Active Magma Chamber Research Articles

Long‐Lived and Continual Volcanic Eruptions, Tectonic Activity, Pit Chains Formation, and Boulder Avalanches in Northern Tharsis Region: Implications for Late Amazonian Geodynamics and Seismo‐Tectonic Processes on Mars

AbstractTharsis is the largest volcanic province on Mars. It consists of a few large shield volcanoes, one of which is Alba Mons (AM), located in northern Tharsis, with the Ceraunius Fossae (CF) graben system to the south of it. Previous workers argued in favor of episodic building of Tharsis volcanoes. In contrast, we show that volcanism in the AM and CF regions occurred continually, at least in the last few hundred million years, based on ages of >1,000 stratigraphically young lava flows. Central volcanism in AM terminated around 200 Ma, while subsequent young volcanism, fed by both small shields and fissures, migrated to the south in CF. Several generations of grabens hosting thousands of simple to complex pit chains were formed by magmatic intrusion through feeder dikes, and these activities also migrated to the south during the Late Amazonian epoch. Thousands of boulder avalanches mark the slopes of grabens, pit chains, and impact craters, with ages ranging from a few tens to thousands of years. The local geological settings of the boulder falls suggest that both tectonic and volcanic marsquakes triggered the boulder avalanches. The broad zone of boulder falls in northern Tharsis defines the potential epicentral zone of (MW 4.1) 18 September 2021 marsquake. We infer that long‐lived and active magma chambers and subcrustal magma underplating driven by a mantle plume are present beneath the AM volcano and that these probably migrated south toward CF in the Late Amazonian epoch.

Crustal structure in and around the East Anatolian volcanic belt by using receiver functions stacking

This study aims to explore the crustal seismic properties across the East Anatolian volcanic belt. Teleseismic receiver functions obtained through deconvolution in the time domain are used to estimate crustal discontinuities and Moho interface. Further, slant stacked method, employed for the first time in the study region, is applied to identify the arrival times of P to S conversion and reverberations (Ps and PpPms) that give Vp/Vs ratio beneath each station. For slant stacking method, radial receiver functions are calculated using deconvolution based on frequency domain. Inversion results indicate that Moho depth varies from at most ∼40 km near the Bitlis-Zagros thrust zone to 45 km in the northeastward of Quaternary volcanic centers. The southwest of study region is characterized overall by a low average crustal Vp/Vs ratio (less than 1.71), implying a felsic crustal composition. A high average crustal Vp/Vs ratio of 1.82–1.87 is found near volcanic centers, characterized by predominantly a mafic crustal composition. The prominent crustal low-velocity zones especially around the Nemrut and Ararat volcanoes are identified below ∼20 km depth and likely imply the presence of partial melt zones or active magma chambers in the region. As a result, the findings of this study provide useful new constraints to the crustal thickness and composition in the study region.

Constraints of magmatic differentiation on epithermal mineralization at Dongan, NE China: Insights from zircon geochronology, elements and Sr–Hf–Nd isotope geochemistry

Low-sulfidation (LS) epithermal deposits formed under relatively low temperature and pressure conditions are generally associated with volcanic activity. To better understand magmatic processes prior to epithermal mineralization, field geology, petrology, UPb zircon geochronology, whole-rock major, and trace element geochemistry along with HfNd isotopic geochemistry of the cogenetic volcanic rocks at Dongan were assessed. The Early Cretaceous (109.8 to 107.0 Ma) volcanic suite consists of high-K calc-alkaline andesitic-rhyolitic tuff and granite porphyry dikes or/and stocks. Arc magma affinity characterizes the suite with enrichments in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depletions in high field intensity elements (HFSEs) and heavy REEs (HREEs). The extrusive andesitic rocks showing enriched mantle features (εNd(t) = + 0.76 to +1.83, εHf = − 3.4 to +3.7, TDM2 = 933 to 1124 Ma) are the result of the hybridization between the mantle and lower crust melts with the contamination of upper crustal materials during eruption or ascending of melts. The intrusive granite porphyry (εNd(t) = +3.81 to +3.9, εHf = −0.3 to +3.7, TDM2 = 901 to 1187 Ma) is geochemically comparable with andesitic rocks and was likely formed by the fractional crystallization of plagioclase and amphibole minerals from cooling andesitic melts at middle–upper crustal levels. Note that the rhyolitic magmas show distinct features of lower εNd(t) (−3.05 to – 2.78), εHf (−0.6 to +1.6) and total REE values (54.78 to 70.09 ppm), older TDM2 (1464 to 1662 Ma) and obvious negative europium anomalies (0.34 to 0.37), suggesting an origin of upper crustal remelting (compositionally equivalent to water-poor felsic granulite or tonalite) that results from the heating of the underlying andesitic magmas. In summary, the sequential emplacement of the ore-related andesitic and granitic melts is attributed to the magmatic processes of the active magma chamber in a continental extensional setting, and the metal-rich bubbles focused at the tops of this chamber due to prolonged differentiation are released to produce epithermal veins at shallower levels (< 1.5 km).

