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1.
Hongchang Hu Genxu Wang Guangsheng Liu Taibing Li Dongxing Ren Yibo Wang Huiyan Cheng Junfeng Wang 《Environmental Geology》2009,57(6):1391-1397
The alpine ecosystem is very sensitive to environmental change due to global and local disturbances. The alpine ecosystem
degradation, characterized by reducing vegetation coverage or biomass, has been occurring in the Qinghai–Tibet Plateau, which
alters local energy balance, and water and biochemical cycles. However, detailed characterization of the ecosystem degradation
effect is lack in literature. In this study, the impact of alpine ecosystem degradation on soil temperature for seasonal frozen
soil and permafrost are examined. The vegetation coverage is used to indicate the degree of ecosystems degradation. Daily
soil temperature is monitored at different depths for different vegetation coverage, for both permafrost and seasonal frozen
soils. Results show that under the insulating effort of the vegetation, the freezing and thawing process become quicker and
steeper, and the start of the freezing and thawing process moves up due to the insulating effort of the vegetation. The influence
of vegetation coverage on the freezing process is more evident than the thawing process; with the decrease of vegetation coverage,
the integral of frozen depth increases for seasonal frozen soil, but is vice versa for permafrost. 相似文献
2.
As a part of the MONTBLEX-90 observational programme, Kytoon and Doppler sodar observations were taken at Kharagpur. These
data are analysed to study the turbulent characteristics of the atmospheric boundary layer in terms of stability, temperature
structure function (C
T
2
) and velocity structure function (C
v
2
).C
T
2
follows aZ
−4/3 law on most of the days, whereas the variation ofC
V
2
is not systematic.C
V
2
andC
T
2
values are found to vary between 10−5−10−1 m4/3s−2 and 10−5−10−2°C2 m−2/3 respectively. 相似文献
3.
Nadia Curetti Davide Levy Alessandro Pavese Gabriella Ivaldi 《Physics and Chemistry of Minerals》2006,32(10):670-678
The elastic properties of coexisting natural 3T and 2M
1 phengite samples (Cima Pal, Sesia Zone; Val Savenca; Western Alps, Italy) with similar chemical compositions have been studied by room temperature–high pressure powder diffraction, using synchrotron radiation on the ID9A beam-line at ESRF (Grenoble, France). The P–V curves have been modelled by the Birch–Murnaghan model; a third-order expansion fitted to the experimental data yields for 3T and 2M
1 K
0=60.4(±0.7) GPa, K′=5.79(±0.11) at V
0=703.8851 Å3, and K
0=57.3(±1.0) GPa, K′=6.97(±0.24) at V
0=938.8815 Å3, respectively. The relative stability of 3T vs. 2M
1 has been explored as a function of pressure and temperature in terms of configuration and deformation contributions to the Gibbs energy, using the elastic properties determined here and other thermodynamic parameters from earlier investigations. The results presented agree with the hypothesis of stability of the 3T polytype in the high pressure regime. 相似文献
4.
The relationship between sensible and latent heat flux and diurnal variation in soil surface temperature and moisture under
four freeze/thaw soil conditions was investigated using observed soil temperature and moisture and simulated sensible and
latent heat flux. The diurnal range of latent heat flux had a similar temporal change pattern as that of unfrozen soil water
at depths of 0–3 cm during the freezing stage. Also, there was a better relationship with the diurnal range of unfrozen soil
water at depths of 3–6 cm during the thawing stage. Diurnal variation in latent heat flux was significant and depended mostly
on solar radiation during the completely thawed stage. However, while diurnal variation in solar radiation during the completely
frozen stage was significant, for latent heat flux it was quite weak due to low unfrozen soil water content. Thus, diurnal
variation in latent heat flux depended mostly on unfrozen soil water content during this stage. During the freezing and thawing
stages, diurnal variation in latent heat flux was also significant and depended mostly on diurnal variation in unfrozen soil
water content. However, the impacts of air temperature change from solar radiation on latent heat flux could not be ignored. 相似文献
5.
