Mass Of Magma Research Articles

Combined gravimetry in the observation of volcanic processes in Southern Italy

High precision measurements of the gravity field are currently carried out on active volcanoes of Southern Italy to single out the ascent of magmatic masses which could trigger a pre-eruptive state. Different aspects of the gravity field are considered to improve the understanding of the volcano dynamics. The volcanoes considered in this paper are Mt. Vesuvius, in the Neapolitan area, and the isle of Vulcano (Aeolian Islands, Sicily). Both are affected by significant gravity changes in time and space occurring with different patterns. At Mt. Vesuvius, a recording gravity station is also operating. It is aimed to monitor the time variations of the gravity field and tidal parameters, possibly reflecting changes of the physical state of the volcano. The migrating fluids may belong in part to an active geothermal system and in part to hydrological system which control the local level of the water table. At the isle of Vulcano, vertical gravity gradient measurements are also periodically carried out on six stations of the gravity network to detect changes of the mass occurring in shallower layers. The gravity field of the isle of Vulcano is characterized by short- and long-term gravity changes, partly confirmed by the results of repeated absolute gravity measurements. They might be attributed to the migration of fluids through shallow levels of the crust, which seems to be also supported by the repeated measurements of the vertical gravity gradient. At Mt. Vesuvius, the field of the gravity changes appears to be very disturbed on the short-term scale, while a quite clear geometry results on the long-term, evidencing an area with decreasing gravity values in the western and upper parts of the volcano and an area with increasing values at the southern–eastern base. The gravity changes are believed to result from the migration of fluids probably occurring through pre-existing fissures. A decrease of up to 60 μGal has been observed from 1985 to 1994 as also consistently detected by absolute measurements. Continuous monitoring of the gravity indicates an increase of the amplitudes of the main tidal waves from 1987/1990 to 1994, which could be due to a change of the elastic properties of the medium.

Intracrustal origin of Arenal basaltic andesite in the light of solid–melt interactions and related compositional buffering

The origin of Arenal basaltic andesite can be explained in terms of fractional crystallization of a parental high-alumina basalt (HAB), which assimilates crustal rocks during its storage, ascent and evolution. Contamination of this melt by Tertiary calc-alkalic intrusives (quartz–diorite and granite, with 87 Sr/ 86 Sr ratios ranging 0.70381–0.70397, nearly identical with those of the Arenal lavas) occurs at upper crustal levels, following the interaction of ascending basaltic magma masses with gabbroic–anorthositic layers. Fragments of these layers are found as inclusions within Arenal lavas and tephra and may show reaction rims (1–5 mm thick, consisting of augite, hypersthene, bytownitic–anorthitic plagioclase, and granular titanomagnetite) at the gabbro–lava interface. These reaction rims indicate that complete `assimilation' was prevented since the temperature of the host basaltic magma was not high enough to melt the gabbroic materials (whose mineral phases are nearly identical to the early formed liquidus phases in the differentiating HAB). Olivine gabbros crystallized at pressure of about 5–6 kbar and equilibrated with the parental HAB at pressures of 3–6 kbar (both under anhydrous and hydrous conditions), and temperatures ranging 1000–1100°C. In particular, `deeper' interactions between the mafic inclusions and the hydrous basaltic melt (i.e., with about 3.5 wt.% H 2O) are likely to occur at 5.4 (±0.4) kbar and temperatures approaching 1100°C. The olivine gabbros are thus interpreted as cumulates which represent crystallized portions of earlier Arenal-type basalts. Some of the gabbros have been `mildly' tectonized and recrystallized to give mafic granulites that may exhibit a distinct foliation. Below Arenal volcano a zoned magma chamber evolved prior the last eruptive cycle: three distinct andesitic magma layers were produced by simple AFC of a high-alumina basalt (HAB) with assimilation of Tertiary quartz–dioritic and granitic rocks. Early erupted 1968 tephra and 1969 lavas (which represent the first two layers of the upper part of a zoned magma chamber) were produced by simple AFC, with fractionation of plagioclase, pyroxene and magnetite and concomitant assimilation of quartz–dioritic rocks. Assimilation rates were constant ( r 1=0.33) for a relative mass of magma remaining of 0.77–0.80, respectively. Lavas erupted around 1974 are less differentiated and represent the `primitive andesitic magma type' residing within the middle–lower part of the chamber. These lavas were also produced by simple AFC: assimilation rates and the relative mass of magma remaining increased of about 10%, respectively ( r 1=0.36, and F=0.89). Ba enrichment of the above lavas is related to selective assimilation of Ba from Tertiary granitic rocks. Lava eruption occurred as a dynamic response to the intrusion of a new magma into the old reservoir. This process caused the instability of the zoned magma column inducing syneruptive mixing between portions of two contiguous magma layers (both within the column itself and at lower levels where the new basalt was intruded into the reservoir). Syneruptive mixing (mingling) within the middle–upper part of the chamber involved fractions of earlier gabbroic cumulitic materials (lavas erupted around 1970). On the contrary, within the lower part of the chamber, mixing between the intruded HAB and the residing andesitic melt was followed by simple fractional crystallization (FC) of the hybrid magma layer (lavas erupted in 1978–1980). By that time the original magma chamber was completely evacuated. Lavas erupted in 1982/1984 were thus modelled by means of `open system' AFCRE (i.e., AFC with continuous recharge of a fractionating magma batch during eruption): in this case assimilation rates were r 1=0.33 and F=0.86. Recharge rates are slightly higher than extrusion rates and may reflect differences in density (between extruded and injected magmas), together with dynamic fluctuations of these parameters during eruption. Ba and LREE (La, Ce) enrichments of these lavas can be related to selective assimilation of Tertiary granitic and quartz–dioritic rocks. Calculated contents for Zr, Y and other REE are in acceptable agreement with the observed values. It is concluded that simple AFC occurs between two distinct eruption cycles and is typical of a period of repose or mild and decreasing volcanic activity. On the contrary, magma mixing, eventually followed by fractional crystallization (FC) of the hybrid magma layer, occurs during an ongoing eruption. Open-system AFCRE is only operative when the original magma chamber has been totally replenished by the new basaltic magma, and seems a prelude to the progressive ceasing of a major eruptive cycle.