Formation of rapakivi feldspar in the Deer Isle Granite Complex, coastal Maine: in situ lead isotope and trace-element analysis

Rapakivi and alkali feldspar phenocrysts from the Devonian Deer Isle Granite Complex, Maine were investigated to see if magma mixing or isothermal decompression was responsible for their formation. Pb-isotope and trace-element analyses of alkali feldspar ovoids and their plagioclase inclusions were obtained by LA-ICP-MS to determine the characteristics of the initial magma; these were compared with the plagioclase mantles. Rapakivi grains show a decrease in Pb-isotopic values and an increase in Ba, La, and Ce concentrations from the ovoids and inclusions to the innermost plagioclase mantles adjacent to the ovoids. These variations, along with CL imagery of quartz phenocrysts, indicate an open system with compositional changes in the magma chamber occurring during rapakivi feldspar growth. Repeated episodes of magma mixing/replenishment by Ba and LREE enriched magmas with lower Pb-isotopic ratios (along with hybridized variations with the host magma) created localized dissolution of alkali feldspar and quartz. Alkali feldspar phenocrysts proximal to these zones of thermal perturbation were first resorbed and then mantled by plagioclase that records the more primitive signature of the more mafic melt. Pb-isotopic values and Ba, La, and Ce concentrations trend back to the initial magma composition at the mantle rims as the effects of the mixing event dissipated. For grains that were somewhat more distal to these zones of mixing, resorption of the alkali feldspar crystals did not occur, yet the fresh supply of Ba resulted in Ba-rich alkali feldspar mantles over Ba-poor alkali feldspar cores. Other alkali feldspar crystals were too far from the site of mixing; hence, they lack any evidence of open system crystallization. As crystallization of the chamber continued along solidification fronts, batches of cooler crystal-rich magmas settled en masse to the floor. Disaggregation of these batches during settling, and subsequent accumulation on the reservoir floor, brought grains with disparate crystallization histories together. Filter pressing of the cumulate pile flushed highly evolved melts/fluids out from interstitial pores to the high silica, upper portions of the chamber. The crystallization, transportation, and juxtaposition of disparate types of feldspar phenocrysts suggest that a relatively large and active magma chamber, periodically recharged by localized batches of melt, must have existed. In this case, it is clear that rapakivi mantled feldspars are the result of magma mixing; such interpretations may apply to many other occurrences as well.

Thermal fluid circulation around the Karliova triple junction: Geochemical features and volcano-tectonic implications (Eastern Turkey)

The Karlıova triple junction (KTJ) in eastern Turkey has been subjected to incremental deformation resulting in complex kinematic and mechanical interactions throughout the upper crust. These interactions have generated tectonic inversions and uplift, extensive seismicity and volcanism. The regional tectonics generate local stresses, some of which are favorable to magma transport and thermal water circulation throughout the lithosphere. Here we evalauate hydrogeochemical, geological and numerical results relating to the mechanism of thermal fluid circulation around the KTJ. Hydrogeochemistry of the samples indicates that the thermal water springs are probably heated by steam. Volcanic rocks at the site appear to be the host rock owing to the enrichment of Na+ and Cl− ions in water and the abundance of these elements in minerals of the volcanic rocks. In addition, it is clear that the thermal fluids are sourced from depth and migrate through permeable networks of faults. The effects of crustal heterogeneities, in particular the geometry and mechanical properties of many faults and layers, on thermal fluid circulation in relation to active magma chambers were investigated under a variety of different mechanical conditions. The numerical results indicate very close relationships between the stress field causing faulting and thermal fluid movement in the KTJ. The effect of thermal transfer was modeled with depth throughout the crust and along the the crustal surface. The models show that some faults encourage thermal fluid circulation below the Varto and Özenç volcanoes. Hydrogeochemical, geological and numerical results suggest that magmas residing beneath both the Varto caldera and the Özenç volcano are the main heat source for thermal fluid in the Varto region. Fluid-solid interactions and fluid circulation models show that the permeable faults are important factors affecting heat transport and fluid circulation. In a series of thermal fluid flow models we probe the mechanism for fluid and gas transport from the 900 °C ‘hot’ zone around the deep magma chambers and investigate how heat is lost throughout the crust on the way to the surface and so eventually creates water channels of temperatures between 50 to 60 °C.

