Pb Abundances Research Articles

Demise of a harbor: a geochemical chronicle from Ephesus

At the end of the first century BC, Ephesus became the Roman capital of Asia Minor and the most important commercial, religious, and cultural center of the region. In order to evaluate the status of anthropogenic fluxes in the port of Ephesus, a 12 m long sediment core drilled in the Roman basin was investigated to shed light on the paleo-environmental evolution of the harbor using grain size distribution analysis, 14C ages, major and trace element geochemistry, and Pb isotope compositions. With the help of complementary sedimentological data and Principal Component Analysis, five distinct units were identified which, together, reflect the different stages of water history in the harbor. Among the major disruptive events affecting the port were earthquakes and military events, both of which were particularly effective at destroying the water distribution system.Seasonal floods of the Cayster River (Küçük Menderes) were the major source of the silt that progressively infilled the harbor. Silting in was further enhanced by the westward migration of the river mouth. A single major disruptive event located at 550 cm core depth and heralding the development of anoxia in the harbor marks the end of the dynamic regime that otherwise controlled the harbor water throughout the Roman Empire period. This remarkable event may correspond to a major disruption of the aqueduct system or to a brutal avulsion of the Cayster River bed. It clearly represents a major disturbance in the history of life at Ephesus. It is poorly dated, but probably occurred during the reign of Augustus or shortly after. Lead isotope and trace metal evidence suggest that in the four bottom units pollution was subdued with respect to other Pb metal inputs, presumably those from aqueducts and natural karstic springs. Near the top of the core, which coincides with harbor abandonment and the more recent period, anthropogenic Pb contamination is clearly visible in both Pb abundances and isotopic compositions.

The lead discrepancy in intrinsicallys-process enriched post-AGB stars in the Magellanic Clouds

Context: Our understanding of the s-process nucleosynthesis in asymptotic giant branch (AGB) stars is incomplete. AGB models predict, for example, large overabundances of lead (Pb) compared to other s-process elements in metal-poor low-mass AGB stars. This is indeed observed in some extrinsically enhanced metal-poor stars, but not in all. An extensive study of intrinsically s-process enriched objects is essential for improving our knowledge of the AGB third dredge-up and associated s-process nucleosynthesis. Aims: We compare the spectral abundance analysis of the SMC post-AGB star J004441.04-732136.4 with state-of-the-art AGB model predictions with a main focus on Pb. The low S/N in the Pb line region made the result of our previous study inconclusive. We acquired additional data covering the region of the strongest Pb line. Methods: By carefully complementing re-reduced previous data, with newly acquired UVES optical spectra, we improve the S/N of the spectrum around the strongest Pb line. Therefore, an upper limit for the Pb abundance is estimated from a merged weighted mean spectrum using synthetic spectral modeling. We then compare the abundance results from the combined spectra to predictions of tailored AGB evolutionary models from two independent evolution codes. In addition, we determine upper limits for Pb abundances for three previously studied LMC post-AGB objects. Results: Although theoretical predictions for J004441.04-732136.4 match the s-process distribution up to tungsten (W), the predicted very high Pb abundance is clearly not detected. The three additional LMC post-AGB stars show a similar lack of a very high Pb abundance. Conclusion: From our study, we conclude that none of these low-mass, low-metallicity post-AGB stars of the LMC and SMC are strong Pb producers. This conflicts with current theoretical predictions.

Origin of Basalts in a Hot Subduction Setting: Petrological and Geochemical Insights from Mt. Baker, Northern Cascade Arc

