"Welcome in my backyard"…but on my terms: making sense of homeless exclusion from renewed urban spaces in Copenhagen

Based on a Danish case study of urban renewal in Copenhagen’s Sundholm District, this paper examines, (a) how present urban regeneration efforts in a historically welfare-driven, but increasingly neoliberalized state context, is contributing to more or less spatial exclusion of the homeless, and (b) to what extent this may be associated with a revanchist/punitive stance of the Danish state. Using an urban political ecological lens, this paper highlights a relational understanding of the apparently dualistic/competing public and civic discourses shaping the Danish urban regeneration program. Revealing the complex ways that public socio-environmental policies and middle-class civic environmentalism/activism intersect with state-entrepreneurialism within such regeneration efforts, this paper presents an instance of a historically and geographically distinct process of neoliberal disciplining of the poor in Sundholm District. The paper shows that while such disciplining of the homeless is not driven by a purely punitive state, it results in soft, green-coded, nonetheless exclusionary implications for the homeless and their socio-spatial practices within the renewed urban spaces.     

Gold Dissolution in Dry Salt Melts in the Presence of SiO<Subscript>2</Subscript> as a Function of Р(O<Subscript>2</Subscript>)

The solubility of gold was measured in dry NaCl salt melt at 860°С in closed systems with SiO₂ (silica glass). The reactions do not occur in a closed system without oxidizer. Reaction of SiO₂ with salt in the presence of an oxidizer (KClO₄) results in the formation of water-soluble sodium silicates (a mixture of meta-, ortho-, and pyrosilicates). Gold mobilization by a salt melt is limited by the diffusion of Na in SiO₂. In a closed system with the addition of a strong oxidizer (dry KClO₄ salt), the solubility of gold increase with increasing amount of KClO₄ and the saturation level is estimated to be ~3 wt % Au. For ampoule configurations used in our experiments, 5.5 g of gold dissolved per 1 g of KClO₄. Only cheap, non-toxic reagents were used in our model experiments on gold dissolution in a salt melt, which did not require elevated pressures. The solubility of 30 g Au per 1 kg NaCl will eliminate geochemical problems associated with the compact leaching of gold ores using cyanide.     

Phase equilibria in the MgO–SiO<Subscript>2</Subscript>–ZrO<Subscript>2</Subscript> system

The results of study of phase equilibria in the MgO–SiO₂–ZrO₂ system at 1450–1550°C are reported. The studied system contains two eutectic points and six fields: (I) MgSiO₃ + SiO₂; (II) MgSiO₃ + ZrO₂; (III) ZrSiO₄ + SiO₂; (IV) MgSiO₃ + Mg₂SiO₄; (V) ZrO₂ + MgO; (VI) ZrSiO₄ + ZrO₂. The presence of fields (II) and (III) on the diagram shows that zircon in equilibrium with olivine and pyroxene crystallizes at very low concentrations of ZrO₂ in the system. This provides a solution for one of the most important problems in zirconology of dunites: the probability of the formation and preservation of zircon in the course of the formation and evolution of dunite.     

Mechanisms and rates of quartz dissolution in melts in the CMAS (CaO–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>) system

The dissolution rate of minerals in silicate melts is generally assumed to be a function of the rate of mass transport of the released cations in the solvent. While this appears to be the case in moderately to highly viscous solvents, there is some evidence that the rate-controlling step may be different in very fluid, highly silica undersaturated melts such as basanites. In this study, convection-free experiments using solvent melts with silica activity from 0.185–0.56 and viscosity from 0.03–4.6 Pa s show that the dissolution rate is strongly dependent on the degree of superheating, silica activity and the viscosity of the solvent. Dissolution rates increase with increasing melt temperature and decreasing silica activity and viscosity. Quartz dissolution in melts with viscosity <0.59–1.9 Pa s and silica activity <0.47 is controlled by the rate of interface reaction as shown by the absence of steady state composition and silica saturation in the interface melts. Only in the most viscous melt with the highest silica activity is quartz dissolution controlled by the rate of diffusion in the melt and only after a long initiation time. The results of this study indicate that although a diffusion-based model may be applicable to dissolution in viscous magmas, a different approach that combines the interplay between the degree of undersaturation of the melt and its viscosity is required in very fluid melts.This revised version was published online September 2004 with a correction to Figure 8.     

