The glasses from Mesozoic sediments of Anabar region (Arctic Siberia, Russia)

The glasses were found in the tuffisites of Orto-Yiarginskoe field (Anabar region, Arctic Siberia). The composition of the glasses is comparable with the composition of the glasses from the Popigai impact structure and the gneisses of the Archean basement of the Anabar folded system. The age of the studied glasses is comparable with the age of the glasses of the Popigai impact structure. Mineralogical studies were carried out using scanning electron microscopy (MIRA 3 LMU, Tescan Ltd) with the INCA Energy 450+ XMax 80 microanalysis system (Oxford Instruments Ltd). The age of the glasses is determined by the 40Ar-39Ar method in the Collective Use Center of the IGM SB RAS.


Introduction
Rich quaternary alluvial diamond placers (with admixtures of gold and platinum group minerals), as well as placer deposits of relict Pliocene erosion-karst valleys have been discovered, developed, and these are successfully mined in the Anabar diamondiferous area (ADA). Motherlodes of diamond and noble metals so far have not reliably established on this territory. The revealed bodies of the Kuonam kimberlite complex of the Triassic age are non-diamondiferous or poorly diamond-bearing.
After discovery of the largest placer of the Ebelyakh River, in the course of exploration and prospecting works for diamonds, it has been established in the ADA the wide spread occurrence of karst among dolomites of the Anabar suite of the Upper Cambrian and diamond potential of some horizons of sediments filling karst. As a whole, low productive for diamonds sandy and loamy sediments dominate in the Cretaceous sediments of funnel sinks.
At the same time, in the carbonate base, a part of cape-shaped and funnel-shaped depressions oval in plane view in cross-section) are filled with specific red clays with intense limonitization and green sideritized clays. It is these rocks in which diamonds finds are known. When prospecting and evaluation works were performed, such rocks were often related to karst sediments of the Cretaceous age. At the same time, a number of researchers proved that these sediments were crusts of weathering of endogenous rocks: tuffs of alkaline basalts and lamproites and these crusts could be a source of diamonds in the Cenozoic placers. The chemical composition of the rocks is characterized by high contents of iron and aluminum oxides, until appearance of free alumina.
In core-samples of certain core holes in the Anabar region on the Orto-Yiarginskoe field, fragments of fresh glasses were found ( Figure 1). According to the data of sporo-pollen analysis, the age of rocks

Geological setting
The Orto-Yiarginskoe field field is confined to the eastern flank of a large tectonic structure of the Siberian platform (Anabar anticlise). Within the scope of the anteclise, two structural stages sre distinguished: the lower Archean stage, which is the platform basement, and the upper one corresponding to the sedimentary cover. The basement depth in the region of the Yiarginsky field is 1-2 km [1]. The platform basement was formed over a long period from 3.5 billion years onwards and ending with 1.9 billion years [2,3]. The upper structural stage is composed of Cambrian carbonate series and overlying Late Cretaceous terrigenous sediments.
Fragments (rock debris) of fresh glasses were found in cores of some core holes in the Late Cretaceous (Post-Cretaceous) sediments of the Anabar region (Orto-Yiarginskoe field) dated early by a sporo-pollen method. The most large-scale manifestation of this complex is established in the locality in the upper reaches of the Otordur stream. Within the bounds of the studied site, rocks of the Mesozoic age are located among carbonate sediments of the Lower and Middle Cambrian. From the surface, these rocks are overlapped only by thin (0-9 m, in the average, 4.1 m) quaternary poorly consolidated sediments.

