Origin of iodine in volcanic fluids: I results from the Central American Volcanic Arc     

Glen T Snyder  Udo Fehn 《Geochimica et cosmochimica acta》2002,66(21):3827-3838

The largest reservoir of crustal iodine is found in marine sediments, where it is closely associated with organic material. This presence, together with the existence of a long-lived, cosmogenic radioisotope ¹²⁹I (t_1/2 = 15.7 Ma), make this isotopic system well suited for the study of sediment recycling in subduction zones. Reported here are the results of ¹²⁹I/I ratios in volcanic fluids, collected during a comprehensive study of fluids and gases in the Central American Volcanic Arc. ¹²⁹I/I ratios, together with I, Br, and Cl concentrations, were determined in 79 samples from four geothermal centers and a number of crater lakes, fumaroles, hot springs, and surface waters in Costa Rica, Nicaragua, and El Salvador. Geothermal and volcanic fluids were found to have iodine concentrations substantially higher than values in seawater or meteoric waters. ¹²⁹I/I ratios in most of the geothermal fluids are below the preanthropogenic input ratio of 1500 × 10⁻¹⁵, demonstrating that recent anthropogenic additions are largely absent from the volcanic systems. The majority of the ¹²⁹I/I ratios are between 500 and 800 × 10⁻¹⁵. These ratios indicate minimum iodine ages between 25 and 15 Ma, in good agreement with the age of subducted sediments in this region. In all four geothermal systems, however, a few samples were found with iodine ages older than 40 Ma—that is, considerably below the expected age range for subducted sediments from the Cocos Plate. These samples probably reflect the presence of iodine derived from sediments in older accreted oceanic terraines. The iodine ages indicate that the magmatic end member for the volcanic fluids originates in the deeper parts of the subducted sediment column, with small additions from older iodine mobilized from the overlying crust. The high concentrations of iodine in geothermal fluids, combined with the observed iodine ages, demonstrate that remobilization in the main volcanic zone (and probably also in the forearc area) is an important part in the overall marine cycle of iodine and similar elements.  相似文献   

Provenance analysis and tectonic setting of the Ordovician clastic deposits in the southern Puna Basin, NW Argentina   总被引：9，自引：0，他引：9  

Udo Zimmermann  Heinrich Bahlburg† 《Sedimentology》2003,50(6):1079-1104

Provenance studies on Early to Middle Ordovician clastic formations of the southern Puna basin in north-western Argentina indicate that the sedimentary detritus is generally composed of reworked crustal material. Tremadoc quartz-rich turbidites (Tolar Chico Formation, mean composition Q_t89 F7 L4) are followed by volcaniclastic rocks and greywackes (Tolillar Formation, mean Q_t33 F42 L25). These are in turn overlain by volcaniclastic deposits (mean Q_t24 F30 L46) of the Diablo Formation (late Arenig–early Llanvirn) that are intercalated by lava flows. All units were deformed in the Oclóyic Orogeny during the Middle and Late Ordovician. Sandstones of the Tolar Chico Formation are characterized by Th/Sc ratios > 1, La/Sc ratios ≈ 10, whereas associated fine-grained wackes show slightly lower values for both ratios. LREE (light rare earth elements) enrichment of the arenites is ≈ 50× chondrite, Eu/Eu* values are between 0·72 and 0·92, and flat HREE (heavy rare earth elements) patterns indicate a derivation from mostly felsic rocks of typical upper crustal composition. The ε_{Nd(t = sed)} values scatter around −11 to −9. The calculated Nd-T_DM residence ages vary between 1·8 and 2·0 Ga indicating contribution by a Palaeoproterozoic crustal component. The Th/Sc and La/Sc ratios of the Tolillar Formation are lower than those of the Tolar Chico Formation. Normalized REE (rare earth elements) patterns display a similar shape to PAAS (post-Archaean average Australian shale) but with higher abundances of HREEs. Eu/Eu* values range between 0·44 and 1·17, where the higher values reflect the abundance of plagioclase and feldspar-bearing volcanic lithoclasts. Average ε_{Nd(t = sed)} values are less negative at −5·1, and Nd-T_DM are lower at 1·6 Ga. This is consistent with characteristics of regional rocks of upper continental crust composition, which most probably represent the sources of the studied detritus. The rocks of the Diablo Formation have the lowest Th/Sc and La/Sc ratios, lower LREE abundances than the average continental crust and are slightly enriched in HREEs. Eu/Eu* values are between 0·63 and 1·17. The Nd isotopes (ε_{Nd(t = sed)} = −3 to −1; T_DM = 1·2 Ga) indicate that one source component was less fractionated than both the underlying Early Ordovician and the overlying Middle Ordovician units. Synsedimentary vulcanites in the Diablo Formation show the same isotopic composition. Our data indicate that the sedimentary detritus is generally composed of reworked crustal material, but that the Diablo Formation appears to contain ≈ 80% of a less fractionated component, derived from a contemporaneous continental volcanic arc. There are no data indicating an exotic detrital source or the accretion of an exotic block at this part of the Gondwana margin during the Ordovician.  相似文献   

