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Article Radon Over Kimberlite Pipes: Estimation of the Emanation Properties of Rocks (Lomonosov Diamond Deposit, NW Russia) Evgeny Yakovlev * and Andrey Puchkov N. Laverov Federal Centre for Integrated Arctic Research of Ural Branch of the Russian Academy of Sciences, 109 Severnoj Dviny Emb., 163000 Arkhangelsk, Russia; vp‐andrew@list.ru * Correspondence: evgeny.yakovlev@fciarctic.ru; Tel.: +7‐931‐401‐41‐08; Fax: +7‐8182‐28‐76‐36 Abstract: In this paper, using the example of the Lomonosov diamond deposit, experimental stud‐ ies of rocks were carried out to assess the main radiation and physical factors affecting the for‐ mation of the radon field over the kimberlite pipes of the Arkhangelsk diamondiferous province. For various types of rocks, represented by vent kimberlites, tuffaceous‐sedimentary rocks of the crater and enclosing and overlying sediments, the following were studied: porosity, density, activ‐ ity of radium‐226, activity of radon in a free state, level of radon production, and emanation coef‐ ficient. The research results showed that the greatest amount of radon in a free state is produced by rocks of the near‐pipe space, represented by the enclosing Vendian V2 deposits and character‐ ized by high values of the emanation coefficient, radium activity, radon production level and po‐ rosity. This fact is associated with the structural and geological features of the near‐pipe space, which was exposed to the impact of kimberlite magma on the host rocks. The lowest values of these parameters are characteristic of the kimberlites of the vent facies, which limits the formation Citation: Yakovlev, E.; Puchkov, A. Radon Over Kimberlite Pipes: of free radon in the body of the pipe. The results of the experimental studies create prospects for Estimation of the Emanation the development of emanation methods for searching for kimberlite pipes in the conditions of the Properties of Rocks (Lomonosov Arkhangelsk diamondiferous province. Diamond Deposit, NW Russia). Appl. Sci. 2021, 11, 6065. Keywords: radon field; kimberlites; Arkhangelsk diamondiferous province; emanation coefficient; https://doi.org/10.3390/app11136065 radon production rate; porosity; density Academic Editor: Giuseppe Lacidogna 1. Introduction Received: 20 April 2021 Radon is part of the decay chain of the uranium‐238 radioactive family and is con‐ Accepted: 24 June 2021 Published: 29 June 2021 tinuously formed in natural environments during the radioactive decay of its parent isotope, radium‐226, the half‐life of which is about 1600 years [1–5]. Being an inert gas Publisher’s Note: MDPI stays neu‐ with a relatively long half‐life (3.82 days), in terms of its physical and chemical proper‐ tral with regard to jurisdictional ties, it acts as an optimal indicator for studying many processes occurring in the envi‐ claims in published maps and insti‐ ronment [4,6–8], including several important geological processes [9,10]. This is due to tutional affiliations. the fact that radon gas shows the ability to easily migrate in the geological environment in the gas phase or dissolved in pore waters [3,11–13], forming a radon field with the appearance of emanation anomalies in the near‐surface horizons of rocks and soils [3,14,15]. In this regard, emanation methods began to be widely used in geochemical, Copyright: © 2021 by the authors. geophysical and geodynamic studies to track geological processes [4]. Emanation meth‐ Licensee MDPI, Basel, Switzerland. ods are also used in the search for mineral deposits, including kimberlites [16–19]. Ema‐ This article is an open access article nation studies carried out in the territory of the kimberlite fields of the Arkhangelsk di‐ distributed under the terms and amondiferous province (Russian Federation) showed that, in the soil horizons above the conditions of the Creative Commons kimberlite bodies, there are increased values of the volumetric activity of radon that are Attribution (CC BY) license several times higher than the background values [20]. It was shown that the nature of (http://creativecommons.org/licenses the distribution of radon over kimberlite pipes has a subcircular structure, which is as‐ /by/4.0/). Appl. Sci. 2021, 11, 6065. https://doi.org/10.3390/app11136065 www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 6065 2 of 26 sociated with the development of faults and fracturing in the enclosing rocks of the near‐ pipe space [19,21]. Previously obtained data have shown high prospects for the applica‐ tion of emanation methods in the search for kimberlite pipes in the territory of the Ar‐ khangelsk diamondiferous province [22]. However, for a more complete understanding of the specifics of locating explosion pipes in the field of radon volumetric activity and the development of emanation meth‐ ods to search for kimberlites, it is necessary to conduct detailed studies of the mecha‐ nisms and conditions of radon formation in the rocks that compose the kimberlite body. The purpose of this study was to study the emanation properties of rocks that make up the kimberlite pipes of the Arkhangelsk diamondiferous province, so the radiation and physical parameters of the kimberlites that overlap and the host rocks of the Lomonosov diamond deposit of the Arkhangelsk diamondiferous province were studied. The fol‐ lowing radiation and physical parameters were studied: activity of radium‐226, activity of radon‐222, emanation coefficient, radon rate production, average and true density, and porosity. An assessment of the main factors causing the formation of the increased activity of radon over the pipes has been carried out. 2. Material and Methods The Lomonosov diamond deposit of the Arkhangelsk diamondiferous province is