Central Volcanic Complex Research Articles

Prediction of vertical gradient of gravity and its significance for volcano monitoring – example from Teide volcano

Abstract We present a detailed calculation of the topographic contribution to the vertical gradient of gravity (VGG) based on high-resolution digital elevation model (DEM) and new developed software (Toposk) for the purpose of predicting the actual VGGs in the field. The calculations presented here were performed for the Central Volcanic Complex (CVC) of Tenerife. We aimed at identifying the most extreme VGGs within the CVC, as well as predicting the VGGs at benchmarks of the former microgravity/deformation network set up to monitor the 2004/5 unrest. We have carried out an observational campaign in June 2016 to verify the predicted VGG values, both the extreme ones and those at the benchmarks. The comparison between the predicted and the in-situ verified VGGs is presented here. We demonstrate the sensitivity of the VGG prediction to the choice of the topo-rock density, which is inherent to the volcanic areas with high variability of rock densities. We illustrate the significance of the use of actual VGG in volcano monitoring microgravimetric surveys on a couple of benchmarks of the CVC network.

Deformation induced topographic effects in inversion of temporal gravity changes: First look at Free Air and Bouguer terms

Abstract We review here the gravitational effects on the temporal (time-lapse) gravity changes induced by the surface deformation (vertical displacements). We focus on two terms, one induced by the displacement of the benchmark (gravity station) in the ambient gravity field, and the other imposed by the attraction of the masses within the topographic deformation rind. The first term, coined often the Free Air Effect (FAE), is the product of the vertical gradient of gravity (VGG) and the vertical displacement of the benchmark. We examine the use of the vertical gradient of normal gravity, typically called the theoretical or normal Free Air Gradient (normal FAG), as a replacement for the true VGG in the FAE, as well as the contribution of the topography to the VGG. We compute a topographic correction to the normal FAG, to offer a better approximation of the VGG, and evaluate its size and shape (spatial behavior) for a volcanic study area selected as the Central Volcanic Complex (CVC) on Tenerife, where this correction reaches 77% of the normal FAG and varies rapidly with terrain. The second term, imposed by the attraction of the vertically displaced topo-masses, referred to here as the Topographic Deformation Effect (TDE) must be computed by numerical evaluation of the Newton volumetric integral. As the effect wanes off quickly with distance, a high resolution DEM is required for its evaluation. In practice this effect is often approximated by the planar or spherical Bouguer deformation effect (BDE). By a synthetic simulation at the CVC of Tenerife we show the difference between the rigorously evaluated TDE and its approximation by the planar BDE. The complete effect, coined here the Deformation Induced Topographic Effect (DITE) is the sum of FAE and TDE. Next we compare by means of synthetic simulations the DITE with two approximations of DITE typically used in practice: one amounting only to the first term in which the VGG is approximated by normal FAG, the other adopting a Bouguer corrected normal FAG (BCFAG).

On inverting gravity changes with the harmonic inversion method: Teide (Tenerife) case study

Abstract Here we investigate the applicability of the harmonic inversion method to time-lapse gravity changes observed in volcanic areas. We carry out our study on gravity changes occured over the period of 2004–2005 during the unrest of the Central Volcanic Complex on Tenerife, Canary Islands. The harmonic inversion method is unique in that it calculates the solution of the form of compact homogeneous source bodies via the mediating 3-harmonic function called quasigravitation. The latter is defined in the whole subsurface domain and it is a linear integral transformation of the surface gravity field. At the beginning the seeds of the future source bodies are introduced: these are quasi-spherical bodies located at the extrema of the quasigravitation (calculated from the input gravity data) and their differential densities are free parameters preselected by the interpreter. In the following automatic iterative process the source bodies change their size and shape according to the local values of quasigravitation (calculated in each iterative step from the residual surface gravity field); the process stops when the residual surface gravity field is sufficiently small. In the case of inverting temporal gravity changes, the source bodies represent the volumetric domains of temporal mass-density changes. The focus of the presented work is to investigate the dependence of the size and shape of the found source bodies on their differential densities. We do not aim here (yet) at interpreting the found solutions in terms of volcanic processes associated with intruding or rejuvenating magma and/or migrating volatiles.

