Eruptive Units Research Articles

Tephrostratigraphy and glass compositions of post-15 kyr Campi Flegrei eruptions: implications for eruption history and chronostratigraphic markers

Volcanic ash (tephra) erupted from the frequently active Campi Flegrei volcano forms layers in many palaeoenvironmental archives across Italy and the Mediterranean. Proximal deposits of 50 of the post-15 ka eruptions have been thoroughly sampled and analysed to produce a complete database of glass compositions (>1900 analyses) to aid identification of these units. The deposits of individual eruptions are compositionally diverse and this variability is often greater than that observed between different units. Many of the tephra units do not have a unique glass chemistry, with compositionally similar tephra often erupted over long periods of time (1000s years). Thus, glass chemistry alone is not enough to robustly correlate most of the tephra from Campi Flegrei, especially in the last 10 kyrs. In order to reliably correlate the eruption units it is important to take into account the stratigraphy, chronology, magnitude, and dispersal of the eruptions, which has been collated to aid identification. An updated chronology is also presented, which was constrained using Bayesian analysis (OxCal) of published radiocarbon dates and 40Ar/ 39Ar ages. All the data presented can be employed to help correlate post-15 ka tephra units preserved in archaeological and Holocene palaeoenvironmental archives. The new database of proximal glass compositions has been used to correlate proximal volcanic deposits through to distal tephra layers in the Lago di Monticchio record ( Wulf et al., 2004, 2008) and these correlations provide information on eruption stratigraphy and the tempo of volcanism at Campi Flegrei.

Integrating records of explosive and effusive activity from proximal and distal sequences: Mt. Taranaki, New Zealand

A complete record of eruption activity is required for a robust volcanic hazard assessment, characterising the types and magnitude of possible activity and the relative frequency of these different types of eruptions. This paper documents a record of Holocene volcanic eruptions from Mt. Taranaki, New Zealand, with the aim of integrating the records of explosive and effusive style eruptions from this volcano. Discriminant function analysis of titanomagnetites from Mt. Taranaki is used to correlate a set of well-dated tephras from lake cores with comparatively poorly dated eruption units that outcrop on the flanks of the cone. Using this process the stratigraphic sections reveal that much of the current edifice was constructed between 6000 and 5000 Cal. BP by prolific lava effusion events. The period of intensive effusive activity followed edifice modification by a major flank-collapse and debris avalanche. This demonstrates that trends in the frequency of effusive activity may vary independently of explosive eruption patterns. Examining the range of processes controlling the occurrence of a range of eruption types and the distribution of their products is essential for the production of robust probabilistic-based eruption forecasting models. In addition, the geochemical data on the widely dispersed Taranaki tephras presented here, tied to this new high-resolution eruption chronology, can be used to constrain future studies of landscape and climatic evolution across large areas of the North Island of New Zealand.

Tephrostratigraphy and petrological study of Chikurachki and Fuss volcanoes, western Paramushir Island, northern Kurile Islands: Evaluation of Holocene eruptive activity and temporal change of magma system

A tephrostratigraphic and petrological study of the Chikurachki (1816 m)-Tatarinov-Lomonosov volcanic chain (CTL volcanic chain) and Fuss (1772 m), located at the southern part of Paramushir Island in the northern Kurile Islands, was carried out to reveal the explosive eruption history during the Holocene and the temporal change of the magma systems of these active volcanoes. Tephra successions were described at 54 sites, and more than 20 major eruptive units were identified, consisting of pumice fall, scoria fall and ash fall deposits, each of which are separated by paleosol or peat layers. The source volcano of each recognized tephra layer was confirmed by correlation with proximal deposits of each eruption center with respect to petrography and whole-rock and glass chemistry. The age of each layer was determined by radiocarbon dating and the stratigraphic relationship with the dated, widespread tephra from Kamchatka according to the thickness of paleosols bracketed between tephra layers. The Holocene activity in this region was initiated by eruptions from the Tatarinov and Lomonosov volcanoes. After the eruptions, the Fuss and Chikurachki volcanoes started their explosive activities at ca. 7.5 ka BP, soon after the deposition of widespread tephra from the Kurile Lake caldera in southern Kamchatka. Compared with Fuss located on the back-arc side, Chikurachki has frequent, repeated explosive and voluminous eruptions. Whole-rock compositions of the rocks of the CTL volcanic chain and Fuss are classified into medium-K and high-K groups, respectively. These suggest that magma systems beneath the CTL volcanic chain and Fuss differ from each other and have been independently constructed. The rocks of the Chikurachki volcano are basalt-basaltic andesite and have gradually evolved their chemical compositions; when graphed on a SiO 2-oxide diagram, these form smooth trends from mafic to more felsic. This suggests that the magma system evolved mainly by fractional crystallization. In contrast, matrix glass chemistries for Fuss pumices are distinct for each eruption and show different K 2O levels on a SiO 2-K 2O diagram. This implies that the magma system of Fuss has been frequently replaced. Both volcanoes have been active under the same subduction system. However, the Chikurachki volcano will continue eruptive activity under a stable magma system with a higher magma discharge rate, whereas Fuss may continue construction with an intermittent supply of distinct, small magma batches.