Seasonal variations in sulfur isotopic composition of dissolved SO<Subscript>4</Subscript><Superscript>2−</Superscript> in the Aha Lake,Guiyang and their implications

The Aha Lake is a seasonal anoxic water system in the southwest of Guiyang City, Guizhou Province, China. Seasonal variations in SO42- concentrations and their isotopic compositions in lake water as well as in the tributaries were investigated in this study. The results showed that sulfate concentrations in river water range from 0.94 to 6.52 mmol/L and their δ34S values range from -14.9‰ and 0.9‰, while lake water has sulfate concentrations ranging from 1.91 to 2.79 mmol/L, and δ34S values from -9.8‰ to -5.9‰. It is suggested that coal mining drainage is the major source of SO42- in the Aha Lake. Rainfall, sewage discharge, sulfide oxidation and gypsum dissolution have made only limited contributions. Different depth-dependent distributions of dissolved SO42- and δ34S were de-veloped for both DB and LJK in summer and winter. Due to water overturn, δ34S values display homogenous vertical distributions in winter and spring. While in summer and autumn, significant positive shifts of δ34S were clearly ob-served in epilimnion and bottom strata as a result of water stratification. High δ34S values in epilimnion may result from the retention of rainwater during water stratification. Dissimilatory sulfate reduction by bacteria was thought to be responsible for the increase of δ34S value in hypolimnion.     

Titanium solubility in olivine in the system TiO<Subscript>2</Subscript>–MgO–SiO<Subscript>2</Subscript>: no evidence for an ultra-deep origin of Ti-bearing olivine

The finding of ilmenite rods in olivine from orogenic peridotites has sparked a discussion about the processes of incorporation and exsolution of titanium in olivine. We have experimentally investigated the solubility of Ti in olivine as a function of composition, temperature and pressure in the synthetic TiO₂–MgO–SiO₂ system. Experiments at atmospheric pressure in the temperature range 1,200–1,500°C showed that the highest concentration of TiO₂ is obtained when olivine coexists with spinel (Mg₂TiO₄). The amount of TiO₂ in olivine in the assemblages olivine + spinel + periclase and olivine + spinel + ilmenite at 1,500°C was 1.25 wt.%. Changes in the coexisting phases and decreasing temperature result in a significant reduction of the Ti solubility. Olivine coexisting with pseudobrookite (MgTi₂O₅) and a Ti–Si-rich melt at 1,500°C displays a fourfold lower TiO₂ content than when buffered with spinel. A similar decrease in solubility is obtained by a decrease in temperature to 1,200°C. There is a negative correlation between Ti and Si and no correlation between Ti and Mg in Ti-bearing olivine. Together with the established phase relations this suggests that there is a direct substitution of Ti for Si at these temperatures, such that the substituting component has the stoichiometry Mg₂TiO₄. The unit cell volume of olivine increases systematically with increasing TiO₂ content demonstrating that the measured TiO₂ contents in olivine are not caused by micro-inclusions but by incorporation of Ti in the olivine structure. Least squares fitting of 20 olivine unit cell volumes against the Ti content yield the relation: V (ų)=290.12(1) + 23.67(85) N_Ti. The partial molar volume of end-member Mg₂TiO₄ olivine (N_Ti=1) is thus 47.24±0.13 cm³. The change of the Ti solubilty in olivine coexistent with rutile and orthopyroxene with pressure was investigated by piston cylinder experiments at 1,400°C from 15 to 55 kbar. There is no increase in TiO₂ contents with pressure and in all the experiments olivine contains ~0.2 wt.% TiO₂. Moreover, a thermodynamic analysis indicates that Ti contents of olivine coexisting with rutile and orthopyroxene should decrease rather than increase with increasing pressure. These data indicate that the ilmenite exsolution observed in some natural olivine does not signify an ultra-deep origin of peridotite massifs.     

