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171.
Unusual exsolution phenomena in omphacite and partial replacement of phengite by phlogopite + kyanite in an eclogite from the Erzgebirge 总被引:4,自引:0,他引:4
Esther Schmädicke Wolfgang Friedrich Müller 《Contributions to Mineralogy and Petrology》2000,139(6):629-642
In the Erzgebirge Crystalline Complex, eclogites occur in three different high pressure (HP) units (1, 2 and 3) recording
contrasting pressure (P)–temperature (T) conditions. Eclogites from HP-unit 1 experienced peak metamorphic conditions in the
coesite stability field at about 33 kbar/850 °C. Commonly, these eclogites from HP-unit 1 are all very similar, with an eclogitic
peak assemblage of omphacite–garnet–coesite–K-feldspar, rarely accompanied by kyanite, and omphacites systematically deviating
from a stoichiometric composition. In contrast, an eclogite recently found near Blumenau, is mineralogically and geochemically
different from the typical eclogites of HP-unit 1. This unusual eclogite reveals the eclogitic equilibrium assemblage omphacite–garnet–coesite–phengite–phlogopite–kyanite,
and yields metamorphic peak conditions of 870 °C and >29 kbar. There is clear textural evidence of the formation of phlogopite
and kyanite under partial consumption of phengite and garnet. Moreover, the omphacite is stoichiometric and contains abundant
exsolution lamellae, the thickest of which were identified as quartz by the electron microprobe. The finer lamellae were studied
by transmission electron microscopy (TEM). Oligoclase was identified as an exsolution phase. Other lamellae proved to consist
of K-white mica, also interpreted as exsolution. Prior to exsolution, the omphacite composition must have been cation-deficient,
as that of the other, common HP-unit 1 eclogites. These non-stoichiometric compositions are ascribed to partial substitution
by the Ca-Eskola pyroxene component, which calculates to an average of 8 mol% for omphacite in HP-unit 1 eclogites. According
to experiments, this substitution becomes significant at P > 30 kbar. Exsolution of K-white mica may indicate hydroxyl defects
in the original omphacite, also favoured by high pressure. Oligoclase and K-white mica exsolution from Ca-Eskola-rich clinopyroxene
has not previously been reported. The omphacite has a disordered C2/c structure; and in just one case very small (a few tens
of nanometres) antiphase domains, resulting from the C2/c to P2/n transformation, are present. These features may indicate
a brief thermal history and rapid tectonic processes.
Received: 4 January 1999 / Accepted: 20 April 2000 相似文献
172.
Identifying causes of ground-penetrating radar reflections using time-domain reflectometry and sedimentological analyses 总被引:5,自引:0,他引:5
Ground-penetrating radar (GPR) is a geophysical technique widely used to study the shallow subsurface and identify various sediment features that reflect electromagnetic waves. However, little is known about the exact cause of GPR reflections because few studies have coupled wave theory to petrophysical data. In this study, a 100- and 200-MHz GPR survey was conducted on aeolian deposits in a quarry. Time-domain reflectometry (TDR) was used to obtain detailed information on the product of relative permittivity (ɛr ) and relative magnetic permeability (μr ), which mainly controls the GPR contrast parameter in the subsurface. Combining TDR data and lacquer peels from the quarry wall allowed the identification of various relationships between sediment characteristics and ɛr μr . Synthetic radar traces, constructed using the TDR logs and sedimentological data from the lacquer peels, were compared with the actual GPR sections. Numerous peaks in ɛr μr , which are superimposed on a baseline value of 4 for dry sand, are caused by potential GPR reflectors. These increases in ɛr μr coincide with the presence of either organic material, having a higher water content and relative permittivity than the surrounding sediment, or iron oxide bands, enhancing relative magnetic permeability and causing water to stagnate on top of them. Sedimentary structures, as reflected in textural change, only result in possible GPR reflections when the volumetric water content exceeds 0·055. The synthetic radar traces provide an improved insight into the behaviour of radar waves and show that GPR results may be ambiguous because of multiples and interference. 相似文献
173.
Since 1980 when F. Lippmann's seminal paper appeared, ourunderstanding of solubility equilibria involving ionic solidsolutions has been advanced by theoretical considerations as wellas careful experimental studies designed to determine excess Gibbsfunctions. A unified theory of solid-solution aqueous-solutionequilibria as well as the thermodynamic background of thephenomenon of ``stoichiometric saturation' are reviewed.It is shown that Lippmann diagrams effectively summarize thethermodynamic basis of solid-solute aqueous-solution equilibria ofsparingly soluble metal carbonate systems. Clearly, the predictivepower of these diagrams may be limited due to kineticrestrictions. Only when dissolution and precipitation areessentially reversible, favourable conditions to synthesizehomogeneous solid phases can be derived from studies of equilibria. 相似文献
174.
175.
