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1.
The partitioning of Fe and Mg between coexisting garnet and olivine has been studied at 30 kb pressure and temperatures of 900 ° to 1,400 °C. The results of both synthesis and reversal experiments demonstrate that K D (= (Fe/Mg)gt/(Fe/Mg)OI) is strongly dependent on Fe/Mg ratio and on the calcium content of the garnet. For example, at 1,000 °C/30 kb, K D varies from about 1.2 in very iron-rich compositions to 1.9 at the magnesium end of the series. Increasing the mole fraction of calcium in the garnet from 0 to 0.3 at 1,000 ° C increases K D in magnesian compositions from 1.9 to about 2.5.The observed temperature and composition dependence of K D has been formulated into an equation suitable for geothermometry by considering the solid solution properties of the olivine and garnet phases. It was found that, within experimental error, the simplest kind of nonideal solution model (Regular Solution) fits the experimental data adequately. The use of more complex models did not markedly improve the fit to the data, so the model with the least number of variables was adopted.Multiple linear regression of the experimental data (72 points) yielded, for the exchange reaction: 3Fe2SiO4+2Mg3Al2Si3O12 olivine garnet 2Fe2Al2Si3O12+3Mg2SiO4 garnet olivine H ° (30kb) of –10,750 cal and S ° of –4.26 cal deg–1 mol–1. Absolute magnitudes of interaction parameters (W ij ) derived from the regression are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the following differences between solution parameters and these differences are fairly well constrained: W FeMg ol -W FeMg gt 800 cal W CaMg gt -W CaFe gt 2,670 cal.The geothermometer is most sensitive in the temperature and composition regions where K D is substantially greater than 1. Thus, for example, peridotitic compositions at temperatures less than about 1,300 ° C should yield calculated temperatures within 60 °C of the true value. Iron rich compositions (at any temperature) and magnesian compositions at temperatures well above 1,300 °C could not be expected to yield accurate calculated temperatures.For a fixed K D the influence of pressure is to raise the calculated temperature by between 3 and 6 °C per kbar.  相似文献   

2.
We have measured the mixing properties of Mn-Mg olivine and Mn-Mg garnet at 1300° C from a combination of interphase partitioning experiments involving these phases, Pt-Mn alloys and Mn-Mg oxide solid solutions. Activity coefficients of Mn dilute in Pt-Mn alloys at 1300° C/1 atm were measured by equilibrating the alloy with MnO at known f O 2. Assuming that the log f O 2 of the Mn-MnO equilibrium under these conditions is-17.80 (Robie et al. 1978), we obtain for Mn: logMn = –5.25 + 3.67 XMn + 11.41X2 Mn Mixing properties of (Mn,Mg)O were determined by reversing the Mn contents of the alloys in equilibrium with oxide at known f O 2. Additional constraints were obtained by measuring the maximum extent of immiscibility in (Mn,Mg)O at 800 and 750° C. The data are adequately described by an asymmetric (Mn,Mg)O solution with the following upper and lower limits on nonideality: (a) WMn = 19.9kj/Mol; WMg = 13.7kj/Mol; (b) WMn = 19.9kj/Mol; WMg = 8.2kj/Mol; Olivine-oxide partitioning was tightly bracketed at 1300° C and oxide properties used to obtain activity-composition relations for Mn-Mg olivine. Despite strong M2 ordering of Mn in olivine, the macroscopic properties are adequately described by a symmetric model with: Wol = 5.5 ± 2.5 kj/mol (1-site basis) Using these values for olivine, garnet-olivine partitioning at 27 kbar/1300° C leads to an Mn-Mg interaction parameter in garnet given by: Wgt = 1.5 ± 2.5kJ/mol (1-site basis) Garnet-olivine partitioning at 9 kbar/1000° C is consistent with the same extent of garnet nonideality and the apparent absence of excess volume on the pyrope-spessartine join indicates that any pressure-dependence of WGt must be small. If olivine and garnet properties are both treated as unknown and the garnet-olivine partitioning data alone used to derive WOl and WGt, by multiple linear regression, best-fit values of 6.16 and 1.44 kJ/mol. are obtained. These are in excellent agreement with the values derived from metal-oxide, oxide-olivine and olivine-garnet equilibria.  相似文献   

