Kinetic rate laws derived from order parameter theory IV: Kinetics of Al,Si disordering in Na-feldspars     

Bernd Wruck  Ekhard K. H. Salje  Ann Graeme-Barber 《Physics and Chemistry of Minerals》1991,17(8):700-710

The kinetic rate laws of Al-Si disordering under dry conditions (T = 1353K, 1253 K, 1223 K, 1183 K) and in the presence of water (p = 1 kbar, T = 1023 K, 1073 K, 1103 K) were studied both experimentally and theoretically. A gradual change of the degree of order was found under dry conditions. For intermediate degrees of order broad distributions of the order parameter Q _od occur. The variations of Q _od are correlated with structural modulations as observed in the transmission electron microscope. The time evolution of the mean value of Q _od can be well described by the rate law: $$\frac{{dQ_{od} }}{{dt}} = - \frac{\gamma }{{RT}}\exp \sum\limits_{i = 1}^n {X_i^2 } \left[ {\frac{{ - (G_a^0 + \varepsilon (\Delta Q_{od} )^2 )}}{{RT}}} \right]\frac{{dG}}{{dQ_{od} }}$$ with the excess Gibbs energy G and G _a ⁰ = 433.8 kJ/mol, ?= -27.4 kJ/mol, γ = 1.687 · 10¹⁴ h ^?1. Under wet conditions, two processes were found which occur simultaneously. Firstly, some material renucleated with the equilibrium degree of order. Secondly, the bulk of the material transformed following the same rate law as under dry conditions but with the reduced activation energy G _a ⁰ = 332.0 kJ/mol and ? = -43.0 kJ/ mol, γ = 1.047 · 10¹³ h^?1. The applicability of the kinetic theory is discussed and some ideas for the analysis of geological observations are evolved.  相似文献   

The reaction of carbonates in contact with laser-generated,superheated silicate melts: Constraining impact metamorphism of carbonate-bearing target rocks     

Christopher Hamann  Saskia Bläsing  Lutz Hecht  Sebastian Schäffer  Alex Deutsch  Jens Osterholz  Bernd Lexow 《Meteoritics & planetary science》2018,53(8):1644-1686

We simulated entrainment of carbonates (calcite, dolomite) in silicate impact melts by 1-bar laser melting of silicate–carbonate composite targets, using sandstone, basalt, calcite marble, limestone, dolomite marble, and iron meteorite as starting materials. We demonstrate that carbonate assimilation by silicate melts of variable composition is extremely fast (seconds to minutes), resulting in contamination of silicate melts with carbonate-derived CaO and MgO and release of CO₂ at the silicate melt–carbonate interface. We identify several processes, i.e., (1) decomposition of carbonates releases CO₂ and produces residual oxides (CaO, MgO); (2) incorporation of residual oxides from proximally dissociating carbonates into silicate melts; (3) rapid back-reactions between residual CaO and CO₂ produce idiomorphic calcite crystallites and porous carbonate quench products; (4) high-temperature reactions between Ca-contaminated silicate melts and carbonates yield typical skarn minerals and residual oxide melts; (5) mixing and mingling between Ca- or Ca,Mg-contaminated and Ca- or Ca,Mg-normal silicate melts; (6) precipitation of Ca- or Ca,Mg-rich silicates from contaminated silicate melts upon quenching. Our experiments reproduce many textural and compositional features of typical impact melts originating from silicate–carbonate targets. They reinforce hypotheses that thermal decomposition of carbonates, rapid back-reactions between decomposition products, and incorporation of residual oxides into silicate impact melts are prevailing processes during impact melting of mixed silicate–carbonate targets. However, by comparing our results with previous studies and thermodynamic considerations on the phase diagrams of calcite and quartz, we envisage that carbonate impact melts are readily produced during adiabatic decompression from high shock pressure, but subsequently decompose due to heat influx from coexisting silicate impact melts or hot breccia components. Under certain circumstances, postshock conditions may favor production and conservation of carbonate impact melts. We conclude that the response of mixed carbonate–silicate targets to impact might involve melting and decomposition of carbonates, the dominant response being governed by a complex variety of factors.  相似文献   

