Active Magmatic System Research Articles

Reconciling Li and O diffusion in zircon with protracted magmatic crystal residence

Lithium and O in zircon have strong potential as diffusion chronometers for assessing thermal histories of magmatic and metamorphic systems, but their diffusion mechanisms remain controversial. Zircon populations erupted from El Chichon volcano, Chiapas, Mexico, often contain xenocrysts of detrital origin with juvenile overgrowth forming a natural diffusion boundary within individual crystals. High-spatial resolution O and Li isotope analyses, U–Pb and U–Th geochronology, and Li (along with Al and Y) scanning ion imaging were acquired to compare diffusion model timescales with those derived from radiometric dating. Oxygen isotopes in El Chichon zircon xenocrysts preserve source heterogeneities, with xenocrystic and juvenile domains of zircon being in O-isotopic disequilibrium with the melt at the time of eruption. Preservation of these heterogeneities is consistent with recently determined O-isotope diffusion parameters when crystal residence at magmatic temperatures is assumed over the 103–104 year timescales indicated by U–Th geochronology. Lithium concentration gradients across the same boundaries are sharp within the ~ 2–5 µm lateral resolution of the ion beam, whereas Li isotopic compositions for xenocrystic cores are homogeneous at δ7Li = + 7.8 ± 2.3‰ (1 standard error) and different from their putative source. Diffusive equilibration of Li isotopes in zircon is consistent with protracted magmatic crystal residence, but preservation of Li abundance heterogeneities is difficult to reconcile with evidence for recurrent high-temperature exposure of zircon in a highly active magmatic system. We speculate that Li partitioning in zircon is in part coupled to slow-diffusing trivalent cations, which can cause severe underestimation of diffusion timescales extracted from Li diffusion modeling.

A Recent Volcanic Eruption Discovered on the Central Mariana Back-Arc Spreading Center

Submarine volcanic eruptions are difficult to detect because they are hidden from view at the bottom of the ocean and far from land-based sensors. However, most of Earth’s volcanic activity is in the oceans along tectonic plate boundaries, and modern tools of oceanography now allow us to find and study recent eruptions in the deep sea. The first known historical eruption on the Mariana back-arc spreading center was discovered in December 2015 during exploration of the southern back-arc for new hydrothermal vent sites. A water-column survey along the axis of the back-arc showed hydrothermal plumes over the site characterized by low particle concentrations and relatively high reduced chemical anomalies. A dive with the autonomous underwater vehicle Sentry collected high-resolution (1 m) multibeam sonar bathymetry over the site, followed by a near-bottom photographic survey of a smaller area. The photo survey revealed the presence of a pristine, dark, glassy lava flow on the seafloor with no sediment cover. Venting of milky hydrothermal fluid indicated that the lava flow was still warm and therefore very young. A comparison of multibeam sonar bathymetry collected by R/V Falkor in December 2015, to the most recent previous survey of the area by R/V Melville in February 2013, revealed large depth changes in the same area, effectively bracketing the timing of the eruption within a window of less than 3 years. The bathymetric comparison shows the eruption produced a string of lava flows with maximum thicknesses of 40–138 m along a distance of 7.3 km (from latitude 15∘22.3′ to 15∘26.3′N) between depths of 4050–4450 m bsl (meters below sea level), making this the deepest known historical submarine volcanic eruption on Earth. The cross-axis width of the lava flows is 200–800 m. The Sentry bathymetry shows that the new lava flows are constructed of steep-sided hummocky pillow mounds and are surrounded by older flows with similar morphology. In April and December 2016, two dives were made on the new lava flows by remotely operated vehicles Deep Discoverer and SuBastian. The pillow lavas have many small glassy buds on the steep flanks of the mounds, locally thick accumulations of hydrothermal sediment near the tops of mounds, and small cones of radiating pillows at their summits. The 2015–2016 observations show a rapidly declining hydrothermal system on the lava flows, suggesting that the eruption had occurred only months before its discovery in December 2015. The morphology of the pillow lavas is similar to other historical eruption sites, so the greater depth and ambient pressure of this site had no apparent effect on the processes of lava extrusion and emplacement. This study reveals that some segments of the Mariana back-arc have active magmatic systems despite the relatively low spreading rate, and that other eruptions are possible in the near future.

