Volcanic Record Research Articles

Understanding the isotopic and chemical evolution of Yellowstone hot spot magmatism using magmatic-thermomechanical modeling

Large-scale melting and reworking of the crust and the upper mantle occurs in areas of mantle plume-crust interaction, generating voluminous and bimodal basaltic and silicic magmatism. Continental hot spot track magmas preserve geochemical and temporal records of these processes. Here, we apply and further develop the large scale (crust and the upper mantle) I2VIS magmatic-thermomechanical modeling code to investigate the potential origins of chemical and isotopic trends in the Yellowstone hot spot track, perhaps the best studied and constrained subcontinental mantle plume system on Earth. We model the propagation of melt via dikes in solid crust and via percolation in partially molten crust by using Lagrangian markers which also track the chemical and isotopic compositions of the melts that eventually erupt at the surface. Confirming the results of earlier geophysical and geochemical studies, we show that the eruptive activity at Yellowstone is best explained in terms of the formation of a ~15 km-thick mid-crustal mafic sill complex with its top at a depth of ~8 km that develops over a period of ~3 Myr. This sill complex releases rhyolitic fractionates and creates additional rhyolites by melting the surrounding crust, driving the voluminous rhyolitic volcanism which characterizes the Yellowstone system.We recognize three key trends in the evolution of a continental hot spot volcanic center such as Yellowstone: (1) frequent, small eruptions results in in larger erupted rhyolite volumes compared to rare and large eruptions, (2) erupted magmas tend to be initially comprised of a large degree of crustal melt, and the relative contribution of fractionates of basalts from the mantle increases with time, and (3), the initial depth of the crust which melts to produce eruptible rhyolites becomes shallower with time. The first of these trends is simply a function of the continuous supply of new basalt to the crust from the mantle. The latter trend, which is responsible for low-δ18O rhyolites, is produced by a combination of progressive melting of shallower crust as the system becomes hotter, repeated caldera collapses advecting shallow crust to a depth where it can melt, and the overplating and burial of shallow crust by repeated intrusions of basalt. The mid- and lower crust is not a significant source of erupted rhyolitic melts, as both the crustal melting and basalt fractionation takes place in the sill complex which forms near the bottom of the upper crust. The resulting modeled isotopic evolution of erupted magmas are a good match with the actual stable and radiogenic isotopic record of Yellowstone hot spot track volcanism preserved in zircon phenocrysts, and further replicates the results of recent geophysical imaging campaigns, giving us confidence that the assumptions and results of these thermomechanical models are broadly correct.

Athapuscow aulacogen revisited: Geochronology and geochemistry of the 2046 Ma Union Island Group mafic magmatism, East Arm of Great Slave Lake, Northwest Territories, Canada

The Slave craton underwent widespread extension during the Paleoproterozoic. The East Arm basin of Great Slave Lake formed during this period and preserves a protracted record of sedimentation and volcanism in the southeastern Slave craton. Within the East Arm basin package, the Union Island Group represents voluminous mafic volcanism with subordinate sedimentary strata. We report the first high precision U–Pb zircon and baddeleyite dates for the Union Island Group; 2045.8 ± 1.0 Ma for a volcaniclastic unit from the lower basalt formation and 2042.7 ± 3.0 Ma for a diabase intrusion within volcaniclastic horizons in the lower basalt formation. We fundamentally revise the lower basin stratigraphy and demonstrate that the Union Island Group is the oldest identified supracrustal package in the East Arm basin, ∼120 Myr older than previously thought. This revised stratigraphy is supported by detrital zircon provenance age distribution in two Union Island Group sedimentary samples, which is dominated by 2.76–2.56 Ga ages, reflecting prominent input from the local Archean Slave basement.The Union Island Group contains two geochemically distinct mafic volcanic packages. The lower basalt package is predominantly alkaline, characterized by an OIB-like chemical signature with high and variable incompatible element contents (108–438 ppm Zr, 13–62 ppm Nb). In contrast, the upper basalt package is tholeiitic, contains much lower and uniform incompatible element contents (83–101 ppm Zr, 2–4 ppm Nb), and has an E-MORB-like chemical signature. Both packages display overlapping, depleted time-integrated εNd(i) values (+1.1 to +3.2). Petrological modeling suggests the lower package originated as OIB-like melts from the asthenosphere that interacted with a depleted reservoir; the upper package was produced by larger degree of partial melting of a similar mantle source, in addition to possible lithospheric input. The petrogenesis established in this study is consistent with a passive continental rift origin for the Union Island Group magmatism and is further supported by geochemical similarities with Proterozoic and Phanerozoic rift successions. We therefore propose that the 2046 Ma Union Island Group represents an incipient rift basin sequence formed during Paleoproterozoic extension that marked the initial breakup of the southern margin of the Slave craton from a pre-Laurentia supercontinent.

