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David M. Jenkins - One of the best experts on this subject based on the ideXlab platform.
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Experimental Investigation of the Upper Thermal Stability of Mg-rich Actinolite; Implications for Kiruna-Type Iron Deposits
Journal of Petrology, 2007Co-Authors: Haroldo Lledó, David M. JenkinsAbstract:The occurrence of Actinolite in magnetite deposits of possible mag-matic origin has prompted an experimental investigation of theupper thermal stability of Mg-rich Actinolite to determine how thestability of Actinolite changes with increasing Fe content.Experiments were carried out primarily on the compositionalre-equilibration of natural tremolite [molar Fe/(FeþMg)¼Fe-number ¼0 014] in the presence of synthetic clinopyroxene(Ca
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Cation ordering in synthetic low-calcium Actinolite
American Mineralogist, 2005Co-Authors: James Driscall, David M. Jenkins, M. Darby Dyar, Krassimir N. BozhilovAbstract:A series of low-Ca (1.8–1.65 atoms Ca per formula unit) Actinolites were synthesized in the system CaO-FeO-MgO-SiO2-H2O with bulk compositions of 0, 10, 20, and 30 mol% ferro-Actinolite when projected onto the tremolite–ferro-Actinolite join for the purpose of determining the cation site occupancies as a function of bulk composition. Syntheses were done by multiple treatments of the oxide-metal mixtures in internally heated gas vessels at 600–800 °C, 6 kbar, f O2 near the Co-CoO buffer, which is sufficiently low to prevent the appearance of Fe3+ in this study, and for total durations up to 774 hours. Bulk compositions of the synthesis products were assessed by electron microprobe analysis (EMPA), analytical transmission electron microscopy (AEM), and by using unit-cell dimensions (CD method), whereas the site occupancies were assessed using powder X-ray diffraction (XRD) Rietveld refinements, Fourier-transform infrared (FTIR) spectroscopy, and Mossbauer spectroscopy. Selected-area electron-diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) images were obtained to study the structure and defects of the samples. A maximum of about 2% non-amphibole chain-multiplicity faults (CMFs) were observed in the sample with 10 mol% ferro-Actinolite component, indicating the strong reduction in CMFs that occurs with only a small addition of ferro-Actinolite to tremolitic amphiboles. This study, as with earlier studies, found no significant preferential partitioning of Fe and Mg relative to the bulk Fe content of the sample at the M1 and M3 sites, weak partitioning of Mg relative to Fe into the M2 site, and strong partitioning of Fe relative to Mg at the M4 site. No ferric iron was detected. The partitioning of Fe and Mg at the M4 site was modeled by a simple ideal-activity, two-site exchange reaction using the Actinolites from this study and those of previous studies with bulk Fe/(Fe + Mg) 0.3 and with low Al, Mn, and Na contents using 1 bar and 298K values of Δ G ° = 62 kJ and Δ S ° = 54 J/K. The FTIR data were analyzed for the presence of short-range order of Fe and Mg at the M1 and M3 sites but none was found.
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Stability and thermodynamic properties of ferro-Actinolite: A re-investigation
American Journal of Science, 2003Co-Authors: David M. Jenkins, Krassimir N. BozhilovAbstract:A re-investigation of the synthesis, characterization, and upper-thermal stability of ferro-Actinolite has been performed in the system CaO-FeO-SiO2-H2O. Synthesis experiments were done over the range of 2 to 10 kbar and 420° to 600°C for several bulk compositions along the join Ca2Fe5Si8O22(OH)2 - Fe7Si8O22(OH)2 and at oxygen fugacities (fO2) defined by the vessel (∼Ni-NiO), Co-CoO, and iron-magnetite (IM) buffers. The highest yields of Actinolite were obtained at 2 kbar, 420°C, and at the relatively oxidizing conditions of the Ni-NiO buffer. Grinding and retreatment of the sample was found to be important for increasing the yield of amphibole up to a maximum of about 70 weight percent; however, complete yields of Actinolite were never obtained indicating some physical barrier or incorrect chemical (fO2) conditions preventing complete reaction. The highest-yield Actinolite syntheses were characterized using a high-resolution transmission electron microscope (TEM) equipped with an energy-dispersive spectrometer. A survey of representative grains using the TEM revealed the presence of very few chain-multiplicity faults (A′(2) = 0.98 –0.99); however, a rather large spread of compositions was observed, with crystals having Ca contents that range from as little as 0.5 up to 2.0 Ca atoms per formula unit (apfu). There was no clear correlation between Actinolite crystal composition