Low-temperature crystallization of granites and the implications for crustal magmatism.

The structure and composition of granites provide clues to the nature of silicic volcanism, the formation of continents, and the rheological and thermal properties of the Earth's upper crust as far back as the Hadean eon during the nascent stages of the planet's formation1-4. The temperature of granite crystallization underpins our thinking about many of these phenomena, but evidence is emerging that this temperature may not be well constrained. The prevailing paradigm holds that granitic mineral assemblages crystallize entirely at or above about 650-700 degrees Celsius5-7. The granitoids of the Tuolumne Intrusive Suite in California tell a different story. Here we show that quartz crystals in Tuolumne samples record crystallization temperatures of 474-561 degrees Celsius. Titanium-in-quartz thermobarometry and diffusion modelling of titanium concentrations in quartz indicate that a sizeable proportion of the mineral assemblage of granitic rocks (for example, more than 80 per cent of the quartz) crystallizes about 100-200 degrees Celsius below the accepted solidus. This has widespread implications. Traditional models of magma formation require high-temperature magma bodies, but new data8,9 suggest that volcanic rocks spend most of their existence at low temperatures; because granites are the intrusive complements of volcanic rocks, our downward revision of granite crystallization temperatures supports the observations of cold magma storage. It also affects the link between volcanoes, ore deposits and granites: ore bodies are fed by the release of fluids from granites below them in the crustal column; thus, if granitic fluids are hundreds of degrees cooler than previously thought, this has implications for research on porphyry ore deposits. Geophysical interpretations of the thermal structure of the crust and the temperature of active magmatic systems will also be affected.

The Acoculco Caldera Complex magmas: Genesis, evolution and relation with the Acoculco geothermal system

The Acoculco Caldera Complex (ACC) is located in the eastern part of the Trans-Mexican Volcanic Belt, México. The ACC was formed 2.7 Ma, and since then, volcanic activity has persisted until 0.06 Ma, through the emission of domes, cinder cones, fissure lava flows and two ignimbrite eruptions dated at 1.2 and 0.65 Ma. The pervasive hydrothermal alteration in the central part of the ACC has motivated considerable geothermal exploration, however, none of it assessed the relation between the magmatic heat supply and the evolution of magmas, including their reservoir depth.Magmatic processes were investigated with petrography, major oxides-trace-element geochemistry, mineral chemistry and isotopic analysis. In addition, we performed a series of hydrothermal experiments in order to constrain the storage depth for the magma tapped 1.2 Ma during a reactivation of the caldera. We investigated the origin of magmas tapped during the eruptive history of the ACC, and the magmatic processes (magma mixing, assimilation and crystallization) that modified these magmas.After the caldera collapse 2.7 Ma, the local stress field was probably modified and allowed the ascent of peralkaline magmas through new plumbing systems. Such magmas mixed with calc-alkaline magmas and formed the post-caldera volcanism. Influx of new magma is considered as the probable heat supply that maintains the active magmatic system, whereas assimilation has been negligible, but yields insights about the depth at which processes have occurred. Our work suggests that there are either shallow storage zones (200–500 bars) where magmas crystallize and either erupt or stall and cool, or there are intrusive magmas unrelated to the ACC suite that have accumulated nearby and heat the reservoir magmas.

Seismic evidence for arc segmentation, active magmatic intrusions and syn-rift fault system in the northern Ryukyu volcanic arc

Tectonic and volcanic structures of the northern Ryukyu arc are investigated on the basis of multichannel seismic (MCS) reflection data. The study area forms an active volcanic front in parallel to the non-volcanic island chain in the eastern margin of the Eurasian plate and has been undergoing regional extension on its back-arc side. We carried out a MCS reflection experiment along two across-arc lines, and one of the profiles was laid out across the Tokara Channel, a linear bathymetric depression which demarcates the northern and central Ryukyu arcs. The reflection image reveals that beneath this topographic valley there exists a ~ 3-km-deep sedimentary basin atop the arc crust, suggesting that the arc segment boundary was formed by rapid and focused subsidence of the arc crust driven by the arc-parallel extension. Around the volcanic front, magmatic conduits represented by tubular transparent bodies in the reflection images are well developed within the shallow sediments and some of them are accompanied by small fragments of dipping seismic reflectors indicating intruded sills at their bottoms. The spatial distribution of the conduits may suggest that the arc volcanism has multiple active outlets on the seafloor which bifurcate at crustal depths and/or that the location of the volcanic front has been migrating trenchward over time. Further distant from the volcanic front toward the back-arc (> 30 km away), these volcanic features vanish, and alternatively wide rift basins become predominant where rapid transitions from normal-fault-dominant regions to strike-slip-fault-dominant regions occur. This spatial variation in faulting patterns indicates complex stress regimes associated with arc/back-arc rifting in the northern Okinawa Trough.

Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey

Los Humeros Geothermal Field (LHGF) is one of four geothermal fields currently operating in Mexico, in exploitation since 1990. Located in a caldera complex filled with very low-permeability rhyolitic ignimbrites that are the reservoir cap-rock, recharge of the geothermal field is both limited and localized. Because of this, planning of any future geothermal exploitation must be based on a clear understanding of the fluid circulation. To this end, a first noble gas survey was carried out in which twenty-two production wells were sampled for He, Ne, Ar, Kr, and Xe isotope analysis. Air-corrected 3He/4He ratios (Rc) measured in the fluid, normalized to the helium atmospheric ratio (Ra; 1.384×10−6), are consistently high across the field, with an average value of 7.03±0.40 Ra. This value is close to that of the sub-continental upper mantle, indicating that LHGF mines heat from an active magmatic system. Freshwater recharge does not significantly affect He isotopic ratios, contributing 1–10% of the total fluid amount. The presence of radiogenic 40Ar* in the fluid suggests a fossil fluid component that might have circulated within the metacarbonate basement with radiogenic argon produced from detrital dispersed illite. Solubility-driven elemental fractionation of Ne/Ar, Kr/Ar, and Xe/Ar confirm extreme boiling in the reservoir. However, a combined analysis of these ratios with 40Ar/36Ar reveals mixing with an air component, possibly introduced by re-injected geothermal fluids.

3D gravity inversion and thermodynamic modelling reveal properties of shallow silicic magma reservoir beneath Laguna del Maule, Chile

Active, large volume, silicic magma systems are potentially the most hazardous form of volcanism on Earth. Knowledge of the location, size, and physical properties of silicic magma reservoirs, is therefore important for providing context in which to accurately interpret monitoring data and make informed hazard assessments. Accordingly, we present the first geophysical image of the Laguna del Maule volcanic field magmatic system, using a novel 3D inversion of gravity data constrained by thermodynamic modelling. The joint analysis of gravity and thermodynamic data allows for a rich interpretation of the magma system, and highlights the importance of considering the full thermodynamic effects on melt density, when interpreting gravity models of active magmatic systems. We image a 30 km3, low density, volatile rich magma reservoir, at around 2 km depth, containing at least 85% melt, hosted within a broader 115 km3 body interpreted as wholly or partially crystallised (>70% crystal) cumulate mush. Our model suggests a magmatic system with shallow, crystal poor magma, overlying deeper, crystal rich magma. Even though a large density contrast (−600 kg/m3) with the surrounding crust exists, the lithostatic load is 50% greater than the magma buoyancy force, suggesting buoyancy alone is insufficient to trigger an eruption. The reservoir is adjacent to the inferred extension of the Troncoso fault and overlies the location of an intruding sill, driving present day deformation. The reservoir is in close proximity to the 2.0 km3 Nieblas (rln) eruption at 2–3 ka, which we calculate tapped approximately 7% of the magma reservoir. However, we suggest that the present day magma system is not large enough to have fed all post-glacial eruptions, and that the location, or size of the system may have migrated or varied over time, with each eruption tapping only a small aliquot of the available magma. The presence of a shallow reservoir of volatile rich, near liquidus magma, in close proximity to regional scale faulting, has important implications for volcano monitoring and hazard mitigation.

