Page 45 - Caldera Volcanism Analysis, Modelling and Response

20 Fidel Costa

Figure 5 Location of theYellowstone magmatic system with the tectonic features related to
the three caldera collapse cycles (I, II, III; hachure towards collapse) and their respective tu¡s.
I is related to Huckleberry, II to Mesa Falls, and III to Lava Creek. Also shown are the two
resurgent domes of Yellowstone caldera in gray. HF, Henry’s Fork caldera. Figure modi¢ed after
Christiansen (2001).

3.3.1. Residence times of the caldera-forming Huckleberry, Mesa Falls and
Lava Creek magmas
The residence times of the main tuffs (Figure 6 and Table 5) were obtained from the
eruption dates and the zircon ages reported by Bindeman et al. (2001). The ages of
these two events overlap for the Huckleberry Tuff, although the error of the U–Pb
zircon age still allows 77–234 ky of residence time. The data on the Mesa Falls
magma gives a residence time of ca. 165 ky, and ca. 60 ky for the Lava Creek
4 3 1
magma. The calculated cooling rates are 0.4 10 –2 10 Ky , and magma
3 1
production rates vary between 0.002 and 0.01 km y (Table 5 and Figure 3).

3.3.2. Residence times of the post-caldera magmas (mainly o0.6 Ma)
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The relatively close agreement between the U–Pb and 40 Ar/ Ar ages for the three
major tuffs contrasts with the variety and spread of ages for the presumably small
3
(o50 km ?) post-caldera magmas (Figure 6). For example, three units erupted
between 0.51 and 0.48 Ma contain zircons with ages ranging from those of eruption
but can be as old as the Huckleberry Tuff (Bindeman et al., 2001). This gives
residence times that are incredibly long (450 ky–1.6 My). The simplest explanation
is that some of the zircons in these post-caldera units are recycled from the major
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tuffs. This is in accord with the d O disequilibrium between minerals and glass of
the post-caldera units found by Bindeman and Valley (2001). They proposed that
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the change towards low d O values after caldera collapse was due to complete
melting of hydrothermally altered rocks that were part of the down-dropped block.
Bindeman and Valley (2001) used a diffusion model to obtain 500–5,000 years for