Partitioning of SO2 between dacitic melt and aqueous fluid phases

Hobbs, L.K.; Keppler, H.

Abstract

Sulphur dioxide has significant effects on global climate. By combining with atmospheric water molecules to form H2SO4 aerosols, which strongly backscatter sunlight, it can cause a potentially global net reduction in temperature following a large explosive eruption. Previous research indicates that SO2 emissions are far higher than predicted from the mass of magma erupted (Wallace and Gerlach, 1994). This suggests that “extra” sulphur might be extracted from the entire magma reservoir by hydrous fluid at the top of the chamber, which is then erupted. Partition coefficients from studies on haplogranitic melt indicate that this is the case (Keppler, 1999).

The eruptive products of most extremely large explosive eruptions (e.g. Toba, 74 ka; Taupo, 27 ka) are rhyolitic in composition, however more moderately large eruptions can be more mafic and are therefore likely to have a higher sulphur content. Two such events in the last 30 years were the dacitic Mount St. Helens (1980) and Pinatubo (1991) eruptions. Table 1 shows that the mass of SO2 emitted by these events in comparison to the Toba 74 ka eruption could be as much as an order of magnitude higher than expected by comparison of erupted volume alone.

Eruption
and composition Erupted tephra volume (km3) Size relative
to Toba 74 ka SO2 emissions
(megatons) SO2 emissions relative to Toba 74 ka
Toba 74 ka (rhyolite)
6325
1.00
460 - 2300
1.00
Helens 1980 (dacite)
1.3
2.06 x 10-4
2 4.35 x 10-3 to
8.7 x 10-4
Pinatuno 1991 (dacite)
13
2.06 x 10-3
17 3.7 x 10 2 to
7.39 x 10-3
Table 1. Erupted tephra volume and SO2 mass of selected eruptions.

Glasses were synthesised according to published compositonal data for these eruptions, and subjected to partitioning experiments in TZM rapid-quench autoclaves using H2SO4 of varying concentration as the source of sulphur, and Re-ReO2 and magnetite-haematite external buffers to control oxygen fugacity. Gold was used as the capsule material as it is both inert with respect to sulphur and sufficiently permeable for hydrogen to allow transfer between inner and outer capsules to control the redox conditions. Each experiment was run for 7 days at 950oC and 1000 bar pressure. Electron microprobe analysis of run products for sulphur content allowed a partition coefficient (D) between the melt and fluid to be calculated by mass balance.

The Re-ReO2 buffer experiments produced a result of D = 81.9 ± 40, with D = 35.7 ± 8.5 for the MH buffer. The more oxidising conditions of the MH buffer allow anhydrite to form in the glass, causing less sulphur to partition into the fluid. As the conditions of the Re-ReO2 buffer are closest to the oxygen fugacity of the pre-eruptive magma of both eruptions, D = 81.9 is the most plausible of the two results. This suggests that sulphur is extracted from dacitic melt by aqueous fluid in the magma chamber, explaining the “excess” sulphur erupted relative to magma mass.

References
Keppler, H. (1999): Experimental evidence for the source of excess sulfur in explosive volcanic eruptions. Science, 284, 1652-1654
Wallace, P.J., Gerlach, T.M. (1994): Magmatic vapor source for sulfur dioxide released during volcanic eruptions: Evidence from Mount Pinatubo. Science, 265, 497-499.

Citation

Hobbs, L., & Keppler, H. (2009, June). Partitioning of SO2 between dacitic melt and aqueous fluid phases

• Presentation Conference Type: Poster
• Conference Location: Bayreuth, Germany
• Start Date: Jun 17, 2009
• End Date: Jun 19, 2009
• Acceptance Date: May 11, 2009
• Peer Reviewed: Not Peer Reviewed
• Keywords: partitioning, SO2, dacitic melt, aqueous fluid phases
• Public URL: https://uwe-repository.worktribe.com/output/996175
• Additional Information: Title of Conference or Conference Proceedings : GRC-BGI Workshop on Deep Earth Mineralogy