Sulfur isotope studies in early Archaean sediments from Isua,West Greenland: Implications for the antiquity of bacterial sulfate reduction

The sulfur contents and sulfur isotope ratios (δ³⁴S) have been measured for samples collected from the Isua area of West Greenland in an effort to place narrower limits on the time of the rise of sulfate respirers during the Precambrian.The δ³⁴S values of the Isua sediments (3.7 × 10⁹ yr old) including the various facies of the banded iron-formations have their mean values close to zero %. (CDT) (±0.5%.) with a standard deviation of less than 1%.. This comes extremely close to the respective means yielded by the presumed tuffaceous amphibolites (+ 0.3 ± 0.9%.) and by the somewhat younger, between 3.1 and 3.7 ± 10⁹ yr, basaltic Ameralik dykes of the region (+ 0.6 ± 1.1%.).In view of the regional distribution of the Isua banded iron-formation sediments, the variety of environmental conditions under which the various facies were deposited and the complete absence of isotopic evidence for sulfate reducers, in contrast to the banded iron-formations of the middle Archaean (δ³⁴Svariesfrom ?20 to +20%.), it seems most unlikely that evidence for 'sulfate reducers' existed or will be found in other sediments of Isua age.The very small spread in δ³⁴S values for the Isua sediments is interpreted as due to minor fractionation during the passage of endogenic sulfur phases to their present sites of emplacement within the sedimentary succession.     

Surveying the anthropogenic impact of the Moldau river sediments and nearby soils using magnetic susceptibility

Measuring magnetic susceptibility is a method which is used to estimate the amount of magnetic particles in soils, sediments or dusts. Changes in magnetic susceptibility can be due to various reasons: input from different sources of sediments, e.g. from different soils or rocks, atmospheric fallout of anthropogenic dusts containing magnetic particles produced by fossil fuel combustion, steel production or road traffic. In the case of river sediments, input from the catchment is of primary significance. The main aim of this investigation was to test the potential of magnetic susceptibility screening in identifying the effect and significance of anthropogenic activities in an area with complex geological conditions. We investigated the magnetic susceptibility of riverbed sediments of the largest river of the Czech Republic, the Moldau river. Besides that, the magnetic signal of nearby topsoils as well as of outcropping bedrocks in the vicinity of the river was examined. In the upper 300 km of the river, the magnetic enhancement of the river sediments can be linked to anthropogenic activities. Positive correlations were found in the river sediments between the contents of Cu and Zn and magnetic susceptibility, while Fe, Mn and Ni did not show a correlation with magnetic susceptibility. However, the major geogenic magnetic anomaly in the area around the Slapy dam has made it impossible to unambiguously interpret the magnetic signal in terms of anthropogenic impact in the last 80 km downstream.     

Dependence of microbial magnetite formation on humic substance and ferrihydrite concentrations

