Magma Storage Zone Research Articles

The proximal marine record of the Marsili Seamount in the last 7 ka (Southern Tyrrhenian Sea, Italy): Implications for the active processes in the Tyrrhenian Sea back-arc

The volcanism of back-arc basins resembles that of oceanic spreading centers, rifts, and, in vanishing stages, extensional arcs, depending on the amount and rate of the dynamic processes associated to the subduction. Marsili Seamount (MS) represents the axial ridge of the Southern Tyrrhenian Sea back-arc basin, which is connected to the slab roll-backing processes affecting the Calabrian Arc (Italy). The Southern Tyrrhenian Sea back-arc is characterized by a significant decline in the spreading rate with time (2.8–3.1mm/a to less than 1.8mm/a in the last 0.78–1Ma). MS develops between about 1Ma and 3ka and mainly consists of lava flows erupted from central and fissural vents. The MS products belong to the calcalkaline association and range in composition from basalts to trachytes. We present new stratigraphic, geochronological, and geochemical data (glass shards and minerals) of tephra from a 2.35m long gravity core (Marsili1 core) recovered on MS at 943m b.s.l. We recognize five tephras [M1 (top of the core) to M5 (bottom)] represented by poorly to highly vesiculated ashes. The lowermost tephra M5 emplaced between ca. 7 and 26ka B.P.; it represents the less evolved distal counterpart of the Unit D related to the Biancavilla–Montalto products of Mount Etna (Sicily). The M1 to M4 tephras emplaced between 2.1 and 7.2ka B.P. and are related to strombolian-like submarine eruptions of NNE–SSW aligned MS vents. The composition of the M1–M4 glasses ranges from basaltic trachyandesites to andesites and trachytes. The M1 to M4 magmas mainly originated by crystal fractionation from a heterogeneous mantle source with varying LILE enrichments by subduction-related fluids. The degree of evolution of the MS magmas increases with decreasing time. The formation of vertically stacked magma storage zones at the crust/mantle interface and within MS is related to the vanishing Southern Tyrrhenian Sea opening, which implies the rapid (<1Ma) evolution from a slow spreading back-arc setting to an arc system.

Monitoring of the volcanic rock compositions during the 2012–2013 fissure eruption at Tolbachik volcano, Kamchatka

Abstract Here we present the results from monitoring of the composition of rocks produced during the 2012–2013 fissure eruption at Tolbachik volcano (FTE). Major and trace element concentrations in 75 samples are reported. Products of this eruption are represented by high alumina basaltic trachyandesites with higher alkalis and titanium contents than in all previously studied rocks of the Tolbachik monogenetic volcanic field. Rocks erupted during the first three days (27–30 November) from the northern (also called Menyailov) group of vents are the most silica- and alkali-rich (SiO2 concentrations up to 55.35 wt.% and K2O up to 2.67 wt.%). From December onwards, when the eruptive activity switched from the Menyailov vents to the southern (Naboko) group of vents, silica content dropped by 2 wt.%, concentrations of MgO, FeO, TiO2 and Mg# increased, and K2O and Na2O concentrations and K2O/MgO ratio decreased. For the rest of the eruption the compositions of rocks remained constant and homogeneous; no systematic compositional differences between lava, bombs and scoria samples are evident. Trace element distributions in the rocks of the Menyailov and Naboko vent lavas are relatively uniform; Menyailov lavas have slightly higher Th, Nb, Hf, Y, and HREE concentrations than the Naboko vent lavas at more or less constant element ratios. We explain the initial change in geochemistry by tapping of a slightly cooler and fractionated (~ 3% Mt and 8% Cpx) upper part of the magma storage zone before the main storage area began to feed the eruption. Thermodynamic constraints show that apparent liquidus temperatures varied from 1142 °C to 1151 °C, and thermodynamic modeling shows that variations in compositions are consistent with a high degree of low pressure (100–300 MPa), nominally anhydrous fractionation of a parent melt compositionally similar to the 1975 Northern Breakthrough high-Mg basalt. Geochemistry, petrological observations and modeling are in agreement with the newly erupted material being derived from remnant high-Al magma from the 1975–76 Southern Breakthrough eruption with only slight amounts of cooling (less than 1 °C per year) during the intervening 36 years.

