The oxygen isotope geochemistry of komatiites; evidence for water-rock interaction

Abstract

Most known komatiites, including all of the Archaean examples, have been subjected to hydrothermal activity of one sort or another, as evidenced by the presence of secondary hydrous minerals such as chlorite, serpentine and tremolite. A combined oxygen isotopic and petrographic study of a single komatiite flow in Munro Township indicates that the degree of mineralogical alteration is inversely correlated with δ¹⁸O. All of the samples lie on a mixing line between fresh komatiites (δ¹⁸O = +5.7 ± 0.3) and serpentinite (δ¹⁸O ≃ +3.0). The fresh komatiite value is similar to that of Phanerozoic ultramafic rocks, and suggest that δ¹⁸ of the Earth's mantle has not changed appreciably through geologic history. Using a final metamorphic temperature of 300-330°C, the serpentinite value suggests that the hydrothermal fluid had δ¹⁸O values between -2 and +2. This fluid was very likely a metamorphic pore fluid that evolved from Archaean sea water during burial metamorphism of the greenstone belt. The water:rock rations during oxygen isotopic exchange appear to have been high (>1). Although some of this water-rock interaction may have taken place immediately upon extrusion of the komatiite into the ocean, most of the alteration was accomplished by sea water circulating deeply into the volcanic pile as the komatiites were buried and heated.

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