The northern Cascade arc is an end-member ‘hot’ subduction zone where slab dehydration may be essentially complete prior to reaching sub-arc depths, presenting a potential problem for a flux melting origin for arc basalts. Nevertheless, mafic lavas from the Mt. Baker volcanic field, the most magmatically productive volcano in the northern Cascade arc, record subduction input and compositions typical of calc-alkaline magmas. Relative to normal mid-ocean ridge basalt, the most primitive lavas at Mt. Baker show elevated abundances of large ion lithophile elements (LILE), Pb and light rare earth elements (LREE). High field strength elements (HFSE) and heavy REE (HREE) are depleted relative to LILE. Reconstructed primary magmas define three groups: Group I calcalkaline basalts (Sulphur Creek, Lake Shannon, Hogback) have lower SiO2, (La/Yb)N and LILE/HFSE, and are olivineand plagioclase-phyric; Group II high-Mg basaltic andesites (Tarn Plateau, Cathedral Crag) also contain clinopyroxene phenocrysts and have higher H2O, SiO2, (La/Yb)N and LILE/HFSE; Group III low-K olivine tholeiite (Park Butte) has the lowest (La/Yb)N but highest LILE/HFSE.The redox states of all groups lie between QFM (quartz^fayalite^magnetite) and QFM þ1·4. Pb, Sr and Nd isotope compositions are similar among all the analysed samples and are consistent with a depleted mantle source modified by input from the subducting slab. Trace element^isotope modeling indicates a subduction component composed predominantly of metabasaltderived fluid with lesser amounts of sediment melt and metabasalt melt. Group I basalts record the smallest melt fractions ( 5^7%), lowest water contents (1·5^2·1wt %), and highest temperatures and pressures of mantle segregation (up to 13548C, 1·5 GPa) from lherzolitic ( spinel) residues. Group II basaltic andesites show the greatest extents of mantle metasomatism, the highest water contents (2·7^3·7 wt %) and partial melt fractions (10^12%), and segregated from harzburgite at 12708C, 1GPa, consistent with pooling of melts at the Moho. Group III records P^Tconditions similar to Group I ( 1·4 GPa, 13268C) but melt fractions (12%) and mantle residues (harzburgite) are more similar to Group II, and H2O contents ( 2·1wt %) are intermediate. Melting beneath Mt. Baker was initiated by dehydration melting of amphibole peridotite at 95 km and 10208C, within the stability field of garnet lherzolite. Initial melt fractions were small ( 1^2%) and near water-saturation. Phase equilibria and trace element modeling show no evidence for garnet-bearing mantle residues, indicating that progressive melting during ascent of diapirs through the hot core of the convecting mantle wedge reduced H2O contents and erased any residual garnet signature. Because fluid release from the slab is restricted to the forearc, mantle hydrated at shallow depths in the serpentine and/or chlorite stability fields must be down-dragged to the region of amphibole stability to initiate dehydration melting.

Highly accurate isotope composition measurements by a miniature laser ablation mass spectrometer designed for in situ investigations on planetary surfaces

An experimental procedure for precise and accurate measurements of isotope abundances by a miniature laser ablation mass spectrometer for space research is described. The measurements were conducted on different untreated NIST standards and galena samples by applying pulsed UV laser radiation (266nm, 3ns and 20Hz) for ablation, atomisation, and ionisation of the sample material. Mass spectra of released ions are measured by a reflectron-type time-of-flight mass analyser. A computer controlled performance optimiser was used to operate the system at maximum ion transmission and mass resolution. At optimal experimental conditions, the best relative accuracy and precision achieved for Pb isotope compositions are at the per mill level and were obtained in a range of applied laser irradiances and a defined number of accumulated spectra. A similar relative accuracy and precision was achieved in the study of Pb isotope compositions in terrestrial galena samples. The results for the galena samples are similar to those obtained with a thermal ionisation mass spectrometer (TIMS). The studies of the isotope composition of other elements yielded relative accuracy and precision at the per mill level too, with characteristic instrument parameters for each element. The relative accuracy and precision of the measurements is degrading with lower element/isotope concentration in a sample. For the elements with abundances below 100ppm these values drop to the percent level. Depending on the isotopic abundances of Pb in minerals, 207Pb/206Pb ages with accuracy in the range of tens of millions of years can be achieved.

Petrography and geochemistry of the Shilu Fe–Co–Cu ore district, South China: Implications for the origin of a Neoproterozoic BIF system

The Shilu Fe–Co–Cu ore district is situated in the western Hainan Province of south China. This district consists of the upper Fe-rich layers and the lower Co–Cu ores, which are mainly hosted within the Neoproterozoic Shilu Group, a dominantly submarine siliciclastic and carbonate sedimentary succession that generally has been metamorphosed to greenschist facies. Three facies of metamorphosed BIFs, the oxide, the silicate–oxide and the sulfide–carbonate–silicate, have been identified within the Shilu Group. The oxide banded iron formation (BIF) facies (quartz itabirites or Fe-rich ores) consists of alternating hematite-rich and quartz-rich microbands. The silicate–oxide BIF facies (amphibolitic itabirites or Fe-poor ores) comprises alternating millimeter to tens of meter scale, magnetite–hematite-rich bands with calc-silicate-rich macro- to microbands. The sulfide–carbonate–silicate BIF facies (Co–Cu ores) contain alternating cobaltiferous pyrite, cobaltiferous pyrrhotite and chalcopyrite macrobands to microbands mainly with dolomite–calcite, but also with minor sericite–quartz bands. Blasto-oolitic, pelletoidal, colloidal, psammitic, and cryptocrystalline to microcrystalline textures, and blasto-bedding structures, which likely represent primary sedimentation, are often observed in the Shilu BIF facies.The Shilu BIFs and interbedded host rocks are generally characterized by relatively low but variable ∑REE concentrations, LREE depletion and/or MREE enrichment relative to HREE, and no Ce, Gd and Eu anomalies to strongly positive Ce, Gd and Eu anomalies in the upward-convex PAAS-normalized REY patterns, except for both the banded or impure dolostones with nil Ce anomaly to negative Ce anomalies and negative La anomalies, and the minor sulfide–carbonate–silicate BIF facies with moderately negative Eu anomalies. They also contain relatively low but variable HFSE abundances as Zr, Nb, Hf, Th and Ti, and relatively high but variable abundances of Cu, Co, Ni, Pb, As, Mn and Ba. The consistently negative εNd(t) values range from −4.8 to −8.5, with a TDM age of ca. 2.0Ga. In line with the covariations between Al2O3 and TiO2, Fe2O3+FeO and SiO2, Mn and Fe, Zr and Y/Ho and REE, and Sc and LREE, the geochemical and Sm–Nd isotopic features suggest that the precursors to the Shilu BIFs formed from a source dominated by seafloor-derived, high- to low temperature, acidic and reducing hydrothermal fluids but with variable input of detrital components in a seawater environment. Moreover, the involved detrital materials were sourced dominantly from an unknown, Paleoproterozoic or older crust, with lesser involvement from the Paleo- to Mesoproterozoic Baoban Group underlying the Shilu Group.The Shilu BIFs of various facies are interpreted to have formed in a shallow marine, restricted or sheltered basin near the rifted continental margin most likely associated with the break-up of Rodinia as the result of mantle superplume activity in South China. The seafloor-derived, periodically upwelling metalliferous hydrothermal plume/vent fluids under anoxic but sulfidic to anoxic but Fe2+-rich conditions were removed from the plume/vent and accumulated in the basin, and then variably mixed with terrigenous detrital components, which finally led to rhythmic deposition of the Shilu BIFs.

