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1.
Paragneisses of the Ivrea-Verbano zone exhibit over a horizontal distance of 5 km mineralogical changes indicative of the transition from amphibolite to granulite facies metamorphism. The most obvious change is the progressive replacement of biotite by garnet via the reaction: a $${\text{Biotite + sillimanite + quartz }} \to {\text{ Garnet + K - feldspar + H}}_{\text{2}} {\text{O}}$$ which results in a systematic increase in the modal ratio g = (garnet)/(garnet + biotite) with increasing grade. The systematic variations in garnet and biotite contents of metapelites are also reflected by the compositions of these phases, both of which become more magnesian with increasing metamorphic grade. The pressure of metamorphism has been estimated from the Ca3Al2Si3O12 contents of garnets coexisting with plagioclase, sillimanite and quartz. These phases are related by the equilibrium: b $$\begin{gathered} 3 CaAl_2 {\text{Si}}_{\text{2}} {\text{O}}_{\text{8}} \rightleftharpoons Ca_3 Al_2 {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} + 2 Al_2 {\text{SiO}}_{\text{5}} + {\text{SiO}}_{\text{2}} \hfill \\ plagioclase garnet sillimanite quartz \hfill \\ \end{gathered} $$ which has been applied to these rocks using the available data on the mixing properties of plagioclase and garnet solid solutions. Temperature and f H 2O estimates have been made in a similar way using thermodynamic data on the biotite-garnet reaction (a) and the approximate solidus temperatures of paragneisses. Amphibolite to granulite facies metamorphism in the Ivrea-Verbano zone took place in the P-T ranges 9–11 kb and 700–820 °C. The differences in temperature and pressure of metamorphism between g= 0 and g = 1 (5 kms horizontal distance) were less than 50° C and approximately 1 kb. Retrogression and re-equilibration of garnets and biotites in the metapelites extended to temperatures more than 50° C below and pressures more than 1.5 kb below the peak of metamorphism, the degree of retrogression increasing with decreasing grade of the metamorphic “peak”. The pressure and temperature of the peak of metamorphism are not inconsistent with the hypothesis that the Ivrea-Verbano zone is a slice of upthrusted lower crust from the crust-mantle transition region, although it appears that the thermal gradient was too low for the zone to represent a near-vertical section through the crust. The most reasonable explanation of the granulite facies metamorphism is that it arose through intrusion of mafic rocks into a region already undergoing recrystallisation under amphibolite facies conditions. 相似文献
2.
Takeshi Ikeda 《Contributions to Mineralogy and Petrology》1998,132(4):321-335
The garnet-cordierite zone, the highest-grade zone of the Ryoke metamorphic rocks in the Yanai district, SW Japan, is defined
by the coexistence of garnet and cordierite in pelitic rocks. Three assemblages in this zone are studied in detail, i.e. spinel
+ cordierite + biotite, garnet + cordierite + biotite and garnet + biotite, all of which contain quartz, K-feldspar and plagioclase.
The Mg/(Fe + Mg) in the coexisting minerals decreases in the following order: cordierite, biotite, garnet and spinel. Two
facts described below are inconsistent with the paragenetic relation in the K2OFeOMgOAl2O3SiO2H2O (KFMASH) system in terms of an isophysical variation. First, garnet and biotite in the last assemblage have Mg/(Fe + Mg)
higher than those in the second. Second, the first two assemblages are described by the reaction,
while they occur in a single outcrop. The addition of MnO, ZnO and TiO2 to the system can resolve the inconsistencies as follows. The assemblage garnet + biotite can consist of garnet and biotite
higher in Mg/(Fe + Mg) than those in garnet + cordierite + biotite as long as they are enriched in spessartine and depleted
in Al, respectively. The assemblage garnet + cordierite + biotite becomes stable relative to spinel + cordierite + biotite
with increasing spessartine content or decreasing gahnite content and the Ti content of biotite. The constituent minerals
of the assemblages, spinel + cordierite + biotite and garnet + cordierite + biotite, preserve several reaction microstructures
indicative of prograde reactions,
and
together with retrograde reactions,
and
This suggests that the pressure-temperature path of the rocks includes an isobaric heating and an isobaric or decompressional
cooling. The high-grade areas consisting of the K-feldspar-cordierite zone, sillimanite-K-feldspar zone and garnet-cordierite
zone have prograde paths involving isobaric heating and show a southwards increase in pressure with a thermal maximum in the
middle. These high-grade zones are closely associated with the gneissose granitic rocks, suggesting that the Ryoke metamorphism,
one of the typical low-pressure type, is caused by the heat supply from the syn-tectonic granitic rocks that emplaced at the
middle level of the crust.