Petrogenesis of plagiogranites from the Troodos Ophiolite Complex, Cyprus

Small volumes of felsic melt, commonly known as oceanic plagiogranites, appear as melt pockets or dikes within the gabbroic section and the sheeted dikes root zone of the oceanic crust. Plagiogranites from the Troodos Ophiolite Complex on Cyprus are among the best exposures of felsic rocks that are embedded in a complete section of obducted oceanic lithosphere. Nevertheless, their exact petrogenesis is still a matter of debate, largely due to limited high-quality trace element and radiogenic isotope data. Previously proposed models for Troodos plagiogranites have included both low-pressure dehydration melting and fractional crystallisation at deeper levels of the oceanic crust. To evaluate both models, oceanic plagiogranites from the Troodos Ophiolite Complex were analysed in this study for their major and trace elements, and for the first time also for Hf–Nd–Sr isotope compositions. The trace element measurements also include for the first time high-precision measurements of high-field-strength element (HFSE) abundances that now permit to better unravel the petrogenesis of the Troodos plagiogranites and their possible mantle sources. In general, the Troodos plagiogranites exhibit a narrow range of Nb/Ta and Zr/Hf that overlap the compositions of mid-ocean ridge basalts (MORB). In line with earlier studies, three compositional groups can be identified: two groups formed by either fractional crystallisation or combined fractional crystallisation and wall rock assimilation, and one group derived from partial melting of slightly altered oceanic crust. The majority of the Troodos plagiogranites (Main Group) are the product of extensive fractional crystallisation of ambient arc-tholeiitic mafic melts. A second group of plagiogranites (Spilia Group) is generated by fractional crystallisation of boninitic precursor melts and the assimilation of arc-tholeiitic crustal material. Variable HFSE concentrations and diagnostic Hf–Nd isotope signatures that are unique to both suites allow discriminating between the two parental melts and fractionation processes. A small group of plagiogranites (Zoopigi Group) is interpreted to derive from partial melting in the conductive layer of active magma chamber lenses (AML). Elevated Nb/Ta, Zr/Hf, and light rare-earth element (LREE) enrichments in these rocks are in support of this model. Collectively, our data suggest that low-pressure fractional crystallisation (also in combination with assimilation of wall rocks) might be the predominant process controlling the formation of felsic rocks on Cyprus, whereas dehydration melting appears to be less important. If compared to Archean tonalitic–trondhjemitic–granodioritic suites (TTGs), compositions of plagiogranites from Cyprus mirror shallow-level processes in thin oceanic crust, which is illustrated by their narrow, MORB-like range of HFSE ratios and their distinct enrichment in heavy rare-earth elements (HREE) that distinguishes them from the Archean TTGs.

The development of divergent margins: Insights from the North Volcanic Zone, Iceland

Iceland is a natural laboratory where plate separation and mid-ocean ridge formation can be studied directly in the field. In this setting, it is not fully understood how magma interacts with faulting and fissuring, and how rift zones propagate. In order to improve the understanding of how these processes operate, we conducted a detailed field study about the kinematics and propagation of 33 Holocene faults along the N–S Theistareykir rift (North Volcanic Zone) and mapped all tension fractures. This rift hosts the Holocene Theistareykir central volcano, for which ground deformation data (InSAR and GPS) indicate an inflating shallow magma chamber throughout 2006–2008. Analyses of the cumulative fault slip distribution at 696 sites show two opposite directions of fault/rift propagation: The first, more developed, is directed northwards and affects the area located north of Theistareykir volcano, whereas south of the volcano, the rift propagates southwards. Tension fractures increase in frequency, with respect to faults, outwards from the volcano, along a N–S to NNE–SSW direction. The presence of aligned vents, dykes and topographic bulging along some of the N–S structures, and the active magma chamber below Theistareykir volcano, suggest that faults and tension fractures propagate following repeated dyke intrusions from the magma chamber outward along the plate margin. The fact that the rift is more developed north of the volcano than south of it, is interpreted as the effect of buttressing from another, coterminous magma chamber sitting below a neighbouring central volcano. Other causes for the general observed pattern such as tectonic stresses, mechanical interaction of faults, and changes in the rheological characteristics of rocks, are discussed as well.

