Calc-alkaline Volcanoes Research Articles

Detecting multiscale periodicity from the secular effusive activity at Santiaguito lava dome complex (Guatemala)

Santiaguito, Guatemala, represents one of the best cases of active lava dome complex in the world, producing lava flow effusion, weak explosive activity, and cycles of lava dome extrusion over varying timescales. Since the inception in 1922, it has shown a remarkable constant eruptive activity, characterized by effusion of blocky domes and lava flows punctuated by moderate explosions of gas-and-ash and pyroclastic flows. In this study, we reconstruct the time evolution of discharge rates of Santiaguito across one entire century, from 1922 to 2021, combining, for the more recent activity, new satellite thermal data. By using discrete Fourier transform (DFT) and Morlet wavelet analyses, we identify three fundamental periodicities in subsets of the 1922–2021 time-series: (i) long term (ca. 10 years), (ii) intermediate term (ca. 3.5 years), and (iii) short term (from ca. 1 year to ca. 3 months), which are comparable with those observed at other lava dome eruptions at calc-alkaline volcanoes. Such inferred periodicities provide a powerful tool for the interpretation of the non-linear eruptive behaviour and represent a pivotal benchmark for numerical modelling aimed to reconstruct the dynamics of the magma feeding system based on a time-averaged discharge rate dataset.Graphical

Understanding eruptive style variations at calc-alkaline volcanoes: the 1913 eruption of Fuego de Colima volcano (Mexico)

The 1913 sub-Plinian eruption of Fuego de Colima volcano (Mexico) occurred after almost 100 years of effusive and (minor) Vulcanian explosive activity, which modulated dome growth and destruction. Dome extrusion persisted from 1869 to 1913. The transition to explosive eruption started on 17 January 1913, and it progressed in three phases: (1) opening, with the generation of block-and-ash flows, (2) vent clearing, with strong explosions that destroyed the summit dome and decompressed the magmatic system, and (3) sustained column (sub-Plinian fallout) with final collapse producing pyroclastic density currents. Because of this succession of events, the 1913 activity represents an excellent case-study for investigating the eruptive style changes at calc-alkaline volcanoes. We investigated the conditions that led to eruptive style transition from effusive (dome growth) to explosive (the final sub-Plinian fallout) through steady-state numerical simulations, using subsurface data and independently inferred (from volcanological data) mass discharge rates as constraints. Results show good matches for hybrid geometrical settings of the shallow conduit-feeding system (i.e., dyke developing into a shallower cylindrical conduit), and the magma chamber top at 6 km of depth. The fragmentation level was shallower than 2 km, as inferred from the lithics contained in the sub-Plinian fall deposits of Phase (3). The most likely solution is represented by a dyke having major axis between 200 and 2000 m and the minor axis of 40 m. The dyke-cylinder transition was set at a depth of 500 m, with a cylinder diameter of 40 m. It emerges that at least two main mechanisms may be responsible for the effusive to explosive transition that led to the Phase (3) of the 1913 eruption: (i) an increase in magma chamber overpressure (magmatic triggering) or (ii) decrease of lithostatic stress acting on the volcano (non-magmatic triggering). The former implies arrival into the magma chamber of a batch of fresh magma, which can have volume between 10 and 200 × 106 m3, depending on the values of magma and host rock compressibility. The latter requires decompression-induced emptying of at least the equivalent of 1000 m of the magma column to produce the necessary unloading of the conduit-feeding system. A sudden jerk in the lateral spreading of the Fuego de Colima cone would be a reliable mechanism for decompressing the upper conduit and driving fragmentation processes over a time period of few hours. The results are not conclusive on an internal (magma chamber overpressure), external (lowering of lithostatic load), or mixed (internal and external) trigger of the observed eruptive style transition. This work highlights how different processes can have non-linear cascade effects on close-to-equilibrium volcanic systems like Fuego de Colima volcano.