and the bulk composition of the starting material. By combining these analyses with three other techniques for determining the composition of the synthetic Actinolite (that is, volume-composition relationships, Rietveld refinement of Ca and Fe on the M4 site, and mass balancing involving bulk-composition and mineral-proportions), the average composition of the synthetic Actinolite was determined to be Ca1.67Fe5.33Si8O22(OH)2. Experimental reversals were obtained on reaction (1): 2 ferro-Actinolite = 4 hedenbergite + 3 fayalite + 5 quartz + 2 water over the range of 1 to 5 kbar and 500° to 525°C with the fO2 defined by the Co-CoO buffer. A parallel set of experiments on reaction (1) was done in the presence of grunerite and the iron-magnetite/wustite-magnetite (IM/WM) buffers. These latter experiments allowed the direct comparison of the upper-thermal stability of Actinolite via reaction (1) and of Ca-saturated grunerite via reaction (2): 2 grunerite = 7 fayalite + 9 quartz + 2 water. Reaction (2) was found to lie 20° to 60°C above reaction (1). Modeling the miscibility gap between Actinolite and grunerite as a regular solution with W = 15.3 kJ, a thermodynamic analysis of the internal consistency of the experimental reversals for reaction (1) was performed, from which the best-fit values of 696.4 J/K · mol and –10,518.2 kJ/mol were derived for the S° and ΔHfo of pure ferro-Actinolite, respectively. Calculation of the invariant array of curves involving the phases ferro-Actinolite, grunerite, hedenbergite, fayalite, quartz, and water indicate that reaction (1) is metastable at all geologically relevant pressures and that the stable reaction defining the stability limit of ferro-Actinolite is 7 ferro-Actinolite = 14 hedenbergite + 3 grunerite + 4 quartz + 4 water.
Hideo Ishizuka - One of the best experts on this subject based on the ideXlab platform.
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Metamorphic evolution of the Susunai metabasites in southern Sakhalin, Russian Republic
Journal of Metamorphic Geology, 2008Co-Authors: Masayuki Sakakibara, Makoto Okamura, Hideo Ishizuka, H. Ofuka, C. Kimura, Sumio Miyashita, O. A. MelinikovAbstract:The Susunai Complex of southeast Sakhalin represents a subduction-related accretionary complex of pelitic and basic rocks. Two stages of metamorphism are recognized: (1) a local, low-P/T event characterized by Si-poor calcic amphiboles; (2) a regional, high-P/T event characterized by pumpellyite, Actinolite, epidote, sodic amphibole, sodic pyroxene, stilpnomelane and aragonite. The major mineral assemblages of the high-P/T Susunai metabasites contain pumpellyite + epidote + Actinolite + chlorite, epidote + Actinolite + chlorite, epidote + Na-amphibole + Na-pyroxene + chlorite-(-haematite. The Na-amphibole is commonly magnesioriebeckite. The Na-pyroxene is jadeite-poor aegirine to aegirine-augite. Application of empirically and experimentally based thermobarometers suggests peak conditions of T= 250–300C, P= 4.7–6 kbar. Textural relationships in Susunai metabasite samples and a petrogenetic grid calculated for the Fe3+-rich basaltic system suggest that pressure and temperature increased during prograde metamorphism.
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subdivision of the sanbagawa pumpellyite Actinolite facies region in central shikoku southwest japan
Island Arc, 2008Co-Authors: Masumi Sakaguchi, Hideo IshizukaAbstract:The mineral assemblages of the pumpellyite–Actinolite facies such as pumpellyite + Actinolite + epidote + chlorite or Actinolite + epidote + hematite + chlorite occur in the Sanbagawa low-grade metamorphic region, central Shikoku, southwest Japan. Chemical compositions of these minerals from the eight newly studied areas were analyzed in order to evaluate the areal extent and thermal structure of the region. In the buffered assemblage of pumpellyite + Actinolite + epidote + chlorite, the Fe3+/(Fe3+ + Al) values of epidote decrease slightly with decreasing Fe2+/(Fe2+ + Mg) values for chlorite. The changes in these values show a general correlation with temperature. The presence of this relationship implies that the Fe3+/(Fe3+ + Al) values of epidote can be used to divide the Sanbagawa low-grade metamorphic region into low-, medium- and high-grade subzones. The areal distribution of these subzones indicates that: (i) the temperature seems to decrease in the same sense as envisaged by the zonal mapping of the higher-grade pelitic schists; and (ii) there is no significant gap of metamorphic conditions through the boundary between the two structural units (Besshi and Oboke units). It follows that the Sanbagawa low-grade metamorphic region decreases in temperature going up the structural section, and tectonic discontinuities have not affected the thermal structure.
