Niobian ilmenite, hydroxylapatite and sulfatian monazite; alternative hosts for incompatible elements in calcite kimberlite from Internatsional'naya, Yakutia
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Ilmenite
Hydroxylapatite
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Ultramafic rock
Chromite
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Enstatite
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Lithophile
Planetary differentiation
Peridotite
Refractory (planetary science)
Post-perovskite
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Accessory REE minerals occur in a small metamorphic magnetite ore deposit at Bacuch, Veporic Superunit, central Slovakia. We distinguish two populations of monazite. Monazite I forms subhedral to euhedral crystals associated with magnetite. It contains ≤12 wt.% ThO2, ≤2.7% UO2, ≤0.85% SO3, with low Ca and Sr contents. Compared to the common monazite-(Ce) I, monazite-(Nd) I (≤26.1% Nd2O3) occasionally occurs with an atomic ratio Nd:Ce up to 1.17. Monazite II is present as irregular aggregates with hingganite in younger hydrothermal quartz – albite – chlorite veinlets, or as rim zones on monazite I. Monazite II is depleted in Th and U and has an unusually high content of S (≤11.3% SO3, 0.31 apfu S) and Sr (≤8.7% SrO, 0.18 apfu Sr). This composition indicates a (Ca,Sr)S(REE,Y)−1P−1 substitution as a dominant mechanism of Sr and S entry into the monazite structure. Some monazite II crystals display an elevated Eu content (≤1.2% Eu2O3). Xenotime-(Y) forms subhedral crystals, in association with monazite-(Ce) I, magnetite, pyrite transformed to goethite (?) and quartz. Gadolinite-group minerals at Bacuch are represented by hingganite with an atomic value of X □/ X (□ + Fe) in the range 0.51–0.72. Neodymium is locally the most abundant REE (17.8–18.7% Nd, ~0.56 apfu ), and an Nd-dominant member of the gadolinite group was identified. The composition of hingganite-(Y) was also determined. The principal mechanism of substitution in hingganite is Fe2+O2□−1(OH)−2. Primary monazite I and xenotime are most likely products of regional metamorphism, together with magnetite mineralization. On the contrary, Sr- and S-rich monazite II and hingganite originated during a younger (Alpine) metamorphic-hydrothermal overprint in a fluid-rich regime.
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Monazite is a light rare earth element (LREE)-bearing phosphate mineral that is present in a wide variety of rock types, has an extremely variable composition reflecting host rock conditions, and is a robust geochronometer that can preserve crystallization ages through a long history of geological events. Monazite crystals typically contain distinct compositional domains that represent successive generations of monazite, which in turn, can provide a detailed record of the geologic history of its host rocks. The electron microprobe can be used to characterize the geometry of compositional domains, analyze the composition of each domain, and, when carefully configured, determine the U-Th-total Pb age for domains as small as 5 μm in width. These data allow the monazite to be linked with, and place timing constraints on, silicate processes in the host rocks. Current applications span a broad range of geologic processes in igneous, metamorphic, hydrothermal, and sedimentary rocks.
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Magnesite
Phlogopite
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Abstract The distribution of REE minerals in metasedimentary rocks was investigated to gain insight into the stability of allanite, monazite and xenotime in metapelites. Samples were collected in the central Swiss Alps, along a well‐established metamorphic field gradient that record conditions from very low grade metamorphism (250 °C) to the lower amphibolite facies (∼600 °C). In the Alpine metapelites investigated, mass balance calculations show that LREE are mainly transferred between monazite and allanite during the course of prograde metamorphism. At very low grade metamorphism, detrital monazite grains (mostly Variscan in age) have two distinct populations in terms of LREE and MREE compositions. Newly formed monazite crystallized during low‐grade metamorphism (<440 °C); these are enriched in La, but depleted in Th and Y, compared with inherited grains. Upon the appearance of chloritoid (∼440–450 °C, thermometry based on chlorite–choritoid and carbonaceous material), monazite is consumed, and MREE and LREE are taken up preferentially in two distinct zones of allanite distinguishable by EMPA and X‐ray mapping. Prior to garnet growth, allanite acquires two growth zones of clinozoisite: a first one rich in HREE + Y and a second one containing low REE contents. Following garnet growth, close to the chloritoid–out zone boundary (∼556–580 °C, based on phase equilibrium calculations), allanite and its rims are partially to totally replaced by monazite and xenotime, both associated with plagioclase (± biotite ± staurolite ± kyanite ± quartz). In these samples, epidote relics are located in the matrix or as inclusions in garnet, and these preserve their characteristic chemical and textural growth zoning, indicating that they did not experience re‐equilibration following their prograde formation. Hence, the partial breakdown of allanite to monazite offers the attractive possibility to obtain in situ ages, representing two distinct crystallization stages. In addition, the complex REE + Y and Th zoning pattern of allanite and monazite are essential monitors of crystallization conditions at relatively low metamorphic grade.
Allanite
Staurolite
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