3.
Summary The crystal structures of a dolomite and of an iron rich ankerite were refined from three-dimensional X-ray data by least squares methods toR0.03. The angle of rotation of the CO3 group is 6.35(5)o in dolomite but only 5.28(5)o in ankerite, a consequence of the different sizes of the (Mg, Fe)O6 octahedra. The carbonate group deviates very slightly from planarity in both minerals. The Ca–O distances are somewhat larger in both minerals than in calcite; the Mg–O distance in dolomite, on the contrary, is somewhat smaller than in magnesite.
Verfeinerung und Vergleich der Kristallstrukturen eines Dolomits und eines Fe-reichen Ankerits
Zusammenfassung Die Kristallstrukturen eines Dolomits und eines eisenreichen Ankerits wurden aus dreidimensionalen Röntgendaten mittels der Methode der kleinsten Quadrate aufR0,03 verfeinert. Der Verdrehungswinkel der CO3-Gruppe ist im Dolomit 6,35(5)o, im Ankerit nur 5,28(5)o; das ist eine Folge der unterschiedlichen Größen der (Mg, Fe)O6-Oktaeder. Die Gestalt der Karbonatgruppe weicht in beiden Mineralen ganz leicht von planar ab. Die Ca–O-Abstände sind in beiden Mineralen etwas größer als im Calcit, der Mg–O-Abstand ist hingegen im Dolomit etwas kleiner als im Magnesit.

With 1 Figure  相似文献   

4.
The development of orthopyroxene-Fe/Mg ferrite symplectites associated with olivine is discussed with respect to the chemical reactions by which they form. Previously proposed reactions are presented graphically and the differences between them are reviewed. With the exception of exsolution, these are all discontinuous reactions in the sense that olivine is replaced by the two-phase symplectite assemblage.Olivine-hosted symplectites developed in the margins of lherzolite xenoliths from Kauai, Hawaii, demonstrate a reaction mechanism which has not been previously documented from natural samples. Original Fo90 olivine in these samples oxidized to a new assemblage consisting of orthopyroxene (En92–95)-Fe/Mg ferrite (Mf35–50) symplectites developed within more magnesian olivine (Fo92–96) hosts. Thus, by this mechanism, olivine of a different composition persists as part of a final three-phase assemblage. As oxidation advanced, the compositions of all three product phases became continuously more magnesian and the stoichiometric coefficients of the orthopyroxene and Fe/Mg ferrite continuously increased, whereas those of the product olivine decreased in the mass-balance equations. These characteristics demonstrate that the reaction was controlled by oxygen diffusion into the xenoliths from the highly oxidized alkali picrite melt in which they were entrained. Thermodynamic calculations suggest that a gradient in oxygen fugacity of 100.9 bars existed across the xenolith rims and resulted in compositional gradients of 4 mol% fayalite and ferrosilite and 15 mol% magnetite.  相似文献   

5.
Calculated phase equilibria among the minerals amphibole, chlorite, clinopyroxene, orthopyroxene, olivine, dolomite, magnesite, serpentine, brucite, calcite, quartz and fluid are presented for the system CaO–FeO–MgO–Al2O3–SiO2–CO2–H2O (CaF-MASCH), with chlorite and H2O–CO2 fluid in excess and for a temperature range of 440°C–600°C and low pressures. The minerals chosen in CaFMASCH represent the great majority of phases encountered in metamorphosed ultramafic rocks. The changes in mineral compositions in terms of FeMg-1 and (Mg, Fe)SiAl-1Al-1 are related to variations in the intensive parameters. For example, equilibria at high in the presence of chlorite involve minerals which are relatively aluminous compared with those at low . The calculated invariant, univariant and divariant equilibria are compared with naturally-occurring greenschist and amphibolite facies ultramafic mineral assemblages. The correspondence of sequences of mineral assemblages and the compositions of the minerals in the assemblages is very good.  相似文献   

