Magma storage conditions beneath Dabbahu Volcano (Ethiopia) constrained by petrology, seismicity and satellite geodesy

A variety of methods exist to constrain sub-volcanic storage conditions of magmas. Petrological, seismological and satellite geodetic methods are integrated to determine storage conditions of peralkaline magmas beneath Dabbahu Volcano, Afar, Ethiopia. Secondary ion mass spectrometry (SIMS) analysis of volatile contents in melt inclusions trapped within phenocrysts of alkali feldspar, clinopyroxene and olivine from pantellerite obsidians representing the youngest eruptive phase (<8?ka) show H₂O contents ≤5.8?wt.% and CO₂ contents generally below 500?ppm, although rarely as high as 1,500 ppm. Volatile saturation pressures (at 679–835°C) are in the range 43–207?MPa, consistent with published experimental data for similar pantellerites, which show that the phenocryst assemblage of alkali feldspar + cpx + aenigmatite ± ilmenite is stable at 100 to 150?MPa. Inferred magma storage depths for these historic eruptions are ~1–5?km below sea-level, consistent with the depths of earthquakes, associated with magma chamber deflation following a dyke intrusion in the period Oct 2005–Apr 2006. Interferometric synthetic aperture radar (InSAR) data for the same period reveal a broad ~20?km diameter area of uplift. Modelling of different geometries reveals that a series of stacked sills over a 1–5?km depth range best matches the InSAR data. The consistency of depth estimates based on petrological study of ancient eruptions and the seismicity, inflation and deflation of Dabbahu observed in relation to the dyking event of 2005, suggest a small but vertically extensive and potentially long-lived magma storage region.     

Breadcrust bombs as indicators of Vulcanian eruption dynamics at Guagua Pichincha volcano,Ecuador

Vulcanian eruptions are common at many volcanoes around the world. Vulcanian activity occurs as either isolated sequences of eruptions or as precursors to sustained explosive events and is interpreted as clearing of shallow plugs from volcanic conduits. Breadcrust bombs characteristic of Vulcanian eruptions represent samples of different parts of these plugs and preserve information that can be used to infer parameters of pre-eruption magma ascent. The morphology and preserved volatile contents of breadcrust bombs erupted in 1999 from Guagua Pichincha volcano, Ecuador, thus allow us to constrain the physical processes responsible for Vulcanian eruption sequences of this volcano. Morphologically, breadcrust bombs differ in the thickness of glassy surface rinds and in the orientation and density of crack networks. Thick rinds fracture to create deep, widely spaced cracks that form large rectangular domains of surface crust. In contrast, thin rinds form polygonal networks of closely spaced shallow cracks. Rind thickness, in turn, is inversely correlated with matrix glass water content in the rind. Assuming that all rinds cooled at the same rate, this correlation suggests increasing bubble nucleation delay times with decreasing pre-fragmentation water content of the melt. A critical bubble nucleation threshold of 0.4–0.9 wt% water exists, below which bubble nucleation does not occur and resultant bombs are dense. At pre-fragmentation melt H₂O contents of >∼0.9 wt%, only glassy rinds are dense and bomb interiors vesiculate after fragmentation. For matrix glass H₂O contents of ≥1.4 wt%, rinds are thin and vesicular instead of thick and non-vesicular. A maximum measured H₂O content of 3.1 wt% establishes the maximum pressure (63 MPa) and depth (2.5 km) of magma that may have been tapped during a single eruptive event. More common H₂O contents of ≤1.5 wt% suggest that most eruptions involved evacuation of ≤1.5 km of the conduit. As we expect that substantial overpressures existed in the conduit prior to eruption, these depth estimates based on magmastatic pressure are maxima. Moreover, the presence of measurable CO₂ (≤17 ppm) in quenched glass of highly degassed magma is inconsistent with simple models of either open- or closed-system degassing, and leads us instead to suggest re-equilibration of the melt with gas derived from a deeper magmatic source. Together, these observations suggest a model for the repeated Vulcanian eruptions that includes (1) evacuation of the shallow conduit during an individual eruption, (2) depressurization of magma remaining in the conduit accompanied by open-system degassing through permeable bubble networks, (3) rapid conduit re-filling, and (4) dome formation prior to the subsequent explosion. An important part of this process is densification of upper conduit magma to allow repressurization between explosions. At a critical overpressure, trapped pressurized gas fragments the nascent impermeable cap to repeat the process.     

