Massimo D'Antonio

Volcanism and deformation since 12,000 years at the Campi Flegrei caldera Italy

Abstract The Campi Flegrei caldera is a restless, nested structure resulting from two major collapses related to the Campanian Ignimbrite (37,000 years BP) and the Neapolitan Yellow Tuff (12,000 years BP) eruptions, respectively. Detailed stratigraphical, structural, volcanological and 14 C (AMS) geochronological studies, devoted to the reconstruction of the volcanic and deformational history of the Campi Flegrei caldera in the past 12,000 years have been carried out. The results of these studies show that in this time span, intense both volcanic and volcano-tectonic activity was confined inside the Neapolitan Yellow Tuff caldera. Volcanism was concentrated in epochs of intense activity, alternating to periods of quiescence. The I epoch lasted from 12,000 to 9500 years BP giving rise to 34 explosive eruptions, each every 70 years on average. During the II epoch, dated between 8600 and 8200 years BP, six explosive eruptions took place at an average interval of 65 years. The III epoch lasted from 4800 to 3800 years BP and produced 16 explosive and four effusive eruptions which followed each other at mean intervals of 50 years. Eruption vents of the I epoch were located mostly along the marginal faults of the Neapolitan Yellow Tuff caldera, while those of the II epoch aligned on the northeastern sector of this margin. During the III epoch volcanism was confined in the northeastern sector of the Neapolitan Yellow Tuff caldera floor. The caldera floor is disjointed in blocks with variable vertical movements by fault and fracture systems mainly trending NE–SW and NW–SE. The still active resurgence of the caldera floor began soon after its collapse. Onset of both II and III epoch of activity coincides with increase in resurgence rate of La Starza marine terrace, the most uplifted part of the resurgent block.

Chemical and Sr-isotopical evolution of the Phlegraean magmatic system before the Campanian Ignimbrite and the Neapolitan Yellow Tuff eruptions

Abstract New geochronological, geochemical, and Sr-isotopic data on volcanics erupted before the Campanian Ignimbrite (CI, 37 ka) and the Neapolitan Yellow Tuff (NYT, 12 ka) caldera-forming eruptions at Campi Flegrei (CF) have allowed us to investigate the behavior and temporal evolution of the Phlegraean magmatic system. The most prominent feature of the CF magmatic system was the existence of a large, trachytic magma chamber, episodically recharged, which fed eruptions for tens of thousands years before the CI and NYT eruptions. During the pre-CI caldera activity, magmas were episodically erupted from vents located outside the present caldera structure. These magmas ranged in composition from trachyte to alkali-trachyte, with Sr-isotope ratios increasing through time, and becoming identical to that of the CI magma, at about 44 ka ago. This suggests that the Phlegraean magmatic system before the CI eruption was acting as an open system. It was being progressively replenished by new batches of magma that mixed with the resident less radiogenic, fractionating trachytic magmas and was periodically tapped. The magma chamber evolution culminated in the catastrophic eruption of the voluminous (150 km3 DRE), chemically and isotopically zoned CI trachytic magmas, and in the resultant CI caldera formation. Subsequent to the CI eruption, during a period of moderate subaereal volcanic activity of about 20 ka duration, magmas predominantly trachytic to alkali-trachytic in composition and isotopically similar to the last emitted CI magma were erupted from vents located inside the CI caldera. The temporal trend shown by Sr-isotope ratios provides evidence for a new input of alkali-trachytic magma, at ca. 15 ka, with 87 Sr / 86 Sr ratio identical to that of the alkali-trachytic magma feeding the first phase of the NYT eruption. These data testify to the arrival in a short time span of a new trachytic to alkali-trachytic magma in the system, isotopically distinct from the CI magma, that gave rise about 3 ka later to eruption of the NYT (40 km3 DRE).

