Preservation of high‐P rocks coupled to rock composition and the absence of metamorphic fluids     

Alasdair Skelton  Alexandre Peillod  Johannes Glodny  Iwona Klonowska  Carolina Mnbro  Karin Lodin  Uwe Ring 《Journal of Metamorphic Geology》2019,37(3):359-381

Eclogites, blueschists and greenschists are found in close proximity to one another along a 1‐km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2–1.9 GPa and 410–530°C followed, at 43–38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–2.1 GPa and 520–580°C. We explain co‐existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10–100‐m and veins on a scale of 1–10‐cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high‐P nappe stack consisting of belts in which high‐P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension.  相似文献   

Experimental study of a biotite-bearing granitic system under water-saturated and water-undersaturated conditions   总被引：6，自引：0，他引：6  

Jacek Puziewicz  Wilhelm Johannes 《Contributions to Mineralogy and Petrology》1990,104(4):397-406

Phase relationships in haplogranitic melts containing ferromagnesian minerals were investigated in the temperature range 760–850°C, atP _total=2 kbar, =0.25 1.00, and at NNO buffer conditions. The compositions of coexisting biotite, orthopyroxene, magnetite and granitic melt were determined by electron microprobe. Biotite was the only Fe–Mg mineral in the starting material. It disappeared in water-undersaturated melts at temperatures above 830°C and in melts saturated with water above 810°C. Magnetite was present in all runs. Orthopyroxene became stable between 780 and 800°C. The Mg/(Mg+Fe) ratios of biotite (0,63–0,72) and orthopyroxene (0,63–0,71) are similar in water-saturated and undersaturated melts at comparable temperatures and increase slightly with increasing temperature. The solubility of Mg in haplogranitic melts is very low (0,07–0,24 wt% MgO). It increases slightly with temperature and is independent of the prevailing water activity. The solubility of Fe is low (0,91–1,37 wt% FeO); it also increases with increasing temperature, and it is higher in water-saturated than in water-undersaturated melts.  相似文献   

Co-use of existing scenario sets to extend and quantify the shared socioeconomic pathways     

Guillaume Rohat  Johannes Flacke  Hy Dao  Martin van Maarseveen 《Climatic change》2018,151(3-4):619-636

More often than not, assessments of future climate risks are based on future climatic conditions superimposed on current socioeconomic conditions only. The new IPCC-guided alternative global development trends, the shared socioeconomic pathways (SSPs), have the potential to enhance the integration of future socioeconomic conditions—in the form of socioeconomic scenarios—within assessments of future climate risks. Being global development pathways, the SSPs lack regional and sectoral details. To increase their suitability in sectoral and/or regional studies and their relevance for local stakeholders, the SSPs have to be extended. We propose here a new method to extend the SSPs that makes use of existing scenario studies, the (re)use of which has been underestimated so far. Our approach lies in a systematic matching of multiple scenario sets that facilitates enrichment of the global SSPs with regional and sectoral information, in terms of both storylines and quantitative projections. We apply this method to develop extended SSPs of human vulnerability in Europe and to quantify them for a number of key indicators at the sub-national level up to 2050, based on the co-use of the matched scenarios’ quantitative outputs. Results show that such a method leads to internally consistent extended SSPs with detailed and highly quantified narratives that are tightly linked to global contexts. This method also provides multiple entry points where the relevance of scenarios to local stakeholders can be tested and strengthened. The extended SSPs can be readily employed to explore future populations’ vulnerability to climate hazards under varying levels of socioeconomic development.  相似文献   

Land restoration in food security programmes: synergies with climate change mitigation     

Dominic Woolf  Dawit Solomon  Johannes Lehmann 《Climate Policy》2018,18(10):1260-1270

Food-insecure households in many countries depend on international aid to alleviate acute shocks and chronic shortages. Some food security programmes (including Ethiopia’s Productive Safety Net Program–PSNP – which provides a case study for this article) have integrated aid in exchange for labour on public works to reduce long-term dependence by investing in the productive capacity and resilience of communities. Using this approach, Ethiopia has embarked upon an ambitious national programme of land restoration and sustainable land management. Although the intent was to reduce poverty, here we show that an unintended co-benefit is the climate-change mitigation from reduced greenhouse gas (GHG) emissions and increased landscape carbon stocks. The article first shows that the total reduction in net GHG emissions from PSNP’s land management at the national scale is estimated at 3.4 million?Mg?CO₂e?y^?1 – approximately 1.5% of the emissions reductions in Ethiopia’s Nationally Determined Contribution for the Paris Agreement. The article then explores some of the opportunities and constraints to scaling up of this impact.

Key policy insights

• Food security programmes (FSPs) can contribute to climate change mitigation by creating a vehicle for investment in land and ecosystem restoration.

• Maximizing mitigation, while enhancing but not compromising food security, requires that climate projections, and mitigation and adaptation responses should be mainstreamed into planning and implementation of FSPs at all levels.

• Cross-cutting oversight is required to integrate land restoration, climate policy, food security and disaster risk management into a coherent policy framework.

