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1.
Ninety-seven mineral phases consisting of ten chloritoids, fifteen epidotes, sixteen garnets, four sphenes, seven rutiles, seven pyroxenes, thirteen blue amphiboles, two green amphiboles, eleven phengites, two paragonites, a mariposite, seven chlorites, and two specimens of albite were obtained from the metamorphic rocks of Île de Groix, and their chemical, physical, optical and X-ray properties determined. The chloritoids are all optically positive, monoclinic polymorphs with large 2V, moderate refractive indices and characterized by high densities. Their fluorine contents have been used to propose a new upper limit for OHF substitution in the chloritoid structure, suggesting that partial pressure of fluorine might modify the stability of chloritoids from that determined in pure H2O. The epidotes belong to the Al-Fe epidote series and are epidote sensu stricto. The almandine-rich garnets and the chloromelanites are metastable relics in the glaucophane schists. The grossular contents of the calcareous schist garnets are believed to have become depressed under high CO2 pressure and the low Tschermak's contents of the pyroxenes are to be explained by equilibria involving epidote at high and low temperature when the Tschermak's components will break down to epidote group minerals. The sphenes contain appreciable amounts of combined water, fluorine substituting for oxygen and aluminium substituting for silicon and titanium. The presence of H3O+ is suspected in a specimen of blue amphibole. The barroisite has a composition between glaucophane and hornblende. On account of its high Fe3+ content it is believed to have formed under higher P O 2 than the blue amphiboles. The paragonites which occur in the ohloritoid veins are unstable in the potassium-rich aluminous schists. The phengites show a tendency towards sericitic composition due to post-glaucophanisation readjustments under the lower pressure conditions of the greenschist facies. Some of the Fe3+ contents of the chlorites are interpreted as due to oxidation of ferrous iron, e.g. 2 [Fe(OH)2]2FeOOH + H2. The minerals show strong chemical control of the host rock and their Mn contents are directly related to those of the minerals from which they have evolved through retrogression.Chloritoids and epidotes that are not associated with garnets contain higher amounts of manganese; similarly, the two blue amphiboles with the highest FeMg ratios were obtained from rocks in which garnet has not appeared. It is therefore believed that ottrelite and piemontite would be stable only at the lowest subfacies of the greenschist facies. Also, the ironrich amphiboles must have evolved from low-grade iron-aluminium chlorites, since on the appearance of garnet in a schist iron-aluminium chlorites react with quartz to give almandine and Mg-rich chlorites. The Fe2+Mg ratios of the blue amphiboles therefore reflect the grade of the original schist in which the minerals formed.  相似文献   

2.
In pelitic rocks, under conditions of low f O 2 and low f H 2 O, the stability of the mineral pair cordierite-garnet is limited by five univariant reactions. In sequence from high pressure and low temperature to high temperature and low pressure these are: cordierite+garnet hypersthene+sillimanite+quartz, cordierite+garnet hypersthene+sapphirine+quartz, cordierite+garnet hypersthene+spinel+quartz and cordierite+garnet olivine+spinel +quartz. In this sequence of reactions the Mg/Mg+Fe2+ ratio of all ferro-magnesian minerals involved decreases continuously from the first reaction to the fifth. The five univariant boundaries delimit a wide P-T range over which cordierite and garnet may coexist.Two divariant equilibria in which the Mg/Mg+ Fe2+ ratio of the coexisting phases are uniquely determined by pressure and temperature have been studied in detail. P-T-X grids for the reactions cordierite garnet+sillimanite+quartz and cordierite+hypersthene garnet+quartz are used to obtain pressure-temperature estimates for several high grade metamorphic areas. The results suggest temperatures of formation of 700–850° C and load pressures of 5–10 kb. In rare occasions temperatures of 950–1000° C appear to have been reached during granulite metamorphism.On the basis of melting experiments in pelitic compositions it is suggested that Ca-poor garnet xenocrysts found in calc-alkaline magmas derive from admixed pelitic rocks and did not equilibrate with the calc-alkaline magma.  相似文献   