Gianluca Iezzi Mario Tribaudino Giancarlo Della Ventura Fabrizio Nestola Fabio Bellatreccia 《Physics and Chemistry of Minerals》2005,32(7):515-523
The synthetic amphibole Na0.95(Li0.95Mg1.05)Mg5Si8O22(OH)2 was studied in situ at high-T, using IR OH-stretching spectroscopy and synchrotron X-ray powder diffraction. At room-T the sample has P21
/m symmetry, as shown by the FTIR spectrum. It shows in the OH region two well-defined and intense absorptions at 3,748 and
3,712 cm−1, respectively, and two minor bands at 3,667 and 3,687 cm−1. The main bands are assigned to the two independent O–H groups in the primitive structure. The two minor bands evidencing
the presence of small amount of vacant A-site (A□0.05). With increasing T, these bands shift continuously and merge into a unique absorption at high temperature. A change as a function of increasing
T is revealed by the evolution of the refined unit-cell parameters, whose trend shows a transition to C2/m at about 320–330°C. The spontaneous scalar strain, fitted with a tricritical 2–6 Landau potential, gives a T
c of 325(10)°C (β parameter = 0.27). Comparison with the second-order P21
/m ⇔ C2/m phase transition at 255°C for synthetic amphibole ANa0.8B(Na0.8Mg1.2)CMg5Si8O22(OH)2 indicates that the substitution of Na with Li at the B-sites strongly affects the thermodynamic character and the T
c of the phase transition. The comparison of LNMSH amphiboles with cummingtonitic ones shows that the high-T thermodynamic behaviour is affected by A-site occupancy. 相似文献
6.
7.
Lukáš Ackerman Jiří Zachariáš Marta Pudilová 《International Journal of Earth Sciences》2007,96(4):623-638
Fluid inclusions, mineral thermometry and stable isotope data from two types of mineralogically and texturally contrasting
pegmatites, barren ones and lithium ones, from the Moldanubian Zone of the Bohemian Massif were studied in order to constrain
P–T conditions of their emplacement, subsolidus hydrothermal evolution and to estimate composition of the early exsolved fluid
and that of the parental melt. Despite the fact that the lithium pegmatites are abundant throughout the crystalline units
of the Bohemian Massif, data similar to this paper have not been published yet. The studied pegmatites are hosted by iron-rich
calcic skarn bodies. This specific setting allowed scavenging of calcium, fluorine and some other elements from the host rocks
into the pegmatitic melts and post-magmatic fluids. Such contamination process was important namely in the case of barren
pegmatites, as can be deduced from the variation in anorthite contents in plagioclase and from the presence of fluorite, hornblende
(with F content) or garnet in the contact zones of pegmatite dykes. Fluid inclusions were studied mostly in quartz, but also
in fluorite, titanite and apatite. Early aqueous–carbonic and late aqueous fluids were identified in both pegmatite types.
The P–T conditions of crystallization as well as the detailed composition of exsolved magmatic fluid, however, particularly differ.
The magmatic fluids associated with barren pegmatites correspond to H2O–CO2 low salinity fluids, composition of which evolved from 20 to 23 to <5 mol% CO2, and from 2 to 4–6 mol% NaCl eq. Sudden decrease in the CO2 content of the post-magmatic fluids (<5 mol% CO2) seems to coincide with the enrichment of the fluid in calcium (from the contamination process) and resulted in precipitation
of calcites (frequently found as trapped solid phases in fluid inclusions). The fluids associated with lithium pegmatites
are more complex (H2O–CO2/N2–H3BO3–NaCl). The CO2 content of early exsolved fluid is 26–20 mol% CO2 and remains the same in the next fluid generation. The main difference between the magmatic and the first post-magmatic fluids
is the presence of 7–9 wt% of H3BO3 (identified as daughter mineral sassolite) in the former. The second post-magmatic fluids are again CO2-poor (∼4 mol%) and more saline (∼4 mol% NaCl eq.). The composition of exsolved fluid was further used to constrain volatile
composition and content of the parental melts. Finally, P–T conditions of pegmatite crystallization are constrained: 600–640°C and 420–580 MPa for the barren pegmatites and 500–570°C
and 310–430 MPa for the lithium pegmatite. While the emplacement of the former occurred in thermal equilibrium with the Moldanubian
host rock environment, the emplacement of the later suggests substantial thermal disequilibrium. 相似文献
8.
