2. Metamorphic rocks and processes
2.1. Geodynamic and geological evolution of the Arctic
When a platform-edge meets a platform: 3D sedimentary architecture of a large-scale prograding paralic system, SW Edgeøya, Svalbard
Ingrid Anell1, Daniela Röhnert1, Alvar Braathen1, Kei Ogata2, Per Terje Osmundsen3, Aleksandra Smyrak-Sikora4, Harmon Maher5, Snorre Olaussen4, Gareth Lord6, Ivar Midtkandal1, Mark Mulrooney1 and Simon Buckley7
1Department of Geosciences, University in Oslo, Norway, 2Vrije University, Amsterdam, Holland, 3Geological Survey of Norway, Trondheim, Norway, 4Department of Arctic Geology, University Centre in Svalbard, Norway, 5University of Nebraska at Omaha, USA, 6Tyr Exploration AS, Trondheim, Norway, 7Uni Research AS, Bergen, Norway
The study combines facies analysis from sedimentary logs with pseudo 3D photogrammetric observations to provide a large-scale regional understanding of the Carnian depositional development around SW Edgeøya, which was filled by strongly tidally influenced deltaic systems. Prism-scale NW advancing Triassic clinoforms are documented in seismic data on the northern Barents Shelf. The sigmoidal clinoforms progressively form low-angle, linear tabular geometries, as the prograding system approaches Edgeøya. The studied succession documents very low angle infill from deep basin to coastal plain reflecting the advance of the clinoform system across the high. The study provides understanding of how the scale of the system, sandbody geometry and sedimentary processes were affected by more limited accommodation and increased tidal reworking.
The studied succession documents an early base-level fall during which large amounts of sand were arrested in fault-bounded half-grabens at Kvalpynten, while bypass and erosion is apparent towards the East. Accretion during a subsequent transgression documents continued high sedimentary input during relative sea-level rise. Paleocurrent and shaling out attest to local influx from an E/NE source around Kvalpynten, while at Svartpynten the regional NW trend is apparent. Svartpynten reveals a more shore-line proximal location, and following transgression, tidal bars indicate proximity to the fluvial source. At Kvalpynten laterally extensive subtidal sand-sheets grade into distinctly flat-based lens-shaped sand-bodies, likely shoaling barrier sands. A variety of channel-fills is documented in the overlying succession, which consists predominately of highly bioturbated sheltered intertidal mud-flats grading and interfingering with vegetated coastal plain.
Late Triassic sandstone provenance of the Barents Shelf; a regional comparison of preliminary detrital zircon geochronological and petrographic data
Trond S. Harstad1, Mai Britt E. Mørk1 and Trond Slagstad2
1Norwegian University of Science and Technology (NTNU), 2Geological Survey of Norway (NGU)
Several scientific contributions have been published in the last years on Triassic sandstone provenance in the Barents Shelf and Arctic areas. A southern or eastern Uralide component is well established in parts of the Triassic, in particular in south-eastern areas of the shelf. More recently, possible sediment contributions from a possible northern Uralide source area in the Late Triassic has been discussed. In our study, we aim to identify and compare sandstone provenance signatures in selected locations of the Upper Triassic De Geerdalen and Snadd formations in a north – south transect from Svalbard to the Nordkapp Basin. Petrographic observations reveal lithic and feldspathic sandstone compositions, notably with plagioclase dominating over K-feldspar, and characterised by lithic fragments of volcanic, metasedimentary and sedimentary origin. The polycrystalline quartz content represents deformed quartzite as well as chert and spiculites. Chromium spinel, apatite, garnet, rutile, tourmaline and zircon heavy minerals are observed in varying abundance. Preliminary detrital zircon age spectra from Svalbard, are similar to published zircon spectra of the larger Arctic region, including Triassic peaks of nearly syn-depositional ages. A dominance of Permian to Carboniferous zircons is compatible with an earlier inferred Uralide provenance to the east. However, a significant Caledonian age component has also been identified, which suggests a possible contribution from western areas. Currently, petrographic analysis of central and southern areas of the transect will be compared with results in progress from ongoing zircon analysis, and current regional geological knowledge, for better constraining alternative source area signatures.