Eruptive history of South Sister, Oregon Cascades

South Sister is southernmost and highest of the Three Sisters, three geologically dissimilar stratovolcanoes that together form a spectacular 20 km reach along the Cascade crest in Oregon. North Sister is a monotonously mafic edifice as old as middle Pleistocene, Middle Sister a basalt–andesite–dacite cone built between 48 and 14 ka, and South Sister is a basalt-free edifice that alternated rhyolitic and intermediate modes from 50 ka to 2 ka (largely contemporaneous with Middle Sister). Detailed mapping, 330 chemical analyses, and 42 radioisotopic ages show that the oldest exposed South Sister lavas were initially rhyolitic ~ 50 ka. By ~ 37 ka, rhyolitic lava flows and domes (72–74% SiO 2) began alternating with radially emplaced dacite (63–68% SiO 2) and andesite (59–63% SiO 2) lava flows. Construction of a broad cone of silicic andesite–dacite (61–64% SiO 2) culminated ~ 30 ka in a dominantly explosive sequence that began with crater-forming andesitic eruptions that left fragmental deposits at least 200 m thick. This was followed at ~ 27 ka by growth of a steeply dipping summit cone of agglutinate-dominated andesite (56–60.5% SiO 2) and formation of a summit crater ~ 800 m wide. This crater was soon filled and overtopped by a thick dacite lava flow and then by > 150 m of dacitic pyroclastic ejecta. Small-volume dacite lavas (63–67% SiO 2) locally cap the pyroclastic pile. A final sheet of mafic agglutinate (54–56% SiO 2) – the most mafic product of South Sister – erupted from and drapes the small (300-m-wide) present-day summit crater, ending a summit-building sequence that lasted until ~ 22 ka. A 20 kyr-long-hiatus was broken by rhyolite eruptions that produced (1) the Rock Mesa coulee, tephra, and satellite domelets (73.5% SiO 2) and (2) the Devils Chain of ~ 20 domes and short coulees (72.3–72.8% SiO 2) from N–S vent alignments on South Sister's flanks. The compositional reversal from mafic summit agglutinate to recent rhyolites epitomizes the frequently changing compositional modes of the South Sister locus throughout its lifetime. South Sister is part of a reach of the Cascades unusually active in the last 50 kyr, characterized by high vent density, N–S vent alignments, and numerous eruptive units of true rhyolite (≥ 72% SiO 2) that distinguishes it from much of the Quaternary Cascade arc; these are eruptive expressions of the complex confluence of arc and intraplate magmatic–tectonic regimes.

Geochemical identification of late Quaternary cryptotephra layers from marine sediments, southwest Japan, using INAA

New compositional data were obtained from glass shards from cryptotephra layers in five marine sediment cores from the Ulleung Basin in the East (Japan) Sea. The shards were extracted from cryptotephra layers that were distinguished in a previous study by INAA core scanning, and analysed by EDS-SEM for major elements and INAA for trace elements. Most of the glass shards have alkaline, trachyte–phonolite, compositions, and have similar major and trace element compositions to eruptive units on Ulleung Island to the west. Stratigraphic inferences indicate the presence of at least four alkaline tephras (referred to in order of increasing age as: U-3, Post U-Oki, U-Oki, Pre-U-Oki) in the interval between the K–Ah rhyolitic tephra (7.3 cal ka BP) and the TL1 regional dark sediment layer (12.4 cal ka BP). The new geochemical data indicate minor compositional distinctions between these tephras that help to infer fine-scale regional correlations. Paleoceanographic conditions in the East (Japan) Sea changed drastically during the late Quaternary, linked to global environmental changes, and the development of a more comprehensive alkaline tephrostratigraphical framework for this region will greatly aid in the reconstruction and chronology of regional stratigraphic variations. The recognition of new alkaline cryptotephra in the marine sediments of the Ulleung Basin also contributes towards a better understanding of the volcanic history of Ulleung Island and nearby seamounts.

Geochemical fingerprinting of the widespread Toba tephra using biotite compositions

Toba caldera, Sumatra, is one of the largest and most explosive volcanoes on Earth, erupting some of the most voluminous volcanic deposits. Chronologically these eruptions are ∼790 ka Older Toba Tuff, ∼500 ka Middle Toba Tuff, and ∼74 ka Younger Toba Tuff. Ash from these eruptions is dispersed over the entire region from India, through Malaysia to Indonesia where it forms isochronous markers that have been documented in a variety of sedimentary sequences including palaeoclimate archives and archaeological sites. The chemistry of the volcanic glass shards usually enables distal volcanic ash units to be correlated to a known eruption. However, the magmas involved in the three largest Toba eruptions are compositionally similar and consequently the glass chemistry that is used to differentiate eruption units cannot be used to identify distal deposits. Here we show that the composition of biotite crystals, which occur with glass in the distal deposits, can be used to fingerprint the deposits of the Younger Toba Tuff. Biotite in Younger Toba Tuff has a lower FeO/MgO (2.1–2.6) than in the products of older eruptions (2.8–3.7). Correlations using these distinct biotite compositions indicate that the ash found in Malaysia and in archaeological sites in India was from the ∼74 ka Younger Toba Tuff eruption.