Abiotic regulators of soil respiration in desert ecosystems 总被引：1，自引：0，他引：1
Soil temperature and soil moisture are the most important environmental factors controlling soil respiration in mesic ecosystems.
However, soil respiration and associated abiotic regulators have been poorly studied in desert ecosystems. In this study,
soil respiration was measured using an automated CO2 efflux system (LI-COR 8100), and the effects of soil temperature and moisture on the rate of soil respiration were examined
in six desert sites [three communities—Haloxylon ammodendron, Halostachys caspica and Anabasis aphylla at high (B) and low (A) vegetation coverage respectively]. It was found that soil respiration was significantly and positively
correlated with soil surface temperature. A multi-variable model of soil temperature and soil moisture could explain 61.9%
of temporal variation in soil CO2 efflux at a larger scale. There were significantly negative correlations between soil respiration and soil moisture in Haloxylon ammodendron B and Halostachys caspica B sites, which represented the driest and wettest sites, respectively. The results also showed that soil respiration displayed
obvious diurnal and seasonal patterns during the growing season. The Q10 values for Haloxylon ammodendron A and B, Halostachys caspica A and B, and Anabasis aphylla A and B sites were 1.3, 1.34, 1.58, 1.65, 1.31 and 1.17, respectively, with a cross-site average of 1.39. The results showed
that soil respiration was not positively correlated with soil moisture unlike in most mesic ecosystems. However, soil respiration
in desert ecosystems is less sensitive to temperature variation than most mesic ecosystems as indicated by the lower Q10 values possibly due to energy limitation. 相似文献
9.
青藏高原季节冻土区土壤冻融过程水热耦合特征 总被引：8，自引：5，他引：3
青藏高原被誉为“中华水塔”, 其广泛分布的多年冻土和季节冻土在保证我国水资源安全上具有重要的地位。基于2015年7月 - 2016年6月青海海北站季节冻土的水热监测数据（土壤含水量为未冻水含量）, 分析了冻结深度的季节变化和冻融过程水热运移特征。结果表明： 各土层土壤温度与土壤水分含量变化均表现为“U”型。土壤温度变化规律与日平均气温基本一致, 但滞后于日平均气温的变化, 滞后时间取决于土层深度。与多年冻土冻融规律不同, 海北站季节冻土表现为单向冻结、 双向融化特征, 冻融过程大致可划分为三个阶段： 冻结初期、 冻结稳定期和融化期。同时, 季节冻土消融速率大于冻结速率, 且融化过程中以浅层土壤融化为主。在冻结过程中, 土壤水分沿上、 下两个方向分别向冻结锋面迁移, 各土层土壤含水量迅速下降。而在融化过程中, 各土层土壤含水量逐渐增加, 且在浅层土壤形成一个土壤水分的高值区。土壤冻融过程中未冻水含量与各土层土壤温度具有较好的相关关系, 且浅层土壤拟合效果优于深层土壤。本研究对揭示高原关键水文过程以及寒区水热耦合模型构建具有重要意义。 相似文献
10.
11.
The empirical linear relation between volume and logarithm of bulk modulus of a material, discovered by Grover, Getting and
Kennedy is taken as the basis for our equation of state. Using the latest experimental information on the adiabatic bulk modulus,
the equation of state is applied to the three polymorphs of Mg2SiO4 to develop a consistent dataset of their thermodynamic properties in the temperature range of 200–2273 K and a pressure range
of 0.1 MPa–30 GPa. The results imply that the bulk sound velocity contrast (v
β−v
α)/v
α increases with temperature along the α–β phase boundary and reaches the value 8.9% at 13.5 GPa, a pressure equivalent to
410 km depth in the Earth. The bulk sound velocity contrast (v
γ−v
β)/v
β decreases with temperature along the β–γ phase boundary and becomes less than 0.7% at temperatures and pressures equivalent
to those associated with the 520-km seismic discontinuity in the Earth.