The pre-eruptive volatile contents of recent basaltic and pantelleritic magmas at Pantelleria (Italy)

Pantelleria Island, located in the Sicily Channel Rift Zone (Italy), is the type locality for the peralkaline rhyolitic rocks called pantellerites. In the last 50 ka, after the large Green Tuff caldera-forming eruption, volcanic activity at Pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows, lava domes and poorly dispersed pantelleritic pumice fall deposits. Basaltic cinder cones and lava flows are only present outside the caldera in the NW sector of the island. The most recent basaltic (Cuddie Rosse, ～ 20 ka) and pantelleritic (Cuddia Randazzo and Cuddia del Gallo, ～ 6 ka) pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions.The melt inclusions in pyroxene and olivine phenocrysts of Cuddie Rosse scoriae have an alkali basalt composition. The dissolved volatiles comprise 0.9–1.6 wt.% H₂O, several hundred ppm of CO₂, 1600–2000 ppm of sulphur and 500–900 ppm of chlorine. The water–carbon dioxide couple gives a confining pressure ～ 2 kbar prior to the eruption. This result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption. The melt inclusions in feldspar, fayalite and aenigmatite phenocrysts of Cuddia del Gallo and Cuddia Randazzo pumice have a pantelleritic composition (Agpaitic Indices 1.3–2.1), up to 4.4 wt.% H₂O, 8700 ppm Cl, 6000 ppm F, and CO₂ below the detection limit. Sulphur averaging 420 ppm has been measured in Cuddia Randazzo melt inclusions. These data indicate relatively high volatile contents for these low-energy Strombolian-type eruptions. Melt inclusions in Cuddia del Gallo pumice show the most evolved composition (Agpaitic Indices 2–2.1) and the highest volatile content, in agreement with fluid saturation conditions in the magma chamber prior to the eruption. This implies a confining pressure of ～ 1 kbar for the top of the pantelleritic reservoir. The composition of melt inclusions and mineralogical assemblage of Cuddia Randazzo pumice indicate that it has a lower evolutionary degree (Agpaitic Indices 1.3–1.8) and lower pre-eruptive Cl and H₂O contents than Cuddia del Gallo pumice. An increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the Cuddia del Gallo explosive eruption. Evidence of widespread pre-eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the Cuddia Randazzo event.     

Effects of eruption and lava drainback on the H2O contents of basaltic magmas at Kilauea Volcano

 Lava drainback has been observed during many eruptions at Kilauea Volcano: magma erupts, degasses in lava fountains, collects in surface ponds, and then drains back beneath the surface. Time series data for melt inclusions from the 1959 Kilauea Iki picrite provide important evidence concerning the effects of drainback on the H₂O contents of basaltic magmas at Kilauea. Melt inclusions in olivine from the first eruptive episode, before any drainback occurred, have an average H₂O content of 0.7±0.2 wt.%. In contrast, many inclusions from the later episodes, erupted after substantial amounts of surface degassed lava had drained back down the vent, have H₂O contents that are much lower (≥0.24 wt.% H₂O). Water contents in melt inclusions from magmas erupted at Pu'u 'O'o on the east rift zone vary from 0.39–0.51 wt.% H₂O in tephra from high fountains to 0.10–0.28 wt.% H₂O in spatter from low fountains. The low H₂O contents of many melt inclusions from Pu'u 'O'o and post-drainback episodes of Kilauea Iki reveal that prior to crystallization of the enclosing olivine host, the melts must have exsolved H₂O at pressures substantially less than those in Kilauea's summit magma reservoir. Such low-pressure H₂O exsolution probably occurred as surface degassed magma was recycled by drainback and mixing with less degassed magma at depth. Recognition of the effects of low-pressure degassing and drainback leads to an estimate of 0.7 wt.% H₂O for differentiated tholeiitic magma in Kilauea's summit magma storage reservoir. Data for MgO-rich submarine glasses (Clague et al. 1995) and melt inclusions from Kilauea Iki demonstrate that primary Kilauean tholeiitic magma has an H₂O/K₂O mass ratio of ∼1.3. At transition zone and upper mantle depths in the Hawaiian plume source, H₂O probably resides partly in a small amount of hydrous silicate melt. Received: 31 March 1997 / Accepted: 17 November 1997     

