Cumulate nodules as evidence for convective fractionation in a phonolite magma chamber     

Stephen R. Tait  Gerhard Wrner  Paul Van Den Bogaard  Hans-Ulrich Schmincke 《Journal of Volcanology and Geothermal Research》1989,37(1)

The physical mechanism by which chemical zonation develops in magma chambers has been controversial partly because unambiguous geological constraints have been lacking. The 11,000 years B.P. eruption of Laacher See Volcano produced a zoned tephra deposit and also ejected crystal-rich nodules which provide a snapshot of the materials crystallising at the magma chamber margins. New data on petrography and chemical compositions of nodules, their cumulate minerals and interstitial glasses are used to deduce the chemical evolution of the phonolite melt due to fractional crystallisation of the mineral assemblages. These data, together with those on the vertical zonation of the melt in the bulk of the chamber, are shown to be consistent with a model of stratification of the chamber by convective fractionation, in which a thin boundary layer of residual melt from fractional crystallisation ascends at the chamber side and accumulates at the roof. Crystallisation could have provided buoyancy to drive convection by enriching incompatible volatile components (mainly water) in the residual melt. Available fluid dynamic studies of single- and double-diffusive boundary layers are used to assess convection in the Laacher See chamber. The boundary layer is likely to have been: (1) laminar, which implies that the density gradient in the chamber steepened upwards; (2) in the counterflow regime, in which compositional and thermal layers flow in opposite directions; and (3) thin ( 10 cm), estimated from theory for a flat wall, suggesting that wall morphology could be important in determining boundary layer characteristics. Estimates of mass transfer rates due to this mechanism suggest that the chamber could have become stratified in a time of the order of 10³ years.  相似文献   

Mixing of rhyolite,trachyte and basalt magma erupted from a vertically and laterally zoned reservoir,composite flow P1, Gran Canaria   总被引：1，自引：0，他引：1  

Armin Freundt  Hans-Ulrich Schmincke 《Contributions to Mineralogy and Petrology》1992,112(1):1-19

The 14.1 Ma composite welded ignimbrite P1 (45 km³ DRE) on Gran Canaria is compositionally zoned from a felsic lower part to a basaltic top. It is composed of four component magmas mixed in vertically varying proportions: (1) Na-rhyolite (10 km³) zoned from crystal-poor to highly phyric; (2) a continuously zoned, evolved trachyte to sodic trachyandesite magma group (6 km³); (3) a minor fraction of Na-poor trachyandesite (<1 km³); and (4) nearly aphyric basalt (26 km³) zoned from 4.3 to 5.2 wt% MgO. We distinguish three sites and phases of mixing: (a) Mutual mineral inclusions show that mixing between trachytic and rhyolitic magmas occurred during early stages of their intratelluric crystallization, providing evidence for long-term residence in a common reservoir prior to eruption. This first phase of mixing was retarded by increasing viscosity of the rhyolite magma upon massive anorthoclase precipitation and accumulation. (b) All component magmas probably erupted through a ring-fissure from a common upper-crustal reservoir into which the basalt intruded during eruption. The second phase of mixing occurred during simultaneous withdrawal of magmas from the chamber and ascent through the conduit. The overall withdrawal and mixing pattern evolved in response to pre-eruptive chamber zonation and density and viscosity relationships among the magmas. Minor sectorial variations around the caldera reflect both varying configurations at the conduit entrance and unsteady discharge. (c) During each eruptive pulse, fragmentation and particulate transport in the vent and as pyroclastic flows caused additional mixing by reducing the length scale of heterogeneities. Based on considerations of magma density changes during crystallization, magma temperature constraints, and the pattern of withdrawal during eruption, we propose that eruption tapped the P1 magma chamber during a transient state of concentric zonation, which had resulted from destruction of a formerly layered zonation in order to maintain gravitational equilibrium. Our model of magma chamber zonation at the time of eruption envisages a basal high-density Na-poor trachyandesite layer that was overlain by a central mass of highly phyric rhyolite magma mantled by a sheath of vertically zoned trachyte-trachyandesite magma along the chamber walls. A conventional model of vertically stacked horizontal layers cannot account for the deduced density relationships nor for the withdrawal pattern.  相似文献   

Contributions to the Deformation Analysis in Germany Based on Precise and Continuous GPS Measurements     

Sibylle Goebell  Hans-Ulrich Wetzel 《Natural Hazards》2006,38(1-2):177-197

Modern space geodetic techniques enable deformation monitoring of continental plate interiors with high spatial and temporal coverage. Resolving data and results are currently evaluated for their application for the integrated assessment of seismic hazard and risk in Germany. This goes especially for regions where earthquakes are generally rare but high magnitudes are still not unrealistic while vulnerability of today’s society is steadily growing. The present contribution deals with the continuous monitoring of tectonic fracture systems in Germany using the GPS. The estimation of the station velocities with GPS and the resulting geodetic strain is supposed to provide additional input to the earthquake hazard assessment. Unfortunately, the low expected and currently seen velocities (<1–2 mm/year) make it extremely difficult to distinguish between noise and a tectonic signal. Because of the short observation interval the velocity uncertainties are about 2 mm/year in the horizontal components. The essential goal of this program is to provide and model highly precise deformation data and to discuss its needs for a better assessment of geological hazard, especially for the most active tectonic regions in Germany, the Rhine-Graben, the Swabian Alb, the Alpine foreland, and the Vogtland. Here we present preliminary results from 2 years of measurements at currently 150 GPS stations throughout Germany. The time span of this program has proven to be too short and the density of the station network to be not dense enough yet for reliable significant horizontal station velocities and supporting the earthquake hazard assessment.  相似文献   

Flow directions in Columbia River Basalt flows and paleocurrents of interbedded sedimentary rocks,South-Central Washington     

Hans-Ulrich Schmincke 《International Journal of Earth Sciences》1967,56(1):992-1020

More than 150 flow directions measured mainly in inclined pipe vesicles and, more rarely, vesicle cylinders, spiracles, and inclined foreset beds in pillowpalagonits-complexes) of the upper 12 widespread Yakima Basalt flows on the Columbia River Plateau in south-central Washington indicate rather uniform transport directions from southeast to northwest. This strongly supports former ideas that the large Grande Ronde-Cornucopia dike swarm in northeast Oregon fed the huge lava floods. It is shown that all previous evidence indicating eruption centers for the basalts in the Cascada Range to the west is inconclusive. Cross-bedding directions, maximum pebble sizes, and distribution of heavy mineral suites in sedimentary deposits interbedded with the basalt sheets delineate in more detail two main paleoslopes whose deposits interfinger:

1. a)
   the Cascade paleoslope to the west with east-southeast paleocurrents and a predominance of volcanic heavy minerals (hornblende, oxyhornblende, hypersthene, and clinopyroxene).  相似文献