Sabatini Volcanic District Research Articles

Fluid geochemistry and geothermometry in the western sector of the Sabatini Volcanic District and the Tolfa Mountains (Central Italy)

A geochemical survey of 197 fluid discharges (cold and thermal waters and bubbling pools) and 15 gas emissions from the western sector of the Sabatini Volcanic District and the Tolfa Mountains (Latium, Central Italy) was carried out in 2007–2008. The chemical and isotopic compositions of the fluid discharges indicate the occurrence of two main sources: 1) relatively shallow aquifers with Ca(Na,K)–HCO3 and Ca(Mg)–HCO3 compositions when trapped in volcanic and sedimentary formations, respectively; and 2) a deep reservoir, which is hosted in the Mesozoic carbonate sequence, rich in CO2 and having a Ca–SO4(HCO3) composition. Dissolution of a CO2-rich gas phase into the shallow aquifers produces high-TDS and high-pCO2 cold waters, while oxidation of deep-derived H2S to SO42− generates low-pH (< 4) sulfate waters. The δ13C–CO2 values for gas emissions (from − 2.8 to + 2.7‰ vs. VPDB) suggest that the origin of CO2 associated with the deep fluids is mainly related to thermo-metamorphic reactions within the carbonate reservoir, although significant mantle contribution may also occur. However, R/Ra values (0.37–0.62) indicate that He is mainly produced by a crustal source, with a minor component from a crust-contaminated mantle. On the basis of the δ13C–CH4 and δD–CH4 values (from − 25.7 to − 19.5‰ vs. VPDB and from − 152 to − 93.4‰ vs. VSMOW, respectively) CH4 production is associated with thermogenic processes, possibly related to abiogenic CO2 reduction within the carbonate reservoir. The δ34S–H2S values (from + 9.3 to + 10.4‰ vs. VCDT) are consistent with the hypothesis of a sedimentary source of sulfur from thermogenic reduction of Triassic sulfates. Geothermometric evaluations based on chemical equilibria CO2–CH4 and, separately, H2S suggest that the reservoir equilibrium temperature is up to ~ 300 °C. The δD and δ18O data indicate that water recharging both the shallow and deep aquifers has a meteoric origin. Fluid geochemistry, coupled with gravimetric data and tectonic lineaments, supports the idea that significant contributions from a deep-seated geothermal brine are present in the Stigliano thermal fluid discharges. Exploration surveys investigated this area during 70's–90's for geothermal purposes. Nevertheless, presently the area is still under-exploited. The presence of thermal waters and anomalous heat flow together with the demographic growth of the last years, makes this site a suitable location for direct applications of the geothermal resource.

Geochronology of the most recent activity in the Sabatini Volcanic District, Roman Province, central Italy

The presence of young, highly explosive volcanoes in the densely populated Roman area (central Italy), makes the evaluation of the long-term volcanic hazard a crucial point. The Sabatini Volcanic District (SVD), part of the potassic Quaternary Roman Province, extends over an area of 1800 km 2 including the city of Rome. Here we report twenty-six new 40Ar/ 39Ar age analyses of pyroclastic and lava units in order to put time constraints on the most recent SVD eruptive activity (i.e. post-dating 400 ka), as a groundwork for a detailed reconstruction of the whole volcano-tectonic evolution. Integrated geochronological and field evidence shows that SVD volcanism maintained a typical areal character through the whole history: both effusive and explosive activity phases from different source areas and volcano-tectonic settings overlapped in time and space, while climactic phases migrated repeatedly through time. In particular, new geochronological data better constrain a climactic phase of activity at around 300 ka, related to the Bracciano volcano-tectonic depression and the Sacrofano caldera, and point out that the most recent eruptive activity in the SVD took place from time–space-clustered hydromagmatic centres at least up to 90 ka. In addition, we remark some parallelism among major activity phases in the SVD and the nearby districts of the Roman Province, with possible implications on Quaternary regional tectonics. Finally, new age data put constraints on tephrostratigraphic correlations through the Apennine Chain and the Adriatic Sea.