• Institutional barriers to optimal implementation should be addressed, such as incentive mechanisms that reward effort rather than results, and lack of centralized monitoring and evaluation of impacts on the physical environment.

• Project implementation can often be improved by adopting best management practices, such as using productive living livestock barriers where possible, and increasing the integration of agroforestry and non-timber forest products into landscape regeneration.

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Diffusion versus recrystallization processes in Rb-Sr geochronology: Isotopic relics in eclogite facies rocks, Western Gneiss Region, Norway   总被引：1，自引：0，他引：1  

Johannes Glodny  Alexander Kühn  Håkon Austrheim 《Geochimica et cosmochimica acta》2008,72(2):506-525

Rb-Sr and U-Pb isotopic data for granulite facies rocks, forming textural relics with respect to eclogite facies metamorphism in the Western Gneiss Region (WGR) of Norway, highlight the importance of textures and mineral reaction kinetics for the interpretation of geochronological data. Studied rocks from Bårdsholmen, southern WGR, were subjected to granulite facies metamorphism at 955 ± 3 Ma (U-Pb, zircon). Later on, they experienced a subduction-related, kinetically stranded eclogitization (T > 650 °C at ∼20 kbar) at 404 ± 2 Ma (Rb-Sr multimineral internal isochron data), followed by exhumation through amphibolite facies conditions. Full conversion of granulite to eclogite was restricted to zones of fluid infiltration and deformation. Despite the fact that metamorphic temperatures vastly exceeded the commonly assumed ‘closure temperature’ for Rb-Sr in submillimeter-sized biotite for several Ma during eclogite facies overprint, Sr-isotopic signatures of relic biotite have not been fully reset. Large biotite crystals nearly record their Sveconorwegian (Grenvillian) crystallization age. Sr signatures of other granulite facies phases (feldspar, pyroxenes, amphibole) remained unchanged, with the exception of apatite. The results imply that isotopic signatures much closer correspond to the P, T conditions of formation recorded by a dated phase and its paragenesis, than to a temperature history. In texturally well-equilibrated high-grade rocks which experienced no mineral reactions and remained devoid of free fluids during exhumation, like granulites or eclogites, isotopic resetting during cooling is either kinetically locked, or restricted to sluggish intermineral diffusion which demonstrably does not lead to full isotopic homogenization. In texturally unequilibrated rocks, textural relics are likely to represent isotopic relics. It is shown that for both high-grade rocks and for rocks with textural disequilibria, geologically meaningful isotopic ages based on isochron methods can only be derived from sub-assemblages in isotopic equilibrium, which have to be defined by analysis of all rock-forming minerals. Conventional two-point ‘mica ages’ for such rocks are a priori geochronologically uninterpretable, and valid multimineral isochron ages a priori do not record cooling but instead date recrystallization-inducing processes like fluid-rock interaction.  相似文献   

Application of ray-traced tropospheric slant delays to geodetic VLBI analysis     

Armin Hofmeister  Johannes Böhm 《Journal of Geodesy》2017,91(8):945-964

The correction of tropospheric influences via so-called path delays is critical for the analysis of observations from space geodetic techniques like the very long baseline interferometry (VLBI). In standard VLBI analysis, the a priori slant path delays are determined using the concept of zenith delays, mapping functions and gradients. The a priori use of ray-traced delays, i.e., tropospheric slant path delays determined with the technique of ray-tracing through the meteorological data of numerical weather models (NWM), serves as an alternative way of correcting the influences of the troposphere on the VLBI observations within the analysis. In the presented research, the application of ray-traced delays to the VLBI analysis of sessions in a time span of 16.5 years is investigated. Ray-traced delays have been determined with program RADIATE (see Hofmeister in Ph.D. thesis, Department of Geodesy and Geophysics, Faculty of Mathematics and Geoinformation, Technische Universität Wien. http://resolver.obvsg.at/urn:nbn:at:at-ubtuw:1-3444, 2016) utilizing meteorological data provided by NWM of the European Centre for Medium-Range Weather Forecasts (ECMWF). In comparison with a standard VLBI analysis, which includes the tropospheric gradient estimation, the application of the ray-traced delays to an analysis, which uses the same parameterization except for the a priori slant path delay handling and the used wet mapping factors for the zenith wet delay (ZWD) estimation, improves the baseline length repeatability (BLR) at 55.9% of the baselines at sub-mm level. If no tropospheric gradients are estimated within the compared analyses, 90.6% of all baselines benefit from the application of the ray-traced delays, which leads to an average improvement of the BLR of 1 mm. The effects of the ray-traced delays on the terrestrial reference frame are also investigated. A separate assessment of the RADIATE ray-traced delays is carried out by comparison to the ray-traced delays from the National Aeronautics and Space Administration Goddard Space Flight Center (NASA GSFC) (Eriksson and MacMillan in http://lacerta.gsfc.nasa.gov/tropodelays, 2016) with respect to the analysis performances in terms of BLR results. If tropospheric gradient estimation is included in the analysis, 51.3% of the baselines benefit from the RADIATE ray-traced delays at sub-mm difference level. If no tropospheric gradients are estimated within the analysis, the RADIATE ray-traced delays deliver a better BLR at 63% of the baselines compared to the NASA GSFC ray-traced delays.  相似文献   