Time-dependent displacements during and after the April 2007 eruption of Piton de la Fournaise, revealed by interferometric data

From March 30 to May 1 2007, Piton de la Fournaise, La Réunion Island, experienced a major eruptive crisis, characterized by the largest emitted lava volume (210×106m3) of the 20th and 21st centuries, and by a 340m deep caldera collapse. The event was captured by InSAR data from the ENVISAT and ALOS-1 satellites. From this data, we computed the EW and vertical components of the displacement that occurred on the entire edifice during the co-eruptive period and in the following months. Our results reveal unusually large and time-dependent displacements of the Central Cone and the Eastern Flank of Piton de la Fournaise, both of which continued to deform for at least a year after the end of the eruption. The analysis of InSAR displacement combined with other geophysical and field observations allows us to propose conceptual models to explain the Central Cone and the Eastern Flank displacements. We propose that the April 2007 caldera collapse induced a sudden decompression of the hydrothermal system, which had been previously pressurized and heated by temporary sealing of its upper part. This sudden decompression resulted in a strong centripetal subsidence. This then decreased exponentially as poro-elastic compaction and creep of the Central Cone propagated from the collapsed rock column to more distal parts of the hydrothermal system. For the Eastern Flank, we propose that the displacement is related to an intrusion within the Grandes Pentes. While propagating to the surface, the intrusion may have encountered a pre-existing structural discontinuity, intruded it and activated it as a detachment surface. It is likely that the detachment slip, by reducing the minimum principal stress close to the summit, allowed the injection of the dyke that fed the brief March 30–31 eruption. Then it may have promoted the migration of magma from the main magma storage zone, beneath the Central Cone, to the distal April 2 eruption site. The EW extensional stress field resulting from the slip of the detachment during the co-eruptive period may have activated a set of normal faults on which aseismic creep controlled the response of the edifice during the post-eruptive period.

Oxygen isotopes in melt inclusions and glasses from the Askja volcanic system, North Iceland

Primitive melt inclusions trapped during the earliest stages of fractional crystallisation are able to preserve oxygen isotope ratios inherited from mantle-derived melts. However, assimilation of low-δ18O hydrothermally altered crustal material and mixing with magmas held in shallow reservoirs may exert a strong control on the δ18O of melt inclusions trapped during later stages of crystallisation. Oxygen isotope ratios in olivine- and plagioclase-hosted melt inclusions and glasses from tephra samples collected from the Askja central volcano and Askja volcanic system indicate significant differences in the mechanisms of magma supply and storage between the northern and southern segments of the Askja volcanic system. Melt inclusions from the Holuhraun fissure eruption, ∼20km south of Askja, mostly preserve δ18O signatures of +4.1‰ to +5.4‰, suggesting that this magma underwent minimal modification by magma mixing or crustal assimilation prior to its eruption. By contrast, melt inclusions and glasses from the Nýjahraun fissure eruption, ∼60km north of Askja, have δ18O between +3.1‰ and +4.0‰. These relatively evolved melt inclusions (∼3.9–4.3wt.% MgO) were probably trapped during late-stage fractional crystallisation in a shallow magma storage zone. Melt inclusions from two phreatomagmatic tuff sequences within the Askja caldera have δ18O between +2.1‰ and +5.2‰, and this variability cannot be explained by mixing with low-δ18O rhyolitic or andesitic contaminants in the upper crust. Instead, mixing of the ascending magmas with hydrated, low-δ18O basaltic magmas is invoked, thus acquiring a low δ18O signature with minimal modification to the magma’s bulk composition. Such magma bodies are likely to be found throughout the upper 11km of the crust beneath Askja. Assimilation of low-δ18O meta-basalt in the upper crust is also likely to affect the δ18O of ascending magmas.