Received: 22 August 1997 / Accepted: 11 May 1998 相似文献
3.
Dr. Achim Hirschberg Helmut G. F. Winkler 《Contributions to Mineralogy and Petrology》1968,18(1):17-42
The stability relations between cordierite and almandite in rocks, having a composition of CaO poor argillaceous rocks, were experimentally investigated. The starting material consisted of a mixture of chlorite, muscovite, and quartz. Systems with widely varying Fe2+/Fe2++Mg ratios were investigated by using two different chlorites, thuringite or ripidolite, in the starting mixture. Cordierite is formed according to the following reaction: $${\text{Chlorite + muscovite + quartz}} \rightleftharpoons {\text{cordierite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} + {\text{H}}_{\text{2}} {\text{O}}$$ . At low pressures this reaction characterizes the facies boundary between the albite-epidotehornfels facies and the hornblende-hornfels facies, at medium pressures the beginning of the cordierite-amphibolite facies. Experiments were carried out reversibly and gave the following equilibrium data: 505±10°C at 500 bars H2O pressure, 513±10°C at 1000 bars H2O pressure, 527±10°C at 2000 bars H2O pressure, and 557±10°C at 4000 bars H2O pressure. These equilibrium data are valid for the Fe-rich starting material, using thuringite as the chlorite, as well as for the Mg-rich starting mixture with ripidolite. At 6000 bars the equilibrium temperature for the Mg-rich mixture is 587±10°C. In the Fe-rich mixture almandite was formed instead of cordierite at 6000 bars. The following reaction was observed: $${\text{Thuringite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Experiments with the Fe-rich mixture, containing Fe2+/Fe2++Mg in the ratio 8∶10, yielded three stability fields in a P,T-diagram (Fig.1):
- Above 600°C/5.25 kb and 700°C/6.5 kb almandite+biotite+Al2SiO5 coexist stably, cordierite being unstable.
- The field, in which almandite, biotite and Al2SiO5 are stable together with cordierite, is restricted by two curves, passing through the following points:
- 625°C/5.5 kb and 700°C/6.5 kb,
- 625°C/5.5 kb and 700°C/4.0 kb.
- At conditions below curves 1 and 2b, cordierite, biotite, and Al2SiO5 are formed, but no garnet.
4.
The biotite zone assemblage: calcite-quartz-plagioclase (An25)-phengite-paragonite-chlorite-graphite, is developed at the contact between a carbonate and a pelite from British Columbia. Thermochemical data for the equilibrium paragonite+calcite+2 quartz=albite+ anorthite+CO2+H2O yields: $$\log f{\text{H}}_{\text{2}} {\text{O}} + \log f{\text{CO}}_{\text{2}} = 5.76 + 0.117 \times 10^{ - 3} (P - 1)$$ for a temperature of 700°K and a plagioclase composition of An25. By combining this equation with equations describing equilibria between graphite and gas species in the system C-H-O, the following partial pressures: \(P{\text{H}}_2 {\text{O}} = 2572{\text{b, }}P{\text{CO}}_2 = 3162{\text{b, }}P{\text{H}}_2 = 2.5{\text{b, }}P{\text{CH}}_4 = 52.5{\text{b, }}P{\text{CO}} = 11.0{\text{b}}\) are obtained for \(f{\text{O}}_2 = 10^{ - 26}\) . If total pressure equals fluid pressure, then the total pressure during metamorphism was approximately 6 kb. The total fluid pressure calculated is extremely sensitive to the value of \(f{\text{O}}_2\) chosen. 相似文献
5.
