V. A. Nivin

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.

Radiogenic helium isotope fractionation: the role of tritium as 3He precursor in geochemical applications

Abstract Reduced 4 He/ 3 He ratios, e.g., down to ≈1/100 times those expected from radiogenic production, were observed in sedimentary rocks. Formation and history of these rocks eliminate a contribution of mantle 3 He-bearing fluid. To explain the difference between the observed and the calculated production 4 He/ 3 He ratios Loosli et al. (1995) and Tolstikhin et al. (1996) suggested a different behaviour of helium and tritium in damage tracks produced by emission of these nuclides. Generally, the tracks cross grain boundaries or some imperfections within a rock or mineral allowing a fast loss of noble 4 He and 3 He atoms. However, radiogenic 3 He has the precursor 3 H, generated in the exothermic 6 Li(n t , α) 3 H + 4.5 MeV reaction. The energetic tritons produce damage tracks comparable with those from α-decay of U and Th series. If 3 H is chemically bound within a track, and the track is able to recover via some diagenetic process before the 3 H decay, then 3 H and daughter 3 He atoms are trapped within the recovered track. This mechanism would explain the shorter residence time of 4 He in the rocks/minerals than of 3 He; therefore, 4 He/ 3 He ratios could decrease through time. To check this mechanism 4 He, 3 H, and 3 He (from 3 H-decay) were produced by the above reaction in special targets, consisting of layered composites of thin sections of quartz, sample, Li-bearing cover, sample, and quartz. The samples were the same rocks in which reduced 4 He/ 3 He ratios have been previously observed. Each target was placed in a quartz ampoule, which was then pumped out, sealed off, and then exposed to the flux of thermal neutrons in a reactor. After irradiation and cooling down (total duration 145 days), the nuclides produced during ( 3 H, 3 He, 4 He) and after ( 3 He) irradiation were measured in the gas phase above the targets and compared with their total quantities expected from the Li abundance and the integrated neutron flux. The ratios obtained were 3 H(gas)/ 3 H(total) 3 He(gas)/ 3 He(total) varying from 0.2 to 0.9. The average residence times τ of 3 H and 3 He, respectively, were estimated to be ≈16 and ≈0.25 yr for this first time interval, which included the irradiation of the targets. After these first measurements, the targets were kept in a vacuum system under room temperature for 210 days and the amounts of 3 H and 3 He, which accumulated above the targets during this second time interval under fully controlled conditions, were also measured. Much slower rates of gas loss from the same targets with average residence times of τ( 3 H) ≈ 600 yr and τ(He) ≈ 1.6 yr resulted for this second time interval. Probably these longer residence times are closer to those in the relevant natural environments, the 3 H residence time being much longer than the 3 H half-life. In all cases the inequality τ( 3 He) ≪ τ( 3 H) is valid. This confirms the proposed scenario envisaging longer retention of 3 H than He in damage tracks. Within the frame of this scenario the life-time of 3 H gives a time constraint on diagenetic processes; at least one to several newly formed atomic layers should appear during ∼10 yr to recover the tracks.

New Information About Natural Gases of Alkaline Massifs

Abstract The use of gas-geochemical study results for decisions of various problems of mining and geological character was shown by example of the alkaline nepheline syenite massifs in the eastern part of Baltic Shield. Two principal morphologic types of gases (freely evolved and occiuded or microincluded) are known in the considered rocks. The gas composition is: hydrogen-hydrocarbons, mixed with He, N2, sometimes CO, and CO2. Discovering gas zoning, relative time of generation and space distribution regularities of gases concerning ore deposits assume the participation of reduced fluids and postmagmatic processes in the formation of certain ores, the origin of which was considered magmatic. In some cases gas-geochemical peculiarities of rock and minerals can be employed in the capacity of indicators of ore mineralization in search works. However studies of gases (compositions, distribution, release conditions and intensity) have most practical significance for guarantee of ore deposits gas-safe exploita...

Helium and Argon Isotope Abundances in Rocks of Lavozero Alkaline Massif

Abstract He and Ar isotope abundances in alkaline rocks of the Lovozero layered ore-bearing complex are used to discuss the origin of hydrogen-hydrocarbon gases (HHG). Three procedures to extract - crushing, milling and melting - and the chemical analysis of these gases have been described, and then the isotope analysis of the ratios of 3He/4He and 40Ar/36Ar is related. It is suggested, that (4He/40Ar)rad ratios and 4He concentrations decrease and 3He/4He increase with the distance from the ore zone, the former ratio being the most contrast indicator of ore mineralization. A direct correlation has been revealed between helium isotopic ratios and methane concentrations. The results of measurements confirm the idea on the crustal origin of HHG.