Benedetto De Vivo

Geochemical and isotopic (Nd-Pb-Sr-O) variations bearing on the genesis of volcanic rocks from Vesuvius, Italy

Abstract Alkaline volcanism produced by Monte Somma–Vesuvius volcano includes explosive plinian and subplinian activity in addition to effusive lava flows. Pumice, scoria, and lava (150 samples) exhibit major- and trace-element gradients as a function of SiO2 (58.9–47.2 wt%) and MgO (0–7.8 wt%); Mg# values are 10 with decreasing age for the Vesuvius system as a whole, yielding similar compositions in the least evolved rocks (low-silica, high-MgO, incompatible element-poor) erupted at the end of each cycle. Internal variations within individual eruptions also systematically changed generally toward a common mafic composition at the end of each cycle, thus reflecting the dominant volume in the magma chamber. At the start of a new eruptive cycle, the rocks are relatively enriched in incompatible elements; younger groups also contain higher abundances than older groups. N-MORB-normalized multielement diagrams exhibit selective enrichments of Sr, K, Rb, Th, and the light rare-earth elements; deep Nb and Ta negative anomalies commonly seen in rocks generated at orogenic margins are absent in our samples. Sr isotopic compositions are known to be variable within some of the units, in agreement with our data ( 87 Sr / 86 Sr ~0.70699 to 0.70803) and with contributions from several isotopic components. Isotopic compositions for δ 18 O (7.3 to 10.2‰), Pb for mineral separates and whole rocks ( 206 Pb / 204 Pb ~18.947 to 19.178, 207 Pb / 204 Pb ~15.617 to 15.769, 208 Pb / 204 Pb ~38.915 to 39.435), and Nd ( 143 Nd / 144 Nd ~0.51228 to 0.51251) also show variability. Oxygen isotope data show that pumices have higher δ 18 O values than cogenetic lavas, and that δ 18 O values and SiO2 are correlated. Radiogenic and stable isotope data plot within range of isotopic compositions for the Roman comagmatic province. Fractional crystallization cannot account for the radiogenic isotopic compositions of the Vesuvius magmas. We favor instead the combined effects of heterogeneous magma sources, together with isotopic exchange near the roof of the magma chamber. We suggest that metasomatized continental mantle lithosphere is the principal source of the magmas. This kind of enriched mantle was melted and reactivated in an area of continental extension (incipient rift setting) without direct reliance on contemporaneous subduction processes but possibly with input from mantle sources that resemble those that produce ocean island basalts.

Experimental and modeled solubilities of chlorine in aluminosilicate melts, consequences of magma evolution, and implications for exsolution of hydrous chloride melt at Mt. Somma-Vesuvius

Abstract Solubility experiments were conducted with forty-one aluminosilicate rock compositions to determine how extensively Cl dissolves in hydrous chloride melt- ± vapor-saturated silicate melts containing low to moderate water contents at 2000 bars. Chlorine solubility in most silicate melts is dominated by the abundances of Mg ≈ Ca > Fe > Na > K > network-forming Al > Li ≈ Rb ≈ Cs, but Ti, F, and P also have strong influences. The relationship of composition to Cl solubility is more complex in peraluminous and peralkaline felsic melts, because network-modifying Al, Na, and K have a greater influence than their network-forming counterparts. Also, the effects of Ca, Mg, and Al in mafic melts characterized by high (Ca + Mg + Al)/(Na + K + Li) are much greater than their effects in silica-enriched melts. Association coefficients that express the influence of each ion on Cl solubility were determined, and the solubility data and coefficients were employed to develop a model that predicts Cl solubility at 2000 bars for water-undersaturated melts ranging from rhyolite to basalt. The coefficients were also used to investigate the predominant chloride complexes in melt, and the bulk of the solubility data are consistent with the interaction of Cl with alkaline-earth metals that provide charge balance for network-forming Al. The Cl solubility model is applied to Mt. Somma-Vesuvius magmas as they evolved from phonotephrite to phonolitic compositions, via fractional crystallization, to investigate the role of Cl in magmatic degassing. The results clearly demonstrate that Cl solubility was dramatically reduced by subtle changes in melt composition. Decreasing abundances of Ca, Mg, and Fe in the residual melt induced a dramatic reduction in Cl solubility that occurred simultaneously with gradual increases in the abundance of volatiles in melt due to crystallization of volatile-free minerals. The increasing abundance of volatiles and concurrent reduction in Cl solubility may have forced the exsolution of a hydrous chloride melt directly from the Cl-enriched mafic magmas. It is likely that the exsolution of hydrous chloride melt may occur in other Cl-enriched magmas, because Cl solubility depends so strongly on melt composition.

