Anne-Magali Seydoux-Guillaume

Experimental resetting of the U–Th–Pb systems in monazite

Abraded fragments (200–400 µm) of a large, chemically homogeneous, and non-metamict Brazilian monazite crystal, characterised by a concordant U–Pb ages of 474 +/- 1 Ma (208Pb/206Pb = 19.5), were hydrothermally treated at varying temperatures with solutions of different compositions. Product monazites were analysed with Scanning Electron Microscope (SEM), Electron Microprobe (EMP), Secondary Ion Mass Spectrometer (SIMS) and Isotope Dilution–Thermal Ionisation Mass Spectrometer (ID-TIMS). Experiments with pure water over a temperature range of 800–1200°C, at 700 MPa and durations ranging from 5 to 60 days showed that even at 1200°C any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain. Neither Pb diffusion at the EMP scale, nor significant discordancy were observed. We performed experiments at 800 and 1000°C for different durations using different fluid compositions at quartz saturation: a 10 wt.% CaCl2 fluid, a 10 wt.% SrCl2 fluid, a 10 wt.% NaCl fluid and a fluid containing NBS 982 Pb standard (208Pb/206Pb = 1). For all runs, EMP traverses revealed no Pb-diffusion profiles. Significant overgrowths of newly formed monazite are documented by SEM analyses. They occurred only in the 1000°C experiments when either CaCl2 or Pb-bearing fluids were present. In the CaCl2 experiment, two zones could be distinguished within the crystal: a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution/precipitation, which was enriched in Ca and Pb-free. ID-TIMS dating of single grains treated with SrCl2 and CaCl2 solutions at 1000°C are significantly discordant. Experiments employing the NBS Pb-standard produced sub-concordant monazite, for which the 207Pb/206Pb apparent age has become older than prior to the experiment (544 Ma at 800°C and 495 Ma at 1000°C). The newly grown monazite rim had obviously incorporated Pb from the fluid. None of our reaction products contained a detectable diffusion profile. The only resetting mechanism we detected involved dissolution/precipitation. The extent of the dissolution/precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks.

Annealing radiation damage and the recovery of cathodoluminescence

The structural recovery upon heat treatment of a highly metamict, actinide-rich zircon (U~6000 ppm) has been studied in detail using a range of techniques including X-ray powder diffraction, Raman spectroscopy, SHRIMP ion probe, electron microprobe, transmission electron microscopy and cathodoluminescence analysis. The structural regeneration of the amorphous starting material depends on random nucleation. It starts between 800 and 950°C when amorphous ZrSiO4 decomposes to form crystalline ZrO2 and amorphous SiO2. At around 1100°C, well-crystallised ZrSiO4 grows at the expense of the oxides. U has been retained in the newly grown zircon whereas Pb was evaporated during the heat treatment. This process is in marked opposition to the reconstitution of moderately metamict minerals, which experience a gradual recovery controlled by the epitaxial growth at the crystalline–amorphous boundaries. Both of these recovery processes are not the direct inverse of metamictisation. The structural regeneration was found to be connected with a significant increase in the emission of CL. In all cases (annealing heavily damaged zircon and moderately damaged zircon and monazite), we observe that the final, wellcrystallised annealing products emit more intense CL than their radiation-damaged starting minerals, although having almost identical elemental composition. Our observations are taken as evidence that the CL is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres.

Transmission electron microscope study of polyphase and discordant monazites: Site-specific specimen preparation using the focused ion beam technique

Electron-microprobe (EMP) U-Th-Pb dating on polyphase and discordant monazites from polymetamorphic granulites of the Andriamena unit (north-central Madagascar) reveals inconsistent chemical ages. To explain these drastic variations, transmission electron microscopy (TEM) foils were prepared directly from thin sections by using the focused ion beam technique. The most important result of the TEM study is the demonstration of the presence of small (~50 nm) Pb-rich domains where large variations in EMP ages occur. We suggest that radiogenic Pb was partially reincorporated in monazite during the recrystallization at 790 Ma. Because the excited volume of EMP is ~4 µm3, U-Th-Pb dating yielded various apparent older ages without geological significance. In addition, TEM analysis of the foils revealed the presence of an ~150-nm-wide amorphous zone along the grain boundary of monazite and its host quartz. This Fe-Si-Al–rich phase may have formed as a result of fluid activity at 500 Ma, and the phases amorphous state may be due to the irradiation from U and Th decay in the monazite. This demonstrates for the first time the enormous potential of the TEM investigations on site-specific specimens prepared with the focused ion beam technique for the interpretation of geochronological data.