located ~90 km northeast of Arkhangelsk, forming a chain about 20 km long with a strike north–north‐east. The deposit consists of six kimberlite pipes: Arkhangelskaya, Karpinsky‐1, Karpinsky‐2, Pionerskaya, Lomonosovskaya, and Pomorskaya (Verzhak et al. 2008). Currently, only two pipes (Arkhangelskaya and Karpinsky‐1) are being devel‐ oped by quarries. The location and geological structure of the Lomonosov diamond de‐ posit is shown in Figure 1. The sampling of rocks was carried out in the quarries of the Arkhangelskaya (N65.2913, E41.0209) and Karpinsky‐1 (N65.3018, E41.0246) kimberlite pipes, together with the geologist of the mining plant (Figure 1). In total, 30 rock samples were taken, weighing up to 2 kg each. Rocks are represented by various types of deposits from quarries up to 186 meters deep (from top to bottom): overlying Quaternary (Q) and Carboniferous rocks (C2), Vendian host rocks (V2), tuffaceous sedimentary crater rocks (iD3‐C2) and vent facies kimberlites (iD3‐ C2). A detailed description of the geological structure of the Lomonosov diamond deposit was presented by Yakovlev (2020) [22]. Typical geology cross‐section of kimberlite pipe of the Lomonosov diamond deposit is in Appendix B. After delivery to the laboratory, rock samples were dried in a BINDER E28 drying oven at 105 °C to an air‐dry state. The determi‐ nation of the radiation and physical parameters of the rocks was carried out in the laboratory of environmental radiology, which complies with the accreditation criteria for testing labora‐ tories established in ISO/IEC 17025. The analytical procedures are described below. Typical Types of the Studied Rocks from the Lomonosov Diamond Deposit are shown in Appendix C. Appl. Sci. 2021, 11, 6065 3 of 26 Figure 1. The location of the study area and the geological structure of the Lomonosov diamond deposit. 1—Kimberlite pipe; 2—Quaternary fluvioglacial sediments; 3–5—Middle Carboniferous sediments: 3—Olmugo–Okunev suite; 4—Voevchensky suite; 5—Ursug suite; 6—Kimberlites: Upper Devonian–Middle Carboniferous; 7—Upper Vendian sediments; 8—Upper Riphean: Tok‐ minskaya suite; 9—Archean: Belozersky complex. 2.1. Gamma Spectrometry Measurements The method of gamma spectrometry is widely used to measure gamma radiation from radionuclides of natural origin, including Ra, because it is a universal, non‐ destructive and easy‐to‐use method, especially at the stages of sample preparation and in the measurement process [23,24]. To determine radionuclide Ra in rocks, we used a semiconductor gamma‐spectrometric complex with nitrogen cooling ORTEC with a GEM 10 P4‐70 HPGe detector (Ametek Ortec, Oak Ridge, TN, USA) complete with lead shielding. The resolution of the gamma spectrometer along the 1.33 MeV ( Co) line is 1.75 keV, and the relative efficiency is 15%. The measurement geometry was a 1‐L Mari‐ nelli vessel (counting sample). The activity of the Ra radionuclide, taking into account the accumulation, was de‐ termined from the radionuclide Pb (351.93 keV with a quantum yield of 35.60%) and Bi (609.32 keV with a quantum yield of 45.49%, 1120.29 keV with a quantum yield of 14.92%, 1764.49 keV yield—15.3%). The values of the activity of Ra, determined from its daughter decay products, and the activity of Ra, experimentally determined from its own gamma radiation (186.21 keV), coincided within at least 90%. Both methods of the registration of the Ra radionuclide (by its own gamma radiation and by the gamma radiation of its decay products) have their own advantages and disadvantages [25]. In the experimental determination of the specific activity of the Ra radionuclide from its gamma radiation, the presence of the uranium‐235 radionuclide, with its gam‐ ma radiation energy of 185.72 keV, was taken into account, the activity of which can be Appl. Sci. 2021, 11, 6065 4 of 26 determined from the lines 63.29 keV and 92.80 keV ( Th). This experimental method for the determination of Ra is applicable only for natural environments that are almost 235 238 always characterized by a constant ratio of U/ U radionuclides and the presence of a 238 234 radioactive equilibrium between U and Th radionuclides [26]. 2.2. Radiometric (Emanation) Measurement Method The determination of the volumetric activity of radon was carried out using a measuring complex for monitoring radon, thoron and their daughter products “Alfarad plus” (NPP “Doza”, Moscow, Russia). The measurement geometry was a 5‐L plastic cyl‐ inder (counting sample). The measurement of the volumetric activity of Rn using this measuring instru‐ ment is based on the electrostatic deposition of charged ions Po (RaA) from an air sample onto the surface of a semiconductor detector. Electrical impulses generated un‐ der the influence of alpha particles on the detector are amplified by a charge‐sensitive preamplifier, fed to the input of the converter and then processed by the built‐in single‐ board PC computer. The volumetric activity of Rn is determined by the number of reg‐ istered alpha particles during the decay of RaA atoms deposited on the detector [27]. 