Formation and Origin determination by geochemical fingerprinting of ruby and pink sapphire from the Fiskenæsset complex, Greenland
Nynke Keulen1, Tonny B. Thomsen1, John C. Schumacher2, Majken D. Poulsen3, Per Kalvig1, Torsten Vennemann4 and Rita Salimi1
1Geological Survey of Denmark and Greenland (GEUS), Denmark, 2Department of Geology, Portland State University, Portland, 3Geological Survey of Denmark and Greenland (GEUS), Denmark & Greenland, 4Institute of Earth Surface Dynamics, University of Lausanne, Géopolis, Switzerland
Metamorphic petrology observations on rubies found in-situ in their host-rock are combined with geochemical measurements and optical microscopy observations on the same rubies, with the aim to connect the ruby-forming metamorphic reaction to a unique fingerprint for these minerals. The Fiskenæsset complex in Greenland is used as an area of case study. Isochemical pressure-temperature sections were calculated based on electron microprobe and whole-rock geochemistry analyses, and compared to field observations. Rubies were formed resulting from a reaction between olivine/serpentine and anorthite, triggered by the intrusion of a pegmatite. Al is sourced from the anorthite, the pegmatite reacts to anthophyllite, Cr3+ is mobile in the pegmatitic fluid, giving colour to the rubies. The ruby-forming reaction occurs at 640 °C and 7 kbar. In order to establish the unique fingerprint for this ruby-bearing ultramafic complex, laser-ablation inductively-coupled-plasma mass-spectrometry trace element measurements, oxygen isotope compositions, optical microscopy and scanning electron microscopy were applied. Due to the setting in an ultramafic rock-anorthosite-leucogabbro complex, the fingerprint of the rubies from the Fiskenæsset complex is rather unique. Fiskenæsset complex rubies contain high Cr, intermediate Fe, and low V, Ga, and Ti concentrations, low oxygen isotope values (1.6 to 4.2 ‰) and a rarely-observed combination of optical growth features and mineral inclusions like anthophyllite+biotite. Results for other Greenlandic localities are presented and discussed as well. Even though these are derived from ultramafic rock settings too, they record different trace element ratios and oxygen isotope values, resulting from variations in the ruby-forming reaction.
Norsk Polar Navigasjon’s archive of old exploratory wells on Svalbard
1Christian Michelsen Research AS
During a 30 year period from the 1960’s to 1990’s several deep exploratory wells for oil drilled on Svalbard . The deepest well at Ishøgda was 3304m deep. Norsk Polar Navigasjon (NPN), the first Norwegian oil company, was involved in nine of the drillings. NPNs archive with information from the drillings has been made available for this study.
Though oil or gas was detected in the wells at Ishøgda, Tromsøbreen and Sarstangen, all wells was plugged and abandoned.
Geological and geophysical information from the wells are limited, but there are interested information of borehole stratigraphy, gas hydrates and geothermal gradients that will be presented and discussed
DEVELOPMENT OF THE AMERASIA BASIN: WHERE ARE WE NOW?
Victoria Pease1, Hemin Koyi2 and Faramarz Nilfouroushan3
1Stockhkolm University, 2Uppsala University, 3University of Gävle
The Amerasia Basin is separated into the Canada Basin and the Makarov-Povodnikov basins by the Alpha-Mendeleev Ridges. Published data supports a conjugate relationship between the Alaskan and Canadian Arctic margins, in which counterclockwise rotation of Arctic Alaska from Arctic Canada resulted in the opening of the Canada Basin. Thus the tectonic development of the Canada Basin is ‘broadly’ understood, although the precise timing and the role of the Chukchi Plateau remain disputed. This leaves the Amerasia Basin and we identify two significant barriers to understanding its tectonic development: i) The northward extent of the Canada Basin fossil spreading ridge, and ii) the role of LIP magmatism. In assessing the former, we simulated extension in a series of two-plate analogue models with properties homologous of homogeneous continental crust around a common rotation axis. In all models, a triangular (ocean) basin forms between the two ‘diverging’ plates, however, depending on the mode of opening and initial plate configuration transpressive, transtensive, and ‘pure’ strike-slip structures can be generated and account for the following first order observations: i) curvature in the fossil ridge, ii) transcurrent margins of opposite motion, and ii)asymmetry of the basin. These results elucidate the consequences of sea-floor spreading in the Amerasia Basin and help to constrain opening scenarios.
