Zimmermann U.,University of Stavanger |
Tait J.,Grant Institute |
Crowley Q.G.,NERC Isotope Geosciences Laboratory |
Crowley Q.G.,Trinity College Dublin |
And 2 more authors.
International Journal of Earth Sciences | Year: 2011
The Witputs section of the Gariep Belt (S Namibia) comprises a sequence of clastic and chemical sediments, which have been interpreted as representative of a Late Neoproterozoic global or near-global ice age event, and recent biostratigraphic work in the upper rocks of the Witputs suggest a Late Ediacaran age. To further characterise this sequence and provide additional age constraints, a detailed sedimentological and detrital zircon study has been carried out. The petrographic, sedimentological and geochemical characteristics of the Witputs diamictite determined in this study are homogenous and indicative of debris flow or palaeo-valley infill sediments, deposited in an oxic environment with no glaciogenic evidence. This homogeneity is also reflected in the detrital zircon age spectra with most ages falling between 1.0 and 1.3 Ga, representing the local geology, with the youngest grain at 1030.2 ± 10.9 (2σ) (n = 92 <10% discordance), despite the fact that mid and Late Neoproterozoic volcanic activity is known in the local region. The overlying carbonate rocks, often considered to be 'cap carbonates', show high Mn (up to 60% MnO), with base metal precipitation (Zn, V, Co), and are recrystallised. Their δ 13C VPDB isotope ratios are homogeneous at around-3. Major and trace element ratios reach values which indicate that C-O isotopes may be disturbed and might not reflect primary global seawater composition, thus questioning their use for global correlation and comparison with composite chemostratigraphic curves. The contact to the overlying Late Ediacaran Sanddrif Member is not exposed, and the rocks dip in a different direction than the underlying carbonate rocks. The c. 40-m-thick section is characterised by rapid lithology changes including shales, calcareous sandstones and wackes, fine-grained conglomerates and rare clean quartz-rich sandstones, all of which have strikingly similar detrital zircon populations, and the youngest zircon is dated at 1082.8 ± 10 Ma (2σ errors, from 72 grains with <10% discordance). Acritarchs earlier found in the Sanddrif Member, however, indicate a post-570 Ma depositional age. If the diamictites are glacio-marine deposits, then an interesting conclusion is that the clastic sediments can display a very immature geochemical signature, indicating a localised provenance, with derivation purely from the local basement rocks, which is also reflected in the detrital zircon populations. However, we would hesitate to assign a glacial origin to the deposits as no glacial indicators, other than a diamictitic texture, were observed. Clearly, far more work on the detailed mapping and sedimentology of the Neoproterozoic Gariep Belt deposits is required, particularly as many are currently used for global correlation. Age constraints derived from extensive detrital zircon work can only constrain the deposits as being post 1.03 Ma with the detritus being purely locally derived. © 2010 Springer-Verlag.
Zack T.,University Mainz |
Stockli D.F.,University of Kansas |
Luvizotto G.L.,Max Planck Institute for Chemistry |
Barth M.G.,University Mainz |
And 3 more authors.
Contributions to Mineralogy and Petrology | Year: 2011
Rutile is a common accessory mineral that occurs in a wide spectrum of metamorphic rocks, such as in blueschists, eclogites, and granulites and as one of the most stable detrital heavy minerals in sedimentary rocks. The advent of rutile trace element thermometry has generated increased interest in a better understanding of rutile formation. This study documents important analytical advances in in situ LA-ICP-MS U/Pb geochronology of rutile: (1) Matrix matching, necessary for robust in situ dating is fulfilled by calibrating and testing several rutile standards (R10, R19, WH-1), including the presentation of new TIMS ages for the rutile standard R19 (489.5 ± 0.9 Ma; errors always stated as 2 s). (2) Initial common lead correction is routinely applied via 208Pb, which is possible due to extremely low Th/U ratios (usually <0.003) in most rutiles. Employing a 213 nm Nd:YAG laser coupled to a quadrupole ICP-MS and using R10 as a primary standard, rutile U/Pb concordia ages for the two other rutile standards (493 ± 10 Ma for R19; 2640 ± 50 Ma for WH-1) and four rutile-bearing metamorphic rocks (181 ± 4 Ma for Ivrea metapelitic granulite; 339 ± 7 Ma for Saidenbach coesite eclogite; 386 ± 8 Ma for Fjortoft UHP metapelite; 606 ± 12 Ma for Andrelandia metepelitic granulite) always agree within 2% with the reported TIMS ages and other dating studies from the same localities. The power of in situ U/Pb rutile dating is illustrated by comparing ages of detrital rutile and zircon from a recent sediment from the Christie Domain of the Gawler Craton, Australia. While the U/Pb age spectrum from zircons show several pronounced peaks that are correlated with magmatic episodes, rutile U/Pb ages are marked by only one pronounced peak (at ca 1,675 Ma) interpreted to represent cooling ages of this part of the craton. Rutile thermometry of the same detrital grains indicates former granulite-facies conditions. The methods outlined in this paper should find wide application in studies that require age information of single spots, e. g., provenance studies, single-crystal zoning and texturally controlled dating. © 2011 Springer-Verlag.