Hydrogeology of the Krafla geothermal system, northeast Iceland

AbstractThe Krafla geothermal system is located in Iceland's northeastern neovolcanic zone, within the Krafla central volcanic complex. Geothermal fluids are superheated steam closest to the magma heat source, two‐phase at higher depths, and sub‐boiling at the shallowest depths. Hydrogen isotope ratios of geothermal fluids range from −87‰, equivalent to local meteoric water, to −94‰. These fluids are enriched in 18O relative to the global meteoric line by +0.5–3.2‰. Calculated vapor fractions of the fluids are 0.0–0.5 wt% (~0–16% by volume) in the northwestern portion of the geothermal system and increase towards the southeast, up to 5.4 wt% (~57% by volume). Hydrothermal epidote sampled from 900 to 2500 m depth has δD values from −127 to −108‰, and δ18O from −13.0 to −9.6‰. Fluids in equilibrium with epidote have isotope compositions similar to those calculated for the vapor phase of two‐phase aquifer fluids. We interpret the large range in δDEPIDOTE and δ18OEPIDOTE across the system and within individual wells (up to 7‰ and 3.3‰, respectively) to result from variable mixing of shallow sub‐boiling groundwater with condensates of vapor rising from a deeper two‐phase reservoir. The data suggest that meteoric waters derived from a single source in the northwest are separated into the shallow sub‐boiling reservoir, and deeper two‐phase reservoir. Interaction between these reservoirs occurs by channelized vertical flow of vapor along fractures, and input of magmatic volatiles further alters fluid chemistry in some wells. Isotopic compositions of hydrothermal epidote reflect local equilibrium with fluids formed by mixtures of shallow water, deep vapor condensates, and magmatic volatiles, whose ionic strength is subsequently derived from dissolution of basalt host rock. This study illustrates the benefits of combining phase segregation effects in two‐phase systems during analysis of wellhead fluid data with stable isotope values of hydrous alteration minerals when evaluating the complex hydrogeology of volcano‐hosted geothermal systems.

High-Precision U-Pb Zircon Chronostratigraphy of the Mount Read Volcanic Belt in Western Tasmania, Australia: Implications for VHMS Deposit Formation

Research Article| March 01, 2015 High-Precision U-Pb Zircon Chronostratigraphy of the Mount Read Volcanic Belt in Western Tasmania, Australia: Implications for VHMS Deposit Formation James K. Mortensen; James K. Mortensen † 1Pacific Centre for Isotopic and Geochemical Research, Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, Canada V6T 1Z4 †Corresponding author: e-mail, jmortensen@eos.ubc.ca Search for other works by this author on: GSW Google Scholar J. Bruce Gemmell; J. Bruce Gemmell 2CODES ARC Centre of Excellence in Ore Deposits, Private Bag 126, University of Tasmania, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar Andrew W. McNeill; Andrew W. McNeill 2CODES ARC Centre of Excellence in Ore Deposits, Private Bag 126, University of Tasmania, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar Richard M. Friedman Richard M. Friedman 1Pacific Centre for Isotopic and Geochemical Research, Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, Canada V6T 1Z4 Search for other works by this author on: GSW Google Scholar Economic Geology (2015) 110 (2): 445–468. https://doi.org/10.2113/econgeo.110.2.445 Article history received: 20 Aug 2012 accepted: 19 Jun 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James K. Mortensen, J. Bruce Gemmell, Andrew W. McNeill, Richard M. Friedman; High-Precision U-Pb Zircon Chronostratigraphy of the Mount Read Volcanic Belt in Western Tasmania, Australia: Implications for VHMS Deposit Formation. Economic Geology 2015;; 110 (2): 445–468. doi: https://doi.org/10.2113/econgeo.110.2.445 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The Middle Cambrian Mount Read Volcanics of western Tasmania, Australia, host several world-class volcanic-hosted massive sulfide (VHMS) deposits, representing a wide range of deposit styles. Although the deposits and their host sequences are variably deformed and locally preserve spectacular examples of primary textures and structures, rapid lateral and vertical facies changes, and faults with uncertain sense and magnitude of displacement, have made it impossible to correlate stratigraphy across the belt. Previous dating studies in the area have yielded relatively imprecise crystallization ages. We have employed the chemical abrasion isotope dilution thermal ionization mass spectroscopy (ID-TIMS) U-Pb zircon method to obtain highly precise crystallization ages for a total of 18 samples of volcanic and intrusive rock units from throughout the Mount Read Volcanics and underlying mafic-ultramafic complexes. The new data permit detailed resolution of age relationships within the belt. The study establishes an age of 516.0 ± 0.9 Ma for the McIvor Hill gabbro, which is part of a mafic-ultramafic complex interpreted to underlie the Mount Read Volcanics. Magmatism in the central Mount Read Volcanics lasted at least 12.7 m.y., from 506.8 ± 1.0 Ma for a massive dacite unit in the lower part of the Central Volcanic Complex to 496.0 ± 0.9 Ma for a welded ignimbrite in the lower Tyndall Group. Together with previous age constraints, results of the study provide a precise chronostratigraphic framework for magmatism and VHMS deposit formation within the Mount Read Volcanics. The precise age data indicate that, north of the Henty fault, magmatism occurred in three discrete pulses, at least two of which were separated by periods of sedimentation. We demonstrate that VHMS deposits in the Mount Lyell, Roseberry-Hercules, and Que-Hellyer districts, comprising the majority of the known, significant VHMS deposits in the belt, formed within a narrow time interval at ~500 ± 1 Ma, at a relatively late stage in the evolution of the belt. Some of the larger intrusions in the belt (e.g., the Bonds Range porphyry; 500.4 ± 0.8 Ma) were emplaced contemporaneously with VHMS deposit formation; however, other bodies such as the Murchison granite (497.3 ± 0.9 Ma) are younger than the deposits and are unlikely to have been involved in their genesis as has previously been suggested. These conclusions will aid explorers in targeting VHMS mineralization in the Mount Read Volcanics. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Alteration Styles in the Orion Central Volcanic Complex, Fort a la Corne Kimberlite Field, Saskatchewan, and Their Effects on Primary Volcaniclastic Textures: Implications for Facies Mapping and Diamond Exploration