3.
Applications of the different formulations of garnet-clinopyroxene geothermometers to crustal granulites and particularly to metamorphosed manganese formations show a wide scatter of estimates. This is primarily related to the uncertainty associated with the formulation of the activity-composition relationship in garnet solid solution besides that in the clinopyroxene solid solution. The largest uncertainty appears to have resulted from the adopted Mg-Mn interaction parameter in garnet which is yet to be experimentally determined. A fresh statistical regression of eleven pairs of garnet and clinopyroxene equilibrated at identical P-T conditions from a manganese formation in India yields W Mg-Mn Gt (=WMn-Mg) value of 1600±500 cal/mols. An extension of the Ellis and Green (1979) geothermometer, known to work well in normal granulites, incorporating terms associated with the activity coefficients of the exchangeable cations, now yields consistent temperature estimation from normal granulites as well as from metamorphosed manganese formations. The geothermometer, however, will not work well for garnet having XFe0.65 and clinopyroxenes containing appreciable Na, Al and/or Fe3+.  相似文献   

4.
Ferric iron contents of coexisting ortho- and clinopyroxene from spinel lherzolite xenoliths were measured with Mössbauer spectroscopy and found to be significant. In orthopyroxene, the range in Fe3+/Fe is from 0.04 to 0.14; in clinopyroxene, the range is from 0.12 to 0.24. Reactions involving coexisting olivine, orthopyroxene, and clinopyroxene, where either the esseneite (CaFe3+ AlSiO6) or the acmite (NaFe3+Si2O6) component in the clinopyroxene is considered, are used to calculate oxygen fugacities. These oxygen fugacities agree well with those calculated with the olivine-orthopyroxene-spinel oxybarometer. Because these reactions do not involve garnet, spinel, or plagioclase, they may be applied to lherzolites to give internally-consistent oxygen fugacities across the pressure-dependent facies boundaries between plagioclase, spinel, and garnet lherzolite. Another application of this method is to predict the Fe3+/Fe in clinopyroxene coexisting with olivine and orthopyroxene given pressure, temperature, , and the compositions of the coexisting phases in either experimental or natural assemblages. At values of equal to those of the synthetic fayalite-magnetite-quartz buffer, for example, 15–35% of the iron in the clinopyroxenes from these xenoliths would be ferric. The simplifying assumption that all Fe is divalent in silicate phases at geologically — reasonable oxygen fugacities must be re-evaluated.  相似文献   

5.
Natural Fe2+, Fe3+-bearing spinel solid solutions from the spinel s.s.-hercynite and gahnite-hercynite series were analyzed and studied by electronic absorption spectroscopy in the spectral range 30000–3500 cm–1 in the temperature and pressure ranges 77 TK 600 and 10–4 PGPa 11.0. Two crystals were light-violet in color (type I) and six green or bluish-green (type II). The spectra of both types of spinels are dominated by an UV-absorption edge near 28000 to 24000 cm–1, depending on the iron contents, and a very intense band system in the NIR centered around 5000 cm–1, which is caused by spin-allowed dd-transition of tetrahedral Fe2+, derived from 5 E5 T2. The strong band is in all spinels studied, split into four sub-bands, which can only be observed in very thin platelets. Between the UV-edge and the high-energy wing of the NIR-band there occur a number of very weak bands in type I spinels while the green type II spinels show some of these with significantly enhanced intensity. The intensity of the very weak bands is nearly independent from temperature. Such bands are attributed to spin-forbidden electronic transitions of IVFe2+. Temperature and pressure dependence of the intensity enhanced bands of spinels type II indicate that they are caused by IVFe2+ and VIFe3+. They are attributed to spin-forbidden transitions 6A1g4A1g, 4Eg, 4T2g and 4T1g of VIFe3+, the two latter being strongly intensified by exchange-coupling interaction with adjacent IVFe2+. The pressure dependence of IVFe2+ dd-band system in the NIR caused by spin-allowed 5 E5 T2 transition noticeably differs from that of octahedral Fe2+, an effect which is attributed to a dynamic Jahn-Teller effect of IVFe2+ in the spinel structure.
Monika Koch-MüllerEmail: Phone: +49-331-288-1492/1402Fax: +49-331-288-1492/1402
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6.
The crystal structure of a synthetic CaFe3+Al-SiO6 pyroxene (20 kb, 1,375° C) with unit cell dimensions a=9.7797(16), b=8.7819(14), c=5.3685(5) Å, =105.78(1), space group C2/c has been refined by the method of least squares to an R-factor of 0.025 based on 812 reflections measured on an automatic single crystal diffractometer. The octahedral M1 site is occupied by 0.82 Fe3+ and 0.18 Al3+. Within the tetrahedral T site, Si4+ (0.50), Al3+ (0.41) and Fe3+ (0.09) ions are completely disordered, although submicroscopic domains with short-range order are very likely. The octahedral site preference energy of the Fe3+ ions with respect to Al3+ ions in CaFe3+AlSiO6 is about 10 kcal/mole, which is much higher than that found in Y3Al x Fe5–2O12 garnets. Topologically the structure of CaFe3+AlSiO6 is intermediate between that of diopside and calcium Tschermak's pyroxene, CaAlAlSiO6. For CaM3+ AlSiO6 clinopyroxenes an increase in the size of the M1 octahedron is accompanied by an increase in the average M2-0, bridging T-0 and 03-03 distances and kinking of the tetrahedral chain.  相似文献   

