New data on the abundance of palladium in meteorites

The concentration of Pd in 7 carbonaceous chondrites, 18 ordinary chondrites, 3 achondrites, 29 iron meteorites and other samples has been determined by stable isotope dilution using solid source mass spectrometry. The Cl chondrite Orgueil gives a ‘cosmic’ abundance for Pd of 1.5 (Si = 10⁶ atoms), in good agreement with the currently accepted value.The concentration of Pd shows little variation among the carbonaceous chondrites, but in ordinary chondrites decreases from the H to L to LL groups. Pd in achondrites is approx 100 times lower than in chondrites. Data for iron meteorites plot around the ‘cosmic’ $\text{Pd}\text{Ni}$ ratio; however the Pd data falls into distinct groups, corresponding to the chemical group classification. These results support the hypothesis that at least two fractionation processes have occurred during the formation of iron meteorites.     

Terrestrial weathering rates defined by extraterrestrial materials

In studies attempting to quantify the effects of climate on weathering rate, meteorites offer a number of advantages over terrestrial rocks—material from one outcrop (in this case, an asteroid parent body) delivers samples of similar composition and texture over the whole of the Earth's surface, in all climatic regimes; the starting composition prior to weathering is known in detail; the terrestrial residence time may be accurately determined from cosmogenic radionuclides; and the degree of weathering can be tracked by measuring the abundance of ferric iron. When analysing meteorites found in different deserts, we can discriminate a range of weathering rates. We observe a rapid initial weathering phase with the majority of weathering occurring in the first few hundred years after fall. Although the overall degree of weathering and the suite of weathering products observed will be specific to meteorites, the relative differences in weathering rate between different sites should be similar for terrestrial rocks.     

Re-Os同位素体系在陨石研究中的应用

铁陨石中的Re ,Os含量反映其结晶分异历史。通过铁陨石定年修正187Re的衰变常数为 :λ(187Re) =1 6 6 6× 10 -11a-1。ReOs同位素测年法可以直接用于对铁陨石的定年 ,结果表明天然铁陨石大体同时形成 ,但ReOs定年技术已有可能揭示不同化学群铁陨石形成年代的序列 ,但研究尚需深入。这些方法也可以用来探讨铁陨石和石铁陨石的形成源区、冷却历史和后期变化。虽然在石陨石中Re ,Os同位素的浓度很低 ,但也有了探索性研究成果。随着技术的不断发展 ,ReOs同位素体系在天体化学中的作用将愈加明显和重要。     

Trace elements in primitive meteorites—V. Abundance patterns of thirteen trace elements and interelement relationships in enstatite chondrites

Neutron activation analysis was used to determine As, Au, Bi, Cd, Co, Cu, Ga, In, Sb, Se, Te, Tl and Zn in 11 samples representing 9 chondrites of grades E4–6. These chondrites exhibit systematic intra- and inter-grade differences particularly for highly-variable elements, the differences being E4 ? E3 > E6 ? E5. The abundance pattern for these 13 and an additional 16 elements in E3-6 chondrites differs from those of other primitive meteorites—the carbonaceous and unequilibrated ordinary chondrites. A search for statistically-significant interelement relationships among the 13 elements (for grades E4–6) reveal that 40 elementpairs are linearly and/or exponentially correlated. Similar consideration of data for 37 elements in 12 chondrites (grades E3–6) reveals that 191 element-pairs exhibit such relationships, 170 involving linear and/or exponential correlations, the remainder involving anti-correlations. The patterns depicting these relationships—i.e. the correlation profiles—and elemental abundance patterns, factor analysis and two-element correlation diagrams are consistent with all enstatite chondrites representing a single evolutionary sequence. The primary process responsible for the chemical trends of these chondrites involved thermal fractionation accompanied by geochemical fractionation of sulfide-plus-metal from silicate, probably during condensation and accretion of solid material from the solar nebula. Chalcophile elements may have been fractionated during condensation or, after accretion, during thermal metamorphism in the parent body. No genetic model proposed thus far accounts for the detailed chemical trends, although the constrained equilibrium theory and two-component condensation theories qualitatively seem most satisfactory. The correlation profiles of enstatite, carbonaceous and unequilibrated ordinary chondrites are distinctly different, pointing to major differences in the formation conditions of these different sorts of primitive meteorites.     

