A detailed and comprehensive TEM, EPMA and Raman characterization of high-metamorphic grade monazites and their U-Th-Pb systematics (the Góry Sowie Block, SW Poland)
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Eleven monazite grains, two from a migmatitic gneiss and nine from two felsic granulites from the Góry Sowie Block (SW Poland) were studied with transmission electron microscopy (TEM), electron probe microanalysis (EPMA), Raman microspectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Th-Pb analysis in order to assess processes affecting U-Th-Pb age record. Two monazite grains from the migmatitic gneiss are patchy zoned in BSE imaging and overgrown by allanite, whereas Raman results indicate moderate radiation damage. Monazite in the corresponding TEM foils shows twins and nanoinclusions of fluorapatite, thorianite, goethite, titanite, chlorite and CaSO4. Furthermore, monazite is partially replaced by secondary monazite, forming ca. 100 nm-thick layers, and calcite along grain boundaries. The submicron alterations had little or no effect on the Pb/U and Pb/Th dates, when compared to earlier age constraints on the metamorphism in the studied region. In contrast, monazite from both granulites is homogeneous in eight investigated TEM foils, contains no solid or fluid nanoinclusions or any signs of fluid-induced alterations, with only one exception of a ca. 140 nm-thick crack filled with monazite. The 206Pb/238U and marginally older 208Pb/232Th mean dates pulled for all data show good coherence. However, the 207Pb/235U isotopic record is disturbed due the presence of common Pb within the entire monazite grain in one granulite and in the cores of two monazite grains in the second granulite, where the UPb data of the rims are not compromised and concordant. Due to lack of TEM evidence for fluid-mediated alterations, the age discordance has to be related to addition of common Pb in the monazite lattice or in the micro-cracks. To summarize, the 208Pb/232Th data reveal the most accurate ages, which are consistent with previous geochronological studies in the region. Therefore, the Pb/Th chronometer, which is less compromised by age disturbance compared to Pb/U ages, is recommended for monazite geochronology. Application of the submicron scale investigations using TEM is recommended to evaluate potential presence of the submicron inclusions of Pb-bearing phases or compositional alterations of monazite that can remain unnoticed by using standard microanalytical instruments.Keywords:
Allanite
Metamictization
Fluorapatite
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°C and pressures of 500 and 1000 MPa. Fluids used included pure H2O, various NaCl, KCl, and CaCl2 brines (salt/H2O = 50/50, 30/70, or 10/90), and either 90/10 CO2/H2O or 40/60 CO2/H2O mix. Monazite formed in the fluorapatite + H2O, fluorapatite + 40/60 CO2/H2O, and the fluorapatite + KCl brine experiments. At 900°C and 1000 MPa, monazite formed both as inclusions within the fluorapatite and externally on its surface. Below 900°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°C and 1000 MPa, is compositionally heterogeneous, specifically with respect to the Th content (ThO2 = 4-38 wt%). Whereas the reactant fluorapatite in the pure H2O experiments remained unzoned at lower temperatures, three coupled zones with different (LREE+Si+Na) abundances developed at 900°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 CaCl2 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
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The phosphate mineral monazite (LREE,Y,Th,Ca,Si)PO 4 occurs as an accessory phase in peraluminous granites and Ca-poor meta-psammopelites. Due to negligible common Pb and very low Pb diffusion rates at high temperatures, monazite has received increasing attention in geochronology. As the monazite grain sizes are mostly below 100 μm in upper greenschist to amphibolite facies meta-psammopelites, and rarely exceed 250 μm in granulite facies gneisses and in migmatites, microstructural observation and mineral chemical analysis need the investigation by scanning electron microscope and electron probe microanalyzer, with related routines of automated mineralogy. Not only the microstructural positions, sizes and contours of the grains, but also their internal structures in backscattered electron imaging gray tones, mainly controlled by the Th contents, can be assessed by this approach. Monazite crystallizes mostly euhedral to anhedral with more or less rounded crystal corners. There are transitions from elliptical over amoeboid to strongly emarginated grain shapes. The internal structures of the grains range from single to complex concentric over systematic oszillatory zonations to turbulent and cloudy, all with low to high contrast in backscattered electron imaging gray tones. Fluid-mediated partial alteration and coupled dissolution-reprecipitation can lead to Th-poor and Th-rich rim zones with sharp concave boundaries extending to the interior. Of particular interest is the corona structure with monazite surrounded by apatite and allanite, which is interpreted to result from a replacement during retrogression. The satellite structure with an atoll-like arrangement of small monazites may indicate re-heating after retrogression. Cluster structures with numerous small monazite grains, various aggregation structures and coating suggest nucleation and growth along heating or/and enhanced fluid activity. Microstructures of monazite fluid-mediated alteration, decomposition and replacement are strongly sutured grain boundaries and sponge-like porosity and intergrowth with apatite. Garnet-bearing assemblages allow an independent reconstruction of the pressure-temperature evolution in monazite-bearing meta-psammopelites. This provides additional potential for evaluation of the monazite microstructures, mineral chemistry and Th-U-Pb ages in terms of clockwise and counterclockwise pressure-temperature-time-deformation paths of anatectic melting, metamorphism and polymetamorphism. That way, monazite microstructures serve as unique indicators of tectonic and geodynamic scenarios.