7.
New experimental data on compositions of garnets in two-pyroxene — garnet assemblages in the system CaO –MgO –Al2O3 –SiO2 (CMAS) are presented for conditions between 1,100 and 1,570° C and 30 to 50 kb. Garnets in these assemblages become less calcic with increasing pressure. Garnet-orthopyroxene barometry (Al-solubility-barometry) pertinent to geobarometry for garnet lherzolites has been evaluated with a set of experimental data covering the range 900 to 1,570° C and 15 to 100 kb. Various formulations of this barometer work well to 75 kb. Phase equilibria are not sufficient to positively verify the thermodynamic validity of any of such models. Empirical garnet-orthopyroxene barometry at least in the system CMAS can be formulated to obtain a pressure estimate without previous temperature estimation (P(kb)=34.4-19.175 1n X Al M1 +17.702 1n X Ca M2 ). The potential application of an analogous garnetclinopyroxene equilibrium is limited because the amount of Ca-Tschermaks in natural clinopyroxenes is usually quite small in garnet lherzolites and many eclogites. The Ca-Mg exchange between garnet and clinopyroxene appears however sufficiently sensitive to pressure to allow calibration of a CMAS barometer. The reaction 3CaMgSi2O6+Mg3Al2Si3O12=3Mg2Si2O6+Ca3Al2Si3O12 has a V o of 3.5 cm3. The total pressure dependency of this reaction is however closer to a theoretical V o of about 5 cm3 when excess volume properties of the phases involved are taken into account. We have calibrated such a barometer (mean error of estimate 2.8 kb) for assemblages with pyrope-rich (py>80) garnets and orthopyroxenes. This may provide the basis for a geobarometer for eclogites from kimberlites.Abbreviations Used in the Text CaTs Ca-tschermak's molecule, CaAl2SiO6 - cpx clinopyroxene - di diopside, CaMgSi2O6 - en enstatite, Mg2Si2O6 - gr grossular, Ca3Al2Si3O12 - gt garnet - MgTs Mg-Tschermak's molecule, MgAl2SiO6 - opx orthopyroxene - px pyroxene - py pyrope, Mg3Al2Si3O12 - a i j activity of component i in phase j - activity coefficient - G(I) molar Gibbs free energy difference of reaction (I) at standard state unless specified otherwise - H(I), (H I) molar enthalpy (difference) of phase (reaction) (I) at standard state unless specified otherwise - S (I), (S I) molar entropy (difference) of phase (reaction) (I) at standard state unless specified otherwise - V o, (V I o) molar volume (difference) of phase (reaction) (I) at standard state - X i j mole fraction of component i in phase j  相似文献   

