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Journal ArticleDOI

An experimental study of the effect of Ca upon garnet-clinopyroxene Fe-Mg exchange equilibria

01 Nov 1979-Contributions to Mineralogy and Petrology (Springer-Verlag)-Vol. 71, Iss: 1, pp 13-22
TL;DR: In this paper, a series of basaltic compositions and compositions within the simple system CaO-MgO-FeO-Al2O3-SiO2 have been crystallized to garnet-clinopyroxene bearing mineral assemblages in the range 24-30 kb pressure, 750°-1,300° C temperature.
Abstract: A series of basaltic compositions and compositions within the simple system CaO-MgO-FeO-Al2O3-SiO2 have been crystallized to garnetclinopyroxene bearing mineral assemblages in the range 24–30 kb pressure, 750°–1,300° C temperature. Microprobe analyses of coexisting garnet and clinopyroxene show that K D(Fe2+/MgG+/Fe2+/MgCpx) for the Fe-Mg exchange reaction between coexisting garnet and clinopyroxene is obviously dependent upon the Ca-content and apparently independent of the Mg/(Mg+Fe) content of the clinopyroxene and garnet. The Ca-effect is believed to be due to a combination of non-ideal Ca-Mg substitutions in the garnet and clinopyroxene. Our data and interpretation reconciles previous inconsistencies in the temperature dependence of K D − values determined in experimental studies of simple systems, complex basalt, grospydite and garnet peridotite compositions. Previous differences between the effect of pressure upon K Das predicted from simple system theory (Banno, 1970), and that observed in experiments on multicomponent natural rock compositions (Raheim and Green, 1974a) can now be resolved. We have determined K Das a function of P, T, and X Gt Ca (grossular) and derived the empirical relation $$T\left( {^\circ {\text{K}}} \right) = \frac{{3104X_{{\text{Ca}}}^{{\text{Gt}}} + 3030 + 10.86P\left( {{\text{kb}}} \right)}}{{\ln K_{\text{D}} + 1.9034}}$$ . This empirical relationship has been applied to garnet-clinopyroxene bearing rocks from a wide range of geological environments. The geothermometer yields similar estimates for garnet-clinopyroxene equilibration for neighbouring rocks of different composition and different K Dvalues. In addition, temperature estimates using the above relationship are more consistent with independent temperature estimates based on other geothermometers than previous estimates which did not correct for the Ca-effect. An alternative approach to the above empirical geothermometer was attempted using regular solution models to derive Margules parameters for various solid solutions in garnets and clinopyroxenes. The derived Margules parameters are broadly consistent with those determined from binary solution studies, but caution must be exercised in interpreting them in terms of actual thermodynamic properties of the relevant crystalline solid solutions because of the assumptions which necessarily have to be made in this approach.
Citations
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Journal ArticleDOI
TL;DR: In this article, the uplift history of eclogite in the Dabie Mountains can be subdivided into two stages: (1) fast uplift driven by thrust during continental-continental collision and deep subduction (at 221 Ma) of the continental crust; (2) later gentle uplift with rise of the Dabbie Mountains in the late Jurassic and Cretaceous (at 134 Ma).

987 citations

Journal ArticleDOI
TL;DR: A thorough survey of over 90 granulite terranes or occurrences reveals that over 50% of them record P-T conditions outside the 7.5 ± 1 kbar and 800 ± 50 °C average granulites regime preferred by many authors as discussed by the authors.
Abstract: Although many recent reviews emphasize a uniformity in granulite pressure–temperature (P–T) conditions and paths, granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation. A thorough survey of over 90 granulite terranes or occurrences reveals that over 50% of them record P–T conditions outside the 7.5 ± 1 kbar and 800 ± 50 °C average granulite regime preferred by many authors. In particular, an increasing number of very high temperature (900−1000 °C) terranes are being recognized, both on the basis of distinctive mineral assemblages and geothermobarometry. Petrogenetic grid and geothermobarometric approaches to the determination and interpretation of P–T histories are both evaluated within the context of reaction textures to demonstrate that the large range in P–T conditions is indeed real, and that both near-isothermal decompression (ITD) and near-isobaric cooling (IBC) P–T paths are important. Amphibolite–granulite transitions promoted by the passage of CO2-rich fluids, as observed in southern India and Sri Lanka, are exceptional and not representative of fluid-related processes in the majority of terranes. It is considered, on the contrary, that fluid-absent conditions are typical of most granulites at or near the time of their recorded thermal maxima.ITD granulites are interpreted to have formed in crust thickened by collision, with magmatic additions being an important extra heat source. Erosion alone is not, however, considered to be the dominant post-collisional thinning process. Instead, the ITD paths are generated during more rapid thinning (1−2 mm/yr exposure) related to tectonic exhumation during moderate-rate or waning extension. IBC granulites may have formed in a variety of settings. Those which show anticlockwise P–T histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion, with or without additional crustal extension. IBC granulites at shallow levels (< 5 kbar) may also be formed during extension of normal thickness crust, but deeper-level IBC requires more complex models. Many granulites exhibiting IBC at deep crustal levels may have formed in thickened crust which underwent very rapid (5 mm/yr) extensional thinning subsequent to collision. It is suggested that the preservation of IBC paths rather than ITD paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust, and that hybrid ITD to IBC paths should also be observed.Most IBC granulites, and probably many ITD granulites, have not been exposed at the Earth's surface as a result of the tectonic episodes which produced them, but have resided in the middle and lower crust for long periods of time (100−2000 Ma) following these events. The eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation.