Tungsten isotopes in mantle plumes: Heads it's positive, tails it's negative

The lowermost mantle is driven to Earth's surface by mantle plumes, providing a volcanic record of its structure and composition. Plumes comprise a head and tail, which melt to form large igneous provinces (LIPs) and ocean island basalts (OIBs), respectively. Recent analyses have shown that LIPs and OIBs exhibit tungsten (W) isotope heterogeneity that was created in the first ∼60 million years of our solar system's evolution. Moreover, the isotopic signature found in LIPs differs to that found in OIBs, revealing that the melt products of plume heads must be dominated by a different ancient mantle reservoir to that of plume tails. However, existing geodynamical studies suggest that plume heads and tails sample the same deep-mantle source region and, therefore, cannot account for any systematic differences in composition. Here, we present a suite of numerical simulations of thermo-chemical plumes and an isotopic model for W sources in the mantle. Our results demonstrate that the W isotope systematics of LIPs and OIBs can, under certain conditions, arise as a dynamical consequence of plumes forming in a heterogeneous, thermo-chemical boundary layer. We find that ultra low-velocity zones (ULVZs), which sit on the core–mantle boundary (CMB), likely contribute to the chemical diversity observed in OIBs but not LIPs, while any dense components residing inside large low shear-wave velocity provinces (LLSVPs) may contribute to both. This study places geochemical observations from Earth's surface in a geodynamically consistent framework and illuminates their relationship with seismically imaged features of the deep mantle.

A cryptotephra record from the Lake Victoria sediment core record of Holocene palaeoenvironmental change

The sediment record from Lake Victoria is an important archive of regional environmental and climatic conditions, reaching back more than 15,000 cal. years before present (15 ka BP). As the largest lake by area in East Africa, its evolution is key to understanding regional palaeohydrological change during the late Pleistocene and Holocene, including controls on the Nile River flow. As well as important palaeoenvironmental proxies, the lake contains a unique record of explosive volcanism from the central Kenyan Rift, in the form of fine-grained volcanic ash (tephra) layers, interpreted as airfall deposits. In the V95-1P core, collected from the central northern basin of the lake, tephra layers vary in concentration from 10s to 10s of 1000s of glass shards per gram of sediment. None of the tephra are visible to the naked eye, and have only been revealed through careful laboratory processing. Compositional analyses of tephra glass shards has allowed the tephra layers to be correlated to previously unrecognized eruptions of Eburru volcano around 1.2 and 3.8 ka, and Olkaria volcano, prior to 15 ka. These volcanoes lie ~300 km east of the core site in the Kenyan Rift. Our results highlight the potential for developing cryptotephra analysis as a key tool in East African palaeolimnological research. Tephra layers offer opportunities for precise correlation of palaeoenvironmental sequences, as well as windows into the eruption frequency of regional volcanoes and the dispersal of volcanic ash.

