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The chemical and mineral composition of volcanic ash in the Upper Carboniferous deposits of the Usolka section (South Ural, Russia)

Volcanic ash from the Usolka section, which is proposed as an international standard for global correlation of the lower boundaries of the Kasimovian and Gzhelian Stages, is considered. The content of rock-forming components in ashes is given. It is shown that the ashes of the Gzhelian Stage differ from older deposits in higher contents of Al, Ca, Mn and lower contents of Si, Ti, Fe. An increase in the content of some microcomponents (Cu, Cr, Ba, S) to the lower boundary of the Kasimovian Stage was noted. For the first time, information has been obtained on the mineral composition of ashes, which allows them to be used for correlation.


Introduction
The Usolka section is located on the right bank of the Yuryzan Usolka River in the Republic of Bashkortostan in the Southern Urals (Fig. 1). The coordinates of the section: 53 ° 55´28´´N, 56 ° 31´47´´E. This section has been proposed as a candidate of the Global Boundary Section Stratotype and Point (GSSP) for the base of the Gzhelian Stage in the International Stratigraphic Chart. The Usolka section was demonstrated as a potential GSSP at the summer of 2015 to participants in the field excursion of the XVIII International Congress on the Carboniferous and Permian [5]. The section is represented by terrigenous-carbonate rocks of Carboniferous (Moscovian, Kasimovian, Gzhelian Stages) and Permian (Asselian, Sakmarian Stages) with intercalations of volcanic ash. The Usolka section is represented by terrigenous-carbonate rocks of Carboniferous (Moscovian, Kasimovian, Gzhelian Stages) and Permian (Asselian and Sakmarian Stages) with intercalations of volcanic ash. This allowed to obtain here detailed dates on the absolute age of zircons from ash horizons by the mass spectrometric method with thermal ionization and isotopic dilution -ID-TIMS [4]. Today, these data are used in the International Chronostratigraphic Scale [3], because they provide high resolution (~ 0.1 Ma) for the Carboniferous and Permian interval as an important complement to the global biostratigraphic correlation. Detailed biostratigraphic characteristics of the Usolka section are given in [1,2,[7][8][9].
This research is devoted to the analysis of the chemical, mineral, and particle size composition of the ash horizons of the Usolka section. Volcanic ashes are found in the form of interlayers of small thickness -from 1-2 to 8 cm, they have yellowish to dark brown shades that contrastly distinguish them against the background of gray and light gray host rocks (Fig. 1). The interlayers of the volcanic ashes are composed of finely dispersed clay particles, sometimes cemented by iron hydroxides (Fig. 1, below). We selected 17 samples from the ash horizons of Carboniferous deposits (Fig. 2): the Upper Moscovian, Kasimovian and Lower Gzhelian Stages. The absolute age of this interval is ~ 302-307.5 million years for U-Pb dating of zircons in the Usolka section [3,4].

Methodology
To determine the chemical composition of the samples, X-ray spectral fluorescence analysis was performed using a S8 Tiger wave dispersive X-ray fluorescence spectrometer (Bruker, Germany). To establish the mineral composition of ash horizons, an analysis was performed using a Bruker D2 Phaser X-ray diffractometer (Germany) with a copper and cobalt anode. Granulometric analysis of samples was performed. All analyzes were performed in the laboratories of KFU, analysts G. M. Eskina, B. I. Gareev, G. A. Batalin. Processing the results of chemical analyzes in the work was carried out by methods of mathematical statistics using the software packages STATISTICA and Excel.

Results and Discussions
According to the results of X-ray phase analysis, kaolinite, montmorilionite, and quartz are present in all ash samples ( fig. 3). Often calcite, muscovite (absent in sample U-3) and albite (absent in sample U-10) are common. Clinochlor and microcline are found in three samples (U-1, U-3, U-10). In 2 samples, dolomite (U-1 and U-13) and hornblende (U-10 and U-16) were found. Based on the results obtained, we can conclude that in the upper part of the Moscovian and Kasimovian Stages there is a wider range of minerals, which can be used as a mineralogical correlation criterion.
Analysis of the particle size distribution showed that in the ashes dominated (about 70%) particles of the silt fraction (0.1-0.01 mm). The clay fraction (<0.01 mm) is 20-30%, while the sand fraction (> 0.1 mm) accounts for 10-15%.    . 6) was also noted, which can be used as a geochemical criterion for correlation of sections.