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Application News No. P106 Electron Probe Microanalyzer Trace Element Mapping of Plagioclase Introduction The minerals contained in rocks record physical and chemical information from the time when the rocks were formed. Shimadzu EPMA is useful for investigating the precise chemical composition and 2-dimensional distribution of the elements in these rock-forming minerals. Here, element mapping of plagioclase in lava from Mt. Unzen volcano in Japan was carried out using a Shimadzu EPMA™ electron probe microanalyzer (EPMA-8050G, hereinafter, EPMA). As a result, it was possible to discover the microstructure in the rim part of the plagioclase. The thin-slice specimens used here were provided by Mr. Hideto Yoshida (Technical Director, Technical Division, School of Science) Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo. R. Ogawa, T. Ono Element Mapping of Plagioclase Plagioclase, the principal component mineral of volcanic rock, is a solid solution consisting of anorthite (CaAl2Si2O8) and albite (NaAlSi3O8). In the cooling process of high-temperature magma, first, crystals with a large anorthite component form, and then, as cooling proceeds, the composition changes to one with a large albite component. Areas where the chemical composition changes discontinuously appear as stripes. This is referred to as “zonal texture.” Fig. 1 shows the result of a wide-area EPMA mapping analysis of the principal elements of plagioclase, O, Na, Al, Si, and Ca, and the trace elements Mg, K, and Fe. Concentric zoning can be seen, and as a general tendency, the core area has a large anorthite content, while the rim area has an albite-rich composition. However, the outermost rim has an anorthite-rich composition, which is termed “reverse zoning,” and a reaction rim can be observed (Fig. 2). These facts suggest that the temperature, pressure, and abundance of H2O changed during the crystal cooling process, and interaction with other magma also occurred. Fig. 1 Wide-Area Element Mapping of Plagioclase 500 μm COMPO 15.0 kV 3.0×2.3 mm 500 μm OKa 15.0 kV 3.0×2.3 mm 3340 (counts) 140 2940 2540 2140 1740 1340 940 540 500 μm Na Ka 15.0 kV 3.0×2.3 mm 222 (counts) 30 198 174 150 126 102 78 54 500 μm Si Ka 15.0 kV 3.0×2.3 mm 1620 (counts) 20 1420 1220 1020 820 620 420 220 500 μm Al Ka 15.0 kV 3.0×2.3 mm 1700 (counts) 100 1500 1300 1100 900 700 500 300 500 μm Mg Ka 15.0 kV 3.0×2.3 mm 246 (counts) 6 216 186 156 126 96 66 36 500 μm KKa 15.0 kV 3.0×2.3 mm 532 (counts) 20 468 404 340 276 212 148 84 500 μm Ca Ka 15.0 kV 3.0×2.3 mm 1340 (counts) 20 1175 1010 845 680 515 350 185 500 μm FeKa 15.0 kV 3.0×2.3 mm 260 (counts) 20 230 200 170 140 110 80 50
Application News No. P106 First Edition: Feb. 2020 For Research Use Only. Not for use in diagnostic procedures. This publication may contain references to products that are not available in your country. Please contact us to check the availability of these products in your country. The content of this publication shall not be reproduced, altered or sold for any commercial purpose without the written approval of Shimadzu. Shimadzu disclaims any proprietary interest in trademarks and trade names used in this publication other than its own. See http://www.shimadzu.com/about/trademarks/index.html for details. The information contained herein is provided to you "as is" without warranty of any kind including without limitation warranties as to its accuracy or completeness. Shimadzu does not assume any responsibility or liability for any damage, whether direct or indirect, relating to the use of this publication. This publication is based upon the information available to Shimadzu on or before the date of publication, and subject to change without notice. © Shimadzu Corporation, 2020 www.shimadzu.com/an/ Mapping of Rim Area of Plagioclase In the rim area of phenocrysts in rocks, various phenomena occur as a result of contact with other magma following once cooling. To elucidate those changes, the rim area of the plagioclase was mapped under higher magnification. As a result, it was found that counts of Na decreases in the rim area, while counts of Al, Ca, and trace elements Mg, K, and Fe increase (Fig. 2). Further enlargement of the rim area revealed that the contents of the principal elements of plagioclase, namely, Na, Al, and Ca are small in parts where the K content is large, and in contrast, particles of Mg and Fe had collected in those parts (Fig. 3). Fig. 2 Element Mapping of Rim Area of Plagioclase Fig. 3 Element Mapping of Rim Area of Plagioclase (Enlarged Views) Trace Element Mapping of Rim Area of Plagioclase High magnification mapping revealed that Mg- and Fe- containing inclusions that concentrated in parts of the rim with high K contents existed at a number of scattered points as microscopic particles with a maximum size of several 100 nm and a minimum size of several 10 nm or less (Fig. 4). As this experiment shows, it was possible to obtain valuable information for understanding the history of interaction between phenocrysts and magma by high resolution mapping with the Shimadzu EPMA. Fig. 4 Mapping of Inclusions Consisting of Mg and Fe EPMA is a trademark of Shimadzu Corporation in Japan and/or other countries. 500 μm COMPO 15.0 kV 3.0×2.3 mm 40 μm COMPO 15.0 kV 180×135 μm 40 μm Na Ka 15.0 kV 180×135 μm 40 μm Mg Ka 15.0 kV 180×135 μm 40 μm FeKa 15.0 kV 180×135 μm 40 μm KKa 15.0 kV 180×135 μm 40 μm Ca Ka 15.0 kV 180×135 μm 40 μm Al Ka 15.0 kV 180×135 μm (counts) 70 (counts) 6 62 54 46 38 30 22 14 180 (counts) 20 160 140 120 100 80 60 40 (counts) 150 10 133 115 98 80 63 45 28 605 45 535 465 395 325 255 185 115 1000 (counts) 40 880 760 640 520 400 280 160 (counts) 1360 280 1225 1090 955 820 685 550 415 40 μm COMPO 15.0 kV 180×135 μm 5 μm COMPO 10.0 kV×6000 5 μm Na Ka 10.0 kV×6000 60 (counts) 4 53 46 39 32 25 18 11 5 μm Mg Ka 10.0 kV×6000 30 (counts) 6 27 24 21 18 15 12 9 5 μm FeKa 10.0 kV×6000 45 (counts) 5 40 35 30 25 20 15 10 5 μm KKa 10.0 kV×6000 85 (counts) 5 75 65 55 45 35 25 15 5 μm Ca Ka 10.0 kV×6000 216 (counts) 24 192 168 144 120 96 72 48 5 μm Al Ka 10.0 kV×6000 418 (counts) 114 380 342 304 266 228 190 152 1 μm COMPO 15.0 kV×25000 1 μm Mg Ka 15.0 kV×25000 18 (counts) 2 16 14 12 10 8 6 4 1 μm FeKa 15.0 kV×25000 90 (counts) 12 80 71 61 51 41 32 22
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