Sample description
Maximum thickness of sediments (more than 200 m) dated back to the Cretaceous period are defined in karst cavities of funnel-shaped and tabular-shaped. Intervals of rocks with glasses are not large in the thickness: these are the first tens of centimeters along boreholes. Material of these rocks was obtained by specialists of OJSC Almazy Anabara in the process of performing exploratory drilling works on the verification of geophysical anomalies in the territory of the Orto-Yiarginsky field. Macroscopically, the rock containing glasses is a fine detrital (2-30 mm) breccia ( Figure. 2), the amount of fragments being about 20-40%. The relative share of fragments of bubbly and foamy glasses is no more than 60%. The rock also contents fragments of biotite-feldspathic crystalline rocks, quartz sandstone, Cambrian carbonate rocks, and carbonized vegetable remains. The binder mass is fine-grained, ash-grey in colour. with the massive texture and specific macro porosity.
The glasses are heterogeneous: from opaque, foamy, pumice-like, to transparent greenish-yellow and brown in colour. Almost black glasses with variegated tarnish on walls of bubbles are less common.  Figure 2. Fragments of the core from the borehole 708s-1, the depth -27.7 m; a -contact (transition) of ash-gray psammitic tuffisite and small-clastic breccia with fragments of bubbly and foamed glass; b -glasses of the first and second varieties in a single fragment.
Glasses include relics of quartz, in some cases with planar elements. The viscous-plastic flow textures are peculiar to glasses. The lithological characteristic of these sediments in the section obtained as a result of drilling is listed in Table 1. Two samples of glasses for isotopic probing were taken from samples 708с-1.3 and 708с-1.4. The samples were taken from core hole (№ 708с-1) at depths 27.7m and 29.7m, respectively. Rocks enclosing "layers" with glasses are represented by very peculiar, mainly sandy, and to a lesser extent, breccia-like sediments. Based on the enormous volumes of geological prospecting and exploration performed by geologists of many organizations, it has been established that rocks of the complex are uniquely associated with rocks repeatedly described within Popigai ring (or impact) structure as allogenic breccia and psammitic-aleuritic coptoclastites by reference to geological, petrographic, and mineralogical-geochemical peculiarities. The overwhelming majority of geologists who have studied these rocks are beyond any doubt that the rocks were formed due to invasion of mobile fluid-saturated mixture into upper horizons of the Lower Paleozoic carbonate cover and weakly bound Cretaceous terrigenous sediments overlying this cover. Intervals of such rocks are contrast with Mesozoic sediments by their structure, composition, porosity of rocks, the presence of large number of inclusions (xenoliths and xenoblocks) of rocks of different ages, inconsistency of age dating obtained by different methods (from Cretaceous to Eocene-Oligocene), and by the presence of glass inclusions ( Figures. 2, 3, 4). But these rocks are especially contrastively distinguished by their forms of occurrence with specific bodies represented by zones crosscutting rocks of the older age (from Proterozoic to Cretaceous), and also in the form of conical bodies facing by their wide part the earth's surface with the tilt angles of walls from of 15 to 45о.Most of geologists studied these rocks (especially from a practical point of view as sources of high-quality diamonds as well as, to a certain extent, gold and platinum group metals) consider that these rocks should be attributed to fluidizates and tuffisites. The average thickness of fluidizates within the explored acreage (150 hectares) is 37.9 m (according to the data from 28 boreholes). Based on the results of the study, two petrographic varieties of rocks have been distinguished: sandy tuffisites and fluidizate breccias. In a quantitative terms, sandy tuffizites are sharply dominant. These are massive rocks with a psammitic texture. The rocks are characterized by marvelous consistency of the material composition and textural-structural features just as in plane, so in section. The deposits are represented by massive fine-grained rocks ash-grey in colour containing scattered fragments (xenoliths) of various rocks ranging in size from the first millimeters to 20 cm ( Figures. 1, 2, 3). Xenoblocks up to 10 cm in size are also recognized. The volume fraction of debris, as a rule, is not large (2-15%).
The presence of macropores in the ground mass, which occupy 1-5%, in some places up to 10% of the rock volume. Pores (voids) have the form of a sphere of a triaxial ellipsoid up to 10 mm in size, which sometimes line up in chains.

