Alban Hills Volcano Research Articles

Elemental and Sr–Nd isotopic evidence for unravelling the origin of the low-temperature geothermal fluids of Tivoli Terme (Latium, central Italy) between erosional S4 and S3 phases (upper Pleistocene) and neotectonics implications

Sr and Nd isotopic ratios along with (REE + Y) and other trace elements were measured in bedded travertines from three quarries at Tivoli Terme, Latium, aimed at deciphering their chemical and isotopic characteristics for the origin and evolution of fluids. Travertines, which were deposited between ∼81 and ∼54 ka BP, display high Sr and low REY contents along with REYPAAS patterns, supporting their fluid origin mainly from marine rocks, in particular the Upper Triassic Burano Evaporite Fm. This has been confirmed by the narrow ranges of δ87Sr and εNd values, close to those of that formation and other Mesozoic marine carbonates from central Italy. A sample of today's thermal water from Tivoli Terme provided a REYPAAS pattern and a δ87Sr value comparable with travertines, both features supporting that today's water is analog to past hydrothermal fluids. The overlap of the chemical and Sr–Nd isotopic compositions of coeval travertines and thermal waters from Tivoli and Acquasanta Terme, Marche, suggests a common fluid origin, ruling out any significant contribution from magmatic fluids/rocks to Tivoli travertines, yet deposited at the periphery of the Alban Hills volcano. Lastly, the geochemical results provide implications for regional neotectonics and the role of CO2-rich fluid pressure in fault activation, even in mildly active to almost inactive tectonic domains.

Asbestiform tremolite within the Holocene late pyroclastic deposits of Colli Albani volcano (Latium, Italy): occurrence and crystal chemistry

This work relates the occurrence and the characterization of fibrous tremolite within the latest pyroclastic deposits of the Colli Albani (Alban Hills) volcano, to the south-east of Rome (Italy). These mineralizations were observed during a systematic rock sampling undertaken to complete the geological survey for the new 1:50,000 map of this volcanic area. The examined specimens were collected inside distal deposits correlated to the last Albano Maar activity, which are geographically located within the boundaries of the Nemi community. Tremolite occurs within both carbonate ejecta and the host pyroclastic rocks. It shows up as whitish to light grey-coloured aggregates of crystals with fibrous aspect and sericeous brightness. Due to the extremely small crystal dimensions, never exceeding 0.5 μm in diameter, the micro-chemical composition of the fibres could be obtained only by combining P-XRD, SEM–EDX and FTIR methods. Infrared spectroscopy, in particular, proved to be a valuable technique to characterize the studied amphibole. The composition determined is that of a Fe-free F-rich (c. 53 %) tremolite with significant (c. 20 %) richterite components in solid solution. The occurrence of fibrous tremolite in an inhabited place, occurring as natural geological material rather than being due to anthropogenic pollution, should be examined with concern, because it implies complex health and legal responsibilities in the case of mobilization due to extreme climatic events.

Interaction between ultrapotassic magmas and carbonate rocks: Evidence from geochemical and isotopic (Sr, Nd, O) compositions of granular lithic clasts from the Alban Hills Volcano, Central Italy