The high-pressure stability of chlorite and other hydrates in subduction mélanges: experiments in the system Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O

The solubility of chromium in chlorite as a function of pressure, temperature, and bulk composition was investigated in the system Cr₂O₃–MgO–Al₂O₃–SiO₂–H₂O, and its effect on phase relations evaluated. Three different compositions with X _Cr = Cr/(Cr + Al) = 0.075, 0.25, and 0.5 respectively, were investigated at 1.5–6.5 GPa, 650–900 °C. Cr-chlorite only occurs in the bulk composition with X _Cr = 0.075; otherwise, spinel and garnet are the major aluminous phases. In the experiments, Cr-chlorite coexists with enstatite up to 3.5 GPa, 800–850 °C, and with forsterite, pyrope, and spinel at higher pressure. At P > 5 GPa other hydrates occur: a Cr-bearing phase-HAPY (Mg_2.2Al_1.5Cr_0.1Si_1.1O₆(OH)₂) is stable in assemblage with pyrope, forsterite, and spinel; Mg-sursassite coexists at 6.0 GPa, 650 °C with forsterite and spinel and a new Cr-bearing phase, named 11.5 Å phase (Mg:Al:Si = 6.3:1.2:2.4) after the first diffraction peak observed in high-resolution X-ray diffraction pattern. Cr affects the stability of chlorite by shifting its breakdown reactions toward higher temperature, but Cr solubility at high pressure is reduced compared with the solubility observed in low-pressure occurrences in hydrothermal environments. Chromium partitions generally according to \(X_{\text{Cr}}^{\text{spinel}}\) ? \(X_{\text{Cr}}^{\text{opx}}\) > \(X_{\text{Cr}}^{\text{chlorite}}\) ≥ \(X_{\text{Cr}}^{\text{HAPY}}\) > \(X_{\text{Cr}}^{\text{garnet}}\). At 5 GPa, 750 °C (bulk with X _Cr = 0.075) equilibrium values are \(X_{\text{Cr}}^{\text{spinel}}\) = 0.27, \(X_{\text{Cr}}^{\text{chlorite}}\) = 0.08, \(X_{\text{Cr}}^{\text{garnet}}\) = 0.05; at 5.4 GPa, 720 °C \(X_{\text{Cr}}^{\text{spinel}}\) = 0.33, \(X_{\text{Cr}}^{\text{HAPY}}\) = 0.06, and \(X_{\text{Cr}}^{\text{garnet}}\) = 0.04; and at 3.5 GPa, 850 °C \(X_{\text{Cr}}^{\text{opx}}\) = 0.12 and \(X_{\text{Cr}}^{\text{chlorite}}\) = 0.07. Results on Cr–Al partitioning between spinel and garnet suggest that at low temperature the spinel- to garnet-peridotite transition has a negative slope of 0.5 GPa/100 °C. The formation of phase-HAPY, in assemblage with garnet and spinel, at pressures above chlorite breakdown, provides a viable mechanism to promote H₂O transport in metasomatized ultramafic mélanges of subduction channels.     

Residence time distribution in a large unconfined–semiconfined aquifer in the Argentine Pampas using <Superscript>3</Superscript>H/<Superscript>3</Superscript>He and CFC tracers

The Pampa region in Argentina includes vast unconfined–semiconfined aquifers that local economies depend upon, but detailed knowledge of the associated water resources is still lacking. The Pampeano aquifer in the Pampa plain of Argentina covers around 1.5 million km². In order to achieve a better understanding of the hydrogeological system through the estimation of mean residence times (MRT), water samples were taken from 12 monitoring wells, drilled at different depths in four locations, and analyzed for environmental tracers. The concentrations of ³H, tritiogenic ³He and chlorofluorocarbons (CFCs) can be explained by mixtures of young waters adjusted to exponential piston flow models (EPM) or dispersion models (DM), and different proportions of tracer-free waters (dead water). The sampling site located very close to the water divide shows a dominance of young waters: 85 % of water best represented by a DM model with a MRT of 3 years. For the shallow wells at other sites, best-fitting models result in a DM with MRT between 20 and 35 years, and proportions of dead water between 40 and 60 %. These results lead to important updates in the conceptual model of the Pampeano aquifer. Large proportions of dead water at a few meters depth can be the consequence of upward flows in a multilayered aquifer or diffusive retardation in the inter-bedded clay layers.     

