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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.
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   

3.
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   

4.
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   

5.
Partition of Fe2+ and Mg between coexisting (Mg, Fe)2SiO4 spinel and (Mg, Fe)SiO3 pyroxene was investigated at pressures 80 and 90 kbar and at temperatures 840 and 1050° C, using tetrahedral-anvil type of high pressure apparatus. Olivine-spinel solid solution equilibria in the system Mg2SiO4-Fe2SiO4 were discussed in the light of the partition reaction. Partition of Fe2+ and Mg in both olivine-spinel and pyroxene-spinel systems can not be regarded as that between ideal solid solutions. By applying the simple solution model for the partition of Fe2+ and Mg, sign of the heat of mixing was estimated to be positive for all olivine, spinel and pyroxene. Relative concentration of Fe2+ in spinel in the pyroxene-spinel system is likely to cause some change in the chemical composition of modified spinel () or spinel () in the transition zone of the mantle. A considerable change is also expected in the transition pressure of to ( + ) and ( + ) to .Presented at the symposium Recent Advances in the Studies of Rocks and Minerals at High Pressures and Temperatures held in Montreal, 1972. Jointly sponsored by the International Mineralogical Association and the Commission on Experimental Petrology.  相似文献   

6.
Some of the garnets in eclogites within the quartzo-feldspathic gneisses of Nordfjord, West Norway, are zoned with higher calcium, iron and manganese in the cores and more magnesium at the rims. The zoning is discussed in terms of the apparent distribution coefficients of Fe2+/Mg between garnet and clinopyroxene (which will be aberrantly high for the garnet cores) and in terms of the metamorphic evolution of the eclogites.Publication nr. 32 in The Norwegian geotraverse project.  相似文献   

7.
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   

8.
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
  相似文献   

9.
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   

10.
This paper describes a suite of peridotite xenoliths. some carrying diamonds at high grades, from the richly diamondiferous early Proterozoic (1180 Ma) Argyle (AK1) lamproite pipe, in northwestern Australia. The peridotites are mostly coarse garnet lherzolites but also include garnet harzburgite, chromite — garnet peridotite, a garnet wehrlite, and an altered spinel peridotite with extremely Cr-rich chromite. In all cases the garnet has been replaced by a kelyphite-like, symplectic intergrowth of Alrich pyroxenes, Al-spinel and secondary silicates. The peridotites have refractory compositions characterized by high Mg/(Mg+Fe) and depletion in lithophile elements (Al2O3 and CaO < 1%, Na2O0.03%) and high field strength cations such as Ti, Zr, Y, and Yb. Olivines have high Mg/(Mg+Fe) (Mg 91–93 ) and, like olivine inclusions in diamonds from the Argyle pipe, contain detectable amounts of Cr2O3 (0.03%–0.07%) but have very low CaO contents (typically 0.04%–0.05%). Enstatites (Mg 92–94 ) have comparatively high Cr2O3 (0.2%–0.45%) and Na2O (up to 0.18%) but very low Al2O3 contents (0.5%–0.7%). Diopsides (Mg 92–94 , Ca/(Ca+Mg+Fe)=0.37–0.43) are Cr-rich (0.7%–1.9% Cr2O3) and have low Al2O3 (0.7%–2.2%) and Na2O (0.5%–1.6%) contents. Many have high K2O contents, typically 0.1%–0.4% but up to 1.3% K2O in one xenolith. The chromite coexisting with former garnet is Mg-and Cr-rich [Mg/(Mg+Fe2+)=0.68–0.72, Cr/(Cr+Al)=0.72–0.79] whereas chromite in the spinel peridotite is even more Cr-rich (65% Cr2O3, Cr/(Cr+Al)=0.85, resembling inclusions in diamond. One highly serpentinized former garnet peridotite contains a Cr-rich (up to 13% Cr2O3) titanate resembling armalcolite but containing significant K2O (1%–2.5%), CaO (0.6%–2.2%), ZrO2 (0.1%–0.8%), SrO (0.1%–0.3%), and BaO (up to 0.58%): this appears to have formed as an overprint of the primary mineralogy. Temperatures and pressures estimated from coexisting pyroxenes and reconstructed garnet compositions indicate that the garnet lherzolites equilibrated at 1140°–1290° C and 5.0–5.9 GPa (160–190 km depth), within the stability field of diamond. Oxygen fugacties within the diamond forming environment are estimated from spinel-bearing assemblages to be reducing, with f O2 between MW and IW. The presence of significant K in the diopsides from the peridotite xenoliths and in diopsides from heavy mineral concentrate from the Argyle pipe implies metasomatic enrichment of the subcontinental lithosphere within the diamond stability field. The P-T conditions estimated for the Argyle peridotites demonstrate that diamondiferous lamproite magmas incorporate mantle xenoliths from similar depths to kimberlites in cratonic settings, and imply that Proterozoic cratonized orogenic belts can have lithospheric roots of comparable thickness to beneath Archaean cratons. These roots lie at the base of the lithosphere within the stability field of diamond. The xenoliths, the calcic nature of chrome pyropes from heavy mineral concentrate, and the diamond inclusion assemblage indicate that the lighosphere beneath the Western Australian lamproites is mostly depleted lherozolite rather than the harzburgite commonly found beneath Archaean cratons. Nevertheless, the dominance of eclogitic paragenesis inclusions in Argyle diamonds indicates a significant proportion of diamondiferous eclogite is also present. The form, mineral inclusion assemblage, and the C-isotopic composition of diamonds in the peridotite xenoliths suggest that disaggregated diamondiferous peridotites are the source of the planar octahedral diamonds which constitute a minor component of the Argyle production. These diamonds are believed to have formed from mantle carbon in reduced, refractory peridotite (Iherzolite-harzburgite) in contrast to the predominant strongly 13C-depleted eclogitic suite diamonds which contain a recycled crustal carbon component. The source region of the lamproites has undergone long-term (2 Ga) enrichment in incompatible elements.  相似文献   

