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1.
The continuous coniferous forest in the Tianshan Mountains primarily consists of Picea schrenkiana. The forest forms an unbroken 1,000 km forest zone extending from west to east on the northern slope of the Tianshan Mountains, where environmental conditions such as precipitation, temperature and terrain vary greatly. To gain insight into the differences between soil properties at the upper and lower limits of P. schrenkiana, soil underlying the forests was studied along 4 transects located at Jinghe, Manasi, Fukang, and Balikun, where the natural conditions are typical and therefore representative of western, mid-western, mid-eastern and eastern portions of the Tianshan Mountains. The soil type under the coniferous forest on the northern slope of the Tianshan Mountains is gray cinnamon forest soil. For each transect, soil samples were collected from three different depths (0–5, 5–30 and 30–70 cm) in the area near the upper and lower limits of P. schrenkiana. A total of 48 soil samples were collected from 16 soil profiles and analyzed in two different laboratories. The soil pH, organic matter (OM), total N, total P, CaCO3, and electrical conductivity (EC) were analyzed using common pedological methods. The continuous distribution of P. schrenkiana on the northern slope of the Tianshan Mountains was caused by combined water-heat conditions and pedological factors. At the upper and lower tree limits, where differences existed in the soil parent materials, precipitation, topography, combined water-heat conditions, and local atmospheric pattern, the soil properties showed a close relationship with the local environmental conditions. Due to the presence of high precipitation over the Manasi transect, the soil was fertile and had a low CaCO3 content. In contrast, low soil fertility with high CaCO3 was observed in the soil along the Jinghe and Balikun transects, where there was low precipitation. Although the soil properties varied on the northern slope of the Tianshan Mountains due to the presence of different environmental conditions across the 1,000-km forest zone, this region was suitable for the growth of P. schrenkiana. Taken together, the results of this study indicated that soil played an important role in controlling the continuous distribution of the P. schrenkiana along the northern slope of the Tianshan Mountains.  相似文献   

2.
Subduction carries atmospheric and crustal carbon hosted in the altered oceanic crystalline basement and in pelagic sediments back into the mantle. Reactions involving complex carbonate solid solutions(s) lead to the transfer of carbon into the mantle, where it may be stored as graphite/diamond, in fluids or melts, or in carbonates. To constrain the thermodynamics and thus reactions of the ternary Ca–Mg–Fe carbonate solid solution, piston cylinder experiments have been performed in the system CaCO3–MgCO3–FeCO3 at a pressure of 3.5 GPa and temperatures of 900–1,100°C. At 900°C, the system has two miscibility gaps: the solvus dolomite–calcite, which closes at X MgCO3 ~0.7, and the solvus dolomite–magnesite, which ranges from the Mg to the Fe side of the ternary. With increasing temperature, the two miscibility gaps become narrower until complete solid solutions between CaCO3–Ca0.5Mg0.5CO3 is reached at 1,100°C and between CaCO3–FeCO3 at 1,000°C. The solvi are characterized by strong compositional asymmetry and by an order–disorder mechanism. To deal with these features, a solid solution model based on the van Laar macroscopic formalism has been calculated for ternary carbonates. This thermodynamic solid solution model is able to reproduce the experimentally constrained phase relations in the system CaCO3–MgCO3–FeCO3 in a broad P–T range. To test our model, calculated phase equilibria were compared with experiments performed in carbonated mafic protolithes, demonstrating the reliability of our solid solution model at pressures up to 6 GPa in complex systems.  相似文献   

3.
Resulting from static experiments performed to study the phase state of CaCO3, it was found that its melting is congruent at 20–22 GPa and 3500 K. The obtained experiment data show that the field of congruent melting of calcium carbonate is rather broad (form 2300 to 3500–3800 K at 20–22 GPa). However, the potential presence of a high-temperature phase boundary at which CaCO3 is decomposed into CaO and CO2 is not ruled out. The existence of a wide area of congruent melting of calcium carbonate (a common primary inclusion in diamonds of the transition zone and lower mantle of the Earth) allow one to consider deep-seated melts as potential parental media for ultradeep diamonds.  相似文献   

