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Magnetite is common in various magmatic and hydrothermal ore deposit types, and its trace element geochemistry has become increasingly used in ore genesis studies and mineral exploration. While fractional crystallization has been shown to influence the chemistry of igneous magnetite, the extent to which this process regulates the trace element composition of hydrothermal magnetite remains poorly understood. In this study, we analyzed trace elements in hydrothermal magnetite from the Baijian Fe-(Co) skarn deposit in eastern China and used Rayleigh and equilibrium fractionation modeling to demonstrate the importance of magnetite precipitation in controlling fluid and magnetite chemistry during Fe skarn mineralization. The Baijian Fe-(Co) skarn deposit has three stages of magnetite. From early Mag-1 to later Mag-2 and Mag-3, the concentrations of compatible elements (Ni and V) decrease, whereas those of incompatible elements (Zn, Mn, and Co) increase. There are obvious trends of increasing incompatible/compatible element ratios (e.g., Co/Ni, Zn/V, and Zn/Ni) and decreasing compatible/incompatible element ratios (e.g., V/Mn, Ni/Mn, and V/Co) from Mag-1 to Mag-3, with strong correlations between each of these ratios. Such systematic trace element variations in successive stages of magnetite can be best explained by increasing degrees of fractional crystallization with time. The wide range of incompatible/compatible element ratios (spanning 2-4 orders of magnitude) in Mag-2 and Mag-3 suggests that magnetite crystallization follows a process akin to Rayleigh fractionation. Results from this study highlight the significant role that magnetite crystallization during skarn formation has on the trace element chemistry of this mineral. Moreover, as the crystallization of magnetite progresses, the Co/Fe ratio of residual hydrothermal fluids is elevated, which favors the precipitation of Co in late-stage sulfides. This process helps to explain why some Fe skarn deposits, as well as magnetite-rich iron oxide-apatite and iron oxide-copper-gold deposits, are potentially important economic sources for Co, currently necessary as one component in Li-ion batteries. [ABSTRACT FROM AUTHOR]

Apatite is a ubiquitous phase in granite plutons and in most adjacent country rocks, thus contamination of a granite magma with wall-rock material results in two genetic types of apatite in the magma: cognate and foreign. These two textural and chemical varieties of apatite undergo textural and compositional changes to reach physical and chemical equilibrium (perfect assimilation) in the melt. Our experiments replicate the conditions in such contaminated granites. The starting materials consist of a peraluminous synthetic SiO2-Al2O3-Na2O-K2O (SANK 1.3) granite gel with A/NK of 1.3, synthetic F-apatite, synthetic Cl-apatite, and natural Durango apatite. Initial experiments in cold-seal hydrothermal pressure vessels at magmatically realistic temperatures of 750 °C and pressures of 200 MPa produced negligible reactions, even after run times of 2000 h. Instead, we used an argon-pressurized internally heated pressure vessel with a rapid-quench setup at temperatures of 1200 °C, pressure of 200 MPa, and run durations of 192 h. An advantage of this high temperature is that it exceeds the liquidus for quartz and feldspar; therefore, apatite is the only solid phase in the run products. The starting composition of each run was 90 wt% SANK 1.3 granite gel and 10 wt% crushed apatite (consisting of one, two, or three varieties), with and without 4 wt% added H2O. Run products were examined by SEM for texture and by EMPA and LA-ICP-MS for composition. The starting synthetic granite composition contains no Ca, F, Cl, or REEs thus, in every run, apatite was initially undersaturated in the melt. In all experiments, most large apatite grains consisted of anhedral shards with rounded corners, most small apatite grains were round, and a small proportion of apatite grains developed one or more crystal faces. In experiments with two or three apatite compositions, the run-product apatite grains had compositions intermediate between those of the starting-material grains, and they were homogeneous with respect to Cl, and probably F, but not with respect to REEs. The processes to reach textural equilibrium consist of dissolution until the melt is saturated in apatite, followed by Ostwald ripening to eliminate small grains and to develop crystal faces on larger ones. The processes to reach chemical equilibrium consist of dissolution of apatite, diffusion of cations (Ca, P, REE) and anions (F, Cl, OH) through the silicate melt, and solid-state diffusion in the undissolved apatite grains. The halogens approached chemical equilibrium in all experiments, but in the experiments containing Durango apatite, the REEs have not. Models involving radial diffusion into spherical apatite grains at the temperatures of the experiments show complete re-equilibration of the halogens, but changes in the REE concentrations affecting only the outer few micrometers. We conclude that the rate of chemical equilibrium for the halogens is greater than the rate of physical equilibrium for texture, which in turn is greater the rate of chemical equilibrium for REEs. We illustrate these processes with a natural example of contaminated granite from the South Mountain Batholith in Nova Scotia. Given that all granites are contaminated rocks, we propose that future petrogenetic studies focus on developing techniques for a minerals-based quantitative estimation of contamination (QEC). [ABSTRACT FROM AUTHOR]