Deep explosive focal depths during maar forming magmatic-hydrothermal eruption: Baccano Crater, Central Italy

We describe the eruptive activity of the Pleistocene composite Baccano maar crater in the Sabatini Volcanic Complex (Central Italy) combining stratigraphy, grain size/componentry and rare earth element and Yttrium (REY) composition of its eruptive products with the stratigraphy and geothermal data derived from deep wells drilled on the Baccano structural high. The main lithological characteristics of the basal Baccano maar pyroclastic deposit, composed of more than 60% wt of non-thermometamorphosed lithic clasts from the sedimentary basement, show that the first eruption was magmatic-hydrothermal in nature. The lithology of the sedimentary lithic clasts indicates that the fragmentation level was at a depth of −1,000 to −1,200 m, with fragment depth verified by deep well stratigraphy. The 15% wt juvenile non-vesicular glass components suggest that magma played a minor role in powering the eruption. Assuming that the high-salinity hot hydrothermal fluids (365 −1,400 m) by the aggressive hydrothermal fluids enriched in acid components (HF, HCl, and H2SO4) may have contributed to increased CO2 partial pressure that helped to drive the hydrofracturing. This could have caused rapid vapour separation and pressure drop, allowing the almost simultaneous breaking of the aquifer cover and brecciation of the calcareous units down to −1,000 to −1,200 m depth. The relative abundance of calcareous lithics in the basal part of the first Baccano eruptive unit, representing about the upper 200 m of stratigraphy below the top of the Baccano structural high, reveals the descent of the piezometric surface during the eruption. Combining deep well information and maar product stratigraphy, using also REY data from maar pyroclastic fragments and the basement rocks we draw an interpretative model for the Baccano maar-forming eruption, concluding that a) magmatic-hydrothermal eruptions may originate deeper than previously thought, and b) hydrothermal fluids circulating in limestone aquifers may play an important role in triggering such eruptions.

Textural and geochemical constraints on eruptive style of the 79 ad eruption at Vesuvius

The 79 ad Plinian eruption of Vesuvius produced first a white pumice fallout from a high steady eruptive column, and then a grey pumice fallout originating from an oscillatory eruptive column with several partial column collapse events after which there was a total column collapse. This first total collapse was followed by renewed Plinian activity and produced the last grey pumice (GP) fallout deposit of the eruption. Textural characteristics (vesicularity and microcrystallinity) of a complete sequence of the pumice fallout deposits are presented along with the major element compositions and residual volatile contents (H2O, Cl) to constrain the degassing processes and the eruptive dynamics. Large variations in residual volatile contents exist between the different eruptive units. Textural features also strongly differ between white and grey pumices, but also within the grey pumices. The degassing processes were thus highly heterogeneous. We propose a new model of the 79 ad eruption in which pre-eruptive conditions (H2O saturation, magma temperature and viscosity) are the critical controls on the diversity of the syn-eruptive degassing processes and hence the eruptive dynamics. Cl contents measured in melt inclusions show that only the white pumice and the upper part of the grey pumice magma were H2O saturated prior to eruption. The white pumice eruptive units represent a typical closed-system degassing evolution, whereas the first grey pumice one, stored under similar pre-eruptive saturation conditions, follows a particular open-system degassing evolution. We suggest that the oscillatory regime that dominated the grey pumice eruptive phase is linked to pre-eruptive water undersaturation of most of the grey magma, and the associated time delays necessary for H2O exsolution. We also suggest that the high residual H2O content of the last grey pumice, deposited after the renewal of Plinian activity following the first total column collapse event, is due to syn-eruptive saturation of GP magma and reduced H2O exsolution efficiency resulting from speciation of dissolved H2O in the melt.