The Agnano-Monte Spina eruption 4100 years BP in the restless / Campi Flegrei caldera Italy

Abstract The Agnano–Monte Spina tephra (AMST), dated at 4100 years BP by 40 Ar / 39 Ar and 14 C AMS techniques, is the product of the highest-magnitude eruption in the Campi Flegrei caldera (CFc) during its last epoch of activity (4800–3800 years BP). The sequence alternates magmatic and phreatomagmatic pyroclastic-fallout, -flow and -surge beds and bedsets. Two main pumice-fallout deposits with variable easterly-to-northeasterly dispersal axes are about 10 cm thick at 42 km from the vent area. High particle concentration pyroclastic currents were confined to the caldera depression; lower concentration flows overtopped the morphological boundary of the caldera and traveled at least 15 km over the surrounding plain. The unit is subdivided into six members, named A through F in stratigraphic sequence, based upon their sedimentological characteristics. Isopachs and isopleths maps suggest a vent location in the Agnano plain. A volcano-tectonic collapse begun during the course of the eruption, took place along the faults of the northeastern sector of the resurgent block within the CFc, and generated the Agnano plain. The early erupted trachytic magma had a homogeneous alkali–trachytic composition, whereas later-erupted magma shows small-scale hetereogeneities. Trace elements and Sr-isotope compositions, indicate that two isotopically distinct magmas, one alkali–trachytic and the other trachytic, were tapped and partially mixed during the eruption. The small volume (1.2 km3 DRE) of erupted magma and the structural position of the vent suggest that the eruption was fed by a dyke intruded along a normal fault in the sector of the resurgent block under a tensional stress regime.

The Neapolitan Yellow Tuff, a large-magnitude trachytic phreatoplinian eruption: eruptive dynamics, magma withdrawal and caldera collapse

The results of a detailed stratigraphical and volcanological study has allowed the definition of the Neapolitan Yellow Tuff (NYT) as the largest known trachytic phreatoplinian eruption with an estimated volume of not less than 30 km3 DRE. The NYT sequence can be subdivided in a lower and an upper member. Sedimentological and textural variations in time and space indicate that different eruptive and depositional mechanisms operated during the eruption. The lower member eruptive phase was characterized by alternating phreatoplinian and magmatic explosions while the upper member eruptive phase was characterized by a random sequence of phreatomagmatic and magmatic explosions. Particles produced by the same explosion were deposited contemporaneously by fallout and surge mechanisms either at different or at the same distance from the vent. Compositional variations show that the NYT sequence cannot be modeled as an inverted compositionally zoned magma body with magma becoming more basic in the course of the eruption. The chamber, composed of three geochemically distinct magma layers, was tapped at different depths according to the eruption dynamics. Each phreatoplinian explosion tapped concurrently all the magma layers while magmatic explosions tapped only one of them. The onset of the NYT caldera collapse is well constrained between lower and upper member eruptive phases. The collapse, which is only gravimetrically detectable, occurred inside the larger Phlegrean Fields caldera, which therefore can be defined as a nested caldera.

The present state of the magmatic system of the Campi Flegrei caldera based on a reconstruction of its behavior in the past 12 ka

