Elena Balaganskaya

Plume-derived rare gases in 380 Ma carbonatites from the Kola region (Russia) and the argon isotopic composition in the deep mantle

In an effort to document the source of the parental melts to carbonatites, we have measured rare gases in 380 Ma carbonatites and associated mineral assemblages from the Kola Peninsula, eastern part of the Baltic shield in Russia. These series were emplaced during widespread Devonian magmatism when several large ultrabasic–alkaline–carbonatite massifs were formed. 4He/3He ratios vary from 1×106 to 1×107 in the bulk He extracted by melting of samples from three localities, including the large Kovdor massif. A comparison of measured abundances of 3He and 4He with those expected from in-situ production revealed a large (up to 105 times) excess of 3He, implying a significant contribution from a mantle-derived 3He-bearing fluid. Crushing of these samples allowed extraction of fluids with 4He/3He ratios down to 38,000, lower than those of mid-ocean ridge basalts and in the range of 4He/3He observed in 3He-rich ocean island basalts (OIBs) related to mantle plumes. 20Ne/22Ne up to 12.1±0.2 are higher than the atmospheric value of 9.80, implying the occurrence of primordial (solar-type) neon in the carbonatite source. 20Ne/22Ne and 21Ne/22Ne ratios display a good correlation, with the regression line close to (slightly to the right of) the Loihi Seamount correlation. Extrapolation of the regression to solar 20Ne/22Ne of 13.8 gives a 21Ne/22Ne of 0.045 for the plume end-member, well below the mid-ocean ridge basalt (MORB) source (upper mantle) end-member of 0.07. The measured 40Ar/36Ar ratios up to 2790 correlate very well with the Ne isotopic ratios, and the best estimate of the 40Ar/36Ar ratio of the plume source is within 5000±1000. Although the 3He/22Ne ratio in the plume source appears to be comparable to the solar value within a factor of 2, the 22Ne/36Ar ratio, computed from Ne–Ar isotope correlation, is two orders of magnitude lower than the solar value. Such difference is unlikely to be due to magmatic fractionation since the observed 4He/40Ar* ratios are close to values expected for radiogenic production and accumulation in the mantle source. It may rather represent a characteristic of the plume source. The isotope composition of light noble gases in samples from ultrabasic–alkaline rocks of the Kola Peninsula, and associated carbonatites, indicate a contribution of material with lower time-integrated (U + Th)/(3He, 22Ne) and (40K/36Ar) ratios than those in the asthenospheric upper mantle, the subcontinental lithosphere, and the continental crust. The location of such material is likely to be below the convective mantle supplying MORB magmas, and reflects the contribution of a plume source material to Kola carbonatitic magmatism. These data support models which advocate a structure of the Earth heterogeneous in its refractory/volatile content.

Rare gas isotopes and parent trace elements in ultrabasic-alkaline-carbonatite complexes, Kola Peninsula: identification of lower mantle plume component

