The importance of defining chemical potentials,substitution mechanisms and solubility in trace element diffusion studies: the case of Zr and Hf in olivine

The diffusion, substitution mechanism and solubility limits of Zr and Hf in synthetic forsterite (Mg₂SiO₄) and San Carlos olivine (Mg_0.9Fe_0.1)₂SiO₄ have been investigated between 1,200 and 1,500 °C as a function of the chemical potentials of the components in the system MgO(FeO)–SiO₂–ZrO₂(HfO₂). The effect of oxygen fugacity and crystallographic orientation were also investigated. The solubilities of Zr in forsterite are highest and diffusion fastest when the coexisting three-phase source assemblage includes ZrSiO₄ (zircon) or HfSiO₄ (hafnon), and lower and slower, respectively, when the source assemblage includes MgO (periclase). This indicates that Zr and Hf substitute on the octahedral sites in olivine, charge balanced by magnesium vacancies. Diffusion is anisotropic, with rates along the crystal axes increasing in the order a < b < c. The generalized diffusion relationship as a function of chemical activity (as \(a_{{{\text{SiO}}_{2} }}\)), orientation and temperature is: \(logD_{\text{Zr}} = \frac{1}{4}loga_{{{\text{SiO}}_{2} }} + logD_{0} - \left( {\frac{{368 \pm 17\;{\text{kJ}}\;{\text{mol}}^{ - 1} }}{{2.303\;{\text{RT}}}}} \right)\) where the values of log D ₀ are ?3.8(±0.5), ?3.4(±0.5) and ?3.1(±0.5) along the a, b and c axes, respectively. Most experiments were conducted in air (fO₂ = 10^?0.68 bars), but one at fO₂ = 10^?11.2 bars at 1,400 °C shows no resolvable effect of oxygen fugacity on Zr diffusion. Hf is slightly more soluble in olivine than Zr, but diffuses slightly slower. Diffusivities of Zr in experiments in San Carlos olivine at 1,400 °C, fO₂ = 10^?6.6 bars are similar to those in forsterite at the same conditions, showing that the controls on diffusivities are adequately captured by the simple system (nominally iron-free) experiments. Diffusivities are in good agreement with those measured by Spandler and O’Neill (Contrib Miner Petrol 159:791–818, 2010) in San Carlos olivine using silicate melt as the source at 1,300 °C, and fall within the range of most measurements of Fe–Mg inter-diffusion in olivine at this temperature. Forsterite–melt partitioning experiments in the CaO–MgO–Al₂O₃–SiO₂–ZrO₂/HfO₂ show that the interface concentrations from the diffusion experiments represent true equilibrium solubilities. Another test of internal consistency is that the ratios of the interface concentrations between experiments buffered by Mg₂SiO₄ + Mg₂Si₂O₆ + ZrSiO₄ or Mg₂SiO₄ + ZrSiO₄ + ZrO₂ (high silica activity) to those buffered by Mg₂SiO₄ + MgO + ZrO₂ (low silica activity) agree well with the ratios calculated from thermodynamic data. This study highlights the importance of buffering chemical potentials in diffusion experiments to provide constraints on the interface diffusant concentrations and hence validate the assumption of interface equilibrium.     

Climate sensitivity estimated from ensemble simulations of glacial climate

The concentration of greenhouse gases (GHGs) in the atmosphere continues to rise, hence estimating the climate system’s sensitivity to changes in GHG concentration is of vital importance. Uncertainty in climate sensitivity is a main source of uncertainty in projections of future climate change. Here we present a new approach for constraining this key uncertainty by combining ensemble simulations of the last glacial maximum (LGM) with paleo-data. For this purpose we used a climate model of intermediate complexity to perform a large set of equilibrium runs for (1) pre-industrial boundary conditions, (2) doubled CO₂ concentrations, and (3) a complete set of glacial forcings (including dust and vegetation changes). Using proxy-data from the LGM at low and high latitudes we constrain the set of realistic model versions and thus climate sensitivity. We show that irrespective of uncertainties in model parameters and feedback strengths, in our model a close link exists between the simulated warming due to a doubling of CO₂, and the cooling obtained for the LGM. Our results agree with recent studies that annual mean data-constraints from present day climate prove to not rule out climate sensitivities above the widely assumed sensitivity range of 1.5–4.5°C (Houghton et al. 2001). Based on our inferred close relationship between past and future temperature evolution, our study suggests that paleo-climatic data can help to reduce uncertainty in future climate projections. Our inferred uncertainty range for climate sensitivity, constrained by paleo-data, is 1.2–4.3°C and thus almost identical to the IPCC estimate. When additionally accounting for potential structural uncertainties inferred from other models the upper limit increases by about 1°C.     