Experimental determination of Thorium partitioning between monazite and xenotime using analytical electron microscopy and X-ray diffraction Rietveld analysis

The Thorium distribution between monazite and xenotime has been determined experimentally using the coupled substitution Th + Si=REE + P. Experiments have been conducted in standard cold seal hydrothermal and internally heated pressure vessels at 200MPa in the range of 600-1100°C. Starting mixtures were prepared from gels composed of equal amounts of CePO4 and YPO4 with addition of 10, 20 and 50 mol%ThSiO4. The grain sizes of the run products were in the range of a few microns. Analytical electron microscopy (AEM) methods were applied to obtain reliable chemical compositions of the reaction products. Lattice parameters of run products were determined using Rietveld analysis. For runs with 10 and 20 mol% ThSiO4 component in the bulk the ThSiO4 component distributes almost exclusively into monazite at all temperatures. The amount of the YPO4 component in monazite increases relative to the Th-free system if significant amounts of ThSiO4 are present within the structure. ThSiO4 favours incorporation of YPO4 resulting in a shift of the monazite limb and the shrinkage of the monazite-xenotime miscibility gap in the CePO4-YPO4-ThSiO4 ternary diagram. Thermometric calculations based on monazite-xenotime equilibria must be corrected for this effect. For runs with 50 mol% ThSiO4 in the bulk, thorite formed as an additional phase at 600 to 900°C but was absent at higher temperatures. At high Xbulk ThSiO4 and low T, the system is three-phase. The three-phase stability field strongly shrinks with increasing temperature. A tentative phase diagram of the ternary system CePO4-YPO4-ThSiO4 is proposed and discussed in the light of monazite-xenotime-thorite-bearing assemblages in natural rocks.

Contrasting response of ThSiO4 and monazite to natural irradiation

In order to compare the irradiation-induced behaviour of monazite and ThSiO 4 , a large single crystal of monazite from Norway (Arendal monazite) containing several ThSiO 4 inclusions was investigated. The estimated theoretical self-irradiation dose received by monazite near those inclusions was approximately 3 x 10 19 α-decay/g. Transmission electron microscopy (TEM) analyses were performed on both host and inclusion. The TEM samples were prepared by the focused ion beam (FIB) technique. Monazite and ThSiO 4 were found to have very different textures, including the presence of an amorphous zone between them. Crystalline monazite showed mottled diffraction contrast, which is characteristic of irradiation damage in this mineral. Observations suggest that there is a maximum defect density in monazite which cannot be exceeded. It is unlikely that some natural monazite can surpass this defect density. In contrast, ThSiO 4 is completely amorphous and exhibits an unusual spherical, bubble-like texture. The size of these spheres is in the range of 10 to 200 nm. It is proposed that ThSiO 4 is composed of aggregated spheres very similar to a gel. The zone between these two phases was certainly amorphized due to irradiation induced by α-decay of Th and U. It could be a preferential alteration zone and act as high-diffusive pathway for elements.

Partial resetting of the U-Th-Pb systems in experimentally altered monazite: Nanoscale evidence of incomplete replacement

Alteration experiments on natural monazite crystals (Manangotry standard, Madagascar) under alkali conditions at 300, 400, 500 and 600 °C and 200 MPa were conducted to clarify mechanisms behind incomplete resetting of U-Th-Pb geochronological systems in monazite replaced by dissolution and precipitation processes. Above 400 °C, experimental products show typical replacement textures: a compositionally distinct monazite rim, referred as altered rim, surrounds the primary monazite (Mnz1). Isotopic and electron microprobe U-Th-Pb in situ dating of the altered rim yields intermediate ages between pristine monazite (555 Ma) and complete experimental resetting (0 Ma). Lead is systematically detected in altered rims, with concentration decreasing from 400 °C to 600 °C. The origin of incomplete resetting is elucidated with transmission electron microscope images that reveal an incomplete replacement of Mnz1 by a secondary monazite (Mnz2) within the altered rim. With increasing temperature, the size and volume of the Mnz2 within the altered rim become more important. Because no structural Pb or Pb nanoinclusions were observed, Pb in the altered rim is attributed to the Mnz1 component. Partial resetting of U-Th-Pb systems depends on the nanomixture of different Mnz1 proportions in the analyzed volume, and explains the higher rejuvenation at 600 °C than at lower temperatures. Although microanalytical techniques have the spatial resolution to date micrometer-sized rims, they are unable to resolve a nanoscale mixture of pristine and secondary monazite that could occur in altered rims formed by fluid-driven replacement, especially at low temperatures. Porosity and/or inclusions and complex age scattering in zoned monazite are significant markers that can indicate a possible nano-sized partial replacement.

In situ characterization of infrared femtosecond laser ablation in geological samples. Part A: the laser induced damage

Infrared femtosecond laser induced damage has been studied in order to determine, with analytical protocols, the processes involved in laser ablation in this regime. Transmission Electron Microscopy (TEM) coupled with Focused Ion Beam (FIB) milled cross-sections of natural ablated monazite were used. Craters were formed using N = 1 and 3 shots, E0 = 0.1 and 0.8 mJ per pulse and τ = 60 fs. Observations revealed that laser settings induce little changes in the nature and size of damaged structures. The crater bottom forms a ∼0.5 μm layer composed of melted and recrystallized monazite grains, and spherical ∼10 nm voids. The underlying sample shows lattice distortions, progressively attenuated with depth, typical of mechanical shocks (thermoelastic relaxation and plasma recoil pressure). No chemical difference appears between these two domains, excluding preferential vaporization and thus laser induced chemical fractionation. Correlations with existing molecular dynamics (MD) simulations indicate that the deep distorted lattice probably undergoes spallation whereas the upper layer rather goes through homogeneous nucleation. Nevertheless, these processes are not pushed forward enough to induce matter removal in the present conditions. In consequence, photomechanical fragmentation and vaporization, requiring higher energy density states, would rather be the main ablation mechanisms. This hypothesis was supported by an additional study focused on the laser produced aerosols. Further links to LA-ICP-MS measurements can then be developed.

