The Taihua group on the southern margin of the North China craton: further insights from U–Pb ages and Hf isotope compositions of zircons

The "Taihua Group" is a collective term for a series of old terranes scattered along the southern margin of the North China Craton. The timing of formation and thermal overprinting of the Taihua Group have long been contentious, and its relationship with the Qinling orogenic belt has been unclear. In this study, new data from integrated in-situ U–Pb dating and Hf isotope analysis of zircons from an amphibolite (from the Xiong’ershan terrane) and a biotite gneiss (from the Lantian-Xiaoqinling terrane) indicate that the Upper Taihua Group formed during the Paleoproterozoic (2.3–2.5 Ga) and thus was originally part of the southern edge of North China Craton, detached during the Mesozoic Qinling orogeny and displaced about 100 km north from its original location. This suggests that the Taihua Group became part of the tectonic terrane associated with the Qinling orogeny and now forms part of the overthrust basement section of the Qinling belt. Before the Qinling orogeny, the Taihua Group was metamorphosed at 2.1 Ga. The initial Hf-isotope compositions of zircons, together with positive εNd(t) values for the whole-rocks, imply that the original magmas were derived from a juvenile source with some assimilation of an Archean crustal component.     

Improved Platinum-Group Element Extraction by NiS Fire Assay from Chromitite Ore Samples Using a Flux Containing Sodium Metaphosphate

Many chromite-rich rocks contain relatively high concentrations of the platinum-group elements (PGE). In many cases, the phases carrying PGE occur as either platinum-group minerals (PGM) or as base metal sulfides in solid solution in sulfides. In some cases, such as the UG-2 unit of the Bushveld Complex, the PGM are occluded inside chromite grains. Chromites are notably difficult to dissolve in most fluxes and if the chromite contains some PGM the possibility exists that not all the PGE will be recovered during fusion. In this work, shortcomings in published methods of analysis based on the nickel sulfide fire assay procedure were investigated and a new procedure developed based on the addition of sodium metaphosphate to the fusion mixture. Optimum composition of the fusion mixture was found to be 10 g sodium metaphosphate and 9 g silica to 10 g sample, 15 g sodium carbonate, 30 g lithium tetraborate, 7.5 g nickel and 4.5 g sulfur to achieve complete dissolution of chromite grains. The new flux mixture was evaluated by the analysis of reference material CHR-Pt+ (which is known to contain PGM inside chromite grains) and no undissolved chromite grains were found in the glassy slag. Analysis of the nickel sulfide beads from this fire assay using neutron activation analysis showed similar results for Rh and Ru when compared with published conventional true (or accepted) values, while Au, Ir, Os, Pd and Pt values determined here were 10 to 30% higher than the corresponding published conventional true values. It was concluded that the addition of sodium metaphosphate improved chromite dissolution in the flux and appears to improve PGE recovery.     

The kinetic studies of the cobalt ion removal from aqueous solutions by dolomite-based sorbent

The kinetics of Co(II) ions adsorption on thermally activated dolomite was studied with respect to the calcination temperature of natural dolomite. The sorption of Co(II) onto all samples is reasonably fast: The first 30–35 min accounts for approximately 70–80 % of Co(II) removal from feed solutions. In order to select the main rate-determining step in the overall uptake mechanism, a series of experiments were performed and data obtained were interpreted in terms of film diffusion control, intraparticle diffusion, pseudo-first-order and pseudo-second-order models. From the modeling of kinetic data, it can be concluded that adsorption of Co(II) ions from aqueous solution by heat-treated dolomite is a complex phenomenon and occurs in a mixed diffusion mode—the kinetic data are well described by the pseudo-second-order equation. The possible multistage sorption mechanism involving film diffusion and intraparticle diffusion control steps as well as chemical interaction between Co(II) ions and calcined dolomite is proposed.     