Bulk chemical and mineral analyses were carried out on a progressiveseries of low-pressure metamorphic pelites and psammites ofthe Bavarian Forest. The variation of rock compositions in thelower grade (=sillimanite-K-feldspar) zone with coexisting biotite+sillimanite(+K-feldspar+quartz) is essentially the same as that in thehigher grade (=cordierite-K-feldspar) zone with coexisting biotite+cordierite±sillimanite(+K-feldspar+quartz), so that nearly isochemical conditionscan be assumed for the metamorphic processes. The two metamorphiczones are related to each other through the multivariant reaction: biotite+sillimanite+quartz = cordierite+K-feldspar+H2O, but analyses of coexisting biotites and cordierites indicatethat metamorphism continues to increase even within the cordierite-K-feldsparzone. This increase is signalized through a continuous shiftof the 3-phase AFM field cordierite-biotite-sillimanite fromMg-rich to more Fe-rich compositions according to the abovereaction. At the highest grade detected the coexistence of biotite+sillimanitein the presence of quartz+K-feldspar is discontinued in favourof cordierite+garnet. Comparison with other metamorphic areas exhibiting the sameAFM assemblages leads to the tenative conclusion that the shiftdetected here is mainly due to increasing temperatures of metamorphism,whereas increasing pressures would shift the 3-phase AFM fieldin the opposite direction, that is towards more Mg-rich compositions.Thus the position of the biotite-sillimanite-cordierite fieldwithin the AFM plot can be used as indicator of metamorphicconditions in seemingly similar cordierite-sillimanite-biotite-quartz-K-feldspargneisses of variable provenance. Assuming water pressure toequal total pressure the conditions that lead to the cordierite-potashfeldspar zone studied here are estimated as 2-3 kb, 650-700°C. 相似文献
6.
A thermobarometer for sphene (titanite) 总被引：9，自引：0，他引：9
Leslie A. Hayden E. Bruce Watson David A. Wark 《Contributions to Mineralogy and Petrology》2008,155(4):529-540
Sphene and zircon are common accessory minerals in metamorphic and igneous rocks of very different composition from many different
geological environments. Their essential structural constituents, Ti and Zr, are capable of replacing each other to some degree.
In this paper we detail the results of high pressure–temperature experiments as well as analyses of natural sphene crystals
that establish a systematic relationship between temperature, pressure and Zr concentration in sphene. Calibrations of the
temperature and pressure relationships are presented as a thermobarometer. Synthetic sphene crystals were crystallized in
the presence of zircon, quartz and rutile at 1–2.4 GPa and 800–1,000°C from hydrothermal solutions. Crystals were analyzed
for Zr by electron microprobe (EMP). The experimental results define a log-linear relationship between equilibrium Zr content
(ppm by weight), pressure (GPa) and reciprocal absolute temperature:
The incorporation of Zr into sphene was found to be rather sensitive to pressure effects and also to the effects of kinetic
disequilibrium and growth entrapment that result in sector zoning. The Zr content of sphene is relatively insensitive to the
presence of both REEs and F-Al substitutions in sphene. To supplement and test the experimental data, sphenes from seven rocks
of well-constrained origin were analyzed for Zr by both EMP and ion microprobe (IMP). The sphene thermobarometer records crystallization
temperatures that are consistent with independent thermometry. When applied to natural sphene of unknown origin or growth
conditions, this thermobarometer has the potential to estimate temperatures with an approximate uncertainty of ±20°C over
the temperature range of interest (600–1,000°C). The Zr-in-sphene thermobarometer can also be used in conjunction with the
Zr-in-rutile thermobarometer to estimate both pressure and temperature of crystallization.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
相似文献
| Leslie A. HaydenEmail: |
7.
Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer 总被引：11，自引：3，他引：8
Rutile is an important carrier of high field strength elements (HFSE; Zr, Nb, Mo, Sn, Sb, Hf, Ta, W). Its Zr content is buffered in systems with quartz and zircon as coexisting phases. The effects of temperature (T) and pressure (P) on the Zr content in rutile have been empirically calibrated in this study by analysing rutile–quartz–zircon assemblages of 31 metamorphic rocks spanning a T range from 430 to 1,100°C. Electron microprobe measurements show that Zr concentrations in rutile vary from 30 to 8,400 ppm across this temperature interval, correlating closely with metamorphic grade. The following thermometer has been formulated based on the maximum Zr contents of rutile included in garnet and pyroxene:
No pressure dependence was observed. An uncertainty in absolute T of ±50°C is inherited from T estimates of the natural samples used. A close approach to equilibrium of Zr distribution between zircon and rutile is suggested based on the high degree of reproducability of Zr contents in rutiles from different rock types from the same locality. At a given locality, the calculated range in T is mostly ±10°C, indicating the geological and analytical precision of the rutile thermometer. Possible applications of this new geothermometer are discussed covering the fields of ultrahigh temperature (UHT) granulites, sedimentary provenance studies and metamorphic field gradients. 相似文献
8.
Eberhard Seidel Martin Okrusch Hans Kreuzer Helmut Raschka Wilhelm Harre 《Contributions to Mineralogy and Petrology》1976,57(3):259-275
The uppermost unit of the Cretan nappe system contains a variegated series of high-grade metamorphic rocks. In the Léndas area, amphibolites are present characterized by the assemblagewhile associated metapelitic gneisses consist of
Judging from relevant experimental data for the gneiss assemblage including the Fe/Mg distribution on coexisting garnet and cordierite, the P-T conditions of metamorphism are estimated at about 700° C and 5 kb water vapour pressure.K/Ar determinations on hornblendes from three amphibolites yielded cooling ages of 71.3, 71.2, and 71.1 (±1.7) m.y. respectively; biotites from three paragneisses gave 70.2 ± 1.4, 69.7 ± 1.2, and 67.9 ± 1.4 m.y. respectively. Assuming a sealing temperature against argon diffusion of 300° C, for biotite, and 500° C, for hornblende, a cooling rate of 100–200° C/m.y. is calculated. Thus a late Cretaceous (eo-Alpine) metamorphic event is established in the post-Cretaceous nappes of Crete. 相似文献
$$\text{brown}\;\text{hornblende}\;+\;\text{diopside}\;+\;\text{plagioclase}\;\text{(An 50)}$$
$$\text{garnet}\;+\;\text{cordierite}\;+\;\text{biotite}\;+\;\text{sillimanite (andalusite)}\;\pm\;\text{K-feldspar}\;+\;\text{plagioclase (An 40-50)}\;+\;\text{quartz}.$$
9.
TitaniQ: a titanium-in-quartz geothermometer 总被引：21，自引：10，他引：11
Titanium is one of many trace elements to substitute for silicon in the mineral quartz. Here, we describe the temperature dependence of that substitution, in the form of a new geothermometer. To calibrate the “TitaniQ” thermometer, we synthesized quartz in the presence of rutile and either aqueous fluid or hydrous silicate melt, at temperatures ranging from 600 to 1,000°C, at 1.0 GPa. The Ti contents of quartz (in ppm by weight) from 13 experiments increase exponentially with reciprocal T as described by:
Application of this thermometer is straightforward, typically requiring analysis of only one phase (quartz). This can be accomplished either by EPMA for crystallization temperatures above 600°C, or by SIMS for temperatures down to at least 400°. Resulting temperature estimates are very precise (usually better than ±5°C), potentially allowing detailed characterization of thermal histories within individual quartz grains. Although calibrated for quartz crystallized in the presence of rutile, the thermometer can also be applied to rutile-absent systems if TiO2 activity is constrained. 相似文献
10.