6.
Fluid-solid-solid dihedral angles in the NaCl-H2O-CO2-calcite-dolomite-magnesite system have been determined at pressures ranging from 0.5 to 7 kbar and temperatures from 450°C to 750°C. At 1 kbar and 650°C, both dolomite and magnesite exhibit a dihedral angle minimum for intermediate H2O-CO2 fluids similar to that previously determined by the present authors for calcite, but the depth of the minimum is smaller, being above the critical value of 60° for both dolomite and magnesite for all fluid compositions. Calcite-calcite-brine dihedral angles at 650°C have been determined in the pressure range 1–5 kbar. Angles decrease with increasing salt content of the fluid, tending towards a constant value of about 65° for strong brines at pressures above 2 kbar. There is a general increase of angle with increasing pressure which is most marked for strong brines. A positive correlation of angle with pressure is also observed in calcite-H2O-CO2 fluids, the position of the minimum moving towards higher angles and towards H2O-rich fluids with increasing pressure. The permeability window previously observed by the present authors at 1 kbar and intermediate fluid compositions closes at about 1.5 kbar. The results demonstrate that the permeability of carbonates to grain edge fluid flow is only possible at low pressures and for fluids of restricted H2O-CO2-NaCl compositions. However, geochemical evidence from metamorphic terrains suggests that pervasive infiltration does occur under conditions where impermeability is predicted. From examination of published studies of infiltrated carbonates we conclude that deformation plays a critical role in enhancing carbonate permeability. Possible mechanisms for this include shear-enhanced dilatancy (micro-cracking), fluid inclusion drag by deformation-controlled grain boundary migration, and dynamically maintained transient grain boundary fluid films.  相似文献   

7.
In contrast to Ferry (1980) (X Ca)-values in garnet even lower than 0.1 have a significant effect on the calculated equilibrium temperature using the experimental calibration of the Fe and Mg paritioning between garnet and biotite. Garnet compositions and Mg/Fe — distribution coefficients from samples of the Eoalpine staurolite — in zone in the southern Ötztal are related by the quadratic regression equation: InK D= -1.7500 (±0.0226) + 2.978 (±0.5317)X Ca Gt -5.906(±2.359)(X Ca Gt )2 Temperatures derived by the Ferry and Spear (1978) calibration using chemistry — correctedK D values are petrologically realistic.Analysis of our data supports non ideal mixing of grossular with almandine — pyrope solid solution. The derived excess mixing energies are quite small for the almandine — pyrope solution (W FeMg= –133 cal/mole) and about +2775 cal/mole for the difference between pyrope-grossular and almandine-grossular solutions (W MgCa —W FeCa) at metamorphic conditions of 570° C and 5,000 bar. The mixing parameters proposed by Ganguly and Saxena (1984) are not confirmed by our data as they would result in significantly lower temperatures.  相似文献   

8.
Mg–Fe interdiffusion rates have been measured in wadsleyite aggregates at 16.0–17.0 GPa and 1230–1530 °C by the diffusion couple method. Oxygen fugacity was controlled using the NNO buffer, and water contents of wadsleyite were measured by infrared spectroscopy. Measured asymmetric diffusion profiles, analyzed using the Boltzmann–Matano equation, indicate that the diffusion rate increases with increasing iron concentration and decreasing grain size. In the case of wadsleyite containing 50–90 weight ppm H2O, the Mg–Fe interdiffusion coefficients at compositions of Mg/(Mg + Fe)=0.95 in the coarse-grained region (about 60 m) and 0.90 in the fine-grained region (about 6 m) were determined to be a DXmg = 0.95 (m2 s–1)=1.24 × 10–9 exp[–172 (kJ mol–1)/RT] and DXmg = 0.90 (m2 s–1)=1.77 × 10–9 exp[–143 (kJ mol–1)/RT], respectively. Grain-boundary diffusion rates were estimated to be about 4 orders of magnitude faster than the volume diffusion rate. Grain-boundary diffusion dominates when the grain size is less than a few tens of microns. Results for the nominally dry diffusion couple in the present study are roughly consistent with previous studies, taking into account differences in pressure and grain size, although water contents of samples were not clear in previous studies. We observed that the diffusivity is enhanced by about 1 order of magnitude in wadsleyite containing 300–2100 wt. ppm H2O at 1230 °C, which is almost identical to the enhancement associated with a 300 °C increase in temperature. It is still not conclusive that a jump in diffusivity exists between olivine and wadsleyite because water contents of olivine in previous diffusion studies and effects of water on the olivine diffusivity are uncertain.  相似文献   

