Rhyolitic Eruptions Research Articles

Crystal scale anatomy of a dying supervolcano: an isotope and geochronology study of individual phenocrysts from voluminous rhyolites of the Yellowstone caldera

A voluminous (>600 km3) and long-lived (~520–75 ka) phase of rhyolitic eruptions followed collapse of the Yellowstone caldera 640 ka. Whether these eruptions represent a dying cycle, or the growth of a new magma chamber, remains an important question. We use new U–Th zircon ages and δ18O values determined by ion microprobe, and sanidine Pb isotope ratios determined by laser ablation, to investigate the genesis of voluminous post-caldera rhyolites. The oldest post-caldera rhyolites, erupted between ~520 and 470 ka, exhibit extreme age and oxygen isotopic heterogeneity, requiring derivation from individual parcels of low-δ18O melts. We find a progressive increase in zircon homogeneity for rhyolite eruptions from ~260 to 75 ka, with homogeneous low-δ18O zircon values of 2.7–2.8‰ that are in equilibrium with low-δ18O host melts for the majority of the youngest eruptions. New sanidine Pb isotope data define separate arrays for post-caldera rhyolites and the caldera-forming tuffs that preceded them, indicating that they were not sourced from a mushy Lava Creek Tuff batholith that remained after caldera collapse. Rather, our new age and isotopic data indicate that the post-caldera rhyolites were generated by remelting of a variety of intracaldera source rocks, consisting of pre-Lava Creek Tuff volcanic and plutonic rocks and earlier erupted post-Lava Creek Tuff rhyolites. Batch assembly of low-δ18O melts starting at ~260 ka resulted in progressive homogenization, followed by differentiation and cooling up until the last rhyolite eruption ~75 ka, a trend that we interpret to be characteristic of a dying magma reservoir beneath the Yellowstone caldera.

Timing and conditions of formation of granitoid clasts erupted in recent pyroclastic deposits from Tarawera Volcano (New Zealand)

Granitoid clasts in pyroclastic deposits of the 0.7ka (Kaharoa) eruption from the intra-caldera Tarawera volcano of the Okataina Volcanic Centre (OVC), New Zealand, provide an insight to the plutonic processes beneath one of the most productive Quaternary rhyolite centers on Earth. SIMS U–Th data for 79 granitoid zircon crystals from six clasts reveal a unimodal age spectrum yielding a weighted average model age of 208±4ka (MSWD=1.2; n=72). The remaining crystals are at secular equilibrium and U–Pb analyses indicate that a few of these outliers date back to ~750ka, a period significantly longer than the known volcanic record at OVC (probably ~550ka). In contrast, zircon crystallization in co-erupted pumice and lava of the 0.7ka Kaharoa event, and that of the three preceding rhyolite eruptions, occurred mostly during 0–50ka, reflecting a separate magmatic history. Brittle deformation features, incipient alteration, and low-δ18O whole-rock compositions (+3‰) are consistent with a shallow solid intrusion that has interacted with meteoric hydrothermal fluids. However, δ18O SIMS analyses of zircons (+5.4±0.2‰; n=11) are consistent with magmatic compositions, and thus meteoric interaction occurred post-emplacement. The Kaharoa granitoids contrast with those ejected in the ~40–60ka caldera-forming Rotoiti event that were partly molten and display zircon age spectra indistinguishable from those for co-erupted pumices, suggesting the latter were derived from contemporaneous crystal mush. The 0.7ka Kaharoa case shows that zones of the magmatic system remained solid despite frequent and voluminous magma production that included caldera-forming events. Such long-lived sub-solidus zones in magma systems would act as barriers to melt connectivity and interaction with country rock, but are also a potential source for antecrysts/xenocrysts in subsequent eruptions. This exemplifies how plutonic and volcanic evolution can diverge even in close proximity.