Pressure evolution in shallow magma chambers upon buoyancy‐driven replenishment

AbstractThe invasion of active magma chambers by primitive magma of deeper provenance is a frequent occurrence in volcanic systems, and it is commonly associated with pressurization. Chamber replenishment is driven by pressure and buoyancy forces that cause magma ascent towards shallow depths. We examine the end‐member case of pure buoyancy‐driven (natural) convection in crustal reservoirs deriving from the presence of degassed, dense magma at shallow level, that can originate a gravitational instability. Space‐time‐dependent numerical simulations of magma dynamics in composite underground systems reveal highly nonlinear pressure evolution dominated by decompression at shallow depths. This counterintuitive result originates from the compressible nature of multiphase magmas and their complex convection and mixing dynamics. Shallow magma chamber decompression on replenishment is favored by large volatile contents of the uprising magma, resulting in large density contrasts among the resident and the incoming components. These results show that the intuitive concept of magma chamber pressurization upon replenishment may not always hold in real situations dominated by buoyancy, and provide new perspectives for the interpretation of geophysical records at active volcanoes.

Subduction-related hot spring-type gold mineralization in the central Tengchong block, southwestern China

Numerous present-day hot springs and associated ancient Lianghe gold deposit occur in the central Tengchong block, SW China, associated with oceanic crust subduction. From the center outwards, the alteration zones in the hot spring of the Rehai consist of alunite and silica in a breccia zone, a kaolinite and illite zone, and a chlorite, illite, kaolinite zone. The gas phase in the hot spring is dominated by CO2 with δ13C ranging from −4.5‰ to −7.2‰. These features indicate a major magmatic origin of the hot spring gas. The 3He/4He ratios in the gas from central Tengchong vary from 1.56 to 5.88, approaching to those of the MORB. This is probably related to the ongoing subduction of the Indian plate beneath the Tengchong block. The hot spring waters in the central Tengchong block have much higher content of Au and other elements, e.g., Ag, As, Sb, and Tl, than other typical hot springs, such as Hasbrouck, Sulfer, etc. in the world. The hot springs with more mantle gas contribution and higher water temperature show more elevated Au contents, which implies that the metal in the hot spring was most likely contributed from the magmatic gas. The compiled D-O isotopes of the hot spring waters from previous studies suggest that they are primarily derived from meteoric water. A genetic model for the hot spring system in the central of Tengchong has been tentatively established. The gold-charged hot spring is genetically connected to an active magma chamber, which was produced by subduction of oceanic crust. The chamber is driving long-term surface meteoric circulation and constantly releasing magmatic volatiles enriched with metals. As hot spring water mixed with magmatic volatiles, causing decompression-boiling and forming hydraulic fractures to penetrate the roof of siliceous sinter, which induced the continuing metal deposition and formed the Lianghe hot spring.

Repeated, multiscale, magmatic erosion and recycling in an upper-crustal pluton: Implications for magma chamber dynamics and magma volume estimates

The Tuolumne Intrusive Complex, an upper-crustal (7–11 km emplacement depths), incrementally constructed (95–85 Ma growth history) plutonic complex (~1100 km 2 ), preserves evidence from several data sets indicating the repeated, multiscale, magmatic erosion of older units occurred and that some eroded material was recycled into younger magma batches. These include: (1) map patterns of internal contacts (hundreds of kilometers) that show local hybrid units, truncations, and evidence of removal of older units by younger; (2) the presence of widespread xenolith and cognate inclusions (thousands), including “composite” inclusions; (3) the presence of widespread enclaves (millions), including “composite” enclaves, plus local enclave swarms that include xenoliths and cognate inclusions; (4) the presence of widespread schlieren-bound magmatic structures (>9000) showing evidence of local (meter-scale) truncations and erosion; (5) antecrystic zircons (billions) and other antecrystic minerals from older units now residing in younger units; (6) whole-rock geochemistry including major element, REE, and isotopic data; and (7) single mineral petrographic and geochemical studies indicating mixing of distinct populations of the same mineral. Synthesis of the above suggests that some erosion and mixing occurred at greater crustal depths, but that thousands of “erosion events” at the emplacement site resulted in removal of ~35–55% of the original plutonic material from the presently exposed surface with some (~25%?) being recycled into younger magmas and the remainder was either erupted or displaced downward. The driving mechanisms for mixing/recycling are varied but likely include buoyancy driven intrusion of younger batches into older crystal mushes, collapse and avalanching along growing and over-steepened solidification fronts within active magma chambers (1 to >500 km 2 in size), and local convection in magma chambers driven by internal gradients (e.g., buoyancy, temperature, and composition).