Colloidal origin of colloform-banded textures in the Paleogene low-sulfidation Khan Krum gold deposit, SE Bulgaria

We studied both colloform-banded macro- and micro-textures as well as the composition of electrum from the epithermal Khan Krum (or Ada Tepe) gold deposit, Bulgaria (5 Mt at 5.1 g/t Au + 2.7 g/t Ag) using optical and electron scanning microscopy, vibration spectroscopy, electron micro-probe analysis, and LA-ICP-MS. The individual textural bands differ in grain size, porosity, quartz-to-adularia ratio, and abundance of electrum, pyrite, and scattered dusty opaque micro-inclusions. The individual macro-bands have formed successively via intermittent fault dilation from episodic hydrothermal pulses, as a result of regional extension and most likely originated from colloidal solutions formed by vigorous boiling during rapid pressure drop and supersaturation with respect to amorphous silica in a closed or quasi-closed hydrothermal system. Electrum is highly concentrated in the finest quartz–adularia colloform micro-bands, which fill joints with widths up to 1–2 mm. We presume that the joints have filled in with more concentrated colloidal solutions than those in the macro-bands, most likely due to extreme boiling of fluids in open or quasi-open hydrothermal system. Electrum in the micro-bands forms transverse dendrite- and chain-like aggregates as well as oval clots along the banding, all of globular morphology. We explain these locations of electrum with reorientation of aggregated electrum globules during plastic deformation of a mixed electrum–silicate gel and in result from the consequent crystallization of silicates. At the same time, electrum is not present in pores and cracks of syneresis, which indicates that it had not been a soluble phase at the time of syneresis, rather colloidal particles. Electrum also forms dense sprinkles of globular morphology deposited in open space on the surfaces of some quartz–adularia bands, due to a condensation of gaseous phase, separated during the boiling of fluids. We found that the electrum-rich quartz–adularia micro-banding is enriched (in descending order) in Bi, Te, Cu, Fe, Pb, Au, As, U, Ba, Zn, Mg, Cr, Al, Tl, Na, K, and Th, and is slightly depleted in Si, Ti, Se, Ag, and Sb in comparison with the electrum-poor macro-bands. Ca displayed equal abundances in both macro- and micro-bands. The highest grades of electrum correspond to the highest abundances of Fe, As, Cu, Pb, Zn, Bi, and Te, which have deposited as sulfides and tellurides, thus revealing the voluminous electrum deposition in response to a significant decrease of sulfur and tellurium activities, and transport of gold and silver in the paleofluids in the form of sulfuric and telluric complexes. The epithermal Khan Krum deposit is Au dominant, Bi and Te rich, thus indicating its relationship to an igneous source.