Tectonostratigraphic atlases: further perspective in Arctic research
Oleg Petrov1, Sergey Shokalsky1, Nikolay Sobolev1, Sergey Kashubin1, Eugeny Petrov1, Denis Leontiev1 and Tatiana Tolmacheva1
1Russian Geological Research Institute (VSEGEI)
Intensive study in the Arctic during last 15 years has resulted in the accumulation of new data on the geology of the region, including those obtained during geophysical sounding of the ocean floor, examination of bottom rock material, studying geology of islands and the continental part of the Russian Arctic. One of the international projects is the Atlas of Geological Maps of the Circumpolar Arctic at a scale of 1:5,000,000, which includes geological and tectonic maps, potential field maps, and mineral resources map. Modern level of knowledge of the Arctic allows the analysis of tectonostratigraphy and litho-geodynamics of the region and interpretation of geological complexes in terms of tectonic settings of stratigraphic sequences formation based. The Atlas “Geological History of the Barents Sea” (2009) by Russia and Norway was first experience in the implementation of this work.
In 2014, geological surveys of the Arctic states complied the “Tectonostratigraphic Atlas of the North-East Atlantic region”. Currently, the Russian Geological Research Institute (VSEGEI) actively works on compilation of the “Tectonostratigraphic Atlas”, covering the eastern Russian Arctic continental part and eastern Eurasian basin.
Compilation of the Atlas as a set of systematic modern geological information is of great importance for understanding the geology and history of tectonic development of this insufficiently studied region.
Together with the NPD and the NGU, we are going to compile the Tectonostratigraphic atlas for the Barents-Kara region. The work at Tectonostratigraphic atlases is a new stage in the development of the international cooperation in the Arctic.
Crustal structure of the Eurekan Orogen on Ellesmere Island, Arctic Canada
Christian Schiffer1 and Randell Stephenson2
1Department of Earth Science, Durham University, 2School of Geosciences, University of Aberdeen
Receiver function analysis of new broadband-seismological data provides images of crustal structure along an approx. 500 km long transect across Ellesmere Island. The gravity response of this crustal transect is consistent with regional gravity anomalies. The new data are, furthermore, integrated with existing crustal information to produce regional maps. Distinct crustal features are identified, such as crustal thickness variations, the presence of high velocity/density lower crust, thick metasedimentary layers as well as shallow younger sedimentary successions.
Moho depths can be interpreted in terms of Eurekan (Cenozoic) and Ellesmerian (Palaeozoic) deformation. A zone of thick metasedmentary layers in central Ellesmere Island correlates with areas of dominantly Ellesmerian accretion. A WSW-ENE orientated zone of shallow Moho (Hazen Stable Block) underlies crust strongly deformed in the Palaeozoic but essentially undeformed in the Cenozoic. A block of thick crystalline crust in the north of Ellesmere Island is clearly separated from the North American-Greenland Craton by the deep (meta-)sedimentary successions, possibly a hint for a northern microcontinent. High velocity lower crust may be related to igneous activity during several rift episodes and/or impact by High Arctic Large Igneous Province magmatism. A correlation appears to exist between topography, Moho depth and the location of dykes in Nansen Sound suggesting the same tectono-magmatic origin of these features. Lincoln Sea shows consistently thin crust likely related to rifting.
Details on the Cretaceous ocean formation in the High Arctic based on satellite gravity data
Arne Døssing1, Carmen Gaina2, Ole Baltazar Andersen1 and H. Ruth. Jackson3
1DTU Space, Denmark, 2CEED, University of Oslo, 3(4) Geological Survey of Canada Atlantic, Dartmouth, Nova Scotia
Understanding the evolution of ocean basins, critical for global studies in plate tectonics, mantle dynamics, and sea-level through time, relies on identifiable tectonic plate boundaries. Based on the latest generation of global satellite gravity models, recent marine geophysical data and vintage aeromagnetic data, we document consistent tectonic details on the remote and ill-defined Canada Basin spreading system: the oldest ocean system in the High Arctic and part of the long-disputed greater Amerasia Basin. We interpret two distinct phases of possibly Cretaceous sea-floor spreading: the early stage being sub-orthogonal and intermediate-spreading (~50 mm/yr) of possible Barremian-Aptian age, while the late stage being highly oblique, segmented, and slow-spreading (20–40 mm/yr) of possible Aptian age. We further demonstrate that the southern part of the Canada Basin spreading system may serve as an ancient analogue of modern mid-ocean ridge propagation into continental crust.
Soapstone from Nuuk: fingerprinting the origin and geological history of soapstone (steatite) suitable for carving in the Godthåbsfjord-Ameralik region (southern West-Greenland).