Abstract Magma–carbonate rock interaction is investigated through a geochemical and Sr–Nd–O isotope study of granular lithic clasts (ejecta) from the Alban Hills ultrapotassic volcano, Central Italy. Some samples (Group-1) basically represent intrusive equivalents of Alban Hills magmas. A few samples (Group-2) are ultramafic, have high MgO (∼30 to 40 wt%) and δ18O‰, and originated by accumulation of mafic phases that crystallised from ultrapotassic melts during assimilation of dolomitic rocks. Group-3 ejecta consist of dominant K-feldspar, and show major element compositions similar to phonolites, which, however, are absent among the Alban Hills volcanics. Finally, another group (Group-4) contains corroded K-feldspars, surrounded by a microgranular to porphyritic matrix, made of igneous minerals (K-feldspar, foids, clinopyroxene, phlogopite) plus wollastonite, garnet, and some cuspidine. Group-4 ejecta are depleted in SiO2 and enriched in CaO with respect to Group-3. The analysed ejecta have similar 143Nd/144Nd (0.51204–0.51217) as the Alban Hills lavas, whereas 87Sr/86Sr (0.70900–0.71067) is similar to lower. Whole rocks δ18O‰ ranges from +7.0 to +13.2, reaching maximum values in ultramafic samples. A positive correlation with CaO is observed in single rock groups. Large Ion Lithophile Element (LILE) abundances and REE fractionation are generally high, and extreme values of Th, U and LREE are found in some Group-3 and Group-4 rocks. Mineralogical, petrological and geochemical data reveal extensive interaction between magma and carbonate wall rocks, involving both dolostones and limestones. These processes had dramatic effects on magma compositions, especially on phonolites, which were transformed to foidites. Evidence of such a process is found in Group-4 samples, in which K-feldspar is observed to react with a matrix that represents strongly undersaturated melts formed by interaction between silicate magma and carbonates. Trace element data also testify to a very important role for F–CO2–H2O–S fluids during magma–wall rock interaction. Fluid transfer was responsible for extreme enrichments in Th, U, and LREE especially observed in Group-3 and Group-4 rocks. Implications of these processes for potassic magma evolution in Central Italy are discussed.

Mid‐Pleistocene pozzolanic volcanic ash in ancient Roman concretes

AbstractThe extraordinarily durable concretes of Imperial Age (27 B.C. through 3rd century A.D.) monument construction in Rome contain scoriaceous, highly potassic, altered volcanic ash from the Pozzolane Rosse ignimbrite, erupted at 456±3ka from the Alban Hills volcano as pozzolanic mortar aggregate. Stratigraphic, micromorphological, and chemical investigations demonstrate that during the relatively warm, humid period preceding marine isotope stage 11, intensive hydrolytic pedogenesis produced dense illuvial clay coatings in the upper horizon of the ignimbrite. Moderately altered ash of the Pozzolane Rosse transitional Bt to Bw soil horizon has opal coatings overlain by limpid, illuvial halloysite coatings. Less weathered ash from a lower C horizon apparently was altered in ground water. Where the ignimbrite filled certain paleovalleys, zeolitic alteration produced phillipsite and chabazite textures. Builders selected ash from these intermediate and least altered horizons for the highest‐quality mortars of the Imperial Age, as for the Forum and Markets of Trajan (A.D. 107 to 113). The alumina‐ and alkali‐rich compositions of cementitious complexes give some preliminary insights as to why the reaction of hydrated lime with the altered, alkali‐rich Pozzolane Rosse ash produced pozzolanic cements that have remained resistant to decay for nearly 2000 years. The results of the geological analyses fully confirm empirical observations made by Esther B. Van Deman in 1912 regarding the durability of the ancient mortars and the technical choices of Roman builders. © 2009 Wiley Periodicals, Inc.

Gas geochemistry of natural analogues for the studies of geological CO2 sequestration

Geological sequestration of anthropogenic CO2 appears to be a promising method for reducing the amount of greenhouse gases released to the atmosphere. Geochemical modelling of the storage capacity for CO2 in saline aquifers, sandstones and/or carbonates should be based on natural analogues both in situ and in the laboratory. The main focus of this paper has been to study natural gas emissions representing extremely attractive surrogates for the study and prediction of the possible consequences of leakage from geological sequestration sites of anthropogenic CO2 (i.e., the return to surface, potentially causing localised environmental problems). These include a comparison among three different Italian case histories: (i) the Solfatara crater (Phlegraean Fields caldera, southern Italy) is an ancient Roman spa. The area is characterised by intense and diffuse hydrothermal activity, testified by hot acidic mud pools, thermal springs and a large fumarolic field. Soil gas flux measurements show that the entire area discharges between 1200 and 1500 tons of CO2 per day; (ii) the Panarea Island (Aeolian Islands, southern Italy) where a huge submarine volcanic-hydrothermal gas burst occurred in November, 2002. The submarine gas emissions chemically modified seawater causing a strong modification of the marine ecosystem. All of the collected gases are CO2-dominant (maximum value: 98.43 vol.%); (iii) the Tor Caldara area (Central Italy), located in a peripheral sector of the quiescent Alban Hills volcano, along the faults of the Ardea Basin transfer structure. The area is characterised by huge CO2 degassing both from water and soil. Although the above mentioned areas do not represent a storage scenario, these sites do provide many opportunities to study near-surface processes and to test monitoring methodologies.

