I. V. Chernyshev

High-precision Pb isotope analysis by multicollector-ICP-mass-spectrometry using 205Tl/203Tl normalization: Optimization and calibration of the method for the studies of Pb isotope variations

The development of the MC-ICP-MS method, which was launched about one decade ago and was largely stimulated by the need to solve geological problems, has opened a new avenue in isotope mass spectrometry. One of the advantages of this method is the possibility of applying a newly developed approach to the correction of analytical results for the effect of mass discrimination by normalizing the measured isotope ratios of an element to a reference (standard) isotope ratio of another element. This makes it possible to overcome the main disadvantage of conventional thermal ionization mass spectrometry (TIMS), in which the effect of mass discrimination cannot be fully taken into account during isotope analysis, and thus to implement a highly accurate method for the analysis of Pb-isotope composition. In application to the capability of the NEPTUNE MC-ICP mass spectrometer, we optimized and calibrated a method for high-accuracy Pb isotope analysis in solutions spiked with Tl, with all currently measured Pb-isotope ratios normalized to the standard 205Tl/203Tl ratio (TLN-MC-ICP-MS). The factors affecting the random and systematic analytical errors were examined, and the optimal operating regime and analytical conditions were determined. Much attention was paid to the correlation of the measurement results and the mass discrimination effect determined from the 205Tl/203Tl ratio. The value of the 205Tl/203Tl normalizing ratio was analytically determined through isotope analyses of the NIST SRM 981, and SRM 982 standard samples of Pb-isotope composition. The data obtained for two mixtures Tl + Pb (SRM 982) and Tl + Pb (SRM 981) in ten replicate analyses were 2.38898 ± 12 and 2.38883 ± 20, respectively. These results are in good mutual agreement, and their general mean 205Tl/203Tl = 2.3889 ± 1 coincides (within the error) with the recently published values of 2.3887 ± 7 [Collerson et al., 2002] and 2.3889 ± 1 [Thirlwall, 2002]. The precision of the method (±2SD), which was assayed by the long-term reproducibility of the results of replicate analyses of SRM 981 and seven galena samples (90 analyses) was 0.016–0.018% for the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios and 0.005 and 0.009% for the 207Pb/206Pb and 208Pb/206Pb ratios, respectively. The precision of the isotope analysis of common Pb was significantly improved (by factors of 6–10 for various isotope ratios) compared with the precision of TIMS techniques acceptable in isotope studies during three decades. The described method was applied to examine the Pb-isotope composition of approximately 250 samples of galena, scheelite, and pyrite from a number of well known (including large) gold, sulfied, and base-metal deposits. The precision of the method (0.01–0.02%) makes it possible to study small inter-and intra-phase differences in Pb-isotope ratios in hydrothermal and magmatic rocks, to assay the scale of regional and variations in the isotope composition of ore Pb, and to correlate the Pb-isotope composition of rocks and ores and reveal its evolutionary trends.

CeRu2Al10 with the YbFe2Al10 structure type

The structure of cerium diruthenium decaaluminium, CeRu2Al10, is characterized by seven crystallographic sites in space group Cmcm, viz. Ce in 4c, Ru in 8a, two Al atoms in 8g, two Al atoms in 8f and one Al atom in 8e. The structure can be interpreted as a stacking of alternating columns running along [001], each formed by only one type of Ru cuboid with composition RuAl6 or CeRuAl4.

Geochronology of eruptions and parental magma sources of Elbrus volcano, the Greater Caucasus: K-Ar and Sr-Nd-Pb isotope data

Complex geochronological and isotope-geochemical studies showed that the Late Quaternary Elbrus volcano (Greater Caucasus) experienced long (approximately 200 ka) discrete evolution, with protracted periods of igneous quiescence (approximately 50 ka) between large-scale eruptions. The volcanic activity of Elbrus is subdivided into three phases: MiddleNeopleistocene (225–170 ka), Late Neopleistocene (110–70 ka), and Late Neopleistocene-Holocene (less than 35 ka).Petrogeochemical and isotope (Sr-Nd-Pb) signatures of Elbrus lavas point to their mantle-crustal origin. It was shown that hybrid parental magmas of the volcano were formed due to mixing and/or contamination of deep-seated mantle melts by Paleozoic upper crustal material of the Greater Caucasus. Mantle reservoir that participated in the genesis of Elbrus lavas as well as most other Neogene-Quaternary magmatic rocks of Caucasus was represented by the lower mantle “Caucasus” source. Primary melts generated by this source in composition corresponded to K-Na subalkali basalts with the following isotopic characteristics: 87Sr/86Sr = 0.7041 ± 0.0001, ƒNd = +4.1 ± 0.2, 147Sm/144Nd = 0.105–0.114, 206Pb/204Pb = 18.72, 207Pb/204Pb = 15.62, and 208Pb/204Pb = 38.78. The temporal evolution of isotope characteristics for lavas of Elbrus volcano is well described by a Sr-Nd mixing hyperbole between “Caucasus” source and estimated average composition of the Paleozoic upper crust of the Greater Caucasus. It was shown that, with time, the proportions of mantle material in the parental magmas of Elbrus gently increased: from ∼60% at the Middle-Neopleistocene phase of activity to ∼80% at the Late Neopleistocene-Holocene phase, which indicates an increase of the activity of deep-seated source at decreasing input of crustal melts or contamination with time. Unraveled evolution of the volcano with discrete eruption events, lacking signs of cessation of the Late Neopleistocene-Holocene phase, increasing contribution of deep-seated mantle source in the genesis of Elbrus lavas with time as deduced from isotope-geochemical data, as well as numerous geophysical and geological evidence indicate that Elbrus is a potentially active volcano and its eruptions may be resumed. Possible scenarios were proposed for evolution of the volcano, if its eruptive activity were to continue.

