THE APPLICATION OF STABLE Fe ISOTOPES TO MAGMATIC SULFIDE SYSTEMS: CONSTRAINTS ON THE Fe ISOTOPE COMPOSITION OF MAGMATIC PYRRHOTITE

We present the first systematic characterization of the Fe isotope composition of magmatic pyrrhotite for the application of Fe isotopes to the origin and evolution of magmatic sulfide deposits. Iron isotopes provide constraints on redox, temperature, fluid exsolution, fractional crystallization, and intermineral diffusion at magmatic or subsolidus conditions. Paired with S isotopes, Fe isotopes have proven to be useful tracers of crustal contamination in bulk magmatic sulfide. Relative and specific Fe isotope values exist for major Fe-bearing igneous minerals; however, there is a paucity of well-constrained Fe isotope data for sulfides. Recent studies indicate that pyrrhotite will strongly influence the Fe isotope systematics of magmatic systems, yet there are few published delta Fe-56 values (delta Fe-56/Fe-54 in the sample relative to IRMM-14) of natural pyrrhotite. To provide this fundamental information, we report the first dataset for magmatic pyrrhotite from nine deposits of various origins and ages. The delta Fe-56 value of pyrrhotite samples (n = 17) ranges from -0.55 +/- 0.04 parts per thousand to +0.05 +/- 0.03 parts per thousand, and reflects the composition of the sulfide, variable degrees of assimilation, and crystallization history of each deposit. Only the impact-related Sudbury deposit shows especially light delta Fe-56 values for pyrrhotite (-0.89 +/- 0.04 parts per thousand; (-0.62 +/- 0.04 parts per thousand; n = 2). A modeling approach using these new data confirms the potentially strong influence of pyrrhotite on the Fe isotope systematics of a magmatic system. A global dataset of pure individual sulfide minerals will aid in more accurately tracing the processes at play in the formation and evolution of deposits containing magmatic sulfide.