Huttonite

Abstract: A systematic study with laser ablation—ICP-MS, scanning electron microscopy and electron microprobe revealed that ∼70–95 wt% of REE (except Eu), Y, Th and U in granite rocks and crustal protoliths reside within REEYThU-rich accessories whose nature, composition and associations change with the rock aluminosity. The accessory assemblage of peraluminous granites, migmatites and high-grade rocks is composed of monazite, xenotime (in low-Ca varieties), apatite, zircon, Thorthosilicate, uraninite and betafite-pyrochlore. Metaluminous granites have allanite, sphene, apatite, zircon, monazite and Thorthosilicaie. Peralkaline granites have aeschinite, fergusonite, samarskite, bastnaesite, fluocerite, allanite, sphene, zircon, monazite, xenotime and Th-orthosilicate. Granulite-grade garnets are enriched in Nd and Sm by no less than one order of magnitude with respect to amphibolite-grade garnets. Granulitegrade feldspars are also enriched in LREE with respect to amphibolite-grade feldspars. Accessories cause non-Henrian behaviour of REE, Y, Th and U during melt—solid partitioning. Because elevated fractions of monazite, xenotime and zircon in common migmatites are included within major minerals, their behaviour during anatexis is controlled by that of their host. Settling curves calculated for a convecting magma show that accessories are too small to settle appreciably, being separated from the melt as inclusions within larger minerals. Biotite has the greatest tendency to include accessories, thereby indirectly controlling the geochemistry of REE, Y, Th and U. We conclude that REE, Y, Th and U are unsuitable for petrogenetical modelling of granitoids through equilibrium-based trace-element fractionation equations.

Abstract: Monazite and xenotime, the RE(P04) dimorphs, are the most ubiquitous rare earth (RE) minerals, yet accurate structure studies of the natural phases have not been reported. Here we report the results of high-precision structure studies of both the natural phases and the synthetic RE(P04) phases for all individual stable rare earth elements. Monazite is monoclinic, P2/n, and xenotime is isostructural with zircon (space group 14/amd). Both atomic arrangements are based on [001] chains of intervening phosphate tetrahedra and RE polyhedra, with a REOgpolyhedron in xenotime that accommodates the heavy lanthanides (Tb-Lu in the synthetic phases) and a RE09 polyhedron in monazite that preferentially incorporates the larger light rare earth elements (La-Gd). As the structure "transforms" from xenotime to monazite, the crystallographic properties are comparable along the [00I] chains, with structural adjustments to the different sizes of RE atoms occurring principally in (00I). There are distinct similarities between the structures that are evident when their atomic arrangements are projected down [001]. In that projection, the chains exist in (100) planes, with two planes per unit cell. In monazite the planes are offset by 2.2 A along [010], relative to those in xenotime, in order to accommodate the larger light RE atoms. The shift of the planes in monazite allows the RE atom in that phase to bond to an additional 02' atom to complete the RE09 polyhedron.

Abstract: A series of monazite dissolution experiments was conducted in a hydrous (1–6 wt%) granitic melt at 8 kbar over the temperature range 1,000–1,400° C A polished cube of monazite was immersed in a natural obsidian melt and allowed to partially dissolve Electron microprobe traverses perpendicular to the crystal-melt interface revealed concentration gradients in the LREEs and P Diffusivities of the LREEs and P were calculated from these profiles, yielding the following Arrhenius relations for the LREEs: D=023 exp(−601 kcal mol−1/RT) at 6% water D=230×107 exp(−1221 kcal mol−1/RT) at 1% water These results demonstrate the importance of dissolved water on REE diffusion Phosphorus diffusivities are nearly identical to those of the rare-earths, suggesting that P diffusion charge-compensates REE diffusion The concentration of LREEs required for monazite saturation in these melts is given by the level of dissolved LREEs at the crystal-melt interface These values also show a dependence on dissolved water, with LREEsat=60 ppm at 6% H2O when extrapolated down to 700° C, and LREEsat=30 ppm at 1% H2O Calculated dissolution rates based on the above parameters indicate that minute ( 2% H2O), whereas larger (> 50 μm) crystals will likely be residual over the duration of an anatectic event The low solubility of monazite in this melt suggests that the LREE depletion observed in some felsic differentiation suites may be the result of monazite crystallization Limited experimental and geochemical/petrologic evidence indicates that compositional changes in the melt accompanying differentiation decrease the solubility of monazite drastically Kinetic and chemical constraints may also lead to localized monazite saturation and inclusion in major phases or even other accessories Variations in the REE composition of monazite from different parageneses probably reflects the REE pattern of the parent melt, and may be due to gradational differences in the stability of individual or subgroup REE-complexes as a function of melt composition Particularly important in this regard seems to be the lime+alkali/alumina balance of the melt and its volatile content

Abstract: Monazite [(Ce,LREE,Th,U,Ca)(P,Si)O4], with complex zoning in Th and other elements, is commonly observed in metamorphic and igneous rocks. The hypothesis that this alteration is a product of fluid-mediated element mass transfer has been tested in the piston-cylinder press (CaF2 assembly, cylindrical graphite oven) at 1,000 MPa and 900°C and in cold seal autoclaves on a hydrothermal line at 500 MPa and 600°C. Experiments included a relatively homogeneous monazite-(Ce) (7–8 wt% ThO2) from a heavy mineral sand plus a series of alkali-bearing fluids including 2N NaOH, 2N KOH, and Na2Si2O5 + H2O. Experiments were conducted using BSE imaging, EMP analysis, and both TEM and HRTEM. A subset of monazite grains from each experiment show evidence of partial alteration in the form of areas enriched in Th + Si with sharp curvilinear compositional boundaries extending from the grain rim into the monazite interior. These ThSiO4-enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic rocks. In the Na2Si2O5 + H2O experiments, scarce inclusions of britholite formed in the altered monazite. The altered monazite is also characterised by strong depletion in Pb, Ca, and Y. Thorium and Si mobility, coupled with the formation of britholite inclusions, during partial alteration in the monazite grain is considered to be the product of fluid-aided coupled dissolution–reprecipitation as opposed to solid-state diffusion. Since other fluids, including NaCl and KCl brines, do not result in the formation of these textures, the experimental replication of ThSiO4-enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced, specifically by alkali-bearing fluids. If true, complex metasomatically induced textures in monazite could yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event, presuming that all the original radiogenic Pb has been removed.