Statistical analyses and characteristics of volcanic tremor on Stromboli Volcano (Italy)

 A study of volcanic tremor on Stromboli is carried out on the basis of data recorded daily between 1993 and 1995 by a permanent seismic station (STR) located 1.8 km away from the active craters. We also consider the signal of a second station (TF1), which operated for a shorter time span. Changes in the spectral tremor characteristics can be related to modifications in volcanic activity, particularly to lava effusions and explosive sequences. Statistical analyses were carried out on a set of spectra calculated daily from seismic signals where explosion quakes were present or excluded. Principal component analysis and cluster analysis were applied to identify different classes of spectra. Three clusters of spectra are associated with two different states of volcanic activity. One cluster corresponds to a state of low to moderate activity, whereas the two other clusters are present during phases with a high magma column as inferred from the occurrence of lava fountains or effusions. We therefore conclude that variations in volcanic activity at Stromboli are usually linked to changes in the spectral characteristics of volcanic tremor. Site effects are evident when comparing the spectra calculated from signals synchronously recorded at STR and TF1. However, some major spectral peaks at both stations may reflect source properties. Statistical considerations and polarization analysis are in favor of a prevailing presence of P-waves in the tremor signal along with a position of the source northwest of the craters and at shallow depth. Received: 15 December 1996 / Accepted: 31 March 1998     

Fluid circulation at Stromboli volcano (Aeolian Islands, Italy) from self-potential and CO2 surveys

This work addresses the study of fluid circulation of the Stromboli island using a dense coverage of self-potential (SP) and soil CO₂ data. A marked difference exists between the northern flank and the other flanks of the island. The northern flank exhibits (1) a typical negative SP/altitude gradient not observed on the other flanks, and (2) higher levels of CO₂. The general SP pattern suggests that the northern flank is composed of porous layers through which vadose water flows down to a basal water table, in contrast to the other flanks where impermeable layers impede the vertical flow of vadose water. In the Sciara del Fuoco and Rina Grande–Le Schicciole landslide complexes, breccias of shallow gliding planes may constitute such impermeable layers whereas elsewhere, poorly permeable, fine-grained pyroclastites or altered lava flows may be present. This general model of the flanks also explains the main CO₂ patterns: concentration of CO₂ at the surface is high on the porous north flank and lower on the other flanks where impermeable layers can block the upward CO₂ flux. The active upper part of the island is underlain by a well-defined hydrothermal system bounded by short-wavelength negative SP anomalies and high peaks of CO₂. These boundaries coincide with faults limiting ancient collapses of calderas, craters and flank landslides. The hydrothermal system is not homogeneous but composed of three main subsystems and of a fourth minor one and is not centered on the active craters. The latter are located near its border. This divergence between the location of the active craters and the extent of the hydrothermal system suggests that the internal heat sources may not be limited to sources below the active craters. If the heat source strictly corresponds to intrusions at depth around the active conduits, the geometry of the hydrothermal subsystems must be strongly controlled by heterogeneities within the edifice such as craters, caldera walls or gliding planes of flank collapse, as suggested by the correspondence between SP–CO₂ anomalies and structural limits. The inner zone of the hydrothermal subsystems is characterized by positive SP anomalies, indicating upward movements of fluids, and by very low values of CO₂ emanation. This pattern suggests that the hydrothermal zone becomes self-sealed at depth, thus creating a barrier to the CO₂ flux. In this hypothesis, the observed hydrothermal system is a shallow one and it involves mostly convection of infiltrated meteoric water above the sealed zone. Finally, on the base of CO₂ degassing measurements, we present evidence for the presence of two regional faults, oriented N41° and N64°, and decoupled from the volcanic structures.     

Unusual sedimentary deposits on the SE side of Stromboli volcano, Italy: products of a tsunami caused by the ca. 5000 years BP Sciara del Fuoco collapse?