Nynke Keulen1, Majken D. Poulsen2, Robert Frei3, Kisser Thorsøe4 and Rebekka J. Knudsen5
1Geological Survey of Denmark and Greenland (GEUS), Nuuk GLD & Copenhagen, DK, 2Geological Survey of Denmark and Greenland (GEUS) Nuuk, GLD & Dept of Geosciences Nat.Res., KU, DK, 3Department of Geosciences and Natural Resource Management, Copenhagen University, Denmark, 4Geological Survey of Denmark and Greenland (GEUS) Nuuk, GLD, 5Greenland Perspective, Natural History Museum of Denmark, Copenhagen University, DK
Soapstone was mined and traded by the Saqqaq, Dorset, Norse, and Morovian cultures living in Greenland through time, and still has a good potential for small-scale mining as a raw material for carving of art products and for the tourist industry. We collected soapstone from outcrops around Nuuk, which hold Greenland’s best quality of soapstone, and analysed their whole-rock geochemistry, Sm-Nd and Rb-Sr isotopes for fingerprinting for geological and archaeological purposes, such that historic artefacts that were traded far away from Nuuk through time, might be traced back to the Nuuk area. Fieldwork in the Nuuk area showed that the quality of the soapstone in the Godthåbsfjord-Ameralik region actually varies widely; however, a correlation exists with the age and/or the metamorphic history of the rocks. The best quality soapstone occurs in a broad band cutting through the fjords in the Nuuk area. This area roughly covers the area with occurrence of Eo-Archaean gneisses. However, the same area was also subject to intense metasomatism of the soapstone precursor-rocks associated with fluid activity along the Ivinnguit fault zone. Further work will be directed to investigate which of those processes played the major role. Expectedly, one or both factors will provide a good fingerprint of the Nuuk soapstone. Additionally, this information will narrow down the area with good soapstone occurrences considerably, such that citizens wanting to investigate for undiscovered localities with good quality soapstone will know to which part of the Godthåbsfjord-Ameralik region their investigations for new small-scale mining areas should be directed.
A large Devonian ultra-high-pressure province in northeastern Greenland
Thorsten Nagel1, Kathrin Fassmer2, Nikolaus Froitzheim2, Peter Sprung3 and Carsten Münker3
1Department of geoscience, Aarhus, Denmark, 2Steinmann-Institut, Bonn University, Germany, 3Department for Geology and Mineralogy, Cologne University, Germany
The Caledonian orogen in northeastern Greenland consists of a 1200 km long, west-vergent nappe pile. It has traditionally been viewed as the retro-wedge of the Scandinavian Caledonides. This concept, however, was challenged by the finding of widely distributed eclogites as well as the large amount of horizontal shortening accommodated in the nappe pile. Existing U-Pb-zircon- and Sm-Nd-garnet ages for high-pressure metamorphism in the central section of the Greenlandic Caledonides scatter around 420 to 390 Ma. A single UHP-metamorphic location in a structurally internal position yielded an exceptionally young zircon age of 360 ±5 Ma and a Sm-Nd-garnet age of around 330 Ma interpreted to record peak-pressure conditions and cooling, respectively (Gilotti et al. 2004). We present new petrologic and Lu-Hf garnet age data from three locations. Samples from Danmarkshavn record ultra-high-pressure metamorphic conditions in eclogites and grt-pyroxenites by means of SiO2-exsolutions in cpx, opx-exsolutions in cpx and thermobarometric estimations. An eclogite yielded a Lu-Hf garnet-whole-rock age of 357.5 ±2.9 Ma thus confirming the existing zircon age for UHP metamorphism obtained 140 kilometers to the north. Samples from two other locations preserve the typical high-temperature-moderate-pressure eclogite-facies conditions and yield ages of 383 ± 17 Ma and 398 ± 14 Ma, respectively. Results suggest that besides the 400 Ma event, Devonian ultrahigh-pressure rocks in northeastern Greenland may be much wider distributed than previously known creating major puzzles for tectonic reconstructions.
