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УДК 56.016.3 DOI: 10.19110/2221-1381-2016-3-26-32
XMT MICROPALAEONTOLOGICAL STUDY OF THE SILICITES
A.V. Zhuravlev1, A. I. Gerasimova2
1Institute of Geology Komi SC UB RAS, Syktyvkar micropalaeontology@gmail.com 2GeoInvestProekt Inc., St-Petersburg jiiin-94@mail.ru
The article deals with possibilities of the computed X-ray microtomography (XMT) in micropalaeontological (conodonts) study of the siliceous and clayey deposits. Using of the traditional methods in this type of deposits is difficult.
The work is based on collection of silicites (lithological type xa1), clayey silicites (lithological type xa2), cherty argillites (lithological type xa3), and limy silicites (lithological type xa4) from the deep-water Devonian and Carboniferous sequences of the Subpolar Urals and NE Pay-Khoy.
Thirty-eight samples (about 20.4 cm3) were studied with X-ray microtomographs SkyScan 1272 and SkyScan 1173 in the NMSU «Gornyi» (Saint-Petersburg, Russia) with spatial resolution of 4—7 um. About 190 conodont elements and their fragments were found in the tomograms.
The possibility of recognition of the conodont elements depends on lithological type of the host rock. The best contrast between conodont elements and host rock is detected in lithological types xa1 and xa3. These lithological types demonstrate maximal concentration of the conodont elements in the rock as well. The concentration of conodonts is up to 90 spec./cm3. The present study shows that XMT method is useful for search and identification of the conodonts in siliceous and clayey deposits. Taxonomic identification is possible just in tomograms of high spatial resolution (5—7 um).
Keywords: conodonts, X-ray microtomography, siliceous rocks.
РЕНТГЕНОТОМОГРАФИЧЕСКНН МЕТОД МНКР0ПАЛЕ0НТ0Л0ГНЧЕСК0Г0 ИЗУЧЕНИЯ КРЕМНИСТЫХ ПОРОД
А. В. Журавлев1, А. И. Герасимова2
1Институт геологии Коми НЦ УрО РАН, Сыктывкар 2 ООО «ГеоИнвестПроект», Санкт-Петербург
Статья посвящена возможностям использования рентгенотомографического метода для микропалеонтологического (конодонты) изучения кремнистых и кремнисто-глинистых отложений, в которых применение традиционных методов затруднено.
Материалом для данной работы послужили образцы различных силицитов и кремнистых аргиллитов из глубоководных отложений девонского и раннекаменноугольного возраста Приполярного Урала и Северо-Восточного Пай-Хоя.
Рентгенотомографическое изучение 38 образцов общим объемом 20.4 см3 проводилось на томографах SkyScan 1272 и SkyScan 1173 в НМСУ «Горный» (Санкт-Петербург) с пространственным разрешением 4—7 мкм.
Полученные результаты показали, что наиболее контрастны по рентгеновской плотности и хорошо опознаются на томограммах конодонтовые элементы в чистых силицитах и кремнистых аргиллитах. В этих же породах отмечена максимальная концентрация конодонтовых элементов. Рентгенотомографический метод может использоваться для поиска и идентификации конодонтовых элементов, вне зависимости от формы их сохранности, в сложно дезинтегрируемых кремнистых и глинисто-кремнистых породах. Таксономическая диагностика возможна лишь при высоком пространственном разрешении томограмм (размер элемента изображения меньше 5—7 мкм).
Ключевые слова: конодонты, рентгеновская микротомография, кремнистые породы.
Introduction
The deep water and volcanoclastic deposits yield the siliceous rocks. Biostratigraphical study of this wide spreading type of sediments is difficult due to taphonomical and palae-oecological reasons. Rare occurrence of the carbonate remains of macro- and microfauna is accompanied with difficulty in extraction of siliceous and phosphate microfossils from the siliceous deposits. Traditionally three methods are used for conodont study in the siliceous and clayey-siliceous rocks [10, 8]: hydrofluoric acid disintegration of the rock; search of the
bed-plane remains of conodont elements; conodont investigation in the thin and polished sections [13, 9, 11, 4]. The acid disintegration is the most effective method; however this reagent is very toxic and leads to conodont element damage in some cases [8]. The search of the conodont elements on the bedding planes is useful just for the microlaminated (clayey) silicites. The study of conodonts in thin and polished sec -tions needs a high abundance of conodont elements, and allows just rough taxonomic diagnostics, especially for the morphologically complex taxa [9, 4].
