The volcanic history of Ruapehu during the past 2 millennia based on the record of Tufa Trig tephras

 Tufa Trig Formation comprises a sequence of at least 19 andesitic tephras erupted from Mt. Ruapehu (Tongariro Volcanic Centre, New Zealand). Tephras of Tufa Trig Formation are the most recent eruptives from Ruapehu, dated between ca. 1850 years B.P. and the present. Members of the Formation show restricted dispersals, principally to the east of Mt. Ruapehu. Volumes calculated for the most widespread members are all less than 0.1 km³. Compared with other Mt. Ruapehu eruptives, Tufa Trig Formation tephras represent small eruptions that have contributed little tephra to the ring plain. They do, however, show a greater frequency of eruption with one event occurring on average every 100 years. Tufa Trig Formation members Tf3–Tf18 are black to dark grey, vitric, coarse-ash and lapilli-grade tephras which mantle the relief. They contain juvenile vitric particles which exhibit varying degrees of vesicularity, together with free crystals of pyroxene and feldspar, and few lithic fragments. Several morphological types of vitric pyroclasts are recognised in these tephras, the dominant type being of equant blocky morphology with fracture-bound surfaces (type-1 morphology). Field characteristics, tephra distributions, and the morphologies and textures of constituent pyroclasts suggest that these members (Tf3–Tf18) are the products of small-volume hydrovolcanic eruptions resulting from the interaction of fresh magma and meteoric water. We propose that a source of this water was an ancestral crater lake which formed within the late Holocene ca. 3000 years B.P. The morphological, compositional, and chemical (major-element) characteristics of three Tufa Trig Formation Tephras are compared with those of two new tephras erupted from Ruapehu Volcano during the October 1995 eruptions which comprise part of a newly defined member (Tf19) of Tufa Trig Formation. The comparisons support our interpretation that the majority of the Tufa Trig Formation tephras are primarily the products of hydrovolcanic eruptions. Other members of the Formation (Tf1 and Tf2) are coarse-grained scoriaceous tephras and are interpreted to be the products of strombolian events. Received: 14 September 1996 / Accepted: 6 June 1997     

Petrology and sulfur and chlorine emissions of the 1963 eruption of Gunung Agung,Bali, Indonesia

 The 1963 eruption of Gunung Agung produced 0.95 km³ dense rock equivalent (DRE) of olivine±hornblende-bearing, weakly phyric, basaltic andesite tephra and lava. Evidence for magma mixing in the eruptive products includes whole-rock compatible and incompatible trace element trends, reverse and complex compositional zoning of mineral phases, disequilibrium mineral assemblages, sieve-textured plagioclase phenocrysts, and augite rims on reversely zoned orthopyroxene. Basalt magma mixed with pre-existing andesite magma shortly before eruption to yield basaltic andesite with a temperature of 1040–1100  °C at an assumed pressure of 2 kb, f O₂>NNO, and an average melt volatile content (H₂O±CO₂) of 4.3 wt.%. Magma-mixing end members may have provided some of the S and Cl emitted in the eruption. Glass inclusions in phenocrysts contain an average of 650 ppm S and 3130 ppm Cl as compared with 70 ppm and 2220 ppm, respectively, in the matrix glass. Maximum S and Cl contents of glass inclusions approach 1800 and 5000 ppm, respectively. Application of the petrologic method to products of the 1963 eruption for estimating volatile release yields of 2.5×10¹²g (Mt) of SO₂ and 3.4 Mt of Cl released from the 0.65 km³ of juvenile tephra which contributed to stratospheric injection of H₂SO₄ aerosols on 17 March and 16 May, when eruption column heights exceeded 20 km above sea level. An independent estimate of SO₂ release from atmospheric aerosol loading (11–12 Mt) suggests that approximately 7 Mt of SO₂ was injected into the stratosphere. The difference between the two estimates can be most readily accounted for by the partitioning of S, as well as some Cl, from the magma into a water-rich vapor phase which was released upon eruption. For other recent high-S-release eruptions of more evolved and oxidized magmas (El Chichón, Pinatubo), the petrologic method gives values two orders of magnitude less than independent estimates of SO₂ emissions. Results from this study of the Agung 1963 magma and its volatile emissions, and from related studies on eruptions of more mafic magmas, suggest that SO₂ emissions from eruptions of higher-S-solubility magma may be more reliably estimated by the petrologic method than may those from more-evolved magma eruptions. Received: 29 June 1994 / Accepted: 25 April 1996     

Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan

During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (<5 vol% phenocrysts, 51.4–52.2 wt% SiO₂), whereas magma issued as volcanic bombs from the summit caldera on 18 August is plagioclase-phyric basalt (20 vol% phenocrysts, 50.8–51.3 wt% SiO₂). The submarine spatter contains two types of crystal-clots, A-type and A-type (andesitic type). The phenocryst assemblages (plagioclase, pyroxenes and magnetite) and compositions of clinopyroxene in these clots are nearly the same, but only A-type clots contain Ca-poor plagioclase (An < 70). We consider that the A-type clots could have crystallized from a more differentiated andesitic magma than the A-type clots, because FeO*/MgO is not strongly influenced during shallow andesitic differentiation. The summit bombs contain only B-type (basaltic type) crystal-clots of Ca-rich plagioclase, olivine and clinopyroxene. The A-type and B-type clots have often coexisted in Miyakejima lavas of the period 1469–1983, suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas. According to the temporal variations of mineral compositions in crystal-clots, the andesitic magma became less evolved, and the basaltic magma more evolved, over the past 500 years. We conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period, causing the andesitic magma to become less evolved with time. The mineral chemistries in crystal-clots of the submarine spatter and 18 August summit bombs of the 2000 eruption fall on the evolution trends of the A-type and B-type clots respectively, suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters. The 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber; as the collapse occurred, it may have caused disruption of crustal cumulates which then contaminated the ascending, deeper basalt. Thus, porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera. The historical magma plumbing system of Miyakejima was probably destroyed during the 2000 eruption, and a new one may now form.Editorial responsibility: S Nakada, T Druitt     

Eruption episodes and magma recharge events in andesitic systems: Mt Taranaki,New Zealand

Eruption episodes, where a series of eruption events are generically related, can include the eruption of a wide spectrum of volcanic activity over decadal periods. This paper concentrates on the opening phases of an eruption episode which occurred approximately 1800 yrs BP from Mt Taranaki, New Zealand. These events spanned the eruption of differing bulk compositions and styles from two distinct vent locations; an andesitic sub-plinian eruption from the summit vent and a scoria cone-building eruption of basaltic magma from a satellite vent. Compositional profiles and zoning textures of plagioclase, amphibole and clinopyroxene phenocrysts from the opening andesitic event show evidence of magma mixing and subsequent crystallisation just prior to the initiation of the eruption episode. Titanomagnetite grain morphology and Ti variation suggest that the magma mixing event occurred within a few days to weeks before the eruption acting as a trigger for it. We present a magmatic model which is constrained by the petrological observations and eruptions of the episode. In this model magma differentiation at depth causes its rise and recharging of a mid-crustal magma storage area at 5–7 km. Although the recharging magma differed slightly in oxygen fugacity and temperature, it was compositionally and physically similar enough to the residing andesitic magma to allow efficient mixing. The petrological characteristics described here can be readily observed and enable identification of mixing events in other recent eruption episodes.     

Heterogeneous andesite–dacite ejecta in 26–16.6 ka pyroclastic deposits of Tongariro Volcano,New Zealand: the product of multiple magma-mixing events