2.3. Calculation Methods for Assessing the Coefficient of Emanation The emanation coefficient of radon was determined by two methods: the gamma‐ spectrometric and radiometric (emanation) methods. First the radionuclide Ra was measured using a gamma spectrometer. The meth‐ od consists of measuring the gamma activity of samples at various intervals after they are placed in a hermetically sealed container. In this experiment, counting samples were prepared in the form of a crushed sample with a grain size of less than 0.5 mm in a 1‐L Marinelli vessel. Samples were measured daily for 21 days after they were sealed in the Marinelli vessel. Based on the results of these measurements, the following were determined: the activity of Ra, without taking into account the accumulation of its decay products; the activity of Ra, taking into account the accumulation of the daughter products of its de‐ cay (DPR); the emanation coefficient; and the period during which the DPR of Ra enter a state of radioactive equilibrium. After the experiment, the counting sample was de‐ pressurized, and after 1–2 days, the specific activity of Ra was measured again. Based on the results of the experiment, the emanation coefficient of Rn (its free state) was determined using the following formula: 𝐴 𝑜𝑛𝑛𝑟𝑖𝑢𝑚𝑒𝑞𝑢𝑖𝑙𝑖𝑏 𝐾 1 100 (1) where А226Ra (non‐equilibrium) is the activity of Ra (in a nonequilibrium state), determined −1 as the average value of the results of the first and last measurements (unsealed), Bq∙kg ; and А226Ra (equilibrium) is the specific activity of Ra (in an equilibrium state), determined as the −1 average value of the results of the last 5 measurements in a sealed state, Bq∙kg . At the second stage, rock samples were pre‐dried at a temperature of 40 °C for at least 24 h. Then the samples were sealed in plastic cylinders with a volume of 5 L for a period of at least 21 days, and using a radon radiometer, the volumetric activity of Rn in the airspace of this cylinder was determined. To prevent the leakage of radon, the joints between each component of the apparatus were sealed with silicone rubber and glycerin prior to experimentation. This stage was necessary to assess the comparability with the results of gamma‐spectrometric measurements and calculations of the level of radon production, as well as to confirm the fact of the release of Rn from rocks in quantities corresponding to its free state. The volumetric activity of Rn in a cylinder at this stage in the airspace of this cyl‐ inder was calculated according to the following formula: 𝑒𝑞𝑢𝑖𝑙𝑖𝑏𝑟𝑖𝑢𝑚 Appl. Sci. 2021, 11, 6065 5 of 26 𝐴 𝑉 𝐴 1 (2) 2000 𝑉 where ARn_container is the activity of Rn in the free volume of the plastic container, Bq/container; ARn_volume is the volumetric activity of Rn, calculated from the results of measurements of the counting sample using a radon radiometer (in this case, the meth‐ −3 od was used for a geometry of a plastic vessel with a volume of 1 L), Bq∙m ; Vcontainer is the free volume of a plastic container after filling it with a rock sample, l; and Vchamber is the volume of the chamber of the radon radiometer, l. According to the results of measurements at the second stage, the emanation coeffi‐ cient of Rn (its free state) was determined according to the following formula: 𝐴 𝑉 𝐾 (3) 1𝑒 𝑀 𝐴 where KRn is the coefficient of radon emanation; A222Rn is the volumetric activity of radon −3 3 in a free state in a sealed container, Bq∙m ; V is the free volume of the container, m ; λ = −6 −1 2.1 × 10 s is the constant of radon decay; λl is the coefficient of radon leakage under the conditions of the experiment; M is the total mass of the sample in the container; t is the time of sealing of the counting sample; and A226Ra is the activity of Ra in a counting −1 sample, Bq∙kg . 2.4. Calculation of Radon Production Rate −3 −1 We also calculated the radon production rate, P (Bq∙m ∙h ), using the following formula [28,29]: 𝑃𝜆𝐾 𝐴 ρ (4) −6 −1 −3 where λ is the decay constant for radon (2.1∙10 s ) and ρb is the bulk density, kg∙m . 2.5. Assessment of Radon Leakage One of the important values of the uncertainty of measurements of radon activity in our experiment is the parameter of radon leakage from the container [23,30]. The estima‐ tion of the radon leakage parameter was carried out in the same geometry as the main experiment to determine the emanation coefficient. Two plastic containers with a vol‐ ume of 5 L were connected through one hole with a valve. A source of radionuclide Ra with high activity was placed in one container. The “inlet” and “outlet” fittings were lo‐ cated on the second container. A system of two containers through fittings was connect‐ ed into one complex with a radon radiometer, an air blower and a dryer. For the first stage, the experiment was carried out without a valve. The Ra source 222 226 was placed in container 1 for two weeks. Rn formed as a result of the decay of Ra was distributed in a system of two containers. For the second stage, the opening connecting the two containers was closed with a valve, after which the first container with the Ra source was removed. Then a series of measurements of the volumetric activity of radon in the second container was carried out for 10 days every day. Based on the measurement results, the parameter of radon leakage, λl, was determined in a plastic container using the following formula [30]: (5) 𝐴 𝐴 𝑒 where λ is the decay constant of radon. The experimental facility for assessing radon leakage is shown in Figure 2. Appl. Sci. 2021, 11, 6065 6 of 26 Figure 2. Experimental facility for assessing radon leakage. 2.6. Assessment of Density and Porosity Parameters The determination of the parameters of porosity and the average (bulk) and true density of the selected samples was carried out according to the methods below, based on the works [29,31]. Average (bulk) density was used as a physical quantity determined by the ratio of the mass of the material to the entire volume it occupies, including pores and voids. True density is related to the densities of the minerals in the sample and their relative abundances. 