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Geowissenschaften

Die Erforschung der Erdgeschichte erfüllt den grundlegenden menschlichen Drang zu wissen, wie die Welt entstanden ist. Im Laufe ihrer 4,6 Milliarden Jahre langen Geschichte hat die Erde eine enorme Transformation durchlaufen, und die Stabilisotopenanalyse steht bei der Erforschung dieser Veränderungsprozesse an vorderster Front. Durch das Verständnis der makro- und mikrozyklischen Mechanismen, welchen die leichten stabilen Isotope in der Lithosphäre durch natürliche Prozesse unterliegen, wächst auch unser Wissen über die Erdgeschichte kontinuierlich.

Paläoklima

Eine der ersten Anwendungen der Stabilisotopenmassenspektrometrie leichter Elemente war die Rekonstruktion des Paläoklimas auf Grundlage der Sauerstoff-Isotopenzusammensetzung (δ18O) von sehr alten, konservierten marinen Carbonaten. Auch heute noch ist dies eine der weitverbreitesten Methoden zur Erforschung der Klimageschichte der Erde. Dieses Paläothermometer basiert auf dem Grundprinzip, dass die Fraktionierung von Sauerstoffisotopen in Calcit temperaturabhängig ist. Das isoprime precisION mit Dual Inlet und MultiCarb ermöglicht eine sehr präzise und zuverlässige, automatisierte Messung von Carbonaten, ideal zur Erzeugung hochauflösender Paläotemperaturaufzeichnungen.

Kosmologie

Die Schwefelisotopenanalyse von extraterrestrischen Sulfidmineralien, z.B. von Meteoriten, hat eine Vielzahl von Erkenntnissen über den Ursprung der Erde und des Sonnensystems ermöglicht. Die Isotopenzusammensetzung von meteoritischem Schwefel (Canyon Diabolo Triolite) wurde auch als der ursprüngliche Bezugspunkt für Erdkruste und -mantel verwendet, von der ausgehend die globalen Fraktionen im Schwefelkreislauf evaluiert werden. Schwefelhaltige Mineralien lassen sich leicht mit unserem vielseitigen Spektrum von EA-IRMS-Systemen analysieren, welche die Advanced Purge and Trap Technologie (APT) für eine unschlagbare Trennung und Peakfokussierung von SO2 nutzen, um eine perfekte Auflösung und eine unvergleichliche Empfindlichkeit für die Mineralschwefelanalyse zu gewährleisten.

Stratigraphie

δ13C-Fluktuationen im Laufe geologischer Zeitalter werden durch Änderungen des Gleichgewichts im Kohlenstoffkreislauf bewirkt. Da die Verweilzeit im Kohlenstoffkreislauf kurz ist (10 ka), werden Änderungen im Kohlenstofffluss im Sediment sehr genau global aufgezeichnet. Als solche sind gelegentliche Spikes im globalen δ13C als stratigraphische Marker für die Chemostratigraphie nützlich, besonders während des Paläozoikums. Der iso FLOW nutzt unsere neuartige UltiTrap-Technologie und ermöglicht so eine präzise, durchgängige Continuous Flow Analyse von Massencarbonaten, um solche Isotopenabweichungen zu identifizieren.

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248 Ergebnisse:

Origins of elements building travertine and tufa: New perspectives provided by isotopic and geochemical tracers
Sedimentary Geology (2016)
P.-A. Teboul, C. Durlet, E.C. Gaucher, A. Virgone, J.-P. Girard, J. Curie, B. Lopez, G.F. Camoin