Sedimentary inputs to the Nankai subduction zone: The importance of dispersed ash

We examine the importance of dispersed volcanic ash as a critical component of the aluminosilicate sediment entering the Nankai Trough, located south of Japan’s island of Honshu, via the subducting Philippine Sea plate. Multivariate statistical analyses of an extensive major, trace, and rare earth element data set from bulk sediment and discrete ash layers at Integrated Ocean Drilling Program (IODP) Sites C0011 and C0012 quantitatively determine the abundance and accumulation of multiple aluminosilicate inputs to the Nankai subduction zone. We identify the eolian input of continental material to both sites, and we further find that there are an additional three ash sources from Kyushu and Honshu, Japan and other regions. Some of these ash sources may themselves represent mixtures of ash inputs, although the final compositions appear statistically distinct. The dispersed ash comprises 38 ± 7 weight percent (wt%) of the bulk sediment at Site C0011, and 34 ± 4 wt% at Site C0012. When considering the entire sediment thickness at Site C0011, the dispersed ash component supplies 38000 ± 7000 g/cm2 of material to the Nankai subduction system, whereas Site C0012 supplies 20000 ± 3000 g/cm2. These values are enormous compared to the ~2500 g/cm2 (C0011) and ~1200 g/cm2 (C0012) of ash in the discrete ash layers. Therefore, the mass of volcanic ash and chemically equivalent alteration products (e.g., smectite) that are dispersed throughout the stratigraphic succession of bulk sediment appears to be up to 15–17 times greater than the mass of discrete ash layers. The composition of the dispersed ash component at Site C0011 appears linked to that of the discrete layers, and the mass accumulation rate for dispersed ash correlates best with discrete ash layer thickness. In contrast, at Site C0012 the mass accumulation rate for dispersed ash correlates better with the number of ash layers. Together, the discrete ash layers, dispersed ash, and clay-mineral assemblages present a complete record of volcanism and erosion of volcanic sources; and indicate that mass balances and subduction factory budgets should include the mass of dispersed ash for a more accurate assessment of volcanic contributions to large-scale geochemical cycling.

New tephrostratigraphic data from Lake Emakat (northern Tanzania): Implications for the eruptive history of the Oldoinyo Lengai volcano

The northern Tanzanian sector of the Gregory Rift is an area of an active continental rifting, in which sedimentation processes are strongly affected by volcanism. Due to limited stratigraphic exposure, the volcanic record of the region is rather sparse, and assigning volcanic centres for the individual eruptions is difficult. This study presents new data on the tephrostratigraphy of the sedimentary sequence of Lake Emakat, Empakaai Crater, northern Tanzania. Seven volcanic ash layers are identified and described from a 1.1-m core of lake sediments. Geochemical, mineralogical, petrographic and magnetic analyses show that: (1) all ash layers are products of highly explosive eruptions of melilite-bearing magmas; (2) most of the eruptions originate from a complex magmatic system; (3) all ash horizons are very well preserved in the lake environment; and (4) there are significant fluctuations of the bulk magnetic susceptibility of the lacustrine sediments which is related to microtephra from additional eruptions, the result of detritus, washed from the shore during periods of strong lake level fluctuations or periods of high erosion rates, or simply by the contamination by the material from the ash layers. Based on geochemistry and mineralogy of the seven identified ash layers in Lake Emakat, combined with the eruption ages from 14C datings, we can pinpoint Oldoinyo Lengai volcano as the source of these specific layers. The combination of this new data with existing chronological data from Ryner et al. (2007), retrieved from the same core, provides precise ages of the voluminous highly explosive eruptions in this region of East Africa during the Pleistocene-Holocene transition.

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

Late Holocene volcanic activity and environmental change in Highland Guatemala

We present a record of late Holocene volcanic eruptions with elemental data for a sequence of sampled tephras from Lake Amatitlan in Highland Guatemala. Our tephrochronology is anchored by a Bayesian P_Sequence age-depth model based on multiple AMS radiocarbon dates. We compare our record against a previously published study from the same area to understand the record of volcanism and environmental changes. This work has implications for understanding the effects of climate and other environmental changes that may be related to the emission of volcanic aerosols at local, regional and global scales.

