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71.
The products of the transformation of natural (Mg0.83Fe0.17)2SiO4 olivine have been prepared at various high pressures (between 25 GPa and 90 GPa), and high temperature in a laser-heated diamond-anvil cell (DAC). Studies of the high-pressure phases have been made by transmission electron microscopy (TEM), and X-ray microanalysis. The olivine/spinel boundaries exhibit all the characteristics of a diffusionless shear transition, having a finely sheared structure and a constant orientation relationship between the close-packed planes of the two structures ((100)ol∥(111)sp). The TEM observations of zones where olivine (or spinel) transforms into post-spinel phases show that the transformation possesses the features of an eutectoïdal decomposition, leading to a lamellar intergrowth of magnesiowüstite (Mg,Fe)O and perovskite (Mg,Fe)SiO3. With increasing temperature and/or decreasing pressure, the grain size of the high-pressure phases increases and obeys an Arrhenius law with an activation volume equal to zero. (Mg,Fe)O grains exhibit a very high density of dislocations (higher than 1011cm?2), whereas (Mg,Fe)SiO3 grains exhibit no dislocations but systematic twinning. The composition plane of the twins is (112) of the GdFeO3-type perovskite, corresponding to the {110} plane of the cubic lattice of ideal perovskite. 相似文献
72.
73.
In situ X-ray diffraction experiments at high pressure were carried out up to 8.9 GPa and 1100 °C to study phase transformations
of iron and two iron-silicon alloys Fe0.91Si0.09 and Fe0.83Si0.17. For iron, the transformation from the bcc phase to the fcc phase was observed at pressures 3.8–8.2 GPa and temperatures
that are consistent with previous in situ X-ray diffraction studies. Reversal of the transformation of iron was found to be
sensitive to temperature; hysteresis of the transformation increased from 25 °C at 3.8 GPa to 100 °C at 7.0 GPa, primarily
because the bcc-fcc phase boundary has a negative Clayperon slope. In the binary system Fe-Si, the observations of the present
study indicate that the ferrite (bcc phase)-stabilizing behavior of silicon persists at high pressures and that the maximum
solubility of silicon in the fcc phase increases with increasing pressure: (1) the transformation from the bcc phase to the
fcc phase was observed in Fe0.91Si0.09 at 6.0, 7.4 and 8.9 GPa and the temperatures measured at the onset of the transformations were 300 °C higher than those in
iron at similar pressures, (2) the transformation rate in Fe0.91Si0.09 was extremely sluggish compared to that of iron, and (3) the bcc-fcc phase transformation was not observed in Fe0.91Si0.09 at 4.7 GPa up to 1000 °C and in Fe0.83Si0.17 at 8.2 GPa and 1100 °C.
Received: 1 June 1998 / Revised, accepted: 9 October 1998 相似文献
74.
We have carried out an in situ synchrotron X-ray diffraction study on iron and an iron-silicon alloy Fe0.91Si0.09 at simultaneously high pressure and temperature. Unit-cell volumes, measured up to 8.9 GPa and 773 K on the bcc phases of
iron and Fe0.91Si0.09, are analyzed using the Birch-Murnaghan equation of state and thermal pressure approach of Anderson. Equation of state parameters
on iron are found to be in agreement with results of previous studies. For both iron and Fe0.91Si0.09, thermal pressures show strong dependence on volume; the (∂KT/∂T)V values are considerably larger than those previously reported for other solids. The present results, in combination with
our previous results on ɛ-FeSi, suggest a small dependency of the room-temperature bulk modulus upon the silicon content,
less than 0.3 GPa for 1 wt.% silicon. We also find that substitution of silicon in iron would not appreciably change the thermoelastic
properties of iron-rich Fe−Si alloys. If this behavior persists over large pressure and temperature ranges, the relative density
contrast between iron and iron-rich Fe−Si alloys at conditions of the outer core of the Earth could be close to that measured
at ambient conditions, i.e., 0.6% for 1 wt.% Si.
Received: 13 January 1998 / Revised, accepted: 8 May 1998 相似文献
75.