Metal island growth and dynamics on molybdenite surfaces     

Udo Becker  Kevin M. Rosso  Michele Warren 《Geochimica et cosmochimica acta》2003,67(5):923-934

In order to understand the adsorption mechanism of metal atoms to semiconducting surfaces, we have studied, as a model system, the vapor phase adsorption of Ag, Au, and Cu on the (001) surface of molybdenite (MoS₂) and the subsequent surface diffusion of these adsorbates. Our scanning tunneling microscopy (STM) images show that, depending on the type of metal atom that is adsorbed, islands of a characteristic size (2 nm for Ag, 8 to 10 nm for Cu, two distinct sizes of 2 nm and 8 to 10 nm for Au), shape (well rounded in the lateral extension) and thickness (one monolayer for Ag, 1 to 1.5 nm for Cu) are formed during the initial stages of deposition. Whole islands are observed to surface diffuse without loss of size or shape. Despite the relatively large size of the copper islands on molybdenite, these islands surface diffuse extensively, suggesting that the Cu-S interaction is weak. Surface diffusion is only hindered once individual islands start to coalesce. As copper islands accumulate, the size and shape of the original islands can still be recognized, supporting the conclusion that these characteristics are constant and that monolayer growth occurs by the aggregation of islands across the surface.The strength and the nature of the Ag-S(MoS₂) bond were further investigated by using molecular orbital calculations, ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). By applying quantum mechanical approaches using a two-dimensional periodic molybdenite slab and hexagonal MoS₂ clusters of different sizes with metal atoms adsorbed to them, it is possible to calculate the electron transfer between the mineral surface and the metal atom as well as the adsorption energy as a function of surface coverage. In addition, we used the results from the quantum mechanical runs to derive empirical potentials that model the characteristics of the forces within the crystal, within the adsorbed islands, and the metal and mineral surface. The combination of quantum mechanical calculations and empirical force field calculations explain the electronic structure and the highest stability of Ag islands that have seven atoms in diameter, which exactly agrees with the size of experimentally observed islands. UPS results also suggest that a specific new state is formed (approximately 4.5 eV into the valence band) which may describe the Ag-S bond because it does not occur in pure silver or molybdenite.This study shows how the combination of microscopic (STM), spectroscopic (STS, UPS), compositional (X-ray photoelectron spectroscopy, XPS) and molecular modeling (quantum mechanical and empirical) techniques is a useful approach to understand the nature of the metal to sulfide bond. Further insights may be gained concerning the natural association of certain metals with sulfides.  相似文献   

Natural fracking and the genesis of five-element veins     

Gregor Markl  Mathias Burisch  Udo Neumann 《Mineralium Deposita》2016,51(6):703-712

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Statistical analyses of Global U-Pb Database 2017     

Stephen J.Puetz  Carlos E.Ganade  Udo Zimmermann  Glenn Borchardt 《地学前缘(英文版)》2018,9(1):121-145

The method of obtaining zircon samples affects estimation of the global U-Pb age distribution.Researchers typically collect zircons via convenience sampling and cluster sampling.When using these techniques,weight adjustments proportional to the areas of the sampled regions improve upon unweighted estimates.Here,grid-area and modern sediment methods are used to weight the samples from a new database of 418,967 U-Pb ages.Preliminary tests involve two age models.Model-1 uses the most precise U-Pb ages as the best ages.Model-2 uses the~(206)Pb/~(238)U age as the best age if it is less than a1000 Ma cutoff,otherwise it uses the~(207)Pb/~(206)Pb age as the best age.A correlation analysis between the Model-1 and Model-2 ages indicates nearly identical distributions for both models.However,after applying acceptance criteria to include only the most precise analyses with minimal discordance,a histogram of the rejected samples shows excessive rejection of the Model-2 analyses around the1000 Ma cutoff point.Because of the excessive rejection rate for Model-2,we select Model-1 as the preferred model.After eliminating all rejected samples,the remaining analyses use only Model-1 ages for five rock-type subsets of the database:igneous,meta-igneous,sedimentary,meta-sedimentary,and modern sediments.Next,time-series plots,cross-correlation analyses,and spectral analyses determine the degree of alignment among the time-series and their periodicity.For all rock types,the U-Pb age distributions are similar for ages older than 500 Ma,but align poorly for ages younger than 500 Ma.The similarities(500 Ma)and differences(500 Ma)highlight how reductionism from a detailed database enhances understanding of time-dependent sequences,such as erosion,detrital transport mechanisms,lithification,and metamorphism.Time-series analyses and spectral analyses of the age distributions predominantly indicate a synchronous period-tripling sequence of~91-Myr,~273-Myr,and~819-Myr among the various rock types.  相似文献   