11.
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   

12.
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   

13.
A thermodynamic solution model is developed for minerals whose compositions lie in the two binary systems Mg2SiO4-Fe2SiO4 and Mg2Si2O6-Fe2Si2O6. The formulation makes explicit provision for nonconvergent ordering of Fe2+ and Mg2+ between M1 and M2 sites in orthopyroxenes and non-zero Gibbs energies of reciprocal ordering reactions in both olivine and orthopyroxene. The calibration is consistent with (1) constraints provided by available experimental and natural data on the Fe-Mg exchange reaction between olivine and orthopyroxene ± quartz, (2) site occupancy data on orthopyroxenes including both crystallographic refinements and Mössbauer spectroscopy, (3) enthalpy of solution data on olivines and orthopyroxenes and enthalpy of disordering data on orthopyroxene, (4) available data on the temperature and ordering dependence of the excess volume of orthopyroxene solid solutions, and (5) direct activity-composition determinations of orthopyroxene and olivine solid solutions at elevated temperatures. Our analysis suggests that the entropies of the exchange [Mg(M2)Fe(M1)Fe(M2)Mg(M1)] and reciprocal ordering reactions [Mg(M2)Mg(M1)+ Fe(M2)Fe(M1)Fe(M2)Mg(M1)+Mg(M2)Fe(M1)] cannot differ significantly (± 1 cal/K) from zero over the temperature range of calibration (400°–1300° C). Consideration of the mixing properties of olivine-orthopyroxene solid solutions places tight constraints on the standard state thermodynamic quantities describing Fe-Mg exchange reactions involving olivine, orthopyroxene, pyralspite garnets, aluminate spinels, ferrite spinels and biotite. These constraints are entirely consistent with the standard state properties for the phases-quartz,-quartz, orthoenstatite, clinoenstatite, protoenstatite, fayalite, ferrosilite and forsterite which were deduced by Berman (1988) from an independent analysis of phase equilibria and calorimetric data. In conjunction with these standard state properties, the solution model presented in this paper provides a means of evaluating an internally consistent set of Gibbs energies of mineral solid solutions in the system Mg2SiO4-Fe2SiO4-SiO2 over the temperature range 0–1300° C and pressure interval 0.001–50 kbars. As a consequence of our analysis, we find that the excess Gibbs energies associated with mixing of Fe and Mg in (Fe, Mg)2SiO4 olivines, (Fe, Mg)3Al2Si3O12 garnets, (Fe, Mg)Al2O4 and (Fe, Mg)Fe2O4 spinels, and K(Mg, Fe)3AlSi3O10(OH)2 biotites may be satisfactory described, on a macroscopic basis, with symmetric regular solution type parameters having values of 4.86±0.12 (olivine), 3.85±0.09 (garnet), 1.96±0.13 (spinel), and 3.21±0.29 kcals/gfw (biotite). Applications of the proposed solution model demonstrate the sensitivity of petrologic modeling to activity-composition relations of olivine-orthopyroxene solutions. We explore the consequences of estimating the activity of silica in melts forming in the mantle and we develop a graphical geothermometer/geobarometer for metamorphic assemblages of olivine+orthopyroxene+quartz. Quantitative evaluation of these results suggests that accurate and realistic estimates of silica activity in melts derived from mantle source regions,P-T paths of metamorphism and other intensive variables of petrologic interest await further refinements involving the addition of trace elements (Al3+ and Fe3+) to the thermodynamic formulation for orthopyroxenes.  相似文献   