Abstract New geochemical and Sr-isotope data have been acquired on samples representative of volcanic units erupted inside the resurgent Campi Flegrei caldera (CFc) over the past 12 ka. These data, integrated with previous published petrological, and with newly acquired geochronological, volcanological and geothermal data, shed light on the nature and timing of the processes that controlled the evolution of the Phlegraean magmatic system. In the past 12 ka, three isotopically and geochemically distinct magmatic components were erupted at the CFc as either homogeneous or mixed magma batches. One component, Campanian Ignimbrite component (CIc) ( 87 Sr / 86 Sr =0.70735–0.70740), is similar to the trachytic magma extruded during the first phase of the Campanian Ignimbrite (CI) eruption (37 ka). A second component, Neapolitan Yellow Tuff component (NYTc) ( 87 Sr / 86 Sr =0.70750–0.70757), is similar to the latitic–alkali–trachytic magma batches extruded during the course of the Neapolitan Yellow Tuff (NYT) eruption (12 ka). A third component, Minopoli component (MIc) ( 87 Sr / 86 Sr ≈0.7086), is similar to the trachybasaltic magma of the Minopoli 2 (MI) eruption (9.7 ka). These components were erupted as either single batches of magma, or mixed CI–NYT or MI–NYT batches of magma, through vents located either along the structural boundary of the NYT caldera or inside the NYT caldera, mainly on portions of the resurgent block under extensional stress. The CI and NYT components represent residual portions of older, large-volume magma reservoirs which have fed eruptions since about 60 and 15 ka, respectively. The least-evolved MI component was erupted only during the 12–9.5 ka and 8.6–8.2 ka epochs of activity, through vents located on a NE–SW regional fault system. This component could represent a deeper reservoir tapped by the NE–SW regional fault system reactivated after the NYT caldera collapse. Deeper MI and shallower CI and NYT magmatic systems interacted by mixing among batches of magma during their rise to surface. Overall, the data suggest that the CFc magmatic system today is characterized by the presence of two larger, independent reservoirs, filled by residual portions of the CI and NYT magmas. These generated many smaller, shallower pockets of evolved magma, that fed most of the eruptions that occurred in the CFc over the past 12 ka. Moreover, a deeper reservoir (MI), tapped by the NE–SW regional fault system, provided batches of less-evolved magma that mixed with magma present in the shallower pockets.

Step-filling and development of a three-layer magma chamber: the Neapolitan Yellow Tuff case history

Abstract The Neapolitan Yellow Tuff, the product of the largest known trachytic phreatoplinian eruption, gives a good opportunity to investigate the filling mechanisms and internal dynamics of a trachytic magma chamber. A detailed study of the geochemical, mineralogical and isotopical features of the deposit was carried out to investigate the behaviour of the magma chamber before the eruption. The collected data show three distinct compositional groups separated by gaps. Single depositional units contain glass shards formed contemporaneously. Although each of these shards is homogeneous they display the same compositional variations and gaps detected in pumice clasts. This feature is taken as an evidence for interpreting the detected compositional gaps as real gaps in the chamber. Therefore the chamber was filled by three distinct magma bodies separated by compositional gaps. The uppermost magma was alkali-trachyte and highly homogeneous, likely a consequence of vigorous convection. The intermediate magma was trachyte with a slight and continuous compositional variation, likely resulting from less intense convection. The lowermost magma was compositionally zoned from alkali-trachyte to latite downward. This compositional zonation was most likely acquired during uprise from a deeper reservoir. The three magmas entered the chamber sequentially from the uppermost to the lowermost. The latter entered the chamber short before the beginning of the eruption. Its input was interpreted as a possible triggering factor for the eruption. The results of this study strongly support a step-filling mechanism for the Neapolitan Yellow Tuff magma chamber and allow definition of the temporal succession of input of magma batches. Furthermore they also suggest that the magma bodies did not mix although, at least the uppermost two, coexisted inside the chamber for a time long enough to allow internal homogeneization by convection.

Thermal model of the Vesuvius magma chamber

A thermal modeling of the Vesuvius is presented, based on its magmatic and volcanic history. A 2D numerical scheme has been developed to evaluate the heat transfer inside and around a magma body, the latent heat of crystallization and the inputs of magma from the asthenosphere to a crustal reservoir. Assuming a ratio >1 between velocities of magma ascending in the conduit and magma laterally displaced in the reservoir, the results indicate that, after 40 ka, the reservoir is vertically thermally zoned. As a consequence it hosts magma batches that can individually differentiate, mix and be contaminated by the crust, and produce the spectrum of isotopic compositions of the Vesuvian products. The thermal model reproduces the geothermal gradient and the brittle-ductile transition (250–300°C) at 6 km of depth (the maximum depth of earthquake foci) only after 0.5–1 Ma, implying a long lived magma chamber below the volcano.

