Thermal history of Phlegraean Fields (Italy) in the last 50,000 years: A schematic numerical model

An attempt is made to reproduce by numerical simulation the last 50,000 years of the Phlegraean Fields volcanic history in terms of a magma chamber with volume progressively reduced by magma extraction to the surface and without refillings from depth. Since the aim is just to verify the plausibility of such an assumption, attention is focused on the major volcanic events, and a rather crude model is adopted. It turns out that the main features of the Phlegraean Fields thermal history, namely the Campanian Ignimbrite, the yellow tuffs emission and the high temperatures measured in the geothermal wells drilled inside the caldera, can reasonably be reproduced under the not-refilled-system assumption. The magmatic body is predicted to have an average temperature of 1000°C at present.     

A laboratory model of a replenished magma chamber

It has recently been suggested that periodic influxes of hot but heavy magma into the base of a basaltic magma chamber can remain isolated from the rest of the chamber while the new magma cools and crystallization proceeds. When thermal equilibrium is almost complete, the suspended crystals settle out and the residual, less dense liquid can then mix with the fluid above. In the present paper the basic fluid-dynamical processes underlying this model have been investigated in laboratory experiments using aqueous solutions. The lower layer was hot KNO₃ solution, for which saturated solutions become less dense as the temperature decreases. With a cold, deeper layer of less dense NaNO₃ or K₂CO₃ above the lower layer, there was strong convective transfer of heat through a sharp interface separating the layers, at a rate which is predicted here drawing on previous studies carried out with oceanographic applications in mind. Once crystallization began, non-equilibrium effects became important and the observed temperatures differ somewhat from those predicted. In the experiments crystals grew mainly from the bottom rather than while in suspension, but this is not an essential aspect of the model. The important fact is that the density of the residual liquid in the lower layer decreased until it became equal to that of the upper layer, and then the interface broke down so that the two layers mixed thoroughly together, leaving a layer of KNO₃ crystals at the base. No crystallization at all occurred when the hot input liquid was forced to mix initially with the cold solution already in the chamber.     

Magnetotelluric survey in the bradyseismic area of Phlegraean Fields

A magnetotelluric survey was carried out in order to contribute to the definition of the deep structure of the Phlegraean Fields area affected by the bradyseismic crisis begun in the summer 1982. In a preliminary campaign carried out in autumn 1983, the MT measurements were made in eight sites, located between 0.3 and 5 km from the Harbour of Pozzuoli and affected by different degrees of uplift.In a second campaign (autumn, 1984) seven of the first eight MT soundings were repeated and four new sites were investigated, one of which located in a stable zone 10 km away from Pozzuoli.MT measurements show the presence of a thick conductive layer (<1 m) thinned underneath the town of Pozzuoli, where its top reaches the minimum depth (1.2–2 km). The upper horizon of this layer has a greater concavity than that produced by surface deformation. Below the conductive layer, the substratum became progressively more resistive with depth. This resistive basement is well defined in the soundings located far away from Pozzuoli. Here, on the contrary, a zone or body much more conductive than its surroundings is detected. This conductive body could correspond to a high temperature magmatic intrusion, probably already solidified, with a rough cylindrical shape with a base diameter of 1.5±0.5 km.Introducing into the obtained model the depth of the conductive horizon desumed from the deep geothermal wells, the top of the intrusive body would lye at 3–4 km depth.A small uplift of this body related to tectonic readjustments eventually induced by strong earthquakes recently occurred in near zones could explain the surface deformation observed during the present bradyseismic crisis.     

Error analysis in hypocentral locations at Phlegraean Fields

The location reliability of the earthquakes occurred at Phlegraean Fields has been analyzed, and the theoretical errors, inferred from the diagonal elements of the covariance matrix, have been estimated. Using only first P-phase arrivals to the local network (22 stations) and assuming a reading error of 0.05 sec., the average error on the spatial coordinates is estimated to be of the order of 0.2 km.Shallow events (depth<1 km) are very poorly constrained in depth at the borders of the network. The use of both P and S arrival times, recorded by a smaller three component network (10 stations), improves the depth determination.Further analysis has been performed on a set of about 350 selected earthquakes, using two different velocity models.Differences in depth considerably greater than the theoretical errors, and showing highly different patterns have been found.Tests with artificial events, randomly distributed in space, indicate that the observed depth distribution is essentially due to the used velocity model.     

