A PRELIMINARY-STUDY OF METHANE INCLUSIONS IN ALKALINE IGNEOUS ROCKS OF THE KOLA IGNEOUS PROVINCE, RUSSIA - IMPLICATIONS FOR THE ORIGIN OF METHANE IN IGNEOUS ROCKS

Fluid inclusions from the Kovdor, Khibina and Lovozero complexes of th e Kola alkaline igneous province have been investigated using microthe rmometric techniques and PVTX modelling. CH4 inclusions, found in all three complexes, are always secondary in origin and frequently occur i n curvilinear arrays associated with secondary H2O-dominant inclusions and, in Khibina, CH4-H2O inclusions. The secondary nature of the CI-L i-bearing inclusions appears to preclude a direct magmatic origin for the methane, which is supported by preliminary C-isotope data which in dicate an abiogenic origin. Isochore projections for primary CO2- and H2O-dominant inclusions intersect the volatile-saturated, agpaitic, ne pheline syenite solidus (at 700 degrees C), at pressures between 3.5 a nd 5.5 kbar, compatible with magmatic exsolution of these fluids at cr ustal depths of between 11-18 km. By contrast, fluid-phase equilibria and the shallow slopes of isochores derived from the PVTX modeling of CH4-dominant and related inclusions are indicative of trapping conditi ons at pressures of 0.5 to 1.8 kbar. The close association of CH4 incl usions with magnetite and late-stage hydrated phases suggests evolutio n of CH4 during hydration within a reducing environment. This evolutio n involved subsolidus Fischer-Tropsch reactions of the type CO2 + 4H(2 ) --> CH4 + 2H(2)O, with the water-produced driving hydration reaction s which generated magnetite and more H-2. Methane production is thus a function of vapour-mineral reactions in the C-H-O system operative du ring late-stage hydrothermal processes and, in particular, is due to t wo self-reinforcing reaction types linked through a positive feedback mechanism.