Origin of middle Miocene leucogranites and rhyolites on the Tibetan Plateau: Constraints on the timing of crustal thickening and uplift of its northern boundary

Leucogranites play a significant role in understanding crustal thickening, melting within continental collisional belts, and plateau uplift. Field investigations show that Miocene igneous rocks from the Hoh Xil Lake area mainly consist of two-mica leucogranites and rhyolites. We studied the Bukadaban two-mica leucogranites and the Kekao Lake, Malanshan and Hudongliang rhyolites by zircon U-Pb, muscovite and sanidine 40Ar/39Ar geochronology, and whole-rock geochemical and Sr-Nd isotopic analysis. Results yielded crystallization and cooling ages for the Bukadaban leucogranites of 9.7±0.2 and 6.88±0.19 Ma, respectively. Extrusive ages of the Kekao Lake and Malanshan rhyolites are 14.5±0.8 and 9.37±0.30 Ma, respectively. All rocks are enriched in SiO2 (70.99%–73.59%), Al2O3 (14.39%–15.25%) and K2O (3.78%–5.50%) but depleted in Fe2O3 (0.58%–1.56%), MgO (0.11%–0.44%) and CaO (0.59%–1.19%). The rocks are strongly peraluminous (A/CNK=1.11−1.21) S-type granites characterized by negative Eu anomalies (δEu=0.18−0.39). In also considering their Sr-Nd isotopic compositions (87Sr/86Sri=0.7124 to 0.7143; e Nd (9 Ma) =−5.5 to −7.1), we propose that these igneous rocks were generated through dehydration melting of muscovite in the thickened middle or lower crust of northern Tibet. Melting was probably triggered by localized E-W stretching decompression in the horse tails of Kunlun sinistral strike-slip faults. Reactivation of the Kunlun strike-slip faults, accompanied by emplacement of leucogranite and eruption of rhyolite in the Hoh Xil Lake area, indicates that large-scale crustal shortening and thickening in northern Tibet mainly occurred before 15 Ma. In addition, these findings suggest that the northern Tibetan Plateau attained its present elevation (∼5000 m) at least 15 Ma ago.

Rift-Related Transition from Andesite to Rhyolite Volcanism in the Taupo Volcanic Zone (New Zealand) Controlled by Crystal–melt Dynamics in Mush Zones with Variable Mineral Assemblages

The Taupo Volcanic Zone (TVZ), located in the North Island of New Zealand, represents part of a magmatic arc that is at present undergoing active extension. Around 0·9 Myr ago, an acceleration in rifting was followed by a progressive transition in the composition of volcanic products (until 0·7 Ma) from typical arc-type andesite into overwhelmingly large, caldera-forming rhyolitic eruptions with subordinate basalt and dacite in the Central TVZ. Despite an obvious compositional gap in the erupted products in the Central TVZ within the last 0·7 Myr (little to no erupted products with SiO2 contents between 55 and 65 wt %), phenocryst minerals (plagioclase, amphibole, pyroxene) show an uninterrupted compositional record that suggests crystallization from a continuum of melt compositions. Coupled with radiogenic isotope evidence, the whole-rock and mineral chemistry data are consistent with magmatic differentiation controlled by crystal fractionation of primary mantle-derived magmas accompanied by some assimilation of local wall-rocks. In the Southern TVZ and in the early part of the Central TVZ, magmatic differentiation was dominated by the lower crustal evolution of relatively dry ( 1wt % H2O) arc basalts, crystallizing a pyroxene^plagioclase-dominated assemblage. However, the conditions of crystallization in the lower crust appear to have changed within the last million years in the Central TVZ, with amphibole and oxides appearing earlier in the crystallization sequence. In this framework and using numerical simulations coupling crystallization kinetics and multiphase fluid dynamics of magma reservoirs, we show that melts extracted from crystal mushes within an optimal ‘extraction window’ ( 50 and 80 vol. % crystals) match those erupted at the surface. Lower crustal mushes fed by basalt with 1wt % H2O (dominated by a pyroxene^plagioclase assemblage) release andesitic melts at the extraction window. These melts then erupt at the surface to form the observed andesitic part of the arc.With a slightly higher water content ( 2 wt %) in the basalt, the melt composition at the extraction window from lower crustal mushes is dacitic rather than andesitic. Although some dacitic melts will reach the surface, most will be trapped in the upper crust and crystallize to form a silicic mush. Extraction of the interstitial liquid after450% crystallization from this upper crustal reservoir produces the large volumes of rhyolitic magma erupted over the past 0·7 Myr (44000 km from ignimbrite-forming eruptions).

