Page 64 - Caldera Volcanism Analysis, Modelling and Response

Residence Times of Silicic Magmas Associated with Calderas 39

Figure 11 Single-reservoir evolution diagram where the absolute time is recalculated to the
caldera collapse episode and plotted vs. magma residence time. If the residence time of the
caldera collapse magma is known, it is possible to calculate when the reservoir started to exist
(e.g., the minerals used to obtain the residence time begin to crystallise).This line has a slope
of one and any pre- or post-caldera magma erupted from the same reservoir as the caldera
forming magma should plot on this line. If the magma reservoir is growing, shorter residence
times could also be expected from newly crystallised melt after of each magma increment.
After caldera collapse the main reservoir will cool down and crystallise if there is also a
decrease in magma input. No time units are shown on the axes. As long as the units are equal
on either axis, any temporal unit is possible.

residence times should increase with eruption (or absolute) time. The maximum
residence time of any batch erupted from a common reservoir should fall on a line
with a slope of 1; the minimum times of the different deposits do not need to be
related since they might reflect progressive magma additions to the reservoir. After
caldera collapse the remaining magma might eventually solidify. Even if post-caldera
lavas are not erupted from the same caldera left-over magma (e.g., erupted after
the solidification limit), if their residence times fall in the time evolution of the
main reservoir the sources of their crystals are partly recycled from caldera-related
material (Figure 11).
This diagram has been constructed for the Long Valley, Taupo and Yellowstone
caldera systems (Figure 12). The long residence times of some small pre-caldera
volcanics in Long Valley and Taupo systems (Oruanui Tuff) seem to be consistent
with them being from the same magma reservoir as the caldera collapse magma.
Such a relation was also proposed by the geochemical similarity between the pre-
and caldera-related rocks (e.g., Long Valley and Taupo sections). Other pre-caldera
magmas clearly deviate from the single reservoir evolution (e.g., old domes in Long
Valley). The majority of the post-caldera deposits have residence times that despite
being long for their volumes, are short compared to the time evolution lines of the
single reservoir. Note that in Taupo (Figure 12b), the very long residence times of