The eruptive history and chemical stratigraphy of a post-caldera,steady-state volcano: Yasur,Vanuatu

The persistent activity of Yasur volcano, a post-caldera scoria cone in the southern Vanuatu Arc, along with the uniformity exhibited by its eruptive products, indicates that it is a “steady-state” volcano. This implies that rates of magma replenishment and tapping are in equilibrium. Examination of recently exposed tephra sequences suggests that Strombolian-style activity at Yasur has persisted in its current form for the last 630–850 years. Eruption of tephra with uniform grain size and texture throughout this period indicates invariant eruption magnitude and style. Based on tephra accumulation rates, a uniform, time-averaged eruption flux of ～410–480 m³ days^?1 is estimated. Major and trace element analyses of glass shards and mineral grains from these tephra deposits show limited variation in magma composition throughout that time, consistent with a chemically buffered magma reservoir and models for steady-state volcanism. Similarly, mineral crystallisation temperature estimates are within error, suggesting the magma reservoir has retained a constant temperature through this time, while pressure estimates suggest shallow crystallisation. Eruptions appear to be driven by gas release, with small fluctuations in magma chemistry and eruptive behaviour governed by perturbations in volatile flux. This period of steady-state activity was preceded by ～600 years of higher-magnitude, lower-frequency eruptions during which less evolved compositions were erupted. Variation between these two styles of eruptive behaviour may be explained by a shift from a periodically closed to fully opened conduit, allowing more regular magma release and changes to degassing regimes. New radiocarbon ages suggest a period of irregular eruptive behaviour extending >1,400 year B.P. Overall, a transition from an irregular to a very steady magmatic system has occurred over the past ～2 kyr. Previously determined tectonic indicators for caldera resurgence in the area suggest revived magma replenishment after a hiatus following the caldera-forming Siwi eruption. This replenishment, while now supplying today’s constant activity, has not yet manifested itself in variations in composition or style/magnitude of eruptions.     

Microstructures of coarse-grained marbles,analyzed using a new technique based on the bireflectance of calcite

A new approach to quantify microstructures of coarse-grained marbles is presented. This technique is based on the intensity of light reflectance in dependence of the crystallographic orientation of calcite grains. The technique setup consists of a high-resolution camera, a strong light source and the polished sample in reflection position. Microstructural data obtained with this method are comparable with those obtained by light microscopy or by secondary electron microscopy. However, the new approach reflects the grain size distribution even more accurately than in the case of other techniques, because it allows an easy quantification of larger sample surfaces. Therefore, in contrast to established techniques, microstructural analysis and statistics can also be performed for coarse-grained samples with grains that exceed diameters of 1 cm. With this new technique, coarse-grained mylonitic microstructures from Naxos were quantified, which in turn allowed relating different microfabrics to different strain localization episodes within a large-scale shear zone complex.     

Baustil und Regionalmetamorphose im präkambrischen Grundgebirge Brasiliens

Ohne ZusammenfassungMit 2 Karten, 4 Profilen und 2 TextabbildungenHerrn Prof. Dr.K. H. Scheumann zum 80. Geburtstag gewidmet.     

Atmospheric Electrification in Dusty,Reactive Gases in the Solar System and Beyond

Detailed observations of the solar system planets reveal a wide variety of local atmospheric conditions. Astronomical observations have revealed a variety of extrasolar planets none of which resembles any of the solar system planets in full. Instead, the most massive amongst the extrasolar planets, the gas giants, appear very similar to the class of (young) brown dwarfs which are amongst the oldest objects in the Universe. Despite this diversity, solar system planets, extrasolar planets and brown dwarfs have broadly similar global temperatures between 300 and 2500 K. In consequence, clouds of different chemical species form in their atmospheres. While the details of these clouds differ, the fundamental physical processes are the same. Further to this, all these objects were observed to produce radio and X-ray emissions. While both kinds of radiation are well studied on Earth and to a lesser extent on the solar system planets, the occurrence of emissions that potentially originate from accelerated electrons on brown dwarfs, extrasolar planets and protoplanetary disks is not well understood yet. This paper offers an interdisciplinary view on electrification processes and their feedback on their hosting environment in meteorology, volcanology, planetology and research on extrasolar planets and planet formation.     

