Variations in chemical compositions of titanite group minerals from ore skarnes in the Ladoga Lake Region (South Karelia, Russia)

Magmatic evolution and source of a Proterozoic rapakivi granite complex in the North China Craton: New evidence from zircon U−Pb ages, mineral compositions, and geochemistry

Mineral and geochemical compositions, regularities of distribution, and specific formation of ore mineralization of the Rogovik gold-silver deposit (northeastern Russia)

It is one of important subjects in earth and planetary sciences to analyze the textures of rocks quantitatively. For the purpose some digital imaging techniques, including back-scattered electron imaging and characteristic X-ray imaging with a scanning electron microscope (SEM) or a electron probe micro-analyzer (EPMA), and optical imaging with a CCD digital camera or a image-scanner etc., have been applied. We applied here a scanning X-ray analytical microscope (SXAM) for the first time in the field of earth and planetary sciences to obtain XRF images of rocks. In this paper, we report the new method of image processing in which the X-ray maps of a rock are transformed to the maps that show distribution of mineral composition. As a test case, the X-ray maps of the Ryoke granite from Teshima, SW Japan, were processed to make the distribution maps of major minerals. XRF intensities of major elements were assumed to have linear relationship with the composition of major rock-forming minerals in each pixel. The coefficients between XRF intensities and mineral compositions were determined by picking up the some pixels at which a pure mineral exists. The composition of minerals was then calculated by maximum likelihood (ML) method for Gaussian distribution i.e. least-square method. It was shown that it was plausible by a numerical experiment to adopt the least-square method when the operation time of SXAM is sufficiently long. We found the sources of errors of the processed mineral maps depending on statistical errors of X-ray counts, variation of chemical composition in each mineral, and a condition of a sample's surface etc., and propose the way of estimating the errors. As an application, the mineral maps was applied to modal analysis of minerals.

WANG, XIAOXIA, WANG, TAO, HAAPALA, ILMARI and LU, XINXIANG 2002. The Shahewan rapakivi-textured granite – quartz monzonite pluton, Qinling orogen, central China: mineral composition and petrogenetic significance. Bulletin of the Geological Society of Finland 74, Parts 1–2, 133–146. The Mesozoic Shahewan pluton consists of four texturally different types of biotite-hornblende quartz monzonite. In the porphyritic types alkali feldspar occurs as euhedral or ovoidal megacrysts that are often mantled by one or more plagioclase shells, and as smaller grains in the groundmass. Quartz, plagioclase (An20–28), biotite, and hornblende occur as inclusions in the alkali feldspar megacrysts and, more abundantly, in the groundmass. Euhedral quartz crystals in the groundmass are not as common and well developed as in typical rapakivi granite. Compared to typical rapakivi granites, the mafic minerals (biotite and hornblende) are rich in Mg and poor in Fe, and the whole rock is low in Si, K, F, Ga, Zr, LREE, Fe/Mg, and K/Na. The rocks of the Shahewan pluton are thus regarded as rapakivi-textured quartz monzonites and granites but not true rapakivi granites.

A geochemical model for the transformation of gabbro into vesuvianite-bearing rodingite

St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure–temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent instantaneous solid compositions from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An≤100) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in cumulate xenoliths and lavas are An97 and An95, respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions (XH2O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950–1025 °C, and fO2 = NNO − 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt KdCa–Na and dissolved H2O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% H2O) basaltic andesites that crystallised over a wide pressure range (1.5–6 kbar). Comparison of experimental and natural (lava, xenolith) mafic mineral composition reveals that whereas olivine in lavas is predominantly primocrysts precipitated at low-pressure, pyroxenes and spinel are predominantly xenocrysts formed by disaggregation of plutonic mushes. Overall, St. Kitts xenoliths and lavas testify to mid-crustal differentiation of low-MgO basalt and basaltic andesite magmas within a trans-crustal, magmatic mush system. Lower crustal ultramafic cumulates that relate parental low-MgO basalts to primary, mantle -derived melts are absent on St. Kitts.