Near Infra Red femtosecond Laser Ablation: the influence of energy and pulse width on the LA-ICP-MS analysis of monazite

We studied the influence of pulse energy (E0) and pulse width (τ) of Near Infra Red femtosecond Laser Ablation coupled to Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). A particular emphasis was put on the 206Pb/238U and 208Pb/232Th repeatability from Moacyr monazite (Itambe, Brazil). Synthetic glass (Standard Reference Material) NIST610 was used as a reference material, as well as a monazite from Manangotry (Madagascar). Pulse energy was investigated in the range E0 = 0.03 to 0.8 mJ/pulse (τ = 60 fs) while pulse width has been studied from τ = 60 fs to 300 fs (E0 = 0.1 mJ/pulse). Data suggest that pulse width has no detectable influence on the accuracy and repeatability of measured elemental ratios in the range of 60–300 fs. Observed measurements repeatabilities are 2.5%RSD and 1.8%RSD for 206Pb/238U and 208Pb/232Th, respectively. At 60 fs, the 0.03–0.8 mJ/pulse energy range studied does not induce any detectable change in data accuracy and repeatability. Uncertainties of 1.1–2.8%RSD were obtained for 206Pb/238U. In the range of E0 = 0.1–0.8 mJ/pulse, matrix matched calibration using Manangotry monazite gives a similar good repeatability of 2.4%RSD for 206Pb/238U. No clear matrix effect could be highlighted.

Dominance of mechanical over thermally induced damage during femtosecond laser ablation of monazite

Effects of infrared femtosecond laser ablation (800 nm, 60 fs, 5 Hz, 85 μJ/pulse, objective × 15) of a well-characterized monazite on its micro- and nano-structure were investigated. Craters were produced by single and multiple pulses ( N = 10, 20, 50, 150 and 300) to follow the evolution of laser-induced damage in monazite using Scanning Electron Microscope (SEM), and Transmission Electron Microscope (TEM) coupled with Focused Ion Beam (FIB) sample preparation, in order to characterize this damage. Voids are observed within craters from the first pulse and cracks appear already after 10 pulses, at the sample surface; radial cracks are well-defined for 50 pulses, and become conchoidal after 150 pulses, indicating high-strain fields in the vicinity of craters. After the first pulse, the monazite lattice is highly strained to depths greater than ~1 μm with a spotty ring diffraction pattern demonstrating that the damaged monazite is a mosaic crystal. Under this area monazite is moderately strained over 6 μm in depth. Crack formation within the crystal is observed from the first pulse. Cracks formed at the surface and propagated over 2 μm into the crystal. Their number increased notably after 10 pulses, with some cracks propagating 8 μm into the crystal. Increasing lattice defects (mosaic crystal, twins) and fracture intensities demonstrate that a cumulative effect exists. Part of the energy carried by the laser is stored within the crystal and used in the formation of defects. This study highlights the intense damages that are created during a femtosecond laser ablation in monazite. Mechanical effects dominate thermal ones, limited to a thin layer (200 nm–1 pulse) of resolidified monazite, and are induced by high-pressure shock wave from plasma expansion.

Fluid-mediated alteration of (Y,REE,U,Th)–(Nb,Ta,Ti) oxide minerals in granitic pegmatite from the Evje-Iveland district, southern Norway

We document the textural relations and chemical composition of (Y,REE,U,Th)–(Nb,Ta,Ti) oxide minerals in a granitic pegmatite from the Evje-Iveland district, southern Norway, using a combination of scanning and transmission electron microscopy, electron probe micro-analysis and infrared absorption spectroscopy. The (Y,REE,U,Th)–(Nb,Ta,Ti) oxide mineral is euxenite, which is strongly radiation damaged and surrounded by radial fractures. Within euxenite grains, three domains of distinct composition comprising unaltered, intermediate and altered euxenite, have been identified. In most cases pyrochlore occurs as corroded grain boundaries around euxenite and within relict fractures. Intermediate and altered euxenite are depleted in U, Pb, Ti, Nb, and Y, but enriched in Si and Ca relative to unaltered euxenite. Pyrochlore is also enriched in Fe, Pb, Zr and LREE relative to all euxenite phases. Altered domains of euxenite have deficient analytical totals and contain O-H. These domains are metamict and contain nanopores and nanodomains enriched in U and Ca. We suggest that as radiation damage accumulated in euxenite, radial fractures developed around the euxenite grains, thus allowing fluid infiltration. In the presence of fluid, euxenite was replaced by secondary euxenite then pyrochlore, owing to dissolution-precipitation and diffusion reactions. During alteration, U and the strategic metals Nb, Ti, and REE were mobilized at both the nanoscale and the scale of the pegmatite.