932 citations

Journal ArticleDOI
TL;DR: In this article, new SHRIMP and TIMS zircon ages, 40Ar/39Ar ages, and eclogite locations contribute significantly to our understanding of the ultrahigh-pressure Dabie Shan.

894 citations


Cites methods from "An experimental study of the effect..."

  • ...Temperatures calculated using Fe–Mg exchange [26] between garnet and omphacite inclusions within the garnet suggest peak temperatures of 725oC, while cores of discrete garnet and omphacite grains yield temperatures of 600oC....

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Journal ArticleDOI
22 Feb 1990-Nature
TL;DR: In this article, the authors reported the occurrence of diamonds in situ in crustal rocks: highly retrograded high-pressure metamorphic garnet-pyroxene and pyroxene-carbonate-garnet rocks, biotite gneisses and schists from the Kokchetav massif, northern Kazakhstan, USSR.
Abstract: DIAMONDS commonly occur in kimberlites, lamproites and alluvial sediments derived from these rocks. More recently, diamonds (or their graphite pseudomorphs) have been discovered in ultramafic massifs1 and picrites2. Here we report the occurrence of diamonds in situ in crustal rocks: highly retrograded high-pressure metamorphic garnet–pyroxene and pyroxene–carbonate–garnet rocks, biotite gneisses and schists from the Kokchetav massif, northern Kazakhstan, USSR. The diamonds are cubo-octahedral, averaging 12 μm in size, and occur in zircons, and with euhedral graphite as inclusions in unzoned garnets. We believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite. Mica, rutile, titanite, clinopyroxene, kyanite and zircon also occur as inclusions in garnet, often intergrown with the diamonds. Equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of ⩾40 kbar and temperatures >900–1,000 °C.

782 citations

Journal ArticleDOI
TL;DR: In this article, a fried ice cream model was proposed to explain the rapid processes of both plate subduction and exhumation, with a short residence time of the UHP slab at mantle depths.

767 citations

References
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Journal ArticleDOI

1,797 citations


"An experimental study of the effect..." refers background in this paper

  • ...Eclogites occur in a wide variety of geological environments (e.g. Coleman et al., 1965) and the stability fields of various eclogites have been experimentally determined in several laboratories (e.g., Yoder and Tilley, 1962; Green and Ringwood, 1967; Ito and Kennedy, 1968)....

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Journal ArticleDOI
TL;DR: In this article, a semi-empirical equation of state for the diopside enstatite miscibility gap has been proposed for ortho-and clinopyroxene solid solutions.
Abstract: Simple mixing models have been applied to ortho- and clinopyroxene solid solutions and a semi-empirical equation of state extracted from the available experimental data for the diopside-enstatite miscibility gap. This equation successfully reproduces the miscibility gap over a temperature range of 800 °C to 1700 °C and is apparently also applicable to aluminous pyroxenes in the system CaSiO3-MgSiO3-Al2O3. The effect of iron solubility in the pyroxenes has been calibrated empirically using most of the available experimental data for multicomponent pyroxenes. This semi-empirical model reproduces most of the experimental data within 70 °C. Temperatures calculated for naturally equilibrated Mg-rich two-pyroxene assemblages deviate markedly from those estimated using the thermometer of Wood and Banno (1973). These discrepancies can be attributed to large inaccuracies in the thermometer of Wood and Banno (1973) for Mg-rich compositions.

1,467 citations

Journal ArticleDOI
TL;DR: In this paper, a variety of basaltic compositions have been studied experimentally in the pressure range from 1 bar to 30 kb at temperatures above 1000°C and below the basalt solidus and the mineral assemblages are characterized by coexistence of garnet, clinopyroxene, plagioclase and, commonly, quartz.

818 citations


"An experimental study of the effect..." refers background or methods in this paper

  • ...The simple system glasses and seeds were prepared in appropriate proportions so as to produce under the P, T conditions of the eclogite facies as defined by Green and Ringwood (1967), the following mineral assemblages with increasing Ca-contents of the garnets (Fig....

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  • ...Experimental techniques are similar to those described by Green and Ringwood (1967)....

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  • ...Eclogites occur in a wide variety of geological environments (e.g. Coleman et al., 1965) and the stability fields of various eclogites have been experimentally determined in several laboratories (e.g., Yoder and Tilley, 1962; Green and Ringwood, 1967; Ito and Kennedy, 1968)....

