Lucia Civetta

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).

Short-term ground deformations and seismicity in the resurgent Campi Flegrei caldera Italy/: an example of active block-resurgence in a densely populated area

Abstract The Campi Flegrei caldera is a complex structure in which resurgence occurs through a simple shearing mechanism. The most uplifted block inside the caldera is the La Starza block. The caldera has shown signs of unrest since 1969 with two bradyseismic events which generated a net vertical ground displacement of 3.5 m around the town of Pozzuoli. The first event occurred between 1969 and 1972, when the ground was uplifted of 1.74 m, while the second took place between 1982 and 1984 and produced a vertical displacement of 1.79 m. These events have been followed by subsidence with small uplifts of few centimeters. Uplifting events, despite the amount of ground displacement, are always accompanied by seismicity which is absent during subsidence. We have performed an analysis of the ground deformation and seismic data collected since 1970. The results obtained on the two independent sets of data are very much concordant. The ground deformation is not quasi bell-shaped as previously suggested, instead its geometry is strongly constrained by structural discontinuities. Seismicity is confined in a rectangular area, NE–SW elongated and centered on the La Starza block. The margins of this area are marked by earthquakes whose hypocenters describe vertical planes which reach depths of about 5 km. The hypocenters of the earthquakes located in the La Starza block are shallower than 3 km. Focal mechanisms of the earthquakes whose hypocenters describe a NNW–SSE fault plane in the Pozzuoli bay show reverse fault solutions, while those of the earthquakes located on the opposite side of the La Starza block show normal fault solutions. All these data well constraints the hypothesis that resurgence occurs through a simple shearing mechanism (Orsi et al., 1996). We have also performed a finite elements modeling of the ground deformation data. This modeling shows that the detected deformation can be accounted for by an overpressure of 75 MPa in a source with a radius of 1 km, located at 5 km depth in a medium characterized by the detected structural discontinuities.

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.

Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)

Petrological and geochemical data are reported for volcanic rocks from Vulcano island. The subaerial volcanism (120 ka to present) built up a NW-SE elongated composite structure, affected by two intersecting multistage calderas. Volcanics older than 20 ka consist mostly of high-K calc-alkaline (HKCA) to shoshonitic (SHO) mafic rocks. These magmas interacted significantly with the continental crust, which generated variable Sr isotopic ratios (0.70412–0.70520). However, a major role was also played by input of parental liquids into the magma chamber, which prevented further evolution of the magmas. HKCA, SHO, and potassic (KS) rocks formed from 20 to 8 ka, display a much larger range of SiO2 (from shoshonites to rhyolites) and higher concentrations of incompatible elements with respect to the previous stage. Sr isotopic ratios show small variations (0.70448–0.70486). Mixing of silicic and mafic liquids and fractional crystallization processes (FC) were the main evolutionary processes during this stage. Volcanics younger than 8 ka consist of SHO and leucite-bearing KS mafic rocks, with abundant intermediate and silicic products. Mafic and intermediate rocks display similar incompatible element abundances and Sr isotopic ratios as the previous stage volcanics, whereas higher 87Sr/86Sr (0.70494–0.70583) are observed in some rhyolites. These products originated from a complex interplay of FC, crustal assimilation, and magma mixing processes. The most mafic rocks show increasing incompatible element abundances, Rb/Sr, Rb/Ba, Mg/Al, Mg/Ca, and a decrease in large ion lithophile to high field strength element ratios, passing from older HKCA-SHO to the younger SHO-KS volcanics. These variations suggest a shifting of magma sources from a slightly metasomatized asthenosphere (fertile peridotite) to a more strongly metasomatized lithospheric mantle (residual peridotite). Time-related petrological and geochemical variations have been used to develop a model for the evolution of the Vulcano plumbing system.

Geology, geochronology and chemical evolution of the island of Pantelleria

Potassium–argon dating, field relations, geochemical and strontium-isotope compositions are reported for the island of Pantelleria (Strait of Sicily, Italy). These data support the following model for the genesis and evolution through time of the volcanic system: the peralkaline rocks originated from mantle-derived parental magmas; the trachytic magma differentiated in a low pressure magma chamber by crystal–liquid fractionation. This process led to a chemically zoned magma chamber tapped at different levels by successive eruptions. During low-pressure differentiation the 87 Sr/ 86 Sr ratios of some of the most evolved Sr-poor rhyolitic magmas increased from 0.703 up to 0.708 by contamination with crustal material. The chemical variation displayed by the products of each of the defined eruptive cycles in the last 50000 years suggests an open system behaviour of the magma chamber which is episodically refilled by more mafic parent magma, differentiated at high rate and episodically erupted.

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.