H 2O- and temperature-zoning in magma chambers: The example of the Tufo Giallo della Via Tiberina eruptions (Sabatini Volcanic District, central Italy)

Textural and chemical variations of juvenile clasts are widely observed in pyroclastic deposits. In particular, the co-existence of whitish, pumiceous, and dark grey, scoriaceous, juvenile clasts has been observed in many eruptive units of well-known volcanoes (i.e., Somma–Vesuvius, Vulsini, Colli Albani, Stromboli). Here we report the example of the Tufo Giallo della Via Tiberina (TGVT) pyroclastic succession, which comprises two eruptive units emplaced at ca. 561 and 548 ka, during the early explosive activity of the Sabatini Volcanic District (SVD; Roman Province, central Italy). TGVT deposits, as well as underlying pyroclastic products (FAD, ca. 582 ka), are characterized by coexisting whitish pumice and black–grey scoria clasts showing common phonolitic composition but different textural features: white pumice is highly vesicular, vitrophyric, and contains scarce, > 50 µm-sized, feldspar and clinopyroxene crystals, while black–grey scoria is poorly vesicular, highly crystallized, and contains diffuse leucite phenocrysts. The latter records crystallization under H 2O-undersaturated conditions, as opposed to the vitrophyric texture of white pumice indicating higher temperature and H 2O concentration. On these grounds, a thermally and H 2O-zoned pre-eruptive system has been modelled for the phonolitic magma chambers feeding the early SVD events, in which white pumice and black–grey scoria represent the inner and peripheral portions of the reservoirs, respectively. Extensive leucite + clinopyroxene crystallization in the H 2O-undersaturated, peripheral portions of the reservoirs, resulted in water flux toward the inner zones, where the higher temperature and increasing H 2O content acted to delay crystallization in the white pumice-feeder magma. The withdrawal of white pumice at the eruption onset produced decompression of the peripheral magma, triggering black–grey scoria eruption during the late phases of explosive events.

Conifer extinction in Quaternary Italian records

Pollen analysis is often not definitive in identifying conifer pollen taxa, because pollen of many genera are indistinguishable through the light microscope. An integration of pollen and macrofossils analyses is needed, as woods, leaves, and seeds are often identified with greater taxonomic precision than pollen. The identification of Keteleeria pollen is doubtful, but the local presence of this tree is documented in permineralized fossil woods from the Sabatini volcanic district (Latium) in rocks younger than 700 ka, the beginning of substantial volcanic activity of the area. During the Pliocene and the Early Pleistocene, “Taxodiaceae” forests developed several times in the Italian peninsula. A permineralized wood of Taxodium was found in the “Tufi litoidi” (Auct.) of the “Peperino” formation of Via Flaminia (Rome), dated 450 ka, together with many other permineralized fossil woods and leaves, some of which belonging to taxa not distinguishable by pollen. They document the presence of several conifer taxa: Amentotaxus, Cephalotaxus, Torreya, Abies, Pinus, Cupressus, Juniperus, Taxus. Considering the pollen record, Tsuga, Cathaya, and Cedrus are at present extinct in Italy. Picea and Abies, which were well represented in several phases of Middle and Upper Pleistocene age around Rome, are no longer included in the flora of Latium.

Plinian activity during the early eruptive history of the Sabatini Volcanic District, Central Italy

The early activity of the Sabatini Volcanic District (SVD; central Italy) was characterised by highly explosive eruptions that produced widespread subplinian and plinian fall deposits. In this study, four major eruptive units—informally named as units A, B, C and D—were recognised in the 514–449 ka age interval. In particular, unit D was emplaced during the early phase of the 449 ka Tufo Rosso a Scorie Nere pyroclastic flow-forming eruption, the most important event in the whole SVD activity history. Estimates of relevant eruptive parameters indicate tephra fall volumes up to 4 km 3 for individual units, peak eruption column heights in the range of 14–29 km and corresponding mass eruption rates of 7.8×10 6–1.3×10 8 kg/s. Isopach and isopleth maps of fallout deposits—as well as the distribution of the proximal lag-breccia of the Tufo Rosso a Scorie Nere—consistently indicate a common vent area, which does not correspond to any volcanic centre identified up to now in the SVD. This was located along NE–SW-trending tectonic lineaments that also controlled the location of the other major volcanic centres of the SVD. The characterisation by means of field aspects, grain size, componentry and density and chemical composition of juvenile clasts, renders the studied fall deposits as valuable stratigraphic markers for the SVD and well beyond it. In fact, their wide areal dispersals toward the E and SE may allow correlations on a regional scale for the Quaternary successions of intermountain basins of the Central Apennine and the Adriatic Sea basin successions. Finally, the correct identification of distal tephra from plinian and co-ignimbrite plumes and their attribution to specific explosive eruptions of the SVD and the other volcanic districts of the Roman Province—rather than to local intra-Apennine centres—provides crucial implications for geodynamic reconstructions.