Influence of oxygen fugacity on the solubility of carbon and hydrogen in FeO-Na<Subscript>2</Subscript>O-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> melts in equilibrium with liquid iron at 1.5 GPa and 1400°C

Equilibria in the model melt (NaAlSi₃O₈(80) + FeO(20))-C-H₂ system were experimentally studied at ΔlogfO₂(IW) from −2.2 to −5.6, a pressure of 1.5 GPa, and a temperature of 1400°C. The experiments were conducted in a piston-cylinder apparatus using Pt capsules. The low fO₂ values were imposed during the experiments by adding 2, 5, and 7 wt % of finely dispersed SiC to NaAlSi₃O₈(80) + FeO(20) powder. The experimental products were investigated by electron microprobe analysis and Raman spectroscopy. The investigations showed that melting at 1.5 GPa and 1400°C in the stability field of a metallic iron phase produces silicate liquids containing both oxidized and reduced H and C species. Carbon and hydrogen are dissolved in the melt as C-H (CH₄) complexes. In addition, OH⁻ groups, molecular hydrogen H₂, and molecular water H₂O were observed in the melts. The proportions of dissolved C and H species strongly depend on oxygen fugacity. With decreasing fO₂, the content of O-H species decreases and that of H-C species increases. The obtained data and previous results (Kadik et al., 2004, 2006) allow us to suppose a fundamental change in the character of magmatic transfer of C-O-H components during the evolution of the redox state of the Earth’s mantle in geologic time toward higher fO₂ in its interiors.     

The formation and recombination kinetics of [Si<Stack><Subscript>4</Subscript><Superscript>5−</Superscript></Stack>] centers in zircon

The radiation sensitivity, time stability and optical sensitivity of [Si₄⁵⁻] paramagnetic centers in natural zircon crystals, particularly those from the Late Neopleistocenic tuff of the Elbrus Volcano, have been studied. Optical bleaching was shown to result in the recombination of [Si₄⁵⁻] centers, while the concentration of centers increases in the absence of light due to the internal radiation background. The data obtained show the infeasibility of using [Si₄⁵⁻] centers in zircons as paleodosimeters in conventional dating methods using electron paramagnetic resonance (EPR) spectroscopy. New specific techniques need to be developed for EPR dating of zircons.     

Structural changes and valence states in the MgCr<Subscript>2</Subscript>O<Subscript>4</Subscript>–FeCr<Subscript>2</Subscript>O<Subscript>4</Subscript> solid solution series

The influence on the structure of Fe²⁺ Mg substitution was studied in synthetic single crystals belonging to the MgCr₂O₄–FeCr₂O₄ series produced by flux growth at 900–1200 °C in controlled atmosphere. Samples were analyzed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption-, infrared- and Mössbauer spectroscopy. The Mössbauer data show that iron occurs almost exclusively as ^IVFe²⁺. Only minor Fe³⁺ (<0.005 apfu) was observed in samples with very low total Fe. Optical absorption spectra show that chromium with few exceptions is present as a trivalent cation at the octahedral site. Additional absorption bands attributable to Cr²⁺ and Cr³⁺ at the tetrahedral site are evident in spectra of end-member magnesiochromite and solid-solution crystals with low ferrous contents. Structural parameters a₀, u and T–O increase with chromite content, while the M–O bond distance remains nearly constant, with an average value equal to 1.995(1) Å corresponding to the Cr³⁺ octahedral bond distance. The ideal trend between cell parameter, T–O bond length and Fe²⁺ content (apfu) is described by the following linear relations: a₀=8.3325(5) + 0.0443(8)Fe²⁺ (Å) and T–O=1.9645(6) + 0.033(1)Fe²⁺ (Å) Consequently, Fe²⁺ and Mg tetrahedral bond lengths are equal to 1.998(1) Å and 1.965(1) Å, respectively.     