Presence and zoning of hydrous components in leucite from the Alban Hills volcano, Rome, Italy

This paper reports a single-crystal FTIR spectroscopic study of leucite, a nominally anhydrous mineral (NAM). Several transparent, inclusion-free samples from different occurrences and localities in the Alban Hills volcanic area (Latium, Italy) were analyzed. The FTIR spectra collected in the 4000-3000 cm -1 H 2 O-stretching region show a broad absorption consisting of overlapping components at 3604, 3500, and 3245 cm -1 . The occurrence of a well-defined combination band at 5225 cm -1 indicates that almost all the examined samples contain hydrous components in the form of structurally bound water molecules. Using the integrated molar absorption coefficient from the working curve of Libowitzky and Rossman (1997), a water content up to >4000 ppm was obtained for the studied specimens. Detailed microspectroscopy mapping shows significant zoning of water in some samples, typically consisting of an anhydrous core mantled by a hydrous rim. The collected data suggest that careful study of the distribution of the volatile content of leucite (and associated volcanic NAMs) may provide a tool to monitor the evolution of the magmatic system where these minerals occur.

Accidental gas emission from shallow pressurized aquifers at Alban Hills volcano (Rome, Italy): Geochemical evidence of magmatic degassing?

Recent studies suggested that Alban Hills (Rome) is a quiescent and not an extinct volcano, as it produced Holocene eruptions and several lahars until Roman times by water overflow from the Albano crater lake. Alban Hills are presently characterized by high P CO2 in groundwaters and by several cold gas emissions usually in sites where excavations removed the superficial impervious cover. Gas consists mostly of CO 2 with minor H 2S and the diffuse CO 2 soil flux is locally very high. Accidental gas blowouts, occurred during shallow well drillings (tens to hundreds m depth) in zones with no surface gas manifestations, indicate the presence of gas pressurized aquifers confined underneath impermeable layers, within both the volcanic rock pile and the underlying Pleistocene loose sediments. Degassing mostly occurs in correspondence of bordering faults of buried horsts cut in the Mesozoic carbonate basement, hosting the main aquifer. Carbon isotopic composition ( δ 13C CO2) suggests that CO 2 is at least partly originated by thermal decarbonation of these limestones. 3He/ 4He isotopic ratio of the gas (up to 1.9 Ra) is the same or even slightly higher than that of olivine and clinopyroxene fluid inclusions of the Alban Hills volcanic products, indicating a possible magmatic source for the gas. Low R/Ra values, compared to MORB and island arc magmas, are characteristic of the potassic Roman Comagmatic Province and reflect a deep involvement of crustal material in the magma genesis. The lack of high temperature fumaroles can be explained by an efficient meteoric cold water penetration and circulation in the volcano permeable terrains.

Coupling between earthquake swarms and volcanic unrest at the Alban Hills Volcano (central Italy) modeled through elastic stress transfer

We study a seismic swarm that occurred in 1989–1990 at the Alban Hills volcano and interpret the seismicity pattern in terms of Coulomb stress changes caused by magma intrusion in a local volcanic source and the extensional tectonic stress field. We first image the three‐dimensional (3‐D) structure of the volcano through a tomographic inversion of P waves and S‐P arrival times recorded by a temporary local network. A high Vp and Vp/Vs body exists beneath the area of most recent volcanic activity, which we interpret as a solidified magma body delimiting the position of the volcanic source. We have relocated 661 events (M ≤ 4.0) using this 3‐D velocity model and we have computed 64 fault plane solutions. Elevation changes, measured between 1951 and 1994 along a 33‐km‐long line crossing the western part of the volcano, reveal an uplift of 0.3 m. We model these data to constrain the position and geometry of the volcanic source. We compute the vertical deformation in a homogeneous half‐space, testing different volcanic sources (spherical magma chamber, sill and dike). We model the Coulomb stress changes caused by the local volcanic source and the regional tectonic stress field. The inflation of magma generates an increase of Coulomb stress larger than 0.5 MPa in the area where the seismicity is located. More than 85% of fault plane solutions are consistent with the stress perturbations induced by the volcanic source. We conclude that microearthquakes at the Alban Hills are promoted by elastic stress changes caused by volcanic unrest episodes.