The before-eruption water content of some high-alumina magmas

An analysis by difference technique yields estimates of H₂O in basaltic and andesitic glasses, which are sufficiently accurate (± 1.4 percent absolute) to be useful. Glass inclusions trapped in large olivine crystals from tephra-rich eruptions have 1 to 5 percent H₂O. The highest H₂O contents are found in basaltic inclusions in magnesium rich olivines from Mount Shasta, California. Andesitic inclusions have less H₂O. It seems probable that tephra-rich high-alumina magmas evolve in a vapor saturated environment at fairly shallow depths (few kilometers). This depth appears to be less for Medicine Lake Highlands than for Mount Shasta. Vapor saturation probably inhibits the rise of magma, thus the initial vapor content of a magma may govern its stagnation level. Volatile-rich parental magmas like Mount Shasta basalt probably tend to stagnate at deeper levels, crystallize early amphibole and produce comparatively calcic differentiates.     

Fumarolic emissions from Mount St. Augustine,Alaska: 1979–1984 degassing trends,volatile sources and their possible role in eruptive style

Gas samples were collected from high-temperature, rooted summit vents at Mount St. Augustine in 1979, 1982, and 1984. All of the gas samples exhibit various degrees of disequilibrium. Thermodynamic restoration of the analyzed gases permits partial or complete removal of these disequilibrium effects and allows inference of equilibrium gas compositions. Long-term (1979–1984) degassing trends within resampled or adjacent vents are characterized by increases (from 97.4 to 99.8 mole%) in the H₂O fraction and major decreases in the residual gases. Over this same period total gas HCl contents decreased by a factor of 3 to 10 while dry gas (H₂O-free recalculated) HCl contents increased by a factor of 1.6 to 3. Dry gas mole proportions at these sites changed from being CO₂-dominated (46% CO₂, 24% H₂ in 1979) to H₂-dominated (49% H₂, 22% CO₂ in 1984). The overall trends in gas chemistry and the stable isotope patterns in gases and condensates from the summit fumaroles can be explained by progressive magmatic outgassing coupled with increasing proportions of seawater in the fumarole emissions.Studies of the gaseous emissions following the 1976 and 1986 Mount St. Augustine eruptions confirmed the Cl- and S-rich nature of the Mount St. Augustine emanations. Seawater, possibly derived from magmatic assimilation or dehydration of near-surface seawater-bearing sediments, could supply a portion of the outgassed Cl and S. Continued seawater influx through subvolcanic fractures or permeable sediments would recharge the seawater-depleted zone and provide a near-surface Cl and S source for the next eruptive cycle,Various lines of evidence support a phreatomagmatic component in the 1976 and 1986 Mount St. Augustine eruptions. We suggest that seawater may interact with magma or volcanic gases during the early explosive phase of Mount St. Augustine eruptions and that it continues to influence high-temperature fumarole emissions as the volcanic system cools.     

Determination of pre-eruptive H2O,F and Cl contents of silicic magmas using melt inclusions: Examples from Taupo volcanic center,New Zealand

Water, F, and Cl contents of melt inclusions in phenocrysts from the 2-ka-old Taupo and Hatepe plinian tephras, and the 22-ka-old Okaia tephra from the Taupo volcanic center, New Zealand, were measured by electron and ion microprobe. Major and trace element chemistry of the inclusions is similar to that of bulk rock, supporting our assumption that volatile contents of inclusions are representative of the magma in which the crystals grew. Inclusions in the 2-ka Taupo plinian tephra contain a mean of 4.3 wt% H₂O, 450 ppm F, and 1700 ppm Cl; from the Hatepe plinian tephra 4.3 wt% H₂O, 430 ppm F, and 1700 ppm Cl; and from the Okaia tephra 5.9 wt% H₂O, 470 ppm F, and 2100 ppm Cl. Sulfur was below the detection limit of 200 ppm. The constant H₂O, F and Cl from a number of stratigraphic horizons in the tephra deposits suggest that the Taupo and Hatepe plinian tephras (>8.2 km³ magma volume) were derived from a magma body that did not contain a strong volatile gradient. By inference, there is no pre-eruptive volatile difference between these plinian eruptions and a phrea-toplinian eruption which occurred between the two. Virtually no major element zonation is seen in this eruptive sequence. Although the Okaia tephra was also erupted from the Taupo volcanic center, probably from a similar vent area, its higher volatile contents and distinct composition as compared to the Taupo tephras show that it was derived from a different, and possibly deeper, magma body.     

