DIffusion of Eu and Gd in basalt and obsidian     

Mordeckai Magaritz  Albrecht W. Hofmann 《Geochimica et cosmochimica acta》1978,42(6):847-858

Tracer diffusion coefficients of ¹⁵³Gd and ¹⁵²Eu in olivine tholeiite have been determined at temperatures between 1150 and 1440°C. The results are identical for both tracers within experimental error. Between 1440 and 1320°C the diffusion coefficients are given by D(Eu, Gd) = 0.058 exp(?40,600/ RT). Between 1320 and 1210°C, the diffusion coefficients are constant at D = (1.4 ± 0.4) × 10^?7 cm²s^?1 and between 1210 and 1150°C, the D values drop irregularly to 4 × 10^?9 cm²s^?1. The liquidus temperature (1270°C) lies within the region of constant D. Such anomalous behavior has not been encountered in previous studies of Ca, Sr, Ba and Co diffusion in basalt. To explain the constant D value near the liquidus, we speculate that the structure of the melt changes as a function of temperature in such a way that the normal temperature dependence of the diffusivity is compensated. For example, the rare earth ions may be displaced from their (high temperature) octahedral coordination sites to other sites where they are more readily dissociated and therefore become progressively more mobile. The behavior below 1210°C may be the result of relatively stable complexes or molecules in the melt or of the formation of a REE bearing crystalline phase that has so far escaped detection. Preliminary results for Eu diffusion in obsidian are D (Eu, 800°C) = 5 × 10^?13 cm² s^?1 and D (Eu, 950°C) = 1.5 × 10^?11 cm² s^?1. These data are consistent with an activation energy of 59 Kcal mole^?1. These low diffusivities indicate that the partitioning of REE in crystallizing intermediate and acidic melts may be controlled by diffusion in the melt rather than equilibrium between the crystal surface and the bulk melt.The diffusion data are applied to partial melting in the mantle, in an attempt to explain how LREE enriched tholeiites may be derived from a LREE depleted mantle source. In this model LREE diffuse from garnet bearing regions that have small melt fractions into garnet free regions that have relatively large melt fractions. REE diffusion is so slow that this process is quantitatively significant only in small partially molten bodies (diameter ～1 km or less) or in larger, but strongly flattened bodies. Internal convective motion during diapiric rise would also increase the efficiency of the process.  相似文献   

Neodymium and Strontium Isotope Data for USGS Reference Materials BCR-1, BCR-2, BHVO-1, BHVO-2, AGV-1, AGV-2, GSP-1, GSP-2 and Eight MPI-DING Reference Glasses   总被引：7，自引：1，他引：7  

Ingrid Raczek  Klaus Peter Jochum  Albrecht W. Hofmann 《Geostandards and Geoanalytical Research》2003,27(2):173-179

We have measured ⁸⁷Sr/⁸⁶Sr and ¹⁴³ Nd/¹⁴⁴ Nd isotope ratios in different batches and aliquots of the new US Geological Survey (USGS) reference materials (RMs) BCR-2, BHVO-2, AGV-2 and GSP-2 and the original USGS RMs BCR-1, BHVO-1, AGV-1 and GSP-1 by thermal ionisation mass spectrometry. In addition, we also analysed the eight Max-Planck-Institut-Dingwell (MPI-DING) reference glasses. Nearly all isotope ratios obtained in the different aliquots and batches agree within uncertainty limits indicating excellent homogeneity of the USGS powders and the MPI-DING glasses. With the exception of GSP-2, the new USGS RMs are also indistinguishable from the ratios found in the original USGS RMs (⁸⁷Sr/⁸⁶Sr: 0.704960, 0.704958 (BCR-1, -2), 0.703436, 0.703435 (BHVO-1, -2), 0.703931, 0.703931 (AGV-1, -2); ¹⁴³ Nd/¹⁴⁴ Nd: 0.512629, 0.512633 (BCR-1, -2), 0.512957, 0.512957 (BHVO-1, -2); 0.512758, 0.512755 (AGV-1, -2)). This means that for normalisation purposes in Sr and Nd isotope geochemistry BCR-2, BHVO-2 and AGV-2 can well replace BCR-1, BHVO-1 and AGV-1 respectively.  相似文献   

The geochemistry of Archaean shales derived from a Mafic volcanic sequence, Belingwe greenstone belt, Zimbabwe: provenance, source area unroofing and submarine versus subaerial weathering   总被引：1，自引：0，他引：1  

