Ion migration in nepheline: a dielectric spectroscopy and computer modelling study

 Using a combination of dielectric spectroscopy and atomistic computer simulation techniques, the dynamical behaviour of the loosely bound (Na⁺ and K⁺) channel ions in nepheline has been investigated. The low-frequency dielectric properties of a natural Bancroft nepheline have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation in the dielectric loss spectrum was observed; the activation energy for this process was determined to be 1.38 ± 0.02 eV. Atomistic simulation techniques were used to elucidate the mechanism and energetics of cation migration. A mechanism involving the hopping of Na⁺ ions between oval sites and partially occupied hexagonal (K⁺) sites, via a bottleneck consisting of a distorted sixfold ring of (Al,Si)O₄ tetrahedra, was found to give a calculated energy barrier in very good agreement with the experimentally determined activation energy. These results confirm the nature of the process responsible for the observed dielectric behaviour. Overall, this study demonstrates the intrinsic, microscopic control of cation diffusion processes in rock-forming minerals. Identifying specific energy barriers and preferred diffusion pathways is fundamental to the prediction of diffusion energetics. Received: 8 May 2000 / Accepted: 21 July 2000     

Long-term deformation and fluid-enhanced mass transport in a Variscan peridotite shear zone in the Ivrea Zone,northern Italy: a microtextural,petrological and geochemical study of a reactivated shear zone

The microtextural, petrological and geochemical study of a ductile shear zone in the phlogopite peridotite of Finero/Ivrea Zone (northern Italy) reveals the long-term deformation of this zone. The zone is divided into a protomylonitic and an ultramylonitic part. Both parts reflect different periods of deformation, although the orientation of the mineral lineations does not change. In the coarse-grained part (period 1) the deformation started under granulite facies conditions (about 775°C). Olivine, ortho- and clinopyroxene and phlogopite recrystallized dynamically. In the ultramylonitic part relics of the granulite facies event and evidence for a continuous or two-stage deformation history under amphibolite facies (minimum 640°C) to upper greenschist facies conditions (maximum 520°C) are preserved (period 2). Amphibolite facies conditions are indicated by olivine recrystallization, the monoclinization of orthopyroxene porphyroclasts and the recrystallization and chemical changes of clinopyroxene. The greenschist facies final stage of period 2 is characterized by decreased X _{CO 2} and the syntectonic formation of antigorite, tremolite and phlogopite at the expense of recrystallized and porphyroclastic olivine and pyroxene. Between both deformation periods a short break in deformation continuity is probable. Continuous deformation or reactivation in shear zones of the Ivrea Zone has not been described so far. During the granulite facies shearing, small amounts of channelized fluid flow led to a slight mass transfer. The shear zone shows a moderate enrichment of Na, Ba, Cu, Cs, H₂O and CO₂ and a small loss of P. The mass balance of the ultramylonite indicates a significant increase in mass transport. A mass gain can be inferred for H, Na, K, Ba, Al, Ti, P, S, Cs, Sr, Rb, C, Zn, Zr, S, Sc, light rare earth elements, Nb, Cl and Au. The zone is depleted in Ca, Cu, Co, F and Ni. Si, Mg, Cr, Mn, Y, Nb and V are constant within analytical error. Deformation and fluid infiltration led to a change in volume which increases during the granulite facies event by 5.7% and during the lower temperature phase by 3.3%. The calculated fluid to rock ratios by standard equations results in unrealistically high values. For the interpretation of highly deformed rocks with drastic grain size reduction it is therefore necessary to consider the enhanced diffusion, which is mainly controlled by the increased grain boundary surface.     

Extracting commuter-specific destination hotspots from trip destination data – comparing the boro taxi service with Citi Bike in NYC

ABSTRACT

Taxi trajectories from urban environments allow inferring various information about the transport service qualities and commuter dynamics. It is possible to associate starting and end points of taxi trips with requirements of individual groups of people and even social inequalities. Previous research shows that due to service restrictions, boro taxis have typical customer destination locations on selected Saturdays: many drop-off clusters appear near the restricted zone, where it is not allowed to pick up customers and only few drop-off clusters appear at complicated crossing. Detected crossings imply recent infrastructural modifications. We want to follow up on these results and add one additional group of commuters: Citi Bike users. For selected Saturdays in June 2015, we want to compare the destinations of boro taxi and Citi Bike users. This is challenging due to manifold differences between active mobility and motorized road users, and, due to the fact that station-based bike sharing services are restricted to stations. Start and end points of trips, as well as the volumes in between rely on specific numbers of bike sharing stations. Therefore, we introduce a novel spatiotemporal assigning procedure for areas of influence around static bike sharing stations for extending available computational methods.     