Background and baseline concentration values of elements harmful to human health in the volcanic soils of the metropolitan and provincial areas of Napoli (Italy)

In this paper, we discuss baseline geochemical maps of elements harmful to human health compiled using concentration data of 982 volcanic soil samples from the metropolitan and provincial areas of Napoli (1171 km2). Each sample was digested in aqua regia and analysed by ICP-MS and ICP-ES. For the compilation of baseline geochemical maps, we apply a recently developed multifractal IDW interpolation method and spectral analysis (S-A) using a new geochemistry-dedicated GIS (GeoDAS). The geochemical baseline in an area of heavy anthropogenic impact, such as the Neapolitan territory, includes the geogenic natural content (background) and the anthropogenic contribution in the soils. The definition and distinction of background (geogenic) values, as opposed to baseline values, are very important in environmental studies because legislation typically fixes the intervention limits for both organic and inorganic substances in soils as a function of local background values. Maps obtained by the S-A method show high baseline values for some metallic elements (Pb, Zn, Sb, Hg, Cd, Cr, Cu) which denote a clear anthropogenic contribution, due to the long period of constant human activities in the study area. These maps also show a high geogenic contribution most probably derived from hydrothermal fluids because the study area is located between two active volcanic fields (Mt. Somma–Vesuvius and Campi Flegrei), where geothermal fluids have been used for spas since Roman times. The maps obtained using the S-A method, implemented in GeoDAS, reflect both the geogenic and anthropogenic contribution, so they must be considered as baseline maps. In order to obtain background values, to be used as reference values for the environmental Italian Law 471/1999, we have applied the concentration–area fractal method (C-A) to classify the pixel values of baseline maps. R-mode factor analysis helped in interpreting data controlled by anthropogenic sources as opposed to those controlled by geogenic sources.

Fluid inclusion studies of ejected nodules from plinian eruptions of Mt. Somma-Vesuvius

Abstract Mt. Somma-Vesuvius (Naples, Italy) has erupted potassium-rich and silica-undersaturated products during a complicated history of plinian and non-plinian events. Coarse-grained cognate nodules are commonly found in the pyroclastics and are upper crustal in origin. We examined cumulate and subeffusive nodules from the 3800 y.B.P. Avellino. A.D. 79 Pompei, and A.D. 472 Pollena eruptions. Silicate-melt and liquid-vapor fluid inclusion studies in clinopyroxene from both types of nodules have been used to assess the fluids attending crystallization and to place constraints on the pressure and temperature of nodule formation. Thermometric and volumetric data from primary and pseudosecondary CO 2 -H 2 O and CO 2 and coeval silicate-melt fluid inclusions indicate that they were trapped at a pressure of ∼1 to ∼2.5 kbar at ∼1200°C. This suggests a crystallization depth of ∼4 to ∼10 km. The H 2 O-bearing fluid inclusions are abundant from plinian eruptions in contrast to non-plinian eruptions where H 2 O-bearing fluid inclusions were rare. The presence of primary H 2 O-CO 2 fluid inclusions indicates that an immiscible, supercritical H 2 O-CO 2 fluid was in the nodule-forming environment. The H 2 O-bearing fluid inclusions in plinian nodules may record a higher pre-eruptive H 2 O content in the bulk magma that is dramatically reflected in the eruption dynamics.