Ephesite,Na(LiAl2) [Al2Si2O10] (OH)2: I. thermal stability and standard state thermodynamic properties     

Udo Warhus  Niranjan D. Chatterjee 《Contributions to Mineralogy and Petrology》1984,85(1):74-79

Ephesite, Na(LiAl₂) [Al₂Si₂O₁₀] (OH)₂, has been synthesized for the first time by hydrothermal treatment of a gel of requisite composition at 300≦T(° C)≦700 and \(P_{H_2 O}\) upto 35 kbar. At \(P_{H_2 O}\) between 7 and 35 kbar and above 500° C, only the 2M₁ polytype is obtained. At lower temperatures and pressures, the 1M polytype crystallizes first, which then inverts to the 2M₁ polytype with increasing run duration. The X-ray diffraction patterns of the 1M and 2M₁ poly types can be indexed unambiguously on the basis of the space groups C2 and Cc, respectively. At its upper thermal stability limit, 2M₁ ephesite decomposes according to the reaction (1) $$\begin{gathered} {\text{Na(LiAl}}_{\text{2}} {\text{) [Al}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{{\text{10}}} {\text{] (OH)}}_{\text{2}} \hfill \\ {\text{ephesite}} \hfill \\ {\text{ = Na[AlSiO}}_{\text{4}} {\text{] + LiAl[SiO}}_{\text{4}} {\text{] + }}\alpha {\text{ - Al}}_{\text{2}} {\text{O}}_{\text{3}} {\text{ + H}}_{\text{2}} {\text{O}} \hfill \\ {\text{nepheline }}\alpha {\text{ - eucryptite corundum}} \hfill \\ \end{gathered}$$ Five reversal brackets for (1) have been established experimentally in the temperature range 590–750° C, at \(P_{H_2 O}\) between 400 and 2500 bars. The equilibrium constant, K, for this reaction may be expressed as (2) $$log K{\text{ = }}log f_{{\text{H}}_{\text{2}} O}^* = 7.5217 - 4388/T + 0.0234 (P - 1)T$$ where \(f_{H_2 O}^* = f_{H_2 O} (P,T)/f_{H_2 O}^0\) (1,T), with T given in degrees K, and P in bars. Combining these experimental data with known thermodynamic properties of the decomposition products in (1), the following standard state (1 bar, 298.15 K) thermodynamic data for ephesite were calculated: H _f,298.15 ⁰ =-6237372 J/mol, S _298.15 ⁰ =300.455 J/K·mol, G _298.15 ⁰ =-5851994 J/mol, and V _298.15 ⁰ =13.1468 J/bar·mol.  相似文献   

A Sr isotope study on fluorite and siderite from post-orogenic mineral veins in the eastern Harz Mountains,Germany   总被引：2，自引：0，他引：2  

Jens?SchneiderEmail author  Udo?Haack  Klaus?Stedingk 《Mineralium Deposita》2003,38(8):984-991

Rb–Sr isotope data for siderite and fluorite from sediment-hosted epithermal mineral veins in the eastern Harz Mountains (Germany) are presented. Several fluorite and siderite-bearing paragenetic stages have been proposed for these veins, with the most important mineralization being related to a quartz–sulfide and a subsequent calcite–fluorite–quartz stage, which occurred at 226±1 and 209±2 Ma, respectively. Our Rb–Sr data do not permit the identification of distinct generations of siderite and fluorite, but rather reveal straight internal mixing relations, reflecting mixing of fluids or differential fluid–rock interaction processes. This indicates merely two significant phases of mineral deposition related to the quartz–sulfide and calcite–fluorite–quartz stages. It is shown that the Paleozoic sedimentary host rocks of the veins are the most likely source for the siderite Sr, whereas fluorite displays a two-component mixture between sedimentary Sr and radiogenic Sr derived from locally occurring Permian metavolcanic rocks. Editorial handling: B. Lehmann  相似文献   

Organochlorverbindungen im Seewasser der westlichen Ostsee Untersuchungsmethode und Ergebnisse der Jahre 1976 und 1978     

Dr. Horst Gaul  Udo Ziebarth 《Ocean Dynamics》1980,33(5):199-209

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Mineral chemistry of gersdorffite – A powerful tool in the differentiation of hydrothermal systems     