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Subdivision of the Sanbagawa pumpellyite–Actinolite facies region in central Shikoku, southwest Japan
Island Arc, 2008Co-Authors: Masumi Sakaguchi, Hideo IshizukaAbstract:The mineral assemblages of the pumpellyite–Actinolite facies such as pumpellyite + Actinolite + epidote + chlorite or Actinolite + epidote + hematite + chlorite occur in the Sanbagawa low-grade metamorphic region, central Shikoku, southwest Japan. Chemical compositions of these minerals from the eight newly studied areas were analyzed in order to evaluate the areal extent and thermal structure of the region. In the buffered assemblage of pumpellyite + Actinolite + epidote + chlorite, the Fe3+/(Fe3+ + Al) values of epidote decrease slightly with decreasing Fe2+/(Fe2+ + Mg) values for chlorite. The changes in these values show a general correlation with temperature. The presence of this relationship implies that the Fe3+/(Fe3+ + Al) values of epidote can be used to divide the Sanbagawa low-grade metamorphic region into low-, medium- and high-grade subzones. The areal distribution of these subzones indicates that: (i) the temperature seems to decrease in the same sense as envisaged by the zonal mapping of the higher-grade pelitic schists; and (ii) there is no significant gap of metamorphic conditions through the boundary between the two structural units (Besshi and Oboke units). It follows that the Sanbagawa low-grade metamorphic region decreases in temperature going up the structural section, and tectonic discontinuities have not affected the thermal structure.
T. F. Emmett - One of the best experts on this subject based on the ideXlab platform.
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Diffusion in coronas around clinopyroxene: modelling with local equilibrium and steady state, and a non-steady-state modification to account for zoned Actinolite-hornblende
Contributions to Mineralogy and Petrology, 1992Co-Authors: J. R. Ashworth, J. J. Birdi, T. F. EmmettAbstract:Retrograde coronas of Caledonian age, between clinopyroxene and plagioclase in the Jotun Nappe Complex, Norway, illustrate the effects of diffusion kinetics on mineral distributions among layers and on the compositions of hornblende-Actinolite. One corona type comprises a symplectite of epidote + quartz adjacent to plagioclase, and a less well-organized intergrowth of amphibole + quartz replacing clinopyroxene. The observed mineral proportions imply an open-system reaction, but the similarity of Al/Si ratios in reactant plagioclase and product symplectite indicates approximate conservation of Al2O3 and SiO2. The largest inferred open-system flux is a loss of CaO, mostly derived from consumption of clinopyroxene. The approximate layer structure, Pl|Ep + Qtz|Hbl + Qtz|Act±Hbl + Qtz|Cpx, is modelled using the theory of steady-state diffusion-controlled growth with local equilibrium. To obtain a solution, it is necessary to use a reactant plagioclase composition which takes into account aluminous (epidote) inclusions. The results indicate that, in terms of Onsager diffusion coefficients Lii, Ca is more mobile than AL (LCaCa/LAlAl≳3.) (where ≳ means greater than or approximately equal to). This behaviour of Ca is comparable with that of Mg in previously studied coronas around olivine. Si is non-diffusing in the present modelling, because of silica saturation. Oxidation of some Fe2+ to Fe3+ occurs within the corona. Mg diffuses towards its source (clinopyroxene) to maintain local equilibrium. Other coronas consist of two layers, hornblende adjacent to plagioclase and zoned amphibole + quartz adjacent to clinopyroxene. In the zoned layer, actinolitic hornblende forms relict patches, separated from quartz blebs by more aluminous hornblende. A preliminary steady-state, local-equilibrium model of grain-boundary diffusion explains the formation of low-Al and high-Al layers as due to Al immobility. Zoning and replacement are qualitatively explained in terms of evolution of Actinolite to more stable aluminous compositions. This is modelled by a non-steady-state modification of the theory, retaining local equilibrium in grain boundaries while relatively steep zoning profiles develop in grain interiors through slow intracrystalline diffusion. Replacement of Actinolite by hornblende does not require a change in P−T conditions if Actinolite is a kinetically determined, non-equilibrium product. The common preservation of a sharp contact between hornblende and actionolite layers may be explained by ineffectiveness of intracrystalline diffusion: according to the theory, given sufficient grain-boundary Al flux, a metastable Actinolite + quartz layer in contact with hornblende may be diffusionally stable and may continue to grow in a steady state.
Osamu Sato - One of the best experts on this subject based on the ideXlab platform.
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Weathering processes and landslides occurrences of the Mikabu Greenstone Belt, central Shikoku, southwest Japan (1)
Landslides, 2000Co-Authors: Masatake Yube, Makoto Okamura, Ryuiti Yatabe, Norio Yagi, Kimitada Yokota, Osamu SatoAbstract:The Mikabu Greenstone Belt is characterized by many creeping landslides with comparatively small slip angles. Landslide distribution in this belt corresponds to the distribution of schistose greenstone and obscures in massive greenstone. Schistose greenstone has well-developped cracks and cleavage, which cause an easy ground water permeation. Also, schistose greenstone being mainly composed of Actinolite and chlorite can be easily hydrated. Weathering process of schistose greenstone begins with dissolution of Ca2+ at first, and formation of smectite then takes place after dissolution of Mg2+ from Actinolite and chlorite. Thus, the physicochemical weathering processes seems to have progressively changed the petromineralogic properties of the rock.