Effect of syneruptive decompression path on shifting intensity in basaltic sub-Plinian eruption: Implication of microlites in Yufune-2 scoria from Fuji volcano, Japan

To constrain the timing and conditions of syneruptive magma ascent that are responsible for shifting eruption intensity, we have investigated a basaltic sub-Plinian eruption that produced Yufune-2 scoria in Fuji volcano 2200 years ago. We deduced magmatic decompression conditions from groundmass microlite textures, including decompression path (i.e. evolution in decompression rate) and approximate decompression rate, in order to relate them to eruption intensity. The microlites revealed decompression conditions after water saturation at 700–1100 m depth. The temporal change in scoria size indicates that the magma discharge rate and resultant eruption intensity increased from unit a to unit b, and then declined toward ending units d and e. The overall decompression rate in each eruptive unit has a positive correlation with eruption intensity. The variation in decompression rate was enlarged in the final units, where the maximum remained the same as the peak through the eruption (0.13–0.22 MPa/s for units b and c), while the minimum was 0.025 MPa/s. The large variation here is due to 1) variation in flow velocity across conduit and 2) part of the erupted magma in unit d experienced remarkably slow decompression (0.002–0.003 MPa/s) resulting from decreased overpressure in the reservoir following the major eruption of unit b. Furthermore, crystal size distribution (CSD) of microlites implied that the earliest erupted magma (unit a) had once been decompressed slowly (0.005–0.012 MPa/s), having been arrested by material in the conduit–vent system, which was followed by an increase in decompression rate due to removal of the material at the initiation of the eruption. In addition, the magma that had been ascending slowly before the unit-d eruption may record the increase in decompression rate. This increased rate resulted from being pushed up by the successive magma at the start of that eruption. Two factors had a major impact on eruption intensity. First, magma decompression rate determined the degree of gas-phase separation from ascending magma. Judging from CSD, different decompression rates had been generated at least at the start of microlite crystallization. The second factor is the conduit radius that, in combination with magma ascent rate, controlled the magma discharge rate. Before the major eruption of unit b, the conduit radius likely increased, as evidenced by xenoliths of basaltic lava and lithic fragments with the same petrography as the xenoliths in unit a. In unit e, the conduit radius decreased through inward development of high-density magma from the conduit margin.

Diverse patterns of ascent, degassing, and eruption of rhyolite magma during the 1.8 ka Taupo eruption, New Zealand: Evidence from clast vesicularity

The 22 km 3 (DRE) 1.8 ka Taupo eruption ejected chemically uniform rhyolite in a wide range of eruptive styles and intensities. The 7 eruptive units include the ‘type examples’ of phreatoplinian (units 3 and 4) and ultraplinian fall (unit 5) deposits, and low-aspect-ratio ignimbrite (unit 6). Contrasts in bulk vesicularity, vesicle (and microlite) number densities and the size distributions of bubbles (and crystals) in the Taupo ejecta can be linked to the influence of shallow conduit processes on volatile exsolution and gas escape, before and during eruption, rather than changes in pre-eruptive chemistry. Existing work has modeled the individual phases of this complex eruption but not fully explained the abrupt shifts in style/intensity that occur between phases. We link these rapid transitions to changes in vent position, which permitted contrasts in storage, conduit geometry, and magma ascent history. Samples in the study show that coalescence of bubbles was a late-stage process in the pre-fragmentation degassing of even the most rapidly ascending magma, but, in most cases, nucleation of new bubbles continued until close to fragmentation. In the two phases of the Taupo eruption linked to dome or cyptodome formation, we can also recognize an influence of onset of permeability, partial outgassing, limited syn-eruptive crystallization, and bubble collapse. Post-fragmentation expansion or contraction of vesicles was only marked in some pumices within the deposits of the two heat-retentive yet nonwelded pyroclastic density currents (units 5B and 6). We can recognize three different types of history of ascent, bubble nucleation, and degassing for the Taupo eruption. Units 1, 2 3, 5, and 6 involved magma that ascended rapidly under the southern and central portions of the vent system and underwent late-stage closed system, coupled vesiculation without syn-eruptive crystallisation of microlites. The limited contrasts in textures amongst these units reflect only slightly different ascent histories, including slower rise permitting extended bubble coalescence (unit 1), rapid, accelerating ascent (units 2, 3 and 5), and exceptionally rapid decompression and ascent at the time of highest mass discharge rates (unit 6). In contrast to all these units, the unit 4 magma also rose rapidly initially but then underwent some degree of limited and shallow storage, permitting variable degrees of prolonged bubble maturation (growth and coalescence), development of permeability and outgassing, under the northern portion of the 10-km-long vent system, probably beginning at the time of eruption of units 1–3 from the southern portion of the fissure. Finally, the magma which formed the late-stage dome (unit 7), remained deeper in the plumbing system for an extended time permitting limited growth of microphenocrysts, (perhaps in response to partial depressurisation occurring during the earlier phases of the eruption), before ascending and continuing to outgas in equilibrium fashion.