Modelling of surface ground deformations in the Phlegraean Fields volcanic area,Italy

A preliminary finite elements model of the ground deformations observed at Phlegraean Fields is proposed. The model assumes an oblate-spheroid magma chamber at the depth of 5.4 km with major semiaxis of 1.5 km and minor semiaxis of 0.75 km. The dimensions of the magma chamber have been evaluated by using a thermal model based on the assumptions that a progressively cooling huge magmatic body is responsible for the volcanic activity at Phlegraean Fields in the last 35,000 years. Surface deformations caused by an over-pressure of 30 MPa in the magma chamber have been calculated. Constant, and temperature-dependent elastic parameters of the surrounding medium have been considered. Vertical displacements of the order of those presently observed at Phlegraean Fields can be obtained only with temperature-dependent elastic properties of the medium.     

A fluid-filled fracture as possible mechanism of ground deformation at Phlegraean Fields,Italy

Two uplift episodes have recently been recorded at Phlegraean Fields, a volcanic region near Naples (south-central Italy). The first episode occurred in 1970 and lasted up to 1972; the second has begun at the end of 1982 and is still in progress.An attempt to model the ground deformations which occurred during the 1970–1972 event is made in this paper. The source has been assumed to be a two-dimensional fluid-filled fracture, similar to a sill. A good fit with experimental data has been obtained for a short (1–2 km long) shallow (3 km deep) source and a driving pressure ranging from 60 to 210 bars. Rigidity values have been fixed at 3.5–4.0 × 10¹⁰ c.g.s., with Poisson ratio equal to 0.3.This solution which differs from previous approaches byMogi (1958) andWalsh andDecker (1971) is non-unique, but the present results are in good agreement with observed horizontal and vertical displacements. Notwithstanding the oversimplification made in assuming a homogeneous elastic semi-infinite medium, the predicted stress field seems to be in agreement with the fault-plane solutions and the pattern of epicenters determined for the uplift-seismic swarm episode that is still in progress.     

Fracture system in Phlegraean Fields (Naples,southern Italy)

During the 1983 seismic crisis in the Phlegraean Fields bradyseismic region (southern Italy), a structural analysis of the area was carried out.With a detailed field survey based on a net of 34 measure stations, a total of 536 fractures (mainly joints and a few normal faults) were measured on a 10 × 10 km area in volcanites capable of memorizing post depositional stress activity by fracturing.The analysis of the collected data was performed with the data bank of the University of Rome computer facilities. The azimuthal analysis of total fractures showed a nonrandom distribution with 5 major sets: N13°E, N45°E, N14°W, N55°W and E-W. These preferential orientations have been detected with an automatic fitting of gaussian curves (bell curves) on the azimuthal histograms. The areal distribution showed that all these fracture sets are in general present in the main collapse area. An azimuthal analysis performed by selecting the data collected for rocks older than 4,600 y BP showed a possible youngest age for the N14°W set (domain) (E-W extension). Fractures with an «opening» wider than 1 cm presented the same 5 azimuthal sets and fit fairly well with a concentric distribution around the main collapse area. The presence of an analogous radial pattern is not evident. A tentative interpretation model relates the superficial fracture sets to two possible causes: volcanic activity, including doming and collapsing, and propagation of active tensile deformations in the sedimentary basement due to regional stress trajectories.Contribution of «Centro di Studio per la Geologia dell'Italia Centrale», CNR, Roma.     

A preliminary study of stress pattern at Phlegraean Fields as inferred from focal mechanisms

Thirty-seven selected earthquakes which occurred in Phlegraean Fields from April to December 1983 are investigated from the focal mechanism point of view in order to attempt a preliminary representation of the stress pattern in the area. From 37 earthquakes, 15 reliable solutions were obtained. Ten mechanisms for events located around the Solfatara crater are of tensional type; 2 located in the Gulf of Pozzuoli seem to be compressive. No predominant orientation ofP andT axes seem to occur. Evidently, a more accurate analysis with a greater number of data — in particular for compressive events — is necessary in order to resolve a detailed map of stress pattern.     

