Insights into the Geochemical Evolution of the Youngest Toba Tuff Magma Chamber using Zircon-hosted Melt Inclusion

Abstract:

Zircon crystals can provide multidimensional insights into magmatic evolution. We combine zircon U-Th dating and trace element chemistry, with zircon-hosted melt inclusion compositions and volatile abundances to investigate the magmatic evolution preceding the 74 ka Youngest Toba Tuff (YTT) eruption.

Zircon have U-Th crystallization ages spanning from eruption age to > 375 ka, reinforcing earlier findings that the YTT system was long-lived. A progressive increase of U in zircon (from < 500 to ~1500 ppm) indicates that the YTT system, or a portion of it, became highly fractionated between 130-200 ka. A possible lull in zircon crystallization is contemporaneous with a previously recognized increase in chemical diversity of allanite ~110-130 ka, suggesting a period of enhanced thermal input into the system.

We identify two main populations of zircon-hosted melt inclusions. A low-MgO type is more evolved (> 280 ppm Rb, ~125 ppm Ba, 25-30 ppm Sr, < 0.03 wt% MgO) and has high water contents (3.8-5.7 wt% H₂O), consistent with formation and storage in a highly fractionated crystal mush ~ 4-9 km deep. A high-MgO type (250-260 ppm Rb, 160-450 ppm Ba, 35-55 ppm Sr, 0.04-0.07 wt% MgO) has compositions similar to matrix glasses, and is typically less hydrous (0.5-3.5 wt% H₂O), suggesting storage and degassing in a somewhat more primitive melt < 3 km deep. Melt inclusions dated via U-Th measurements of surrounding zircon zones, show no clear temporal differences between the two MgO populations. Rather, melt inclusions entrapped throughout the entire YTT history have relatively invariant major element chemistries, and have no temporal trends in volatile abundances.

Zircon-hosted melt inclusions (particularly the low-MgO type) of many ages occur within sealed reentrant melt-channels. A number of zircon grains have actively open melt channels rimmed with 1-3 μm of low-U zircon growth. These dissolution/regrowth features are texturally similar to dissolution zones with high-T overgrowths described previously in YTT quartz; collectively, these textures provide evidence of a major thermal perturbation(s) 10’s-100’s of years before the YTT eruption. We conclude that high-T recharge events occurred throughout the pre-eruptive YTT evolution, and suggest that a large recharge event triggered the cataclysmic YTT eruption.