H 2O- and temperature-zoning in magma chambers: The example of the Tufo Giallo della Via Tiberina eruptions (Sabatini Volcanic District, central Italy)

Textural and chemical variations of juvenile clasts are widely observed in pyroclastic deposits. In particular, the co-existence of whitish, pumiceous, and dark grey, scoriaceous, juvenile clasts has been observed in many eruptive units of well-known volcanoes (i.e., Somma–Vesuvius, Vulsini, Colli Albani, Stromboli). Here we report the example of the Tufo Giallo della Via Tiberina (TGVT) pyroclastic succession, which comprises two eruptive units emplaced at ca. 561 and 548 ka, during the early explosive activity of the Sabatini Volcanic District (SVD; Roman Province, central Italy). TGVT deposits, as well as underlying pyroclastic products (FAD, ca. 582 ka), are characterized by coexisting whitish pumice and black–grey scoria clasts showing common phonolitic composition but different textural features: white pumice is highly vesicular, vitrophyric, and contains scarce, > 50 µm-sized, feldspar and clinopyroxene crystals, while black–grey scoria is poorly vesicular, highly crystallized, and contains diffuse leucite phenocrysts. The latter records crystallization under H 2O-undersaturated conditions, as opposed to the vitrophyric texture of white pumice indicating higher temperature and H 2O concentration. On these grounds, a thermally and H 2O-zoned pre-eruptive system has been modelled for the phonolitic magma chambers feeding the early SVD events, in which white pumice and black–grey scoria represent the inner and peripheral portions of the reservoirs, respectively. Extensive leucite + clinopyroxene crystallization in the H 2O-undersaturated, peripheral portions of the reservoirs, resulted in water flux toward the inner zones, where the higher temperature and increasing H 2O content acted to delay crystallization in the white pumice-feeder magma. The withdrawal of white pumice at the eruption onset produced decompression of the peripheral magma, triggering black–grey scoria eruption during the late phases of explosive events.

Chronology and Evolution of Caldera-forming and Post-caldera Magma Systems at Okataina Volcano, New Zealand from Zircon U–Th Model-age Spectra

U–Th disequilibrium model-age data are presented for zircons from four young eruptive units from Okataina volcano, New Zealand. These data highlight contrasts in the time-scales over which eruptible silicic magma bodies are generated and accumulated below a highly active rhyolite volcano prior to and following the ~61 ka caldera-forming Rotoiti eruption (80 – 120 km 3 magma). The Rotoiti event was followed by ≥12 explosive eruptions of the Mangaone Subgroup between ~45 and 30 ka. A change in eruptive styles between ~30 and 25 ka brackets the major Oruanui eruption of Taupo volcano ~80 km away; subsequently eight rhyolitic, lava-dominated and two basaltic explosive eruptions occurred between 25 ka and 1886 CE. We present (238U–230Th) zircon model-age data determined by secondary-ion mass spectrometry (SIMS) and thermal ionization mass spectrometry (TIMS) from the Rotoiti, Ngamotu (Unit B) (~45 ka), Mangaone (Unit I) (33 ka) and Rotorua (15•4 ka) eruptive units. We couple these data with published and new compositional information to trace magma crystallization and storage patterns. Population density curves of SIMS model ages from zircons in the two pumice types (biotite-free and biotite-bearing) from the Rotoiti eruption differ. Zircons from the former yield a model-age peak coincident with eruption age, whereas zircons from the latter show a peak at 70–90 ka and little variation in TIMS model-age values from different crystal size fractions. Concentration weighted means of model ages from the two pumices, however, are the same within 1 SD error, and their Sr isotopic values overlap at 2 SD precision, suggesting that they are genetically linked. Model-age spectra for the Ngamotu, Mangaone and Rotorua pumices are complex, indicating recycling of crystals from multiple older populations that largely pre-date the 61 ka caldera-forming eruption. Superimposed on the older age spectra are variably developed younger pre-eruptive suites of ages reflecting varying amounts of crystallization prior to each eruption. A lack of commonality in these younger peaks in the post-caldera eruptive rocks, along with compositional and isotopic differences between (and sometimes within) the eruptive units, collectively precludes their origin from a single melt-bearing mush system. Okataina contrasts with Taupo, where comparable-age eruption deposits have simpler age spectra, consistent with larger-scale crystallization cycles and thermal events in the magma chambers there. When compared with other caldera-related silicic systems for which suitable data are available, Okataina differs in lacking a simple pre-eruptive history prior to its caldera-forming event and having a complex, non-coherent magmatic history of post-caldera eruptions.

Linking experimental and natural vesicle textures in Vesuvius 79AD white pumice

Vesicle populations in volcanic pumice provide a partial record of shallow magma ascent and degassing. Here we compare pumice textures from the well-characterized 79AD Vesuvius eruption to those generated during isothermal decompression experiments. Three series of experiments were conducted using starting material from the first two phases of the eruption (eruptive units EU1 and EU2). Samples were decompressed from 100 or 150 MPa to final pressures of 10–25 MPa using conditions appropriate for simulating eruption conditions ( T = 850 °C, d P/d T = 0.25 MPa/s). The experiments differed not only in starting material but also in temperature at which samples were annealed prior to decompression, which determined the initial number of crystals present in the melt. Results show that experiments approach the vesicle number densities and sizes of pumice samples, but show narrower size distributions. The wider size range of pumice samples suggests continuous, rather than instantaneous nucleation, which may reflect non-linear rates of decompression. All experiments exhibited equilibrium degassing, a process that was probably aided by heterogeneous bubble nucleation on oxide microlites. We conclude that delayed bubble nucleation cannot explain the explosivity of the Vesuvius eruption, which instead appears to require high rates of magma decompression.