The pre-eruptive magma plumbing system of the 2007–2008 dome-forming eruption of Kelut volcano, East Java, Indonesia

Kelut volcano, East Java, is an active volcanic complex hosting a summit crater lake that has been the source of some of Indonesia’s most destructive lahars. In November 2007, an effusive eruption lasting approximately 7 months led to the formation of a 260-m-high and 400-m-wide lava dome that displaced most of the crater lake. The 2007–2008 Kelut dome comprises crystal-rich basaltic andesite with a texturally complex crystal cargo of strongly zoned and in part resorbed plagioclase (An47–94), orthopyroxene (En64–72, Fs24–32, Wo2–4), clinopyroxene (En40–48, Fs14–19, Wo34–46), Ti-magnetite (Usp16–34) and trace amounts of apatite, as well as ubiquitous glomerocrysts of varying magmatic mineral assemblages. In addition, the notable occurrence of magmatic and crustal xenoliths (meta-basalts, amphibole-bearing cumulates, and skarn-type calc-silicates and meta-volcaniclastic rocks) is a distinct feature of the dome. New petrographical, whole rock major and trace element data, mineral chemistry as well as oxygen isotope data for both whole rocks and minerals indicate a complex regime of magma-mixing, decompression-driven resorption, degassing and crystallisation and crustal assimilation within the Kelut plumbing system prior to extrusion of the dome. Detailed investigation of plagioclase textures alongside crystal size distribution analyses provide evidence for magma mixing as a major pre-eruptive process that blends multiple crystal cargoes together. Distinct magma storage zones are postulated, with a deeper zone at lower crustal levels or near the crust-mantle boundary (>15 km depth), a second zone at mid-crustal levels (~10 km depth) and several magma storage zones distributed throughout the uppermost crust (<10 km depth). Plagioclase-melt and amphibole hygrometry indicate magmatic H2O contents ranging from ~8.1 to 8.6 wt.% in the lower crustal system to ~1.5 to 3.3 wt.% in the mid to upper crust. Pyroxene and plagioclase δ18O values range from 5.4 to 6.7 ‰, and 6.5 to 7.6 ‰, respectively. A single whole rock analysis of the 2007–2008 dome lava gave a δ18O value of 7.6 ‰, whereas meta-basaltic and calc-silicate xenoliths are characterised by δ18O values of 6.2 and 10.3 ‰, respectively. Magmatic δ18O values calculated from individual pyroxene and plagioclase analyses range from 5.7 to 7.0 ‰, and 6.2 to 7.4 ‰, respectively. This range in O-isotopic compositions is explained by crystallisation of pyroxenes in the lower to mid-crust, where crustal contamination is either absent or masked by assimilation of material having similar δ18O values to the ascending melts. This population is mixed with isotopically distinct plagioclase and pyroxenes that crystallised from a more contaminated magma in the upper crustal system. Binary bulk mixing models suggest that shallow-level, recycled volcaniclastic sedimentary rocks together with calc-silicates and/or limestones are the most likely contaminants of the 2007–2008 Kelut magma, with the volcaniclastic sediments being dominant.

Consequences of volcano sector collapse on magmatic storage zones: Insights from numerical modeling

Major volcano flank collapses strongly affect the underlying magmatic plumbing system. Here, we consider the magma storage zone as a liquid pocket embedded in an elastic medium, and we perform numerical simulations in two-dimensional axisymmetric geometry as well as in three dimensions in order to evaluate the consequences of a major collapse event. We quantify the pressure decrease induced within and around a magma reservoir by a volcano flank collapse. This pressure reduction is expected to favor replenishment with less evolved magma from deeper sources. We also estimate the impact of the magma pressure decrease, together with the stress field variations around the reservoir, on the eruptive event associated with the edifice failure. We show that, for a given magma reservoir geometry, the collapse of a large strato-volcano tends to reduce the volume of the simultaneous eruption; destabilization of large edifices may even suppress magma emission, resulting in phreatic eruptions instead. This effect is greater for shallow reservoirs, and is more pronounced for spherical reservoirs than for vertically-elongated ones. It is reduced for compressible magmas containing a large amount of volatiles. Over a longer time scale, the modification of pressure conditions for dyke initiation at the chamber wall may also explain an increase in eruption rate as well as an apparent change of magma storage location.