Axel Hofmann  Robert Bolhar  Hielke Jelsma 《Geochimica et cosmochimica acta》2003,67(3):421-440

Shales of the ∼2.7 Ga Zeederbergs Formation, Belingwe greenstone belt, Zimbabwe, form thin (0.2-2 m) horizons intercalated with submarine lava plain basalts. Shales of the overlying Cheshire Formation, a foreland basin sedimentary sequence, form 1-100 m thick units intercalated with shallow-water carbonates and deep-water, resedimented basalt pebble conglomerates. Zeederbergs shale is characterized by high contents of MgO and transition metals and low concentrations of K₂O and LILE as compared to average Phanerozoic shale, indicative of an ultramafic to mafic source terrain. Cheshire shales have similar major and trace element contents, but MgO and transition metals are less enriched and the LILE are less depleted. Zeederbergs shales have smoothly fractionated REE patterns (La_N/Yb_N = 2.84-4.45) and no significant Eu anomaly (Eu/Eu* = 0.93-0.96). REE patterns are identical to those of the surrounding basaltic rocks, indicating local derivation from submarine reworking. Cheshire shales have rather flat REE patterns (La_N/Yb_N = 0.69-2.19) and a small, negative Eu anomaly (average Eu/Eu* = 0.85), indicative of a mafic provenance with minor contributions of felsic detritus. A systematic change in REE patterns and concentrations of transition metals and HFSE upwards in the sedimentary succession indicates erosion of progressively more LREE-depleted basalts and ultramafic volcanic rocks, followed by unroofing of granitoid crust. Weathering indices confirm the submarine nature of Zeederbergs shale, whereas Cheshire shale was derived from a source terrain subjected to intense chemical weathering.  相似文献   

The sediment record of the past 200 years in a Swiss high-alpine lake: Hagelseewli (2339 m a.s.l.)     

A.F. Lotter  P.G. Appleby  R. Bindler  J.A. Dearing  J.-A. Grytnes  W. Hofmann  C. Kamenik  A. Lami  D.M. Livingstone  C. Ohlendorf  N. Rose  M. Sturm 《Journal of Paleolimnology》2002,28(1):111-127

Sediment cores spanning the last two centuries were taken in Hagelseewli, a high-elevation lake in the Swiss Alps. Contiguous 0.5 cm samples were analysed for biological remains, including diatoms, chironomids, cladocera, chrysophyte cysts, and fossil pigments. In addition, sedimentological and geochemical variables such as loss-on-ignition, total carbon, nitrogen, sulphur, grain-size and magnetic mineralogy were determined. The results of these analyses were compared to a long instrumental air temperature record that was adapted to the elevation of Hagelseewli by applying mean monthly lapse rates.During much of the time, the lake is in the shadow of a high cliff to the south, so that the lake is ice-covered during much of the year and thus decoupled from climatic forcing. Lake biology is therefore influenced more by the duration of ice-cover than by direct temperature effects during the short open-water season. Long periods of ice-cover result in anoxic water conditions and dissolution of authigenic calcites, leading to carbonate-free sediments.The diversity of chironomid and cladoceran assemblages is extremely low, whereas that of diatom and chrysophyte cyst assemblages is much higher. Weak correlations were observed between the diatom and chrysophyte cyst assemblages on the one hand and summer or autumn air temperatures on the other, but the proportion of variance explained is low, so that air temperature alone cannot account for the degree of variation observed in the paleolimnological record.Analyses of mineral magnetic parameters, spheroidal carbonaceous particles and lead suggest that atmospheric pollution has had a significant effect on the sediments of Hagelseewli, but little effect on the water quality as reflected in the lake biota.  相似文献   

Sub-Milankovitch and Milankovitch forcing on a model Mesozoic carbonate platform – the Latemar (Middle Triassic, Italy)     