Recent advances in the dating of monazite using the electron microprobe indicate that ThSi[O.sub.4]- and/ or CaTh[(P[O.sub.4]).sub.2]-enriched intergrowths in monazite can give ages younger than the original monazite. The morphology of the intergrowths suggests that the original monazite grain has been partly altered by a fluid in which Th, Si, and Ca are mobile. This hypothesis has been tested in the piston-cylinder apparatus at 1000 MPa and 900[degrees]C utilizing a natural, unzoned, Th- and Pb-bearing monazite-(Ce) and [Na.sub.2][Si.sub.2][O.sub.5]+[H.sub.2]O. During the experiment, a subset of the monazite grains acquired ThSi[O.sub.4]-enriched areas with sharp compositional boundaries devoid of Pb and show evidence of being a pseudomorphic partial replacement of the monazite rather than an overgrowth of the monazite. These experiments support the hypothesis that similar Th-enriched or Th-depleted patches with sharp compositional boundaries observed in natural monazite could be the result of fluid-induced alteration via coupled dissolution-reprecipitation. If so, such altered regions would yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of single or multiple metasomatic events assuming that all pre-existing radiogenic Pb is removed during alteration. Keywords: Monazite, huttonite, alkali-bearing fluids, dissolution-reprecipitation, geochronology, experimental petrology, high-grade metamorphism, Th-U-Pb dating DOI: 10.2138/am.2010.3525

Solution calorimetric measurements have been made on 17 synthetic fluorapatite-chlorapatite crystalline solutions at 50[degrees]C in 20.0 wt% HCl under isoperibolic conditions. Analysis of the calorimetric data indicates that heats of mixing across the series may reach values as high as 8.3 kJ/mol. Normally such a high degree of thermodynamic nonideality would be associated with immiscibility, yet no such miscibility gap is indicated by either synthetic or natural fluor-chlorapatite specimens. Based on full chemical analyses, all Cl-rich samples ([X.sub.Cl] > 0.65) of this study have halogen deficiencies that imply the presence of 4-11 mol% vacancies in the anion sites, which are interpreted to be associated with oxyapatite substitution. Separate analysis of data for the vacancy-free samples produces a linear fit for enthalpy of solution vs. composition, which yields an alternative interpretation of thermodynamic ideality. Together these models define the limits of enthalpy behavior for the fluor-chlorapatite system. Keywords: Enthalpies of mixing, fluorapatite, chlorapatite, thermodynamics DOI: 10.2138/am.2010.3485

Thorite and uraninite inclusions are documented in metasomatized regions in three xenotime crystals and one monazite crystal collected from three granitic pegmatites hosted within an anorthosite intrusion located on the island of Hidra, southwestern Norway. Formation of these inclusions is demonstrated to be the result of dissolution-reprecipitation processes that occurred in an effectively closed chemical system. The reaction was initiated by interaction between the early crystallizing orthophosphate minerals and the evolving pegmatite fluid. This fluid was dominated by [H.sub.2]O, but also contained F and minor amounts of C1, and significant quantities of dissolved [Na.sup.+] and [K.sup.+]. This is an example of auto-metasomatism and highlights a natural case where Th-U-Si-enriched orthophosphate minerals have been partially altered such that the Th, U, and Si have been converted to secondary inclusions within the now Th-U-Si-depleted orthophosphate via reaction with fluids rich in alkali elements. Keywords: Monazite, xenotime, thorite, uraninite, dissolution-reprecipitation, pegmatite, alkali-rich fluids

The formation of sillimanite, under metastable conditions, relative to corundum and quartz has been defined experimentally, approximately 700 to 800 MPa inside the kyanite stability field thereby allowing for the approximate location of the metastable Sil = Cnd + Qtz equilibrium to be outlined in P-T space from 600 to 800 [degrees]C. Experiments involved using a NaCl assembly with a graphite furnace in a two-piston-cylinder apparatus. The thermocouple tip was in direct contact with the flat-lying, folded Pt capsule thereby minimizing thermal gradients to Keywords: Sillimanite, kyanite, corundum, quartz, rate of reaction, experimental petrology, Rietveld refinement, aluminosilicates