Analytical techniques
Fragments of glass samples (708с-1.3 and 708с-1,4) were extracted from tuffisites, placed in checkers and covered with epoxy resin. After the resin hardened, the samples were opened and leveled by a b grinding, and then coated with Super Cement glue, followed by polishing with diamond pastes. Polished glasses were studied by ore microscopy using an AxioSkop A1 microscope and SEM methods. The composition and morphology of the glasses grains were investigated using a MIRA 3 LMU (Tescan Orsay Holding) scanning electron microscope with an attached INCA Energy 450 XMax 80 (Oxford Instruments Nanoanalysis) microanalysis energy-dispersive system at the X-ray Laboratory of the Institute of Geology and Mineralogy, Siberian Branch RAS (analysts N.S. Karmanov, M.V. Khlestov). We employed an accelerating voltage of 20 kV, a beam current of 1600 pA, an energy resolution (MIRA) of 126-127 eV at the Mn Kα line, and a region (3-5 μm), depending on the average atomic number of the sample and the wavelength of analytical line. 40 Ar/ 39 Ar measurements were performed at the Analytical Centre of Multi elemental and Isotope Investigations at IGM SB RAS. Samples and biotite MCA-11 (K/Ar standard OSO no. 129-88), which was used as the mineral monitor, were wrapped in Al foil and vacuum-sealed in quartz vials. Biotite MCA-11 was certified as an 40 Ar/ 39 Ar monitor with use of the muscovite Bern 4m and biotite LP-6 internationally certified standards [4]. The quartz ampoules with samples were irradiated in the Cdcoated channel of a research reactor (VVR-K type) at the Tomsk Polytechnic University, Tomsk, Russia. The neutron gradient did not exceed 0.5 % at the sample size. 40 Ar/ 39 Ar step heating experiments were undertaken using a quartz vial heated by external furnace. Released argon was purified by exposure to Zr-Al SAES-getters. The isotopic composition of Ar was measured on a Noble Gas 5400 mass spectrometer. The 40 Ar blank at 1200°C, measured over a period of 10 min, did not exceed 5 * 10 -10 ncm 3 . The reported analytical errors are ± 1σ.

Mineralogical, geochemical and petrochemical features of glass from sample 708с-1.3.
Fragment of glass more than 2 cm in in size along the long axis was studied. In the marginal parts of a grain, the glass is opaque, light grey to white, foamed (froth-like) pumiceous. The central part of the sample has the fluidal texture, tobacco colour with round pores (bubbles) up to 1 mm in size. The results of petrographic and electron microscopic study are shown in Figure. 5 (а, b, c) and are listed in Table 2.
The rock consists of glass having a fluidal texture and structure, which is emphasized by jets and and oriented stripes of bubbly glasses of different composition, often curved with signs of flow. .In addition, areas with flow and dissolution structures are in evidence. Glass contains subangular quartz grains, mostly fractured, cataclased, with numerous fluid and mineral inclusions, less often plagioclase, potassium feldspar is hardly found, although local areas of glass enriched in potassium are recognized. Most of not completely melted grains are represented by quartz with zonal rims of glasses of the different composition, which also penetrate trough cracks. Small pores in the glass are round, while large ones are elongated, which also indicates substance flow. Often there are areas near melted quartz grains saturated with gas-liquid and mineral inclusions with sulfide globules, which on the ground of these characteristics can be attributed to lechatelierite. Sulfide globules ranging in size from several microns to tens ones are presented by a monosulfide solid solution containing iron, sulfur. nickel, and copper. Expressed in terms of 100% sulfide, the contents are as follow: 53.5-62% Fe, 34.3-40.1% S, 0.6-9.6% Ni, and 0,5-2.56% Cu. Sphene, zircon, pyrrhotite are less common in the glass, and monazite, barite, and sulfides of iron, zinc, and copper are found in gas cavities. Albite and two hypersthene grains were found on one microsite of the sample, which may indicate involvement of hypersthene-containing gneisses as a substrate in the process of glass formation. It is known that the complexes of the Archean Anabar folded system are represented, in particular, by rocks of the Khapchansky series: garnet-hypersthene, graphite-biotite-hypersthene, and biotite-hypersthene gneisses [5]. In this case, it is not surprising that not only hypersthene, but also graphite are present in the gneisses.  Based on the local analysis of sample 708с-1.3, two types of glasses are distinguished, which differ in the composition. The first type is characterized by the concentrations: 50-66% SiO 2 , 17% Al 2 O 3 , 7.0 % FeO, 0.9-1.0% TiO 2 , 3.3-3.5% K 2 O, 2.5% Na 2 O, and 3-3.3% MgO; concentrations of the second type are as follows: 60-61.9% SiO 2 , 17.5-17.8% Al 2 O 3 , 8-9 FeO, 0.8-0.9% TiO 2 , 3.0-3.3% K 2 O, 2.3-2.4% Na 2 O, and 3.5-4.3% MgO. Thus, as a whole, the glass correspond to andesite in the composition with variations of alkalies, silicic acid, and iron. The analysis data of the glass structure indicate its occurrence as a result of melting of rocks of the medium-acid composition given that preferentially quartz remained in "relic" minerals in glass.

