The structure of monomeric and dimeric uranyl adsorption complexes on gibbsite: A combined DFT and EXAFS study

We investigated the structure of uranyl sorption complexes on gibbsite (pH 5.6-9.7) by two independent methods, density functional theory (DFT) calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy at the U-L_III edge. To model the gibbsite surface with DFT, we tested two Al (hydr)oxide clusters, a dimer and a hexamer. Based on polarization, structure, and relaxation energies during geometry optimization, the hexamer cluster was found to be the more appropriate model. An additional advantage of the hexamer model is that it represents both edges and basal faces of gibbsite. The DFT calculations of (monomeric) uranyl sorption complexes show an energetic preference for the corner-sharing versus the edge-sharing configuration on gibbsite edges. The energy difference is so small, however, that possibly both surface species may coexist. In contrast to the edge sites, sorption to basal sites was energetically not favorable. EXAFS spectroscopy revealed in all investigated samples the same interatomic distances of the uranyl coordination environment (R_U-Oax ≈ 1.80 Å, R_U-Oeq ≈ 2.40 Å), and towards the gibbsite surface (R_U-O ≈ 2.87 Å, R_U-Al ≈ 3.38 Å). In addition, two U-U distances were observed, 3.92 Å at pH 9.7 and 4.30 Å at pH 5.6, both with coordination numbers of ∼1. The short U-U distance is close to that of the aqueous uranyl hydroxo dimer, UO₂(OH)₂, reported as 3.875 Å in the literature, but significantly longer than that of aqueous trimers (3.81-3.82 Å), suggesting sorption of uranyl dimers at alkaline pH. The longer U-U distance (4.30 Å) at acidic pH, however, is not in line with known aqueous uranyl polymer complexes. Based on the EXAFS findings we further refined dimeric surface complexes with DFT. We propose two structural models: in the acidic region, the observed long U-U distance can be explained with a distortion of the uranyl dimer to form both a corner-sharing and an edge-sharing linkage to neighboring Al octahedra, leading to R_U-U = 4.150 Å. In the alkaline region, a corner-sharing uranyl dimer complex is the most favorable. The U-O path at ∼2.87 Å in the EXAFS spectra arises from the oxygen atom linking two Al cations in corner-sharing arrangement. The adsorption structures obtained by DFT calculations are in good agreement with the structural parameters from EXAFS analysis: U-Al (3.394 Å), U-U (3.949 Å), and U-O (2.823 Å) for the alkaline pH model, and U-Al (3.279 Å), U-U (4.150 Å), and U-O (2.743 Å) for the acidic pH model. This work shows that by combining EXAFS and DFT, consistent structural models for uranyl sorption complexes can be obtained, which are relevant to predict the migration behavior of uranium at nuclear facilities.     

焦家式金矿水-岩交换作用——蚀变岩石氢氧同位素组成研究

焦家式台上蚀变岩型金矿地表及浅部绢英岩δ18O值平均在12.5‰，δD值在- 75‰，分别比新鲜原岩花岗闪长岩高出2‰~3‰和20‰。垂向1000 m以下，即沿矿体倾斜方向1500 m以下，绢英岩的δ18O和δD值分别降低为+8‰和-95‰左右，显示出比原岩亏损18O，但是D含量几乎不变。相应地，石英和绢云母蚀变矿物对的δ18O值分别从14‰和12‰左右，降为深部的11‰和9‰左右。综合资料表明，成矿流体是由大气降水与深部中基性岩呈面型淋滤相互作用，温度为350～400℃，有效W/R比为0.001左右时形成的演化大气降水含矿储体热液，随之上升进入破碎带充填呈线型扩散水/岩相互作用，有效W/R比超过5.0，降温的缓冲开放体系中矿化沉积。     