9.
Published phase diagrams for the siliceous carbonate system CaO–MgO–SiO2–CO2–H2O are contradictory because of different estimates of the relative stability of magnesite. Experimental data on magnesite are too ambiguous to determine the validity of these estimates. Therefore, field evidence is used to select the correct phase diagram topology for siliceous carbonate and carbonate ultramafic rocks at pressures of about 2–5 kbar. The primary selection criterion is provided by the existence of the stable assemblage talc+dolomite+forsterite+tremolite+antigorite, which occurs in the Bergell contact aureole and Swiss Central Alps. Field evidence also is used to argue that the reaction magnesite+quartz=enstatite must occur at lower temperature than the reaction dolomite+quartz=diopside. T-X CO 2 and P CO 2-T phase diagrams consistent with these observations are calculated from experimental and thermo-dynamic data. For antigorite ophicarbonate rocks, remarkable agreement is obtained between the spatial distribution of low variance mineral assemblages and the calculated diagrams.  相似文献   

10.
 The partitioning of Mg and Fe between magnesiowüstite and ringwoodite solid solutions has been measured between 15 and 23 GPa and 1200–1600 ∘C using both Fe and Re capsule materials to vary the oxidation conditions. The partitioning results show a clear dependence on the capsule material used due to the variation in Fe3+ concentrations as a consequence of the different oxidation environments. Using results from experiments performed in Fe capsules, where metallic Fe was also added to the starting materials, the difference in the interaction parameters for the two solid solutions (W FeMg mw−W FeMg ring) is calculated to be 8.5±1 kJ mol−1. Similar experiments performed in Re metal capsules result in a value for W FeMg mw−W FeMg ring that is apparently 4 kJ higher, if all Fe is assumed to be FeO. Electron energy-loss near-edge structure (ELNES) spectroscopic analyses, however, show Fe3+ concentrations to be approximately three times higher in magnesiowüstite produced in Re capsules than in Fe capsules and that Fe3+ partitions preferentially into magnesiowüstite, with K D Fe3+ ring/mw estimated between 0.1 and 0.6. Using an existing activity composition model for magnesiowüstite, a least–squares fit to the partitioning data collected in Fe capsules results in a value for the ringwoodite interaction parameter (W FeMg ring) of 3.5±1 kJ mol−1. The equivalent regular interaction parameter for magnesiowüstite (W FeMg mw) is 12.1±1.8 kJ mol. These determinations take into account the Fe3+ concentrations that occur in both phases in the presence of metallic Fe. The free energy change in J mol−1 for the Fe exchange reaction can be described, over the range of experimental conditions, by 912 + 4.15 (T−298)+18.9P with T in K, P in kbar. The estimated volume change for this reaction is smaller than that predicted using current compilations of equation of state data and is much closer to the volume change at ambient conditions. These results are therefore a useful test of high pressure and temperature equation of state data. Using thermodynamic data consistent with this study the reaction of ringwoodite to form magnesiowüstite and stishovite is calculated from the data collected using Fe capsules. Comparison of these results with previous studies shows that the presence of Fe3+ in phases produced in multianvil experiments using Re capsules can have a marked effect on apparent phase relations and determined thermodynamic properties. Received: 13 September 2000 / Accepted: 25 March 2001  相似文献   

11.
Published experimental data including garnet and clinopyroxene as run products were used to develop a new formulation of the garnet–clinopyroxene geothermometer based on 333 garnet–clinopyroxene pairs. Only experiments with graphite capsules were selected because of difficulty in estimating the Fe3+ content of clinopyroxene. For the calibration, a published subregular‐solution model was adopted to express the non‐ideality of garnet. The magnitude of the Fe–Mg excess interaction parameter for clinopyroxene (WFeMgCpx), and differences in enthalpy and entropy of the Fe–Mg exchange reaction were regressed from the accumulated experimental data set. As a result, a markedly negative value was obtained for the Fe–Mg excess interaction parameter of clinopyroxene (WFeMgCpx = ? 3843 J mol?1). The pressure correction is simply treated as linear, and the difference in volume of the Fe–Mg exchange reaction was calculated from a published thermodynamic data set and fixed to be ?120.72 (J kbar?1 mol?1). The regressed and obtained thermometer formulation is as follows: where T = temperature, P = pressure (kbar), A = 0.5 Xgrs (Xprp ? Xalm ? Xsps), B = 0.5 Xgrs (Xprp ? Xalm + Xsps), C = 0.5 (Xgrs + Xsps) (Xprp ? Xalm), Xprp = Mg/(Fe2+ + Mn + Mg + Ca)Grt, Xalm = Fe/(Fe2+ + Mn + Mg + Ca)Grt, Xsps = Mn/(Fe2+ + Mn + Mg + Ca)Grt, Xgrs = Ca/(Fe2+ + Mn + Mg + Ca)Grt, XMgCpx = Mg/(Al + Fetotal + Mg)Cpx, XFeCpx = Fe2+/(Al + Fetotal + Mg)Cpx, KD = (Fe2+/Mg)Grt/(Fe2+/Mg)Cpx, Grt = garnet, Cpx = clinopyroxene. A test of this new formulation to the accumulated data gave results that are concordant with the experimental temperatures over the whole range of the experimental temperatures (800–1820 °C), with a standard deviation (1 sigma) of 74 °C. Previous formulations of the thermometer are inconsistent with the accumulated data set; they underestimate temperatures by about 100 °C at >1300 °C and overestimate by 100–200 °C at <1300 °C. In addition, they tend to overestimate temperatures for high‐Ca garnet (Xgrs ≈ 0.30–0.50). This new formulation has been tested against previous formulations of the thermometer by application to natural eclogites. This gave temperatures some 20–100 °C lower than previous formulations.  相似文献   