Body and surface wave reconstruction from seismic noise correlations between arrays at Piton de la Fournaise volcano

AbstractBody wave reconstruction from ambient seismic noise correlations is an important step toward improving volcano imaging and monitoring. Here we extract body and surface waves that propagate in Piton de la Fournaise volcano on La Réunion island using ambient noise cross correlation and array‐processing techniques. Ambient noise was continuously recorded at three dense arrays, each comprising 49 geophones. To identify and enhance the Green's function from the ambient noise correlation, we apply a double beamforming (DBF) technique between the array pairs. The DBF allows us to separate surface and body waves, direct and reflected waves, and multipathing waves. Based on their azimuths and slownesses, we successfully extract body waves between all the combinations of arrays, including the wave that propagates through the active magmatic system of the volcano. Additionally, we identify the effects of uneven noise source distribution and interpret the surface wave reflections.

3-D analysis and interpretation of magnetotelluric data from the Aluto-Langano geothermal field, Ethiopia

The Main Ethiopian Rift Valley encompasses a number of volcanoes, which are known to be actively deforming with reoccurring periods of uplift and setting. One of the regions where temporal changes take place is the Aluto volcanic complex. It hosts a productive geothermal field and the only currently operating geothermal power plant of Ethiopia. We carried out magnetotelluric (MT) measurements in early 2012 in order to identify the source of unrest. Broad-band MT data (0.001-1000 s) have been acquired at 46 sites covering the expanse of the Aluto volcanic complex with an average site spacing of 1 km. Based on this MT data it is possible to map the bulk electrical resistivity of the subsurface down to depths of several kilometres. Resistivity is a crucial geophysical parameter in geothermal exploration as hydrothermal and magmatic reservoirs are typically related to low resistive zones, which can be easily sensed by MT. Thus by mapping the electrical conductivity one can identify and analyse geothermal systems with respect to their temperature, extent and potential for production of energy. 3-D inversions of the observed MT data from Aluto reveal the typical electrical conductivity distribution of a high-enthalpy geothermal system, which is mainly governed by the hydrothermal alteration mineralogy. The recovered 3-D conductivity models provide no evidence for an active deep magmatic system under Aluto. Forward modelling of the tippers rather suggest that occurrence of melt is predominantly at lower crustal depths along an off-axis fault zone a few tens of kilometres west of the central rift axis. The absence of an active magmatic system implies that the deforming source is most likely situated within the shallow hydrothermal system of the Aluto-Langano geothermal field.

Melt evolution and residence in extending crust: Thermal modeling of the crust and crustal magmas

Tectonic extension and magmatism often act in concert to modify the thermal, mechanical, and chemical structure of the crust. Quantifying the effects of extension and magma flux on melting relationships in the crust is fundamental to determining the rate of crustal melting versus fractionation, magma residence time, and the growth of continental crust in rift environments. In order to understand the coupled control of tectonic extension and magma emplacement on crustal thermal evolution, we develop a numerical model that accounts for extension and thermal-petrographic processes in diverse extensional settings. We show that magma flux exerts the primary control on melt generation and tectonic extension amplifies the volume of melt residing in the crustal column. Diking into an extending crust produces hybrid magmas composed of 1) residual melt remaining after partial crystallization of basalt (mantle-derived melt) and 2) melt from partial melting of the crust (crustal melt). In an extending crust, mantle-derived melts are more prevalent than crustal melts across a range of magma fluxes, tectonic extension rates, and magmatic water contents. In most of the conditions, crustal temperatures do not reach their solidus temperatures to initiate partial melting of these igneous lithologies. Energy balance calculations show that the total enthalpy transported by dikes is primarily used for increasing the sensible heat of the cold surrounding crust with little energy contributing to latent heat of melting the crust (maximum crustal melting efficiency is 6%). In the lower crust, an extensive mush region develops for most of the conditions. Upper crustal crystalline mush is produced by continuous emplacement of magma with geologically reasonable flux and extension rates on timescales of 106 yr. Addition of tectonic effects and non-linear melt fraction relationships demonstrates that the magma flux required to sustain partially molten regions in the upper crust is within the range of estimates of magmatic flux in many rifting regions (∼10−4 to 10−3 km3/yr) and at least an order of magnitude lower than previous modeling estimates. Our results demonstrate the importance of tectonics in augmenting melt production, composition, and crustal evolution in active magmatic systems.