Magmatic diversity of western Mexico as a function of metamorphic transformations in the subducted oceanic plate

This geochemical study of the Mexican subduction zone elucidates how metamorphic and dehydration reactions affecting the subducted oceanic plate at different depths can influence magmatic diversity. In the western Trans-Mexican Volcanic Belt, there is a narrow potassic volcanic front running in parallel to the Middle American Trench that becomes replaced by intraplate-like high-Nb rocks to the north, and by more typical calc-alkaline products to the southeast. Potassic rocks have high MgO and are enriched in incompatible trace elements, but have lower heavy rare earth element contents than more evolved calc-alkaline and high-Nb magmas, and slightly more enriched Sr, Nd and Pb isotopes. Potassic magmas also have higher Rb/Cs and Ba/Cs ratios than the calc-alkaline and high-Nb suites, and extend to unusually high Nb/Ta ratios that correlate positively with Rb/Ta, Zr/Ta, La/Ta and Gd/Yb. These chemical variations are inconsistent with different extents of melting of a peridotitic source, but are also incompatible with melting of a phlogopite-rich mantle (vein-plus-wall-rock relationship), unless mica is totally consumed during melting, and a titaniferous phase such as rutile remains in the residue together with garnet. This assemblage is unlikely in the source region of primitive hydrous magmas, but it is what would be expected during dissolution of phengite and monazite/allanite in the subducted slab, with the concurrent formation of an anhydrous rutile-bearing eclogite. The magmatic diversity of western Mexico can thus be explained by invoking contributions of chemically different subduction agents as a function of slab depth and residual mineralogy: a low-pressure/temperature aqueous fluid would induce melting of the peridotitic mantle wedge and form typical calc-alkaline volcanoes, whereas a deeper and hotter slab-derived melt (or supercritical liquid) would contribute to the formation of potassic magmas due to phengite/monazite/allanite disintegration. In this context, intraplate-like magmas derive from decompression melting of the upper mantle as a natural consequence of subduction geodynamics.

Intra-sill magmatic evolution in the Cummings Complex, Abitibi greenstone belt: Tholeiitic to calc-alkaline magmatism recorded in an Archaean subvolcanic conduit system

The stratigraphy of the Abitibi greenstone belt in the Chibougamau area (southern Superior Province, Québec), is dominated by 2 cycles of mafic–felsic metavolcanic and metasedimentary rocks constituting the Roy Group, which is riddled by metagabbroic sills (25%). The Doré Lake Complex (DLC, 2728 Ma) is emplaced into the lower Roy Group. The Cummings Complex sills (2717 Ma) were injected between the Bruneau member and Blondeau Formations of the 2nd Roy Group cycle. The sills of the Cummings Complex (Roberge, Ventures and Bourbeau Sills) contain metaperidotite, metapyroxenite, metagabbro and metagranophyric facies. The trace element contents of melts in equilibrium with these metacumulate rocks were calculated and are compared to Roy Group lavas to clarify the regional magmatic history. Many DLC model melts have fractionated trace element profiles, with LILE-LREE-enrichment, HREE-depletion, and negative Nb–Ta-anomalies suggesting that the DLC formed largely from calc-alkaline melts extracted from garnet-bearing residues. The DLC is coeval with, and shows geochemical resemblances to Waconichi Formation tuffs (the felsic cap of the 1st Roy Group cycle), suggesting it could represent the Waconichi's shallow magma chamber. Meta-anorthosite rafts from the para-autochtonous zone of the Grenville province yield model melts closely resembling those of the DLC and are correlated on this basis. Most Roy Group sills yield model melts with trace element patterns typical of Archaean tholeiites, suggesting they fed the regionally-dominant tholeiitic volcanic plain lavas of the Roy Group. Models for the Cummings Complex imply that it contained two types of magma. Model melts from the Roberge Sill have strongly fractionated calc-alkaline-like trace element patterns, while those of the Ventures and Bourbeau Sills are mostly flat, N-MORB-normalized tholeiitic-like patterns that cannot be derived from the Roberge Sill melts by fractional crystallization. The Roberge Sill must have a separate magmatic stem, and represents the feeder system for a calc-alkaline volcano, possibly represented in part by Blondeau Formation rhyolites, andesites and basalts? If this is correct, the Roberge may be older than the Ventures and Bourbeau Sills, which would have fed the tholeiitic base of a third magmatic cycle, since eroded.