During the Devonian magmatism (370 Ma ago) ∼20 ultrabasic-alkaline-carbonatite complexes (UACC) were formed in the Kola Peninsula (north-east of the Baltic Shield). In order to understand mantle and crust sources and processes having set these complexes, rare gases were studied in ∼300 rocks and mineral separates from 9 UACC, and concentrations of parent Li, K, U, and Th were measured in ∼70 samples. 4He/3He ratios in He released by fusion vary from pure radiogenic values ∼108 down to 6 × 104. The cosmogenic and extraterrestrial sources as well as the radiogenic production are unable to account for the extremely high abundances of 3He, up to 4 × 10−9 cc/g, indicating a mantle-derived fluid in the Kola rocks. In some samples helium extracted by crushing shows quite low 4He/3He = 3 × 104, well below the mean ratio in mid ocean ridge basalts (MORB), (8.9 ± 1.0) × 104, indicating the contribution of 3He-rich plume component. Magnetites are principal carriers of this component. Trapped 3He is extracted from these minerals at high temperatures 1100°C to 1600°C which may correspond to decrepitation or annealing primary fluid inclusions, whereas radiogenic 4He is manly released at a temperature range of 500°C to 1200°C, probably corresponding to activation of 4He sites degraded by U, Th decay. Similar 4He/3He ratios were observed in Oligocene flood basalts from the Ethiopian plume. According to a paleo-plate-tectonic reconstruction, 450 Ma ago the Baltica (including the Kola Peninsula) continent drifted not far from the present-day site of that plume. It appears that both magmatic provinces could relate to one and the same deep-seated mantle source. The neon isotopic compositions confirm the occurrence of a plume component since, within a conventional 20Ne/22Ne versus 21Ne/22Ne diagram, the regression line for Kola samples is indistinguishable from those typical of plumes, such as Loihi (Hawaii). 20Ne/22Ne ratios (up to 12.1) correlate well with 40Ar/36Ar ones, allowing to infer a source 40Ar/36Ar ratio of about 4000 for the mantle end-member, which is 10 times lower than that of the MORB source end-member. In (3He/22Ne)PRIM versus (4He/21Ne)RAD plot the Kola samples are within array established for plume and MORB samples; almost constant production ratio of (4He/21Ne)RAD ≅ 2 × 107 is translated via this array into (3He/22Ne)PRIM ∼ 10. The latter value approaches the solar ratio implying the non-fractionated solar-like rare gas pattern in a plume source. The Kola UACC show systematic variations in the respective contributions of in situ-produced radiogenic isotopes and mantle-derived isotopes. Since these complexes were essentially plutonic, we propose that the depth of emplacement exerted a primary control on the retention of both trapped and radiogenic species, which is consistent with geological observations. The available data allow to infer the following sequence of processes for the emplacement and evolution of Kola Devonian UACC: 1) Ascent of the plume from the lower mantle to the subcontinental lithosphere; the plume triggered mantle metasomatism not later than ∼700 to 400 Ma ago. 2) Metasomatism of the lithosphere (beneath the central part of the Kola Peninsula), including enrichment in volatile (e.g., He, Ne) and in incompatible (e.g., U, Th) elements. 3) Multistage intrusions of parental melts, their degassing, and crystallisation differentiation ∼370 Ma ago. 4) Postcrystallisation migration of fluids, including loss of radiogenic and of trapped helium. Based on model compositions of the principle terrestrial reservoirs we estimate the contributions (by mass) of the plume material, the upper mantle material, and the atmosphere (air-saturated groundwater), into the source of parent melt at ∼2%, 97.95%, and ∼0.05%, respectively.

Petrological and geochemical (trace elements and Sr–Nd isotopes) characteristics of the Paleozoic Kovdor ultramafic, alkaline and carbonatite intrusion (Kola Peninsula, NW Russia)

The Kovdor intrusion belongs to the Paleozoic (380–360 Ma) Kola alkaline and carbonatite province (NW Russia). It displays a complete sequence of rocks that include in order of intrusion, ultramafic rocks, melilitolites, alkaline silicate rocks of the melteigite–ijolite series, phoscorites and carbonatites, and late nepheline syenite dykes (the latter were not studied). The ultramafic sequence (dunite–peridotite–clinopyroxenite) consists of olivine–clinopyroxene cumulates (Mg#=86–70) with intercumulus phlogopite and magnetite and late calcite. Melilitolites, with up to 35 wt.% CaO (melilite cumulates), have a magmatic rather than metasomatic origin. Rocks of the melteigite–ijolite series are very heterogeneous (variations of grain size, mineralogy and modal proportions) and show disequilibrium textures (core resorption and complex zoning of the clinopyroxenes) suggesting either magma mixing or contamination. All the rocks have strong incompatible element enrichments in multi-element spidergrams. The rare earth element (REE) patterns are steep with (La/Yb)N>20; they are subparallel and do not show any Eu anomaly. The variations of Nb/Ta ratios and the REE distributions suggest that the carbonatites and the rocks of the melteigite–ijolite series are not conjugate immiscible liquids. Most rocks (ultramafics, melilitolites, carbonatites and phoscorites) plot in the depleted mantle quadrant of the Nd–Sr diagram with low ()i ratios (0.70332 to 0.70377) and positive eNd(t) values (+5.2 to +0.6). The fairly large range of isotopic compositions is not in favour of a simple, closed system magmatic evolution; it suggests a complex evolution implying several magma batches derived either from an isotopically heterogeneous mantle source or from various mixing proportions of two mantle reservoirs. The isotopic composition of the melteigites–ijolites requires a slightly enriched component that could be similar to that of the Kandalaksha ultramafic lamprophyres and of the Tersky Coast and Arkhangelsk kimberlites.

