A. I. Buikin

Light element geochemistry of the Chelyabinsk meteorite

The spectacular arrival of a huge meteorite in central Siberia on February 15th 2013 was the largest event of its kind for more than a hundred years. Oxygen isotope analysis reveals the object involved was an ordinary chondrite of type LL. Petrological examination of the material analysed shows two main lithologies, metamorphic grade 5, were present both having veins of shock produced glass. All three types of material were investigated for carbon, nitrogen and noble gas content and isotopic compositions. The relatively low abundance of carbon and nitrogen indicate that Chelyabinsk is uncontaminated by comparison with other samples in meteorite collections so that indigenous components may be recognized. All the samples studied contained minimal amounts of cosmogenic and primordial noble gases, evidence that the pre-atmospheric size of the object was large, greater than two metres in diameter and that the explosion and break-up was accompanied by catastrophic degassing. The implications for other major meteorite falls and the Tunguska event are discussed.

Composition and sources of volatiles and noble gases in fluid inclusions in pyroxenites and carbonatites of the Seblyavr Massif, Kola Peninsula

Carbonatites and related pyroxenites from the Seblyavr alkaline-ultramafic massif were analyzed for isotopic composition and concentrations of carbon (in carbon dioxide), nitrogen, and noble gases using the stepwise crushing technique. The C isotopic composition in crushing steps of calcite from the carbonatite varies from −6.6 to −15.0‰ (PDB) with average values from −8.5 to −10.5‰, which is lower than the mantle range for

PVT parameters of fluid inclusions and the C, O, N, and Ar isotopic composition in a garnet lherzolite xenolith from the Oasis Jetty, East Antarctica

delta ^{13} C_{(CO_2 )}

The fluid phase evolution during the formation of carbonatite of the Guli massif: Evidence from the isotope (C, N, Ar) data

(from −3 to −5‰) and can likely be explained by long-term isotopic exchange between the carbon of CO2 in inclusions and their host Ca carbonate. The 40Ar/36Ar ratios in the crushing extractions of the calcites vary from the atmospheric value of 296 to 3200. Diopside from the pyroxenite has these ratios as high as 26000–33000 (such high values for pyroxenite in the Kola alkaline-ultramafic province have been obtained for the first time), which corresponds to the values obtained for MORB chilled glasses. Nitrogen in the samples is isotopically heavy, δ15N from +1 to +2 on average, which is consistent with earlier data on carbonatite massifs in the Kola alkaline province (Dauphas and Marty, 1999) and carbonatites of the Guli Massif (Buikin et al., 2011). The N2 content in the crushing extractions is correlated with the 36Ar concentration, which is an indicator of atmospheric contamination and suggests the dominance of the crustal N component in the samples, likely as a result of subduction or penetration of the ancient meteoric water into the magma chamber or a metasomatic source. The variations in the isotopic and elemental composition of the gas components between crushing steps suggest that the investigated samples contain inclusions of at least two populations.

Light noble gas data in Guli massif carbonatites reveal the subcontinental lithospheric mantle as primary fluid source

Orthopyroxene, clinopyroxene, and olivine from a metasomatized mantle xenolith of garnet lherzolite in alkaline rocks at the Jetty Oasis, East Antarctica, contain numerous carbon dioxide-dominated composite melt-fluid and fluidized sulfide-silicate (±carbonate) inclusions. Although the maximum pressure under which the inclusions were captured by rock-forming minerals was evaluated at 13 kbar, its actual value should have been much higher, judging by the fact that the inclusions have lost part of their material (decrepitated) when the xenolith was brought to the surface. Two major fluid populations are distinguished. The fluids entrapped during the earlier episode have a more complicated composition. Dominated by CO2, these fluids contain much N2 (0.1–0.2 mole fractions), H2S, and perhaps, also H2O and are hosted by sulfide-silicate (±carbonate) inclusions produced by liquid immiscibility. As these inclusions evolved, they enriched in CO2 and depleted in H2S and N2. Although the concentrations of N2, H2S, and H2O were generally relatively low, these components played an important role in mantle metasomatism, as is reflected in the geochemistry of the derived magmas. The fluids of the younger episode (pressures lower than 7 kbar) are notably richer not only in CO2 but also in H2O (up to the appearance of inclusions with a liquid aqueous phase and the formation of CO2 gas hydrate when cooled in a cryometric stage by liquid N2). The effect of fluids on the mantle source in two discrete episodes is also confirmed by isotopic-geochemical data. Isotopic data on gases obtained immediately from fluid inclusions in minerals by the stepwise crushing technique provide evidence of the evolution of elemental and isotopic ratios of the gases in the course of the metasomatic processes. The high-pressure fluid inclusions of the earlier episode have low C/N2, C/Ar, and N2/Ar ratios, isotopically heavy N2, and somewhat elevated (to 530) 40Ar/36Ar ratios. The younger fluids typically have higher (by two to three orders of magnitude) C/N2 and C/Ar ratios, lower δ13C of CO2, and N2/Ar and 40Ar/36Ar ratios close to the atmospheric values. The nitrogen and argon isotopic compositions and elemental ratios suggest that the younger fluids could have been produced by two-component mixing in the mantle-atmosphere system. Comprehensive analysis of the data and in particular the 40Ar/36Ar ratios, which are atypical of the mantle, and an increase in the H2O concentration, suggests a subduction-related nature of the fluids.

