Diffusion-controlled development of silica-undersaturated domains in felsic granulites of the Bohemian Massif (Variscan belt of Central Europe)

Plagioclase rims around metastable kyanite crystals appear during decompression of high-pressure felsic granulites from the high-grade internal zone of the Bohemian Massif (Variscan belt of Central Europe). The development of the plagioclase corona is a manifestation of diffusion-driven transfer of CaO and Na₂O from the surrounding matrix and results in isolation of kyanite grains from the quartz- and K-feldspar-bearing matrix. This process establishes Si-undersaturated conditions along the plagioclase–kyanite interface, which allow crystallization of spinel during low-pressure metamorphism. The process of the plagioclase rim development is modeled thermodynamically assuming local equilibrium. The results combined with textural observations enable estimation of equilibration volume and diffusion length for Na and Ca that extends ∼400–450 and ∼450–550 μm, respectively, around each kyanite crystal. Low estimated bulk diffusion coefficients suggest that the diffusion rate of Ca and Na is controlled by low diffusivity of Al across the plagioclase rim.     

Decay mechanism of indoor porous opuka stone: a case study from the main altar located in the St. Vitus Cathedral,Prague (Czech Republic)

A carving of the indoor main altar of St. Vitus Cathedral in Prague (Czech Republic) is made of the opuka stone—a clayey–calcareous silicite—which now exhibits the development of decay phenomena such as the formation of salt-laden case-hardened subsurface layer (approx. 150 mm thick), with detachment of the case-hardened layer manifested by blistering and/or flaking. Formation of this gypsum-rich layer is linked to the reaction of components (SO₂) from polluted air (both outdoor and indoor) and from the rock itself (calcium ion from calcite). Development of brittle damage in the subsurface layer and underlying stone is interpreted based on the results from previous environmental monitoring in the Cathedral’s interior, which indicated highly fluctuating temperature and humidity, resulting in a hygrothermal stress in the material described by the “double-layer sandwich” model. The sensitivity of the studied stone to the above-mentioned processes is evidenced by its microstructural properties, specifically parameters of the pore spaces which indicate an extremely high susceptibility to damage by the actions of freezing water and/or salt crystallisation.     

Extreme P-, Bi-, Nb-, Sc-, U- and F-rich zircon from fractionated perphosphorous granites: The peraluminous Podlesí granite system, Czech Republic

The strongly peraluminous and P-rich, protolithionite and zinnwaldite leucogranites from Podlesí, western Krušné Hory Mts., Czech Republic, contain accessory zircon with extraordinary enrichment of several elements, which constitute trace elements in common zircon. Elements showing a not yet reported anomalous enrichment include P (up to 20.2 wt.% P₂O₅; equivalent to 0.60 apfu, formula calculated on the basis of 4 oxygen atoms), Bi (up to 9.0 wt.% Bi₂O₃; 0.086 apfu), Nb (up to 6.7 wt.% Nb₂O₅, 0.12 apfu), Sc (up to 3.45 wt.% Sc₂O₃; 0.10 apfu), U (up to 14.8 wt.% UO₂; 0.12 apfu) and F (up to 3.81 wt.% F; 0.42 apfu). Strong enrichment of P preferentially involved the berlinite-type substitution (2 Si⁴⁺  P⁵⁺ + Al³⁺) implying that significant Al may enter the Si position in zircon. Incorporation of other exotic elements is primarily governed by the xenotime (Si⁴⁺ + Zr⁴⁺  P⁵⁺ + Y³⁺), pretulite (Sc³⁺ + P⁵⁺  Zr⁴⁺ + Si⁴⁺), brabantite-type (Ca²⁺ + (U, Th)⁴⁺ + 2P⁵⁺  2Zr⁴⁺ + 2Si⁴⁺), and ximengite-type (Bi³⁺ + P⁵⁺  Zr⁴⁺ + Si⁴⁺) substitution reactions. One part of the anomalous zircons formed late-magmatically, from a strongly peraluminous, P–F–U-rich hydrous residual melt that gave rise to the zinnwaldite granite. Interaction with aggressive residual fluids and metamictization have further aided in element enrichment or depletion, particularly in altered parts of zircon contained in the protolithionite granite. Most of the zircon from F-rich greisens have a composition close to endmember ZrSiO₄ and are chemically distinct from zircon in its granite parent. This discrepancy implies that at Podlesí, granitic zircon became unstable and completely dissolved during greisenization. Part of the mobilized elements was reprecipitated in newly grown, hydrothermal zircon.     

How well can we simulate complex hydro‐geomorphic process chains? The 2012 multi‐lake outburst flood in the Santa Cruz Valley (Cordillera Blanca,Perú)

Changing high‐mountain environments are characterized by destabilizing ice, rock or debris slopes connected to evolving glacial lakes. Such configurations may lead to potentially devastating sequences of mass movements (process chains or cascades). Computer simulations are supposed to assist in anticipating the possible consequences of such phenomena in order to reduce the losses. The present study explores the potential of the novel computational tool r.avaflow for simulating complex process chains. r.avaflow employs an enhanced version of the Pudasaini ( 2012 ) general two‐phase mass flow model, allowing consideration of the interactions between solid and fluid components of the flow. We back‐calculate an event that occurred in 2012 when a landslide from a moraine slope triggered a multi‐lake outburst flood in the Artizón and Santa Cruz valleys, Cordillera Blanca, Peru, involving four lakes and a substantial amount of entrained debris along the path. The documented and reconstructed flow patterns are reproduced in a largely satisfactory way in the sense of empirical adequacy. However, small variations in the uncertain parameters can fundamentally influence the behaviour of the process chain through threshold effects and positive feedbacks. Forward simulations of possible future cascading events will rely on more comprehensive case and parameter studies, but particularly on the development of appropriate strategies for decision‐making based on uncertain simulation results. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.