Crystal chemistry of kimzeyite from Anguillara, Mts. Sabatini, Italy

A single garnet crystal of kimzeyite from Anguillara Sabazia, Sabatini Volcanic District (Rome), has been studied by electron probe microanalysis (EPMA), single crystal X-ray diffraction (XRD) and Raman spectroscopy (RS). X-ray intensities for the structure refinement were collected by means of both a conventional four circle diffractometer (CD) and an area detector (AD) diffractometer. Salient crystallographic data from AD measurements are: a = 12.397(4) A, V = 1905.24 A 3 , μ= 25 cm −1 , crystal dimension ∼ 40 μm, D x =3.83 g cm −3 . The chemical analyses allowed to ascertain that Ca, Ba, Fe, Al, Mg, REE, Zr, Si and Ti are present in the structure. The transition element distribution is: Fe 3+ (Y) ≪ Fe 3+ (Z), and Ti as Ti 4+ occurs only at Y. No hydrogen atoms were detected. The problem of the correct distribution of Ba and REE (essentially Eu and Gd) is dealt with, but not fully solved in the present study. Because of the very small amount of sample available and of the strong dilution of Ba and REE (∼0.03 and 0.02 atoms per formula unit respectively) the correct distribution of these atoms over the X and Y sites is problematical. The radius ratio rule and electrostatic requirements suggest Ba at X and REE at Y. From these principles, the following site distribution can be deduced: Ca 2+ and Ba 2+ at X; Mg 2 +, REE 3+ , Zr 4+ , Ti 4+ , Fe 3+ at Y; Si 4+ , Al 3+ , Fe 3+ at Z. On the other hand, structural details and the best fit between EPMA and XRD site populations indicate REE at X and Ba at Y. Bond strength theory arguments support this hypothesis. The main substitution mechanisms can be summed up as follows: 1) Ti 4+ (Y) + Fe 3+ (Z) ↔ Si 4+ (Z) + M 3+ (Y) [schorlomite substitution]; 2) Zr 4+ (Y) + M 3+ (Z) ↔ Si 4+ (Z) + M 3+ (Y) [kimzeyite substitution]; 3) Mg 2+ (Y) + Zr 4+ (Y) ↔ 2M 3+ (Y) [morimotoite-like substitution], where M 3+ = (Al, Fe). In addition, following the bond strength theory indications, another mechanism which possibly involves the X and Y sites is also taken into account: 4) REE 3+ (X) + Ba 2+ (Y) ↔ Ca 2+ (X) + M 3+ (Y).

Sanidine holocrystalline ejecta from central sabatini volcanic district, latium (Italy). II. Intergranular ejecta and minerogenetic deductions

The sanidine holocrystalline ejecta with intergranular texture from the central Sabatini Volcanic District contain high-T feldspars, out-of-equilibrium K- and Na- Ca-feldspar, and two pyroxene generations formed at different depths, as well as many mineral phases which indicate highf(H2O) and/orf(O2). A simple evolution model by fractional crystallization, in a plutonic or hypoabissal domain, has been derived from the mineralogical assemblages present in both the isotropic and intergranular type of ejecta. The intergranular type, the crystallization of which began at deeper levels than the isotropic one, underwent multiple pyroxene-liquidus re-equilibrations during a quick magma ascent to shallower levels.

Sanidine holocrystalline ejecta from central Sabatini Volcanic District, latium (Italy). I. Introduction and isotropic ejecta

The chemical analysis of main mineralogical constituents of the sanidine holocrystalline ejecta with isotropic texture found in the central area of the Sabatini Volcanic District leads to their petrological characterization. The application of the most advanced graphical geothermometers to coexisting feldspar pairs of various samples provides a temperature of equilibration ofT≤900°C. A chronologically intermediate event of vesiculation is evident in the crystallization sequence. Products of a peculiar late magmatic fluid circulation also occur.

Petrology and geochemistry of the ultrapotassic rocks from the Sabatini Volcanic District, central Italy: the role of evolutionary processes in the genesis of variably enriched alkaline magmas