Nearly contemporaneous evolution of the A- and S-type fractionated granites in the Krušné hory/Erzgebirge Mts., Central Europe

The Krušné hory/Erzgebirge Mountains is one of the best-known provinces of granite magmatism and associated metallogenesis in Europe. Variscan magmatic activity in the Krušné hory/Erzgebirge area spanned the period from 330 to 295Ma. For much of this time, two types of magma were generated and emplaced in close proximity in this province. Strongly peraluminous P-rich (S-type) melts were formed alongside slightly peraluminous P-poor melts (A-type). Two suites of strongly peraluminous P, F, Li, Rb, Cs, U, Sn-rich and Zr, Th, Y, HREE-poor magmas were intruded in rapid succession over a period of about 10Ma (about 330–320Ma) over the entire region of the Krušné hory. Several peraluminous rhyolitic dykes near Gottesberg mark an isolated event at about 295Ma. Slightly peraluminous F, Li, Rb, Cs, Sn, Zr, Y, HREE-rich and P-poor magmas were intruded in several events separated in space and time (325–295Ma). Both magmatic episodes culminated with strongly fractionated subvolcanic granite intrusions, accompanied by explosive brecciation and followed by Sn+W mineralization of greisen type. Successive volcanic eruption of rhyolites and dacites of S-type, and rhyolites of A-type form the fill of the Altenberg-Teplice Caldera (ATC), thus demonstrating the close temporal and spatial relationship between these two types of magma. When these two geochemically distinct types of rock are in direct contact, then the rocks of A-type are always younger. The absolute time difference between the two types can range from less than 1Ma (the first eruption of the Teplice rhyolite versus the Schönfeld dacite in the ATC) to about 15Ma (eruption of rhyolites at Gottesberg versus emplacement of the Eibenstock granite). The assumed protolith of all late-Variscan granites in the area of Krušné hory/Erzgebirge is a mixture of fertile quartzo-feldspathic rocks with micaceous metapelites enriched in LILE and Sn+W. The differences in chemical composition of S- and A-type melts originated from local changes in the proportion of quartzo-feldspathic and pelitic lithologies, different pT-conditions of melting, the degree of melting, and the degree of metamorphic dehydratation of the protolith prior to the melting.

The role of dyking and fault control in the rapid onset of eruption at Chaitén volcano, Chile

Rhyolite is the most viscous of liquid magmas, so it was surprising that on 2 May 2008 at Chaitén Volcano, located in Chile's southern Andean volcanic zone, rhyolitic magma migrated from more than 5 km depth in less than 4 hours (ref. 1) and erupted explosively with only two days of detected precursory seismic activity. The last major rhyolite eruption before that at Chaitén was the largest volcanic eruption in the twentieth century, at Novarupta volcano, Alaska, in 1912. Because of the historically rare and explosive nature of rhyolite eruptions and because of the surprisingly short warning before the eruption of the Chaitén volcano, any information about the workings of the magmatic system at Chaitén, and rhyolitic systems in general, is important from both the scientific and hazard perspectives. Here we present surface deformation data related to the Chaitén eruption based on radar interferometry observations from the Japan Aerospace Exploration Agency (JAXA) DAICHI (ALOS) satellite. The data on this explosive rhyolite eruption indicate that the rapid ascent of rhyolite occurred through dyking and that melt segregation and magma storage were controlled by existing faults.

The Irosin co-ignimbrite ash-fall deposit: A widespread tephra marker in the Bicol arc, south Luzon, Philippines

The Irosin caldera, which is located in the province of Sorsogon, southern Luzon, Philippines, represents the largest extrusion of highly silicic magmas in the Bicol arc at ca. 41 cal ka BP. The 41 cal ka BP rhyolitic eruption led to a collapse and formation of the 11 km–wide Irosin caldera. This paper presents the results of the stratigraphy, grain assemblage, morphology, and geochemistry of the recently discovered rhyolitic fine ash which is exposed in the crater of Inascan scoria cone, 80 km from Irosin caldera. The morphology of glass shards obtained at Inascan cone is not only pumice-type but also bubble-wall-type glass shards which are typical of a co-ignimbrite ash. The mineral assemblage of the fine ash is quite similar to those of Irosin pumice. The refractive index, measured using the thermal immersion method, together with geochemical analyses of glass shards from the fine rhyolitic ash deposits and ignimbrite deposits from Irosin caldera, both indicate a strong geochemical similarity between the ignimbrite and fine ash deposits. Thus, the tephra sequence at Inascan scoria cone is interpreted as co-ignimbrite ash falls sourced from the 41 cal ka BP catastrophic eruption that formed the Irosin caldera. The Irosin co-ignimbrite ash-fall deposit, which measures 1.3 m thick and 80 km away from its source volcano, represents the most explosive eruption in the Bicol arc. The identification of the Irosin co-ignimbrite ash-fall deposit is a valuable contribution to the establishment of a chronological framework of widespread tephra in the Philippines as well as a potential regional tephra marker.