8.
Three small intrusions in Ketilidian gneisses near Julianehaab comprise sheets and veins of olivine-magnetite-grunerite or magnetite-amphibole rocks partly surrounded by garnetiferous hornblende-biotite granitoid rock. The latter skin locally widens out into diffuse bodies of fayalite-orthopyroxene-quartz syenite or monzonite and biotite granite, which show layering similar to that resulting from gravity settling of crystals. Near the intrusions the country rocks lose their foliation and have been partially melted. Intrusion probably occurred at the close of regional metamorphism 1,750–1,780 m.y. ago, just prior to emplacement of the rapakivi granite suite of South Greenland. The mafic minerals of the intrusions are markedly enriched in iron and, in the case of olivine, orthopyroxene, grunerite and garnet, in manganese as well: olivine Fa90Te5Fo5; orthopyroxene (inverted pigeonite) Ca2Fe77Mn6Mg15; calciferous amphiboles are typically hastingsitic; biotites generally have Fe/Fe+Mg ratios of 0.8; garnets are almandine-grossularite-spessartine mixtures; essentially pure magnetite is the dominant oxide mineral and ilmenite is only moderately manganiferous. Crystallization of the mafic rocks appears to have followed the trend of the quartz-fayalite-magnetite buffer curve from perhaps 800°C to <550°C at pressures, calculated from thermodynamic considerations, of 4 to 5 kb. However, the presence of Mn makes estimates of pressure and temperature uncertain. Comparison with other late- to post-orogenic intrusions—the South Greenland and Finnish rapakivi granite suites, the Labrador adamellite complex and the Pikes Peak batholith of Colorado—reveals both similarities and differences, particularly with respect to mineral parageneses, depth of emplacement and manganese enrichment.  相似文献   

9.
Late Variscan granites intruded Brioverian (Upper Proterozoic) and Lower Paleozoic pelitic sequences to the north of the South Armorican shear zone. In the vicinity of the granites, Brioverian garnet micaschists contain pre/syn-S2 assemblages with garnet + staurolite and post-S2 assemblages with staurolite ± andalusite. Andalusite appeared pre/syn- and post-S2 in garnet-free micaschists. The garnets in the Brioverian micaschists are zoned with increasing Mg and decreasing Mn and Ca from core to inner rim. Only poor garnet zonations occur in Paleozoic hornfelses of enclaves in the Rostrenen granite. The results of a microstructurally controlled application of garnet–biotite geothermometers and garnet–plagioclase geobarometers are similar to P–T trends obtained by the Gibbs method of garnet zonation modelling in the system NCFMnMASH. The P–T paths of a pre/syn-S2 regional metamorphism are clockwise between 500–550°C/8 kbar and 700°C/5 kbar, followed by cooling decompression. They contrast with isobaric contact metamorphism between 500 and 700°C at 2.5–3 kbar in Paleozoic hornfelses. This points to a two-stage Variscan metamorphism with a pre-granitic pressure-dominated event in the Brioverian micaschists, followed by Late Variscan contact metamorphism, and suggests the existence of a pre-granitic tectonic boundary between the micaschists and overlying low-grade sequences.  相似文献   