The campi flegrei (Italy) geothermal system: A fluid inclusion study of the mofete and San Vito fields

Abstract A fluid inclusion study of core from the Mofete 1, Mofete 2, Mofete 5, San Vito 1, and San Vito 3 geothermal wells (Campi Flegrei, Campania, Italy) indicates that the hydrothermal minerals were precipitated from aqueous fluids ( ±CO 2 ) that were moderately saline (3–4 wt.% NaCl equiv.) to hypersaline (> 26 wt.% NaCl equiv.) and at least in part, boiling. Three types of primary fluid inclusions were found in authigenic K-feldspar, quartz, calcite, and epidote: (A) two-phase [ liquid (L) + vapor (V) ], liquid-rich inclusions with a range of salinity; (B) two-phase ( L + V ), vaporrich inclusions with low salinity; and (C) three-phase [ L + V + crystals (NaCL) ], liquid-rich inclusions with hypersalinity. Results of microthermometric and crushing studies are reported for twenty drill core samples taken from the lower portions of the five vertical wells. Data presented for selected core samples reveal a general decrease in porosity and increase in bulk density with increasing depth and temperature. Hydrothermal minerals commonly fill fractures and pore-spaces and define a zonation pattern, similar in all five wells studied, in response to increasing depth (pressure) and temperature. A greenschist facies assemblage, defined by albite + actinolite , gives way to an amphibolite facies, defined by plagioclase (andesine) + hornblende , in the San Vito 1 well at about 380°C. The fluid inclusion salinity values mimic the saline and hypersaline fluids found by drilling. Fluid inclusion V/L homogenization temperatures increase with depth and generally correspond to the extrapolated down-hole temperatures. However, fluid inclusion data for Mofete 5 and mineral assemblage data for San Vito 3, indicate fossil, higher-temperature regimes. A limited 87 Sr/ 86 Sr study of leachate (carbonate) and the leached cores shows that for most samples (except San Vito 3) the carbonate deposition has been from slightly 87 Sr-enriched fluids and that Sr isotopic exchange has been incomplete. However, San Vito 3 cores show an approach to fluid/rock Sr equilibrium with a fluid similar to modern ocean water in 87 Sr/ 86 Sr ratio. The Campi Flegrei volcanic system has evolved undersaturated products, mostly trachyte, and defines a large ( ≈ 12 km ) caldera. The hydrothermal system developed in this location can be used as an analog for fossil systems in similar trachytic environments. The potential for ore mineralization is expressed by the recognition, from fluid inclusion and drilling data, of ore-forming environments such as boiling and brine stratification.

Quantitative model for magma degassing and ground deformation (bradyseism) at Campi Flegrei, Italy: Implications for future eruptions

Campi Flegrei (Phlegrean Fields) is an active volcanic center near Naples, Italy. Numerous eruptions have occurred here during the Quaternary, and repeated episodes of slow vertical ground movement (bradyseism) have been documented since Roman times. Here, we present a quantitative model that relates deformation episodes to magma degassing and fracturing at the brittle-ductile transition in a magmatic-hydrothermal environment. The model is consistent with fi eld and laboratory observations and predicts that uplift between 1982 and 1984 was associated with crystallization of ~0.83 km 3 of H 2 O-saturated magma at 6 km depth. During crystallization, ~6.2 ◊ 10 10 kg of H 2 O and 7.5 ◊ 10 8 kg of CO 2 exsolved from the magma and generated ~7 ◊ 10 15 J of mechanical (PΔV) energy to drive the observed uplift. For comparison, ~10 17 J of thermal energy was released during the 18 May 1980 lateral blast at Mount St. Helens.

Sampling and major element chemistry of the recent (A.D. 1631–1944) Vesuvius activity

