L. N. Kogarko

The Kola Alkaline Province of the CIS and Finland: Precise RbSr ages define 380–360 Ma age range for all magmatism

Abstract Twenty-one new RbSr mineral and biotite-whole rock isochrons are presented for intrusive rock complexes of the Palaeozoic Kola Alkaline Province in the CIS and Finland. Samples from the ultrabasic-alkaline and nepheline-syenite complexes Khibina, Lovozero, Afrikanda, Turiy Mys, Ozernaya Varaka, Sokli and Iivaara all yield ages in the very narrow range from 361.7 to 377.3 Ma, i.e. late Middle Devonian to Upper Devonian. These data represent crystallization rather than intrusion ages, but are nevertheless argued as marking very closely the time of intrusion. There is no indication of bimodal age distribution. The emplacement of the alkaline magmas is related temporally to the subsidence of the Kontozero Graben of the Central Kola Peninsula. Sr initial ratios of the agpaitic nepheline-syenites of the giant Khibina and Lovozero intrusions scatter between 0.703 and 0.704. Because of high Rb Sr ratios of the rocks, these isotope ratios exclude long-term pre-enrichment of alkalis in the sources of the magmas prior to segregation and intrusion, and thus episodic enrichment during earlier rifting events in the northern Baltic Shield. The magma formation may be related to wrench movements in the surroundings of the major Trollfjord-Komagelv Fault, which forms part of the Late Caledonian Arctic—North Atlantic Megashear System.

Physicochemical conditions of magma formation at the base of the Siberian plume: Insight from the investigation of melt inclusions in the meymechites and alkali picrites of the Maimecha-Kotui province

Based on the compositions of melt inclusions and coexisting minerals from meymechites and alkali picrites, the temperatures and pressures of the ascending material of the Siberian plume were estimated at the level of the lithosphere-asthenosphere boundary. The melts trapped in olivine show high contents of titanium and other incompatible elements. The rocks crystallized under high oxygen fugacity conditions. The calculated compositions of primary magmas are similar to the compositions of near-solidus melts derived from a dry fertile lherzolite at 7 GPa. The estimated potential temperature is close to 1650°C, which is much higher than the potential temperature of plumes that generate the primary basaltic magmas of mid-ocean ridges. The obtained data show that, during the activity of the giant magma-generating system of the Siberian trap province, hot peridotite masses ascended probably from the core-mantle boundary up to the base of the continental lithosphere. Our results are at odds with the suggestion that the basalt flows of the Siberian and other large igneous provinces are not related to mantle plumes.

Spheroidal structures in arfvedsonite lujavrite, Ilı́maussaq alkaline complex, South Greenland—an example of macro-scale liquid immiscibility

Abstract Lujavrites are meso- to melanocratic agpaitic nepheline syenites and are the most highly evolved rocks of the Ilimaussaq complex, South Greenland. Spheroidal bodies measuring up to 20 cm in diameter occur locally in arfvedsonite lujavrite. They consist of a core rich in analcime and arfvedsonite and a rim rich in analcime, aegirine and a low temperature potassium feldspar. The host lujavrite is dominated by albite and arfvedsonite. The igneous lamination of the host continues undisturbed through rim and core of spheroids and the arfvedsonite crystals in host, rim and core have identical chemical compositions. There are sharp contacts between host and rim and rim and core. Host, rim and core contain the same accessory minerals and have almost identical chemical compositions, the main differences are high H 2 O and Na 2 O but low SiO 2 in the core and high Fe 2 O 3 in the rim. The spheroids are proposed to have been formed by separation of immiscible interstitial H 2 O-rich globules of magma from the host lujavrite magma at a late stage in the crystallization of the lujavritic melt.

Redox potential of mantle magmatic systems

A new variant of the olivine-clinopyroxene-spinel oxygen barometer was developed on the basis of the equilibrium 3CaMgSi2O6(Cpx) + 2Fe3O4(Spl) = 3CaMgSiO4(Ol) + 3Fe2SiO4(Ol) + O2. Oxygen fugacity was estimated for the mineral assemblages of meymechites, olivine-bearing rocks of the Guli intrusion, and olivine and clinopyroxene microphenocrysts from interstitial glasses in mantle xenoliths containing metal alloys from Sal Island, Cape Verde Archipelago. It was shown that oxygen fugacity may vary in mantlederived magmatic systems by 7–8 orders of magnitude. Thermodynamic analysis showed that the low water activity in the lower part of the subcratonic lithosphere prevents the formation of hydrocarbons even at the presence of elemental carbon and low oxygen fugacity. The most probable mechanism of diamond formation is the reduction of carbonate components in the composition of near-solidus melts coming into the lithosphere from ascending mantle plumes.

Rare metal mineralization of calcite carbonatites from the Cape Verde Archipelago

Rare metal mineralization of oceanic carbonatites was studied for the first time by the example of calcite carbonatite from Fogo Island in the Cape Verde Archipelago. The following evolutionary sequence of rare metal minerals was established: zirconolite-Th-calciobetafite-betafite + Th-pyrochlore-thorite + Ti-Zr-Nb silicates + zircon.Schematic reactions were proposed for zirconolite transformation to secondary phases: (Ca,Th,U)Zr(Ti,Nb)2O7 (zirconolite) + SiO2 + Ca(F,OH)2 → ZrSiO4 (zircon) + (Ca,Th,U)2(Ti,Nb)2O6(OH,F) (Th-calciobetafite) and (Ca,Th,U)2(Ti,Nb)2O6(OH,F) + Na2Si2O5 → ThSiO4 (thorite) + (Ca,Na,Th)2(Nb,Ti)2O6(OH,F) (Th-pyrochlore), where SiO2, Ca(F,OH)2, and Na2Si2O5 are the components of melt-solution coexisting with the carbonatite.It was shown that the distribution and behavior of rare and radioactive elements in oceanic carbonatites show the same tendencies as in continental carbonatites. The contents and distribution of Ti, Ta, and Th in zirconolites and pyrochlores from oceanic and continental carbonatites are different: the minerals of oceanic carbonatites are enriched in Ti and Th and strongly depleted in Ta.

Trace element and PGE distribution in layered ferromanganese crusts

A collection of layered ferromanganese ores (27 samples) from the Atlantic and Pacific oceans was studied. Trace element and PGE contents were determined layer-by-layer (up to 10 microlayers) in 13 of these samples.The trace, rare earth, and platinum group element distributions, including their layer-to-layer variations, were compared in hydrogenic and hydrothermal crusts from different regions. It was found that the main PGE variations (by a factor of 10–50) are related to their layer-to-layer variations within a given ore field.The distributions of PGE and trace elements are strongly heterogeneous, which is related, first, to different contents of the elements in the layers of different age in ferromanganese crusts (FMC) and, second, to the observed regional heterogeneity and influence of hydrothermal fluids. Geochemical data indicate that CFC formation was mainly caused by the hydrochemical precipitation of material from seawater. This process was accompanied by diagenetic phenomena, water-rock interaction, and influence of volcanic and hydrothermal sources.

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 )}

New data on the age of the Guli intrusion and implications for the relationships between alkaline magmatism in the Maymecha-Kotuy province and the Siberian superplume: U-Th-Pb isotopic systematics

(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.

A new version of the spinel-olivine-pyroxene oxybarometer and extreme redox differentiation in magmatic systems of mantle sources

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.