Calcium-isotope fractionation in selected modern and ancient marine carbonates     

Thomas Steuber  Dieter Buhl 《Geochimica et cosmochimica acta》2006,70(22):5507-5521

The calcium-isotope composition (δ^44/42Ca) was analyzed in modern, Cretaceous and Carboniferous marine skeletal carbonates as well as in bioclasts, non-skeletal components, and diagenetic cements of Cretaceous and Carboniferous limestones. In order to gain insight in Ca²⁺_aq-CaCO₃-isotope fractionation mechanisms in marine carbonates, splits of samples were analyzed for Sr, Mg, Fe, and Mn concentrations and for their oxygen and carbon isotopic composition. Biological carbonates generally have lower δ^44/42Ca values than inorganic marine cements, and there appears to be no fractionation between seawater and marine inorganic calcite. A kinetic isotope effect related to precipitation rate is considered to control the overall discrimination against ⁴⁴Ca in biological carbonates when compared to inorganic precipitates. This is supported by a well-defined correlation of the δ^44/42Ca values with Sr concentrations in Cretaceous limestones that contain biological carbonates at various stages of marine diagenetic alteration. No significant temperature dependence of Ca-isotope fractionation was found in shells of Cretaceous rudist bivalves that have recorded large seasonal temperature variations as derived from δ¹⁸O values and Mg concentrations. The reconstruction of secular variations in the δ^44/42Ca value of seawater from well preserved skeletal calcite is compromised by a broad range of variation found in both modern and Cretaceous biological carbonates, independent of chemical composition or mineralogy. Despite these variations that may be due to still unidentified biological fractionation mechanisms, the δ^44/42Ca values of Cretaceous skeletal calcite suggest that the δ^44/42Ca value of Cretaceous seawater was 0.3-0.4‰ lower than that of the modern ocean.  相似文献   

Geochemistry of oceanic carbonatites compared with continental carbonatites: mantle recycling of oceanic crustal carbonate   总被引：11，自引：2，他引：11  

Kaj Hoernle  George Tilton  Mike J. Le Bas  Svend Duggen  Dieter Garbe-Schönberg 《Contributions to Mineralogy and Petrology》2002,142(5):520-542

Major and trace element and Sr-Nd-Pb-O-C isotopic compositions are presented for carbonatites from the Cape Verde (Brava, Fogo, Sáo Tiago, Maio and Sáo Vicente) and Canary (Fuerteventura) Islands. Carbonatites show pronounced enrichment in Ba, Th, REE, Sr and Pb in comparison to most silicate volcanic rocks and relative depletion in Ti, Zr, Hf, K and Rb. Calcio (calcitic)-carbonatites have primary (mantle-like) stable isotopic compositions and radiogenic isotopic compositions similar to HIMU-type ocean island basalts. Cape Verde carbonatites, however, have more radiogenic Pb isotope ratios (e.g. ²⁰⁶Pb/²⁰⁴Pb=19.3-20.4) than reported for silicate volcanic rocks from these islands (18.7-19.9; Gerlach et al. 1988; Kokfelt 1998). We interpret calcio-carbonatites to be derived from the melting of recycled carbonated oceanic crust (eclogite) with a recycling age of ~1.6 Ga. Because of the degree of recrystallization, replacement of calcite by secondary dolomite and elevated ‘¹³C and ‘¹⁸O, the major and trace element compositions of the magnesio (dolomitic)-carbonatites are likely to reflect secondary processes. Compared with Cape Verde calcio-carbonatites, the less radiogenic Nd and Pb isotopic ratios and the negative Ɨ/4 of the magnesio-carbonatites (also observed in silicate volcanic rocks from the Canary and Cape Verde Islands) cannot be explained through secondary processes or through the assimilation of Cape Verde crust. These isotopic characteristics require the involvement of a mantle component that has thus far only been found in the Smoky Butte lamproites from Montana, which are believed to be derived from subcontinental lithospheric sources. Continental carbonatites show much greater variation in radiogenic isotopic composition than oceanic carbonatites, requiring a HIMU-like component similar to that observed in the oceanic carbonatites and enriched components. We interpret the enriched components to be Phanerozoic through Proterozoic marine carbonate (e.g. limestone) recycled through shallow, subcontinental-lithospheric-mantle and deep, lower-mantle sources.  相似文献   

