Daniel E. Harlov

Fluid-induced nucleation of (Y + REE)-phosphate minerals within apatite: Nature and experiment. Part I. Chlorapatite

Abstract This study investigates chlorapatite [ideally Ca10(PO4)6Cl2)] from two spatially separated locations in the Ødegårdens 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 μm) 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 °C) and pressure (1000-500 MPa) for a series of fluid compositions (pure H2O, a 50/50 molar mix of H2O and CaF2, and CO2 with 1-2 wt% H2O) were conducted on unaltered chlorapatite from the Ødegårdens Verk. In the H2O 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 CaF2-H2O 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+ + (Y + REE)3+ = 2 Ca2+ and Si4+ + (Y + REE)3+ = P5+ + Ca2+. In the case of DL136 and H2O-fluid experiments, Na+ and Si4+ went into solution, whereas the released (Y + REE) contributed to the growth of monazite and xenotime within the metasomatized apatite structure. For sample TN174 and the 50/50 CaF2 + H2O-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.

Apatite-monazite relations in the Kiirunavaara magnetite-apatite ore, northern Sweden

The magnetite–apatite ores in the Kiruna area, northern Sweden, are generally considered to be of magmatic origin formed in a subvolcanic–volcanic environment during the early Proterozoic. They are thought to have crystallised from volatile-rich iron oxide magmas derived by immiscibility in calc-alkaline to slightly alkaline parental magmas. Three major morphological types of the magnetite–apatite ore (primary, brecciated, and banded) have been investigated for textural relations and mineral chemistry using transmitted light, back-scattered electron imaging (BSE), electron microprobe analysis (EMPA), and laser ablation–inductively coupled plasma-mass spectrometry (LA–ICPMS). In all three types, Th- and U-poor monazite is present as small inclusions in the apatite. Larger (up to 150 μm) recrystallised monazite grains, both along apatite grain boundaries and intergrown with magnetite and silicate minerals, are present in the brecciated and banded samples. Primary apatite grains, without monazite inclusions, are generally enriched in light rare earth elements (LREEs) together with Na and Si. Petrological and mineralogical evidence suggest that the Kiruna magnetite–apatite ore experienced successive stages of fluid–rock interaction. The first stage occurred under high-temperature conditions (700–800 °C) shortly after emplacement and crystallisation of the ore magmas and involved concentrated, probably Cl-dominated brines expelled from the magma. This fluid is held to be responsible for the nucleation of the numerous small monazite inclusions within the apatite due to high-temperature leaching of Na and Si, while the LREEs were concentrated in the monazite. The large monazite grains in the brecciated and banded samples are proposed to be the product of recrystallisation from the much smaller monazite inclusions. During greenschist-facies metamorphism (T=300–400 °C), fluids from the surrounding country rock caused strong (LREE+Na+Si) depletion along apatite grain boundaries and cracks in the apatite. LREEs were either redeposited as monazite grains along apatite grain boundaries or were flushed out of the ore. This fluid interaction also introduced the silicate components responsible for the interstitial formation of allanite, talc, tremolite, chlorite, serpentine, muscovite, quartz, and carbonates along apatite grain boundaries.

Partial high-grade alteration of monazite using alkali-bearing fluids: Experiment and nature

Abstract Recent advances in the dating of monazite using the electron microprobe indicate that ThSiO4- and/ or CaTh(PO4)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 °C utilizing a natural, unzoned, Th- and Pb-bearing monazite-(Ce) and Na2Si2O5+H2O. During the experiment, a subset of the monazite grains acquired ThSiO4-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 dissolutionreprecipitation. 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

Experimental determination of stability relations between monazite, fluorapatite, allanite, and REE-epidote as a function of pressure, temperature, and fluid composition

