共查询到20条相似文献,搜索用时 15 毫秒
1.
The high albite (Ab)?jadeite (Jd)+quartz(Q) reaction (1) and the quartz(Q)?coesite (Cs) transformation (2) have been determined within the temperature range of 1000–1100°C and 1000–1400°C respectively under variable pressures using an anvil-with-hole apparatus. The equilibrium curves for the two reactions as a function ofP andT are as follows: P=?1·33+0·0296T (reaction 1);P=18·949+0·0111T(reaction 2). These two lines intersect at 31·1±0·5kb and 1096°C. The possibility of using an anvil-with-hole apparatus for conducting current investigations is discussed in this paper. 相似文献
2.
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
3.
Sediment Melts at Sub-arc Depths: an Experimental Study 总被引:14,自引:0,他引:14
The phase and melting relations in subducted pelites have beeninvestigated experimentally at conditions relevant for slabsat sub-arc depths (T = 600–1050°C, P = 2·5–4·5GPa). The fluid-present experiments produced a dominant paragenesisconsisting of garnet–phengite–clinopyroxene–coesite–kyanitethat coexists with a fluid phase at run conditions. Garnet containsdetectable amounts of Na2O (up to 0·5 wt%), P2O5 (upto 0·56 wt%), and TiO2 (up to 0·9 wt%) in allexperiments. Phengite is stable up to 1000°C at 4·5GPa and is characterized by high TiO2 contents of up to 2 wt%.The solidus has been determined at 700°C, 2·5 GPaand is situated between 700 and 750°C at 3·5 GPa.At 800°C, 4·5 GPa glass was present in the experiments,indicating that at such conditions a hydrous melt is stable.In contrast, at 700°C, 3·5 and 4·5 GPa, asolute-rich, non-quenchable aqueous fluid was present. Thisindicates that the solidus is steeply sloping in P–T space.Fluid-present (vapour undersaturated) partial melting of thepelites occurs according to a generalized reaction phengite+ omphacite + coesite + fluid = melt + garnet. The H2O contentof the produced melt decreases with increasing temperature.The K2O content of the melt is buffered by phengite and increaseswith increasing temperature from 2·5 to 10 wt%, whereasNa2O decreases from 7 to 2·3 wt%. Hence, the melt compositionschange from trondhjemitic to granitic with increasing temperature.The K2O/H2O increases strongly as a function of temperatureand nature of the fluid phase. It is 0·0004–0·002in the aqueous fluid, and then increases gradually from about0·1 at 750–800°C to about 1 at 1000°C inthe hydrous melt. This provides evidence that hydrous meltsare needed for efficient extraction of K and other large ionlithophile elements from subducted sediments. Primitive subduction-relatedmagmas typically have K2O/H2O of 0·1–0·4,indicating that hydrous melts rather than aqueous fluids areresponsible for large ion lithophile element transfer in subductionzones and that top-slab temperatures at sub-arc depths are likelyto be 700–900°C. KEY WORDS: experimental petrology; pelite; subduction; UHP metamorphism; fluid; LILE 相似文献
4.
Summary High-grade blocks in the Franciscan complex at Tiburon, California, record relatively low temperature eclogite-facies metamorphism
and blueschist-facies overprinting. The eclogite-facies mineral assemblage contains prograde-zoned garnet + omphacite + epidote
± hornblende (katophoritic and barroisitic Ca–Na amphibole) ± glaucophane + phengite (∼3.5 Si p.f.u.) ± paragonite + rutile
+ quartz. The blueschist-facies mineral assemblage contains chlorite + titanite + glaucophane + epidote ± albite ± phengite
(∼3.3 Si p.f.u.). Albite is not stable in the eclogite stage. New calculations based on garnet-omphacite-phengite thermobarometry
and THERMOCALC average-P–T calculations yield peak eclogite-facies P–T conditions of P = 2.2–2.5 GPa and T = 550–620 °C; porphyroclastic omphacite with inclusions of garnet and paragonite yields an average-P–T of 1.8 ± 0.2 GPa at 490 ± 70 °C for the pre-peak stage. The inferred counterclockwise hairpin P–T trajectory suggests prograde eclogitization of a relatively “cold” subducting slab, and subsequent exhumation and blueschist-facies
recrystallization by a decreasing geotherm. Although an epidote-garnet amphibolitic assemblage is ubiquitous in some blocks,
P–T pseudosection analyses imply that the epidote-garnet amphibolitic assemblage is stable during prograde eclogite-facies metamorphism.