Monazite is both partially replaced and overgrown by a ThSi[O.sub.4] phase along grain rims in a series of experiments from 300 to 900 [degrees]C and 200 to 1000 MPa. All experiments consisted of 10 mg of 100-500 [micro]m size, euhedral to subhedral crystals of a natural Th-free monazite-(Ce), 5 mg of Th[(N[O.sub.3]).sub.4]*5[H.sub.2]O, 2.5 mg of Si[O.sub.2], and 5 mg of [H.sub.2]O loaded into 3 mm wide, 1 or 1.3 cm long platinum capsules that were arc welded shut. Experimental conditions were: 300 [degrees]C at 200 and 500 MPa; 300, 400, 500, 600, and 700 [degrees]C at 500 MPa (cold seal hydrothermal autoclave); and 900[degrees]C at 1000 MPa (Catz assembly; pistoncylinder press). Back-scattered electron (BSE) imaging, electron back-scattered diffraction (EBSD) analysis, and transmission electron microscopy (TEM) indicates that in the experiments from 500 to 900[degrees]C, the ThSi[O.sub.4] phase took the form of monoclinic huttonite implying that huttonite, associated with monazite, could exist metastably over a much greater P-T range than previously thought. TEM analysis of a foil cut perpendicular to the monazite-huttonite interface from the 600 [degrees]C, 500 MPa experiment using a focused ion beam (FIB) indicates that the huttonite as well as the interface between the huttonite and monazite is characterized by fluid inclusions. High-resolution TEM analysis indicates that the huttonite-monazite interface is coherent. In the case of replacement of monazite by huttonite, fluid-aided dissolution-reprecipitation is proposed as the most likely mechanism responsible. Keywords: Monazite, huttonite, dissolution-reprecipitation, metastability, metasomatism, EBSD, TEM, pseudomorphism

To elucidate at what pressure and temperature and for what fluid compositions monazite may be induced to form from fluorapatite, the LREE-enriched Durango fluorapatite has been metasomatized experimentally at temperatures of 300, 600, 700, 800, 850, and 900 [degrees]C and pressures of 500 and 1000 MPa. Fluids used included pure [H.sub.2]O, various NaCl, KCl, and Ca[Cl.sub.2] brines (salt/[H.sub.2]O = 50/50, 30/70, or 10/90), and either 90/10 C[O.sub.2]/[H.sub.2]O or 40/60 C[O.sub.2]/[H.sub.2]O mix. Monazite formed in the fluorapatite + [H.sub.2]O, fluorapatite + 40/60 C[O.sub.2]/[H.sub.2]O, and the fluorapatite + KCl brine experiments. At 900[degrees]C and 1000 MPa, monazite formed both as inclusions within the fluorapatite and externally on its surface. Below 900[degrees]C, monazite grew only externally on the fluorapatite, either as euhedral to semi-euhedral crystals or as partial mantles over smaller fluorapatite grains. Monazite, especially at 900[degrees]C and 1000 MPa, is compositionally heterogeneous, specifically with respect to the Th content (TH[O.sub.2] = 4-38 wt%). Whereas the reactant fluorapatite in the pure [H.sub.2]O experiments remained unzoned at lower temperatures, three coupled zones with different (LREE+Si+Na) abundances developed at 900[degrees]C. These zones roughly follow the rim of the fluorapatite enclosing a fourth zone or the core, resembling the original composition. Monazite inclusions formed only in the one zone where the LREE are depleted. In the NaCl brine experiments, the Na replaced Si lost to the solution, which stabilized the LREE, and precluded formation of monazite. Similarly, the high activity of Ca in the Ca[Cl.sub.2] brine caused Ca to replace (LREE+Na) on the Ca site and discouraged the growth of monazite. The fluorapatite recrystallized to a fluor-chlorapatite, which displays oscillatory zoning, specifically with respect to the LREE. The results from this study imply that the presence of monazite inclusions and rim grains associated with fluorapatite (1) can be metasomatically induced; (2) can give insights into the chemistry of the metasomatizing fluids; (3) can provide some information on the grade of the metasomatic overprint; and (4) could indicate the occurrence of one or more metasomatic events.

