Research Activities- NAKAMURA Michihiko -


Activity Report for External Review (2008-2009.10)

Research Projects: (1) Mechanisms of permeable flow degassing of magmas
(2) Role of grain growth on chemical transport in fluid-bearing rocks

Objectives:
(1) Since bubble growth and expansion in magmas are the driving forces of violent volcanic eruptions, the mechanism of degassing (outgassing) has been a central topic in volcanology. In the last two decades, the permeable gas-flow hypothesis has been widely accepted as an explanation of the degassing of viscous silicic magmas, in which bubbles scarcely rise in the time scale of eruptions. As magma ascends, the solubility of volatiles decreases and the bubbles expand due to decompression, resulting in an increase in magma vesicularity and the formation of foam. This foam should be highly permeable for effective degassing. However, mechanisms of the permeable flow degassing have been poorly understood. To address this issue, Nakamura has been carried out integrated study of experimental volcanology, petrography and permeability measurement.
(2) Knowledge of the fundamental mechanism of fluid-rock interaction is indispensable to understand geochemical dynamics. The mechanisms generally assumed to be rate-limiting during chemical exchange are lattice diffusion and dissolution-reprecipitation. This assumption is, however, often difficult to test in natural rock samples because grain-scale chemical traces of fluids can be easily blurred by annealing and late stage geologic events. Previous studies have pointed out that the two conventional processes fail to explain some geochemical observations such as enhanced chemical exchange in rock recrystallization through coarsening, exsolution and stress deformation, but the fundamental processes leading to enhanced exchange remain uncertain. In order to investigate the mechanism of fluid-rock interaction, we have carried out hydrothermal experiments at 1200ºC and 1.2 GPa, where Ni infiltrated into synthesized dunites.

   
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