Rainer Zühlke  Thilo Bechstädt  Roland Mundil 《地学学报》2003,15(2):69-80

ABSTRACT Basic shallowing-upward cycles (shu-cycles) and five-bundled megacycles in the Latemar platform have been widely regarded as a model example for precessional and eccentricity forcing in the Mesozoic. Based on bio-/chronostratigraphic data, recent studies questioned this particular type of Milankovitch forcing on the Latemar cyclic series. We present an integrated model that incorporates (i) new cyclostratigraphic data, (ii) new and existing bio-/chronostratigraphic data and (iii) new spectral analyses. The basic shu-cycles in the Latemar reflect sub-Milankovitch control. Cycle bundles of 1 : 4–5 (megacycles : shu-cycles) indicate precessional forcing. They do not reflect eccentricity superimposed on precessional forcing. Spectral analyses reveal highly significant ratios in the large-scale cycle bundlings. Stacking patterns of 1 : 9.9 and 1 : 24.0 shu-cycles represent obliquity and short eccentricity forcing. Both sub-Milankovitch and Milankovitch forcing potentially controlled shallow subtidal carbonate deposition in Mesozoic greenhouse times. Cyclostratigraphic models require an integrated approach including bio- and chronostratigraphic data.  相似文献   

Holocene glacier fluctuations and migration of Neolithic yak pastoralists into the high valleys of northwest Bhutan     

M.C. Meyer  Ch.-Ch. Hofmann  A.M.D. Gemmell  E. Haslinger  H. Häusler  D. Wangda 《Quaternary Science Reviews》2009,28(13-14):1217-1237

Here we present geomorphologic, palaeoenvironmental and archaeo-botanical data which elucidate the Late Pleistocene and Holocene glacial history of the high, mountain-locked Himalayan valleys in northwest Bhutan and provide one of the earliest proofs of human activity yet known for the High Himalaya range. In this area, difficult to access, close linkage between climatic change, glacier fluctuations and human migration patterns has been discovered. Glacier systems in the studied area are characterized by avalanching and debris mantled glacier snouts, with the significant local influence of the Indian summer monsoon causing decoupling of glacier responses from temperature changes but supporting the idea of monsoonal forcing. Geomorphologic mapping, together with Optically Stimulated Luminescence (OSL) and radiocarbon dating of ice-proximal sediments, has been used to construct a local glacial chronology. Local ice-stream networks developed during the Early Holocene (ca 10,000–9000 a ago) and during the early part of the Mid Holocene (6710 ± 90–4680 ± 155 cal a BP) at which times there were ice advances of about 5 km from the modern glacier termini. At such times, the intensity of pro- and periglacial processes would have intensified and ice-dammed lakes were probably common as well, rendering human colonization of the high valleys in northwest Bhutan impossible. An abrupt shift to dry climatic conditions on the Tibetan Plateau between 5000 and 4500 a BP coincided with glacial decay and the onset of morphodynamically stable conditions on the broad valley floors of the high valleys in this part of the Himalaya. Palynological data suggest that the sudden disappearance of juniper and rhododendron pollen, the immediate onset of pollen input from cereals (confirmed by detailed SEM analysis) and a clear pattern of over-grazing, trampling and peat deterioration can be linked to human arrival in the valleys at ca 4280 ± 130 cal a BP. Extensive charcoal horizons dating to 4745 ± 250 and 4680 ± 155 cal a BP are interpreted as evidence for human use of fire and forest clearances and agree spatially and temporally with the pollen-based picture. Charcoal occurrences as old as 6710 ± 90 cal a BP might be linked to yet earlier exploration of these Himalayan valleys during phases of low glacial activity. We provide an account of the colonization of these high valleys in response to glacial and monsoonal change and argue that the most likely founder societies come from the Tibetan Plateau, where yak and barley based pastoralism and Neolithic settlements are known to have existed since the Mid Holocene.  相似文献   

Niobium-tantalum Fractionation During Slab Subduction:Implications for the Formation of Continental Crust     

Yilin Xiao  Weidong Sun  Jochen Hoefs  Klaus Simon  Zeming Zhang  Shuguang Li 《矿物岩石地球化学通报》2007,26(Z1)

Compared with the oceanic crust, knowledge about the formation of the continental crust (CC) is relatively poor. Although melting of subducted slabs in the early history of the Earth has been considered as the major way that shaped the chemical characteristics of the CC by most geologists, as the CC shares many characteristics with modern adakites, some geologists argued that Archean TTG was formed in the same way as modern arcs rather than slab melting, whereas others proposed that melting at the bottom of the thickened oceanic crust was more important. Recently,the debate is mainly focused on the unique subchondritic Nb/Ta value of the CC, and particularly, how Nb and Ta fractionated from each other and consequently how, in detail, the CC was built.  相似文献   

Mobilization of colloidal ferrihydrite particles in porous media—An inner-sphere complexation approach     

Annette Hofmann  Liyuan Liang 《Geochimica et cosmochimica acta》2007,71(24):5847-5861