Chapter 14 Continuous and campaign-style gravimetric investigations on Montserrat 2006 to 2009

Abstract Gravimetric time series can provide vital clues about subsurface dynamics associated with active volcanism. Here, we report on continuous and campaign-style gravimetric observations on Montserrat between 2006 and 2009. More than 240 days of continuous gravimetric records enabled us to derive a first local joint solid Earth tides and ocean loading model for Montserrat, and we report the tidal harmonics for 14 major wave groups. Compared to global predictions, the new model (MTY11) achieves an up to one order of magnitude better precision over diurnal and semi-diurnal frequencies. We anticipate that the model will help reduce the effects of tidal perturbations on other geodetic time series recorded on Montserrat. Abrupt variations in gravity accompanied Vulcanian explosions and probably reflect the response of a shallow aquifer to stress changes during pressurization and depressurization of the subvolcanic plumbing system. Campaign data enabled the quantification of mass variations during a cycle of activity including dome formation and repose. Both forward and inverse modelling of the spatio-temporal time series indicates that the source of the recorded gravity variations is situated beneath central Montserrat. Our favourite interpretation of the campaign data is that the gravity variations reflect volcano-tectonic interaction beneath the Centre Hills of Montserrat that are triggered by changes in the active magmatic system of Soufrière Hills Volcano (SHV). We also discuss our findings on subsurface mass variations in relation to annual precipitation records and active dome formation. Both continuous and discrete gravimetric observations indicate coupling between the dominant magmatic sources responsible for the ongoing eruption at SHV and shallow-seated local sources such as aquifers and fluid-saturated fault-damage zones. Our investigations demonstrate the value of including gravimetric observations over a wide frequency range for volcanic system characterization in a volcanic island arc setting. Supplementary material: Details on the inversion routine of the explorative source model GROWTH 2.0 and the resulting images from its application to time-lapse gravity data are available at http://www.geolsoc.org.uk/SUP18701 .

Petrological cannibalism: the chemical and textural consequences of incremental magma body growth

The textures of minerals in volcanic and plutonic rocks testify to a complexity of processes in their formation that is at odds with simple geochemical models of igneous differentiation. Zoning in plagioclase feldspar is a case in point. Very slow diffusion of the major components in plagioclase means that textural evidence for complex magmatic evolution is preserved, almost without modification. Consequently, plagioclase affords considerable insight into the processes by which magmas accumulate in the crust prior to their eventual eruption or solidification. Here, we use the example of the 1980–1986 eruptions of Mount St. Helens to explore the causes of textural complexity in plagioclase and associated trapped melt inclusions. Textures of individual crystals are consistent with multiple heating and cooling events; changes in total pressure (P) or volatile pressure ( $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ ) are less easy to assess from textures alone. We show that by allying textural and chemical analyses of plagioclase and melt inclusions, including volatiles (H2O, CO2) and slow-diffusing trace elements (Sr, Ba), to published experimental studies of Mount St. Helens magmas, it is possible to disambiguate the roles of pressure and temperature to reconstruct magmatic evolutionary pathways through temperature–pressure–melt fraction (T– $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ –F) space. Our modeled crystals indicate that (1) crystallization starts at $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ > 300 MPa, consistent with prior estimates from melt inclusion volatile contents, (2) crystal cores grow at $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ = 200–280 MPa at F = 0.65–0.7, (3) crystals are transferred to $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ = 100–130 MPa (often accompanied by 10–20 °C of heating), where they grow albitic rims of varying thicknesses, and (4) the last stage of crystallization occurs after minor heating at $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ ~ 100 MPa to produce characteristic rim compositions of An50. We hypothesize that modeled $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ decreases in excess of ~50 MPa most likely represent upward transport through the magmatic system. Small variations in modeled $$P_{{{\text{H}}_{ 2} {\text{O}}}}$$ , in contrast, can be effected by fluxing the reservoir with CO2-rich vapors that are either released from deeper in the system or transported with the recharge magma. Temperature fluctuations of 20–40 °C, on the other hand, are an inevitable consequence of incremental, or pulsed, assembly of crustal magma bodies wherein each pulse interacts with ancestral, stored magmas. We venture that this “petrological cannibalism” accounts for much of the plagioclase zoning and textural complexity seen not only at Mount St. Helens but also at arc magmas generally. More broadly we suggest that the magma reservoir below Mount St. Helens is dominated by crystal mush and fed by frequent inputs of hotter, but compositionally similar, magma, coupled with episodes of magma ascent from one storage region to another. This view both accords with other independent constraints on the subvolcanic system at Mount St. Helens and supports an emerging view of many active magmatic systems as dominantly super-solidus, rather than subliquidus, bodies.

