Rhyodacite magma storage conditions prior to the 3430 yBP caldera-forming eruption of Aniakchak volcano, Alaska

This study presents a pre-eruptive magma storage model for the rhyodacite and andesite magmas erupted during the 3430 yBP caldera-forming eruption of Aniakchak volcano, Alaska, derived from phase equilibria experiments and petrological data. The compositions of Fe–Ti oxide pairs from the early erupted Plinian rhyodacite pumice yield core temperatures of 871–900°C, with rims up to ∼942°C, and fO₂ from −10.6 to −11.8 log units. Melt inclusions entrapped in plagioclase phenocrysts have H₂O contents between 3 and 5 wt%, estimated by FTIR and electron microprobe volatiles by difference methods, with no detectable CO₂. Assuming water saturation, this corresponds to entrapment pressures between ∼65 and 150 MPa. Phase equilibria results reproduce the natural phase assemblages at of 95–150 MPa at 870–880°C, assuming water saturation. A mismatch in experimental versus natural glass SiO₂ and Al₂O₃, and MELTS models for H₂O-undersaturated conditions indicate that the rhyodacite may not have been H₂O saturated. MELTS models with and P _total of 125–150 MPa at 870–880°C reproduce the natural groundmass glass Al₂O₃ composition best, indicating the magma may have been slightly H₂O undersaturated. Those pressures correspond to storage at 4.5–5.4 km depth in the crust. MELTS models and VBD estimates from melt inclusions in titanomagnetite grains from the andesite indicate pre-eruptive conditions of ∼1,000°C and > 110 MPa, corresponding to a minimum residence depth of ∼4.1 km assuming water saturation or greater if the magma was H₂O undersaturated. Previous geochemical studies indicate separate histories of the two magmas, though they retain some evidence that they are ultimately related through fractional crystallization processes. Analogous to the 1912 Novarupta magmas, the rhyodacite and andesite presumably originated within the same crystal mush zone beneath the edifice, yet were separated laterally and underwent different degrees of crustal assimilation. The andesite must have resided in close proximity, with ascent occurring in response to movement of the rhyodacite, and resulting in extensive syn-eruptive mingling.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Deep saltwater in Chalk of North-West Europe: origin, interface characteristics and development over geological time

High-concentration saltwaters occur in many places in the regional Chalk aquifers of North-West Europe; to investigate deep occurrences, profiles of interstitial porewater chemistry have been studied from three 250–450m deep cores drilled in the eastern parts of Zealand, Denmark. At the studied location, saline water in the Chalk resides at depths from 40 to 80m and salinity increases with depth. Concentrations of chloride up to ca. 30,000ppm have been observed at depths of 400m. Measured vertical hydraulic heads in open boreholes suggest that advective groundwater flow is now restricted in deeper parts of the Chalk formation and diffusive transport is thus the predominant transport mechanism. Laboratory-measured porosity and effective diffusion coefficients were used as input to a numerical 1D diffusion model of the interface between freshwater in an upper, fractured aquifer and modified connate formation water below. The model satisfactorily simulated the observed chloride and δ¹⁸O profiles. The diffusive refreshening of the Chalk formation has been going on for about 0.9 million years. The connate water in the Chalk of parts of the sedimentary basin seems to have been modified by transport of saltwater from underlying Mesozoic and Paleozoic sediments during compaction, which presumably ceased around 4 million years ago.     

Coupled trace element mobilisation and strain softening in quartz during retrograde fluid infiltration in dry granulite protoliths

This study focuses on the retrograde rheological and chemical evolution of quartz and the behaviour of quartzites during retrograde metamorphism following dry high grade metamorphism at 750°C, 7 kbar. SEM-CL and LA-HR-ICP-MS are applied to document quartz texture and chemistry, respectively. Four generations of quartz were distinguished by SEM-CL; Qz1, Qz2, Qz3 and Qz4. Qz1, brecciated and partly dissolved old grains, is enriched in B, Al and Ti when compared with the other types. Qz2, formed during brecciation and partial dissolution of Qz1, has low Al contents (<50 ppm) but, due to rutile inclusions, variable Ti contents when occurring in amphibolite (210–10 ppm) but more consistent values when occurring in quartzites (peak value 32 ppm). Qz3, dark grey luminescent quartz forming fluid migration channels (fluid pathways), has Ti < 5 ppm and Al contents below 10 ppm and B < 1 ppm. Qz4, comprises are group of quartz later than Qz3 filling micron thick cracks and pods with very low luminescent quartz, i.e. darker than Qz3. The textural and chemical evolution of quartz in our study is explained by two major influxes of aqueous fluids during regional uplift and retrogression. They facilitated rehydration and recrystallisation in the otherwise dry high grade quartzites. The first introduction of aqueous fluids was associated with brecciation of the high grade quartz (Qz1) and dissolution/precipitation of quartz (Qz2). Ti in quartz geothermometry (Wark and Watson, Contrib Mineral Petrol 152(6):637–652) gives 626°C in agreement with the retrograde PT-path deduced from phase diagrams. Later fluid influx associated with scapolitisation of amphibolite caused localised recrystallisation (Qz3) and alteration of biotite to muscovite along mm-wide fluid migration channels. During subsequent deformation, Qz3 deformed plastically and recovered by subgrain rotation recrystallisation (SGR), resulting in a reduction of grain size, whereas Qz1 quartz formed micro faults. Qz2 was plastic but did not experience SGR to the same degree as Qz3 quartz. Increased plasticity and recovery rates most likely relate to an increased H₂O fugacity and the depletion in trace elements of the quartz lattice by promoting strain softening processes dislocation climb and recovery. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Weather radar observations of the Hekla 2000 eruption cloud,Iceland