Geodynamics of magmatic Cu-Ni-PGE sulfide deposits; new insights from the Re-Os isotope system

In this study, we reassess crustal contamination and sulfide ore-forming processes in some of the largest magmatic ore deposits, using published Re-Os isotope data and a modeling methodology that incorporates the R factor, defined as the effective mass of silicate magma with which a given mass of sulfide magma has equilibrated, in an Re-Os isotope mixing equation. We show that there is less disparity between conclusions based on Re-Os isotope data compared to other isotopic systems if the R factor is considered. Komatiite-associated Ni sulfide ore systems typically have high Os concentrations. low Re/Os ratios, and near-chondritic initial Os isotope compositions. For magmatic sulfide ores that are interpreted to have experienced relatively low R factors ( 10,000 for Noril'sk-Talnakh and the J-M reef, Stillwater Complex). In these very dynamic magmatic ore systems, crustal contamination processes are more difficult to assess using Re-Os isotopes as the effects of contamination are masked by the R factor process in which sulfide magma equilibrates with extensive amounts of asthenospheric mantle-derived magma. Sulfide protore for these systems may, then, have been very radiogenic and of crustal origin prior to R factor processes that occurred during transport in feeder conduits and in upper crustal magma chambers. This study, therefore, highlights the need for caution when interpreting the Re-Os isotope geochemistry of sulfide ores from dynamic magmatic systems. The results of our reinvestigation of these giant ore deposits suggest that geodynamic processes associated with large magmatic systems, including major lithospheric pathways to the surface, changes in flow regime, coupled magma flow-through and magma mixing (providing enhanced R factors), may be critical to our understanding of the emplacement, localization, and quality of magmatic sulfide deposits. Thus, the timing and exact mechanism of sulfide saturation may be subordinate to dynamic magmatic processes in the localization of economic concentrations of magmatic sulfides.

Isochronal maps at Mt. Etna volcano (Italy): a simple and reliable tool for investigating large-scale heterogeneities

This paper analyses twelve etnean earthquakes which occurred at various depths and recorded at least by eleven stations. The seismic stations span a wide part of the volcanic edifice; therefore each set of direct P-wave arrival times at these stations can be considered appropriate for tracing isochronal curves. Using this simple methodology and the results obtained by previous studies the authors make a reconstruction of the geometry of the bodies inside the crust beneath Mt. Etna. These bodies are interpreted as a set of cooled magmatic masses, delimited by low-velocity discontinuities which can be considered, at present, the major feeding systems of the volcano.

Deep fluids and volatile recycling: crust to mantle

There is now clear evidence that geofluids are involved in many stress release processes at many depths. Modern studies of crustal regions show that surface fluids may penetrate to depths of many kilometres. At subduction zones, volatiles may be recycled to mantle depths. There is increasing evidence for the complexity of subduction structures with flaking off of major fragments which may devolatilize and revolatilize lower crust and upper mantle. The same processes may be associated with massive magma underplating of continental crust. The new techniques of modern surface chemistry open new possibilities for the study of fluid tracks, thin films on mineral grains, which may influence many properties like deep electrical conductivity.