Received: 1 August 2000 / Accepted: 1 March 2001 相似文献
12.
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
13.
P. Tropper I. Deibl F. Finger R. Kaindl 《International Journal of Earth Sciences》2006,95(6):1019-1037
The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An0–15) during stage 4. Calculation of the P–T conditions of the peak assemblage (stage 1) yields 750–840°C, 0.29–0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620–730°C, 0.27–0.36 GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38 GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit. 相似文献
14.
A thermoelastic model for calculating the high-pressure and high-temperature properties of isotropic solids is presented
by extending the formalism by Thomsen and combining the resulting one with the Vinet model for static lattice and the Debye
model for lattice vibration. Applying it to polycrystalline corundum, we have shown that the calculated values of entropy
and heat capacity at constant pressure are in agreement with literature values to 2325 K at zero pressure and that the calculated
values of thermal expansivity agree reasonably with experimental data to 1100 K at zero pressure. The model reproduces experimental
data of sound velocities v
p
and v
s
of compressional and shear waves to 1825 K at zero pressure and those to 62 GPa at room temperature, and it reproduces also
experimental shock-wave equation of state to 150 GPa. The velocity correlation (∂ln v
s
/∂ln v
p
)
S
was found to have weak pressure and temperature dependences and the results under lower mantle conditions are compared with
those of magnesian and calcium silicate perovskites and magnesiowüstite, and the PREM values of the Earth's lower mantle.
Received: 12 February 2000 / Accepted: 15 July 2000 相似文献
15.
M. Akaogi H. Takayama H. Kojitani H. Kawaji T. Atake 《Physics and Chemistry of Minerals》2007,34(3):169-183
The low-temperature isobaric heat capacities (C
p) of β- and γ-Mg2SiO4 were measured at the range of 1.8–304.7 K with a thermal relaxation method using the Physical Property Measurement System.
The obtained standard entropies (S°298) of β- and γ-Mg2SiO4 are 86.4 ± 0.4 and 82.7 ± 0.5 J/mol K, respectively. Enthalpies of transitions among α-, β- and γ-Mg2SiO4 were measured by high-temperature drop-solution calorimetry with gas-bubbling technique. The enthalpies of the α−β and β−γ
transitions at 298 K (ΔH°298) in Mg2SiO4 are 27.2 ± 3.6 and 12.9 ± 3.3 kJ/mol, respectively. Calculated α−β and β−γ transition boundaries were generally consistent
with those determined by high-pressure experiments within the errors. Combining the measured ΔH°298 and ΔS°298 with selected data of in situ X-ray diffraction experiments at high pressure, the ΔH°298 and ΔS°298 of the α−β and β−γ transitions were optimized. Calculation using the optimized data tightly constrained the α−β and β−γ transition
boundaries in the P, T space. The slope of α−β transition boundary is 3.1 MPa/K at 13.4 GPa and 1,400 K, and that of β−γ boundary 5.2 MPa/K at 18.7 GPa
and 1,600 K. The post-spinel transition boundary of γ-Mg2SiO4 to MgSiO3 perovskite plus MgO was also calculated, using the optimized data on γ-Mg2SiO4 and available enthalpy and entropy data on MgSiO3 perovskite and MgO. The calculated post-spinel boundary with a Clapeyron slope of −2.6 ± 0.2 MPa/K is located at pressure
consistent with the 660 km discontinuity, considering the error of the thermodynamic data. 相似文献
16.