The Pomici di Avellino eruption of Somma-Vesuvius (3.9 ka bp). Part I: stratigraphy, compositional variability and eruptive dynamics

The stratigraphic succession of the Pomici di Avellino Plinian eruption from Somma-Vesuvius has been studied through field and laboratory data in order to reconstruct the eruption dynamics. This eruption is particularly important in the Somma-Vesuvius eruptive history because (1) its vent was offset with respect to the present day Vesuvius cone; (2) it was characterised by a distinct opening phase; (3) breccia-like very proximal fall deposits are preserved close to the vent and (4) the pyroclastic density currents generated during the final phreatomagmatic phase are among the most widespread and voluminous in the entire history of the volcano. The stratigraphic succession is, here, divided into deposits of three main eruptive phases (opening, magmatic Plinian and phreatomagmatic), which contain five eruption units. Short-lived sustained columns occurred twice during the opening phase (Ht of 13 and 21.5 km, respectively) and dispersed thin fall deposits and small pyroclastic density currents onto the volcano slopes. The magmatic Plinian phase produced the main volume of erupted deposits, emplacing white and grey fall deposits which were dispersed to the northeast. Peak column heights reached 23 and 31 km during the withdrawal of the white and the grey magmas, respectively. Only one small pyroclastic density current was emplaced during the main Plinian phase. In contrast, the final phreatomagmatic phase was characterised by extensive generation of pyroclastic density currents, with fallout deposits very subordinate and limited to the volcano slopes. Assessed bulk erupted volumes are 21 × 106 m3 for the opening phase, 1.3–1.5 km3 for the main Plinian phase and about 1 km3 for the final phreatomagmatic phase, yielding a total volume of about 2.5 km3. Pumice fragments are porphyritic with sanidine and clinopyroxene as the main mineral phases but also contain peculiar mineral phases like scapolite, nepheline and garnet. Bulk composition varies from phonolite (white magma) to tephri-phonolite (grey magma).

Rhyolites at Kerlingarfjöll, Iceland: the evolution and lifespan of silicic central volcanoes

Kerlingarfjoll central volcano is Iceland’s second largest outcrop of Quaternary rhyolite and is part of the Icelandic Western Rift Zone. Geochemical and Ar/Ar age data show that at least 21 different rhyolite eruptions have taken place at Kerlingarfjoll over the last 350 ka. Ar/Ar dating was carried out on samples of obsidian which showed variable reproducibility, illustrating the difficulty in dating young Icelandic volcanics. Nevertheless, reasonable estimates of eruption age have been derived for a number of eruptive units that are consistent with observed stratigraphy, enabling an understanding of the temporal evolution of Kerlingarfjoll. Two rhyolite magma types are present. The first is an older, low-Nb rhyolite that was erupted episodically along a cryptic curved fracture system, to form a discontinuous ring of rhyolite mountains, between 350 and 250 ka. This discontinuous ring is similar to structures observed at other volcanoes in Iceland, suggesting that the development of a curved fracture that acts as a pathway for episodic silicic eruptions is a feature of central volcano development. The second magma is a younger, high-Nb rhyolite that was erupted episodically between 250 and 68 ka in the northern part of Kerlingarfjoll, forming two clusters, both of which have areas of intense hydrothermal activity. Repose periods for rhyolite volcanism are thought to be on the order of tens of thousands of years, and it is possible that Kerlingarfjoll will erupt rhyolite again in the future.

The Plinian Lower Pumice 2 eruption, Santorini, Greece: Magma evolution and volatile behaviour