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Abstract The textural and chemical evolution of allanite and monazite along a well‐constrained prograde metamorphic suite in the High Himalayan Crystalline of Zanskar was investigated to determine the P–T conditions for the crystallization of these two REE accessory phases. The results of this study reveals that: (i) allanite is the stable REE accessory phase in the biotite and garnet zone and (ii) allanite disappears at the staurolite‐in isograd, simultaneously with the occurrence of the first metamorphic monazite. Both monazite and allanite occur as inclusions in staurolite, indicating that the breakdown of allanite and the formation of monazite proceeded during staurolite crystallization. Staurolite growth modelling indicates that staurolite crystallized between 580 and 610 °C, thus setting the lower temperature limit for the monazite‐forming reaction at ~600 °C. Preservation of allanite and monazite inclusions in garnet (core and rim) constrains the garnet molar composition when the first monazite was overgrown and subsequently encompassed by the garnet crystallization front. Garnet growth modelling and the intersection of isopleths reveal that the monazite closest to the garnet core was overgrown by the garnet advancing crystallization front at 590 °C, which establishes an upper temperature limit for monazite crystallization. Significantly, the substitution of allanite by monazite occurs in close spatial proximity, i.e. at similar P–T conditions, in all rock types investigated, from Al‐rich metapelites to more psammitic metasedimentary rocks. This indicates that major silicate phases, such as staurolite and garnet, do not play a significant role in the monazite‐forming reaction. Our data show that the occurrence of the first metamorphic monazite in these rocks was mainly determined by the P–T conditions, not by bulk chemical composition. In Barrovian terranes, dating prograde monazite in metapelites thus means constraining the time when these rocks reached the 600 °C isotherm.
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Monazite, a typical light rare-earth element (LREE) mineral of S-type granitoids in the Western Carpathians, was found in the peraluminous biotite granodiorite-tonalite in the Tribeč Mountains commonly containing polymineralic inclusions. These inclusions are dominated by anhedral allanite, although allanite also occurs rarely as discrete grains not enclosed by monazite. The monazite studied here is relatively homogeneous and characterized by high Th contents with proportions of huttonite (ThSiO4) and brabantite [CaTh(PO4)2] up to 14.6 and 9.3%, respectively. The discrete allanite grains are highly aluminous with a composition consistent with the peraluminous type of host rock. However, allanite included in monazite is extremely variable in LREE, Al, Fe, and Mg contents. This variation is interpreted to result from entrapment of allanite (+ melt) in monazite before local equilibrium was attained. The change from allanite to monazite as the stable LREE-rich phase is related to an overall decrease in Ca concentration caused by the onset of plagioclase crystallization. The early precipitation of allanite was possible because of the high LREE concentrations in the melt. The crystallization temperature of allanite must have been higher than monazite saturation (>856-845 °C and 798-790 °C for two analyzed samples). The Zr saturation temperature based on zircon solubility and REE thermometry based on monazite solubility reflect an increase in temperature from the edge to the center of the pluton, which coincides with an increase in the huttonite content in monazite. The primary LREE assemblage is accompanied by small grains of late huttonite(?) replacing monazite and brabantite replacing allanite.
