Abstract

Fluid inclusions from the Kovdor, Khibina and Lovozero complexes of the Kola alkaline igneous province have been investigated using microthermometric techniques and PVTX modelling. CH 4 inclusions, found in all three complexes, are always secondary in origin and frequently occur in curvilinear arrays associated with secondary H 2 O-dominant inclusions and, in Khibina, CH 4 -H 2 O inclusions. The secondary nature of the CH 4 -bearing inclusions appears to preclude a direct magmatic origin for the methane, which is supported by preliminary C-isotope data which indicate an abiogenic origin. Isochore projections for primary CO 2 - and H 2 O-dominant inclusions intersect the volatile-saturated, agpaitic, nepheline syenite solidus (at 700 degrees C), at pressures between 3.5 and 5.5 kbar, compatible with magmatic exsolution of these fluids at crustal depths of between 11-18 km. By contrast, fluid-phase equilibria and the shallow slopes of isochores derived from the PVTX modelling of CH 4 -dominant and related inclusions are indicative of trapping conditions at pressures of 0.5 to 1.8 kbar. The close association of CH 4 inclusions with magnetite and late-stage hydrated phases suggests evolution of CH 4 during hydration within a reducing environment. This evolution involved subsolidus Fischer-Tropsch reactions of the type CO 2 +4H 2 -->CH 4 +2H 2 O, with the water-produced driving hydration reactions which generated magnetite and more H 2 . Methane production is thus a function of vapour-mineral reactions in the C-H-O system operative during late-stage hydrothermal processes and, in particular, is due to two self-reinforcing reaction types linked through a positive feedback mechanism.

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