A micro-Mössbauer study of chromites included in diamond and other mantle-related rocks

Oxygen fugacity ( $ f_{{{\text{O}}_{ 2} }} $ f O 2 ) is a fundamental but little known intensive variable in mantle processes. It influences the P/T position of a mantle solidus and the composition of mantle-derived melts and fluids and constrains mantle-core equilibria and a number of geophysical properties of the mantle. An important source of information on oxidation states is the ferric–ferrous iron ratio in mantle spinels. Since the magnetite component is low in mantle spinels, normal analytical errors translate into considerable $ f_{{{\text{O}}_{ 2} }} $ f O 2 uncertainties. In this study, we analyzed the Fe³⁺–Fe_tot ratio of chromites present as inclusions in diamond and other mantle-related occurrences by point-source Mössbauer spectroscopy using single-crystal absorbers as well as conventional Mössbauer spectroscopy using powder absorbers. The studied spinels have been previously analyzed by single-crystal X-ray diffraction and electron microprobe. The ferric–ferrous ratios found are normally similar to the different techniques apart from some samples where a large number of grains have been used for the analyses (powder absorbers). The general agreement between the different techniques allows us to conclude that the studied chromites are stoichiometric. However, conventional Mössbauer spectroscopy on powder absorbers should be conducted with great care, since the method requires a relatively large amount of sample material. Spinel frequently occurs as small grains, and the large number of crystals required may possess different degrees of oxidation/alteration and, consequently, different ferric–ferrous ratio leading to possible errors in the interpretation of the results.     

Atmospheric Pb and Cd input into the Baltic Sea: a new estimate based on measurements

Bulk deposition samples were collected during a summer (1997) and a winter (1998) measurement campaign at four coastal stations along the southern Baltic Sea coast and on the Island of Gotland. The data were used to construct Pb and Cd deposition fields over the Baltic Sea. A weak gradient with decreasing deposition rates from the southwest towards the east and north was obtained for Pb. In the case of Cd, the spatial distribution pattern was characterized by an extreme deposition maximum at the Polish station on the Hel Peninsula. The total atmospheric input of Pb and Cd into the Baltic Sea was 550 and 33 t/year, respectively, and exceeds the riverine input by approximately about 50%. Previous measurement-based estimates were higher by a factor 2–3 and indicate a decrease of the atmospheric deposition during the past 10–15 years. The comparison with modelled deposition data yielded partly large differences and was impaired by the fact that 1990 emission inventories were used whereas our measurements were performed in 1997/1998.Relating our deposition estimate and the Pb/Cd input by rivers to the mean concentrations in Baltic Sea water, residence times of 0.29 and 3.6 years were obtained for Pb and Cd, respectively.     

Anasazi (Pre-Columbian Native-American) Migrations During The Middle-12Th and Late-13th Centuries – Were they Drought Induced?

Severe droughts in the middle-12th and late-13th centuries appear to have affected Anasazi (pre-Columbian Native American) populations. During the first drought most of the great houses in the central San Juan Basin were vacated; the second drought resulted in the abandonment of the Four Corners region. During the first drought, villages may not have been completely abandoned. The multi-year drought periods probably were characterized by reductions in both winter and summer precipitation. Maize is dependent on winter precipitation for its germination and initial growth and on summer (monsoonal) precipitation for its continued growth. Reductions in precipitation are hypothesized to have resulted in low yields of maize, the dietary staple of the Anasazi. A comparison of historic climate data and tree-ring-based reconstructions of precipitation in the Four Corners region with tree-ring-based reconstructions of the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) indicate that severe and persistent drought in the Four Corners region occurs when the PDO is negative and the AMO is positive. Historic climate data from the greater San Juan Basin indicate that a negative PDO is characterized by reductions in both water-year and summer precipitation, reinforcing the concept that at least some multi-year droughts involved weakening of the summer monsoon with attendant decreases in the yields of maize.     