Available geochronologic data combined with our new insight for the maximum pressure suggest an average exhumation rate of
∼5 km/Ma, as rapid as those of some ultrahigh pressure metamorphic terranes. 相似文献
5.
At the Bufa del Diente contact-metamorphic aureole, brine infiltration through metachert layers embedded in limestones produced
thick wollastonite rims, according to Cc+Qz=Wo+CO2. Fluid inclusions trapped in recrystallized quartz hosts include: (1) high salinity four phase inclusions [Th(V-L)=460–573° C; Td(salts)=350–400° C; (Na+K)Cleq=64–73 wt%; X
CO
2≤0.02]; (2) low density vapour-rich CO2-bearing inclusions [Th(L-V)≈500±100° C; X
CO
2=0.22–0.44; X
NaCl≤0.01], corresponding to densities of 0.27± 0.05 gcm−3. Petrographical observations, phase compositions and densities show that the two fluids were simultaneously trapped in the
solvus of the H2O-CO2-salts system at 500–600° C and 700±200 bars. The low density fluid was generated during brine infiltration at the solvus
via the wollastonite producing reaction. Identical fluid types were also trapped as inclusion populations in wollastonite
hosts 3 cm adjacent to quartz crystals. At room temperature, both fluid types additionally contain one quartz and one calcite
crystal, generated by the back-reaction Wo+CO2=Cc+Qz of the host with the CO2-proportion of the fluid during retrogression. All of the CO2 was removed from the fluid. On heating in the microstage, the reaction progress of the prograde reaction was estimated via
volume loss of the calcites. In vapour-rich fluids, 50% progress is reached at 490–530° C; 80% at 520–560° C; and 100% at
540–590° C, the latter representing the trapping temperatures of the original fluid at the two fluid solvus. The progress
is volume controlled. With knowledge of compositions and densities from unmodified inclusions in quartz and using the equation
of state of Duan et al. (1995) for H2O-CO2-NaCl, along with f
CO
2-values extracted from it, the reaction progress curve was recalculated in the P-T-X-space. The calculated progress curve passes through the two fluid solvus up to 380° C/210 bars, continues in the one fluid
field and meets the solvus again at trapping conditions. The P-T slope is steep, most of the reaction occurs above 450° C and there is high correspondence between calculated and measured
reaction progress. We emphasize that with the exception of quartz, back-reactions between inclusion fluids and mineral hosts
is a common process. For almost any prograde metamorphic mineral that was formed by a devolatilization reaction and that trapped
the equilibrium fluid or any peak metamorphic fluid as an inclusion, a fluid-host back-reaction exists which must occur somewhere
along the retrograde path. Such retrograde reactions may cause drastic changes in density and composition of the fluid. In
most cases, however, evidence of the evolving mineral assemblages is not given for they might form submicroscopical layers
at the inclusion walls.
Received: 15 March 1995 / Accepted: 1 June 1995 相似文献
6.