Mineral-fluid equilibria in the system CaO–MgO–SiO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O–CO<Subscript>2</Subscript>–NaCl and the record of reactive fluid flow in contact metamorphic aureoles

Phase equilibria in the system CaO–MgO–SiO₂–CO₂–H₂O–NaCl are calculated to illustrate phase relations in metacarbonates over a wide-range of P–T–X[H₂O–CO₂–NaCl] conditions. Calculations are performed using the equation of state of Duan et al. (Geochim Cosmochim Acta 59:2869–2882, 1995) for H₂O–CO₂–NaCl fluids and the internally consistent data set of Gottschalk (Eur J Mineral 9:175–223, 1997) for thermodynamic properties of solids. Results are presented in isothermal-isobarical plots showing stable mineral assemblages as a function of fluid composition. It is shown that in contact-metamorphic P–T regimes the presence of very small concentrations of NaCl in the fluid causes almost all decarbonation reactions to proceed within the two fluid solvus of the H₂O–CO₂–NaCl system. Substantial flow of magma-derived fluids into marbles has been documented for many contact aureoles by shifts in stable isotope geochemistry of the host rocks and by the progress of volatile-producing mineral reactions controlled by fluid compositions. Time-integrated fluid fluxes have been estimated by combining fluid advection/dispersion models with the spatial arrangement of mineral reactions and isotopic resetting. All existing models assume that minerals react in the presence of a single phase H₂O–CO₂ fluid and do not allow for the effect that fluid immiscibility has on the flow patterns. It is shown that fluids emanating from calc-alkaline melts that crystallize at shallow depths are brines. Their salinity may vary depending mainly on pressure and fraction of crystallized melt. Infiltration-driven decarbonation reactions in the host rocks inevitably proceed at the boundaries of the two fluid solvus where the produced CO₂ is immiscible and may separate from the brine as a low salinity, low density H₂O–CO₂ fluid. Most parameters of fluid–rock interaction in contact aureoles that are derived from progress of mineral reactions and stable isotope resetting are probably incorrect because fluid phase separation is disregarded.     

Chemical and isotopic investigations (δ<Superscript>18</Superscript>O, δ<Superscript>2</Superscript>H, <Superscript>3</Superscript>H, <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr) to define groundwater processes occurring in a deep-seated landslide in flysch

Deep-seated landslides are complex systems. In many cases, multidisciplinary studies are necessary to unravel the key hydrological features that can influence their evolution in space and time. The deep-seated Berceto landslide, in the northern Apennines of Italy, has been investigated in order to define the origin and geochemical evolution of groundwater (GW), to identify the slope system hydrological boundary, and to highlight the GW flow paths, transit time and transfer modalities inside the landslide body. This research is based on a multidisciplinary approach that involves monitoring GW levels, obtaining analyses of water chemistry and stable and unstable isotopes (δ¹⁸O-δ²H, ³H, ⁸⁷Sr/⁸⁶Sr), performing soil leaching tests, geochemical modelling (PHREEQC), and principal component analysis (PCA). The results of δ¹⁸O-δ²H and ⁸⁷Sr/⁸⁶Sr analyses show that the source of GW recharge in the Berceto landslide is local rainwater, and external contributions from a local stream can be excluded. In the landslide body, two GW hydrotypes (Ca-HCO₃ and Na-HCO₃) are identified, and the results of PHREEQC and PCA confirm that the chemical features of the GW depend on water–rock interaction processes occurring inside the landslide. The ³H content suggests a recent origin for GW and appears to highlight mixing between shallow and deep GW aliquots. The ³H content and GW levels data confirm that shallow GW is mainly controlled by a mass transfer mechanism. The ³H analyses with GW levels also indicate that only deep GW is controlled by a pressure transfer mechanism, and this mechanism is likely the main influence on the landslide kinematics.     

