Enthalpies in (Na,Ca)- and (K,Ca)-feldspar binaries: a high-temperature solution calorimetric study

A series of high structural state plagioclases (Ab-An) was crystallized from glasses. By exchanging Na for K in KCl melts, metastable K-plagioclases (Or-An) were prepared which possess the same structural state as the starting plagioclases. Both series were investigated at 980 K by lead borate solution calorimetry. Continuing the ideas of Carpenter and McConnell (1984) and Carpenter (1992a), the results can be interpreted as follows. In the high plagioclase series, the enthalpies of solution, jH_sol, reflect the schemes of Al,Si ordering: (1) analbite-like (C2/m) ordering in the An-poor region 0hX_AnА.2, (2) high albite-like (C1¥) ordering in intermediate plagioclases, and (3) anorthite-like (I1¥) ordering in the An-rich region 0.7AnБ. In regions 1 and 2, jH_sol decreases as a function of X_An, but increases in region 3 as a consequence of the C1¥MI1¥ ordering reaction. Therefore, it is not a mixing effect but a compositionally restricted ordering effect which causes the excess enthalpies, jH^ex, to be positive in the plagioclase binary as a whole. Neglecting the existence of phase transitions at X_An=0.2 and X_An=0.7, jH^ex can be approximated by a two-parameter Margules model yielding W^H_AnAb=14Dž kJ/mol and W^H_AbAn=40Dž kJ/mol. jH_sol values of I1¥ plagioclases (X_An>0.7) can be "corrected" for the C1¥MI1¥ ordering effect (Carpenter 1992a). When combining the corrected values with the jH_sol data which were actually measured on the C1¥ plagioclases (X_An<0.7), negative excess enthalpies are generated in the plagioclase binary. This may be expected when C1¥ ordering occurs relative to topochemically monoclinic reference states of analbite and hypothetical anorthite devoid of I1¥ order. The solution experiments on the K-plagioclases resulted in similar characteristics as those found for the plagioclases. However, in addition to the ordering effects observed in the plagioclase binary, volume mismatch effects contribute to jH^ex in the K-plagioclase series. jH^ex can be represented by a Margules model with W^H_AnOr=60ᆞ kJ/mol and W^H_OrAn=91ᆢ kJ/mol when the phase transitions at X_An=0.2 and X_An=0.7 are again neglected. The contribution of the volume mismatch effect to jH^ex is considerable, as appears from the large difference between the K-plagioclase and the plagioclase Margules parameters. Their difference corresponds to a practically symmetrical dependence of jH^ex_volmism on composition, with W^H_volmism=48ᆡ kJ/mol.     

Weathering of the New Albany Shale, Kentucky: II. Redistribution of minor and trace elements

During weathering, elements enriched in black shale are dispersed in the environment by aqueous and mechanical transport. Here a unique evaluation of the differential release, transport, and fate of Fe and 15 trace elements during progressive weathering of the Devonian New Albany Shale in Kentucky is presented. Results of chemical analyses along a weathering profile (unweathered through progressively weathered shale to soil) describe the chemically distinct pathways of the trace elements and the rate that elements are transferred into the broader, local environment. Trace elements enriched in the unweathered shale are in massive or framboidal pyrite, minor sphalerite, CuS and NiS phases, organic matter and clay minerals. These phases are subject to varying degrees and rates of alteration along the profile. Cadmium, Co, Mn, Ni, and Zn are removed from weathered shale during sulfide-mineral oxidation and transported primarily in aqueous solution. The aqueous fluxes for these trace elements range from 0.1 g/ha/a (Cd) to 44 g/ha/a (Mn). When hydrologic and climatic conditions are favorable, solutions seep to surface exposures, evaporate, and form Fe-sulfate efflorescent salts rich in these elements. Elements that remain dissolved in the low pH (<4) streams and groundwater draining New Albany Shale watersheds become fixed by reactions that increase pH. Neutralization of the weathering solution in local streams results in elements being adsorbed and precipitated onto sediment surfaces, resulting in trace element anomalies.Other elements are strongly adsorbed or structurally bound to solid phases during weathering. Copper and U initially are concentrated in weathering solutions, but become fixed to modern plant litter in soil formed on New Albany Shale. Molybdenum, Pb, Sb, and Se are released from sulfide minerals and organic matter by oxidation and accumulate in Fe-oxyhydroxide clay coatings that concentrate in surface soil during illuviation. Chromium, Ti, and V are strongly correlated with clay abundance and considered to be in the structure of illitic clay. Illite undergoes minimal alteration during weathering and is concentrated during illuvial processes. Arsenic concentration increases across the weathering profile and is associated with the succession of secondary Fe(III) minerals that form with progressive weathering. Detrital fluxes of particle-bound trace elements range from 0.1 g/ha/a (Sb) to 8 g/ha/a (Mo). Although many of the elements are concentrated in the stream sediments, changes in pH and redox conditions along the sediment transport path could facilitate their release for aqueous transport.     