Seismotectonic pattern and the source region of volcanism in the central part of Sunda Arc

The seismotectonic pattern in the central part of the Sunda Arc (Java, Nusa Tenggara) was studied in relation to the distribution of active calc-alkaline volcanoes, using global seismological data. Hypocentral determinations of the International Seismological Centre from the period 1964–1999, as relocated by Engdahl, and Harvard Centroid Moment Tensor Solutions from the period 1976–2003 were used. The following phenomena, which could assist the location of the source region of primary magma for island arc calc-alkaline volcanism, were observed: (1) An aseismic gap without any strong teleseismically recorded earthquakes was found in the Wadati-Benioff zone of the subducting slab along the whole investigated region of the Sunda Arc, forming a continuous strip of laterally variable depth and shape, at depths between 100 and 200 km. The absence of strong earthquakes (with m b>4.0) indicates a significant change in the mechanical properties of the subducting slab at intermediate depths. All active calc-alkaline volcanoes in the Sunda Arc are located above this gap. (2) The majority of earthquakes occurring in the lithospheric wedge of the Eurasian Plate above the subducted slab could be attributed to several deep-rooted seismically active fracture zones of regional extent. All delineated active fracture zones display a thrust tectonic regime as shown by the available fault plane solutions. (3) Clusters of earthquakes were found beneath active volcanoes of western Java, Bali and Nusa Tenggara in the lithospheric wedge above the slab and identified as seismically active columns. These clusters occur only beneath the volcanoes that are located at the outcrops of seismically active fracture zones. We interpret the earthquakes in these clusters beneath volcanoes as events induced by magma transport through the medium of the lithospheric wedge that has been subcritically pre-stressed by the process of plate convergence. (4) Beneath the volcanoes of central Java no seismically active columns were observed. The latter volcanoes are not located at the outcrop of any seismically active fracture zone. The presence of the intermediate-depth aseismic gap in the Wadati-Benioff zone and the presence of seismically active columns above the gap beneath some active volcanoes support the concept that the subducted oceanic lithosphere is the source region for the primary magma for calc-alkaline volcanoes at convergent plate margins. However, for volcanoes without any seismically active columns and fracture zones beneath them, a source region for the primary magma in the lithospheric wedge above the subduction zone cannot be excluded.

Seismicity pattern: an indicator of source region of volcanism at convergent plate margins

The results of detailed investigation into the geometry of distribution of earthquakes around and below the volcanoes Korovin, Cleveland, Makushin, Yake-Dake, Oshima, Lewotobi, Fuego, Sangay, Nisyros and Montagne Pelée at convergent plate margins are presented. The ISC hypocentral determinations for the period 1964–1999, based on data of global seismic network and relocated by Engdahl, van der Hilst and Buland, have been used. The aim of this study has been to contribute to the solution of the problem of location of source regions of primary magma for calc-alkaline volcanoes spatially and genetically related to the process of subduction. Several specific features of seismicity pattern were revealed in this context. (i) A clear occurrence of the intermediate-depth aseismic gap (IDAG) in the Wadati–Benioff zone (WBZ) below all investigated active volcanoes. We interpret this part of the subducted slab, which does not contain any teleseismically recorded earthquake with magnitude greater than 4.0, as a partially melted domain of oceanic lithosphere and as a possible source of primary magma for calc-alkaline volcanoes. (ii) A set of earthquakes in the shape of a seismically active column (SAC) seems to exists in the continental wedge below volcanoes Korovin, Makushin and Sangay. The seismically active columns probably reach from the Earth surface down to the aseismic gap in the Wadati–Benioff zone. This points to the possibility that the upper mantle overlying the subducted slab does not contain large melted domains, displays an intense fracturing and is not likely to represent the site of magma generation. (iii) In the continental wedge below the volcanoes Cleveland, Fuego, Nisyros, Yake-Dake, Oshima and Lewotobi, shallow seismicity occurs down to the depth of 50 km. The domain without any earthquakes between the shallow seismically active column and the aseismic gap in the Wadati–Benioff zone in the depth range of 50–100 km does not exclude the melting of the mantle also above the slab. (iv) Any earthquake does not exist in the lithospheric wedge below the volcano Montagne Pelée. The source of primary magma could be located in the subducted slab as well as in the overlying mantle wedge. (v) Frequent aftershock sequences accompanying stronger earthquakes in the seismically active columns indicate high fracturing of the wedge below active volcanoes. (vi) The elongated shape of clusters of epicentres of earthquakes of seismically active columns, as well as stable parameters of the available fault plane solutions, seem to reflect the existence of dominant deeply rooted fracture zones below volcanoes. These facts also favour the location of primary magma in the subducting slab rather than in the overlying wedge. We suppose that melts advancing from the slab toward the Earth surface may trigger the observed earthquakes in the continental wedge that is critically pre-stressed by the process of subduction. However, for definitive conclusions it will be necessary to explain the occurrence of earthquake clusters below some volcanoes and the lack of seismicity below others, taking into account the uncertainty of focal depth determination from global seismological data in some regions.

Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea: petrology and geochemistry of fluid-metasomatised mantle wedge xenoliths

Ultramafic, mafic and sedimentary xenoliths have been recovered from a recently erupted, shoshonitic submarine cinder cone (Tubaf and Edison volcanoes) from the Tabar–Lihir–Tanga–Feni island arc, located in the New Ireland basin of Papua New Guinea. These samples represent a proxy drill hole that can be re-assembled into an ‘ophiolite-type’ model of oceanic lithosphere. Petrographic and geochemical examination of the gabbroic and depleted mantle xenoliths indicates that the New Ireland fore-arc lithosphere is a fragment of ancient Pacific Plate generated at a mid-ocean ridge spreading centre and transported to the Pacific–Australian Plate margin. Convergent margin processes subjected the harzburgitic mantle wedge to hydrofracturing and hydration metasomatism at T=790–1030°C as a consequence of dewatering of a subducted slab. Advection of a high-density, H 2O-rich fluid containing a substantial dissolved component (alkali aluminosilicate melt and aqueous carbon and sulphur species) through these mantle fractures caused a net transfer of soluble elements from the lower to upper mantle wedge and created a network of oxidised (ΔFMQ≈1.8–2.0) metasomatised peridotite enriched in orthopyroxene, clinopyroxene, phlogopite, amphibole, magnetite, and Fe–Ni sulphides. The vein mineral assemblage magnetite+sulphide indicates precipitation from a hydrous fluid with high SO 2/H 2S, consistent with the hydrous fluid being derived from dehydration of subducted, altered oceanic crust. Preferential partial melting of these metasomatically enriched mantle wedge regions could account for the highly oxidised, sulphur- and alkali-rich nature of the high-K calc-alkaline volcanoes of the Tabar–Lihir–Tanga–Feni island chain.

The origin of composition gaps at South Sister volcano, central Oregon: implications for fractional crystallization processes beneath active calc-alkaline volcanoes

South Sister is an active, calc-alkaline strato-volcano located in west-central Oregon in the high Cascades. South Sister contains lavas that range from basalt (50% SiO2) to rhyolite (75% SiO2). Within this compositional spectrum the lavas are restricted to three distinct populations: basalt/basaltic-andesite, andesite and rhyolite. Compositional, textural and mineralogic criteria indicate that the basaltic-andesites are hybrid mixtures of basalt and andesite magma. This implies that the system is characterized by basalt, andesite and rhyolite magma populations that are separated from one another by composition gaps. Major, minor and trace element modeling suggests that the three magma populations are related to one another primarily through fractional crystallization. We propose a standard fractionation model for generating compositional diversity amongst erupted lavas wherein in situ fractional crystallization occurs within a downward descending solidification front. Extraction and eruption of the interstitial fractionated liquid, now separated from the parental liquid by a composition gap, occurs only after the percentage of crystallization within the solidification front has exceeded the appropriate critical crystallinity value for the given parental magma. We envision a magmatic plumbing system characterized by multiple, independently solidifying bodies of magma. Thus, at any given time, discrete bodies of basalt, andesite and rhyolite magma may exist. Interaction and mixing among these bodies could generate the continuous range of lava compositions that characterize most calc-alkaline volcanic complexes.

The dimensions of magmatic inclusions as a constraint on the physical mechanism of mixing

Silicic lavas often bear inclusions of more mafic lava. Several physical processes have been proposed by which these magmas may become mixed. Measurements of the sizes of mafic inclusions in lavas of two calc–alkaline volcanoes are used here to obtain a characteristic length scale for this type of mixing and provide a means to discriminate between different mixing processes. We quantitatively assess the proposal that, in a stratified reservoir, dynamic interactions of rising gas bubbles with the interface between a lower layer of mafic magma and an upper layer of silicic magma lead to the formation of vesiculated blobs of the lower magma in the upper one. This is due to gravitational instability of a foam layer growing at the interface between the two magmas. Our theoretical calculations of this instability predict both the correct order of magnitude for the characteristic size of inclusions and the way in which inclusion size varies with the physical properties of the magmas. This fluid dynamic framework is used to map out different possible flow regimes and place constraints on the fraction of gas in these systems.