Diamond grade distributions within individual kimberlite bodies are controlled by physical emplacement processes, which are generally determined from volcaniclastic facies characteristics. However, the effects and distribution of secondary alteration styles, which usually mask primary kimberlite facies, have been given little attention. At the Fort a la Corne kimberlite field, Saskatchewan, complex overprinting alteration textures and mineral phases are recognized. This study documents the characteristics, overprinting relationships, and spatial distribution of alteration styles within a crater-filling volcaniclastic deposit, Body 219 of the Orion Central volcanic complex. Data were acquired from macroscopic drill core observations, microscopic petrography, SEM imaging, EDS element mapping, and XRD analyses. Olivine crystals display a mesh fracture pattern and have been replaced by multiple generations of serpentine and magnetite. Late-stage serpentine has migrated into the interstitial medium. Bulk-rock alteration textures include (1) fine-grained crystal-rich domains, which are alteration pseudoclasts and/or relict juvenile pyroclasts; (2) a heterogeneous interstitial medium, varying from early-stage disseminated calcite-rich serpentine to amorphous serpentine and crystalline calcite; and (3) overprinting serpentine, carbonate and magnetite veins, and serpentine nodules. Alteration styles defined by unique alteration characteristics are spatially zoned through the Body 219 volcaniclastic deposit, and this is a result of multiple reaction fronts associated with the influx of fluids of varying composition over time and permeating different parts of the deposit. Recognition of alteration processes, their paragenesis and spatial controls can assist in defining and constraining the distribution of primary kimberlite facies, and by consequence the distribution of diamond grades.

The gravimetric picture of magmatic and hydrothermal sources driving hybrid unrest on Tenerife in 2004/5