Grain boundary migration of water and carbon dioxide during uplift of garnet-zone Alpine Schist, New Zealand

Abstract Deformed quartz veins in garnet-zone schist adjacent to the active Alpine Fault, New Zealand, have fluid inclusions trapped along quartz grain boundaries. Textures suggest that the inclusions formed in their present shapes during annealing of the deformed veins. Many of the inclusions are empty, but some contain carbon dioxide with densities that range from 0.16 to 0.80 g cm⁻³. No water, nitrogen or methane was detected. The inclusions are considerably more CO₂-rich than either the primary metamorphic fluid (<5% CO₂) or fluids trapped in fracture-related situations in the same, or related, rocks (<50% CO₂). Enrichment of CO₂ is inferred to have resulted from selective migration (wicking) of saline water from the inclusions along water-wet grain boundaries after cooling-induced immiscibility of a water-CO₂ mixture. Inclusion volumes changed after loss of water. Non-wetting CO₂ remained trapped in the inclusions until further percolation progressively removed CO₂ in solution. This mechanism of fluid migration dominated in ductile quartz-rich rocks near, but below, the brittle-ductile transition. At deeper levels, hydraulic fracturing is also an important mechanism for fluid migration, whereas at shallower levels advection through open fractures dominates the fluid flow regime.     

Arrested charnockite formation at Kottavattam, southern India

Abstract At Kottavattam, southern Kerala (India), late Proterozoic homogeneous leptynitic garnet–biotite gneisses of granitic composition have been transformed on a decimetric scale into coarse-grained massive charnockite sensu stricto along a set of conjugate fractures transecting the gneissic foliation. Charnockitization post-dates the polyphase deformation, regional high-grade metamorphism and anatexis, and evidently occurred at a late stage of the Pan-African tectonothermal history. Geothermobarometric and fluid inclusion data document textural and chemical equilibration of the gneiss and charnockite assemblages at similar P_lith–T conditions (650–700°C, 5–6 kbar) in the presence of carbonic fluids internally buffered by reaction with graphite and opaque mineral phases (X_CO2= 0.7–0.6; X_H2O= 0.2–0.3; X_N2= 0.1; log f_O2= -17.5). Mineralogical zonation indicates that charnockitization of the leptynitic gneiss involved first the breakdown of biotite and oxidation of graphite in narrow, outward-migrating transition zones adjacent to the gneiss, followed by the breakdown of garnet and the neoblastesis of hypersthene in the central charnockite zone. Compared to the host gneiss, the charnockite shows higher concentrations of K, Na, Sr, Ba and Zn and lower concentrations of Mg, Fe, Ti, V, Y, Zr and the HREE, with a complementary pattern in the narrow transition zones of biotite breakdown. The P_lith–T–X_H2O data and chemical zonation patterns indicate charnockitization through subsolidus-dehydration reaction in an open system. Subsequent residence of the carbonic fluids in the charnockite resulted in low-grade alteration causing modification of the syn-charnockitic elemental distribution patterns and the properties of entrapped fluids. We favour an internally controlled process of arrested charnockitization in which, during near-isothermal uplift, the release of carbonic fluids from decrepitating inclusions in the host gneiss into simultaneously developing fracture zones led to a change in the fluid regime from ‘fluid-absent’in the gneiss to ‘fluid-present’in the fracture zones and to the development of an initial fluid-pressure gradient, triggering the dehydration reaction.     

Alkali ion migration in albite and K-feldspar

As part of a study of ion migration mechanisms in feldspars, the dynamical behaviour of the alkali metal cations ions in albite and K-feldspar has been investigated using a combination of dielectric spectroscopy and atomistic computer simulation techniques. The low-frequency dielectric properties of these minerals have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation in the dielectric loss spectra was observed for both albite and K-feldspar; the activation energy for these processes was determined to be 1.33 eV in both albite and K–feldspar. Atomistic simulation techniques were used to elucidate the mechanism and energetics of the cation migration processes. Mechanisms involving the conventional hopping of Na⁺ and K⁺ ions between cation sites in the (010) plane were found to give calculated energy barriers in good agreement with the experimentally determined activation energies. These results assist in understanding the nature of the processes responsible for the observed dielectric behaviour.     