Fluid/rock interaction represents a major process in the formation of calcitic or aragonitic travertine and tufa (CATT). In most cases, CATT is associated to limestone dissolution somewhere along the hydrogeological pathway. However, a wide array of other substratum (basalts, rhyolites, carbonatites, ultramafics, granites, dolomites, evaporites) can act as potential source of elements involved in the formation of CATT. This study reports on the evaluation of potential geochemical tracers linking CATT to their substratum, and unravelling the origin of elements. A large database was established from available literature data as well as new data acquired in the frame of this study for a set of Modern to Recent CATT (Ligurian ophiolites, Italy; the Chaine des Puys, Limagne graben and Paris Basin, France; Reunion Island, Indian Ocean; Jebel Oust, Tunisia). Four most reliable tracing methods are identified (1) δ13C and δ18O cross-plot allows distinguishing epigean (minδ13C=−27.2‰, maxδ13C=0.9‰, meanδ13C=−12.3‰ for N=314) from hypogean systems (minδ13C=−4‰, maxδ13C=11.7‰, meanδ13C=−2.87‰ for N=198). Very low δ13C values (<−12‰) and δ18O>−4‰ associated to negative δ13C values are specifically indicative of an ultramafic source rock. (2) Barium and strontium cross-plot helps to discriminate different groups of source rocks amongst the hypogean CATT: (i) source rocks composed of mixed limestones, evaporites, and dolomites are characterised by low barium (<100ppm) and high strontium (>400ppm) contents, (ii) mafic and granitic source rocks are undifferentiated and display similar barium (from 15 to 930ppm) and high strontium (>200ppm) contents, (iii) the carbonatite group is characterised by its exceptional high barium and strontium values. In epigean CATT, a pure limestone source rock usually relates to very low barium and strontium contents (<50ppm and <70ppm respectively), whereas mixed limestone, evaporite and dolomite source rocks generally display low strontium content (<580ppm) with higher barium content (>50ppm). (3) Relatively high beryllium content (>30ppm) in CATT seems to indicate a pure granitoid source. (4) High chromium concentrations (>20ppm) are systematically documented in Modern CATT located on an ultramafic substratum. The definition of diagnostic compositional fields for actively forming or recently formed CATT is influenced by many factors including water composition, water temperature, dissolved gas composition and concentration, biological activity, position in the sedimentary body and early diagenesis, in addition to substratum lithology. However, the results of this study illustrate that, despite these many factors, the combined use of Ba, Sr, Be, Cr, δ13C, and δ18O may be valuable to discriminate the rock lithology prevailing in the hydrogeological or palaeo-hydrogeological reservoir of CATT.
Schlagworte: carbon , oxygen , geol , gashead

Carbonatite associated with ultramafic diatremes in the Avon Volcanic District, Missouri, USA: Field, petrographic, and geochemical constraints
Lithos (2016)
Ethan J. Shavers, Abduwasit Ghulam, John Encarnacion, David L. Bridges, Patrick B. Luetkemeyer

Here we report field, petrographic, and geochemical analyses of the southeast Missouri Avon Volcanic District intrusive rocks and present the first combined textural and geochemical evidence for the presence of a primary magmatic carbonatite phase among ultramafic dikes, pipes, and diatremes of olivine melilitite, alnöite, and calciocarbonatite. The δ13CVPDB values measured for primary calciocarbonatite as well as carbonates in olivine melilitite and alnöite rocks range from -3.8‰ to -8.2‰, which are within the typical range of mantle values and are distinct from values of the carbonate country rocks, 0.0‰ -1.3‰. The carbonate oxygen isotope compositions for the intrusive lithologies are in the range of 21.5‰ to 26.2‰ (VSMOW), consistent with post-emplacement low temperature hydrothermal alteration or kinetic fractionation effects associated with decompression and devolatilization. Metasomatised country rock and breccia-contaminated igneous lithologies have carbonate δ13CVPDB values gradational between primary carbonatite values and country rock values. Unaltered sedimentary dolomite breccia and mafic spheroids entrained by calciocarbonatite and the lack of microstratigraphic crystal growth typical of carbonate replacement, also exclude the possibility of hydrothermal replacement as the cause of the magmatic-textured carbonates. Rare earth element (REE) patterns for the alnöite, olivine melilitite, and carbonatite are similar to each other with strong light REE enrichment and heavy REE depletion relative to MORB. These patterns are distinct from those of country rock rhyolite and sedimentary carbonate. These data suggest that rocks of the Avon Volcanic District represent a single ultramafic carbonatite intrusive complex possibly derived from a single mantle source.

