Oxygen Isotope Systematics of Quartz—Magnetite Pairs from Ptecambrian Iron Formations: Evidence for Fluid—Rock Interaction during Diagenesis and Metamorphism

Abstract

The purpose of this paper is to review mineral-pair systematics in light of the types of isotopic effects that can be expected to be recorded by mineral pairs during prograde and retrograde metamorphic events. The oxygen isotopic ratios of minerals in the crust are sensitive to changes in temperature and changes in the isotopic composition of the bulk rock brought about by phase changes induced during metamorphism. In crustal environments, solid-state diffusion rates are slow enough that the presence of a fluid phase is required to promote isotopic exchange on geologic time scales. In rock-buffered systems, mineral pairs will essentially behave as isotopic closed systems, because the fluid composition will be strongly dependent upon local isotopic composition of the rocks. The degree to which fluid—rock interaction is important in a particular metamorphic terrane will be recorded in the stable isotopic systematics first by a failure to satisfy conservation of mass constraints on mineral isotopic composition imposed by protolith bulk composition, and second by the degree and scale over which the isotopic compositions of the minerals are homogenized. Mineral-pair behavior that deviates from closed-system predictions results from the differing exchange rates between minerals and fluids (Gregory and Taylor, in press a, in press b), through the infiltration of fluids whose isotopic composition is externally controlled (Taylor et al., 1963; Rumble et al., 1982; Graham et al., 1983), or combinations of both.KeywordsOxygen IsotopeIron FormationMetamorphic EventExchange TrajectoryMineral PairThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.