Challenges of calculating dunefield mobility over the 21st century

Attention has been directed towards both the impacts of future climate change on environmental systems and dunefield activity in the past, but there has been relatively little consideration of potential dune mobility in a future and possibly warmer world. This paper considers the use and limitations of four Global Circulation Models (GCMs) (Hadcm3, Hadcm2, CSIRO-mk2b and CGCM1), in combination with simple dune mobility indices to predict the activity of the Kalahari dunefield. It is clear that uncertainties surround GCM resolution and accuracy, mobility index robustness for the calculation of intra-annual dune mobility and data collection for mobility index calibration. Macro-scale studies that look at large areas of the world over long time scales are well suited to GCM and mobility index use, but dune mobility can be variable within a dunefield, and it is the extreme sand transporting events, occurring at high temporal resolutions, that are the most important for short term studies. To investigate intra-annual changes in dune mobility over a specific dunefield techniques such as downscaling, weather generators and probability curve fitting can help provide climate predictions for smaller areas over shorter time frames. However, these methods introduce uncertainty of their own, and they often rely on the accuracy of original GCM predictions or the climate parameter relationships observed at present. Analysis of intra-annual changes also requires mobility indices that can model monthly mobility patterns well, although existing indices have only been used for calculating annual dune mobility potential. When they are used for intra-annual predictions, the lack of lag response between precipitation decreases and the assumed vegetation dieback leads to an exaggerated amplitude pattern of dune mobility throughout the year. Calibration of dune mobility indices to dune mobility observed on the ground is therefore important but is hampered by a lack of observed measurements for individual months. Solutions are available to overcome some of the outlined problems, but they can provide their own set of uncertainties, which combine to further reduce the confidence given to future dune mobility predictions.     

Mobility of a remobilised parabolic dune in Kennemerland, The Netherlands

A parabolic dune in the Netherlands was remobilised in December 1998 by removing vegetation and soil. The main aim of the experiment was ecological: to investigate whether permanent rejuvenation at the landscape scale is possible by restoration of natural processes. If processes can be reactivated at coarse scale, periodic rejuvenation of the landscape over the long term is possible, without the need for managers to interfere further. The experiment provides the opportunity to address another important question: can large parabolic dunes in the Netherlands be mobile in the present climate? Mobility of the dune is investigated by means of erosion pins, aerial photography and measurement of cross sections. Activity indices are derived from erosion pin recordings and correlated to weather conditions. From 1999 to 2001, displacement of the dune ranged from 0 to 12 m in east–northeasterly direction. Activity of the dune is related to wind conditions, but the relationship is strongly influenced by precipitation and therefore differs for wet and dry periods. Periods with extreme wind speeds resulted in much less geomorphic change than expected.     

首尔都市区的人口迁居及对中国的启示

中国目前已进入快速的工业化和城市化时期,人口迁居在我国大都市区显得越来越重要。韩国在1960年代开始大量的乡村人口迁移到城市地区,进入1970年代,出现了郊区化现象。首先分析了韩国人口迁居的一般特征,接着从就业机会、住房供给和区域政策三方面分析了影响韩国人口迁居的因素,最后针对我国的具体实际,结合韩国的研究提出了几点建议。     

地电化学提取法的理论与实验研究

根据离子溶液电化学的基本性质和稳定电流场的基本理论,通过理论计算和物理模拟实验,指出：离子在外电场作用下,由所在地点迁移至接收电极所需时间与多种因素有关,当外界条件一定时,加大供电电流强度可减少接收时间;离子淌度值很小,由实测时量曲线计算出的ｕｉ值,一般不超过几个ｃｍ２·ｈ－１·Ｖ－１单位,因此欲提取深部离子,即使在最有利条件下也是非常困难的;由不同供电电流强度测出的时量曲线,计算得到的淌度值是相等的,因此改变供电电流强度并不能改变离子淌度值,但当围岩介质的颗粒度较大时,可使离子淌度值变大;离子的迁移质量与供电电量Ｑ＝Ｉ·ｔ成正比,因此在供电电流强度不变的条件下,采用间断供电方式可取得与连续供电相同的结果;通过浅部已存在的离子晕,用电提取法可以发现深部隐伏矿体。