排序方式： 共有55条查询结果，搜索用时 593 毫秒
11.
Donna L. Whitney Clémentine Hamelin Christian Teyssier Natalie H. Raia Megan S. Korchinski Nicholas C. A. Seaton Brian C. Bagley Anette von der Handt Françoise Roger Patrice F. Rey 《Journal of Metamorphic Geology》2020,38(3):297-327
In orogens worldwide and throughout geologic time, large volumes of deep continental crust have been exhumed in domal structures. Extension-driven ascent of bodies of deep, hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents. The dominant rock type in exhumed domes is quartzofeldspathic gneiss (typically migmatitic) that does not record its former high-pressure (HP) conditions in its equilibrium mineral assemblage; rather, it records the conditions of emplacement and cooling in the mid/shallow crust. Mafic rocks included in gneiss may, however, contain a fragmentary record of a HP history, and are evidence that their host rocks were also deeply sourced. An excellent example of exhumed deep crust that retains a partial HP record is in the Montagne Noire dome, French Massif Central, which contains well-preserved eclogite (garnet+omphacite+rutile+quartz) in migmatite in two locations: one in the dome core and the other at the dome margin. Both eclogites record P ~ 1.5 ± 0.2 GPa at T ~ 700 ± 20°C, but differ from each other in whole-rock and mineral composition, deformation features (shape and crystallographic preferred orientation, CPO), extent of record of prograde metamorphism in garnet and zircon, and degree of preservation of inherited zircon. Rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss at c. 310 Ma, interpreted based on zircon rare earth element abundance in eclogite zircon as the age of HP metamorphism. Dome-margin eclogite zircon retains a widespread record of protolith age (c. 470–450 Ma, the same as host gneiss protolith age), whereas dome-core eclogite zircon has more scarce preservation of inherited zircon. Possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and/or the duration of metamorphic heating and extent of interaction with aqueous fluid, affecting zircon crystallization. Differences in HP deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development. Regardless of differences, both eclogites experienced HP metamorphism and deformation in the deep crust at c. 310 Ma and were exhumed by lithospheric extension—with their host migmatite—near the end of the Variscan orogeny. The deep crust in this region was rapidly exhumed from ~50 to <10 km, where it equilibrated under low-P/high-T conditions, leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome. 相似文献
12.
The effect of chrysotile nanotubes on the serpentine-fluid Li-isotopic fractionation 总被引：1，自引：0，他引：1
Bernd Wunder Fabien Deschamps Anke Watenphul Stéphane Guillot Anette Meixner Rolf L. Romer Richard Wirth 《Contributions to Mineralogy and Petrology》2010,159(6):781-790
We determined the lithium isotope fractionation between synthetic Li-bearing serpentine phases lizardite, chrysotile, antigorite,
and aqueous fluid in the P,T range 0.2–4.0 GPa, 200–500°C. For experiments in the systems lizardite-fluid and antigorite-fluid, 7Li preferentially partitioned into the fluid and Δ7Li values followed the T-dependent fractionation of Li-bearing mica-fluid (Wunder et al. 2007). By contrast, for chrysotile-fluid experiments, 7Li weakly partitioned into chrysotile. This contrasting behavior might be due to different Li environments in the three serpentine
varieties: in lizardite and antigorite lithium is sixfold coordinated, whereas in chrysotile lithium is incorporated in two
ways, octahedrally and as Li-bearing water cluster filling the nanotube cores. Low-temperature IR spectroscopic measurements
of chrysotile showed significant amounts of water, whose freezing point was suppressed due to the Li contents and the confined
geometry of the fluid within the tubes. The small inverse Li-isotopic fractionation for chrysotile-fluid results from intra-crystalline
Li isotope fractionation of octahedral Li[6] with preference to 6Li and lithium within the channels (Li[Ch]) of chrysotile, favoring 7Li. The nanotubes of chrysotile possibly serve as important carrier of Li and perhaps also of other fluid-mobile elements
in serpentinized oceanic crust. This might explain higher Li abundances for low-T chrysotile-bearing serpentinites relative to high-T serpentinites. Isotopically heavy Li-bearing fluids of chrysotile nanotubes could be released at relatively shallow depths
during subduction, prior to complete chrysotile reactions to form antigorite. During further subduction, fluids produced during
breakdown of serpentine phases will be depleted in 7Li. This behavior might explain some of the Li-isotopic heterogeneities observed for serpentinized peridotites. 相似文献
13.