12.
Summary At the northeastern flank of Gebel Yelleq, northern Sinai, pure limestones of Upper Cretaceous age were subjected to a thermal overprint, caused by a c. 80m thick Tertiary olivine dolerite sill. Metasomatic supply of Si, Al, Fe, Mg and Ti was greater to the c. 7m wide upper than to the c. 25m wide lower thermal aureole. The greater width of the lower aureole is possibly due to a longer duration of the thermal overprint at this contact. Mineral assemblages in both aureoles are (from the contact outward):(i) clinopyroxene + garnet ± wollastonite + calcite(ii) garnet ± wollastonite + calcite;(iii) wollastonite + calcite.In places, late stage xenoblasts of apophyllite and witherite overgrow these assemblages. Garnets are grandites to melanites with Grs56–86Adr14–42Sch0–2Sps0–0.2Prp0 in the lower, and Grs29–94Adr5–64Sch0–12Sps0–0.2Prp0–1.7 in the upper aureole. Close to the upper contact, clinopyroxene is virtually pure diopside with X Mg = Mg/(Mg + Fe2+) = 0.97–1.0, whereas clinopyroxenes farther away from the upper contact and in the lower aureole have X Mg-values of 0.49 and 0.53, respectively.The minimum temperatures reached during contact metamorphism in the upper and lower aureole are defined by the lower stability limit of wollastonite. The temperatures are inferred with a calculated T-X(CO2) projection in the system CMASCH and are estimated at c. 290 °C and 380 °C for X(CO2) values of 0.05 and 0.25, respectively. A pressure of roughly 100 bar is estimated for the lower dolerite-limestone contact. As indicated by one-dimensional thermal modelling, a maximum temperature of 695 °C was attained at this contact, assuming a magma temperature of 1150 °C. Further modelling results indicate (i) wollastonite, which occurs first 13 m away from the lower contact, formed at a maximum temperature of c. 575 °C, (ii) there, wollastonite formation lasted for approximately 170 years and, (iii) at the outer rim of the lower aureole, the maximum temperature reached was 480 °C, and temperatures sufficient for wollastonite formation lasted for about 140 years.  相似文献   

13.
The model for the thermodynamic properties of multicomponent pyroxenes (Part I) is calibrated for ortho- and clinopyroxenes in the quadrilateral subsystem defined by the end-member components Mg2Si2O6, CaMgSi2O6, CaFeSi2O6, and Fe2Si2O6. This calibration accounts for: (1) Fe-Mg partitioning relations between orthopyroxenes and augites, and between pigeonites and augites, (2) miscibility gap features along the constituent binary joins CaMgSi2O6-Mg2Si2O6 and CaFeSi2O6-Fe2Si2O6, (3) calorimetric data for CaMgSi2O6-Mg2Si2O6 pyroxenes, and (4) the P-T-X systematics of both the reaction pigeonite=orthopyroxene+augite, and miscibility gap featurs, over the temperature and pressure ranges 800–1500°C and 0–30 kbar. The calibration is achieved with the simplifying assumption that all regular-solution-type parameters are constants independent of temperature. It is predicated on the assumptions that: (1) the Ca-Mg substitution is more nonideal in Pbca pyroxenes than in C2/c pyroxenes, and (2) entropies of about 3 and 6.5 J/K-mol are associated with the change of Ca from 6- to 8-fold coordination in the M2 site in magnesian and iron C2/c pyroxenes, respectively. The model predicts that Fe2+-Mg2+ M1-M2 site preferences in C2/c pyroxenes are highly dependent on Ca and Mg contents, with Fe2+ more strongly preferring M2 sites both in Ca-rich C2/c pyroxenes with a given Fe/(Fe+Mg) ratio, and in magnesian C2/c pyroxenes with intermediate Ca/(Ca+Fe+Mg) ratios.The proposed model is internally consistent with our previous analyses of the solution properties of spinels, rhombohedral oxides, and Fe-Mg olivines and orthpyroxenes. Results of our calibration extend an existing database to include estimates for the thermodynamic properties of the C2/c and Pbca pyroxene end-members clinoenstatite, clinoferrosilite, hedenbergite, orthodiopside, and orthohedenbergite. Phase relations within the quadrilateral and its constitutent subsystems are calculated for temperatures and pressures over the range 800–1700°C and 0–50 kbar and compare favorably with experimental constraints.  相似文献   