浙西典型石煤矿山周边耕地富硒土壤地球化学特征及影响因素

为系统研究石煤矿山周边耕地富硒土壤地球化学特征及影响因素,在浙江省常山县辉埠石煤矿山周边耕地采集了表层土壤样品144件、农产品甘蔗样20件、土壤垂向剖面土壤样6件、岩石样5件和地表水样3件。通过样品Se、有机质、As、Cd、Cu、Pb、Zn、Ni、Hg、Cr等元素含量指标测定和统计,探究了该区富硒土壤地球化学特征。结果表明:区内表层土壤硒含量主要集中于0.47~1.34 mg/kg之间,平均含量0.87 mg/kg,远高于浙江省平均值,且硒含量高值区与辉埠石煤矿的走向一致;硒含量随着土壤深度增加而逐渐下降;89.58%的土壤样品和65%的甘蔗样品达到富硒标准,但存在土壤和农产品Cd等主要重金属元素含量超标的生态风险。土壤硒含量均值在寒武系下统荷塘组最高(1.31 mg/kg),奥陶系中—下统最低(0.64 mg/kg);旱地硒均值略高于水田,但差异不显著(p>0.05);硒含量均值高低变化为粗骨土>石灰岩土>水稻土>红壤。研究认为,土壤硒含量受地质背景、土壤类型和有机质等因素的影响,含石煤层黑色岩系和碳酸盐岩等富硒地层是形成富硒土壤的主要因素,小部分与矿山开采等人类活动有关。     

The distribution of total nitrogen in iron meteorites

Total nitrogen abundances in 123 iron meteorites have been determined by inert carrier-gas fusion extraction-gas chromatography. The median value for the iron meteorites was found to be 18 ppm N. The N contents of Sulfide inclusions are greater, in nine cases out of ten, than the corresponding metallic phase. The N content of the iron meteorites is positively correlated with germanium content. The effects of terrestrial weathering and heat treatment by man are discussed in relation to the N contents measured for certain specimens. A correlation between N and cooling rates was found, with lower cooling rates associated with greater N abundances.     

Nonracemic isovaline in the Murchison meteorite: chiral distribution and mineral association

The enantiomeric and carbon-isotopic composition of the amino acid isovaline have been analyzed in several samples of the Murchison meteorite and one sample of the Murray meteorite. l-Enantiomeric excesses of the amino acid were found to range from 0 to 15.2%, varying significantly both between meteorite stones and at short distances within a single stone. The upper limit of this range is the largest enantiomeric excess measured to date for a biologically rare meteoritic amino acid and raises doubts that circularly polarized light irradiation could have been the sole cause of amino acids chiral asymmetry in meteorites. Individual d- and l-isovaline δ¹³C values ware found to be about +18‰, with no significant differences between the two enantiomers to suggest terrestrial contamination. The amino acid relative abundance also varied between samples, with isovaline/alanine ratios of 0.5 to 6.5. X-ray diffraction analyses of contiguous meteorite fragments suggest a possible correlation between isovaline and hydrous silicates abundances.     

The stable carbon isotopes in enstatite chondrites and Cumberland Falls

The carbon isotopic composition of the total carbon in the enstatite chondrites Indarch, Abee, St. Marks, Pillistfer, Hvittis and Daniel's Kuil and the enstatite achondrite Cumberland Falls has been measured. The empirical relationhip between carbon isotopic composition and total carbon content is distinct from that of carbonaceous and ordinary chondrites. Within the enstatite chondrite group the average ¹³C content increases with petrographic type: E4 < E5 < E6. Daniel's Kuil shows the largest ¹³C enrichment in the bulk carbon of any meteorite. The carbon isotopic composition is most clearly correlated with the abundance of the elements Zn, Cd and In. Insofar as these elements may hold the key to the understanding of enstatite chondrites, more detailed combined carbon isotope and trace element studies of these meteorites will play an important role in the deciphering of their history.     