Permian volcanic rocks from the Apuseni Mountains (Romania): Geochemistry and tectonic constraints

An Early Permian volcanic assemblage is well exposed in the central-western part of the Apuseni Mountains (Romania). The rocks are represented by rhyolites, basalts and subordinate andesites suggesting a bimodal volcanic activity that is intimately associated with a post-orogenic (Variscan) syn-sedimentary intra-basinal continental molasse sequences. The mafic and mafic-intermediate rocks belong to sub-alkaline tholeiitic series were separated in three groups (I–III) showing a high Th and Pb abundances, depletion in Nb, Ta and Sr, and slightly enriched in LREE patterns (LaN/YbN=1.4–4.4). Isotopically, the rocks of Group I have the initial ratios 87Sr/86Sr(i)=0.709351–0.707112, 143Nd/144Nd(i)=0.512490–0.512588 and high positive ɛNd270 values from 3.9 to 5.80; the rocks of Group II present for the initial ratios values 87Sr/86Sr(i)=0.709434–0.710092, 143Nd/144Nd(i)=0.512231–0.512210 and for ɛNd270 the negative values from −1.17 to −1.56; the rocks of Group III display for the initial ratios the values 87Sr/86Sr(i)=0.710751–0.709448, 143Nd/144Nd(i)=0.512347–0.512411 and for ɛNd270 the positive values from 1.64 to 2.35. The rocks resembling continental tholeiites, suggest a mantle origin and were further affected by fractionation and crustal contamination. In addition, the REE geochemistry (1>SmN/YbN<2.5; 0.9>LaN/SmN<2.5) suggests that these rocks were generated by high percentage partial melting of a metasomatized mantle in the garnet peridotite facies. The felsic rocks are enriched in Cs, Rb Th and U and depleted in Nb, Ta, Sr, Eu, and Ti. The REE fractionation patterns show a strong negative Eu anomaly (Eu/Eu*=0.23–0.40). The felsic rocks show the initial ratios the values: 87Sr/86Sr(i)=0.704096–0.707805, 143Nd/144Nd(i)=0.512012–0.512021 and for ɛNd270 the negative values from −5.27 to −5.44. They suggest to be generated within the lower crust during the emplacement of mantle-derived magmas that provided necessary heat to crustal partial melting.The mafic-intermediate and felsic rocks from Apuseni Mts represent a specific example of an association of bimodal volcanic rocks within a post-collisional extensional setting.

Trace element geochemistry of the Mt Vulture carbonatites, southern Italy

The Mt Vulture carbonatites are the only carbonatite occurrence in the southern Apennines. We present new trace element data for these rocks in order to evaluate the factors influencing rare earth element (REE) and other trace element fractionations and their REE grade. This study focuses on massive hyalo-alvikites from two lava flows and one dike, which have different relative abundances of silicate and carbonate (i.e. Si/Ca). These differences are also evident from CaO/(CaO + MgO + FeO(T) + MnO) and Sr/Ba ratios. The REE grade of the Mt Vulture carbonatites is very similar to that of the global average for calcio-carbonatites. R-mode factor analysis shows that most of the trace element variance reflects the relative roles of carbonate and silicate minerals in influencing trace element distributions. Silicates largely control heavy rare earth element (HREE), transition metal, Zr, and Th abundances, whereas carbonate minerals control light rare earth element (LREE), Ba, and Pb abundances. In addition, apatite influences LREE concentrations. Increasing silica contents are accompanied by decreases in (La/Yb)N and (La/Sm)N ratios and less marked LREE enrichment. In contrast, higher carbonate contents are associated with increases in (La/Yb)N and (La/Sm)N. The Si/Ca ratio has little influence on Eu anomalies and middle rare earth element (MREE) to HREE fractionations. Apatite has a negligible effect on inter-REE fractionations amongst the carbonatites.

Mantle source heterogeneity for South Tyrrhenian magmas revealed by Pb isotopes and halogen contents of olivine-hosted melt inclusions

Nineteen olivine-hosted melt inclusions from seven volcanoes (Etna, Stromboli, Vulcano, Ustica, Alicudi, Marsili and Vavilov) from the southern Italy region were analyzed for abundances of major, trace, and volatile elements, along with Pb isotope ratios (207Pb/206Pb, 208Pb/206Pb and 206Pb/204Pb). The systematics of primary volatile (in particular F and Cl) compositions and Pb isotopes identify three main geochemically distinct mantle sources for this region: (1) a component with radiogenic Pb (207Pb/206Pb=0.780) and low volatile element abundances typified by the Etnean melt inclusions, (2) a component with moderately radiogenic Pb (207Pb/206Pb=0.798) and high volatile abundance mainly associated with Sommata magmas, and (3) a component with moderate volatile abundances associated with a MORB-like 207Pb/206Pb (0.830).The variation of F/Nd and Pb isotopes among the southern Italy melt inclusions reflects mixing between a slightly modified depleted mantle (DMM–EMII mixture) and radiogenic (HIMU-like) mantle components expressed in Pb isotope space, superimposed by slab-derived metasomatic components represented by F/Nd (F/Nd>30).