Atmospheric dust from the Pennsylvanian Copacabana Formation (Bolivia): A high-resolution record of paleoclimate and volcanism from northwestern Gondwana

This study documents the occurrence of atmospheric dust from Pennsylvanian carbonates of the Copacabana Formation, recovered in core (Mobil-Oxy Manuripi X-1) from the Madre de Dios basin (Bolivia), within southern mid-latitudes (~35°S) of western Gondwana. The Copacabana Formation spans Pennsylvanian-Early Permian time, and thus formed coeval with and in relative proximity to ice centers and associated glacial deposits located at southern paleolatitudes in adjoining regions of Gondwana (e.g. the Paraná, Tarija, and Paganzo basins in Brazil, southeastern Bolivia, and Argentina, respectively). In Pennsylvanian time carbonate deposition of the Copacabana Formation occurred on a ramp isolated from fluvial-deltaic influx, and thus siliciclastic material in this system reflects atmospheric input. The study interval comprises a series of upwardly shallowing successions 1–3m thick ranging from open marine ramp facies to more restricted inner-ramp facies, commonly capped by horizons of microkarsted and/or red mudstone reflecting subaerial exposure of the carbonate ramp. These horizons mark abnormal exposure and are interpreted to record glacial lowstands.Dust recovered from throughout the study section varies from ~1 to 43wt% in carbonate facies and is quartzo-feldspathic. Grain size modes range from <1 to 97μm, with coarser intervals generally corresponding to peak dust content (wt%), and high-frequency sequence (glacial-stage) boundaries. Provenance indicates two discrete sources of atmospheric input–a western volcanic arc source and eastern continental source, recording both westerly (zonal) and easterly (katabatic) wind directions. The western (volcanic) source records zonal westerlies expected at this mid-latitude (~35°S) locality. In contrast, easterly winds suggest the influence of katabatic winds associated with Gondwanan ice centers. The most likely dust-sourcing regions are the periglacial to proglacial regions of the Gondwanan ice sheets. Non-volcanic peaks in dust occur most commonly associated with subaerial exposure surfaces; this, together with mass accumulation rate estimations suggest that atmospheric dust loading peaked during glacial stages.

Alleged Carboniferous (Viséan) volcanism at the eastern margin of the Moravo-Silesian Basin, Kraków region, Southern Poland

The volcanic rocks, represented by clasts in the breccias occurring within the Visean Culm sequence at Gluchowki, previously were considered to provide evidence of the earliest record of Variscan volcanism on the eastern margin of Moravo-Silesian Basin in the Krakow area. The breccias have been described as the deposits of submarine slides, accompanying the Visean volcanic activity. This paper provides evidence of a different mode of origin. The present study, based on new field observations, combined with petrological and geochemical data, showed that volcanic rocks are major framework constituents of a breccia and are accompanied by clasts of spotty hornfels and Culm mudstone. Some clasts originated in erosion of the nearby Permian Zalas rhyodacite laccolith and the spotty hornfelses of the contact aureole, while other clasts came from the Culm mudstones. The breccia framework components are not rounded and poorly rounded, which reflects their short distance of transport. Therefore, an alternative scenario for the genesis of the breccia is proposed. The breccias represent material deposited in erosional pockets formed along the bedding of the Culm mudstones. The red iron oxide shells around clasts may indicate weathering in a warm continental climate, thus constraining the breccia formation to either the Permian or the Early or Late Triassic. Thus, the only well documented Visean igneous rock in the study area remains the diabase, known from the Klucze borehole. Conversely, no volcanic rocks of this age have been documented in the area under consideration.