The role of sector collapse in the generation of catastrophic volcanigenic tsunami has become well understood only recently, in part because of the problems in the preservation and recognition of tsunami deposits. Tinti et al. [Tinti, S., Bortolucci, E., Romagnoli, C., 2000. Computer simulations of tsunamis due to sector collapse at Stromboli, Italy. J. Volcanol. Geotherm. Res. 96, 103–128] modeled a tsunami produced by the c. 5,000 years BP collapse of the Sciara del Fuoco on the island volcano Stromboli. Although deposits associated with this event are generally lacking on the island, volcaniclastic breccias on the SE side of the island extending to an elevation above 120 m a.s.l. may have been generated by this tsunami. Deposits above 100 m are dominated by coarse breccias comprising disorganized, poorly sorted, nonbedded, angular to subangular lava blocks in a matrix of finer pyroclastic debris. These breccias are interpreted as a water-induced mass flow, possibly a noncohesive debris flow, generated as colluvial material on steep slopes was remobilized by the return flow of the tsunami wave, the run-up of which reached an elevation exceeding 120 m a.s.l. Finer breccias of subrounded to rounded lava blocks cropping out at 15 m a.s.l. are similar to modern high-energy beach deposits and are interpreted as beach material redeposited by the advancing tsunami wave. The location of these deposits matches the predicted location of the maximum tsunami wave amplitude as calculated by modeling studies of Tinti et al. [Tinti, S., Bortolucci, E., Romagnoli, C., 2000. Computer simulations of tsunamis due to sector collapse at Stromboli, Italy. J. Volcanol. Geotherm. Res. 96, 103–128]. Whereas the identification and modeling of paleo-tsunami events is typically based on the observation of the sedimentary deposits of the tsunami run-up, return flow may be equally or more important in controlling patterns of sedimentation.     

Evolution of Br/Cl ratios in fumarolic salammoniac from Vulcano (Aeolian Islands,Italy)

 Bromine/chlorine ratios in salammoniac samples, collected from 1983 to 1994 at the Fossa crater fumaroles of Vulcano (Aeolian Islands), were determined. The data obtained in this work have been compared to those obtained by Coradossi et al. (1985) in order to carry out a systematic study of the Br/Cl ratio variations since 1977. Br/Cl ratios range from 0.04 to 0.29. The Br/Cl ratio variations observed have been interpreted as the result of different contributions among deep and shallow components feeding the fumarolic fluids of Vulcano. The Br/Cl increases, which occurred from 1977 to 1979, in 1983–1984 and in 1992–1993, have been connected to an increased input of magmatic gases. Received: 10 November 1995 / Accepted: 12 July 1996     

A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feeding system

 This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary series, an older one (50–25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo_88–91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements, with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180±20  °C) and similar volatile abundances. The H₂O, S and Cl contents are relatively high, whereas magmatic CO₂ concentrations are very low, probably due to CO₂ loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic mantle. The younger primitive melt is characterized with respect to the older one by higher K₂O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination. Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs. Received: 11 March 1998 / Accepted: 14 July 1998     

Pyroclastic density currents at Stromboli volcano (Aeolian Islands,Italy): a case study of the 1930 eruption

Pyroclastic density currents (PDC) related to paroxysmal eruptions have caused a large number of casualties in the recent history of Stromboli. We combine here a critical review of historical chronicles with detailed stratigraphic, textural, and petrographic analyses of PDC deposits emplaced at Stromboli over the last century to unravel the origin of currents, their flow mechanism and the depositional dynamics. We focus on the 1930 PDC as they are well described in historical accounts and because the 1930 eruption stands as the most voluminous and destructive paroxysm of the last 13 centuries. Stromboli PDC deposits are recognizable from their architecture and the great abundance of fresh, well-preserved juvenile material. General deposit features indicate that Stromboli PDC formed due to the syn-eruptive gravitational collapse of hot pyroclasts rapidly accumulated over steep slopes. Flow channelization within the several small valleys cut on the flanks of the volcano can enhance the mobility of PDC, as well as the production of fine particles by abrasion and comminution of hot juvenile fragments, thereby increasing the degree of fluidization. Textural analyses and historical accounts also indicate that PDC can be fast (15–20 m/s) and relatively hot (360–700 °C). PDC can thus flow right down the slopes of the volcano, representing a major hazard. For this reason, they must be adequately taken into account when compiling risk maps and evaluating volcanic hazard on the Island of Stromboli.     