Large volume phreatomagmatic ignimbrites from the Colli Albani volcano (Middle Pleistocene, Italy)

In this paper we describe four large volume, ash- and accretionary lapilli-rich, phreatomagmatic compound ignimbrite units, mafic in composition, from Colli Albani volcano, south of Rome. The four units, that form the ‘Pisolitic Tuffs’ succession, are separated by paleosols and represent the earliest explosive large volume eruptive episodes from the Quaternary Alban Hills volcano. The occurrence of large volume phreatomagmatic–phreatoplinian eruptions implies the availability of large quantities of water interacting with the rising magma. The paleogeography of the area below the volcano has been reconstructed by the analysis of stratigraphic data from more than a thousand bore-holes distributed around the volcano that allowed to identify NW-trending and NE-trending paleotopographic lows that underlie the central area of the volcano and interpreted as extensional tectonic basins. These lows are filled with Lower to Middle Pleistocene, pre-volcanic lacustrine and fluvial deposits and suggest that at least the central part of the Colli Albani volcanic area hosted a large lake or lagoon. The absence of sedimentary xenoliths in the Pisolitic Tuffs and the low vesicularity of scoria and shards suggest that water interacted with a poorly fragmented magma at very shallow level, triggering the large explosivity of the eruptions. We suggest that water interacting with magma was mostly surficial water related to the presence of the lagoon or lake. In this environment, it is likely that vents were subaqueous allowing a continuous access of water to the conduit. Considering that the minimum calculated volume of products for the Pisolitic Tuffs succession is >37 km 3 and each eruption unit averages approximately around minimum volume of 10 km 3, we suggest that each eruption was related to a caldera collapse, which would have allowed the persistence of a Taupo-like lake in the central area and of phreatoplinian activity. After the last phreatomagmatic eruption, however, the fragmentation style of large volume ignimbrites from Colli Albani became magmatic, suggesting the extinction of the lake after that date. Each of the four units show a basal phreatoplinian fallout level, overlain by a complex association of low aspect ratio surge deposits and ignimbrites. Phreatomagmatic pyroclastic flow deposits are found at distances of more than 40 km from the central area of the volcano, and show important facies variation according to the paleotopography. To the west of the volcano, pyroclastic flows reached the Tyrrhenian coast and emplaced mostly stratified facies on a flat topography, interpreted to reflect both the spreading of pyroclastic flows on an unconfined topography and their interaction with lacustrine and lagoon areas. By contrast, to the east of the volcano, where pyroclastic flows were confined within paleovalleys, the main facies is thick and massive.

Mineral chemistry and petrogenesis of granular ejecta from the Alban Hills volcano (Central Italy)

A suite of lithics (ejecta) collected from the latest erupted pyroclastic products of the Alban Hills volcano (Central Italy) has been studied to determine their mineralogical composition and to investigate their genesis. The ejecta commonly have granular texture and consist of coarse-grained crystals often associated with a fine- to medium-grained matrix. The mineralogical composition is variable and consists of both typical igneous minerals and contact metamorphic phases. Garnet, clinopyroxene K-feldspar are almost ubiquitous, whereas leucite, wollastonite, sodalite-group minerals, phlogopite, nepheline and phillipsite are present in most of the ejecta; minor and accessory phases include cuspidine, amphibole, pyrrhotite, magnetite, apatite, uranpyrochlore, sphene, kalsilite, and melilite; anorthite, zircon and fluorine-bearing Ca, Zr silicate phases, larnite, and baryte are found sporadically. Ca, REE, Th silicophosphates occur in many samples generally disseminated along interstices and fractures of main minerals. Calcite is present as discrete crystals sometimes enclosed in other minerals, as granules in the fine-grained matrix and as late microcrystalline veins. It shows high oxygen and low carbon isotope ratios with δ18O = + 17.96 to + 27.19, and δ13C = −4.74 to −19.57. Clinopyroxene ranges from diopside to compositions strongly enriched with both Ca-Tschermak’s and esseneite components. Feldspars are generally potassic even though Ba and Sr are found in significant concentrations in some samples. K-feldspars from wollastonite-bearing ejecta are often rimmed with elongated felty crystals identified by X-ray diffraction analysis as leucite. These feldspars show a depletion in Si, and enrichment in Al and K from core to rim. Significant compositional variations are also shown by various other phases such as nepheline, apatite, Ca, REE, Th silicophosphate. The occurrence of igneous and contact metamorphic minerals, as well as the chemical variations of clinopyroxenes and feldspars in the investigated ejecta reveal complex genetic processes related to the interaction between potassic magma and wall rocks. The Ca-rich composition of most phases points to a carbonate nature for the wall rocks. Textural evidence suggests that coarse-grained rocks formed at the margin of the magma chamber were invaded by a late, volatile rich potassic liquid which crystallized as a fine-grained matrix and produced disaggregation and reaction of early formed minerals. Fluid phases percolating through the rocks generated infiltration metasomatism and deposited some uncommon phases rich in Ca, REE, Th, U, which are found along cracks and at the margins of early crystallized minerals. Overall, the all spectrum of the minerals found in this study are also typical of carbonatitic rocks. Their presence in the Alban Hills ejecta demonstrates that their genesis can be related to interaction between ultrapotassic melts and carbonate wall rocks, in addition to precipitation from carbonatitic melts.