Sulfur isotope constraints on marine transgression in the lacustrine Upper Cretaceous Songliao Basin, northeastern China
Palaeogeography, Palaeoclimatology, Palaeoecology (2016)
Hansheng Cao, Alan J. Kaufman, Xuanlong Shan, Huan Cui, Guijie Zhang

Organic-rich Cretaceous source rocks of the petroliferous Songliao Basin in northeast China are considered to be lacustrine in origin, but paleontological and organic geochemical evidence suggest episodic marine incursions. As a test of this hypothesis, we applied time-series measurements of elemental and isotopic abundances on core and cutting samples to evaluate fluctuations in the sulfur and carbon cycles across the Santonian-Campanian transition preserved in the upper Yaojia and lower Nenjiang formations. The data reveal a spike in pyrite sulfur abundance and a marked negative excursion in ??34S at the base of the Nenjiang Formation when the basin expanded to its maximal extent. The elemental and isotopic data suggest that flooding was associated with rapid marine transgression that enhanced sulfate concentrations, which promoted microbial sulfate reduction in anoxic bottom waters that were episodically euxinic. Subsequent restriction of the basin and a decline in marine influence is supported by progressive upsection 34S enrichment (up to 30???) in Nenjiang Member I, which are interpreted to reflect the distillation of sulfate through enhanced pyrite burial, followed by a gradual return to lacustrine conditions that prevailed in overlying strata.
Schlagworte: carbon , nitrogen , sulfur , geol , elem

Dynamic anoxic ferruginous conditions during the end-Permian mass extinction and recovery
Nature Communications (2016)
M. O. Clarkson, R. A. Wood, S. W. Poulton, S. Richoz, R. J. Newton, S. A. Kasemann, F. Bowyer, L. Krystyn

The end-Permian mass extinction, ~252 million years ago, is notable for a complex recovery period of ~5 Myr. Widespread euxinic (anoxic and sulfidic) oceanic conditions have been proposed as both extinction mechanism and explanation for the protracted recovery period, yet the vertical distribution of anoxia in the water column and its temporal dynamics through this time period are poorly constrained. Here we utilize Fe–S–C systematics integrated with palaeontological observations to reconstruct a complete ocean redox history for the Late Permian to Early Triassic, using multiple sections across a shelf-to-basin transect on the Arabian Margin (Neo-Tethyan Ocean). In contrast to elsewhere, we show that anoxic non-sulfidic (ferruginous), rather than euxinic, conditions were prevalent in the Neo-Tethys. The Arabian Margin record demonstrates the repeated expansion of ferruginous conditions with the distal slope being the focus of anoxia at these times, as well as short-lived episodes of oxia that supported diverse biota.
Schlagworte: sulfur , geol , elem

Characterization of the water chemistry, sediment 13C and 18O compositions of Kolleru Lake—a Ramsar wetland in Andhra Pradesh, India
Environmental Monitoring and Assessment (2016)
Subrata Das Sharma, D. Sujatha

Schlagworte: carbon , oxygen , geol , mulitcarb

A Southern Hemisphere record of global trace-metal drawdown and orbital modulation of organic-matter burial across the Cenomanian-Turonian boundary (Ocean Drilling Program Site 1138, Kerguelen Plateau)
Sedimentology (2016)
Alexander J. Dickson, Matthew Saker-Clark, Hugh C. Jenkyns, Cinzia Bottini, Elisabetta Erba, Fabio Russo, Olga Gorbanenko, B. David A. Naafs, Richard D. Pancost, Stuart A. Robinson, Sander van den Boorn, Erdem Idiz

Schlagworte: carbon , geol , ocea , gaschrom

Cross-checking groundwater age by 4He and 14C dating in a granite, Tono area, central Japan
Geochimica et Cosmochimica Acta (2016)
Takuma Hasegawa, Kotaro Nakata, Yuichi Tomioka, Kazuyuki Goto, Kouki Kashiwaya, Katsuhiro Hama, Teruki Iwatsuki, Takanori Kunimaru, Masaki Takeda