Experimental constraints on differentiation and H2O abundance of calc-alkaline magmas

Partial melting experiments were conducted on a natural high-alumina basalt from the Higashi-Izu volcanoes (47.4 wt.% SiO2; 8.3 wt.% MgO) with 1 and 2 wt.% total H2O at 0.5 and 1 GPa, 1125-875°C. The partial melts produced segregate by capillary action into crimped parts of the capsule, preventing chemical modification of the melt during quenching. Liquid compositions in 1 wt.% total H2O runs at 0.5 GPa mimic the chemical variations of calc-alkaline andesites and dacites associated with the basalt. The residual phase assemblages from 1 wt.% total H2O experiments are consistent with the observed phenocrysts in two pyroxene andesite and hornblende (hb)-orthopyroxene (opx) dacite. In contrast, a clinopyroxene (cpx)-hb rhyolite assemblage is produced in the 2 wt.% total H2O experiments. Although no such cpx-hb rhyolite is found in either Northeast Japan or the Izu-Mariana arc, this type of rhyolite is found in Central America, which is also characterized by high H2O contents in the associated basaltic melts ([1]).The abundance of SiO2 in residual liquids increased from 49 to 54 wt.% in the 25°C interval between 1125° and 1100°C in the 1 wt.% total H2O system, and from 63 to 71 wt.% in the 50°C interval between 975° and 925°C. The first gap between basalt and andesite is generated by a sudden change in the amount of partial melt, and the second gap between andesite and dacite is produced by the formation of hornblende by the consumption of pyroxene, because the SiO2 abundance of hornblende is less than that of pyroxene. These two reaction relations result in the existence of two compositional gaps, between basalt and andesite, and between andesite and dacite, which are recognized in the Higashi-Izu volcano group and many other calc-alkaline volcanoes.

Petrography and geochemistry of Plio-Quaternary calc-alkaline volcanoes of Southwestern Colombia

The six studied southwestern Colombian Plio-Quaternary volcanoes (Puracé, Doña Juana, Galeras, Azufral, Cumbal and Chiles) are located in the central part of the Northern Volcanic Zone (NVZ) of the Andes. The lavas are predominantly andesites and dacites, with lesser amount of basalt, and range 52% to 70% SiO 2. The two-pyroxenes basic and acid andesites exhibit plagioclase phenocrysts with core composition of An 50–80 and An 40–70 respectively. Hornblende and biotite occur in dacites in association with plagioclase cores ranging An 30–50. The lavas belong to medium- to high-potassic calc-alkaline series, typical of active continental margins. Two types of lavas can be distinguished in the studied area: 1. 1) Northeastern-type lavas which show high TiO 2, K-group elements, LREE, and Ce Yb ratio, low HREE content and a low Ba La ratio 2. 2) Southwestern-type lavas are characterized by low TiO 2, K-group elements, LREE, low Ce Yb , an enrichment in HREE content and high Ba La ratio. The northward increase in incompatible elements is attributed to a higher contribution of the siliceous Pre-Mesozoic crust in the north of the studied area. The younger (Post-Cretaceous) oceanic basement underlying the southwestern volcanoes cannot be responsible for an enrichment in incompatible elements only by contamination processes. Geochemical evidences suggest that the NVZ calc-alkaline lavas were derived from a primitive mantle which was enriched in Ba and partially in other K-group elements by dehydration and/or partial melting of the oceanic slab within the asthenosphere. This primitive melt, induced in response of the hydration processes within the mantle, suffered various stages of assimilation and fractional crystallization at lower to intermediate crustal levels, as revealed by the regional depletion in HREE of the northeastern-type lavas. At an upper crustal level, within the magma chamber, magma mixing between “primitive” up-rising basalt melt and andesitic-dacitic differentiated magma accompanied by fractional crystallization and crustal assimilation processes produced the andesitic series of each volcanoes.

Crystal retention, fractionation and crustal assimilation in a convecting magma chamber, Nisyros Volcano, Greece

