Records of apatite for multiple injections of magmas in adakitic plutons: A case study of Mesozoic plutons in the Shatanjiao region of the Tongling ore cluster, south China
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Fractional crystallization (geology)
Mineral redox buffer
Amphibole
but the results suggest that other mechanisms may also be involved. All four amphiboles exhibited systematically higher ferric-ferrous ratios with increasing 6, of equilibration. Equilibrium R values were achieved relatively rapidly and could be readily restored to onginal values by treatment at the appropriate buffer. In some cases, a metastable equilibrium of ferric-ferrous ratio was achieved before the amphibole decomposed to other Fe3*-bearing phases. Of the four amphiboles, grunerite is apparently the least able to accommodate Fe3* within its crystal structure and decomposes at relatively higher oxygen fugacities. The value of a sin B decreases systematically uniformly as the Fe3* content of tschermakitic hornblende, magnesio-hornblende, and riebeckite increases, reflecting increasing Fe3* in the octahedral cation sites. The variation in a sin B ofgrunerite is significantly less than for the other three amphiboles. The variation in b for the two hornblendes suggests that Fe3* produced by oxidation is not strongly ordered into the M(2) site.
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Amphibole
Mineral redox buffer
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Liquidus
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Analyses of sodic amphiboles from a blueschist metamorphic terrain of constant grade in the Tavşanli area, Northwest Turkey, show that practically the whole range of sodic amphibole compositions is stable in the glaucophane-lawsonite facies. By analysing sodic amphiboles coexisting with magnetite and hematite, it has proved possible to plot for a constant pressure and temperature an oxygen fugacity isopleth. Through the analyses of sodic amphiboles and reference to the extended oxygen fugacity contours drawn on the Miyashiro compositional diagram, relative rock oxygen fugacities can be obtained. The oxygen fugacity isopleth corresponding to the magnetite/hematite buffer from two blueschist terrains with differing P-T regimes, namely Tavşanli in Turkey and Shuksan in Cascades, lies along the same loci on the amphibole diagram indicating that the intrinsic $$fO_{2}$$ slope of sodic amphiboles in the $$fO_{2}$$-T plane is approximately the same as the $$fO_{2}$$ slope of the magnetite/hematite buffer. Changes in the $$fO_{2}$$ during metamorphism may be recorded as zoning in sodic amphiboles, two examples of which are given.
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Mineral redox buffer
Blueschist
Doming
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Mineral redox buffer
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Iron is a common constituent in minerals from the Earth's crust and upper mantle and often occurs in minerals as mixtures of two valence states, Fe3+ or Fe2+. Quantification of the values of Fe3+/FeTotal, where FeTotal = Fe3++Fe2+, in minerals may be necessary to accurately apply certain mineral equilibria to determine equilibrium values of important variables such as temperature (T), pressure (P), and oxygen fugacity (ƒO₂). Most useful would be an analytical technique that permits determination of values of Fe3+/FeTotal within a single mineral grain that is contained within a standard petrographic thin section, and the excellent spatial resolution and relative accessibility of the electron microprobe (EMP) have resulted in various attempts to use this instrument to determine values of Fe3+/FeTotal. These efforts have typically involved quantifying characteristics of the FeLα and/or FeLβ peaks. In this paper, we employ the method of Fialin et al. (2001), who have shown that the location of the FeLα peak changes as a function of Fe content and values of Fe3+/ FeTotal, to determine values of Fe3+/FeTotal in amphiboles.
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The stability of amphiboles on the join MgrSi.Orr(OH)r-FezSisorr(OH), has been hydrothermally investigated at 2 kbar fluid pressure as a function of oxygen fugacity and temperature. Atf;, defined by the MH buffer, the maximum extent of solution of Feend-member in amphibole is l4 and 22mole percent at725 and 630'C respectively; amphibole is unstable below 630'C, being replaced by the assemblage talc + quartz * magnetite + hematite. AtI), defined by the NNO buffer the extent of solid solution expands to 54,62, and 65 mole percent Fe end-member at 7250, 625', and 600C, respectively. Results obtained in this study have been combined with previously published data to produce a I-X section of the upper thermal stability of amphibole at2kbar andl,, defined by the FMQ buffer. Temperatures for the reaction: amphibole + pyroxene + quartz + vapor decrease from -765'C for the pure Mg end-member to -710'C for 62 mole percent Fe endmember. The breakdown reaction: amphibole - olivine + quartz + vapor, was observed for the more iron-rich amphiboles, and takes place at -675C for amphibole of 73 mole percent Fe end-member. Comparison of the experimental results to selected natural cases shows good agreement in maximum iron contents for the appropriate oxygen fugacity range. Estimates of temperatures of crystallization of metamorphic and igneous Fe-Mg amphiboles are also consistent with pnor reports.
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Pyroxene
Fayalite
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Mineral redox buffer
Phenocryst
Liquidus
Fugacity
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