A rapid and precise quantitative electron probe chemical mapping technique and its application to an ultrahigh-pressure eclogite from the Moldanubian Zone of the Bohemian Massif (Nové Dvory, Czech Republic)

0498537 - GLÚ 2019 RIV US eng J - Článek v odborném periodiku
Yasumoto, A. - Yoshida, K. - Kuwatani, T. - Nakamura, D. - Svojtka, Martin - Hirajima, T.
A rapid and precise quantitative electron probe chemical mapping technique and its application to an ultrahigh-pressure eclogite from the Moldanubian Zone of the Bohemian Massif (Nové Dvory, Czech Republic).
American Mineralogist. Roč. 103, č. 10 (2018), s. 1690-1698. ISSN 0003-004X. E-ISSN 1945-3027
Institucionální podpora: RVO:67985831
Klíčová slova: Bohemian Massif * compositional map * electron probe * eclogite * pyroxenite * ultrahigh pressure * X‑ray map
Obor OECD: Mineralogy
Impakt faktor: 2.631, rok: 2018

Quantitative X‑ray mapping using an electron probe enables quantitative evaluation of inhomogeneities within rocks. Recent studies have proposed methods to construct quantitative chemical maps by combining X‑ray maps with referential spot analyses within a mapped area. These approaches address matrix effects by assuming each pixel in the mapped area represents a single phase. In such cases, the spatial resolution of the X‑ray maps must be sufficiently high to separate mineral phases.This study proposes a new procedure to reliably quantify centimeter-scale X‑ray maps even if the maps contain an ineligible number of pixels analyzing multiple phases because of a large mapping probe diameter. Such multi-phase pixels are statistically classified into their constituent phases by introducing a distribution-based clustering analysis. Furthermore, based on referential spot analyses, we implemented corrections for matrix effects and the backgrounds of single- and multi-phase pixels. Our technique, termed QntMap, was developed as an open source R package and distributed on a social coding platform, GitHub (https://github.com/atusy/qntmap).
We applied QntMap to calculate local bulk compositions within an ultrahigh-pressure eclogite from Nové Dvory, Czech Republic. The studied sample is a garnet-rich bimineralic eclogite that includes a 3 mm thick pyroxene-rich layer. A mapped area is approximately 3 × 1 cm in size and oriented normal to the layer. A profile normal to the layer shows increases in Cr₂O₃ (0.0 to 0.3 wt%) and X_Mg [Mg/(Fe+Mg) = 0.5 to 0.8] from the garnet-rich matrix toward the pyroxene-rich layer. A large variation in X_Mg and high-Cr₂O₃ contents in the pyroxene-rich layer are inconsistent with a cumulate origin. We suggest that the pyroxene-rich layer was derived from a pyroxenitic melt that intruded the eclogite.
Trvalý link: http://hdl.handle.net/11104/0290879