4.
The pressure–temperature conditions of the reactions of the double carbonates CaM(CO3)2, where M = Mg (dolomite), Fe (ankerite) and Mn (kutnohorite), to MCO3 plus CaCO3 (aragonite) have been investigated at 5–8 GPa, 600–1,100°C, using multi-anvil apparatus. The reaction dolomite = magnesite + aragonite is in good agreement with the results of Sato and Katsura (Earth Planet Sci 184:529–534, 2001), but in poor agreement with the results of Luth (Contrib Mineral Petrol 141:222–232, 2001). The dolomite is partially disordered at 620°C, and fully disordered at 1,100°C. All ankerite and kutnohorite samples, including the synthetic starting materials, are disordered. The P–T slopes of the three reactions increase in the order M = Mg, Fe, Mn. The shallower slope for the reaction involving magnesite is due partly to its having a higher compressibility than expected from unit-cell volume considerations. At low pressures there is a preference for partitioning into the double carbonate of Mg > Fe > Mn. At high pressures the partitioning preference is reversed. Using the measured reaction positions, the P–T conditions at which dolomite solid solutions will break down on increasing P and T in subduction zones can be estimated.  相似文献   

5.
This paper presents pelagic records of planktic foraminifera, as well as data of stable isotope stratigraphy and carbonate stratigraphy since 3.5 Ma B.P. from site ODP758 in the Ninetyeast Ridge of the Indian Ocean. Based on these data, manifestations and related mechanisms of major tectonic and environmental events such as the rapid uplift of the Himalaya Mountains, “middle Pleistocene climatic transition” and “mid-Brunhes dissolution event” in the region are discussed. According to the analysis and comparison of various indices and changes in terms of foraminifera assemblage, paleotemperature, paleosalinity and themocline from site ODP758, the authors deduce that the paleoclimatic changes might correlate with the mid-Pleistocene transition at 1.4–1.7 Ma B.P. The changes of CaCO3, mass accumulation rates (MAR) of CaCO3 and non- CaCO3 MAR indicate that the loaded terrigenous sediments increased at 1.7 Ma, which is in agreement with the uplift history of the Qinghai-Tibet plateau as shown by the available data. The last two changes coincide with the uplift of the Qinghai-Tibet plateau, hence they are called “Qinghai-Tibet movement” (1.7 Ma), and the “Kunlun-Yellow River movement” (1.2–0.6 Ma). The changes of the CaCO3 content, coarse fraction (> 150 μm) content and planktonic foraminifera biostratigraphy show that strong dissolution of abyssal CaCO3 occurred in the study region during 0.5–0.4 Ma. The event was consistent with the “mid-Brunhes dissolution event” in the sedimentary records of the Atlantic Ocean, Pacific Ocean, Indian Ocean and Nansha sea area of the South China Sea. __________ Translated from Geological Bulletin of China, 2007, 26(12): 1627–1632 [译自: 地质通报]  相似文献   

6.
We report on high-pressure and high-temperature experiments involving carbonates and silicates at 30–80 GPa and 1,600–3,200 K, corresponding to depths within the Earth of approximately 800–2,200 km. The experiments are intended to represent the decomposition process of carbonates contained within oceanic plates subducted into the lower mantle. In basaltic composition, CaCO3 (calcite and aragonite), the major carbonate phase in marine sediments, is altered into MgCO3 (magnesite) via reactions with Mg-bearing silicates under conditions that are 200–300°C colder than the mantle geotherm. With increasing temperature and pressure, the magnesite decomposes into an assemblage of CO2 + perovskite via reactions with SiO2. Magnesite is not the only host phase for subducted carbon—solid CO2 also carries carbon in the lower mantle. Furthermore, CO2 itself breaks down to diamond and oxygen under geotherm conditions over 70 GPa, which might imply a possible mechanism for diamond formation in the lower mantle.  相似文献   

