The Al-Fe(III)epidote miscibility gap in a metamorphic profile through the penninic series of the Tauern window, Austria
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Mineral redox buffer
Ilmenite
The usually medium-grained garnetites form up to several cm thick layers in quartz-magnetite mineralizations of the Desna Unit in the Silesicum. The garnetite mineral assemblage consists of epidote, quartz, garnet, magnetite, biotite, ilmenite, apatite and retrograde chlorite. The mineralogical and petrological results demonstrate that garnetites are metamorphosed submarine exhalites. Garnet from the garnetites has compositions with nearly equal amounts of spessartine and almandine components and can by interpreted as result metamorphic reactions partially consuming of the Mn-rich epidote. Thermobarometric calculations from the garnetites yielded peak amphibolite facie metamorphic conditions ca. 540–600 °C and up to 3 kbar. Chemical composition of fluids estimated for this metamorphic event is H2O = 82 až 95 mol. %, CO2 = 4 až 17 mol %, NaCl = 1 až 2 mol. %. The biotite was partially replaced by chlorite during younger retrograde metamorphic event under greenschist facie.
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Pelite
Wolframite
Ilmenite
Fugacity
Rutile
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Anorthosite
Ilmenite
Baddeleyite
Mineral redox buffer
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Columbite
Layered intrusion
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In this study, heavy mineral analysis was carried out in different size fractions of the Yellow River sediment to extract its end-members. It shows that heavy mineral contents, species, and compositions vary in different grain sizes. Distribution curve of heavy mineral concentration (HMC) and particle size frequency curve are in normal distribution. In most samples, the size fraction of 4.5–5.0 Φ contains the maximum HMC (18% on average). Heavy mineral assemblages of the Yellow River are featured by amphibole + epidote + limonite + garnet. Amphibole content is high in coarse fraction of >3.0 Φ and reaches its peak value in 3.5–4.5 Φ. Epidote is rich in a size fraction of >3.5 Φ, and increase as the particle size becomes fine. Micas content is high in coarse subsamples of <3.0 Φ, but almost absent in fine grains of >4.0 Φ. Metallic minerals (magnetite, ilmenite, hematite, and limonite) increase as the sediment particle size become fine, and reach the peak in silt (>4.0 Φ). Other minerals such as zircon, rutile, tourmaline, garnet, and apatite account for about 15%, and mainly concentrate in fine sediment. Further analysis reveals that similarity value between the most abundant grain size group and wide window grain size group is high (0.978 on average). The grain size of 4.0–5.0 Φ ± 0.5 Φ is suitable to carry out detrital mineral analysis in the Yellow River sediments. Our study helps to eliminate cognitive bias due to narrow grain size strategy, and to provide heavy mineral end-members of the Yellow River sediment for provenance discrimination in the marginal seas of East China.
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Amphibole
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Ilmenite
Mineral redox buffer
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Fractional crystallization (geology)
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The redox state of sillimanite zone (650–700°C, 5–6 kbar) metasediments of the Barrovian type area, Scotland, was investigated using estimates of metamorphic oxygen fugacity (fO2), sulfur fugacity (fS2), and fluid chemistry based on new determinations of mineral and rock compositions from 33 samples. A total of 94% of the samples lack graphite, contain both ilmenite–hematite solid solutions (RHOMOX) and magnetite, and had metamorphic fO2 about 2 log10 units above the quartz–fayalite–magnetite (QFM) buffer. The regional variation in metamorphic fO2 for these rocks was minimal, about ±0·3 log10 units, reflecting either a protolith that was homogeneous with respect to redox state, or an initially variable protolith whose redox state was homogenized by metamorphic fluid–rock interaction. RHOMOX inclusions in garnet porphyroblasts that become richer in ilmenite from the interior to the edge of the host porphyroblast suggest that at least some syn-metamorphic reduction of rock occurred. Significant variations in bulk-rock oxidation ratio (OR) that are probably inherited from sedimentary protoliths are found from one layer to the next; OR ranges mostly between ∼20 and ∼50 [OR = molecular 2Fe2O3 × 100/(2Fe2O3 + FeO)]. These OR variations are uncorrelated with fO2 and do not indicate that large, order-of-magnitude gradients in fO2 and redox state existed or were preserved between layers during metamorphism. The other 6% of the samples contain ilmenite, lack magnetite, and had low fO2 0–1 order of magnitude below QFM in the stability field of graphite. They are characterized by combinations of the following: large fluid HF/H2O; metasomatic, tourmaline-bearing veins; absence or rarity of primary organic matter; and crosscutting late metamorphic shear zones rich in carbonaceous material. Such observations suggest that locally low fO2 conditions may have been related to the influx of reducing fluids from elsewhere in the area.
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Isograd
Fayalite
Pelite
Sillimanite
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Columbite
Muscovite
Pegmatite
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Tourmaline
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Mineral redox buffer
Ilmenite
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