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In the Bergen Arcs rocks that formed the root zone of the Caledonian mountain chain formed by the collision of Laurentia and Baltica at ~400 Ma are exposed. They display a well-exposed, highgrade metamorphic terrane where Sveconorwegian granulites and Caledonian eclogite and amphibolite parageneses coexist spatially and provide challenging problems related to their interrelationships and their geodynamic settings. Some of the ideas that have been proposed for the formation of the granulites and the mechanism of their retrograde hydration during the Caledonian orogeny are reviewed. We make some preliminary microstructural observations on the earliest stages of retrogression and suggest that much progress in understanding metamorphism in terranes such as the Bergen Arcs could be made by greater attention to the relationship between seismicity and metamorphism. The Bergen Arcs provide a natural laboratory in which it should possible to elucidate the extent to which seismicity and cataclasis are a necessary precursor to fluid infiltration and metamorphism, the volume of rock processed by cataclasis during an orogenic event and the role of deviatoric stress in the metamorphic reactions.

Frost weathering is effective in arctic and subarctic climate zones where chemical reactions are limited by the reduced availability of liquid water and the prevailing low temperature. However, small scale mineral dissolution reactions are nevertheless important for the generation of porosity by allowing infiltration of surface water with subsequent fracturing due to growth of ice and carbonate minerals. Here we combine textural and mineralogical observations in natural samples of partly serpentinized ultramafic rocks with a discrete element model describing the fracture mechanics of a solid when subject to pressure from the growth of ice and carbonate minerals in surface-near fractures. The mechanical model is coupled with a reaction–diffusion model that describes an initial stage of brucite dissolution as observed during weathering of serpentinized harzburgites and dunites from the Feragen Ultramafic Body (FUB), SE-Norway. Olivine and serpentine are effectively inert at relevant conditions and time scales, whereas brucite dissolution produces well-defined cm to dm thick weathering rinds with elevated porosity that allows influx of water. Brucite dissolution also increases the water saturation state with respect to hydrous Mg carbonate minerals, which are commonly found as infill in fractures in the fresh rock. This suggests that fracture propagation is at least partly driven by carbonate precipitation. Dissolution of secondary carbonate minerals during favorable climatic conditions provides open space available for ice crystallization that drives fracturing during winter. Our model reproduces the observed cm-scale meandering fractures that propagate into the fresh part of the rock, as well as dm-scale fractures that initiate the breakup of larger domains. Rock disintegration increases the reactive surface area and hence the rate of chemical weathering, enhances transport of dissolved and particulate matter in the weathering fluid, and facilitates CO2 uptake by car

© 2015 Elsevier B.V. Most changes in mineralogy, density, and rheology of the Earth's lithosphere take place by metamorphism, whereby rocks evolve through interactions between minerals and fluids. These changes are coupled with geodynamic processes and have first order effects on the global geochemical cycles of a large number of elements. In the presence of fluids, metamorphic reactions are fast compared to tectonically induced changes in pressure and temperature. Hence, rocks evolve through near-equilibrium states during fluid-producing metamorphism. However, much of the Earth's lower crust, and a significant fraction of the upper mantle do not contain free fluids. These parts of the lithosphere exist in a metastable state and are mechanically strong. When subject to changing temperature and pressure conditions at plate boundaries or elsewhere, these rocks do not react until exposed to externally derived fluids. Metamorphism of such rocks consumes fluids, and takes place far from equilibrium through a complex coupling between fluid migration, chemical reactions, and deformation processes. This disequilibrium metamorphism is characterized by fast reaction rates, dissipation of large amounts of energy as heat and work, and the generation of a range of emergent pore structures and fracture patterns that often control transport properties and thus further reaction progress. Fluid-consuming metamorphism leads to mechanical weakening due to grain size reduction, the formation of sheet silicates, and local heat production. Strain localization in the lower crust and upper mantle is therefore likely to be controlled by the availability of fluids. Fault-controlled migration of meteoric fluids from the brittle crust to the underlying ductile region in areas of compressive stress may provide a spatial and temporal link between localized strain and seismic activity in the upper crust and shear zone controlled deformation below. In a similar way, channelized fluid migration fro

Sublimation at the lower surface of a seasonal sheet of translucent CO2 ice at high southern latitudes during the Martian spring, and rapid outflow of the CO2 gas generated in this manner through holes in the ice, has been proposed as the origin of dendritic 100 m–1 km scale branched channels known as spiders or araneiforms and dark dust fans deposited on top of the ice. We show that patterns very similar to araneiforms are formed in a Hele-Shaw cell filled with an unconsolidated granular material by slowly deforming the upper wall upward and allowing it to return rapidly to its original position to drive air and entrained particles through a small hole in the upper wall. Straight, braided and quasiperiodic oscillating channels, unlike meandering channels on Earth were also formed

Carbonation of ultramafic rocks is being considered as an in situ disposal strategy for anthropogenic CO 2. We present observations from massively carbonated serpentinites at Linnajavri, N-Norway, where serpentinized fragments of the dismembered ophiolite are hydrothermally altered to soapstone and listvenite on a km-scale. Several sharp reaction fronts between the soapstone and the serpentinite can be followed for hundreds of metres. Listvenite is present between the underlying carbonate-mica schist and the soapstone. The O-isotopic compositions of vein minerals in the listvenite and adjacent soapstone indicate an isothermal formation at ~275°C. In agreement with C-isotope signatures, field observations suggest that fluids were derived from overthrusted sediments. Porosimetry data, mass-balance calculations and the perpetuation of serpentinite structures imply an isovolumetric alteration. We conclude that the reactions stopped due to the cessation of the external CO 2 supply. Soapstone formation is associated with sequestration of more than 13 Mt of CO 2. © 2012 Blackwell Publishing Ltd.

Udgivelsesdato: August 11

Replacement processes are common transformation mechanisms in minerals and rocks at a variety of conditions and scales. The underlying mechanisms are, in general, poorly understood, but both mechanical and chemical processes are thought to be important. Replacement of leucite (KAlSi2O6) by analcime (NaAlSi2 O6·H2O) is common in silica-poor igneous rocks. A 10% increase in volume is associated with the replacement process, and this generates stresses that eventually cause fracturing of the reacting leucite. Experimentally reacted leucite samples display characteristic fracturing patterns that include both spalling of concentric 'onion-skin'-like layers near the reacting interface and the formation of cross-cutting, often hierarchically arranged, sets of fractures that divide the remaining leucite into progressively smaller domains. These structures may explain the 'patchy' alteration patterns observed in natural leucite samples and similar, so-called, mesh-textures associated with the serpentinization of olivine grains during hydration of mafic or ultramafic rocks. They are also strikingly similar to larger scale patterns formed during spheroidal weathering processes. A simple discrete element model illustrates the mechanics that control the formation of such systems, and shows how these replacement processes may be accelerated due to the generation of new reactive surface area by hierarchical fracturing processes. © Springer-Verlag 2008.

geochemistry