Gas Emissions From the Western Aleutians Volcanic Arc

The Aleutian Arc is remote and highly active volcanically. Its 4,000 km extent from mainland Alaska to Russia’s Kamchatka peninsula hosts over 140 volcanic centers of which about 50 have erupted in historic times. We present data of volcanic gas samples and gas emission measurements obtained during an expedition to the western-most segment of the arc in September 2015 in order to extend the sparse knowledge on volatile emissions from this remote but volcanically active region. Some of the volcanoes investigated here have not been sampled for gases before this writing. Our data show that all volcanoes host high-temperature magmatic-hydrothermal systems and have gas discharges typical of volcanoes in oceanic arcs. Based on helium isotopes, the western Aleutian Arc segment has minimal volatile contributions from the overriding crust. Volcanic CO2 fluxes from this arc segment are small, compared to the emissions from volcanoes on the Alaska Peninsula and mainland Alaska. The comparatively low CO2 emissions may be related to the lower sediment flux delivered to the trench in this part of the arc.

The Holocene History of the Diatom Community in a Small Water Body on Shemya Island (Aleutian Arc, USA): The Influence of Global and Local Environmental Changes

A diatom analysis of a peat deposit from Shemya Island (Aleutian Arc, USA) is performed, and the dynamics of the diatom community are described. According to the radiocarbon dating, the formation of the deposit began 9300 cal. years BP. Principal component analysis made it possible to relate the dynamics of the diatom community to certain environmental conditions and the factors that influenced the coastal ecosystems during its formation. The following factors (predictors) were considered: the influence of age, zoo- and anthropogenic effects, and changes in climatic conditions. Sea level change was the main driver of the diatom community in the studied water body having a continuous direct and indirect influence on the studied small water body, i.e., by bird colony formation and more humid and coastal conditions. Since 3000–2000 cal. years BP, the anthropogenic factor (hunting depression of the bird colony) also became significant. During the whole water body lifetime and following peat formation, the diatom community was influenced by groups of factors: global factors (e.g., sea level rise) caused gradual change of local factors, which resulted in smooth shifts in community. In contrast, local factor influence (bird colony rise and fall due to human activity) caused abrupt and transient shifts. We can hypothesize that the relatively stable global environmental conditions in the Late Holocene were an auspicious background to see abrupt changes due to influence of the zoogenic and anthropogenic factors. We believe that further works on the material from other islands will make it possible to form a general picture of changes in the diatom communities in the Holocene and interpret it in connection with climatic changes in the region.

No evidence for tephra in Greenland from the historic eruption of Vesuvius in 79 CE: Implications for geochronology and paleoclimatology

Volcanic signatures archived in polar ice sheets provide important opportunities to date and correlate ice-core records as well as to investigate the environmental impacts of eruptions. Only the geochemical characterization of volcanic ash (tephra) embedded in the ice strata can confirm the source of the eruption, however, and is a requisite if historical eruption ages are to be used as valid chronological checks on annual ice layer counting. Here we report the investigation of ash particles in a Greenland ice core that are associated with a volcanic sulfuric acid layer previously attributed to the 79 CE eruption of Vesuvius. Major and trace element composition of the particles indicates that the tephra does not derive from Vesuvius but most likely originates from an unidentified eruption in the Aleutian arc. Using ash dispersal modelling, we find that only an eruption large enough to include stratospheric injection is likely to account for the sizeable (24–85 μm) ash particles observed in the Greenland ice at this time. Despite its likely explosivity, this event does not appear to have triggered significant climate perturbations, unlike some other large extra-tropical eruptions. In light of a recent re-evaluation of the Greenland ice-core chronologies, our findings further challenge the previous dating of this volcanic event to 79 CE. We highlight the need for the revised Common Era ice-core chronology to be formally accepted by the wider ice-core and climate modelling communities in order to ensure robust age linkages to precisely dated historical and paleoclimate proxy records.

