TY - JOUR A1 - De Andrade, V. A1 - Vidal, O. A1 - Lewin, E. A1 - Agard, P. A1 - O'brien, Patrick T1 - Quantification of electron microprobe compositional maps of rock thin sections: an optimized method and examples JF - Journal of metamorphic geology N2 - Quantification of discrete pressure-temperature domains in deformed chlorite + white mica-bearing metapelites was undertaken on mineral compositions derived by two-dimensional microprobe compositional mapping of selected areas of rock thin sections. In order to achieve compositional information at sufficient analytical precision, spatial resolution and sample coverage within a typical analysis time of 1 day, an optimization of measurement methods was necessary. The method presented here allows collection of raw counts for eight different element concentrations at an analytical precision of similar to 1-2 wt%. X-ray intensity multiplane maps (one map per measured chemical element) are translated into concentration multiplane maps, utilizing selected conventionally measured spot analyses combined with the Castaing approximation for each mineral. As this step requires identification of the different minerals present in the mapped area, a statistical clustering technique to identify different groups of composition was developed, guided by simple petrographic inspection of the thin section, to delineate the important minerals in the mapped area. Finally, the compositions of each pixel are translated into a mineral structural formula thus yielding a new kind of image with a high content of petrological information. The reliability of the mineral composition images was emphasized by carrying out precision tests on the analytical data. The possible use of chemical maps to infer the P-T-deformation history of metamorphic rocks is illustrated with two samples from the Spitzbergen and the Sambagawa blueschist facies belts. In both samples, a strong correlation between structures and chemistry is observed. Qualitative estimates of P-T conditions from the Si-content of mica and chlorite are in good agreement with their location in microstructures that formed at different times. Therefore, the combination of chemical maps with microstructural observations is a very powerful approach to understand both the evolution of complex metamorphic rocks and the control by deformation of mineral reactivity. KW - chlorite KW - compositional map KW - electron microprobe KW - mica KW - Sambagawa KW - Spitzbergen KW - X-ray mapping Y1 - 2006 U6 - https://doi.org/10.1111/j.1525-1314.2006.00660.x SN - 0263-4929 VL - 24 SP - 655 EP - 668 PB - Wiley-Blackwell CY - Oxford ER - TY - JOUR A1 - Krmíček, Lukáš A1 - Timmerman, Martin Jan A1 - Ziemann, Martin Andreas A1 - Sudo, Masafumi A1 - Ulrych, Jaromir T1 - 40Ar/39Ar step-heating dating of phlogopite and kaersutite megacrysts from the Železná hůrka (Eisenbühl) Pleistocene scoria cone, Czech Republic JF - Geologica Carpathica N2 - (40)A/Ar-39 step-heating of mica and amphibole megacrysts from hauyne-bearing olivine melilitite scoria/tephra from the Zelezna hurka yielded a 435 +/- 108 ka isotope correlation age for phlogopite and a more imprecise 1.55 Ma total gas age of the kaersutite megacryst. The amphibole megacrysts may constitute the first, and the younger phlogopite megacrysts the later phase of mafic, hydrous melilitic magma crystallization. It cannot be ruled out that the amphibole megacrysts are petrogenetically unrelated to tephra and phlogopite megacrysts and were derived from mantle xenoliths or disaggregated older, deep crustal pegmatites. This is in line both with the rarity of amphibole at Zelezna hurka and with the observed signs of magmatic resorption at the edges of amphibole crystals. KW - Bohemian Massif KW - Zelezna hurka KW - Eisenbuhl KW - argon dating KW - mica KW - amphibole KW - melilitite Y1 - 2020 U6 - https://doi.org/10.31577/GeolCarp.71.4.6 SN - 1335-0552 SN - 1336-8052 VL - 71 IS - 4 SP - 382 EP - 387 PB - Veda CY - Bratislava ER -