Interactions between several silicate and metallic phases are studied by applying a self consistent thermodynamic approach
and using recent thermodynamic data. We compute proportions and compositions of oxidized silicates and of reduced metallic
phase in equilibrium at various temperatures and oxygen fugacities. The empirically observed activity-composition relationships
for ternary metallic alloys are used and their applications to a general thermodynamic expression for a non-regular ternary
system is explicitly discussed. We show that the stability limits of olivines and pyroxenes with respect to precipitation
of metallic phases under reducing conditions are directly related to the presence of nickel impurities. We precisely evaluate
the modifications of the stability limits as a function of nickel content. For typical mantle olivines [Fe/(Fe+Mg) = 0.1]
the stability limits are given for values of x
Ni= Ni/(Ni+Fe+Mg) ranging from 10 ppm to 1% by: ln f
O2=−39.83+ 7.86 ln x
Ni, ln f
O2=−14.68+6.21 ln x
Ni, at 900 K and 1600 K, respectively.
Received: 17 November 1999 / Accepted: 14 May 2000 相似文献
76.
F. Guyot H. Boyer M. Madon B. Velde J. P. Poirier 《Physics and Chemistry of Minerals》1986,13(2):91-95
Raman microprobe (RMP) spectra were produced for each of the olivine and spinel structured phases of Mg2GeO4 and (Mg, Fe)2SiO4. The assembled data show that bands due to the tetrahedra in silicate and germanate olivines shift in a way that indicates a dominant mass effect. This correspondence is difficult to make in spinels due to differences in structural type. Differences in Fe/Mg content of olivine shift the tetrahedral vibration bands only slightly, but their linear shifts could be used to indicate the composition of the phase. 相似文献
77.
J. Zhang I. Martinez F. Guyot P. Gillet S. K. Saxena 《Physics and Chemistry of Minerals》1997,24(2):122-130
P–V–T measurements on magnesite MgCO3 have been carried out at high pressure and high temperature up to 8.6 GPa and 1285 K, using a DIA-type, cubic-anvil apparatus
(SAM-85) in conjunction with in situ synchrotron X-ray powder diffraction. Precise volumes are obtained by the use of data collected above 873 K on heating and
in the entire cooling cycle to minimize non-hydrostatic stress. From these data, the equation-of-state parameters are derived
from various approaches based on the Birch-Murnaghan equation of state and on the relevant thermodynamic relations. With K′0 fixed at 4, we obtain K0=103(1) GPa, α(K−1)=3.15(17)×10−5 +2.32(28)×10−8 T, (∂KT/∂T)P=−0.021(2) GPaK−1, (dα/∂P)T=−1.81×10−6 GPa−1K−1 and (∂KT/∂T)V= −0.007(1) GPaK−1; whereas the third-order Birch-Murnaghan equation of state with K′0 as an adjustable parameter yields the following values: K0=108(3) GPa, K′0=2.33(94), α(K−1)=3.08(16)×10−5+2.05(27) ×10−8 T, (∂KT/∂T)P=−0.017(1) GPaK−1, (dα/∂P)T= −1.41×10−6 GPa−1K−1 and (∂KT/∂T)V=−0.008(1) GPaK−1. Within the investigated P–T range, thermal pressure for magnesite increases linearly with temperature and is pressure (or
volume) dependent. The present measurements of room-temperature bulk modulus, of its pressure derivative, and of the extrapolated
zero-pressure volumes at high temperatures, are in agreement with previous single-crystal study and ultrasonic measurements,
whereas (∂KT/∂T)P, (∂α/∂P)T and (∂KT/∂T)V are determined for the first time in this compound. Using this new equation of state, thermodynamic calculations for the
reactions (1) magnesite=periclase+CO2 and (2) magnesite+enstatite=forsterite+CO2 are consistent with existing experimental phase equilibrium data.
Received September 28, 1995/Revised, accepted May 22, 1996 相似文献
78.
