The effects of metamorphism on O and Fe isotope compositions in the Biwabik Iron Formation, northern Minnesota

The Biwabik Iron Formation of Minnesota (1.9 Ga) underwent contact metamorphism by intrusion of the Duluth Complex (1.1 Ga). Apparent quartz-magnetite oxygen isotope temperatures decrease from similar to 700 degrees C at the contact to similar to 375 degrees C at 2.6 km distance (normal to the contact in 3D). Metamorphic pigeonite at the contact, however, indicates that peak temperatures were greater than 825 degrees C. The apparent O isotope temperatures, therefore, reflect cooling, and not peak metamorphic conditions. Magnetite was reset in delta(18) O as a function of grain size, indicating that isotopic exchange was controlled by diffusion of oxygen in magnetite for samples from above the grunerite isograd. Apparent quartz-magnetite O isotope temperatures are similar to calculated closure temperatures for oxygen diffusion in magnetite at a cooling rate of similar to 5.6 degrees C/kyr, which suggests that the Biwabik Iron Formation cooled from similar to 825 to 400 degrees C in similar to 75 kyr at the contact with the Duluth Complex. Isotopic exchange during metamorphism also occurred for Fe, where magnetite-Fe silicate fractionations decrease with increasing metamorphic grade. Correlations between quartz-magnetite O isotope fractionations and magnetite-iron silicate Fe isotope fractionations suggest that both reflect cooling, where the closure temperature for Fe was higher than for O. The net effect of metamorphism on delta O-18-delta Fe-56 variations in magnetite is a strong increase in delta O-18(Mt) and a mild decrease in delta Fe-56 with increasing metamorphic grade, relative to the isotopic compositions that are expected at the low temperatures of initial magnetite formation. If metamorphism of Iron Formations occurs in a closed system, bulk O and Fe isotope compositions may be preserved, although re-equilibration among the minerals may occur for both O and Fe isotopes.