14.
The exchange of Ca and Mg between olivine and clinopyroxene has been studied in the CFMS system. Experiments were conducted in a piston-cylinder apparatus in the temperature range of 1,100–1,300° C and the pressure range 10–30 kbar. Results confirm the previous suggestion (Adams and Bishop 1982) that this reaction has a sufficiently large V° to be used as a geobarometer in several basic and ultrabasic systems. Experimental results were corrected for compositional effects using recent activity-composition models for quadrilateral pyroxenes and olivines. The corrected results indicate that the exchange reaction has aH 1 bar of 34,900 J, a S° of -7.25J/deg, and a V° of -0.489 J/bar. Corrected results agree well with calculations based on the thermodynamic properties of the endmembers.Application of the olivine-clinopyroxene geobarometer to many systems will require additional calibration of non CFMS components. Preliminary pressure estimates based on simple assumptions about the activity relations of these components have been made for spinel lherzolites from southwestern United States and coarse and porphyroclastic garnet lherzolites from southern Africa. A geotherm calculated from spinel lherzolites near the Rio Grande rift is consistent with a geophysical geotherm based on near-surface heat-flow measurements of 100 mW/m2 or greater. Results on garnet lherzolites yield a southern African geotherm with no inflection which falls at somewhat higher temperatures than pyroxene geotherms calculated for the same area.  相似文献   

15.
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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16.
Garnet-clinopyroxene geothermometry has been applied to numerous rock types from the blueschist and eclogite terrain of the Sesia zone in the Western Alps. Discrepancies in apparent temperatures of up to 600° C have been found suggesting substantial deviation from ideal behaviour of the garnet-clinopyroxene equilibria in certain assemblages. Assemblages containing jadeitic pyroxenes rather than omphacitic or diopsidic pyroxenes yield very low K D (2.0) values and correspondingly high apparent temperatures (> 1,000° C). The garnets coexisting with high-Na pyroxenes have compositions similar to those coexisting with omphacitic pyroxenes. Departure from ideal behaviour of these garnets is likely to be minor and is accomodated in the formulation of the geothermometer. Numerous examples of coexisting garnet-clinopyroxene pairs indicate that there is no obvious relationship of K D with jadeite content in pyroxenes with jadeite content less than 70%, but at higher values, K D varies inversely with the jadeite content. The dependence of K D upon the jadeite content in the high-Na pyroxenes is believed to be due to the preference of M2 sites for Fe2+ in the enstatite-jadeite substitution (Fe2+MgNa–1Al–1). This substitution is usually very limited and has no demonstrable effect upon X Fe Cpx when the M1 (Fe2++Mg) content of the pyroxenes is high as in omphacitic and diopsidic pyroxenes. However, when the M1 (Fe2++Mg) content is low, as in jadeitic pyroxenes, the non-ideality of the enstatite type exchange has a marked effect on X Fe Cpx and consequently on garnet-clinopyroxene calculations.  相似文献   

17.
Iron- and vanadium-bearing kyanites have been synthesized at 900 and 1100° C/20 kb in a piston-cylinder apparatus using Mn2O3/Mn3O4- and MnO/Mn-mixtures, respectively, as oxygen buffers. Solid solubility on the pseudobinary section Al2SiO5-Fe2SiO5(-V2SiO5) of the system Al2O3-Fe2O3(V2O3)-SiO2 extends up to 6.5 mole% (14mole %) of the theoretical end member FeSiO5(V2SiO5) at 900°C/20 kb. For bulk compositions with higher Fe2SiO5 (V2SiO5) contents the corundum type phases M2O3(M = Fe3+, V3+) are found to coexist with the Fe3+(V3+)-saturated kyanite solid solution plus quartz. The extent of solid solubility on the join Al2SiO5-Fe2SiO5 at 1 100°C was not found to be significantly higher than at 900° C. Microprobe analyses of iron bearing kyanites gave no significant indication of ternary solid solubility in these mixed crystals. Lattice constants a 0, b 0, c 0, and V0 of the kyanite solid solutions increase with increasing Fe2SiO5- and V2SiO5-contents proportionally to the ionic radii of Fe3+ and V3+, respectively, the triclinic angles ,, remain constant. Iron kyanites are light yellowish-green, vanadium kyanites are light green. Iron kyanites, (Al1.87 Fe 0.13 3+ )SiO5, were obtained as crystals up to 700 m in length.  相似文献   

18.
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   

19.
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   

20.
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   

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