REE AND Sr-Nd ISOTOPE COMPOSITIONS OF CLINOPYROXENITES, PHOSCORITES AND CARBONATITES OF THE SEBLYAVR MASSIF, KOLA PENINSULA, RUSSIA

Ree and Sr-Nd Isotope Compositions of Clinopyroxenites, Phoscorites and Carbonatites of the Seblyavr Massif, Kola Peninsula, Russia Clinopyroxenites, phoscorites and carbonatites from the Devonian Seblyavr intrusion (Kola Peninsula, Russia) have petrographic characteristics indicating that they are accumulative in origin. Their geochemical (major and rare earth elements) compositions can be accounted for by mixtures of their major rock-forming minerals and accessory phases, i.e. they reflect the record of mineral accumulation. All of the analysed Seblyavr rocks are strongly LREE-enriched with (La/Yb)N mostly ranging from 38 to 189. However, a dolomite carbonatite with hydrothermal LREE-Sr mineralization has an extreme (La/Yb)N value of 1659. Such late-stage dolomite carbonatites were formed by hydrothermal (rather than magmatic) processes. Whole-rock samples of representative magmatic lithologies from Seblyavr have initial 87Sr/86Sr and εNd that fall in a very narrow range from 0.7031 to 0.7033 and +4.9 to +5.9, respectively. We therefore conclude that clinopyroxenites, phoscorites and carbonatites were formed by differentiation and crystallization of a single batch of melt. The parental melt was derived from a depleted upper mantle source that had been meta-somatised prior to melting. REE i izotopy Sr-Nd w klinopiroksenitach, foskorytach i karbonatytach masywu sebliawrskiego z Półwyspu Kola (Rosja) Petrograficzna charakterystyka dewońskich klinopiroksenitów, foskorytów i karbonatytów z masywu sebliawrskiego na Półwyspie Kola (Rosja) wskazuje, że skały te są kumulatami. Ich skład chemiczny (zarówno pierwiastki główne jak i pierwiastki ziem rzadkich) stanowi zapis procesów akumulacyjnych i wynika ze zmieszania głównych minerałów skałotwórczych i faz akcesorycznych. Wszystkie analizowane skały masywu sebliawrskiego są silnie wzbogacone w LREE, a ich stosunek (La/Yb)N waha się przeważnie pomiędzy 38 a 189. Skrajnie wysoki stosunek (La/Yb)N równy 1659 zanotowano w dolomitowym karbonatycie, zawierającym hydrotermalną mineralizację LREE-Sr. Takie karbonatyty zostały utworzone w końcowych etapach powstawania masywu dzięki właoenie procesom hydrotermalnym, a nie magmowym. Próbki reprezentujące skały magmowe masywu wykazują początkowe stosunki izotopów strontu 87Sr/86Sr i wartości parametru εNd zawarte w wąskich przedziałach, odpowiednio od 0,7031 do 0,7033 i od +4,9 do +5,9. Upoważnia to do sformułowania wniosku, że badane klinopiroksenity, foskoryty i karbonatyty utworzyły się w wyniku dyferencjacji i krystalizacji pojedynczej porcji stopu magmowego. Ten macierzysty stop został wygenerowany ze zubożonego górnego płaszcza, a zródłowymateriał uległ metasomatozie jeszcze przed momentem stopienia.