Magma degassing and basaltic eruption styles: a case study of 2000 year BP Xitle volcano in central Mexico

To investigate the relationship between volatile abundances and eruption style, we have analyzed major element and volatile (H₂O, CO₂, S) concentrations in olivine-hosted melt inclusions in tephra from the 2000 yr BP eruption of Xitle volcano in the central Trans-Mexican Volcanic Belt. The Xitle eruption was dominantly effusive, with fluid lava flows accounting for 95% of the total dense rock erupted material (1.1 km³). However, in addition to the initial, Strombolian, cinder cone-building phase, there was a later explosive phase that interrupted effusive activity and deposited three widespread ash fall layers. Major element compositions of olivine-hosted melt inclusions from these ash layers range from 52 to 58 wt.% SiO₂, and olivine host compositions are Fo_84–86. Water concentrations in the melt inclusions are variable (0.2–1.3 wt.% H₂O), with an average of 0.45±0.3 (1σ) wt.% H₂O. Sulfur concentrations vary from below detection (50 ppm) to 1000 ppm but are mostly ≤200 ppm and show little correlation with H₂O. Only the two inclusions with the highest H₂O have detectable CO₂ (310–340 ppm), indicating inclusion entrapment at higher pressures (700–900 bars) than for the other inclusions (≤80 bars). The low and variable H₂O and S contents of melt inclusions combined with the absence of less soluble CO₂ indicates shallow-level degassing before olivine crystallization and melt inclusion formation. Olivine morphologies are consistent with the interpretation that most crystallization occurred rapidly during near-surface H₂O loss. During cinder cone eruptions, the switch from initial explosive activity to effusive eruption probably occurs when the ascent velocity of magma becomes slow enough to allow near-complete degassing of magma at shallow depths within the cone as a result of buoyantly rising gas bubbles. This allows degassed lavas to flow laterally and exit near the base of the cone while gas escapes through bubbly magma in the uppermost part of the conduit just below the crater. The major element compositions of melt inclusions at Xitle show that the short-lived phase of renewed explosive activity was triggered by a magma recharge event, which could have increased overpressure in the storage reservoir beneath Xitle, leading to increased ascent velocities and decreased time available for degassing during ascent.     

Experimental petrology relating to oversaturated peralkaline volcanics: A review

Experiments specifically devoted to problems of oversaturated peralkaline rocks have been primarily concerned with quartz + feldspar + liquid equilibria, and the determination of the low temperature liquids in the feldspar primary phase region. The results are brought together, and compared with natural compositions, by recalculating and plotting in the system Na₂O-K₂O-Al₂O₃-SiO₂ (molecular). The minimum zone in the peralkaline quartz-feldspar cotectic is the synthetic analogue of most comendites and many pantellerites. Peralkaline trachytes and trachytic pantellerites appear to be the natural equivalents of synthetic low temperature liquids in the feldspar primary phase region, but the more peralkaline liquids cannot be a simple evolutionary series controlled only by feldspar fractionation. Experiments have yet to reveal the relating process (or processes) for the series pantelleritic trachyte to pantellerite. Feldspars separating from low temperature synthetic and natural liquids are usually Or₃₅ ±₅ i.e. equivalent to the composition range of the thermal minimum between the anorthoclase and sanidine solid solution loops in the alkali feldspar join. Such liquids may therefore be envisaged as the locus of compositions in the peralkaline system that are in equilibrium with alkali feldspar at the minimum in the solid solution series. Such feldspar, when it separates from the vast majority of peralkaline liquids is fractionating K₂O and Al₂O₃, making the residual liquids more peralkaline and more sodic. Development of the peralkaline condition in natural liquids is commonly ascribed to the « plagioclase effect », but this creates its own dilemma by seeming to be effective only in liquids which are already distinctively alkaline. Furthermore it can only work in a low pressure regime. Examination of the high pressure melting curves of possible mantle minerals shows that acmitic pyroxenes have the lowest melting, in either hydrous or anhydrous conditions, especially at low partial pressures of oxygen. This provides a simple source control by which liquids will either be intrinsically peralkaline (if the melt volume is small) or inherit the potential for low pressure operation of the « plagioclase effect » (most basic magmas). Alkali transfer is well-attested in solid ? vapour experiments and in natural examples of metasomatic aureoles. The mobility of alkalis (and iron) must figure in any realistic scheme of peralkaline petrogenesis. This points up the need for experiments designed to meet the challenge ofopen system magmatism.     