Nisyros island is a calc-alkaline volcano, built up during the last 100 ka. The first cycle of its subaerial history includes the cone-building activity with three phases, each characterized by a similar sequence: (1) effusive and explosive activity fed by basaltic andesitic and andesitic magmas; and (2) effusive andextrusive activity fed by dacitic and rhyolitic magmas. The second eruptive cycle includes the caldera-forming explosive activity with two phases, each consisting of the sequence: (1) rhyolitic phreatomagmatic eruptions triggering a central caldera collapse; and (2) extrusion of dacitic-rhyolitic domes and lava flows. The rocks of this cycle are characteized by the presence of mafic enclaves with different petrographic and chemical features which testify to mixing-mingling processes between variously evolved magmas. Jumps in the degree of evolution are present in the stratigraphic series, accompanied by changes in the porphyritic index. This index ranges from 60% to about 5% and correlates with several teochemical parameters, including a negative correlation with Sr isotope ratios (0.703384–0.705120). The latter increase from basaltic andesites to intermediate rocks, but then slightly decrease in the most evolved volcanic rocks. The petrographic, geochemical and isotopic characteristics can be largely explained by processes occurring in a convecting, crystallizing and assimilating magma chamber, where crystal sorting, retention, resorption and accumulation take place. A group of crystal-rich basaltic andesites with high Sr and compatible element contents and low incompatible elements and Sr isotope ratios probably resulted from the accumulation of plagioclase and pyroxene in an andesitic liquid. Re-entrainment of plagioclase crystals in the crystallizing magma may have been responsible for the lower 87Sr/86Sr in the most evolved rocks. The gaps in the degree of evolution with time are interpreted as due to liquid segregation from a crystal mush once critical crystallinity was reached. At that stage convection halted, and a less dense, less porphyritic, more evolved magma separated from a denser crystal-rich magma portion. The differences in incompatible element enrichment of pre-and post-caldera dacites and the chemical variation in the post-caldera dome sequence are the result of hybridization of post-caldera dome magmas with more mafic magmas, as represented by the enclave compositions. The occurrence of the quenched, more mafic magmas in the two post-caldera units suggests that renewed intrusion of mafic magma took place after each collapse event.

Discovery of a pure rhenium mineral at Kudriavy volcano

KUDRIAVY volcano on Iturup island in the Kuril arc is an active calc-alkaline volcano. It has not erupted this century; its current volcanic activity is characterized by hot (up to 910oC) gas jets which have been stable for at least 30 years. The composition of the gaseous emissions is typical of high-temperature fumaroles, but we report here the discovery of unusual subsurface sublimates associated with one gas jet—a sulphide mineral containing rhenium as the only cation. To our knowledge, this is the first reported occurrence of a pure rhenium mineral. The concentration of rhen-ium in the fumarole gas is only 2–10 p.p.b., so the condensation of pure rhenium sulphide from this gas requires both enrichment of rhenium by eight orders of magnitude and remarkable selectivity. Rhenium is generally believed to exist in only trace amounts at the Earth's surface, but our findings demonstrate that it can be readily mobilized, dispersed and concentrated by degassing magmas.

A simple magma-mixing model for sulphur behaviour in calc-alkaline volcanic rocks: mineralogical evidence from Mount Pinatubo 1991 eruption

The increasing recognition of anhydrite as a primary magmatic mineral in volcanic products, including the important 1991 Pinatubo eruption, places new emphasis on the cycling of sulphur from deep sources including the upper mantle. We present a simple model for the supersaturation of sulphur during mingling and mixing in calc-alkaline magmas due to the quenching and oxidation of the mafic end member, which may have important consequences for predicting the annual flux of igneous sulphur. This model predicts the coexistence of anhydrite and sulphur dioxide gas, and explains the discrepancy between erupted magma volumes and sulphur gas emissions. Although sulphur may exist as a number of species in volcanic emissions, one of its most important phases in volcanic rocks is primary anhydrite (CaS0 4 ). A number of important historic eruptions of calc-alkaline volcanoes have now been found to involve anhydrite-bearing magmas. These include Mount Lamington in Papua New Guinea (Arculus et al. 1983), Nevado del Ruiz in Colombia (Fournelle 1990), and El Chichon in Mexico ( Rose et al. 1984 ). The Ruiz eruption involved documented magma mixing, and both the Nevado del Ruiz and El Chichon eruptions were associated with very high sulphur emissions (both approximately 3.3 x 10 6 tonnes; Krueger 1983 ; Williams et al. 1990 ). We have observed magma-mixing textures in detailed studies of two neighbouring calc-alkaline volcanoes in northern Chile, at Lascar ( Matthews 1992 ) and Tumisa ( Gardeweg 1988 ). Anhydrite has been found associated with these textures in pumice from Lascar Volcano. Both anhydrite and similar disequilibrium textures indicative of magma

