XRF SR application for studying of moonmilk from Botovskaya cave (Eastern Siberia)
Annotation. The abstract presents the data on chemical composition of moonmilk samples from Botovskaya cave, the largest cave in Russia. Concerning with the host rocks, moonmilk is characterized by the increased Ca and Sr and reduced Ti, Mn and Fe contents. The investigation of this type of cave sediments with the use of XRF SR method was done for the first time.
Introduction Cave investigations are of a great interest for modern scientific society. The sediments of the caves are the source of geochemical, geological and paleo environmental information demanding the application of modern research technique. X-ray fluorescence analysis with the use of synchrotron radiation (XRF SR) differs from the other methods by its all-purpose usage. XRF SR method is multi-elemental, has sensitivity sufficient for the determination of rare elements, guarantees the accuracy of measurements and can be used for objects of different origin and composition. Moreover, XRF SR is a non-destructive analysis, allowing to work with a small amount of material, which is an important advantage while examining the samples from such hard-to-access objects as caves. In this abstract, we present the results of studies of two probe of moonmilk sampled from the Botovskaya cave located at Eastern Siberia.
Objects and Methods Botovskaya Cave is the longest in Russia and located on the Verkhnelensky karst plateau within the Central Siberian plateau. The host rocks are represented by algal limestones of the Ust-Kutskaya suite of the Lower Ordovician, which form a layer with observed thickness up to 8 m, grasped between the marine sandstones of the same formation [Filippov, 1994]. The cave is one-storey, labyrinth type, has several entrances, located in the upper part of the left side of the river Boty. The length of the cave passages is 66743 m (according to the data of the speleoclub "Arabica", February 2011). In the cave are widely represented the collapse, water mechanical, organogenic and chemogenic secondary formations, ice is observed in some cave areas. Chemogenic sediments are represented by incrustations, stalactites, stalagmites, subaquatic rims of puddles, incrustations of their bottom, cave pearls, heliсtites and moonmilk. Lunar milk is a special type of chemogenic formations. It is a soft clay-like substance that becomes fluid when touched. The ability of lunar milk to liquefy was called microtixotropy. In the caves, moonmilk occurs in the form of films, streaks, lumps on the walls and on the floor. The composition of lunar milk, depending on the rocks in which the underground cavity is embedded, it can be calcite, gypsum, alumino-silicate and phosphate. The chemical composition of lunar milk was determined by the XRF SR method using the Measurements technique for determining the elemental composition of samples of magmatic, metamorphic, sedimentary-metamorphic and sedimentary rocks by X-ray fluorescence analysis using synchrotron radiation as an x-ray source [Daryin A.V, 2013]. Previously it was approved that the accuracy of the results obtained with the applied measurement technique ensures the reliability of geochemical data [Markova et al., 2012]. The analysis was carried out at SRC Siberian Center for Synchrotron and Terahertz Radiation" (Institute of Nuclear Physics, SB RAS, Novosibirsk) The chemical composition of the enclosing rocks and clay sediments was determined by the method of silicate analysis (analyst Samoilenko M.M) at the CRS Earth's Crust Institute SB RAS in Irkutsk and by the method of spectral analysis (analysts Shcherban VV, Naumova AV and Vorotynova L.V.). The pictures of lunar milk crystals were done with the application of scanning electron microscope CAMskan in Moscow at the Moscow State University by S.E. Mazina and on a scanning electron microscope VEGA 3 LMH with an X-ray energy dispersive microanalysis system INCA Energy 350 / X-max 20 at the Mining Institute of the Ural Branch of the Russian Academy of Sciences in Perm by analyst Korotchenkova O.V.
Results and discussion
In mineral terms, both samples of lunar milk from Botovskaya cave are composed of calcite. The concentrations of the following elements were determined by XRF SR: Ca, Ti, V, Mn, Fe, Cu, Zn, Ga, As, Br, Rb, Sr, Y, Zr, Mo, Ag (Table 1). The moonmilk showed increased contents of Ca, Ti, Mn, Ba and reduced Sr, Fe, V contents in comparison with the carbonate rocks Clarke content [Interpretation of geochemical data, 2001]. The contents of Mo and Ag do not exceed 0.00002 mass. %, so they were not taken into account. Concentrations of the main rock-forming elements in moonmilk were compared with the concentrations of these elements in the host rock composition (only the elements which content exceeded 0.01 mass. % in moonmilk were taken into account). The results are shown on figure (Fig. 1).
Fig. 1. Botovskaya cave’ concentrations of chemical elements in lunar milk, host rocks and clay sediments. 1 - the average composition of lunar milk (2 samples), 2 - the composition of the clay sample, 3 - the average composition of the enclosing rocks (8 samples).
In comparison with the host rocks, the contents of Ti, Mn, Fe, Cu are reduced in the moonmilk and the contents of Ca and Sr are increased. The content of V varies insignificantly. Four main hypotheses of the origin of lunar milk are known [Hill, Forti, 1997], such as: - cryogenic (lunar milk is a residual product from limestone dissolution); - disintegration hypothesis, close to cryogenic (lunar milk is a residual product of dissolution and disintegration of bedrock under the influence of water, respiration of microorganisms and other factors); - biogenic (many bacteria and fungi can precipitate small calcite crystals as a side-product of their activity); - accumulation hypothesis (lunar milk is a product of rapid precipitation substance from a saturated solution simultaneously around many crystallization centers). Taking into account that the samples were selected in the warm part of the cave far from the spread of seasonal and permanent glaciation, the cryogenic formation of these samples is questionable. More over, observed the differences in the morphology of lunar milk crystals and cryogenic calcite powder, taken from the surface of ice bodies (Fig. 2). While cryogenic formations are characterized by lamellar splices and casts, lunar milk is composed of filose crystals forming harp bunches.
Fig. 2. а - cryogenic calcite powder, б, в –lunar milk samples
The lunar milk formation during the disintegration of the bedrock presupposes a similarity of the composition of the enclosing rocks and the moonmilk. Considering the significant differences in the content of Fe, Mn, Ti, we can assume that this milk was formed from a solution saturated with calcium carbonate, and elements mentioned above were removed and precipitated in the residual clay. As the fig.1 shows clay differs in Ti, V, Fe, Cu, Zn contents, which are relatively higher in host rock and moonmilk. If we accept the biogenic hypothesis of mondmilh formation, these elements could be absorbed by microorganisms. To prove or disprove this hypothesis, microbiological studies are necessary. This abstract shows the preliminary results of geochemical cave investigations. XRF SR method was not widely used for studies of cave sedimentations, previously. Probably exactly the application of this method will move the geochemical studies of cave sediments towards a new stage. The authors thank the all the speleologists of "Arabica" club and A.V. Osintsev personally for their help in collecting samples, the analysts Samoilenko M.М., Shcherban V.V., Naumova A.V., Vorotynova L.V., Korotchenkova O.V. for analysts work and Masina S.E. for electron microscope pictures providing.
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