Arc crust formation and differentiation constrained by experimental petrology

We review igneous experimental data designed to understand fractionation processes in the roots of magmatic arcs, focusing on fractional crystallization experiments in dry and hydrous basalts to andesites to simulate the liquid and cumulate lines of descent of primary mantle derived magmas. An evaluation of the major element composition indicates that the cumulate line of descent (CLD) of hydrous systems is fundamentally different from dry systems. Cumulates derived from hydrous experiments display elevated Al2O3 and CaO contents at low SiO2, producing voluminous andesitic to rhyolitic liquids that closely overlap with compositions of natural systems. The mineralogical and chemical composition of cumulates converges for very different hydrous primary mantle magmas, indicating that fundamental phase equilibria under the conditions prevailing in the roots of magmatic arcs exert a strong control on the compositions of derivative andesitic to rhyolitic liquids. Melting experiments on amphibolite or basaltic to andesitic compositions equally produce granitic – rhyolitic liquids of similar composition yet their restites do not present the same variability of the CLD of hydrous fractional crystallization experiments. The comparison of our experimentally produced cumulates with natural rock suites from the Jurassic Talkeetna arc (South Central Alaska) and the Cretaceous Kohistan arc (Pakistan) show that fractionation experiments closely reproduce the natural rock compositions and in particular the cumulate sequences exposed in the roots of island arcs. Both dry and hydrous fractionation series are preserved yet only the hydrous fractionation series produces voluminous ultramafic cumulates (50–70%) that are negatively buoyant with respect to mantle peridotite. Delamination of dense ultramafic roots can explain the andesitic composition of continental crust. Our fractionation experiments show that generation of granites is a natural consequence of fractional crystallization of primary basaltic magmas. Discrepancies of liquid compositions produced in the lower crust and the arc volcanic record can readily be explained by further differentiation, partial assimilation and mingling and mixing processes during ascent and decompression in transcrustal magmatic systems. We therefore favor the interpretation that Earth-like continental crust is primarily produced by fractional crystallization in the lower crust and upper mantle, and that magma mixing, assimilation, metamorphism and crustal melting are important secondary processes that refine and amplify the difference and stratification between mafic lower crust and a granitic upper crust.

Volcanologic and petrologic evolution of Antuco-Sierra Velluda, Southern Andes, Chile

The Andean Southern Volcanic Zone comprises >30 active arc front volcanoes that grew over periods of hundreds of thousands of years. Quantifying the rates at which these volcanoes grow is key to appreciating geological hazards, clarifying petrologic evolution, and exploring possible relationships between volcanism, ice loading, and climate. The integration of precise geochronology and geologic mapping, together with new lava compositions and volume estimates, reveal the evolution of the Antuco-Sierra Velluda volcanic complex at 37.2°S. Thirty-one new 40Ar/39Ar age determinations illuminate a punctuated eruptive history that spans at least 430kyr. Sierra Velluda comprises ~130km3 and began to grow prior to 426.8ka. A lacuna in the volcanic record between 343.5 and 150.4ka coincides with glaciations associated with marine isotope stages (MIS) 10 and 8, although shallow intrusions were emplaced at 207.0 and 190.0ka. Antuco began to grow rapidly on the northeast flank of Sierra Velluda, erupting >60km3 of lava during three phases: (1) an early phase that began at 150.4ka, (2) a post-MIS 2 phase between 16.3 and 6.2ka, and (3) a post-sector collapse phase after 6.2ka. Volcanism has been continuous during the last 100kyr, with an average rate of cone growth during this period of ~0.46km3/kyr that has accelerated by about 50% during the past 6kyr.Whereas Sierra Velluda erupted basaltic andesitic to andesitic (53.5 to 58.7wt% SiO2) lavas, during the last expansion of glaciers between ~130 and 17ka, Early Antuco erupted a wider spectrum of lavas, ranging from basaltic andesite to dacite (52.0 to 64.5wt% SiO2). Notably, eruptions following the last glacial termination at 17ka produced basalts and basaltic andesites (50.9–53.7% SiO2), and following the 6.2ka cone collapse they have been exclusively olivine basalt (50.9–53.0% SiO2) with >5wt% MgO. Thermodynamic and trace element modeling suggests that lavas from Sierra Velluda and Early Antuco reflect extensive fractional crystallization of parental basaltic magmas with low water content (~1wt%) at pressures between 0.9 and 1.5kbar. In contrast, eruptions following rapid deglaciation tapped asthenospheric mantle-derived basalt that has been extensively modified by assimilation of partial melts of lower crustal rocks.