New heat flow measurements from central Arctic Ocean
Grace Shephard1, Steffen Wiers2, Evgenia Bazhenova3, Lara Pérez4, Luz María Mejía5, Carina Johansson6, Martin Jakobsson6 and Matt O’Regan6
1Centre for Earth Evolution and Dynamics (CEED), University of Oslo, 2Department of Earth Sciences, Uppsala University, 3Center for Coastal and Ocean Mapping, University of New Hampshire // St Petersburg State University, 4Department of Geophysics, Geological Survey of Denmark and Greenland (GEUS), 5Geological Institute, Department of Earth Sciences, ETH, Zürich, 6Department of Geological Sciences, Stockholm University
Constraining the thermal evolution of the Arctic Ocean is hampered by notably sparse measurements of heat flow. Furthermore, previous results from the central Lomonosov Ridge and the adjacent central Amundsen Basin reveal variable magnitudes, including those higher than expected considering plate cooling or simple uniform stretching models. Here we present new heat flow results gathered from 17 sediment cores acquired during the “Arctic Ocean 2016” and “SWERUS-C3” expeditions on the Swedish icebreaker Oden. Three sites located in the Amundsen Basin reveal heat flow in the order of 71-95 mW/m2, in line-with or slightly higher (1-21 mW/m2) than expected from oceanic heat flow curves. These values are substantially lower than values of another study, with 104-127 mW/m2 found on similarly aged oceanic crust in the Amundsen Basin located approximately 100-200 km away. Sites from the submerged continental fragments of the Lomonosov Ridge and Marvin Spur recovered heat flow in the order of 53-76 and 51-69 mW/m2 respectively. When considering the potential enhanced surface heat flux from radiogenic heat production in the crust, these variable measurements are broadly in line with predictions from uniform extension models for continental crust. An upper mantle seismic anomaly in the vicinity of the North Pole might therefore have a compositional as well as thermal component. This complexity highlights the difficult in disentangling temporally and spatially evolving crustal, lithospheric and mantle processes to present-day surface heat flow contributions.
2.2. Deformation and metamorphism of rocks: Microstructures, processes, and physical properties
A novel gel-mediated reaction mechanism in metamorphic rocks
Matthias Konrad-Schmolke1, Ralf Halama2, Richard Wirth3, Nico Klitscher3, Luiz Morales4 and Franziska Wilke3
1Earth Science Department, University of Gothenburg, Sweden, 2School of Geography, Geology and the Environment, Keele University, UK, 3GeoForschungsZentrum Potsdam, GFZ, Potsdam, Germany, 4ETH Zürich, Switzerland
Fluid-mediated mineral dissolution and re-precipitation is a key mechanism for mineral reactions in different fields of Earth and Material Science. Mineral reactions in the solid Earth constrain many geodynamic and geochemical processes, such as density changes in subducted oceanic crust, the release of carbon dioxide in subduction zones and the precipitation of ore minerals from percolating fluids. Detailed knowledge about rate-limiting mechanisms during such mineral reactions is fundamental for the quantification of rock transformations and the associated element transport. Unresolved questions regarding fluid-mediated mineral dissolution and re-precipitation are the formation of permeability and the quantification of the element transport during mineral transformation. Here we present examples where the entire cycle of mineral dissolution, element transport and re-precipitation of new equilibrium phases is visualized in high-pressure metamorphic rocks that experienced re-hydration in a subduction zone. The replacement of primary amphibole and pyroxene in our samples occurs via coupled dissolution-re-precipitation that involves the formation of an amorphous alkali-Al-Si-rich material. This amorphous material forms directly from the reacting minerals by de-polymerization of the original crystal lattice. De-polymerization starts along dislocation cores in the crystal lattice that serve as element exchange pathways and are also sites of porosity formation. In our samples, the amorphous material resulting from this de-polymerization occupies large volumes in a syn-metamorphic interconnected porosity network. The direct transformation of the mineral into the amorphous state enables element transfer decoupled from the presence of ionic solutions. Precipitation of secondary amphibole occurs directly by re-polymerization of the amorphous material at the product surface.
Connection between grain shape, chemical zonation and texture as indicator for fabric development in eclogites
Kai Neufeld1, Holger Stünitz2, Ane Kongsro Finstad1 and Jiri Konopásek1
1Department of Geosciences, University of Tromsø, Norway, 2Department of Geosciences, University of Tromsø, Norway; Institut de Sciences de la Terre, Universit
Eclogites are the most important piece of evidence of high pressure conditions in subduction zones. Their fabric development is an interesting topic and may allow to determine the deformation rates in subduction zones. Most previous studies suggested dislocation creep to be the principal process causing the fabric development, because many eclogite fabrics are quite strong. The viability of this may be tested by studying the chemical zonation of garnet and omphacite in order to track and quantify texture and microstructure development in eclogites. The aim of this study is to assess the influence of crystal growth on mineral preferred orientation and therefore its role in fabric development in eclogite-facies rocks.