2.6.1. Assessment of Average (Bulk) Density The average density of the rock sample was determined by coating it with paraffin about 1 mm thick. For this, the sample, dried to constant weight, was immersed in heat‐ ed paraffin and cooled in air. If bubbles or damage were found during cooling on the paraffin film, they were smoothed out using a hot metal plate, knife or wire. The prepared sample was weighed. Then, weighing was carried out on a hydrostat‐ ic balance. The average density of waxed rock samples of various shapes, ρk1 (g/cm ), was de‐ termined by the formula: 𝜌 𝑚 𝑚 𝑚 𝑚 (6) 𝜌 𝜌 where m is the dry weight of the sample, g; m′1 is the mass of the waxed sample in air, g; m′2 is the mass of the waxed sample in water, ρw is the density of water, taken to be −3 equal to 1 g∙cm ; and ρp is the density of paraffin (can be taken to be equal to 0.93 −3 g∙cm ). The result was calculated as the arithmetic mean of the test results of five rock sam‐ ples. 2.6.2. Assessment of True Density Test Preparation Samples, on which the average density was determined, were used for testing. Each marked sample was cleaned to remove dust with a brush, ground to a grain size of 5 mm and then mixed. Then, the sample was reduced by quartering to 150 g. Then, this sample was again ground to a grain size of less than 1.25 mm, mixed and reduced to 30 g. The prepared sample was ground into powder in a porcelain mortar, poured into a weighing glass or into a porcelain cup, dried to constant weight and cooled to room Appl. Sci. 2021, 11, 6065 7 of 26 temperature in a desiccator over concentrated sulfuric acid or anhydrous calcium chlo‐ ride, after which two portions of 10 g each were weighed. Testing Each sample was poured into a clean dried pycnometer, and distilled water was poured in such an amount that the pycnometer was filled to no more than half its vol‐ ume. The pycnometer was placed in a slightly inclined position in a sand or water bath and its contents were boiled for 15–20 min to remove air bubbles. After removing air, the pycnometer was wiped off, cooled to room temperature, refilled to the mark with dis‐ tilled water and weighed. The pycnometer was freed from the contents, washed, filled to the mark with distilled water at room temperature and weighed again. Calculation of test results: −3 The true density ρw, g∙cm , was calculated by the formula: 𝑚𝜌 𝜌 (7) 𝑚𝑚 𝑚 where m is the weight of the sample of powder dried to constant weight, g; ρw is the −3 density of water, taken to be equal to 1 g∙cm ; m1 is the mass of the pycnometer with dis‐ tilled water, g; and m2 is the mass of the pycnometer with a sample and distilled water after removing air bubbles, g. The result was calculated as the arithmetic mean of two parallel tests. 2.6.3. Porosity Assessment The porosity of the rock samples, ϕ, %, was determined by a calculation based on the preset values of the true and average density, using the formula: 𝜑 1 100 (8) −3 where ρk is the average density of the rock, g∙cm ; ρ is the true density of the rock, −3 g∙cm . 3. Results and Discussion Table 1 shows the general characteristics of the samples under study. Depending on the facies they belong to, the samples of the studied rocks are subdivided into the fol‐ lowing types: overlying deposits of Quaternary (Q) and Carboniferous (C2) age, enclos‐ ing rocks of the near‐pipe space (V2), tuffaceous‐sedimentary rocks of the crater facies (iD3‐C2) and kimberlite vents (iD3‐C2). Appl. Sci. 2021, 11, 6065 8 of 26 Table 1. General characteristics of the investigated samples. ID Sample Rock Type Geological Age Overlying Rocks 1 7CA_13 Moraine sand and gravel Q 2 GGS‐2‐10_1_2 Sandstone C2 3 GGS _2_10_20_5 Sandstone C2 4 GGS _2_10_30_0 Sandstone C2 5 GGS _2_10_54_1 Sandstone, siltstone C2 Enclosing Rocks of the Near‐Pipe Space 6 16СА_13 Siltstone V2 7 18СА_13 Siltstone V2 8 СА_5_16 Sandstone V2 9 СА_13_16 Sandstone V2 10 СА_14_16 Sandstone V2 11 СА_21_16 Sandstone V2 12 СА_6_16 Sandstone V2 13 СА_7_16 Sandstone V2 14 СА_8_16 Sandstone V2 15 СА_19_16 Sandstone V2 16 24CA_13 Sandstone V2 17 40CA_13 Sandstone V2 18 11CA_13 Siltstone V2 19 12CA_13 Siltstone V2 20 21CA_13 Siltstone V2 Tuffaceous Sedimentary Rocks of the Peripheral Parts of the Crater 21 30СА_13 Tuffite iD3‐C2 22 31СА_13 Tuffite iD3‐C2 23 СА_10_16 Tuffite iD3‐C2 24 СА_12_16 Tuffite iD3‐C2 25 33CA_13 Tuff iD3‐C2 Vent Facies Kimberlites 26 СА_16_16 Autolithic breccia iD3‐C2 27 СА_17_16 Autolithic breccia iD3‐C2 28 2009_477 Autolithic breccia iD3‐C2 29 2013_262 Autolithic breccia iD3‐C2 30 37CA_13 Autolithic breccia iD3‐C2 3.1. Estimation of the Parameter of Radon Leakage from a Counting Sample The decrease in radon activity in the experimental model is shown in the Figure 3. This process occurs due to radioactive decay, as well as radon leakage through the con‐ −7 −1 tainer. The average value of the level of radon leakage was 9.02 ± 0.28%∙10 s . Appl. Sci. 2021, 11, 6065 9 of 26 Figure 3. Radon leakage parameter assessment results. 3.2. Results of Measurements of Radiation and Physical Parameters As mentioned earlier, in the first stage, the activity of Ra was measured daily for 21 days. Based on the results of these measurements, the following were determined: the activity of Ra, without taking into account the accumulation of its decay products; the activity of Ra, taking into account the accumulation of the daughter products of its de‐ cay; the percentage of radon accumulation (in the “free” state); and the period for which the daughter decay products of Ra enter the state of radioactive equilibrium. After the experiment, the counting sample was depressurized, and after 1–2 days, the specific ac‐ tivity of Ra was measured again. To establish the equilibrium of all members of the Ra series in the samples of the studied rocks, a time interval from 5 to 10 days is sufficient (Figure 4). When carrying out further studies with rocks of other types, we consider it possible to reduce the meas‐ urement period to 15 days. Appl. Sci. 2021, 11, 6065 10 of 26 Figure 4. The nature of the distribution of the activity of Ra in time after sealing. The