The previously poorly documented 26–16.6 ka interval of pyroclastic volcanism from Tongariro Volcano is marked by three distal lapilli fall units (Rt1-3) exposed in ring-plain deposits. The distal Rt1-3 units are tentatively correlated to proximal scoria deposits on the upper slopes of North Crater based on their dispersal patterns, petrography and geochemistry. Lapilli in each of the Rt1-3 deposits are characterised by variable groundmass crystallinity, vesicularity and colour within individual clasts. Matrix glasses are mostly microlite-free, and compositionally diverse across the deposits (SiO₂ = 62–75 wt%), with wide composition ranges occurring within single clasts. The glasses represent different melts that were mingled and mixed shortly before eruption; a finding supported by widely variable Fe–Ti oxide equilibrium temperature estimates (∼830–1,200°C). Ranges of 30–160°C (typically 70°C) occur within individual clasts. Some clinopyroxene crystals display Mg-rich (∼Mg #88) rim zones around homogeneous low-Mg (∼Mg #68) cores, with abrupt transition zones. This zoning is interpreted as resulting from the injection of a more mafic melt into a stagnating, resident magma. Crystal-melt equilibria indicate that several episodes of mafic intrusion occurred, to produce hybrid melts with zoned crystals forming isolated ponds within the resident magma. Variable mixing from the percolation of melts and the coalescence of melt ponds would explain the wide range of melt compositions and equilibrium temperatures observed in the ejecta. The magma heterogeneity was preserved by quenching on prompt eruption, with much of the short-duration chaotic mixing of melts and crystals occurring in the conduit. The Rt1-3 eruptions were from an open magmatic system consisting of one or more long-lived stagnant crystal mush zones, from which eruptions were rapidly triggered by new injections of mafic magmas from greater depths. A similar pattern of magmatic dynamics was observed in the much smaller 1995 eruptions of the neighbouring Ruapehu Volcano.     

Temporal variations in gravity at Mt. Etna (Italy) associated with the 1989 and 1991 eruptions

 Results are presented from 11 microgravity surveys on Mt. Etna between 1987 and 1993, a period including the major 1989 and 1991–1993 flank eruptions and subordinate 1990 activity. Measurements were made with LaCoste and Romberg D-62 and D-157 gravity meters along a network around the volcano between 1000 and 1900 m a.s.l. and, since 1992, a N–S summit profile. Gravity changes of as much as 200 μGal were observed at scales from the size of the summit region to that of the volcano. None was associated with significant changes in ground elevation. The data show an increase in gravity for 2 years before the 1989 eruption. The increase is attributed to the accumulation of magma (0.25–1.7×10⁹m³) in an elongate zone, oriented NNW–SSE, between 2.5 and 6 km below sea level. Part of this magma was injected into the volcanic pile to supply the 1989 and 1990 eruptions. It also probably fed the start of the 1991–1993 eruption, since this event was not preceded by significant gravity changes. A large gravity increase (up to 140 μGal) detected across the volcano between June and September 1992 is consistent with the arrival in the accumulation zone of 0.32–2.2×10⁹m³ of new magma, thus favoring continued flank effusion until 1993. A large gravity decrease (200 μGal) in the summit region marked the closing stages of the 1991–1993 event and is associated with magma drainage from the upper levels of Etna's central feeding system. Received: 15 July 1995 / Accepted: 27 October 1997     

Evidence of magma mixing: Petrological study of Shirouma-Oike calc-alkaline andesite volcano, Japan

Shirouma-Oike volcano, a Quaternary composite volcano in central Japan, consists mostly of calc-alkaline andesitic lavas and pyroclastic rocks. Products of the earlier stage of the volcano (older group) are augite-hypersthene andesite. Hornblende crystallized during the later stage of this older group, whereas biotite and quartz crystallized in the younger group.Assemblages of phenocrysts in disequilibrium, such as magnesian olivine(Fo₃₀)/quartz, iron-rich hypersthene(En₅₅)/iron-poor augite(Wo_43.5, En_42.5, Fs_14.0), and two different types of zoning on the rim of clinopyroxene are found in a number of rocks. Detailed microprobe analyses of coexisting minerals reveal that phenocrysts belong to two distinctly different groups; one group includes magnesian olivine + augite which crystallized from a relatively high-temperature (above 1000°C) basaltic magma; the second group, which crystallized from relatively low temperature (about 800°C) dacitic to andesitic magma, includes hypersthene + hornblende + biotite + quartz + plagioclase + titanomagnetite ± ilmenite (in the younger group) and hypersthene + augite + plagioclase + titanomagnetite ± hornblende (in the older group). The temperature difference between the two magmas is clarified by Mg/Fe partition between clinopyroxene and olivine, and Fe-Ti oxides geothermometer. The compositional zoning of minerals, such as normal zoning of olivine and magnesian clinopyroxene, and reverse zoning of orthopyroxene, indicate that the basaltic and dacitic-andesitic magmas were probably mixed in a magma reservoir immediately before eruption. It is suggested that the basaltic magma was supplied intermittently from a deeper part to the shallower magma reservoir, in in which dacitic-andesitic magma had been fractionating.     