Systematic changes in whole-rock chemistry, mineralogy, mineral textures, and mineral chemistry are seen along a ca. 95-km traverse of late Archean granitoid orthogneisses in the Shevaroy Block, Eastern Dharwar Craton, southern India. The traverse passes from amphibolite-grade gneisses in the north to granulite-grade rocks (charnockite) in the south. Changes include whole-rock depletion of Rb, Cs, Th, and U in the granulite grade rocks as relative to the amphibolite grade gneisses, and oxidation trends regionally from highly oxidised granulite-facies rocks near the magnetite–haematite buffer to relatively reduced amphibolite-facies rocks below the fayalite-magnetite-quartz. Rare earth elements show limited mobility and are hosted a variety of minerals whose presence is dependent on the metamorphic grade ranging from titanite and allanite in the amphibolite-facies rocks to monazite in the vicinity of the orthopyroxene-in isograd to apatite in the granulite-grade charnockite. Cathodoluminescence and back-scattered electron sub-grain imaging and sensitive high-resolution ion microprobe analysis of zircon from 29 samples of dioritic, tonalitic, and granitic orthogneiss from the traverse reveals magmatic zircon cores that record the emplacement of the granitoid protoliths mostly about 2580 to 2550 Ma, along with a few older mid to late Archean tonalites. Protolith zircon was modified during metamorphism by overgrowth and/or replacement. Relative to igneous cores, U-enriched metamorphic zircon, dominant in the amphibolite-grade gneisses, formed at ca. 2530 Ma, predating retrograde titanite growth at ca. 2500 Ma. Uranium-depleted mantles grew on zircon between 2530 and 2500 Ma in granulite-grade samples south of the orthopyroxene-in isograd. In some of these samples, the U-depleted metamorphic zircon is preceded by mantles of U-undepleted zircon, indicating a progression of metamorphic zircon growth with increasingly depleted compositions between 2530 and 2500 Ma. With increasing metamorphic grade (from amphibolite to granulite) and oxidation state, allanite and monazite disappear from the assemblage and zircon became depleted in U and Th. Whole-rock U-Th compositions became decoupled from relict magmatic zircon compositions, reflecting the development of U-depleted metamorphic zircon and indicating that whole-rock chemical differences along the traverse were produced during metamorphism, rather than just reflecting differences in dioritic vs granitic protoliths. Although in situ anatexis and melt extraction may have played a role, whole-rock and zircon depletion of trace elements can be explained by the action of externally derived, oxidising, low-H2O activity hypersaline fluids migrating up through the mid to lower crust. Fluids and element migration during metamorphism may be the end result of subduction related processes that cumulated in the collision and concatenation of island arcs and continental blocks. These tectonic processes assembled the Dharwar Craton at the end of the Archean. [ABSTRACT FROM AUTHOR]

In this study we present a novel method for the automatic detection of minerals and elements using hyperspectral transmittance imaging microscopy measurements of complete thin sections (HyperTIM). This is accomplished by using a hyperspectral camera system that operates in the visible and near-infrared (VNIR) range with a specifically designed sample holder, scanning setup, and a microscope lens. We utilize this method on a monazite ore thin section from Steenkampskraal (South Africa), which we analyzed for the rare earth element (REE)-bearing mineral monazite ((Ce,Nd,La)PO 4), with high concentrations of Nd. The transmittance analyses with the hyperspectral VNIR camera can be used to identify REE minerals and Nd in thin sections. We propose a three-point band depth index, the Nd feature depth index (NdFD), and its related product the Nd band depth index (NdBDI), which enables automatic mineral detection and classification for the Nd-bearing monazites in thin sections. In combination with the average concentration of the relative Nd content, it permits a destruction-free, total concentration calculation for Nd across the entire thin section. [ABSTRACT FROM AUTHOR]

Orientational order-disorder behavior of N[H.sup.+.sub.4] is a well-established phenomenon in certain ammonium halides and ammonium salts. Anticipating similar behavior of ammonium in the interlayer site of tobelite, synthetic N[D.sub.4]/N[H.sub.4] tobelite has been studied by infrared spectroscopy from room temperature down to 20 K. Both N[D.sub.4] and N[H.sub.4] are found to occupy the interlayer site in this mica structure. The spectra show noticeable changes on cooling. Autocorrelation analysis reveals distinct changes in the line width of the central autocorrelation peak. This is attributed to the transition of the N[D.sup.+.sub.4] group from an orientationally disordered state at higher temperatures to a relatively ordered state below a critical temperature of 140 K. The order parameter for the transition follows classical second-order behavior as a function of temperature.