Shear-wave polarizations in the Peter the First Range indicating crack-induced anisotropy in a thrust-fault regime

Summary. Three-component seismograms of small local earthquakes recorded in the Peter the First Range of mountains near Garm, Tadzhikistan SSR, display shear-wave splitting similar to that previously observed near the North Anatolian Fault in Turkey. The Peter the First Range is in a region of compressional tectonics, whereas the North Anatolian Fault is a comparatively simple strike-slip fault. Detailed analysis of the Turkish records suggests that the splitting is diagnostic of crack-induced anisotropy caused by vertical microcracks aligned parallel to the direction of maximum compression. Preliminary examination of paper records from Garm shows that most shear waves arriving within the shear-wave window display shear-wave splitting, and that the polarizations of leading shear-waves are consistently aligned in a NE/SW direction. The area is complicated and the tectonics are not well-understood, but the NE/SW direction is approximately perpendicular to the compressional axis in many of the fault-plane mechanisms of the earthquakes. These earthquakes are usually at depths between 5 and 12 km, although there are some deeper events nearby.
Parallel shear-wave polarizations, such as those observed, are expected to indicate the strike of nearly vertical parallel microcracks, which would be aligned parallel to the direction of maximum compression. Thus the shear-wave polarizations in the Peter the First Range indicate that the directions of principal stress are reversed in the rock above the earthquake foci where thrust faulting is taking place.     

Oxidized and reduced mineral assemblages in greenstone belt rocks of the St. Ives gold camp, Western Australia: vectors to high-grade ore bodies in Archaean gold deposits?

Hydrothermal sulfide–oxide–gold mineral assemblages in gold deposits in the Archaean St. Ives gold camp in Western Australia indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time. Reduced alteration assemblages (pyrrhotite–pyrite) occur in deposits in the southwest of the camp (e.g., Argo, Junction deposits) and moderately to strongly oxidized assemblages (magnetite–pyrite, hematite–pyrite) occur in deposits in the Central Corridor in the northeast (e.g., North Orchin, Revenge deposits). Reduced mineral assemblages flank the Central Corridor of oxidized deposits and, locally, cut across it along E–W trending faults. Oxidized mineral assemblages in the Central Corridor are focused on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1,000 m below present surface. Hydrothermal magnetite predates and is synchronous with early phases of gold-associated albite–carbonate–pyrite–biotite–chlorite hydrothermal alteration. Later-stage, gold-associated pyrite is in equilibrium with hematite. The spatial distribution and temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly synchronous hydrothermal fluids with contrasting redox states. Sulfur isotope constraints support the argument that the different mineral assemblages reflect differences in redox conditions. The δ ³⁴S values for pyrite for the St. Ives gold camp range between −8.4‰ and +5.1‰ with the negative values occurring in oxidized magnetite-rich domains and slightly negative or positive values occurring in reduced, pyrrhotitic domains. Preliminary spatial and paragenetic analysis of the distribution of iron sulfides and oxides in the St. Ives camp suggests that gold grades are highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite–pyrite to relatively oxidized magnetite–pyrite and hematite–pyrite both in space and time. Gold deposition is inferred to have occurred where fluids of contrasting redox state mixed.     

Complex mathematical model of the WIG motion including the take-off mode

This paper describes an effort to develop a predictive tool for the design of a new promising marine transport — the WIG craft. The presented mathematical model of the WIG craft is capable of modeling the aerodynamics of a WIG system including the ground effect, the hydrodynamics of a stepped planing hull with a hydrofoil and also the simulation of motion for the craft. Based on extensive experience using the model, it is shown that the most important and necessary features of WIG aero- and hydrodynamics are taken into account. The results of simulations have been validated through comparison with other theoretical approaches and also with model experiments. The mathematical model is applied to investigate the dynamics of the small manually piloted WIG craft: “Hydrowing VT01”. The numerical study resulted in recommendations allowing the pilot to overcome the pitch-up tendency and also to perform the take-off manoeuvre smoothly. The stability of the WIG with a hydrofoil and also the dynamic properties of anti-collision manoeuvres have been studied and are presented.     

