SO₂ emissions from basaltic eruptions, and the excess sulfur issue

Volcanic SO 2 can affect the Earth's environment. Where no direct measurements of SO 2 in the atmosphere are available, a petrologic method of assessing sulfur release from the magma must be used. However, in studies of arc‐derived eruptions, satellite‐based measurements of SO 2 emissions using Total Ozone Mapping Spectrometer (TOMS) data are orders of magnitude greater than those calculated petrologically, implying that a separate S‐rich gas phase in the magma chamber may be responsible for the excess sulfur. We test whether this applies in other settings. For Icelandic and Hawaiian basalts we find that petrologic SO 2 values are comparable to measurements of SO 2 by TOMS. Thus, for non‐arc basalts, the petrologic method gives reliable estimates of SO 2 released. The implied absence of excess sulfur in non‐arc basaltic magmas is a reflection of source magma conditions, notably lower f O 2 and volatile contents than arc magmas, inhibiting the exsolution of a S‐rich gas prior to eruption.