Discussão dos processos de construção do complexo granítico São Sepé, RS: feições geológicas e petrográficas

A construção do complexo granítico São Sepé (CGSS) é discutida a partir de informações de campo, geológicas e estruturais, complementadas com a avaliação dos sistemas tectônicos regionais integrando sensores remotos (MDE-SRTM e imagem ASTER) e mapas aeromagnetométricos, e com a investigação detalhada de texturas e estruturas. O CGSS, localizado no oeste do Escudo Sul-riograndense (Bloco São Gabriel), representa os níveis epizonais (250-160 MPa) de uma coluna vertical de magma-mush com atividade moderada, controlada por zonas de falha pré-existentes N30-35°E e N55-60°W, cuja atividade foi vinculada aos eventos pós-colisionais da Orogênese Dom Feliciano (640-620 Ma). Sua formação ocorreu no final do Neoproterozóico (570-560 Ma), durante evento tectônico transtensivo ao longo de zonas de falha N15-20°E, responsável pelo soerguimento regional e expressivo magmatismo ácido, que é reflexo de orogenias mais jovens ocorridas a norte. É um pluton composto de dimensão moderada (473 km²), consistindo de monzogranitos centrais e sienogranitos na borda, que exibem grande variedade de texturas. A expansão do sistema magmático com acúmulo de voláteis no final de sua formação, como registrado pela cúpula granítica e ampla margem resfriada félsica, teria promovido a reativação do sistema de falhas NW-SE. Isto permite inferir o papel, mesmo reduzido, de mecanismos de alojamento via o soerguimento do teto. De acordo com o modelo proposto, a construção do CGSS decorre de episódios progressivos e cíclicos de recarga com magma máfico na base de uma soleira sienogranítica precursora, o que é registrado por feições diversificadas de hibridismo nos monzogranitos. Isto acarretou a estratificação do magma residente, que inicia a cristalizar separadamente: monzogranitos hibridizados na base, e sienogranitos a partir de líquidos magmáticos residuais coletados no topo, cuja cristalização foi sucessivamente postergada com a concentração de voláteis (H2 O e F). Episódios subsequentes de recarga máfica, em uma intrusão mais fria e parcialmente solidificada, promoveram o reaquecimento e a consequente maturidade textural e erosão termal das fácies cristalizadas; a remobilização de mushes cristalinos, com reintrusão e co-mingling de magmas hibridizados; e o maior subresfriamento termal no polo máfico. Tais feições apontam para o acúmulo de magmas sienograníticos altamente móveis, que sustentaram uma das câmaras magmáticas alimentadoras do vulcanismo coevo (Formação Acampamento Velho).

Geology, tectonic settings and iron ore metallogenesis associated with submarine volcanism in China: An overview

Submarine volcanogenic iron oxide (SVIO) deposits are one of the most important sources of high-grade iron ores in China. The spatial distribution of the deposits is controlled by the tectonic settings including arc, back-arc and rift environments, with the SVIO deposits mostly concentrated in the western part of China namely, the southwestern Yangtze Craton, Western and Eastern Tianshan, and Altay orogens and the Kaladawan iron ore district in the eastern part of the Altyn Tagh region. The Chinese SVIO deposits range in age from Paleoproterozoic to Mesozoic, and were formed during two main metallogenic epochs in the Proterozoic and Paleozoic. More than 70% of the SVIO deposits formed in the Paleozoic, with three important SVIO-metallogenic provinces recognized, in the Altay, Eastern and Western Tianshan orogens. These SVIO deposits are hosted in lithofacies that are related to submarine magmatism, such as lavas and associated pyroclastic and volcaniclastic-sedimentary rocks. The iron orebodies are hosted in different volcanic lithofacies with different features. Moreover, the different volcanic lithofacies in which the Fe ores are hosted also provide information as to their spatial relationship, ranging from distal to proximal to the eruption center or vent. Many of these deposits are characterized by well developed skarns, and could be interpreted either by a distal position of the ore system in question and/or exposed igneous rocks or active magma chamber, or a relationship to early metamorphism and continuous alteration at relatively high temperature followed by retrograde alteration as temperatures decline. Geological and geochemical evidence suggests that these deposits were formed as a result of submarine magmatic activity, including subaqueous volcanic eruptions, associated volcano-sedimentary lithofacies, and related post-magmatic hydrothermal activity. Iron oxide ore probably formed the hydrothermal fluids which generated the skarns could be a mixture of evolved magma-derived water and convecting sea water driven by the heat from the shallow active magma chamber, whereas volcano-sedimentary deposits could be formed by the fallout of the ore-bearing materials to the sea floor emanating from submarine eruption columns, or fractional precipitation of iron which had been introduced locally into the bottom water by volcanic-origin hydrothermal solutions and by leaching from the relatively iron-rich volcanic rocks. The formation of these various styles of Fe ore deposits is controlled by several key factors, such as magma differentiation, lithofacies of host rocks, temperature and chemical compositions of hydrothermal fluids, as well as the depth of sea water. In combination with their geological characteristics, geodynamic mechanisms and metallogenesis, we propose a genetic model in which the origin of these deposits can be related to the space–time evolution of the submarine volcanism, and their relationship to volcanic lithofacies variation, such as central, proximal and distal environments of ore formation.

Perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon

The formation of oceanic plagiogranite has been attributed primarily to either 1) extreme fractional crystallization of a mantle melt, or 2) partial melting of hydrated mafic crust, with support for the latter from field evidence and recent melting experiments. Remelting of hydrothermally-altered ocean crust could yield rocks (and minerals) with diverse primary magmatic δ18O values forming proximal to the magmatic center where crustal growth is occurring. To constrain the magmatic δ18O of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis, we characterized the δ18O of zircons in 22 plagiogranite samples (tonalite and trondhjemite) from 8 different ophiolites and one dacite sampled along the East Pacific Rise using Secondary Ion Mass Spectrometry (SIMS). The δ18O values of 202 magmatic zircons from ophiolites range from 3.9 to 5.6‰ (n=244 spots; average 4.9±0.6‰; 2SD), extending ~1‰ below typical zircon in equilibrium with mantle and from gabbroic massifs along slow-spreading mid-ocean ridges (4.7–5.9‰). East Pacific Rise dacite zircons range from 4.6 to 5.0‰ (n=12 spots). Plagiogranite from the dike-gabbro transition zone of the northern Oman Ophiolite yield the lowest δ18O(Zrn), with rock-average values of 4.3–5.0‰. The low-δ18O values are best explained by remelting of crust altered by hydrothermal fluids with seawater-like isotopic compositions at high temperatures, possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high-temperature hydrothermal system in the overlying crust. If the partial melt was assimilated into a fractionating melt lens with MORB-like δ18O, as envisioned for km-scale plagiogranite bodies in Oman, up to 20% contamination by a protolith with δ18O=2‰ would be required. Previous oxygen isotope constraints from quartz in Oman plagiogranite suggested melting of both high and low δ18O crust had occurred; comparison of quartz–zircon pairs indicates that quartz has been modified in most samples, and we find no evidence for the involvement of high-δ18O rocks during plagiogranite formation.

Origin and evolution of the Deception Island caldera (South Shetland Islands, Antarctica)

Deception Island has been interpreted variously as a classical ring fault caldera, as a tectonically controlled collapse caldera or as a tectonic depression. Review of previous studies combined with new fieldwork has allowed us to obtain a more precise model of the formation and internal structure of the Deception Island caldera. It formed as a result of the explosive eruption of basaltic-to-andesitic magmas, mostly as pyroclastic density currents representing in total a bulk volume of the order of 90 km3. Caldera collapse occurred rapidly along a polygonal structural network consisting of several pre-existing major normal faults. These faults, which originated as a result of regional tectonics, controlled pre- and post-caldera volcanism on the island. The formation of the caldera generated a very active geothermal system inside its depression, which is responsible for most of the present-day seismic activity and may also have a significant influence on the observed surface deformation. Our results do not support the hypothesis that there is a large but shallow, active magma chamber beneath the current caldera; instead we suggest that recent eruptions have been fed by small batches of deeper-sourced magmas. The intrusive remains of these eruptions and probably of other minor intrusions that have not reached the surface provide the main heat source that sustains the current geothermal system.

PIXE mapping on multiphase fluid inclusions in endoskarn xenoliths of AD 472 eruption of Vesuvius (Italy)

Abstract In this work we report a microthermometric and proton-induced X-ray emission (PIXE) mapping investigation on multiphase fluid inclusions hosted within nepheline and clinopyroxene of endoskarn xenoliths present in the deposits of the AD 472 eruption of Vesuvius. PIXE mapping on magmatic fluid inclusions repesents a useful tool for the characterization of the composition of magma derived fluids, exsolved from active magma chambers. In fluid inclusions we observed the occurrence of widespread solid phases formed by Fe, Pb, Zn, As ± Cu ± Mn, suggesting the good metal transport capability of Vesuvius magmatic fluids, which interacted with carbonate country rocks leading to the formation of endoskarn.