emanation coefficient was calculated using two methods: gamma‐spectrometric and radiometric (emanation). The coefficient of variation between the results of both methods did not exceed 5–10%. So herein when the emanation coefficient is mentioned it refers to the one calculated only by the gamma spectrometric method. In general, the research results are presented in Table 2, in which the following des‐ ignations are used: ARa226—specific activity of Ra under sealing conditions; Kemanation— coefficient of emanation; AVdens—average density; TRdens—true density; Porosity— porosity. Additionally, these designations are used later in the text of the article. In ex‐ panded form the results of the study of radiation and physical parameters are shown in Table A1, Appendix A. Table 2. Results of measurements of radiation and physical characteristics of samples. Radiation and Physical Characteristics ARa226 Kemanation AVdens TRdens Porosity Range Mean Range Mean Range Mean Range Mean Range Mean Overlapping 15.88–30.21 22.19 11.09–24.91 17.41 1.83–2.03 1.93 2.38–2.94 2.54 18.47–32.65 23.77 Rocks Kimberlites 12.42–31.46 17.59 1.76–10.67 7.14 1.74–2.35 2.06 1.83–2.37 2.11 0.46–4.92 2.39 Host Rocks 16.05–63.32 35.52 6.19–29.13 13.94 1.47–2.19 1.89 2.06–2.7 2.42 4.72–40.96 21.11 Tuffaceous Sedimentary 11.45–48.4 28.81 9.82–34.13 22.34 1.36–1.90 1.73 2.12–2.54 2.28 13.21–36.74 24.04 Rocks We have identified a wide range in the Rn emanation coefficient in the studied rock samples from 3 to 40%. The kimberlites of the vent facies, represented by autolithic breccias, are characterized by low values of the emanation coefficient, in the range from Appl. Sci. 2021, 11, 6065 11 of 26 1 to 8%. In only one case, the value of the emanation coefficient exceeded 10%. Kimber‐ lites were also characterized by a low porosity from 0.46 to 4.91%, which agrees with previously obtained results for pipes of the Arkhangelsk diamondiferous province [32]. At the same time, kimberlite pipes in Yakutia have been characterized by a wider range of porosity variation from 3.6 to 12.9% and in rare cases exceeding 20% [33,34]. Vent kimberlites are characterized by the lowest activity of Ra among the studied types of −1 rocks, varying from 12.42–15.89 Bq∙kg (including sealing), with the exception of one 226 −1 sample, in which the specific activity of Ra was much higher 31.46 Bq∙kg (sample ID 37СА‐13). It is known that among the magmatic formations, kimberlites are character‐ ized minimal content of radioactive elements [35]. In addition, the kimberlites of the pipes of the Zolotitsky field have been characterized by an extremely low average con‐ −1 226 centration of uranium, 0.67 mg∙kg [19], which, in terms of the activity of Ra under conditions of radioactive equilibrium in the uranium‐238 chain, does not exceed 10 −1 Bq∙kg . Tuffaceous‐sedimentary rocks were distinguished by a wider range of radiation and physical parameters. The activity of Ra in these rocks varies from 11.45 to 48.40 −1 Bq∙kg , the emanation coefficient from 9.82 to 34.13% and porosity from 13.20 to 36.74%. However, a different picture was observed in the distribution of the studied parameters in the host Vendian and overlying Quaternary and Carboniferous rocks. Most of these rocks were characterized by significant porosity (up to 41%) which are in good agree‐ ment with the data given in [32] concerning the study of the host rocks of the Arkhan‐ gelsk diamondiferous province (from 8 to 40%). The host rocks are distinguished by a 226 −1 high level of specific activity of Ra—up to 63.32 Bq∙kg and the overlying rocks have a high level of the emanation coefficient (up to 28%) (Figure 5). (a) (b) (c) Figure 5. Histogram of radium activity (a), emanation coefficient (b) and porosity (c) for the studied types of rocks. Appl. Sci. 2021, 11, 6065 12 of 26 The rate of radon production was calculated for the different types of rocks (Table 3). This parameter is one of the most important for assessing radon in its free state. Table 3. Radon production rate for the studied types of rocks. −3 −1 Radon Production Rate, Bq∙m ∙h Range Mean Overlapping Rocks 38.88–59.12 52.47 Kimberlites 9.84–18.96 16.15 Host rocks 37.19–132.03 63.88 Tuffaceous Sedimentary Rocks 33.97–139.57 75.39 −3 −1 The minimum level of radon production, from 9.84 to 18.96 Bq∙m ∙h (Table 3) is characteristic of kimberlites of the vent facies. Considering the high values of the specific activity of radium, the emanation coefficient, the level of radon production and porosity, the enclosing rocks of the near‐pipe space stand out among the other types. In the Ven‐ dian host rocks a number of mineralogical, structural and geochemical features were found. Postmagmatic endogenous bleaching associated with the influence of kimberlites was found in the host red rocks of the Vendian [21,36–41]. The zones of bleaching oc‐ curred at the contact with magmatic bodies and in tectonic faults were formed as a result of the action of reducing fluids [42]. These rocks with vein bleaching clarification are characterized by an increased content of uranium, thorium and potassium, and they are also characterized by geochemical anomalies, specific mineralogical associations and stable isotope anomalies [21,22,40]. These rocks with vein endogenous bleaching are characterized by high content of K, Fe, Rb, Zn, Sr, Ni and depleted (light) isotopic com‐ position of calcite carbon (δ С−4.9 and −5.5‰) Among specific mineralogical associa‐ tions the saponite and minerals of smectite group (including montmorillonite and beidellite) are observed [37,40–43]. The contacts of kimberlites and host rocks have local tectonic elements: zones of mylonites, steep fractures and low‐amplitude thrusts [21,40,43–47]. The abundance of fracture zones