We present results from the inversion of gravity changes observed at the central volcanic complex (CVC) of Tenerife, Canary Islands, between May 2004 and July 2005. Marking a period of elevated activity and a reawakening of the volcanic system, the data depict spatial and temporal variations in the sub-surface processes that defined this period of unrest at the Pico Viejo (PV)–Pico Teide (PT) complex, after the last volcanic eruption on Tenerife in 1909. An initial non-linear inversion, based on 3D line segments approximation, yielded three line segments at depths between 1kma.s.l. and 2km b.s.l. Our interpretation of the initial inversion results is that the line segments represent apparent composite sources, a superposition of deep and shallow seated sources. We therefore decomposed the gravity changes into shallow and deep parts (fields) using a procedure based on triple harmonic continuation. The shallow and deep fields could then be inverted separately, using the same inversion methodology. The deep field constrains two connected line segments at the depth of about 6km b.s.l., in the center of the NW seismogenic zone of VT event swarm of the seismic unrest, that we interpret as magma input. The inversion of the shallow field images three weak line segments that are all situated at very shallow, near-surface depths. We interpret the weak segments as hydrothermal sources potentially excited by the deeper magma injection. Our results indicate no significant input into the shallow phonolitic plumbing system of the PV–PT complex, but rather a deeper-seated rejuvenation of the mafic feeder reservoir. The emerging picture from our analysis is that the 2004/5 unrest on Tenerife was of a hybrid nature due to the combination of a deep magma injection (failed eruption?) coupled with fluid migration to shallow depths. The identified causative link between deep and shallow unrest sources indicates the presence of permeable pathways for shallow fluid migration at the CVC.

Hydrothermal system of Central Tenerife Volcanic Complex, Canary Islands (Spain), inferred from self-potential measurements

An extensive self-potential survey was carried out in the central volcanic complex of Tenerife Island (Canary Islands, Spain). A total amount of ~237km of profiles with 20m spacing between measurements was completed, including radial profiles extending from the summits of Teide and Pico Viejo, and circular profiles inside and around Las Cañadas caldera and the northern slopes of Teide and Pico Viejo. One of the main results of this mapping is the detection of well-developed hydrothermal systems within the edifices of Teide and Pico Viejo, and also associated with the flank satellite M. Blanca and M. Rajada volcanoes. A strong structural control of the surface manifestation of these hydrothermal systems is deduced from the data, pointing to the subdivision of Teide and Pico Viejo hydrothermal systems in three zones: summit crater, upper and lower hydrothermal systems. Self-potential maxima related to hydrothermal activity are absent from the proximal parts of the NE and NW rift zones as well as from at least two of the mafic historical eruptions (Chinyero and Siete Fuentes), indicating that long-lived hydrothermal systems have developed exclusively over relatively shallow felsic magma reservoirs. Towards Las Cañadas caldera floor and walls, the influence of the central hydrothermal systems disappears and the self-potential signal is controlled by the topography, the distance to the water table of Las Cañadas aquifer and its geometry. Nevertheless, fossil or remanent hydrothermal activity at some points along the Caldera wall, especially around the Roques de García area, is also suggested by the data. Self-potential data indicate the existence of independent groundwater systems in the three calderas of Ucanca, Guajara and Diego Hernández, with a funnel shaped negative anomaly in the Diego Hernández caldera floor related to the subsurface topography of the caldera bottom. Two other important self-potential features are detected: positive values towards the northwestern Santiago rift, possibly due to the relatively high altitude of the water-table in this area; and a linear set of minima to the west of Pico Viejo, aligned with the northwestern rift and related to meteoric water infiltration along its fracture system.

Evolution of low-18O Icelandic crust

The Krafla central volcano in the neovolcanic zone of Iceland hosts a chemically diverse suite of magmas characterized by anomalously low δ18O values. A rhyolite magma intercepted by the Iceland Deep Drilling Project (IDDP) exploratory well at 2.1km depth provided a unique opportunity to investigate the origins of an unerupted rhyolite melt in the primarily basaltic central volcano at Krafla. Here we compare whole rock hydrogen and oxygen isotopes of this melt to those of lavas within and near the caldera of the Krafla central volcano ranging from recent fissure eruptions to Plio--Pleistocene age (including analyses of 18 new samples, plus previously published values) in order to evaluate the petrogenesis of low-18O magmas within the neovolcanic zone of Iceland.Oxygen isotope values of the IDDP-1 melt (δ18O=+3.2±0.2‰) are within the range of Krafla eruptives that have a bimodal composition of olivine-tholeiite and rhyolite (δ18O=+1.6‰ to +4.5‰). Lavas show significantly more variability in hydrogen isotope values (δD=−161‰ to −92‰) than the IDDP-1 melt (−121±2‰), whose δD is comparable to local hydrothermal epidote (−127 to −108‰), and show significantly lower water contents than IDDP-1 (0.1–1.1wt%, in contrast to ~1.8wt%). Basaltic to dacitic lavas from the proximal Heidarspordur ridge volcanic zone have δ18O between +3.4‰ and +4.2‰ and δD between −105‰ and −99‰.Uniformity of oxygen isotopes in the Heidarspordur ridge lavas suggests that their magmatic compositional variations are a consequence of fractional crystallization. The δD of the glass sampled by IDDP-1 unequivocally identifies the source of silicic low-18O melts like those erupted from within the caldera of the Krafla volcano as anatexis of meteoric-hydrothermally altered basalts resulting from the intrusion of mantle-derived basaltic magma. Finally, mantle-derived basalts in both the Krafla central volcano and Heidarspordur ridge (MgO>5wt%) have δ18O values lower than the typical MORB. There is convincing evidence favoring low-18O compositions in primitive Icelandic basalts as also being the result of assimilating meteoric hydrothermally altered Icelandic crust, particularly in the roots of central volcanic complexes, where high-level intrusions develop. Here hydrothermally altered basalts have a high probability of undergoing partial melting. If this is the case, we hypothesize that there should be a notable secular variation in the δ18O values of upper crustal igneous products of the Iceland mantle plume over the sub-aerial extrusive geologic history of Iceland, in which progressively younger lavas exhibit decreasing δ18O values. Synthesis of published oxygen isotope analyses of unaltered whole rock and mineral separates of lavas from Tertiary age to present lends support to this hypothesis.