Udo Harms 《Chemie der Erde / Geochemistry》2010,70(2):175-184

Gersdorffite from two mineralization types (post-Variscan vein deposits, strata-bound mineralization) was investigated in the Niederberg area Rhenish Massif. In the ternary Ni–Co–Fe space gersdorffite from post-Variscan vein deposits displays a tight cluster with the highest Ni-contents ranging from 0.825 to 0.962 atoms per formula unit (a.p.f.u.). As/S ratios comprise a narrow range from 0.875 to 1.012. In contrast gersdorffite from the strata-bound mineralization displays a substitutional trend. Co and Fe substitute for Ni in a ± fixed ratio. Ni ranges between 0.494 and 0.836 a.p.f.u. As/S ratios (1.025–1.211) display a wider range and indicate higher As-contents relative to gersdorffite from post-Variscan vein deposits. Based on these results, two different hydrothermal fluid systems can be identified in the Niederberg area forming gersdorffite in both mineralization types. The hydrothermal fluids circulating in the post-Variscan vein deposits were homogeneous (high Ni-activities, lower As fugacities) and mixing occurred far away from the site of deposition whereas the fluids of the strata-bound mineralization were more heterogeneous (decreasing Ni-activities) with moderate elevated As fugacities. With respect to the post-Variscan vein deposits in the Niederberg area the results are compatible with earlier findings.Comparison with available gersdorffite analyses from adjacent areas (borehole Viersen, Ramsbeck deposit) reveal that gersdorffite compositions provide a reliable tool in distinguishing between different hydrothermal systems on a regional scale in the northern Rhenish Massif. However, gersdorffite compositions cannot be used to discriminate between Variscan and post-Variscan deposits with confidence.The country rocks in the Niederberg area are possible sources for Ni, Co and Fe during gersdorffite formation of the strata-bound mineralization. However, due to the remarkable homogeneity of gersdorffite compositions of the post-Variscan vein deposits irrespective of age and composition of the immediate adjacent host-rocks it is assumed that these host-rocks are not the source of the metals. Reduced Zechstein sulfate is assumed to be the source of sulfur. The As source remains unknown.Due to conflicting experimental data concerning the gersdorffite solid solution field it is not possible to derive reliable formation temperatures for the strata-bound mineralization. However, gersdorffite compositions of the post-Variscan vein deposits are compatible with low formation temperatures (<300 °C) in accordance with earlier findings.  相似文献   

Origin and age of ultramafic rocks and gabbros in the southern Puna of Argentina: an alleged Ordovician suture revisited     

Udo Zimmermann  Heinrich Bahlburg  Klaus Mezger  Jasper Berndt  Suzanne Mahlburg Kay 《International Journal of Earth Sciences》2014,103(4):1023-1036

Ultramafic rocks and gabbros are exposed in the southern Puna (NW Argentina) in tectonic association with continental arc-related Ordovician (volcano) sedimentary successions and granitoids. The origin of this mafic rock suite has been debated for three decades as either representing an Ordovician terrane suture, primitive Ordovician arc-related rocks or relics of the pre-Ordovician basement in tectonic contact with the Ordovician retro-arc basin successions. We present the first U–Pb ages of primary and inherited zircon from gabbros of this mafic–ultramafic assemblage. LA-ICP-MS analyses on cores and rims of these zircon grains yielded a concordia age of 543.4 ± 7.2 Ma for the gabbroic rocks. Other analysed zircons have Mesoproterozoic, and Early Ediacaran core and rim ages indicating that the magmas also assimilated Meso- and Neoproterozoic crustal material prior to final crystallization. The mafic rocks witnessed higher metamorphic grade than associated Ordovician rocks, which are unmetamorphosed or only affected by anchimetamorphism. The gabbros are mostly tholeiitic and enriched in Zr, Th, as well as other incompatible elements and have εNd _t=540Ma ranging from 1.3 to 7.4 with most of the values between 5 and 7. ¹⁴⁷Sm/¹⁴⁴Nd ratios show evidence of weak crustal contamination. The mafic rocks do not reveal any affinity to mid-ocean ridge basalts in their geochemistry but point instead to an emplacement in an active plate margin arc environment. Chromites from ultramafic rocks show typical Ti, Al, Cr#, Fe³⁺ abundances found in magmatic arc rocks. The formation of the gabbros and the associated ultramafic rocks in the southern Argentine Puna is related to the evolution of the margin of the Pampia terrane, including the Puncoviscana basin, during the Late Neoproterozoic and earliest Cambrian. In contrast to previous interpretations, the rocks predate the Ordovician evolution of the Central proto-Andean active margin. Consequently, interpretations assuming these rocks to represent an oceanic terrane suture of Ordovician age have to be dismissed as much as all palaeotectonic models that define Ordovician terranes in the Central Andes based on assumption that the ultramafic rocks and gabbros exposed in the southern Puna mark plate boundaries.  相似文献