Distinguishing between natural and aquaculture-derived sediment concentrations of heavy metals in the Broughton Archipelago, British Columbia

Marine sediment samples were collected in the Broughton Archipelago, British Columbia, to assess the use of a geochemical normalization technique in the identification of a chemical tracer of aquaculture waste material. Zinc and copper were suggested as tracers of feed pellets, while copper was considered an indicator of anti-foulant agents used on netpen systems. The sediment samples were analyzed for carbon, nitrogen, organic matter, water, trace-element, and free sulfide concentrations, and sediment grain-size distribution. Sediment texture analysis revealed a wide range of substrate types from sand to silty loam categories. Strong relationships between sediment texture, sediment porosity, and organic content were observed across both near-field and far-field stations. Excess zinc and copper sediment concentrations, identified using a lithium-normalization technique, were restricted to near-field sampling stations (0 and 30m from netpen systems). The relationships between these metal tracers and organic content and sulfur concentrations were explored to account for variations in sediment concentrations of zinc and copper.     

Application of stable isotope analysis for improved understanding of the methane budget: comparison of TROICA measurements with TM3 model simulations

Presented is a detailed comparison of CH₄ and δ¹³C–CH₄ measurements with simulations of the global transport model TM3. Experimental data were obtained during campaigns along the Trans-Siberian railroad in the framework of the TROICA project. Two summer (1999 and 2001) and one spring (2003) expeditions are evaluated. Model simulations include sensitivity tests to further investigate the isotopic composition of natural gas and emissions from Siberian wetlands. Comparison of the average mixing ratio of methane and its isotopic composition (δ¹³C) has been performed for different geographic zones, including the European part of Russia, Western Siberia and Central Siberia. Simulations are in reasonable agreement with the measurements for the European part of Russia and confirm a high contribution of natural gas to the observed methane levels. An increase of emission from bogs shifts the simulated methane isotopic composition closer to the observations. The relative importance of the Western Siberia emissions in current inventories is underestimated in comparison with other wetland regions in the former USSR. Simulated average mixing ratios are in a good agreement with the observations in Central Siberia, while ¹³C(CH₄) values tend to be higher than measured in all considered scenarios. These results point to a bias in the modeled source mixture over Russia, which could be repaired by shifting emissions from isotopically heavy methane sources (e.g. coal, oil or biomass burning) to light sources (e.g. wetlands, ruminants, waste treatment). Alternatively, the average isotopic signature of Siberian wetlands may be lighter than expected.     

Rare-earth element fractionation and petrogenetic modelling in charnockitic rocks,southwest Norway

Quantitative analysis of REE distribution in differentiated, clearly intrusive charnockites from southwest Norway, reveal extreme variation in both absolute abundances and fractionation patterns. The pyroxene-bearing, charnockitic facies show uniform REE patterns with slight enrichment of the light-REE (about 150x chondrites) and positive, neutral or negative Eu-anomalies. Subsequent amphibole-bearing, adamellitic facies show progressive, preferential enrichment of light-REE, reaching La-values higher than 500x chondrites, and increasingly negative Euanomalies. Finally, highly differentiated biotite-granites show a marked depletion of ight-REE, ending with chondrite-normalized La/Lu ratios about 1 and Eu/ Eu^* ratios less than 0.2.Using geochemical model calculation, relating major element variations between three main stages of differentiation in terms of refractory mineral assemblages, stepwise quantitative modelling of the REE distributions reproduces the observed changes, and support an origin of the charnockite series as progressively fractionated residual liquids.Close similarity with the REE patterns of charnockite-rapakivi suites elsewhere implies that these may constitute a series of co-magmatic rocks, derived from related more basic source magmas. The fact that the least differentiated members of the series, the charnockites, generally display remarkably uniform REE-patterns, suggests that they equilibrated with a refractory crystal fraction that produces a uniform, bulk partition coefficient. The present analysis suggests that this would be plagioclase and orthopyroxene in a ratio of about 41 and including minor apatite, which in turn points towards affinity with potentially plagioclase-rich cumulates.