Relationships between deformation and mixing processes in lava flows: a case study from Salina (Aeolian Islands,Tyrrhenian Sea)

 The massive unit of a lava flow from Porri volcano (Salina, Aeolian Islands) displays many unusual structures related to the physical interaction between two different magmas. The magma A represents approximately 80% of the exposed lava surface; it has a crystal content of 51 vol.% and a dacitic glass composition (SiO₂=63–64 wt.%). The magma B has a basaltic-andesite glass composition (SiO₂=54–55 wt.%) and a crystal content of approximately 18 vol.%. It occurs as pillow-like enclaves, banding, boudin-like and rolling structures which are hosted in magma A. Structural analysis suggests that banding and boudin-like structures are the result of the deformation of enclaves at different shear strain. The linear correlation between strain and stratigraphic height of the measured elements indicates a single mode of deformation. We deduce that the component B deformed according to a simple shear model. Glass analyses of the A–B boundary indicate that A and B liquids mix together at high shear strain, whereas only mingling occurs at low shear strain. This suggests that the amount of deformation (i.e. forced convection) plays an important role in the formation of hybrid magmas. High shear strain may induce stretching, shearing and rolling of fluids which promote both forced convection and dynamical diffusion processes. These processes allow mixing of magmas with large differences in their physical properties. Received: 15 July 1995 / Accepted: 30 May 1996     

The Holocene Secche di Lazzaro phreatomagmatic succession (Stromboli,Italy): evidence of pyroclastic density current origin deduced by facies analysis and AMS flow directions

The edifice of Stromboli volcano gravitationally collapsed several times during its volcanic history (>100 ka–present). The largest Holocene event occurred during the final stage of the Neostromboli activity (∼13–5 ka), and was accompanied by the emplacement of phreatomagmatic and lahar deposits, known as the Secche di Lazzaro succession. A stratigraphic and paleomagnetic study of the Secche di Lazzaro deposits allows the interpretation of the emplacement and the eruptive processes. We identify three main units within the succession that correspond to changing eruption conditions. The lower unit (UA) consists of accretionary lapilli-rich, thinly bedded, parallel- to cross-stratified ash deposits, interpreted to indicate the early stages of the eruption and emplacement of dilute pyroclastic density currents. Upward, the second unit (UB) of the deposit is more massive and the beds thicker, indicating an increase in the sedimentation rate from pyroclastic density currents. The upper unit (UC) caps the succession with thick, immediately post-eruptive lahars, which reworked ash deposited on the volcano’s slope. Flow directions obtained by Anisotropy of Magnetic Susceptibility (AMS) analysis of the basal bed of UA at the type locality suggest a provenance of pyroclastic currents from the sea. This is interpreted to be related to the initial base-surges associated with water–magma interaction that occurred immediately after the lateral collapse, which wrapped around the shoulder of the sector collapse scar. Upward in the stratigraphy (upper beds of UA and UB) paleoflow directions change and show a provenance from the summit vent, probably related to the multiple collapses of a vertical, pulsatory eruptive column.     

Temporal variations in gravity at Mt. Etna (Italy) associated with the 1989 and 1991 eruptions

 Results are presented from 11 microgravity surveys on Mt. Etna between 1987 and 1993, a period including the major 1989 and 1991–1993 flank eruptions and subordinate 1990 activity. Measurements were made with LaCoste and Romberg D-62 and D-157 gravity meters along a network around the volcano between 1000 and 1900 m a.s.l. and, since 1992, a N–S summit profile. Gravity changes of as much as 200 μGal were observed at scales from the size of the summit region to that of the volcano. None was associated with significant changes in ground elevation. The data show an increase in gravity for 2 years before the 1989 eruption. The increase is attributed to the accumulation of magma (0.25–1.7×10⁹m³) in an elongate zone, oriented NNW–SSE, between 2.5 and 6 km below sea level. Part of this magma was injected into the volcanic pile to supply the 1989 and 1990 eruptions. It also probably fed the start of the 1991–1993 eruption, since this event was not preceded by significant gravity changes. A large gravity increase (up to 140 μGal) detected across the volcano between June and September 1992 is consistent with the arrival in the accumulation zone of 0.32–2.2×10⁹m³ of new magma, thus favoring continued flank effusion until 1993. A large gravity decrease (200 μGal) in the summit region marked the closing stages of the 1991–1993 event and is associated with magma drainage from the upper levels of Etna's central feeding system. Received: 15 July 1995 / Accepted: 27 October 1997     