Geochronology and Genesis of Subalkaline Basaltic Lava Rivers at the Dzhavakheti Highland, Lesser Caucasus: K-Ar and Sr-Nd Isotopic Data

The paper reports newly obtained K-Ar isotopic-geochronological data on the age of three lava flows (Khrami, Mashavera, and Kura), which begin at the Dzhavakheti volcanic highland in southern Georgia. All of the dated rocks, including those from the Kura Flow, which was previously considered as the Pleistocene, are demonstrated to have a Pliocene age. The lavas of the longest Khrami Flow were erupted at 3.25–3.10 Ma, and those of the Kura and Mashavera Flows at 2.20–2.05 Ma, a fact testifying to two pulses of volcanic activity at the Dzhavakheti Highland. The petrogeochemical and isotopic characteristics of the rocks (87Sr/86Sr = 0.7039–0.7042; ∈Nd = 3.4–5.1) indicate that they are subalkaline within-plate basalts formed by the fractional crystallization of a basic mantle melt with the usually discontinuous selective or rarely continuous contamination with material that was not in geochemical equilibrium with the melt. The volcanics of the Khrami Flow are characterized by the less radiogenic Sr isotopic composition and the highest ∈Nd values, while the younger rocks of the Mashavera and Kura Flows have similar and more “crustal” isotopic signatures. The 87Sr/86Sr ratios of the Dzhavakheti subalkaline basalts are close to the initial Sr isotopic ratios of the Quaternary and Middle Pliocene dacite lavas from the same territory. Considered together with petrogeochemical and geological data, this suggests that all young rocks in Southern Georgia were produced in similar tectonic and geodynamic environments.

K-Ar Age and Sr-Nd Characteristics of Subalkali Basalts in the Central Georgian Neovolcanic Region (Greater Caucasus)

Late Cenozoic magmatism in the Greater Caucasus during the last 8.5 Ma was manifested in the Elbrus, Kazbek, and Central Georgian neovolcanic regions. In our work, we present K‐Ar dates obtained for young rocks of Central Georgia, which, in turn, allowed us to determine the age of volcanism in this region and specify its role in the Neogene‐Quaternary magmatic history of the Greater Caucasus. In addition, based on new Sr‐Nd isotopic‐geochemical and petrochemical data, we consider the petrogenesis of volcanics in this region and the problem of material source for young igneous rocks in the Greater Caucasus as a whole. The present work continues our recent geochronological investigations in this region [1‐5]. The Central Georgian neovolcanic region is located

Geochronology of Neogene-Quaternary Volcanism of the Geghama Highland (Lesser Caucasus, Armenia)

This work continues the systemic isotopic‐geochronological studies of Late Cenozoic volcanism in the Lesser Caucasus [1‐5] undertaken to elaborate the general regional scale of magmatic activity in this region during the Neogene‐Quaternary and to establish neovolcanic centers potentially hazardous in terms of possible catastrophic eruptions. The young magmatism in the Caucasian segment of the Alpine belt that developed in the course of collision between the Eurasian and Arabian lithospheric plates is genetically related to the activity of the mantle “hot field” [6], which determined the prevalence of basic rocks among volcanics. The most intense pulses of volcanic activity during the Neogene‐Quaternary are recorded in the Lesser Caucasus and eastern Anatolia, while young igneous rocks are distributed only in small areas of the Greater Caucasus. The Lesser Caucasus volcanic province, which extends in the arcuate manner from Ajaria to Nagornyi Karabakh and includes abundant Neogene‐Quaternary volcanics [7], is divided into six neovolcanic areas (from the northwest to southeast): Erusheti‐Arsiani, Javakheti, Aragats, Geghama, Vardenis, and Syunik. Young volcanics of the Lesser Caucasus are highly variable in terms of their chemical composition with varieties of normal and elevated alkalinity being most abundant. Our previous studies [1‐5] provided ages for Neogene‐Quaternary igneous rocks from several reference volcanic centers of the Aragats and Javakheti areas. These dates allowed us to define regional geochronological scales for young volcanism. In this work, we consider isotopic‐geochronological data obtained for Neogene‐Quaternary igneous rocks from the Geghama neovolcanic area located in the territory of the Republic

Geochronological scale and evolution of late Cenozoic magmatism within the Caucasian segment of the alpine belt