Experimental impact cratering: A summary of the major results of the MEMIN research unit          下载免费PDF全文

Thomas Kenkmann  Alex Deutsch  Klaus Thoma  Matthias Ebert  Michael H. Poelchau  Elmar Buhl  Eva-Regine Carl  Andreas N. Danilewsky  Georg Dresen  Anja Dufresne  Nathanaël Durr  Lars Ehm  Christian Grosse  Max Gulde  Nicole Güldemeister  Christopher Hamann  Lutz Hecht  Stefan Hiermaier  Tobias Hoerth  Astrid Kowitz  Falko Langenhorst  Bernd Lexow  Hanns-Peter Liermann  Robert Luther  Ulrich Mansfeld  Dorothee Moser  Manuel Raith  Wolf Uwe Reimold  Martin Sauer  Frank Schäfer  Ralf Thomas Schmitt  Frank Sommer  Jakob Wilk  Rebecca Winkler  Kai Wünnemann 《Meteoritics & planetary science》2018,53(8):1543-1568

This paper reviews major findings of the Multidisciplinary Experimental and Modeling Impact Crater Research Network (MEMIN). MEMIN is a consortium, funded from 2009 till 2017 by the German Research Foundation, and is aimed at investigating impact cratering processes by experimental and modeling approaches. The vision of this network has been to comprehensively quantify impact processes by conducting a strictly controlled experimental campaign at the laboratory scale, together with a multidisciplinary analytical approach. Central to MEMIN has been the use of powerful two-stage light-gas accelerators capable of producing impact craters in the decimeter size range in solid rocks that allowed detailed spatial analyses of petrophysical, structural, and geochemical changes in target rocks and ejecta. In addition, explosive setups, membrane-driven diamond anvil cells, as well as laser irradiation and split Hopkinson pressure bar technologies have been used to study the response of minerals and rocks to shock and dynamic loading as well as high-temperature conditions. We used Seeberger sandstone, Taunus quartzite, Carrara marble, and Weibern tuff as major target rock types. In concert with the experiments we conducted mesoscale numerical simulations of shock wave propagation in heterogeneous rocks resolving the complex response of grains and pores to compressive, shear, and tensile loading and macroscale modeling of crater formation and fracturing. Major results comprise (1) projectile–target interaction, (2) various aspects of shock metamorphism with special focus on low shock pressures and effects of target porosity and water saturation, (3) crater morphologies and cratering efficiencies in various nonporous and porous lithologies, (4) in situ target damage, (5) ejecta dynamics, and (6) geophysical survey of experimental craters.  相似文献   

High‐Arctic climate conditions for the last 7000 years inferred from multi‐proxy analysis of the Bliss Lake record,North Greenland     

Jesper Olsen  Kurt H. Kjær  Svend Funder  Nicolaj K. Larsen  Anna Ludikova 《第四纪科学杂志》2012,27(3):318-327

The Arctic is more vulnerable to climate change than are mid latitudes. Therefore, palaeolimnological studies from the High Arctic are important in providing insights into the dynamics of the climate system. Here we present a multi‐proxy study from one of the world's northernmost lakes: Bliss Lake, Peary Land, Greenland. The early Holocene (10 850–10 480 cal. a BP) is characterized by increased erosion and gradually more marine conditions. Full marine conditions developed from 10 480 cal. a BP until the lake was isolated at 7220 cal. a BP. From its marine isolation at 7220 cal. a BP Bliss Lake becomes a lacustrine environment. Evidence from geochemical proxies (δ¹³C and total organic carbon) suggests that warmer conditions prevailed between 7220 and 6500 cal. a BP, corresponding to the Holocene thermal maximum, and from 3300 until 910 cal. a BP. From 850 to 500 cal. a BP colder climate conditions persisted. The transition from warmer to colder climate conditions taking place around 850 cal. a BP may be associated with the transition from the Medieval Warm Period to the Little Ice Age. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

