Mafic magma recharge supplies high CO₂ and So₂ gas fluxes from Popocatépetl volcano, Mexico

Since late 1994, open-vent eruptive activity and degassing at Popocatépetl volcano, Mexico, have released large masses of CO₂ and SO₂. Tephra and lava produced by these eruptions show evidence for mixing of mafic and silicic magmas shortly before eruption. We present the first measurements of dissolved CO₂ in the mafic magma end member based on analyses of olivine-hosted melt inclusions that were trapped at pressures as high as ∼400 MPa (∼15 km depth) beneath the volcano. We combine our data with thermodynamic models to show that degassing of mafic magma at ∼150-350 MPa pressure can explain the CO₂/SO₂ mass ratios (1-8) of volcanic gases released from the volcano during 1995-1997. Our results demonstrate that mafic magma recharge was responsible for the high measured fiuxes of CO₂ and SO₂ from 1995 to 1997. The total SO₂ emission of 9 Mt during this period requires intrusion and degassing of a minimum of 0.8 km³ of mafic magma. Only ∼0.3% of this new mafic magma has been erupted in the form of mixed (hybrid) lava and tephra. Our results suggest that the ongoing eruption of Popocatépetl is essentially an intrusive event. More generally, we suggest that intrusion and deep degassing may explain the high gas fiuxes at some other open-vent volcanoes rather than convection of magma in the uppermost parts of subvolcanic conduits.