Vincent P. Milesi Didier Jzquel Mathieu Debure Pierre Cadeau Franois Guyot Grard Sarazin Francis Claret Emmanuelle Vennin Carine Chaduteau Aurlien Virgone Eric C. Gaucher Magali Ader 《Sedimentology》2019,66(3):983-1001
The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l?1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the <2 μm clay fraction indicate the presence of a saponite‐like mineral, a Mg‐rich smectite. The saponite‐like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite‐like mineral instead of Al‐free Mg‐silicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic link may exist between saponite and the development of organic‐rich carbonate rocks, which may be fuelled by the input of CO2‐rich volcanic gases. This provides novel insights into the composition and formation of saponite‐rich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs. 相似文献
79.
Channel and floodplain sediment dynamics in a reach of the tropical meandering Rio Beni (Bolivian Amazonia) 总被引:1,自引:0,他引:1
Emmanuèle Gautier Daniel Brunstein Philippe Vauchel Jean‐Marie Jouanneau Marc Roulet Coral Garcia Jean‐Loup Guyot Marcello Castro 《地球表面变化过程与地形》2010,35(15):1838-1853
The study investigates interactions, water and sediment exchanges, between a rapidly migrating meander and its associated floodplain at fine temporal and spatial scales. The Beni River, an Amazonian free meandering river, makes the transition between Andean ranges and Amazonian lowlands. For the period 2002–2006, an assemblage of tools and methods (water and sediment discharges, topometric and bathymetric surveys, sedimentation rate estimations from unsupported 210Pb and sediment trapping system) was used to jointly analyse the influence on the sediment budget of external factors (mainly water and sediment discharge) and the inherent behaviour of the system. The main issue addressed is the investigation of the complex relationship between ‘morphological conditioning’ of fluvial landform and process. The first part of the study was undertaken with the aim of linking erosion–deposition in an active meander with water and sediment fluxes. The three inter‐annual evolutions are characterized by very unequal sediment budgets; the first two intervals underwent predominant erosion, and the latter slight accumulation. Digital elevation models, evaluated for the active meander, demonstrate that sedimentation on the point bar depends more on external factors than erosion of the concave bank, which fluctuates slightly. The second part of the study, focusing on water and sediment exchanges between active bend and floodplain, examines the respective parts played by overbank flow and by an abandoned channel on the diffusion and sequestration of sediment. The association of short‐ and long‐term estimation of sedimentation rates suggests that floodplain construction is associated with two different processes and rhythms of sediment transportation. Finally, a sediment budget is proposed for the Beni River in the upper part of the Amazonian lowlands. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
80.
Abstract Using daily suspended sediment and water discharge data, we calculated the current mean annual runoff and Specific Suspended Sediment Yield (SSY) for 66 mountainous and piedmont catchments in Chile. These catchments are located from the extreme north of Chile to Southern Patagonia and cover an exceptionally wide range of climates, slopes, and vegetation. The SSY ranges mainly between 0 and 700 t km-2 year-1 with some exceptions as high as 1780 t km-2 year-1. The SSY increases between the extreme north and 33°S and then decreases toward the south. Sediment and water discharge north of 33°S occur mainly during summer. Farther south the contribution of winter precipitation increases and predominates. When the SSY database is correlated with topographic, climatic and vegetation indices, it is found to correlate significantly with runoff and mean slope only. In order to concentrate on erosion processes in the mountain range, 32 mountainous catchments were selected along a strong north–south SSY gradient between 27°S and 40°S. From north to south, SSY increases strongly with runoff and then decreases, even while runoff keeps increasing. In catchments where SSY is low, although runoff is high, the mean slope is less than 40% and the vegetation cover is greater than 8%. For the other catchments, runoff variations explain 67% of the variance in sediment yields. Thus, SSY seems to be controlled by vegetation cover and slope thresholds. In addition, SSY also correlates with glacier cover. However, a correlation between SSY and seismicity, although possible, is ambiguous. Citation Pepin, E., Carretier, S., Guyot, J. L. & Escobar, F. (2010) Specific suspended sediment yields of the Andean rivers of Chile and their relationship to climate, slope and vegetation. Hydrol. Sci. J. 55(7), 1190–1205. 相似文献