Mica Inclusions Research Articles

Disambiguation of pyrope‐rich garnet inclusions in coloured sapphires from Tanzania and identification of other inclusions by Raman spectroscopy

AbstractStudy of inclusions in gems from secondary deposits can give valuable clues on the geologic (and sometimes geographic) origin of these displaced materials. For the present study, seven gem quality fancy‐coloured sapphires from Songea, Tanzania, were selected with, rarely observed in these gems, garnet inclusions in order to better understand their formation. Therefore, Raman spectroscopy was used to study garnet, as well as rutile, aluminosilicate and mica inclusions. Garnet composition calculations were made using the MIRAGEM Raman spectra software, which was further improved for the purpose of this study. A couple of garnet inclusions were also analysed by energy dispersive X‐ray spectroscopy (EDS) attached to a scanning electron microscope (SEM). All garnet inclusions presented similar Raman spectra, thus similar chemical composition. Chemical analysis demonstrates that these are pyrope‐rich (>40%) garnets with over 30% of almandine and over 20% grossular without any detectable titanium, chromium and sodium. Calculations made using the updated/improved MIRAGEM software were able to indicate the same composition as compatible with the obtained chemical analyses. Muscovite and sillimanite were also identified using micro‐Raman spectroscopy. This inclusion scene is not similar to those described in fancy‐coloured gem sapphires from other localities. The exact geological formation of these gems is yet not understood but might be eclogite or upper mantle related.

The Potential of Quartzitic Veins in SW Cameroon for High-Purity Quartz

High-purity quartz (HPQ), the prime source of silica, is increasingly considered a strategic mineral in the world market because it is the raw material for special applications in high-tech industries owing to its unique physical and chemical properties. The expected growth in demand for HPQ implies that more sources have to be found. This is the primary motivation for this study which targets the Supe area, SW Region of Cameroon. The structural occurrences of quartz veins were mapped and samples collected. The chemical nature and textural features of these samples were subsequently analyzed by X-ray tomography, scanning electron microscopy (SEM), and electron microprobe analysis (EMPA). Three deformation events, D1 - D3 are recorded. D1 is marked by the development of the strong S1 foliation defined by the gneissic layering and schistosity. D2 is a NE-SW ductile to brittle sinistral shear that controlled the formation of tension gashes that served as subsequent pathways for quartz-rich hydrothermal fluids to circulate and eventually precipitate quartz. The presence of rutile, tourmaline, and mica inclusions in the Supe veins affects the purity of the quartz. However, quartz can be treated to improve its quality. Taking into consideration the nine determinant trace elements used to classify HPQ, three (Li, P, and B) are below detection limits, Al concentrations are within the HPQ, quartz market standard, and average natural abundance; Ti values are high above all three standards in all except one sample, while Na, Ca, K, and Fe values fluctuate, thus classifying one of the samples as high purity quartz and the others as low purity quartz. These veins define tension gashes which usually occur in an echelon arrangement covering a large surface area, suggesting that more of such veins probably exist in the Supe area. Consequently, the quality and potential quantity of HPQ veins in this area suggest a high potential for HPQ exploration in the region.

Analysis of mica inclusions using LA-ICP-MS: A new approach for sourcing raw material of ceramics

The analytical methods used for determining the origin of archaeological ceramics, such as petrography, X-ray powder diffraction, X-ray fluorescence spectrometry, attain their limits for differentiating bodies of similar composition, especially when they derive from the alteration of granitoid rocks. Our new approach is based on the chemical analysis of biotite inclusions in ceramic bodies using laser ablation-inductively coupled plasma-mass spectrometry, and enables us to identify the geological origin of the clay used. Spot analysis of the biotite inclusions contained in ceramics made from clay derived from the alteration of rocks can be linked to the original granitic massif. The thin section or rock analysis of biotite inclusions is relatively fast. The dosage of trace elements, such as lithium and vanadium establishes a filiation between the rocks and ceramics. Using this geochemical base, it is possible to establish a link between a granitic batholite and early ceramics and to propose a circumscribed production area. This article will focus on occupations located on islands from western France.

