Page 41 - Caldera Volcanism Analysis, Modelling and Response

16 Fidel Costa

older (e.g., Taupo units D and E). If we neglect crystals older than 0.5 My, most
zircons have maximum model ages of up to 200 ka. Moreover, when all the data are
taken together, there are two peaks, at about 35 and 100 ka. These peaks do not
correspond to any eruption and thus if some of the old zircons are inherited from
previous magmas these were never erupted. The exceptions are the 100 ka ages
found in the deposits of three consecutive eruptions of Tihoi, Okaia, and Oruanui.
This might reflect that they share a similar (common?) reservoir or petrogenetic
history (see Section 4).
The residence times were calculated using the oldest and the youngest zircon
peak ages determined by Charlier et al. (2005) either by SIMS or TIMS data, and
they vary between 13 and 100 ky (Table 3). The residence of the Oruanui magma
can be up to ~70 ky, but can also be as short as 4–26 ky if the youngest peak ages as
used (Figure 4). Charlier et al. (2005) argue that the zircon model ages at ca. 100 ka
are from inherited crystals from previous but young intrusive episodes, and they
propose residence times that are somewhat intermediate between the two extremes
noted above. Moreover, they suggest that bimodal (or multi-modal) zircon model
age spectra indicate remobilization of zircons from a crystal mush, rather than a
long-lived magma body. Perhaps the complexities of the data partly reflect the
current limitations in spatial resolution and precision of the analytical techniques.
For example, the detailed in situ zircon age data for the Whakamaru group
ignimbrites (Brown and Fletcher, 1999; their Figure 3) shows crystals with ages
that progressively decrease from core to rim (e.g., with no jumps). If this is the case,
one would expect no modality in the age distribution, and the origin of the
bimodality found by Charlier et al. (2005) in the Whakamaru data is unclear (their
Figure 22a).
The cooling rate of the Oruanui magma obtained from the difference between
the pre-eruptive temperature of 7601C (water content of 4.5 wt% and pressure of
150 MPa; Wilson et al., 2006) and the liquidus (8301C, as calculated by MELTS;
Ghiorso and Sack, 1995) over the range of residence times varies between 2 10 2
1
and 10 3 Ky . This is almost two orders of magnitude higher than for the
Whakamaru ignimbrites discussed above (Table 1). The calculated magma produc-
3 1
tion rates (Figure 4) range from o10 5 for small eruptions up to 0.1 km y
for the Oruani, the highest found in all caldera related magmas (e.g., Table 1).

3.2. Toba Caldera complex

The Toba Caldera complex is located in Sumatra, Indonesia, in a subduction-zone
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setting. During the past 1.2 Ma, at least 3,400 km of magma have been erupted in
four ash-flow units, three from the Toba caldera (Chesner and Rose, 1991;
Chesner, et al., 1991; Chesner, 1998). The tuffs are compositionally zoned from
minor andesite to predominant rhyolite with relatively high crystal content
(12–40 wt%; Chesner, 1998). Toba is the largest resurgent quaternary caldera on
Earth, measuring 100 km by 30 km, and was the source of the voluminous
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(2,800 km ) Youngest Toba Tuff at 73 ka (Table 4). Chesner (1998) reports pre-
eruptive conditions of 701–7801C, 300 MPa, and magmatic water content of