Decoupled crystallization and eruption histories of the rhyolite magmatic system at Tarawera volcano revealed by zircon ages and growth rates

We obtained U–Th disequilibrium age data on zircons from each of the four rhyolite eruptions that built Tarawera volcano in the last 22 ka within the Okataina Volcanic Center (OVC), caldera, New Zealand. Secondary ion mass spectrometry analyses on unpolished euhedral crystal faces that lack resorption features show that crystal growth variously terminated from near-eruption age to ~100 ka prior to eruption. Age-depth profiling of crystals reveals long periods of continuous (~34 ka) and discontinuous growth (~90 ka). Growth hiatuses of up to ~40 ka duration occur, but do not all relate to obvious resorption surfaces. Age differences up to similar magnitude are found on opposing faces of some crystals suggesting episodes of partial exposure to melts. These features are best explained by periodic, complete, or partial, sub-solidus storage and/or inclusion in larger crystal phases, followed by rapid liberation prior to eruption. This is supported by high abundances of U and Th (~500 − >2,000 ppm) in some zircons consistent with periods of high crystallinity (>70%) in the magmatic system, based on crystal/melt partitioning. Contemporaneous but contrasting rim-ward trends of these elements within crystals, even in the same lava hand sample, require synchronous growth in separate melt bodies and little connectivity within the system, but also significant crystal transport and mixing prior to eruption. Many crystals record continuity of growth through the preceding ~60 ka OVC caldera-collapse and subsequent eruptions from Tarawera. This demonstrates a decoupling between eruption triggers, such as shallow crustal extension and mafic intrusion, and the crystallization state of the OVC silicic magmatic system. The data highlights the need to distinguish between the time for accumulation of eruptible magma and the long-term magma residence time based on the age of crystals with high closure temperatures, when assessing the potential for catastrophic eruptions.

Ultra-distal tephra deposits from super-eruptions: Examples from Toba, Indonesia and Taupo Volcanic Zone, New Zealand

Voluminous rhyolitic eruptions from Toba, Indonesia, and Taupo Volcanic Zone (TVZ), New Zealand have dispersed volcanic ash over vast areas in the late Quaternary. The ∼74 ka Youngest Toba Tuff (YTT) eruption deposited ash over the Bay of Bengal and the Indian subcontinent to the west. The ∼340 ka Whakamaru eruption (TVZ) deposited the widespread Rangitawa Tephra, dominantly to the southeast (in addition to occurrences northwest of vent), extending across the landmass of New Zealand, and the South Pacific Ocean and Tasman Sea with distal terrestrial exposures on the Chatham Islands. These super-eruptions involved ∼2500 km3 and ∼1500 km3 of magma (dense-rock equivalent; DRE), respectively.Ultra-distal terrestrial exposures of YTT at two localities in India, Middle Son Valley, Madhya Pradesh, and Jurreru River Valley, Andhra Pradesh, at distances of >2000 km from the source caldera, show a basal ‘primary’ ashfall unit ∼4 cm thick, although deposits containing reworked ash are up to ∼3 m in total thickness. Exposures of Rangitawa Tephra on the Chatham Islands, >900 km from the source caldera, are ∼15–30 cm thick. At more proximal localities (∼200 km from source), Rangitawa Tephra is ∼55–70 cm thick and characterized by a crystal-rich basal layer and normal grading. Both distal tephra deposits are characterized by very-fine ash (with high PM10 fractions) and are crystal-poor.Glass chemistry, stratigraphy and grain-size data for these distal tephra deposits are presented with comparisons of their correlation, dispersal and preservation. Using field observations, ash transport and deposition were modeled for both eruptions using a semi-analytical model (HAZMAP), with assumptions concerning average wind direction and strength during eruption, column shape and vent size. Model outputs provide new insights into eruption dynamics and better estimates of eruption volumes associated with tephra fallout. Modeling based on observed YTT distal tephra thicknesses indicate a relatively low (<40 km high), very turbulent eruption column, consistent with deposition from a co-ignimbrite cloud extending over a broad region. Similarly, the Whakamaru eruption was modeled as producing a predominantly Plinian column (∼45 km high), with dispersal to the southeast by strong prevailing winds. Significant ash fallout of the main dispersal direction, to the northwest of source, cannot be replicated in this modeling. The widespread dispersal of large volumes of fine ash from both eruptions may have had global environmental consequences, acutely affecting areas up to thousands of kilometers from vent.