The Plinian Lower Pumice 2 (LP2) eruption (172 ka) was one of the first major caldera-forming eruptions of the Santorini volcanic complex (Greece). The eruption shows some striking similarities to the caldera-forming Late Bronze Age (Minoan) eruption in terms of field, petrological and geochemical characteristics of its eruptive products, which are used to reveal the storage conditions of the LP2 magmas, pre-eruptive magmatic processes and the behaviour and degassing of volatiles prior to and during eruption. The LP2 eruption comprises four, predominantly rhyodacitic eruptive units (LP2-A, B, C, D). The lowermost unit of the Plinian LP2 deposits (LP2-A) consists of a basal phreatomagmatic bed (LP2-A1), which is overlain by three discrete pumice fall deposits (LP2-A2-1, A2-2, A3), the most prominent of which (LP2-A3) contains abundant, quench-textured scoriae that range in composition from basalt to basaltic andesite. The eruption proceeded with the deposition of pumice-rich pyroclastic flows (LP2-B) characterised by a lower, stratified and cross-bedded ignimbrite (LP2-B1) that may grade into a massive, non-welded ignimbrite (LP2-B2), a lithic-rich pumiceous breccia (LP2-C) and a co-ignimbrite lithic lag breccia (LP2-D). The main volume of rhyodacitic magma, which formed by fractionation of olivine, clinopyroxene, orthopyroxene, plagioclase, amphibole, Fe–Ti oxides, pyrrhotite and apatite from basaltic parental magmas and assimilation of crustal rocks, was held at mid-crustal levels (≤ 16 km depth), magmatic temperatures of 831 ± 12 °C and an oxygen fugacity slightly above the fayalite–magnetite–quartz (FMQ) oxygen buffer. Injection of ∼ 200 °C hotter mafic magma into the rhyodacitic reservoir and subsequent mingling and minor hybridisation with the resident magma helped to remobilise the rhyodacitic host magma and determined the final compositional range of the erupted products. Melt inclusion data show that sulphur concentrations were reduced to < 270 ppm in the rhyodacite, primarily due to partitioning of sulphur into pyrrhotite or, depending on temperature, a FeS-rich melt during magmatic differentiation at oxygen fugacities around the FMQ oxygen buffer. Sulphur concentrations in groundmass glasses of the LP2 pumices suggest that ∼ 43% of the remaining sulphur was released into the atmosphere during the LP2 eruption, the climatic effects of which are considered minor when compared to eruptions of more oxidised silicic arc magmas. Chlorine remained dissolved in the melt during magmatic differentiation prior to and during the LP2 eruption, indicating that chlorine emissions to the atmosphere were negligible.

Geology and evaluation strategy of the Star and Orion South kimberlites, Fort à la Corne, Canada

Diamond-bearing kimberlites in the Fort à la Corne field, east-central Saskatchewan, consist primarily of volcaniclastic deposits which are interstratified with Lower Cretaceous continental, marginal marine and marine sediments. Seventy kimberlite occurrences have been drill-confirmed with the majority of those occurring in the Fort à la Corne main trend. The Star (352 ha) and Orion South (358 ha) kimberlite complexes are located at the south-eastern extremity of the main trend and contain some of the world's best examples of preserved primary volcaniclastic kimberlite deposits. The advanced exploration programs on both the Star and Orion South complexes offer a rare opportunity to examine a wide variety of in situ volcaniclastic kimberlite deposits. Both kimberlite complexes are composed of multiple eruptive units that were deposited episodically over 4 to 6 million years, separated by periods of volcanic quiescence marked by reworking of kimberlite and siliciclastic sedimentation. Detailed logging of core from 100 m spacing grid drilling has revealed well-defined tuff ring, tuff cone, crater and extra crater deposits. Evaluation of these large tonnage, lower grade, coarse diamond size frequency kimberlite complexes requires a methodical and multi-branched exploration strategy utilizing: 1. Grid-pattern core drilling to gain an understanding of the three-dimensional architecture of the kimberlite complexes; 2. Underground bulk sampling to provide appreciable carats for accurate estimates of diamond grade and price; and, 3. Grid-pattern mini-bulk sampling via large diameter drilling to obtain an understanding of the diamond grade across the breadth of each kimberlite.

Rapid Rates of Magma Generation at Contemporaneous Magma Systems, Taupo Volcano, New Zealand: Insights from U–Th Model-age Spectra in Zircons