Iron mineral (trans)formation during microbial Fe(III) reduction is of environmental relevance as it can influence the fate of pollutants such as toxic metal ions or hydrocarbons. Magnetite is an important biomineralization product of microbial iron reduction and influences soil magnetic properties that are used for paleoclimate reconstruction and were suggested to assist in the localization of organic and inorganic pollutants. However, it is not well understood how different concentrations of Fe(III) minerals and humic substances (HS) affect magnetite formation during microbial Fe(III) reduction. We therefore used wet-chemical extractions, magnetic susceptibility measurements and X-ray diffraction analyses to determine systematically how (i) different initial ferrihydrite (FH) concentrations and (ii) different concentrations of HS (i.e. the presence of either only adsorbed HS or adsorbed and dissolved HS) affect magnetite formation during FH reduction by Shewanella oneidensis MR-1. In our experiments magnetite formation did not occur at FH concentrations lower than 5 mM, even though rapid iron reduction took place. At higher FH concentrations a minimum fraction of Fe(II) of 25-30% of the total iron present was necessary to initiate magnetite formation. The Fe(II) fraction at which magnetite formation started decreased with increasing FH concentration, which might be due to aggregation of the FH particles reducing the FH surface area at higher FH concentrations. HS concentrations of 215-393 mg HS/g FH slowed down (at partial FH surface coverage with sorbed HS) or even completely inhibited (at complete FH surface coverage with sorbed HS) magnetite formation due to blocking of surface sites by adsorbed HS. These results indicate the requirement of Fe(II) adsorption to, and subsequent interaction with, the FH surface for the transformation of FH into magnetite. Additionally, we found that the microbially formed magnetite was further reduced by strain MR-1 leading to the formation of either dissolved Fe(II), i.e. Fe²⁺, in HEPES buffered medium or Fe(II) carbonate (siderite) in bicarbonate buffered medium. Besides the different identity of the Fe(II) compound formed at the end of Fe(III) reduction, there was no difference in the maximum rate and extent of microbial iron reduction and magnetite formation during FH reduction in the two buffer systems used. Our findings indicate that microbial magnetite formation during iron reduction depends on the geochemical conditions and can be of minor importance at low FH concentrations or be inhibited by adsorption of HS to the FH surface. Such scenarios could occur in soils with low iron mineral or high organic matter content.     

Strata bound scheelite in the Archean Malene supracrustal belt,West Greenland

In the Archean of Western Greenland a tungsten province ca. 300 km long and up to 120 km wide has been discovered with extensive banded amphibolites containing up to 2% W and 0.16 ppm gold. The tungsten occurs as scheelite which is associated with tourmalinites and strata-bound tourmaline-rich layers in amphibolites of presumed tuffaceous origin and with an iron-formation containing high amounts of tungsten, zinc, copper, lead, molybdenum, and tin. The scheelite is shown to be strata-bound and of submarine exhalative origin.     

北京东郊722土壤垂向剖面重金属污染的磁学响应及其统计意义

对北京城东附近某苗圃内典型的土壤污染剖面进行了环境磁学和地球化学分析,发现某些重金属(Pb、Zn、Sr、Ba、Cu)和磁性参数呈现相同的垂向变化趋势,都在37cm以上显示高值区,磁化率均值达到192.02×10-8m3/kg,如Pb含量达到67.62mg/kg,而在37cm以下,明显属低值区,磁化率均值只有18.38×10-8m3/kg,Pb含量也只有23.43mg/kg.借助于指标聚类分析和主成分分析方法,揭示出各种指标之间的内在联系,表明磁参数与Pb、Zn、Sr、Ba、Cu等元素显著相关,彼此的相关系数都达到0.90以上,属于同一类别的隶属度在80%以上,说明磁指标可以作为这些重金属污染的一种代用指标.利用模糊C均值聚类分析分辨出了土壤上部污染物堆积层和下部未污染土壤背景2种不同的特征段.     

The enhanced element enrichment in the supercritical states of granite–pegmatite systems

In this paper, we show that supercritical fluids have a greater significance in the generation of pegmatites, and for ore-forming processes related to granites than is usually assumed. We show that the supercritical melt or fluid is a silicate phase in which volatiles; principally H₂O are completely miscible in all proportions at magmatic temperatures and pressures. This phase evolves from felsic melts and changes into hydrothermal fluids, and its unique properties are particularly important in sequestering and concentrating low abundance elements, such as metals. In our past research, we have focused on processes observed at upper crustal levels, however extensive work by us and other researchers have demonstrated that supercritical melt/fluids should be abundant in melting zones at deep-crustal levels too. We propose that these fluids may provide a connecting link between lower and upper crustal magmas, and a highly efficient transport mechanism for usually melt incompatible elements. In this paper, we explore the unique features of this fluid which allow the partitioning of various elements and compounds, potentially up to extreme levels, and may explain various features both of mineralization and the magmas that produced them.