Abstract The experimental alteration of monazite to allanite, REE-epidote, fluorapatite, and/or fluorapatitebritholite was investigated at 450 to 610 MPa and 450 to 500 °C. Experiments involved monazite + albite ± K-feldspar + muscovite ± biotite + SiO2 + CaF2 and variety of fluids including H2O, (KCl + H2O), (NaCl + H2O), (CaCl2 + H2O), (Na2Si2O5 + H2O), 1 M HCl, 2 M NaOH, 2 M KOH, 1 M Ca(OH)2, 2 M Ca(OH)2, and (CaCO3 + H2O). The reaction products, or lack thereof, clearly show that the stability relations between monazite, fluorapatite, and allanite or REE-epidote are more dependent on the fluid composition and the ratio of silicate minerals than on the P-T conditions. A high Ca content in the fluid promotes monazite dissolution and the formation of fluorapatite and allanite or REE-epidote. Lowering the Ca content and raising the Na content in the fluid decreases the solubility of monazite but promotes the formation of allanite. Replacing Na with K in the same fluid causes fluorapatite, with a britholite component, to form from the monazite. However, allanite and REE-epidote are not formed. Monazite is stable in the presence of NaCl brines. In KCl brine, monazite shows a very limited reaction to fluorapatite. When the fluid is (Na2Si2O5 + H2O), strong dissolution of monazite occurs resulting in the mobilization of REEs, and actinides to form fluorapatite-britholite and turkestanite. These experimental results are consistent with natural observations of the partial to total replacement of monazite by fluorapatite, REE-epidote, and allanite in fluid-aided reactions involving the anorthite component in plagioclase at mid- to high-grade metamorphic conditions. In contrast, an alkali-bearing environment with excess Na prevents the growth of allanite and eventually promotes the precipitation of secondary monazite. The results from this study provide implications for geochronology and for deducing fluid compositions in metamorphic rocks.

Characterization of fluor-chlorapatites by electron probe microanalysis with a focus on time-dependent intensity variation of halogens

Abstract Prior research has shown that fluorine and chlorine X-ray count rates vary with exposure to the electron beam during electron probe microanalysis (EPMA) of apatite. Stormer et al. (1993) and Stormer and Pierson (1993) demonstrate that the EPMA-operating conditions affect the halogen intensities in F-rich natural Durango and Wilberforce apatites and in a Cl-rich apatite. Following these studies, we investigated the effects of operating conditions on time-dependent X-ray intensity variations of F and Cl in a broad range of anhydrous fluor-chlorapatites. We tested 7, 10, and 15 kV accelerating voltages; 4, 10, and 15 nA beam currents; 2, 5, and 10 μm diameter fixed spot sizes; and the influence of 2 distinct crystal orientations under the electron beam. We find that the halogen X-ray intensity variations fluctuate strongly with operating conditions and the bulk F and Cl contents of apatite. We determined the optimal EPMA operating conditions for these anhydrous fluor-chlorapatites to be: 10 kV accelerating voltage, 4 nA beam current (measured at the Faraday cup), 10 μm diameter fixed spot, and the apatite crystals oriented with their c-axes perpendicular to the incident electron beam. This EPMA technique was tested on a suite of 19 synthetic anhydrous apatites that covers the fluorapatite-chlorapatite solid-solution series. The results of these analyses are highly accurate; the F and Cl EPMA data agree extremely well with wet-chemical analyses and have an R2 value >0.99.

Metasomatic thorite and uraninite inclusions in xenotime and monazite from granitic pegmatites, Hidra anorthosite massif, southwestern Norway: Mechanics and fluid chemistry

Abstract 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 H2O, but also contained F and minor amounts of Cl, and significant quantities of dissolved Na+ and K+. 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.

The relative stability of monazite and huttonite at 300–900 °C and 200–1000 MPa: Metasomatism and the propagation of metastable mineral phases

Abstract Monazite is both partially replaced and overgrown by a ThSiO4 phase along grain rims in a series of experiments from 300 to 900 °C and 200 to 1000 MPa. All experiments consisted of 10 mg of 100.500 μm size, euhedral to subhedral crystals of a natural Th-free monazite-(Ce), 5 mg of Th(NO3)4·5H2O, 2.5 mg of SiO2, and 5 mg of H2O loaded into 3 mm wide, 1 or 1.3 cm long platinum capsules that were arc welded shut. Experimental conditions were: 300 °C at 200 and 500 MPa; 300, 400, 500, 600, and 700 °C at 500 MPa (cold seal hydrothermal autoclave); and 900 °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 °C, the ThSiO4 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 °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.