Boron and Oxygen Isotope Composition of Certified Reference Materials NIST SRM 610/612 and Reference Materials JB-2 and JR-2 总被引：1，自引：0，他引：1
Simone Kasemann Anette Meixner Alexander Rocholl Torsten Vennemann Martin Rosner Axel K. Schmitt Michael Wiedenbeck 《Geostandards and Geoanalytical Research》2001,25(2-3):405-416
We present data on the concentration, the isotope composition and the homogeneity of boron in NIST silicate glass reference materials SRM 610 and SRM 612, and in powders and glasses of geological reference materials JB-2 (basalt) and JR-2 (rhyolite). Our data are intended to serve as references for both microanalytical and wet-chemical techniques. The δ11 B compositions determined by N-TIMS and P-TIMS agree within 0.5% and compare with SIMS data within 2.5%. SIMS profiles demonstrate boron isotope homogeneity to better than δ11 B = 2% for both NIST glasses, however a slight boron depletion was detected towards the outermost 200 μm of the rim of each sample wafer. The boron isotope compositions of SRM 610 and SRM 612 were indistinguishable. Glasses produced in this study by fusing JB-2 and JR-2 powder also showed good boron isotope homogeneity, both within and between different glass fragments. Their major element abundance as well as boron isotope compositions and concentrations were identical to those of the starting composition. Hence, reference materials (glasses) for the in situ measurement of boron isotopes can be produced from already well-studied volcanic samples without significant isotope fractionation. Oxygen isotope ratios, both within and between wafers, of NIST reference glasses SRM 610 and SRM 612 are uniform. In contrast to boron, significant differences in oxygen isotope compositions were found between the two glasses, which may be due to the different amounts of trace element oxides added at ten-fold different concentration levels to the silicate matrix. 相似文献
14.
The geochemical cycle of boron: Constraints from boron isotope partitioning experiments between mica and fluid 总被引：8，自引：0，他引：8
Bernd Wunder Anette Meixner Rolf L. Romer Richard Wirth Wilhelm Heinrich 《Lithos》2005,84(3-4):206-216
The fractionation of boron isotopes between synthetic boromuscovite and fluid was experimentally determined at 3.0 GPa/500 °C and 3.0 GPa/700 °C. For near-neutral fluids Δ11B(mica-fluid) = δ11B(mica) − δ11B(fluid) is − 10.9 ± 1.3‰ at 500 °C, and − 6.5 ± 0.4‰ at 700 °C. This supports earlier assumptions that the main fractionation effect is due to the change from trigonal coordination of boron in neutral fluids to tetrahedrally coordinated boron in micas, clays and melts. The T-dependence of this effect is approximated by the equation Δ11B(mica,clay,melt–neutral fluid) = − 10.69 · (1000/T [K]) + 3.88; R2 = 0.992, valid from 25 °C for fluid–clay up to about 1000 °C for fluid–silicate melt. Experiments at 0.4 GPa that used strongly basic fluids produced significantly lower fractionations with Δ11B(mica–fluid) of − 7.4 ± 1.0‰ at 400 °C, and − 4.8 ± 1.0‰ at 500 °C, showing the reduced fractionation effect when large amounts of boron in basic fluids are tetrahedrally coordinated. Field studies have shown that boron concentrations and 11B/10B-ratios in volcanic arcs systematically decrease across the arc with increasing distance from the trench, thus reflecting the thermal structure of the subducting slab. Our experiments show that the boron isotopic signature in volcanic arcs probably results from continuous dehydration of micas along a distinct P–T range. Continuous slab dehydration and boron transport via fluid into the mantle wedge is responsible for the boron isotopic signature in volcanic arcs. 相似文献
15.
Constitutive model with time-dependent deformations 总被引：4，自引：0，他引：4
Anette Krogsbll 《Engineering Geology》1998,49(3-4):285-292
In many geological and engineering problems it is necessary to transform information from one scale to another. Data collected at laboratory scale are often used to evaluate field problems on a much larger scale. This is certainly true for geological problems where extreme scale differences are common in time as well as size. This problem is addressed by means of a new constitutive model for soils. It is able to describe the behavior of soils at different deformation rates. The model defines time-dependent and stress-related deformations separately. They are related to each other and they occur simultaneously. The model is based on concepts from elasticity and viscoplasticity theories. In addition to Hooke's law for the elastic behavior, the framework for the viscoplastic behavior consists, in the general case (two-dimensional or three-dimensional), of a yield surface, an associated flow rule and a hardening law. The model is formulated in incremental terms and is therefore suitable for computational modeling and it has been implemented in a computer program used for analyzing the depositional history of an oil field in the Danish part of the North Sea. An important part of the problem in this case was the difference in time scale between the geological process of deposition (millions of years) and the laboratory measurements of mechanical properties (minutes or hours). In addition, the time scale relevant to the production history of the oil field was interesting (days or years). 相似文献
16.