14.
The uncommon Mg-rich and Ti-poor Zhaoanzhuang serpentine-magnetite ores within Taihua Group of the North China Craton(NCC) remain unclear whether the protolith was sourced from ultramafic rocks or chemical sedimentary sequences. Here we present integrated petrographic and geochemical studies to characterize the protoliths and to gain insights on the ore-forming processes. Iron ores mainly contain low-Ti magnetite(TiO_2 ~0.1 wt%) and serpentine(Mg#=92.42–96.55), as well as residual olivine(Fo=89–90), orthopyroxene(En=89–90) and hornblende. Magnetite in the iron ores shows lower Al, Sc, Ti, Cr, Zn relative to that from ultramafic Fe-Ti-V iron ores, but similar to that from metamorphic chemical sedimentary iron deposit. In addition, interstitial minerals of dolomite, calcite, apatite and anhydrite are intergrown with magnetite and serpentine, revealing they were metamorphic, but not magmatic or late hydrothermal minerals. Wall rocks principally contain magnesian silicates of olivine(Fo=83–87), orthopyroxene(En=82–86), humite(Mg#=82–84) and hornblende [XMg=0.87–0.96]. Dolomite, apatite and anhydrite together with minor magnetite, thorianite(Th-rich oxide) and monazite(LREE-rich phosphate) are often seen as relicts or inclusions within magnesian silicates in the wall rocks, revealing that they were primary or earlier metamorphic minerals than magnesian silicates. And olivine exists as subhedral interstitial texture between hornblende, which shows later formation of olivine than hornblende and does not conform with sequence of magmatic crystallization. All these mineralogical features thus bias towards their metamorphic, rather than magmatic origin. The dominant chemical components of the iron ores are SiO_2(4.77–25.23 wt%), Fe_2O_3 T(32.9–80.39 wt%) and MgO(5.72–27.17 wt%) and uniformly, those of the wall rocks are also SiO_2(16.34–48.72 wt%), Mg O(16.71–33.97 wt%) and Fe_2O_3 T(6.98–30.92 wt%). The striking high Fe-Mg-Si contents reveal that protolith of the Zhaoanzhuang iron deposit was more likely to be chemical sedimentary rocks. The distinct high-Mg feature and presence of abundant anhydrite possibly indicate it primarily precipitated in a confined seawater basin under an evaporitic environment. Besides, higher contents of Al, Ti, P, Th, U, Pb, REE relative to other Precambrian iron-rich chemical precipitates(BIF) suggest some clastic terrestrial materials were probably input. As a result, we think the Zhaoanzhuang iron deposit had experienced the initial Fe-Mg-Si marine precipitation, followed by further Mg enrichment through marine evaporated process, subsequent high-grade metamorphism and late-stage hydrothermal fluid modification.  相似文献   

