Geochemical Modeling Insights into the Formation of Black Smoker Fluids

Hydrothermal circulation at mid-ocean ridges represents one of the largest points of exchange of energy and chemistry between Earth’s surface and interior. In basalt-hosted systems, black smoker chimneys vent metal-rich fluids at up to ~400°C that fuel unique ecosystems and produce massive sulfide deposits. Recharging seawater reacts with the surrounding basalt at increasing pressure and temperature, drastically changing fluid chemistry. Yet, the extent and shape of hydrothermal recharge pathways remain poorly constrained.

Here, we present a series of geochemical models, investigating these processes through equilibrium thermodynamics: In a system of 16 elements (Si, Ti, Al, Fe, Mg, Cu, Pb, Zn, Ca, Na, K, S, C, Cl, H, O), we test a broad range of hydrothermal recharge pathways with various pressure and temperature profiles and fluid/rock ratios. Multi-pass sequential reactor chain models are set up using xgems (https://github.com/gemshub/xgems), the Python package derived from GEMS [1], and the MINES thermodynamic database [2]. Simplified recharge pathways are varied in circulation depth (1–5 km below seafloor), peak temperature (370–430°C) and integrated fluid/rock ratio. Using fluids derived from these models, a second set of models is run, reproducing the basalt alteration patterns observed in rocks below the TAG hydrothermal field. Based on comparison to measured TAG vent fluids, these models offer three main conclusions:

• For significant metal leaching matching black smoker fluids, peak temperatures of hydrothermal circulation need to exceed 400°C.
• Relatively shallow circulation (< 3 km bsf), and thus shallow heat sources, favorably result in fluid compositions matching black smokers.
• Black smoker fluids only result from rock-buffered reactions. This implies that recharge pathways must contain a significant fraction of fresh basalt throughout the lifetime of a hydrothermal system.

 

References

[1] Kulik, D. A., Wagner, T., Dmytrieva, S. V., Kosakowski, G., Hingerl, F. F., Chudnenko, K. V., & Berner, U. R. (2013). GEM-Selektor geochemical modeling package: revised algorithm and GEMS3K numerical kernel for coupled simulation codes. Computational Geosciences. https://doi.org/10.1007/s10596-012-9310-6

[2] Gysi, A. P., Hurtig, N. C., Pan, R., Miron, D. G., & Kulik, D. A. (2023). MINES thermodynamic database. New Mexico Bureau of Geology and Mineral Resources, Version 23. https://doi.org/10.58799/mines-tdb