Experimental Investigation of the Upper Thermal Stability of Mg-rich Actinolite; Implications for Kiruna-Type Iron Deposits 总被引:2,自引:0,他引:2
The occurrence of actinolite in magnetite deposits of possiblemagmatic origin has prompted an experimental investigation ofthe upper thermal stability of Mg-rich actinolite to determinehow the stability of actinolite changes with increasing Fe content.Experiments were carried out primarily on the compositionalre-equilibration of natural tremolite [molar Fe/(Fe + Mg) =Fe-number = 0·014] in the presence of synthetic clinopyroxene(Ca0·80Fe0·67Mg0·54Si2·00O6), syntheticpigeonite/orthopyroxene (Ca0·08Fe1·19Mg0·70Si2·02O6),quartz, and water to a more Fe-rich actinolite over the rangeof 600–880°C, 1 and 4 kbar, at the Ni–NiO oxygenbuffer, for durations of 1–2 weeks. The bulk compositionof the mineral mixture is close to actinolite with Fe-number= 0·5. These experiments constitute a half-reversal ofthe amphibole composition, which, when approached from a Mg-richstarting composition, provides information on the minimum Fecontent of actinolite at a given temperature. Compositionalchanges were monitored by electron microprobe analysis of amphibolerim compositions and/or overgrowths on the original tremolite.At 4 kbar and 880–800°C, tremolite shows strong re-equilibrationwith overgrowths of an Fe-rich but low-Ca (1·7 > Ca> 1·4) actinolite; Fe-rich cummingtonite (Ca <0·7)begins to nucleate at 860°C. At 800–700°C, tremoliteshows weak compositional re-equilibration but strong nucleationof Fe-rich cummingtonite. Similar results were observed at 1kbar, with tremolite showing strong re-equilibration to low-Caactinolite at 790–600°C with cummingtonite nucleationat 800°C and below. The wide variation in Ca contents ofthe re-equilibrated amphiboles was unexpected. Additional univariantreversal experiments were carried out on the thermal decompositionof a natural actinolite (Fe-number = 0·22) from PleitoMelón, Chile, indicating the breakdown of actinoliteto clinopyroxene, orthopyroxene, quartz, and water at 780°Cand 1 kbar, and 850°C and 4 kbar. Considering only amphiboleswith Ca >1·7 a.p.f.u., the thermal stability of actinoliteis observed to decrease in a linear manner over the P–Trange investigated with a dT/dFe-number slope of –372°C/Fe-numberat 1 kbar and –546°C/Fe-number at 4 kbar. The highthermal stabilities (750–900°C) of actinolites withFe-numbers in the range of 0–0·4 overlap with therange of water-saturated melting for a typical andesite or tonalite.These conditions also overlap the field of experimental Fe–P-richmelt formation, suggesting that actinolite may have an igneousorigin in Kiruna-type ore deposits. KEY WORDS: actinolite; mineral stability; Kiruna deposits, thermodynamic values; cummingonite 相似文献
7.
Martin Morlidge Alison Pawley Giles Droop 《Contributions to Mineralogy and Petrology》2006,152(3):365-373
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
8.
The equilibrium water content of cordierite has been measured for 31 samples synthesized at pressures of 1000 and 2000 bars
and temperatures from 600 to 750° C using the cold-seal hydrothermal technique. Ten data points are presented for pure magnesian
cordierite, 11 data points for intermediate iron/magnesium ratios from 0.25 to 0.65 and 10 data points for pure iron cordierite.
By representing the contribution of H2O to the heat capacity of cordierite as steam at the same temperature and pressure, it is possible to calculate a standard
enthalpy and entropy of reaction at 298.18° K and 1 bar for,
(Mg,Fe)2Al4Si5O18+H2O ⇄ (Fe,Mg)2Al4Si5O18.H2O
Combining the 31 new data points with 89 previously published experimental measurements gives: ΔH
°
r
=–37141±3520 J and ΔS
°
r
=–99.2±4 J/degree. This enthalpy of reaction is within experimental uncertainty of calorimetric data. The enthalpy and entropy
of hydration derived separately for magnesian cordierite (–34400±3016 J, –96.5±3.4 J/degree) and iron cordierite (–39613±2475,
–99.5±2.5 J/degree) cannot be distinguished within the present experimental uncertainty. The water content as a function of
temperature, T(K), and water fugacity, f(bars), is given by n
H2O=1/[1+1/(K ⋅ f
H2O)] where the equilibrium constant for the hydration reaction as written above is, ln K=4466.4/T–11.906 with the standard state for H2O as the gas at 1 bar and T, and for cordierite components, the hydrous and anhydrous endmembers at P and T.