Combining relative and absolute gravity measurements to enhance volcano monitoring

To achieve a balance between uncertainty and efficiency in gravity measurements, we have investigated the applicability of combined measurements of absolute and relative gravity as a hybrid method for volcano monitoring. Between 2007 and 2009, three hybrid gravity surveys were conducted at Mt Etna volcano, in June 2007, July 2008, and July 2009. Absolute gravity data were collected with two absolute gravimeters, which represent the state of the art in recent advances in ballistic gravimeter technology: (1) the commercial instrument FG5#238 and (2) the prototype instrument IMGC-02. We carried out several field surveys and confirmed that both the absolute gravimeters can still achieve a 10 μGal or better uncertainty even when they are operated in severe environmental conditions. The use of absolute gravimeters in a field survey of the summit area of Mt Etna is unprecedented. The annual changes of the gravity measured over 2007–2008 and 2008–2009 provide unequivocal evidence that during the 2007–2009 period, two main phenomena of subsurface mass redistribution occurred in distinct sectors of the volcano, accompanying different eruptive episodes. From 2007 to 2008, a gravity change of −60 μGal was concentrated around the North-East Rift. This coincided with a zone affected by strong extensional tectonics, and hence might have been related to the opening of new voids. Between 2008 and 2009, a North-South elongate feature with a maximum gravity change of +80 μGal was identified in the summit craters area. This is interpreted to indicate recharge of a deep-intermediate magma storage zone, which could have occurred when the 2008–2009 eruption was still ongoing.

Formation of Öskjuvatn caldera at Askja, North Iceland: Mechanism of caldera collapse and implications for the lateral flow hypothesis

The Öskjuvatn caldera at Askja volcano, North Iceland, was formed as a result of an explosive eruption at Askja on 28th–29th March 1875. Öskjuvatn is one of the youngest collapse calderas on Earth, and its initiation and growth were observed and documented by explorers and geologists working in the Askja region between 1875 and 1932. It is an example of caldera formation where the volume of the caldera collapse far exceeds the volume of associated erupted products. The discrepancy between the collapse volume and associated erupted products has been used by previous workers as a justification for the hypothesis that the injection of lateral dykes from Askja fed the February to October 1875 Nýjahraun fissure eruptions at the Sveinagjá graben, some 60km north of Askja. However, historical accounts documenting the caldera formation in sufficient detail show clearly that Öskjuvatn formed by piecemeal collapse, taking over 40years to reach its current form. We use these accounts to undertake a detailed examination of the stages of caldera collapse and to compare them with the known magmatic output of Askja in the years following the 1875 eruption. ArcGIS software was used to calculate the volume of Öskjuvatn during the various stages of collapse, and the volume of material erupted after 1875. While a dyke extending between Askja and Sveinagjá may be implied to account for the ‘missing’ volume, our results demonstrate that the volume of Öskjuvatn in July 1875 closely corresponds to the volume of rhyolitic material erupted on 28th–29th March 1875. In addition, geochemical evidence indicates that the Nýjahraun magmas were sourced from a deeper reservoir rather than by lateral injection from Askja's shallow crustal magma chamber. We therefore suggest that the injection of sills and dykes in a coherent intrusion complex beneath Askja central volcano, combined with background deflation of the magma storage zone beneath Öskjuvatn caldera over 30 to 40years following 1875, satisfactorily accommodates the volume required to form Öskjuvatn caldera.

U–Th–Pb geochronology of zircon and monazite from syenite and pincinite xenoliths in Pliocene alkali basalts of the intra-Carpathian back-arc basin

Phreatomagmatic eruptions in the Lučenec Basin in Slovakia ejected fragments of igneous rocks of unknown age and ranging in composition from gabbro to syenite and orthopyroxene granodiorite – pincinite. Two kinds of syenites with different Fe 3+ contents composed of anorthoclase and interstitial metaluminous trachyte-to-alkali rhyolite glass represent the final products of alkali basalt differentiation at depths of 10 and 15–18 km. Pincinite composed of plagioclase, quartz, orthopyroxene, ilmenite and interstitial peraluminous dacite glass is a quenched supersolidus anatectic melt formed at the depth of 17–20 km by high-temperature dehydration melting of biotite-bearing gneissic protolith. Zircon from pincinite and syenite, and monazite from syenite were dated using excimer laser ablation ICP-MS. The obtained ages cluster at 5.8 ± 0.1 and 5.4 ± 0.1 Ma, thus corroborating the genetic link between the two rock-types and constraining the residence time of at least 300 ± 100 ka of the parental basalt in the deep crustal magma storage zones. The inferred ages overlap the Upper Miocene–Pliocene boundary, and are considerably younger than spatially associated basaltic lava flows dated using the K/Ar method. Consequently, the fluviolacustrine Poltár Formation overlying the maar structure must have sedimented during the Pliocene and not during the Upper Miocene (Pontian) as previously assumed.