Mitsuhiro Toriumi - One of the best experts on this subject based on the ideXlab platform.
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progress of Actinolite forming reactions in mafic schists during retrograde metamorphism an example from the sanbagawa metamorphic belt in central shikoku japan
Journal of Metamorphic Geology, 2005Co-Authors: Atsushi Okamoto, Mitsuhiro ToriumiAbstract:Hydration reactions are direct evidence of fluid-rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde Actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite fi hornblende fi Actinolite in the high-grade zone, and winchite fi Actinolite in the low-grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass-balance relations suggest that: (1) the Actinolite-forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H2O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of Actinolite to total amphibole, Yact, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low (Yact 0.7); and (3) the extent of the winchite breakdown reaction is commonly high (Yact > 0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H2O and the surface area of hornblende. In contrast to high- grade rocks, the winchite breakdown reaction is well advanced in the low-grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low- grade rocks may be due to small size of winchite before the reaction.
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Progress of Actinolite‐forming reactions in mafic schists during retrograde metamorphism: an example from the Sanbagawa metamorphic belt in central Shikoku, Japan
Journal of Metamorphic Geology, 2005Co-Authors: Atsushi Okamoto, Mitsuhiro ToriumiAbstract:Hydration reactions are direct evidence of fluid-rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde Actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite fi hornblende fi Actinolite in the high-grade zone, and winchite fi Actinolite in the low-grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass-balance relations suggest that: (1) the Actinolite-forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H2O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of Actinolite to total amphibole, Yact, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low (Yact 0.7); and (3) the extent of the winchite breakdown reaction is commonly high (Yact > 0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H2O and the surface area of hornblende. In contrast to high- grade rocks, the winchite breakdown reaction is well advanced in the low-grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low- grade rocks may be due to small size of winchite before the reaction.
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Reaction progress and deformation of basic shicsts during retrograde metamorphism
2003Co-Authors: Atsushi Okamoto, Mitsuhiro Toriumi, K. Okamoto, Y. IizukaAbstract:Microboudin structure of amphibole in basic schists is considered to have been developed as the result of synmetamorphic deformation, because the pulled-apart spaces are commonly filled by later amphibole compositions. Separation of fragments gives an estimates of the extensional strain, and the amphibole compostion constrains the P-T condition of the deformation. Basic schists in the Sanbagawa metamorphic belt, Japan, are mainly composed of amphibole, epidote, plagioclase, chlorite and quartz. Amphibole in the Sanbagawa schists commonly shows a compositional zoning and is ofiten broken into several fragments. Coupling of the analyses of a growth zoning and a microboudin structure of amphiboles reveals that the extensional strain was accumulated during Actinolite overgrowth, which corresponds to the late stage of the exhumation of the Sanabagawa belt. For evaluating the extent of the Actinolite-foming reaction, each amphibole grain is divided into two, mantle and core. Mantle is the portion with Actinolite composition (Al 1.5), and core is mainly composed of hornblende in high grade zone rocks. The proportion of the amount of mantle to the amount of whole amphibole was measured from a X-ray maps of EPMA. The average of the porportion among a lot of amphibole grains in a same thin section is defined as Xact. Analyses of Xact, mineral modes, and bulk rock chemistry revealed that (1) there are no systematic change in bulk rock composition among the rocks with different Xact values, and (2) as Xact increases, the amounts of chorite and Actinolite increases while the amount of hornblende decreases. Thus, it is expected that retrograde reaction proceeded at the expense of hornblende. Mass balance relations for the Actinolite-forming reaction was solved to evaluate the condition of rock before the reaction. As the result, it is found that the reaction is distinct hydration, and that the Xact value is available as the indicator of the progress of the overall reaction. Relation between the extensinal strain deduced from amphibole microbudin and the Xact reveals that the deformation proceeded as the Actinolite-forming reaction proceeded. This may be result from the fact that the sepation of amphibole fragments correlates to the average grwoth amount of amphibole grains in the Actinolite-forming stage. A spatial distribution of Xact of basic rocks in the central Shikoku shows that the Actinolite-forming reaction associated with deformation proceeded at the garnet zone and the bottom of the albiti-biotite zone significantly. FeTi-oxide in the garnetite; An experimental constrain on the MORB+H2O system K. OKAMOTO AND Y. IIZUKA