The origin of the potassic rock suite from Batu Tara volcano (East Sunda Arc, Indonesia)

atu Tara is an active potassic volcano in the eastern Sunda arc. Its leucite-bearing rock suite can be subdivided into two groups, one less evolved with Th<20 ppm, the other more evolved with Th>20 ppm. 87Sr/ 86Sr, δ 18O and trace-element systematics in the less evolved group suggests that existence of parental magmas with different mantle origins. The mantle below Batu Tara is most likely heterogeneous and several source components are involved in magma genesis. Trace element and isotopic compositions of Batu Tara and adjacent volcanoes are consistent with the involvement of a subducted sedimentary/crustal component as well as MORB and OIB mantle, the latter with geochemical characteristics comparable to the mantle underlying Muriah (Java). Melt extraction from this complex mixture is envisioned as a two-stage process: partial melts of the crust-contaminated MORB mantle mix in the mantle wedge with partial melts of OIB domains. Different mixtures of these two melts provide the parental magmas that enter the volcanic plumbing system, where crystallization, hybridization and refilling processes occur. The calcalkaline volcanoes in the arc segment show stronger signatures for a subducted crustal component than Batu Tara, which displays a greater influence from the OIB mantle source. The potassium enrichment can therefore be attributed to contributions both from the enriched mantle and from subducted crustal material. Mantle-type δ 18O values of the Batu Tara magmas indicate that the mantle wedge below potassic orogenic volcanoes is not necessarily strongly enriched in 18O.

Magmatic evolution of Pacaya and Cerro Chiquito volcanological complex, Guatemala

Four major phases are distinguished during the building of the Pacaya volcanological complex (Guatemala): (1) the ‘ancestral volcano’, now much eroded, covered by younger deposits and battered by faulting and landslides; (2) the initial cone made up of large lava flows and dated at about 0.5 Ma; (3) andesito-dacitic domes (Cerro Chiquito dome and others) emplaced during an extrusive phase at about 0.16 Ma; and (4) the active Pacaya volcano. Lavas of phases 2 and 4 are basalts and basaltic andesites with almost the same major and trace element compositions. Classical enrichment in LILE and depletion in HFSE are observed. Phase 3 domes show magma-mingling features. The dacitic host rock includes basaltic andestic enclaves, 20 to 30% in volume. According to geochemical and mineralogic data (Mg/Fe ratios of basic minerals higher in dacite, groundmass glasses sodic in dacite and potassic in basaltic andesite), the basaltic andesites and dacites of phase 3 cannot be related by a simple fractional crystallization process. The existence of such differences suggests that magma mingling/mixing processes were involved by a connection between the two magma chambers prior to the extrusion of the andesito-dacitic domes. However, some trace element data clearly suggest that fractional crystallization played a significant role in the differentiation of these lavas. Remelting of amphibole-bearing cumulates from the dacite may also have played a role in the basaltic andesitic liquid genesis. Thermodynamical parameters of each liquid are contrasted. The basaltic andesitic magma, at a high temperature (1037°C) and in relatively small amounts, is embayed in the cooler (905° C) dacitic magma. The former liquid, denser (2.72) and less viscous (103.31 poises for free crystal liquid) may crystallize while the latter, lighter (2.60) and more viscous (104.46 poises), remains still liquid. Isotopic data (0.70383<87Sr/86Sr <0.70400; 0.512785<143Nd/144Nd<0.512908; 18.61<206Pb/204Pb<18.66; 15.56<207Pb/204Pb <15.58; 38.30<208Pb/204Pb<38.40) indicate that all the lavas (from Pacaya as well as from Cerro Chiquito) are cogenetic and derive from the same mantle source. Sr, Nd and Pb isotope ratios are similar to those of OIBs. (230Th/232Th) activity ratios on two historical lavas are respectively 1.2 and 1.3. The Th excess is similar to that of other calcalkaline volcanoes emplaced on a continental crust. These lavas evolved, possibly in separate magma chambers, through processes of fractional crystallization and magma mixing.

Anhydrite-bearing pumices from Mount Pinatubo: further evidence for the existence of sulphur-rich silicic magmas

THE eruption of EI Chichon in 1982 showed that relatively small but sulphur-rich eruptions from calc-alkaline volcanoes can produce long-lived stratospheric clouds of sulphate aerosols1,2, which affect the global climate3,4. Here we report the presence of primary anhydrite (CaSO4) phenocrysts in dacitic pumice clasts from the 14–15 June eruption of Mount Pinatubo, which clearly shows that the Mount Pinatubo magma is also rich in sulphur. The post-eruptive sulphur content of the Pinatubo pumices ranges from 0.37 to 0.48 wt% SO3. The considerable amount of sulphate aerosol injected into the stratosphere by the Mount Pinatubo eruptions5 should lead to a measurable cooling of the Earth's surface over the next few years, and could also trigger heterogeneous chemical reactions leading to stratospheric ozone depletion6. This new eruption of a sulphur-rich silicic magma thus shows that the EI Chichon eruption is not unique, and that climate-modifying eruptions of this type may be more common than previously believed.