Clive M. Rice Richard J. McCoyd Adrian J. Boyce Peter Marchev 《Mineralium Deposita》2007,42(7):691-713
The Madjarovo ore district is centred on the exposed section of a Lower Oligocene volcano and consists of radially disposed
Pb–Zn-precious metal veins and attendant intermediate sulfidation wallrock alteration. Earlier high sulfidation and potassic
porphyry style alterations are found in the centre of the district spatially associated with monzonitic intrusions. The total
duration of all mineralization and alteration was ca. 300 ka. Stable isotope analyses (S, O, H) have been carried out on a
suite of sulfides, sulfates and silicates from the mineralization, high and intermediate sulfidation alterations and a suite
of basement rocks. These data range between the following limits: . We also analysed δD of fluid inclusions in quartz and barite for which we obtained, respectively, the ranges of −43.6 to −78.6 and −58.4 to −67.1‰.
The data show that high sulfidation alteration was dominated by magmatic fluids with minor meteoric water, whereas the fluids
responsible for the intermediate sulfidation alteration were essentially magmatic. The fluids responsible for the intermediate
sulfidation Pb–Zn mineralization were mixed magmatic–meteoric and certainly contained a significant meteoric component. Sulphur
is likely derived from basement and/or igneous sources. The evolution of alteration and mineralization styles from potassic,
porphyry copper style to high sulfidation to intermediate sulfidation can be understood in terms of changing ore fluid composition
resulting from an increasing permeability of the system and an increasingly remote source of magmatic fluid with time. These
changes link directly to the geological evolution of this volcanic centre. 相似文献
11.
During an experimental investigation of the metamorphism of siliceous dolomites the equilibrium data of the heterogeneous bivariant reaction 1 $$3{\text{ dolomite + 4 quartz + 1 H}}_{\text{2}} O \rightleftharpoons + 3 calcite + 3 CO_2 $$ were determined for the total fluid pressures of 1,000, 3,000 and 5,000 bars. The equilibrium conditions were found by experiments in which dolomite, quartz and water react to form talc, calcite and CO2, as well as by experiments with reversible reaction direction. Results are shown on the temperature- \(X_{CO_2 } \) -diagram of Fig. 3. The temperature of formation of talc and calcite depends to a considerable extent on the composition of the CO2-H2O-gas phase; this can be read straight off the isobaric (P f =const.) equilibrium curves in Fig. 3. In addition a strong dependence of the equilibrium temperature on the total pressure P f was established (see Fig. 5). At a total gas pressure of 1,000 bars dolomite and quartz can react, according to the composition of the CO2-H2O-gas phase, to talc and calcite over the whole of the temperature range between about 350° and 490° C. This indicates that at low pressures the formation of talc and calcite takes place in the field of the albite-epidote-hornfels facies. At a pressure of 3,000 bars dolomite and quartz are stable up to about 550° C if the fluid phase is rich in carbon dioxide and correspondingly poor in water. Thus, this paragenesis can occur up to the stability field of staurolite [see annotation (5)] if the partial pressure of CO2 is large. At the higher total gas pressure of 5,000 bars dolomite and quartz react even at medium CO2-concentrations only at about 580° C to give talc and calcite. Therefore it is expected that in regional metamorphism at about 5,000 bars pressure or more the paragenesis dolomite plus quartz exists up to and within the stability field of staurolite and reacts only here to form talc and calcite after reaction (1) or tremolite and calcite after the following reaction (2)1: $$5 dolomite + 8 quartz + 1 H_2 O \rightleftharpoons 1 tremolite + 3 calcite + 7 CO_2 $$ . The exact physico-chemical conditions under which dolomite, quartz and water react on the one hand to form talc, calcite and CO2, and on the other hand to form tremolite, calcite and carbon dioxide, will be discussed later when our experimental investigations on the formation of tremolite are completed. First results were already published in a short note by Metz, Puhan and Winkler (1968). 相似文献
12.
