On the paragenesis of allanite and epidote as rock-forming minerals
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Paragenesis
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Epidote-group minerals, together with albite, quartz, fluorite, Al-poor and Fe-rich phyllosilicates, zircon, and minor oxides and sulphides, are typical hydrothermal phases in peralkaline alkali-feldspar granites from the Corupá Pluton, Graciosa Province, South Brazil. The epidote-group minerals occur as single crystals and as aggregates filling in rock interstices and miarolitic cavities. They display complex recurrent zoning patterns with an internal zone of ferriallanite-(Ce), followed by allanite-(Ce), then epidote-ferriepidote, and an external zone with allanite-(Ce), with sharp limits, as shown in BSE and X-ray images. REE patterns show decreasing fractionation degrees of LREE over HREE from ferriallanite to epidote. The most external allanite is enriched in MREE. LA-ICP-MS data indicate that ferriallanite is enriched (>10-fold) in Ti, Sr and Ga, and depleted in Mg, Rb, Th and Zr relative to the host granite. Allanite has lower Ga and Mn and higher Zr, Nb and U contents as compared to ferriallanite, while epidote is enriched in Sr, U and depleted in Pb, Zr, Hf, Ti and Ga. The formation of these minerals is related to the variable concentrations of HFSE, Ca, Al, Fe and F in fluids remaining from magmatic crystallization, in an oxidizing environment, close to the HM buffer. L-MREE were in part released by the alteration of chevkinite, their main primary repository in the host rocks.
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Epidote, which contains significant amounts of rare earth elements and thorium, and allanite occur in the Sanbagawa pelitic schists of central Shikoku at all the metamorphic grades. Petrographical and chemical studies suggest a metamorphic origin of the REE-bearing epidote and a detrital origin of the allanite. The detrital allanite is the major source of REE and Th in the metamorphic epidote.
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Allanite-(La) is a member of the epidote group. It was sampled at Buca delia Vena mine, near Stazzema, Apuan Alps, Tuscany, Italy. It occurs as prismatic crystals up to 2-3 mm in length along [0101 within barite veins cutting a dolomitic metamorphosed limestone. The mineral is black with brownish reflections, a brown streak and a vitreous luster, transparent to translucent, nonfluorescent, brittle with a conchoidal fracture. The observed forms are {001}, {IOO}, {IOI}, {IOI}, {21O}, {Oil}, and the cleavage is imperfect along ZOO I}. The hardness (Mohs) is about 6, D(obs.) 3.93( I), D(calc.) 3.94 g/cm3. Optically, allanite-(La) is biaxial, with n, = 1.755(5), ny = 1.760(5), n, = 1.765(5), 2V = 90(2)°, very high dispersion and strong pleochroism from greenish to greenish brown. The chemical composition of allanite-(La) has been determined by electron-microprobe analysis and points to the following empirical formula: Ca 1.000[(L~J.288CeO.206Ndo.116PrO.106 Y0.00IhO.7I7Cao.279 Tho.006h: 1.002 (Alo.899Fe3+ 0.10I)l1.000Al 1.000 (Fe2+0669Fe3+o 279Mgo.044Tio 007ho 999 [(Sio.978Aloo22)nooo041 (Si207) 0 (OH). The simplified formula is Ca(REE,Ca)AI2(Fe2+, Fe3+)(Si04)(Si207)O(OH) (2 = 2). The six strongest lines in the X-ray powder-diffraction pattern [d in A(l)(hkl)] are: 3.506(20)(211), 2.90 I (100)(113), 2.860(40)(020), 2.692(60)(013), 2.611 (50)(311), and 2.174(25)(40 I). Allanite-(La) is monoclinic, P211m, with a 8.914(4), h 5.726(1), c 10.132(6) A, 13114.87(5)°, V 469.1(3) A3. The crystal structure of allanite-(La) has been refined to R = 0.0328 for 1985 independent reflections collected via single-crystal X-ray-diffraction measurements. The name allanite-(La) recalls the relationship with allanite-(Ce), the Levinson modifier indicating the dominant REE. The root name honors the Scottish mineralogist Thomas Allan (1777-1883). Although the name allanite-(La) has been already used in the literature, the mineral had not been officially approved. The recognition and complete characterization of allanite-(La) from Buca delia Vena mine, which now has.to be considered the type locality of the mineral, fill a gap in the systematics of the epidote group.
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Hydrothermal experiments were performed on natural epidote and REE-bearing fluids as starting material to understand the formation of allanite from epidote. The experiments were performed at 30 to 100 MPa and at 250, 550 and 800 degrees C with a fluid containing 1000 mu g/g of each of the following elements: Y, La, Ce, Nd, Eu, Yb; and of 10 mu g/g of Th and U. At 550 degrees C, the only new mineral phase containing trivalent elements is located on grain boundaries and cracks of the starting epidote. Its composition corresponds to allanite. Except for the rim of the starting epidote no reaction is observed. At 800 degrees C the same reaction is observed at the rim of the former epidote. However, epidote is completely transformed to zoisite, clinopyroxene, spinel and garnet. The allanite seems to be formed by a solution-precipitation mechanism directly out of the trace element enriched fluid.
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