MULTIPLE SULFUR ISOTOPE CONSTRAINTS ON MASS TRANSFER PROCESSES DURING PYRITE PRECIPITATION AND RECRYSTALLIZATION: AN EXPERIMENTAL STUDY AT 300 AND 350 °C

Conventional ³⁴S/³²S isotope equilibrium fractionation factors have commonly been used to interpret pyrite-H₂S/SO₄^2- sulfur isotope systematics at mid-ocean ridge (MOR) hydrothermal settings. However, currently available fractionation factors may not be accurate as they are empirically determined [1]. Here we use the multiple S isotope system (³²S, ³³S, ³⁴S) to calibrate the pyrite – H₂S/SO₄^{2- 34}S/³²S equilibrium fractionation factors, while also providing the first experimental data on multiple S isotope systematics and the rate of exchange between pyrite – fluid S-reservoirs during pyrite precipitation and recrystallization. Two different experimental techniques were used: (1) multiple S isotope partial exchange experiments conducted at 350 °C and 500 bars to reexamine the equilibrium ³⁴S/³²S fractionation between pyrite and H₂S; and, (2) pyrite precipitation experiments conducted at 300 and 350 °C, 500 bars using flexible gold cell hydrothermal equipment.

The partial exchange experiments indicate an equilibrium ³⁴S/³²S fractionation between pyrite and H₂S, ln³⁴α_Pyrite/H2S, at 350 °C of -2.3 ‰, which is markedly different in magnitude and sign from past equilibrium estimates of ~ +1 ‰ [1]. S isotope data from the pyrite precipitation experiments indicate that pyrite and the aqueous S-species are in gross isotope disequilibrium upon rapid nucleation of pyrite from solution. This disequilibrium is likely attributed to kinetic isotope effects upon precipitation coupled with sluggish exchange rates between pyrite and the fluid S reservoirs. With increasing extents of pyrite recrystallization at 350 °C, the ³⁴S/³²S fractionation between pyrite and H₂S approaches the equilibrium value determined by the exchange experiments. Concurrently, Δ³³S data of pyrite, H₂S, and SO₄^2- converge, indicating that the S isotope system is approaching equilibrium. Comparison of the new experimental data with data from natural hydrothermal systems [2] suggests that pyrite δ³⁴S is closer to equilibrium with vent fluid H₂S than previously thought. However, disequilibrium effects induced by rapid precipitation may persist because the exchange rate between pyrite and the fluid S reservoirs is much slower compared to the fluid residence time in MOR vent systems.

[1] Ohmoto and Rye, 1979 (RMG)

[2] Shanks, 2001 (RMG)