Variscan and Alpine eclogites from two different locations have been studied. In both cases, asymmetric but still concentric chemical zoning developed during prograde garnet growth together with elongated garnet grain shape and can be related to a corresponding prograde (in terms of pressure change) chemical zoning in omphacite grains. Crystal plastic deformation of garnet can be excluded based on chemical zonation patterns. Chemical, microstructural and texture data indicate a direct relationship between the growth of garnet and omphacite grains in the principal stretching direction, causing a strong fabric development during prograde and peak metamorphic conditions. No dynamic recrystallization microstructures were detected. Thus, the textures are interpreted as growth fabrics. Dislocation creep as a possible fabric formation process is excluded. Instead, diffusion creep (grain scale precipitation producing elongation in the stretching direction) is concluded to be the dominant deformation mechanism.
Shear deformation in the transformation of gabbro to eclogite: Potential key to rheological contrasts in the change from HP/UHP conditions to later amphibolite-facies deformation
Peter Robinson1, Michael P. Terry2 and Florian Heidelbach3
1Geological Survey of Norway (NGU) N-7491 Trondheim, Norway, 2PO Box 553, Naples, NY 14512, USA, 3Bayerisches Geoinstut, Universität Bayreuth, D95440 Bayreuth, Germany
The Proterozoic ~1700-1550 Ma Baltican basement of the northern Western Gneiss Region, Norway, consists of coarse granitoid gneisses locally intruded by 1466 and 1250 Ma gabbros. Progressive relative rheological behavior of this diverse assemblage during Scandian (420-375 Ma) subduction to HP/UHP metamorphic conditions (2.5-4 GPa) and exhumation back to near-surface conditions, is key to understanding tectonic development through multiple phases of thrusting, recumbent folding, and extension.
Recent studies of deformation-enhanced reactions en route to eclogite-facies conditions within sheared gabbro have demonstrated progressive destruction of plagioclase in reaction with pyroxene and oxides and growth of garnet, to superplastic behavior in garnet by grain-boundary sliding. Plausibly, during this, rheological differences between deforming gabbro and gneisses were minimal. Thus, gabbro-gneiss contacts are most commonly decorated by 1-10m thick layers of strongly foliated, fine-grained garnet-rich eclogite, and nearby small eclogite bodies are usually similar in texture and composition.
These observations suggest that the gneisses may not have partially melted during HP-UHP conditions, but rather by fluid-absent partial melting of micas during exhumation to <2 GPa. The resulting changed ductility contrast between mafic and felsic rocks produced myriad gabbro and eclogite boudins that characterize the region, allowing access to aqueous fluids facilitating growth of secondary hornblende. Contacts of some gabbros show various processes: Some contacts decorated by fine-grained foliated eclogite, locally retrograded to foliated garnet amphibolite. Other contacts with little eclogitized gabbro, directly sheared against gneiss at a late stage, but converted locally by infiltrating fluid to isotropic hornblende gabbro.
Terry, M. P.and Heidelbach, F. 2004, Superplasticity in garnet from eclogite facies shear zones in the Haram Gabbro, Haramsøya, Norway. Geology 32, 281-284.
Terry, M. P.and Heidelbach, F. 2006, Deformation-enhanced metamorphic reactions and the rheology of high-pressure shear zones,Western Gneiss Region, Norway. Journal of Metamorphic Geology 24, 3-18.
Record of a Lower Crustal Fossil Earthquake Initiated by CO2 Flux and Reaction-driven Strain Softening
Bjørn Eske Sørensen1, Thomas Grant1 and Rune Larsen1
1Department of Earth Science and Petroleum, NTNU, Norway
The ultramafic rocks of the Reinfjord intrusion in the Seiland Igneous Province is cut locally by narrow (mm-cm) thick extensional shearzones, containing extremely fine-grained material with a distinct shape-preferred orientation. These shearzones offset dykes across numerous micro-faults are documented in areas close to the major fault zone cutting through the area. Within the shearzones olivine and clinopyroxene reacts to form orthopyroxene and dolomite at approximately 7-11 kb and 750-850 degrees Celsius according to the reaction:
Olivine + Clinopyroxene + CO2 = Dolomite + Orthopyroxene
As evidenced by coronas of orthopyroxene and dolomite between olivine and clinopyroxene and by large olivine grains proximal to the shearzones displaying a microstructure with subgrain walls decorated by rounded grains of dolomite and more irregular and elongated grains of orthopyroxene. With clinopyroxene at least hundreds of microns away this suggested at least some material transport within the shearzone.