Silicic magma entering a basaltic magma chamber: eruptive dynamics and magma mixing — an example from Salina (Aeolian islands,Southern Tyrrhenian Sea)

The Pollara tuff-ring resulted from two explosive eruptions whose deposits are separated by a paleosol 13 Ka old. The oldest deposits (LPP, about 0.2 km³) consist of three main fall units (A, B, C) deposited from a subplinian column whose height (7–14 km) increased with time from A to C, as a consequence of the increased magma discharge rate during the eruption (1–8x10⁶ kg/s). A highly variable juvenile population characterizes the eruption. Black, dense, highly porphyritic, mafic ejecta (SiO₂=50–55%) almost exclusively form A deposits, whereas grey, mildly vesiculated, mildly porphyritic pumice (SiO₂=56–67%) and white, highly vesiculated, nearly aphyric pumice (SiO₂=66–71%) predominate in B and C respectively. Mafic cumulates are abundant in A, while crystalline lithic ejecta first appear in B and increase upward. The LPP result from the emptying of an unusual and unstable, compositionally zoned, shallow magma chamber in which high density mafic melts capped low density salic ones. Evidence of the existence of a short crystal fractionation series is found in the mafic rocks; the andesitic pumice results from complete blending between rhyolitic and variously fractionated mafic melts (salic component up to 60 wt%), whereas bulk dacitic compositions mainly result from the presence of mafic xenocrysts within rhyolitic glasses. Viscosity and composition-mixing diagrams show that blended liquids formed when the visosities of the two end members had close values. The following model is suggested: 1. A rhyolitic magma rising through the metamorphic basement enterrd a mafic magma chamber whose souter portions were occupied by a highly viscous, mafic crystal mush. 2. Under the pressure of the rhyolitic body the nearly rigid mush was pushed upwards and mafic melts were squeezed against the walls of the chamber, beginning roof fracturing and mingling with silicic melts. 3. When the equilibrium temperature was reached between mafic and silicic melts, blended liquids rapidly formed. 4. When fractures reached the surface, the eruption began by the ejection of the mafic melts and crystal mush (A), followed by the emission of variously mingled and blended magmas (B) and ended by the ejection of nearly unmixed rhyolitic magma (C).     

Crystal size distributions and other quantitative textural measurements in lavas and tuff from Egmont volcano (Mt. Taranaki), New Zealand

 The size, shape and orientation of plagioclase crystals have been quantified in a tuff and series of andesite lavas from the active Egmont volcano (Mt. Taranaki), New Zealand. Linear crystal size distributions (CSDs) show that if the magma had several components, then only one provided the crystals. The slope of the CSD indicates that the earliest lavas measured had a residence time of ∼50 years in the magma chamber for a growth rate of 10^–11 cm/s. Subsequent lavas had slightly longer residence times (50–75 years), but the following series returned to 50-year residence times. The youngest magmas, from both Egmont summit and the parasitic Fantham's Peak, have the shortest residence times of ∼30 years. Variations in residence time may reflect changes in the magma chamber shape or depth, or the temperature of the surrounding rocks. Crystal shapes and zonation suggest that crystallization occurred in a bottle-shape magma chamber, and not in a narrow conduit. If future eruptions use the same magma chamber as the most recent eruptions, then a delay of approximately 30 years can be expected between refilling and eruption. Received: 25 October 1995 / Accepted: 19 April 1996     

Multiple levels of magma storage during the 1980 summer eruptions of Mount St. Helens, WA