in the near‐pipe space of kimberlite bod‐ ies is associated with the process of diatreme formation, which influenced the tectonic structure of the adjacent sediment, resulting in the formation of a system of fractures of various types [48]. The occurrence of zones of fracturing and faults is associated with the formation of diatremes as a result of the significant mechanical effect of penetrating gas‐ es and melts on the host rocks [49,50]. As a result, a system of radial and concentric zones of fracturing and faults with crushing and the vertical movement of blocks of en‐ closing rocks has arisen in the near‐pipe space [51]. The impact of kimberlites on the Vendian rocks, which led to the formation of fault zones in the near‐pipe space, in‐ creased the fracturing and various mineralogical and geochemical changes, influencing the radiation parameters of the host rock. Enrichment with uranium (radium) and the increased fracturing of the near‐pipe space created conditions for the production and advection of radon through the host rock mass. As a result, in the soil horizons above the kimberlite bodies of the Lomonosov diamond deposit, an increased volumetric activ‐ ity of radon can be observed, several times higher than the background values [20,52]. In the course of this study, we found that the main source of radon observed in the soil air above kimberlite pipes is the enclosing Vendian rocks of the near‐pipe space. To understand the relationship between the studied radiation and physical parame‐ ters, a statistical analysis of the data was performed. 3.3. Statistical Analysis To study the features of radon emanation in rocks, a correlation analysis of the main ra‐ diation and physical parameters of the samples was performed (Table 4). The following pa‐ rameter was also added to the correlation matrix: ARn222, the volumetric activity of radon in Appl. Sci. 2021, 11, 6065 13 of 26 −3 the container, Bq∙m . The values of this parameter were obtained as a result of experimental work on the accumulation of radon in a sealed container with test samples. Table 4. Correlation matrix from the studied parameters of the radon emanation. ARa226 Kemanation ARn222 AVdens TRdens Porosity ARa226 1 Kemanation − 0.277 1 ARn222 0.350 0.709 1 AVdens 0.054 −0.666 −0.531 1 TRdens 0.092 0.294 0.335 0.155 1 Porosity 0.043 0.753 0.691 −0.648 0.646 1 Number of samples = 30. Correlation is significant at the 0.01 level. A significant correlation is observed in rocks for Kemanation‐ARn222 (r = 0.709), Kema‐ nation‐AVdens (r = −0.666), Kemanation‐Porosity (r = 0.753), ARn222‐AVdens (r = −0.531), ARn222‐ Porosity (r = 0.691), AVdens‐Porosity (r = −0.648) and TRdens‐Porosity (r = 0.646). Ra activ‐ ity in rock samples has no significant correlations with any of the parameters suggesting that it is not the main parameter influencing the formation of a radon field. The lack of relationships between the content of Ra and the volumetric activity of radon is proba‐ bly due to the form of Ra in the minerals that make up the rocks [30,53]. Radon formed in a solid can enter the surrounding space due to both radioactive recoil and diffusion. In the case of radioactive decay, radon atoms acquire a certain recoil energy, which they subsequently lose when moving. Some of the atoms remain in the solid phase making up so‐called “bound radon”. However, the recoil energy of about 86 keV is enough to re‐ lease atoms outside the crystal lattice, while forming free radon [24]. Taking the above into account, to characterize the territory of Arkhangelsk diamondif‐ erous province according to the distribution of radon, it is advisable to use a complex of two parameters—the activity of Ra in soils and rocks and their emanation coefficient. As can be seen from the correlation data, the main factors in the formation of the radon field are the emanation coefficient (r = 0.709) and the porosity of the rock (r = 0.691). At the same time, an increase in porosity leads to an increase in the emanation coefficient (r = 0.753). There is a negative relationship between the average density and the volumetric activity of radon in a free state (r = −0.531). This is due to the fact that the method for calculating the average density takes into account the presence of pores in the rock. At the same time, the true density is characterized by a weak effect on the ema‐ nation coefficient and the volumetric activity of radon. Additional information for the interpretation of the obtained statistical data on rock samples is provided by the results of factor analysis (Table 5). Table 5. Factor loadings matrix for dataset on the studied parameters of radon emanation. Factor Parameter 1 2 3 ARa226 −0.040 0.036 0.990 Kemanation 0.876 0.271 −0.256 ARn222 0.776 0.288 0.418 AVdens −0.920 0.248 0.028 TRdens 0.071 0.992 0.054 Porosity 0.774 0.572 0.036 % of Variance 52.81 21.15 17.95 Appl. Sci. 2021, 11, 6065 14 of 26 On the diagram of factor loads (Figure 6), three groups of factors are distinguished, which determine the nature of the emanation of radon from rocks. The total variance for the three factors is 91.91%. Figure 6. Plot of factor loadings for studied parameters of radon emanation. The first factor, with a dispersion of 52.81%, includes the radon emanation coeffi‐ cient, the volumetric activity of radon in the counting sample and the porosity of the sample. This is due to the fact that these parameters are the main parameters in the pro‐ cess of forming a radon field. This conclusion was made in the conditions of the experi‐ ment and does not take into account other physical factors that can affect the behavior (gas permeability, humidity, temperature, pressure). The same parameters are involved in calculating the level of radon production. The second