青藏高原多年冻土区高寒草甸土壤水分入渗变化研究 总被引：4，自引：3，他引：1
在多年冻土区典型坡面上,将坡面划分为坡下（L）、坡中（M）和坡顶（H）三个坡位,每个坡位上各选取92%、60%和30%植被盖度为研究对象,用双环入渗仪测定土壤水分入渗过程,对影响土壤入渗过程的环境因子进行了分析,并基于土壤物理特性及土壤水分的测定进行模型模拟。结果表明：研究区不同植被盖度下土壤水分入渗性能在活动层冻融过程中差异明显,初始含水量和初始入渗率具有较好的负相关关系;稳定入渗率大小为：活动层融化期,92%（0.61 mm·min-1） > 60%（0.50 mm·min-1） > 30%（0.29 mm·min-1）;活动层开始冻结期,60%（0.56 mm·min-1） > 30%（0.39 mm·min-1） > 92%（0.26 mm·min-1）。土壤水分入渗速率具有显著的坡位差异并与冻土的冻融循环过程关系紧密。主要表现为,稳定入渗速率随坡位高度的降低依次递减;同一坡位下,开始冻结期入渗速率小于融化期。在整个入渗阶段,坡顶的累积入渗量是最大的,体现了较好的入渗性能。影响高寒草甸土壤水分入渗的环境因子主要有容重,有机质含量及粒径<0.1 mm微粒。通过比较研究得出,在长江源地区,活动层融化期通用经验模型f（t）=a+bt-n更适用于该研究区域高寒草甸土壤水分入渗过程的研究,而在开始冻结期Horton模型f（t）=ic+（i0-ic）e-kt则具有更好的适用性。 相似文献
17.
Selenium (Se), an animal toxicant and aquifer contaminant, occurs in coal mine environments of Wyoming. There is a paucity
of information on solution-phase Se speciation in mine soils. The objectives of this study were to compare Se extraction efficiencies
of various reagents and to characterize SeO2–
3 (selenite), SeO2–
4 (selenate) and organic Se components in these extracts. Forty coal mine soils were extracted using DI (deionized) water,
hot water (0.1% CaCl2), AB-DTPA, NaOH, and KH2PO4. Each solution was analyzed for total dissolved Se, and inorganic and organic Se fractions. Both inorganic and organic Se
fractions were detected in the soil extracts. The order of Se (total, inorganic, and organic) extraction efficiency for different
reagents was DI water < hot water < AB-DTPA < NaOH < KH2PO4. The inorganic–organic Se ratios in DI water, hot water, AB-DTPA, NaOH, and KH2PO4 extracts were 60 : 40, 26 : 74, 61 : 39, 87 : 13, and 52 : 48, respectively, indicating predominance of inorganic Se in all
but the hot water extract. Selenite was the dominant inorganic species in AB-DTPA and KH2PO4 extracts, while SeO2–
4 was the major Se species in the DI water, hot water, and NaOH extracts. Significant correlations (P<0.01) were observed among extractable inorganic Se [NaOH and KH2PO4 (r=0.95); hot water and AB-DTPA (r=0.89)], total soluble Se [DI water with hot water (r=0.98) and AB-DTPA (r=0.95)], and Se species [SeO2–
3 in AB-DTPA with SeO2–
4 in NaOH (r=0.94) and SeO2–
3 in KH2PO4 (r=0.88)]. These correlations are indicative of Se extraction efficiency, thermodynamically predicted chemical transformations
(such as oxidation of SeO2–
3 to SeO2–
4), and probable interconversions between the organic and inorganic Se fractions (r=0.70 in KH2PO4 extracts); as a whole the correlations can be used as statistical validations of possible geochemical processes.
Received: 21 August 1995 · Accepted: 16 October 1995 相似文献
18.