Modeling hydrothermal fluid circulation and gravity signals at the Phlegraean Fields caldera

The Phlegraean Fields caldera is an active volcanic system where episodes of ground deformation are accompanied by significant changes in geochemical and geophysical parameters monitored at the surface. These changes derive from a complex interaction between magmatic system and hydrothermal fluid circulation. We calculate the gravity changes associated with the variable density of hydrothermal fluids. We simulate the multi-phase and multi-component fluid circulation triggered by a pulsating magma degassing, periodically increasing the discharge of CO₂-enriched fluids into the shallow hydrothermal system. The simulated evolution of the hydrothermal system successfully reproduces the observed composition of gas discharged at the surface. At the same time, results indicate that changes in average fluid density generate a detectable gravity signal that is of the same order of magnitude of the observed changes. This contribution to gravity changes can explain the peculiar behavior of gravity data collected at Solfatara, where surface hydrothermal phenomena are present. Simultaneous fitting of two independent sets of monitoring data (gas composition and gravity changes) confirms the conceptual model proposed for the hydrothermal system at Solfatara, and it provides new insights for the interpretation of gravity data.     

Space-time distribution of small earthquakes at Phlegraean Fields,south-central Italy

A preliminary statistical analysis of the space-time distribution of small seismic events in the volcanic area of Phlegraean Fields, south-central Italy, was done on the basis of a catalogue of earthquakes recorded by the local seismic stations in the period January 1, December 31, 1983.The non-random character of the sequence has been tested by matching the observed time-dstribution of seismic events with the theoretical Poisson process.A clustered occurrence of seismic events seems to be the main cause of the departure from a Poisson process as the inter-arrival times distribution clearly shows.Furthermore, the non-random behaviour of the events space-time distribution mainly due to quiescient and clustered recursive seismic phases could be studied by using the method proposed byVon Seggern et al. (1981). The analysis and the space-time diagrams confirm the swarm-type character of the entire seismic sequence.     

Geochemical surveillance of the Solfatara of Pozzuoli (Phlegraean Fields) during 1983

Geochemical surveillance of the Phlegraean Fields area has been intensified since 1983, in response to the increased uplift rate (brady-seismic activity).Fumarolic gases from Solfatara (Pozzuoli) were sampled and analyzed monthly. A Reducing Capacity (RC) monitoring unit was installed at Soffione, the most active fumarole in the Solfatara system.The preliminary analysis of the RC temporal variations suggest they are consistent with the rate of the seismic energy release.The composition of fumarolic gases indicates that the equilibrium temperature and pressure are higher than those of sampling.The observed variations in CH₄ content are explained as an increase of pressure (from 1982 to the end of 1983) of about 70% at an extimated depth of 3–3.5 km.Finally, during 1983, there were no geochemical indications of both rising magma and significant accumulation of energy at shallow depth.     

A mathematical model of magma transport in the asthenosphere

Abstract

We present a mathematical model for the flow of a partial melt through its solid phase. The model is based on the conservation laws of two-phase flow, which reduce to a generalization of porous flow in a permeable medium, when the solid matrix deforms very slowly. The continuity equation for the melt contains a source term (due to melting), which is determined by the energy equation. In addition, the melt fraction is unknown, and a new equation, representing conservation of pore space, is introduced. This equation may also be thought of as a constitutive law for the melt pressure (which is not lithostatic).

The model is non-dimensionalized and simplified. Some simple solutions are considered, and it is suggested that the occurrence of high fluid pressures in the solutions may initiate fractures in the lithosphere, thus providing a starting-up mechanism for magma ascent to the surface.     

Composition of hydrothermal fluids during the bradyseismic crisis which commenced at Phlegraean Fields in 1982