15.
Sodic pyroxene (jadeite content X jd=0.1–0.3) occurs locally as small inclusions within, albite porphyroblasts and in the matrix of hematite-bearing quartz schists in the Sanbagawa (Sambagawa) metamorphic belt, central Shikoku, Japan. The sodic, pyroxene-bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene+subcalcic (or sodic) amphibole+phengitic mica+albite+quartz+hematite+titanite±epidote. Spessartine-rich garnet occurs in Mn-rich samples. Sodic pyroxene in epidote-bearing samples tends to be poorer in acmite content (average X Acm=0.26–0.50) than that in the epidote-free samples (X Acm=0.45–0.47). X Jd shows no systematic relationship to metamorphic grade, and is different among the three sampling regions [Saruta-gawa, Asemi-gawa and Bessi (Besshi)]. The average X Jd of the Saruta-gawa samples (0.21–0.29) is higher than that of the Asemi-gawa (0.13–0.17) and Bessi (0.14–0.23). The P-T conditions of the Asemi-gawa and Bessi regions are estimated at 5.5–6.5 kbar, >360°C in the chlorite zone, 7–8.5 kbar, 440±15°C in the garnet zone and 8–9.5 kbar, 520±25°C in the albite-biotite zone. Metamorphic pressure of the Saruta-gawa region is systematically 1–1.5 kbar higher than that of the Asemi-gawa and Bessi regions, and materials of the Saruta-gawa region have been subducted to a level 3–5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the Asemi-gawa and Bessi regions. Pressure slightly increases toward the north (structurally high levels) through the Sanbagawa belt of central shikoku. Two types of zonal structure were observed in relatively coarse-grained sodic pyroxenes in the matrix. One type is characterized by increasing X Jd from core to rim, the other type by decreasing X Jd from core to rim. Both types of zoned pyroxenes show an increase in X Fe 2+[=Fe2+/(Fe2++Mg)] from core to rim. The first type of zoning was observed in a sample from the chlorite zone of lowest grade, whereas the latter occurs in the garnet and albite-biotite zones of higher grade. The contrast in zonal structure implies that dP/dT during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher-grade zones. The estimated dP/dT of the prograde stage of the chlorite zone is 3.2 kbar/100°C, and that of the garnet and albite-biotite zones is -1.8 to 0.9 kbar/100°C. The variation of dP/dT at shallow and deep levels of a subduction system probably reflects the difference of heating duration and/or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle.  相似文献   

16.
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   

17.
The present study from the Sausar Mobile Belt (SMB) in the southern part of the Central Indian Tectonic zone (CITZ) demonstrates how microdomainal compositional variation of a single garnet porphyroblast in a metapelite granulite sample records the different segments of a near complete P-T path of metamorphic evolution. The microdomainal variation is ascribed to the preservation of growth zoning and heterogeneous distribution of diverse inclusion mineral assemblages. Subsequent mineral reactions under changing P/T conditions were controlled by this compositional heterogeneity. Four stages of metamorphic evolution have been deciphered. An early prograde stage (Mo) is implied by the rare presence of staurolite-biotite-quartz and in places of kyanite inclusion assemblages in other metapelite samples, together with the growth zoning preserved in garnet. The peak metamorphism (M1) at ~9.5 kbar, ~850 °C is consistent with the biotite dehydration melting that produced garnet-K-feldspar and granitic leucosomes. This was followed by near isothermal decompression (M2) at ~6 kbar, ~825 °C, during which different garnet segments behaved as separate microscale bulk compositions and decomposed both internally and externally to produce different retrograde mineral assemblages. In the quartz-bearing domain of almandine-rich and grossular-rich garnet core, grossular components in garnet reacted with included sillimanite and quartz to produce coronal plagioclase (XAn=0.90). By contrast, grossular-rich garnet in quartz-absent domain reacted with included sillimanite to produce layered spinelss {XMg (Mg/Mg+Fe2+) = 0.23–0.26}, XAl (Al/Al+Fe3+)=0.71–0.81}-plagioclase (XAn=0.91)-cordierite {XMg (Mg/Mg+Fe2+) = 0.80–0.83} coronas both in the core and inner rim region of garnet. During post-decompression cooling, reactions occurred at about 600 °C (M3), whereby quartz-bearing, sillimanite-absent microdomains of pyrope-rich, grossular-poor garnet outer rim decomposed to form relatively magnesian assemblages of cordierite-anthophyllite and cordierite-biotite-quartz. M2 spinelss decomposed to polyphase domains of spinel-magnetite±högbomite at this stage. Collating the textural and geothermobarometric results, a clockwise P-T path has been deduced. The deduced P-T loop is consistent with a model of crustal thickening due to continental collision, followed by rapid vertical thinning, which appears to be the general feature of the Sausar Mobile Belt. Using model calculations of the preserved growth and diffusion zoning in garnet, we demonstrate rather short-lived nature of this collision orogeny (in the order of 40–60 Ma).Editorial responsibility: W. Schreyer  相似文献   

