TEMPERATURES AND RESIDENCE TIMES OF THE AMALIA TUFF: RESULTS OF TITANIUM-IN-QUARTZ THERMOMETRY AND DIFFUSION MODELING
We applied Ti-in-quartz thermometry to the Amalia Tuff using an equation from . Using a TiO2 activity (aTiO2) of 0.5 and a pressure of 2.0 kbar, quartz rim temperatures averaged 650°C , at the lower end of the granitic solidus. Because the TiO2 activity of the Amalia Tuff is not well-constrained, we also decided to test the lowest possible aTiO2 of 0.1 which produces average quartz rim temperatures of 750°C.
We then applied diffusion modeling across the rim-core boundary, comparing results using 650°C and 750°C as the temperature input. Two equations for calculating diffusion coefficient were compared— and . Diffusion timescales at 650°C were 944 - 15235 a when using  and 2.3 to 29 Ma when using , which is unrealistically long. Diffusion timescales at 750°C were 29 - 470 a when using  and 0.035 to 0.56 Ma when using . Generally, using diffusion coefficients calculated by  implies short timescales for rim growth, presumably related to a late-stage reheating event initiating eruption of Amalia Tuff, whereas applying  suggests that the rims grew early during the assembly and long-term storage of the system in the upper crust rather than being related to eruption.
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