Received: 2 August 1994/Accepted: 7 February 1996 相似文献
9.
TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz 总被引:5,自引:2,他引:3
Jay B. Thomas E. Bruce Watson Frank S. Spear Philip T. Shemella Saroj K. Nayak Antonio Lanzirotti 《Contributions to Mineralogy and Petrology》2010,160(5):743-759
Quartz and rutile were synthesized from silica-saturated aqueous fluids between 5 and 20 kbar and from 700 to 940°C in a piston-cylinder
apparatus to explore the potential pressure effect on Ti solubility in quartz. A systematic decrease in Ti-in-quartz solubility
occurs between 5 and 20 kbar. Titanium K-edge X-ray absorption near-edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in quartz at all conditions studied. Molecular dynamic simulations support XANES measurements
and demonstrate that Ti incorporation onto fourfold sites is favored over interstitial solubility mechanisms. To account for
the P–T dependence of Ti-in-quartz solubility, a least-squares method was used to fit Ti concentrations in quartz from all experiments
to the simple expression
RTlnX\textTiO 2 \textquartz = - 60952 + 1.520 ·T(K) - 1741 ·P(kbar) + RTlna\textTiO 2 RT\ln X_{{{\text{TiO}}_{ 2} }}^{\text{quartz}} = - 60952 + 1.520 \cdot T(K) - 1741 \cdot P(kbar) + RT\ln a_{{{\text{TiO}}_{ 2} }} 相似文献
10.
Summary The Karimnagar granulite terrain is an integral part of the Eastern Dharwar Craton (EDC). It has received much interest because
of the only reported granulite facies rocks in the EDC. These granulites contain quartz-free sapphirine-spinel-bearing granulites,
kornerupine – bearing granulites, mafic granulites, orthopyroxene-cordierite gneisses, charnockites, amphibolites, dolerite
dykes, granite gneisses, quartzites and banded magnetite quartzite. The orthopyroxene-cordierite gneisses occur as enclaves
within granite-gneiss in association with banded magnetite quartzites, charnockites and amphibolites. The observed reaction
textures, spectacular as they are, have an extraordinary information content within a tiny domain. Coronas, symplectites and
resorption textures are of particular interest as they reflect discontinuous or continuous reactions under changing physical
conditions. The main mineral assemblages encountered in these gneisses are orthopyroxene – cordierite – biotite – plagioclase
– perthite – quartz and garnet – orthopyroxene – cordierite – biotite – quartz – plagioclase – perthite ± sillimanite. Multiphase
reaction textures in conjunction with mineral chemical data in the KFMASH system indicate the following reactions:
Based on chemographic relationships and petrogenetic grids in the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system, a sequence of prograde (early stage), isothermal decompression (middle stage) and retrograde (late stage)
reactions (‘back reactions’ and hydration reactions) are inferred. Relatively lower P–T estimates (0.35 GPa/550–750 °C) obtained from the different geothermobarometers are attributed to late Fe–Mg re-equilibration
during cooling. Therefore, the convergence method has been applied to retrieve simultaneously the P–T conditions of the thermal peak of metamorphism. The near thermal peak condition of metamorphism estimated by the convergence
method are 850 °C/0.62 GPa. The P–T estimates define a retrograde trajectory with substantial decompression. 相似文献
11.
K. K. Podlesskii 《Petrology》2010,18(4):350-368
Consistent thermodynamic data on the properties of pure mineral end members and the mixing properties of solid solutions in
the system FeO-MgO-Al2O3-SiO2 were employed to simulate phase relations of sapphirine, garnet, spinel, orthopyroxene, cordierite, quartz, Al silicates,
and corundum. Compositional variations of the solid solutions with temperature notably modify the topology of the P-T diagrams, which differ from the petrogenetic grids widely used in the literature. It is worth noting that the evaluation
of P-T metamorphic conditions based on reaction relations in sapphirine-bearing assemblages cannot be so far considered reliable
enough. The lower stability limit of the sapphirine + quartz assemblage in the system in question is possibly located at much
lower P-T parameters: at least 835°C and ∼6 kbar. The sapphirine + kyanite assemblage can be stable at temperatures below 860°C and
a pressure of ∼11 kbar, and the stability field of the sapphirine + olivine assemblage is narrow and constrained to temperatures
no higher than ∼800°C. 相似文献
12.