Although natural occurrences of corundum + quartz + aluminosilicate are known, internally consistent thermodynamic databases suggest that they do not represent a stable assemblage. This observation has motivated two sets of experiments. In the first set, the equilibrium kyanite = corundum + quartz has been reversed ([+ or -]5 [degrees] C; [+ or -]25 MPa) at 600 [degrees] C/320 MPa and 700 [degrees] C/535 MPa (externally heated cold-seal hydrothermal autoclaves); 800 [degrees] C/775 MPa (gas apparatus; NaCl and Ca[F.sub.2] furnace assemblies, non end-load piston-cylinder press); and at 900 [degrees] C/1075 MPa and 1000 [degrees] C/1325 MPa (Ca[F.sub.2] furnace assembly, non end-load piston-cylinder press). These reversals imply an enthalpy of formation from the elements for kyanite of -2594.75 kJ/mol. The slope of the equilibrium curve also confirms both volume and experimental [C.sub.p] data for kyanite. These reversals can serve as a useful calibration for the piston-cylinder press using NaCl furnace assemblies in the 700-800 [degrees] C and 500-1000 MPa range and indicate a friction correction for Ca[F.sub.2], furnace assemblies of 75-100 MPa over 800-1000 [degrees] C and 1000-1500 MPa. The second set of experiments (1200 [degrees] C and 2000 MPa) investigated the growth of kyanite along corundum-quartz grain interfaces. In experiments where no fluid was present, except adsorbed [H.sub.2]O, kyanite did not nucleate and grow. In experiments with 2 wt% [H.sub.2]O, kyanite formed and grew preferentially in the pores surrounding the corundum grains parallel to the corundum-quartz interface and along quartz grain boundaries. Due to a large [DELTA][V.sub.R], reaction halos around the corundum grains never become closed to fluid migration. This suggests that in nature, fluids are channeled to these reaction sites via porous reaction halos surrounding the corundum grains and indicates that, under such circumstances, formation of kyanite is self-promoting and probably goes to completion quickly. The stability of sillimanite relative to corundum + quartz is also discussed, from the standpoints of what is predicted by internally consistent mineral databases and what is observed in nature.

This study investigates chlorapatite [ideally [Ca.sub.10][(P[O.sub.4]).sub.6][Cl.sub.2])] from two spatially separated locations in the Odegardens Verk apatite mine, Bamble Sector, SE Norway, which was partially metasomatized under amphibolite-facies conditions. Relative to the original chlorapatite, metasomatized areas in sample DL136 are enriched in OH and F, depleted in Na, Si, and (Y + REE) and contain numerous inclusions (1-15 [micro]m) of monazite and xenotime. In contrast, metasomatized areas in sample TN174 are enriched in F and Si, depleted in Na, show (Y + REE) abundances similar to the original chlorapatite, and host only a few monazite and xenotime grains. In order to define the role of fluids in the formation of (Y + REE)-phosphate minerals associated with apatite, a series of experiments covering wide ranges of temperature (900-300 [degrees] C) and pressure (1000-500 MPa) for a series of fluid compositions (pure [H.sub.2]O, a 50/50 molar mix of [H.sub.2]O and Ca[F.sub.2], and C[O.sub.2] with 1-2 wt% [H.sub.2]O) were conducted on unaltered chlorapatite from the [empty set]degardens Verk. In the [H.sub.2]O experiments, features due to metasomatic alteration [depletion in Na, Si, and (Y + REE) as well as numerous monazite and xenotime inclusions], as observed in the natural sample DL136, were fully reproduced. Monazite and xenotime grains were also observed growing on the surface of the apatite in dissolution embayments in metasomatized areas. In the 50/50 Ca[F.sub.2]-[H.sub.2]0 experiments, metasomatized regions are strongly enriched in F and Si, depleted in Na, show unchanged (Y + REE) abundances, and contain only a very few, small monazite and xenotime grains similar to what was observed in sample TN174. Natural and experimental observation imply the presence of two coupled substitutions: [Na.sup.+] + [(Y + REE).sup.3+] = 2 [Ca.sup.2+] and [Si.sup.4+] + [(Y + REE).sup.3+] = [P.sup.5+] + [Ca.sup.2+]. In the case of DLI36 and [H.sub.2]0-fluid experiments, [Na.sup.+] and [Si.sup.4+] went into solution, whereas the released (Y + REE) contributed to the growth of monazite and xenotime within the metasomatized apatite structure. For sample TN 174 and the 50/50 Ca[F.sub.2] + [H.sub.2]O-fluid experiments, Na in apatite became depleted but Si was enriched thereby stabilizing the (Y + REE) in the now recrystallized fluor-hydroxylapatite structure, which retarded the growth of monazite or xenotime inclusions. The basic conclusion of this study is that nucleation of monazite and xenotime in the metasomatized chlorapatite is principally a function of the composition of the infiltrating fluid and, to a much lesser extent, temperature and pressure.