The structure of metamict zircon: A temperature-dependent EXAFS study

The thermal annealing (300–1700 K) of two metamict zircons (Ampagabe, Madagascar and Näegy, Japan) has been studied using X-Ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure spectroscopy (EXAFS) at Zr K-edge. Two stages of thermal annealing within the aperiodic zircon are evidenced between 293 and 1700 K. The first stage (up to 600° C) shows a decrease of the a ₀-cell parameter from 6.674 (at 300° C) to 6.610 (at 600° C)± 0.005 Å. In that temperature range, the average local environment around Zr (presence of ^VIIZr and d(Zr-Zr) 3.3–3.6 Å) shows a weak, but significant increase of the Zr-Zr correlations located at 3.3–3.4 Å, undetectable by XRD. At temperatures up to 700° C (stage 2), the XRD-Bragg component arising from crystalline zircon increases in magnitude, whereas, Zr-K EXAFS analysis indicates a progressive ^VIIZr^VIIIZr transition, associated with a recovery of the crystalline zircon medium-range environment. For both techniques, the zircon structure is fully recovered at annealing temperatures up to 900° C.Electrostatic modelings suggest that the ^VIIIZr^VIIZr transition observed in zircon with increasing alpha-decay damage creates significantly overbonded oxygen atoms around Zr. With increasing temperature, those oxygen atoms are better bonded to ^VIIZr, due to the thermal expansion of the Zr-O bond. The congruent recovery of the zircon structure should therefore be favoured with increasing temperature. On the other hand, the metamict network can be also partially reorganized around 400–500° C, with the creation of Zr-rich domains, as measured by EXAFS. However, the growth of these domains after 3 hours annealing affects only minor portions of the aperiodic network. This model is corroborated by a similar thermal behaviour observed for a synthetic sol-gel of ZrO₂ · SiO₂ composition.     

焦家式金矿水-岩交换作用——成矿流体氢氧同位素组成研究

焦家式金矿位于胶东半岛西北部，矿体赋存于花岗片麻岩－花岗闪长岩类岩石断裂带内，呈破碎黄铁绢英岩化浸染状矿石。矿体中均伴有富含金的成矿前、后中基性脉岩。成矿作用包括乳白色块状黄铁矿石英和（或）伟晶石英脉阶段、含金硫化物石英阶段和方解石石英阶段。三个阶段石英δ＾１８Ｏ值平均为１２．４‰、１３．２‰、和１４．４‰。计算的石英δ＾１８ＯＨ２ｏ值平均分别为４．７‰、３．５‰和３．５‰。三个阶段石英流体包裹体     

富有机质地质样品Re-Os同位素体系研究进展

Re-Os同位素的研究在过去几十年取得了重要进展。Re、Os能够在各种不同类型的富有机质地质样品中富集,Re-Os同位素体系在各种富有机质样品的研究已成为同位素地球化学研究领域一个新的热点。文章介绍了Re-Os同位素体系研究应用于富有机质地质样品研究的原理及其地质样品种类,从风化淋滤和熟化两方面对Re-Os同位素体系的封闭性进行了说明,并且结合实例论述了Re-Os同位素体系应用于富有机质地质样品的重大意义,还对不同类型富有机质样品的采样和溶样方法进行了归纳。此外,还提出了当前富有机质地质样品Re-Os同位素研究亟待解决的问题,指出富有机质地质样品Re-Os同位素分析将成为解决地质难题一种新的有力工具。     

Exploring Cd,Cu, Pb,and Zn dynamic speciation in mining and smelting-contaminated soils with stable isotopic exchange kinetics

The exchange kinetics of Cd, Cu, Pb, and Zn in seven mining and smelting-contaminated soils and the other two anthropogenically contaminated soils was investigated by using multi-elementary stable isotopic exchange kinetic (SIEK) method, and the experimental results were successfully interpreted by modelling using a sum of pseudo first order kinetics equations. SIEK results show that in the studied soils the isotopic exchange of Cd is a relatively fast process, and the exchange almost reaches an apparent plateau after 3-d equilibration; whereas for Cu, Pb, and Zn, the exchange is more sluggish, suggesting that it is important to understand the time-dependent metal mobility for risk assessment and management of contaminated soils. In most of the soils, the total isotopically exchangeable pool is divided, for all the metals, into two distinct pools: a fast exchangeable pool (E₁) with a kinetic rate constant k₁ having values around 1 min⁻¹ and a much slower exchangeable pool (E₂) with k₂ ranging from 0.0001 min⁻¹ to 0.001 min⁻¹. The distribution of the two exchangeable pools varies significantly among metals. The amount of isotopically exchangeable Cd related to the fast pool is dominant, accounting for on average 60% of total isotopically exchangeable pool in the soils; whereas this pool is smaller for Cu, Zn, and Pb. The sequence of average k₁ values is Cd > Pb ≈ Zn > Cu, consistent with the reported sequence of stability constants of metal-humic substances (HS) complexes while the average k₂ values follow the order: Cd > Pb > Cu > Zn, probably controlled by the slow desorption of metal ions associated with soil organic matter (SOM) fraction. Our results imply that further study on the exchange kinetics of metals on each individual sorption surface in soils, especially SOM, is critical to help understanding the overall exchange kinetics of heavy metals in whole soils.     