Repeated post-Caledonian intra-cratonic rifting in the central North Sea: Evidence from the volcanic record in the Embla oil field

Abstract Intrusive and extrusive mafic igneous rocks in the Embla oil field, central North Sea, testify to repeated post-Caledonian magmatism on the northern flank of the Mid North Sea High. The igneous rocks are highly clay- and carbonate-altered, but retain their High Field Strength element signatures on the sample scale. These signatures are used to group, classify and investigate the tectonic significance of the rocks. Three magmatic events are identified. Late Devonian transitional basalts are interpreted as part of a bimodal volcanic assemblage that includes ca. 375 Ma alkali rhyolites, suggested to record rifting in a proto-Central Graben. Early Permian volcanic and hypabyssal alkaline rocks in the Embla oil field display lamprophyric traits and represent low degree melts. They likely correlate either to ca. 300 Ma lamprophyre magmatism leading up to, or 298-292 Ma alkaline magmatism in the Midland Valley - Southern Uplands coeval with the northwest European magmatic flare-up at the Permo-Carboniferous boundary. Kimmeridgian-Tithonian submarine tholeiitic basalts in the Embla oil field post-date the alkaline continental mid Jurassic magmatism associated with doming and initial rifting in the North Sea, and are suggested to represent a hitherto unknown volcanic expression of the late Jurassic main rift phase in the Central Graben.

High resolution record of Quaternary explosive volcanism recorded in fluvio-lacustrine sediments of the Uwa basin, southwest Japan

The identification of widespread tephras is important, not only for understanding volcanic processes but also for age estimation and establishing stratigraphic relationships. Many widespread tephras have been erupted from six active and buried caldera volcanoes in the SW Japan-Ryukyu volcanic arcs. To reconstruct high-resolution tephrostratigraphy, we analyzed core UT from the Uwa basin on western Shikoku Island and correlated the tephras in the core with the proximal source deposits that we sampled. This 120 m-long core composed of fluvio-lacustrine deposits, spanning the middle Pleistocene to Holocene. We recognized and characterized 52 visible tephra and tephric sediment layers in the core on the basis of their stratigraphy, the petrographic features and the major-element composition of glass shards. We successfully correlated 15 tephras with the following previously identified widespread tephras, mainly from volcanoes on Kyushu: Kikai-Akahoya (7.3 ka), Aira-Tn (29 ka), Aso-4 (87 ka), Kikai-Tozurahara (95 ka), Aso-ABCD (100 ka), Ata (105-110 ka), Aso-3 (112.7 ka, calculated in this study), Aso-2 (140 ka), Ata-Toihama (240 ka), Aso-1 (250-270 ka), Tky-Ng1 (290-300 ka), Kakuto (330-340 ka), Oda (420-450 ka), Hiwaki (570-580 ka), and Yufugawa (600 ka). Above 50 m depth, all known widespread tephras from Kyushu younger than 350 ky were included in the sequence that has a fairly constant sedimentation rate of about 0.14 mm/yr. This high-resolution tephrostratigraphy was correlated with those of regional long-core records from the Lake Biwa in the central Japan and offshore of Shikoku that contributes to establish onshore-onshore and onshore-offshore correlations in southwest Japan. In the sediments, each of the following tephras from caldera-forming eruptions, Aso-4, Aso-ABCD, Ata, Aso-3, Aso-2 and Aso-1 occurred above multiple, compositionally similar tephra layers erupted from the same volcano over a long time period. This high-resolution record is therefore an important contribution to the correlation of regional stratigraphy and reconstruction of both the long-term eruptive history and the magma evolution of volcanoes in the SW Japan and -Ryukyu volcanic arcs.