Arsenic incorporation into authigenic pyrite, Bengal Basin sediment, Bangladesh

Sediment from two deep boreholes (∼400 m) approximately 90 km apart in southern Bangladesh was analyzed by X-ray absorption spectroscopy (XAS), total chemical analyses, chemical extractions, and electron probe microanalysis to establish the importance of authigenic pyrite as a sink for arsenic in the Bengal Basin. Authigenic framboidal and massive pyrite (median values 1500 and 3200 ppm As, respectively), is the principal arsenic residence in sediment from both boreholes. Although pyrite is dominant, ferric oxyhydroxides and secondary iron phases contain a large fraction of the sediment-bound arsenic between approximately 20 and 100 m, which is the depth range of wells containing the greatest amount of dissolved arsenic. The lack of pyrite in this interval is attributed to rapid sediment deposition and a low sulfur flux from riverine and atmospheric sources. The ability of deeper aquifers (>150 m) to produce ground water with low dissolved arsenic in southern Bangladesh reflects adequate sulfur supplies and sufficient time to redistribute the arsenic into pyrite during diagenesis.     

Remote sensing of terrestrial impact craters: The TanDEM‐X digital elevation model

With the TanDEM‐X digital elevation model (DEM), the terrestrial solid surface is globally mapped with unprecedented accuracy. TanDEM‐X is a German X‐band radar mission whose two identical satellites have been operated in single‐pass interferometer configuration over several years. The acquired data are processed to yield a global DEM with 12 m independent posting and relative vertical accuracies of better than 2 m and 4 m in moderate and mountainous terrain, respectively. This DEM provides new opportunities for space‐borne remote‐sensing studies of the entire sample of terrestrial impact craters. In addition, it represents an interesting repository to aid in the search for new impact crater candidates. We have used the TanDEM‐X DEM to investigate the current set of confirmed impact structures. For a subsample of the craters, including small, midsized, and large structures, we compared the results with those from other DEMs. This quantitative analysis demonstrates the excellent quality of the TanDEM‐X elevation data. Our findings help to estimate what can be gained by using the TanDEM‐X DEM in impact crater studies. They may also be beneficial in identifying the regions and morphologies where the search for currently unknown impact structures might be most promising.     

Weathering of the New Albany Shale, Kentucky, USA: I. Weathering zones defined by mineralogy and major-element composition

Comprehensive understanding of chemical and mineralogical changes induced by weathering is valuable information when considering the supply of nutrients and toxic elements from rocks. Here minerals that release and fix major elements during progressive weathering of a bed of Devonian New Albany Shale in eastern Kentucky are documented. Samples were collected from unweathered core (parent shale) and across an outcrop excavated into a hillside 40 year prior to sampling. Quantitative X-ray diffraction mineralogical data record progressive shale alteration across the outcrop. Mineral compositional changes reflect subtle alteration processes such as incongruent dissolution and cation exchange. Altered primary minerals include K-feldspars, plagioclase, calcite, pyrite, and chlorite. Secondary minerals include jarosite, gypsum, goethite, amorphous Fe(III) oxides and Fe(II)-Al sulfate salt (efflorescence). The mineralogy in weathered shale defines four weathered intervals on the outcrop—Zones A–C and soil. Alteration of the weakly weathered shale (Zone A) is attributed to the 40-a exposure of the shale. In this zone, pyrite oxidization produces acid that dissolves calcite and attacks chlorite, forming gypsum, jarosite, and minor efflorescent salt. The pre-excavation, active weathering front (Zone B) is where complete pyrite oxidation and alteration of feldspar and organic matter result in increased permeability. Acidic weathering solutions seep through the permeable shale and evaporate on the surface forming abundant efflorescent salt, jarosite and minor goethite. Intensely weathered shale (Zone C) is depleted in feldspars, chlorite, gypsum, jarosite and efflorescent salts, but has retained much of its primary quartz, illite and illite–smectite. Goethite and amorphous FE(III) oxides increase due to hydrolysis of jarosite. Enhanced permeability in this zone is due to a 14% loss of the original mass in parent shale. Denudation rates suggest that characteristics of Zone C were acquired over 1 Ma. Compositional differences between soil and Zone C are largely attributed to illuvial processes, formation of additional Fe(III) oxides and incorporation of modern organic matter.     

