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[en] The James Bay Lowlands, which is the SE part of the Hudson Bay Lowlands, Canada, and within the Paleozoic limestone terrane, is covered mostly by peatlands. Peat samples were examined in the Attawapiskat area, a region of discontinuous permafrost, where more than 19 kimberlite pipes have been found beneath a cover of peat (2-4 m thick) and Quaternary sediments (up to 20 m thick) of Tyrell Sea clay beds and glacial tills. Pore water at a depth of 40 cm in the peat has a consistently low pH, <4, and high Eh, ∼290 mV, in the areas over limestones far from kimberlites. On the other hand, peat pore water close to kimberlites has a high pH, up to 6.7, and low Eh, down to 49 mV; the values of pH and Eh are inversely correlated. The high pH and low Eh close to kimberlites suggest active serpentinization of olivine in the underlying kimberlites. The bulk compositions of peat indicate precipitation of secondary CaCO3 and Fe-O-OH. The secondary carbonate contains high concentrations of kimberlite pathfinder elements, such as Ni, rare earth elements (REE) and Y. The ratios of metal concentrations extracted by ammonium acetate solution at pH 5 (AA5) to those in a total digestion confirm that a majority of the divalent cations are hosted by the secondary carbonate, whereas tri-, tetra- and penta-valent cations are not. As these charged cations are not leached in Enzyme Leach, they are most likely adsorbed on Fe-O-OH. The compositions of peat show spatial variation with the distribution of kimberlites, suggesting that they are influenced by the underlying rocks even through there are thick layers of tills and sediments between the bedrocks and peat. However, elevated concentrations of pathfinder elements of kimberlites in bulk peat samples and AA5 leach are not necessarily directly above kimberlites. The diffused metal anomalies around kimberlites are attributed to the dissolution-precipitation of secondary phases (carbonates and Fe-O-OH) in acidic and reduced waters in peat, and the movement of waters through peat. This pilot study suggests that peat compositions do reflect the underlying bedrock compositions. For kimberlite exploration, a geochemical survey of peat is useful to discriminate concealed kimberlites from other anomalies defined by geophysical and other techniques; however, such a geochemical survey is not suitable for delineating the shapes of the concealed kimberlites due to broad dispersed anomalies
[en] We report here the first synchrotron radiation (SR)-based X-ray absorption fine structure (XAFS) spectra for Os at LIII-edge in geo- and cosmo-chemical materials. Samples were Negrillos meteorite, a II-A type iron meteorite, and a Pt ore specimen, aggregates of coarse grained Pt-Fe alloys, from the Choco district, Columbia. X-ray absorption near-edge structure (XANES) spectra of Os are presented for Negrillos, which has an Os abundance of 65 ppm, as well as for the platinum-group mineral (PGM). The XANES spectra showed that Os in the iron meteorite and the PGM chiefly exist as a metallic species. The average interatomic distance of Os in the Pt ore from the Choco district are calculated from the EXAFS spectra. The results suggest that the neighboring atoms of Os are not Pt. Instead, they are most likely Os and Ir. This is consistent with the common occurrence of fine lamellae of Os and iridosmine as exsolution products of Pt-Fe alloys. Furthermore, this confirms the limited solubility of Os in Pt-Fe alloys. EXAFS could thus be useful for identifying the host phases of Os in natural samples. This study has verified that the chemical state of Os in natural samples can be determined using XAFS. It is expected that further application of the XAFS technique to Os speciation in natural samples will help to elucidate Re/Os fractionation processes in the Re-Os isotopic system. (author)
[en] Highlights: → Gases released from Xinjiang mud volcanoes are dominated by thermogenic origin. → Secondary microbial activities occurring closer to the surface dramatically changed the δ13CCO2. → The water-rock interaction occurred at deeper level than gas and petroleum reservoir. - Abstract: A comprehensive study was performed to characterize, for the first time, the mud, water, and gases released from onshore mud volcanoes located in the southern margin of the Junggar Basin, northwestern China. Chemical compositions of mud, along with the geology of the basin, suggest that a source of the mud is Mesozoic or Cenozoic shale. Oxygen and H isotope compositions of the released water suggest a local meteoric origin. Combined with the positive Eu anomalies of the water, a large 18O shift of the water suggests extensive interaction with rocks. Gases discharged from the mud volcanoes are predominantly thermogenic hydrocarbons, and the high δ13C values (>+20 per mille VPDB) for CO2 gases and dissolved carbonate in muddy water suggest secondary methanogenesis with CO2 reduction after oil biodegradation. The enrichments of Eu and 18O in water and the low thermal gradient of the area suggest that the water-rock interactions possibly occur deeper than 3670 ± 200 m. On the other hand, considering the relationship to the petroleum reservoir around the mud volcanoes, the depth of the gases can be derived from about 3600 m, a depth that is greater than that generally estimated for reservoirs whose gas is characterized by 13C-enriched CO2. Oil biodegradation with CO2 reduction likely occurs at a shallower depth along the seepage system of the mud volcano. The results contribute to the worldwide data set of gas genesis in mud volcanoes. Moreover, they further support the concept that most terrestrial mud volcanoes release thermogenic gas produced in very deep sediments and may be early indicators of oil biodegradation, an important problem in the petroleum industry.