Natural Gas Hazard (CO2, 222Rn) within a Quiescent Volcanic Region and Its Relations with Tectonics: The Case of the Ciampino-Marino Area, Alban Hills Volcano, Italy

Groundwater surveys were performed by detailed (around 300 sites) grid-analysis of water temperature, pH, redox potential, electrical conductivity, Rn-222, alkalinity and by calculating the pCO(2), throughout the Ciampino and Marino towns in the Alban Hills quiescent volcano (Central Italy). Following several episodes of dangerous CO2 exhalation from soils during the last 20 years and earlier as historically recorded, the work aimed at assessing the Natural Gas Hazard (NGH) including the indoor-Rn hazard. The NGH was defined as the probability of an area to become a site of poisonous peri-volcanic gas exhalations from soils to the lower atmosphere (comprising buildings). CO2 was found to be a "carrier" for the other poisonous minor and in trace components (H2S, CH4, Rn-222, etc.). This assessment was performed by extrapolating in the aquifer CO2 and Rn-222 conditions, and discriminating sectors where future CO2 flux in soils as well as indoor-Rn measurements have to be noted. A preliminary indoor-Rn survey was performed at about 200 sites. The highest values were found in the highest pCO(2) and high Rn-222 values in groundwater. This indicates convection and enhanced permeability in certain sectors of the main aquifer, i.e., along the bordering faults and inside the gas-trap of the Ciampino Horst., where "continuous gas-phase micro-macro seepage mechanism" is invoked to explain the high peri-volcanic gases flux. Language: en

Age of Ancient Monuments by Means of Building Stone Provenance: a Case Study of the Tullianum, Rome, Italy

The construction age of the Tullianum, the oldest portion of the Carcere Mamertino (Mamertine Jail), the early Roman execution site, is not known. Identifying the provenance of the building stone used to construct the Tullianum can aid in determining the approximate age of the structure because the introduction of different types of building materials occurred at known times. The40Ar/39Ar age of the rock used to construct the Tullianum is 36±1ka, indicating that it is Lapis albanus, which crops out only on the flanks of the Alban Hills volcano approximately 20km from Rome. Based on the general belief that the introduction of Lapis albanus as a building stone did not occur before the 4th centurybc , we interpret that the Tullianum was built, or rebuilt, after this time. This age conflicts with the architectural style of the Tullianum, which matches that of ancient cisterns, believed to date to the 7th or 6th centuries bc. This younger age for the Tullianum suggests a unique history for this building, perhaps tied to ancient religious rituals.