Key factors in the genesis of Noril'sk, Sudbury, Jinchuan, Voisey's Bay and other world‐class Ni‐Cu‐PGE deposits: Implications for exploration

Magmatic Ni‐Cu sulfide deposits form as the result of the segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma and the partitioning of chalcophile elements into these from the silicate melt. Sulfide saturation of a magma is not enough in itself to produce a massive concentration of magmatic sulfide. The appropriate physical environment is required to concentrate sulfides from a large mass of magma into one place. The deposits of the Noril'sk region have developed within flat, elongate bodies (15 X 2 X 0.2 km) that intrude argillites, evaporites and coal measures, adjacent to a major, transcrustal fault and immediately below the centre of a 3.5 km‐thick volcanic basin. An anticlinal axis that transects the axis of the basin at a high angle has brought these intrusions to surface to give rise to the two major ore junctions, Noril'sk and Talnakh. Studies of the overlying basalts have shown that basalts forming a 500 m‐thick sequence have lost 75% of their Cu and Ni and more than 90% of their PGE. Basalts above this show a gradual recovery in their chalcophile element concentrations to reach ‘normal’ values 500 m above the highly depleted zone. Two groups of mineralised bodies have been identified as correlative with these basalts: the poorly mineralised Lower Talnakh‐type bodies,which resemble the highly depleted basalts; and the ore‐bearing Noril'sk‐type intrusions which correlate with the overlying, essentially undepleted basalts. The high proportion of sulfide (2–10 wt%) associated with the Noril'sk‐type intrusions, the high PGE content of the ores, the extensive metamorphic aureole (100–400 m around the bodies), and the heavy sulfur isotopic composition of the ores (+ 8 to +12 834S) are explicable if the ore‐bearing bodies are exit conduits from high‐level intrusions, along which magma has flowed en route to surface. The Lower Talnakh bodies are interpreted as intrusions along which magma flow stopped earlier than along those of the Noril'sk type. The first magma to enter the high‐level intrusion reacted with much evaporitic sulfur, at a low ‘R’ value and thus gave rise to sulfides with low metal tenors. Successive pulses of magma through the system progressively enriched the sulfides in the conduits, losing progressively less of their chalcophile metals, and thus accounting for the upward increase in metals in successive lava flows. The flow direction along the conduits is shown by the direction in which the tenor of disseminated sulfides decreases. Sulfides have settled from the moving magma to form separate injections of liquid sulfide, up to 3.5 X 1.5 X 0.05 km in size. Recent reflection seismic studies at Sudbury have shown the Sudbury Igneous complex to have been much more extensive than originally supposed. Nd and Sr isotopic studies on rocks of the Sudbury Igneous Complex and Re‐Os studies on the ores have indicated the incorporation of much country rock gneiss in the complex. Debate has centred around whether the Sudbury Igneous Complex is entirely an impact melt or the consequence of the mixing of primary magma with 50 or more wt% impact melt; the most recent evidence favours the latter hypothesis. The Jinchuan deposit of north‐central China occurs within a 6 km‐long dyke‐like body of peridotite. The compositions of olivine within the dyke, the igneous rocks themselves, and the ore are all inconsistent with derivation of the body from ultramafic magma, as originally supposed, and indicate that the structure forms the keel of a much larger intrusion of magnesian basalt. Flow of magma into the intrusion has resulted in olivine and sulfide being retained within this keel. The Voisey's Bay deposit lies partly within a 30–100 m‐thick sheet of troctolite, which is interpreted as a flat‐lying part of a feeder for an adjacent intrusion, and partly at the base of the intrusion, where the feeder adjoins it. Ore types range from disseminated sulfides in troctolite, which increase downward, grading into massive ore. The latter is underlain by a breccia composed of fragments of gneiss, unmineralised troctolite and peridotite in a troctolitic matrix that, in places, contains appreciable sulfide. When most major Ni‐Cu sulfide deposits, including those at Kambalda, Western Australia, are viewed in the light of studies at Noril'sk, Sudbury, Jinchuan and Voisey's Bay, three factors become apparent: (i) the concentration of sulfides in channels or conduits through which much magma has flowed (feeder conduits for intrusions are much more prospective targets for exploration than the base of the intrusions themselves); (ii) the interaction of the source magma with country rocks, either leading to the incorporation of sulfur, or the felsification of the magma in question; and (iii) fractional crystallisation of sulfide liquid giving rise to Cu‐rich ores which may be far removed from the ‘source’ ore.

