Hot groundwaters that circulate within the Earth’s crust. Involved in formation of ore deposits, chemical alteration of rocks and sediments, and the origin of hot springs and geothermal fields. Hydrothermal fluids are highly reactive - inspired some of the earliest geochemical modeling studies. Hydrothermal Fluids

Hot springs discovered by the submarine ALVIN in 1977 along the Galapogos spreading center. Later expeditions to the East Pacific Rise and Juan de Fuca spreading center found more springs, some discharging fluids as hot as 350°C. Seawater descends into oceanic crust, circulates near magma bodies where it warms and reacts with deep rocks, then discharges back into ocean. Fluid mixes with seawater, cools quickly, and precipitates clouds of fine-grained minerals. Case Study: Seafloor Hot Springs Near the East Pacific Rise

Task 1 — Black Smokers Mix reducing hydrothermal fluid with oxygenated cold seawater. What minerals form in our model? Which were observed near the vents?

Seawater reacted (kg) Volume (cm 3 ) Talc Pyrite Anhydrite Amorphous silica Temperature (°C)

“Smoke” in plume Pyrrhotite, Fe 1-x SAnhydrite, CaSO 4 Sphalerite, ZnSBarite (trace), BaSO 4 Pyrite, FeS 2 Cubanite (trace), CuFe 2 S 3 Unidentified Fe-S-Si phasesWurtzite (trace), ZnS Chalcopyrite, CuFeS 2 Covellite (trace), CuS Amorphous silica, SiO 2 Marcasite (trace), FeS 2 Elemental sulfur, SUnidentified silicates, aluminosilicates Goethite, FeOOH Particles dispersed in local seawater Anhydrite, CaSO 4 Sphalerite, ZnS Pyrite, FeS 2 Elemental sulfur, S Gypsum, CaSO 4. 2H 2 OPyrrhotite, Fe 1-x S Chalcopyrite, CuFeS 2

Seawater reacted (kg) Species concentration (mmolal) H 2 S(aq) HSO 4 - NaSO e–70 1e–60 1e–50 1e–40 1e–30 O 2 (g) fugacity SO 4 2−

Task 2 — Energy Available to Thermophiles Build off of previous example. How does available energy from environment change as fluids mix?

Temperature (°C) mmolal μmolal CH 4 (aq) CH 3 COO − H 2 S(aq) SO 4 −− HCO 3 − Seawater mixing H 2 (aq) O 2 (aq) oxic anoxic

Temperature (°C) Redox Potential (mV) CH 3 COO − – CH 4 HCO 3 − – CH 3 COO − HCO 3 − – CH 4 SO 4 −− – H 2 S O 2 – H 2 O H + – H 2 Seawater mixing biotic

MetabolismDonating couple Accepting couple Limiting reactant Anaerobic (T ≥ 38°C) H 2 -trophic sulfate reductionH 2 – H + SO 4 −− – H 2 SH 2 (aq) H 2 -trophic acetogenesisH 2 – H + CO 2 - CH 3 COO − H 2 (aq) H 2 -trophic methanogenesisH 2 – H + CO 2 – CH 4 H 2 (aq) Ac-trophic sulfate reductionCH 3 COO − – CO 2 SO 4 −− – H 2 SCH 3 COO − Ac-clastic methanogenesisCH 3 COO − – CO 2 CH 3 COO − – CH 4 CH 3 COO − Aerobic (T ≤ 38°C) Sulfide oxidationH 2 S - SO 4 −− O 2 -H 2 OO 2 or H 2 S MethanotrophyCH 4 – HCO 3 − O 2 -H 2 OCH 4 (aq) AcetotrophyCH 3 COO − – HCO 3 − O 2 -H 2 OCH 3 COO −

Thermodynamic Drive,  G r  G r = free energy change, J mol  (or V C mol  ) n = number of electrons transferred F = Faraday constant, 96,485 C mol 

Craig M. Bethke and Brian Farrell © Copyright 2016 Aqueous Solutions LLC. This document may be reproduced and modified freely to support any licensed use of The Geochemist’s Workbench® software, provided that any derived materials acknowledge original authorship.