Magma mixing as a source for Pinatubo sulphur

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Abstract

On 15 June 1991, a huge plinian eruption at Mount Pinatubo discharged 3.7–5.3 km3of pyroclastic material1, along with a minimum of 17 megatonnes of SO2 gas2,3. This represents the largest stratospheric SO2 cloud ever measured, and the SO2 generated in this eruption is believed to have had a significant effect on global climate2,4 and the ozone layer4 for several years after the event. The source for this massive amount of SO2 aerosols remains controversial. Here I present thermodynamic arguments which suggest that the source of the SO2, along with the trigger for the eruption itself, can be attributed to redox reactions accompanying the injection of a reduced sulphide-saturated basaltic magma into an oxidized sulphate-saturated dacitic melt. The proposed mixing event would drive most sulphur out of both dacitic and basaltic liquids and would drive both anhydrite and iron-rich sulphide liquid outside their stability field, thus purging sulphur from all major non-volatile sulphur-bearing phases in the mixed volume. Similar eruptions are possible any time that an oxidized sulphate-saturated magma interacts with a reduced sulphide-saturated magma, and this mechanism may therefore be relevant to recent volcanic activity at Popocatépetl Volcano in Mexico.

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Figure 1: Schematic diagram of the solubility of sulphur in silicate magma, where Xssatis the mole fraction of S at vapour saturation, as a function of oxygen fugacity in ΔFMQ (see text).
Figure 2: Relative contributions of the main sulphur-bearing phases to the >17 Mt total SO2 discharged in the 15 June Pinatubo eruption, based on assumptions given in the text.

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Acknowledgements

I thank M. R. Carroll, B. W. Evans, M. S. Ghiorso, K. Hattori, N. Metrich, B. O. Mysen, B. K. Nelson, C. G. Newhall, J. S. Pallister, D. A. Snyder and P. J. Wallace for criticisms, suggestions, information and reviews.

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Correspondence to Victor Kress.

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Kress, V. Magma mixing as a source for Pinatubo sulphur. Nature 389, 591–593 (1997) doi:10.1038/39299

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