Fig. 6. Potential thermal histories of AT magma determined by inverse modeling.

For each potential temperature-time pathway, the maximum observed Ti diffusion distance in AT quartz (L = 5.21 μm, one side of the complimentary error function) is achieved (using C07). Top figure shows seven examples of plausible temperature-time pathways (legend is in the lower plot). The eruption temperature of 790°C is inferred from Fe-Ti oxide geothermometry (23). Black dots represent the onset of quartz crystallization. The crystal in path 1 begins crystallizing 10,500 years before AT eruption and rapidly cools to subsolidus conditions at 650°C (not visible on the reduced x axis). Although the maximum time in an eruptible state (above 750°C) is 1070 years, cold storage extends the total diffusion time (e.g., paths 1 to 4), and storage at hotter temperatures reduces the total residence time (e.g., paths 5 to 7). The bottom plot shows the cumulative diffusion length for each of the T-t paths above; all converge on the maximum diffusion length observed in AT quartz. Note that timescales of eruptibility are brief, always less than the repose time leading up to the AT eruption.