H2O and CO2 evolution in the Bandelier Tuff sequence reveals multiple and discrete magma replenishments

The sequence of eruptions in the Bandelier magmatic system provides an opportunity to study volatile evolution through different stages of a large silicic magma chamber. The Lower Bandelier Tuff (LBT) and Upper Bandelier Tuff (UBT) eruptions offer a snapshot of a pre-eruptive magma chamber primed for eruption, while the sequence of Valle Toledo Member (VTM) eruptions open a window into the temporal evolution of the chamber’s upper regions between the two super-eruptions. We measured H2O and CO2 concentrations in melt inclusions from the entire sequence of eruptions and identified three peaks in CO2 concentrations: (1) in the middle of the LBT plinian airfall (increase in mean CO2 concentrations from 27 ± 5 ppm at the base of the plinian to 173 ± 5 ppm in the mid-plinian); (2) in VTM group III (mean of 197 ± 5 ppm); and (3) in the middle of the UBT plinian airfall (mean of 54 ± 5 ppm at the base of the plinian to 101 ± 5 ppm in the mid-plinian). We propose that these increased CO2 concentrations are due to injections of fresh magma into the system, whereby CO2-rich vapours exsolved from the injected magma percolated through the magma chamber to increase CO2 levels. Although the sharp increase in the LBT plinian indicates a rapid succession of recharge events in a short period of time, the gradually increasing CO2 levels through the final VTM phase and the UBT plinian indicate that recharge events may have been spread over a longer period of time before the UBT eruption. Based on the theoretical and observed gradients in H2O and CO2 through the LBT and UBT sequence, we calculate a vapour phase equivalent to maximum 6.7 wt% of the magma body was exsolved from the LBT magma chamber; for the less degassed UBT, the exsolved vapour phase was maximum 4.2 wt% of the magma body. Our results indicate that the volatile composition of magmatic systems, with a particular focus on CO2, can record evidence of magmatic recharge into the system and be an important tool in deciphering recharge events.