Geochemistry and Petrology of the Most Recent Deposits from Cotopaxi Volcano, Northern Volcanic Zone, Ecuador

Cotopaxi volcano is located in the Northern Volcanic Zone of the South American Andes. Pyroclastic deposits and lava flows from Cotopaxi comprise basaltic andesites, andesites and rhyolites that have erupted since 13 200 years BP. Nine rhyolite eruptions were produced in at least five separate events, punctuated by intermittent andesite eruptions. High La/Yb (>5) and 230Th excesses in the andesites are consistent with equilibration of magma with garnet-bearing lower crust or mantle, and numerical models show that lower crustal assimilation–fractional crystallization involving crystallization of amphibole and plagioclase is sufficient to create the observed variations in trace elements. The Cotopaxi andesites contain intergrowths of plagioclase + pyroxene, and at least four populations of plagioclase crystals indicate pervasive magma mixing. Sr, Nd and Pb isotopes are consistent with 5–20% assimilation of radiogenic crust, but higher levels of mafic crust (cryptic assimilation) and/or source contamination by volcanogenic sediments are also likely. The Cotopaxi rhyolites formed from compaction and extraction of high-SiO2 melt from an andesitic or dacitic crystal mush. On the basis of U-series data, the residence time of the rhyolitic melts is of the order of 74 kyr. Temporal variations in MgO and Sr of the Cotopaxi andesites reflect frequent recharge of the Cotopaxi system, variable recharge composition and a system that has not become more evolved over time.

Evolution of silicic magmas in the Kos-Nisyros volcanic center, Greece: a petrological cycle associated with caldera collapse

Multiple eruptions of silicic magma (dacite and rhyolites) occurred over the last ~3 My in the Kos-Nisyros volcanic center (eastern Aegean sea). During this period, magmas have changed from hornblende-biotite-rich units with low eruption temperatures (≤750–800°C; Kefalos and Kos dacites and rhyolites) to hotter, pyroxene-bearing units (>800–850°C; Nisyros rhyodacites) and are transitioning back to cooler magmas (Yali rhyolites). New whole-rock compositions, mineral chemistry, and zircon Hf isotopes show that these three types of silicic magmas followed the same differentiation trend: they all evolved by crystal fractionation and minor crustal assimilation (AFC) from parents with intermediate compositions characterized by high Sr/Y and low Nb content, following a wet, high oxygen fugacity liquid line of descent typical of subduction zones. As the transition between the Kos-Kefalos and Nisyros-type magmas occurred immediately and abruptly after the major caldera collapse in the area (the 161 ka Kos Plateau Tuff; KPT), we suggest that the efficient emptying of the magma chamber during the KPT drew out most of the eruptible, volatile-charged magma and partly solidified the unerupted mush zone in the upper crust due to rapid unloading, decompression, and coincident crystallization. Subsequently, the system reestablished a shallow silicic production zone from more mafic parents, recharged from the mid to lower crust. The first silicic eruptions evolving from these parents after the caldera collapse (Nisyros units) were hotter (up to >100°C) than the caldera-forming event and erupted from reservoirs characterized by different mineral proportions (more plagioclase and less amphibole). We interpret such a change as a reflection of slightly drier conditions in the magmatic column after the caldera collapse due to the decompression event. With time, the upper crustal intermediate mush progressively transitioned into the cold-wet state that prevailed during the Kefalos-Kos stage. The recent eruptions of the high-SiO2 rhyolite on Yali Island, which are low temperature and hydrous phases (sanidine, quartz, biotite), suggest that another large, potentially explosive magma chamber is presently building under the Kos-Nisyros volcanic center.

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.