Vent conditions for expected eruptions at Vesuvius

Determining consistent sets of vent conditions for next expected eruptions at Vesuvius is crucial for the simulation of the sub-aerial processes originating the volcanic hazard and the eruption impact. Here we refer to the expected eruptive scales and conditions defined in the frame of the EC Exploris project, and simulate the dynamics of magma ascent along the volcanic conduit for sub-steady phases of next eruptions characterized by intensities of the Violent Strombolian (VS), Sub-Plinian 2 (SP2), and Sub-Plinian 1 (SP1) scale. Sets of conditions for the simulations are determined on the basis of the bulk of knowledge on the past history of Vesuvius [Cioni, R., Bertagnini, A., Santacroce, R., Andronico, D., Explosive activity and eruption scenarios at Somma–Vesuvius (Italy): towards a new classification scheme. Journal of Volcanology and Geothermal Research, this issue.]. Volatile contents (H₂O and CO₂) are parameterized in order to account for the uncertainty in their expected amounts for a next eruption. In all cases the flow in the conduit is found to be choked, with velocities at the conduit exit or vent corresponding to the sonic velocity in the two-phase non-equilibrium magmatic mixture. Conduit diameters and vent mixture densities are found to display minimum overlapping between the different eruptive scales, while exit gas and particle velocities, as well as vent pressures, largely overlap. Vent diameters vary from as low as about 5 m for VS eruptions, to 35–55 m for the most violent SP1 eruption scale. Vent pressures can be as low as less than 1 MPa for the lowest volatile content employed of 2 wt.% H₂O and no CO₂, to 7–8 MPa for highest volatile contents of 5 wt.% H₂O and 2 wt.% CO₂ and large eruptive scales. Gas and particle velocities at the vent range from 100–250 m/s, with a tendency to decrease, and to increase the mechanical decoupling between the phases, with increasing eruptive scale. Except for velocities, all relevant vent quantities are more sensitive to the volatile content of the discharged magma for the highest eruptive scales considered.     

Effect of Miocene basaltic volcanism in Shanwang (Shandong Province, China) on environmental changes

Miocene(16―10 Ma) basalts,together with significantly well-preserved fossils(including animal and plant fossils) in the contemporaneously tephra-rich Maar sediments,are located in Shanwang volcanic region,Shandong Province,China.Distribution area of the basaltic eruption products is about 240 km2.Detailed field observations indicate that most of basaltic rocks are fissure eruptive products and some are central eruptives constrained by linear faults.The well-preserved fossils in the lacustrine deposits have been considered to be a result of mass mortalities.Based on physically volcanologic modeling results,eruption column of the basaltic fissure activities in the Shanwang volcanic region is estimated to have entered the stratosphere.Petrographic observations indicate that the basalts have porphyritic textures with phenocrysts of olivine,pyroxene,plagioclase feldspar and alkali feldspar setting in groundmass of plagioclase feldspar,alkali feldspar,quartz,apatite and glass.Based on observations of tephra,tuff and tuffites collected in the Maar sediments of the Shanwang area,we determined major element oxide concentrations and volatile composition of melt inclusions in phenocrysts and matrix glasses by electron microprobe analysis.Volatile(including S,Cl,F and H2O) concentrations erupted into the stratosphere were estimated by comparing pre-and post-eruptive volatile concentrations.Our determination results show that contents of S,Cl,F and H2O emitted into the stratosphere were 0.18%― 0.24%,0.03%―0.05%,0.03%―0.05% and 0.4%―0.6%,respectively,which was characterized by high-S contents erupted.Amounts of volatiles emitted in the Shanwang volcanic region are much higher than those in eruptions which had a substantial effect on climate and environment.According to the com-positions and amounts of the volatiles erupted from the Miocene basaltic volcanism in Shanwang,we propose a hypothesis that volatile-rich basaltic volcanism could result in the mass mortalities by in-jecting volatiles(e.g.,SO2,H2S,HCl,HF and H2O) into the stratosphere that would have triggered abrupt environmental changes(including formation of acid rain,temperature decline,ozone depletion,etc.) and altered lake chemistry,and subsequently volcanic ash fall buried and covered the dead animals and plants,forming well-preserved fossils in Shanwang Maar sediments.     