The shearzones thus gives a unique view into CO2-Metasomatism of the lower crust. Moreover the shearzones provide a unique insight into the interplay between CO2-metasomatism and reaction accommodated strain softening. This is also confirmed by fractures extending into large olivine grains proximal to the shearzones. The CO2 cracking and mineral reaction also serves to reduce grain-size , making grain-boundary sliding an efficient process, further enhancing the rheological contrast between the shearzone and the host rock. The sudden decrease in rock strength lead to sudden fast deformation and we suggest a relation to pseudotachylites and hence also earth quakes in near proximity of the micro-shearzones.
The effects of confinement on reaction-induced fracturing during an hydration reaction
1University of Oslo
Several geological processes involve mineral transformations where nominally dry rocks transform into hydrated ones when left in contact with water (i.e. eclogitization, serpentinization). In these systems, the transformation may induce density variations which generate stresses controlled by the rock confinement, also called force of crystallization. Here, we study a retrograde metamorphic reaction model, the hydration of periclase, MgO, into brucite, Mg(OH)2, to quantify the coupling between reaction, stress generation, porosity evolution and fracturing. Samples of a microporous MgO ceramic were reacted at 170-210°C, 5-80 MPa confining pressure, 6-95 MPa differential stress and 5 MPa pore fluid pressure. Three-dimensional X-ray microtomography images were acquired in situ every five minutes during the reaction. Results imply that below 30 MPa mean pressure, the hydration reaction coupled with the MgO ceramics fracturing. The transformation rate followed a sigmoidal kinetics curve with a slow initiation, a fast reaction coupled to fracturing and the generation of a transient porosity pulse, and a slow-down until an almost complete transformation of periclase into brucite. Conversely, above 30 MPa, the reaction kinetics was slower and following the commonly admitted reaction curve but without fracturing. Considering the driving force of the hydration reaction, stress generation should be several hundreds MPa, whereas the present experiments show that fracturing occurs only below 30 MPa. One interpretation is that the stress created by the reaction may overcome the disjoining pressure at the grain-grain interface, expelling the water film trapped there and reducing dramatically the kinetics of reaction.
Late magmatic-hydrothermal alteration and deformation of ultramafic rocks and dikes from the Reinfjord Ultramafic Complex
Frederik Valentin Reinhard1, Bjørn Eske Sørensen2, Thomas Wagner1 and Rune Berg-Edland Larsen2
1RWTH Aachen University, 2NTNU Trondheim
This study concerns hydrothermal alteration of ultramafic rocks comprised by dunites and subordinate wehrlite and dykes of ultramafic to mafic and lamprophyric composition in the the Reinfjord Ultramafic Complex the Seiland Igneous Province, northern Norway. The dikes and host ultramfic rocks went through three main alteration stages:
Alt1. Microfaults with orthopyroxene, dolomite and olivine. Alt2. Yellowish magnesite-tremolite-talc alterations, comprised by central thin magnesite veins (orientation 55/110) surrounded by thicker metasomatic sides of tremolite and magnesite and then a thin talc rim towards the host ultramafic host. When intersecting the more pyroxenitic dykes the alteration gives a blueish green color. Alt3 Late serpentinisation intersecting all other structures.The study focuses on alteration type 2. The usually less than cm wide magnesite-bearing veins are surrounded by up to dm wide metasomatic sides. The weathered surface of the veins form massive, creamy yellow to rust red colored magnesite bearing mineral assemblages. The metasomatic sides stand out with a more yellow color than the weathered host rock and is comprised by a magnesite and tremolite bearing assemblage, rimmed by a mm-thin talc rim towards the host rock. Furthermore the veins also crosscut the different generations of dykes, resulting in different alteration assemblages.
The aim of the study is to use different alteration assemblages to constrain PTX-fluid conditions and determine ore remobilization. Furthermore stable isotope data of O, H, S, and C will be used to constrain the fluid source which is thought to be either magmatic or meteoric.Top