Patterns in the recent 2007–2008 activity of Mount Etna volcano investigated by integrated geophysical and geochemical observations

Seismic, deformation, and volcanic gas observations offer independent and complementary information on the activity state and dynamics of quiescent and eruptive volcanoes and thus all contribute to volcanic risk assessment. In spite of their wide use, there have been only a few efforts to systematically integrate and compare the results of these different monitoring techniques. Here we combine seismic (volcanic tremor and long‐period seismicity), deformation (GPS), and geochemical (volcanic gas plume CO2/SO2 ratios) measurements in an attempt to interpret trends in the recent (2007–2008) activity of Etna volcano. We show that each eruptive episode occurring at the Southeast Crater (SEC) was preceded by a cyclic phase of increase‐decrease of plume CO2/SO2 ratios and by inflation of the volcano's summit captured by the GPS network. These observations are interpreted as reflecting the persistent supply of CO2‐rich gas bubbles (and eventually more primitive magmas) to a shallow (depth of 1–2.8 km asl) magma storage zone below the volcano's central craters (CCs). Overpressuring of the resident magma stored in the upper CCs' conduit triggers further magma ascent and finally eruption at SEC, a process which we capture as an abrupt increase in tremor amplitude, an upward (&gt;2800 m asl) and eastward migration of the source location of seismic tremor, and a rapid contraction of the volcano's summit. Resumption of volcanic activity at SEC was also systematically anticipated by declining plume CO2/SO2 ratios, consistent with magma degassing being diverted from the central conduit area (toward SEC).

Magma storage and ascent at Lipari Island (Aeolian archipelago, Southern Italy) at 223–81 ka: the role of crustal processes and tectonic influence

Fluid inclusion studies together with volcanological and petrochemical data allow reconstruction of the magma feeding system of basaltic-andesitic to andesitic activity during the oldest and intermediate stages of development of Lipari Island (223–81 ka). A major magma storage zone is active during the overall investigated time span at depths of 22 km, close to the crust-mantle Moho transition, at which mantle-derived mafic magmas tend to accumulate due to neutral buoyancy conditions. Beneath central-type volcanoes (M. Mazzacaruso, M. S.Angelo, M. Chirica-Costa d’Agosto), a shallower magma reservoir is located within the upper crust at 5.5–3.5 km, associated with a major lithological discontinuity. For fissural-type volcanoes (Timpone Ospedale, Monterosa, M. Chirica), tectonic structures are suggested to influence further magma ascent and storage at mid-crustal depths (∼14 km), with no ponding at shallower levels. Partial crustal melting processes at the roofs of the deep magma reservoirs (∼17 km) are invoked to explain the origin of cordierite-bearing lavas beneath M. S.Angelo and M. Chirica-Costa d’Agosto volcanoes, which were active during the intermediate stages of development of Lipari (105–81 ka). The generation of felsic anatectic melts in the lower crust could have created density and rheologic barriers to impede the passage of mafic melts and promote their ponding, with influence on the subsequent evolution of Lipari volcano.

Ground surface deformation patterns, magma supply, and magma storage at Okmok volcano, Alaska, from InSAR analysis: 2. Coeruptive deflation, July–August 2008

A hydrovolcanic eruption near Cone D on the floor of Okmok caldera, Alaska, began on 12 July 2008 and continued until late August 2008. The eruption was preceded by inflation of a magma reservoir located beneath the center of the caldera and ∼3 km below sea level (bsl), which began immediately after Okmok's previous eruption in 1997. In this paper we use data from several radar satellites and advanced interferometric synthetic aperture radar (InSAR) techniques to produce a suite of 2008 coeruption deformation maps. Most of the surface deformation that occurred during the eruption is explained by deflation of a Mogi‐type source located beneath the center of the caldera and 2–3 km bsl, i.e., essentially the same source that inflated prior to the eruption. During the eruption the reservoir deflated at a rate that decreased exponentially with time with a 1/e time constant of ∼13 days. We envision a sponge‐like network of interconnected fractures and melt bodies that in aggregate constitute a complex magma storage zone beneath Okmok caldera. The rate at which the reservoir deflates during an eruption may be controlled by the diminishing pressure difference between the reservoir and surface. A similar mechanism might explain the tendency for reservoir inflation to slow as an eruption approaches until the pressure difference between a deep magma production zone and the reservoir is great enough to drive an intrusion or eruption along the caldera ring‐fracture system.

