40Ar/39Ar laser probe dating of the Central European tektite‐producing impact event

Abstract— A new ⁴⁰Ar/³⁹Ar data set is presented for tektites from the Central European strewn field (moldavites). This is the only strewn field that is entirely situated in a continental environment and still characterized by scattered ages (14–15.3 Myr). The main objectives of the study were to define more precisely the moldavite formation age and provide a good calibration for a glass standard proposed for fission‐track dating. The laser total fusion ages obtained on chips from 7 individual specimens from the Southern Bohemian and Moravian subfields are restricted to a narrow interval of time, with an average of 14.34 ± 0.08 Myr relative to the 27.95 ± 0.09 Myr of the Fish Canyon Tuff biotite. This result gives a more precise age not only for the tektite field but also for its producing impact. If the genetic link between the moldavites and the Nördlinger Ries impact crater is maintained, then this new age has to be considered a reliable estimate for the Ries crater also. This new value places the formation of Central European tektites within the Lower Serravallian period in the latest geologic timescales. Evidence of their impact products, such as glass spherules or shocked minerals, can, therefore, be sought in sedimentary marine formations in a more precisely defined age interval.     

Neogene exhumation of the Marrakech High Atlas (Morocco) recorded by apatite fission-track analysis

Apatite fission-track (AFT) data have been obtained along a traverse across the Marrakech High Atlas to constrain its tectono-thermal evolution. AFT ages vary between 212 ± 15 Ma and 20 ± 4 Ma. An Early Miocene AFT age accompanied by long mean track length from the central part of the chain has been interpreted as the timing of the main inversion of this region with the creation of relief because of the shortening induced by the interplay between the African and Eurasian plates. Thermal modelling of samples collected south of the South Atlas Fault Zone indicates a Middle-Late Miocene or even later cooling that has been attributed to the component of the uplift of the chain related to the thermal anomaly present beneath the Atlas Mountains.     

Discussion of REE distribution patterns of carbonatites and alkalic rocks

Analytical data on REE, Sr, Ba, and Na of carbonatites, their calcite separates, and associated alkalic rocks from Fen/Norway, Alnö/Sweden and Kaiserstuhl/Germany are discussed. The REE distribution patterns of carbonatites suggest sympathetic behaviour with those of their associated alkalic rocks. This could be interpreted as the result of liquid/liquid separation. The typically steep patterns may be the result of Ca-metasomatism of alkaline-earth rich, but alkali-poor liquids with olivine-rich, alkali bearing, subcrustal rocks. Additionally, during this process incompatible elements could become concentrated in the carbonate rich liquid.     

The impacts of pyroclastic surges on buildings at the eruption of the Soufrière Hills volcano, Montserrat

We investigated the impacts on buildings of three pyroclastic surges that struck three separate villages on 25 June, 21 September and 26 December, 1997, during the course of the andesitic dome building eruption of the Soufrière Hills Volcano, Montserrat, which began on 18 July, 1995. A detailed analysis of the building damage of the 26 December event was used to compare the findings on the flow and behaviour of dilute pyroclastic density currents (PDCs) with the classical reports of PDCs from historical eruptions of similar size. The main characteristics of the PDC, as inferred from the building damage, were the lateral loading and directionality of the current; the impacts corresponded to the dynamic pressure of the PDC, with a relatively slow rate of rise and without the peak overpressure or a shock front associated with explosive blast; and the entrainment of missiles and ground materials which greatly added to the destructiveness of the PDC. The high temperature of the ash, causing the rapid ignition of furniture and other combustibles, was a major cause of damage even where the dynamic pressure was low at the periphery of the current. The vulnerability of buildings lay in the openings, mainly windows, which allowed the current to enter the building envelope, and in the flammable contents, as well as the lack of resistance to the intense heat and dynamic pressure of some types of vernacular building construction, such as wooden chattel houses, rubble masonry walls and galvanised steel-sheet roofs. Marked variability in the level of damage due to dynamic pressure (in a range 1–5 kPa, or more) was evident throughout most of the impact area, except for the zone of total loss, and this was attributable to the effects of topography and sheltering, and projectiles, and probably localised variations in current velocity and density. A marked velocity gradient existed from the outer part to the central axis of the PDC, where buildings and vegetation were razed to the ground. The gradient correlated with the impacts due to lateral loading and heat transfer, as well as the size of the projectiles, whilst the temperature of the ash in the undiluted PDC was probably uniform across the impact area. The main hazard characteristics of the PDCs were very consistent with those described by other authors in the classic eruptions of Pelée (1902), Lamington (1951) and St Helens (1980), despite differences in the eruptive styles and scales. We devised for the first time a building damage scale for dynamic pressure which can be used in research and in future volcanic emergencies for modelling PDCs and making informed judgements on their potential impacts. Editorial responsibility: T. Druitt     

The H contents of quartz from alpine veins from the penninic rocks of the central and western Tauern Window (Austria/Italy)

Summary The hydrogen content of 29 quartz crystals originating from the Penninic series of the central and western Tauern Window (Tyrol, Austria) was determined by IR spectroscopy. The hydrogen contents are between 7 and 66 H/10⁶ Si-atoms. In one sample showing a mimetic lamellar structure H-contents up to 140 H/10⁶ Si-atoms could be found. In most cases no difference exists between the hydrogen contents of margins and cores of the crystals. No clear relationships between the hydrogen content of the quartz lattice and the metamorphic temperature ranging from about 600°C to about 450°C can be found.

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Der H-Gehalt von Quarzen aus alpinen Klüften der Penninischen Gesteine des westlichen Tauernfensters (Österreich/Italien)

Zusammenfassung Der Wasserstoffgehalt von 29 Quarzkristallen aus den alpinen Zerrklüften der penninischen Gesteine des zentralen und westlichen Tauernfensters (Tirol, Österreich) wurde IR-spektroskopisch bestimmt. Der H-Gehalt schwankt zwischen 7 und 66 H/10⁶ Si-Atome. In einer Quarzprobe, die auch eine Lamellenstruktur zeigt, wurde ein H-Gehalt bis zu 140 H/10⁶ Si-Atome gemessen. In den meisten Quarzkristallen konnte kein Unterschied in den H-Gehalten des inneren und des äußeren Bereiches gefunden werden. Es konnte auch kein Zusammenhang zwischen den Metamorphosetemperaturen, die zwischen ca. 600 und 450°C liegen, und dem H-Gehalt der Quarzkristalle gefunden werden.

     

Metamorphic plagioclase crystallization and zones of equal anorthite content in epidote-bearing,amphibole-free rocks of the western Tauernfenster,eastern Alps

The plagioclase in the Penninic, epidote-bearing and amphibole-free gneisses, mica schists and phyllites of the western Hohe Tauern (Austria/Italy) was generated by Alpine crystallization. Reverse zoning in these plagioclase formed during the ‘Tauern Crystallization’ indicate that they were probably produced by a two-stage process of regional metamorphism.Statistical analysis of the regional distribution of plagioclase An-content shows that the highest values are located in the Zillerthal Alps in the central Tauernfenster, whereas An values decrease towards the periphery of the Tauernfenster. Three areas of differing plagioclase composition have been delineated by the regional analysis. In the central region maximum An-contents range from 20 to 29%; in the intermediate region they vary from 14 to 19%; and in marginal areas they do not exceed 6%. In general these three zones are concordant to the major tectonic structures.