Groundwater dating was performed simultaneously by the 4He and 14C methods in granite of the Tono area in central Japan. Groundwater was sampled at 30 packed-off sections of six 1000-m boreholes. 4He concentrations increased and 14C concentrations decreased along a groundwater flow path on a topographic gradient. 4He ages were calculated by using the in situ 4He production rate derived from the porosity, density, and U and Th content of the rock, neglecting external flux. 14C ages were calculated with a noncorrected model in which the initial 14C content was 100 percent of the modern radiocarbon level (Co = 100 pmC), a statistical model using the average 14C content of tritium-bearing samples (Co = 46.4 pmC), and a δ13C model based on the isotopic mass balance. Although the absolute 14C ages calculated by the models were different, the relative 14C ages were almost identical. The relative 14C ages were considered reliable because dissolved inorganic carbon has no significant geochemical reactions in granite. The relation between the 4He ages and the noncorrected 14C ages was [4He age] = 1.15 [14C age] + 7200 (R2 = 0.81), except in the discharge area. The slope of this relation was equivalent to unity, which indicates that the 4He accumulation rate is confirmed by the relative 14C ages. Moreover, the accumulated 3He/4He ratio was equivalent to that derived from the 6Li(α,n)3H reaction in granite. These results show that the accumulated He is of crustal origin, produced in situ without external flux, except in the discharge area. The intercept value of 7200 a implies that the 14C concentrations were diluted due to geochemical reactions. Tritium-bearing samples supported this result. Simultaneous measurements make it feasible to estimate the accumulation rate of 4He and initial dilution of 14C, which cannot be done with a single method. Cross-checking groundwater dating has the potential to provide more reliable groundwater ages. The circulation time of the groundwater flow in the Tono area may be several tens of thousands of years, and the groundwater age calculated from the 4He and 14C ages appears to be consistent with geochemical information such as groundwater types, δD and δ18O.
Schlagworte: hydrogen , oxygen , geol , gashead

Sulfur isotopic characteristics of volcanic products from the September 2014 Mount Ontake eruption, Japan
Earth, Planets and Space (2016)
Kei Ikehata, Teruyuki Maruoka

Components and sulfur isotopic compositions of pyroclastic materials from the 2014 Mt. Ontake eruption were investigated. The volcanic ash samples were found to be composed of altered volcanic fragments, alunite, anhydrite, biotite, cristobalite, gypsum, ilmenite, kaolin minerals, native sulfur, orthopyroxene, plagioclase, potassium feldspar, pyrite, pyrophyllite, quartz, rutile, and smectite, and most of these minerals were likely derived from the acidic alteration zones of Mt. Ontake. The absence of juvenile material in the eruptive products indicates that the eruption was phreatic. The sulfur isotopic compositions of the water-leached sulfate, hydrochloric acid-leached sulfate, acetone-leached native sulfur, and pyrite of the samples indicate that these sulfur species were produced by disproportionation of magmatic SO2 in the hydrothermal system at temperatures of 270–281 °C. This temperature range is consistent with that inferred from the hydrothermal mineral assemblage (e.g., pyrophyllite and rutile) in the 2014 pyroclastic materials (200–300 °C). Except for the sulfur isotopic compositions of anhydrite, which may have been altered by incorporation of sulfate minerals in a fumarolic area with lower sulfur isotopic values into the underground materials during the 1979 eruption, no significant differences in the mineral assemblages and sulfur isotopic compositions of the pyroclastic materials were identified between the products of the 2014 and 1979 Ontake phreatic eruptions, which suggests geochemical similarities in the underlying hydrothermal systems before the 2014 and 1979 eruptions

Multiphase dolomitization of deeply buried Cambrian petroleum reservoirs, Tarim Basin, north-west China
Sedimentology (2016)
Lei Jiang, Chunfang Cai, Richard H. Worden, Stephen F. Crowley, Lianqi Jia, Ke Zhang, Ian J. Duncan

Cambrian dolostone reservoirs in the Tarim Basin, China, have significant potential for future discoveries of petroleum, although exploration and production planning is hampered by limited understanding of the occurrence and distribution of dolomite in such ancient rocks buried to nearly 8 km. The study herein accessed new drill core samples which provide an opportunity to understand the dolomitization process in deep basins and its impact on Cambrian carbonate reservoirs. This study documents the origin of the dolostone reservoirs using a combination of petrology, fluid-inclusion microthermometry, and stable and radiogenic-isotopes of outcrop and core samples. An initial microbial dolomitization event (D1) occurred in restricted lagoon environments and is characterized by depleted δ13C values. Dolomicrite (D2) from lagoonal and sabkha facies, some fabric-retentive dolomite (D3) and fabric-obliterative dolomite (D4) in the peloidal shoal and reef facies show the highest δ18O values. These dolomites represent relatively early reflux dolomitization. The local occurrence of K-feldspar in D2 indicates that some strontium was contributed via terrigenous input. Most fabric-retentive dolomite (D3) may have precipitated from seawater at slightly elevated temperatures, suggested by petrological and isotopic data. Most fabric-obliterative dolomite (D4), and medium to coarse dolomite cement (D5), formed between 90°C and 130°C from marine evaporitic brine. Saddle dolomite (D6) formed by hydrothermal dolomitization at temperatures up to 170°C, and involved the mixing of connate brines with Sr- enriched hydrothermal fluids. Intercrystalline, moldic, and breccia porosities are due to the early stages of dolomitization. Macroscopic, intergranular, vuggy, fracture, and dissolution porosity are due to buial-related dissolution and regional hydrothermal events. This work has shown that old (for example, Cambrian or even Precambrian) sucrosic dolomite with associated anhydrite, buried to as much as 8000 m, can still have a high potential for hosting large hydrocarbon resources and should be globally targeted for future exploration.
Schlagworte: carbon , geol , oilg , gashead