A tephrostratigraphic record for the last glacial–interglacial cycle from Lake Ohrid,Albania and Macedonia     

Hendrik Vogel  Giovanni Zanchetta  Roberto Sulpizio  Bernd Wagner  Norbert Nowaczyk 《第四纪科学杂志》2010,25(3):320-338

Here we present a tephrostratigraphic record (core Co1202) recovered from the northeastern part of Lake Ohrid (Republics of Macedonia and Albania) reaching back to Marine Isotope Stage (MIS) 6. Overall ten horizons (OT0702‐1 to OT0702‐10) containing volcanic tephra have been recognised throughout the 14.94 m long sediment succession. Four tephra layers were visible at macroscopic inspection (OT0702‐4, OT0702‐6, OT0702‐8 and OT0702‐9), while the remaining six are cryptotephras (OT0702‐1, OT0702‐2, OT0702‐3, OT0702‐5, OT0702‐7 and OT0702‐10) identified from peaks in K, Zr and Sr intensities, magnetic susceptibility measurements, and washing and sieving of the sediments. Glass shards of tephra layers and cryptotephras were analysed with respect to their major element composition, and correlated to explosive eruptions of Italian volcanoes. The stratigraphy and the major element composition of tephra layers and cryptotephras allowed the correlation of OT0702‐1 to AD 472 or AD 512 eruptions of Somma‐Vesuvius, OT0702‐2 to the FL eruption of Mount Etna, OT0702‐3 to the Mercato from Somma‐Vesuvius, OT0702‐4 to SMP1‐e/Y‐3 eruption from the Campi Flegrei caldera, OT0702‐5 to the Codola eruption (Somma‐Vesuvius or Campi Flegrei), OT0702‐6 to the Campanian Ignimbrite/Y‐5 from the Campi Flegrei caldera, OT0702‐7 to the Green Tuff/Y‐6 eruption from Pantelleria Island, OT0702‐8 to the X‐5 eruption probably originating from the Campi Flegrei caldera, OT0702‐9 to the X‐6 eruption of generic Campanian origin, and OT0702‐10 to the P‐11 eruption from Pantelleria Island. The fairly well‐known ages of these tephra layers and parent eruptions provide new data on the dispersal and deposition of these tephras and, furthermore, allow the establishment of a chronological framework for core Co1202 for a first interpretation of major sedimentological changes. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using the Greenland NGRIP ice core,and selected auxiliary records     

Mike Walker  Sigfus Johnsen  Sune Olander Rasmussen  Trevor Popp  Jørgen‐Peder Steffensen  Phil Gibbard  Wim Hoek  John Lowe  John Andrews  Svante Björck  Les C. Cwynar  Konrad Hughen  Peter Kershaw  Bernd Kromer  Thomas Litt  David J. Lowe  Takeshi Nakagawa  Rewi Newnham  Jakob Schwander 《第四纪科学杂志》2009,24(1):3-17

The Greenland ice core from NorthGRIP (NGRIP) contains a proxy climate record across the Pleistocene–Holocene boundary of unprecedented clarity and resolution. Analysis of an array of physical and chemical parameters within the ice enables the base of the Holocene, as reflected in the first signs of climatic warming at the end of the Younger Dryas/Greenland Stadial 1 cold phase, to be located with a high degree of precision. This climatic event is most clearly reflected in an abrupt shift in deuterium excess values, accompanied by more gradual changes in δ¹⁸O, dust concentration, a range of chemical species, and annual layer thickness. A timescale based on multi‐parameter annual layer counting provides an age of 11 700 calendar yr b2 k (before AD 2000) for the base of the Holocene, with a maximum counting error of 99 yr. A proposal that an archived core from this unique sequence should constitute the Global Stratotype Section and Point (GSSP) for the base of the Holocene Series/Epoch (Quaternary System/Period) has been ratified by the International Union of Geological Sciences. Five auxiliary stratotypes for the Pleistocene–Holocene boundary have also been recognised. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Holocene environmental history in high‐Arctic North Greenland revealed by a combined biomarker and macrofossil approach     