The Sudbury Structure (Ontario,Canada): a tectonically deformed multi-ring impact basin     

A. Deutsch  R. A. F. Grieve  M. Avermann  L. Bischoff  P. Brockmeyer  D. Buhl  R. Lakomy  V. Müller-Mohr  M. Ostermann  D. Stöffler 《International Journal of Earth Sciences》1995,84(4):697-709

The occurrence of shock metamorphic features substantiates an impact origin for the 1.85 Ga old Sudbury Structure, but this has not been universally accepted. Recent improvements in knowledge of large-scale impact processes, combined with new petrographic, geochemical, geophysical (LITHOPROBE) and structural data, allow the Sudbury Structure to be interpreted as a multi-ring impact structure. The structure consists of the following lithologies: Sudbury Breccia —dike breccias occurring up to 80 km from the Sudbury Igneous Complex (SIC); Footwall rocks and Footwall Breccia — brecciated, shocked crater floor materials, in part thermally metamorphosed by the overlying SIC; Sublayer and Offset Dikes, Main Mass of the SIC and Basal Member of the Onaping Formation (OF) — geochemically heterogeneous coherent impact melt complex ranging from inclusion-rich basal unit through a dominantly inclusion-free to a capping inclusion-rich impact melt rock; Grey Member of OF — melt-rich impact breccia (suevite); Green Member of OF — thin layer of fall back ejecta; Black Member of OF — reworked and redeposited breccia material; Onwatin and Chelmsford Formations — post-impact sediments. Observational and analytical data support an integrated step-by-step impact model for the genesis of these units. Analysis of the present spatial distribution of various impact-related lithologies and shock metamorphic effects result in an estimated original rim-to-rim diameter of the final crater of 200 or even 280 km for the Sudbury Structure, prior to tectonic thrusting and deformation during the Penokean orogeny.  相似文献   

Drastic shift in lava geochemistry in the volcanic-front to rear-arc region of the Southern Kamchatkan subduction zone: Evidence for the transition from slab surface dehydration to sediment melting   总被引：2，自引：0，他引：2  

Svend Duggen  Maxim Portnyagin  Joel Baker  Kaj Hoernle  Nathalie Grassineau 《Geochimica et cosmochimica acta》2007,71(2):452-480

The shift of lava geochemistry between volcanic front to rear-arc volcanoes in active subduction zones is a widespread phenomenon. It is somehow linked to an increase of the slab surface depth of the subducting oceanic lithosphere and increasing thickness of the mantle wedge and new constraints for its causes may improve our understanding of magma generation and element recycling in subduction zones in general. As a case study, this paper focuses on the geochemical composition of lavas from two adjacent volcanic centres from the volcanic front (VF) to rear-arc (RA) transition of the Southern Kamchatkan subduction zone, with the aim to examine whether the shift in lava geochemistry is associated with processes in the mantle wedge or in the subducted oceanic lithosphere or both. The trace element and O-Sr-Nd-Hf-Pb (double-spike)-isotopic composition of the mafic Mutnovsky (VF) and Gorely (RA) lavas in conjunction with geochemical modelling provides constraints for the degree of partial melting in the mantle wedge and the nature of their slab components. Degrees of partial melting are inferred to be significantly higher beneath Mutnovsky (∼18%) than Gorely (∼10%). The Mutnovsky (VF) slab component is dominated by hydrous fluids, derived from subducted sediments and altered oceanic crust, eventually containing minor but variable amounts of sediment melts. The composition of the Gorely slab component strongly points to a hydrous silicate melt, most likely mainly stemming from subducted sediments, although additional fluid-contribution from the underlying altered oceanic crust (AOC) is likely. Moreover, the Hf-Nd-isotope data combined with geochemical modelling suggest progressive break-down of accessory zircon in the melting metasediments. Therefore, the drastic VF to RA shift in basalt chemistry mainly arises from the transition of the nature of the slab component (from hydrous fluid to melt) in conjunction with decreasing degrees of partial melting within ∼15 km across-arc. Finally, systematic variations of key inter-element with high-precision Pb-isotope ratios provide geochemical evidence for a pollution of the Mutnovsky mantle source with Gorely melt components but not vice versa, most likely resulting from trench-ward mantle wedge corner flow. We also present a geodynamic model integrating the location of the Mutnovsky and Gorely volcanic centres and their lava geochemistry with the recently proposed thermal structure of the southern Kamchatkan arc and constraints about phase equilibria in subducted sediments and AOC. Herein, the slab surface hosting the subducted sediments suffers a transition from dehydration to melting above a continuously dehydrating layer of AOC. Wider implications of this study are that an onset of (flush-) sediment melting may ultimately be the main trigger for the VF to RA transition of lava geochemistry in subduction zones.  相似文献   