The radiography was used for conodont study by some authors [8, 12, 6], however with little success due to low radi-odensity contrast between the conodont elements and the host rocks.
Appearance in the late decade of the x-ray microtomogra-phy (XMT) with a high spatial resolution promises new opportunities in conodont study in the siliceous and clayey deposits [7, 5, 1, 2]. This article is aimed to consider the XMT possibilities.
Methods
Computed microtomography (XMT) is X-ray imaging in 3D. It represents true 3D microscopy, where a very fine scale internal structure of objects is imaged non-destructively. The present study was based on XMT systems SkyScan 1173 and SkyScan 1272 (Bruker, Belgium) of National Mineral Resources University (Saint-Petersburg, Russia). The samples were scanned with an isotropic voxel resolution of 3—7 mcm at 70— 90 kV source voltage (Al 0.5 mm and Al 0.5+Cu 0.038 filters). The reconstructions were made with NRecon software (Bruker, Belgium).
Material
The Devonian and Early Carboniferous deep-water silic-ites (cherts) and cherty argillites from the Sub-Polar Urals (Kozhym River and Kharuta River sections), NE Pay-Khoy (Kara River, Khardto Lake, Labsuyakha and Peschanaya rivers sections) were studied. The carbonates were excluded from the studied collection due to low radiodensity contrast between cal-cite, dolomite and conodont bioapatite. The siliceous and clayey deposits possess low radiodensity providing good contrast between the conodont elements and the host rock [1].
The samples of various silicites and cherty argillites containing conodont elements were selected for this study. The 38 rock pieces of 5—15 mm size (0.1—3.9 cm3) were scanned mainly with Skyscan 1272 with an isotropic voxel resolution of 4—7 mkm.
3-dimensional reconstructions and processing were made using NRecon, CTVox, DataViewer (SkyScan, Bruker), Voxler 3 (Golden Software), VolView 3 (Kitware Inc.), 3D Analyser («GeolnvestProekt»), and MeshLab (Visual Computing Lab — ISTI — CNR) software.
All the studied samples correspond to four lithological and genetic types [3]: gray to dark-gray massive or unclear laminated silicites (xa1); clayey gray to dark-gray silicites possessing wavy lamination (xa2); black carbonaceous and cherty laminated argillites (xa3); gray limy silicites demonstrating wavy lamination (xa4).
Results
The quality of the tomographic images of conodont elements depends on the host rock type, because of different relations of the radiodensities of conodont elements and rocks [1, 2]. The highest radiodensity contrast was detected in the pure silicites and cherty argillites. These lithological types demonstrate much lower radiodensity than conodont elements (Fig. 1). Increasing carbonate content in the host rock leads to decreasing contrast due to close radiodensities of the calcite, dolomite and conodont bioapatite.
The radiodensity of conodont element matter depends on the source voltage: it is about 0.035 mm—1 for 90 kV source voltage (Al 0.5 + Cu 0.038 filter) and 0.05—0.06 mm—1 for 70 kV source voltage (Al 0.5 filter) [1]. The radiodensity value can vary significantly depending on digital processing of the tomogram, sample size, and type of the host rock (Fig. 1).
The abundance of conodont elements in the study samples varies from 0 up to 90 elements per cubic cm; some samples was empty (Text-table 1). Total volume of the study samples was 20.4 cm3.
The cherty argillites (lithological type xa3) possess the highest abundance of conodont elements (from 4 up to 90 elements per cubic cm, 24 elements per cubic cm in average (studied volume is 3.6 cm3). The conodont elements and their debris are distributed and oriented along the bedding planes (Fig. 2).
Lithological types xa1 and xa2 demonstrate lower abundance of conodont elements: 0—35 elements per cubic cm, 6 elements per cubic cm in average (studied volume is 14.3 cm3). The conodont elements are scattered in the rock or concentrated along the bedding planes marked by clayey matter and/ or radiolarians (Fig. 3).