U-series disequilibrium study on a gaseous discharge area (Solforata of Pomezia, Alban Hills, Italy): implications for volcanic and geochemical risk

The age and environmental impact of the gaseous discharge at Solforata of Pomezia (Alban Hills, Italy) has been investigated by the application of U-series disequilibrium methods. Chronological data of sulphide mineralization and of a phreatomagmatic deposit close to Solforata fit with a temporal window (26–47 ka BP) of intense volcanic activity which occurred during the final hydromagmatic phase (180–11 ka BP) of the Alban Hills volcano. Radon measurements have allowed an estimation of fluxes of the main gases presently released at Solforata. A model combining Darcy’s law and modified Stoker–Krüger equations has been used in order to predict the upper limit of the gas pressure (19 atmospheres) at the base of the volcanic succession and of the rising time (35 days) of the gas through the volcanic pile. Geochronological and geochemical constraints, obtained by U-series disequilibrium methods, have proved useful in comparing the volcanic and geochemical risks in this area. The comparison shows that, under normal conditions of gas dispersion, the geochemical risk associated with Rn and H 2S exposure is high and it must be tackled immediately with particular surveillance and intervention methods.

Seismic deformation at the Alban Hills volcano during the 1989-1990 seismic sequence

We analysed the one-year-long seismic swarm at the Alban Hills volcano which occurred during 1989-1990. We portray spatial distribution of seismic moment release, better delineating the activated volume during the swarm. The seismic structure is imaged as a 7-km long, 3-km wide, and 3-km thick volume, located between 2 and 5 km depth, and NW-SE striking. Fault plane solutions and scalar seismic moments for the largest earthquakes provide the description of the average strain rate tensor. The principal strain rate axes show a dominant extension in NE-SW direction, a SE-NW direction of compression and a negligible thickening rate. P and T axes direction of the smaller earthquakes suggests that the same mode of deformation is distributed all over the activated volume. These results are discussed in terms of seismic deforming processes active at the Alban Hills volcano, in the frame of magmatic inflation recently invoked to explain the rapid vertical uplift affecting part of the volcano. The observed average deformation is consistent with shear failures occurring on faults connecting stress-oriented dykes in response to an increasing fluid pressure.

Recent uplift of the Alban Hills Volcano (Italy): Evidence for magmatic inflation?

First‐order leveling surveyed in 1951 and 1994 along a line across the Alban Hills volcano reveals maximum vertical uplift of 30 cm (average of 7 mm/yr) in the region of geologically recent (0.020–0.030 Ma) explosive activity. The uplifted region corresponds to an area where seismic activity is concentrated, at depths of 3–6 km. Uplift exceeding 10 cm is observed over a 14‐km wide zone in the center of a 40‐km long line, with the deformation tapering to zero at both ends of the line. Data from a few sparse, older benchmarks measured in 1891 and 1927 suggest that the deformation in the first half of this century was very small compared to the period 1951–1994. The observed uplift, together with increased seismic activity, suggest that recent magma injection has occurred in the upper crust beneath the Alban Hills volcano.

Large teleseismic P-wave residuals observed at the Alban Hills volcano, Central Italy

We collected teleseismic waveforms from a digital microseismic network deployed by the Istituto Nazionale di Geofisica (ING) in collaboration with the U.S. Geological Survey (USGS), on the Alban Hills Quaternary volcano during the 1989-1990 seismic swann. About 50 events were recorded by the network, 30 of them by at least 4 stations. We analysed the data in order to image crustal heterogeneities beneath the volcano. The results show large delay time residuals up to - 1 second for stations located on the volcano with respect to station CP9 of the National Seismic Network located about 20 km to the east, on the Apennines. This suggests that the whole area overlies a broad low-velocity region. Although the ray coverage is not very dense, we model the gross seismic structure beneath the volcano by inverting the teleseismic relative residuals with the ACH technique. The main features detected by tbc inversion are a low-velocity zone beneath the southwestern fiank of tbc volcano, and a high-velocity region beneath the center. The depth extension of these anomalous zones ranges between 5 and 16 km. The correspondence between the low-velocity region and the most recent activity of the volcano (- 0.027 Ma) leads us to infer the presence of a still hot magmatic body in the crust beneath the southwestern side of the volcano, whereas the central part overlies the older and colder high-velocity volcanic roots related to the previous central activity (0.7 to 0.3 Ma).