Distribution of organochlorines in the dissolved and suspended phase of the sea-surface microlayer and seawater in Hong Kong, China

Polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) were measured in the dissolved and suspended particulate phase in the sea-surface microlayer (SML) and subsurface water (SSW) collected from Hong Kong's coastal environment. The concentration ranges (pg/L) for summation sigmaHCHs, summation sigmaDDTs and summation sigmaPCBs in the SSW dissolved phase (DP i.e. sum of truly dissolved and colloidal phase) were 409-940 (mean 602), 774-5583 (mean 1908) and 266-433 (mean 278), respectively. The concentration ranges (pg/L) for summation sigmaHCHs, summation sigmaDDTs and summation sigmaPCBs in SSW suspended particulate matter (SPM) were <5-85 (mean 59), 358-1369 (mean 787) and 85.6-273 (mean 172), respectively. The enrichment factor of PCBs and OCPs in the SML varied between 1.1 and 4.5 for the DP, and 0.4-8.2 for the SPM. The distribution of contaminants between DP and SPM in both the SML and SSW indicates that particulate matter plays an important role in the distribution and fate of DDTs and PCBs, but not for HCH isomers. The Pearl River Estuary is likely to be a major source of contaminants transported to Hong Kong.     

High-resolution vertical profiles of groundwater electrical conductivity (EC) and chloride from direct-push EC logs

Elevated groundwater salinity associated with produced water, leaching from landfills or secondary salinity can degrade arable soils and potable water resources. Direct-push electrical conductivity (EC) profiling enables rapid, relatively inexpensive, high-resolution in-situ measurements of subsurface salinity, without requiring core collection or installation of groundwater wells. However, because the direct-push tool measures the bulk EC of both solid and liquid phases (EC_a), incorporation of EC_a data into regional or historical groundwater data sets requires the prediction of pore water EC (EC_w) or chloride (Cl^?) concentrations from measured EC_a. Statistical linear regression and physically based models for predicting EC_w and Cl^? from EC_a profiles were tested on a brine plume in central Saskatchewan, Canada. A linear relationship between EC_a/EC_w and porosity was more accurate for predicting EC_w and Cl^? concentrations than a power-law relationship (Archie’s Law). Despite clay contents of up to 96%, the addition of terms to account for electrical conductance in the solid phase did not improve model predictions. In the absence of porosity data, statistical linear regression models adequately predicted EC_w and Cl^? concentrations from direct-push EC_a profiles (EC_w = 5.48 EC_a + 0.78, R ² = 0.87; Cl^? = 1,978 EC_a – 1,398, R ² = 0.73). These statistical models can be used to predict EC_w in the absence of lithologic data and will be particularly useful for initial site assessments. The more accurate linear physically based model can be used to predict EC_w and Cl^? as porosity data become available and the site-specific EC_w–Cl^? relationship is determined.     

Scale dependency of regional climate modeling of current and future climate extremes in Germany

A 26-year simulation (1980–2005) was performed with the Weather Research and Forecast (WRF) model over the Volta Basin in West Africa. This was to investigate the ability of a climate version of WRF to reproduce present day temperature and precipitation over the Volta Basin. The ERA-Interim reanalysis and one realization of the ECHAM6 global circulation model (GCM) data were dynamically downscaled using two nested domains within the WRF model. The outer domain had a horizontal resolution of 50 km and covered the whole of West Africa while the inner domain had a horizontal resolution of 10 km. It was observed that biases in the respective forcing data were carried over to the RCM, but also the RCM itself contributed to the mean bias of the model. Also, the biases in the 50-km domain were transferred unchanged, especially in the case of temperature, to the 10-km domain, but, for precipitation, the higher-resolution simulations increased the mean bias in some cases. While in general, WRF underestimated temperature in both the outer (mean biases of ?1.6 and ?2.3 K for ERA-Interim and ECHAM6, respectively) and the inner (mean biases of ?0.9 K for the reanalysis and ?1.8 K for the GCM) domains, WRF slightly underestimated precipitation in the coarser domain but overestimated precipitation in the finer domain over the Volta Basin. The performance of the GCM, in general, is good, particularly for temperature with mean bias of ?0.7 K over the outer domain. However, for precipitation, the added value of the RCM cannot be overlooked, especially over the whole West African region on the annual time scale (mean biases of ?3% for WRF and ?8% for ECHAM6). Over the whole Volta Basin and the Soudano-Sahel for the month of April and spring (MAM) rainfall, respectively, mean bias close to 0% was simulated. Biases in the interannual variability in both temperature and precipitation over the basin were smaller in the WRF than the ECHAM6. High spatial pattern correlations between 0.7 and 0.8 were achieved for the autumn precipitation and low spatial correlation in the range of 0.0 and 0.2 for the winter season precipitation over the whole basin and all the three belts over the basin.     