Stephanie Kusch  Ole Bennike  Bernd Wagner  Matthias Lenz  Ilona Steffen  Janet Rethemeyer 《Boreas: An International Journal of Quaternary Research》2019,48(2):273-286

In this study, we use a combined biomarker and macrofossil approach to reconstruct the Holocene climate history recorded in Trifna Sø, Skallingen area, eastern North Greenland. Chronological information is derived from comparison of lithological, biogeochemical and macrofossil characteristics with a well‐dated record from nearby Lille Sneha Sø. Following local deglaciation around c. 8 cal. ka BP, the local peak warmth occurred between c. 7.4 and 6.2 cal. ka BP as indicated by maximum macrofossil abundances of warmth‐demanding plants (Salix arctica and Dryas integrifolia) and invertebrates (Daphnia pulex and Chironomidae). Warm conditions were dominated by terrestrial organic matter (OM) sedimentation as implied by the alkane‐based P_aq ratio, but increased aquatic productivity is indicated when temperature was highest around 6.5 cal. ka BP. The n‐C₂₉/n‐C₃₁ alkane ratio shows that vegetation in the catchment was dominated by shrubs after deglaciation, but shifted towards relatively more grassy/herbaceous vegetation during peak warmth. After 5.4 cal. ka BP, the disappearance of warmth‐demanding plant and invertebrate macrofossils indicates cooling in the Skallingen area. This cooling was characterized by a significant shift towards dominance of aquatic OM sedimentation in Trifna Sø as implied by high P_aq ratios. Cooling was also associated with a shift in vegetation type from dwarf‐shrub heaths towards relatively more herbaceous vegetation in the catchment, stronger erosion and more oligotrophic conditions in the lake. Our data show that mean air temperatures inferred using branched glycerol dialkyl glycerol tetraethers (brGDGTs) do not seem to accurately reflect the local climatic history. Irrespective of calibration, methylation of branched tetraethers (MBT) palaeothermometry cannot be reconciled with the macrofossil evidence and seems to be biased by either changing brGDGT sources (in situ vs. soil‐derived) or changing species assemblages and/or an unknown physiological response to changing environmental conditions at high latitude.  相似文献   

Deglaciation and catchment ontogeny in coastal south‐west Greenland: implications for terrestrial and aquatic carbon cycling     

Melanie J. Leng  Bernd Wagner  N. John Anderson  Ole Bennike  Ewan Woodley  Simon J. Kemp 《第四纪科学杂志》2012,27(6):575-584

Here we present Holocene organic carbon, nitrogen, sulphur, carbon isotope ratio and macrofossil data from a small freshwater lake near Sisimiut in south‐west Greenland. The lake was formed c. 11 cal ka BP following retreat of the ice sheet margin and is located above the marine limit in this area. The elemental and isotope data suggest a complex deglaciation history of interactions between the lake and its catchment, reflecting glacial retreat and post‐glacial hydrological flushing probably due to periodic melting of local remnant glacial ice and firn areas between 11 and 8.5 cal ka BP. After 8.5 cal ka BP, soil development and associated vegetation processes began to exert a greater control on terrestrial–aquatic carbon cycling. By 5.5 cal ka BP, in the early Neoglacial cooling, the sediment record indicates a change in catchment–lake interactions with consistent δ¹³C while C/N exhibits greater variability. The period after 5.5 cal ka BP is also characterized by higher organic C accumulation in the lake. These changes (total organic carbon, C/N, δ¹³C) are most likely the result of increasing contribution (and burial) of terrestrial organic matter as a result of enhanced soil instability, as indicated by an increase in Cenococcum remains, but also Sphagnum and Empetrum. The impact of glacial retreat and relatively subdued mid‐ to late Holocene climate variation at the coast is in marked contrast to the greater environmental variability seen in inland lakes closer to the present‐day ice sheet margin. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Discharge experiments simulating chemical evolution on the surface of Titan     

