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1

Synneusis: does its preservation imply magma mixing?

Gogoi B., Saikia A.

Mineralogia

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2018

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Vol. 49, iss. 1/4

99--117

EN

The Ghansura Felsic Dome (GFD) occurring in the Bathani volcano-sedimentary sequence was intruded by mafic magma during its evolution leading to magma mixing. In addition to the mafic and felsic rocks, a porphyritic intermediate rock occurs in the GFD. The study of this rock may significantly contribute toward understanding the magmatic evolution of the Ghansura dome. The porphyritic rock preserves several textures indicating its hybrid nature, i.e. that it is a product of mafic-felsic magma mixing. Here, we aim to explain the origin of the intermediate rock with the help of textural features and mineral compositions. Monomineralic aggregates or glomerocrysts of plagioclase give the rock its characteristic porphyritic appearance. The fact that the plagioclase crystals constituting the glomerocrysts are joined along prominent euhedral crystal faces suggests the role of synneusis in the formation of the glomerocrysts. The compositions of the glomerocryst plagioclases are similar to those of plagioclases in the mafic rocks. The results from this study indicate that the porphyritic intermediate rock formed by the mixing of a crystal-rich mafic magma and a crystal-poor felsic melt.

2

Beta dosimetry of potassium feldspars in sediment extracts using imaging microprobe analysis and beta counting

Godfrey-Smith D. I., Scallion P., Clarke M. L.

Geochronometria

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2005

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Vol. 24

7-12

EN

We provide a comparison of two independent methods used to determine the K concentration of potassium feldspar extracts from sand-sized lacustrine, colluvial, and aeolian sediments. The two methods being compared are gas-flow beta counting and imaging microprobe X-ray fluorescence analysis. Imaging analysis demonstrated that the proportion of potassium feldspar grains in a K-feldspar extract varied from a low of 7% to a high of 84%. All extracts included a significant proportion of quartz, and some also contained a few plagioclase feldspar grains. However, the K2O concentration in individual potassium feldspar grains of all eight extracts examined was within the range of 15.5 š 0.7 % K2O by weight. The K2O concentrations in four of five extracts, measured using gas-flow beta counting, were significantly lower than this value. This reflects the dilution effect of significant amounts of non-K bearing grains in these extracts. This difference can result in up to an 80% underestimate in the internal beta dose rate of a potassium feldspar grain. A 14% underestimate in the total dose rate to potassium feldspar grains, and therefore a 14% overestimate in sediment ages determined by luminescence dating of nominally pure potassium feldspar extracts is thus possible. We suggest that gas-flow beta counting is suitable for the determination of K concentrations of bulk sediments and therefore the external beta dose rate, but that internal beta dosimetry is best performed by imaging microprobe X-ray analysis. An alternative, inexpensive approach would be to assume a 15.5 š 0.7 % K2O concentration for all nominal K-feldspar extracts separated using a heavy liquid with specific gravity of 2.58 gźcm-3, and use this value to determine the internal beta dose rate. Internal beta dose rate errors incurred with this approach are expected to be significantly less than those incurred by the small sample gas-flow beta counting method.

3

Experimental investigation of the reaction : An-rich plagioclase + quartz + fluid = analcime-wairakite[ss]

Galbarczyk-Gąsiorowska L., Słaby E.

Acta Geologica Polonica

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2001

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Vol. 51, nr 2

155-162

EN

The reaction An-rich plagioclase + quartz + fluid = analcime (NaAlSi4O12 * 2H2O) - wairakite (CaAl2Si4O12 * 2H2O)[ss] (solid solution series) has been studied experimentally in the temperature range 350-200 stopni C. Distilled water or 0.1 M NaHCO3 solution was used as fluid. All experiments have been performed under low silica activity. Due to the short duration of the experiments the reaction had not approached equilibrium and partly metastable phases were formed. Wairakite nucleated in the laumontite stability field in the system plagioclase-quartz-H2O. Higher sodium activites led to the formation of mixed analcime-wairakite crystals or analcime crystals. The mechanisms of zeolite formation seems to comprise topotaxial growth on the plagioclase surface caused by a dissolution-precipitation process. Some variations in the nucleation and growth kinetics were noticed which appear to be due to different physico-chemical conditions of zeolite formation