Ar and N Isotopic Composition and Elemental Ratios of Ar, N, He, and C in Fluid Inclusions in Ultramafic Rocks of the Gu
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nd N Isotopic Composition and Elemental Ratios of Ar, N, He, and C in Fluid Inclusions in Ultramafic Rocks of the Guli Massif (Polar Siberia) A. I. Buikina, *, A. B. Verchovskyb, A. M. Asavina, and L. N. Kogarkoa aVernadsky
Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia b The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom *e-mail: [email protected] Received April 3, 2020; revised April 28, 2020; accepted May 1, 2020
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. Keywords: nitrogen and argon isotopes, helium, carbon dioxide, fluid inclusions, stepwise crushing, Guli Massif DOI: 10.1134/S0016702920110038
INTRODUCTION The Guli massif is the world’s largest alkaline–carbonatite ultramafic intrusive–volcanic complex and is located in the area between the Maimecha and Kotui rivers on the northern border of the Siberian Platform with the Khatanga Paleorift (Allen et al., 2006). Similarly to many other alkaline–ultramafic intrusions, the Guli Massif is a complex multistage pluton (Egorov, 1991). The complex multiphase evolution of the massif has started with the formation of ultramafic rocks (dunite and kosvite), continued with the formation of rocks of the melilitic series, melteigite, jacupirangite–melteigite, passing to the rocks of the ijolitic and, then, syenitic series, and finally ended up with the formation of a number of veins and stocks of rocks of the phoscorite and carbonatite groups (Egorov, 1991). The presence of the phlogopite ore manifestations and the wide abundance of phlogopi
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