A systematic geochemical surveillance on the fumaroles of Solfatara and the boiling pools of Pisciarelli was carried out by discontinuous monitoring of the chemical composition of the emitted fluids during the Phlegraean Fields bradyseismic crisis which has begun in 1982. The fluids are considered to be produced by the ebullition of shallow aquifers receiving a convective gaseous inflow from the underlying magma chamber.Increased water vapor concentrations at a constant temperature of about 155 °C throughout the investigated period, along with the occurrence of ground deformations and seismic phenomena, are interpreted as resulting from an increased heat supply to the boiling water bodies.Dissolution processes and reactions with the confining rocks can alter the chemical composition of fluids escaping from magma to a large extent. Therefore it does not appear correct to consider the absolute values of any chemical constituent for geochemical surveillance without taking this modifying factor into account. Acid gases will be preferably absorbed by the above mentioned aquifers, while other species like H₂, N₂, O₂, CH₄, will instead increase their relative concentrations. Because of this, water vapour concentrations and the ratios H₂S/CO₂ and H₂/CH₄ in surface thermal manifestations appear to reflect better the varying extent of the observed phenomenon.On the basis of these parameters, and of both the upheaval rate and the intensity of seismic events, maximum values in the convective input of magmatic origin are estimated to have occurred at the beginning of the crisis and in September–October 1983.As long as water bodies at shallow depth are able to buffer the convective flow both thermally and chemically, no important volcanic activity can develop. When the absorbing capacity of these aquifers is exhausted, the increasing temperature and the changing characteristics of fluids towards a magmatic composition will indicate a higher probability of eruptive phenomena.CNR —Centro di studio per la mineralogia e la geochimica dei sedimenti.     

Seismic coda Q and turbidity coefficient at the Phlegraean Fields volcanic area: Preliminary results

Digital recordings of microearthquake codas from shallow seismic events in the Phlegraean Fields region (south-central Italy) were used to calculate the attenuation factor Q_c.A quite unusual frequency dependence was found for the coda attenuation comparable to Hawaii pattern of Q_c. This is interpreted as due to the presence of magma that increases the amount of anelasticity. Amount of scattering at Phlegraean Fields was estimated through the « turbidity » coefficient (Dainty model), that shows a high degree of scattering due to inhomogeneities as compared to Hawaii. Probably this is due to the greater crustal thickness of Phlegraean Fields with respect to Hawaii that produces more scattering.     

Volcanic hazard assessment in the Phlegraean Fields: a contribution based on stratigraphic and historical data

Phenomena occurring since 1982 in the Phlegraean Fields, interpreted as precursors of a potential renewal of volcanic activity, have forced us to anticipate some conclusions of a volcanic-hazard study based on the reconstruction of past eruptions in the area, to serve as basis for civil defense preparedness plans. The eruptive history of the Phlegraean Fields suggests a progressive decrease with time in the strength of eruptive phenomena paralleling a migration of vents towards the center of the Phlegraean caldera. Studies concerning the volcanic risk zonation were therefore concentrated on activities during the last 4,500 years and two eruptions (Monte Nuovo and Agnano Monte Spina), that occurred in 1538 and 4,400 years B.P., respectively were selected as the «reference eruptions» from which possible eruption scenarios were drawn.     

Evolution of double diffusion convection in a felsic magma chamber

The onset of double diffusion convection (DDC) is modeled in a two-dimensional case in respect to magma chambers. The viscosity model for the melt takes into account the effects of temperature and concentration of the dissolved component (H₂O). The upper boundary of the convecting magma chamber is assumed to be anhydrous and at constant temperature, whereas the lower boundary is treated as being hydrous permeable with a temperature greater than that within the upper boundary. The case of positive compositional and thermal buoyancy of melt is studied assuming a H₂O diffusion coefficient small in comparison with thermal diffusivity. The DDC has been modeled using a system of equations solved by the finite difference method on a square grid. The convective pattern evolution has been studied for fixed boundary conditions as well as for cooling and degassing. Due to the higher viscosity in the upper zone, the upper boundary layer is thicker than the lower one. The variation of water concentration in this zone of the convective cell can be significant. In nature, the high gradient of water concentration can be responsible for the observed variations of water content in minerals crystallized from a granite melt (e.g., biotite). Because of a high Lewis number (= 100), temperature variations in the magma chamber decay much faster than the water concentration. In this case the intensive convection can continue at a constant temperature due to the non-zero water content in the chamber. In principle, the effect can be applied to the formation of magmatic bodies. If the cooling and degassing system reaches a uniform temperature distribution prior to the crystallization temperature, water content throughout the body may still remain variable.     

