Controls on sulphur speciation in volcanic and deep magmatic systems

In majority of volcanic and deep magmatic systems, sulphur occurs as a trace element and is present as reduced ‘sulphide’ or oxidized ‘sulphate’ components. Despite its very low concentration in natural rocks, the presence of sulphur has a strong influence on many economic geologic processes, such as the formation of Cu- and Au-ore deposits. Moreover, the variation in sulphur dioxide content in volcanic gas emissions is critical for our biosphere. SO2 emissions are carefully monitored by a global “volcano hazard” program supported by many international institutions, since a sudden release of this gas in the stratosphere can have a global effect on the Earth’s climate system.

Either for deep ore-forming or subsurface and volcanic processes, the speciation of sulphur is considered to be an important aspect to constrain. It is believed that reduced, sulphide-saturated systems are not economically ‘fertile’ in terms of metal enrichment and the formation of hypogene ore deposits. The same is true for volcanic eruptions, where erupted materials from sulphide-saturated magma are often considered not to be hazardous due to the low solubility of S2- in the melt. On the other hand, recent literature and observations of natural samples suggest that besides sulphur speciation, there are many other parameters involved.

In my work I am using a new experimental approach to study the evolution of S in deep magmatic in volcanic environments. I will demonstrate that apart from redox conditions, the S-speciation is a strong function of depth (pressure) and is highly sensitive to the surrounding fluid environment. Examples from the Tambora 1815 and Pinatubo 1991 eruptions as well as modern ore deposits in Chile will be used to demonstrate possible scenarios of S-evolution in modern arc environments.