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 a ssemblages from the Kola Peninsula, eastern part of the Baltic shield in Ru ssia. These series were emplaced during widespread Devonian magmatism when several large ultrabasic-alkaline-carbonatite massifs were formed. He-4/He- 3 ratios Vary from 1 x 10(6) to 1 x 10(7) in the bulk He extracted by melti ng of samples from three localities, including the large Kovdor massif. A c omparison of measured abundances of He-3 and He-4 with those expected from in-situ production revealed a large (up to 10(5) times) excess of He-3, imp lying a significant contribution from a mantle-derived He-3-bearing fluid. Crushing of these samples allowed extraction of fluids with He-4/He-3 ratio s down to 38,000, lower than those of mid-ocean ridge basalts and in the ra nge of He-4/He-3 observed in He-3-rich ocean island basalts (OIBs) related to mantle plumes. Ne-20/Ne-22 up to 12.1 +/- 0.2 are higher than the atmosp heric value of 9.80, implying the occurrence of primordial (solar-type) neo n in the carbonatite source. Ne-20/Ne-22 and Ne-21/Ne-22 ratios display a g ood correlation, with the regression line close to (slightly to the right o f) the Loihi Seamount correlation. Extrapolation of the regression to solar Ne-20/Ne-22 of 13.8 gives a Ne-21/Ne-22 of 0.045 for the plume end-member, well below the mid-ocean ridge basalt (MORB) source (upper mantle) end-mem ber of 0.07. The measured Ar-40/Ar-36 ratios up to 2790 correlate very well with the Ne isotopic ratios, and the best estimate of the Ar-40/Ar-36 rati o of the plume source is within 5000 +/- 1000. Although the He-3/Ne-22 rati o in the plume source appears to be comparable to the solar value within a factor of 2, the Ne-22/Ar-36 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 He-4/Ar-40 * ratios are close to values expected for radiogenic production and accumul ation 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 carbonatite s, indicate a contribution of material with lower time-integrated (U + Th)/ (He-3, Ne-22) and (K-40/Ar-36) ratios than those in the asthenospheric uppe r mantle, the subcontinental Lithosphere, and the continental crust. The lo cation of such material is likely to be below the convective mantle supplyi ng MORE magmas, and reflects the contribution of a plume source material to Kola carbonatitic magmatism. These data support models which advocate a st ructure of the Earth heterogeneous in its refractory/volatile content. (C) 1998 Elsevier Science B.V. All rights reserved.