Generation of Calc-Alkaline Magmas During Crystallization at High Oxygen Fugacity: An Experimental and Petrologic Study of Tephras from Buldir Volcano, Western Aleutian Arc, Alaska, USA

Despite agreement that calc-alkaline volcanism occurs at subduction zones and is responsible for the genesis of continental landmasses, there is no consensus on the source of the Fe-depleted signature hallmark to calc-alkaline volcanism. In this study, we utilize mafic tephras collected from Buldir Volcano to address the genesis of strongly calc-alkaline volcanic rocks (those with a low Tholeiitic Index; ≤0.7) in a segment of the western Aleutian Arc to determine if the eruptions are plausibly part of a liquid line of descent, if they are mixtures of crustal melts and parental magmas, or if they are mixtures of melts of the mantle and the subducting slab. We conducted a series of H2O-saturated phase equilibrium experiments (1175–1000 °C; 100 MPa) in a rapid-quench cold-seal (MHC) apparatus on the most primitive natural lava from Buldir (9.34 wt% MgO) at oxidizing conditions near the Re-ReO2 buffer. We confirmed that all experiments equilibrated 0.3 ± 0.23 log units above the Re-ReO2 buffer (ΔQFM ∼ +2.8) using X-ray Absorption Near Edge Structure (XANES) spectroscopy. Chromite is the liquidus phase, followed by olivine, then plagioclase, then clinopyroxene, and finally hornblende. Once clinopyroxene saturates, spinel composition shifts to magnetite. We compared our experimental results to the major element geochemistry and petrology of six tephras (51.9–54.8 wt% SiO2) from Buldir collected during the 2015 field season of the GeoPRISMS shared platform. Tephras contain olivine + plagioclase + clinopyroxene + spinel ± hornblende; plagioclase comprises most of the crystalline volume, followed by either olivine or hornblende. Spinel is ubiquitous; with Cr- rich spinel inclusions in olivine and hornblende, and magnetite in the groundmass. Variations in phenocryst assemblages and compositions between samples can be attributed to differences in pre-eruptive temperatures, where hotter samples are devoid of hornblende, and contain Fo-rich olivine and plagioclase with lower An-contents, owing to the position of the mineral-in curves at fluid-saturated conditions. Experimental glasses match the depletion in FeOT observed in the tephra whole rock compositions. The continuous depletion in FeOT is attributable to saturation of spinel as a liquidus phase (initially as chromite) and continuous crystallization through the experimental series (changing to magnetite at colder temperatures). In contrast to the natural samples, the experiments show enrichment in TiO2 with decreasing MgO, suggesting that differentiation did not occur at 100 MPa on Buldir. The TiO2 depletion in volcanic rocks from Buldir can be accounted for if hornblende crystallization occurs close to the liquidus of a parental magma; a condition that is met at higher pressures and hydrous conditions. The emerging picture for Buldir Island is that (1) oxidizing conditions are required to drive the observed depletions in FeOT via crystallization of spinel, and (2) elevated H2O contents and high pressures are required to saturate hornblende close to the liquidus to reproduce the entire suite of major elements. Our study provides a mechanism to generate the calc-alkaline trends observed at Buldir without requiring mixing of slab and mantle melts. We conclude that calc-alkaline volcanic rocks with extremely low Tholeiitic Indices (0.7), like those from Buldir, cannot be generated in absence of high oxygen fugacity, even at high pressure and/or elevated water pressures.

Imaging the Tectonic Grain of the Northern Cordillera Orogen Using Transportable Array Receiver Functions

Azimuthal variations in receiver function conversions can image lithospheric structural contrasts and anisotropic fabrics that together compose tectonic grain. We apply this method to data from EarthScope Transportable Array in Alaska and additional stations across the northern Cordillera. The best-resolved quantities are the strike and depth of dipping fabric contrasts or interfaces. We find a strong geographic gradient in such anomalies, with large amplitudes extending inboard from the present-day subduction margin, the Aleutian arc, and an influence of flat-slab subduction of the Yakutat microplate north of the Denali fault. An east–west band across interior Alaska shows low-amplitude crustal anomalies. Anomaly amplitudes correlate with structural intensity (density of aligned geological elements), but are the highest in areas of strong Cenozoic deformation, raising the question of an influence of current stress state. Imaged subsurface strikes show alignment with surface structures. We see concentric strikes around arc volcanoes implying dipping magmatic structures and fabric into the middle crust. Regions with present-day weaker deformation show lower anomaly amplitudes but structurally aligned strikes, suggesting pre-Cenozoic fabrics may have been overprinted or otherwise modified. We observe general coherence of the signal across the brittle-plastic transition. Imaged crustal fabrics are aligned with major faults and shear zones, whereas intrafault blocks show imaged strikes both parallel to and at high angles to major block-bounding faults. High-angle strikes are subparallel to neotectonic deformation, seismicity, fault lineaments, and prominent metallogenic belts, possibly due to overprinting and/or co-evolution with fault-parallel fabrics. We suggest that the underlying tectonic grain in the northern Cordillera is broadly distributed rather than strongly localized. Receiver functions thus reveal key information about the nature and continuity of tectonic fabrics at depth and can provide unique insights into the deformation history and distribution of regional strain in complex orogenic belts.