Non-destructive in situ analysis of garnet by combining scanning electron microscopy and X-ray diffraction techniques (original) (raw)
Related papers
Spectrochimica Acta Part B: Atomic Spectroscopy, 2003
This paper deals with the application of the electron and proton microprobe (EPMA and micro-PIXE) in order to determine major and trace elements in metamorphic garnet samples. The selected garnet samples come from a metapelitic rock belonging to the tectonic unit of Monte Rosa Nappe (Italy). Quantitative spot analysis profiles and compositional X-ray maps of both major and trace (yttrium) elements are reported. Major elements show a smoothed and continuous compositional zoning characterized by concentric variations from core to rim. Yttrium displays a strong enrichment in the core and a flat pattern at the rim. No correlation is shown between major and yttrium distribution. Only a rough correlation may be supposed with manganese. A good agreement was found between EPMA and micro-PIXE yttrium data, for values higher than 80-100 ppm. For lower values micro-PIXE technique is strongly recommended. The combination of X-ray two-dimensional maps and quantitative microanalyses allowed to evaluate the distribution of major and trace elements in a petrologically significative rock-forming mineral, to define the type of the chemical zoning and finally to attest difference in diffusivity between major and trace elements. Because of the slow diffusivity of yttrium, its quantitative determination in garnets is fundamental to reconstruct the temperature path suffered by polymetamorphic garnet-bearing rocks. ᮊ
American Mineralogist, 2005
Gaseous secondary electron (GSE) imaging of eclogite garnets under the environmental scanning electron microscope (ESEM) at low chamber gas pressures reveals detailed image contrast patterns (charge-contrast images, CCI) that are not present in back-scattered or secondary electron images. Image intensity is a function of the amount of surface charge accumulation. Successful acquisition of CCI depends on frame size and beam scan rate at a given chamber gas pressure and beam current. Images are obtained in a few seconds, and are stable and reproducible. CCI patterns do not correlate with cracks or grain boundaries, but do correspond closely to variations in major-element composition, both in the form of concentric (growth) zoning, and branching, linear features interpreted as cracks that have been healed by new garnet growth. Causes of CCI are not yet well understood, but may be related to variations in lattice defect density and their infl uence on charge-trapping and dissipation. These in turn infl uence the rate of charge build-up at or very close to the specimen surface. One interesting possibility is that CCI images detect vacancies related to non-homovalent coupled substitutions involving, for example, REE and hydroxyl, so the method offers a way of imaging the distribution of these trace species in garnets. The CCI images are rich in microstructural detail and offer the potential for rapid, high-resolution, low-noise reconnaissance mapping of intragranular microstructure and compositional variation in both natural and synthetic garnets.
EPMA Major and Trace Element Analysis in Garnet and its Petrological Application
Mikrochimica Acta, 2002
A comparison between major and trace element concentrations in garnet performed by electron microprobe (EPMA) technique is reported. Quantitative spot analyses and X-ray maps of major elements (Fe, Mg, Mn, Ca) and the trace element yttrium in garnets from metamorphic rocks are presented. The selected garnet samples come from meta-pelitic and meta-basic specimens belonging to the tectonic unit of the Monte Rosa Nappe (Western Alps). In the metapelites, the quantitative Y distribution maps display a prominent increase at the core, the Y abundance varying by over two orders of magnitude, from about 80 ppm (rim) to over 2100 ppm (core). The Y profiles show well defined patterns with sharp features that do not correlate with major element distributions. A roughly comparable pattern can be supposed only with Mn. The Y distribution suggests that the diffusion of Y through the garnet is very slow compared to the major elements, thus the Y results are suitable for geothermometric estimates. In the metabasites, the Y spatial distribution is characterised by an increasing content from the core to the rim, displaying a zoning pattern opposite to the metapelite garnet. Quantitative EPMA analyses range from 1100 ppm at the rim to values lower than the detection limit at the core. Therefore, the Y content in the garnet can be related to several chemical and physical variables such as the bulk rock composition and the phase assemblage. In particular, in the xenotime-bearing metapelitic system the Y distribution seems to be correlated with metamorphic peak temperature.