Genesis of silicic magmas during tertiary continental rifting in Yemen

The magmatism in Yemen associated with Oligo-Miocene continental rifting in the Red Sea and the Gulf of Aden comprises a bimodal mafic-felsic suite represented by flows, dykes and plutons. This paper reports the chemical and Nd-Sr isotopic compositions of the felsic rocks (trachytes, syenites, rhyolites and granites) associated with the Yemen Trap Series and deals with the controversy in regard to the crustal or mantle origin of A-type magmas generated in tensional contexts. The overall age of the studied rocks ranges from 26.5 to 16 Ma. The major and trace element compositions of the felsic rocks can partly be explained by fractional crystallization from some of the basic magmas. This process was more extensive in the case of the rhyolites and some of the granites than that of the trachytes and the syenite. In addition, the trachytes differ from the rhyolites by featuring a negative Nb anomaly that suggests contamination by continental crust. The trachytes have much lower 143Nd 144Nd ratios, and higher 87Sr 86Sr ratios than the associated basalts, whereas the rhyolites have much higher 87Sr 86Sr ratios, but 143Nd 144Nd ratios identical to those of the basalts. The plutons span the whole compositional range of isotopic compositions displayed by the trachytes and the rhyolites. The isotopic compositions of the felsic rocks do not support their genesis by direct melting of continental crust. Rather, these rocks were generated in an open magma system that evolved by a combination of fractional crystallization and crustal contamination. Mixing and AFC processes involving heterogeneous crust are both required to explain the trachytic and rhyolitic compositions where various amounts of crystal fractionation and crustal contamination were involved. The genetic differences between the trachytes and the rhyolites are related to their respective ages. The highest degree of contamination observed in the trachytes occurred contemporaneously with the emplacement of the traps and could have been related to a high thermal gradient in the crust due to massive magma transfer at that time. This stage presumably corresponds to the impingement of the Afar plume head against the lithosphere. The less contaminated and more fractionated rhyolites, which formed later during continental rifting, are consistent with a lower thermal gradient.

Intrusion and Cooling of Magma. Comparison between 2D and 3D Analyses.

Steady thermal field associated with magma mass intrusion is studied in two-dimensional (2D) and three-dimensional (3D) spaces. It is proved that the latter model leads to surface heat flow substantially smaller than that for the former. A numerical approach based on a relaxation technique for solving the Laplace equation is made use of. Transient heat conduction is solved when a lava dome makes an appearance on the earth's surface. Converting the differential equation that governs the problem into a difference equation, 2D and 3D solutions are obtained. It becomes apparent that the 2D analysis results in a cooling rate considerably smaller than that for the 3D analysis. A similar transient problem related to the filling of a reservoir with high-temperature magma is also studied. Even in this case, a 2D analysis leads to cooling appreciably slower than that for a 3D analysis. The mode of heat flow anomaly development at the earth's surface for the 2D analysis is noticeably different from that for the 3D analysis. In short, 3D analyses of the geothermal problems are important for obtaining realistic results although 2D analyses have so far been customary in many cases.

Assimilation of felsic crust by basaltic magma: Thermal limits and extents of crustal contamination of mantle-derived magmas

Basaltic magma is the most widely used probe of mantle chemistry and structure. Chemical characterization of the sources for mantle-derived magma is possible only after quantitative evaluation of possible contamination by crustal wall rocks. The most commonly invoked contamination mechanisms are the combined processes of crustal assimilation and magmatic fractional crystallization (AFC). Because of the energetic link between these processes, predicting assimilation and crystallization progress requires modeling both heat and mass transfer in the magma–country-rock system. Using the MELTS software package, we modeled isenthalpic (heat-balanced) AFC between two common types of basaltic magma and several types of felsic crust at 1 kbar. Our simulations show that during the early stages of isenthalpic AFC, the ratio of rates of assimilation to crystallization (r) may be substantially greater than one (2.0–2.7), allowing assimilation of a mass of country rock up to 5%–18% of the initial magma mass with only 3%–7% crystallization. The second stage of AFC, beginning with plagioclase and/or pyroxene saturation, is characterized by lower r (0.5–1.0). The initial high- r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses. Under certain conditions, even small degrees of crystallization of olivine alone, coupled with cooling of the magma, can accommodate relatively large amounts of crustal assimilation, and cause large shifts in isotopic and trace element geochemical indices with little differentiation.

Earth tides and Etnean volcanic eruptions: an attempt at correlation of the two phenomena during the 1983, 1985 and 1986 eruptions

An attempt has been made to correlate volcanic events on Mt. Etna and Moon-Sun attraction during the eruptions of 28 March–6 August 1983, 12 March–13 July 1985 and 30 October 1986. The possibility of a ‘trigger’ effect has been supposed for eruptive phenomena that originate by small and continuous external forces under particular stress conditions on the volcanic structure. The parameters compared, for the two phenomena, are the hourly mean tremor amplitude recorded at Serra La Nave station and the hourly value of the vertical and of the two partial horizontal north-south and east-west components of the tidal acceleration induced by the Moon-Sun system. The relationship between two series of values (the former for the tremor amplitude and the latter for one of the tidal acceleration components) were then tested by statistical methods based on the information theoretic measure of interdependence between any pair of signal sequences. The results show an effect of the horizontal tidal components on well-known tectonic structures, which may be widened or closed; in contrast, there is little evidence that the uprise of magma masses might depend on the vertical tidal attraction.