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Journal ArticleDOI
TL;DR: In this article, a comparison of eclogites from different geologic occurrences but with similar bulk compositions demonstrates variation in Ca-Mg partition between coexisting garnet and pyroxene.
Abstract: Eclogites are divisible into three groups based on mode of occurrence: Group A, inclusions in kimberlites, basalts, or layers in ultramafic rocks; Group B, bands or lenses within migmatite gneissic terrains; Group C, bands or lenses within alpine-type metamorphic rocks. The compositions range from olivine basalt for Group A to tholeiitic basalts for Group C. New analytical data on six eclogites from glaucophane schist terrains in California and New Caledonia now permit comparisons among the three eclogite types. The pyrope content of the garnets is distinctive for each group as follows: Group A, greater than 55 per cent py; Group B, 30–55 per cent py; Group C, less than 30 percent py. Pyroxenes coexisting with these garnets also reflect a compositional change related to their occurrence. The jadeite content progressively increases from Group A through Group B, whereas the diopside content decreases. A comparison of eclogites from different geologic occurrences but with similar bulk compositions demonstrates variation in Ca-Mg partition between coexisting garnet and pyroxene. The Ca/Mg ratio increases in garnet and decreases in pyroxene from Group A through Group B eclogites. This obvious difference in the Ca-Mg partition between coexisting garnet-pyroxene in eclogites of the same bulk composition indicates a broad range of pressure-temperature conditions obtained during crystallization. Experimental synthesis of eclogite-like material at high pressures and temperatures demonstrates that some eclogites may form in the earth's mantle, but naturally occurring Group C eclogites have coexisting garnet-pyroxene with distinct Ca/Mg ratios when compared to Group A or B eclogites of similar bulk composition. This difference in the Ca/Mg ratio must reflect the pressure-temperature conditions characterizing the glaucophane schist facies. The formation of eclogites within different metamorphic facies is strong evidence of the divergent pressure-temperature conditions that allow basalts to recrystalhze into garnet-pryoxene rocks. In view of the rather compelling field evidence, it would seem advisable to discontinue the concept of an eclogite metamorphic facies.

638 citations


"An experimental study of the effect..." refers background in this paper

  • ...…that eclogites from different geological environments have different K D values (Kv=(Fe /Mg)a t / (Fe /Mg) cpx where Fe=Fe 2+ (e.g., Banno, 1970; Coleman et al., 1965; Lovering and White, 1969; Mysen and Heier, 1972) and these differences have been interpreted in terms of different P, T…...

    [...]

  • ...Eclogites occur in a wide variety of geological environments (e.g. Coleman et al., 1965) and the stability fields of various eclogites have been experimentally determined in several laboratories (e.g., Yoder and Tilley, 1962; Green and Ringwood, 1967; Ito and Kennedy, 1968)....

    [...]

Journal Article
TL;DR: In this article, the authors compared the structural properties of eight natural garnets (Cr-pyrope, almandine, spessartine, Mngrossular, grossular, uvarovite, goldmanite, and andradite) with that of synthetic pyrope to determine the effects of substituent cations on polyhedral interactions, bond lengths, and angles.
Abstract: Refined structures of eight natural garnets [Cr-pyrope, almandine, spessartine, Mngrossular, grossular, uvarovite, goldmanite, and andradite] are compared with that of synthetic pyrope to determine the effects of the substituent cations on polyhedral interactions,bond lengths, and angles. The Si-O bonds in these garnets constitute two populations whichcan be related to (r!X!), the mean radius of the X dodecahedra] cation. If (r{X}) is lessthan 1.0 A, then Si-O= 1.635 :1:0.005A, but if (r!X\) exceeds 1.0 A, then Si-O = 1.650 to.005 A. However, the X-O and Y-O bond lengths and the O-X-O, O-Y-O, and O-Si-O anglesare linearly dependent on (r{X!) and on (r[Y]) (where (r[Y])= the mean radius of the Y octahedral cation). A multiple linear regression analysis indicates the positional parameters of these garnets to be related to (1'{X} ) and (1'[ V]) using Shannon and Prewitt's effectiveradii as follows: x = 0.006 + 0.022(r{X} > + 0.014(r[Y]) y = 0.051 - 0.023(r{X}) + 0.037(r[Y]) z = 0.643 - 0.009(r{X}) + 0.034(r[Y]). The positional parameters of Fe-pyrope calculated with these equations [x=0.0336; y =0.0491; z=0.6530] are in statistical agreement with the observed [x=0.0339(5); y =0.0491(6);z=0.6535(6)] (Euler and Bruce, 1965). Furthermore, using a cell edge calculated from an equation obtained by regression analysis of 56 well characterized silicate garnets [a=9.04+1.61(r{XD+1.89(r[YJ)], the predicted positional parameters give bond lengthsand angles that are statistically identical with those observed. These equations were used to predict the structural details of over 200 hypothetical cubic silicate garnet compounds by assigning (r{XI) values between 0.80 and 1.50 A and (r[Y]) values between 0.50 and 1.15 A (at 0.05 A intervals). Using criteria based on reasonable 0-0, Si-O, X-O, and Y-O distances, a diagram was prepared in which the structural "stability" field of silicate garnets is delineated as a function of (r{X}) and (r[Y]).

483 citations