The greenschist to amphibolite transition as modeled by the reaction zoisite+tremolite + quartz= anorthite+diopside+water
has been experimentally investigated in the chemical system H2O−CaO− MgO−Al2O3−SiO2 over the range of 0.4–0.8 GPa. This reaction is observed to lie within the stability fields of anorthite + water and of zoisite
+ quartz, in accord with phase equilibrium principles, and its position is in excellent agreement with the boundary calculated
from current internally-consistent data bases. The small dP/dT slope of 0.00216 GPa/K (21.6 bars/K) observed for this reaction supports the pressure-dependency of this transition in this
chemical system. Experimental reversals of the Al content in tremolitic amphibole coexisting with zoisite, diopside, quartz,
and water were obtained at 600, 650, and 700°C and indicated Al total cations (atoms per formula unit, apfu) of only up to 0.5±0.08 at the highest temperature. Thermodynamic
analysis of these and previous compositional reversal data for tremolitic amphibole indicated that, of the activity/composition
relationships considered, a two-site-coupled cation substitution model yielded the best fit to the data and a S
0 (1 bar, 298 K) of 575.4±1.6 J/K · mol for magnesio-hornblende. The calculated isopleths of constant Al content in the amphibole
are relatively temperature sensitive with Al content increasing with increasing temperature and pressure. Finally, several
experiments in the range of 1.0–1.3 GPa were conducted to define the onset of melting, and thus the upper-thermal limit, for
this mineral assemblage, which must involve an invariant point located at approximately 1.05 GPa and 770°C.
Received: 24 January 1997 / Accepted: 2 October 1997 相似文献
13.
Characteristics of Corona Textures in the Huangtuling Granulite in the Dabie Comples,China and Their Implications for Tectonic Settings 总被引:3,自引:0,他引:3
Widely developed in the Dabie complex are various disequilibrium textures which provide direct evidence for the evolution of metamorphism and late-stage uplifting history.The typical mineral assemblasge in the Opx-Gt-Pl-Bi gneiss in Huangtuling,Luotian County,Hubei Province,Is Opx(I) Gt Pl(I) Bi(I) Q.The corona composed of cordierite and orthopyroxene(Ⅱ)growing around garnets in the granulite makes it clear that there occurres the following metamorphic reaction:Gt Q→Cd Opx(Ⅱ).It is estimated that the gran ulite-forming temperature(T)and pressure(P)are 857-998℃ and 1.18-1.23Gpa,respectively,and the corona was formed under the following conditions:T=829-911℃ and P=0.52-0.59GPa.The above results indicate that There occurred a rapid and nearly adiabatic uplifting event and a decompressional metamorphism in the Dabie complex after the formation of granulite.As compared with the granulites worldwidely distributed in 90 locations(Harley 1989),the Huangtuling granulite should belong to the high-pressure type,which represents the composition of the crust at a depth of more than 40 kilometers. 相似文献
14.
Daisuke Yamazaki Eiji Ito Yoshinori Tange Takashi Yoshino Shuangmeng Zhai Hiroshi Fukui Anton Shatskiy Tomoo Katsura Ken-ichi Funakoshi 《Physics and Chemistry of Minerals》2007,34(4):269-273
In situ X-ray observations of the phase transition from ilmenite to perovskite structure in MnGeO3 were carried out in a Kawai-type high-pressure apparatus interfaced with synchrotron radiation. The phase boundary between
the ilmenite and perovskite structures in the temperature range of 700–1,400°C was determined to be P (GPa) = 16.5(±0.6) − 0.0034(±0.0006)T (°C) based on Anderson’s gold pressure scale. The Clapeyron slope, dP/dT, determined in this study is consistent with that for the transition boundary between the ilmenite and the perovskite structure
in MgSiO3. 相似文献
15.
