Cosmic-ray exposure ages of four acapulcoites and two differentiated achondrites and evidence for a two-layer structure of the acapulcoite/lodranite parent asteroid

We determined the He, Ne, and Ar isotopic abundances in the four acapulcoites Dhofar (DHO) 125, DHO 290, DHO 312, and Graves Nunataks 98028, the metal-rich diogenite Northwest Africa (NWA) 1982, and a unique achondrite, NWA 1058, that resembles the acapulcoites in its chemical composition. The noble gases in these meteorites consist of three components: trapped gases, cosmic-ray produced nuclides, and nuclides produced by K, Th, and U decay. The four acapulcoites yield cosmic-ray exposure (CRE) ages in the range of 5.0-5.7 Ma and confirm earlier conclusions concerning break-up of all acapulcoites from a common S-type parent asteroid, possibly in three events 4.9, 5.9, and 14.8 Ma ago. We also discuss the other characteristics (mineralogy, chemistry, formation ages, and oxygen and trapped noble gas isotopes) of all other acapulcoites and their relatives, the lodranites. We propose that the acapulcoite/lodranite parent asteroid had a shell structure similar to that of the H chondrites: The less metamorphosed acapulcoites correspond to the H3 and H4 chondrites and originate from the exterior layers, whereas the more severely metamorphosed lodranites, similar to the H5 and H6 chondrites, represent the inner regions of their parent body. Ungrouped achondrite NWA 1982, probably a diogenite, shows a CRE age of 18.9 ± 2.0 Ma that falls on the major exposure age cluster of the diogenites. The unique achondrite NWA 1058 differs in cosmic-ray exposure age (38.9 ± 4.0 Ma) and in oxygen-isotopic composition from the acapulcoites and lodranites and is probably a winonaite.     

Temperature independent thermal expansivities of sodium aluminosilicate melts between 713 and 1835 K

The thermal expansivities of eight sodium aluminosilicate liquids were derived from the slope of new volume data at low temperatures (713−1072 K) combined with the high temperature (1300−1835 K) volume measurements of Stein et al. (1986) on the same liquids. Melt compositions range from 47−71 wt% SiO₂, 0−31 wt% A1203, and 17−33 wt% Na₂O; the volume of albite supercooled liquid at 1092 K was also determined. The low temperature volumes were derived from measurements of the glass density of each sample at 298 K, followed by measurements of the glass thermal expansion coefficient from 298 K to the respective glass transition interval. This technique takes advantage of the fact that the volume of a glass is equal to the volume of the corresponding liquid at the limiting fictive temperature (T′_f), and that T′_f can be approximated as the onset of the rapid rise in thermal expansion at the glass transition in a heating curve (Moynihan, 1995). No assumptions were made regarding the equivalence of enthalpy and volume relaxation through the glass transition. The propagated error on the volume of each supercooled liquid at T′_f is 0.25%. Combination of these low temperature data with the high temperature measurements of Stein et al. (1986) allowed a constant thermal expansivity of each liquid to be derived over a wide temperature interval (763−1001 degrees) with a fitted 1σ error of 0.6–4.6%; in every case, no temperature dependence to dV/dT^liq could be resolved. Calibration of a linear model equation leads to fitted values ± 1σ (units of cm³/mole) for (26.91 ± .04), (37.49 ± .12), (26.48 ± .06) at 1373 K, and (7.64 ± .08 × 10^-3 cm³/mole-K). The results indicate that neither Si02 nor Al₂O₃ contribute to the thermal expansivity of the liquids, and that dV/dT^liq is independent of temperature between 713–1835 K over a wide range of liquid composition. Calculated volumes based on this model recover both low and high temperature measurements with a standard deviation <0.25%, whereas values of dV/dT^liq can be predicted within 5.6%.     

Oxidation and Origin of Organic Matter in Surficial Eastern Mediterranean Hemipelagic Sediments