Climate and cave control on Pleistocene/Holocene calcite-to-aragonite transitions in speleothems from Morocco: Elemental and isotopic evidence

The occurrence of aragonite in speleothems has commonly been related to high dripwater Mg/Ca ratios, because Mg is known to be a growth inhibitor for calcite. Laboratory aragonite precipitation experiments, however, suggested a more complex array of controlling factors. Here, we present data from Pleistocene to Holocene speleothems collected from both a dolostone and a limestone cave in northern Morocco. These stalagmites exhibit both lateral and stratigraphic calcite-to-aragonite transitions. Aragonite fabrics are well-preserved and represent primary features. In order to shed light on the factors that control alternating calcite and aragonite precipitation, elemental (Mg, Sr, Ba, U, P, Y, Pb, Al, Ti and Th) abundances were measured using LA-ICP-MS, and analysed with Principal Component Analysis. Samples were analyzed at 100–200μm resolution across stratigraphic and lateral transitions. Carbon and oxygen isotope ratios were analysed at 100μm resolution covering stratigraphic calcite-to-aragonite transitions. Results show that the precipitation of aragonite was driven by a decrease in effective rainfall, which enhanced prior calcite precipitation. Different geochemical patterns are observed between calcite and aragonite when comparing data from the Grotte de Piste and Grotte Prison de Chien. This may be explained by the increased dripwater Mg/Ca ratio and enhanced prior aragonite precipitation in the dolostone cave versus lower dripwater Mg/Ca ratio and prior calcite precipitation in the limestone cave. A full understanding for the presence of lateral calcite-to-aragonite transitions is not reached. Trace elemental analysis, however, does suggest that different crystallographic parameters (ionic radius, amount of crystal defect sites, adsorption potential) may have a direct effect on the incorporation of Sr, Mg, Ba, Al, Ti, Th and possibly Y and P.

Post-AGB stars in the SMC as tracers of stellar evolution: the extreme s-process enrichment of the 21 μm star J004441.04-732136.4

By virtue of their spectral types, favourable bolometric corrections as well as their constrained distances, post-AGB stars in external galaxies offer unprecedented tests to AGB nucleosynthesis and dredge-up predictions. We focus here on one object J004441.04-732136.4, which is the only known 21 micrometer source of the SMC. Spectral abundance results reveal J004441.04-732136.4 to be one of the most s-process enriched objects found up to date, while the photospheric C/O ratio of 1.9 +- 0.7, shows the star is only modestly C-rich. J004441.04-732136.4 also displays a low [Fe/H] = -1.34 +- 0.32, which is significantly lower than the mean metallicity of the SMC. From the SED, a luminosity of 7600 +- 200 solar luminosities is found, together with E(B-V) = 0.64 +- 0.02. According to evolutionary post-AGB tracks, the initial mass should be approximately 1.3 solar masses. The photometric variability shows a clear period of 97.6 +- 0.3 days. The detected C/O as well as the high s-process overabundances (e.g. [Y/Fe] = 2.15, [La/Fe] = 2.84) are hard to reconcile with the predictions. The chemical models also predict a high Pb abundance, which is not compatible with the detected spectrum, and a very high 12C/13C, which is not yet constrained by observations. The predictions are only marginally dependent on the evolution codes used. We show that our theoretical predictions match the s-process distribution, but fail in reproducing the detected high overabundances and predict a high Pb abundance which is not detected. Additionally, there remain serious problems in explaining the observed pulsational properties of this source.

Non-LTE effects on the lead and thorium abundance determinations for cool stars

Knowing accurate Pb abundances of metal-poor stars provides constraints on the Pb production mechanisms in the early Galaxy. Accurately deriving Th abundances permits a nucleo-chronometric age determination of the star. We improve the calculation of the Pb I and Th II lines in stellar atmospheres based on non-LTE line formation and evaluate the influence of departures from LTE on Pb and Th abundance determinations through a range of stellar parameters. Comprehensive model atoms for Pb I and Th II are presented. The departures from LTE lead to systematically depleted total absorption in the Pb I lines and positive abundance corrections. Non-LTE removes the discrepancy between the solar and the meteoritic Pb abundance. With the Holweger & Mueller (1974) solar model atmosphere, log eps(Pb, non-LTE) = 2.09. We revise the Pb and Eu abundances of the strongly r-process enhanced (r-II) stars CS 31082-001 and HE 1523-0901 and the Roederer et al. (2010) stellar sample. Our results provide strong evidence for universal Pb/Eu relative r-process yields during course of the Galaxy evolution. The stars with -2.3<[Fe/H]< -1.4 have, on average, 0.51 dex higher Pb/Eu ratios compared with that of the r-II stars suggesting that the s-process synthesis of Pb started as early as the time when Galactic metallicity had grown to [Fe/H] = -2.3. The average Pb/Eu ratio of the -1.4<[Fe/H]< -0.59 stars is close to the solar value, in line with the predictions of Travaglio et al. (2001) that AGB stars with [Fe/H] ~ -1 provided the largest contribution to the solar s-nuclei of Pb. Non-LTE leads to weakened Th II lines. Overall, the abundance correction does not exceed +0.2 dex when collisions with H I atoms are taken into account in non-LTE calculations.