Age and origin of the Nigerian mesozoic granites: A Rb-Sr isotopic study

Sixty-four Rb-Sr and two K-Ar isotopic measurements from seven ring complexes in central Nigeria provide evidence for a systematic age trend along a 200 km zone ranging from 174±5 m.y. in the north to 154±4 m.y. in the south. A peak of anorogenic magmatism occurred in the Jos Plateau region about 164±4 m.y. ago. Although a small syenitetrachyte complex at Zaranda, near Bauchi, gives an age of 190±15 m.y., unpublished ages of 290–330 m.y. for the southern Niger ring complexes confirm the existence of an overall southerly decreasing age trend in the Niger-Nigeria province of West Africa. Isotopic measurements on two small, oversaturated syenite intrusions at Zaranda and Pankshin suggest that syenitic liquids had initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7048—not significantly different from the mantle range of values, but that related peralkaline silicic variants from the same complexes are depleted in total Sr and have higher ⁸⁷Sr/⁸⁶Sr initial ratios characteristic of the earth's crust. This variation of initial ⁸⁷Sr/⁸⁶Sr ratios in syenite-related granitic liquids of the peralkaline spectrum has also been noted at the Shere Hills, near Jos, and at Liruei, near Kano, and may be representative for all syenite-granite occurrences in the Nigerian Younger Granite province. Such isotopic variations in the initial ⁸⁷Sr/⁸⁶Sr ratio may be attributed to “crustal enrichment” of syenitic liquids whose source lies in the mantle. Coarse-grained, peraluminous biotite granites have consistently low initial ⁸⁷Sr/⁸⁶Sr ratios in the range 0.706–0.709 (similar to the ca. 600 m.y. Pan-African granites of the basement), and may represent further modifications of originally syenitic liquids in the crust, or the granites may have originated from an independent source within a “dioritio” lower crust. Although the magmatic trends show small variations in the initial ⁸⁷Sr/⁸⁶Sr ratio, much higher initial ratios are recorded in granites which have been modified within their roof zone by deuteric (autometamorphic) and/or metasomatic processes.     

Influence of pore system characteristics on limestone vulnerability: a laboratory study

Hyblean limestone of Oligo-Miocene age was widely used as a construction material in the architectural heritage of Eastern Sicily (Italy). Among them, the so-called Pietra Bianca di Melilli (Melilli limestone) and Calcare di Siracusa (Syracuse limestone) were prized for their attractive appearance, ease of quarrying, and workability. Syracuse limestone shows general weathering, whereas Melilli limestone is better preserved, and only differential erosion or superficial exfoliation can be detected in monuments. The cause of the different behavior of these two limestones was investigated from the petrographic and petrophysical points of view. The saturation coefficient is higher in Melilli limestone, and ultrasound measurements indicate that it is less compact than Syracuse limestone, so that Melilli limestone could deteriorate more easily than Syracuse limestone. However, pore interconnections and the size of very small pores play the main role in the durability of both materials. The “irregularity” of the Syracuse pore system and its greater number of micropores hinder water flow through the exterior, promote stress in pore structure, and favor the development of scaling, as confirmed by salt crystallization tests. In Melilli limestone, the low concentration of micropores and fast water evaporation allow solutions to reach the surface more easily, resulting in less damaging efflorescence.     

A combined Sm-Nd, Rb-Sr, and U-Pb isotopic study of Mg-suite norite 78238: Further evidence for early differentiation of the Moon