Wolfgang Hillebrandt 《Astrophysics and Space Science》2002,281(1-2):173-182
Recent developments in theoretical model-calculations for the synthesis of the chemical elements during late stages of stellar
evolution are reviewed. Special emphasis is put on a discussion of various astrophysical sites, including core-collapse and
thermonuclear supernovae, and the physics of turbulent reactive fluids. Results of numerical simulations are presented and
discussed, together with new results concerning solar-system abundances as well as abundances observed in very metal-poor
stars, in the context of searches for constraints on the still rather uncertain nuclear physics data and astrophysical models.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
176.
Fukun Chen Xiang-Hui Li Xiu-Li Wang Qiu-Li Li Wolfgang Siebel 《International Journal of Earth Sciences》2007,96(6):1179-1194
The Baoshan block of the Tethyan Yunnan, southwestern China, is considered as northern part of the Sibumasu microcontinent.
Basement of this block that comprises presumably greenschist-facies Neoproterozoic metamorphic rocks is covered by Paleozoic
to Mesozoic low-grade metamorphic sedimentary rocks. This study presents zircon ages and Nd–Hf isotopic composition of granites
generated from crustal reworking to reveal geochemical feature of the underlying basement. Dating results obtained using the
single zircon U–Pb isotopic dilution method show that granites exposed in the study area formed in early Paleozoic (about
470 Ma; Pingdajie granite) and in late Yanshanian (about 78–61 Ma, Late Cretaceous to Early Tertiary; Huataolin granite).
The early Paleozoic granite contains Archean to Mesoproterozoic inherited zircons and the late Yanshanian granite contains
late Proterozoic to early Paleozoic zircon cores. Both granites have similar geochemical and Nd–Hf isotopic charateristics,
indicating similar magma sources. They have whole-rock T
DM(Nd) values of around 2,000 Ma and zircon T
DM(Hf) values clustering around 1,900–1,800 and 1,600–1,400 Ma. The Nd–Hf isotopic data imply Paleoproterozoic to Mesoproterozoic
crustal material as the major components of the underlying basement, being consistent with a derivation from Archean and Paleoproterozoic
terrains of India or NW Australia. Both granites formed in two different tectonic events similarly originated from intra-crustal
reworking. Temporally, the late Yanshanian magmatism is probably related to the closure of the Neotethys ocean. The early
Paleozoic magmatism traced in the Baoshan block indicates a comparable history of the basements during early Paleozoic between
the SE Asia and the western Tethyan belt, such as the basement outcrops in the Alpine belt and probably in the European Variscides
that are considered as continental blocks drifting from Gondwana prior to or simultaneously with those of the SE Asia. 相似文献
177.
Cosmas Kongnyuy Shang Muharrem Satir Emmanuel Nkonguin Nsifa Jean-Paul Liégeois Wolfgang Siebel Heiner Taubald 《International Journal of Earth Sciences》2007,96(5):817-841
We present a geochemical and isotopic study that, consistent with observed field relations, suggest Sangmelima late Archaean
high-K granite was derived by partial melting of older Archaean TTG. The TTG formations are sodic-trondhjemitic, showing calcic
and calc-alkalic trends and are metaluminous to peraluminous. High-K granites in contrast show a potassic calc-alkaline affinity
that spans the calcic, calc-alkalic, alkali-calcic and alkalic compositions. The two rock groups (TTG and high-K granites)
on the other hand are both ferroan and magnesian. They have a similar degree of fractionation for LREE but a different one
for HREE. Nd model ages and Sr/Y ratios define Mesoarchaean and slab-mantle derived magma compositions respectively, with
Nb and Ti anomalies indicating a subduction setting for the TTG. Major and trace element in addition to Sr and Nd isotopic
compositions support field observations that indicate the derivation of the high-K granitic group from the partial melting
of the older TTG equivalent at depth. Geochemical characteristics of the high-K granitic group are therefore inherited features
from the TTG protolith and cannot be used for determining their tectonic setting. The heat budget required for TTG partial
melting is ascribed to the upwelling of the mantle marked by a doleritic event of identical age as the generated high-K granite
melts. The cause of this upwelling is related to linear delamination along mega-shear zones in an intracontinental setting. 相似文献
178.
Further Information of the Associated Li Deposits in the No.6 Coal Seam at Jungar Coalfield,Inner Mongolia,Northern China 总被引:2,自引:0,他引:2
SUN Yuzhuang ZHAO Cunliang LI Yanheng WANG Jinxi ZHANG Jiany JIN Zhe LIN Mingyue Wolfgang KALKREUTH 《《地质学报》英文版》2013,87(4):1097-1108
Total 138 coal samples and 14 parting samples were taken from the No. 6 Seam of the Jungar Coalfield, Inner Mongolia. These samples were analysed by optical microscopy, sequential chemical extraction procedure (SCEP), inductively coupled plasma mass spectrometry (ICP-MS), X-ray powder diffraction (XRD), and scanning electron microscope in conjunction with an energy-dispersive X-ray spectrometer (SEM-EDX) analysis. The results indicate that the Li contents have reached the industrial grade of the coal associated Li deposit, and the total Li reserves have reached 2406600 tons, that is, 5157000 tons Li2O in the No. 6 seam in the Jungar Coalfield. The sequential chemical extraction procedure results suggest that the Li concentration is mainly related to inorganic matter. The minerals in the coals consist of kaolinite, boehmite, chlorite-group mineral, quartz, calcite, pyrite, siderite and amorphous clay material. Some Li could be absorbed by clay minerals in the Li-bearing coal seam. The chlorite phase?could be?most likely the host for a part of Li. The Yinshan Oldland should be the most possible source of Li of the coal. 相似文献
179.