Mica Inclusions inside Host Mica Grains from the Sutlej Section of the Higher Himalayan Crystallines, India-Morphology and Constrains in Genesis

Biotite and muscovite inclusions inside mica host minerals from the Sutlej section of the Higher Himalayan Crystalline were studied under an optical microscope. These inclusions formed possibly by local recrystallization of mica grains during regional prograde metamorphism, with some affected by top-to-SW shear leading to parallelogram shapes. Recrystallization may have been assisted by solution transfer along the cleavage planes of the host grains. The relative competency of deformed phyllosilicate inclusions with the same or different composition to the host depends on the size and orientation of (001) cleavage planes of the inclusions relative to the host. Shearing of mica inclusions led to their parallelogram geometries within the contained mica inclusions. Some of the sheared inclusions deflect cleavage planes in the host minerals and define flanking microstructures. Trapezoid-shaped inclusions are a new finding that deserves more attention for their genesis. These structurally anisotropic inclusions did not originate from sub-grains, secondary infillings or retrogression. These inclusions are also not related to pseudomorphism, isomorphism, folding of the bulk rock etc. Some of the inclusions formed by recrystallization of the host mineral during top-to-SW ductile shear.

Ochre Use in the Middle Stone Age at Sibudu, South Africa: Grinding, Rubbing, Scoring and Engraving

Many Middle Stone Age (MSA) sites have evidence of the regular collection and use of ochre. Sibudu (KwaZulu- Natal, South Africa) has a large MSA ochre assemblage of over 9000 pieces from layers dating between ~77 ka and ~38 ka. There are 682 pieces with signs of use. All usetraces were examined and activity categories were defined based on published ochre experiments. The most frequent markings on ochre pieces are grinding striations that are smoothed by subsequent rubbing. Grinding and rubbing also occur independently on many pieces. Scored pieces are rare, but are more common in the pre-Still Bay (~77 ka) industry than elsewhere in the sequence. Some scored pieces may represent deliberate engravings. Markings acquired during powder-production are most numerous in the assemblage. Powder was mostly produced from bright-red pieces, but scoring was mainly performed on brown-red pieces. Pieces with mica inclusions are not common, but were favoured for powder production. Ochre powder was used as an aggregate in hafting adhesives, but other possible applications are as paint or as a substance to aid hide tanning.

Quantitative evaluation of mineral grains using automated SEM–EDS analysis and its application potential in optically stimulated luminescence dating

Optically stimulated luminescence (OSL) dating allows constraining the depositional age of sediments with good accuracy and precision. A fundamental requirement in OSL dating is to use purified sub-samples (i.e. mono-mineralogic aliquots composed of e.g. quartz or potassium feldspar only), because of the different OSL properties and dosimetry characteristics of each mineral phase. Where multiple mineral phases are present on an aliquot, a mixed OSL signal might be obtained, with potentially adverse effects on the robustness of the resulting optical ages. Detailed evaluation of the mineralogical composition of the hundreds or even thousands of individual mineral particles that constitute an aliquot in OSL dating has – until recently – not been reasonably feasible with current analytical techniques.Here we report on the use of an automated mineralogy system that combines scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) and facilitates ultra-fast analysis of particulate mineral phases with a spatial resolution on the micron scale. The method is applied to mono-mineralogic coarse-grained (100–250 μm) and poly-mineral fine-grained (4–11 μm) OSL samples, respectively and cross-checked with electron probe micro analysis (EPMA). It is shown that (i) some coarse-grained mineral extracts that underwent standard physico-chemical preparation to isolate quartz for OSL dating, still suffer from mineralogical contamination, mainly in the form of feldspar and mica inclusions and, that (ii) polymineral fine-grained samples reveal a complex mineralogical composition with a significant percentage of mica (mainly muscovite). Implications of these quantitative mineralogical observations for OSL dating are discussed. QEMSCAN is further used to examine the efficiency of different physico-chemical preparation strategies to isolate a restricted range of mineralogies and to optimize single preparation steps. We conclude that the clear advantage of automated SEM–EDS systems lies in the rapidity with which accurate high-resolution maps of hundreds or even thousands of mineral particles can be generated, i.e. at a level statistically representative of the bulk OSL sample. Automated SEM–EDS techniques might thus be helpful in OSL dating for quality assurance and investigation of problematic OSL samples.