Geochronology and geochemistry of submarine volcanic rocks in the Yamansu iron deposit, Eastern Tianshan Mountains, NW China: Constraints on the metallogenesis

The Yamansu skarn iron deposit is hosted in Early Carboniferous submarine lava flow and volcaniclastic rocks of the Yamansu Formation in Eastern Tianshan Mountains, NW China. The lava flows are predominantly basaltic, with minor andesites. Laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) U–Pb zircon dating of the basalts and skarns yields almost coeval ages of 324.4±0.94 and 323.47±0.95Ma, respectively. The basalts contain clinopyroxene and plagioclase phenocrysts with a considerable amount of Fe–Ti oxide minerals in the groundmass as interstitial phases, probably suggesting that olivine–, clinopyroxene- and plagioclase fractionated within the magma chamber. Geochemically, the basalts are characterized by slight variations in SiO2 (42.90–46.61wt.%), P2O5 (0.08–0.12wt.%), MnO (0.35–0.97wt.%) and TiO2 (0.74–0.82wt.%), and relatively large variations in CaO (6.93–15.13wt.%), Al2O3 (14.71–19.93wt.%), total Fe2O3 (8.14–12.66wt.%) and MgO (4.96–8.52wt.%). They possess flat to light rare earth element (REE)-depleted patterns and display variable degrees of depletions in high field-strength elements (HFSE), suggesting a transitional feature between MORB and arc volcanic rocks, and indicating a back-arc tectonic setting. Furthermore, the geochemical signature also suggests that the volcanic rocks of Yamansu Formation were produced by partial melting of the spinel-facies, asthenospheric mantle peridotite which had been metasomatized by slab-derived fluids. The broadly overlapping ages of the basalts and skarn mineralization suggests that the skarn formation in the Yamansu deposit is related to subaqueous volcanism. In combination with the available information including fluid inclusions and stable isotope data, we infer that the hydrothermal fluids that generated the skarns could be a mixture of evolved magma-derived fluids and convecting sea water driven by the heat from the shallow active magma chamber. The Yamansu basalts provided the source of iron for the skarn mineralization. We envisage the submarine volcanism, skarn alteration and iron mineralization in the Yamansu iron deposit as a continuous process, different from either conventional intrusion-related skarn type or submarine volcanic exhalation sedimentation type.

First insights on the metallogenic signature of magmatic fluids exsolved from the active magma chamber of Vesuvius (AD 79 “Pompei” eruption)

Fluid inclusions hosted in sanidine from xenoliths, representing the crystallization front of the AD 79 magma chamber of Vesuvius, offer a rare opportunity to track the composition of magmatic fluids exsolving at the peripheral part of a growing K-alkaline magma chamber disrupted by an explosive eruption. This work demonstrates that the exsolving magmatic fluids have a complex composition taking in evidence the great capability of metal transport by means of these solutions. Inclusions are particularly enriched in Zn, Mo, W, Sb, Bi, Tl and Pb. Metals display two different positive correlations: 1) Pb–Zn–Tl–Bi; 2) Mo–W. The different trends may reflect the different complexing agent for these elements (probably chloride for Pb–Zn–Tl–Bi and probably hydroxy for Mo–W). The occurrence of a “silicothermal” component in these fluids may furthermore favor the metal transport of these solutions. This is probably a major, previously underestimated process in the enrichment of some metals in magmatic–hydrothermal systems. This hypothesis may challenge conventional ideas on fluid-phase mass transport of components by means of magmatic–hydrothermal fluids, and should be taken into account in future works on these topics.