Lunar Mg-suite norite 78238 was dated using the Sm-Nd, Rb-Sr, and U-Pb isotopic systems in order to constrain the age of lunar magma ocean solidification and the beginning of Mg-suite magmatism, as well as to provide a direct comparison between the three isotopic systems. The Sm-Nd isotopic system yields a crystallization age for 78238 of 4334 ± 37 Ma and an initial value of −0.27 ± 0.74. The age-initial (T-I) systematics of a variety of KREEP-rich samples, including 78238 and other Mg-suite rocks, KREEP basalts, and olivine cumulate NWA 773, suggest that lunar differentiation was completed by 4492 ± 61 Ma assuming a Chondritic Uniform Reservoir bulk composition for the Moon. The Rb-Sr isotopic systematics of 78238 were disturbed by post-crystallization processes. Nevertheless, selected data points yield two Rb-Sr isochrons. One is concordant with the Sm-Nd crystallization age, 4366 ± 53 Ma. The other is 4003 ± 95 Ma and is concordant with an Ar-Ar age for 78236. The ²⁰⁷Pb-²⁰⁶Pb age of 4333 ± 59 Ma is concordant with the Sm-Nd age. The U-Pb isotopic systematics of 78238 yield linear arrays equivalent to younger ages than the Pb-Pb system, and may reflect fractionation of U and Pb during sample handling. Despite the disturbed nature of the U-Pb systems, a time-averaged μ (²³⁸U/²⁰⁴Pb) value of the source can be estimated at 27 ± 30 from the Pb-Pb isotopic systematics. Because KREEP-rich samples are likely to be derived from source regions with the highest U/Pb ratios, the relatively low μ value calculated for the 78238 source suggests the bulk Moon does not have an exceedingly high μ value.     

Metamictization and U-PB isotopic discordance in single zircons: a combined Raman microprobe and SHRIMP ion probe study

Summary We report results of a combined SHRIMP ion microprobe and Raman microprobe study of the correlation between metamictization and U-Pb isotopic discordance of zircon. The spatial resolution of the SHRIMP and Raman probe used are about 200 m³ and 80 m³, respectively. This allows a direct comparison of U-Pb isotopic discordance and metamictization of small areas within zircon crystals. We show that the impact of the oxygen ion beam on the zircon during the SHRIMP analysis does not cause significant amorphization or recrystallization in the remaining zircon on a scale of 1 m. Consequently, it is possible to determine the initial degree of metamictization of zircon within and adjacent to a SHRIMP analytical spot by Raman microprobe measurements after performing SHRIMP analyses. A combination of the two microprobe techniques gives information on the concentration and distribution of radionuclides as well as the degree of metamictization and its heterogeneity and the lateral age distribution within the grain. We found that the degree of U-Pb isotopic discordance correlates closely with the degree of metamictization within single zircon grains, on a scale of 15 m, which is consistent with previous results on the scale of single and multiple zircon grains showing that metamictization enhances the relative potential for secondary loss of radiogenic lead in zircon.

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Metamiktisierung und U-Pb-Isotopendiskordanz in Einzelzirkonen: eine komplexe Untersuchung mittels Ramanmikrosonde und SHRIMP Ionensonde

Zusammenfassung Wir präsentieren die Ergebnisse einer Untersuchung des Zussammenhangs von Metamiktisierung und U-Pb Isotopendiskordanz in Zirkonen mittles kombinierter SHRIMP-Ionenmikrosonden- und Ramanmikrosondenanalysen. Die räumliche Auflösung der verwendeten Analysensysteme, experimentell mit 200 m³ (SHRIMP) und 80 m³ (Ramansonde) bestimmt, gestattet den direkten Vergleich der in Mikrobereichen innerhalb von Zirkon-Einkristallen gemessenen Isotopendiskordanzen und Metamiktisierungsgrade. Wir zeigen, daß der während der SHRIMP-Analyse auf den Zirkon einwirkende Sauerstoffionenstrahl weder signifikante Amorphisierung noch Rekristallisation des benachbarten Zirkonmaterials im 1 m-Bereich hervorruft. Es ist deshalb möglich, den Grad der Metamiktisierung eines Zirkon-Mikrobereiches auch in den SHRIMP-Analysengrübchen, d.h. erst nach dem Analysieren dieses Bereiches mit der Ionensonde, mittels hochauflösender Ramanmessungen unverfälscht zu bestimmen. Der Einsatz beider Mikromethoden liefert kombinierte Informationen zu den Gehalten und Verteilungen radioaktiver Elemente, zum Grad der Metamiktisierung und seiner Heterogenität und zur lateralen Altersverteilung innerhalb von Einzelkörnern. Wir weisen eine deutliche Korrelation des Grades der U-Pb-Isotopendiskordanz mit dem Grad der Metamiktisierung im Mikrobereich (15 m Auflösung) nach. Dies stimmt mit früheren, an Einzelzirkonen und Populationen getätigten Beobachtungen überein, daß die Metamiktisierung von Zirkon dessen Potential zu sekundären Bleiverlusten erhöht.

