The Skaergaard liquid line of descent revisited

There is a fundamental conflict between the suggestion that the iron content of Skaergaard liquids increases during Fe–Ti oxide fractionation and the observation that at the same time oxygen fugacity () drops by two log-units below the fayalite-magnetite-quartz oxygen buffer (FMQ). A new petrographic study of average Skaergaard gabbros shows that the total modal content of Fe–Ti oxides is about 22% in the early LZc and markedly decreases to below 5% in the UZc. Forward modeling based on these modal constraints, as well as experimental results on Skaergaard-related dikes, predicts that fractionation of troctolitic LZa gabbros drives the derivative liquid towards a high-iron content. Strong iron enrichment continues, together with a small decline in silica, during LZb crystallization due to the appearance of augite as a fractionating phase. The fractionation of Fe–Ti oxides in the LZc initially suppresses iron enrichment and reverses the silica trend to one of slight enrichment. However, continued evolution into the UZ produces liquids with maximum UZc FeO* content of 23–25 wt.% and SiO₂ content of 53 wt.% (FeO* is total iron as FeO). The maximum in FeO* is dependent on several factors of which the Fe–Ti oxide mode has the strongest effect. The during crystallization of the LZc is widely thought to have been at, or slightly below, the fayalite-magnetite-quartz oxygen buffer (FMQ). Under closed system evolution, incorporation of ferric iron into augite during formation of the LZb restricts the increase in to about 0.1 log-units above FMQ (=0.1 ΔFMQ). Likewise, crystallization of the LZc through the UZa, involving Fe–Ti oxide minerals, leads to a decline in of less than 0.1 ΔFMQ. Crystallization of the UZb-c gabbros results in oxidation to a maximum of 0.5 ΔFMQ. This behavior can account for the iron-rich character of the UZ gabbros, as well as, the low modal content of Fe–Ti oxides. Thus, evolved Skaergaard liquids are high in iron and contain a modest amount of SiO₂. Our modeling result do not account for a strong drop in through the layered series. Such a drop would require an unacceptably high proportion of Fe–Ti oxides and high-magnetite content in the fractionating assemblage. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

New insights into the genesis of volcanic-hosted massive sulfide deposits on the seafloor from numerical modeling studies

Numerical computer simulations have been used to gain insight into the evolution of marine hydrothermal systems and the formation conditions of massive sulfide deposits in ancient and modern submarine volcanic terrains. Simulation results have been used to gain a better understanding of the formation of massive sulfide ore deposits, their location, zonation, size, and occurrence in various geotectonic settings.Most hydrothermal fluid discharging at the seafloor exhibits temperatures ranging from 200 °C to about 410 °C and average fluid discharge velocities of 1 to 2 m/s in agreement with seafloor observations. Mass calculations imply that average massive sulfide deposits may form in ~ 5000 years while giant deposits take longer than 5000 years to accumulate; supergiant deposits either need much longer time to form (> 35,000 years) or at least 100 ppm of metal in solution. Results indicate that supergiant deposits may only form in certain geotectonic environments where longevity and preservation potential of the hydrothermal system are high. An additional process (mineral precipitate cap) is proposed here to explain the zinc content of massive sulfide deposits. This cap would prevent the widespread dissolution of anhydrite and the ‘wash-out’ of zinc by subsequent hydrothermal fluid discharge.     

Early Ordovician continental break-up in Variscan Europe: Nd---Sr isotope and trace element evidence from bimodal igneous associations of the Southern Massif Central, France

The high-grade Marvejols Group and the low-grade Albigeois-Cévennes sedimentary sequence contain bimodal igneous rocks of Early Ordovician age which are representative of a widespread thermal event in the European Variscides. Comparison of their Nd---Sr isotope and trace-element characteristics provides additional evidence for their origin in an ensialic extensional setting. As an alternative to the back-arc model proposed by previous authors, we propose that these associations record a continental break-up episode unrelated to contemporaneous subduction. In this model, the widespread Early Paleozoic bimodal magmatism marks the birth of a Mid-European oceanic arm. We propose that the break-up was controlled by both transtensional processes and mantle-plume activity.

High grade associations, such as the Marvejols Group, and low-grade bimodal associations, such as the Albigeois-Cévennes, might be regarded as representing opposite rifted passive margins. The “northern”, Marvejols-type margin was involved in a Late Ordovician-Silurian subduction-like process, with HP-HT metamorphism. In contrast, the “southern” Albigeois/Cévennes margin remained largely unaffected, possibly as part of the overriding plate of the subduction zone.