U–Pb and fission‐track dating of a submarine pyroclastic rock from southwest Japan

AbstractThe Okinoshima Formation crops out on Okinoshima Island and comprises a thick sequence (&gt; 200 m) of pyroclastic rocks and alternating beds of sandstone and mudstone. Because Okinoshima Island is located between Honshu and Tsushima Island, the Okinoshima Formation potentially provides an important record of volcanism during the opening of the Japan Sea in northwest Kyushu, as well as a record of the formation of the present Genkai Sea region. In consideration of the lack of previous geochronological work, dating (fission‐track and U–Pb) of igneous zircons extracted from the Okinoshima Formation were undertaken and studied the clay mineral alteration in the pyroclastic material in order to reveal its thermal history. These data are used to constrain the age of the Okinoshima Formation and the present Genkai Sea region. Our results show that no thermal event has reset the fission‐track age after deposition of the pyroclastic rocks, and that the Okinoshima Formation was deposited at 16.2 Ma. The present Genkai Sea region is a deep‐sea basin, and its formation at 16.2 Ma was accompanied by submarine volcanism and rapid subsidence that marked the climactic stage of Japan Sea formation. After 16 Ma, the tectonic setting of the present Genkai Sea region changed from one of extension (related to the formation of the Japan Sea) to one of compression, with uplift occurring under the influence of the clockwise rotation of southwest Japan. Consequently, after 16 Ma the present Genkai Sea region became isolated from the forming processes of the Japan Sea.

40Ar/39Ar geochronology and revised stratigraphy of the late Eocene Taxco volcanic field, southern Mexico

The late Eocene Taxco volcanic succession constitutes an important record of explosive silicic volcanism in the north-central Sierra Madre del Sur of southern Mexico. Detailed stratigraphic studies with age dating of individual units in this area were lacking in spite of it being an important mining district. Re-examination of the stratigraphy together with geochronologic studies were used to define the eruptive style and constrain the age of the main episode of silicic activity. Major element data show a rhyolitic composition for all the units analyzed. The volcanic succession records silicic explosive eruptions during which ash-flows, surge and ash-fall deposits, lava flows and domes were emplaced. Earliest activity was contemporary with the end of sediment accumulation of the continental Balsas Group. The first main episode of volcanic activity produced massive, moderately welded, crystal-poor lithic ignimbrites and ash-fall deposits, which evolved into crystal-rich, densely welded ignimbrites with flattened pumice clasts (San Gregorio ignimbrite) thought to represent erupted mush related to caldera collapse during emptying of the magma chamber. A second episode of non-welded, vapor-phase, indurated ignimbrites and surge deposits (Sombrerito ignimbrite), which are rich in pumice but crystal-poor, suggest volatile-rich magmatism and replenishment of the magma chamber by evolved magmas. Rhyolite emplaced in volcanic units of both episodes point to volatile-poor magmas during which rhyolite flows and domes formed.40Ar/39Ar dating of the units, from the crystal-rich San Gregorio ignimbrite near the base of the succession to rhyolites at the top, has constrained the age of the Taxco volcanic field to a relatively short period between ∼36 and 34.5 Ma. Most of the sanidine ages obtained are indistinguishable within error: the densely welded San Gregorio ignimbrite (34.75 ± 0.26, 35.22 ± 0.26 Ma) and the overlying Peral ignimbrite (35.29 ± 0.41, 35.68 ± 0.20). Plagioclase of the Tenería Formation rhyolites gave the same age within error (34.45 ± 1.10, 34.94 ± 0.34 Ma), documenting the end of the main volcanic episodes. A younger plagioclase age of 30.89 ± 1.45 Ma of a dome near Tetipac gives evidence of later silicic magmatism in the area.The source of the ignimbrites of the Taxco region is inferred to be located in the Tenería area, where abundant rhyolite dome structures seem to obliterate the main caldera structure. Differences in age, mineralogy and geochemistry between the Taxco succession and nearby volcanic zones where caldera structures have been identified preclude sources in these areas.

New Volcanic Island Unveils Explosive Past

A recent volcanic eruption near Tonga in the southwest Pacific created a new island, giving scientists a rare opportunity to explore the volcanic record of this remote region.