Crampin S.,British Geological Survey |
Crampin S.,Grant Institute |
Gao Y.,China Earthquake Administration
Journal of Asian Earth Sciences | Year: 2012
Rock is weak to shear-stress and the energy released by the 26th December, 2004, M≈ 9.2 Sumatra-Andaman Earthquake, the largest earthquake for four decades, must have accumulated over enormous volumes of crust and mantle, certainly plate-wide, possibly world-wide. Here we report evidence for plate-wide stress accumulation. Changes in seismic shear-wave splitting monitor stress-induced changes in the geometry of the microcrack distributions in almost all rocks in the Earth's crust. Such changes observed in Iceland show stress-accumulation beginning at least four years before the Sumatra-Andaman Earthquake. These changes were recognised as monitoring stress-accumulation before an impending large earthquake and 10 'stress-forecasts' were emailed to Iceland Meteorological Office for some 29. months forecasting an impending large earthquake. The remarkable sensitivity of critical-systems of microcrack geometry to miniscule changes of stress had not been recognised at that time and the stress-accumulation was expected to lead to a M. 7 earthquake somewhere in Iceland. Only now is it recognised that the changes in shear-wave splitting were monitoring stress-accumulation which would eventually lead to the Sumatra-Andaman Earthquake at a distance of some 10,500. km on the opposite side of the Eurasian Plate. This extreme sensitivity confirms the critical nature of fluid-saturated stress-aligned microcracks in the Earth's crust. © 2011 Elsevier Ltd.
MacDonald J.M.,University of Liverpool |
MacDonald J.M.,Imperial College London |
Wheeler J.,University of Liverpool |
Harley S.L.,Grant Institute |
And 4 more authors.
Contributions to Mineralogy and Petrology | Year: 2013
Zircon is a key mineral in geochemical and geochronological studies in a range of geological settings as it is mechanically and chemically robust. However, distortion of its crystal lattice can facilitate enhanced diffusion of key elements such as U and Pb. Electron backscatter diffraction (EBSD) analysis of ninety-nine zircons from the Lewisian Gneiss Complex (LGC) of northwest Scotland has revealed five zircons with lattice distortion. The distortion can take the form of gradual bending of the lattice or division of the crystal into subgrains. Zircon lattices are distorted because of either post-crystallisation plastic distortion or growth defects. Three of the five distorted zircons, along with many of the undistorted zircons in the population, were analysed by ion microprobe to measure U and Pb isotopes, Ti and REEs. Comparison of Th/U ratio, 207Pb/206Pb age, REE profile and Ti concentration between zircons with and without lattice distortion suggests that the distortion is variably affecting the concentration of these trace elements and isotopes within single crystals, within samples and between localities. REE patterns vary heterogeneously, sometimes relatively depleted in heavy REEs or lacking a Eu anomaly. Ti-in-zircon thermometry records temperatures that were either low (~700 °C) or high (>900 °C) relative to undistorted zircons. One distorted zircon records apparent 207Pb/206Pb isotopic ages (-3.0 to +0.3 % discordance) in the range of ~2,420-2,450 Ma but this does not correlate with any previously dated tectonothermal event in the LGC. Two other distorted zircons give discordant ages of 2,331 ± 22 and 2,266 ± 40 Ma, defining a discordia lower intercept within error of a late amphibolite-facies tectonothermal event. This illustrates that Pb may be mobilised in distorted zircons at lower metamorphic grade than in undistorted zircons. These differences in trace element abundances and isotope systematics in distorted zircons relative to undistorted zircons are generally interpreted to have been facilitated by subgrain walls. Trace elements and isotopes would have moved from undistorted lattice into these subgrain walls as their chemical potential is modified due to the presence of the dislocations which make up the subgrain wall. Subgrain walls provided pathways for chemical exchange between crystal and surroundings. Only five per cent of zircons in this population have lattice distortion suggesting it will not have a major impact on zircon geochronology studies, particularly as three of the five distorted zircons are from strongly deformed rocks not normally sampled in such studies. However, this does suggest there may be a case for EBSD analysis of zircons prior to geochemical analysis when zircons from highly deformed rocks are to be investigated. © 2013 Springer-Verlag Berlin Heidelberg.