Experimental evidence of decompression melting during exhumation of subducted continental crust 总被引：16，自引：2，他引：14
E. Auzanneau D. Vielzeuf M. W. Schmidt 《Contributions to Mineralogy and Petrology》2006,152(2):125-148
Experiments have been carried out on a metagreywacke at 800, 850 and 900°C, in the pressure range 0.5–5 GPa to locate the solidus and the eclogite/amphibolite facies transition in felsic rocks, identify the nature of the reactions responsible for major mineralogical changes, and determine the proportions of phases as a function of pressure. The mineral assemblage phengite + clinopyroxene + garnet + quartz/coesite is stable above 2.3 GPa while biotite + plagioclase + garnet + quartz is stable below 2 GPa. The model reaction for the eclogite/amphibolite facies transition in metagreywackes is:
with melt on the low pressure–high temperature side of the reaction. The modal proportion and calcium content of garnet change with pressure. Both decrease from 5 to 2.5 GPa, then increase at the eclogite/amphibolite facies transition, and finally decrease with decreasing pressure below 2.3 GPa. The grossular content in garnet is thus a potential marker of the eclogite/amphibolite facies transition during retrogression. The modal proportion of melt progressively increases with decreasing pressure from 5 to 2.5 GPa, then shows a sudden and marked increase between 2.5 and 2.3 GPa, and finally decreases between 2.3 and 1 GPa. Thus, a melting pulse occurs at the eclogite/amphibolite facies transition during decompression of subducted continental crust. A survey of the main UHP metamorphic regions and the P–T paths followed during their geotectonic history indicates that partial melting may have played a role during their exhumation. A striking feature of retrogressed UHP felsic rocks is that garnet rims are commonly enriched in grossular. Our experiments explain this observation and demonstrate that a grossular-rich growth zone in garnet is not necessarily indicative of highest pressures reached during metamorphism but may correspond to a decompression stage.
相似文献
| E. AuzanneauEmail: |
13.
Stefano Poli 《Contributions to Mineralogy and Petrology》1991,106(4):399-416
The transition from feldspar amphibolite to eclogite is a very wide P-T field that extends from some-where close to 5 kbar where the garnet-amphibole pair starts to appear, to 10–20 kbar at albite-out reaction, then up to 25–30 kbar where an hydrated phase such as amphibole can be stable with pyroxene and garnet. Thus the assemblage garnet (py)+ amphibole (tr)+epidote (cz)±plagioclase (ab)±clinopyroxene (di)±quartz (qz)±fluid is commonly reported in a large number of metamorphic terrains. These mineral phases are complex solid-solutions which adapt to variations in environmental conditions mainly by means of continuous reactions. The reaction space, introduced by. Thompson in 1982a, provides a very elegant and powerful tool to approach these high-variance assemblages. The reactions:
|
相似文献
14.
矿物或流体包体可以有效保存寄主矿物生长过程中的化学环境、温度、压力等信息。在高压变质过程中,石榴子石中石英包体的分子振动与环境压力之间具有良好的相关性。根据这一特性,前人提出了石英包体拉曼偏移压力计,其原理是利用显微激光拉曼光谱仪分析石英包体在室温常压下保留的残余压力,再结合石英与寄主矿物的弹性物理特性,恢复石英包体被捕获时的压力,属于矿物物理光谱学压力计,本文介绍了利用石英包体的拉曼位移计算变质压力的基本原理和方法,并对不同方法的适用条件和使用局限做了简要分析。石英包体拉曼偏移压力计是恢复变质岩石形成温度和压力条件的有效方法,具有广泛的应用前景。 相似文献
15.