Compositional re-equilibration of garnet: the importance of sub-grain boundaries
European Journal of Mineralogy, 2007
Garnets from meta-granitoid high pressure rocks (Sesia Zone, Western Alps) show complex internal sub-grain textures in electron forescatter images. All investigated garnets consist of a large number of sub-grains with different shapes and sizes. Some garnets exhibit a sub-texture with very fine-grained (< 20 µm) sub-grains in their cores overgrown by palisade-like subgrains in the rims. Sub-grain boundaries in these garnets have enabled diffusive element exchange between the garnet core and the surrounding matrix. Compositional mapping reveals zonation patterns of Mg that indicate modification of the garnet composition during prograde metamorphism. The extent of diffusional re-equilibration is dependent on sub-grain size and element diffusivities. Our samples show that X Mg is strongly influenced by diffusion along the sub-grain boundaries, whereas apparently slow diffusing elements, such as Ca, Ti and Y preserve their original concentric zonation pattern. This differential re-equilibration leads to very complex chemical zonation that cannot be easily interpreted in terms of simple prograde growth zonation or of normally-applied spherical diffusion models. The observation that almost all garnets in the investigated samples exhibit a sub-grain pattern suggests this might be a common feature in high pressure/low temperature rocks.
Three-dimensional distribution of primary melt inclusions in garnets by X-ray microtomography
American Mineralogist
X-ray computed microtomography (X-μCT) is applied here to investigate in a non-invasive way the three-dimensional (3D) spatial distribution of primary melt and fluid inclusions in garnets from the metapelitic enclaves of El Hoyazo and from the migmatites of Sierra Alpujata, Spain. Attention is focused on a particular case of inhomogeneous distribution of inclusions, characterized by inclusion-rich cores and almost inclusion-free rims (i.e., zonal arrangement), that has been previously investigated in detail only by means of 2D conventional methods. Different experimental X-μCT configurations, both synchrotron radiation-and X-ray tube-based, are employed to explore the limits of the technique. The internal features of the samples are successfully imaged, with spatial resolution down to a few micrometers. By means of dedicated image processing protocols, the lighter melt and fluid inclusions can be separated from the heavier host garnet and from other non-relevant features (e.g., other mineral phases or large voids). This allows evaluating the volumetric density of inclusions within spherical shells as a function of the radial distance from the center of the host garnets. The 3D spatial distribution of heavy mineral inclusions is investigated as well and compared with that of melt inclusions. Data analysis reveals the occurrence of a clear peak of melt and fluid inclusions density, ranging approximately from 1 ⁄3 to 1 ⁄2 of the radial distance from the center of the distribution and a gradual decrease from the peak outward. Heavy mineral inclusions appear to be almost absent in the central portion of the garnets and more randomly arranged, showing no correlation with the distribution of melt and fluid inclusions. To reduce the effect of geometric artifacts arising from the non-spherical shape of the distribution, the inclusion density was calculated also along narrow prisms with different orientations, obtaining plots of pseudo-linear distributions. The results show that the core-rim transition is characterized by a rapid (but not step-like) decrease in inclusion density, occurring in a continuous mode. X-ray tomographic data, combined with electron microprobe chemical profiles of selected elements, suggest that despite the inhomogeneous distribution of inclusions, the investigated garnets have grown in one single progressive episode in the presence of anatectic melt. The continuous drop of inclusion density suggests a similar decline in (radial) garnet growth, which is a natural consequence in the case of a constant reaction rate. Our results confirm the advantages of high-resolution X-μCT compared to conventional destructive 2D observations for the analysis of the spatial distribution of micrometer-scale inclusions in minerals, owing to its non-invasive 3D capabilities. The same approach can be extended to the study of different microstructural features in samples from a wide variety of geological settings.