Transitions in eruptive style—explosive to effusive, sustained to pulsatory—are a common aspect of volcanic activity and present a major challenge to volcano monitoring efforts. A classic example of such transitions is provided by the activity of Mount St. Helens, WA, during 1980, where a climactic Plinian event on May 18 was followed by subplinian and vulcanian eruptions that became increasing pulsatory with time throughout the summer, finally progressing to episodic growth of a lava dome. Here we use variations in the textures, glass compositions and volatile contents of melt inclusions preserved in pyroclasts produced by the summer 1980 eruptions to determine conditions of magma ascent and storage that may have led to observed changes in eruptive activity. Five different pyroclast types identified in pyroclastic flow and fall deposits produced by eruptions in June 12, July 22 and August 7, 1980, provide evidence for multiple levels of magma storage prior to each event. Highly vesicular clasts have H₂O-rich (4.5–5.5 wt%) melt inclusions and lack groundmass microlites or hornblende reaction rims, characteristics that require magma storage at P≥160 MPa until shortly prior to eruption. All other clast types have groundmass microlites; P_H20 estimated from both H₂O-bearing melt inclusions and textural constraints provided by decompression experiments suggest pre-eruptive storage pressures of ∼75, 40, and 10 MPa. The distribution of pyroclast types within and between eruptive deposits can be used to place important constraints on eruption mechanisms. Fall and flow deposits from June 12, 1980, lack highly vesicular, microlite-free pyroclasts. This eruption was also preceded by a shallow intrusion on June 3, as evidenced by a seismic crisis and enhanced SO₂ emissions. Our constraints suggest that magma intruded to a depth of ≤4 km beneath the crater floor fed the June eruption. In contrast, eruptions of July and August, although shorter in duration and smaller in volume, erupted deep volatile-rich magma. If modeled as a simple cylinder, these data require a step-wise decrease in effective conduit diameter from 40–50 m in May and June to 8–12 m in July and August. The abundance of vesicular (intermediate to deep) clast types in July and August further suggests that this change was effected by narrowing the shallower part of the conduit, perhaps in response to solidification of intruded magma remaining in the shallow system after the June eruption. Eruptions from July to October were distinctly pulsatory, transitioning between subplinian and vulcanian in character. As originally suggested by Scandone and Malone (1985), a growing mismatch between the rate of magma ascent and magma disruption explains the increasingly pulsatory nature of the eruptions through time. Recent fragmentation experiments Spieler et al. (2004) suggest this mismatch may have been aided by the multiple levels at which magma was stored (and degassed) prior to these events.Editorial responsibility: J Stix     

An experimental facility for the investigation of magma fragmentation by rapid decompression

 A special experimental facility has been developed to investigate the fragmentation of vesicular magma undergoing rapid decompression. The facility operates in a regime similar to that of shock tubes and at temperatures up to 950  °C and pressures up to 200 bar. Cylindrical samples (diameter ca. 17 mm, length ca. 50 mm) undergo rapid decompression in a high-temperature, high-pressure section of the facility following the disruption of a diaphragm separating that section from a low-pressure, low-temperature section. Actual vesicular magma samples have been experimentally fragmented at elevated temperatures and pressures corresponding to those observed during explosive volcanic eruptions and the resulting pyroclastics have been photographically resolved in flight and collected for physical characterization. The results of these experiments show that the rapid decompression of highly viscous vesicular magma can generate pyroclastic ejecta via rapid and complete fragmentation of magma at high temperature. This new fragmentation facility is presented as a tool for experimental volcanology under well-constrained conditions. Received: 19 March 1996 / Accepted: 25 August 1996     

1998 Eruption at Volcán Cerro Azul, Galápagos Islands: I. Syn-Eruptive Petrogenesis

The 1998 eruption of Volcán Cerro Azul in the Galápagos Islands produced two intra-caldera vents and a flank vent that erupted more than 1.0×10⁸ m³ of lava. Lava compositions changed notably during the 5-week eruption, and contemporaneous eruptions in the caldera and on the flank produced different compositions. Lavas erupted from the flank vent range from 6.3 to 14.1% MgO, nearly the entire range of MgO contents previously reported from the volcano. On-site monitoring of eruptive activity is linked with petrogenetic processes such that geochemical variations are evaluated in a temporal context. Lavas from the 1998 eruption record two petrogenetic stages characterized by progressively more mafic lavas as the eruption proceeded. Crystal compositions, whole rock major and trace element compositions, and isotope ratios indicate that early lavas are the product of mixing between 1998 magma and remnant magma of the 1979 eruption. Intra-caldera lavas and later lavas have no 1979 signature, but were produced by the 1998 magma incorporating olivine and clinopyroxene xenocrysts. Thus, early magma petrogenesis is characterized by mixing with the 1979 magma, followed by the magma progressively entraining wehrlite cumulate mush.Editorial Responsibility: M.R. Carroll     