A direct approach to estimating the number of potential fatalities from an eruption: Application to the Central Volcanic Complex of Tenerife Island

One of the critical issues in managing volcanic crises is making the decision to evacuate a densely-populated region. In order to take a decision of such importance it is essential to estimate the cost in lives for each of the expected eruptive scenarios. One of the tools that assist in estimating the number of potential fatalities for such decision-making is the calculation of the FN-curves. In this case the FN-curve is a graphical representation that relates the frequency of the different hazards to be expected for a particular volcano or volcanic area, and the number of potential fatalities expected for each event if the zone of impact is not evacuated. In this study we propose a method for assessing the impact that a possible eruption from the Tenerife Central Volcanic Complex (CVC) would have on the population at risk. Factors taken into account include the spatial probability of the eruptive scenarios (susceptibility) and the temporal probability of the magnitudes of the eruptive scenarios. For each point or cell of the susceptibility map with greater probability, a series of probability-scaled hazard maps is constructed for the whole range of magnitudes expected. The number of potential fatalities is obtained from the intersection of the hazard maps with the spatial map of population distribution. The results show that the Emergency Plan for Tenerife must provide for the evacuation of more than 100,000 persons.

Three-dimensional geometry of concentric intrusive sheet swarms in the Geitafell and the Dyrfjöll volcanoes, eastern Iceland

Sheet intrusions (inclined sheets and dykes) in the deeply eroded volcanoes of Geitafell and Dyrfjoll, eastern Iceland, were studied at the surface to identify the location, depth, and size of their magmatic source(s). For this purpose, the measured orientations of inclined sheets were projected in three dimensions to produce models of sheet swarm geometries. For the Geitafell Volcano, the majority of sheets converge toward a common focal area with a diameter of at least 4 to 7 km, the location of which coincides with several gabbro bodies exposed at the surface. Assuming that these gabbros represent part of the magma chamber feeding the inclined sheets, a source depth of 2 to 4 km below the paleoland surface is derived. A second, younger swarm of steeply dipping sheets crosscuts this gabbro and members of the first swarm. The source of this second swarm is estimated to be located to the SE of the source of Swarm 1, below the present-day level of exposure and deeper than the source of the first swarm. For the Dyrfjoll Volcano, we show that the sheets can be divided into seven different subsets, three of which can be interpreted as swarms. The most prominent swarm, the Njardvik Sheet Swarm, converges toward a several kilometers wide area in the Njardvik Valley at a depth of 1.5 to 4 km below the paleoland surface. Two additional magmatic sources are postulated to be located to the northeast and southwest of the main source. Crosscutting relationships indicate contemporaneous, as well as successive activity of different magma chambers, but without a resolvable spatial trend. The Dyrfjoll Volcano is thus part of a complex volcanic cluster that extends far beyond the study area and can serve as fossil analog for nested volcanoes such as Askja, whereas in Geitafell, the sheet swarms seem to have originated from a single focus at one time, thus defining a single central volcanic complex, such as Krafla Volcano.