Fluid inclusion and stable isotopic constraints on fluid sources and evolution of the Luojiahe Cu deposit in the southern margin of the North China Craton
Ore Geology Reviews (2016)
Yuhang Jiang, Hecai Niu, Yan Zhao, Zhiwei Bao, Ningbo Li, Qiang Shan

The Luojiahe Cu deposit in the Zhongtiaoshan region is located in the southern margin of the North China Craton. The orebodies are hosted in the mafic volcanic-sedimentary sequences of the metamorphosed (greenschist-facies) Neoarchean Songjiashan Group. The Luojiahe Cu mineralization can be divided into the primary volcanogenic massive sulfide (VMS) mineralization stage (Stage I, banded or stockwork ores) and the subsequent metamorphic remobilization stage (Stage II, coarse-vein ores). Three types of quartz selected for fluid inclusion (FI) studies were collected from the Stage I banded (Q1) and stockwork (Q2) ores and Stage II coarse-vein (Q3) ores. Four types of FIs were identified: (1) liquid-rich FIs (L-type), (2) pure vapor and vapor-rich FIs (V-type), (3) daughter mineral-bearing FIs (S-type), and (4) CH4-H2O FIs (C-type). Systematical microthermometric and H-O isotopic studies show that the Stage I ore-forming fluids consist predominantly of high salinity evolved seawater (125–220°C; 23.9–27.9wt.% NaCl equiv.) and some magmatic-hydrothermal fluids (249–339°C; 34.5–42.2wt.% NaCl equiv.). The two fluid end-members are represented by the L-type FIs in Q1 and the S- and V-type FIs in Q2. The temperature- and salinity variation trends of the L-type FIs in Q1 indicate a mixing process between the hot evolved seawater and cold seawater at Stage I. Furthermore, the V- and S-type FI coexistence in Q2 and their microthermometric data suggest that fluid unmixing has occurred in original magmatic fluids at Stage I. In contrast, the Stage II ore-forming fluids consist of CH4-rich metamorphic fluids (192–350°C; 10.6–43.2wt.% NaCl equiv.). Carbon isotopic analysis of the Stage II calcite (−4.58 to −10.83‰) and graphite (−32.01 to −39.16‰) in the ore-hosting chlorite schist indicates that the metamorphic ore-forming fluids had exchanged carbon isotope with graphite. The generation of CH4 may have resulted from the interaction between H2O (released by metamorphic devolatilization) and graphite. The continuous consumption of H2O in the hydrothermal fluid system may have increased the fluid salinity and triggered fluid unmixing in the CH4-NaCl-H2O system. In addition, the VMS metallogenic environment is generally favorable for microbial communities. It is considered that the graphite at Luojiahe may have been derived from sedimentary organic matter formed in seafloor hydrothermal vent systems, as also supported by carbon isotopic data. We propose that at Stage I, the main mineralization may have been resulted from 1) fluid mixing of hot evolved seawater and cold seawater in the near-surface environment; and 2) fluid unmixing caused by the percolation of magmatic fluids into syn-volcanic faults, forming the stockwork ores. At Stage II, the interaction between H2O and graphite may have resulted in the reduction of ore-forming fluids and Cu precipitation, and fluid unmixing in the CH4-NaCl-H2O system may have further promoted the Cu mineralization.
Schlagworte: hydrogen , oxygen , geol , gashead