Abstract Detailed sampling of the Vesuvius lavas erupted in the period A.D. 1631–1944 provides a suite of samples for comprehensive chemical analyses and related studies. Major elements (Si, Ti, Al, Fetotal, Mn, Mg, Ca, Na, K and P), volatile species (Cl, F, S, H2O+, H2O− and CO2), and ferrous iron (Fe2+) were determined for one hundred and forty-nine lavas and five tephra from the A.D. 1631–1944 Vesuvius activity. The lavas represent a relatively homogeneous suite with respect to SiO2, TiO2, FeOtotal, MnO and P2O5, but show systematic variations among MgO, K2O, Na2O, Al2O3 and CaO. The average SiO2 content is 48.0 wt.% and the rocks are classified as tephriphonolites according to their content of alkalis. All of the lavas are silica-undersaturated and are nepheline, leucite, and olivine normative. There is no systematic variation in major-element composition with time, over the period A.D. 1631–1944. The inter-eruption and intra-eruption compositional differences are the same magnitude. The lavas are highly porphyritic with clinopyroxene and leucite as the major phases. Fractionation effects are not reflected in the silica content of the lavas. The variability of MgO, K2O, Na2O, and CaO can be modelled as a relative depletion or accumulation of clinopyroxene.

Pre-eruptive volatile content, melt-inclusion chemistry, and microthermometry of interplinian Vesuvius lavas (pre-A.D. 1631)

Silicate-melt inclusions from lavas and pyroclastics from a selected suite of pre-A.D. 1631 interplinian Mt. Somma-Vesuvius lavas and scoria have been experimentally homogenized and studied by microthermometry, electron microprobe (EMPA) and secondary-ion mass spectrometry (SIMS) to examine pre-eruptive volatile content and magma evolution. The melt inclusions have a bubble about 0.06% their volume, uncommonly contain non-condensable gas but do not contain any dense fluid phases. Clinopyroxene-hosted inclusions yield homogenization temperatures (T h ) from 1170 to 1260°C, most between 1220 and 1240°C; plagioclase-hosted inclusions have T h from 1210 to 1230°C; these values are typical for the Vesuvius environment. The dominant factor controlling major element variability in the inclusions is clinopyroxene fractionation; MgO varies from 5 to 3 wt%, SiO 2 varies from 60 to 48 wt%, total alkalis vary from 15 to 4 wt%, and CaO varies from 13 to 5 wt%. H 2 O varies from 2.7 to 0.6 wt% and is decoupled from incompatible element evolution suggesting vapor saturation during trapping. Chlorine and F vary from 1.0 wt% to 0 and 0.63 to 0 wt%, respectively. Bulk rock and limited matrix glass analyses show that the lavas lost about half of their F and Cl content except for the A.D. 472-1631 lava which contains similar Cl abundances as the bulk rock. SO 3 varies from 0.5 to 0 wt% and compared with matrix glass and bulk rock demonstrate that the lavas have lost essentially all sulfur. The samples can be classified into three age groups, > 25,000 yr B.P., 25,000-17,000 yr B.P., and A.D. 472-1631. There is a systematic increase in some components, e.g., total alkalis, SO 3 , Cl, Li, B, and Sr with the youth of the sample and a decrease in others, e.g., Zr and Y. However, on average these samples seem less evolved than later A.D. 1631-1944 lavas.

Pre-eruptive volatile contents of Vesuvius magmas: constraints on eruptive history and behavior. I - The medieval and modern interplinian activities

The eruptive history of Mt. Vesuvius is characterized by a large variation in eruptive styles during the last 2000 years. As part of an extensive investigation on the pre-eruptive magma geochemistry, we have analyzed, by EMPA and SIMS, silicate melt inclusions (MI), hosted in clinopyroxene phenocrysts, to gain insight into the eruptive processes and history of this significant magmatic system. The rock samples analyzed are lavas and scoriae erupted during two intervals of interplinian volcanic activity: 472 to 1139 A.D. and 1631 to 1944 A.D. The compositions of MI from samples of the modern and medieval periods are consistent with petrological and geochemical evidence of melt evolution via fractional crystallization. Although fractional crystallization played a decisive role in the evolution of medieval and modern magmas, other processes may also have been active. In MI from the medieval eruptive products, the fixed Cl contents, and relationships involving other elements soluble in aqueous volatile phases ( e.g. H 2 O, S, B, Be, Li) indicate crystallization of magmatic vapor-phase-saturated silicate melt(s). Conversely, MI from the modern eruptive activity are characterized by linear increases in the abundances of Cl, SO 2 , and K 2 O in fractions of residual melt. Furthermore, no evidence was found that volatile abundances were fixed or buffered by the presence of an aqueous vapor (or liquid), which is consistent with fractional crystallization of a magma under volatile-phase-absent conditions. The modern and medieval periods, separated by the violent 1631 eruption, exhibit dramatically different eruptive behavior. The medieval magmas erupted less often, but with comparatively greater violence, which may have been facilitated by the presence of a pre-eruptive volatile phase in magma. The modern magmas erupted more frequently and in a relatively passive manner, which is consistent with the absence of textural or geochemical evidence for fluid saturation before eruption.