He, Ne, Ar stepwise crushing data on basalt glasses from different segments of Bouvet Triple Junction

The first data on variations of the isotope composition and element ratios of carbon, nitrogen, and argon in carbonatites of different generations and ultrabasic rocks of the Guli massif obtained by the method of step crushing are reported. It is shown that early carbonatite differs significantly from the later ones by the concentration of highly volatile components, as well as by the isotope compositions of carbon (CO2), argon, and hydrogen (H2O). The data obtained allow us to conclude that the mantle component predominated in the fluid at the early stages of formation of rocks of the Guli massif, whereas the late stages of carbonatite formation were characterized by an additional fluid source, which introduced atmospheric argon, and most likely a high portion of carbon dioxide with isotopically heavy carbon.

Prospects of the method of stepwise crushing as a source of information on the fluid phase of rocks and minerals

For better understanding of the fluid phase sources of carbonatites of Guli alkaline-ultrabasic intrusion (Maymecha-Kotuy complex) we have studied isotope composition of He and Ne in the carbonatites of different formation stages. The data definitely point to the subcontinental lithospheric mantle (SCLM) as a primary source of fluid phase of Guli carbonatites. The absence of plume signature in such a plume-like object (from petrological point of view) could be explained in terms that Guli carbonatites have been formed at the waning stage of plume magmatic activity with an essential input of SCLM components.

A new injection technique of microquantity of water from fluid inclusions into mass spectrometer for measurement of hydrogen and oxygen isotope compositions

Here we present the first data on He, Ne, Ar isotopic and elemental composition in fluid phases of tholeiitic chilled glasses from the Bouvet Triple Junction (BTJ). The chilled glasses from several dredging stations situated at different segments of BTJ have been investigated: Spiess Ridge, Mid Atlantic Ridge (MAR) and in a valley of the Southwest Indian Ridge (SWIR). The data allow to distinguish within BTJ three segments characterized by different geochemical behavior of He, Ne and Ar. MAR and Spiess samples contain MORB-like helium and neon while SWIR is characterized by addition of plume type He and Ne. The strong atmospheric contamination is typical of all segments, but for MAR it is less pronounced. The Ne-Ar isotope systematics suggests that the atmospheric component was most probably introduced into the mantle source of the fluids with fragments of oceanic crust/sediments.

40Ar-39Ar dating of volcanic rocks from the Fernando de Noronha Archipelago

This paper illustrates opportunities provided by the method of stepwise crushing for the investigation of the fluid phase of geologic objects. Owing to the efficient separation of gases from fluid inclusions of different generations trapped during mineral growth and/or subsequent alteration (metasomatic and hydrothermal), stepwise crushing allows us to obtain the isotopic characteristics of end-members and, thus, reliably establish the source and evolution of fluids in magmatic and postmagmatic processes; this method provides clues to a better understanding of interaction of global reservoirs, such as the mantle, crust, and atmosphere. The importance of information obtained by this method is exemplified here by the results of the investigation of mantle rocks and minerals from various geologic environments (MORB, SCLM, and carbonated mantle). It was shown that the multi-isotope approach yields most comprehensive data on the genetic features and evolution of the fluid phase. The importance of combining isotope geochemical and microthermometric fluid inclusion data is demonstrated by the example of a mantle xenolith of garnet lherzolite from the Jetty Oasis. Together with the microthermometric investigation of fluid inclusions and developing laser techniques for opening of individual inclusions, the method of stepwise crushing provides a means for solving one of the most important practical problems—obtaining information on the geochemical features and physicochemical parameters of mineral-forming (and ore-forming) processes.

On the Separation Efficiency of Entrapped and in situ-Produced Noble Gas Components at Sample Crushing in Vacuum

The study of fluid inclusions is very important for understanding the physicochemical conditions of magmatic system during their entrapment by minerals, as well as for obtaining information on sources and evolution of fluid phase. Fluids are one of the most important agents of alteration of mantle rocks, in particular, carbonatites and peridotites. They cause mantle metasomatism and enrichment of a deep protolith in trace elements, which later reflects in the geochemical features of generated magmas and related ore deposits. The finds of fluid inclusions in rock-forming minerals indicate the presence of a free fluid phase in the lower crust and upper mantle. The study of the fluid inclusions makes it possible to trace the variations in volatile composition depending on temperature and pressure, as well as the fluid regime of the system. Isotope studies of gases extracted from fluid inclusions in rocks and minerals provide insight into fluid source and gas evolution during metasomatic events.