeo Green Tuff (MEGT) resulted in caldera collapse across all or part of the island. It is shown to comprise at least two thick intracaldera ignimbrite flowunits, separated by volcaniclastic sediments that were deposited during a pause in the eruption. Extracaldera deposits of the MEGT include a pumice fall deposit emplaced during the opening phases of the eruption, a widespread lithic lag breccia outcropping across much of Ischia and Procida, and a distal ignimbrite in south-west Campi Flegrei. During this period the style and magnitude of volcanism was dictated by the dynamics of a large differentiated magma chamber, which was partially destroyed during the MEGT eruption. This contrasts with the small-volume Holocene and historical effusive and explosive activity on Ischia, the timing and distribution of which has been controlled by the resurgence of the Monte Epomeo block. The new data contribute to a clearer understanding of the long-term volcanic and magmatic evolution of Ischia.200 33 - PublicationOpen AccessThe 2013 eruption of Chaparrastique volcano (El Salvador): Effects of magma storage, mixing, and decompression(2017)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ;On December 29, 2013, an isolated vulcanian-type eruption occurred at Chaparrastique volcano (El Salvador) after 12 years of inactivity. The eruption was classified as VEI 2 and produced an ash plume with a maximum height of ~9 km. Textural and compositional data fromphenocrysts fromthe erupted products have been inte- grated with geochemical and isotopic information frombulk rocks to elucidate the magmatic processes respon- sible for the reawakening of volcanic activity. Phenocrysts consist of Fo-rich poikilitic olivines hosting high-Mg titanomagnetites, and Fo-poor olivines coexisting with low-Mg titanomagnetites.Mineral-melt equilibria suggest an origin for the distinct phenocryst populations by mixing between a high-T (~1130–1150 °C), basaltic magma with fO2 (NNO buffer) typical of the lower crust inarc systems and a low-T (~1060–1080 °C), basaltic andesiticmagma with fO2 (NNO+1 buffer) commonly encountered in shallower, more oxidized crustal reservoirs. Thermobarometry based on Fe-Mg ex- change between orthopyroxene and clinopyroxene constrains the crystallization before eruption at relative low-P (~150–250MPa) and low-T (~1000–1050 °C).Mixing between twochemically distinctmagmas is also ev- idenced by the occurrence of reverse zoned plagioclase phenocrysts with resorbed sodic cores and re-growth of sieve-textured calcicmantles. Conversely, plagioclase rims exhibit disequilibriumcompositions addressed to de- compression kinetics (~10−3 MPa/s) driven by rapidmagma ascent to the surface (~0.03 m/s). Major and trace element modelling excludes fractional crystallization as the primarymechanismcontrolling the bulk rock variability,whereas geochemical data align along amixing trend between two end-members represen- tative of the primitive basalt and the differentiated basaltic andesite. Trace element and isotope data indicate that the primary source of magmatism is an enriched MORB-like mantle with the contribution of fluxes ofmetaso- matic fluids and/ormelts produced by the subducted slab. The roleplayed by slab-fluid inputs of carbonate origin and slab-melts fromthe hemipelagic sediments seems to beminimal. Assimilation/contamination processes of magmas by crustal rocks are also negligible. In contrast, the geochemical signature of magmas is greatly influ- enced by slab-derived aqueous fluids produced prevalently by progressive dehydration of marine sediments and altered basaltic crust356 98 - PublicationOpen AccessInsights into the explosive eruption history of the Campanian volcanoes prior to the Campanian Ignimbrite eruptionThe Campanian Volcanic Zone (CVZ) comprises multiple active volcanoes and includes the highly productive Campi Flegrei and Ischia caldera systems. These caldera volcanoes have produced probably the largest eruptions in Europe in the past 200 ka, such as the Monte Epomeo Green Tuff (MEGT; Ischia) at ca. 56 ka and the Campanian Ignimbrite (CI; Campi Flegrei) at ca. 40 ka, which form widespread isochrons across the Mediterranean region. These closely-spaced volcanic centres erupt phonolitic to trachytic glass compositions that are similar, and thus it can be challenging to correlate tephra deposits to specific volcanic sources. Here we present a detailed tephrostratigraphy for pre-CI eruption activity using the units preserved within a sequence at the coastal Acquamorta outcrop, on the western side of the CI caldera rim. Both the MEGT and CI units are present in the section, and they bracket twelve eruption units that were logged and sampled. New major and trace element glass chemistry data have been acquired for these Acquamorta tephra deposits. Three eruption deposits from Ischia and nine from Campi Flegrei are identified, which helps constrain the tempo of volcanic activity of these centres between the large caldera-forming eruptions. The three Ischia tephra deposits between the MEGT and the CI are indistinguishable based on both major and trace element glass chemistry and cannot be correlated to a specific or known eruption in this interval, such as the Schiappone tephra. The compositional variations between the Campi Flegrei eruptions reveal temporal shifts in the composition of the tephra deposits that reflect changes in the magmatic system prior to the CI eruption. These deposits indicate that there were at least nine eruptions at Campi Flegrei within 16 ka of the enormous CI eruption, and suggest that there was no significant period of repose before the caldera generating eruption.
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