7.
 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   

8.
We performed multi-anvil experiments in the system MgO-SiO2 ± H2O at 13.0–13.7 GPa and 1,025–1,300°C and in the system MgO-FeO-SiO2 ± H2O, under reducing conditions, at 11.0–12.7 GPa and 1,200°C, to depict the effect of H2O on the P-T-x coordinates of the 410-km discontinuity, i.e. the olivine–wadsleyite phase boundary. The charges were investigated with Electron Microprobe (EMP), Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), Secondary Ion Mass Spectrometry (SIMS) and Electron Energy Loss Spectroscopy (EELS). We observe in the MgO-SiO2-H2O system at 1,200°C a 0.6 GPa shift of the phase boundary to lower pressure compared to dry conditions, due to the stronger water fractionation into wadsleyite (wad) rather than in olivine (ol). In the MgO-FeO-SiO2-H2O system, we reproduced the triple point, i.e. observed coexisting hydrous ol, wad and ringwoodite (ring). SIMS H quantifications provided partitioning coefficients for water: D\textwad/ol\textwater D_{\text{wad/ol}}^{\text{water}}  ~ 3.7(5) and D\textring/ol\textwater D_{\text{ring/ol}}^{\text{water}}  ~ 1.5(2) and D\textwad/ring\textwater D_{\text{wad/ring}}^{\text{water}}  ~ 2.5(5). For a bulk composition of x Fe = 0.1, our data indicate only a slight difference in the width of the loop of the two phase field ol–wad under hydrous conditions compared to dry conditions, i.e. no broadening with respect to composition but a shift to lower pressures. For bulk compositions of x Fe > 0.2, i.e. in regions where wad–ring and ol–ring coexist, we observe, however, an unexpected broadening of the loops with a shift to higher iron contents. In total, the stability field of hydrous wad expands in both directions, to lower and higher pressures. Fe3+ concentrations as determined by EELS are very low and are expected to play no role in the broadening of the loops.  相似文献   

9.
Partitioning of oxygen and silicon between molten iron and (Mg,Fe)SiO3 perovskite was investigated by a combination of laser-heated diamond-anvil cell (LHDAC) and analytical transmission electron microscope (TEM) to 146 GPa and 3,500 K. The chemical compositions of co-existing quenched molten iron and perovskite were determined quantitatively with energy-dispersive X-ray spectrometry (EDS) and electron energy loss spectroscopy (EELS). The results demonstrate that the quenched liquid iron in contact with perovskite contained substantial amounts of oxygen and silicon at such high pressure and temperature (P–T). The chemical equilibrium between perovskite, ferropericlase, and molten iron at the P–T conditions of the core–mantle boundary (CMB) was calculated in Mg–Fe–Si–O system from these experimental results and previous data on partitioning of oxygen between molten iron and ferropericlase. We found that molten iron should include oxygen and silicon more than required to account for the core density deficit (<10%) when co-existing with both perovskite and ferropericlase at the CMB. This suggests that the very bottom of the mantle may consist of either one of perovskite or ferropericlase. Alternatively, it is also possible that the bulk outer core liquid is not in direct contact with the mantle. Seismological observations of a small P-wave velocity reduction in the topmost core suggest the presence of chemically-distinct buoyant liquid layer. Such layer physically separates the mantle from the bulk outer core liquid, hindering the chemical reaction between them.  相似文献   

10.
 Planewave pseudopotential calculations of supercell total energies were used as bases for first-principles calculations of the CaCO3–MgCO3 and CdCO3–MgCO3 phase diagrams. Calculated phase diagrams are in qualitative to semiquantitative agreement with experiment. Two unobserved phases, Cd3Mg (CO3)4 and CdMg3(CO3)4, are predicted. No new phases are predicted in the CaCO3–MgCO3 system, but a low-lying metastable Ca3Mg(CO3)4 state, analogous to the Cd3Mg(CO3)4 phase is predicted. All of the predicted lowest-lying metastable states, except for huntite CaMg3(CO3)4, have dolomite-related structures, i.e. they are layer structures in which A m B n cation layers lie perpendicular to the rhombohedral [111] vector. Received: 6 May 2002 / Accepted: 23 October 2002 Acknowledgements This work was partially supported by NSF contract DMR-0080766 and NIST.  相似文献   

11.
High arsenic (As) concentrations, >900 μg/L, were measured in Ca–Mg–SO4 waters from springs and drainages in the village of Pesariis in the Carnic Alps (NE Italy). Oxidation of the outcropping arsenian marcasite ore deposits of the area is proposed as the mechanism for As release into oxygenated waters during runoff. Nevertheless, the limited extension of the ore deposit and the relatively low As content of the mineralization suggest that sulfide weathering might not be the only process responsible for the highest As concentration in groundwaters. An additional mechanism involves As adsorption onto ferric iron particulate during oxidation, the drawdown in reducing environment at depth during water infiltration, and the release of ferrous iron and sorbed arsenic to the water columns by reductive dissolution of hydrous ferric oxides (HFO). This yields the observed Fe–As correlation. Newly formed HFO precipitates when groundwaters discharge to aerated conditions, leading to the removal of As, which strongly partitions into the iron-rich sediments, adsorbed onto the surface of amorphous Fe2O3·xH2O. The calculated and measured As concentration in sediments exceeds 10% by weight. Furthermore, geochemical and isotopic data indicate that the As-rich reservoir partly mixes with shallower aquifers, commonly tapped for drinking supply, representing a natural hazard for inhabitants.  相似文献   