Accurate measurement of silver isotopic compositions in geological materials including low Pd/Ag meteorites

Very precise silver (Ag) isotopic compositions have been determined for a number of terrestrial rocks, and high and low Pd/Ag meteorites by utilizing multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The meteorites include primitive chondrites, the Group IAB iron meteorites Canyon Diablo and Toluca, and the Group IIIAB iron meteorite Grant. Silver isotopic measurements are primarily of interest because ¹⁰⁷Ag was produced by decay of the short-lived radionuclide ¹⁰⁷Pd during the formation of the solar system and hence the Pd-Ag chronometer has set constraints on the timing of early planetesimal formation. A 2σ precision of ±0.05‰ can be obtained for analyses of standard solutions when Ag isotopic ratios are normalized to Pd, to correct for instrumental mass discrimination, and to bracketing standards. Caution must be exercised when making Ag isotopic measurements because isotopic artifacts can be generated in the laboratory and during mass spectrometry. The external reproducibility for geological samples based on replicate analyses of rocks is ±0.2‰ (2σ).All chondrites analyzed have similar Ag isotopic compositions that do not differ significantly (>0.3‰) from the ‘terrestrial’ value of the NIST SRM 978a Ag isotope standard. Hence, they show no evidence of excess ¹⁰⁷Ag derived from ¹⁰⁷Pd decay or, of stable Ag isotope fractionation associated with volatile element depletion within the accretion disk or from parent body metamorphism. The Group IAB iron meteorite samples analyzed show evidence of complex behavior and disturbance of Ag isotope systematics. Therefore, care must be taken when using this group of iron meteorites to obtain chronological information based on the Pd-Ag decay scheme.     

Cadmium in Zinc Deposits: Economic Geology of a Polluting Element

The chief host of cadmium in zinc deposits is sphalerite, the cadmium content of which depends on the type of deposit. Sphalerite from Mississippi Valley-type (MVT) deposits has high cadmium concentrations whereas sphalerite from exhalative deposits has low cadmium concentrations. The Cd content of sphalerite depends on the Cd/Zn ratio, ligand activities, and temperature of the ore-forming fluids. The combined effect of variation of temperature, pH, total activity of reduced sulfur, and activity of Cl- cannot by itself account for either Cd depletion (exhalative deposits) or Cd enrichment (MVT deposits). Variations in the Cd/Zn ratio of the fluid have a significant effect in determining that of sphalerite. Basinal brines, which can considered to be the recent equivalents of MVT fluids, have high Cd/Zn ratios, and active exhalative systems are characterized by low Cd/Zn ratios. Probably the differences in sphalerite composition between the different deposit types are less a function of temperature and ligand activities than Cd/Zn ratio of the ore-forming fluids. In the hydrothermal environment, the Cd/Zn ratio is generally not high enough to allow crystallization of cadmium sulfides (greenockite or hawleyite). The abundance of greenockite in the supergene alteration zone of hydrothermal zinc deposits can be explained on the basis of Zn scavenging by crystallizing smithsonite.     

Geochemistry of Cadmium in Mesozoic Phosphorites of the East European Platform

Cadmium is the most toxic admixture in mineral fertilizers. The Cd concentration in Mesozoic phosphorites, which are widespread in the East European Platform, has not been investigated. The present study was stimulated by the scanty and contradictory nature of the published data on this issue. We determined Cd concentration in 21 phosphorite samples from major deposits and checked the reliability of obtained results by external replicate analyses. It has been established that the Cd concentration in phosphorites varies from 5 ppm in the Late Jurassic–Early Cretaceous basin to 2 ppm in the Late Cretaceous basin. Cadmium does not enter the structure of phosphate and sulfide minerals. The Cd concentration is independent of the phosphorus abundance. However, all studied samples show a positive correlation of Cd with organic matter mainly contained in phosphates, supporting the biophilic nature of Cd. Mesozoic phosphorites of the East European Platform accumulated in epicontinental basins. They are significantly depleted in Cd relative to Mesozoic–Cenozoic phosphorites in pericontinental basins of the southern margin of the Tethys Ocean. The Cd concentration is more stable in Mesozoic phosphorites than in Mesozoic–Cenozoic deposits.     