Coupled degassing and crystallization: experimental study at continuous pressure drop, with application to volcanic bombs

 Experiments on degassing of water-saturated granite melts with a pressure drop from 100 and 450 MPa to 40 and 120 MPa, respectively, at temperatures close to feldspar liquidus (750–700  °C), were carried out to determine the modality of water exsolution and vesicle formation at the liquidus temperature. Pressure-drop rates as small as approximately 100 bar/day were used. Uniform space distributions of bubbles of exsolved water were obtained with starting glass containing a small fraction (≈0.5 vol.%) of trapped air bubbles. Volume crystallization of feldspar was observed in degassed melts supplied with seeds. Bubble size distributions (BSD) measured in granite glasses after degassing are presented. Data on vesicle characteristics (number, radius, area, elongation) were acquired on images digitized with standard software, while the reconstruction of size distributions was performed with the Schwartz-Saltikov "unfolding" procedure. Bubble size distributions of size classes in the range 5–1000 μm were acquired with proper magnification and satisfactory statistical reliability of determined number densities. The BSDs of the experimental samples are compared with the results of measurements of rapidly degassed products of Mt. Etna and Vulcano Island. Many particular features of the bubble nucleation and growth can be distinguished in an individual BSD. However, the general BSD of the whole data set, including natural ones, can be relatively well described with linear regression in bilogarithmic coordinates. The slope of this regression is approximately 2.8±0.1. This dependence is in striking contrast with distributions theoretically predicted with classical nucleation models based on homogeneous nucleation of vesicles. The theoretical distribution requires the occurrence of strong maxima that are not observed in our experimental and natural samples, thus arguing for heterogeneous nucleation mechanisms. Received: 1 October 1998 / Accepted: 25 June 1999     

Petrologic investigations of the 1995 and 1996 eruptions of Ruapehu volcano, New Zealand: formation of discrete and small magma pockets and their intermittent discharge

 Ruapehu volcano erupted intermittently between September and November 1995, and June and July 1996, producing juvenile andesitic scoria and bombs. The volcanic activity was characterized by small, sequential phreatomagmatic and strombolian eruptions. The petrography and geochemistry of dated samples from 1995 (initial magmatic eruption of 18 September 1995, and two larger events on 23 September and 11 October), and from 1996 (initial and larger eruptions on 17–18 June) suggest that episodes of magma mixing occurred in separate magma pockets within the upper part of the magma plumbing system, producing juvenile andesitic magma by mixing between relatively high (1000–1200  °C)- and low (∼1000  °C)- temperature (T) end members. Oscillatory zoning in pyroxene phenocrysts suggests that repeated mixing events occurred prior to and during the 1995 and 1996 eruptions. Although the 1995 and 1996 andesitic magmas are products of similar mixing processes, they display chronological variations in phenocryst clinopyroxene, matrix glass, and whole-rock compositions. A comparison of the chemistry of magnesian clinopyroxene in the four tephras indicates that, from 18 September through June 1996, the tephras were derived from at least two discrete high-temperature (high-T) batches of magma. Crystals of magnesian clinopyroxene in the 23 September and 11 October tephras appear to be derived from different high-T magma batches. Whole-rock and matrix-glass compositions of all tephras are consistent with their derivation from distinct mixed melts. We propose that, prior to 1995 there was a shallow low-temperature (low-T) magma storage system comprising crystal-rich mush and remnant magma from preceding eruptive episodes. Crystal clots and gabbroic inclusions in the tephras attest to the existence of relict crystal mush. At least two discrete high-T magmas were then repeatedly injected into the mush zone, forming discrete and mixed magma pockets within the shallow system. The intermittent 1995 and 1996 eruptions sequentially tapped these magma pockets. Received: 1 April 1998 / Accepted: 22 December 1998     

Silicic magma entering a basaltic magma chamber: eruptive dynamics and magma mixing — an example from Salina (Aeolian islands,Southern Tyrrhenian Sea)

