Retrieval of P–T information from shear zones: thermobarometric consequences of changes in plagioclase deformation mechanisms

Changes in deformation mechanism coupled with spatial and temporal variations in reaction rates can result in preservation of disequilibrium mineral compositions in rocks affected by synmetamorphic shearing. Thermobarometric calculations on such rocks may thus yield meaningless results. We use Garbenschiefer samples from a shear zone in the Eastern Alps to study the effects of different deformational processes on calculated pressures and temperatures in samples that experienced the same overall PTt history. We focus on plagioclase, which accommodates strain by a variety of deformation mechanisms and is a key mineral in many thermobarometers. Plagioclase that deformed largely via dislocation creep mechanisms shows concentric chemical zoning, whereas plagioclase that experienced dissolution-precipitation creep preserves complex zoning. Rim compositions in the latter domains are not necessarily the youngest compositions, nor did they typically equilibrate with other phases in the assemblage. The timing of hornblende breakdown reactions relative to changes in plagioclase deformation mechanism also affected chemical zoning. Samples that escaped shear strain while near the thermal maximum yield internally consistent thermobarometric results, whereas samples that experienced shearing near the thermal maximum yield scattered results. Some of the variability in the results likely represents real differences in the P–T conditions at which equilibration occurred during deformation. However, much of the variability represents spurious results obtained by pairing mineral compositions that were never in equilibrium with one another. Extraction of useful P–T information from samples that experienced synmetamorphic deformation requires careful documentation of the relationships between deformation mechanisms and chemical zoning in order to select appropriate mineral compositions for thermobarometric calculations.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

On the reconstruction of structural and functional properties in random heterogeneous media

Flow and transport in porous media is determined by its structure. Beside spatial correlation, especially the connectivity of heterogeneous conductivities is acknowledged to be a key factor. This has been demonstrated for well defined random fields having different topological properties. Yet, it remains an open question which morphological measures carry sufficient information to actually predict flow and transport in porous media. We analyze flow and transport in classical, two-dimensional random fields showing different topology and we determine a selection of structural characteristics including classical two-point statistics, chord-length distribution and Minkowski functions (four-point statistics) including the Euler number as a topological measure. Using the approach of simulated annealing for global optimization we generate analog random fields that are forced to reproduce one or several of theses structural characteristics. Finally we evaluate in how far the generated analogons reproduce the original flow and transport behavior as well as some more elaborate structural characteristics including percolation probabilities and the pair connectivity function. The results confirm that two-point statistics is insufficient to capture functional properties since it is not sensitive to connectivity. In contrast, the combination of Minkowski functions and chord length distributions carries sufficient information to reproduce the breakthrough curve of a conservative solute. Hence, global topology provided by the Euler number together with local clustering provided by the chord length distribution seems to be a powerful condensation of structural complexity with respect to functional properties.     

A history of violence: magma incubation,timing and tephra distribution of the Los Chocoyos supereruption (Atitlán Caldera,Guatemala)

The climactic Los Chocoyos (LCY) eruption from Atitlán caldera (Guatemala) is a key chronostratigraphic marker for the Quaternary period given the extensive distribution of its deposits that reached both the Pacific and Atlantic Oceans. Despite LCY tephra being an important marker horizon, a radioisotopic age for this eruption has remained elusive. Using zircon (U–Th)/He geochronology, we present the first radioisotopically determined eruption age for the LCY of 75 ± 2 ka. Additionally, the youngest zircon crystallization ²³⁸U–²³⁰Th rim ages in their respective samples constrain eruption age maxima for two other tephra units that erupted from Atitlán caldera, W-Fall (130 ⁺¹⁶/₋₁₄ ka) and I-Fall eruptions (56 ^+8.2/_−7.7 ka), which under- and overlie LCY tephra, respectively. Moreover, rim and interior zircon dating and glass chemistry suggest that before eruption silicic magma was stored for >80 kyr, with magma accumulation peaking within ca. 35 kyr before the LCY eruption during which the system may have developed into a vertically zoned magma chamber. Based on an updated distribution of LCY pyroclastic deposits, a new conservatively estimated volume of ~1220 ± 150 km³ is obtained (volcanic explosivity index VEI > 8), which confirms the LCY eruption as the first-ever recognized supereruption in Central America.     

Direct 40Ar/39Ar K‐feldspar dating of Late Permian—Early Triassic brittle faulting in northern Norway

While the offshore post‐Caledonian extensional history of the north Norwegian passive margin is well constrained, the tectonic relationship between onshore and offshore regions is less clear because of limited age constraints on the timing of rifting onshore. ⁴⁰Ar/³⁹Ar dating of K‐feldspar from hydrothermally altered fault rocks in a Precambrian gneiss complex in northern Norway was used to study the timing of extensional faulting onshore. In addition, ⁴⁰Ar/³⁹Ar dating of K‐feldspar from the host rock provided insight into the regional rock cooling history prior to brittle deformation. Results indicated a dominant Late Permian–Early Triassic (~265–244 Ma) faulting event and found no evidence for later reactivation, which has been documented offshore. The region cooled to below the closure temperature for ⁴⁰Ar/³⁹Ar K‐feldspar in the Carboniferous to Early Permian, prior to the main brittle faulting event. ⁴⁰Ar/³⁹Ar dating of fault zone K‐feldspar products provided a means to date brittle faulting events.     

GIS Teacher Training: Empirically-Based Indicators of Effectiveness

In spite of various actions, the implementation of GIS (geographic information systems) in German schools is still very low. In the presented research, teaching experts as well as teaching novices were presented with empirically based constraints for implementation stemming from an earlier survey. In the process of various group discussions, the participants developed ideas for overcoming the constraints in the field of continuing teacher education among others. These ideas were used to create empirically based strategies for the future design of training activities in continuing education of teachers while taking these constraints into account. These strategies were later validated externally by comparing them to empirical findings on effects of training activities in continuing teacher education in general and empirical findings on GIS implementation from other studies.     

Implementation of Particle-scale Rotation in the 3-D Lattice Solid Model

In this study, 3-D Lattice Solid Model (LSMearth or LSM) was extended by introducing particle-scale rotation. In the new model, for each 3-D particle, we introduce six degrees of freedom: Three for translational motion, and three for orientation. Six kinds of relative motions are permitted between two neighboring particles, and six interactions are transferred, i.e., radial, two shearing forces, twisting and two bending torques. By using quaternion algebra, relative rotation between two particles is decomposed into two sequence-independent rotations such that all interactions due to the relative motions between interactive rigid bodies can be uniquely decided. After incorporating this mechanism and introducing bond breaking under torsion and bending into the LSM, several tests on 2-D and 3-D rock failure under uni-axial compression are carried out. Compared with the simulations without the single particle rotational mechanism, the new simulation results match more closely experimental results of rock fracture and hence, are encouraging. Since more parameters are introduced, an approach for choosing the new parameters is presented.