Variscan-late Variscan-early Alpine progressive extensional collapse of central Spain

AbstractThe Variscan—late Variscan—early Alpine evolution of Central Spain is characterized by a progressive transition from dominant compressional conditions, towards a generalized extensional regime. This evolution involved four tectonomag-matic stages: 1) a Dl Variscan stage, associated to widespread compression with E-vergent structural elements, except to the NW, where extension and coeval plutonic activity occurred; 2) a D2 Variscan stage, characterized by increasing extension and associated massive magmatism in the center, with compression localized to the margins; 3) a D3 late Variscan stage involving a NS-directed gravitational collapse of the whole area through EW-oriented detachments, within an oval-shaped plutonometamorphic core complex; and, 4) a final D4 early Alpine stage characterized by EW extension, giving rise to a Basin & Range-type asymmetric rift to the E.

Crustal anatexis during the influx of mantle volatiles

Many data show that large masses of silicic magma can be formed by crustal anatexis under the influence of volatiles, possibly enriched in primary water derived from a mantle source. In the present paper, a model of crustal anatexis, accompanied by a limited influx of volatiles is suggested. The water influx is not excessive, no independent fluid water phase appearing in the magma. Convective diffusion is assumed to be the dominant mechanism of volatile transport within the silicic melt. This mechanism increases the volatile flux by several orders of magnitude compared with the diffusion flux in a non-convecting system. A high convective flux may be generated only at specific stages of magma-chamber formation. In this paper, a mathematical formulation of the conditions favouring this type of anatexis is given. The most plausible source of deep-seated, water-bearing volatiles beneath large, silicic, crust-derived magma masses are mantle-derived magmas which maintain vigorous convection. It is shown that convective-diffusion influx of volatiles from a lower layer of mafic magma into an upper layer of silicic magma leads to a quasi-equilibrium situation (“transient two-liquid eguilibrium”; Watson, 1976). In this situation, the distribution coefficient of water between adjacent silicic and mafic magmas is proportional to the ratio of water solubilities in these magmas and amounts to about 1.4 by mass. The silicic magma overlying the mafic magma can contain up to 6 wt.% water, which is particularly true of the latest stages of cryatallization of the mafic magma, when its water content rises and diffusion of water across the mafic/silicic melt boundary becomes more efficient. On the basis of our results, the different stages of formation of a silicic magma are discussed, the geological consequences are analyzed and some new regularities in the interpretation of geological and petrological data relevant to granitoid petrogenesis are proposed.

Siderophile and chalcophile metals as tracers of the evolution of the Siberian Trap in the Noril'sk region, Russia

In this study Cu, Ni, and platinum-group elements (PGE) were determined in a sequence of basaltic and picritic lavas from the Siberian Trap in the Noril'sk area of Russia to constrain genetic relationships between the basalts and the petrogenesis of Ni-Cu-PGE sulfide deposits associated with the Talnakh and Noril'sk intrusions. In the most primitive basalts (8–19 wt% MgO) of the Tuklonsky (Tk) suite, Pt and Pd concentrations range from 4–13 ppb, increasing with decreasing MgO content; whereas Ir contents decrease with MgO from 0.8-0.05 ppb. The contrasting behavior of these elements, which all have very high sulfide-silicate partition coefficients, as well as the primitive mantle-like ratios of Cu/Y and Pd/Y, suggests that these magmas were not sulfide-saturated. The high PGE abundances imply that their parental magmas were also not sulfide saturated during partial melting in the mantle. Due to sulfide segregation, the overlying basalts of the Nadezhdinsky (Nd) series are low in Cu and Ni (52 and 38 ppm, respectively); highly depleted in all PGE; and have very low Cu Y , Pd Y , and Pd Cu ratios. However, in stratigraphically higher levels, Cu, Ni, and PGE concentrations increase systematically through the Morongovsky (Mr) suite to reach a concentration plateau in the uppermost Mokulaevsky (Mk) suite (Pt 8 ppb; Pd: 9 ppb; Ir: 0.12 ppb; Rh: 0.4 ppb). At the same time, ratios such as Cu Y increase and approach primitive mantle values. However, ratios involving PGE, such as Pd Y , remain low, suggesting the removal of small amounts of sulfide (0.01–0.03%). The compositional variations in the basalts and the sulfide liquids can be quantitatively described by fractional segregation of a sulfide liquid in an open- or closed-system magma chamber. The latter model suggests that the basalts represent the eruption products of a zoned magma chamber in which light magma, with crustal components contaminated, overlies less contaminated, denser magma. Crustal contamination caused sulfide saturation, and the resulting sulfide liquids settled through a magma column and accumulated at the bottom of the chamber. In this model, the sulfide liquid is not in equilibrium with the whole magma mass, and sulfide segregation is compared with the zone-refining process of metallurgy. The sulfides become more enriched as they move through the magma; and although the magma left behind is depleted in PGE, Cu, and Ni, their concentrations also increase with depth. Eventually, the magma chamber is emptied from the top to the bottom, producing the flood basalt sequence and the associated intrusions and ore deposits. In the open-system model, sulfide saturation was initially caused by assimilation of crustal material by the Tuklonsky magma. Continuous and simultaneous replenishment, assimilation, and crystallization processes formed the lower Nd lavas. The concurrent removal of 0.5-1% sulfide strongly depleted these magmas in chalcophile and siderophile metals. Due to the continuous replenishment of the magma chamber with uncontaminated PGE-rich magma, succeeding lavas (Mr, Mk) show diminishing signs of crustal contamination and become less sulfide-saturated, as indicated by the increasing Ni, Cu, and PGE abundances. During the evolution of the chamber, the magma remained sulfur-saturated, and sulfides accumulated at the base. The composition of the sulfide ores could be regarded as a mixture consisting of low Ni-, Cu-, and PGE-sulfides derived with a low silicate/sulfide ratio (100) from the Tk-Nd magma and high Ni-, Cu-, and PGE-sulfides formed with a high ratio (10,000) from the Mr-Mk magma.