Lava bubble-wall fragments formed by submarine hydrovolcanic explosions on Lō'ihi Seamount and Kīlauea Volcano

 Glassy bubble-wall fragments, morphologically similar to littoral limu o Pele, have been found in volcanic sands erupted on Lō'ihi Seamount and along the submarine east rift zone of Kīlauea Volcano. The limu o Pele fragments are undegassed with respect to H₂O and S and formed by mild steam explosions. Angular glass sand fragments apparently form at similar, and greater, depths by cooling-contraction granulation. The limu o Pele fragments from Lō'ihi Seamount are dominantly tholeiitic basalt containing 6.25–7.25% MgO. None of the limu o Pele samples from Lō'ihi Seamount contains less than 5.57% MgO, suggesting that higher viscosity magmas do not form lava bubbles. The dissolved CO₂ and H₂O contents of 7 of the limu o Pele fragments indicate eruption at 1200±300 m depth (120±30 bar). These pressures exceed that generally thought to limit steam explosions. We conclude that hydrovolcanic eruptions are possible, with appropriate pre-mixing conditions, at pressures as great as 120 bar. Received: 22 December 1998 / Accepted: 16 July 1999     

Phenocryst-hosted melt inclusions record stalling of magma during ascent in the conduit and upper magma reservoir prior to vulcanian explosions, Soufrière Hills volcano, Montserrat, West Indies

The mechanics of explosive eruptions influence magma ascent pathways. Vulcanian explosions involve a stop–start mechanism that recurs on various timescales, evacuating the uppermost portions of the conduit. During the repose time between explosions, magma rises from depth and refills the conduit and stalls until the overpressure is sufficient to generate another explosion. We have analyzed major elements, Cl, S, H₂O, and CO₂ in plagioclase-hosted melt inclusions, sampled from pumice erupted during four vulcanian events at Soufrière Hills volcano, Montserrat, to determine melt compositions prior to eruption. Using Fourier transform infrared spectroscopy, we measured values up to 6.7 wt.% H₂O and 80 ppm CO₂. Of 42 melt inclusions, 81 % cluster between 2.8 and 5.4 wt.% H₂O (57 to 173 MPa or 2–7 km), suggesting lower conduit to upper magma reservoir conditions. We propose two models to explain the magmatic conditions prior to eruption. In Model 1, melt inclusions were trapped during crystal growth in magma that was stalled in the lower conduit to upper magma reservoir, and during trapping, the magma was undergoing closed-system degassing with up to 1 wt.% free vapor. This model can explain the melt inclusions with higher H₂O contents since these have sampled the upper parts of the magma reservoir. However, the model cannot explain the melt inclusions with lower H₂O because the timescale for plagioclase crystallization and melt inclusion entrapment is longer than the magma residence time in the conduit. In Model 2, melt inclusions were originally trapped at deeper levels of the magma chamber, but then lost hydrogen by diffusion through the plagioclase host during periodic stalling of the magma in the lower conduit system. In this second scenario, which we favor, the melt inclusions record re-equilibration depths within the lower conduit to upper magma reservoir.     

Gas release rates from pyroclastic flows: a assessment of the role of fluidisation in their emplacement

Estimates are presented for the rates of release of dissolved water from the particles in a pyroclastic flow by diffusion. Velocities of gas escaping from pyroclastic flows of different thicknesses are calculated. For quite small residual gas contents (0.2 to 0.8% H₂O), gas velocities of 10 to over 100 cm/sec during the first 10³ sec of release are estimated for flows of thickness 1 to 20 m, which experimental studies demonstrate are the velocities required to fluidise fine to medium ash. Flows with high residual gas contents or large volume (thick flows) are likely to be substantially fluidised by exsolving gas. 30% to over 60% of the particles in such flows are predicted to be fluidised. Fluidisation is thus believed to be an important mechanism in the flow and in determining the mobility of the large magnitude, prehistoric pyroclastic flows which formed extensive ignimbrite sheets. Small pyroclastic flows, however, of the magnitude observed in several historic eruptions are not believed to be fluidised, because of their low residual gas contents, small volume, and the substantial amount of cooling that occurs during their emplacement.     

Variations of near-surface atmospheric CO2 and H2O concentrations during summer on Muztagata

Expeditions during the summers of 2002 and 2003 implemented continuous monitoring of near-surface (2 m height) atmospheric CO₂ and H₂O concentrations at the 4500 m elevation on Muztagata. The resultant data sets reveal a slight decrease of CO₂ concentrations (of about 5 μmol·mol^-1) and changes in the diurnal variations from the end of June to the middle August. The daily maximum CO₂ concentrations occur between 02:30-05:30 AM (local time) and the minimum levels occur between 12:00-15:30 PM. The atmospheric CO₂ concentrations in the summer of 2002 were around 5 μmol·mol^-1 lower than those during the same period of 2003, whereas the diurnal amplitude was higher. In contrast, we found that the daily mean atmospheric H₂O content in 2003 was much lower than that in 2002 and there exists a striking negative correlation between CO₂ and H₂O concentrations. We therefore suggest that the near-surface atmospheric CO₂ concentration is affected not only by photosynthesis and respiration, but also by the air H₂O content in the glaciated region around Muztagata.     

