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11.
Metamorphism of Greater and Lesser Himalayan rocks exposed in the Modi Khola valley, central Nepal 总被引：3，自引：1，他引：2
Aaron J. Martin Jibamitra Ganguly Peter G. DeCelles 《Contributions to Mineralogy and Petrology》2010,159(2):203-223
Thermobarometric estimates for Lesser and Greater Himalayan rocks combined with detailed structural mapping in the Modi Khola
valley of central Nepal reveal that large displacement thrust-sense and normal-sense faults and ductile shear zones mostly
control the spatial pattern of exposed metamorphic rocks. Individual shear zone- or fault-bounded domains contain rocks that
record approximately the same peak metamorphic conditions and structurally higher thrust sheets carry higher grade rocks.
This spatial pattern results from the kinematics of thrust-sense faults and shear zones, which usually place deeper, higher
grade rocks on shallower, lower grade rocks. Lesser Himalayan rocks in the hanging wall of the Ramgarh thrust equilibrated
at about 9 kbar and 580°C. There is a large increase in recorded pressures and temperatures across the Main Central thrust.
Data presented here suggest the presence of a previously unrecognized normal fault entirely within Greater Himalayan strata,
juxtaposing hanging wall rocks that equilibrated at about 11 kbar and 720°C against footwall rocks that equilibrated at about
15 kbar and 720°C. Normal faults occur at the structural top and within the Greater Himalayan series, as well as in Lesser
Himalayan strata 175 and 1,900 m structurally below the base of the Greater Himalayan series. The major mineral assemblages
in the samples collected from the Modi Khola valley record only one episode of metamorphism to the garnet zone or higher grades,
although previously reported ca. 500 Ma concordant monazite inclusions in some Greater Himalayan garnets indicate pre-Cenozoic
metamorphism. 相似文献
12.
Abstract— Among the calcium‐aluminum‐rich inclusions (CAIs), excess 41K (41K*), which was produced by the decay of the short‐lived radionuclide 41Ca (t1/2 = 0.1 Myr), has so far been detected in fassaite and in two grains of melilites. These observations could be used to provide important constraints on the thermal history and size of the planetesimals into which the CAIs were incorporated, provided the diffusion kinetic properties of K in these minerals are known. Thus, we have experimentally determined K diffusion kinetics in the melilite end‐members, åkermanite and gehlenite, as a function of temperature (900–1200 °C) and crystallographic orientation at 1 bar pressure. The closure temperature of K diffusion in melilite, Tc(K:mel), for the observed grain size of melilite in the CAIs and cooling rate of 10–100 °C/Myr, as calculated from our diffusion data, is much higher than that of Mg in anorthite. The latter was calculated from the available Mg diffusion data in anorthite. Assuming that the planetesimals were heated by the decay of 26Al and 60Fe, we have calculated the size of a planetesimal as a function of the accretion time tf such that the peak temperature at a specified radial distance rc equals Tc(K:mel). The ratio (rc/R)3 defines the planetesimal volume fraction within which 41K* in melilite grains would be at least partly disturbed, if these were randomly distributed within a planetesimal. A similar calculation was also carried out to define R versus tf relation such that 26Mg* was lost from ?50% of randomly distributed anorthite grains, as seems to be suggested by the observational data. These calculations suggest that ?60% of melilite grains should retain 41K* if ?50% of anorthite grains had retained 26Mg*. Assuming that tf was not smaller than the time of chondrule formation, our calculations yield minimum planetesimal radius of ?20–30 km, depending on the choice of planetesimal surface temperature and initial abundance of the heat producing isotope 60Fe. 相似文献
13.
14.
Approximate mixing properties of the end-member components of the quarternary garnet solid solution, (Fe,Mg,Ca,Mn)3Al2Si3O12, have been derived through theoretical analysis of observational data, combined with certain experimental results and crystal chemical considerations. The results suggest that the mixing of pyrope with grossularite, spessartite, and almandine would involve significant positive excess free energies of mixing leading to the critical mixing temperatures of 694±55, 535±140, and 479±63 °C respectively. Spessartite would mix with almandine nearly ideally, and with grossularite with small positive deviation from ideality. The quarternary solution reduces essentially to a ternary mixture of pyrope, grossularite, and almandine + spessartite. The solid solubility relation, and tie line coordinates in this ternary system has been calculated as a function of temperature; the solid solution is found to be intrinsically stable for practically all ternary compositions at 600 °C. 相似文献
15.