1 Introduction
Recent improvements in the precision of Li and B isotope measurements have demonstrated the potential of these elements in tracing a wide range of geological processes. The Li and B isotope systematics of ultrahigh-pressure (UHP) metamorphic rocks provides a unique opportunity to investigate the behaviour of Li and B during fluid-rock interaction at high temperatures and very high pressures and to constrain the fluid budget and the recycling of subducted crustal materials into the mantle during UHP metamorphism. 相似文献
17.
Cassandra R. Fenton Reginald L. Hermanns Lars H. Blikra Peter W. Kubik Charlotte Bryant Samuel Niedermann Anette Meixner Mirjam M. Goethals 《Quaternary Geochronology》2011,6(5):437-452
Two rock avalanches in Troms County – the Grøtlandsura and Russenes – were selected as CRONUS-EU natural cosmogenic 10Be production-rate calibration sites because they (a) preserve large boulders that have been continuously exposed to cosmic irradiation since their emplacement; (b) contain boulders with abundant quartz phenocrysts and veins with low concentrations of naturally-occurring 9Be (typically < 1.5 ppb); and (c) have reliable minimum radiocarbon ages of 11,424 ± 108 cal yr BP and 10,942 ± 77 cal yr BP (1σ), respectively. Quartz samples (n = 6) from these two sites contained between 4.28 × 104 and 5.06 × 104 at 10Be/g using the 1.387 Myr 10Be half-life. Determination of these concentrations accounts for topographic and self-shielding, and effects on nuclide production due to isostatic rebound are shown to be negligible. Persistent, constant snow and moss cover cannot be proven, but if taken into consideration they may have reduced 10Be concentrations by 10%. Using the 10Be half-life of 1.387 Myr and the Stone scaling scheme, and accounting for snow- and moss-cover, we calculate an error-weighted mean total 10Be production rate of 4.12 ± 0.19 at/g/yr (1σ). A corresponding error-weighted mean spallogenic 10Be production rate is 3.96 ± 0.16 at/g/yr (1σ), respectively. These are in agreement within uncertainty with other 10Be production rates in the literature, but are significantly, statistically lower than the global average 10Be production rate. This research indicates, like other recent studies, that the production of cosmogenic 10Be in quartz is lower than previously established by other production-rate calibration projects. Similarly, our findings indicate that regional cosmogenic production rates should be used for determining exposure ages of landforms in order to increase the accuracy of those ages. As such, using the total 10Be production rate from our study, we determine an error-weighted mean surface-exposure age of a third rock avalanche in Troms County (the Hølen avalanche) to be 7.5 ± 0.3 kyr (1σ). This age suggests that the rock avalanche occurred shortly after the 8.2 kyr cooling event, just as the radiocarbon ages of the Grøtlandsura and Russenes avalanches confirm field evidence that those rock-slope failures occurred shortly after deglaciation. 相似文献
18.
The chromium number of spinel Cr#sp (atomic ratio of Cr/(Cr+Al)) is an important geochemical parameter for the estimation of the degree of partial melting, temperatures,
and provenance in peridotites. In this study, a model has been developed in order to determine the effect of subsolidus reactions
on the Cr#sp in ultramafic rocks. The final model includes temperature-dependent distribution coefficients of relevant reactions as well
as solubility data and has been applied to lithologies common in mid-ocean ridge settings. Significant changes in the Cr#sp are predicted from the application of this model during cooling from 1300 to 800°C at mantle pressures. For spinel lherzolites
and harzburgites, the Cr#sp is predicted to decrease proportional to the absolute values of the Cr#sp at (constantly) increasing spinel mass. Cpx-dunites show the same trend, although to a lower extent. Websterites show a different
behavior with a slight increase in the Cr#sp due to their lack of olivine. Modal abundance of spinel correlates with the magnitude in Cr#sp change, too. Finally, these results were tested for possible effects on the calculated degree of partial melting as function
of the Cr#sp. Application of the Cr#sp from a peridotite equilibrated down to 800°C would result in an underestimation of only 1.5 % in the degree of melting, justifying
the use of Cr#sp for estimations of this parameter. 相似文献
19.
20.
Anette Reenberg Trine Louring Nielsen Kjeld Rasmussen 《Global Environmental Change》1998,8(4):309-327
The paper addresses the dynamic relationship between the human use of land and alterations in the biophysical environment, demographic pressure or socio-economic conditions. An empirical study from the Sahelian zone in northern Burkina Faso illustrates the dynamics of cultivation pattern at the village level and the changing priorities given to different landscape units over time. Field measurements, aerial photos and satellite images from seven successive years provide information on land use pattern changes from 1945 to 1995. A household survey illustrates how socio-economic and cultural parameters enable and constrain land use strategies at the farm level. 相似文献