Replenishment of a mafic magma in a zoned felsic magma chamber beneath Rishiri Volcano, Japan

The trachytic Tanetomi lava from Rishiri Volcano, northern Japan, provides useful information concerning how a replenished mafic magma mixes with a compositionally zoned felsic magma in a magma chamber. The Tanetomi lava was erupted in the order of Lower lava 1 (LL1, 59.2-59.8 wt.% in SiO₂), Lower lava 2 (LL2, 58.4-59.1 wt.%), and Upper lava (UL, 59.9-65.1 wt.%). Evidence for mixing with a mafic magma is observed only in the LL2, in which a greater amount of crystals derived from the mafic magma occurs in rocks with higher SiO₂ content. The whole-rock compositional trend of the Tanetomi lavas is fairly smooth except for the LL2 lava composition, which scatter along the main composition trend. There is no reasonable composition of basaltic magma on the extrapolation of the LL2 composition trend, and the trend cannot be explained by a simple two-component magma mixing. Before the replenishment, the felsic magma was zoned in composition (58-65 wt.% in SiO₂) and temperature (1030-920°C) in the magma chamber located at the pressure of ~2 kbar. The compositional variation of the main felsic magma was produced by extraction of a fractionated interstitial melt from mush zones along the chamber walls and its subsequent mixing with the main magma (boundary layer fractionation). The LL1 magma tapped the magma chamber soon after the replenishment, before the mafic magma mixed with the overall felsic magma. Then the basalt magma mixed heterogeneously with the upper part of the felsic magma by forced convection as a fountain during injection. The mixing of the basalt magma with compositionally zoned felsic magma resulted in the characteristic composition trend of the LL2. The fraction of basaltic magma in the LL2 magma is estimated to be at most 10%. Despite such a small proportion, the basalt magma was mixed completely with the felsic magma, probably because the crystallinity of undercooled basalt magma was low enough to behave as a liquid.     

The messina earthquake of 1908 and the magma chamber of Etna

Comparison between observed and theoretical seismic intensities of the Messina earthquake of 1908 shows an area of negative anomalies, which seems to correspond to the «shadow» of the Etna magma chamber. This magma chamber has probably an elongated form, being 40 km wide and about 100 km long; the mean depth appears to be close to 5 km, but the thickness cannot be estimated from the present data.     

长白山天池火山岩浆系统分析

针对外媒报道长白山天池火山在近2年内有可能喷发的言论,在长白山天池火山区布测了一条长度约为103 km的二维大地电磁测深观测剖面,对火山区深部电性结构进行探测研究.由于研究区内不明来源的电磁干扰非常强,对数据采用了远参考处理、Robust处理、Rhoplus分析、张量阻抗分解和基于一维层状介质电阻率与相位互算方法等先进处理技术,获取到一批在强干扰区质量较为可靠的电磁数据,利用数据计算分析了长白山天池火山区二维构造走向和感应矢量特征,采用NLCG二维反演技术对资料进行了二维反演解释,并将反演结果与前人探测结果进行了对比分析.探测结果表明:在天池火山口下方存在明显的直立型岩浆通道,岩浆通道在下方约5~8 km位置形成关闭;在火山口下方往北方向附近,在埋深位置约7 km深处存在一个明显的低阻异常体,电阻率小于10 Ωm,且与岩浆通道对接,推测其可能是地表浅部发育的岩浆囊;在长白山山门附近C07-C09号测点之间和C04-C05号测点之间,在埋深约7~17 km深处发现近直立型低阻带,低阻带与下方低阻体直接相连,推测低阻带内赋存有活动的岩浆;随着埋深的增加,从天池火山口南部约20 km位置往北方向,在埋深13~30 km之间壳内广泛发育明显的低阻异常体,推测其可能是活动的岩浆囊.反演结果与前人探测结果整体电性特征相似,但又局部不同.     

A geochemical model of mid-ocean ridge magma chambers

A computer model of mid-ocean ridge basalt generation using trace element geochemistry has been developed. The model simulates a periodically replenished, continually cooled and fractionated magma chamber, with periodic lava extrusion. Primitive basalts from the ocean floor are used to generate likely evolution paths for the magma chamber. The steady state variant of this model has led to the isolation of several variables which critically affect the basalt composition. Although the fraction of cumulates is an important parameter, other variables such as the volume of incoming magma batches, their frequency, and the volume of the mixing cell, play a critical part especially on slow-spreading ridges. The growing magma chamber model uses random number generators to simulate the initiation and growth of a chamber. This model predicts a rapid increase in incompatible element concentrations, immediately after chamber initiation on a fast-spreading ridge. This would occur in situations such as propagating rifts and may help in the understanding of ferrobasalt generation.