The results of recent investigations on the stability limitsof staurolite have been combined together with those of thepresent study to develop a semi-quantitative model of the P–T–fo2–Xrelations of staurolite±quartz±magnetite. Theproblem with respect to the hydroxyl content of staurolite hasbeen analysed; it is concluded that no evidence has yet beenmustered to discount the idealised stoichiometry proposed byNaray-Szabó & Sasvari (1958), at least as a limitingcomposition. The stability limits of staurolite±magnetitehave been calculated from the experimental data for the equilibriainvolving quartz. Also the conditions over which the assemblagecordierite+magnetite+quartz could be stable, as well as a quantitativemodel for the fo2-P stability of almandine ± quartz havebeen deduced theoretically. An analysis is presented of the paragenetic relations of staurolitein common pelitic schists. It is suggested that the formationof staurolite at the expense of either chloritoid or chlorite,rather than the unqualified first appearance of staurolite asproposed by Winkler (1970), should define a ‘staurolite-in’isograd in the range of 500–575 °C. In regional metamorphism,chloritoid, staurolite, and aluminum silicates should, underequilibrium conditions, be unstable relative to almandine ingraphitic pelitic schists involving magnetite (chloritoid/staurolite/Al2SiO6+magnetite+quartzalmandine+O2+H2O).The limits of P-T conditions over which staurolite and cordieritemay coexist in natural assemblages have been deduced; it isrestricted, almost entirely within the field of andalusite,between 500–700 °C, and 2–6 kbars, thus definingthe range of P-T conditions for the ‘low-pressure intermediate’—or ‘Buchan’–type amphibolite facies discussedby Miyashiro (1961). In assemblages involving staurolite andandalusite, cordierite rather than almandine should usuallybe stable; the reverse holds for assemblages involving stauroliteand sillimanite. 相似文献
16.
Equilibrium Compositions of Coexisting Garnet and Orthopyroxene: Experimental Determinations in the System FeO-MgO-Al2O3-SiO2, and Applications 总被引：1，自引：0，他引：1
We have determined the Fe-Mg fractionation between coexistinggarnet and orthopyroxene at 20–45 kb, 975–1400?C,and the effect of iron on alumina solubility in orthopyroxeneat 25 kb, 1200?C, and 20 kb, 975?C in the FMAS system. The equilibriumcompositions were constrained by experiments with crystallinestarting mixtures of garnet and orthopyroxene of known initialcompositions in graphite capsules. All iron was assumed to beFe2+. A mixture of PbO with about 55 mol per cent PbF2 provedvery effective as a flux. The experimental results do not suggest any significant dependenceof KD on Fe/Mg ratio at T 1000?C. The lnKD vs. l/T data havebeen treated in terms of both linear and non-linear thermodynamicfunctional forms, and combined with the garnet mixing modelof Ganguly & Saxena (1984) to develop geothermometric expressionsrelating temperature to KD and Ca and Mn concentrations in garnet. The effect of Fe is similar to that of Ca and Cr3+ in reducingthe alumina solubility in orthopyroxene in equilibrium withgarnet relative to that in the MAS system. Thus, the directapplication of the alumina solubility data in the MAS systemto natural assemblages could lead to significant overestimationof pressure, probably by about 5 kb for the relatively commongarnetlherzolites with about 25 mol per cent Ca+Fe2+ in garnetand about 1 wt. per cent Al2O3 in orthopyroxene. 相似文献
17.
We have developed a thermodynamic model for the determination of the closure temperature (TC) at which the minerals defining an internal isochron in RbSr, or similar, geochronological system were set with a geochronological clock. It is shown that the equilibrium fractionation of87Rb and87Sr between a pair of minerals at TC [KD(87Rb87Sr)C] is given by the ratio of the quantity (87Rb/86Sr) in the two minerals as measured at the present time. KD(87Rb-87Sr), which equals the element distribution coefficient KD(RbSr) under equilibrium condition, can be calibrated as a function of temperature, and compared with the retrieved value of KD(87Rb87Sr)C in a natural pair to obtain TC. The various mineral pairs defining an internal isochron will yield concordant or discordant values of TC depending on whether or not they closed simultaneously with respect to the diffusion of Rb and Sr. Both types of results are expected, and are important in the analyses of the evolutionary history of the host rocks. Preliminary analyses of the published data in the RbSr system suggest a fairly wide range of TC even for the same mineral pair, reflecting differences in the cooling rates and physico-chemical environments of the host rocks. 相似文献
18.