Some remarks on volcanic vent evolution during plinian eruptions

The magma mass discharge rate governs the eruption magnitude of plinian eruptions, and depends strongly on conduit geometry. The presence of lithic clasts in pumice deposits results largely from conduit erosion, and the magma discharge rate and the lithic abundance in the resulting deposit are parameters linked by the overall vent widening rate. Many pumice fall deposits have lithic-enriched basal and top sections; pumice flow deposits tend to carry more lithics than the preceding fall layer. Theoretical models of vent evolution for a cylindrical conduit with radius R indicate an R 4 dependency of the magma discharge rate, which would lead to rapidly decreasing lithic abundances from bottom to top in plinian fall deposits, and from fall to flow deposits, contrary to observations. To simulate observed lithic abundance patterns, a model of conduit evolution with a stepped geometry was developed, in which the lower conduit section remains at approximately constant radius, while the upper conduit section widens. The mass discharge rate during an eruption then increases with R 2 top , and predicted lithic abundance patterns agree qualitatively with field data. Field evidence for conduit widening in the upper conduit section only is presented from plinian pumice beds from Nisyros, Greece. The change in eruption style from plinian fall to surge and ash flow is sometimes accompanied by a dramatic change in lithic abundance and lithic type. In these cases, this transition in deposition style is not the result of a gradual increase in mass discharge rate because of overall vent widening, but is caused by catastrophic rupture of the lower vent section during incipient caldera collapse, with resulting changes in magma discharge rates, lithic abundance and possibly water/magma interaction.

Heat flow variations in continental rifts

In the terrestrial thermal field, continental rifts stand out as rather narrow regional thermal anomalies in which augmented heat flow is unevenly distributed. Heat flow variations are discussed on the example of the Baikal, East African, Cordilleran and Rhine-Lybian rift systems. Rift-related disturbance of the asthenosphere/lithosphere system is accompanied by a general increase in the surficial heat flow. Regional thermal anomalies are mainly caused by conductive heat transfer from the asthenosphere. Local thermal anomalies are caused by magmatic mass transfer into the crust and by fluid convection along porous fracture systems.

Mid-Pleistocene lavas from the Seguam volcanic center, central Aleutian arc: closed-system fractional crystallization of a basalt to rhyodacite eruptive suite