Asian Orogeny And Continental Tectonics From Geochemical Perspectives: A Special Issue in Memory of Professor Bor-ming Jahn for His Scientific Contributions and Service to JAES (Part 2)

This contribution presents complimentary mineralogical and whole-rock chemical data for the Mullnäset rapakivi massif. Three rock types occur: gabbro, syenite with associated granite and “normal” rapakivi granite, here denoted rapakivi-textured granite. In the syenite-associated granite, extremely Mg-poor clinopyroxene and fayalite are the most important mafic minerals, whereas ferrohornblende and strongly chloritized biotite are most important in the rapakivi-textured granite. All mafic minerals are Fe-extreme. The two rapakivi granites are metaluminous straddling the boundary between calc-alkaline and alkaline granite, whereas the neighbouring, older Sörvik granite is peraluminous and typically calc-alkaline. The Mullnäset rapakivi granite is A-type. It is lower in incompatible trace elements than the Sörvik granite. The HFSE patterns differ between the two granites. In both the rapakivi and Sörvik granites, Archaean rocks form part of the source rocks. The Archaean input seems to be more important in the Mullnäset (and other rapakivi granites from the same general area) granite than in the Sörvik granite. No data contradict the suggested formation of rapakivi granite magma: upwelling mantle melts under a thickened continental crust triggering melting of the continental crust.

We have developed a novel analytical technique for diamond-trap experiments to directly analyze high-pressure, high-temperature fluid and melt compositions in equilibrium with mantle material. Experiments were conducted at a pressure of 6 GPa and temperatures between 900-1200 °C in a multi-anvil apparatus with a synthetic K-free eclogite doped with 860 ppm Cs, ~20 wt% H 2 O, and a layer of diamond aggregates serving as a fluid/melt trap. Experiments at identical conditions were analyzed with two different methods. In the new, “freezing,” approach, the capsule was frozen prior to opening and kept frozen during laser-ablation ICP-MS analysis, thus ablating the quenched fluid (precipitates together with water that unmixed upon quenching) in a solid state. Cesium, fractionating completely into the fluid or melt phase, was used as an internal standard for calculating the fluid compositions. Calculated uncertainties on H2O content in the fluid composition are 0.7-2.5%. In the conventional “evaporation” approach, water from the unmixed fluid was first evaporated from the capsule, then the remaining fluid precipitates were analyzed by LA-ICP-MS. The compositions of the residual eclogitic minerals were measured by electron microprobe, and the fluid composition was then determined by mass-balance. Uncertainties in mineral compositions lead to poor precision in fluid composition in this latter approach. Results of the two methods of fluid analysis were found to be in good agreement. Because the “freezing” approach analyzes the entire fluid directly and does not rely on mass balance for calculating fluid compositions, our new method provides a superior means for determining the composition of fluids. Secondly, it avoids loss of cations that remain soluble in water (e.g., Cs, K) after quenching the experiment.

The so-called “Secondary Shirasu (lacustrine or marine sediments of pyroclastics)” is distributed in separate forms and in small scale, and occurs in association with the socalled “Shirasu or Primary Shirasu (pyroclastic flow deposits)” which is widely distributed over southern Kyushu. Mineral and chemical compositions, and optics of constitutent minerals of the Secondary Shirasu, which was derived from the Aira Shirasu, have been studied, and are compared with those of the Primary Shirasu, i. e. the Aira Shirasu which came from the Aira volcanic caldera. Mineral and chemical compositions show that the Secondary Shirasu is characterized by lower content of mafic minerals and by higher silica and lower ferrous iron oxide and magnesia contents than the Primary Shirasu. The differences in chemical composition are consistent with those in mineral composition between the Secondary and Primary Shirasu. The nature of the Secondary Shirasu is reffected in these differences.