Submarine silicic volcanism of the Healy caldera,southern Kermadec arc (SW Pacific): I – volcanology and eruption mechanisms

The submarine Healy volcano (southern Kermadec arc), with a 2-2.5 km wide caldera, is pervasively mantled with highly vesicular silicic pumice within a water depth of 1,150-1,800 m. Pumices comprise type 1 white-light grey pumice with ⢾ mm vesicles and weak-moderate foliation, type 2 grey pumice with millimetre-scale laminae, flow banded foliation, including stretched vesicles ⣗ mm in length, and a minor finely vesicular type 3 pumice. All types are sparsely porphyritic, with undevitrified glassy groundmass (68-70% SiO₂), which is microlite and lithic free. Coexisting pyroxenes yield magma temperatures of ~950 °C. Pumice density is А.5 g cm^-3 and vesicularity is 78-83%. Vesicle size distributions for types 1 and 2 pumice, range from ~20 µm to >20 mm, with a strong power-law relation (with d=-2.5ǂ.4) for vesicles <1-2 mm. Larger vesicles have variable size modes. The vesicle size distribution and packing indicates rapid magma decompression and ascent. Consideration of the pressure dependent, solubility of H₂O at a magma temperature of 𙧶 °C and water content of Ж wt%, with pumice petrography and vesicle granulometry, strongly suggests a pyroclastic eruption. Reconstructions of the submarine edifice between water depths of 1,000 and 550 m constrain the ambient hydrostatic pressure to ~6-9 MPa. Pressures >~9 MPa will limit vesicularity to less than the observed 78-83%, whereas pressure <~6 MPa require a more shallower reconstruction of the edifice and larger-volume syn-eruptive collapse. Uniformly high vesicularity is interpreted as evidence of insulation within an eruption column comprising steam and hot pyroclasts. Most pyroclasts cool, condensing and ingesting water into steam-inflated vesicles, and then sink. Progression into pyroclastic mode would expand the eruption column, displace ambient water, reduce the hydrostatic load, and further promote vesiculation and fragmentation. Pyroclasts within the column would quench at these reduced pressures. We argue that Healy eruptions deeper than ~1,000 m cannot be pyroclastic. Volumes for the lower and upper bounds of edifice size are 2.36 and 3.58 km³, respectively, but do not account for intra-caldera pumice fill. These volumes are considered to be predominantly primary eruption output, as shown by a dearth of accessory lithics in all pumice, yielding (at an average 81% vesicularity) eruptive pumice volumes of between 10 and 15 km³. Some pyroclasts may have risen to the sea surface and be a correlative of the sea-rafted Loisels pumice; the latter occurs in some New Zealand Holocene beach sequences and has a estimated age of 590ᇤ calendar years.     

Consequences of artificially opening coastal lagoons on their fish assemblages

The consequences of artificially opening lagoon entrances on the fish assemblages of coastal lagoons in south-eastern Australia were investigated. Impacts of lagoon opening were assessed by comparing changes in the fish assemblages of Werri and Shellharbour Lagoons, both of which intermittently opened to the sea with that of Lake Illawarra, a third coastal lagoon which remained closed during the study period. Numbers of fish species and individuals did not differ significantly after lagoon openings at Werri Lagoon and Shellharbour Lagoon. However, higher numbers of species and individuals were found in Lake Illawarra before the other lagoons had opened. Abundances of some economically significant species (namelyMugil cephalus, Myxus elongatus andLiza argentea) significantly increased after openings, mainly due to small juveniles (<40 mm FL) recruiting to the seagrass beds. In contrast, these species were caught as larger juveniles (>70 mm FL) in Lake Illawarra. Furthermore, their abundance in Lake Illawarra declined during successive months, coinciding with declining salinity. Abundances of resident species (such asPseudomugil olorum andAfurcagobius tamarensis) remained largely unchanged after openings. Although it was clear that marine-spawning species must benefit from lagoon openings, the number and composition of species able to move into lagoons from adjacent waters may be dependent upon the frequency, duration and time of year that the entrance is connected to the sea. The present study thus provides data which may be useful for fisheries management in intermittently open lagoons. However, these results should not be cited as reason to artificially open lagoon entrances as further detailed studies of other aspects of lagoon biota, such as their invertebrate and avian faunas, are also needed to provide sufficient basis for broader management frameworks.     

Circular moraine features on the Varanger Peninsula, northern Norway, and their possible relation to polythermal ice sheet coverage

This paper introduces a new hypothesis about the formation of circular moraine features (CMFs) on felsenmeer-covered plateaux of the Varanger Peninsula, northern Norway. CMFs and glacial lineations in the northwestern part of the Varanger Peninsula were mapped from aerial photographs. In two CMF-rich areas, Base œrro-Plateau and Båtsfjordfjellet, the CMFs were investigated during fieldwork. The aim was to investigate possible entrainment, transport and deposition mechanisms of the CMFs in connection to ice sheet properties.The CMFs were deposited in connection with a polythermal ice cover. During glaciations, the ice sheet was cold-based on the boulder field plateaux and warm-based in valleys, especially in ice drainage pathways towards fjords. The CMF material was probably entrained en masse, on the Varanger Peninsula, in transition zones between warm- and cold-based ice. The debris transport took place close to the ice base. During deglaciation, the debris formed dirt cones, slowing down the melting of underlying dead-ice blocks. As material slid down the margins of the cones, the typical CMF shape was formed.