10.
The authors report a redox profile based on Mössbauer data of spinel and garnet to a depth of 210 km from mantle xenoliths of the northern (N) and southeastern (SE) Slave craton (northern Canada). The profile transects three depth facies of peridotites that form segments of different bulk composition, represented by spinel peridotite, spinel–garnet peridotite, low-temperature garnet peridotite, high-temperature garnet peridotite, and pyroxenite. The shallow, more depleted N Slave spinel peridotite records lower oxygen fugacities compared to the deeper, less depleted N Slave spinel–garnet peridotite, consistent with their different spinel Fe3+ concentrations. Garnet peridotites show a general reduction in log fO2 (FMQ)s with depth, where values for garnet peridotites are lower than those for spinel–garnet peridotites. There is a strong correlation between depletion and oxygen fugacity in the spinel peridotite facies, but little correlation in the garnet peridotite facies. The strong decrease in log fO2 (FMQ) with depth that arises from the smaller partial molar volume of Fe3+ in garnet, and the observation of distinct slopes of log fO2 (FMQ) with depth for spinel peridotite compared to spinel–garnet peridotite strongly suggest that oxygen fugacity in the cratonic peridotitic mantle is intrinsically controlled by iron equilibria involving garnet and spinel.
C. McCammonEmail: Phone: +49-921-553709Fax: +49-921-553769
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11.
This paper attempts to illustrate the chemical variations of metamorphic hornblendes regarding host rocks and prograde variations. Changes related to bulk chemistry (orthoamphibolites) mainly concern Si, Al, Mg, Fetot and Ca. The Mg, Fe2+ and Fe3+ contents of hornblendes are, however, not strictly related to host rook compositions and Mg enrichments are correlated with increasing Fe3+ contents in the amphiboles. Thus, variations of oxygen fugacity may control the Mg contents of the Ca amphiboles studied but this does not show clear relations with the prograde metamorphism. The most sensitive but irregular variation related to the metamorphic conditions is the prograde enrichment of the alkalis into the A vacant position and an increase of the (Na+K)tot/Na+K+Ca ratios of the amphiboles. Increasing Ti and AlIV contents as well as decreasing AlVI concentrations are also, but much less evidently, related to increasing T and P. A variation trend from tschermakitic to edenitic hornblendes may be drawn using Shido's end members calculation; this tendency and the relative deficiency of AlVI contents in the low-grade members suggests that the amphiboles studied were subjected to conditions of a low-pressure metamorphism type. Such a conclusion is in agreement with the occurrence of andalusite-cordierite/sillimanite-cordierite associations in the metapelitic rocks, and the absence of Fe-rich garnet and epidote from the orthoamphibolites of the amphibolite facies at Aracena. Comparisons with Ca amphiboles from other metamorphic areas show, in agreement with various authors, that Abukuma hornblendes are similar to those encountered in high-grade thermal aureoles and tonalitic intrusives but different from the hornblendes of Barrovian metamorphism types.  相似文献   

12.
D.A Carswell 《Lithos》1974,7(2):113-121
The chemistry of the pyroxenes suggests that the garnet lherzolites enclosed in the Norwegian basal gneisses have equilibrated at depths greater than 70 kilometres along an expected sub-continental geotherm. Such depths are somewhat shallower than the apparent depths of origin of most garnet lherzolite xenoliths in kimberlite pipes. Distribution coefficients for Fe2+/Mg2+ and Mn2+/Mg2+ between coexisting clinopyroxenes and garnets support the slightly lower equilibration temperatures deduced for the Norwegian garnet lherzolites compared with the xenolithic garnet lherzolites in kimberlites.The pressure-temperature equilibration conditions deduced for the Norwegian garnet lherzolites (800–1020°C at 22–37 kbs) contrast with previous estimates (625 ± 30° at 14 kbs) for basic eclogite masses in the Norwegian gneisses. This suggests a possible dual paragenesis of the Norwegian eclogites, with the garnet lherzolites being tectonic slices of the sub-continental upper mantle and the basic eclogites deep crustal metamorphic rocks.  相似文献   

13.
Kokchetavite, a new polymorph of K-feldspar (KAlSi3O8), has been identified as micrometer-size inclusions in clinopyroxene and garnet in a garnet-pyroxene rock from the Kokchetav ultrahigh-pressure terrane, Kazakhstan. Kokchetavite has a hexagonal structure with a =5.27(1) Å, c=7.82(1) Å, V=188.09 Å3, Z=1, and is found to be associated with phengite + /-cristobalite (or quartz) + siliceous glass ± phlogopite/titanite/calcite/zircon, occurring as multi-phase inclusions in clinopyroxene and garnet. It is concluded that kokchetavite could not be an exsolution phase in host minerals. Instead, it might be metastably precipitated from an infiltrated K-rich melt during rock exhumation. Alternatively, although less likely, kokchetavite might be derived from dehydration of K-cymrite, which, in turn, was formed at high pressures. In either case, kokchetavite is a metastable polymorph of K-feldspar.  相似文献   