On the Magma Chambers beneath Klyuchevskoi Volcano,Kamchatka

Numerous summit and parasitic eruptions of moderate potassium magnesian and high-alumina basalts and basaltic andesites, their mineralogic and geochemical features, and the composition of in situ chilled melt inclusions in the olivine of cinder lapilli discharged by Klyuchevskoi Volcano all provide evidence of the presence of magma chambers beneath the volcano. This is also supported by a dualism in the variation of CaO and Al₂O₃ concentrations in olivine and clinopyroxene during crystallization. The mineralogic features in the high-alumina basalts that were discharged by all parasitic eruptions of Klyuchevskoi provide evidence of magnesian magma being emplaced from a deeper chamber into a shallow high-alumina chamber. The distribution of incoherent elements in the volcano’s magnesian and aluminiferous rocks shows that they came from a single mantle source. The geochemical and mineralogic data are in good agreement with the results of geophysical surveys that concern the structure and properties of the lithosphere beneath Klyuchevskoi.     

Erratum to Compositional variations and magma mixing in the 1991 eruptions of Hudson volcano,Chile

The August 1991 eruptions of Hudson volcano produced ~2.7 km³ (dense rock equivalent, DRE) of basaltic to trachyandesitic pyroclastic deposits, making it one of the largest historical eruptions in South America. Phase 1 of the eruption (P1, April 8) involved both lava flows and a phreatomagmatic eruption from a fissure located in the NW corner of the caldera. The paroxysmal phase (P2) began several days later (April 12) with a Plinian-style eruption from a different vent 4 km to the south-southeast. Tephra from the 1991 eruption ranges in composition from basalt (phase 1) to trachyandesite (phase 2), with a distinct gap between the two erupted phases from 54–60 wt% SiO₂. A trend of decreasing SiO₂ is evident from the earliest part of the phase 2 eruption (unit A, 63–65 wt% SiO₂) to the end (unit D, 60–63 wt% SiO₂). Melt inclusion data and textures suggest that mixing occurred in magmas from both eruptive phases. The basaltic and trachyandesitic magmas can be genetically related through both magma mixing and fractional crystallization processes. A combination of observed phase assemblages, inferred water content, crystallinity, and geothermometry estimates suggest pre-eruptive storage of the phase 2 trachyandesite at pressures between ~50–100 megapascal (MPa) at 972 ± 26°C under water-saturated conditions (log fO₂ –10.33 (±0.2)). It is proposed that rising P1 basaltic magma intersected the lower part of the P2 magma storage region between 2 and 3 km depth. Subsequent mixing between the two magmas preferentially hybridized the lower part of the chamber. Basaltic magma continued advancing towards the surface as a dyke to eventually be erupted in the northwestern part of the Hudson caldera. The presence of tachylite in the P1 products suggests that some of the magma was stalled close to the surface (<0.5 km) prior to eruption. Seismicity related to magma movement and the P1 eruption, combined with chamber overpressure associated with basalt injection, may have created a pathway to the surface for the trachyandesite magma and subsequent P2 eruption at a different vent 4 km to the south-southeast. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mixed deposits of complex magmatic and phreatomagmatic volcanism: an example from Crater Hill,Auckland, New Zealand

 A series of alternating phreatomagmatic ("wet") and magmatic ("dry") basaltic pyroclastic deposits forming the Crater Hill tuff ring in New Zealand contains one unit (M1) which can only be interpreted as the products of mixing of ejecta from simultaneous wet and dry explosions at different portions of a multiple vent system. The principal characteristics of M1 are (a) rapid lateral changes in the thicknesses of, and proportions in juvenile components in individual beds, and (b) wide ranges of juvenile clast densities in every sample. M1 appears to have been associated with an elongate source of highly variable and fluctuating magma : water ratios and magma discharge rates. This contrasts with the only other documented mixed (wet and dry) basaltic pyroclastic deposits where mixing from two point sources of quite different but stable character has been inferred. Received: July 11, 1995 / Accepted: February 13, 1996     

Magma-reservoir crystallization processes: small-scale dikes in cumulate gabbros, Mauna Kea Volcano, Hawaii