A sequence of mineral associations was examined in eclogitized basites of the Krasnaya Guba dike field in the Belomorian Mobile
Belt. Two morphological types of eclogite and eclogite-like rocks were recognized: (1) eclogite rocks that developed after
ferrogabbro dikes and completely replaced these dikes from contact to contact and (2) eclogite-like rocks that developed after
gabbronorites in zones of ductile deformations and shearing. According to data mineral geothermobaromety, both rock types
were formed within temperature and pressure ranges corresponding to high-pressure and high-temperature amphibolite facies
at T = 700 ± 40°C and P = 10.0 ± 0.5 kbar. The peak metamorphic parameters of the host gneisses are analogous. The decompressional stage, which is
unambiguously identified by reaction textures, occurred at 630–660°C and 7.9–8.2 kbar. As the temperature and, first of all,
pressure decreased, the SiO2 activity in the fluid systematically varied. The eclogitization of the basites took place locally in relation to fluid fluxes,
which were restricted to zones of intense deformations, at variable SiO2 activity. The rocks show evidence of two stages of post-eclogite amphibolization. Older amphibolization 1 was coeval with
the late prograde metamorphic stage (T = 650°C, P = 10–11 kbar). Younger amphibolization 2 affected eclogitized basite dikes and unaltered gabbronorites (together with their
host gneisses) over large areas. This process coincided with decompression (T = 580°C, P = 7–8 kbar) and was likely accompanied by the exhumation of deep zones of BMB to upper-middle crustal levels. 相似文献
16.
Link between convection and meridional gradient of sea surface temperature in the Bay of Bengal 总被引:2,自引:0,他引:2
We use daily satellite estimates of sea surface temperature (SST) and rainfall during 1998–2005 to show that onset of convection
over the central Bay of Bengal (88–92°E, 14–18°N) during the core summer monsoon (mid-May to September) is linked to the meridional
gradient of SST in the bay. The SST gradient was computed between two boxes in the northern (88–92°E, 18–22°N) and southern
(82–88°E, 4–8°N) bay; the latter is the area of the cold tongue in the bay linked to the Summer Monsoon Current. Convection
over central bay followed the SST difference between the northern and southern bay (ΔT) exceeding 0.75°C in 28 cases. There was no instance of ΔT exceeding this threshold without a burst in convection. There were, however, five instances of convection occurring without
this SST gradient. Long rainfall events (events lasting more than a week) were associated with an SST event (ΔT ≥ 0.75°C); rainfall events tended to be short when not associated with an SST event. The SST gradient was important for the
onset of convection, but not for its persistence: convection often persisted for several days even after the SST gradient
weakened. The lag between ΔT exceeding 0.75°C and the onset of convection was 0–18 days, but the lag histogram peaked at one week. In 75% of the 28 cases,
convection occurred within a week of ΔT exceeding the threshold of 0.75°C. The northern bay SST, T
N
, contributed more to ΔT, but it was a weaker criterion for convection than the SST gradient. A sensitivity analysis showed that the corresponding
threshold for T
N
was 29°C. We hypothesise that the excess heating (∼1°C above the threshold for deep convection) required in the northern
bay to trigger convection is because this excess in SST is what is required to establish the critical SST gradient. 相似文献
17.
Philippe Bertrand David J. Ellis David H. Green 《Contributions to Mineralogy and Petrology》1991,108(1-2):55-71
Phase relations and mineral chemistry involving the phases garnet (Gt), spinel (Sp), hypersthene (Hy), sapphirine (Sa), cordierite
(Cd), sillimanite (Sil) and quartz (Qz) have been experimentally determined in the system FMAS (FeO−MgO−Al2O2−SiO2) under low fO2 and for various H2O/CO2 conditions. Several compositions were studied with 100 (Mg/Mg+Fe) ratio ranging from 64 to 87 with excess quartz and sillimanite.