Prediction of colloid release and transport as affected by reactive species remains a significant challenge for field applications. In this paper, we report experimental and modeling results of ferrihydrite colloid release under the influence of citrate species. Using a 3-plane surface complexation model, equilibrium constants were obtained for the three proposed inner-sphere complexes by fitting a citrate adsorption isotherm on ferrihydrite at pH 4, and a pH adsorption envelop with 0.64 mM citrate. The constants were used in a reactive transport model for simulating reaction fronts of dissolved species during injection of citrate in ferrihydrite-coated quartz columns. Simulation results show that sorption alone may not adequately describe the breakthrough curves. Inclusions of ferrihydrite dissolution and re-adsorption of Fe(III) improve the prediction of dissolved species transport. Additionally, matrix diffusion may be needed for improved prediction. For the release of colloidal iron oxides it was shown that both oxide dissolution and interfacial repulsion controlled the process during complete breakthrough. However, the peak release of colloids, which occurred during the actual breakthrough of dissolved species, was mainly brought about by electric double layer forces. These particles underwent detachment-deposition-detachment cycles along the flow path, and emerged in the effluent with the major reaction front. To quantitatively predict colloid release, a semi-empirical linear correlation was established, linking the calculated electric potential to experimental colloid release rates. The model may be applied to the prediction and scaling of aquifer remediation studies involved in the injection of organic ligands to mobilize particle bound contaminants.  相似文献   

The transport of Osmium and Strontium isotopes through a tropical estuary     

M. Sharma  K. Balakrishna  A.W. Hofmann 《Geochimica et cosmochimica acta》2007,71(20):4856-4867

We have determined the concentration and isotopic composition of Os and Sr in the estuarine waters from the Godavari delta in Peninsular India. Additionally, we have obtained the concentration and isotopic composition of Os and Al concentration in selected suspended particulate matter recovered on 0.45 μm filters. The Na, K, Mg, and Ca concentrations of water samples obtained along salinity gradients from two distributary channels in the delta display a general two component mixing between river- and sea-water. The data also reveal that Al behaves non-conservatively and is affected by interactions with suspended particulates. The ⁸⁷Sr/⁸⁶Sr ratio of the riverine end member is 0.716303 and shows a linear decrease with salinity to seawater value and Sr isotope systematics indicate that its behavior is conservative in the estuary.The ¹⁸⁷Os/¹⁸⁸Os ratio of the Godavari river end-member is 1.24 and within error of the average eroding upper continental crust. The concentration and isotopic composition of Os through the two salinity transects shows that its behavior in the Godavari estuary is complex and non-conservative. By comparing the Al/Os ratios and Os isotopes in the waters with those of the suspended particulate we find that both Os gains and losses occur in the water column. However, in one of the distributaries (Vasishta) the Os concentration of suspended load increases and that of dissolved load decreases with increasing salinity towards the Bay of Bengal end-member. We infer that there is removal of seawater Os at higher salinities. The estimated mean residence time of Os in the oceans is 37 ± 14 (2σ) kyr. A comparison of the Os concentration of the Bay of Bengal and the Indian Ocean waters indicates that the rainout rate of Os in Bay of Bengal is 30% faster than that in the open ocean and suggests that the observed discrepancy between the mean residence time calculated from mass balance considerations and that estimated from the relaxation of the Os isotopic ratio in marine record may not be real as the relaxation time experiments likely estimate the residence time for a basin/sub-basin and not for the entire ocean.  相似文献   

Adaptive Optics on a 3.6-Meter Telescope. The ADONIS System.     

J.-L. Beuzit  L. Demailly  E. Gendron  P. Gigan  F. Lacombe  D. Rouan  N. Hubin  D. Bonaccini  E. Prieto  F. Chazallet  D. Rabaud  P.-Y. Madec  G. Rousset  R. Hofmann  F. Eisenhauer 《Experimental Astronomy》1997,7(4):285-292

ADONIS is an adaptive optics (AO) user friendly instrument offered to the European astronomical community on the ESO 3.6-m telescope at La Silla. It is an upgraded version of COME-ON-PLUS, the VLT AO prototype, which already produced significative astrophysical results in a wide range of fields, from planetology to extragalactic astrophysics. ADONIS is now allowing the astronomer to use adaptive optics as a common user instrument thanks to the implementation of an open artificial intelligence software that handles the large number of parameters needed to optimise the AO correction. We will describe the ADONIS system, including the two dedicated infrared cameras, summarize its performances and discuss the observing procedures.  相似文献