Kanjanapayont P.,Chulalongkorn University |
Grasemann B.,University of Vienna |
Edwards M.A.,Grant Institute |
Fritz H.,University of Graz
Journal of Structural Geology | Year: 2012
The NNE trending Khlong Marui shear zone has a strong geomorphic signal with marked fault-strike parallel topographic ridges. The lithologies within the strike-slip zone mainly consist of vertical layers of mylonitic meta-sedimentary rocks associated with orthogneisses, mylonitic granites, and pegmatitic veins. The pegmatitic veins concordantly intrude the mylonitic foliation but were sheared at the rims indicating syn-kinematic emplacement. Microstructures and mineral assemblages suggest that the rocks in the area have been metamorphosed at amphibolite facies and low to medium greenschist facies by the first deformation. The Khlong Marui shear zone was deformed under dextral simple shear flow with a small finite strain. The ductile-to-brittle deformation involves a period of exhumation of lenses of higher grade rocks together with low grade fault rocks probably associated with positive flower structures. The final stage brittle deformation is reflected by normal faulting and formation of proto-cataclasites to cataclasites of the original mylonitic meta-sedimentary host rock. Although clear age-constraints are still missing, we use regional relationships to speculate that earlier dextral strike-slip displacement of the Khlong Marui shear zone was related to the West Burma and Shan-Thai collision and subduction along the Sunda Trench in the Late Cretaceous, while the major exhumation period of the ductile lens was tectonically influenced by the early India-Asia collision. The changing stress field has responded by switching from dextral strike-slip to normal faulting in the Khlong Marui shear zone, and is associated with "escape tectonics" arising from the overall India-Asia collision. © 2011 Elsevier Ltd.
Kanjanapayont P.,Chulalongkorn University |
Klotzli U.,University of Vienna |
Thoni M.,University of Vienna |
Grasemann B.,University of Vienna |
Edwards M.A.,Grant Institute
Journal of Asian Earth Sciences | Year: 2012
In southern Thailand, the Khlong Marui shear zone is dominated by a NNE-SSW striking high topographic lozenge shaped area of ca. 40. km long and 6. km wide between the Khlong Marui Fault and the Bang Kram Fault. The geology within this strike-slip zone consists of strongly deformed layers of mylonitic meta-sedimentary rocks associated with orthogneisses, mylonitic granites, and pegmatitic veins with a steeply dipping foliation. The strike-slip deformation is characterized by dextral ductile deformation under amphibolite facies and low to medium greenschist facies. In situ U-Pb ages of inherited zircon cores from all zircons in the Khlong Marui shear zone indicate that they have the same material from the Archean. Late Triassic to Late Cretaceous ages obtained for zircon outer cores of the mylonitic granite are probably related to a period of magmatic activity that was significantly influenced by the West Burma and Shan-Thai collision and the subduction along the Sunda Trench. The early dextral ductile deformation phase of the Khlong Marui shear zone in the Early Eocene suggested by U-Pb ages of zircon rims, and the later dextral transpressional deformation in the Late Eocene indicated by mica Rb-Sr ages. Rb-Sr, Sm-Nd, and U-Pb dating correlation implies that the major exhumation period of the ductile lens was in the Eocene. This period was tectonically influenced in the SE Asia region by the early India-Asia collision. © 2012 Elsevier Ltd.
France J.L.,Royal Holloway, University of London |
King M.D.,Royal Holloway, University of London |
MacArthur A.,Grant Institute
Icarus | Year: 2010
Dusty water-ice snowpacks on Mars may provide a habitable zone for DNA based photosynthetic life. Previous work has over estimated the depths and thicknesses of such photohabitable zones by not considering the effect of red dust within the snowpack. For the summer solar solstice, at 80°N and a surface albedo of 0.45, there is a calculated photohabitable zone in the snowpack between depths of 5.5 and 7.5 cm. For an albedo of 0.62, there is a calculated photohabitable zone in the snowpack between depths of 8 and 11 cm. A coupled atmosphere-snow radiative-transfer model was set to model the Photosynthetic Active Radiation and DNA dose rates through water-ice snow at the north polar region of Mars. The optical properties of the polar caps were determined by creating a laboratory analogue to the Mars north polar deposits, and directly measuring light penetration and albedo. It is important for future exobiology missions to the polar regions of Mars to consider the implications of these findings, as drilling to depths of ∼11 cm should be sufficient to determine whether life exists within the martian snows, whether it is photosynthetic or otherwise, as at this depth the snow cover will provide a permanent protection from DNA damaging UV radiation. © 2009 Elsevier Inc. All rights reserved.
