Russian Journal of Earth Sciences
Vol 1, No. 2, December 1998
Translated February 1999

Low mantle plume component in 370 Ma old Kola ultrabasic-alkaline-carbonatite complexes: Evidences from rare gas isotopes and related trace elements

I. N. Tolstikhin, I. L. Kamensky, V. A. Nivin, V. R. Vetrin, E. G. Balaganskaya, S. V. Ikorsky, M. A. Gannibal, and Yu. M. Kirnarsky

Geological Institute, Kola Scientific Centre, Russian Academy of Sciences, Apatity 184200, Russia

B. Marty

Centre National de la Recherche Scientifique Centre de Recherches Petrographiques et Geochimiques(C.R.P.G.) B.P. 20, 54501 Vandoeuvre-Nancy Cedex, France

D. Weiss, A. Verhulst, and D. Demaiffe

Petrologie et Geodynamique Chimique Department des Sciences de la Terre et de l`Environnement Faculte des Sciences Universite Libre de Bruxelles Avenue F. D. Roosevelt 50 B-1050 Bruxelles, Belgique



During Devonian pulse of alkaline magmatism (370 Ma ago) 18 ultrabasic alkaline-carbonatite complexes were formed on the Kola Peninsula (the north-eastern segment of the Baltic Shield). Rare gas isotope abundances were studied in ~300 samples from 8 complexes and also from Devonian dikes. 4He/ 3He ratio in bulk He released by melting vary widely from pure radiogenic values of sim 108 down to 6 times 104. A comparison of expected from in-situ closed-system production (calc) and measured (meas) abundances has shown that 4He meas le 4He calc whereas in some rocks and minerals 3He meas exceeded 3He calc up to 1,000 times indicating occurrence of a mantle fluid in a majority of samples. Gas extraction by milling of rock/mineral samples liberated fluid-related helium with as low 4He/ 3He ratios as 3times104. These values, well below the mean MORB ratio of (8.9 pm 1) times 104, show a contribution of 3He-rich plume-like component. A reason for highly variable 4He/ 3He ratios in similar rocks selected from different massifs is not well understood yet. The following processes appear to play major roles: (i) different degree of melt degassing, trapping and subsequent retention of trapped (initially homogeneous) fluid and (ii) various contributions of radiogenic in-situ produced He. Isotope composition of Ne supports the occurrence of plume component: 20Ne/ 22Ne ratio varies from 10.4 to 12.0 and 20Ne/ 22Ne versus 21Ne/ 22Ne correlation (arising from mixing of the mantle and atmospheric components) is quite similar to that observed for Loihi hot spot, Hawaii. 20Ne/ 22Ne versus 40Ar/ 36Ar correlation and K-Ar systematics give initial 40Ar/ 36Ar ge 3,000 for the mantle end-member, which is by an order of magnitude lower than that accepted for the upper mantle, source of MORB. Contamination of parent UAC melts by air argon can not be ruled out. A comparison of these data with rare gas systematic in the mantle enable us to suggest a contribution of material from the less degassed deep mantle reservoir, source of primordial rare gases, to Devonian ultrabasic-alkaline and carbonatite rocks on the Kola Peninsula. Available model characterisations of principle terrestrial reservoirs allow contributions (by mass) of the lower mantle, the upper mantle, and the atmosphere (an air-saturated groundwater) to be evaluated: 2%, 97.95%, and 0.05%, respectively.

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