Ar and N Isotopic Composition and Elemental Ratios of Ar, N, He, and C in Fluid Inclusions in Ultramafic Rocks of the Guli Massif (Polar Siberia)

Abstract

The first data on the isotopic composition and elemental ratios of nitrogen, carbon, and noble gases in samples from the early stages of the formation of the Guli Complex (Maimecha-Kotui igneous province, Polar Siberia), obtained using the method of stepwise crushing, are reported. The 40Ar/36Ar ratios in the crushing steps vary from the values close to atmospheric (~296) in the meimechite sample to 5000–6000 in pyroxenite and melilitolite; the latter correspond to the estimate for the mantle source of carbonatite from the Guli massif, according to the Ne–Ar systematics. The bulk nitrogen isotopic composition (δ15N) in the studied rocks varies from +3.7 to –5.9‰, showing a regular increase in δ15N with increasing nitrogen concentration; the range of δ15N variations in crushing steps is even wider: from +6.7‰ in pyroxenite to –15.4‰ in meimechite. The data obtained for the ultrabasic rocks of the Guli Massif suggest a significantly lower contribution of the subducted nitrogen of organic origin in comparison with the alkaline–ultramafic rocks of the Kola alkaline province and associated carbonatite, as well as with the Guli carbonatite and some Indian alkaline–ultramafic complexes with carbonatite. The N/36Ar ratios in the trapped mantle component of the ultramafic rocks are 4–8 times lower than that in the associated carbonatite. This, most likely, indicates different fluid sources, or the preferred escape of nitrogen from the fluid phase at the early stages of the formation of the Guli Massif.