Iron in Precambrian rocks: implications for the global oxygen budget of the ancient Earth

Banded iron formations (BIF) are prominent in sediments older than 2 Ga. However, little is known about the absolute abundance of BIF in Archean and Early Proterozoic sediments, and the source of the Fe is still somewhat uncertain. Also unknown is the role that Fe may have played in the maintenance of low oxygen pressures in the Archean and Early Proterozoic atmosphere. An analysis of the chemical composition of Precambrian rocks provides some insight into the role of Fe in Precambrian geochemical cycles. The Fe content of igneous rocks is well correlated with their Ti content. Plots of Fe vs. Ti in Precambrian sandstones and graywackes fall very close to the igneous rock trend. Plots of Fe vs. Ti in Precambrian shales also follow this trend but show a definite scatter toward an excess of Fe. Phanerozoic shales and sandstones lie essentially on the igneous rock trend and show surprisingly little scatter. Mn/Ti relations show a stronger indication of Precambrian Mn loss, perhaps due to weathering under a less oxidizing early atmosphere. These data show that Fe was neither substantially added to nor significantly redistributed in Archean and early Proterozoic sediments. Enough hydrothermal Fe was added to these sediments to increase the average Fe content of shales by at most a factor of 2. This enrichment would probably not have greatly affected the near-surface redox cycle or atmospheric oxygen levels. Continued redistribution of Fe and mixing with weathered igneous rocks during the recycling of Precambrian sediments account for the excellent correlation of Fe with Ti in Phanerozoic shales and for the similarity between their Fe/Ti ratio and that of igneous rocks.     

Geochemistry of basic dikes in the Lanzo massif (Western Alps): Petrogenetic and geodynamic implications

The Alpine peridotite massif of Lanzo (Italy) contains three generations of basic dikes (gabbros and basalts). The older gabbros are plagioclase-rich mantle segregates while the younger gabbro dikes are cumulates very similar in chemical composition to recent oceanic gabbros and gabbros from ophiolitic complexes. They both were derived from the N-type mid-ocean ridge basalt (MORB) magmas which were progressively more depleted in incompatible elements and were probably generated during a dynamic melting of a rising mantle diapir. The basaltic dikes are the N-type MORB and closely resemble the Alpine-Apennine ophiolitic basalts. They were derived from a different upper mantle source than the parental magmas of the gabbros. The source of the basalts was less depleted in light REE. The presence of basic magmas with N-type MORB affinities in the Lanzo massif is consistent with the close genetic relationship between the Alpine peridotite body and the ophiolites of the Liguro-Piemontese basin.     

Infrared reflectivity spectra of single crystal cassiterites

The infrared reflectivity spectra of two natural cassiterite single crystals from Portugal and Vietnam have been studied and analysed in the frequency range of 15–4000 cm^–1 at room temperature. The optical mode parameters are deduced by simulation of the experimental spectra using the factorised form of the dielectric function. The difference with synthetic SnO₂ spectra is a dip near 500 cm^–1 understood as the activation of an infrared forbidden E _g mode due to the amount of Fe³⁺ and Ti⁴⁺ impurities. Another result of this work is the derivation of the correct values of the static dielectric constant of cassiterite.     

Simple trace determination of platinum in geological materials

A direct, sensitive and simple method for the determination of sub-microgram amounts of platinum in acid leachates from geological materials is described. The sample is sized to plus 80-mesh and roasted at 700°C. During this operation the most important interfering elements such as C, S, Hg, Se, Te and As are eliminated. Anions and small amounts of most cations found in strongly acid leachates from rock samples do not interfere. Hydrogen peroxide is used as the oxidant during leaching with 5–7 M HCI. Platinum is reacted with Sn²⁺ after filtration. Addition of Rhodamine 6G precipitates a red quaternary complex. The colloidal suspension is suitable for spectrophotometric determination of platinum. While the reagent absorbance at 530 nm decreases, a peak due to the colloid appears at 565 nm. Beer's law is obeyed up to 0.6 absorbance unit, which corresponds to about 1.7 μg/ml of Pt. The sensitivity (3 times the noise level) corresponds to 0.01 μg/ml of Pt. A test procedure is described for serial analysis of 1–3-g powdered samples which enables an operator to perform from 30 to 50 analyses per man-day in the concentration range of 0.1–17 ppm of platinum. The procedure has been validated against standard samples and a recovery of 90% has been obtained with unspiked samples.     