Upper crustal structure of an active volcano from refraction/reflection tomography, Montserrat, Lesser Antilles

To better understand the volcanic phenomena acting on Montserrat, the SEA-CALIPSO seismic experiment (Seismic Experiment with Airgun-source – Caribbean Andesitic Lava Island Precision Seismo-geodetic Observatory) was conducted in 2007 December with the aim of imaging the upper crust and the magmatic system feeding the active Soufriére Hills Volcano. The 3-D survey covered an area of about 50 × 40 km and involved the deployment of 247 land stations and ocean-bottom seismometers (OBSs). A subset of the data, recorded by four OBSs and four land stations on a southeast to northwest line, has been analysed, and traveltimes have been inverted to obtain a 2-D seismic velocity model through the island. Inverted phases include crustal and sediment P waves and wide-angle reflections. The resulting velocity model reveals the presence of a high velocity body (3.5–5.5 km s-1) beneath the island, with highest velocities beneath the Soufriére and Centre Hills, corresponding primarily to the cores of these volcanic edifices, built of a pile of andesite lava domes and subsequent intrusions. In the offshore region, velocities in the surficial sediment layer vary from 1.5 to 3.0 km s-1, consistent with a mainly calcareous and volcaniclastic composition. A wide-angle reflector is observed at a depth of ∼1200 m below the seabed, and appears to deepen beneath the island. The upper crust beneath this reflector has velocities of 4.0–6.0 km s-1 and is inferred to correspond to plutonic and hypabyssal rocks and sedimentary material of the old arc. The high velocity region beneath the island, extends into the crust to a depth of at least 5 km, and is believed to be caused by an intrusive complex, possibly of intermediate composition. A low velocity zone, as would be expected in the presence of an active magma chamber, was not observed perhaps due to the limited resolution beneath ∼5 km depth. Our results so far provide the first wide-angle seismic constraints on the upper crustal structure of the island to a depth of 10 km, and will help understanding the processes that drive volcanism at Montserrat and other island arc volcanoes.

The Mammoth Peak sheeted complex, Tuolumne batholith, Sierra Nevada, California: a record of initial growth or late thermal contraction in a magma chamber?

The Mammoth Peak sheeted intrusive complex formed in the interior of a ~7–10 km deep magma chamber, specifically in the Half Dome granodiorite of the Tuolumne batholith, central Sierra Nevada, CA (USA). The sheets consist of fractionated melts with accumulated hornblende, biotite, magnetite, titanite, apatite, and zircon. The accumulation, especially of titanite, had a profound effect on minor and trace elements (Nb, Ta, Ti, REE, U, Th, P, Zr, Hf, etc.), increasing their contents up to five to six times. Our thermal–mechanical modeling using the finite element method shows that cooling-generated tensile stresses resulted in the inward propagation of two perpendicular sets of dilational cracks in the host granodiorite. We interpret the sheeted complex to have formed by a crack-seal mechanism in a high strength, crystal-rich mush, whereby outward younging pulses of fractionated magma were injected into these syn-magmatic cracks at the margin of an active magma chamber. Thermal–mechanical instabilities developed after the assembly of the sheeted complex, which was then overprinted by late ~NW–SE magmatic foliation. This case example provides a cautionary note regarding the interpretation that sheeted zones in large granitoid plutons imply a diking mechanism of growth because the sheeted/dike complexes in plutons (1) may display inverse growth directions from the growth of the overall intrusive sequence; (2) need not record initial chamber construction and instead may reflect late pulsing of magma within an already constructed magma chamber; (3) have an overprinting magmatic fabric indicating the continued presence of melt after construction of sheeted complexes and thus a prolonged thermal history as compared to dikes; and (4) because the scale of the observed sheeted complexes may be small (<1%) in comparison to large homogenous parts of plutons, in which there is no evidence for sheeting or diking. Thus, where extensive dike complexes in plutons are absent, such as in much of the Tuolumne batholith, the application of an incremental diking model to explain chamber construction is at best speculative.

Magmatic erosion of the solidification front during reintrusion: the eastern margin of the Tuolumne batholith, Sierra Nevada, California

The Tuolumne batholith, Sierra Nevada, California, consists of several nested granitoid units and is an example of upper-crustal normally zoned intrusions. The two outermost units of the batholith are separated by a wide gradational contact in what is interpreted to represent a large magma chamber. In the Potter Point area near the eastern margin of the batholith, the gradational contact is cross-cut by a network of interconnected mafic–felsic sheets, which grade into zones of magmatic erosion by stoping where the host granodiorite between the sheets was entirely removed and replaced by younger enclave-rich quartz diorite. We interpret these features to record disruption of a steep solidification front, which migrated inwards from the eastern batholith margin and separated the mushy to solidified margin from the remaining active magma chamber. When intersecting the gradational contact, the solidification front started to break up via a network of tectonically driven fractures accompanied by simultaneous injection of localized magma pulses. The solidification front break-up is interpreted here as an initial stage of a “recycling” process, whereby older magma mush is disrupted and incorporated into younger magma batches, a process we propose to have been widespread along internal contacts in the Tuolumne magma chamber.