Kristof Plankensteiner  Bernd M. Rode  Tomas Mikoviny  Armin Hansel  Georg Fischer  Helmut O. Rucker 《Icarus》2007,187(2):616-619

Saturn's moon Titan has been considered as one of the few places in our Solar System, where atmospheric and surface conditions could have produced organic compounds essential as precursors for an evolution of life. The Cassini-Huygens mission has provided new data on Titan's atmosphere and surface, which enabled us to simulate the chemical processes occurring under these conditions. Possible lightning events on Titan cannot only produce higher hydrocarbons, but also allow surface water ice to participate in the reaction scenario, resulting in CHO, CHN, and CHON compounds including several molecules relevant for the formation of amino acids and nucleic acids.  相似文献   

Paralytic shellfish toxin profiles in net phytoplankton samples from Bahía Concepción, Gulf of California, Mexico     

Gárate-Lizárraga I  Bustillos-Guzmán JJ  López-Cortes DJ  Hernández-Sandoval F  Erler K  Luckas B 《Marine pollution bulletin》2006,52(7):800-806

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青藏高原东北部冬给措纳湖湖区冰缘环境探讨   总被引：3，自引：3，他引：0  

罗栋梁  金会军  杨思忠  Bernd Wünnerman 《冰川冻土》2010,32(5)

冬给措纳湖位于青藏高原多年冻土区东北部,湖区高寒沼泽、冰缘环境、冻土分布及其演化在全球气候变暖条件下,有其自身特点.为探讨冬给措纳湖湖区冻土分布及冰缘环境,2009年5月9—26日,对冬给措纳湖进行了考察,发现冻融草丘、冻胀丘遗迹、寒冻裂缝、热融洼地、古冻融褶皱等现代冰缘及古冰缘现象在湖区普遍存在.湖东、西岸的沼泽湿地中,热喀斯特发育明显,形成热融洼地和冻融草丘.湖北岸阶地及草场中存在零星和岛状冻土,经钎探表明,当时融化深度为0.3～0.8m,0.4m深度处冻土有胶结冰发育.湖东冲积平原的沼泽湿地中,冻融草丘和热融洼地存在;冻胀丘遗迹、寒冻裂缝在沼泽湿地边缘泥炭覆盖地存在.湖北岸二级阶地剖面发现古冻融褶皱.  相似文献   

The OH site in topaz: an IR spectroscopic investigation     

Anke Watenphul  Eugen Libowitzky  Bernd Wunder  Matthias Gottschalk 《Physics and Chemistry of Minerals》2010,37(9):653-664

The OH site in topaz is investigated by IR spectroscopy depending on the OH concentration and temperature. The two OH bands that can be distinguished are due to the local ordering of F and OH in opposite sites of the crystal structure. The first typical sharp band stems from OH groups with fluorine in the opposite (=acceptor) site. The second band occurs as a shoulder on the low-energy wing and is related to two opposite OH groups. The degree of local OH–OH ordering depends on the OH concentration and, due to statistical F/OH distribution, can be predicted by probability calculations. The substitution of OH for F has a non-linear effect on the increase of the lattice parameters. An autocorrelation analysis of the IR spectra revealed two temperature-induced phase transitions. At −135°C, the local symmetry changes from P1 to Pbn2₁, although this change involves only the H atoms. The transition from Pbn2₁ to Pbnm at 160°C is caused by changes of the local F/OH ordering in the crystal structure.  相似文献