This paper introduces the software solution Bingo‐Antidote for thermodynamic calculations at equilibrium based on iterative thermodynamic models. It describes a hybrid strategy combining the strength of Gibbs energy minimization (GEM) and inverse thermobarometry models based on the comparison between the modelled and observed mineral assemblage, modes and compositions. The overall technique relies on quantitative compositional maps acquired by electron probe micro‐analyser for obtaining a mutually consistent set of observed data such as bulk rock and mineral compositions. Thus it offers the opportunity to investigate metamorphic rocks on a microscale. The scoring part Bingo integrates three statistical model quality factors for the assemblage, for the mineral modes, for the mineral compositions combined in a global evaluation criterion that quantifies how the model reproduces the observations for the investigated volume. The input parameters of GEM affecting the model quality such as pressure, temperature and eventually some components of the bulk composition (e.g. the molar amount of hydrogen, carbon or oxygen) or activity variables of fluids and gases (e.g. , , f(O2)) can be optimized by inversion in Antidote using several mapping stages followed by a direct search optimization. Examples of iterative models based on compositional maps processed with Bingo‐Antidote demonstrate the utility of the program. In contrast to the qualitative interpretation of phase diagrams, the inversion maximizes the benefits of GEM and permits the derivation of statistically ‘optimal’ pressure–temperature conditions for well‐equilibrated samples. In addition, Bingo‐Antidote opens new avenues for petrological investigations such as the generation of chemical potential landscape maps.

Partitioning of Cu between mafic minerals, Fe–Ti oxides and intermediate to felsic melts

Determination by X-ray diffraction of the relative proportions of component minerals in sediments and rocks involves comparison of the integrated intensity of selected reflections from component minerals in the unknown mixture to standard calibration curves. Although the analysis may be rapid and precise, a number of factors contribute to poor accuracy. The role of one of these, variation in the chemical composition of the component minerals, is not generally appreciated. Calculations of the relative integrated intensities of the two principal reflections from calcite, magnesite, siderite, rhodochrosite, dolomite, ankerite, and kutnahorite show that the strongest reflection, {211} ({1014} referred to hexagonal axes), is highly sensitive to isomorphous substitution of the cations. It is a poor reflection to use for analysis if there is any significant variation in the composition of the minerals in the mixture. One important example of compositional variation is the substitution of magnesium for calcium in the biogenic calcite of Recent sediments. The {210} ({1123} referred to hexagonal axes) reflection is the second strongest for most compositions of the rhombohedral carbonates. It is practically independent of solid-solution variation and makes an ide l choice for the analysis of mixtures. Calibration curves involving an internal standard show that if a component mineral comprises more than about eight weight percent of a sample, the {210} reflection will be strong enough to utilize. Solid-solution variation is common and extensive in carbonate sediments and rocks. An investigator should be aware of its effect on intensities before beginning X-ray analysis. The use of {210} is recommended for most natural mixtures, with cerric oxide as an internal standard.