Petrologic, chemical, and isotopic evaluation of pitchstones in the Samho area, southwestern Okcheon Belt, South Korea

The Samho area in the southwestern Okcheon Belt, Korean Peninsula, contains spatially related eruptions of lapilli tuff, rhyolite, and pitchstone, considered to be of Late Cretaceous age. The pitchstones, which are the youngest rocks in this volcanic suite, are chemically highly evolved, strongly metaluminous to weakly peraluminous, and calc-alkaline. They have moderately fractionated chondrite-normalized rare earth element (REE) patterns, with relative enrichment of light REEs and Eu/Eu* values of 0.53–0.57. A distinct negative Nb anomaly is observed on a primitive mantlenormalized trace element diagram. These chemical features correspond to an I-type granite derived from a continental/oceanic arc. The pitchstones have Zr contents of 45–92 ppm, and zircon thermometry yields temperatures of 650–743 °C (mean value, 722 °C). The pitchstones have an initial 87Sr/86Sr isotopic ratio of 0.712, an initial 143Nd/144Nd ratio of 0.5120, and aNd(t) values of −10.6 to −12.0. The mean K-Ar whole-rock ages of the lapilli tuff, rhyolite, and pitchstone are 81.9 ± 2.4 Ma, 65.5 ± 1.9 Ma, and 12.7 ± 0.9 Ma, respectively. The depleted mantle model ages (TDM) range from 1.46 to 1.58 Ga. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb isotopic ratios are 18.477–18.569, 15.672–15.702, and 38.889–38.999, respectively. The Sr and Nd isotopic compositions of the pitchstones are relatively enriched compared with depleted MORB mantle (DMM), indicating the involvement of lower crustal components in their source. We conclude that the pitchstones in the Samho area formed in a continental arc setting by partial melting of a probable Mesoproterozoic parental source. The magma series, petrogenesis, and tectonic setting of Late Cretaceous magmatism in the Samho area can be correlated with those of the Haenam volcanic fields located east of the present study area.

Large-volume Rhyolite Genesis in Caldera Complexes of the Snake River Plain: Insights from the Kilgore Tuff of the Heise Volcanic Field, Idaho, with Comparison to Yellowstone and Bruneau–Jarbidge Rhyolites

Generation of large-volume rhyolites in the shallow crust is an important, yet enigmatic, process in the Snake River Plain and worldwide. Here, we present data for voluminous rhyolites from the 6·6–4·5 Ma Heise volcanic field in eastern Idaho. Heise is arguably the best site to evaluate shallow rhyolite genesis in the Snake River Plain; it is the youngest complete record of caldera cluster volcanism along the Yellowstone hotspot track and it culminated with the eruption of the most voluminous low-δ 18 O rhyolite known on Earth: the 1800 km 3 Kilgore Tuff (δ 18 O = 3·4‰). Such low-δ 18 O values fingerprint meteoric waters, and thus the shallow crust. New oxygen isotope data for phenocrysts, obtained by laser fluorination, correspond to a low-δ 18 O magma value of 3·4 ± 0·1‰ (2 standard error) for Kilgore Tuff samples erupted >100 km apart; however, ion microprobe data for single zircon crystals show significant diversity, with δ 18 O values that range from –1·3‰ to 6·1‰. U–Pb zircon ages, mineral chemistry, whole-rock major and trace element geochemistry, Sr and Nd isotope data, and magmatic (liquidus) temperatures are similar and/or overlapping for all studied samples of the Kilgore Tuff. Normal-δ 18 O Heise tuff units that preceded the Kilgore Tuff define a temporal compositional trend in trace element concentrations, trace element ratios, and Sr and Nd isotope ratios that is consistent with fractional crystallization from a common reservoir, whereas low-δ 18 O Kilgore cycle units have compositions that define a sharp reversal in the temporal trend back towards the composition of the first normal-δ 18 O Heise tuff (6·62 Ma Blacktail Creek Tuff). The data support derivation of the voluminous low-δ 18 O Kilgore Tuff from remelting of hydrothermally altered ( 18 O depleted) intracaldera and subvolcanic portions of the Blacktail Creek Tuff. Single pockets of melt with variable low-δ 18 O values were assembled and homogenized on a caldera-wide scale prior to the climactic Kilgore Tuff eruption, and the best record of this process is provided by the δ 18 O diversity in Kilgore Tuff zircons. Temporal trends of oxygen isotopic depletion and recovery in rhyolite eruptions of the Heise volcanic field are clearly linked to caldera collapse events, and remarkably consistent with trends in the Yellowstone Plateau volcanic field. At Heise and Yellowstone, magmatic δ 18 O values can be predicted on the basis of cumulative eruptive volumes, with a decrease in δ 18 O by ∼1‰ for every ∼1000 km 3 of erupted rhyolite. The Kilgore Tuff of the Heise volcanic field has the same timing, magnitude of δ 18 O depletion, and cumulative eruptive volume as the youngest phase of voluminous rhyolitic eruptions in the Yellowstone Plateau volcanic field, indicating that the Kilgore Tuff may serve as a useful analog for these and perhaps other large-volume low-δ 18 O rhyolites on Earth.