Mechanisms of fine extinction band development in vein quartz: new insights from correlative light and electron microscopy

Fine extinction bands (FEBs) (also known as deformation lamellae) visible with polarized light microscopy in quartz consist of a range of nanostructures, inferring different formation processes. Previous transmission electron microscopy studies have shown that most FEB nanostructures in naturally deformed quartz are elongated subgrains formed by recovery of dislocation slip bands. Here we show that three types of FEB nanostructure occur in naturally deformed vein quartz from the low-grade metamorphic High-Ardenne slate belt (Belgium). Prismatic oriented FEBs are defined by bands of dislocation walls. Dauphiné twin boundaries present along the FEB boundaries probably formed after FEB formation. In an example of two sub-rhombohedral oriented FEBs, developed as two sets in one grain, the finer FEB set consists of elongated subgrains, similar to FEBs described in previous transmission electron microscopy studies. The second wider FEB set consists of bands with different dislocation density and fluid-inclusion content. The wider FEB set is interpreted as bands with different plastic strain associated with the primary growth banding of the vein quartz grain. The nanometre-scale fluid inclusions are interpreted to have formed from structurally bounded hydroxyl groups that moreover facilitated formation of the elongate subgrains. Larger fluid inclusions aligned along FEBs are explained by fluid-inclusion redistribution along dislocation cores. The prismatic FEB nanostructure and the relation between FEBs and growth bands have not been recognized before, although related structures have been reported in experimentally deformed quartz.     

Illite mineral synthesis at surface temperatures

The synthesis of illite mixed-layer minerals at surface conditions is possible through precipitation of Al hydroxides from Si-, Mg- and K-containing solutions. It has been shown that amorphous hydroxides of Al, Fe, etc. are capable of coprecipitating silica even from very dilute solutions. By aging of these X-ray amorphous hydroxide—silica precipitates under certain conditions, clay minerals can be synthesized at low temperatures. The presence of Mg particularly favors the formation of three-layer clay minerals. Mg-rich Al hydroxide—silica precipitates permit formation of tri- and di-octahedral smectite, illite and chlorite. The formation of three-layer clay minerals is only possible when the precipitates contain at least 6% MgO. The precipitates stay amorphous if the Mg content is lower. The adsorption of Mg and K on the hydroxide—silica precipitate controls the illite or montmorillonite portion in the mixture of the three-layer silicates. There is a competition for K and Mg adsorption on the hydroxide—silica precipitates. Higher K concentration inhibits the three-layer mineral formation through the lowering of the Mg content in the precipitates. Illite mineral formation is favored under certain K/Mg ratios. Higher NaCl contents do not favor the three-layer mineral formation.The enrichment of Mg and K in the precipitates is not as large as the enrichment of Si in the hydroxides. This means that the illite mineral formation is only possible from solutions with a high-salt content like seawater.     

Weathering, dust, and biocycling effects on soil silicon isotope ratios

Silicon isotope ratios (δ³⁰Si) of bulk mineral materials in soil integrate effects from both silicon sources and processing. Here we report δ³⁰Si values from a climate gradient of Hawaiian soils developed on 170 ka basalt and relate them to patterns of soil chemistry and mineralogy. The results demonstrate informative relationships between the mass fraction of soil Si depletion and δ³⁰Si. In upper (<1 m deep) soil horizons along the climate gradient, Si depletion correlates with decreases of residual δ³⁰Si values in low rainfall soils and increases in high rainfall soils. Strong positive correlation between soil δ³⁰Si and dust-derived quartz and mica content show that both trends are largely controlled by the abundance of these weathering-resistant minerals. The data also lend support to the idea that fractionation of Si isotopes in secondary phases is controlled by partitioning of silicon between dissolved and precipitated products during the initial weathering of primary basalt. Secondary mineral δ³⁰Si values from lower (>1 m deep) soil horizons generally correlate with the isotope fractionation predicted by a study of dissolved Si in basalt-watershed rivers and driven by preferential ²⁸Si removal from the dissolved phase during precipitation. In contrast, after correcting for the influence of dust, secondary mineral Si depletion and δ³⁰Si values in shallow (<1 m deep) soil horizons showed evidence of biocycling induced Si redistribution and substantially lower δ³⁰Si values than predicted. Low δ³⁰Si values in shallow soil horizons compared to predictions can be attributed to repeated fractionation as secondary minerals undergo additional cycles of dissolution and precipitation. Primary mineral weathering, secondary mineral weathering, dust accumulation, and biocycling are major processes in terrestrial Si cycling and these results demonstrate that each can be traced by δ³⁰Si values interpreted in conjunction with mineralogy and measures of Si depletion.     

Degradation of chlorinated ethylenes by Fe0: inhibition processes and mineral precipitation

. Granular zero-valent iron was used for the treatment of groundwater pollution caused by chlorinated ethylenes, mainly TCE, cis-DCE and VC at an industrial site. The rapidly decreasing rates of de-chlorination in the initial phase were attributed to the precipitation of carbonates and the development of hydrogen by anaerobic corrosion. After 70 pore volumes, sulphate was reduced by bacteria. From this point in time, the degradation of TCE was slightly accelerated whereas the de-chlorination rates of the other chlorinated ethylenes decreased only slowly. This relative improvement was assumed to be caused by the uptake of electron-transfer-blocking hydrogen by bacteria. Because the overall trend of the degradation rates is negative we conclude that the inhibitive effect of carbonate precipitation and hydrogen formation cannot be compensated for by the positive influence of the activity of sulphate-reducing bacteria.