Chung-Cherng Lin 《Physics and Chemistry of Minerals》2013,40(2):157-166
The elastic properties of calcite have been determined by Brillouin spectroscopy for temperatures up to 600 °C. The results reveal that the variations of the aggregate bulk (K VRH) and shear (G VRH) moduli of calcite with respect to temperature can be approximately expressed as follows: $$\begin{aligned} K_{{{\text{VRH}}}} ({\text{GPa}}) & = 79.57-0.0230\;T\, (T\;{\text{in}}\;^{^\circ } {\text{C}}) \\G_{{{\text{VRH}}}} ({\text{GPa}}) & = 32.23 - 0.0097\;T. \\\end{aligned}$$ This indicates a nearly constant Poisson’s ratio (0.322) for calcite from 22 to 600 °C. A further analysis shows that the compressibility along the c axis (β ||) and that perpendicular to the c axis have comparable contributions to the volume compressibility of calcite, although the contribution of β || decreases with an increase in the temperature. 相似文献
16.
Progressive sequence of reactions of the Ryoke metamorphism in the Yanai district, southwest Japan: the formation of cordierite 总被引：2，自引：0，他引：2
T. IKEDA 《Journal of Metamorphic Geology》1998,16(1):39-52
The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low- to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K-feldspar have a higher K component in an A'KF diagram than those in rocks lacking K-feldspar. This fact reflects an increase in the K
At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low-pressure type metamorphism. Some textures in one of the high-grade areas, the K-feldspar-cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K-feldspar+water and andalusite=sillimanite.quartz=cordierite+K-feldspar+water This implies that this zone probably has a P–T path involving isobaric heating. 相似文献 17.
A phengite-talc-chloritoid-chlorite-kyanite-quartz assemblage is reported from a nearly undeformed quartz-rich metapelite found in the Monte Rosa massif (Western Alps). Chloritoid contains up to 74 mol % of the Mg end member and is the most magnesian ever reported. Textural relationships and mineral compositions suggest equilibrium and therefore a low-variance assemblage which represents the high-pressure stability limit of chlorite+quartz according to the terminal reaction $${\text{chlorite + quartz }} \rightleftarrows {\text{ talc + chloritoid + kyanite + H}}_{\text{2}} {\text{O}}{\text{.}}$$ Mineral compositions combined with new experimental data on the stability of the Mg-chloritoid end member lead, for a temperature close to 500° C, to a pressure estimate of 16 kbar and a water activity of 0.6 which is supported by fluid inclusions study. Chloritoid composition is in fact a fine metamorphic indicator which opens new ways for barometry in high-grade blueschists. It demonstrates here the existence of a high-pressure metamorphism in the Monte Rosa massif. The assemblage remained mineralogically unaffected during the subsequent lower-pressure evolution. Two size fractions of the single phengite generation were analysed by the 39Ar-40Ar incremental release method. Both spectra are identical with a plateau at 110±3 Ma representing over 96% of the 39Ar degassed. The ages of the first heating steps are discordant and increase with increasing temperature from values near 70 Ma to the plateau age. Isotope correlation diagrams show two 36Ar components, one released at high temperature and correlated with 40Ar and 39Ar, the other released at low temperature in a mixture of atmospheric argon and of a loosely held argon of 70 Ma apparent age. The 110 Ma plateau age may reflect the presence of homogeneously incorporated excess argon, the 70 Ma value might then be a true age. However we favour the alternative hypothesis that the 110 Ma plateau age is a true age, implying that the internal crystalline massifs of the Western Alps have endured high-pressure metamorphism as early as mid-Cretaceous. Whatever the interpretation chosen, the preserved high-pressure mineral assemblage remained isotopically unaffected during the low-pressure mid-Tertiary event which is recorded by the 37 Ma plateau age of phengite from a foliated, recrystallised quartzite collected in the same, westernmost part of the massif. The contrasting behaviour of the two samples shows that even at temperatures as high as 400–450° C deformation and recrystallisation are also major controlling factors of isotope mobility. 相似文献
18.