Degassing of the H2O-rich rhyolites of the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand

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Magmatic perturbations in the Okataina Volcanic Complex, New Zealand at thousand-year timescales recorded in single zircon crystals

Using single crystals to trace the chemical evolution of a magmatic system has long been a goal of igneous petrology. Crystals utilized for in situ dating hold great potential to link compositional and temporal information to better understand the evolution of a magmatic system. If micron-scale zoning of trace elements within a single zircon can be directly associated with volcanic events in magmatic systems, then new insights into long-term maintenance and storage of eruptible magma can be unlocked. This study presents new data that directly links the geochemical history recorded in individual zircon crystals with over 50,000 yrs of changing physical and chemical conditions within a magma reservoir. Trace elements (Hf and Y) in zircon have not diffusively equilibrated with their host melt so that they store information about the melt when that zone crystallized. Thus, different stages of growth can be associated with discrete time periods in the magmatic system. Zircon from the two most recent rhyolite eruptions (Kaharoa and Whakatane) within the Okataina Volcanic Complex (OVC), New Zealand, have both isotopic (age) and trace element signatures that correspond with distinct changes in the temperature and phase assemblage of pulses of erupted magma within the caldera system. This discrete change is associated with caldera collapse and reflects a change from cold, wet source rhyolite to a relatively hotter and drier source rhyolite and back. The Kaharoa and Whakatane eruptions are separated by 4000 yrs and 15 km distance, yet the zircon populations record the same distinct thermal and chemical pulse that occurred following caldera collapse, suggesting an interconnected magmatic system that houses at least small volumes of rhyolite melt for timescales of 10 3 to 10 5 yrs that is periodically extracted during eruption.

Tephra in deglacial ocean sediments south of Iceland: Stratigraphy, geochemistry and oceanic reservoir ages

AbstractIcelandic tephra layers within deglacial ocean sediment cores from south of Iceland have been detected and their timing with respect to the climate shifts of the last deglaciation constrained. Geochemical analysis of the tephra allowed the likely source volcanic systems to be identified. The previously known Saksunarvatn ash and Vedde ash are recognised and described. Several other major tephra layers are examined: basaltic eruption(s) of Katla at ∼8.4 ka; a basaltic eruption of Katla at ∼12.6 ka; a rhyolitic eruption of Katla at ∼13.6 ka producing tephra similar in appearance and composition to the Vedde ash; a basaltic eruption of Katla at ∼14.0 ka; and two basaltic eruptions of Grímsvötn at ∼14.6 ka and ∼15.0 ka. Abundant rhyolitic ash with a similar appearance and chemistry to the Vedde ash is found throughout the deglacial interval, predating the Vedde ash by up to 3000 years, supporting previous suggestions that there were pre‐Vedde ash eruptions of rhyolite that may have contributed to North Atlantic Ash Zone 1.This study expands the tephro‐stratigraphic framework of the North Atlantic and provides a marine archive in which the timing of tephra layers (useful as isochrons) can be directly compared to major ocean and climate events of the last deglaciation. Furthermore, by correlating tephra layers and abundance changes in the polar foraminifera, Neogloboquadrina pachyderma (sinistral), to equivalent tephra events and inferred abrupt cooling/warming in Greenland ice‐cores, contemporaneous 14C‐dated planktonic foraminifera have been used to estimate changes in the surface radiocarbon reservoir age south of Iceland. Consistent with previous studies, larger surface reservoir ages are calculated during late Heinrich Stadial 1 and the Younger Dryas (∼2000 years and ∼800–1900 years respectively). Copyright © 2011 John Wiley &amp; Sons, Ltd.