The Pollara tuff-ring resulted from two explosive eruptions whose deposits are separated by a paleosol 13 Ka old. The oldest deposits (LPP, about 0.2 km³) consist of three main fall units (A, B, C) deposited from a subplinian column whose height (7–14 km) increased with time from A to C, as a consequence of the increased magma discharge rate during the eruption (1–8x10⁶ kg/s). A highly variable juvenile population characterizes the eruption. Black, dense, highly porphyritic, mafic ejecta (SiO₂=50–55%) almost exclusively form A deposits, whereas grey, mildly vesiculated, mildly porphyritic pumice (SiO₂=56–67%) and white, highly vesiculated, nearly aphyric pumice (SiO₂=66–71%) predominate in B and C respectively. Mafic cumulates are abundant in A, while crystalline lithic ejecta first appear in B and increase upward. The LPP result from the emptying of an unusual and unstable, compositionally zoned, shallow magma chamber in which high density mafic melts capped low density salic ones. Evidence of the existence of a short crystal fractionation series is found in the mafic rocks; the andesitic pumice results from complete blending between rhyolitic and variously fractionated mafic melts (salic component up to 60 wt%), whereas bulk dacitic compositions mainly result from the presence of mafic xenocrysts within rhyolitic glasses. Viscosity and composition-mixing diagrams show that blended liquids formed when the visosities of the two end members had close values. The following model is suggested: 1. A rhyolitic magma rising through the metamorphic basement enterrd a mafic magma chamber whose souter portions were occupied by a highly viscous, mafic crystal mush. 2. Under the pressure of the rhyolitic body the nearly rigid mush was pushed upwards and mafic melts were squeezed against the walls of the chamber, beginning roof fracturing and mingling with silicic melts. 3. When the equilibrium temperature was reached between mafic and silicic melts, blended liquids rapidly formed. 4. When fractures reached the surface, the eruption began by the ejection of the mafic melts and crystal mush (A), followed by the emission of variously mingled and blended magmas (B) and ended by the ejection of nearly unmixed rhyolitic magma (C).     

Deformation patterns in a high-viscosity lava flow inferred from the crystal preferred orientation and imbrication structures: an example from Salina (Aeolian Islands,southern Tyrrhenian Sea,Italy)

Shape-preferred orientation and imbrication structures of crystals have been measured on samples representative of the base, centre and top of a highly viscous lava flow on Salina (Aeolian Islands, southern Tyrrhenian Sea). The data allow zones with different deformation patterns to be identified. In the base and top of the flow, deformation leads to the development of discrete preferred orientation and imbrication of the elongate crystals. The sense of shear is right-lateral at the base and left-lateral at the top of the flow. Shear strain can be estimated by the analysis of crystal preferred orientation. Deformation increases from the flow centre to the outer, more viscous boundary layers. Random orientation of crystals in the inner zone supports the presence of plug flow in a pseudoplastic lava. The textural features of the studied lava may be related to different mechanisms (i.e. lateral expansion). We conclude that the observed crystal alignments and imbrication structures may be related to a plug flow moving between two non-deforming walls. The walls are represented by the solidified, broken upper and basal crust of the flow. The low shear strain values calculated in the outer margins of the flow are indicative of the last deformation event. Crystal preferred orientation and imbrication structures may be related to the occurrence of velocity gradients existing between the inner zone of the flow and its solidus or near-solidus outer margins.     

Chemical and isotopic variations in fumarolic discharge and thermal waters at Vulcano Island (Aeolian Islands, Italy) during 1996: evidence of resumed volcanic activity