Nanoinclusions in microdiamonds from Neogenic sands of the Ukraine (Samotkan’ placer): A TEM study

Mineral and fluid inclusions in seven microdiamonds (ca. 0.2 mm in diameter) from the Neogene, Samotkan’ placer of the Ukrainian shield were investigated by TEM. Various types of submicron- or nanometre-sized inclusions such as olivine and orthopyroxene, Fe,Sn oxide and assemblages of mineral multi-phases were observed.In an octahedral microdiamond crystal numerous olivine and enstatite micro- and nanocrystals were observed. Their composition is close to that of other olivine and enstatite inclusions found in kimberlitic diamonds worldwide. An unusual feature of these minerals in the samples studied, however, is a relatively high NiO content: 0.50–0.80 wt.% in olivine (Fo91.3–92.1) and 0.40–0.70 wt.% in enstatite (Fo94.1–95.4). These peridotitic mineral associations represent the host (possibly lherzolitic mantle) in which the diamonds grew. The mean Mg# values of olivine and enstatite inclusions in Samotkan’s microdiamond are, respectively, 91.43 and 94.83, which are closer to Mg#-values of these minerals in lherzolite, rather than in harzburgite assemblages (included in diamond). In addition, enstatite–clinoenstatite, K-richterite, graphite, and Fe,Sn oxide were also identified as nanoinclusions in Samotkan’ diamond; the Fe,Sn oxides and magnetite were found in two microdiamonds of the transitional {111}+{110} form. The K-richterite and graphite have been attributed to epigenetic inclusions. The origin of Fe,Sn oxide is not clear.Assemblages of multi-phase, minerals were observed in four microdiamonds of variable morphology: octahedron, transitional {111}+{110} form, cube and cube-coated diamond. These phases belong to fluid-bearing inclusions and are enclosed in cavities which, as a rule, are not filled completely by the solid phases. The remainder of the volume is occupied by a fluid that for the most part has been released during sample preparation. In the majority of cases the multi-phase assemblages consist of carbonate, mica, rutile, ilmenite, apatite and sylvite. Carbonates are the most abundant phases in the fluid inclusions; they are calcium-rich carbonates, frequently with admixtures of Fe, Mg and Sr. The mica inclusions are enriched in silicon, with Si varying from ca. 6.7 to 7.0 a.p.f.u. The multi-phase mineral assemblages present in Samotkan’s microdiamond are similar to fluid-bearing, microinclusions found previously in other diamond types (e.g., fibrous diamonds, the fibrous coat of coated diamonds or the internal clouds of octahedral diamonds) from kimberlites of different provinces worldwide. They represent the carbonate-, alkali-, and chlorine-rich mantle fluid composition present during crystallisation of the Samotkan’s microdiamond. Judging from the composition of the multi-phase assemblages it may be assumed that the Samotkan’s microdiamond grew from a carbonatitic to a slightly silicic melt, rich in alkali and volatile components.

Evidence for multistage evolution in a xenolith of diamond-bearing eclogite from the Udachnaya kimberlite pipe

We present the results of a detailed investigation of large (18 cm long, 6.9 kg) diamondiferous eclogite xenolith from the Udachnaya kimberlite pipe. We have studied the spatial distribution of diamonds, their relationship with the matrix minerals, mineral inclusions and their major- and trace-element compositions. The xenolith is composed of pale green clinopyroxene, round garnet grains and phlogopite, and is characterized by irregular distribution of minerals and development of zones enriched in phlogopite and garnet. Minerals in diamondiferous xenoliths show major-element compositions transitional between Udachnaya eclogites and those from peridotites. The host clinopyroxene are enriched relative to pyroxenes from Group A eclogites and websterites in respect K 2O and LREE. We believe that such peculiarities reflect the interaction between mantle peridotites and LREE-enriched melts. The sources of the melts are uncertain. The distribution of diamonds (1 to 5 mm in size) in the xenolith is extremely irregular: they are more abundant in the garnet- and phlogopite-enriched zones. Diamonds often make up veins and schlieren. Nitrogen aggregation characteristics suggest that the xenolith contains diamonds of at least two generations. High degrees of aggregation in colorless diamonds of the earlier generation indicates that they formed at least 1 Ga before kimberlite eruption, whereas yellow diamonds of the second generation (type IaA) were formed only a short time before kimberlite emplacement. The presence of K-Clinopyroxene, coesite, CaCO 3, and high-silica mica inclusions in diamonds of the second generation, and their internal and external morphology, suggest that these diamonds crystallized during interaction of the eclogitic substrate with high-density fluids compositionally intermediate between carbonatitic and hydrous-silicic end members rich in Si, K, Ti.

40Ar/39Ar dating of pyrite from gold deposits in low grade terranes.