14.
Applying Fe2+–Mg exchange geothermometers to natural samples may lead to incorrect temperature estimates if significant Fe3+ is present. In order to quantify this effect, high-pressure experiments were carried out in a belt apparatus in a natural system close to CFMAS at 5 GPa and 1,100–1,400 °C. The oxygen fugacity in the experiments was at or below the Re–ReO2 buffer. This is at significantly more oxidized conditions than in previous experiments, and, as consequence, higher Fe3+/Fe2+ ratios were generated. The Fe3+ content of garnet in the experiments was quantified by electron microprobe using the flank method. Making the usual assumption that Fetotal = Fe2+, the two-pyroxene thermometer of Brey and Köhler (J Pet 31:1353–1378, 1990) reproduced the experimental temperature to ±35 °C and the garnet–clinopyroxene Fe2+–Mg exchange thermometer of Krogh (Contrib Miner Pet 99:44–48, 1988) overestimated the temperatures on average by only 25 °C. On the other hand, application of the garnet–olivine (O’Neill and Wood in Contrib Miner Pet 70:59–70, 1979) and garnet–orthopyroxene (Harley in Contrib Miner Pet 86:359–373, 1984) exchange geothermometers yielded an underestimation in calculated temperatures of >200 °C. However, making explicit accounting for Fe3+ in garnet (i.e. using only measured Fe2+) leads to a vast improvement in the agreement between calculated and experimental temperatures, generally to within ±70 °C for the garnet–orthopyroxene geothermometer as well as noticeable improvement of calculated temperatures for the garnet–olivine geothermometer. Our results demonstrate that the two-pyroxene and garnet–clinopyroxene thermometers are rather insensitive to the presence of Fe3+ whilst direct accounting of Fe3+ in garnet is essential when applying the garnet–olivine and garnet–orthopyroxene thermometers.  相似文献   

15.
A general model has been developed to calculate changes of 18O of minerals in addition to their composition and modal abundance in metamorphic systems. A complete set of differential equations can be written to describe any chemical system in terms of the variables dP, dT, dX, dM, and d18O (X, M, and 18O refer to the chemical composition, number of moles, and oxygen isotope composition of each phase respectively). This set is composed of the differentials of five subsets of equations: (1) conditions of heterogeneous equilibrium; (2) compositional stoichiometry for each mineral; (3) mass balance for each oxide component; (4) oxygen isotope partitioning between phases; (5) conservation of the oxygen isotope ratio of the system. The variance of the complete set of equations is 2, and changes of 18O, composition, and modal abundance for each mineral can be calculated for arbitrary changes of P and T. Applications to a typical pelitic bulk composition at amphibolite and lower granulite facies conditions suggest that for systems dominated by continuous reactions such as: (a) chlorite + quartz = garnet+H2O; (b) staurolite + biotite = garnet + muscovite + H2O; or (c) garnet + muscovite = sillimanite + biotite, isopleths of mineral 18O are nearly independent of pressure, and have a spacing of about 0.1 per 10–20°C. For nearly discontinuous reactions such as: (d) garnet + chlorite + muscovite = biotite + staurolite+H2O; (e) staurolite + muscovite = biotite + aluminosilicate + garnet+H2O; or (f) muscovite + quartz = sillimanite + K-feldspar+H2O, isopleths of mineral 18O have slopes more nearly parallel to endmember reaction boundaries and 18O of phases can have a greater temperature dependence (e.g., 0.1 per 2°C for reaction d). This behavior results from relatively large amounts of reaction progress for small changes of P or T. However, the calculated exhaustion of a reactant within 0.1–5°C ensures that the predicted effects of such reactions on mineral 18O will not exceed 0.25 for typical bulk compositions. Models that allow for fractional crystallization of garnet suggest that prograde garnet zoning in pelitic assemblages will be relatively smooth until staurolite becomes unstable. At higher temperatures, garnet may develop a step of as much as 0.6 in its core-rim zoning as a result of combined garnet resorption during the continuous reaction garnet + muscovite = sillimanite + biotite and repartitioning of the garnet rim composition to relatively heavy 18O. The models are insensitive to the degree to which garnet fractionally crystallizes and to the isotope fractionation factors used; only extreme changes in modal abundance or bulk composition for a given mineral assemblage can produce significant changes in the predicted trends. In the absence of infiltration, isotopic shifts resulting from net transfer reactions for minerals in typical amphibolite, eclogite, and lower granulite facies metapelites and metabasites are inferred from the models to be 1 or less for 150°C of heating.  相似文献   