Gas samples from some fumaroles at ‘La Fossa' crater and Baia di Levante on Vulcano Island and from a diffuse soil gas emission were analysed during 1995–1996, along with water samples from thermal wells in the area of Vulcano Porto. During 1996, we observed a significant increase both in the gas/steam ratio and in the CO₂ concentration, as well as strong variations in δ¹³C_CO2, δD_H2O and δ¹⁸O_H2O of fumarolic gases. These variations are probably related to an increased inflow of deep fluids of magmatic origin. The temperatures of fumaroles did not show remarkable variations except for fumarole F11. In this case, temperature increased by about 80°C from February to August 1996. During the same period, remarkable variations in temperature, phreatic level and chemical and isotopic composition of water were also recorded in one of the geothermal wells in the Vulcano Porto area (Camping Sicilia; T60°C). The observed variations in this well are probably related to a pressure build-up, occurring at least in the surficial part of the system, because of increased gas flux and/or decreased permeability of the fumarolic degassing system. Chemical and isotopic composition of the water showed that during this evolutionary phase, the content of fumarolic condensate in this well was about 80 to 90%. Based on the observation of physical and chemical variables of the Camping Sicilia fluids, during this phase of activity, it is concluded that this area is affected by a phreatic eruption hazard if a volcanic episode with high energy discharge in a limited time span occurs. It follows that this well may be considered as a preferential point for volcanic activity monitoring, both in the case of normal routine surveillance and in the case of inaccessibility to the crater area.     

Geochemistry of gases and waters discharged by the mud volcanoes at Paternò, Mt. Etna (Italy)

 Approximately 20 km south of Mt. Etna craters, at the contact between volcanic and sedimentary formations, three mud volcanoes discharge CO₂-rich gases and Na–Cl brines. The compositions of gas and liquid phases indicate that they are fed by a hydrothermal system for which temperatures of 100–150  °C were estimated by means of both gas and solute geothermometry. The hydrothermal system may be associated with CO₂-rich groundwaters over a large area extending from the central part of Etna to the mud volcanoes. Numerous data on the He, CH₄, CO₂ composition of the gases of the three manifestations, sampled over the past 5 years, indicate clearly that variations are due to separation processes of a CO₂-rich gas phase from the liquid. The effects of these processes have to be taken into account in the interpretation of the monitoring data collected for the geochemical surveillance of Etna volcano. Received: 4 September 1995 / Accepted: 14 February 1996     

Lead isotope composition of the sublimates from the fumaroles of Vulcano (Aeolian Islands,Italy): inferences on the deep fluid circulation

The fumarolic fluids of Vulcano (Aeolian Islands, Italy) consist of a mixture of both deep and shallow components. The final products, the fumarolic gases and the sublimates associated with them, provide information on the complex interactions that occur at depth. As radiogenic isotopes do not undergo fractionation after they are incorporated in a fumarolic gas, they can be used directly to characterize the components that mixed. Lead isotopes are particularly suitable, because seawater, which plays an important part in the formation of the fumarolic fluids of Vulcano, contains only negligible amounts of it (10^-12 g/g). Therefore, the lead present in the fumarolic gases (and sublimates) is derived from the magmatic component and a water-rock interaction process. The lead isotope compositions of the lead sulfosalt sublimates collected from the Fossa Crater of Vulcano in 1924, and between 1989 and 1993, are given. The lead isotope ratios of most of the samples are the same within the range of analytical error, regardless of their collection date. The only samples that display slight variations are those collected in 1993. On the whole, the compositional trend of the lead isotopes of the sublimates coincides with that of the latitic-rhyolitic activity of Fossa and differs substantially from that of the pre-Fossa trachy-basaltic activity. The lead composition of the sublimates is very different from that of the Calabrian basement rocks. The data presented here show that the magma presently degassing at Vulcano has the same lead isotopic composition as the products of the recent activity of Fossa, whereas the fumarolic fluid circulation of Vulcano has not involved basement rocks similar to the Calabrian metasediments.     

Structure of a hypovolcanic acid complex inferred from magnetic susceptibility anisotropy measurements: the Western Red Hills granites (Skye,Scotland, Thulean Igneous Province)

 The geochemistry and the injection mechanism of hypovolcanic ring dykes have been extensively studied, but such is not the case for their internal fabric. The Tertiary Western Red Hills epigranites of the Isle of Skye are a classic example of such intrusions. Using anisotropy of magnetic susceptibility measurements, we present the first structural data of their internal magmatic fabric. The magnetic foliations, equated with the magmatic flow planes, have strikes which roughly follow the walls of the different intrusions. They dip steeply toward the convex wall of each intrusion. The lineations, or maximal magnetic susceptibility axes, generally have shallow plunges, except in the latest granite intrusion. These structures appear to be related to the compressional deformation of each intrusion toward the end of its crystallization. This shortening would be a consequence of a radial and compressive stress field acting after each injection of magma. This radial stress field is interpreted as the effect of high magma pressures originating from the acid magma chamber underlying the ring-dyke complex at a shallow depth. Received: 10 October 1995 / Accepted: 4 June 1997     