U-Pb systematics in iron meteorites: Uniformity of primordial lead

Pb isotopic compositions and U-Pb abundances were determined in the metal phase of six iron meteorites: Canyon Diablo IA, Toluca IA, Odessa IA, Youndegin IA, Deport IA and Mundrabilla An. Prior to complete dissolution, samples were subjected to a series of leachings and partial dissolutions. Isotopic compositions and abundances of the etched Pb indicate a contamination by terrestrial Pb which is attributable to previous cutting of the meteorite. Pb isotopic compositions measured in the decontaminated samples are identical within 0.2% and essentially confirm the primordial Pb value defined by Tatsumotoet al. (1973). These data invalidate more radiogenic Pb isotopic compositions published for iron meteorites, which are the result of terrestrial Pb contamination introduced mainly by analytical procedure. Our results support the idea of a solar nebula which was isotopically homogeneous for Pb 4.55 Ga ago. The new upper limit for U-abundance in iron meteorites, 0.001 ppb, is in agreement with its expected thermodynamic solubility in the metal phase.     

Sulfur contents of the parental metallic cores of magmatic iron meteorites

Magmatic iron meteorites are thought to be samples of the central metallic cores of asteroid-sized parent bodies. Sulfur is believed to have been an important constituent of these parental cores, but due to the low solubility of S in solid metal, initial S-contents for the magmatic groups cannot be determined through direct measurements of the iron meteorites. However, experimental solid metal-liquid metal partition coefficients show a strong dependence on the S-content of the metallic liquid. Thus, by using the experimental partition coefficients to model the fractional crystallization trends within magmatic iron meteorite groups, the S-contents of the parental cores can be indirectly estimated. Modeling the Au, Ga, Ge, and Ir fractionations in four of the largest magmatic iron meteorite groups leads to best estimates for the S-contents of the parental cores of 12 ± 1.5 wt% for the IIIAB group, 17 ± 1.5 wt% for the IIAB group, and 1 ± 1 wt% for the IVB group. The IVA elemental fractionations are not adequately fit by a simple fractional crystallization model with a unique initial S-content. These S-content estimates are much higher than those recently inferred from crystallization models involving trapped melt. The discrepancy is due largely to the different partition coefficients that are used by the two models. When only partition coefficients that are consistent with the experimental data are used, the trapped melt model, and the low S-contents it advocates, cannot match the Ge and Ir fractionations that are observed in IIIAB iron meteorites.     

成都市某蔬菜基地重金属污染状况分析

分别用原子吸收和原子荧光光谱法，研究了成都市蔬菜基地蔬菜中Cd、Pb、Zn、Cu、Hg、As的含量和相应的土壤中Cu、Zn、Pb、Cd的含量。研究结果表明蔬菜中重金属含量均低于国家食品卫生标准。蔬菜中Cd、Zn、Cu、Hg主要来自土壤，Pb、As主要来自大气降尘。温室种植能有效地避免蔬菜Pb、As的污染。温室无土栽培应注液培养基中的重金属含量，以免导致二次污染。     

Comparative stable isotope geochemistry of Ni, Cu, Zn, and Fe in chondrites and iron meteorites

High-precision Ni isotopic variations are reported for the metal phase of equilibrated and unequilibrated ordinary chondrites, carbonaceous chondrites, iron meteorites, mesosiderites, and pallasites. We also report new Zn and Cu isotopic data for some of these samples and combine them with literature Fe, Cu, and Zn isotope data to constrain the fractionation history of metals during nebular (vapor/solid) and planetary (metal/sulfide/silicate) phase changes.The observed variations of the ⁶²Ni/⁵⁸Ni, ⁶¹Ni/⁵⁸Ni, and ⁶⁰Ni/⁵⁸Ni ratios vary linearly with mass difference and define isotope fractionation lines in common with terrestrial samples. This implies that Ni was derived from a single homogeneous reservoir. While no ⁶⁰Ni anomaly is detected within the analytical uncertainties, Ni isotopic fractionation up to 0.45‰ per mass-difference unit is observed. The isotope compositions of Ni and Zn in chondrites are positively correlated. We suggest that, in ordinary chondrites, exchange between solid phases, in particular metal and silicates, and vapor followed by mineral sorting during accretion are the main processes controlling these isotopic variations. The positive correlation between Ni and Zn isotope compositions contrasts with a negative correlation between Ni (and Zn) and Cu isotope compositions, which, when taken together, do not favor a simple kinetic interpretation. The observed transition element similarities between different groups of chondrites and iron meteorites are consistent with the genetic relationships inferred from oxygen isotopes (IIIA/pallasites and IVA/L chondrites). Copper is an exception, which we suggest may be related to separate processing of sulfides either in the vapor or during core formation.     