Influence of surface load variations on eruption likelihood: application to two Icelandic subglacial volcanoes, Grímsvötn and Katla

SUMMARY We investigate how surface load variations around volcanoes act on shallow magma chambers. Numerical calculations are carried out in axisymmetric geometry for an elliptical chamber embedded in an elastic medium. Magma compressibility is taken into account. For variable chamber shape, size and depth, we quantify how unloading events induce magmatic pressure change as well as variation of the threshold pressure required for dyke initiation at the chamber wall. We evaluate the triggering effect of these surface events on onset of eruptions and find it depends strongly on the surface load location and the magma chamber shape. We apply this model to two active Icelandic subglacial volcanoes: Gr´ omsv¨ otn and Katla. The 2004 eruption of Gr´ omsv¨ otn was immediately preceded by a j¨ okulhlaup, a glacial outburst flood of 0.5 km 3 . We show that this event may have triggered the eruption only if the system was very close to failure conditions. Katla volcano is covered by the M´ yrdalsj¨ okull ice cap. An annual cycle, with up to 6 m change in snow thickness, occurs from winter to summer. As the seasonal snow load is reduced, a pressure decrease of the same order of magnitude as the load is induced within the magma storage zone. Our model predicts that, in the case of a spherical or horizontally elongated magma chamber, eruptions are more likely when the snow cover is smallest, which appears consistent with the fact that all the last nine major historical eruptions at Katla occurred during the summer period. The model predicts an increase in Coulomb stress around the caldera, up to 7 km from its centre, during unloading periods, enough to trigger earthquakes. Stress due to snow load variations, with focusing of it in weak zones near the caldera boundary, is considered a contributing factor to seasonal seismicity observed beneath M´ yrdalsj¨ okull.

Microcrysts Record Transient Convection at Piton de la Fournaise Volcano (La Reunion Hotspot)

We studied the morphologies of olivine, clinopyroxene and plagioclase microcrysts in olivine-rich basalts from the December 2005 eruption at Piton de la Fournaise, to reconstruct the thermal history of the magma prior to reaching the surface. Olivine microcrysts and their melt inclusions show textural and compositional evidence for rapid growth caused by successive cooling and reheating cycles below the liquidus of olivine (1230°C). Clinopyroxene crystals are corroded, which indicates reheating above the liquidus of clinopyroxene (1140°C). Clinopyroxene crystallization–dissolution is also evidenced by the composition of melt inclusions in olivine microcrysts. We interpret the cooling–heating cycles experienced by olivine and clinopyroxene microcrysts as evidence for thermal convection in the shallow magma storage zone (2–2·5 km depth) of the volcano. Plagioclase microcrysts do not record temperature fluctuations, and hence probably crystallized in the conduit. Given the small size of the olivine and clinopyroxene microcrysts and their rapid growth, it is likely that convection preceded the December 2005 eruption by only a few days at most. Our results suggest that convection is transient and proceeds in small-scale cells at low Rayleigh number (\textless 104), rather than vigorous and continuous convection at high Rayleigh number.

VP Structure of Mount St. Helens, Washington, USA, imaged with local earthquake tomography

We present a new P-wave velocity model for Mount St. Helens using local earthquake data recorded by the Pacific Northwest Seismograph Stations and Cascades Volcano Observatory since the 18 May 1980 eruption. These data were augmented with records from a dense array of 19 temporary stations deployed during the second half of 2005. Because the distribution of earthquakes in the study area is concentrated beneath the volcano and within two nearly linear trends, we used a graded inversion scheme to compute a coarse-grid model that focused on the regional structure, followed by a fine-grid inversion to improve spatial resolution directly beneath the volcanic edifice. The coarse-grid model results are largely consistent with earlier geophysical studies of the area; we find high-velocity anomalies NW and NE of the edifice that correspond with igneous intrusions and a prominent low-velocity zone NNW of the edifice that corresponds with the linear zone of high seismicity known as the St. Helens Seismic Zone. This low-velocity zone may continue past Mount St. Helens to the south at depths below 5 km. Directly beneath the edifice, the fine-grid model images a low-velocity zone between about 2 and 3.5 km below sea level that may correspond to a shallow magma storage zone. And although the model resolution is poor below about 6 km, we found low velocities that correspond with the aseismic zone between about 5.5 and 8 km that has previously been modeled as the location of a large magma storage volume.