Genesis of collision volcanism in Eastern Anatolia, Turkey

Late Cenozoic volcanism in Eastern Anatolia extends in a broad SW - NE trending belt across the Arabia - Eurasia collision zone, from the Arabian foreland basin in the southwest and to the Kars Plateau and Lesser Caucasus in the northeast. Foreland volcanism is dominated by basaltic shield and fissure eruptions of transitional tholeiitic - alkaline composition. Volcanism on the thickened crust north of the Bitlis Thrust Zone varies from the mildly alkaline volcano, Nemrut, and older Mus volcanics in the south, through the transitional calc-alkaline/alkaline volcanoes Bingöl and Süphan and the alkaline volcano Tendürek to the calc-alkaline volcano Ararat and older Kars plateau volcanics in the north. Isotope (Sr, Nd) and trace element systematics indicate that the lavas from the foreland were derived from the mantle lithosphere of the Arabian continent which had been enriched by small volumes of asthenospheric melts over a period of time; and that lavas from the alkaline volcanic area around Mus, and the volcanoes Nemrut and Tendürek north of the Bitlis Thrust Zone were derived from a lithospheric source of similar composition, either from the same, underthrust, Arabian continent or from the Bitlis Massif microcontinent. By contrast, the transitional lavas from Bingöl and Süphan and the calc-alkaline lavas from Ararat and Kars were derived from lithosphere carrying a distinct subduction signature inherited from pre-collision subduction events. Positive correlations between 87Sr/ 86Sr and SiO 2 and Rb/Nb and SiO 2 in the alkaline and transitional lavas suggest that combined assimilation and fractional crystallization was an important process within at least part of the thickened crust of the collision zone. Trace element covariation diagrams such as Y-Rb indicate the importance of hornblende crystallization at depth (and orthopyroxene at shallow levels) within the calc-alkaline provinces, in contrast to the consistently anhydrous crystallization sequences of the alkaline lavas. Trace element diagrams, based on the covariation of compatible and incompatible elements, point to moderate - low degrees of partial melting with residual clinopyroxene throughout, and residual garnet in the foreland province. Consideration of mineral stabilities, mantle solidi and geothermal gradients before and after collision suggest that lithospheric thickening should both increase the thickness of metasomatized lithosphere and depress the metasomatized zone to greater depths, probably beneath the amphibole and dolomite breakdown curves. Perturbation of the thickened lithosphere by delamination of the thermal boundary layer, perhaps coupled with local stretching associated with pull-apart basins on strike-slip fault systems, is then sufficient to generate melt, the composition of that melt being largely dependent on the enrichment history of the lithosphere in question. In Eastern Anatolia, volcanism appears to have started between about 8 and 6 Ma ago, some 5 Ma after the start of rapid uplift of the East Anatolian Plateau.

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

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

Ordovician volcanism in the Welsh Borderland

AbstractBasaltic, andesitic and rhyodacitic/rhyolitic volcanism was widespread during Ordovician time in the Welsh Basin. New chemical data are presented for Llanvirn to Caradoc lavas and tuffs from the Welsh Borderland which, during Ordovician time, formed the southeastern margin of the Welsh Basin. In view of the observed chemical alteration, immobile elements are used in the interpretation of the original geochemical character. The data indicate that the Llanvirn Stapeley volcanic group of the Shelve inlier was a bimodal basalt/basaltic andesite – rhyodacite/rhyolite association. The basalts have trace element contents of tholeiitic associations with a subduction-related character. The Caradoc Whittery and Hagley volcanic groups of the Shelve inlier comprise lavas and tuffs of calc-alkaline andesite. Blocks sampled from the Breidden Hills show that these were also derived from a calc-alkaline volcano. Associated Caradoc pumice- and ash-flow deposits from the Breidden Hills are probably of altered calc-alkaline rhyodacite/rhyolite composition. The Sibdon Carwood basalt flow, the only known example of Ordovician volcanism east of the Pontesford–Linley and Church Stretton lineaments, has transitional tholeiitic to alkaline character, with trace element contents influenced by subduction-related processes. The overall tholeiitic to calc-alkaline nature of the magmatism is consistent with the view that, during Llanvirn to Caradoc time, the Welsh Basin was an ensialic marginal basin.