Our data do not show any differences in Gt−Cd stability and composition as a function of H2O, CO2 and H2O−CO2 (±CH4) content, in good agreement with a previous experimental study at lower temperature (Aranovich and Podlesskii 1983). At 1,000°
C and 11 kbar, under CO2-saturated conditions, cordierite grew from a crystalline mix unseeded with cordierite. Thus, under water-absent conditions,
cordierite will have a high-P stability field in the presence of CO2. If water has a pressure stabilizing effect on cordierite, then our results would indicate that the effects of H2O and CO2 are of the same magnitude at high temperature. Our data support the theoretical P-T grid proposed by Hensen (1986) for high-T metapelites and are largely consistent with the high-temperature experimental data of Hensen and Green (1973). The univariant
boundary Gt+Cd=Hy+Sil+Qz, which marks the disappearance of Hy−Sil−Qz assemblages, has a negative dP/dT slope above 1,000° C and a positive one below this temperature. Extrapolation of our data to iron-free systems shows that
the high-P breakdown limit of Mg-cordierite has a negative slope in the range 1,025–1,300° C and probably positive below 1,000° C. This
indicates a maximum of stability for Mg-cordierite at around 1,000° C and 13 kbar. Because of the curvature of the univariant
reactions En+Sil=Py+Qz, Mg−Cd=En+Sil+Qz and Gt+Cd=Hy+Sil+Qz, the iron-free invariant point involving the phases Py, En, Cd,
Sil and Qz probably does not exist. Sapphirine—Qz-bearing assemblages are stable only at temperatures above 1,050° C. At 1,075°
C, the joint Gt−Sa is stable up to 11 kbar. At higher pressure, garnet, sapphirine and quartz react according to the reaction
Gt+Sa+Qz=Hy+Sil. Reequilibrated sapphirines are more aluminous than the theoretical endmember Mg2Al4SiO10 due to AlAl=MgSi substitutions [100(Al2O3/Al2O3+FeO+MgO) in experimental sapphirines ranges from 50.5 to 52.2]. Sapphirine in the assemblage Sa−Cd−Sil−Qz shows a decrease
in Al content with decreasing temperature and pressure, such that the alumina isopleths for sapphirine have a slight negative
dP/dT slope. A similar decrease in Al content of sapphirine with temperature is also observed in Sa−Sil−Qz assemblages. 相似文献
18.
Matteo Alvaro Fernando Cámara M. Chiara Domeneghetti Fabrizio Nestola Vittorio Tazzoli 《Contributions to Mineralogy and Petrology》2011,162(3):599-613
A natural Ca-poor pigeonite (Wo6En76Fs18) from the ureilite meteorite sample PCA82506-3, free of exsolved augite, was studied by in situ high-temperature single-crystal
X-ray diffraction. The sample, monoclinic P21/c, was annealed up to 1,093°C to induce a phase transition from P21/c to C2/c symmetry. The variation with increasing temperature of the lattice parameters and of the intensity of the b-type reflections (h + k = 2n + 1, present only in the P21/c phase) showed a displacive phase transition P21/c to C2/c at a transition temperature T
Tr = 944°C, first order in character. The Fe–Mg exchange kinetics was studied by ex situ single-crystal X-ray diffraction in
a range of temperatures between the closure temperature of the Fe–Mg exchange reaction and the transition temperature. Isothermal
disordering annealing experiments, using the IW buffer, were performed on three crystals at 790, 840 and 865°C. Linear regression
of ln k
D versus 1/T yielded the following equation:
ln k\textD = - 3717( ±416)/T(K) + 1.290( ±0.378); (R2 = 0.988) \ln \,k_{\text{D}} = - 3717( \pm 416)/T(K) + 1.290( \pm 0.378);\quad (R^{2} = 0.988) . The closure temperature (T
c) calculated using this equation was ∼740(±30)°C. Analysis of the kinetic data carried out taking into account the e.s.d.'s