A Rhyolite Compositional Continuum Governed by Lower Crustal Source Conditions in the Taupo Volcanic Zone, New Zealand

Rhyolites generated in the modern Taupo Volcanic Zone (TVZ), New Zealand, have previously been interpreted as having evolved by a combination of extensive fractional crystallization of mantle-derived mafic magmas and limited crustal assimilation (up to 25%). Polytopic vector analysis (PVA), a form of multivariate statistical analysis, of the major-element compositions of over 475 basaltic to rhyolitic bulk-rock samples, representing over 600 kyr of volcanism within the TVZ, has provided a robust platform for rhyolite characterization and new insights into rhyolite petrogenesis. There is a continuum of compositions between two rhyolite end-member magma types (EM1 and EM2), which have been identified on the basis of the PVA and which have distinct petrological and geochemical characteristics, as follows. EM1: crystal-rich (up to 45%), hydrous phases (± hornblende ± biotite ± cummingtonite), high Aluminum Saturation Index [ASI; molar Al 2 O 3 /(CaO + Na 2 O + K 2 O)], low FeO*/MgO (calc-alkaline series), depleted abundances of middle rare earth elements (MREE) and Y, and high Sr; EM2: crystal-poor (<10%), anhydrous phases (orthopyroxene ± clinopyroxene), high FeO*/MgO (tholeiitic series), low ASI, less depleted MREE and Y, and low Sr. The range of ASI values, and relative depletion in MREE and Y in the rhyolites is consistent with the results of experiments to constrain the partial melting behaviour of amphibolite at crustal pressures. The major- and trace-element data are also consistent with 50-60% equilibrium crystallization of a crustally contaminated, hornblende-bearing andesite to produce the TVZ rhyolites. Distinct major- and trace-element variations along the continuum between the two rhyolite end-member types can be effectively modelled by simulating changes in the temperature-fO 2 -fH 2 O conditions in the lower crust where mantle-derived mafic magmas are stored and differentiate. Low T and high fO 2 and fH 2 O in the crustal magma storage zone promote abundant hornblende crystallization and suppress plagioclase crystallization, which produces the EM1 type rhyolite. By increasing the temperature and/or lowering fO 2 and fH 2 O in the magma storage region, plagioclase becomes more dominant and hornblende crystallization is suppressed, producing more EM2-like rhyolitic magma types.

The 2007 eruption of Stromboli volcano: Insights from real-time measurement of the volcanic gas plume CO 2/SO 2 ratio

The recent eruption of Stromboli in February–April 2007 offered a unique chance to test our current understanding of processes driving the transition from ordinary (persistent Strombolian) to effusive activity, and the ability of instrumental geophysical and geochemical networks to interpret and predict these events. Here, we report on the results of two years of in-situ sensing of the CO 2/SO 2 ratio in Stromboli's volcanic gas plume, in the attempt to put constraints on the trigger mechanisms and dynamics of the eruption. We show that large variations of the plume CO 2/SO 2 ratio (range, 0.9–26) preceded the onset of the eruption (since December 2007), interrupting a period of relatively-steady and low ratios (time-averaged ratio, 4.3) lasting from at least May to November 2006. By contrasting our observations with numerical simulations of volcanic degassing at Stromboli, derived by use of an equilibrium saturation model, we suggest that the pre-eruptive increase of the ratio reflected an enhanced supply of deeply-derived CO 2-rich gas bubbles to the shallow-plumbing system. This larger-than-normal ascent of gas bubbles was likely sourced by a 1–3 km deep gas–melt separation region (probably a magma storage zone), and caused faster convective overturning of magmas in the shallow conduit; an increase in the explosive rate and in seismic tremor, and finally the collapse of the la Sciara del Fuoco sector triggering the effusive phase. The high CO 2/SO 2 ratios (up to 21) observed during the effusive phase, and particularly in the days and hours before a paroxysmal explosion on March 15, 2007, indicate the persistence of the same gas source; and suggest that de-pressurization of the same 1–3 km deep magma storage zone could have been the trigger mechanism for the paroxysm itself.