 Gabbroic xenoliths that represent cumulate environments within Mauna Kea Volcano are, in rare examples, penetrated by small-scale (<7 cm) dikes. We examined four dike/host composite xenoliths to establish how this evidence for magma seemingly injected into cumulate gabbro fits into the evolution of igneous processes in shield volcano magma reservoirs. Olivine, clinopyroxene, and plagioclase compositions in both host gabbros and dikes are characteristically tholeiitic and evolved (Fo_71–66, cpx-Mg # 79–77, An_72–51) with respect to Hawaiian magmatism. Dikes, however, when compared with their host gabbros, have slightly greater abundances of some incompatible elements and slightly more evolved olivine compositions (e.g., Fo₆₈ vs Fo₇₁₎. Compared with Mauna Kea lava compositions, both host gabbros and dikes have lower incompatible-element abundances, positive Eu anomalies, and, notable for dikes, major-element compositions unlike those of lavas (e.g., SiO₂<46 wt.%). The small-scale dikes, therefore, also have cumulate characteristics. We interpret them as representing late-stage liquids (e.g., <5 wt.% MgO, based on <Fo₇₀) "squeezed" from solidifying cumulus piles of evolved (e.g., ∼Fo₇₀) gabbroic assemblages. The compositions of the dikes, however, do not match those of the most evolved liquids expected in reservoirs because they appear to have lost interstitial liquids (e.g., positive Eu anomalies, low abundances of some trace elements). Because minerals in the dikes were in equilibrium with highly evolved liquids, conditions for small-scale dike formation in cumulate environments apparently occur only at the last stages of reservoir magma differentiation and solidification. Received: 25 February 1997 / Accepted: 14 June 1997     

Short time scales of magma-mixing processes prior to the 2011 eruption of Shinmoedake volcano, Kirishima volcanic group, Japan

We estimated time scales of magma-mixing processes just prior to the 2011 sub-Plinian eruptions of Shinmoedake volcano to investigate the mechanisms of the triggering processes of these eruptions. The sequence of these eruptions serves as an ideal example to investigate eruption mechanisms because the available geophysical and petrological observations can be combined for interpretation of magmatic processes. The eruptive products were mainly phenocryst-rich (28 vol%) andesitic pumice (SiO₂ 57 wt%) with a small amount of more silicic pumice (SiO₂ 62–63 wt%) and banded pumice. These pumices were formed by mixing of low-temperature mushy silicic magma (dacite) and high-temperature mafic magma (basalt or basaltic andesite). We calculated the time scales on the basis of zoning analysis of magnetite phenocrysts and diffusion calculations, and we compared the derived time scales with those of volcanic inflation/deflation observations. The magnetite data revealed that a significant mixing process (mixing I) occurred 0.4 to 3 days before the eruptions (pre-eruptive mixing) and likely triggered the eruptions. This mixing process was not accompanied by significant crustal deformation, indicating that the process was not accompanied by a significant change in volume of the magma chamber. We propose magmatic overturn or melt accumulation within the magma chamber as a possible process. A subordinate mixing process (mixing II) also occurred only several hours before the eruptions, likely during magma ascent (syn-eruptive mixing). However, we interpret mafic injection to have begun more than several tens of days prior to mixing I, likely occurring with the beginning of the inflation (December 2009). The injection did not instantaneously cause an eruption but could have resulted in stable stratified magma layers to form a hybrid andesitic magma (mobile layer). This hybrid andesite then formed the main eruptive component of the 2011 eruptions of Shinmoedake.     

Magma mixing in the Aleutian arc: evidence from cognate inclusions and composite xenoliths

The complexity of igneous processes in the Aleutian calc-alkaline magma series can be inferred from study of xenolithic fragments. Composite xenoliths and cognate inclusions provide direct evidence for magma—magma and wall-rock—magma mixing processes. Using distributions of Cr in clinopyroxene, compositional endmembers involved in mixing are identified within the xenoliths. The basaltic mixing endmember is more mafic than calc-alkaline lavas in the arc. Magma mixing and wall-rock assimilation within calc-alkaline basaltic to andesitic magmas is identified in phenocrystic assemblages as well as in xenoliths, and appears to be a widespread phenomenon in Aleutian calc-alkaline magmas.     