Mineralogical, geochemical and petrochemical features of glass from sample 708с-1.4.
A fragment of porous glass (sample size was about 10 mm) is transparent, relatively homogeneous, tobacco (brown green) in colour has been studied. The results of petrographic and electron microscopic studies are given in Figures 5, 6 (d, e, f)) and in Table 2. The rock consists of glass (to a large extent, it is close to that described above) of the fluidal texture and structure, which are emphasized by jets and oriented strips of bubbly glasses of different compositions often curved and with signs of flow. In addition, there are areas with flow and dissolution structures, as well as with porphyritic texture. Subangular quartz grains are enclosed in glass mostly fractured, cataclased, with numerous fluid and mineral inclusions, more rarely plagioclase, and potassium feldspar is hardly found, although local areas of glass enriched in potassium are recognized. Most of not completely melted grains are represented by quartz with zonal rims of glasses of the different composition. Small pores in the glass are rounded, while large ones are slightly elongated, which also indicates substance flow. Often there are glass areas near fritted quartz grains saturated with gas-liquid and mineral inclusions with sulfide globules, which, on the ground of these characteristics, can be attributed to lechatelierite. Zircon, magnetite, and pyrrhotite are rarely found in glass, and in gas cavities, apatite, barite, titanium magnetite, chromite, calcium carbonate (with magnesium admixture), monazite barite, and sulfides of iron, zinc and copper are met. Sulfide globules ranging in size from several microns to tens ones are represented by a monosulfide solid solution (mss) containing iron, sulfur, nickel, and copper (rare cobalt) expressed in terms of 100% sulfide, the contents are as follow: 55.3-74.1% Fe,11.7-36.2% S, 1.4-7.8% Ni, and 1.35-10.1% Cu.
Based on the local analysis of sample 708с-1.4, two types of glasses differing in composition are distinguished. The first type is characterized by the concentrations: 64-66% SiO 2, 15.5-16.0% Al 2 O 3 , 7% FeO, 0.8-0.9%TiO 2 , 3.0-3.2% K 2 O, 2.0-2.3% Na 2 O, 3.0-3/7% MgO; The concentrations of the second type are as follows: 59-61% SiO 2 , 14.5-17.0% Al 2 O 3 , 8-9% FeO, 0/6-0.9%TiO 2 , 1.8-2.2% K 2 O, 1.5-2.0% Na 2 O, and 4.7-7.4% MgO. The presented data were obtained as a result of calculation for 100% of glass (carbon was not taken into account). Thus, glasses in sample 708c-1.4 are represented mainly by two types: andesitic and dacitic compositions. But at several points, the analytical data show the concentrations corresponding to andesite-basalt and even basalt occurring also due to melting of rocks of the medium-acid composition taking into consideration that mainly quartz remained in glass in "relic" minerals in glass.  Figure 6. SEM photographs of a glass grains No. 708c-1.3 (a, b, c) and No. 708c-1.4 (d, e, f). a -at the point 14 is quartz, with a large number of globules and relics of planar elements, through which preferential melting takes place. At point 9 is monazite spherule located in a cavity filled with . Lechatelierite is possible inside a quartz grain near a large crack. 1melted from the edges, broken-down quartz grain; 2, 6 silicate glass; 3, 4, 5 -monosulfide solid solution consisting of iron, nickel and copper sulfides; at point 8, a grain was found similar to grain at point 14, which contains barium, zinc, iron and sulfur.

The 40 Ar-39 Ar age of glass
For glass sample No 708-1.3, an age spectrum was obtained in which a plateau is distinguished consisting of three stages characterized by 63.4 % of evolved 39 Ar and the age value 35.3 ± 1.0 Ma old (Figure 7). On the isochronous diagram, points form a regression with age value of 34.5 ± 1.1 Ma, which is consistent with the plateau age. Thus, it is logical to assume the age calculated by the plateau method to be commeasure with the sample formation time.
For the glass sample No708-1.4, an age spectrum of the saddle-shaped form (Figure 7) was obtained, which is often interpreted as a sign of the presence of radiogenic 40 Ar. In this case, the sample age should not be older than the minimum value in the middle part of the spectrum. Consequently, the sample was formed not earlier than 40.0 ± 1.3 Ma back. On the other hand, in the isochronous diagram, three point form a linear regression characterized by the value 66.3 ± 1.7 Ma with MSWD = 4. Based on the data set, it seems unlikely that the isochronous age for this glass sample has any geological meaning.