Magma Ascent Rates

Volcanoes can erupt explosively, generating high columns of ash and occasional pyroclastic flows, or they can erupt slowly forming lava flows and domes. This variation reflects significant differences in mass eruption rate (e.g., Wilson et al. 1980). Mass eruption rates in turn are controlled by the rates of magma ascent through the volcanic conduit, and the conduit size. The magma ascent rate itself is a function of the pressure in the magma storage region, the physical properties of the magma, such as its density, viscosity and crystallinity, and the resistance to flow in the conduit that connects the magma storage zone to the surface (Papale and Dobran 1994; Mastin and Ghiorso 2001; Pinkerton et al. 2002; Sparks et al. 2006). A number of important characteristics of volcanic eruptions are affected or controlled by the rate at which magma ascends from depth. For example, the bubble content (i.e., vesicularity) and the degree of crystallization that develop in the melt phase can be significantly different in rapidly vs. slowly ascended magma. In fact, the inability of rapidly ascending magma to effectively lose exsolved gas may be one factor causing an eruption to change from effusive to explosive behavior, as recently documented for the Soufriere Hills eruption on Montserrat in the West Indies (Sparks et al. 1998). Therefore, in order to better understand the processes involved and the changes that occur in volcanic eruptions, it is important to quantify the rates at which different magmas rise to the surface. The fact that the rate of magma ascent controls a number of reactions that occur in volatile-rich magmas suggests several ways to study magma ascent rates using reaction data and theoretical flow models. One reaction that is a direct result of magma ascent is the exsolution of volatiles from the melt …

Petrology of ultramafic nodules from São Tomé Island, Cameroon Volcanic Line (oceanic sector)

Ultramafic xenoliths were found in recent alkali basalts from São Tomé Island. These include spinel peridotites (lherzolites, harzburgites and dunites) and pyroxenites (orthopyroxenites and clinopyroxenites). Textures and mineral compositions indicate that pyroxenites originated from crystal/liquid separation processes operating on magmas similar to those giving rise to their present host rocks whereas spinel peridotite xenoliths had an accidental origin; Fo (>89) and Ni (>0.36 wt.%) contents in olivines, Mg# (91–95) of orthopyroxenes and low Ti in clinopyroxene (primary crystals: TiO 2<0.06 wt.%) and in spinel (TiO 2<0.1 wt.%) are within the range reported for abyssal peridotites, indicating São Tomé spinel peridotites represent refractory residues of melting. Nevertheless, the lack of correlation between mineral chemistry and modal composition suggests that spinel peridotite xenoliths are not simple residues and were affected by infiltration of fluid/melts within the mantle. The wide temperature range obtained for spinel peridotites (700 to >1150 °C) is compatible with a long period of pre-entrainment cooling supporting Fitton's [Tectonophysics 94 (1983) 473] hypothesis that proposes oceanic lithosphere uprising in the Cameroon Volcanic Line prior to the initiation of the current thermal regime, related to São Tomé magmatism. The association of upper mantle (peridotite) xenoliths with igneous cumulates (pyroxenites) suggests that the spinel peridotite suite originated in the uppermost mantle above the São Tomé magma storage zone(s), probably in a region of high strain rate, near the boundary between the mantle and the overlying oceanic crust.

Temperature of Vesuvius magmas

Melt inclusions in Ca-pyroxenes from pyroclasts of several Vesuvius eruptions show that pyroxenes (Wo 46–48 -En 45–50 to Wo 50–52 -En 20–25 ) formed in shallow reservoirs from K-basalt and/or K-tephrite to K-phonolite magmas over a wide temperature range (800–1200 °C). The data set reflects the general modalities of magma evolution at Vesuvius, where the highly evolved compositions are generally reached under closed-conduit conditions, forcing magmatic crystallization under increasing activity of dissolved volatiles. These data allow the establishment of two empirical geothermometric laws that link temperature either to the equilibrium constant of melt-pyroxene pairs or to a compositional parameter of the melt (e.g., CaO). They are used to reconstruct the thermal and compositional evolution of Vesuvius magmas. This evolution is often affected by syneruptive mixing processes in the magma storage zone that alter preeruptive conditions. The method can have general applicability for the definition of empirical thermometric laws.