Geochemistry and argon thermochronology of the Variscan Sila Batholith, southern Italy: source rocks and magma evolution

The Sila batholith is the largest granitic massif in the Calabria-Peloritan Arc of southern Italy, consisting of syn to post-tectonic, calc-alkaline and metaluminous tonalite to granodiorite, and post-tectonic, peraluminous and strongly peraluminous, two-mica±cordierite±Al silicate granodiorite to leucomonzogranite. Mineral 40Ar/39Ar thermochronologic analyses document Variscan emplacement and cooling of the intrusives (293–289 Ma). SiO2 content in the granitic rocks ranges from ∼57 to 77 wt%; cumulate gabbro enclaves have SiO2 as low as 42%. Variations in absolute abundances and ratios involving Hf, Ta, Th, Rb, and the REE, among others, identify genetically linked groups of granitic rocks in the batholith: (1) syn-tectonic biotite±amphibole-bearing tonalites to granodiorites, (2) post-tectonic two-mica±Al-silicate-bearing granodiorites to leucomonzogranites, and (3) post-tectonic biotite±hornblende tonalites to granodiorites. Chondrite-normalized REE patterns display variable values of Ce/Yb (up to ∼300) and generally small negative Eu anomalies. Degree of REE fractionation depends on whether the intrusives are syn- or post-tectonic, and on their mineralogy. High and variable values of Rb/Y (0.40–4.5), Th/Sm (0.1–3.6), Th/Ta (0–70), Ba/Nb (1–150), and Ba/Ta (∼50–2100), as well as low values of Nb/U (∼2–28) and La/Th (∼1–10) are consistent with a predominant and heterogeneous crustal contribution to the batholith. Whole rock δ18O ranges from ∼+8.2 to +11.7‰; the mafic cumulate enclaves have the lowest δ18O values and the two-mica granites have the highest values. δ18O values for biotite±honblende tonalitic and granodioritic rocks (9.1 to 10.8‰) overlap the values of the mafic enclaves and two-mica granodiorites and leucogranites (10.7 to 11.7‰). The initial Pb isotopic range of the granitic rocks (206Pb/204Pb ∼18.17–18.45, 207Pb/204Pb ∼15.58–15.77, 208Pb/204Pb ∼38.20–38.76) also indicates the predominance of a crustal source. Although the granitic groups cannot be uniquely distinguished on the basis of their Pb isotope compositions most of the post-tectonic tonalites to granodiorites as well as two-mica granites are somewhat less radiogenic than the syn-tetonic tonalites and granodiorites. Only a few of the mafic enclaves overlap the Pb isotope field of the granitic rocks and are consistent with a cogenetic origin. The Sila batholith was generated by mixing of material derived from at least two sources, mantle-derived and crustal, during the closing stages of plate collision and post-collision. The batholith ultimately owes its origin to the evolution of earlier, more mafic parental magmas, and to complex intractions of the fractionating mafic magmas with the crust. Hybrid rocks produced by mixing evolved primarily by crystal fractionation although a simple fractionation model cannot link all the granitic rocks, or explain the entire spectrum of compositions within each group of granites. Petrographic and geochemical features characterizing the Sila batholith have direct counterparts in all other granitic massifs in the Calabrian-Peloritan Arc. This implies that magmatic events in the Calabrian-Peloritan Arc produced a similar spectrum of granitic compositions and resulted in a distinctive type of granite magmatism consisting of coeval, mixed, strongly peraluminous and metaluminous granitic magmas.