Redox-sensitive trace elements document chemical depositional environment and post-depositional oxidation of the Ediacaran Biri Formation, Southern Norway

2014 Summer.Includes bibliographical references.The Ediacaran Biri Formation comprises carbonate and silisiclastic facies including a ~ 70 m thick organic-rich shale facies exposed 9 km west of Biri, Norway in a steep bedrock stream channel at Djupdalsbekken. This outcrop is overlain by ~ 30 m of coarse-clasitic conglomeritic facies of the Ring Formation, deposited in the southern and western portion of the Hedmark basin as prograding subaerial and submarine delta fans. Concentrations and distributions of some redox-sensitive trace elements, specifically molybdenum and uranium, within the Biri Formation shale indicate deposition under sub-oxic to anoxic conditions. Pyrite framboid size distribution corroborates trace element evidence and suggests that sulfidic conditions existed within the sediment with a chemocline at or near the sediment-water interface. An attempt to date the Biri Formation shale by Hannah et al. (2007) found disturbed Re-Os isotope systematics from samples in the first 8 meters of the exposure, while data obtained from samples further down section were undisturbed. Here, an attempt to understand these disturbed and undisturbed sections using redox-sensitive trace element chemistry suggests the disturbed data was a result of post-depositional re-oxygenation within the upper few meters of the Biri shale. This is indicated by concentration peaks in trace element profiles that result from remobilization and subsequent re-fixation of these elements at different locations in the shale. A well constrained hypothesis constructed using uranium and molybdenum as proxies for rhenium shows that rhenium was likely remobilized after deposition of the Biri Formation and either subsequently re-deposited, or flushed out of the system. In this scenario, the post-depositional remobilization of rhenium (and most likely osmium also) resulted in disturbed Re-Os isotope systematics described by Hannah et al. (2007). Trace element geochemistry, petrographic, and δ13C and δ18O stable isotope evidence document post-depositional re-oxygenation of the Biri Formation shale. Re-oxygenation occurred either synchronous to deposition of the overlying Ring Formation or during a later event, the Caledonian orogeny (~ 440 Ma) being the most likely candidate. While the geochemical evidence does not preclude one time period or the other, disturbed Re-Os isotope systematics and resulting dates given by Hannah et al. (2007) can only be supported by re-oxygenation of the Biri Formation shale during the Caledonian orogeny

Zone Leveling Crystal Growth of Thermoelectric PbTe Alloys with Sb_(2)Te_3 Widmanstätten Precipitates

Unidirectional solidification of PbTe-rich alloys in the pseudobinary PbTe-Sb_(2)Te_3 system using the zone leveling technique enables the production of large regions of homogeneous solid solutions for the formation of precipitate nanocomposites as compared with Bridgman solidification. (PbTe)_(0.940)(Sb_(2)Te_3)_(0.060) and (PbTe)_(0.952)(Sb_(2)Te_3)_(0.048) alloys were successfully grown using (PbTe)_(0.4)(Sb_(2)Te_3)_(0.6) and (PbTe)_(0.461)(Sb_(2)Te_3)_(0.539) as seed alloys, respectively, with 1 mm h^(–1) withdrawal velocity. In the unidirectionally solidified regions of both alloys, Widmanstatten precipitates are formed due to the decrease in solubility of Sb_(2)Te_3 in PbTe. To determine the compositions of the seed alloys for the zone leveling experiments, the solute distribution in solidification in the PbTe-richer part of the pseudobinary PbTe-Sb_(2)Te_3 system has been examined from the concentration profiles in the samples unidirectionally solidified by the Bridgman method

Size control of Sb_2Te_3 Widmanstätten precipitates in thermoelectric PbTe

The number density and area per unit volume of Sb_2Te_3 Widmanstätten plates in thermoelectric PbTe were controlled through two types of heat treatments of (PbTe)_(1−x)-(Sb_2Te_3)_x, where x = 0.04 and 0.06: isothermal annealing at various temperatures and cooling from a solid-solution regime to a two-phase region with various rates. The microstructure was quantified by image analysis of scanning electron micrographs and Rietveld refinements of X-ray diffraction profiles. Isothermal annealing of (PbTe)_(0.94)-(Sb_2Te_3)_(0.06), results in increasing number density and area per volume of precipitates with decreasing temperature. In controlled cooling rate experiments, faster cooling rates or smaller x result in higher number density and area per volume. These trends are discussed using phase transformation theories. Overall the number density and area per volume of precipitates were controlled in the ranges from 0.4 to 44 μm^(−3) and from 0.5 to 1.8 μm^(−1), respectively. Isothermal annealing was performed for time periods from 10 to 166 h at 723 K to check the stability of the microstructure at the (PbTe)_(0.94)-(Sb_2Te_3)_(0.06) composition. While the Boyd and Nicholson model of the Greenwood–Lifshitz–Slyozov–Wagner theory for the average diameter of plates gives a reasonable value for peripheral interfacial energy, the time dependence was found to decelerate more than the t^(1/3) rule. It has also been found that the coarsening mechanism involves the elongation of plates