Timothy P. Loomis Jibamitra Ganguly Stephen C. Elphick 《Contributions to Mineralogy and Petrology》1985,90(1):45-51
Data from experimentally-induced diffusion profiles at approximately 40 Kbar, 1,300–1,500° C in spessartine-almandine couples and a pyrope-almandine couple at 40 Kbar, 1,440° C, described in Part I, were used to derive tracer diffusion coefficients (D
*) of Fe, Mn and Mg in garnet. The experimental data were fitted by numerical simulations that model multicomponent, compositionally-dependent difussion, including the effects of nonideal thermodynamic mixing. The simulations use the formalism of irreversible thermodynamics and an eigenvector technique of solution. We were able to fit the asymmetrical spessartine-almandine profiles using constant D
* and either the Darken/Hartley-Crank or Manning-Lasaga models relating D
* and interdiffusion coefficients, and both models yielded D
Mg
*
consistent with the direct measurement of D
Mg
*
in by Cygan and Lasaga (1985) at lower temperatures (750–900° C). The results (equations 4.1–4.3 and Table 1) indicate that D
Fe
*
D
Mg
*
<D
Mn
*
and Q
FeQ
Mg>Q
Mn, where Q is the activation energy. In contrast, the asymmetry of pyrope-almandine profiles is too great to fit with either tracer model assuming constant D
* and indicates that D
Mg
*
is similar to its value in spessartine-almandine couples but D
Fe
*
is an order of magnitude less. The fit also suggests that D
Ca
*
< D
Fe
*
Mg
*
in pyrope-almandine couples. Synthesis of data from the two types of diffusion couples suggests that D
Mg
*
is insensitive to compositional changes, whereas D
Fe
*
is affected by Mn/Mg and Fe/Mg ratios and probably by other factors. These compositional effects on tracer coefficients are compatible with those documented by Morioka (1983) for cation diffusion in olivine. 相似文献
19.
We have determined Cr diffusion coefficients (D) in orthopyroxene parallel to the a-, b-, and c-axial directions as a function temperature at f(O2) corresponding to those of the wüstite-iron (WI) buffer. Diffusion is found to be significantly anisotropic with D(//c) > D(//b) > D(//a), conforming to an earlier theoretical prediction. Increase of f(O2) from WI buffer conditions to 4.5 log unit above the buffer at 950 and 1050 °C leads to decrease of D(Cr) by a factor of two to three, possibly suggesting significant contribution from an interstitial diffusion mechanism. We have used the diffusion data to calculate the closure temperatures (Tc) of the Mn-Cr decay system in orthopyroxene as a function of initial temperature (T0), grain size (a) and cooling rate for spherical and plane sheet geometries. We also present graphical relations that permit retrieval of cooling rates from knowledge of the resetting of Mn-Cr ages in orthopyroxene during cooling, T0 and a. Application of these relations to the Mn-Cr age data of the cumulate eucrite Serra de Magé yields a Tc of 830-980 °C, and cooling rates of 2-27 °C/Myr at Tc and ∼1-13 °C/Myr at 500 °C. It is shown that the cooling of Serra de Magé to the closure temperature of the Mn-Cr system took place at its original site in the parent body, and thus implies a thickness for the eucrite crust in the commonly accepted HED parent body, Vesta, of greater than 30 km. This thickness of the eucrite crust is compatible only with a model of relatively olivine-poor bulk mineralogy in which olivine constitutes 19.7% of the total asteroidal mass. 相似文献
20.
Massimiliano Tirone Jibamitra Ganguly Ralf Dohmen Richard Hervig 《Geochimica et cosmochimica acta》2005,69(9):2385-2398
We determined the diffusion coefficient of Sm in almandine garnet as function of temperature at 1 bar and fO2 corresponding to that of wüstite-iron buffer, and to a limited extent, that of a few other selected rare earth elements in almandine and pyrope garnets. Both garnets were demonstrated to have metastably survived the diffusion annealing at conditions beyond their stability fields. The experimental diffusion profiles were analyzed by secondary ion mass spectrometry, and in addition, by Rutherford back scattering spectroscopy for two samples. Transmission electron microscopic study of an almandine crystal that was diffusion-annealed did not reveal any near-surface fast diffusion path. Using reasonable approximations and theoretical analysis of vacancy diffusion, the experimental data were used to develop an expression of rare earth element (REE) diffusion coefficient in garnet as a function of temperature, pressure, fO2, ionic radius, and matrix composition. Calculation of the closure temperature for the Sm-Nd decay system in almandine garnet in a metamorphic terrain shows very good agreement with that constrained independently. Modeling of the REE evolution in melt and residual garnet suggests that for dry melting condition, the REE pattern in the melt should commonly conform closely to that expected for equilibrium melting. However, for much lower solidus temperatures that would prevail in the presence of a H2O-CO2 fluid, the concentration of light REE in the melt could be significantly lower than that under equilibrium melting condition. A reported core and rim differences in the REE content of a garnet crystal in a mantle xenolith in kimberlite have been reproduced by assuming that the REE zoning was a consequence of entrapment in a magma derived from an external source for ∼32,000 yr before the eruption. 相似文献