     

The origin and compositions of Mesoarchean oceanic crust: Evidence from the 3075 Ma Ivisaartoq greenstone belt, SW Greenland

The Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt contains well-preserved primary magmatic structures, such as pillow lavas, volcanic breccias, and clinopyroxene cumulate layers (picrites), despite the isoclinal folding and amphibolite facies metamorphism. The belt also includes variably deformed gabbroic to dioritic dykes and sills, actinolite schists, and serpentinites. The Ivisaartoq rocks underwent at least two stages of post-magmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. These alteration processes resulted in the mobilization of many major and trace elements. The trace element characteristics of the least altered rocks are consistent with a supra-subduction zone geodynamic setting and shallow mantle sources. On the basis of geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq greenstone belt represents a relic of dismembered Mesoarchean supra-subduction zone oceanic crust. This crust might originally have been composed of a lower layer of leucogabbros and anorthosites, and an upper layer of pillow lavas, picritic flows, gabbroic to dioritic dykes and sills, and dunitic to wehrlitic sills.

The Sm–Nd and U–Pb isotope systems have been disturbed in strongly altered actinolite schists. In addition, the U–Pb isotope system in pillow basalts appears to have been partially open during seafloor hydrothermal alteration. Gabbros and diorites have the least disturbed Pb isotopic compositions. In contrast, the Sm–Nd isotope system appears to have remained relatively undisturbed in picrites, pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (ε_Nd = + 4.23 to + 4.97) than pillow lavas, gabbros, and diorites (ε_Nd = + 0.30 to + 3.04), consistent with a variably depleted, heterogeneous mantle source.

In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (> 90%) of Ca-rich plagioclase and are interpreted as anorthositic cumulates brought to the surface by upwelling gabbroic magmas. The anorthositic cumulates have significantly higher initial ε_Nd (+ 4.8 to + 6.0) values than the surrounding gabbroic matrix (+ 2.3 to + 2.8), consistent with different mantle sources for the two rock types.     

Gyromagnetic remanence acquired by greigite (Fe3S4) during static three-axis alternating field demagnetization

A magnetic study was carried out on lacustrine sediments from the Zoigê basin, Tibetan Plateau, in order to obtain a better understanding of palaeoclimatic changes there. Gyromagnetic remanence (GRM) acquisition is unexpectedly observed during static three-axis alternating field (AF) demagnetization in about 20 per cent of a large number of samples. X-ray diffraction (XRD) analysis on a magnetic extract clearly shows that greigite is the dominant magnetic mineral carrier. Scanning electron microscopy (SEM) reveals that the greigite particles are in the grain size range of 200–300  nm, possibly in the single-domain state. Greigite clumps of about 3  μm size are sealed by silicates. Fitting of XRD peaks yields a crystalline coherence length of about 15  nm, indicating that the particles seen in the SEM are polycrystalline.
  GRM intensities of most samples are of the same order as the NRM, while others show much stronger GRM although their magnetic properties are similar. Variation of the demagnetization sequence confirms that GRM is mainly produced perpendicular to the AF direction. The anisotropy direction can be derived from GRM, but more systematic studies are needed for detailed conclusions. An attempt to correct for GRM failed due to high GRM intensities and because smaller GRM acquisition was also found along the demagnetization axis. Behaviours of acquisition and AF demagnetization of GRM are comparable with those of NRM, ARM, IRM, indicating fine grain sizes of remanence carriers.     

On the early archaean isua iron-formation,West Greenland

In the pre-3.7 Ga old Isua supracrustal belt, West Greenland, a banded iron-formation occurs. The iron-formation can be subdivided into different facies according to composition and mineralogy, and these facies resemble the facies subdivision of younger Archaean and Precambrian iron-formations. The geochemistry of the Isua iron-formation indicates that the secular variation in the contents of phosphorus, calcium and aluminium as well as the Na/K ratios of Precambrian iron-formations can be extended into the Early Archaean. A remarkable feature of the Isua iron-formation is the high chalcopyrite/iron-sulphide ratio. Field relationships and geochemical evidence indicate that the iron-formation is mainly of submarine-exhalative origin from brines of basaltic pedigree.