Petrology of lavas from the Puu Oo eruption of Kilauea Volcano: III. The Kupaianaha episode (1986–1992)

 The Puu Oo eruption has been remarkable in the historical record of Kilauea Volcano for its duration (over 13 years), volume (>1 km³) and compositional variation (5.7–10 wt.% MgO). During the summer of 1986, the main vent for lava production moved 3 km down the east rift zone and the eruption style changed from episodic geyser-like fountaining at Puu Oo to virtually continuous, relatively quiescent effusion at the Kupaianaha vent. This paper examines this next chapter in the Puu Oo eruption, episodes 48 and 49, and presents new ICP-MS trace element and Pb-, Sr-, and Nd-isotope data for the entire eruption (1983–1994). Nearly aphyric to weakly olivine-phyric lavas were erupted during episodes 48 and 49. The variation in MgO content of Kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of Kilauea, both of which probably were controlled by variations in Kilauea's magma supply rate. These lavas contain euhedral olivines which generally are in equilibrium with whole-rock compositions, although some of the more mafic lavas which erupted during 1990, a period of frequent pauses in the eruption, accumulated 2–4 vol.% olivine. The highest forsterite content of olivines (∼85%) in Kupaianaha lavas indicates that the parental magmas for these lavas had MgO contents of ∼10 wt.%, which equals the highest observed value for lavas during this eruption. The composition of the Puu Oo lavas has progressively changed during the eruption. Since early 1985 (episode 30), when mixing between an evolved rift zone magma and a more mafic summit reservoir-derived magma ended, the normalized (to 10 wt.% MgO) abundances of highly incompatible elements and CaO have systematically decreased with time, whereas ratios of these trace elements and Pb, Sr, and Nd isotopes, and the abundances of Y and Yb, have remained relatively unchanged. These results indicate that the Hawaiian plume source for Puu Oo magmas must be relatively homogeneous on a scale of 10–20 km³ (assuming 5–10% partial melting), and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinopyroxene component as the eruption continued. The rate of variation of highly incompatible elements in Puu Oo lavas is much greater than that observed for Kilauea historical summit lavas (e.g., Ba/Y 0.09 a^–1 vs ∼0.03 a^–1). This rapid change indicates that Puu Oo magmas did not mix thoroughly with magma in the summit reservoir. Thus, except for variable amounts of olivine fractionation, the geochemical variation in these lavas is predominantly controlled by mantle processes. Received: 8 March 1996 / Accepted: 30 April 1996     

Paleomagnetism of Jurassic to Miocene sediments from the Apenninic carbonate platform (southern Apennines, Italy): evidence for a 60° counterclockwise Miocene rotation

The tectonic evolution of the Apennine belt/southern Tyrrhenian Sea system is addressed through a paleomagnetic study of Lias to Langhian sediments from the Apenninic carbonate platform (southern Apennines, Italy). Reliable paleomagnetic data gathered from 21 sites document a regional-scale post-Langhian 80° counterclockwise (CCW) rotation. Since previous studies of the Plio-Pleistocene clays spread over the orogen had shown a ∼20°CCW rotation, we conclude that the southern Apennines rotated by 60° during Middle-Late Miocene. Our data provide evidence that the southeastward drift of Calabrian block (and synchronous spreading of the southern Tyrrhenian Sea) induced ‘saloon door’ like deformation of the southern Apennines and Sicily, which underwent similar magnitude (although opposite in sign) orogenic rotations. A paleomagnetically derived paleogeographic reconstruction shows that at 15 Ma (Late Langhian) the Alpine-Apennine belt collided with a NNE-oriented carbonate platform corridor surrounded by oceanic basins. We speculate that both the end of the Corsica-Sardinia rotation and the eastward jump of the locus of back-arc extension (from the Liguro-Provençal to the Tyrrhenian Sea) may have been consequences of this event.