of the atomic fractions used to define the Fe–Mg degree of order, performed according to Mueller’s model, allowed us to retrieve
the disordering rate constants C
0
K
dis+ for all three temperatures yielding the following Arrhenius relation:
ln( C0 K\textdis + ) = ln K0 - Q/(RT) = 20.99( ±3.74) - 26406( ±4165)/T(K); (R2 = 0.988) \ln \left( {C_{0} K_{\text{dis}}^{ + } } \right) = \ln \,K_{0} - Q/(RT) = 20.99( \pm 3.74) - 26406( \pm 4165)/T(K);\quad (R^{2} = 0.988) . An activation energy of 52.5(±4) kcal/mol for the Fe–Mg exchange process was obtained. The above relation was used to calculate
the following Arrhenius relation modified as a function of X
Fe (in the range of X
Fe = 0.20–0.50):
ln( C0 K\textdis + ) = (21.185 - 1.47X\textFe ) - \frac(27267 - 4170X\textFe )T(K) \ln \left( {C_{0} K_{\text{dis}}^{ + } } \right) = (21.185 - 1.47X_{\text{Fe}} ) - {\frac{{(27267 - 4170X_{\text{Fe}} )}}{T(K)}} . The cooling time constant, η = 6 × 10−1 K−1 year−1 calculated on the PCA82506-3 sample, provided a cooling rate of the order of 1°C/min consistent with the extremely fast late
cooling history of the ureilite parent body after impact excavation. 相似文献
19.
We determined the forward rate constant (K+) for the Fe2+–Mg order–disorder between the M2 and M1 sites of orthopyroxene (OPx), which is described by the homogeneous reaction Fe2+ (M2) + Mg(M1) ↔ Mg(M2) + Fe2+ (M1), by both ordering and disordering experiments at isothermal condition and also by continuous cooling experiments. The
rate constant was determined as a function of temperature in the range of 550–750°C, oxygen fugacity between quartz–fayalite–iron
and Ni–NiO buffers, and at compositions of 16 and 50 mol% ferrosilite component. The K+ value derived from disordering experiment was found to be larger than that derived from ordering experiment at 550°C, while
at T>580°C, these two values are essentially the same. The fO2 dependence of the rate constant can be described by the relation K+ α (fO2)
n
with n=5.5–6.5, which is compatible with the theoretically expected relation. The Arrhenius relation at the WI buffer condition
is given by
20.
Anurag Sharma 《Contributions to Mineralogy and Petrology》1996,125(2-3):263-275
The stability of pargasite in the presence of excess quartz has been determined in the range of 0.5–6.0 kbar and 500–950 °C
in the system Na2O– CaO–MgO–Al2O3–SiO2–H2O, using synthetic minerals. The experimental results from this study indicate the presence of two distinct mineral assemblage
regions: (1) a high temperature supersolidus region containing tremolitic amphibole+melt+quartz; (b) a low temperature subsolidus region consisting of Al-rich amphibole+plagioclase+enstatite+quartz. Compositional reversals have been determined for the
following three equilibria:
(a) 2 pargasite+9 quartz=tremolite+4 plagioclase (An50)+1.5 enstatite+H2O, (b) 2 pargasite+10 quartz=tremolite+4 plagioclase (An50)+talc, and (c) pargasite+diopside+5 quartz=tremolite+2 plagioclase (An50). These experiments indicate a continuous change of amphibole composition from pargasite to tremolite with increasing temperature,
and an opposite effect with increasing pressure. The third equilibria is used to constrain a site-mixing model for the pargasitic
amphiboles, which favor a single-coupled NaA-AlT1 site mixing. The thermochemical data for pargasite estimated from the reversal data of the three equilibrium reactions is
estimated as for ΔG
0
f
,Pg=−12022.11±5.2 kJ mole-1, and S
0
Pg=591.7 ±7.9 JK-1 mole-1.
Received: 31 July 1995/Accepted: 3 June 1996 相似文献
|