     

松散砂粒孔隙结构、孔隙分形特征及渗透率研究

多孔介质孔隙结构特征是决定流体微观运移机制的重要方面。文章用环氧树脂固结松散石英砂磨制出二维多孔介质薄片,并用孔隙结构图像处理方法提取出孔隙结构参数信息;根据孔隙结构参数,建立描述孔隙结构的分形几何模型,分析松散地层孔隙结构的分形特征;进行多孔介质渗透率的实验测量和分形计算。结果表明:多孔介质的孔隙率和孔径与粒径之间存在线性相关性,拟合度分别达到-0.976 5和0.996 6;在方格边长ε和含有孔隙的格子总数N(ε)的双对数坐标系中,lnε-lnN(ε)数据点近似成直线,且不同样品的孔隙分布分维数值比较接近;而在孔径r与大于该孔径的孔隙总数N(r)的双对数坐标系中,lnr-lnN(r)的数据点必须进行分段回归分析,以便能更好地反映孔隙结构的实际情况;孔隙分布分维数和孔径分维数与粒径也存在较高的对应关系;中砂渗透率的实验测得值是5.19×10-5mm2,孔径分维数分段回归法计算的大孔隙多孔介质渗透率为5.75×10-5mm2,测量值与计算值较为接近,孔径分维数可以计算多孔介质的渗透率。     

Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism

In situ oxygen isotopic measurements of primary and secondary minerals in Type C CAIs from the Allende CV3 chondrite reveal that the pattern of relative enrichments and depletions of ¹⁶O in the primary minerals within each individual CAI are similar to the patterns observed in Types A and B CAIs from the same meteorite. Spinel is consistently the most ¹⁶O-rich (Δ¹⁷O = −25‰ to −15‰), followed by Al,Ti-dioside (Δ¹⁷O = −20‰ to −5‰) and anorthite (Δ¹⁷O = −15‰ to 0‰). Melilite is the most ¹⁶O-depleted primary mineral (Δ¹⁷O = −5‰ to −3‰). We conclude that the original melting event that formed Type C CAIs occurred in a ¹⁶O-rich (Δ¹⁷O  −20‰) nebular gas and they subsequently experienced oxygen isotopic exchange in a ¹⁶O-poor reservoir. At least three of these (ABC, TS26F1 and 93) experienced remelting at the time and place where chondrules were forming, trapping and partially assimilating ¹⁶O-poor chondrule fragments. The observation that the pyroxene is ¹⁶O-rich relative to the feldspar, even though the feldspar preceded it in the igneous crystallization sequence, disproves the class of CAI isotopic exchange models in which partial melting of a ¹⁶O-rich solid in a ¹⁶O-poor gas is followed by slow crystallization in that gas. For the typical (not associated with chondrule materials) Type C CAIs as well for as the Types A and B CAIs, the exchange that produced internal isotopic heterogeneity within each CAI must have occurred largely in the solid state. The secondary phases grossular, monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace, but in one case (CAI 160) grossular is ¹⁶O-enriched (Δ¹⁷O = −10‰ to −6‰) relative to melilite (Δ¹⁷O = −5‰ to −3‰), meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration. This isotopic exchange in melilite and anorthite likely occurred on the CV parent asteroid, possibly during fluid-assisted thermal metamorphism.     

AUSTRALIS: A new tool for the study of isotopic systems and geochronology in mineral systems

AUSTRALIS (AMS for Ultra Sensitive TRAce eLement and Isotopic Studies) is a microbeam accelerator mass spectrometry (AMS) system designed for in situ microanalysis of geological samples for trace elements and radiogenic and stable isotope data. The AMS method eliminates molecular and isobaric interferences in in situ mass spectrometric measurements, opening up new opportunities in geochronology and tracer applications. Tests have been carried out for measurements of Pb, S and Os isotopes, conducted mainly at 1.5 MV accelerating voltage. In Pb and S tests, precision as high as 0.3‰ has been obtained, made possible by a fast isotope switching system to counter the effect of instabilities in the ion source and beam transport system. In trace‐element analysis, a detection limit for Au at the sub‐ppb level was obtained.     