Potassium enrichment by magma mixing and vapor exsolution: an example in the miocene volcanism of eastern Morocco

Miocene volcanism in eastern Morocco is comprised of potash rich calc-alkalic and alkaline rocks. In the southern part of the area, at the base of the Guilliz massif, basic inclusions are found in latites. From a petrological, geochemical and mineralogical study of both latites and their inclusions, it appear that the inclusions represent a basic liquid quenched in the latitic magma, inside the magma chamber. As a result of the drop in pressure and crystallization, a K-rich vapour phase separated from the inclusion-forming liquid and percolated the latitic magma, increasing the K₂O content of the latter and possibly triggering eruptions. The mixing process between the two magmas seems supported by density and viscosity estimations. Calculations show that for temperatures ranging from 700 °C to 1000 °C and H₂O contents from 1.6 to 5%, the inclusion forming liquid is less dense than the latitic liquid and can therefore ascend into the latitic magma by interface disequilibrium and flotation.     

Water contents of clinopyroxenes from sub‐arc mantle peridotites

One poorly constrained reservoir of the Earth's water budget is that of clinopyroxene in metasomatised, mantle peridotites. This study presents reconnaissance Sensitive High‐Resolution, Ion Microprobe–Stable Isotope (SHRIMP–SI) determinations of the H₂O contents of (dominantly) clinopyroxenes in rare mantle xenoliths from four different subduction zones, i.e. Mexico, Kamchatka, Philippines, and New Britain (Tabar‐Feni island chain) as well as one intra‐plate setting (western Victoria). All of the sub‐arc xenoliths have been metasomatised and carry strong arc trace element signatures. Average measured H₂O contents of the pyroxenes range from 70 ppm to 510 ppm whereas calculated bulk H₂O contents range from 88 ppm to 3 737 ppm if the variable presence of amphibole is taken into account. In contrast, the intra‐plate, continental mantle xenolith from western Victoria has higher water contents (3 447 ppm) but was metasomatised by alkali and/or carbonatitic melts and does not carry a subduction‐related signature. Material similar to the sub‐arc peridotites can either be accreted to the base of the lithosphere or potentially be transported by convection deeper into the mantle where it will lose water due to amphibole breakdown.     

Chlorine in submarine glasses from the Lau Basin: seawater contamination and constraints on the composition of slab-derived fluids

Measurements of chlorine concentrations in matrix glasses from 18 primitive (>6 wt% MgO) and eight evolved lavas from active spreading centers in the Lau Basin back-arc system provide insight into the processes which control chlorine concentrations in subduction-related magmas, and can be used to investigate chlorine enrichment related to fluids derived from the underlying subducted slab. Chlorine contents of the glasses are highly variable (0.008-0.835 wt%) and generally high with respect to uncontaminated mid-ocean ridge basalt. Chlorine contents are highest in fractionated lavas from propagating ridge tips and lowest in more primitive basaltic lavas. Two different styles of enrichment in chlorine (relative to other incompatible elements) are recognized. Glasses from the Central Lau Spreading and Eastern Lau Spreading Center typically have low Ba/Nb ratios indicating minimal input of slab-derived components, and high to very high ratios of chlorine relative to K₂O, H₂O, and TiO₂. This style of chlorine enrichment is highest in the most fractionated samples and is consistent with crustal assimilation of chlorine-rich altered ocean crust material. Data from the literature suggest that contamination by chlorine-rich seawater-derived components also characterizes the Woodlark Basin and North Fiji Basin back-arc systems. The second style of chlorine enrichment reflects input from slab-derived fluid(s) to the mantle wedge from the adjacent Tonga subduction zone. Basaltic glasses from the Valu Fa Ridge and Mangatolu Triple Junction show correlations between ratios of chlorine and K₂O, H₂O, and TiO₂ and indices of slab-derived fluid input such as Ba/Nb, Ba/Th and U/Th, consistent with chlorine in these lavas originating from a saline fluid added to the mantle wedge. Within the Valu Fa Ridge the measured range of chlorine contents equates to a chlorine flux of 224-1120 kg/m/yr to the back-arc crust. Using a simple melting model and additional data from other back-arc and arc sample suites we conclude that chlorine is a major component within the slab fluids that contribute to many arc and back-arc melting systems, and probably plays an important role in regulating trace element transport by slab fluids in the mantle wedge. For the back-arc suites we have examined the estimated Cl/H₂O and Cl/K₂O ratios in the slab fluid component correlate with proximity to the arc front, suggesting that progressive dehydration of the slab and/or re-equilibration and transport within the mantle wedge may influence the overall degree of chlorine enrichment within the slab fluid component. The degree of chlorine enrichment observed in most back-arc lavas also appears too great to be explained solely by melting of amphibole, phlogopite or apatite within the mantle source and suggests that chlorine must be present in another phase, possibly a chlorine-rich fluid.     