16.
Two xenoliths of garnet harzburgite from the Finsch kimberlite, South Africa, have been found to contain diamond. One of the xenoliths has mineral compositions typical of low-T coarse textured garned peridotites, whereas minerals in the other are similar but not identical to most peridotite-suite minerals included in diamonds, especially in the low-CaO content of garnet. Geothermobarometric calculations show both xenoliths equilibrated at temperatures above 1,100°C and pressures>55 kbar, which is near the low-pressure end of the range of equilibration conditions for diamond-free garnet lherzolites and garnet harzburgites from Finsch. The chemistries of the minerals in the two rocks are distinctly different to most of the mineral inclusions in Finsch diamonds. This, as well as the different 13C compositions between xenolith diamonds (-2.8 to-4.6) and diamonds in the kimberlite (generally<-4.3) suggest different origins or sources for the diamonds.  相似文献   

17.
In order to evaluate rates of tectonometamorphic processes, growth rates of garnets from metamorphic rocks of the Tauern Window, Eastern Alps were measured using Rb-Sr isotopes. The garnet growth rates were determined from Rb-Sr isotopic zonation of single garnet crystals and the Rb-Sr isotopic compositions of their associated rock matrices. Garnets were analyzed from the Upper Schieferhülle (USH) and Lower Schieferhülle, (LSH) within the Tauern Window. Two garnets from the USH grew at rates of 0.67 –0.13 +0.19 mm/million years and 0.88 –0.19 +0.34 mm/million years, respectively, indicating an average growth duration of 5.4±1.7 million years. The duration of growth coupled with the amount of rotation recorded by inclusion trails in the USH garnets yields an average shear-strain rate during garnet growth of 2.7 –0.7 +1.2 ×10-14 s-1. Garnet growth in the sample from the USH occurred between 35.4±0.6 and 30±0.8 Ma. The garnet from the LSH grew at a rate of 0.23±0.015 mm/million years between 62±1.5 Ma and 30.2±1.5 Ma. Contemporaneous cessation of garnet growth in both units at 30 Ma is in accord with previous dating of the thermal peak of metamorphism in the Tauern Window. Correlation with previously published pressure-temperature paths for garnets from the USH and LSH yields approximate rates of burial, exhumation and heating during garnet growth. Assuming that theseP — T paths are applicable to the garnets in this study, the contemporaneous exhumation rates recorded by garnet in the USH and LSH were approximately 4 –2 +3 mm/year and 2±1 mm/year, respectively.  相似文献   

18.
Germanate garnets are often used as isostructural analogues of silicate garnets to provide insight into the crystal chemistry and symmetry of the less accessible natural garnet solid solutions. We synthesised two series of germanate garnets at 3 GPa along the joinVIIICa3VI(CaGe)IVGe3O12–VIIICa3VIFe2IVGe3O12 at 900 °C and 1,100 °C. Samples with compositions close to the CaGeO3 end-member consist of tetragonal garnet with a small amount of triclinic CaGe2O5. Samples with nominal compositions between XFe=0.4 and 1.0 consist of a mixture of tetragonal and cubic garnets; whereas, single-phase cubic garnets were obtained for compositions with XFe>1.2 (XFe gives the iron content expressed in atoms per formula unit, and varies between 0 and 2 along the join). Run products which were primarily single-phase garnet were investigated using Mössbauer spectroscopy. Spectra from samples synthesised at 1,100°C consist of one well-resolved doublet that can be assigned to Fe3+ in the octahedral site of the garnet structure. A second doublet, present primarily in samples synthesised at 900°C, can be assigned to Fe2+ at the octahedral sites of the garnet structure. The relative abundance of Fe2+ decreases with increasing iron content. Transmission electron microscopy analyses confirm this tendency and show that the garnets are essentially defect-free. The unit-cell parameters of tetragonal VIIICa3VI(CaGe)IVGe3O3 garnet decrease with increasing synthesis temperature, and the deviation from cubic symmetry becomes smaller. Cubic garnets show a linear decrease of unit-cell parameter with increasing iron content. The results are discussed in the context of iron incorporation into VIIIMg3VI(MgSi)IVSi3O3 majorite.  相似文献   