Three-dimensional velocity structure and earthquake relocation in the Alban Hills volcano, Central Italy

Abstract The Alban Hills is a Quaternary volcano located along the Tyrrhenian margin of the Apennines (Central Italy), which has erupted from 0.7 to 0.027 Ma. The high density of population around the volcano as well as the proximity to Rome (∼20 km) spurred us to investigate the three-dimensional (3D) crustal structure beneath the volcano, with particular emphasis to the identification of volcano-related phenomena. Hydrothermal circulation and frequent seismic activity characterize the present state of the volcano. A seismic swarm from April 1989 to March 1990 consisted of shallow seismicity (depths less than 6 km), located beneath the western side of the volcano, where the phreatomagmatic eruptions (0.03 Ma) occurred. In this paper, we present new results obtained from seismic tomography using the recorded local earthquakes. We imaged the earthquake distribution jointly with the 3D velocity structure by inverting 1314 P and 1185 S arrival times from 163 selected events using the method developed by Thurber (1983). We progressively focused toward the well-sampled crustal volume, decreasing the model gridspacing in subsequent inversion. Detailed resolution analysis and comparison with results of a synthetic test ensure that the computed models are reliable, at least in the volume where the ray coverage is sufficient. The calculated model improves the hypocentral locations of the 1989-1990 earthquakes, allowing us to visualize the structures that generated the seismic swarm. The relocated earthquakes are confined between 3 and 6 km in depth, and they surround a body with anomalously high P -wave velocity that is probably uplifted and metamorphosed carbonate rocks.

Mineralogical and geochemical study of granular xenoliths from the Alban Hills volcano, Central Italy: bearing on evolutionary processes in potassic magma chambers

Granular xenoliths (ejecta) from pyroclastic deposits emplaced during the latest stages of activity of the Alban Hills volcano range from ultramafic to salic. Ultramafic types consist of various proportions of olivine, spinel, clinopyroxene and phlogopite. They show low SiO2, alkalies and incompatible element abundances and very high MgO. However, Cr, Co and Sc are anomalously low, at a few ppm level. Olivine is highly magnesian (up to Fo%=96) and has rather high CaO (1% Ca) and very low Ni (around a few tens ppm) contents. These characteristics indicate a genesis of ultramafic ejecta by thermal metamorphism of a siliceous dolomitic limestone, probably with input of chemical components from potassic magma. The other xenoliths have textures and compositional characteristics which indicate that they represent either intrusive equivalents of lavas or cumulates crystallized from variably evolved ultrapotassic magmas. One sample of the former group has major element composition resembling ultrapotassic rocks with kamafugitic affinity. Some cumulitic rocks have exceedingly high abundances of Th (81–84 ppm) and light rare-earth elements (LREE) (La+Ce=421–498 ppm) and extreme REE fractionation (La/Yb=288–1393), not justified by their modal mineralogy which is dominated by sanidine, leucite and nepheline. Finegrained phases are dispersed through the fractures and within the interstices of the main minerals. Semiquantitative EDS analyses show that Th and LREE occur at concentration levels of several tens of percent in these phases, indicating that their presence is responsible for the high concentration of incompatible trace elements in the whole rocks. The interstitial position of these phases and their association with fluorite support a secondary origin by deposition from fluorine-rich fluids separated from a highly evolved potassic liquid. The Nd isotopic ratios of the cjecta range from 0.51182 to 0.51217. 87Sr/86Sr ratios range from 0.70900 to 0.71036. With the exception of one sample, these values are lower than those of the outcropping lavas, which cluster around 0.7105±3. This indicates either the occurrence of several isotopically distinct potassic magmas or a variable interaction between magmas and wall rocks. However, this latter hypothesis requires selective assimilation of host rocks in order to explain isotopic and geochemical characteristics of lavas and xenoliths. The new data indicate that the evolutionary processes in the potassic magmas of the Alban Hills were much more complex than envisaged by previous studies. Interaction of magmas with wall rocks may be an important process during magmatic evolution. Element migration by gaseous transfer, often invoked but rarely constrained by sound data, is shown to have occurred during the latest stages of magmatic evolution. Such a process was able to produce selective enrichment of Th, U, LREE and, to a minor degree, Ta and Hf in the wall rocks of potassic magma chamber. Finally, the occurrence of xenoliths with kamafugitic composition points to the existence of this type of ultrapotassic magma at the Alban Hills.

TH-230 dating of the last products of the Alban Hills volcano (Rome)

TH-230 dating of the last products of the Alban Hills volcano (Rome)