In contrast to adjacent volcanic centers of the modern central Aleutian arc, Seguam Island developed on strongly extended arc crust. K-Ar dates indicate that mid-Pleistocene, late-Pleistocene, and Holocene eruptive phases constitute Seguam. This study focuses on the petrology of the mid-Pleistocene, 1.07–07 Ma, Turf Point Formation (TPF) which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst-poor andesite to rhyodacite lavas. Increasing whole-rock FeO*/MgO from basalt to dacite, the anhydrous Plag+Ol+Cpx±Opx±Mt phenocryst assemblage, groundmass pigeonite, and the reaction Ol+Liq=Opx preserved in the mafic lavas indicate a tholeiitic affinity. Thermometry and comparison to published phase equilibria suggests that most TPF basalts crystallized Plag+Ol+Cpx±Mt at ≥1160°C between about 3–5 kb (±1–2% H2O), andesites crystallized Plag+Cpx+Opx±Mt at ≥1000°C between 3–4 kb with 3–5% H2O, and dacites crystallized Plag +Cpx±Opx±Mt at 1000°C between 1–2 kb with 2–3% H2O. All lavas crystallized at f o 2 close to the NNO buffer. Mineral compositions and textures indicate equilibrium crystallization of the evolved lavas; petrographic evidence of open-system mixing or assimilation is rare. MgO, CaO, Al2O3, Cr, Ni, and Sr abundances decrease and K2O, Na2O, Rb, Ba, Zr, and Pb increase with increasing SiO2 (50–71%). LREE enrichment [(Ce/Yb)n=1.7±0.2] characterizes most TPF lavas; total REE contents increase and Eu anomalies become more negative with increasing SiO2. Relative to other Aleutian volcanic centers, TPF basalts and basaltic andesites have lower K2O, Na2O, TiO2, Rb, Ba, Sr, Zr, Y, and LREE abundances. 87Sr/86Sr ratios (0.70361–0.70375) and ratios of 206Pb/204Pb (18.88–18.97), 207Pb/204Pb (15.58–15.62), 208Pb/204Pb (38.46–38.55) are the highest measured for any suite of lavas in the oceanic portion of the Aleutian arc. Conversely, eNd values (+5.8 to+6.7) are among the lowest from the Aleutians. Sr, Nd, and Pb ratios are virtually constant from basalt through rhyodacite, whereas detectable isotopic heterogenity is observed at most other Aleutian volcanic centers. Major and trace element, REE, and Sr, Nd, and Pb isotopic compositions are consistent with the basaltic andesitic, andesitic, dacitic, and rhyodacitic liquids evolving from TPF basaltic magma via closed-system fractional crystallization alone. Fractionation models suggest that removal of ∼80 wt% cumulate (61% Plag, 17% Cpx, 12% Opx, 7% Ol, and 3% Mt) can produce 20 wt% rhyodacitic residual liquid per unit mass of parental basaltic liquid. Petrologic and physical constraints favor segregation of small batches of basalt from a larger mid-crustal reservoir trapped below a low-density upper crustal lid. In these small magma batches, the degree of cooling, crystallization, and fractionation are functions of the initial mass of basaltic magma segregated, the thermal state of the upper crust, and the magnitude of extension. Tholeiitic magmas erupted at Seguam evolved by substantially different mechanisms than did calc-alkaline lavas erupted at the adjacent volcanic centers of Kanaga and Adak on unextended arc crust. These variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew. At Seguam, intra-arc extension promoted eruption of voluminous basalt and its differentiates, unmodified by interaction with lower crustal or upper mantle wallrocks.

Plume heads, continental lithosphere, flood basalts and tomography

Abstract High-resolution uppermantle tomographic models are interpreted in terms of plate tectonics, hotspots and plume theories. Ridges correlate with very low velocity areas to a depth of 100 km, probably a result of passively induced upwelling and partial melting. Past positions of ridges also exhibit very low seismic velocities in the uppermantle. At depths greater than 100 km, some low velocity anomalies (LVA) may record past positions of migrating ridges. Buoyant upwellings induced by spreading do not track the migration of surface ridges; they lag behind. At depths greater than about 150 km many LVA (Atlantic and Indian oceans) are more closely related to hotspots, and past positions of ridges than to current ridge locations. In the upper 200 km of the mantle, back-arc and continental extension areas are generally slower than hotspot mantle, possibly reflecting partially molten and/or hydrous mantle. The Pacific ocean ridges tend to be LVA, and probably hot, to about 400 km depth. The surface locations of hotspots, ridges and continental basaltic magmatism seem to require a combination of hot uppermantle and suitable lithospheric conditions, presumably the existence of tensile stresses. The high-velocity regions of the upper 200 km of the mantle correlate with Archaean cratons. Below 300 km the regions of generally fast seismic velocity, and therefore cold mantle, correlate with regions probably underlain by ancient slabs, where the uppermantle may be cooled from below. A moving plate, overriding a hot region, and being put into tension, will behave as if it were being impacted from below by a giant plume head. At sublithospheric depths there are very large LVA (VLVA) in the Pacific and Indian oceans and in the North and South Atlantics. The large continental and oceanic flood basalt provinces seem to have formed over these large, presumably hot, regions. These VLVA do not appear to be plume heads nor is there any obvious damage to the lithosphere under the present locations of flood and plateau basalt provinces. The uppermantle does not appear to be isothermal; the LVA are not restricted to hotspot locations. We suggest that LVA are hotcells in the uppermantle which reflect, in part, the absence of subduction cooling. Plate tectonic induced rifting causes massive magmatism if the break occurs over hotcells, i.e. low-seismic velocity regions. Flood basalts (CFB) may result from the upwellings of already hot, even partially molten, mantle. In contrast to plume heads and plume tails, hotcells are robust features which are fixed relative to one another. They are most pronounced in parts of the mantle that have not been cooled by subduction. There is a close relationship between CFB initiation sites, LVA and ridges and, we believe, hotcells.