The Abundances of some Trace-elements in the First-ever Reported Sample of Spinifex-Textured Komatiite from Ghatti Hosahalli, Karnataka

Abstract The paper reports wavelength-dispersive x-ray fluorescence spectrometric data on the abundances of Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, Pb, and Th in the first-ever reported sample of spinifex-textured komatiite from India at Ghatti Hosahalli in Karnataka. With some exceptions, these abundances are similar to those reported for the spinifex-textured komatiite from the Barberton greenstone belt in South Africa. The values for some alteration-resistant element ratios — Ti/Zr, Ti/Y, Ti/Sc, Ti/V, Zr/Y, Zr/Sc, Sc/Y, V/Zr, and V/Sc — for chondrite and for spinifex-textured komatiites from Ghatti Hosahalli (India), Barberton (South Africa), Munro (Canada), and Yilgarn (Australia) reveal that, except for the Ti/Zr ratio for the Ghatti Hosahalli komatiite, the other ratios from the four terrains define a unique trend that is almost superimposed on the trend defined by these ratios for chondrite. This suggests that the processes of formation of komatiitic lavas from the four far-separated terrains were similar, and that, the source regions from which these lavas formed had a chondritic composition.

Trace element geochemistry of nyerereite and gregoryite phenocrysts from natrocarbonatite lava, Oldoinyo Lengai, Tanzania: Implications for magma mixing

The abundances of Li, P, Cl, V, Mn, Rb, Sr, Y, Cs, Ba, Pb, Th, U and REE, within and between, phenocrysts of nyerereite and gregoryite occurring in natrocarbonatite lavas erupted from the active volcano Oldoinyo Lengai (Tanzania) have been determined by electron microprobe, LA-ICP-MS and SIMS. These data show that, in general, nyerereite is enriched in Rb (71–137ppm), Sr (14,485–23,240ppm), Y (2.0–8.9ppm), Cs (1.6–5.3ppm), Ba (4000–11,510ppm), but poorer in Li (21–91ppm), P (820–1900ppm) and V (5.1–47ppm) relative to gregoryite (Rb=43–106; Sr=4255–7275; Y=0.3–4.0; Cs=0.6–5.1; Ba=1125–7052; Li 84–489; P=6790–15,860; V=33–155ppm). Nyerereite is highly enriched in REE (La=236–973; Ce=395–1044ppm) relative to gregoryite (La=59–309; Ce=59–301ppm). Chondrite normalized REE distribution patterns for nyerereite and gregoryite are parallel and linear with no Eu anomalies. They show extreme enrichment in light REE and depletion in heavy REE (nyerereite La/YbCN=1759–7079; gregoryite La/YbCN=1051–10,247). Significant differences exist in the abundances of trace elements within and between coexisting crystals occurring in diverse natrocarbonatite flows, although there do not appear to be any significant secular variations in phenocryst compositions in lavas erupted from a given vent. It is concluded that both major, minor and trace element compositional data for nyerereite and gregoryite phenocrysts occurring in natrocarbonatite lavas are derived by the crystallization of several different batches of magma in a continuously replenished fractionating magma chamber. Natrocarbonatite lavas are considered to be hybrids formed by the mixing of both crystals and melts formed from several batches of natrocarbonatite magma; thus bulk rock compositions cannot represent the compositions of the primary magma composition before the onset of fractionation. Differentiation of natrocarbonatite melts leads to enrichment of residua in Ba and Mg.

Trace metal concentrations in tidal flat coastal sediments, Yamaguchi Prefecture, southwest Japan

Geochemical investigations of tidal flat coastal sediments at Ogori, Ozuki, and Kasado in Yamaguchi Bay of southwest Japan were conducted to determine their metal concentrations and to assess contamination levels, compared with sediment quality guidelines (SQG) and several pollutant indicators. Selected major oxides, trace elements, and total sulfur (TS) were determined by X-ray fluorescence. pH values of most samples were alkaline, indicating anoxic conditions. Average abundances of As, Pb, Zn, Cu, Ni, and Cr in Ozuki sediments were 11, 27, 109, 21, 19, and 52 mg/kg, respectively, compared to 9, 29, 80, 16, 18, and 42 mg/kg at Ogori and 12, 27, 151, 34, 30 and 80 mg/kg at Kasado, respectively. Average concentrations of As, Zn, and Cu in all samples and TiO(2), Fe(2)O(3), and P(2)O(5) at Kasado were greater than those of the upper continental crust. Contamination levels were assessed based on SQG, contamination factors (CF), pollution load index (PLI), enrichment factor (EF), and index of geoaccumulation (Igeo). According to the SQG of the US EPA, the sediments were heavily polluted with respect to As, whereas Zn, Cu, Ni, and Cr were classed as moderately polluted. The elevated CF values of As, Pb, and Zn identify moderate to considerable contamination, indicating that these metals are potentially toxic in the study area. Based on PLI and EF, the study sites are moderate to moderately severe polluted with As and Pb, moderately polluted with Zn, and weakly contaminated to noncontaminated with Cu, Ni, and Cr. The highest Igeo values for As, Pb, and Zn in the surface and core sediments reflected the tendency of metal contamination that seems to be related to their fine-grained nature, organic matter-rich sediments, and anthropogenic point sources. Trace metal contents were strongly correlated with Fe(2)O(3) and TiO(2), suggesting that Fe oxyhydroxides and detrital clastic load play a role in controlling abundances in the study area.