Low enthalpy geothermal energy resources in Denmark     

Niels Balling  Svend Saxov 《Pure and Applied Geophysics》1978,117(1-2):205-212

The deep oil exploration drillings in Denmark have shown that especially the Danish Embayment contains low enthalpy geothermal resources associated with warm aquifers. The most promising reservoirs have been found in highly permeable Upper Triassic sand and sandstone beds, which cover at least 5000 km² at depths of 2000–3000 m and at temperatures of 60–100°C. The porosity of the main reservoir is of 15–25%, and the permeability is presumed to be approximately 1 darcy (10^–12 m²) or higher. A layer thickness of 30–60 m has been observed on a number of localities. Also the Middle Jurassic and the Lower Triassic contain reservoirs of interest. A major geothermal exploration work is planned with seismic investigations, drillings to depths of 2000–4000 m and probably establishment of pilot district heating plants.  相似文献   

Rb/1bSr isochrons for the granitic plutons around Farsund,southern Norway     

Svend Pedersen  Torgeir Falkum 《Chemical Geology》1975,15(2):97-101

The granitic and charnockitic intrusions in the Precambrian migmatites around Farsund in southern Norway have previously been shown to be successively intruded as well as being geochemically different. Rb- and Sr-isotope data support the conclusion that they are intruded with a separate time interval and that the Farsund charnockite is younger (852±41 m.y.) than the Lyngdal hornblende granite (932±38 m.y.). Furthermore, the different initial ratios (0.7128 and 0.7054, respectively) disprove a comagmatic origin unless large-scale contamination has occurred.  相似文献   

Lower and Middle Ordovician conodonts of Laurentian affinity from blocks of limestone in the Rosroe Formation,South Mayo Trough,western Ireland and their palaeogeographic implication     

Svend Stouge  David A. T. Harper  George D. Sevastopulo  Darren O'Mahony  John Murray 《Geological Journal》2016,51(4):584-599