The “Pomici di mercato” Plinian eruption of Somma-Vesuvius: magma chamber processes and eruption dynamics

The Pomici di Mercato (PdM, 8,010 ± 40 a), also known in the literature as Pomici Gemelle or Pomici di Ottaviano, is one of the oldest Plinian eruptions of Somma-Vesuvius. This eruption occurred after the longest (7 ka) quiescence period of the volcano and was followed by more than 4 ka of repose. The erupted magma is phonolitic in composition. All the products have very low phenocrysts content (less than 3%) and show evidence of mineralogical disequilibria. They contain K-feldspar ± clinopyroxene (salite and diopside) ± plagioclase ± garnet ± biotite ± amphibole ± apatite ± Fe-Ti oxides. Pumice fragments collected at different stratigraphic heights are slightly less evolved and more enriched in radiogenic Sr composition upsection. The glass composition is fairly homogeneous in single pumice fragment and among pumice fragments from different layers. Glass separated from pumice fragments collected at different stratigraphic heights is homogeneous in the Sr-isotope composition (around a value of 0.70717). Glass is in isotopic equilibrium with salite throughout the entire sequence and with diopside at the base of the sequence. Diopside becomes more radiogenic upsection, reaching a value of 0.707458 ± 7, whereas feldspar is consistently slightly less radiogenic than glass. Nd-isotope composition is fairly uniform (ca. 0.51247) through the whole sequence. The isotopic disequilibria among glass, feldspar and diopside, together with the homogeneous isotopic composition of pumice glass in equilibrium with salite, and the mineralogical disequilibria between plagioclase and K-feldspar, imply that most of the diopside and plagioclase crystals are xenocrysts incorporated into the phonolitic magma during residence in a magma chamber and/or during ascent towards the surface. The PdM Tephra are compositionally and isotopically similar to the phonolitic, first-erupted products of the subsequent Pomici di Avellino Plinian eruption. On the basis of this similarity, we suggest that the magma feeding both eruptions resulted from the tapping of a unique magma chamber. Prior to the PdM eruption, this chamber was formed by a large and homogeneous phonolitic magma body. After the PdM eruption, as a consequence of new arrivals of more radiogenic in Sr, less-differentiated magma batches, the magma chamber progressively developed a slightly stratified phonolitic uppermost portion, capping a tephriphonolitic layer, both emitted during the subsequent Pomici di Avellino eruption.     

Sub-volcanic crystallization at Stromboli (Aeolian Islands, southern Italy) preceding the Sciara del Fuoco sector collapse: evidence from monzonite lithic suite

 We describe texture, mineralogy and whole-rock composition of cognate monzonite sub-volcanic clasts within debris flow deposits related to the 5000 years catastrophic phreatomagmatic eruption probably linked to the Sciara del Fuoco sector collapse. The debris flows are at the top of accretionary lapilli-rich ash deposits overlying potassic (KS, shoshonites) lavas of the Neostromboli period. The monzonites are inferred to be crystallized in situ, at low P, at the side walls and/or roof margins of a shallow magma chamber and to be cogenetic with the KS Neostromboli extrusives. They can be considered "ideal orthocumulates" since they approximately retain a bulk liquid composition and possibly represent "slowly cooled equivalents" of their KS shoshonite host rock. The "closure temperature" of final solidification of the monzonite lithic suite was estimated through ternary-feldspar geothermobarometry, plagioclase–K-feldspar and K-feldspar–biotite equilibria and is in the range of 750–790  °C with a maximum –logfO₂ around 15.1–15.3. The estimated pressure of crystallization is <0.5 kbar. Potassic lavas and dikes, previously emplaced during the Neostromboli period, also resemble the monzonites in both major trace elements and mineral chemistry. The cogenetic relationship between KS Neostromboli extrusives and the monzonite host-rock magma from which the sub-volcanic clasts were derived is clear evidence that a shallow magma chamber existed between the caldera collapse of the Vancori period and the Sciara del Fuoco sector collapse (i.e. between 13 000 and 5000 years). The monzonite clasts were derived from crystallization at very shallow depth (ca. 1 km) and strongly support the hypothesis of violent decompression of the shallow magmatic plumbing system during the Sciara del Fuoco sector collapse. Climax of the regressive landslide event, with maximum disruption of the chamber walls, took place during emplacement of the debris flows, i.e. during the late stage of the Neostromboli phreatomagmatic eruption. Received: 15 September 1996 / Accepted: 5 May 1997