Characterising unrest during the reawakening of the central volcanic complex on Tenerife, Canary Islands, 2004–2005, and implications for assessing hazards and risk mitigation

Increased onshore seismic activity in April 2004 marked the first documented renewal of tectonic unrest on Tenerife, Canary Islands, Spain, since the island's last volcanic eruption in 1909. Events included tremors, felt earthquakes and the occasional emission of a visible gas plume from the central 3718 m high Teide volcano, and an increased diffuse emission of CO 2. Here, we evaluate results from seismic and microgravimetric observations in addition to other available data obtained between April 2004 and July 2005, in order to shed light on the source of these events. We discuss the information to assess whether collectively the phenomena qualify to be termed “volcanic unrest”, and the socio-economic implications of the phenomena, and critically examine the ensuing scientific response. We also evaluate the potential volcanic-eruption precursory character of the data. Suggestions for the establishment of improved volcano monitoring programmes, early warning systems and civil response protocols for volcanic crises on Tenerife are proposed.

Reconstruction of the differentiated long-term exhumation history of Fuerteventura, Canary Islands, Spain, through fission-track and (U-Th–Sm)/He data

Miocene Intrusives and Lower Cretaceous siliciclastic sedimentary rocks from the Basal Complex in western-Fuerteventura were analyzed with low-temperature thermochronometric methods such as fission-track, and (U–Th–Sm)/He dating, in order to reveal the evolution of the island’s exhumation history. The obtained thermochronometric data yields a very slow rate of cooling in the order of 1.5–3°C/Myr from ~50 to 20 Ma for the Early Cretaceous siliciclastic rocks. These sedimentary units have never been heated significantly above 240°C after deposition and still record the submarine onset of the island’s formation in the Eocene. Intrusive bodies associated with the early Miocene magmatic activity of the central volcanic complex of the island show rapid initial cooling rates of 50–70°C/Myr from ~20 to 14 Ma. Contemporaneous with the intrusions the cooling rate of the Cretaceous sedimentary units increased to 25–35°C/Myr and it is inferred that this increase is associated with enhanced uplift and erosion of the Central Volcanic Complex. After ~14 Ma rates slowed down to 3–6°C/Myr. Palaeosols overlying the sedimentary units are themselves covered by Pliocene basalt flows and reveal that the sedimentary rocks reached the surface before ~5 Ma. The thermochronometric data obtained in this study for central Fuerteventura is difficult to reconcile with the cooling history derived from previously obtained fission-track and K–Ar data from the north-western part of the island. This inconsistency is likely to indicate that the exhumation history of Fuerteventura is more complex and regionally subdivided than previously believed.

Shallow sub-surface structure of the central volcanic complex of Tenerife, Canary Islands: implications for the evolution and the recent reactivation of the Las Cañadas caldera

We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain. The high-density core of the CVC and the pronounced gravity low centred in the Las Cañadas caldera (LCC) in greater detail than previously available. Mathematical construction of a subsurface model from the local anomaly data, employing a 3-D inversion enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture of the complex, and shedding light on its evolution.

Shallow structure beneath the Central Volcanic Complex of Tenerife from new gravity data: Implications for its evolution and recent reactivation

We present a new local Bouguer anomaly map of the Central Volcanic Complex (CVC) of Tenerife, Spain, constructed from the amalgamation of 323 new high precision gravity measurements with existing gravity data from 361 observations. The new anomaly map images the high-density core of the CVC and the pronounced gravity low centred in the Las Cañadas caldera in greater detail than previously available. Mathematical construction of a sub-surface model from the local anomaly data, employing a 3D inversion based on “growing” the sub-surface density distribution via the aggregation of cells, enables mapping of the shallow structure beneath the complex, giving unprecedented insights into the sub-surface architecture. We find the resultant density distribution in agreement with geological and other geophysical data. The modelled sub-surface structure supports a vertical collapse origin of the caldera, and maps the headwall of the ca. 180 ka Icod landslide, which appears to lie buried beneath the Pico Viejo–Pico Teide stratovolcanic complex. The results allow us to put into context the recorded ground deformation and gravity changes at the CVC during its reactivation in spring 2004 in relation to its dominant structural building blocks. For example, the areas undergoing the most significant changes at depth in recent years are underlain by low-density material and are aligned along long-standing structural entities, which have shaped this volcanic ocean island over the past few million years.