The limy silicites (xa4) possess the lowest abundance of conodont elements: 2—8 elements per cubic cm, 3—4 elements per cubic cm in average (studied volume is 2.56 cm3). The conodont elements show both the chaotic and ordered along the bedding planes distribution in the rock (Fig. 4).
The close relation between the conodont element abundance and concentration of the radiolarians was detected in the study samples (Fig. 5). The concentration of radiolarians was evaluated on the basis of study of the thin and polished sections. The rocks containing more than 7 % of radiolarians show low concentration of conodont elements (less than 10 elements per cm3). Wide variations of the conodont element abundance are characteristic for rocks containing less than 7 % of radiolar-ians (Fig. 5). This tendency is true not only for the cherty silicites but for all the lithological types (Fig. 5).
It is interesting to note that debris of conodont elements are common in the studied rocks. The debris/element ratio is up to 2 in the clayey silicites and cherty argillites. Thus we can suppose that conodont element fragmentation, observed in the conodonts extracted from the rock, rather depends on preservation, than on sample processing.
Some samples of cherty argillites and clayey silicites demonstrate specific mode of preservation of conodont elements as molds. XMT allows reconstructing morphology of the cono-dont elements on the basis of the molds (Fig. 2).
The XMT data of high spatial resolution (voxel size less than 4 mkm) allow taxonomic diagnostics of the conodont elements. E.g. species of Siphonodella, Ancyrodella, and Polygnathus were recognized in cherty argillites, and species of Siphonodella, Palmatolepis, and Polygnathus were distinguished in silicites (Fig. 6). The lower spatial resolution (voxel size larger than 7 mkm) makes it impossible to determine the conodonts at both the species and genus levels.
Discussion
The XMT investigations of the cherty rocks demonstrate possibility of using of this method for search and study of the conodont elements in rock. The method is effective in case if conodont abundance is larger than 2000 elements per kg (about 5—6 elements/cm3). In this case sample size of 0.5—1 cm3, which can be scanned in the micro-tomograph with high resolution, provides presence of some conodont elements. Cherty argillites, clayey and pure silicites may possess such abundance of conodont elements according the data obtained. Additionally it is difficult to process these lithological types with traditional methods (acid disintegration), and these rocks have low radiodensity providing visibility of the conodont elements in tomograms.
Fig. 1. Radiodensity of the conodont elements (in red) and the host rock (in blue) in different lithological types and source voltage compared.
Radiodensity is in cm-1
Рис. 1. Соотношение рентгеновских плотностей конодонтовых элементов (красная заливка на гистограммах) и вмещающей породы (синяя заливка на гистограммах) для различных литологических типов и ускоряющих напряжений. Значения рентгеновской плотности приведены в см-1
Tectonically altered cherts and shales, containing fractured conodont elements (Fig. 6), represent appropriate objects for XMT study. For example, XMT study of the siliceous deposits of the lower part of the Silovayakha Fm. (D3—C1 sl) in the NE Pay-Khoy (Labsuyakha River, Amderma district) allows to spread biostratigraphical characteristics of this part of sequence, where traditional methods supplied just debris of conodont elements. The Pa-elements of Palmatolepisgracilis (Fig. 6), characteristic for the upper Famennian, were recognized in the tomogram. XMT was the only way to study conodonts in the clayey silicites of the Gromashor Fm. in the Kara River section (Pay-Khoy). The Pa-element ofgenus Ancyrodella was recognized in the tomogram (Fig. 6).
The tomography supplies interesting data about distribution of conodont elements in the clayey and cherty sediments. A high conodont abundance, observed in the cherty argillites, is probably caused by concentrating of the conodont elements on the bed planes corresponding to diastems. Uniform sedimentation rate of the silicites caused chaotic distribution of the conodont elements and less conodont abundance, especially in the radiolarian-reach sediments possessing high sedimentation rate.
XMT demonstrates some difficulties in practice. It is necessary to use a high spatial resolution ensuring visibility of con-
odont elements in tomograms. The high resolution leads to small size of the samples under processing. Optimal sample size is about 1 cm3, maximal 4 cm3. Additionally some lithological types with a high radiodensity (e. g. carbonates, limy silicites and argillites) are inapplicable for the conodont study with XMT due to low radiodensity contrast making conodonts invisible in tomograms (Fig. 1).