12.
The temperature (T) evolution of the barium carbonate (BaCO3) structure was studied using Rietveld structure refinements based on synchrotron X-ray diffraction and a powdered synthetic sample. BaCO3 transforms from an orthorhombic, Pmcn, α phase to a trigonal, R3m, β phase at 811°C. The orthorhombic BaCO3 structure is isotypic with aragonite, CaCO3. In trigonal R3m BaCO3, the CO3 group occupies one orientation and shows no rotational disorder. The average <Ba–O> distances increase while the <C–O> distances decrease linearly with T in the orthorhombic phase. After the 811°C phase transition, the <Ba–O> distances increase while C–O distances decrease. There is also a significant volume change of 2.8% at the phase transition.  相似文献   

13.
Rising atmospheric pCO2 and ocean acidification originating from human activities could result in increased dissolution of metastable carbonate minerals in shallow-water marine sediments. In the present study, in situ dissolution of carbonate sedimentary particles in Devil’s Hole, Bermuda, was observed during summer when thermally driven density stratification restricted mixing between the bottom water and the surface mixed layer and microbial decomposition of organic matter in the subthermocline layer produced pCO2 levels similar to or higher than those levels anticipated by the end of the 21st century. Trends in both seawater chemistry and the composition of sediments in Devil’s Hole indicate that Mg-calcite minerals are subject to selective dissolution under conditions of elevated pCO2. The derived rates of dissolution based on observed changes in excess alkalinity and estimates of vertical eddy diffusion ranged from 0.2 mmol to 0.8 mmol CaCO3 m−2 h−1. On a yearly basis, this range corresponds to 175–701 g CaCO3 m−2 year−1; the latter rate is close to 50% of the estimate of the current average global coral reef calcification rate of about 1,500 g CaCO3 m−2 year−1. Considering a reduction in marine calcification of 40% by the year 2100, or 90% by 2300, as a result of surface ocean acidification, the combination of high rates of carbonate dissolution and reduced rates of calcification implies that coral reefs and other carbonate sediment environments within the 21st and following centuries could be subject to a net loss in carbonate material as a result of increasing pCO2 arising from burning of fossil fuels.  相似文献   

14.
 The interpretation of metamorphically induced U-Pb isotopic discordance requires a thorough understanding of zircon-fluid interactions. With this aim we have studied the behaviour of metamict and crystalline zircon phases and their U-Pb systems by cathodoluminescence after treatment by 2M Na2CO3 solution at T = 200–800 °C and P = 1–5 kbar for 3–14 days, X-ray diffraction, microprobe and isotope dilution analysis. The data indicate that zircon transformation under hydrothermal conditions depends on the experimental conditions and the degree of structural damage. Reconstitution of defective and impurity-enriched zones of metamict zircon (homogenization of impure element concentrations and increase of crystallinity) was observed at 400 °C and P = 1 kbar. Considerable lead and uranium loss occurred under these conditions. As a result of zircon dissolution, newly formed baddeleyite accommodating U from 2M Na2CO3 solution and Zr-Na-silicate were recognized. This process intensified with increasing pressure. Study of crystalline zircon indicates that migration of U and Pb took place only during dissolution of zircon at T above 650 °C. In the presence of carbonate-ions essential U and Pb amounts are lost from metamict zircon at a lower P-T than is typical for greenschist facies metamorphism. Received: 4 October 1997 / Accepted: 6 December 1999  相似文献   