Copper(II) humate mobility in kaolinite soil

An investigation of the influence of humate on the mobility of copper(II) ions in a kaolinite soil using leaching tests and electrokinetic experiments is reported. The data are interpreted in terms of humate–copper–clay interactions and humate electrical charge. Humate is mostly immobile below pH8 but is more mobile in alkaline conditions (sorption to kaolinite reduces its mobility in neutral conditions). Copper humate complexes are mobile in both acidic and alkaline conditions, but not in neutral conditions where they are sorbed. The dissolved copper humate complexes that form in acidic conditions are positively charged. The net effect of humate is to increase cupric ion mobility in kaolinite soil, especially in alkaline conditions.     

Geochemistry and geochronology of proterozoic tholeiite dykes of east antarctica: evidence for mantle metasomatism

Two episodes of tholeiite dyke emplacement have been identified in Archaean high-grade metamorphics of the Napier Complex in Enderby Land. Middle Proterozoic Amundsen dykes are typical continental tholeiites and most of the chemical variation in individual suites can be explained in terms of different degrees of partial melting and low-pressure crystal fractionation. Group I Amundsen tholeiites were derived from a relatively homogeneous source region 1,190±200 m.y. ago, whereas that of the group II Amundsen tholeiites was chemically and isotopically heterogeneous. Group II dykes have various degrees of enrichment in incompatible elements, and commonly show normalised trace element abundance patterns with negative Nb anomalies. These features imply variable metasomatism of the source region by a volatile-rich fluid phase (rather than a melt of any observed igneous composition) enriched in K, Rb, Ba, Th, and possibly La and Ce.Early Proterozoic (2,350±48 m.y.) tholeiites were emplaced at considerable depths in the crust during the waning stages of granulite-facies metamorphism and include a high-Mg suite of possible komatiitic affinity, ranging in composition from hypersthene-rich tholeiite (norite) to quartz-rich tholeiite. They tend to have higher ratios of highly to moderately incompatible elements (e.g., K/Zr, K/Ce), and larger Nb anomalies (i.e., higher K/Nb) compared with middle Proterozoic tholeiites, suggesting derivation from more enriched source regions. Isotopic data are not compatible with significant crustal contamination, but constrain source metasomatism to a time immediately before emplacement. Metasomatism of the source region of the much younger group I tholeiites may have been contemporaneous with that of the high-Mg suite.     

汉诺坝玄武岩Re-Os同位素地球化学——Re的挥发性丢失和壳-幔相互作用的证据

本文报道了采自汉诺坝玄武岩区周坝和白龙硐剖面以及白布洛张20井等地29个玄武岩样品的Re、Os含量和~(187)Os/~(188)Os比值。Os含量为11×10~(-12)～314×10~(-12),Re含量为40×10~(-12)～238×10~(-12),Re和Os含量有正相关趋势。碱性玄武岩(AK)的Re、Os含量高于拉斑玄武岩(TH)和过渡玄武岩(TR),玄武岩Os含量变化与分离结晶作用有关,玄武岩的低Re含量与地面喷发的火山岩浆脱气过程中Re的挥发性丢失作用有关。玄武岩的~(187)Os/~(188)Os比值为0.14735～0.61136,AK的~(187)Os/~(188)Os比值比TH和TR低且变化小。玄武岩的~(187)Os/~(188)Os比值与Os含量有负相关性。随着Os含量降低到小于75×10~(-12),~(187)Os/~(188)Os比值迅速升高,反映了地壳混染在TH和TR成因中的贡献。在以往的研究中没有观察到类似的地壳混染作用,说明了Re-Os同位素体系在示踪壳源物质上的优势。一些Os含量较高的TH的~(187)Os/~(188)Os比值表明其地幔源区既非亏损的又非经交代富集的SCLM,可能是混入了地壳俯冲物质的"Marble cake"型地幔。总之,汉诺坝玄武岩的Re-Os同位素地球化学研究支持了以往研究的主要成果,两类玄武岩地球化学差异性和异源成因论;分离结晶和部分熔融过程在玄武岩成因中的重要作用;碱性玄武岩的成因与地幔柱的关系等。同时揭示了一些新的现象:汉诺坝玄武岩形成中存在少量的地壳混染作用;地面喷发的火山熔岩在脱气过程中Re的挥发性丢失;拉斑玄武岩的源区更有可能为"Marble cake"型地幔。