Volatile budget variations along and across the arc in northern Japan
Monday, 14 December 2020: 14:45
Virtual
Raimundo Brahm1, Georg F Zellmer2, Takeshi Kuritani3, Naoya Sakamoto4, Mitsuhiro Nakagawa3, Hisayoshi Yurimoto5, Daniel Coulthard Jr1 and Eiichi Sato6, (1)Massey University, Palmeston North, New Zealand, (2)Massey University, Volcanic Risk Solutions, Palmeston North, New Zealand, (3)Hokkaido University, Sapporo, Japan, (4)Hokkaido University, Isotope Imaging Laboratory, Sapporo, Japan, (5)Hokkaido University, Division of Earth and Planetary Sciences, Sapporo, Japan, (6)Hokkaido Education University, Sapporo, Japan
Abstract:
The main engine of magma genesis in subduction zones is the volatile release from the subducted slab into the overlying mantle wedge. Volatiles like H2O and halogen elements are mainly transported in hydrous mineral phases (e.g., amphibole, serpentine, chlorite), which are carried in the hydrated lithospheric mantle, altered MORB and sediment cover layers of the slab. There is still much debate on the processes involved during slab-fluid release (slab dehydration or melting) and the nature of the fluid (aqueous, supercritical or silicate liquid). Halogen elements (F and Cl) have the potential to track slab dehydration and fluid-flux melting processes, as they are not easily partitioned during early magmatic differentiation processes. In addition, the contrasting partitioning behaviour of F and Cl between hydrous minerals and fluids/melts can assist with the identification of the controlling processes and mineral phases involved during volatile release from the slab.
We estimate primary magma compositions from olivine-hosted melt inclusions collected from tephra samples of seven volcanic systems along and across the arc in northern Japan. Arc front volcanoes show coupling behaviour of F and Cl, suggesting that the mineral phases and volatile release processes are similar for all arc volcanoes in this area.
There is a general increase of halogen contents towards the back-arc, correlated to slab surface temperature below the volcanic systems, while maintaining similar F/Cl ratios. This may indicate that the nature of slab-fluid is the same between arc and back-arc and the volatile increase could be related to the interplay between the fluid influx and the degree of partial melting. There is a change of behaviour in the volcano located furthest from the trench, with higher F/Cl. This contrasting behaviour can be linked to a change in the volatile release process, from dehydration in the arc and back-arc volcanoes closer to the trench, to melting in the far back-arc.
