Sulfur Yield of the 1600 Eruption of Huaynaputina Determined by Apatite Compositions

Aerosols from the VEI 6 eruption of Huaynaputina in 1600 have been implicated as a major factor in producing the coldest summers in the Northern Hemisphere in the last 500 years. We have previously estimated the stratospheric sulfur (S) input to range from 16 to 32 Mt based on ice core data, while others have estimated S in the magma to have been ~2 to 4 Mt. Another approach has suggested a fluid phase with ~24 to 51 Mt of S in equilibrium with magma. Here we report on S contents measured in apatite grains in the pumice from the plinian eruption. We invert these results to obtain independent estimates of the magmatic S contents and reassess the magnitude and source of the atmospheric loading. Apatite occurs as distinct microphenocrysts or as inclusions in biotite and amphibole ranging in size up to 200 microns. Most of the examples we analyzed exhibited basal sections. Microprobe analyses yielded apatite SO3 concentrations of 0.09 to 0.17 wt % with no systematic difference between microphenocrysts and inclusions. Multiple analyses of single grains demonstrated homogeneity. Residual glass contains SO3 below detection limits, <100 ppm, which suggests it was almost completely degassed. Other phase equilibria yield magmatic conditions of 840±21 °C and log fO2 of ~-11.5 (considerably less oxidized than recent anhydrite bearing dacites). Using these data, we calculate a total magmatic S content of ~4.1 Mt for the 9 km3 of magma erupted during the plinian eruption. Subtracting a conservative estimate of ~100 ppm S remaining dissolved in the melt, the total erupted sulfur is ~2.1 Mt. This value is almost identical to the range of erupted S calculated from experimental equilibria (~2 to 4 Mt) and supports the idea that much of the stratospheric sulfur input from Huaynaputina was from a fluid phase in equilibrium with the melt. Our work also demonstrates that weakly oxidized dacitic magmas that never stabilized anhydrite can have climatically significant atmospheric loading.