Predicting the activity of Cd and Zn in soil pore water from the radio-labile metal fraction

Cadmium and zinc were added at 3 and 300 mg kg⁻¹, respectively, to 23 soils and incubated at 16°C and 80% field capacity for 818 d. Following addition of metal, changes in the radio-labile concentrations of both elements were examined on seven separate sampling occasions over 818 d. At each sample time, soil pore water was extracted using Rhizon soil solution samplers, and concentrations of Cd, Zn, dissolved organic carbon, and major cations and anions were determined. The chemical speciation program WHAM 6 was used to determine free metal ion activity, (M²⁺). Similar measurements were made on a set of historically contaminated soils from old mining areas, sewage sludge disposal facilities, and industrial sources. The two data sets were combined to give a range of values for p(Cd²⁺) and p(Zn²⁺) that covered 5 and 4 log₁₀ units, respectively. A pH-dependent Freundlich model was used to predict Zn²⁺ and Cd²⁺ ion activity in soil pore water. Total and radio-labile metal ion concentration in the solid phase was assumed to be adsorbed on the “whole soil,” humus, or free iron oxides to provide alternative model formats. The most successful models assumed that solubility was controlled by adsorption on soil humus. Inclusion of ionic strength as a model variable provided small improvements in model fit. Considering competition with Ca²⁺ and between Zn²⁺ and Cd²⁺ produced no apparent improvement in model fit. Surprisingly, there was little difference between the use of total and labile adsorbed metal as a model determinant. However, this may have been due to a strong correlation between metal lability and pH in the data set used. Values of residual standard deviation for the parameterized models using labile metal adsorbed on humus were 0.26 and 0.28 for prediction of p(Cd²⁺) and p(Zn²⁺), respectively. Solubility control by pure Zn and Cd minerals was not indicated from saturation indices. However there may have been fixation of metals to non-radio-labile forms in CaCO₃ and Ca-phosphate compounds in the soils in the higher pH range. Independent validation of the Cd model was carried out using an unpublished data set that included measurements of isotopically exchangeable Cd. There was good agreement with the parameterized model.     

A study of the U-Pb isotopic systematics and its application in uranium geology

The main purpose of this paper is to present a fundamental concept of U-Pb isotopic spectrum-system and its application to the interpretation of the mechanism of uranium ore formation. It is evident from a number of examples of uranium deposits that uranium deposits formed in various processes are characterized by different types of U-Pb isotopic spectrum-system.     

Microscale oxygen isotopic exchange and magnetite formation in the Ningqiang anomalous carbonaceous chondrite

We report in situ measurements of O-isotopic compositions of magnetite, olivine and pyroxene in chondrules of the Ningqiang anomalous carbonaceous chondrite. The petrographic setting of Ningqiang magnetite is similar to those in oxidized-CV chondrites such as Allende, where magnetite is found together with Ni-rich metal and sulfide in opaque assemblages in chondrules. Both magnetite and silicate oxygen data fall close to the carbonaceous-chondrite-anhydrous-mineral line with relatively large ranges in δ¹⁸O in magnetite (−4.9 to +4.2‰) and in silicates (−15.2 to −4.5‰). Magnetite and silicates are not in O-isotopic equilibrium: the weighted average Δ¹⁷O (=δ¹⁷O − 0.52 × δ¹⁸O) values of magnetite are 1.7 to 3.6‰ higher than those of the silicates in the same chondrules. The petrological characteristics and O-isotopic disequilibrium between magnetite and silicates suggest the formation of Ningqiang magnetite by the oxidation of preexisting metal grains by an aqueous fluid during parent body alteration. The weighted average Δ¹⁷O of −3.3 ± 0.3‰ is the lowest magnetite value measured in unequilibrated chondrites and there is a positive correlation between Δ¹⁷O values of magnetite and silicates in each chondrule. These observations indicate that, during aqueous alteration in the Ningqiang parent asteroid, the water/rock ratio was relatively low and O-isotopic exchange between the fluid and chondrule silicates occurred on the scale of individual chondrules.     

