TY - JOUR

T1 - H2O and CO2 evolution in the Bandelier Tuff sequence reveals multiple and discrete magma replenishments

AU - Waelkens, Clara M.

AU - Stix, John

AU - Eves, Erin

AU - Gonzalez, Carla

AU - Martineau, David

N1 - Funding information: This work was funded by a NSERC Discovery grant to John Stix, the Multidisciplinary Applied Geochemistry Network (MAGNET) NSCERC-Create program and McGill University. The authors would like to thank Gregor Lucic of Picarro Inc. for his guidance and assistance with sample preparation and FTIR analyses. This research was supported by a Natural Sciences and Engineering Research Council of Canada Discovery grant to John Stix and by the Multidisciplinary Applied Geochemistry Network (MAGNET).

PY - 2021/12/4

Y1 - 2021/12/4

N2 - 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 H 2O and CO 2 concentrations in melt inclusions from the entire sequence of eruptions and identified three peaks in CO 2 concentrations: (1) in the middle of the LBT plinian airfall (increase in mean CO 2 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 CO 2 concentrations are due to injections of fresh magma into the system, whereby CO 2-rich vapours exsolved from the injected magma percolated through the magma chamber to increase CO 2 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 CO 2 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 H 2O and CO 2 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 CO 2, can record evidence of magmatic recharge into the system and be an important tool in deciphering recharge events.

AB - 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 H 2O and CO 2 concentrations in melt inclusions from the entire sequence of eruptions and identified three peaks in CO 2 concentrations: (1) in the middle of the LBT plinian airfall (increase in mean CO 2 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 CO 2 concentrations are due to injections of fresh magma into the system, whereby CO 2-rich vapours exsolved from the injected magma percolated through the magma chamber to increase CO 2 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 CO 2 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 H 2O and CO 2 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 CO 2, can record evidence of magmatic recharge into the system and be an important tool in deciphering recharge events.

KW - Magmatic recharge

KW - Melt inclusions

KW - Silicic magma systems

KW - Supereruption

KW - Volatiles

UR - http://www.scopus.com/inward/record.url?scp=85120724628&partnerID=8YFLogxK

U2 - 10.1007/s00410-021-01866-6

DO - 10.1007/s00410-021-01866-6

M3 - Article

VL - 177

JO - Contributions to Mineralogy and Petrology

JF - Contributions to Mineralogy and Petrology

SN - 0010-7999

IS - 1

M1 - 1

ER -