Comments on the paper ‘Subduction of a fossil slow–ultraslow spreading ocean: a petrology-constrained geodynamic model based on the Voltri Massif,Ligurian Alps,NW Italy’ by G. B. Piccardo

Analyses were made of samples of the several classes of iron meteorites: (hexahedrites, octahedrites, ataxites, and troilite inclusions) in further study of the isotopic composition of primordial lead and toward establishing correlation between the distribution of lead among the mineral inclusions and the nickel-iron mass of the meteorite. Two groups of iron meteorites can be distinguished on the basis of isotopic composition lead suggesting two ages for the parent bodies of common iron meteorites. The distribution of lead in iron meteorites ranges markedly but no relation could be found between isotopic composition of lead and the several structures and compositions. The content of lead in troilites are one or two orders of magnitude higher than in the nickel-iron phase.-- M. Russell.     

火焰原子吸收光谱法快速测定铁矿石中铅锌铜

将ISO、国家标准等方法中同一铁矿石样品分别称样,用不同的方法溶样,再分别测定改进为对同一样品一次称样,盐酸、氢氟酸一次性溶样,应用空气-乙炔火焰原子吸收光谱法连续测定同一进口铁矿石中铅、锌和铜含量,可简化操作,减少误差。方法准确、可靠,提高了工作效率和经济效益。     

大吉山稀有金属花岗岩中富镉闪锌矿的矿物学特征

电子探针研究表明江西大吉山矿床二云母花岗岩和白云母花岗岩中副矿物闪锌矿为富镉闪锌矿(Cd,4.34—19.89％,wt％,下同),铁含量低(Fe,1.03—4.24),尤以具W、Be、Ta、Nb矿化的白云母花岗岩中闪锌矿镉含量为高,并随岩体由下到上(标高的增大)而增高。矿物中Cd+Fe与Zn之间存在良好的线性负相关关系,指示了主要类质同象替代发生在Cd、Fe与Zn之间。矿物中可能存在不等位替代现象和四面体空位。富镉闪锌矿晶胞参数为0.54250—0.54440nm,随矿物中Cd含量的增加而增大。对照表明,当矿物中镉含量特别高时,Skinner(1961)晶胞参数计算公式估计值偏大,需作重新修正。最后,文章对富镉闪锌矿的成因意义进行了讨论。     

Identification of the projectile component in the impact structures Rochechouart, France and Sääksjärvi, Finland: Implications for the impactor population for the earth

A set of 11 impact melt rock samples from the Rochechouart impact structure, France and nine impact melt rock samples from Sääksjärvi impact structure, Finland were studied for their major and trace element compositions, including the abundances of the platinum group elements. The main goal of this study was to identify the projectile type(s) responsible for the formation of these two impact structures. The results confirmed previous studies that suggested extraterrestrial contamination in both the Rochechouart and Sääksjärvi impact melt rocks. The projectile types found for Rochechouart and Sääksjärvi are quite similar, and compatible with the composition of non-magmatic iron meteorites (IA and IIIC). This interpretation is based on: identical platinum group element patterns as well as peculiar Ni/Cr, Ni/Ir and Cr/Ir ratios, which can be explained by mixing of the different components of non-magmatic iron meteorites. This result indicates that, besides ordinary chondrites, non-magmatic iron may be among the most common material impacting the Earth, as they also represent the majority of the projectiles for craters smaller that 1.5 km. The abundance of non-magmatic irons as projectiles as well as their composition (olivine, pyroxene and iron) supports the assumption that a fraction of the S-type asteroids could by related to this type of material.     

Mass spectrometric isotope dilution analyses of tellurium in meteorites and standard rocks

A stable isotope dilution technique using solid source mass spectrometry has been used to determine the elemental abundance of Te in 25 chondrites, 3 achondrites, 1 tektite, and 12 standard rocks. Mean values for the C1, C2, and CV3 meteorites are 2.34, 1.48, and 1.03 ppm, respectively; or atomic abundances for Te (normalized to Si = 10⁶ atoms) of 4.84, 2.49, and 1.46. The atomic abundance obtained for the C1 chondrite Orgueil is significantly lower than the accepted value of 6.42. As a consequence we recommend that the ‘cosmic’ abundance of Te and Xe should be re-examined. The depletion ratio for Te in ordinary chondrites of 0.10, is about the same as that for Zn. Elemental abundances of Te in 12 standard rocks are in the ppb range.