March 31, 2021

Team Profile:
Sulfur in Supercritical Conditions

Terry Seward

photo credit:
Terry Seward

Despite the importance of sulfur in New Zealand’s high temperature-high pressure Earth systems, there are major inadequacies in our knowledge of sulfur chemistry in hydrothermal solutions, especially at supercritical conditions, where fluids have a wide range of densities, from gas-like to liquid-like.

Sulfur occurs ubiquitously throughout the Earth’s crust and plays a fundamental role in the ongoing chemical evolution of the Earth’s crust. It is intimately involved in metal transport and precipitation reactions by hydrothermal solutions, and associated with element fractionation and sequestration during magmatism, volcanism and metamorphism. In addition, deep sulfur is transferred to the Earth’s hydrosphere, biosphere and atmosphere via seafloor and subaerial geothermal systems and active volcanism.

At high temperatures and pressures in the Earth’s crust, the chemistry of sulfur is generally considered to comprise sulfur dioxide (SO2) and hydrogen sulfide (H2S). However, aqueous sulfur chemistry at temperatures greater than 250-300°C is more complicated, because of the presence of the thiozonide species (S3-), which has been largely ignored by Earth scientists.

Intriguingly, the only “geological” evidence for the thiozonide species in the deep crust is manifested by the gem stone, lapis lazuli (lazurite). The intense blue colour of this gem stone is due to the thiozonide ion chromophore - a chemical group that absorbs light at a specific frequency and imparts colour to a molecule. The thiozonide ion chromophore is trapped in the lazurite structure (in the sodalite cage) during formation at supercritical conditions in the Earth's crust.

Lapis Lazuli,Badakhshan, Afghanistan (5 x 9 cm) with pyrite (brassy coloured).

Importantly, the thiozonide species is not stable in water at lower temperatures (i.e. t ≤250°C) and is thus undetectable in geothermal discharge fluids at ambient temperature. There are currently no reliable thermodynamic data permitting a rigorous evaluation of thiozonide’s importance in high temperature-high pressure systems in the Earth’s crust.

A key question of the GNG project is: under what conditions in the Earth’s crust does the thiozonide species predominate and hence, what role does it play in the reactivity of supercritical fluids with host rocks and in the precipitation of sulfate minerals, such as anhydrite (CaSO4), in deep geothermal reservoir environments?

Such mineral precipitation is of fundamental importance in reservoir permeability and scaling in energy producing geothermal systems.

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