Results of the isotope-geochronological studies of the Late Cenozoic magmatism of Caucasus have been considered. The Neogene-Quaternary volcanic activity is found to have evolved during the last 15 m. y. being most intensive in the Middle-Late Pliocene. Within separate neovolcanic areas of the Caucasus region, magmatism was of a clearly discrete character when intense eruption periods interchanged with prolonged (up to several million years) times of quiet conditions. Four stages of young magmatism of the Caucasus are recognized: the Middle Miocene (15–13 Ma), the Late Miocene (9–5 Ma), the Pliocene (4.5–1.6 Ma), and the Quaternary (less than 1.5 Ma). However, for certain areas the time limits of these stages were shifted relative to each other and overlap the whole age range from the mid-Miocene to the end of the Quaternary period. Therefore, within the collision zone, the Neogene-Quaternary magmatism evolved almost continuously during almost the last 9 m. y., but in the time interval of 13–9 Ma in the Caucasian segment, volcanic activity was possibly low. No evidence of directed lateral migration of volcanic activity within the entire Caucasus region was found. At the same time, in the Lesser Caucasus the young magmatism commenced earlier (∼15 Ma), compared to the Greater Caucasus (∼8 Ma).

The Strel’tsovka uranium district: Isotopic geochronological (U-Pb, Rb-Sr, Sm-Nd) characterization of granitoids and their place in the formation history of uranium deposits

An isotopic geochronological study of Russia’s largest Strel’tsovka uranium district has been carried out. Polychronous granite generation, which determined the structure of the pre-Mesozoic basement, had important implications for the formation of volcanotectonic structural elements bearing economic uranium mineralization. The study of U-Pb, Rb-Sr, and Sm-Nd isotopic systems of whole-rock samples and minerals of granitic rocks allowed us to estimate the deportment of these systems in spatially conjugated granite-forming and hydrothermal processes differing in age and gave grounds for revising the age of granites pertaining to the Urulyungui Complex and refining the age of the Unda Complex.

Lead isotopic composition from data of high-precession MC-ICP-MS and sources of matter in the large-scale Sukhoi Log noble metal deposit, Russia

The lead isotopic composition of 33 sulfide samples from orebodies of the Sukhoi Log deposit was studied by high-precession MC-ICP-MS with a precision of ±0.02% (±2SD from 120 analyses of the SRM 981 standard sample). The deposit is located in the Bodaibo gold mining district in the northern Baikal-Patom Highland. Gold mineralization is hosted in Neoproterosoic black slates. Variations of lead isotope ratios of the Sukhoi Log sulfides are generally typical of Phanerozoic deposits and ore fields. They are significant for 206Pb/204Pb (17.903–18.674), moderate for 208Pb/204Pb (37.822–38.457), and relatively narrow for 207Pb/204Pb (15.555–15.679). In the Pb-Pb isotope diagrams, the data points of pyrite and galena constitute a linear trend. The points corresponding to pyrite from metasomatic ore occupy the left lower part of the trend. Galena from late gold-quartz veins shows more radiogenic Pb, and corresponding data points are located in the upper part of the trend. According to the Stacey-Kramers model, the end points of the trend, which is regarded as a mixing line, have μ2 = 9.6 and μ2 = 13.2 and model Pb-Pb ages 455 and 130 Ma, respectively. The isotope characteristics of ore lead, their relationships in pyrite and galena, and the mixing trend of Pb isotopic compositions are clearly tied to two Paleozoic stages in the formation of the Sukhoi Log deposit (447 ± 6 and 321 ± 14Ma) and testify to the leading role of crustal sources, which are suggested as being the Neoproterozoic black-shale terrigenous-carbonate rocks.

Sources of material for massive sulfide deposits in the Urals: Evidence from the high-precision MC-ICP-MS isotope analysis of Pb in galena

This work reports the results of one of the pioneer investigation of the Pb isotope geochemistry based on modern high-precision multichannel mass spectrometry with inductively coupled plasma (MC-ICP-MS). We studied the main mineral and age groups of massive sulfide deposits in the Urals: superlarge Gai deposit (~9 Mt); large deposits, such as Sibai (Cu + Zn, ~3 Mt), Uchaly (~7 Mt), and Saf’yanov (~2 Mt); and eight medium and small deposits. The lead isotopic compositions of these deposits were studied in 54 galena samples. According to Albarede [1], the MC-ICP-MS method has opened “a new era in isotope geochemistry,” particularly in the study of the Pb isotopic composition [2, 3]. Until recently, the method of thermoionization mass spectrometry (TIMS) dominated in this field. The overwhelming majority of Pb isotope data reported in the world during the last 50 years is based on the TIMS measurement of 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb ratios with an error of 0.1‐0.2% or more. Application of the MC-ICP-MS method decreased the error by 5‐ 10 times. Therefore, it has become possible to investigate minor local and regional variations in the Pb isotopic composition, decrease significantly (from ± 35 to ± 4 Ma) the analytical error of the model Pb‐Pb age calculation, and unravel “short” correlation trends of the isotopic characteristics of Pb. We have developed and applied a special MC-ICPMS version based on the NEPTUNE (ThermoFinnigan) device at the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (Moscow). The method consists in the high-precision isotopic analysis of Pb from Tl-doped solutions and the normalization of all current Pb isotope data relative to the