Organotin compounds in the environment — an overview

Tin has a larger number of its organometallic derivatives in commercial use than any other element. This has given rise to an increase of the worldwide production of organotin compounds during the last 50 years. Due to the wide industrial applications considerable amounts of the organotins have entered various ecosystems. While Sn in its inorganic form is considered to be non-toxic, the toxicological pattern of the organotin compounds is complex. Depending on the nature and the number of the organic groups bound to the Sn cation, some organotins show specific toxic effects to different organsims even at very low concentrations. Therefore, specific determination of the individual organotin compounds is required. In recent years new sensitve analytical techniques have been developed for the detection of organotin compounds in various environmental samples. High amounts of the toxic tributyltin and some other organotin derivatives can be found not only in water and sediments, but also various aquatic organisms and tissues of mammals and birds are contaminated by these compounds. First investigations of human blood and livers show enhanced concentrations for some organotin derivatives. In spite of legislative regulations of usage for organotins in numerous countries, these pollutants represent a risk for aquatic and terrestric ecosystems.     

Single-crystal X-ray diffraction study of cation distribution in MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript>–MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel solid solution

Synthesis experiments in the system MgAl₂O₄–MgFe₂O₄ [MgAl_2–xFe_xO₄ (0 x 2)] were carried out using a PbF₂ flux. The crystalline products synthesized in the compositional range of 0.6 <x 1.2 consisted of two spinel phases, whereas those synthesized in the compositional ranges of 0.0 x 0.6 and 1.2 < x 2.0 crystallized as single spinel phases. Structure refinements of the spinel single crystals, which grew in the ranges of 0.0 x 0.6 and 1.2 < x 2.0, show that the degree of randomness of cation distribution between A and B sites increases as x approaches the two-phase region. This means that the degree of the size mismatch among Mg²⁺, Fe³⁺ and Al³⁺occupying each equivalent mixing site increases as x approaches the two-phase region. Consequently, if the coexistence of two spinels observed in the intermediate compositions reveals the existence of a miscibility gap at low temperatures, this increase in the degree of the size mismatch among the three cations is suggested as a factor of energetic destabilization to form the miscibility gap.     

Phase relations of potassium-bearing clinopyroxene in the system CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>-KAlSi<Subscript>2</Subscript>O<Subscript>6</Subscript> at 7 GPa

The pseudo-binary system CaMgSi₂O₆-KAlSi₂O₆, modeling the potassium-bearing clinopyroxene (KCpx) solid solution, has been studied at 7 GPa and 1,100–1,650 °C. The KCpx is a liquidus phase of the system up to 60 mol% of KAlSi₂O₆. At higher content of KAlSi₂O₆ in the system, grossular-rich garnet becomes a liquidus phase. Above 75 mol% of KAlSi₂O₆ in the system, KCpx is unstable at the solidus as well, and garnet coexists with kalsilite, Si-wadeite and kyanite. No coexistence of KCpx with kyanite was observed. Above the solidus, KAlSi₂O₆ content of the KCpx coexisting with melt increases with decreasing temperature. Near the solidus of the system (about 1,250 °C) KCpx contains up to 5.6 wt% of K₂O, i.e. about 22–26 mol% of KAlSi₂O₆. Such high concentration of potassium in KCpx is presumably the maximal content of KAlSi₂O₆ in the Fe-free clinopyroxene at 7 GPa. In addition to the major substitution Mg^M1C²Al¹K², the KCpx solid solution contains Ca-Eskola and only minor Ca-Tschermack components. Our experimental results indicate that the natural assemblage KCpx+grossular-rich garnet might be a product of crystallization of the ultra-potassic SiO₂-rich alumino-silicate mantle melts (>200 km).Editorial responsibility: J. Hoefs     

δ<Superscript>18</Superscript>O and δD variations in Holocene massive ice in the Sabettayakha river mouth,northern Yamal Peninsula

The conditions of formation of massive ice near the South Tambey gas-condensate field in northern Yamal Peninsula are studied. It is shown that massive ice bodies up to 4.5 m thick occur in the Holocene deposits of the high laida and the first terrace. Therefore, they cannot be the remains of glaciers; they are ground ice formations. All three types of massive ice have quite various isotopic compositions: the values of δD range from–107 to–199.7, and δ¹⁸O from–15.7 to–26.48‰. Such a significant differentiation in isotopic composition is a result of cryogenic fractionation in a freezing water-saturated sediment. The most negative isotope values are even lower in this Holocene massive ice than in the Late Pleistocene ice-wedge ice of Yamal Peninsula.     