The Middle Ordovician Rosroe Formation consists of some 1350 m of coarse, mainly siliciclastic to volcaniclastic sedimentary rocks, deposited in a submarine fan environment, and is restricted to the southern limb of the South Mayo Trough, western Ireland. Discrete allochthonous blocks, reaching 5 m in size, are present in the formation at several localities. Conodonts recovered from these blocks, collected from two separate locations, are of late Early and mid Mid Ordovician age. The conodonts have high conodont‐alteration indices (CAI 5) indicative of temperatures as high as 300^o to max. 480 °C; some found in the Lough Nafooey area have abnormally high indices (CAI 6), which correspond to temperatures of about 360^o to max. 550 °C. The oldest fauna is dominated by Periodon aff. aculeatus and characterized by Oepikodus evae typical of the Oepikodus evae Zone (Floian Stage; Stage Slices Fl2–3, Lower Ordovician). The younger conodont assemblage, characterized by Periodon macrodentatus associated with Oistodella pulchra, is referred to the P. macrodentatus conodont Biozone (lower Darriwilian; Stage Slices Dw1–2). The Rosroe conodont assemblages are of Laurentian affinity; comparable faunas are well known from several locations along the east to south‐eastern platform margin of Laurentia and the Notre Dame subzone of central Newfoundland, Canada. The faunal composition from the limestone blocks suggests a shelf edge to slope (or fringing carbonate) setting. The faunal assemblages are coeval with, respectively, the Tourmakeady Formation (Floian–Dapingian) and Srah Formation (Darriwilian) in the Tourmakeady Volcanic Group in the eastern part of the South Mayo Trough and probably are derived from the same or similar laterally equivalent short‐lived carbonate successions that accumulated at offshore ‘peri‐Laurentian’ islands, close to and along the Laurentian margin. During collapse of the carbonate system in the late Mid Ordovician, the blocks were transported down a steep slope and into deep‐water by debris flows, mixing with other rock types now found in the coarse polymict clastics of the Rosroe Formation. The faunas fill the stratigraphical ‘gap’ between the Lower Ordovician Lough Nafooey Volcanic Group and the upper Middle Ordovician Rosroe Formation in the South Mayo Trough and represent a brief interval conducive to carbonate accumulation in an otherwise siliciclastic‐ and volcaniclastic‐dominated sedimentary environment. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

A major element,PGE and Re–Os isotope study of Middle Atlas (Morocco) peridotite xenoliths: Evidence for coupled introduction of metasomatic sulphides and clinopyroxene     

Nadine Wittig  D. Graham Pearson  Joel A. Baker  Svend Duggen  Kaj Hoernle 《Lithos》2010,115(1-4):15-26

We present major element and PGE (platinum-group-element) abundances in addition to Re–Os isotope data for 11 spinel-facies whole rock peridotites from a single maar from the Middle Atlas Mountains, Morocco.Major element systematics of these xenoliths are generally correlated with indices of depletion. FeO–MgO systematics appear to suggest spinel-facies melting in the range of 5 to 25%. However, Al₂O₃ abundances in these xenoliths appear elevated relative to primitive mantle (Prima). The Al₂O₃ abundances in conjunction with other major elements require distinct re-enrichment of the Middle Atlas continental mantle root due to melt/rock reaction and precipitation of amphibole and/or clinopyroxene from passing silicate melts akin to MORB or OIB that evolved in reverse direction along the melting curves in e.g. FeO–MgO space. Sc and V confirm the range of apparent depletion and also indicate that the currently preserved fO₂ in these peridotites is distinctly different from fO₂ conditions observed in subduction zones.The majority of these xenoliths have low Os and Ir (I-PGEs) concentrations relative to Prima and modelled sulphide- and clinopyroxene-depleted residues of mantle melting under low fO₂, mid-ocean ridge-like conditions. Moreover, Pt and Pd (P-PGE) abundances are elevated when compared to their expected abundances after substantial melt extraction. Importantly, the systematically low Ir abundances in the majority of samples show well-correlated trends with Al₂O₃, MgO and Cu that are inconsistent with established melting trends. Os isotopes in the Middle Atlas xenoliths range from ¹⁸⁷Os/¹⁸⁸Os = 0.11604 to 0.12664 although most samples are close to chondritic. The Os isotope ratios are decoupled from ¹⁸⁷Re/¹⁸⁸Os but, together with Re abundances, also exhibit a good correlation with Al₂O₃, MgO and Cu.The major element, I-PGE and Os isotope correlations suggest that the initial melt depletion led to the exhaustion of sulphide and clinopyroxene (20 to 30%) without significant stabilization of I-PGE-rich alloys. During later modal metasomatism of the refractory Middle Atlas continental mantle root with silicate melts akin to MORB or OIB the introduction of clinopyroxene/amphibole reduced the volume of the melt inducing sulphur saturation in these melts causing precipitation of secondary sulphides. This coupled crystallization of pyroxenes and sulphides (chalcopyrite) resulted in the two-component mixing systematics exhibited by I-PGEs, Os isotopes with major elements and Cu preserved in the Middle Atlas continental mantle root.  相似文献