MAGMA MIXING AS A SOURCE FOR PINATUBO SULFUR

On 15 Tune 1991, a huge plinian eruption at Mount Pinatubo discharged 3.7-5.3 km(3) of pyroclastic material(1), along with a minimum of 17 m egatonnes of SO2 gas(2,3). This represents the largest stratospheric S O2 cloud ever measured, and the SO2 generated in this eruption is beli eved to have had a significant effect on global climate(2,4) and the o zone layer(4) for several years after the event. The source for this m assive amount of SO2 aerosols remains controversial. Here I present th ermodynamic arguments which suggest that the source of the SO2, along with the trigger for the eruption itself, can be attributed to redox r eactions accompanying the injection of a reduced sulphide-saturated ba saltic magma into an oxidized sulphate-saturated dacitic melt. The pro posed mixing event would drive most sulphur out of both dacitic and ba saltic liquids and would drive both anhydrite and iron-rich sulphide l iquid outside their stability field, thus purging sulphur from all maj or non-volatile sulphur-bearing phases in the mixed volume. Similar er uptions are possible any time that an oxidized sulphate-saturated magm a interacts with a reduced sulphide-saturated magma, and this mechanis m may therefore be relevant to recent volcanic activity at Popocatepet l Volcano in Mexico.