Unusual lapilli tuff ejecta erupted at Stromboli during the 15 March 2007 explosion shed light on the nature and thermal state of rocks forming the crater system of the volcano

Textural and mineralogical study of high-temperature, angular blocks erupted during the Stromboli explosion of 15 March 2007 was used to make inferences on the nature and thermal state of rocks forming the subsurface of the volcano' summit crater terrace. The studied ejecta consist of lapilli tuff that formed as a result of the transformation and high temperature induration (sintering) of the basaltic scoriae, lapilli and ash originally accumulated as loose tephra during the current activity of the volcano. The main processes leading to the tephra transformation were investigated through microstructural observations, mineral and glass analyses (SEM-EDS and EMP analyses). Investigations revealed that subsolidus reactions and partial melting of the tephra occurred, at temperatures higher than 600°C and under variable fO2 conditions from QFM to HM buffering curves. In some blocks, evidence of high-T reheating and partial melting at the expense of secondary hydrothermal minerals was also observed. In order to track the subsolidus reheating history of the basaltic pyroclasts, a detailed study of the pseudomorphic phases and reactions after olivine, driven by iron oxidation under high-T conditions, was performed. The observed mineralogical transformation suggests that the lapilli tuff material, originating from the burial of tephra routinely accumulated by persistent Strombolian explosions within the crater terrace, were in some cases altered by the circulation of acidic fluids and were in any case reheated due to isotherm rise forced by high heat flux and gas streaming delivered by the underlying magma system. It is worth noting that the ejection of these unusual volcanic lithotypes was possible because a few days before the 15 March 2007 event, the craters were clogged with lapilli tuff material that slid into the crater bottom between 7 and 9 March. Findings of this study suggest that the scattered permanently active vents and shallow conduits of Stromboli are surrounded sideways and underneath the crater terrace, by a fairly large volume of high temperature rocks with variable degree of compaction, sintering up to partially melted. Such a spectrum of rock types is in good agreement with the conceptual model of prominent thermal zoning all around (sideway and upwards) the active magmatic system. We speculate that continuous migration upwards of isotherms led to transformation and partial melting of the normal Strombolian tephra.

Fluid geochemistry of hydrothermal systems in the Arica-Parinacota, Tarapacá and Antofagasta regions (northern Chile)

We investigate the chemical and isotopic composition of water and gas thermal discharges from six hydrothermal systems in the Tarapacà and Antofagasta regions (northern Chile): Surire, Puchuldiza-Tuja, Pampa Lirima, Pampa Apacheta, El Tatio and Torta de Tocorpuri, to determine the chemical–physical conditions at the fluid source. The chemical facies of the thermal discharges vary from Na +–Cl − (El Tatio, Puchuldiza-Tuja and part of Surire where SO 4 2 −-rich waters also occur) to Na +(Ca 2+)–Cl −(SO 4 2 −) (Pampa Lirima), Ca 2+–SO 4 2 −(HCO 3 −) (Torta de Tocorpuri) and Ca 2+–SO 4 2 − (Pampa Apacheta). The gas seeps are characterized by the dominance of CO 2, H 2S and CH 4 with N 2/Ar ratios that occasionally are up to 450. Significant amounts of SO 2 and HCl are present at Pampa Apacheta. Water and gas geothermometry suggests that El Tatio and Puchuldiza-Tuja fluid reservoirs have relatively high reservoir equilibrium temperature (up to 270 °C). Gases from Pampa Apacheta apparently equilibrated at unusually high temperature for hydrothermal fluids (up to 350 °C), being likely related to an active magmatic system, as testified by the presence of highly acidic gas compounds such as SO 2 and HCl. On the contrary, low equilibrium temperatures were calculated for the Surire fluids (< 200 °C), suggesting interactions of deep-originated fluids with meteoric water at shallow depth. Shallow, low temperature aquifers mask any signal of deep fluids in the Pampa Lirima and Torta de Tocorcupi thermal discharges. Despite the variations in temperatures, He isotopic compositions and redox conditions of the fluid reservoirs consistently indicate a significant contribution from magmatic degassing. Although geophysical studies are necessary to better constrain the geothermal potential of northern Chile, the geothermometric estimations carried out in the present study indicate that El Tatio, Pampa Apacheta, Surire and Puchuldiza-Tuja can be regarded as the most promising areas and deserve more detailed geological investigations.