Last glacial tephra layers in the Talos Dome ice core (peripheral East Antarctic Plateau), with implications for chronostratigraphic correlations and regional volcanic history

Tephra isochrons offer considerable potential for correlating diverse palaeoarchives and highlighting regional climatic differences. They are especially useful when applied to polar ice records encompassing the last glacial, as these clearly portray the pronounced millennial-scale climate variability that characterised this period. Here we present the continuous record of primary fallout tephra layers in the East Antarctic Talos Dome ice core (72°49′S, 159°11′E), developed upon examination of the core sections spanning the glacial period 16.5 to 71 ka. A total of ca. 45 discrete tephra deposits precisely positioned stratigraphically relative to the temperature record for the core and dated using the AICC2012 timescale, were identified. Quantitative grain size, particle morphology, major and trace element composition using Coulter Counter, SEM, EPMA-WDS, and LA-ICP-MS analytical methods were studied as diagnostic features for tephra characterisation. The tephrostratigraphic framework provides a reference for future precise comparison between ice and sediment sequences across the Antarctic continent. Indeed, several potential markers characterised by distinct volcanic glass geochemistry and/or particular stratigraphic location (e.g., a 17.6-ka ash layer deposited during the well-known major acidity event) are now available for the direct linkage of palaeoclimatic archives. The Talos Dome tephra sequence, dominated by mid-distal pyroclastic products from the nearby Northern Victoria Land volcanoes, also represents the most comprehensive and best time-constrained record of regional Antarctic volcanism yet developed. It documents nearly continuous sustained explosive activity during the considered time interval and, combined with previous ice-core tephra results for the last and the current interglacial periods, suggests progressive compositional shift through time.

The architecture of submarine monogenetic volcanoes – insights from 3D seismic data

AbstractMany prospective sedimentary basins contain a variety of extrusive volcanic products that are ultimately sourced from volcanoes. However, seismic reflection‐based studies of magmatic rift basins have tended to focus on the underlying magma plumbing system, meaning that the seismic characteristics of volcanoes are not well understood. Additionally, volcanoes have similar morphologies to hydrothermal vents, which are also linked to underlying magmatic intrusions. In this study, we use high resolution 3D seismic and well data from the Bass Basin, offshore southern Australia, to document 34 cone‐ and crater‐type vents of Miocene age. The vents overlie magmatic intrusions and have seismic properties indicative of a volcanic origin: their moderate–high amplitude upper reflections and zones of “wash‐out” and velocity pull‐up beneath. The internal reflections of the vents are similar to those found in lava deltas, suggesting they are composed of volcaniclastic material. This interpretation is corroborated by data from exploration wells which penetrated the flanks of several vents. We infer that the vents we describe are composed of hyaloclastite and pyroclasts produced during submarine volcanic eruptions. The morphology of the vents is typical of monogenetic volcanoes, consistent with the onshore record of volcanism on the southern Australian margin. Based on temporal, spatial and volumetric relationships, we propose that submarine volcanoes can evolve from maars to tuff cones as a result of varying magma‐water interaction efficiency. The morphologies of the volcanoes and their links to the underlying feeder systems are superficially similar to hydrothermal vents. This highlights the need for careful seismic interpretation and characterization of vent structures linked to magmatic intrusions within sedimentary basins.

Regional variability in the frequency and magnitude of large explosive volcanic eruptions

Quantifying the frequency at which volcanic eruptions of different size occurs is impor- tant for hazard assessment. Volcanic records can be used to estimate the recurrence rate of large-magnitude eruptions (magnitude ≥4), but recording biases that impact data complete- ness complicate analysis. To overcome these biases, we conceptualize the volcanic record as a series of individual and unique time series associated by a common behavior. Thus, we approach issues of completeness on a volcano-by-volcano basis and use a hierarchical Bayes- ian approach to characterize the common frequency-magnitude (f-M) behavior for different groups of volcanoes. We identify variations in the f-M relationship between different volcano types and between different volcanic arcs. By accounting for systematic under-recording in the volcanic record, we also calculate the global recurrence rates for large-magnitude erup- tions during the Holocene, which are similar to previous estimates. However, higher recur- rence rates for smaller-magnitude events are observed, which is a result of our adjustments for data completeness. Quantifying how the f-M relationship varies between different groups of volcanoes provides an opportunity to understand how the tectonic setting in uences f-M behavior, which is important to quantify long-term regional volcanic hazard.