Holocene eruptive history of Ksudach volcanic massif,South Kamchatka: evolution of a large magmatic chamber

The combination of geological, tephrochronological and geochemical studies is used to reconstruct the Holocene eruptive history of Ksudach volcanic massif, South Kamchatka and to trace the evolution of its magma. Ksudach is located in the frontal volcanic zone of Kamchatka. From Early Holocene till AD 240, the volcano had repetitive voluminous caldera-forming eruptions. Later they gave way to frequent moderate explosive–effusive eruptions that formed the Shtyubel' stratovolcano inside the nested calderas, and then to frequent larger explosive eruptions. Holocene eruptive products are low-K₂O two pyroxene–plagioclase basaltic andesite to rhyodacite. Mineralogical, geochemical and isotopic data suggest that all the rock varieties originated as a result of fractionation of an initial mafic melt, with insignificant contamination and assimilation. Intensive mixing of the fractionating melts prior to, and during the course of the eruptions, is ubiquitous. The eruptions might have been triggered by repetitive injections of new mafic melt into the silicic chamber. Crystallization of the andesitic and rhyodacitic melts is estimated to have occurred at temperatures of 970–1010°C and 890–910°C, respectively, P_H2O 1.5–2.0 kbar and f_O2 close to the NNO buffer. According to the experimental data, such P_H2O corresponds to 4.5%–5.5% of water in the melt, that is close to the content of water in the silicic hornblende-bearing magmas of the rear zone of the Kuril–Kamchatka arc. Hence, we suggest that the transition from pyroxene phenocryst associations of the frontal zone to the hornblende-bearing ones of the rear zone might be interpreted as reflecting higher temperatures of crystallization of the melts from the frontal zone rather than increasing water content in the rear zone magmas.     

Volcán Las Navajas,a Pliocene-Pleistocene trachyte/peralkaline rhyolite volcano in the northwestern Mexican volcanic belt

Volcán Las Navajas, a Pliocene-Pleistocene volcano located in the northwestern portion of the Mexican volcanic belt, erupted lavas ranging in composition from alkali basalt through peralkaline rhyolite, and is the only volcano in mainland Mexico known to have erupted pantellerites. Las Navajas is located near the northwestern end of the Tepic-Zacoalco rift and covers a 200-m-thick pile of alkaline basaltic lavas, one of which has been dated at 4.3 Ma. The eruptive history of the volcano can be divided into three stages separated by episodes of caldera formation. During the first stage a broad shield volcano made up of alkali basalts, mugearites, benmoreites, trachytes, and peralkaline rhyolites was constructed. Eruption of a chemically zoned ash flow then caused collapse of the structure to form the first caldera. The second stage consisted of eruptions of glassy pantellerite lavas that partially filled the caldera and overflowed its walls. This stage ended about 200 000 years ago with the eruption of pumice falls and ash flows, which led to the collapse of the southern portion of the volcano to form the second caldera. During the third stage, two benmoreite cinder cones and a benmoreite lava flow were emplaced on the northwestern flank of the volcano. Finally, the calc-alkaline volcano Sanganguey was built on the southern flank of Las Lavajas. Alkaline volcanism continued in the area with eruptions of alkali basalt from cinder cones located along NW-trending fractures through the area. Although other mildly peralkaline rhyolites are found in the rift zones of western Mexico, only Las Navajas produced pantellerites. Greater volumes of basic alkaline magma have erupted in the Las Navajas region than in the other areas of peralkaline volcanism in Mexico, a factor which may be necessary to provide the initial volume of material and heat to drive the differentiation process to such extreme peralkaline compositions.