Coda-Wave Attenuation Imaging of Galeras Volcano, Colombia

Abstract The spatial variation of S -wave coda attenuation () in the Galeras volcano region is analyzed. The study region is at present an active magmatic system situated in the southwestern Colombian Andes. is estimated using seismograms from 435 volcano-tectonic earthquakes recorded at 31 stations of the Galeras seismograph network. is then imaged using a 3D spatial stacking procedure. The technique is based on the assumption of uniform distribution of over a spheroidal shell with its volume determined by the associated source–receiver distance. The resultant tomograms also show frequency dependence, which is interpreted in terms of the scale of the heterogeneities producing the scattering. High-attenuation anomalies are detected at high frequencies. Synthetic tests indicate the validity of the inversion technique.

Multiple inflation and deflation events at Kenyan volcanoes, East African Rift

The presence or absence of magma exerts a fundamental control on the distribution of strain in continental rift zones, yet the time scales of magma intrusion remain unconstrained. Using more than a decade of measurements from interferometric synthetic aperture radar (InSAR), we detected geodetic activity at four of the eleven central rift volcanoes in the Kenyan sector of the East African Rift. Subsidence of 2–5 cm occurred at Suswa and Menengai over the period 1997–2000, ~9 cm of uplift was recorded at Longonot in 2004–2006, and ~21 cm of uplift occurred at Paka during 2006–2007. The deformation is episodic, and no deformation was observed at these volcanoes during other time periods. Preceding the infl ation at Paka, we observed transient uplift and subsidence of a second, nearby source, likely associated with fl ow through a complex plumbing system. The best-fi tting source models for each episode include infl ation or defl ation of a horizontal penny-shaped crack at a depth of 2–5 km. The episodic nature of the activity, its lack of correlation with seasons, and the preferred source geometry are all consistent with activity in the volatile-rich cap to a crystal-rich magma chamber beneath each of the four volcanoes. The presence of active magmatic systems beneath more than 40% of the volcanoes, and the 2007–2008 explosive eruptions at nearby Oldoinyo Lengai volcano, have implications for models of continental rifting, caldera volcanoes, geothermal resources, and volcanic and seismic hazard in rift zones.

Implications of incremental emplacement of magma bodies for magma differentiation, thermal aureole dimensions and plutonism–volcanism relationships

Field observations and geophysical data indicate that many igneous bodies grow by amalgamation of successive magma pulses that commonly take the shape of horizontal sheets (sills). Emplacement styles and emplacement rates of magma bodies have fundamental implications on magma differentiation, country rock metamorphism and assimilation, and for the formation of large magma chambers in the upper crust. When a magma body begins to grow by slow accretion of sills, each successive intrusion solidifies before the injection of the next one. When the system is thermally mature, sill temperatures equilibrate above the solidus, melts accumulate and older sills can re-melt. The time needed for each magma injection to cool down and equilibrate with its surrounding is short relatively to the total emplacement time of the body. The transition from a mafic crystal-poor magma to a partially molten rock that retains a highly differentiated melt is fast, whereas the resulting evolved residual melt can reside in the crust for protracted periods. As long as temperatures in the system are relatively low, highly differentiated melts are generated, which may explain the bi-modal character and the absence of intermediate compositions in some magmatic provinces. The level of emplacement of successive magma pulses controls the shape of the thermal anomaly associated with the magma body growth. Metamorphism, partial melting and assimilation of the country rock are favoured if successive magma sheets are emplaced at or close to the country rock–magma body boundary. If the magma emplacement rate is low, the size of the thermal aureole is controlled by the size of one pulse and not by the size of the entire igneous body. Understanding emplacement of magma bodies is fundamental for our understanding of the plutonism–volcanism relationship. Magma emplacement rates of several centimetres per year are needed for a magma body to evolve into a large magma chamber able to feed large silicic explosive eruptions. The time-averaged emplacement rates of plutons are lower than this critical emplacement rate. Eruptions of 100s to 1000s cubic kilometres of silicic products show that such high volumes of magmas can accumulate in the upper crust. This suggests that the emplacement of magma bodies is a multi-timescale process with the development of large magma chambers corresponding to the highest magma fluxes. Because they control magmatic processes and the impact of magma intrusion on the country rock, future studies should focus on magma emplacement rates and on magma emplacement geometries. These studies should integrate field observation on plutons and geophysical data on active magmatic systems, coupled with laboratory experiments and numerical simulations.