Biogeochemical evolution of a landfill leachate plume, Norman, Oklahoma

Leachate from municipal landfills can create groundwater contaminant plumes that may last for decades to centuries. The fate of reactive contaminants in leachate-affected aquifers depends on the sustainability of biogeochemical processes affecting contaminant transport. Temporal variations in the configuration of redox zones downgradient from the Norman Landfill were studied for more than a decade. The leachate plume contained elevated concentrations of nonvolatile dissolved organic carbon (NVDOC) (up to 300 mg/L), methane (16 mg/L), ammonium (650 mg/L as N), iron (23 mg/L), chloride (1030 mg/L), and bicarbonate (4270 mg/L). Chemical and isotopic investigations along a 2D plume transect revealed consumption of solid and aqueous electron acceptors in the aquifer, depleting the natural attenuation capacity. Despite the relative recalcitrance of NVDOC to biodegradation, the center of the plume was depleted in sulfate, which reduces the long-term oxidation capacity of the leachate-affected aquifer. Ammonium and methane were attenuated in the aquifer relative to chloride by different processes: ammonium transport was retarded mainly by physical interaction with aquifer solids, whereas the methane plume was truncated largely by oxidation. Studies near plume boundaries revealed temporal variability in constituent concentrations related in part to hydrologic changes at various time scales. The upper boundary of the plume was a particularly active location where redox reactions responded to recharge events and seasonal water-table fluctuations. Accurately describing the biogeochemical processes that affect the transport of contaminants in this landfill-leachate-affected aquifer required understanding the aquifer's geologic and hydrodynamic framework.     

Geochemistry of arsenic during low-temperature water–rock interaction

High arsenic water has been a global focus of both scientists and water supply managers because of its serious adverse impact on human health and wide distribution in the world. Processes of redox, sorption, precipitation, and dissolution release arsenic in both natural systems and in environments intensely modified by human activities. In natural systems, groundwater arsenic is controlled by lithologic geochemistry, sedimentation conditions, hydrogeologic setting and groundwater chemistry. However, in the intensely human-affected systems (such as mining and tilling areas), arsenic mobilization is dependent on the composition of the primary materials, treatment methods, storage design, and local climate. Well-designed experimental systems aid in characterizing sorption, precipitation, and redox processes associated with arsenic dynamics during water-rock interaction. Continued investigations of field sites will further refine understanding of the processes favoring arsenic mobility in the range of natural and man-made systems. The combination of field and experimental studies will lead to better understanding of arsenic cycling in all systems and sustainable management of water resources in arsenic-affected areas.     

Stratovolcano stability assessment methods and results from Citlaltépetl,Mexico

Citlaltépetl volcano is the easternmost stratovolcano in the Trans-Mexican Volcanic Belt. Situated within 110 km of Veracruz, it has experienced two major collapse events and, subsequent to its last collapse, rebuilt a massive, symmetrical summit cone. To enhance hazard mitigation efforts we assess the stability of Citlaltépetl's summit cone, the area thought most likely to fail during a potential massive collapse event. Through geologic mapping, alteration mineralogy, geotechnical studies, and stability modeling we provide important constraints on the likelihood, location, and size of a potential collapse event. The volcano's summit cone is young, highly fractured, and hydrothermally altered. Fractures are most abundant within 5–20-m wide zones defined by multiple parallel to subparallel fractures. Alteration is most pervasive within the fracture systems and includes acid sulfate, advanced argillic, argillic, and silicification ranks. Fractured and altered rocks both have significantly reduced rock strengths, representing likely bounding surfaces for future collapse events. The fracture systems and altered rock masses occur non-uniformly, as an orthogonal set with N–S and E–W trends. Because these surfaces occur non-uniformly, hazards associated with collapse are unevenly distributed about the volcano. Depending on uncertainties in bounding surfaces, but constrained by detailed field studies, potential failure volumes are estimated to range between 0.04–0.5 km³. Stability modeling was used to assess potential edifice failure events. Modeled failure of the outer portion of the cone initially occurs as an "intact block" bounded by steeply dipping joints and outwardly dipping flow contacts. As collapse progresses, more of the inner cone fails and the outer "intact" block transforms into a collection of smaller blocks. Eventually, a steep face develops in the uppermost and central portion of the cone. This modeled failure morphology mimics collapse amphitheaters present at many of the world's stratovolcanoes that have experienced massive failure events.Editorial responsibility: H. Shinohara