Sulphur Isotopes Geochemistry

Sulphur Isotopes

There are four stable isotopes of sulphur:

• ³²S: 95.02%
• ³³S: 0.75%
• ³⁴S: 4.21%
• ³⁶S: 0.02%

The ratio between the two most abundant isotopes, ³⁴S/³²S, is essential in geochemistry. It is expressed in parts per thousand relative to the troilite (FeS) reference standard from the Canyon Diablo iron meteorite (CDT) as follows:

34S/32S=( 34S/32S (sample) - 34S/32S (standard) ) / 
34S/32S (standard) x 1000

Sulphur isotope ratios are measured using SO₂ gas. The precision during mass spectrometry is approximately 0.02‰, with accuracy around 0.1‰. The in situ analysis of fine-grained sulphide intergrowths is now feasible using the ion microprobe (precision: 1.5 to 3.0‰) and the laser microprobe (precision: ± 1.0‰ for δ³⁴S).

The Distribution of Sulphur Isotopes in Nature

Naturally occurring sulphur-bearing species include native sulphur, sulphate and sulphide minerals, gaseous H₂S and SO₂, and a range of oxidized and reduced sulphur ions in solution. The isotopic compositions of major rock types are diverse, influenced by three isotopically distinct reservoirs:

• Mantle-derived sulphur: δ³⁴S values range from 0 ± 3‰.
• Seawater sulphur: δ³⁴S today is around +20‰, with historical variations.
• Strongly reduced (sedimentary) sulphur: Exhibits large negative δ³⁴S values.

The best estimate for the δ³⁴S composition of the primitive mantle relative to CDT is +0.5‰, slightly but significantly different from that of chondritic meteorites (0.2 ± 0.2‰). MORB values, indicative of depleted mantle, fall within δ³⁴S = +0.3 ± 0.5‰. Island-arc volcanic rocks exhibit a wider range of δ³⁴S (-0.2 to +20.7‰). Granitic rocks are highly variable (-10 to +15‰), compared to the average continental crust value of δ³⁴S = +7.0‰.

Sulphur Isotope Fractionation in Igneous Rocks

Sulphur isotope fractionation in igneous rocks can occur through two main mechanisms:

1. Crystal-melt fractionation: Fractionation between primary sulphide minerals and magma is minor, usually 1-3‰.
2. Solid-gas fractionation: Occurs during the degassing of SO₂ from lavas. This fractionation is controlled by the sulphate/sulphide ratio of the melt, which depends on temperature, pressure, water content, and oxygen activity. SO₂ outgassed by basic lavas is enriched in δ³⁴S relative to the melt.

Sulphur Isotope Fractionation in Sedimentary Rocks

The sedimentary sulphur cycle involves several fractionation processes:

1. Bacterial reduction of sulphate to sulphide: This is the principal low-temperature control (below 50°C) on sulphur isotope fractionation, producing sulphide depleted by up to 22‰ relative to the seawater source.
2. Bacterial oxidation of sulphide to sulphate: Produces minimal fractionation, except possibly in the Archaean.
3. Crystallization of sedimentary sulphate from seawater: During evaporite formation, results in a small δ³⁴S enrichment of 1.65 ± 0.12‰.
4. Non-bacterial reduction of sulphate to sulphide: Occurs at higher temperatures, potentially involving inorganic reduction with ferrous iron.

Sulphur Isotope Fractionation in Hydrothermal Systems

At very high temperatures (T > 400°C), the dominant sulphur species in hydrothermal systems are H₂S and SO₂. The isotopic composition of the fluid can be approximated by the following equation:

δ34Sfluid = δ34SH2S * XH2S + δ34SSO2 * XSO2

At lower temperatures (T < 350°C), sulphate and H₂S become the dominant species. The isotopic fractionation between these species depends on several physicochemical conditions, including oxygen activity, sulphur activity, pH, and the activity of cations associated with sulphate. These variables significantly influence the δ³⁴S values of minerals formed in hydrothermal systems.