Exploring the impact of diagenesis on (isotope) geochemical and microstructural alteration features in biogenic aragonite 下载免费PDF全文
Ann‐Christine Ritter Vasileios Mavromatis Martin Dietzel Ola Kwiecien Felix Wiethoff Erika Griesshaber Laura A. Casella Wolfgang W. Schmahl Jennifer Koelen Rolf D. Neuser Albrecht Leis Dieter Buhl Andrea Niedermayr Sebastian F. M. Breitenbach Stefano M. Bernasconi Adrian Immenhauser 《Sedimentology》2017,64(5):1354-1380
For the Quaternary and Neogene, aragonitic biogenic and abiogenic carbonates are frequently exploited as archives of their environment. Conversely, pre‐Neogene aragonite is often diagenetically altered and calcite archives are studied instead. Nevertheless, the exact sequence of diagenetic processes and products is difficult to disclose from naturally altered material. Here, experiments were performed to understand biogenic aragonite alteration processes and products. Shell subsamples of the bivalve Arctica islandica were exposed to hydrothermal alteration. Thermal boundary conditions were set at 100°C, 175°C and 200°C. These comparably high temperatures were chosen to shorten experimental durations. Subsamples were exposed to different 18O‐depleted fluids for durations between two and twenty weeks. Alteration was documented using X‐ray diffraction, cathodoluminescence, fluorescence and scanning electron microscopy, as well as conventional and clumped isotope analyses. Experiments performed at 100°C show redistribution and darkening of organic matter, but lack evidence for diagenetic alteration, except in Δ47 which show the effects of annealing processes. At 175°C, valves undergo significant aragonite to calcite transformation and neomorphism. The δ18O signature supports transformation via dissolution and reprecipitation, but isotopic exchange is limited by fluid migration through the subsamples. Individual growth increments in these subsamples exhibit bright orange luminescence. At 200°C, valves are fully transformed to calcite and exhibit purple‐blue luminescence with orange bands. The δ18O and Δ47 signatures reveal exchange with the aqueous fluid, whereas δ13C remains unaltered in all experiments, indicating a carbonate‐buffered system. Clumped isotope temperatures in high‐temperature experiments show compositions in broad agreement with the measured temperature. Experimentally induced alteration patterns are comparable with individual features present in Pleistocene shells. This study represents a significant step towards sequential analysis of diagenetic features in biogenic aragonites and sheds light on reaction times and threshold limits. The limitations of a study restricted to a single test organism are acknowledged and call for refined follow‐up experiments. 相似文献
180.
Wolfgang D. Maier Sarah-Jane Barnes Julian S. Marsh 《Contributions to Mineralogy and Petrology》2003,146(1):44-61
Concentrations of the platinum-group elements have been determined in several suites of southern African flood-type basalts and mid-ocean ridge basalt (MORB), covering some 3 Ga of geologic evolution and including the Etendeka, Karoo, Soutpansberg, Machadodorp, Hekpoort, Ventersdorp and Dominion magmas. The magmas cover a compositional range from 3.7 to 18.7% MgO, 26–720 ppm Ni, 16–250 ppm Cu, and <1–255 ppb total platinum-group elements (PGE). The younger basalts (Etendeka, Karoo) tend to be depleted in PGE relative to Cu, while most of the older basalts (Hekpoort, Machadodorp, Ventersdorp, Dominion) show no PGE depletion relative to Cu. Further, the younger basalts tend to have lower average Pt/Pd ratios than the older basalts, and the MORBs have lower average Pt/Pd than the continental basalts within the broad groupings of "old" and "young" basalts. This may reflect (1) a decreasing degree of mantle melting through geologic time, and (2) source heterogeneity, in that the MORBs are derived from predominantly asthenospheric mantle, whereas the continental basalts also contain a lithospheric mantle component enriched in Pt. In addition to these factors, some PGE fractionation also occurred during differentiation of the magmas, with Pd showing incompatible behaviour and the other PGE variably compatible behaviour. The examined southern African flood-type basalts and MORB appear to offer limited prospects for magmatic sulfide ores, largely because they show little evidence for significant chalcophile metal depletion that could be the result of sulphide extraction during ascent and crystallization.Editorial responsibility: I. Parsons 相似文献