Spatio-temporal variations of δ<Superscript>2</Superscript>H and δ<Superscript>18</Superscript>O in precipitation and shallow groundwater in the Hilly Loess Region of the Loess Plateau,China

Groundwater is of utmost significance to socio-economic development and ecological recovery for the Loess Plateau. However, studies regarding the mechanism governing groundwater recharge over this area appear to be inadequate. This study is to examine the spatio-temporal variations of δ²H and δ¹⁸O in precipitation and shallow groundwater. On the basis of this, the mechanisms governing shallow groundwater recharge were explored. Precipitation and groundwater were sampled monthly from May to October during the period 2004–2006 at 13 sites in the Chabagou Catchment (187 km²). In the Caopingxigou Experimental Watershed (0.1 km²), meteorological variables were observed and rainfall larger than 5 mm was sampled immediately after each rain event. Across the area, 90% of the precipitation occurred from May to September primarily in the form of heavy rains or rainstorms with great spatial variability. There were about 30 localized rains in each year. It was indicated that there existed notable seasonality and pronounced spatial variability in precipitation isotopic compositions. Contributing factors and indications of isotopic compositions, as well as their climatic indications such as monsoon intensities and mixing processes of water vapor, were investigated. The δ²H–δ¹⁸O relation of groundwater was found to be δ²H = 3.22 × δ¹⁸O − 38.1, deviating from the local meteoric water line δ²H = 7.57 × δ¹⁸O + 3.9. The range of δ values in groundwater is shrunken to be 15–21% of that in individual precipitations, and groundwater in the middle reaches shows a wider range of δ values. Isotopic results showed that groundwater originates from precipitation with hydrogen and oxygen isotopic compositions being −69 and −9.7‰, respectively, and most groundwater experiences serious evaporation and adequate mixing with old water during infiltration or percolation in the aerated zone. It was also founded that obvious fluctuations of isotopic compositions in groundwater mainly appear in the middle reaches especially at sites that are close to valleys, suggesting varying sources of groundwater from precipitation, precipitation runoff, isotopically enriched surface water and/or lateral recharge of adjacent groundwater.     

Effect of KCl on melting in the Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript>–MgSiO<Subscript>3</Subscript>–H<Subscript>2</Subscript>O system at 5 GPa

To examine the effect of KCl-bearing fluids on the melting behavior of the Earth’s mantle, we conducted experiments in the Mg₂SiO₄–MgSiO₃–H₂O and Mg₂SiO₄–MgSiO₃–KCl–H₂O systems at 5 GPa. In the Mg₂SiO₄–MgSiO₃–H₂O system, the temperature of the fluid-saturated solidus is bracketed between 1,200–1,250°C, and both forsterite and enstatite coexist with the liquid under supersolidus conditions. In the Mg₂SiO₄–MgSiO₃–KCl–H₂O systems with molar Cl/(Cl + H₂O) ratios of 0.2, 0.4, and 0.6, the temperatures of the fluid-saturated solidus are bracketed between 1,400–1,450°C, 1,550–1,600°C, and 1,600–1,650°C, respectively, and only forsterite coexists with liquid under supersolidus conditions. This increase in the temperature of the solidus demonstrates the significant effect of KCl on reducing the activity of H₂O in the fluid in the Mg₂SiO₄–MgSiO₃–H₂O system. The change in the melting residues indicates that the incongruent melting of enstatite (enstatite = forsterite + silica-rich melt) could extend to pressures above 5 GPa in KCl-bearing systems, in contrast to the behavior in the KCl-free system.