Nocturnal Near-Surface Temperature,but not Flow Dynamics,can be Predicted by Microtopography in a Mid-Range Mountain Valley     

Lena Pfister  Armin Sigmund  Johannes Olesch  Christoph K. Thomas 《Boundary-Layer Meteorology》2017,165(2):333-348

We investigate nocturnal flow dynamics and temperature behaviour near the surface of a 170-m long gentle slope in a mid-range mountain valley. In contrast to many existing studies focusing on locations with significant topographic variations, gentle slopes cover a greater spatial extent of the Earth’s surface. Air temperatures were measured using the high-resolution distributed-temperature-sensing method within a two-dimensional fibre-optic array in the lowest metre above the surface. The main objectives are to characterize the spatio-temporal patterns in the near-surface temperature and flow dynamics, and quantify their responses to the microtopography and land cover. For the duration of the experiment, including even clear-sky nights with weak winds and strong radiative forcing, the classical cold-air drainage predicted by theory could not be detected. In contrast, we show that the airflow for the two dominant flow modes originates non-locally. The most abundant flow mode is characterized by vertically-decoupled layers featuring a near-surface flow perpendicular to the slope and strong stable stratification, which contradicts the expectation of a gravity-driven downslope flow of locally produced cold air. Differences in microtopography and land cover clearly affect spatio-temporal temperature perturbations. The second most abundant flow mode is characterized by strong mixing, leading to vertical coupling with airflow directed down the local slope. Here variations of microtopography and land cover lead to negligible near-surface temperature perturbations. We conclude that spatio-temporal temperature perturbations, but not flow dynamics, can be predicted by microtopography, which complicates the prediction of advective-heat components and the existence and dynamics of cold-air pools in gently sloped terrain in the absence of observations.  相似文献   

Multi-Scale and Multi-Resolution Stochastic Modeling of Subsurface Heterogeneity by Tree-Indexed Markov Chains     

Michel Dekking  Amro Elfeki  Cor Kraaikamp  Johannes Bruining 《Computational Geosciences》2001,5(1):47-60

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Beginning of melting and composition of first melts in the system Qz-Ab-Or-H2O-CO2   总被引：1，自引：1，他引：1  

A. Ebadi  W. Johannes 《Contributions to Mineralogy and Petrology》1991,106(3):286-295

Solidus temperatures have been determined for minimum melt compositions in the system Qz(SiO₂)-Ab(NaAlSi₃O₈)-Or(KAlSi₃O₈) at P(fluid)=2,5 and 10 kbar and at various water activities. The dry solidus was investigated in a dry argon environment. Water activities (aH₂O) between 0.0 and 1.0 were obtained by using H₂O-CO₂ mixtures. The Or/Ab+Or ratio of first melts increases considerably with decreasing water activity. At 10 kbar it is 0.28 in the water-saturated system and 0.56 at water activity 0.1. The Qz-content does not change with changing water activities. The Ab-content of minimum melts formed at high pressures and low aH₂O may remain almost constant in ascending magmas that are cooling and crystallizing. Qz-content increases at the expense of the Or-component. Solidus temperatures decrease considerably when aH₂O increases slightly from zero. At 10 kbar, the temperature difference between dry melting and the solidus for aH₂O=0.1 is 120°C. The influence of pure CO₂ on the solidus is very limited in the investigated P-T range. The solidus is approximatively 760°C at aH₂O=0.5 between 2 and 10 kbar and approximatively 830°C at aH₂O=0.3. This means that melting of quartz-feldspar assemblages may induce dehydration reactions at P-T conditions of the granulite facies.  相似文献   

AN EMPIRICAL STUDY OF THE ELECTRON TEMPERATURE AND HEAVY ION VELOCITIES IN THE SOUTH POLAR CORONAL HOLE     

Ko  Yuan-Kuen  Fisk  Lennard A.  Geiss  Johannes  Gloeckler  George  Guhathakurta  Madhulika 《Solar physics》1997,171(2):345-361

The solar wind ions flowing outward through the solar corona generally have their ionic fractions freeze-in within 5 solar radii. The altitude where the freeze-in occurs depends on the competition between two time scales: the time over which the wind flows through a density scale height, and the time over which the ions achieve ionization equilibrium. Therefore, electron temperature, electron density, and the velocity of the ions are the three main physical quantities which determine the freeze-in process, and thus the solar wind ionic charge states. These physical quantities are determined by the heating and acceleration of the solar wind, as well as the geometry of the expansion. In this work, we present a parametric study of the electron temperature profile and velocities of the heavy ions in the inner solar corona. We use the ionic charge composition data observed by the SWICS experiment on Ulysses during the south polar pass to derive empirically the electron temperature profile in the south polar coronal hole. We find that the electron temperature profile in the solar inner corona is well constrained by the solar wind charge composition data. The data also indicate that the electron temperature profile must have a maximum within 2 solar radii. We also find that the velocities of heavy ions in their freeze-in regions are small (<100 km s^-1) and different elements must flow at different velocities in the inner corona.  相似文献