吉林省土壤冻融的逐日变化及与气温、 地温的关系 总被引：2，自引：1，他引：1
土壤冻融过程对气候和生态环境演变有重要影响。为了研究季节冻土区土壤冻融过程及其对气候变化的响应,利用2014-2017年吉林省典型代表观测站逐日冻土、气温和地温数据,研究土壤冻融的逐日变化及其与气温、地温的关系。结果表明：在土壤冻结和融化完整过程中,冻土上限呈直线上升趋势变化,下限呈先增大后减小的三次曲线趋势变化,即从稳定冻结初日起,冻土深度逐渐加深,在达到最大值后,缓慢变浅。冻土融化包括下限和上限融化两个过程,具有“两头化”的变化特征。冻土上限融化与下限同时开始或者晚于下限,但冻土上限融化的日变化量要大于下限。在土壤冻结过程中,活动积温、0 cm地积温、10 cm地积温与冻结深度呈三次曲线变化关系,随负积温的增加,冻结深度加深。在冻土上限融化过程中,活动积温、0 cm地积温、10 cm地积温与冻土上限深度呈三次曲线变化关系,随正积温的增加,上限融化深度加深。在冻土下限融化过程中,活动积温、0 cm地积温、160 cm地积温与冻土下限深度呈显著的直线趋势,随正积温的增加,下限融化深度变浅。 相似文献
19.
Wenjun Yong E. Dachs A. C. Withers E. J. Essene 《Physics and Chemistry of Minerals》2007,34(2):121-127
A multi-anvil device was used to synthesize 24 mg of pure γ-Fe2SiO4 crystals at 8.5 GPa and 1,273 K. The low-temperature heat capacity (C
p) of γ-Fe2SiO4 was measured between 5 and 303 K using the heat capacity option of a physical properties measurement system. The measured
heat capacity data show a broad λ-transition at 11.8 K. The difference in the C
p between fayalite and γ-Fe2SiO4 is reduced as the temperature increases in the range of 50–300 K. The gap in C
p data between 300 and 350 K of γ-Fe2SiO4 is an impediment to calculation of a precise C
p equation above 298 K that can be used for phase equilibrium calculations at high temperatures and high pressures. The C
p and entropy of γ-Fe2SiO4 at standard temperature and pressure (S°298) are 131.1 ± 0.6 and 140.2 ± 0.4 J mol−1 K−1, respectively. The Gibbs free energy at standard pressure and temperature (ΔG°
f,298) is calculated to be −1,369.3 ± 2.7 J mol−1 based on the new entropy data. The phase boundary for the fayalite–γ-Fe2SiO4 transition at 298 K based on current thermodynamic data is located at 2.4 ± 0.6 GPa with a slope of 25.4 bars/K, consistent
with extrapolated results of previous experimental studies. 相似文献
20.
A. V. Bobrov Yu. A. Litvin L. Bindi A. M. Dymshits 《Contributions to Mineralogy and Petrology》2008,156(2):243-257
The pseudo-binary system Mg3Al2Si3O12–Na2MgSi5O12 modelling the sodium-bearing garnet solid solutions has been studied at 7 and 8.5 GPa and 1,500–1,950°C. The Na-bearing garnet
is a liquidus phase of the system up to 60 mol% Na2MgSi5O12 (NaGrt). At higher content of NaGrt in the system, enstatite (up to ∼80 mol%) and then coesite are observed as liquidus phases.
Our experiments provided evidence for a stable sodium incorporation in garnet (0.3–0.6 wt% Na2O) and its control by temperature and pressure. The highest sodium contents were obtained in experiments at P = 8.5 GPa. Near the liquidus (T = 1,840°C), the equilibrium concentration of Na2O in garnet is 0.7–0.8 wt% (∼6 mol% Na2MgSi5O12). With the temperature decrease, Na concentration in Grt increases, and the maximal Na2MgSi5O12 content of ∼12 mol% (1.52 wt% Na2O) is gained at the solidus of the system (T = 1,760°С). The data obtained show that most of natural diamonds, with inclusions of Na-bearing garnets usually containing
<0.4 wt% Na2O, could be formed from sodium-rich melts at pressures lower than 7 GPa. Majoritic garnets with higher sodium concentrations
(>1 wt% Na2O) may crystallize at a pressure range of 7.0–8.5 GPa. However the upper pressure limit for the formation of naturally occurring
Na-bearing garnets is restricted by the eclogite/garnetite bulk composition. 相似文献