Explosive rhyolite tuya formation: classic examples from Kerlingarfjöll, Iceland

Rhyolite eruptions in Iceland mostly take place at long-lived central volcanoes, examples of which are found associated with each of the present-day rift-zone ice caps. Subglacial eruptions at Kerlingarfjöll central volcano produced rhyolite tuyas that are notable for their exposures of early-erupted pyroclastic material. Observations from a number of these edifices are synthesised into a general model for explosive rhyolite tuya formation. Eruptions begin with violent phreatomagmatic explosions that generate massive tuff (mT), but the influence of water quickly declines, leading to the formation of massive lapilli-tuffs (mLT) containing magmatically-fragmented vesicular pumice and ash. These are deposited rapidly near the vent, probably by moist pyroclastic density currents, confined by ice but not within a meltwater lake. The explosive-effusive transition is controlled by the ascent rate and gas content of the magma. An unusual obsidian-rich massive lapilli-tuff lithofacies (omLT) is identified and interpreted as pyroclastic material that was intruded into gas-fluidised deposits at the explosive-effusive transition. The effusive phase of eruption involves the emplacement of intrusions and lava caps. Intrusions of lava into the early-erupted phreatomagmatic deposits are characterised by peperitic margins and the formation of hyaloclastite. Intrusions into stratigraphically higher levels of the pyroclastic material show more limited interaction with the host tephra and have microcrystalline cores. Large lava bodies with columnar-jointed margins cap the tuyas and have intrusive basal contacts with the tephras. The main influence of the ice is to confine the rhyolite eruptive products to immediately above the vent region. This is in contrast to subglacial basaltic tuya-forming eruptions, which are characterised by the formation of meltwater lakes, phreatomagmatic fragmentation and subaqueous deposition. The lack of meltwater storage may reduce the potential for large jökulhlaups.

Reconstruction of the most recent volcanic eruptions from the Valles caldera, New Mexico

Products of the latest eruptions from the Valles caldera, New Mexico, consist of the El Cajete Pyroclastic Beds and Battleship Rock Ignimbrite, a sequence of pyroclastic fall and density current deposits erupted at ~ 55 ka, capped by the later Banco Bonito Flow erupted at ~ 40 ka, and collectively named the East Fork Member of the Valles Rhyolite. The stratigraphy of the East Fork Member has been the subject of conflicting interpretations in the past; a long-running investigation of short-lived exposures over a period of many years enables us to present a more complete event stratigraphy for these eruptions than has hitherto been possible. The volume of rhyolitic magma erupted during the 55 ka event may have been more than 10 km 3, and for the 40 ka event can be estimated with rather more confidence at 4 km 3. During the earlier event, plinian eruptions dispersed fallout pumice over much of the Valles caldera, the southern Jemez Mountains, and the Rio Grande rift. We infer a fallout thickness of several decimeters at the site of the city of Santa Fe, and significant ash fall in eastern New Mexico. In contrast, pyroclastic density currents were channeled within the caldera moat and southwestward into the head of Cañon de San Diego, the principal drainage from the caldera. Simultaneous (or rapidly alternating) pyroclastic fallout and density current activity characterized the ~ 55 ka event, with density currents becoming more frequent as the eruption progressed through two distinct stages separated by a brief hiatus. One early pyroclastic surge razed a forest in the southern caldera moat, in a similar manner to the initial blast of the May 18, 1980 eruption of Mt. St. Helens. Ignimbrite outflow from the caldera through the drainage notch may have been restricted in runout distance due to steep, rugged topography in this vicinity promoting mixing between flows and air, and the formation of phoenix clouds. Lavas erupted during both the ~ 55 and ~ 40 ka events were largely confined to the caldera moat. Any future rhyolitic eruptions of similar magnitude in the southern or western parts of the Valles caldera will likely affect similar areas.

Interdisciplinary Studies of Eruption at Chaitén Volcano, Chile

High‐silica rhyolite magma fuels Earth's largest and most explosive eruptions. Recurrence intervals for such highly explosive eruptions are in the 100‐ to 100,000‐year time range, and there have been few direct observations of such eruptions and their immediate impacts. Consequently, there was keen interest within the volcanology community when the first large eruption of high‐silica rhyolite since that of Alaska's Novarupta volcano in 1912 began on 1 May 2008 at Chaitén volcano, southern Chile, a 3‐kilometer‐diameter caldera volcano with a prehistoric record of rhyolite eruptions [Naranjo and Stern, 2004semi; Servicio Nacional de Geología y Minería (SERNAGEOMIN), 2008semi; Carn et al., 2009; Castro and Dingwell, 2009; Lara, 2009; Muñoz et al., 2009]. Vigorous explosions occurred through 8 May 2008, after which explosive activity waned and a new lava dome was extruded.