“Pseudo-calc-alkaline” signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break-up episodes. In this case, negative Nb anomalies are produced by the addition of crustal components enriched in Th and LREE relative to Nb. This inferred alternative origin of Nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data.     

Experimental evidence of fast groundwater responses in a hillslope/floodplain area in the Black Forest Mountains,Germany

Analysis of water flow pathways from hillslopes to streams is essential for the optimal protection of water resources as well as for ecohydrological studies. This study addresses runoff generation processes at a hillslope and near‐stream shallow groundwater system in the Black Forest Mountains, southwestern Germany. The changing spatial and temporal flow patterns during differing hydrological situations were examined using a combined hydraulic and hydrochemical approach. Groundwater levels at 10 wells, discharge at a near‐stream saturated area, and several natural tracers (deuterium, dissolved silica, and major anions and cations) were observed at different locations during high and low flows. The importance of the groundwater component during flood formation was clearly demonstrated: its contribution was about 80% during a double peak flood event at the saturated area. In addition, a rapid change of the shallow groundwater levels was observed along two transects of groundwater wells in the floodplain. This led to an enhanced groundwater discharge into the saturated area located at the end of one study transect. The amount of groundwater additionally activated during the event was about 30% of total discharge recorded at the outlet of the saturated area. Two alternative hypotheses are discussed to explain this phenomenon: the establishment of locally confined conditions and the development of a pressure wave (hypothesis A), or the significant change of the three‐dimensional groundwater flow lines that caused a large increase of the groundwater catchment at the saturated area during the investigated event (hypothesis B). Even if the exact flow paths and mechanisms could not be clearly identified, the importance of rapid responding hillslope groundwater was undoubtedly demonstrated by a combination of tracer and hydrometric methods. Copyright © 2004 John Wiley & Sons, Ltd.     

Application of nondestructive testing methods to study the damage zone underneath impact craters of MEMIN laboratory experiments

Abstract– Within the framework of the Multidisciplinary Experimental and Modeling Impact Research Network (MEMIN) research group, the damage zones underneath two experimentally produced impact craters in sandstone targets were investigated using several nondestructive testing (NDT) methods. The 20 × 20 × 20 cm sandstones were impacted by steel projectiles with a radius of 1.25 mm at approximately 5 km s^?1, resulting in craters with approximately 6 cm diameter and approximately 1 cm depth. Ultrasound (US) tomography and vibrational analysis were applied before and after the impact experiments to characterize the damage zone, and micro‐computer tomography (μ‐CT) measurements were performed to visualize subsurface fractures. The newly obtained experimental data can help to quantify the extent of the damage zone, which extends to about 8 cm depth in the target. The impacted sandstone shows a local p‐wave reduction of 18% below the crater floor, and a general reduction in elastic moduli by between approximately 9 and approximately 18%, depending on the type of elastic modulus. The results contribute to a better empirical and theoretical understanding of hypervelocity events and simulations of cratering processes.     

Groundwater Modeling in Integrated Water Resources Management—Visions for 2020

Groundwater modeling is undergoing a change from traditional stand-alone studies toward being an integrated part of holistic water resources management procedures. This is illustrated by the development in Denmark, where comprehensive national databases for geologic borehole data, groundwater-related geophysical data, geologic models, as well as a national groundwater-surface water model have been established and integrated to support water management. This has enhanced the benefits of using groundwater models. Based on insight gained from this Danish experience, a scientifically realistic scenario for the use of groundwater modeling in 2020 has been developed, in which groundwater models will be a part of sophisticated databases and modeling systems. The databases and numerical models will be seamlessly integrated, and the tasks of monitoring and modeling will be merged. Numerical models for atmospheric, surface water, and groundwater processes will be coupled in one integrated modeling system that can operate at a wide range of spatial scales. Furthermore, the management systems will be constructed with a focus on building credibility of model and data use among all stakeholders and on facilitating a learning process whereby data and models, as well as stakeholders' understanding of the system, are updated to currently available information. The key scientific challenges for achieving this are (1) developing new methodologies for integration of statistical and qualitative uncertainty; (2) mapping geological heterogeneity and developing scaling methodologies; (3) developing coupled model codes; and (4) developing integrated information systems, including quality assurance and uncertainty information that facilitate active stakeholder involvement and learning.     

Allgemeines über die Geologie von Mittel-Bünden

Ohne Zusammenfassung