Metalliferous Manganese Oxide Mineralization Associated with the Boleo Cu-Co-Zn District, Mexico

Apart from its large reserves of Cu-Co-Zn mineralization contained within stratiform ore beds or mantos, the Boleo district of central Baja, Mexico, also contains significant concentrations of Mn oxides (typically 2–3 wt % Mn) interspersed with hydrothermal sulfides and both hydrothermal and weathering-related metal oxide minerals. Within the mantos, Mn oxides occur as concentric nodules, disk-shaped concretions, laminae, granules or aggregates, veins, dendrites, and various replacements. The oxides are composed of varying amounts of hollandite, cryptomelane, todorkite, romanechite, pyrolusite, and X-ray amorphous Mn oxide. Chemically, the manto oxides are unique owing to their extraordinarily high abundances of Co, Cu, Pb, and Zn (typically 2–5 wt %, but up to 23 wt %). Existing mineralogical and geochemical classifications are ambiguous regarding the genetic classification of Mn oxides within the mantos. A new MnO 2 -Fe 2 O 3 t-MgO diagram is proposed in order to discriminate among highly metalliferous Mn oxides. Based on the new classification scheme, manto Mn oxides record a complex interaction between hydrothermal and diagenetic processes. To better understand the mechanism of oxide deposition, radiogenic isotopes (Sr and Pb) and REE geochemistry were used to delineate fluid and metal sources. Isotopic and REE constraints indicate that the Boleo Mn oxides are largely hydrothermal in origin; however, they show varying degrees of modification resulting from diagenetic processes. Hydrothermal Mn oxides are syngenetic (synsedimentary to syndiagenetic) in origin and were precipitated in association with sulfides, reflecting localized variations in redox conditions within the manto sequence, due to the downward infiltration of metal-bearing brines. Apart from morphological and stratigraphic evidence (e.g., occurrence of nodular crusts along the upper contact of the manto), a syngenetic origin is supported by a single K-Ar date of 7.0 ± 0.2 Ma, which is within the estimated age range for deposition of Boleo Formation clastic sediments. Interaction between brine and seawater or brine and pore fluid, as it infiltrates the sedimentary pile, produced hydrothermal precipitates with certain hydrogenetic attributes (i.e., positive Ce anomaly, LREE enrichment). On the other hand, oxides with hydrogenetic and with mixed hydrothermal-hydrogenetic affinities represent diagenetic alteration and remobilization of earlier formed hydrothermal oxides by meteoric fluids. By far the greatest degree of remobilization occurred in response to compactional dewatering of the manto, where the dewatering pore waters enriched the uppermost manto strata in Mn, Fe, Zn, Co, and to lesser degrees, Cu. Later diagenetic modifications coincided with the development of a supergene overprint, which typifies Boleo Formation strata at elevations above the present-day water table.

First stars

Context. A small fraction of extremely metal-poor (EMP) stars exhibit moderate to extreme excesses of heavy neutron-capture elements produced in the r -process. The production site(s) of these elements in the early Galaxy remain(s) unclear, as is the reason for their occasional enhancement in the otherwise regular pattern of abundances of elements up to the iron peak. The detailed abundance pattern of the heaviest elements in EMP stars provides insight into their origin and role in the chemical enrichment of the early Galaxy and in radioactive nucleochronology. Aims. The EMP giant star CS 31082-001 ([Fe/H] ~ −2.9) exhibits an extreme enhancement of neutron-capture elements ([r/Fe] ~ +1.7) with U and Th enhanced by a further ~+0.7 dex, and a minimum of blending by molecular lines such as CH or CN. A rich inventory of r -process element abundances was established previously from optical spectra. Here we aim to supplement these data with abundances from near-UV spectroscopy of the third-peak neutron-capture elements, which are crucial for understanding the synthesis of the heaviest elements. Methods. Near-UV spectra from HST/STIS were analysed with LTE model atmospheres and spectrum synthesis calculations to derive new abundances of Os, Ir, Pt, Au, Bi and Pb in CS 31082-001. Results. Together with earlier data, the resulting abundance pattern for the r -process elements provides improved constraints on the nature of the r -process. The observed U and Th abundances and the initial production ratio place CS 31082-001 as one of the oldest stars in the Galaxy, consistent with its extreme metal deficiency. Comparison with the heaviest stable reference elements and with the daughter nuclides Pb and Bi provides a consistency check on this age determination. Finally, the existence of such r -element rich stars indicate that the early chemical evolution of the Galaxy was localised and inhomogeneous.