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Abstract Fluorapatite grains with monazite inclusions and/or rim grains are described in two of four samples from a set of granulite-facies metapelites collected from the Variscan Schwarzwald, southern Germany. Fluorapatite in all four samples appears to have experienced some dissolution in the partial granitic melt formed during granulite-facies metamorphism. Monazite inclusions and rim grains are highly deficient in Th and are presumed to have formed from fluorapatite in association with partial melting during granulite-facies metamorphism. Monazite inclusions range from very small (<1 μm) and very numerous to small (1–2 μm), sometimes elongated, and less numerous; both types are evenly distributed throughout the fluorapatite grain interior. Monazite rim grains tend to be 1–10 μm. The formation of monazite inclusions is proposed to be due to dissolution-reprecipitation of the fluorapatite by the aqueous fluids inherent in the granitic melt. We propose that an increase in inclusion size coupled with a decrease in inclusion number is due to Ostwald ripening (interfacial energy reduction), which is greatly facilitated by the presence of an interconnected, fluid-filled porosity in the metasomatized fluorapatite. We further propose that monazite rim grains formed principally during partial dissolution of the fluorapatite in the granitic melt and to a lesser extent by partial dissolutionreprecipitation of the fluorapatite grain rim area allowing for the partial removal of (Y+ REE ). We conclude that fluorapatite, with monazite inclusions and rim grains, experienced partial dissolution in a H2O-rich peraluminous granitic melt compared to fluorapatite with monazite rim grains and no inclusions which reacted with a similar, relatively less H 2 O-rich melt. In contrast, monazite-free fluorapatite experienced partial dissolution in a comparatively H 2 O-poor, subaluminous, possibly peralkaline melt.
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Allanite-fluorapatite reaction coronas around monazite are abundant in metamorphic rocks. We report here special cases where a new generation of “satellite” monazite grains formed within these coronas. Using examples from different P-T regions in the eastern Alps, we examine the origin and the petrological significance of this complex mineralogical association by means of the electron microprobe utilizing Th-U-Pb monazite dating and high-resolution BSE imaging. Satellite monazite grains form when a monazite-bearing rock is metamorphosed in the allanite stability field (partial breakdown of first generation monazite to fluorapatite plus allanite), and is then heated to temperatures that permit a back reaction of fluorapatite plus allanite to secondary satellite monazite grains surrounding the remaining original first generation monazite. Depending on the whole-rock geochemistry satellite monazites can form under upper greenschist- as well as amphibolite-facies conditions. In each of the three examples focused on here, the inherited core monazite was resistant to recrystallization and isotopic resetting, even though in one of the samples the metamorphic temperatures reached 720 °C. This shows that in greenschist- and amphibolite-facies polymetamorphic rocks, individual grains of inherited and newly formed monazite can and often will occur side by side. The original, inherited monazite will preferentially be preserved in low-Ca, high-Al lithologies, where its breakdown to allanite plus fluorapatite is suppressed. Conversely, a medium- or high-Ca, monazite-bearing rock will become particularly fertile for secondary monazite regrowth after passing through a phase of strong retrogression in the allanite stability field. Based on this knowledge, specific sampling strategies for monazite dating campaigns in polymetamorphic basement can be developed.
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The REE enrichment process in fluorapatite and the REE redistribution among fluorapatite, monazite, and allanite were studied in a series of three sets of experimental runs at P-T conditions of 0.5 to 4 GPa and 650 to 900 °C. The first two sets of experimental runs utilized fluorapatite as a P-source, synthetic monazite or allanite as the REE sources, albite, quartz, and NaF-H2O or NaCl-H2O. The third set of runs was carried out with powdered Ca3(PO4)2, allanite, quartz, (±Al2O3), and a NaF-H2O solution.
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The Sin Quyen deposit in northwestern Vietnam is composed of Fe-Cu-LREE-Au ore bodies hosted in Proterozoic metapelite. There are massive and banded replacement ores with variable amounts of monazite-(Ce) and chevkinite-(Ce) crystals, which have undergone fluid-induced alteration. Monazite-(Ce) and chevkinite-(Ce) were deposited from high-temperature fluids in the early ore-forming stage, but became thermodynamically unstable, and thus were altered to other phases in later ore-forming stages. The alteration of monazite-(Ce) formed a three-layered corona texture, which commonly has relict monazite-(Ce) in the core, newly formed fluorapatite in the mantle, and newly formed allanite-(Ce) in the rim. In some cases, the original monazite-(Ce) was completely consumed, forming a core of polygonal fluorapatite crystals rimmed by allanite-(Ce) crystals. The formation of allanite-(Ce) and fluorapatite at the expense of monazite-(Ce) indicates that the later-stage fluids had high Ca/Na ratios and relatively low temperatures. Chevkinite-(Ce) was variably replaced by an assemblage of allanite-(Ce) + aeschynite-(Ce) ± bastnäsite-(Ce) ± columbite-(Fe) ± ilmenite. The replacement of chevkinite-(Ce) by mainly allanite-(Ce) and aeschynite-(Ce) required low-temperature, Ca-, LREE-, and Nb-rich metasomatic fluids, probably with relatively low fo2.
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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.
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