Geochemical characteristics of potassic volcanics from Mts. Ernici (Southern Latium, Italy)

Major elements, trace elements and 87Sr/86Sr data are reported for the Quaternary potassic alkaline rocks from the Mts. Ernici volcanic area (Southern Latium — Italy). These rocks are represented by primitive types which display high Mgv, low D.I., variable degrees of silica undersaturation and different K2O contents which allowed the distinction of a potassium series (KS) and a high potassium series (HKS). All the analyzed samples have high LIL element contents and high 87Sr/86Sr which ranges between 0.707–0.711. They also have fractionated REE patterns. The KS rocks have lower LIL element concentrations and 87Sr/86Sr ratios than the HKS rocks with a large compositional gap between the two series. Minor but still significant isotopic and trace element variations are also observed within both KS and HKS. The genesis cannot be completly explained either by crystal liquid fractionation, mixing or assimilation processes or by different degrees of equilibrium partial melting from a homogeneous source, thus indicating that both the KS and HKS consist of several geochemically and isotopically distinct magma types. The data suggest that the KS and HKS magmas originated by low degrees of melting of a garnet peridotite mantle heterogeneously enriched in LIL elements and radiogenic strontium, possibly accompanied by disquilibrium melting of some accessory phases. The occurrence of a geochemical anomaly within the mantle is believed to be due to fluid metasomatism probably generated by dehydration of a lithospheric slab subducted during the Late Tertiary development of the Apennine Chain.

The eruptive history of Pantelleria (Sicily channel) in the last 50 ka

Six silicic eruptive cycles have been recognized in the last 50 ka at Pantelleria. The products of each cycle exhibit a compositional variation from pantellerite to less peralkaline rhyolite or to trachyte. The relationships between the range of chemical variation, the erupted volume and the time of eruptions, allow us to estimate an average differentiation rate of 5% crystal fractionation per 1000 years and a constant long-term rate of magma discharge of 0.1 km3 per 1000 years. Pressure increase in the magma chamber caused by the addition of new magma, accumulation of highly-differentiated, volatile-rich magma in the roof zone and a concomitant build-up of a vapour phase, is postulated as a possible triggering mechanism for eruptions.

Magma mixing and convective compositional layering within the Vesuvius magma chamber

The pumice-fall deposits of the last two Plinian eruptions of Vesuvius-a.d. 79 “Pompei” and 3700 b.p. “Avellino”-show a marked vertical compositional variation from white phonolite at the base to grey tephritic phonolite at the top. In both Avellino and Pompei sequences a compositional gap separates white from grey pumice. Grey and white pumice have distinct Sr and Nd isotopic compositions (grey pumice: 87Sr/36Sr=0.70749-56, 143Nd/144Nd=0.512507 for Pompei; 0.70760-69, 0.512504 for Avellino; white pumice: 0.70757-78 for Pompei; 0.70729-42 for Avellino). K-feldspar separated from both grey and white pumice has, in all cases, a “white” 87Sr/86Sr ratio (0.70766-79 for Pompei, 0.70728-33 for Avellino). The observed variations are interpreted as reflecting a pre-eruptive zonation of the magma chamber. Although mineralogical evidence of interaction between magma and calcareous country rocks exists in both eruptions, crustal contamination has not significantly modified the isotopic signatures of the erupted products. Petrographic and isotopic evidence of syneruptive magma mingling occur in Pompei grey pumice as well as in Avellino white and grey pumice, but they do not fully explain all the observed geochemical and isotopic variations. These variations are related to the complex refilling history of the magmatic system and result by fractional crystallization and mixing processes acting within the magma chamber. Preliminary data from other Plinian and subplinian sruptions of the Somma-Vesuvius point out the repeticive behaviour of 87Sr/86Sr variation in the last 25 000 years, hence suggesting a single magma chamber and continuity of the feeding system.

Volcanic ash layers illuminate the resilience of Neanderthals and early modern humans to natural hazards

Marked changes in human dispersal and development during the Middle to Upper Paleolithic transition have been attributed to massive volcanic eruption and/or severe climatic deterioration. We test this concept using records of volcanic ash layers of the Campanian Ignimbrite eruption dated to ca. 40,000 y ago (40 ka B.P.). The distribution of the Campanian Ignimbrite has been enhanced by the discovery of cryptotephra deposits (volcanic ash layers that are not visible to the naked eye) in archaeological cave sequences. They enable us to synchronize archaeological and paleoclimatic records through the period of transition from Neanderthal to the earliest anatomically modern human populations in Europe. Our results confirm that the combined effects of a major volcanic eruption and severe climatic cooling failed to have lasting impacts on Neanderthals or early modern humans in Europe. We infer that modern humans proved a greater competitive threat to indigenous populations than natural disasters.