Petrogenesis of the Late Cretaceous tholeiitic magmatism in the passive margins of northeastern Madagascar

New chemical and Sr-Nd isotopic data on the Late Cretaceous mafic dike swarm intruding the Archean-Proterozoic crystalline basement in the Tamatave-Sainte Marie Island sector (northeast coast passive margin), and on lavas and dikes of the northeastern part of the Mahajanga sedimentary basin (passive margin after the opening of the Jurassic-Cretaceous Somali basin), allow better knowledge of the chemical variations observed in the northern part of the Madagascan igneous province. Two distinct basalt groups have been identified. Group 1 basalts have low light to heavy rare earth element (REE) ratios [(La/Yb) n = 2.2-2.9], low Zr/Y and Nb/Y (4-6 and 0.2-0.4, respectively), low ( 87 Sr/ 86 Sr) 88 (0.7034-0.7042), and high to moderate e Nd (88) (+5.1 to +1.5). Subgroup la comprises basalts with the same light to heavy REE ratios [(La/Yb)n = 2.7-3], Zr/Y and Nb/Y (4.5-5.8 and 0.2-0.3, respectively), and slightly high ( 87 Sr/ 86 Sr) 88 (0.7042-0.7048) at the same e Nd (88) (+5.4 to +4.4) of the group 1 basalts. Group 2 basalts have high light to heavy REE ratios [(LaNb) n = 5.3-7.8], high Zr/Y and Nb/Y (7-11 and 0.5-0.8, respectively), relatively high ( 87 Sr/8 6 Sr) 88 (0.7045-0.7057), and low e Nd (88) (+3.8 to +1). The basalts of the groups 1, la, and 2 cannot be linked by closed-system magma-differentiation processes, and require distinct mantle sources. The major and trace element variations of the Tamatave dikes of the group 1-la are compatible with moderate degrees of crystal fractionation (∼60%) from the least (MgO = 7.3 wt%) to the most evolved compositions (MgO = 4.2 wt%), involving the separation of plagioclase, augite, pigeonite, and minor oxides, perhaps accompanied by crustal contamination or differences in the 87 Sr/ 86 Sr ratios. The mantle sources of the group 1-la basalts seem to be located well within the spinel stability field, whereas a larger contribution of melts derived from garnet-bearing residual mantle is observed in the geochemistry and in the melting models of the group 2 basalts. The chemical and isotopic composition of both rock groups indicate their ultimate provenance from variably enriched lithospheric mantle sources; there is no clear evidence of a hotspot component like that found in the present-day lavas of the Marion-Prince Edward archipelago. The sources of this volcanism seem to be significantly similar to those of the Mahableshwar and Ambenali basalts of the later erupted Deccan Traps, located on formerly contiguous parts of the Gondwana lithosphere.

Tracking plumbing system dynamics at the Campi Flegrei caldera, Italy: High-resolution trace element mapping of the Astroni crystal cargo

Abstract The Campi Flegrei caldera (southern Italy) is one of the most hazardous volcanic systems on Earth, having produced >60 eruptions in the past 15 ka. The caldera remains active and its potential for future eruptions is high, posing a danger to the dense population living nearby. Despite this, our understanding of pre-eruptive processes and the architecture of the sub-volcanic system are poorly constrained. Here, we combine established petrological techniques, geothermobarometric evaluation, and high-resolution trace element crystal mapping, to present a multifaceted, coherent reconstruction of the complex pre-eruptive dynamics and eruption timescales of Astroni volcano located in the eastern sector of Campi Flegrei caldera. The Astroni volcano is an important case study for investigating plumbing system processes and dynamics at Campi Flegrei caldera because it produced the most recent (ca. 4 ka ago) Plinian eruption within the caldera (Astroni 6); current long-term forecasting studies postulate that a similar sized event in this location is a probable future scenario. Geothermobarometric results indicate interaction between an evolved, shallow magma chamber, and a less evolved, deeper pocket of magma, in agreement with previous studies focused on the Astroni 6 eruption products. In addition, a range of textural and trace element zoning patterns point to a complex evolution of both magmas prior to their subsequent interaction. High-resolution trace element crystal maps reveal discrete zonations in compatible elements. These zonations, combined with knowledge of K-feldspar growth rates, highlight a recharge event in the shallow plumbing system a few hours to days before the Astroni 6 eruption.