Journal of Metamorphic Geology, 2011
The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re-equilibration of these garnet crystals and to reconstruct the pressure-temperature-fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100-200 lm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell-shaped profile of spessartine component (7-3 mol.%), an increase in grossular from 14 to 22 mol.% and d 18 O values between 9.5 ± 0.3& and 10.4 ± 0.2&. Garnet inner rims (90-130 lm) are fluid inclusion-rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The d 18 O values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2&. The outer rims (<10 lm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four-stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re-equilibration by coupled dissolution-reprecipitation at the temperature peak (630 < T < 650°C). This causes the creation of porosity as the coupled dissolution-reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rimÕs domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolutionreprecipitation is a possible re-equilibration process for garnet in metamorphic rocks and that intramineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements.
Some systematics of the garnet structure
Journal of Solid State Chemistry, 1981
Equations relating the positional parameters of the anion in the oxide garnets to the mean constituent ionic radii of the cations occupying the {X}, [Y], and (Z) sites have been derived from published garnet structures using multiple regression analysis:
Geochemistry, Mineralogy and Petrology, 2006
Abstract. In order to obtain informative data on the crystal chemistry of garnets from metamorphic rocks of the Zhulti Chal and Ustrem Formations in the rim of the Sakar pluton (SE Bulgaria), their chemical composition, trace elements, unit cell parameters and compositional zoning were investigated. All garnets are almandine rich. For samples from the Zhulti Chal Formation the molar percentage of almandine range from 70.2 to 79.0, of grossular – from 4.2 to 16.5, of pyrope – from 5.0 to 14.0 and of spessartine – from 3.6 to 11.4. For samples from the Ustrem Formation the molar percentage of almandine range from 72.7 to 74.8, of grossular – from 9.8 to 14.5, of pyrope – from 8.0 to 10.4 and of spessartine – from 3.2 to 6.4. The values of the unit cell parameter of garnets from the Zhulti Chal Formation range between 11.544(4) and 11.597(3) Å while those from the Ustrem Formation – between 11.552(3) and 11.583(3) Å. The (FeO+MgO) / (CaO+MnO) oxide ratio and the unit cell parameters allow suggesting that samples from the Zhulti Chal Formation were formed in more variable temperature conditions of metamorphism in comparison with samples from the Ustrem Formation. The following trace elements were determined in the studied garnets: P2O5 (0.03-0.20 wt.%), SO3 (<0.03-0.37 wt.%), Ba (10-92 ppm), Co (<10-18 ppm), Cr (63-128 ppm), Ni (<10-40 ppm), Sr (6-36 ppm), V (56-115 ppm), Zn (104-218 ppm), Zr (15-31 ppm). The zoning paths of the studied garnets, except sample No 2, show normal type of compositional zoning. It is of prograde genesis. Only in the garnet rims the retrograde features were observed. The examination of the growth zoning features of sample No 2 from the region of the Orlov Dol village shows complex compositional zoning, which is a possible indication for growth under polymetamorphic conditions. Key words: garnet, chemical composition, trace elements, unit-cell parameter, metamorphism.
Journal of Structural Geology, 2002
The microstructures of two contrasting garnet grains are mapped using automated electron backscatter diffraction. In both cases there is a very strong crystallographic preferred orientation, with measurements clustered round a single dominant orientation. Each garnet grain is divided into domains with similar orientations, limited by boundaries with misorientations of 28 or more. In both samples most of misorientation angles measured across orientation domain boundaries are signi®cantly lower than those measured between random pairs of orientation domains. One sample is a deformed garnet that shows considerable distortion within the domains. Lines of orientation measurements within domains and across domain boundaries show small circle dispersions around rational crystallographic axes. The domain boundaries are likely to be subgrain boundaries formed by dislocation creep and recovery. The second sample is a porphyroblast in which the domains have no internal distortion and the orientation domain boundaries have random misorientation axes. These boundaries probably formed by coalescence of originally separate garnets. We suggest that misorientations across these boundaries were reduced by physical relative rotations driven by boundary energy. The data illustrate the potential of orientation maps and misorientation analysis in microstructural studies of any crystalline material. q