15.
Solidus temperatures of quartz–alkali feldspar assemblages in the haplogranite system (Qz-Ab-Or) and subsystems in the presence of H2O-H2 fluids have been determined at 1, 2, 5 and 8 kbar vapour pressure to constrain the effects of redox conditions on phase relations in quartzofeldspathic assemblages. The hydrogen fugacity (f H2) in the fluid phase has been controlled using the Shaw membrane technique for moderately reducing conditions (f H2 < 60 bars) at 1 and 2 kbar total pressure. Solid oxygen buffer assemblages in double capsule experiments have been used to obtain more reducing conditions at 1 and 2 kbar and for all investigations at 5 and 8 kbar. The systems Qz-Or-H2O-H2 and Qz-Ab-H2O-H2 have only been investigated at moderately reducing conditions (1 and 5 kbar) and the system Qz-Ab-Or-H2O-H2 has been investigated at redox conditions down to IW (1 to 8 kbar). The results obtained for the water saturated solidi are in good agreement with those of previous studies. At a given pressure, the solidus temperature is found to be constant (within the experimental precision of ± 5°C) in the f H2 range of 0–75 bars. At higher f H2, generated by the oxygen buffers FeO-Fe3O4 (WM) and Fe-FeO (IW), the solidus temperatures increase with increasing H2 content in the vapour phase. The solidus curves obtained at 2 and 5 kbar have similar shapes to those determined for the same quartz - alkali feldspar assemblages with H2O-CO2- or H2O-N2-bearing systems. This suggests that H2 has the behaviour of an inert diluent of the fluid phase and that H2 solubility in aluminosilicate melts is very low. The application of the results to geological relevant conditions [HM (hematite-magnetite) > f O2 > WM] shows that increasing f H2 produces a slight increase of the solidus temperatures (up to 30 °C) of quartz–alkali feldspar assemblages in the presence of H2O-H2 fluids between 1 and 5 kbar total pressure. Received: 4 March 1996 / Accepted: 22 August 1996  相似文献   

16.
Experimental data on Fe-CaCO3 interaction at 6 GPa and 1273–1873 K are presented. The system models the hypothetical redox interaction in subducting slabs at the contact with the reduced mantle and a putative process at the core-mantle boundary. The reaction is accompanied by carbonatite melt formation. It also produces Fe3C and calcium wustite, which form solid or liquid phases depending on experimental conditions. In iron-containing systems at 6 GPa, calcium carbonate melts in the range 1473–1573 K, which is consistent with aragonite disappearance from complex carbonate systems. The composition of calcium carbonate liquid is not influenced by metallic Fe. It corresponds to nearly pure CaCO3. Along the mantle adiabat or at slightly higher temperatures, nearly pure CaCO3 coexists with metallic iron or calcium wustite. This hypothesis explains the coexistence of metallic iron and carbonate inclusions in lithospheric and superdeep diamonds.  相似文献   

17.
This work considers the studies of melt and fluid inclusions in spinel of ultramafic rocks in the mantle wedge beneath Avacha volcano (Kamchatka). The generations of spinel were identified: 1 is spinel (Sp-I) of the “primary” peridotites, has the highest magnesium number (#0.69–0.71), highest contents of Al2O3 and lowest contents of Cr2O3 (26.2–27.1 and 37.5–38.5 wt %, respectively), and the absence in it of any fluid and melt inclusions; 2 is spinel (Sp-II) of the recrystallized peridotites, has lower magnesium number (Mg# 0.64–0.61) and the content of Al2O3 (18–19 wt %), a higher content of Cr2O3 (45.4–47.2 wt %) and the presence of primary fluid inclusions; 3 is spinel (Sp-III) that is characterized by the highest content of Cr2O3 (50.2–55.4 wt %), the lowest content of Al2O3 (13.6–16.6 wt %), and the presence of various types of primary melt inclusions. The data obtained indicate that metasomatic processing of “primary” peridotites occurred under the influence of high concentrated fluids of mainly carbonate-water-chloride composition with influx of the following petrogenic elements: Si, Al, Fe, Ca, Na, K, S, F, etc. This process was often accompanied by a local melting of the metasomatized substrate at a temperature above 1050°C with the formation of melts close to andesitic.  相似文献   

18.
Release of CO2 from surface ocean water owing to precipitation of CaCO3 and the imbalance between biological production of organic matter and its respiration, and their net removal from surface water to sedimentary storage was studied by means of a quotient θ = (CO2 flux to the atmosphere)/(CaCO3 precipitated). θ depends not only on water temperature and atmospheric CO2 concentration but also on the CaCO3 and organic carbon masses formed. In CO2 generation by CaCO3 precipitation, θ varies from a fraction of 0.44 to 0.79, increasing with decreasing temperature (25 to 5°C), increasing atmospheric CO2 concentration (195–375 ppmv), and increasing CaCO3 precipitated mass (up to 45% of the initial DIC concentration in surface water). Primary production and net storage of organic carbon counteracts the CO2 production by carbonate precipitation and it results in lower CO2 emissions from the surface layer. When atmospheric CO2 increases due to the ocean-to-atmosphere flux rather than remaining constant, the amount of CO2 transferred is a non-linear function of the surface layer thickness because of the back-pressure of the rising atmospheric CO2. For a surface ocean layer approximated by a 50-m-thick euphotic zone that receives input of inorganic and organic carbon from land, the calculated CO2 flux to the atmosphere is a function of the CaCO3 and Corg net storage rates. In general, the carbonate storage rate has been greater than that of organic carbon. The CO2 flux near the Last Glacial Maximum is 17 to 7×1012 mol/yr (0.2–0.08 Gt C/yr), reflecting the range of organic carbon storage rates in sediments, and for pre-industrial time it is 38–42×1012 mol/yr (0.46–0.50 Gt C/yr). Within the imbalanced global carbon cycle, our estimates indicate that prior to anthropogenic emissions of CO2 to the atmosphere the land organic reservoir was gaining carbon and the surface ocean was losing carbon, calcium, and total alkalinity owing to the CaCO3 storage and consequent emission of CO2. These results are in agreement with the conclusions of a number of other investigators. As the CO2 uptake in mineral weathering is a major flux in the global carbon cycle, the CO2 weathering pathway that originates in the CO2 produced by remineralization of soil humus rather than by direct uptake from the atmosphere may reduce the relatively large imbalances of the atmosphere and land organic reservoir at 102–104-year time scales.  相似文献   