Formation of harzburgite by pervasive melt/rock reaction in the upper mantle

Many mantle peridotite samples are too rich in Si02 (in the form of orthopyroxene) and have ratios of light to heavy rare earth elements that are too high to be consistent with an origin as the residuum of partial melting of the primitive mantle. Trace element studies of melt/rock reaction zones in the Trinity peridotite provide evidence for reaction of the mantle lithosphere with ascending melts, which dissolved calcium-pyroxene and precipitated orthopyroxene as magma mass decreased. This process can account for the observed major and trace element compositions of lithospheric mantle samples, and may accordingly be prevalent in the upper mantle.

Geochemical investigations of submarine volcanic exhalations to the east of Panarea, Aeolian Islands, Italy

Abstract Results are presented on scubadiving investigations carried out on thermal manifestations in the area of Panarea (Aeolian Islands). The area investigated falls inside a caldera which extends from the main island to the group of islets located to the northeast. The distribution of the gaseous manifestations is regulated by the NE-SW, NW-SE and N-S regional tectonic directrices, through which the more recent basic magma intruded, giving rise to dikes and pillow lavas. f O 2 -temperature relation of the gases sampled in the investigated area was calculated to be: log f O 2 = 11−24,593/ T which indicates that a buffering mechanism acted on the gases as they cooled down during their ascent. The high 3 He/ 4 He ratio (6 × 10 −6 ) and the δ 13 C = −3.2%. (PDB), suggest the presence of a magmatic component in the gas feeding the investigated manifestations. The above relations and the almost constant high He/N 2 ratio suggest that all the fumaroles are fed by the same deep hot fluids. On the basis of both the chemical characters of the fluids and the geothermo-barometric data, a deep geothermal body, having a temperature of about 240°C, is recognized. Two other shallower thermal aquifers, with a temperature of 170–210°C, are identified. A circulation pattern of the geothermal fluids is also proposed. On the basis of calculations regarding the convective energy released by the geothermal system of Panarea, and the magmatic mass responsible for the positive gravimetric anomaly of the area, it was estimated that the last volcanic activity took place less then 10,000 years ago.

Renewal of explosive activity at Vesuvius: models for the expected tephra fallout

One of the major problems concerning the assessment of volcanic hazard at Vesuvius is to determine the type and size of the eruptive event that will characterize the volcano when it becomes active once again. During its history, Somma-Vesuvius has exhibited different types of activity, ranging from quiet lava emission to moderate strombolian activity, to catastrophic plinian eruptions. Available data support a behavior model characterized by the increasing size and explosiveness of the eruptions with increasing repose time, as a consequence of a roughly constant periodic supply of deep basic magma to a shallow magma chamber and differentiation and mixing in the chamber. After the A.D. 79 eruption, a homogeneous HK (high potassium) nature of erupted products was reflected by a magma alimentation rate roughly estimated at 1.5–2.0 millions of cubic meters per year. Assuming no major changes have occurred in the feeding system of the volcano after its last eruption in 1944, a volume of 40–70 × 10 6 m 3 magma could be considered presently available for a renewal of activity at Vesuvius. The emission of such a mass of magma during a single eruption would result into the largest event since the highly disruptive 1631 subplinian eruption. Presently, no possibility exists to forecast the eruptive character of such an eruption, and either a “ultrastrombolian” or a “subplinian” case appear equally possible. The latter possibility implies the highest potential hazard. This paper provides the numerical simulations of the main eruptive phenomenon that probably will occur during this “maximum expected event”: the fallout of tephra from a high, sustained eruption column. After the initial explosive opening of the vent, the scenario consists of the formation of a high convective column with lee-side fallout of pumice and lithic fragments, accompanied and followed by column collapses generating pyroclastic flows and surges. The column behavior was numerically simulated by using the Wilson and Walker column model with the following input parameters: mass eruption rate between 0.4 and 2.0 × 10 7 kg s −1, gas content between 5 and 10 wt.%, lithic content 20 wt.%, basal radius of the column larger than the minimum value for subsonic initial velocity of the jet. The resulting maximum column height and maximum gas thrust height were respectively 11–16 km and below 1000 m. The tephra fallout was modeled by a continuity equation solved on a nonuniform three-dimensional grid. The input parameters to the model are: seasonal wind velocity profiles; eddy diffusion coefficients of 3000 m 2 s −1 (horizontal) and 50 m 2 s −1 (vertical); particles whose nature and grain size are inferred after data on pyroclastic deposits from ancient Vesuvius eruptions distributed along a vertical linear source. The hazard implications of the different modeled cases are briefly discussed.