Aerobic mineralisation of C_org in surface sedimentsof the deep (>2000 m water depth) eastern Mediterranean Sea has been quantified by analysis of detailedbox core C_org concentration versus depth profiles and the modelling environment for early diageneticproblems MEDIA. The reactive fraction comprises 60–80% of the total C_org reachingthe sediments and is largely oxidised within the surficial 10 cm. A non-reactive C _orgfraction (G_NR) dominates at depths >10 cm, and makes up20–40% of the total C _org flux to the sediments. First-order rateconstants for decomposition of the reactive fraction calculated from theC _org profiles range from 5.4 × 10^-3 to8.0 × 10^-3 y^-1 to 8.0 × 10^-3 y^-1. Total mineralization rates in thesurface sediment are between 1.7 and 2.6 mol C cm^-2 y^-1 and thus are typical for oligotrophic, deep-seaenvironments. The low fluxes and rapid remineralisation of C _org are accompanied by²¹⁰Pb_excess surface mixed layers which are only 2 cm deep, among the thinnest reported for oxygenated marine sediments.Model results indicate a mismatch between the C _org profiles and O₂ microprofileswhich were measured onboard ship. This can be attributed to a combination of decompression artefactsaffecting onboard measurement of the O₂ profiles or the leakage ofoxygen into the core during handling on deck. Furthermore, the used D_b values, based on ²¹⁰Pb, may not befully appropriate; calculations with higher D_b values improve the O₂ fits. The surficial sediment¹³C _org values of -22 become less negative with increasing depth and decreasing C _orgconcentrations. The major ¹³C change occurs in the top 3 to 4 cm and coincides with the interval weremost of the organic carbon oxidation takes place. This indicates that the reactive fractionof organic matter, commonly assumed to be marine, has a more negative ¹³C _orgthan the refractory fraction, usually held to be terrestrial. Palaeoproductivity estimates calculated from thesediment data by means of literature algorithms yield low surface productivities(12–88 gC m^-2 y^-1), which are in good agreement with field measurements of primary productivity in otherstudies. Such values are, however, significantly lower than those indicated by recent productivitymaps of the area derived from satellite imagery (>100 gC m^-2 y^-1).     

Hydrogen incorporation in orthopyroxene: interaction of different trivalent cations

The incorporation of hydrogen into ferrosilite, Fe-bearing enstatite and orthopyroxene containing different trivalent cations (Cr³⁺ and Al³⁺, Cr³⁺ and Fe³⁺) was investigated experimentally at 25 kbar. Hydrogen concentration was determined by FTIR-spectroscopy on oriented crystal sections and by secondary ion mass spectroscopy, whereas Mößbauer spectroscopy and optical spectroscopy were used to characterise the valence state of Fe in orthopyroxene. Results suggest that hydrogen incorporation in ferrosilite is achieved by a similar mechanism as in pure enstatite. In Cr-bearing samples, however, hydrogen incorporation is reduced by the presence of other trivalent cations by an increased tendency to form Tschermaks substitutions, e.g. Si _T ⁴⁺ + Mg _M1 ²⁺ ? Al _T ³⁺ + Cr _M1 ³⁺ . Thus, hydrogen solubility in natural orthopyroxenes from the Earth’s mantle, containing significant amounts of Cr³⁺, Al³⁺, and Fe³⁺, may be much more limited than expected from their trivalent cation content, as a large fraction of the trivalent cations does not participate in H-incorporating reactions as 2 Mg _M1 ²⁺ ? M _M1 ³⁺ + V_M1 + H _i ⁺ .     

Malaco temperature reconstructions and numerical simulation of environmental conditions in the southeastern Carpathian Basin during the Last Glacial Maximum

We investigate the glacial climate conditions in the southeastern Carpathian Basin (Vojvodina, Serbia) based on the reconstruction of malacological palaeotemperatures and results from a high-resolution regional climate simulation. Land snail assemblages from eight loess profiles are used to reconstruct July temperatures during the Last Glacial Maximum (LGM). The malacological reconstructed temperatures are in good agreement with the simulated LGM July temperatures by the Weather Research and Forecast model. Both methods indicate increasing temperatures from the northwestern towards the southeastern parts of the study area. LGM aridity indices calculated based on the regional climate model data suggest more arid conditions in the southeastern parts compared with more humid conditions in the northwestern parts. However, for present-day conditions, the moisture gradient is reversed, exhibiting more humid (arid) conditions in the southeast (northwest). An explanation for the reversed LGM aridity pattern is provided by an analysis of the prevailing wind directions over the South Banat district (Serbia). The prevailing moist northwesterly winds during summer are not able to compensate for the annual lack of moisture induced by the dry winds from the southeast that are more frequent during the LGM for the other seasons.     