Heavy metal distribution and enrichment in sediments of Mejillones Bay (23° S), Chile: a spatial and temporal approach

Concentrations of Ni, Cd, Pb, Cu, and Zn were measured in the surface sediments of Mejillones Bay at 32 sites ranging in depth from 10 to 95 m. A similarity dendrogram analysis separated the study area into two large sedimentary facies: a coastal zone with low metal concentrations and a deep zone with high metal concentrations. The abundances of Zn, Ni, and Pb increased between two sampling periods (5 years apart) in association with greater anthropogenic activity along the coastline of the bay at the present. The results were analyzed with the index of geoaccumulation and by applying the sediment quality guideline. Although similar to preindustrial levels, current Cu and Ni concentrations pose a likely threat to communities of benthic organisms. Concentrations of Zn and Pb are enriched, probably in connection with the industrial development of the zone, but do not reach dangerous levels for the benthic communities. Cd concentrations are toxic to benthic life, even though those measured herein are similar to preindustrial levels. Thus, these Cd levels are a result of the bay's natural characteristics (e.g., high biological productivity, the microxic water-sediment interface). The results of the Chilean aquatic sediment quality standards and the international standard (sediment quality guideline) were similar but differed from those found using the index of geoaccumulation, which suggests that the proposed reference values for Chilean aquatic sediment should be reviewed.

Chemical enrichment mechanisms inωCentauri: clues from neutron-capture elements

In the complex picture of multiple stellar populations in globular clusters (GCs), a special role is played by NGC 5139 Omega Centauri. At variance with the majority of GCs, Omega Cen exhibits significant star-to-star variations in metallicity and in relative neutron-capture element abundance ratios with respect to Fe, along with split evolutionary sequences as revealed from colour-magnitude diagrams. Combining information from photometry and spectroscopy, several studies suggested that an age spread of several Gyr has to be invoked to explain (at least partially) some of the observed features. However, a comprehensive understanding of the formation, evolution and chemical enrichment processes is still not at hand. Relatively metal-rich Omega cen stars display neutron-capture abundance distributions dominated by contributions from the s-process, but it is not clear what roles have been played by the so-called main and weak s-process components in generating these abundances. To gain better insight into this question we derived lead (Pb) abundances for several cluster members, because this element can only be produced by the main s-process. We analysed high-resolution UVES@VLT spectra of a sample of twelve red-giant branch stars, deriving abundances of Pb and also of Y, Zr, La, Ce, Eu, and the C+N+O sum. Spectral synthesis was applied to all features, taking into account isotopic shifts and/or hyperfine structure as needed. We measured for the first time the Pb content in Omega Cen, discovering a clear hint for a Pb production occurring at [Fe/H]>-1.7 dex. Our data suggest that the role of the weak component in the production of s-process elements is negligible. Moreover, evidence gathered from the abundances of other elements indicates that the main component occurring in this GC is peculiar and shifted towards higher mass polluters than the standard one.

Rubidium isotopes in primitive chondrites: Constraints on Earth's volatile element depletion and lead isotope evolution

The bulk silicate Earth (BSE) shows substantial deficits in volatile elements compared to CI-chondrites and solar abundances. These deficits could be caused by pre-accretionary depletion in the solar nebula during condensation of solids, or by later heat-driven evaporation during collision of small bodies that later accreted to form the Earth. The latter is considered to result in isotope fractionation for elements with low condensation temperatures that correlates with the degree of depletion. Here, we report first high-precision isotope ratio measurements of the moderately volatile and lithophile trace element Rb. Data from seventeen chondrite meteorites show that their Rb isotope abundances are nearly indistinguishable from Earth, not deviating more than 1 per mil in their 87Rb/85Rb. The almost uniform solar system Rb isotope pool suggests incomplete condensation or evaporation in a single stage is unlikely to be the cause of the volatile element deficit of the Earth. As Rb and Pb have similar condensation temperatures, we use their different degrees of depletion in the BSE to address the mechanisms and timing of terrestrial volatile depletion. The Rb isotope data are consistent with a scenario in which the volatile budget of the Earth was generated by a mixture of a highly volatile-element depleted early Proto-Earth with undepleted material in the course of terrestrial accretion. Observed Pb and Rb abundances and U–Pb and Rb–Sr isotope systematics suggest that volatile addition occurred at approximately the same time at which last core–mantle equilibration was achieved. In line with previous suggestions, this last equilibration involved a second stage of Pb (but not Rb) depletion from the BSE. The timing of this second Pb loss event can be constrained to ~110Ma after the start of the solar system. This model supports a scenario with core storage of Pb in the aftermath of a putative Moon forming giant impact that also delivered the bulk of the volatile elements to the Earth.