Bond A.E.,Quintessa Ltd. |
Benbow S.,Quintessa Ltd. |
Wilson J.,Quintessa Ltd. |
Mcdermott C.,Grant Institute |
English M.,Grant Institute
Mineralogical Magazine | Year: 2012
The Nuclear Decommissioning Authority Radioactive Waste Management Directorate have been participating in the current DECOVALEX-2011 project (development of coupled models and their validation against experiments) one task of which has been examining the Mont Terri Ventilation Experiment (VE). This long-term (>9 years), field-scale experiment in the Opalinus Clay near the Swiss-French border, was designed to examine the coupled hydraulic - mechanical - chemical changes caused in the tunnel and in the surrounding geology, by the controlled ventilation of a 1.65 m diameter micro-tunnel. In contrast to many conventional benchmarking and validation exercises, a key aspect of the VE as examined in DECOVALEX was that some data were held back and participants were required to make predictions of key metrics for the future evolution of the system. This paper presents an overview of the work conducted by the Quintessa and University of Edinburgh team including selected results. The coupled models developed include multiphase flow, elastic deformation and chemical processes in both detailed and upscaled geometries. The models have been able to replicate the observed desaturation around the tunnel, tunnel deformation and localized failure, vapour migration in the tunnel, and the transition in redox conditions into the host rock. © 2012 The Mineralogical Society.
Haszeldine R.S.,Grant Institute |
Scott V.,Grant Institute
Issues in Environmental Science and Technology | Year: 2014
To re-establish global climate balance, it is necessary to remove large amounts of fossil carbon emitted by humans, which is currently located in the atmosphere and the upper ocean. Although great attention is given to technologies of capture, the ability to store immense tonnages of carbon stock for geologically long time periods, isolated from atmosphere and ocean interaction, is equally important. In this chapter, the multiple storage locations for carbon stocking on and below land, also within and below the ocean, are evaluated. The evaluation shows that carbon dioxide reduction (CDR) is useful for mitigation, but cannot balance the rate of new emissions from fossil fuel exploitation. Many CDR methods have large uncertainty in their quantity, life-cycle, global impact and engineered feasibility. Competition for biomass and land usage is inevitable. Pathways and reservoirs of carbon in the ocean are complex and interlocked. Engineered storage of carbon will also be expensive, resource intensive and cannot substitute for a greatly reduced usage of fossil carbon. Human industrial and economic activity must "move beyond hydrocarbons" to be sustainable beyond 2050. © The Royal Society of Chemistry 2014.
Ufkes E.,VU University Amsterdam |
Kroon D.,Grant Institute
Palaeontology | Year: 2012
The last one million years are important in terms of climate development during the so-called Mid-Pleistocene Transition when amplification of the glacial-interglacial cycles occurred. This study describes abundance changes in fossil planktonic foraminifera in sediments from Core T89-40, retrieved from the Walvis Ridge in the south-east Atlantic, across this time period. Cycles between upwelling and subtropical planktonic foraminiferal assemblages are shown to mirror changes between glacial and interglacial periods, respectively. During interglacial marine isotopic stages (MIS) 9, 11 and 31, however, anomalously high abundances of the polar left-coiled Neogloboquadrina pachyderma occur, presumably linked to unusual seasonal upwelling waters. The planktonic foraminiferal abundance record shows 41-ky cyclic variations in the regional oceanography linked to cycles in insolation influenced by changes in the Earth's axial tilt (obliquity). These orbitally induced oscillations in oceanographic change occurred throughout the entire record. The most conspicuous feature of the planktonic foraminiferal record is the near absence of left-coiled Globorotalia truncatulinoides between 960 and 610ka (MIS 26-15). The abrupt disappearance of this species is synchronous with the onset of the Mid-Pleistocene Transition in MIS 26. © The Palaeontological Association.