The Sabatini Volcanic District (SVD) is a large volcanic field characterised by the lack of any major volcanic center. Its activity, spread over a wide area, started at 0.6 Ma and developed through five main phases, during which several calderas and the Bracciano lake volcano-tectonic depression were formed. All the volcanic rocks belong to the Roman-type ultrapotassic series (HKS), ranging from leucite tephrites to leucite and haüyne phonolites. Although the major- and compatible-element contents indicate a single series of evolution, there are differences in the incompatible trace-element abundances. A high-Ba series (HBaS) has been distinguished from a low-Ba series (LBaS), with the former also enriched in all other incompatible elements (e.g., REE, Nb, Zr, Th) except Rb. The HBaS rocks are plagioclase-free, leucite-bearing lavas and were abundantly outpoured from the Bracciano volcanoes during the third and fifth phase of activity. Plagioclase- and phlogopite-bearing rocks constitute the LBaS and were erupted during the other phases generally from smaller and eccentric volcanic centers. The initial 87Sr/ 86Sr values are higher in the HBaS rocks and do not vary significantly with magma evolution (0.71047–0.71080), but cover a wider range in the LBaS rocks (0.70944–0.71038), with the lowest Sr isotope ratios occurring in the least evolved lavas. The higher Ca content in the olivine and Ti and Al IV in the clinopyroxene, and the lower ulvöspinel content of the Ti-magnetites of the HBaS rocks suggest an evolution at lower pressure and higher temperature for this magma. The observed petrologic characteristics suggest that the HBaS magma evolved at lower depths by processes of refilling, tapping, fractionation and probably assimilation (RTFA), where the crystallisation rate of clinopyroxene+leucite±olivine dominates over the input rate of the fresh magma. The LBaS magma evolved at slightly higher pressure, in separate and small magma bodies, by fractional crystallisation of clinopyroxene+plagioclase±phlogopite±olivine that was often associated with crustal assimilation (AFC). It has been suggested that RTFA processes with high input rate/crystallisation rate ratios could also be responsible for the differentiation between the HBaS and LBaS. The different processes of evolution undergone by the HBaS and LBaS could have been related to the different volumes of magma rising from the source.

Structural evolution of the Bracciano volcano-tectonic depression, Sabatini Volcanic District, Italy

Abstract The Latian volcanoes of Central Italy form several K-rich volcanic districts, some of which are characterized by large volcano-tectonic depressions filled with lakes. The development of these large depressions (&gt;8 km in diameter) occurred from 0.4–0.3 Ma bp , during a climax of regional extension. Analysis of geophysical and deep drilling data and detailed field analysis, allows reconstruction of the evolution of the Bracciano volcanotectonic depression in the Sabatini Volcanic District, northwest of Rome. This depression developed during the Upper Pleistocene inside a NE-trending half-graben structure. The evolution of the half-graben has been driven by NE-trending regional faults which were reactivated during volcanism. Magma rose along these faults and came to rest at 4–7 km depth, probably as a laccolithic body, and was then erupted with emplacement of large volume ignimbrites. A lava plateau recognized by drilling data in the southern and western sectors of the depression is interpreted as being associated with NE-striking swarms of feeder dykes. The final downsagging was caused by NE and NW-trending regional faults after 0.17 Ma. The eastern depression morphology was modified by late Pleistocene N-trending strike-slip faults which were accompanied by intense hydromagmatic activity. Hydromagmatic craters are distinguished in the eastern margin of the area and in the southeastern part of Lake Bracciano in high-frequency seismic sections.

Contact metasomatic and hydrothermal minerals in the SH2 deep well, Sabatini volcanic district, Latium, Italy

Metasomatic and hydrothermal minerals were logged throughout the SH2 geothermal well, which reached a depth of 2498 m in the Sabatini volcanic district. Below 460 m of volcanics, where the newly formed minerals were mainly chlorite, calcite and zeolites (mostly phillipsite), drilling entered the Allochthonous Flysch Complex. Evidence of the “Cicerchina facies” was found down to 1600 m depth. Starting from 1070 m, down to hole bottom, a contact metasomatic complex was defined by the appearance of garnet. Garnet together with K-feldspar, vesuvianite, wilkeite, cuspidine, harkerite, wollastonite and apatite prevail in the top part of the contact metasomatic complex. Vesuvianite and phlogopite characterize the middle part. Phlogopite, pyroxene, spinel and cancrinite predominate in the bottom part. The 1500 m thick metasomatic complex indicates the presence at depth of the intrusion of a trachytic magma which released hot fluids involved in metasomatic mineral-forming reactions. Minerals such as harkerite, wilkeite, cuspidine, cancrinite, vesuvianite and phlogopite indicate the intrusive melt had a high volatile content which is in agreement with the very high explosivity index of this volcanic district. The system is at present sealed by abundant calcite and anhydrite. It is proposed that most, if not all, of the sulphates formed after reaction of SO 2 with aqueous calcium species rather than from sulphates being remobilized from evaporitic (Triassic) rocks as previously inferred. The hypothesis of a CO 2-rich deep-derived fluid ascending through major fracture systems and contrasting cooling in the hottest areas of Latium is presented.