New and/or enlarged datasets of U–Th disequilibrium model ages from secondary ionization mass spectrometry (SIMS) analyses of zircons in eight eruptive units from the area of Taupo volcano, New Zealand, highlight the behavioural contrasts of two closely adjacent, contemporaneous but independent magma chambers. One yielded closely similar crystal-poor (‘Oruanui-type’) rhyolites, sampled in three small precursor eruptions (Tihoi, ‘New plinian’, Okaia) from ∼45 to 30 ka, then the major 27 ka Oruanui eruption. Three of the four eruptions had vents within the modern Lake Taupo, whereas the fourth (‘New plinian’) was sourced ∼20 km NNE of the other vents, fed by lateral magma migration. Samples from all four eruptions share a common model-age peak at ∼95 ka of antecrystic zircons. However, three of the four differ in younger pre-eruptive model-age peaks that require their parental melt-dominant bodies to have been physically extracted independently from a common mush zone represented by the ∼95 ka peak. A sample from a fifth eruption (‘New phreatoplinian’, also at ∼45 ka) shares an older 80–100 ka peak but has numerous older grains and distinctly contrasting Sr-isotopic characteristics to the ‘Oruanui-type’ magmas. The 530 km 3 Oruanui melt-dominant body was produced in at most ∼3000 years as shown by differences in zircon model-age spectra and average ages between it and the 30 ka Okaia eruption, despite their coincidence in vent locations. The second suite of eruptions at ∼47, 28 and 16 ka ejected moderately crystal-rich biotite rhyolites from a second source chamber, which vented over a 15 km wide area NE of Taupo (overlapping with Maroa volcano). This second chamber is inferred to have comparable horizontal dimensions to the vent spacing. The three biotite rhyolites show unimodal model-age spectra that peak at 30, 15–25 and 6 kyr prior to each eruption, respectively, and underwent single cycles of melt generation and eruption with no recycling of significantly older antecrysts or xenocrysts ( 5 m 3 /s (Oruanui) and effectively drained the mush of melt in doing so (Oruanui vs post-Oruanui activity), probably mediated by active rifting processes and tectonic disruption of the mush pile. Comparisons of ‘magma residence times’ and discussion of the growth histories of large silicic chambers represented by volcanic or plutonic rocks are self-limited by the uncertainties in the respective SIMS analyses. Growth times of Miocene and older plutons dated by SIMS U–Pb techniques are comparable with the 2 Myr lifetime of the whole Taupo Volcanic Zone, and the associated 1σ SIMS analytical uncertainties exceed the lifetime of a volcano such as Taupo. Subtle details that indicate the rapidity of magma accumulation and recycling of crystals in the young Taupo system cannot be discerned in most pre-300 ka silicic systems. Averaging of SIMS model-age data further obscures subtle details that would allow discrimination of newly crystallized versus recycled zircons. Discussions of volcano–plutonic relationships and accumulation rates for large silicic melt-dominant bodies cannot rely on age data in isolation, but require knowledge of the stratigraphic and compositional settings.

The ∼ 1245 yr BP Asososca maar eruption: The youngest event along the Nejapa–Miraflores volcanic fault, Western Managua, Nicaragua

The Asososca Tephra comprises the asymmetrically distributed, well-stratified phreatomagmatic products of the 1245 + 125/− 120 yr BP Asososca maar eruption West of Managua, Nicaragua. It is the youngest eruptive unit related to the monogenetic volcanism concentrated along the N–S trending Nejapa–Miraflores active fault and intersecting E–W to NE–SW and NW–SE minor faults. The close relationship between monogenetic volcanism and the structural-local tectonic framework is evidenced by the lineament domains and vent alignment patterns identified by remotely sensed images and DEMs of the research area. The Asososca maar lake bathymetry suggests that the crater was formed by coalescing vents migrating during the eruptive event. The Asososca Tephra dominantly consists of accidental lithics disrupted from the underlying stratigraphic units observed inside and around the maar crater. Dry base-surge bedsets are dominant throughout the eruptive unit, showing facies variation from proximal, cross-stratified beds to mid-distal plane-parallel and wavy-parallel beds. Grain-size analyses indicate that base surges transported dominantly − 1, 0, and 1ϕ sized particles at the bedload in form of traction carpets, while fine ash material in continuous suspension was minimal. Juvenile fragments were only identified in the finer fractions, varying from vesiculated scoriae to dense sideromelane fragments. SEM analyses suggest that the Asososca eruption resulted from a highly efficient fuel–coolant interaction between a tholeiitic basaltic melt and an aquifer hosted in a shallow level (between 50 and 200 m) of olivine-bearing scoria ash and lapilli beds. The explosion triggered the country-rock comminution and the production of moss-like, fused-shaped, and blocky ash-shards with stepped surfaces, quenching cracks, pitting, alteration skins, and adhered particles, all indicative of phreatomagmatic fragmentation. The very recent age of the Asososca maar eruption confirms that the densely inhabited Managua area is volcanically active and that hazard studies should focus on the potential for phreatomagmatic eruptions.

U–Pb dating of magmatic and xenocryst zircons from Mangakino ignimbrites and their correlation with detrital zircons from the Torlesse metasediments, Taupo Volcanic Zone, New Zealand

Mangakino ignimbrites are a sequence of voluminous eruptive units in the rhyolitic Mangakino Volcanic Centre (MVC) caldera complex, Taupo Volcanic Zone, New Zealand. U–Pb dating of 113 zircons extracted from four Mangakino ignimbrites reveals magmatic and xenocrystic zircons. Magmatic zircon ages from three ignimbrites are slightly older than K/Ar and 40Ar/ 39Ar ages. Magmatic zircon age-spectra are tightly clustered and support a simple crystallisation history for these three ignimbrites. It is possible that the assimilation of zircon antecrysts leads to magmatic zircon spectra for the Ahuroa ignimbrite being ≥ 70 ka older than the 40Ar/ 39Ar age. Xenocrystic zircon age spectra correlate with spectra of detrital zircons from the underlying Mesozoic Torlesse metasediments. Our data are consistent with significant assimilation of metasediments and the incorporation of zircon prior to eruption. Systematic variations in the age spectra of xenocrystic zircons in successive Mangakino ignimbrites may be related to the structural development of the North Island, New Zealand.