CONDUIT CONVECTION VS. DEEP DEGASSING AT OPEN VENT VOLCANOES: MELT INCLUSION EVIDENCE FROM POPOCATEPETL VOLCANO, MEXICO

An unsolved problem in understanding volcano degassing and eruptive behavior is the so-called open-vent degassing that characterizes many volcanoes such as Etna and Stromboli in Italy. This style of activity involves long-term quiescent (non-eruptive) release of large masses of gas punctuated by much shorter episodes of eruptive activity. Open-vent degassing behavior requires large masses of magma to be degassed at depth, but where and how gas is separated from magma is poorly understoood. Since late 1994, open-vent activity at Popocatépetl volcano, Mexico, has released large amounts of CO2 and SO2. Tephra and lava produced by this activity shows evidence for mixing of mafic and silicic magmas shortly before eruption. Analysis of H2O and CO2 in olivine-hosted melt inclusions indicate trapping pressures as high as ~400 MPa (~17 km depth) beneath the volcano. We combine our data with thermodynamic models to show that degassing of mafic magma at ~150 to 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 fluxes of CO2 and SO2 from 1995-1997. 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 fluxes at some other open-vent volcanoes rather than convection of magma in the uppermost parts of subvolcanic conduits.