New results on the resistivity structure of Merapi Volcano (Indonesia), derived from three-dimensional restricted inversion of long-offset transient electromagnetic data

Author(s): Commer, Michael; Helwig, Stefan, L.; Hordt, Andreas; Scholl, Carsten; Tezkan, Bulent | Abstract: Three long-offset transient electromagnetic (LOTEM) surveys werecarried out at the active volcano Merapi in Central Java (Indonesia) during the years 1998, 2000, and 2001. The measurements focused on the general resistivity structure of the volcanic edifice at depths of 0.5-2 km and the further investigation of a southside anomaly. The measurements were insufficient for a full 3D inversion scheme, which could enable the imaging of finely discretized resistivity distributions. Therefore, a stable, damped least-squares joint-inversion approach is used to optimize 3D models with a limited number of parameters. The mode ls feature the realistic simulation of topography, a layered background structure, and additional coarse 3D blocks representing conductivity anomalies. Twenty-eight LOTEM transients, comprising both horizontal and vertical components of the magnetic induction time derivative, were analyzed. In view of the few unknowns, we were able to achieve reasonable data fits. The inversion results indicate an upwelling conductor below the summit, suggesting hydrothermal activity in the central volcanic complex. A shallow conductor due to a magma-filled chamber, at depths down to 1 km below the summit, suggested by earlier seismic studies, is not indicated by the inversion results. In conjunction with an anomalous-density model, derived from arecent gravity study, our inversion results provide information about the southern geological structure resulting from a major sector collapse during the Middle Merapi period. The density model allows to assess a porosity range andthus an estimated vertical salinity profile to explain the high conductivities on a larger scale, extending beyond the foothills of Merapi.

New evidence for the reawakening of Teide volcano

Geophysical signals accompanying the reactivation of a volcano after a period of quiescence must be evaluated as potential precursors to impending eruption. Here we report on the reactivation of the central volcanic complex of Tenerife, Spain, in spring 2004 and present gravity change maps constructed by time‐lapse microgravity measurements taken between May 2004 and July 2005. The gravity changes indicate that the recent reactivation after almost a century of inactivity was accompanied by a sub‐surface mass addition, yet we did not detect widespread surface deformation. We find that the causative source was evolving in space and time and infer fluid migration at depth as the most likely cause for mass increase. Our results demonstrate that, even in the absence of previous baseline data and ground deformation, microgravity measurements early in developing crises provide crucial insight into the dynamic changes beneath a volcano.

Interpretation of long‐offset transient electromagnetic data from Mount Merapi, Indonesia, using a three‐dimensional optimization approach

In the years 1998, 2000, and 2001, long‐offset transient electromagnetic (LOTEM) surveys were carried out at the active volcano Merapi in Central Java. The measurements investigated the conductivity structure of the volcanic edifice. Our area of interest, which is below the summit and the upper flanks, was investigated using horizontal and vertical magnetic field time derivative data from seven transmitter‐receiver setups. Because of topography and a three‐dimensional (3‐D) underground structure, a 3‐D interpretation is used. The method optimizes few parameters of a 3‐D model by a stable least squares joint inversion of the data, providing sufficient resolution capability. Reasonable data fits are achieved with a nonhorizontally layered model featuring a very conductive basement below depths of 1.5 km. While hydrothermal alteration is also considered, we tentatively explain the high conductivities by aqueous solutions with relatively high salt contents. A large magma body or a small superficial reservoir below Merapi's central volcanic complex, as discussed by other authors, cannot be resolved by the LOTEM data.