40Ar/39Ar dating of fine-grained white micas from gold deposits, formed under low-grade metamorphic conditions, is often complicated by multiple mica populations, recoil loss/redistribution of 39ArK and loss of 40Ar. Recent studies have suggested that pyrite armours enclosed mica inclusions against argon loss and, therefore, may provide improved age information in low grade terranes. This premise is tested by applying the 40Ar/39Ar pyrite dating method to three gold deposits in Victoria, namely Stawell, Fosterville and Walhalla. These analyses indicate that, although pyrite behaves as a partially closed system, the method is able to ?see through? younger hydrothermal overprints. At the same time, 40Ar/39Ar analyses of pyrite from the Fosterville gold mine failed to constrain the timing of gold deposition due to the encapsulation of detrital mica grains by the pyrite. These results emphasize the importance of petrological control on sample selection and combining 40Ar/39Ar analyses of pyrite and matrix mica.

Halogen and Ar–Ar age determinations of inclusions within quartz veins from porphyry copper deposits using complementary noble gas extraction techniques

Extension of Ar–Ar methodology has been used to determine mineralisation ages from mica inclusions and to simultaneously evaluate the noble gas and halogen composition of inclusion fluids within irradiated quartz vein samples from five porphyry copper deposits. Samples have been collected from the potassic and propylitic zones of Bingham Canyon, Utah, and four Arizonan deposits; Silverbell, Globe-Miami, Pinto Valley and Ray. Data obtained using three noble gas extraction techniques (laser ablation, in vacuo crushing and stepped heating) are compared with each other. Laser ablation provides a means for the analysis of individual fluid inclusions but is limited by blank levels and detection limits. Stepped heating and in vacuo crushing are bulk extraction techniques that preferentially release gases from solid and fluid inclusion phases, respectively, and can be used in combination to obtain accurate and meaningful ages of mineralisation. Ages obtained for porphyry copper deposits are as follows: Bingham Canyon 37.1±0.5, Ray 65.3±1.5, Globe-Miami 61.7±3.4, Pinto Valley 63.2±8.0 and Silverbell 55.8±1.8. The age of mineralisation at Ray is of particular interest as it enables a reported discrepancy in K–Ar ages to be understood. Halogen data for the Bingham Canyon samples gives insight into the partitioning of the heavy halogens (Br and I) between solid and liquid phases.

Cyclic fluid flow through a regionally extensive fracture network within the Kodiak accretionary prism

Two types of periodic textures observed in veins from the Kodiak accretionary prism attest to cyclic fluid flow through a regionally extensive fracture network buried at 8–12 km depth: (1) crack‐seal microstructures with bands of mica inclusions and (2) collapse microstructures with jagged bands of residue embedded within euhedral crystals of quartz. The difference in texture reflects the closure of cracks: crack‐seal microstructures record the complete chemical sealing of the crack after each fracture event, whereas the collapse features record longer fluid‐filled periods followed by more rapid draining of fractures. Collapse features consist of pressure solution selvages trapped within veins and in the wall rock adjacent to euhedral growth terminations; the high concentrations of immobile elements in these selvages indicate that these fractures closed by collapse and penetration of quartz crystals into wall rock. Analysis of chemical composition on either side of four large euhedral growth veins and whole rock analysis of slates across the Kodiak Formation reveal local depletion of silica adjacent to veins but no evidence for long‐distance silica transport within the system. Both crack‐seal and collapse textures are observed in a regionally extensive vein system that displays a regular geometry, with thin, closely spaced (0.5–3 cm), near‐vertical crack‐seal veins that connect vertically and laterally with thicker euhedral growth veins arranged in widely spaced (∼500 mm) southeast dipping en echelon sets. The mesoscopic distribution and textural variability of the vein network suggests that the development of the vein system involved early nucleation and growth of vertical hydrofractures. As the fracture density increased, arrays of fractures locally provided zones of weakness and southeast dipping brittle‐ductile shear zones nucleated. These en echelon cracks remained open and provided small reservoirs of fluid. Textures show that en echelon fractures remain open but periodically grow by upward and downward propagation. Crack tips are then sealed with locally derived silica, and fluid drains back into en echelon fracture arrays. This local fluid movement is punctuated by less frequent events where the system links up over a greater distance, fractured reservoirs become interconnected, and the fluid within reservoirs is drained upward or laterally. Periodic inflation and deflation of en echelon arrays may reflect periodic slip on crosscutting faults and rupture of the seals that separate reservoirs.