Petrologic evidence for K-feldspar metasomatism in granulite facies rocks

Abstract We present evidence for K-feldspar metasomatism in charnockitic granulites from two well-known terranes: the Shevaroy Hills Massif, S. India (750°C, 8 kbars) and the Bamble Sector, S.E. Norway (790°C, 7.5 kbars) in the form of K-feldspar veins principally along plagioclase and quartz grain boundaries and in the form of highly variable antiperthitic patches of K-feldspar in an uneven scattering of plagioclase grains. With one exception, orthopyroxene or amphibole grains in contact with these K-feldspar veins show no alteration to secondary biotite, indicating that the H 2 O activity of the fluids responsible for these veins must have been relatively low. A high Ba concentration in these veins also suggests a metasomatic origin. Point counted, back-scattered electron photomicrographs, along with microprobe analyses, provide reintegrated K-feldspar and plagioclase compositions for 8 to 12 predetermined random areas per thin section for three samples from Bamble and three samples from the Shevaroy Hills. These reintegrated feldspar compositions plot over a range of temperatures on the feldspar ternary for each sample, and indicate saturation temperatures above the mean temperature for either region with a few reintegrated compositions above the 1000°C isotherm and with the lower cut-off temperatures at 700°C and 600°C for the Shevaroy and Bamble samples, respectively. These patterns suggest that exsolution alone could not have been responsible for the formation of the K-feldspar veins and patches in these rocks. We suggest that these veins are due to the influx of complex, supercritical, low H 2 O activity brines shortly after peak metamorphic conditions, that this influx continued during the initial phases of post-peak metamorphic uplift and that these fluids represent the first stage in a series of fluid influxes in which the H 2 O activity increased as uplift continued. Reaction of these brines with potassium-undersaturated plagioclase grains formed K-feldspar veins along grain boundaries and fractures, as well as diffusing into the plagioclase, which became supersaturated, and exsolved K-feldspar as antiperthite patches during uplift and cooling. Formation of secondary biotite halos around orthopyroxene in one sample associated with these K-feldspar veins can be explained by heightened H 2 O activity in the brines due to enrichment in H 2 O, emplacement of the veins at lower pressure during uplift or both compared to the other Shevaroy and Bamble samples.

Stability of corundum + quartz relative to kyanite and sillimanite at high temperature and pressure

Abstract 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 (±5 °C; ±25 MPa) at 600 °C/320 MPa and 700 °C/535 MPa (externally heated cold-seal hydrothermal autoclaves); 800 °C/775 MPa (gas apparatus; NaCl and CaF2 furnace assemblies, non end-load piston-cylinder press); and at 900 ∞C/1075 MPa and 1000 °C/1325 MPa (CaF2 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 CP data for kyanite. These reversals can serve as a useful calibration for the piston-cylinder press using NaCl furnace assemblies in the 700-800 ∞C and 500- 1000 MPa range and indicate a friction correction for CaF2 furnace assemblies of 75-100 MPa over 800-1000 °C and 1000-1500 MPa. The second set of experiments (1200 °C and 2000 MPa) investigated the growth of kyanite along corundum-quartz grain interfaces. In experiments where no fluid was present, except adsorbed H2O, kyanite did not nucleate and grow. In experiments with 2 wt% H2O, 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 DVR, 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.

Solution calorimetric investigation of fluor-chlorapatite crystalline solutions

Abstract Solution calorimetric measurements have been made on 17 synthetic fluorapatite-chlorapatite crystalline solutions at 50°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 (XCl > 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