Characterization of geosynthetic clay liner bentonite using micro-analytical methods

In barrier design, familiarity of the structure and composition of the soil material at the micron scale is necessary for delineating the retention mechanisms of introduced metals, such as the formation of new mineral phases. In this study, the mineralogical and chemical makeup of the bentonite from a geosynthetic clay liner (GCL) was extensively characterized using a combination of conventional benchtop X-ray diffraction (XRD) and micro X-ray diffraction (μXRD) with synchrotron-generated micro X-ray fluorescence (μXRF) elemental mapping and μXRD (S-μXRD). These methods allow for the non-destructive, in situ investigation of a sample, with μm spatial resolution. Synchrotron-based hard X-ray microprobes are specifically advantageous to the study of trace metals due to higher spatial resolution (<10 μm) and higher analytical sensitivity (femtogram detection) than is possible using normal laboratory-based instruments. Minerals comprising less than 5% of the total bentonite sample such as gypsum, goethite and pyrite were identified that were not accessible by other conventional methods for the same GCL bentonite. Two dimensional General Area Diffraction Detector System (GADDS) images proved to be particularly advantageous in differentiating between the microcrystalline clay, which appeared as homogeneous Debye rings, and the ‘spotty’ or ‘grainy’ appearance of primary, more-coarsely-crystalline, accessory minerals. For S-μXRD, the tunability of the synchrotron X-rays allowed for efficient distinction of both clay minerals at low scattering angles and in identifying varying Fe oxide minerals at higher angles. GCL samples permeated with metal-bearing mining solutions were also examined in order to consider how mechanisms of metal attenuation may be identified using the same techniques. In addition to the cation exchange capacity from the montmorillonite clay, tests showed how minerals comprising only 1–2% of the bentonite such as goethite could potentially play a significant role in sequestering a range of metals, specifically Ni, Zn and Cu.     

Evaluation of a spatial rainfall generator for generating high resolution precipitation projections over orographically complex terrain

Space–time variability of precipitation plays a key role as driver of many environmental processes. The objective of this study is to evaluate a spatiotemporal (STG) Neyman–Scott Rectangular Pulses (NSRP) generator over orographically complex terrain for statistical downscaling of climate models. Data from 145 rain gauges over a 5760-km² area of Cyprus for 1980–2010 were used for this study. The STG was evaluated for its capacity to reproduce basic rainfall statistical properties, spatial intermittency, and extremes. The results were compared with a multi-single site NRSP generator (MSG). The STG performed well in terms of average annual rainfall (+1.5 % in comparison with the 1980–2010 observations), but does not capture spatial intermittency over the study area and extremes well. Daily events above 50 mm were underestimated by 61 %. The MSG produced a similar error (+1.1 %) in terms of average annual rainfall, while the daily extremes (>50-mm) were underestimated by 11 %. A gridding scheme based on scaling coefficients was used to interpolate the MSG data. Projections of three Regional Climate Models, downscaled by MSG, indicate a 1.5–12 % decrease in the mean annual rainfall over Cyprus for 2020–2050. Furthermore, the number of extremes (>50-mm) for the 145 stations is projected to change between ?24 and +2 % for the three models. The MSG modelling approach maintained the daily rainfall statistics at all grid cells, but cannot create spatially consistent daily precipitation maps, limiting its application to spatially disconnected applications. Further research is needed for the development of spatial non-stationary NRSP models.     

Do carbonate liquids become denser than silicate liquids at pressure? Constraints from the fusion curve of K<Subscript>2</Subscript>CO<Subscript>3</Subscript> to 3.2 GPa

Brackets on the melting temperature of K₂CO₃ were experimentally determined at 1.86 ± 0.02 GPa (1,163–1,167°C), 2.79 ± 0.03 GPa (1,187–1,195°C), and 3.16 ± 0.04 GPa (1,183–1,189°C) in a piston-cylinder apparatus. These new data, in combination with published experiments at low pressure (<0.5 GPa), establish the K₂CO₃ fusion curve to 3.2 GPa. On the basis of these experiments and published thermodynamic data for crystalline and liquid K₂CO₃, the high-pressure density and compressibility of K₂CO₃ liquid were derived from the fusion curve. The pressure dependence of the liquid compressibility (K′₀ = dK ₀/dP, where K ₀ = 1/β₀) is between 16.2 and 11.6, with a best estimate of 13.7, in a third-order Birch–Murnaghan equation of state (EOS). This liquid K′₀ leads to a density of 2,175 ± 36 kg/m³ at 4 GPa and 1,500°C, which is ∼30% lower than that reported in the literature on the basis of the falling-sphere method at the same conditions. The uncertainty in the liquid K′₀ leads to an error in melt density of ± 2% at 4 GPa; the error decreases with decreasing pressure. With a K′₀ of 13.7, the compressibility of K₂CO₃ at 1,500°C and 1 bar (K ₀ = 3.8 GPa) drops rapidly with increasing pressure ( ), which prevents a density crossover with silicate melts, such as CaAlSi₂O₈ and CaMgSi₂O₆, at upper mantle depths.