Variations in microtextures, strain, whole-rock chemistry, mineralogy, fluid inclusion characteristics, and fracture network properties record complex interactions between deformation and fluid-rock processes during progressive growth of a shear zone within crystalline basement rocks. This shear zone formed at a depth of about 15 kilometers, T ≈ 350°C, and at elevated fluid pressures. The shear zone (SZ) has a 10 meter thick core of highly deformed phyllonite, and is bounded by a 3 to 8 meter thick transition zone (TZ) of variably fractured chloritic gneiss, which grades outward into relatively undeformed granitic gneiss. Granitic gneiss consists mostly of coarser-grained feldspar and quartz; chloritic gneiss consists of varying amounts of quartz, feldspar, and fine-grained micaceous matrix produced by a mixture of cataclastic, plastic, and alteration processes; and phyllonite consists mostly of very fine-grained quartz-mica-rich matrix produced by pervasive plastic deformation and alteration processes. Estimated strain ratios are 10:1 within the SZ. Whole-rock chemical compositions record significant depletion of Ca and Na, and enrichment of Mg and H2O during progressive alteration of granitic gneiss into phyllonite. Similar average contents of Al, Ti, and Fe, as well as Si, indicate that alteration was about isovolumetric at outcrop scale, although variations between individual samples record up to ±20 percent local volume change. Alteration produced significant changes in mineral abundances and compositions, with conversion of feldspar to muscovite, mafic minerals to Mg-rich chlorite, and dissolution/precipitation of quartz. Fluid inclusion characteristics and mineral compositions indicate that SZ fluids were moderately saline, and became depleted in Mg during alteration. Changes in mineralogy and fluid composition record large influxes of reactive fluids, with geochemical fluid-rock ratios on the order of 102 to 103. Variably deformed, cross-cutting veins, fractures, and microcracks record repeated episodes of cataclasis, fluid influx, and sealing along complex networks. Fluid flow appears to have been concentrated within the SZ along grain-scale and vein networks, with a component of outward flow into the TZ along fracture and microcrack networks. Estimated average fluid fluxes are on the order of 10−7 to 10−10 m/s in the SZ, and 10−9 to 10−12 m/s in the TZ, consistent with average permeabilities of 10−14 to 10−17 m2 in the SZ and 10−16 to 10−19 m2 in the TZ for moderate fluid pressure gradients. These average permeabilities are consistent with observed grain-scale and fracture network properties for a range of sealing and fluid pressure histories. Permeabilities may have been transiently greater during episodes of very high fluid pressure, but decreased as fluid pressure gradients equilibrated and fractures sealed. Strain softening in the SZ was likely produced by reaction softening, grain size reduction, and hydrolytic weakening of quartz. A model of shear zone growth involves: (1) initial fracturing of relatively strong, coarser-grained, quartz-feldspar-rich rock; (2) episodic influx of reactive fluids during periods of fracturing and high fluid pressure, with intervening periods of sealing and reduced fluid pressure; (3) progressive alteration of feldspar and mafic minerals to micas and recrystallization of quartz to form relatively weak, fine-grained matrix; and (4) concentrated deformation and focused fluid flow in a growing SZ core.

Object of research. Spinelides of izrandites of the Aleksandrov polymetamorphic complex in the Southern Urals. Purpose of research. Studying of composition of spinel group minerals and coexisting ilmenite in single grains and in different phases in unmixing structures after decomposition of solid solutions, reconstruction of primary compositions of oxide minerals and comparison with the same minerals from Ural-Alaskan-type complexes having an ankaramine affinity. Methods. The study was performed on a Tescan Mira scanning electron microscope at the “Geoanalitic” Center of Common Use (Ekaterinburg). The images were obtained in backscattered electron mode. The composition of minerals was determined in points and using an area scanning facilities of SEM for the unmixing structures of spinels. Results. Chrome spinel containing more than 25 wt % Cr2O3 and corresponding to the earliest stage of crystallization has been discovered in izrandites of the Alexandrovsky polymetamorphic complex in the Southern Urals. The several stages of Cr-Fe-Ti-oxide and rock-forming silicates crystallization were determined. It was shown that during cooling and subsolidus transformation, oxide minerals undergo complex multistage decomposition of the solid solution with the formation of phases enriched in aluminum and ferric iron in equilibrium with ilmenite. The compositions of these phases are distributed along the Cr-spinel solvus. The earliest primary hypersolvus spinels form inclusions in olivine and clinopyroxene. They are characterized by 3–4 wt % of TiO2 and 15–20 wt % of Cr2O3. The late spinel forms inclusions in kaersutite and are situated in the intergranular space. Their compositions are poor in Cr2O3 < 7%, but rich in TiO2 10–25 wt %, corresponding to titanomagnetite and ulvospinel. Conclusions. The composition of rocks, silicate minerals and Cr-Fe-Ti-oxides confirm the similarity of izrandites with ankaramites and tilaites from complexes of Ural-Alaskan-type. High titanium content in izrandites in comparison with similar rocks of the Ural Platinum Belt reflect the geochemical peculiarities of the primary melt which was formed by melting of the metasomatically transformed Mesoproterozoic mantle under the influence of a plume.