Dariusz Dobrzyński 《Aquatic Geochemistry》2007,13(3):197-210
A wide set of aqueous chemistry data (574 water analyses) from natural environments has been used to testify and validate
of the solubility of synthetic hydroxyaluminosilicate (HASB), Al2Si2O5(OH)4. The ground and surface waters represent regolith and/or fissure aquifers in various (magmatic, sedimentary and metamorphic)
bedrocks in the Sudetes Mts. (SW Poland). The solubility of HASB in natural waters was calculated using the method proposed by Schneider et al. (Polyhedron 23:3185–3191, 2004). Results confirm
usefulness and validity of this method. The HASB solubility obtained from the field data (logKsp = −44.7 ± 0.58) is lower than it was estimated (logKsp = −40.6 ± 0.15) experimentally (Schneider et al. Polyhedron 23:3185–3191, 2004). In the waters studied the equilibrium with
HASB is maintained at pH above 6.7 and at [Al3+] ≤ 10−10. Silicon activity (log[H4SiO4]) ranges between −4.2 and −3.4. Due to the calculation method used, the Ksp mentioned above cannot be considered as a classical solubility constant. However, it can be used in the interpretation of
aluminium solubility in natural waters. The HASB has solubility lower than amorphous Al(OH)3, and higher than proto-imogolite. From water samples that are in equilibrium with respect to HASB, the solubility product described by the reaction,
is calculated to be logKsp = 14.0 (±0.7) at 7°C. 相似文献
19.
Wilhelm Johannes 《Contributions to Mineralogy and Petrology》1984,86(3):264-273
The beginning of melting in the system Qz-Or-Ab-An-H2 O was experimentally reversed in the pressure range
kbar using starting materials made up of mixtures of quartz and synthetic feldspars. With increasing pressure the melting
temperature decreases from 690° C at 2 kbar to 630° C at 17 kbar in the An-free alkalifeldspar granite system Qz-Or-Ab-H2O. In the granite system Qz-Or-Ab-An-H2O the increase of the solidus temperature with increasing An-content is only very small. In comparison to the alkalifeldspar
granite system the solidus temperature increases by 3° C (7° C) if albite is replaced by plagioclase An 20 (An 40). The difference
between the solidus temperatures of the alkalifeldspar granite system and of quartz — anorthite — sanidine assemblages (system
Qz-Or-An-H2O) is approximately 50° C.
With increasing water pressures plagioclase and plagioclase-alkalifeldspar assemblages become unstable and are replaced by
zoisite+kyanite+quartz and zoisite+muscovite-paragonitess +quartz, respectively. The pressure stability limits of these assemblages are found to lie between 6 and 16 kbar at 600°
C. At high water pressures (10–18 kbar) zoisite — muscovite — quartz assemblages are stable up to 700 and 720° C. The solidus
curve of this assemblage is 10–20° C above the beginning of melting of sanidine — zoisite — muscovite — quartz mixtures.
The amount of water necessary to produce sufficient amounts of melt to change a metamorphic rock into a magmatic looking one
is only small. In case of layered migmatites it is shown that 1 % of water (or even less) is sufficient to transform portions
of a gneiss into (magmatic looking) leucosomes. High grade metamorphic rocks were probably relatively dry, and anatectic magmas
of granitic or granodioritic composition are usually not saturated with water. 相似文献
20.
The chemical potential of oxygen (µO2) in equilibrium with magnesiowüstite solid solution (Mg, Fe)O and metallic Fe has been determined by gas-mixing experiments at 1,473 K supplemented by solid-cell EMF experiments at lower temperatures. The results give:
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