Crack–reaction veins from the Hodgkinson Formation, North Queensland, Australia

AbstractThin, subplanar or parallel-sided syntectonic quartz veins from the Hodgkinson Province, North Queensland, Australia contain mica inclusions that are identical to wallrock mica textures. These ghost textures can only be produced by incomplete replacement of the wallrock by quartz. The planar form of the veins indicates that there was a planar control on their formation. We propose that the veins formed around syntectonic fractures that localized silica micrometasomatism of the fracture wall. The formation of the veins can be explained by a model which involves fracture of the wallrock, reaction between the fracture wall and the ambient metamorphic fluid and eventual sealing of the fracture by precipitation from the fluid. The new vein is then a locus of further fracture. Replacement of wallrock micas occurs via a two-stage process where biotite is replaced by muscovite, which is in turn replaced by quartz. We propose the term crack–reaction to describe the resulting cyclic process because it can be compared with the crack–seal model of vein formation. Crack–seal and crack–reaction are different only in the relative amounts of metasomatism and precipitation that occur subsequent to fracture and can be envisaged as end members of a more general vein-forming process.

The effects of sub-blocking temperature metamorphism on the K/Ar systematics of hornblendes: 40Ar/39Ar dating of polymetamorphic garnet amphibolite from the Franciscan Complex, California

The effects of sub-blocking temperature metamorphism on the K/Ar system in hornblende, as revealed by 40Ar/39Ar release spectra, have been studied in a polymetamorphic “knocker” from the Franciscan Complex (FC), California. A primary amphibolite assemblage of horn-blende +rutile+epidote+apatite±garnet±sphene is variably overprinted by a blueschist facies assemblage of blue amphibole +lawsonite+chlorite+white mica+pumpellyite±sphene. The secondary assemblage formed at a temperature of ≤370° C, below that at which rapid Ar diffusion is expected in hornblende. Hornblendes from three, variably-altered samples of garnet amphibolite yield total gas ages of 147 to 161 Ma, but the corresponding plateau ages of 163.0±2.8, 160.6±2.2, and 161.8±2.2 Ma are identical within error. Hornblende separates with lower total gas ages come from more highly overprinted rocks, have “excess” K compared to that expected on the basis of electron probe analyses, and exhibit anomalously high K/Ca ratios in the low-temperature fractions of their incremental heating spectra. The reduced total gas ages result from the presence of thin (∼2 μm) sheets of younger white mica in hornblendes from the moderately and highly altered amphibolites. The secondary micas are difficult to detect because of their small size and low abundance (≤2%), but because their K content is 50 to 100 times that of the host hornblende, they contribute significantly to the K and Ar budgets of the sample. The mica intergrowths are not removed by normal sample preparation, but because the mica inclusions degas at lower temperatures than hornblende during vacuum extraction, incremental heating analyses can provide precise cooling ages for the hornblendes as well as useful estimates of the age of the mica inclusions. The hornblende separate from the most altered sample contained 20±10% younger blue amphibole replacing hornblende, but its plateau age was not significantly affected. This is consistent with a replacement process in which K and Ar loss from the hornblende are coupled, leaving the K/Ar system undisturbed in relict primary grains. The K and Ar budgets of the sample are not strongly affected by the blue amphibole because of its very low K content. Because partial replacement of primary amphibole by high-K phyllosilicates occurs in many geological environments, effects like those described here could be widespread.

X-Ray Fluorescence Analysis Using Synchrotron Radiation

Synchrotron radiation (SR) has several unique properties which cause it to be used for many purposes in science and technology. It is continuous spectrally, collimated spatially, short-pulsed yet continuously-operating temporally, as well as polarized( 1). Many applications of SR have already been demonstrated. These range from production of fine scale structures by lithography to numerous studies of materials. Determination of atomic structure by diffraction and other scattering experiments and by absorption spectroscopy (EXAFS), and emission spectroscopic studies of electronic structure have been given most attention(2). Only limited attention has been paid to determination of materials composition by x-ray fluorescence analysis (XRF) employing synchrotron radiation. Sparks et al.(3-5) have performed the most notable experiments, examining mica inclusions for the presence of primordial super heavy elements and irradiating two National Bureau of Standards Standard Reference Materials (SRM 1571, Orchard Leaves and SRM 1632, Coal) to measure the fluorescent x—ray intensity from the trace elements.