19.
 Pore water concentration profiles of sediments at a site on the Amazon Fan were investigated and simulated with the numerical model CoTReM (column transport and reaction model) to reveal the biogeochemical processes involved. The pore water profiles for gravity core GeoB 4417-7 showed a distinct sulfate–methane transition zone in which deep sulfate reduction occurs. Only a small sulfide peak could be observed at the reaction zone. Due to high amounts of iron minerals, the produced sulfide is instantaneously precipitated in form of iron sulfides. We present a simulation which starts from a steady state system with respect to pore water profiles for methane and sulfate. Furthermore, sulfide, iron, pH, pE, calcium and total inorganic carbon (TIC) were included in the simulation. The program calculated mineral equilibria to mackinawite, iron sulfides (more stable than mackinawite), iron hydroxides and calcite via saturation indices (SI) by a module incorporating the program PHREEQC (Parkhurst 1995). The measured sulfide and iron profiles are obtained in the simulation output by using a constant SI (=0) for mackinawite and calcite, while a depth dependent SI distribution is applied for the PHREEQC phases “Pyrite” and “Fe(OH)3(a)”, representing a composition and the kinetics of different iron sulfides and iron hydroxides. These SI distributions control the results of sulfide and iron pore water profiles, especially conserving the sulfide profile at the reaction zone during the simulation. The results suggest that phases of iron hydroxides are dissolved, mackinawite is precipitated within, and other iron sulfides are precipitated below the reaction zone. The chemical reactivity of iron hydroxides corresponds to the rate of sulfide production. The system H2O–CO2–CaCO3 is generally successfully maintained during the simulation. Deviations to the measured pH profile suggest that further processes are active which are not included in the simulation yet. Received: 9 November 1998 / Accepted: 26 October 1999  相似文献   

20.
The rates of grain growth of stoichiometric dolomite [CaMg(CO3)2] and magnesite (MgCO3) have been measured at temperatures T of 700–800°C at a confining pressure P c of 300 MPa, and compared with growth rates of calcite (CaCO3). Dry, fine-grained aggregates of the three carbonates were synthesized from high purity powders by hot isostatic pressing (HIP); initial mean grain sizes of HIP-synthesized carbonates were 1.4, 1.1, and 17 μm, respectively, for CaMg(CO3)2, MgCO3, and CaCO3, with porosities of 2, 28, and 0.04% by volume. Grain sizes of all carbonates coarsened during subsequent isostatic annealing, with mean values reaching 3.9, 5.1, and 27 μm for CaMg(CO3)2, MgCO3, and CaCO3, respectively, in 1 week. Grain growth of dolomite is much slower than the growth rates of magnesite or calcite; assuming normal grain growth and n = 3 for all three carbonates, the rate constant K for dolomite (≃5 × 10−5 μm3/s) at T = 800°C is less than that for magnesite by a factor of ~30 and less than that for calcite by three orders of magnitude. Variations in carbonate grain growth may be affected by differences in cation composition and densities of pores at grain boundaries that decrease grain boundary mobility. However, rates of coarsening correlate best with the extent of solid solution; K is the largest for calcite with extensive Mg substitution for Ca, while K is the smallest for dolomite with negligible solid solution. Secondary phases may nucleate at advancing dolomite grain boundaries, with implications for deformation processes, rheology, and reaction kinetics of carbonates.  相似文献   

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