The Hemlo gold deposit,Ontario: A geochemical and isotopic study

The Hemlo deposit, near Marathon, Ontario, is one of the largest gold deposits in North America. It is stratiform within Archean metamorphosed volcano-sedimentary rocks. The main ore zone is composed of pyritic, sericitic schist, and massive barite. This is the first report of stratiform barite in the Archean of North America, but other occurrences have since been found west of Hemlo. The mineralization is substantially enriched in Au, Mo, Sb, Hg, Tl and V and lacks carbonate. Because of metamorphism and deformation of the body its genesis is uncertain.${}^{87}\text{Sr}{}^{86}\text{Sr}$ of .7017 for barite from the deposit is similar to that of the sedimentary barite west of Hemlo and to initial ratios of contemporaneous volcanic rocks. At the base of the main ore zone, barite with δ^34S of +8 to +12%. was deposited with ~0%. pyrite. Upward, both barite and pyrite get isotopically lighter, with minimum values for pyrite, to ?17.5%, in non-baritic schist forming the upper part of the ore zone. In drill section, Au grades correlate with the isotopic composition of pyrite. This, and the association of fractionated sulphide with sulphate, suggests that Au, pyrite and barite were deposited contemporaneously. The linked, asymmetric distributions of S minerals and isotopic distributions, which are continuous from section to section, and the isotopic similarity of the Hemlo and western barites are consistent with a syngenetic depositional model.Two sources for the S minerals are considered. In the first, exogenous sulphate from a restricted basin were partially reduced in a geothermal system to form ³⁴S-depleted sulphide. In the second, the sulphate and sulphide are of magmatic-hydrothermal origin. Sulphate and fractionated sulphide are uncommon in Archean rocks, but one or both occur with unusual frequency in major Archean gold deposits. Hydrothermal fluids of moderately high , containing sulphate and permitting isotopic fractionation between oxidized and reduced S species, may have favoured the dissolution, transport and precipitation of Au.     

Constraints on the U-Pb isotopic systematics of Mars inferred from a combined U-Pb, Rb-Sr, and Sm-Nd isotopic study of the Martian meteorite Zagami

Uranium-lead, Rb-Sr, and Sm-Nd isotopic analyses have been performed on the same whole-rock, mineral, and leachate fractions of the basaltic martian meteorite Zagami to better constrain the U-Pb isotopic systematics of martian materials. Although the Rb-Sr and Sm-Nd systems define concordant crystallization ages of 166 ± 6 Ma and 166 ± 12 Ma, respectively, the U-Pb isotopic system is disturbed. Nevertheless, an age of 156 ± 6 Ma is derived from the ²³⁸U-²⁰⁶Pb isotopic system from the purest mineral fractions (maskelynite and pyroxene). The concordance of these three ages suggest that the ²³⁸U-²⁰⁶Pb systematics of the purest Zagami mineral fractions have been minimally disturbed by alteration and impact processes, and can therefore be used to constrain the behavior of U and Pb in the Zagami source region. The μ value of the Zagami source region can be estimated, with some confidence from the ²³⁸U-²⁰⁶Pb isochron, to be 3.96 ± 0.02. Disturbance of the U-Pb isotopic systems means that this represents a minimum value. The μ value of the Zagami source is significantly lower than the μ values estimated for most basaltic magma sources from Earth and the Moon. This is surprising given the high initial ⁸⁷Sr/⁸⁶Sr ratio (0.721566 ± 82) and low initial ε_Nd value (−7.23 ± 0.17) determined for Zagami that indicate that this sample is derived from one of the most highly fractionated reservoirs from any known planetary body. This suggests that Mars is characterized by a low bulk planet U/Pb ratio, a feature that is consistent with its relatively volatile-rich nature.The leachates contain terrestrial common Pb that was probably added to the meteorite during handling, curation, or sawing. The mineral fractions, particularly those with significant amounts of impact melt glass, contain a second contaminant. The presence of this contaminant results in Pb-Pb ages that are older than the crystallization age of Zagami, indicating that the contaminant is characterized by a high ²⁰⁷Pb/²⁰⁶Pb ratio. Such a contaminant could be produced by removal of single-stage Pb from a relatively high μ martian reservoir before ∼1.8 Ga, and therefore could be an ancient manifestation of hydrous alteration of martian surface material.     

A comparative isotopic study of the Lady Loretta zinc-lead-silver deposit

Measurements of the ³⁴S/³²S ratios in sulphides from the slightly metamorphosed Lady Loretta deposit show the sphalerite and galena to be in isotopic equilibrium. Pyrite in immediate association with these sulphides is not isotopically related. A similar distribution of sulphur isotopes had previously been noted in the even less altered McArthur deposit, for which a dual sulphur source was postulated. This fresh isotopic evidence from Lady Loretta now suggests that such a genesis for stratiform sulphides from the Proterozoic is not uncommon.