POSSIBLE SUBMARINE ADVANCED ARGILLIC ALTERATION AT THE BASIN LAKE PROSPECT, WESTERN TASMANIA, AUSTRALIA

The Basin Lake copper-gold prospect lies in western Tasmania’s Mount Read Volcanics and is hosted in a series of calc-alkaline andesites, quartz-feldspar porphyries, mudstones, carbonates, and sandstones between the Tyndall Group and the Central Volcanic Complex. Alteration at the Basin Lake prospect occurs over a strike length of 1.4 km and includes thin, strata-bound pyrophyllite-quartz-paragonite-kaolinite-pyrite-alunite alteration zones, up to 12 m wide and containing up to 50 wt percent pyrophyllite, with local fluorite veining. These zones grade out to paragonite-muscovite-kaolinite-quartz-pyrite and muscovite-carbonate-chlorite alteration zones. Extensive propylitic alteration (chlorite-carbonate-epidote) affects most other rocks outside these zones. Mineralization consists of thin strata-bound zones of massive and vein pyrite, tennantite, and chalcopyrite, with trace covellite and galena, hosted mainly within an intensely silicified core of the pyrophyllite-quartzsericite alteration zone. Pyrite has δ34S values of –1.4 to +6.9 per mil, although marginal vein pyrite in the propylitic zone has δ34S values around 12.4 per mil. Large silicified glacial erratic boulders at surface contain massive and vein pyrite, enargite, and tennantite, with minor barite, and trace covellite, stannoidite, and mawsonite. Pyrite and enargite have δ34S values of 1.7 to 6.8 per mil; barite has δ34S values around 35.2 per mil with 87Sr/86Sr around 0.7108. The alteration and mineralization at the Basin Lake prospect is similar to that associated with high-sulfidation copper-gold systems formed by acidic, relatively oxidized fluids. A new geochemical vector, here termed the “advanced argillic alteration index” [AAAI = 100 (SiO2)/(SiO2 + 10MgO + 10CaO + 10Na2O)], has been devised to help quantify the intensity of alteration. The values of the AAAI at Basin Lake are similar to those of several high-sulfidation epithermal deposits. The low sulfide δ34S values are similar to those at other sulfide occurrences in the Mount Read Volcanics that have previously been considered to be barren, are lower than those of nearby volcanic-hosted massive sulfide deposits, and may indicate a magmatic fluid component. However, the δ34S and 87Sr/86Sr values of Basin Lake barite at the assumed highest exposed level of the system and higher δ34S values in pyrite from marginal veins are similar to those of Cambrian volcanic-hosted massive sulfide systems, indicating the involvement of reduced seawater sulfate at these locations. Calcite carbon and oxygen isotope values, silicate oxygen isotope values, and the unusual abundance of carbonate close to advanced argillic alteration indicate fluid mixing and suggest that acidic, magmatic fluids were likely neutralized by seawater. This occurrence strengthens the case for prospecting the Mount Read Volcanics and other similar submarine volcanic belts for copper-gold and gold-only deposits that formed by the actions of hyperacid oxidized fluids.

Zonation of Alteration Facies at Western Tharsis: Implications for the Genesis of Cu-Au Deposits, Mount Lyell Field, Western Tasmania

The Western Tharsis deposit, located in the Mount Read volcanic belt of western Tasmania, is one of 22 mainly Cu-Au deposits in the Mount Lyell district. Although the orebody at western Tharsis is characterized mainly by disseminated pyrite-chalcopyrite like most other deposits in the Mount Lyell district, it also contains bornite-rich ores characteristic of a second, less common type of deposit. The deposit is stratiform and occurs within intermediate and felsic volcanic rocks of the ~500 Ma Central Volcanic Complex. Alteration is developed broadly symmetrically around the ore zone, with an ore-bearing core characterized by a quartz-chlorite ± sericite assemblage. The core is enveloped by a quartz-pyrophyllite ± topaz ± fluorite ± zunyite ± woodhouseite assemblage with local bornite-bearing ore zones. This zone, in turn, is enveloped by a pyritic quartz-sericite assemblage. The outermost quartz-chlorite-carbonate-sericite ± albite assemblage is only developed in the stratigraphic footwall more than 200 m below the ore position. The hypogene pyrophyllite-bearing assemblage is more characteristic of high sulfidation than volcanic-hosted massive sulfide (VHMS)-related alteration assemblages. Moreover, the close relationship of this alteration assemblage with the more prevalent quartz-chlorite ± sericite assemblage associated with disseminated pyrite-chalcopyrite mineralization suggests that these two types of mineralization are two stages of one mineralizing event, not the two separate events suggested by previous authors (e.g., Arnold and Carswell, 1990). Geologic relationships and radiogenic isotope data are consistent with an Ordovician (~460 Ma) timing for Mount Lyell Cu-Au mineral deposits; this mineralization may be related to the geologic event that formed the local Haulage unconformity. These observations indicate a potential new exploration model for mineralization in the Mount Read volcanic belt and in surrounding Ordovician rocks.