18.
The kinetic problems associated with the experimental determination of reactions among complex solidsolution phases at low temperatures have hindered our understanding of the phase relations in greenschist facies rocks. In the absence of reliable experimental data, we have used the new, expanded internally-consistent thermodynamic dataset of Holland and Powell (1990), to present calculated phase equilibria for the system CaO–FeO–MgO–Al2O3–SiO2–H2O–CO2 (CaFMASCH) with quartz in eccess, in the range 400°–500°C at low to intermediate pressures, involving the minerals amphibole, chlorite, anorthite, clinozoisite, dolomite, chloritoid, garnet, margarite, andalusite, and calcite. By solving independent sets of non-linear equations formed from equilibrium relationships, we calculate not only the loci of reactions in pressuretemperature-x(CO2) space, but also the compositions of coexisting minerals in terms of the substitutions, FeMg-1 and (Fe,Mg)SiAl-1Al-1. Invariant, univariant and divariant equilibria are calculated and discussed in relation to naturally-occurring greenschist facies metabasic and siliceous dolomitic mineral assemblages. We thus avoid the use of activity-corrected curves so commonly presented in the literature as a substitute for genuine univariant phase diagram boundaries.  相似文献   

19.
Summary The chemical composition of olivine phenocrysts was determined for 13 basaltic samples taken from the central part of the Cameroon Volcanic line to estimate the compositions of the primary olivine phenocrysts and the primary magmas. The Mg/(Mg + Fe) ratios of the olivines attain 0.91, though many of the magnesian olivines are identified on a textural basis as xenocrysts from peridotite inclusions. Most magnesian olivine phenocrysts have Mg/(Mg + Fe) ratios of 0.87–0.88 and NiO contents of 0.32–0.35 wt %. The NiO versus Mg/(Mg + Fe) ratios of the olivines can be used to estimate the compositions of the primary phenocrysts in a range of Mg/(Mg + Fe) = 0.88–0.90. Assuming the Fe(3)/(total Fe) ratios of the magmas are 0.17–0.25, which is estimated from the Fe-Mg partitioning between plagioclase and groundmass, the compositions of the primary magmas were obtained from the Fe2+-Mg partition equilibrium with the primary olivine phenocrysts. The primary magmas of the Cameroonian basalts have 13.5 ± 4.0 wt MgO, which is similar to the compositions of basalts with highest MgO contents reported so far from the Cameroon volcanic line.
Olivin Kristalle in einigen Basalten aus Kamerun: Hinweise auf die Zusammensetzung des primitiven Magmas
Zusammenfassung Die chemische Zusammensetzung von Olivin Phenokristallen wurde in 13 Basalt-Proben, die vom Zentralteil .des Kamerun-Vulkangürtels stammen, bestimmt, um Hinweise auf die Zusammensetzung der primären Olivineinsprenglinge and des primären Magmas zu erlangen. Die Mg/(Mg + Fe) Verhältnisse der Olivine reichen bis 0.91, obwohl viefle dieser Mg-Olivine auf Grund textureller Kriterien als aus Peridotit-Einschlüssen stammende Xenokristalle zu identifizieren sind. Die Mg/(Mg + Fe) Verhältnsse der Hauptmasse der Olivin-Phenokristalle liegen im Bereich von 0.87 bis 0.88, die Ni0 Gehalte zwischen 0.32 und 0.35 Gew %. Die Ni0 gegen Mg/(Mg + Fe) Verhältnisse eignen sich zur Abschätzung der Zusammensetzung der primären Phenokristalle, solange die Mg/(Mg + Fe) Verhältnisse im Bereich von 0.88 bis 0.90 liegen. Die Zusammensetzung des Primärmagmas wurde mittels der Fe2+-Mg-Verteilung zwischen dem Magma und den sich mit dem Magma im Gleichgewicht befindlichen primären Olivinkristallen unter der Annahme eines Fe3+/Fe total Verhältnisses von 0.17–0.25, welches sich aus der Mg-Fe Verteilung zwischen Plagioklas und Grundmasse ergibt, bestimmt. Demnach ergibt sich für das primäre Magma der Kamerun Basalte ein MgO Gehalt von 13.5 + 4.0 Gew%. Dieser Wert ist mit der Zusammensetzung von Basalten mit höchsten MgO-Gehalten, wie sie bisher vom Kamerun-Vulkangürtel berichtet worden sind, vergleichbar.

With 6 Figures  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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