factor, with a dispersion of 21.15%, combines two parameters—true density and porosity. The relationship between these parameters is due to the fact that the porosity is a calculated value and is determined based on the density of the sample. The third factor is represented by one parameter—the activity of radionuclide Ra. The weak determination of the second and third factors is probably associated with more complex interaction mechanisms during the formation of the radon field; to fully understand them, additional data are required to determine the geochemical and miner‐ alogical compositions [14]. Based on the results of measuring the volumetric activity of radon in a sealed con‐ tainer and calculating the level of radon production, we built a regression model (Figure 7), which is a linear function of the dependence of two parameters (dependent variable and regressor) and is characterized by regression coefficients (i.e., slope, coefficient of determination). Appl. Sci. 2021, 11, 6065 15 of 26 Figure 7. Regression model of the relationship between the volumetric activity of radon and the level of radon production. In our experiment, the regression model has the form y = 19.138x − 32.696 and is characterized by a positive slope equal to 19.138 and a coefficient of determination R = 0.8786. The positive slope indicates that with an increase in the level of radon production −3 by 1 unit, the volumetric activity increases by 19.138 Bq∙m . This value is theoretical and can vary depending on a number of parameters. For this model, it is calculated based on the results of the measurements of parameters. The value of the constant a = −32.696 in this case is not taken into account, because under ideal conditions (for example, the ab‐ −3 −1 sence of extraneous sources of radon) at a level of radon production P = 0 Bq∙m h , the −3 volumetric activity will also be equal to 0 Bq∙m . The coefficient of determination shows that the change in the volumetric activity of radon in the container (dependent variable) by 87.9% is described by the independent variable (regressor)—the level of radon production—which indicates a sufficient justifi‐ cation for choosing this model. This model more clearly predicts the distribution of ra‐ don based on the results of calculating the radon production rate. 4. Conclusions Experimental studies were carried out to assess the main radiation and physical fac‐ tors affecting the formation of the radon field over the kimberlite pipes of the Arkhan‐ gelsk diamondiferous province. For this purpose, samples of kimberlites and rocks were taken from the quarries of the Arkhangelskaya and Karpinsky‐1 pipes of the Lomonosov diamond deposit. The samples were represented by the main types of rocks exposed by quarries to a depth of 186 m: overlying Quaternary (Q) and Carboniferous rocks (C2), host rocks of the Vendian (V2), tuffaceous‐sedimentary rocks of the crater (iD3‐C2) and kimberlites of the vent facies (iD3‐C2). 226 222 Radiation (activity of Ra, activity of Rn in a free state, emanation coefficient, ra‐ don production rate) and physical (average and true density, porosity) parameters were determined. Appl. Sci. 2021, 11, 6065 16 of 26 Among the studied types of rocks, it was found that kimberlites of the vent facies are characterized by the lowest values of the emanation coefficient, porosity, specific ac‐ tivity of Ra and the radon production rate. The lowest values of these parameters limit the formation of free radon in the pipe body. The largest amount of radon in a free gas state has been produced by rocks of the near‐pipe space, represented by the enclosing Vendian V2 deposits and characterized by high values of the emanation coefficient, specific activity of radium, radon production rate and porosity. This is due to the structural and geological features of the near‐pipe space, which has signs of the impact of kimberite magma on the host rocks: the devel‐ opment of fracturing as well as structural, mineralogical and geochemical changes. The overlying and tuffaceous‐sedimentary rocks of the crater are characterized by interme‐ diate values of the studied parameters. This study advances the knowledge of using emanation methods for prospecting for kimberlite pipes in the territory of the Arkhangelsk diamondiferous province. Author Contributions: All authors contributed to the study conception and design. Conceptual‐ ization, validation, and writing—original draft preparation were performed by E.Y. and A.P. Methodology was performed by E.Y. and A.P. Formal analysis and investigation were performed by E.Y. and A.P. Writing—review and editing were performed by E.Y. and A.P. Funding acquisi‐ tion were performed by E.Y. Project administration and resources were performed by E.Y. Super‐ vision and visualization were performed by E.Y. All authors have read and agreed to the pub‐ lished version of the manuscript. Funding: The reported study was funded by RFBR, project number 20‐35‐70060. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The datasets presented in this study can be obtained upon request to the corresponding author. Acknowledgments: The authors are grateful to the staff of the Laboratory of Environmental Radi‐ ology of N. Laverov Federal Center for Integrated Arctic Research for their invaluable help and support at all stages of the study. Conflicts of Interest: The authors declare that they have no conflicts of interest. Appl. Sci. 2021, 11, 6065 17 of 26 Appendix A Table A1. Radiation and physical characteristics of studied rocks. Radiation Parameters Physical Parameters Radium‐226 Activity Radium‐226 Activity Radium‐226 Activity Volumetric Activity of Average Sample ID Emanation Radon Production True Density **, (Unsealed Sample), (Sealed Sample), (after Opening the Radon in the Container, Density **, Porosity ***, % −3 −1 −3 Coefficient, % Rate *, Bq∙m ∙h g∙cm −1 −1 −1 −3 −3 Bq∙kg Bq∙kg Sample), Bq∙kg Bq∙m g∙cm Overlying Rocks 7CA_13 16.43 ± 3.94 20.44 ± 4.91 18.83 ± 5.08 13.75 ± 5.46 658 ± 197 38.88 1.83 2.38 23.11 GGS‐2‐10_1_2 22.33 ± 4.69 26.40 ± 4.22 23.17 ± 4.40 13.83 ± 3.78 1116 ± 335 54.64 1.98 2.94 32.65 GGS_2_10_20_5 16.12 ± 3.71 18.01 ± 3.06 17.57 ± 4.57 23.47 ± 8.02 1120 ± 336 59.12 1.85 2.39 22.59 GGS_2_10_30_0 12.21 ± 3.42 15.88 ± 4.13 11.64 ± 3.38 24.91 ± 10.67 834 ± 250 58.32 1.95 2.50 22.00 GGS_2_10_54_1 26.59 ± 4.52 30.21 ± 4.53 27.13 ± 3.53 11.09 ± 2.42 1637 ± 491 51.41 2.03 2.49 18.47 Enclosing Rocks of the Near‐Pipe Space 16СА_13 37.37 ± 5.98 39.34 ± 5.51 36.44 ± 5.10 6.19 ± 1.35 755 ± 227 37.19 2.02 2.12 4.72 18СА_13 20.05 ± 4.21 22.68 ± 4.08 19.53 ± 5.08 12.74 ± 4.43 802 ± 241 42.38 1.94 2.38 18.49 СА_5_16 11.01 ± 3.19 16.05 ± 3.85 11.74 ± 3.29 29.13 ± 11.82 1235 ± 371 51.96 1.47 2.49 40.96 СА_13_16 22.44 ± 4.94 29.67 ± 6.23 23.42 ± 5.15 22.72 ± 7.60 1822 ± 547 97.85 1.92 2.66 27.82 СА_14_16 25.90 ± 5.96 30.73 ± 4.30 27.82 ± 6.40 12.60 ± 3.73 1105 ± 332 57.37 1.96 2.38 17.65 СА_21_16 56.01 ± 8.96 59.84 ± 7.78 55.69 ± 6.13 6.67 ± 1.25 1059 ± 318 57.33 1.90 2.62 27.48 СА_6_16 35.79 ± 6.80 40.69 ± 6.10 36.55 ± 5.85 11.11 ± 2.68 1186 ± 356 59.12 1.73 2.06 16.02 СА_7_16 21.90 ± 4.16 26.84 ± 5.64 23.51 ± 4.70 15.41 ± 4.92 1241 ± 372 68.48 2.19 2.55 14.12 СА_8_16 25.35 ± 5.83 30.82 ± 4.93 24.30 ± 5.10 19.45 ± 5.65 1624 ± 487 87.46 1.93 2.59 25.48 СА_19_16 28.33 ± 6.80 31.46 ± 5.66 27.75 ± 5.27 10.87 ± 3.13 906 ± 272 50.67 1.96 2.70 27.41 24CA_13 32.01 ± 6.08 35.55 ± 5.33 32.60 ± 4.24 9.13 ± 1.99 1008 ± 302 45.64 1.86 2.12 12.26 40CA_13 32.44 ± 5.84 40.71 ± 6.11 30.30 ± 3.94 22.94 ± 5.01 2367 ± 710 132.03 1.87 2.69 30.48 11CA_13 58.40 ± 9.93 63.32 ± 7.60 59.9 ± 6.59 6.59 ± 1.18 1109 ± 333 64.99 2.06 2.23 7.62 12CA_13 23.37 ± 4.91 26.66 ± 5.60 23.45 ± 4.92 12.19 ± 3.98 962 ± 289 44.96 1.83 2.38 23.11 21CA_13 34.41 ± 6.19 38.47 ± 5.39 33.74 ± 4.72 11.42 ± 2.49 1283 ± 385 60.78 1.83 2.38 23.11 Tuffaceous Sedimentary Rocks of the Peripheral Parts of the Crater 30СА_13 17.85 ± 4.28 27.34 ± 5.19 18.17 ± 5.09 34.13 ± 12.70 2561 ± 768 95.94 1.36 2.15 36.74 31СА_13 19.52 ± 5.08 21.61 ± 3.89 18.92 ± 5.49 11.06 ± 4.15 792 ± 238 33.97 1.88 2.23 15.70 СА_10_16 8.94 ± 3.40 11.45 ± 2.98 8.20 ± 3.53 25.15 ± 13.90 872 ± 262 41.36 1.90 2.54 25.20 СА_12_16 23.13 ± 4.86 35.25 ± 5.29 25.13 ± 5.28 31.55 ± 8.96 3146 ± 944 139.57 1.66 2.35 29.36 33CA_13 44.08 ± 7.93 48.40 ± 6.78 43.21 ± 5.19 9.82 ± 1.99 1288 ± 386 66.11 1.84 2.12 13.21 Appl. Sci. 2021, 11, 6065 18 of 26 Vent Facies Kimberlites СА_16_16 11.93 ± 3.10 13.14 ± 3.42 12.21 ± 3.54 8.14 ± 3.49 135 ± 41 15.93 1.97 2.02 2.48 СА_17_16 10.84 ± 3.04 12.42 ± 3.48 11.35 ± 3.52 10.67 ± 4.90 102 ± 31 17.43 1.74 1.83 4.92 2009_477 14.11 ± 3.81 15.89 ± 4.13 15.27 ± 3.82 7.55 ± 3.00 212 ± 64 18.96 2.09 2.16 3.24 2013_262 13.64 ± 3.41 15.03 ± 4.06 14.14 ± 4.10 7.58 ± 3.30 98 ± 29 18.60 2.16 2.17 0.46 37CA_13 31.17 ± 4.99 31.46 ± 4.40 30.64 ± 4.29 1.76 ± 0.38 56 ± 17 9.84 2.35 2.37 0.84 Notes: * The radon production rate was calculated based on the results of measurements of the activity of radium‐226, average density, emanation coefficient; ** The standard devia‐ −3 tion was no more than 0.02 g∙cm ; *** The porosity parameter is calculated based on the results of measurements of the average and true density. Appendix B Figure A1. Typical geology cross‐section of kimberlite pipe of the Lomonosov diamond deposit. 1—Quaternary fluvioglacial sediments; 2—Middle Carboniferous sediments (Ursug suite); 3—Upper Vendian sediments (host rocks of the near‐pipe space); 4—Tuffaceous‐sedimentary rocks of the crater facies; 5—Kimberlites of the vent facies. Appl. Sci. 2021, 11, 6065 19 of 26 Appendix C. Typical Types of the Studied Rocks from the Lomonosov Diamond Deposit. Figure A2. Moraine sand and gravel mix (sample ID 7CA‐13), overlying Quaternary fluvioglacial sediments (Q), Lomonosov diamond deposit. Appl. Sci. 2021, 11, 6065 20 of 26 Figure A3. Gray sandstone with calcareous and clay cement (sample ID GGS _2_10_20), overlying Middle Carboniferous rocks, Ursug suite (C2ur), Lomonosov diamond deposit. Appl. Sci. 2021, 11, 6065 21 of 26 Figure A4. Red sandstone (sample ID 24CA_13), Vendian enclosing rocks of the near‐pipe space (V2), Arkhangelskaya pipe. Appl. Sci. 2021, 11, 6065 22 of 26 Figure A5. Tuff (sample ID 33CA_13), Tuffaceous sedimentary rocks of the crater, Upper Devonian—Middle Carboniferous (iD3‐C2), Arkhangelskaya pipe, Lomonosov diamond de‐ posit. Appl. Sci. 2021, 11, 6065 23 of 26 Figure A6. Autolithic breccia (sample ID 37CA_13), Vent Facies Kimberlites (diatreme), Upper Devonian—Middle Carboniferous (iD3‐C2), Arkhangelskaya pipe. Appl. Sci. 2021, 11, 6065 24 of 26 References 1. Sabbarese, C.; Ambrosino, F.; D’Onofrio, A.; Pugliese, M.; La Verde, G.; D’Avino, V.; Roca, V. The first radon potential map of the Campania region (southern Italy). Appl. Geochem. 2021, 126, 10.1016/j.apgeochem.2021.104890. 2. Giustini, F.; Ciotoli, G.; Rinaldini, A.; Ruggiero, L.; Voltaggio, M. Mapping the geogenic radon potential and radon risk by using Empirical Bayesian Kriging regression: A case study from a volcanic area of central Italy. Sci. Total Environ. 2019, 661, doi:10.1016/j.scitotenv.2019.01.146. 3. Miklyaev, P.; Petrova, T. Studies of emanation of clay rocks by radon. 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Applied Sciences – Multidisciplinary Digital Publishing Institute
Published: Jun 29, 2021
Keywords: radon field; kimberlites; Arkhangelsk diamondiferous province; emanation coefficient; radon production rate; porosity; density
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