Conclusions
XMT can be used for search and identification of the con-odont elements in the silicites and cherty argillites. Tomograms with high spatial resolution (5—7 mcm voxel size) provide possibility of the taxonomic diagnostics of conodonts. The cherty argillites and pure silicites are the most promising lithological types for XMT study. These litotypes possess rather a high con-odont abundance in association with high radiodensity contrast between the conodont elements and the host rock.
Acknowledgements. A. A. Shtyrlyaeva (NMSU «Gornyi») helped to take the tomograms. The investigations were partly supported by the project «Devonian deep-water domains as a key toward understanding biodiversity crises» (Silesian University, Poland).
XMT data on samples studied Ретнгенотомографические данные по образцам
Section Разрез Sample Образец Lithological type Литотип Age Возраст Volume (cm3) Объем (см3) Total elements and debris Общее количество элементов и обломков Abundance of conodont elements in the rock (specimen/cm3) Содержание конодонтовых элементов (шт/куб.см) Radiolarians content (%) Содержание радиолярий в породе (%)
Konstantinov Creek Tn-23/99 xal c,t2 0.20 7 35.0 7
руч. Константинов Tn-27/99 xal C,t2 4.09 10 2.9 7
Tz-78-1 xa3 C, t2 0.64 5 7.8 0
80-1/87 хаЗ c, t2 1.41 51 36.2 1
84-1/87 xa2 C, t2 1.21 14 11.6 0
1215/23 xa4 C,t, 0.40 3 7.5 20
Kozhym River p. Кожым 1215/23 xa2 c,t, 1.40 5 3.6 7
1215/23 хаЗ C, t, 0.10 5 50.0 2
1215/26 xa2 C,t, 2.30 2 0.9 10
1215/33-1 xa4 c,t2 0.86 2 2.3 0
1215/33-1 хаЗ c, t2 0.07 4 57.1 0
1215/39 xal c,t2 0.30 2 6.7 20
76-1 хаЗ C,t, 0.03 16 484.8 0.3
Kharuta River p. Харута 32-11 xal C,t, 0.30 1 3.30 1
32-21 xa2 C,t2 0.56 15 26.8 1
32-18 (fl) xa2 c,t2 0.32 6 18.5 0
Peschanaya River p. Песчаная 2037/23(1) xa2 C,v2 0.48 0 0 0
Khardto Lake 2044/2 xa2 D3 fm2 0.23 5 21.7 1
оз. Хардто 2044/3 xal D3fm3 0.35 5 14.3 7
Labsuyakha River p. Лабсуйяха 2048/5 xa4 D3 fm3 1.30 4 ЗЛО 10
2032/5 хаЗ D, f 0.35 6 17.1 0
2032/1 xa2 D3f 1.17 3 2.6 0
Taripatayakha River 6053-2 xal D| em 0.30 4 13.3 2
p. Тарипатаяха 7043/1 xal C, t 0.31 9 29.0 5
Kara River p. Кара 7388/1 хаЗ D3f 0.98 4 4.1 15
Moreizyakha River p. Мореизъяха 2074/2 xal D3 0.84 0 0 10
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Fig. 2. Conodont elements in the tomograms of cherty argillite (lithological type xa3): 1, 2 — sample 80-1/87, the Kozhym River, Subpolar Urals, Tournaisian, Siphonodella quadruplicata Zone; 3, 4 — sample 76-1/87, the same locality and age
Рис. 2. Конодонтовые элементы на томограммах кремнистого аргиллита (литотип xa3): 1, 2 — обр. 80-1/87, р. Кожым, Приполярный Урал, турнейский ярус, зона Siphonodella quadruplicata; 3, 4 — обр. 76-1/87, местонахождение то же
Fig. 3. Conodont elements in the tomograms of silicite (lithological type xa1), sample Tn-23, the Konstantinov Creek, Kozhym River, Subpolar
Urals, Tournaisian, Siphonodella quadruplicata Zone
Рис. 3. Конодонтовые элементы на томограммах силицита (литотип xa1), обр. Tn-23, руч. Константинов, р. Кожым,
Приполярный Урал, турнейский ярус, зона Siphonodella quadruplicata
Fig. 4. Conodont elements in the tomograms of limy silicite (lithological type xa4): 1 — sample 1215-33, Kozhym River, Subpolar Urals, Tournaisian, Siphonodella quadruplicata Zone; 2 — sample 2048/5, Labsuyakha River, Pay-Khoy, Famennian, expansa Zone
