Links between thermochemical state in silicic systems and near-eruption growth record of major and accessory phases
Monday, 8 January 2018
Salon Maule (Hotel Quinamavida)
Casey R Tierney, Northern Arizona University, School of Earth Sciences and Environmental Sustainability, Flagstaff, AZ, United States, Mary R Reid, Northern Arizona University, Flagstaff, AZ, United States, Dale H Burns, Stanford University, Palo Alto, CA, United States, Craig A Chesner, Eastern Illinois University, Geology/Geography, Charleston, IL, United States and Fidel Costa, Nanyang Technological University, Earth Observatory of Singapore, Singapore, Singapore
Abstract:
Much of the magma reservoir responsible for the enormous (2800 km
3) and compositionally zoned (68-77 wt.% SiO
2) 74 ka Youngest Toba Tuff (YTT) was characterized by a granite minimum (eutectoid) composition and mineral assemblage including quartz and two feldspars. As determined by U-Th dating, only ~half of the melt-hosted YTT zircon in crystal-poor (<25% phenocrysts) pumices have detectable (>0.5 µm) rims that are within error of eruption; surface ages on others range to >150 kyr before eruption. This is in contrast to other voluminous silicic eruptions with non-eutectoid compositions, for which most zircon surfaces are either eruption-aged or typically no more than a few tens of kyr older than eruption. Throughout the YTT, Ti zoning and diffusion chronometry reveals quartz rim growth only 15 to 100 years before eruption, the nature of which depends on the thermochemical system state. Quartz in pumices with eutectoid compositions is found to be relatively thin and less distinct in Ti concentrations from adjacent interior zones than in quartz in pumices that are non-eutectoid (they lack sanidine).
We combined r-MELTS and phase saturation modeling to link these seemingly disparate observations and infer the role that thermochemical state plays in the near-eruption growth record of minerals in silicic systems. The consistent timing for the onset quartz rim growth throughout the YTT likely records the effect of a marked and temporally discrete magmatic/recharge event that affected the entire YTT system, but the thermally buffered eutectoid portion of the system was subjected to negligible temperature variations during crystallization, resulting in limited contrasts in quartz Ti contents (temperature-sensitive). Detectable growth of zircon is also not expected at eutectoid conditions unless significant major phase crystallization occurs, in which case, zircon growth is proportional to that of major phases and to the attendant increases in the concentration of key components (e.g. Zr) in the melt. In contrast, in non-eutectoid systems such as Yellowstone or Taupo the lack of thermochemical buffering reduces the likelihood that older accessory phase crystals will survive recharge events, but also enhances the growth potential when accessory phases are saturated – both contribute to the younger age distribution.