The Hekla eruption cloud on 26–27 February 2000 was the first volcanic cloud to be continuously and completely monitored advecting above Iceland, using the C-band weather radar near the Keflavík international airport. Real-time radar observations of the onset, advection, and waning of the eruption cloud were studied using time series of PPI (plan-position indicator) radar images, including VMI normal, Echotop, and Cappi level 2 displays. The reflectivity of the entire volcanic cloud ranges from 0 to >60 dBz. The eruption column above the vent is essentially characterised by VMI normal and Cappi level 2 values, >30 dBz, due to the dominant influence of lapilli and ash (tephra) on the overall reflected signal. The cloud generated by the column was advected downwind to the north-northeast. It is characterised by values between 0 and 30 dBz, and the persistence of these reflections likely result from continuing water condensation and freezing on ash particles. Echotop radar images of the eruption onset document a rapid ascent of the plume head with a mean velocity of ~30 to 50 m s^–1, before it reached an altitude of ~11–12 km. The evolution of the reflected cloud was studied from the area change in pixels of its highly reflected portions, >30 dBz, and tied to recorded volcanic tremor amplitudes. The synchronous initial variation of both radar and seismic signals documents the abrupt increase in tephra emission and magma discharge rate from 18:20 to 19:00 UTC on 26 February. From 19:00 the >45 dBz and 30–45 dBz portions of the reflected cloud decrease and disappear at about 7 and 10.5 h, respectively, after the eruption began, indicating the end of the decaying explosive phase. The advection and extent of the reflected eruption cloud were compared with eyewitness accounts of tephra fall onset and the measured mass of tephra deposited on the ground during the first 12 h. Differences in the deposit map and volcanic cloud radar map are due to the fact that the greater part of the deposit originates by fallout off the column margins and from the base of the cloud followed by advection of falling particle in lower level winds.Editorial responsibility: P. Mouginis-Mark     

Chelation of divalent metal ions by brown algal polyphenols

High-molecular-weight, nondialyzable polyphloroglucinols from the marine brown algae Ascophyllum nodosum and Fucus vesiculosus (Phaeophyceae, Fucales) chelate the divalent metal ions Sr²⁺, Mg²⁺, Ca²⁺, Be²⁺, Mn²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, Pb²⁺ and Cu²⁺ in weakly acidic aqueous solution. Values of the relative selectivity coefficients k have been calculated relative to Mn²⁺; the ionotropic series with the algal polyphenols are similar to those with catechol and pyrogallol. These studies may serve as a guide for the examination of metal chelation by brown algal polyphenols both in situ and after exudation into seawater.     

Experimental constraints on syneruptive magma ascent related to the phreatomagmatic phase of the 2000<Emphasis Type="SmallCaps">AD</Emphasis> eruption of Usu volcano,Japan

We experimentally studied the dacitic magma ejected during the first event in the Usu 2000 eruption to investigate the conditions of syneruptive magmatic ascent. Geophysical data revealed that the magma reached under West Nishiyama, the location of the event’s craters, after rising beneath the summit. Prior study of bubble-size distributions of ejecta shows two stages (stage 1 and stage 2) with different magma ascent rates, as the magma accelerated beneath West Nishiyama with the start of the second stage. To simulate ascent of stage 1 from the main reservoir, which was located at a depth of 4–6 km (125 MPa) to 2 km (50 MPa) beneath West Nishiyama, decompression experiments were conducted isothermally at 900°C following two paths. Single step decompression (SSD) samples were decompressed rapidly (0.67 MPa/s) to their final pressure and held for 12 to 144 hours. Multiple step decompression (MSD) samples were decompressed stepwise to their final pressure and quenched instantly. In MSD, the average decompression rates and total experimental durations varied between 0.01389 to 0.00015 MPa/s and 1.5 to 144 hours, respectively. Syneruptive crystallization was confined to stage 1, and the conditions of ascent were determined by documenting similarities in decompression-induced crystallization between ejecta and experiments. Core compositions, number densities, and shapes of experimental microlites indicate that ascent to 2 km depth occurred in less than 1.5 h. Volumes and number densities of experimental microlites from the SSD experiments that best replicate the decompression rate to 2 km indicate that the magma remained at 2 km for approximately 24 h before the eruption. Stagnation at a depth of 2 km corresponds with horizontal transport through a dike from beneath the summit to West Nishiyama, according to geodetic results. The total magma transport timescale including stage 2 is tens of hours and is shorter than the timescale of precursory seismicity (3.5 days), indicating that the erupted magma did not move out of the reservoir for the first 2 days. This is consistent with the temporal change in numbers of earthquakes, which reached a peak after 2 days.     

Open boundaries in short wave simulations — A new approach

A new way of implementing radiation boundary conditions in finite difference schemes is reported. Instead of prescribing the incident field at the model boundary, waves are generated inside the model boundary. All outgoing waves are absorbed at open boundaries using so-called “sponge” layers.