19.
Na2O contents were determined by electron microprobe analysis in 124 garnets from diamonds, xenoliths of peridotites, eclogites from kimberlitic pipes and metamorphic complexes. Na2O content ranges between 0.01 and 0.22% with the limit of detection at about 0.01%. In the garnets of diamond-bearing eclogites and orange garnets from diamonds a regular increase in the Na2O content has been established, varying from 0.09 to 0.22, as compared to garnets from eclogites of metamorphic complexes (range 0.01 to 0.06). It is assumed that the increased Na2O content in the garnets of eclogites is mainly connected with higher pressure, whereas isomorphism of sodium is connected with the initial stages of the transition from Si4 to Si6 in the garnet structure: CaAlNaSi.The study of the sodium content of garnets has shown that all the orange-coloured garnets from diamonds so far studied are related to eclogite assemblage. Determination of the Na2O content of individual inclusions of chrome pyropes from diamonds permits a conclusion on the type of assemblage (with or without clinopyroxene). Proceeding from these data, the importance of garnet-olivine paragenesis within the stability field of diamond has been revealed.Some clear distinctions in the sodium content of the garnets from xenoliths of the kyanite eclogites from the Zagadochnaya pipe in Yakutia and the Roberts Victor mine in South Africa confirm the relation of these eclogites to different subfacies.A conclusion is drawn as to the possibility of utilizing the Na/Na+Ca distribution in the garnets and pyroxenes of eclogites of especially deep-seated origin as a pressure indicator and to the necessity for experimental testing of the dependence of the distribution of these elements in garnets and pyroxenes on pressure, presumably in the range of 30–100 kbars.  相似文献   

20.
Some inclusions from Salt Lake Crater are essentially single-phase subcalcic clinopyroxenites whose original clinopyroxenes, prior to extensive unmixing, were tschermakitic subcalcic varieties with compositions close to Ca34Mg54Fe12. In addition to copious amounts of orthopyroxene, very minor garnet and spinel also were exsolved from the subcalcic clinopyroxenes.The genesis of the garnet pyroxenite suite at Salt Lake Crater has been examined in terms of three models, namely: (i) cumulates from alkali basaltic magmas; (ii) fractional fusion of basanitic garnet clinopyroxenite; and (iii) anatexis of upper mantle lherzolites. Field, mineralogical, chemical and experimental data collectively favour model (iii) and indicate that the nodules are genetically unrelated to their nephelinitic hosts. The Salt Lake garnet pyroxenites can be closely equated with the garnet pyroxenites in magmatictype layers in certain alpine-type ultramafic massifs and they are also similar to many garnet pyroxenite xenoliths in alkaline volcanics from other localities.Liquids produced by anhydrous partial melting of spinel Iherzolite at pressures of approximately 20 kb commonly have picritic chemistries. The crystallization behaviour of picritic liquids at elevated pressures ( 20 kb) indicates that the initial crystallization products may be either essentially single-phase subcalcic clinopyroxenites (with minimal high pressure fractionation) or a range of olivine-aluminous orthopyroxene-aluminous subcalcic clinopyroxene-garnet-(spinel) assemblages with variable 100 Mg/(Mg+Fe) ratios (when fractionation has been operative). All these assemblages may be subsequently modified by subsolidus exsolution and recrystallization.  相似文献   

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