PhD Oral Defense of Weiran Li: Eruptive styles of subduction-zone volcanoes: Investigation of magmatic volatile budgets and ascent rates using apatite
- Oral Defense
About the Event:
The most explosive and dangerous volcanoes around the world are located at subduction zones. Understanding the changes in their eruption dynamics has great significance for mitigating volcanic hazards. In this thesis I investigate the role of volatiles on volcanic eruptions using a combination of thermodynamic, experimental, and observational approaches. First, I propose a new thermodynamic model of the partition behaviour of F-Cl-OH between apatite and melt to estimate the melt water contents. This model includes a treatment of the thermodynamically non-ideal properties of the ternary apatite at magmatic temperatures, and has been developed as a free web-based calculator. Second, I use the thermodynamic model and various crystal chemistry data to calculate the melt volatile budgets of two eruptions of distinct styles at Merapi volcano (Indonesia). I characterize for the first time the CO2, H2O, S, Cl and F contents of the deep melts (near the Moho) below this volcano. Fluxing of these deep volatiles to the shallower magmatic system is likely the cause of the large and explosive 2010 eruption. Third, I report new experimental results for the diffusion coefficients of F, Cl, OH and S in apatite at magmatic temperatures. I find that these volatiles re-equilibrate via a multicomponent diffusion mechanism. Application of the new diffusion data to zoning of volatiles in natural apatite allows determination of the timescales of magma ascent. The results from this Ph.D. thesis provide new insights into the plumbing system, magmatic volatile budgets and magma ascent rates of subduction-zone volcanoes, and help to improve the interpretation of volcanic monitoring data that is critical for eruption forecasting.
Oral Examination Committee:
- Assoc. Prof. Richard Webster, Nanyang Technological University, Singapore (Chairman)
- Assoc. Prof. Xianfeng Wang, Nanyang Technological University, Singapore (Oral Examiner)
- Assoc. Prof. Fidel Costa, Nanyang Technological University, Singapore (Main Supervisor)
- Prof. Tim White, Nanyang Technological University, Singapore (Co-Supervisor)
- Assoc. Prof. Emma Hill, Nanyang Technological University, Singapore (Mentor)
About the Speaker:
Weiran Li (Alex) is a volcanologist and petrologist by training. She received her B.S. (in Geochemistry) and B.A. (in Art History) in 2012 from Peking University (PKU), China. During 2012-2014, she was a master student at PKU working on using the40Ar/39Ar isochron method to date Quaternary volcanic rocks from Changbaishan volcano (China). In August 2014, she joined the Interdisciplinary Graduate School and Earth Observatory of Singapore at NTU, and started to work on the pre-eruptive magmatic processes of subduction-zone volcanoes, under the supervision of Professor Fidel Costa. In the third year of her PhD, she received the Dr. Stephen Riady’s geoscience scholarship, and had a three-month visit at the Ruhr University at Bochum (Germany) to perform diffusion experiments. With the work from this project, she received the Best Student Presentation Award from the EMPG conference in Clermont-Ferrand (France) in 2018.
Weiran’s PhD research focuses on using mineral apatite to evaluate the contributions of magmatic volatile budgets and ascent rates to different types of eruptions. Her work combines a variety of approaches from field observation, through petrography, bulk and in-situ chemical analyses, diffusion experiments, thermodynamic calculations, and numeric modelling. She has also converted her models into free online calculators for the convenience of application by other researchers. Her goal is to become an expert in volatile behaviour in subduction-zone magmatism, and to explore the volatile cycles between the atmosphere and mantle.