Shallow magma pathway geometry at Mt. Etna volcano

A fundamental goal of volcano seismology is to understand the dynamics of active magmatic systems in order to assess eruptive behavior and the associated hazard. Imaging of magma conduits, quantification of magma transport, and investigation of long‐period seismic sources, together with their temporal variations, are crucial for the comprehension of eruption‐triggering mechanisms. At Mt. Etna volcano, several intense episodes of tremor activity were recorded during 2007, in association with strombolian activity and/or intense fire fountaining episodes occurring from the South East Crater (SEC). The locations of the tremor sources and of the long‐period seismic events are used here to constrain both the area and the depth range of magma degassing, highlighting the geometry of the shallow conduits feeding SEC. The imaged conduits consist of two connected resonating dike‐like bodies, NNW‐SSE and NW–SE oriented, extending from sea level to the surface. In addition, we show how tremor, long‐period (LP), and very‐long‐period (VLP) event locations and signatures reflect pressure fluctuations in the plumbing system associated with the ascent/discharge of gas‐rich magma linked to the lava fountains. The evidence here reported, also corroborated by ground deformation variations, can help develop a better prediction and early warning system for those eruptions (effusive or explosive) that apparently manifest no clear precursors.

WHITE, J. D. L., SMELLIE, J. L. &amp; CLAGUE, D. A. (eds) 2005. Explosive Subaqueous Volcanism. Geophysical Monograph Series Vol. 140. x + 379 pp. Washington DC: American Geophysical Union. Price US $90.00 (hard covers); AGU members' price US $63.00. ISBN 0 87590 999 X

WHITE, J. D. L., SMELLIE, J. L. &amp; CLAGUE, D. A. (eds) 2005. Explosive Subaqueous Volcanism. Geophysical Monograph Series Vol. 140. x + 379 pp. Washington DC: American Geophysical Union. Price US 63.00. ISBN 0 87590 999 X - Volume 143 Issue 6

The Managua Graben and Las Sierras-Masaya volcanic complex (Nicaragua); pull-apart localization by an intrusive complex: results from analogue modeling

There is a well-documented association between pull-apart basins in strike-slip zones and large volcanic caldera complexes. Las Sierras-Masaya volcanic complex, Nicaragua, is a large basaltic lava and ignimbrite shield with nested calderas. The widest caldera is partly underlain by a large and dense intrusive complex, evidenced by a positive gravity anomaly. The Las Sierras Caldera, relating to this cumulate complex, is less than 30,000 years old and is probably still hot and ductile. The inner Masaya Caldera hosts an active magmatic system. The volcanic complex is in a dextral transtensional tectonic context of the Nicaraguan Depression. The highly active Managua Graben is on the northern part of the volcano. We speculate that the graben and volcano are linked tectonically, with the graben initiating in response of a regional stress field modified around the dense, ductile intrusive complex. Previous field work and reappraisal of structures with digital elevation model morphological analysis show that the volcano is surrounded by a rhombic fault pattern that may form a nascent pull-apart basin. We have done scaled analogue models to test the effect of intrusion density and rheology anomalies on strike-slip fault geometries. The models show that intrusion density variations alone do not significantly change fault patterns. In contrast, ductile rocks (silicone in the models as analogue for hot mafic intrusive rocks) markedly alter strike-slip fault patterns. In transtension the presence of a ductile intrusion causes the formation of a pull-apart, while in pure strike-slip or transpression, uplift and thrusting is generated. Pull-apart and uplift structures are rhomb-shaped even when the ductile inclusion is circular. We conclude that a pull-apart is developing at Las Sierras-Masaya volcanic complex in response to the transtensive regional deformation regime and the dense, ductile intrusive complex. We suggest that the volcano and graben are one dynamic system and should be monitored as one entity.