Рис. 4. Конодонтовые элементы на томограммах карбонатного силицита (литотип xa4): 1 — обр. 1215-33, р. Кожым, Приполярный Урал, турнейский ярус, зона Siphonodella quadruplicata; 2 — обр. 2048/5, р. Лабсуйяха, Пай-Хой, фаменский ярус, зона
Expansa
Fig. 5. Conodont element content versus content of radiolarians
Рис. 5. Зависимость содержания конодонтовых элементов в породе от содержания радиолярий
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Fig. 6. Conodont elements from the clayey and siliceous rocks (tomographic images): 1 — Siphonodella cf. hassi Ji, sample 1215-33, lithological type xa3, Kozhym River, Subpolar Urals, Tournaisian, Siphonodella quadruplicata Zone; 2 — Ancyrodella sp., sample 7388/1, lithological type xa3, Kara River, Pay-Khoy, Frasnian; 3 — Polygnathus purus Voges, sample 80-1/87, lithological type xa3, Kozhym River, Subpolar Urals, Tournaisian, Siphonodella quadruplicata Zone; 4, 5 — Palmatolepisgracilis Branson et Mehl, sample 2048/5, lithological type xa4, Labsuyakha River, Pay-Khoy, Famennian, Expansa Zone; 6 — Siphonodella cf. obsoleta Hass, sample Tn-23, lithological type xa1, Konstantinov Creek, Kozhym River, Subpolar Urals, Tournaisian, Siphonodella quadruplicata Zone; 7 — Siphonodella crenulata (Cooper), sample Tn-23, lithological type xa1, Konstantinov Creek, Kozhym River, Subpolar Urals, Tournaisian, Siphonodella quadruplicata Zone; 8, 9 — Siphonodella quadruplicata (Branson et Mehl) surrounded by undiagnostic fragments of platform and ramiform elements, sample 80-1/87, lithological type xa3, Kozhym River, Subpolar Urals, Tournaisian, Siphonodella quadruplicata Zone; 10 — fragmented S-element of Palmatolepis, sample 7388/1, Kara River, Pay-Khoy, Frasnian; 11 — fragmented M-element of Siphonodella, sample Tn-23, Konstantinov Creek, Kozhym River, Subpolar Urals,
Tournaisian, Siphonodella quadruplicata Zone
Рис. 6. Конодонтовые элементы из кремнисто-глинистых пород, томографические изображения. 1 — Siphonodella cf. hassi Ji, обр. 1215-33, литотип xa3, р. Кожым, Приполярный Урал, турнейский ярус, зона Siphonodella quadruplicata; 2 — Ancyrodella sp., обр. 7388/1, литотип xa3, р. Кара, Пай-Хой, франский ярус; 3 — Polygnathus purus Voges, обр. 80-1/87, литотип xa3, р. Кожым, Приполярный Урал, турнейский ярус, зона Siphonodella quadruplicata; 4, 5 — Palmatolepis gracilis Branson et Mehl, обр.2048/5, литотип xa4, р. Лабсуйяха, Пай-Хой, фаменский ярус, зона Expansa; 6 — Siphonodella cf. obsoleta Hass, обр. Tn-23, литотип xa1, руч. Константинов, Приполярный Урал, турнейский ярус, зона Siphonodella quadruplicata; 7 — Siphonodella crenulata (Cooper), обр. Tn-23, литотип xa1, руч. Константинов, р. Кожым, Приполярный Урал, турнейский ярус, зона Siphonodella quadruplicata; 8, 9 — Siphonodella quadruplicata (Branson et Mehl) в окружении недиагностируемых фрагментов платформенных и рамиформных элементов, обр. 80-1/87, литотип xa3, р. Кожым, Приполярный Урал, турнейский ярус, зона Siphonodella quadruplicata; 10 — фраг-ментированный S-элемент Palmatolepis, обр. 7388/1б р. Кара, Пай-Хой, франский ярус; 11 — фрагментированный M-элемент Siphonodella, обр. Tn-23, руч. Константинов, р. Кожым, Приполярный Урал, турнейский ярус, зона Siphonodella quadruplicata
