Possible sources for CH4-rich fluid inclusions in olivine from troctolitic rocks of the Bathtub intrusion, Duluth Complex, U.S.A.

Abstract

Cumulus olivine from the Babbitt deposit in the troctolitic Bathtub intrusion (Duluth Complex) contains abundant CH4-rich fluid inclusions with common hydrous silicate and carbonate precipitates at their inclusion walls. Given the active rifting environment in which magma of the Duluth Complex (1.1 Ga) has intruded as repeated pulses, fluid inclusions in olivine are traceable throughout an entire stratigraphic drill core section with comparable textural features that enable a classification of three generations: (1) primary CH4-rich fluid inclusions oriented parallel to growth planes of olivine or arranged in clusters; (2) intragranular fluid inclusion planes of CH4-rich fluid inclusions extending towards grain boundaries and (3) transgranular fluid inclusion planes of CH4-rich fluid inclusions, closely associated with serpentinization. The hydrous minerals and carbonates contained in fluid inclusions of generations (1) and (2) indicate that original constituents of the early homogenous magmatic fluid must have been H2O and CO2. Upon cooling (below 700 °C), at fixed Pfluid (2 kbar) and fO2fluid at or below QFM conditions, an increase of XCH4 at the expense of CO2 results in the CH4-rich chemistry of the fluid. The formation of late secondary fluid inclusion planes of generation (3) is related to extensive magma-country rock interaction. These inclusions contain no solids and their composition reflects the reduction of CO2 to CH4 during serpentinization in an upper stability limit of 620 °C. This study emphasizes fluid inclusions as powerful indicators in remodeling chemical changes of the magmatic fluid during crystallization of cumulus phases, magma-country rock interaction and subsequent alteration.