Page 37 - Caldera Volcanism Analysis, Modelling and Response

12 Fidel Costa

3.1.1. Eruptive products between 1.2 Ma and 300 ka
Brown and Smith (2004) reported zircon ages from the Ongatiti ignimbrite
3
(300–1,000 km ; erupted at 1.21 Ma) ranging from 1.44 Ma to that of eruption,
with a peak at 1.31 Ma. Although no systematic age differences between crystal
cores and rims were found, the largest difference between eruption and the zircon
ages is about 240 ky, and this is the maximum residence time (Table 3 and Figure 4).
There are no rocks erupted at ca 1.40 Ma (Houghton et al., 1995) which means that
if the old ages are partly inherited zircons, their sources cannot be identified on
the surface. In a similar study, Brown and Fletcher (1999) reported the ages of
3
zircons from the Whakamaru group ignimbrites (300–1,000 km , erupted at ca.
340 ka). In this case the age of the crystal rims overlapped with that of eruption, but
cores extended up to 608 ka, indicating residence times of up to 250 ky. These old
ages cannot be correlated with any other erupted rock in the Taupo volcanic zone.
The spectacular zoning in age from core to rim in these zircons is probably the best
evidence for long magma residence times (see Figure 3 of Brown and Fletcher,
1999). However, there is also the possibility that some of the old ages are inherited
from previous (but unerupted) magmas (Brown and Fletcher, 1999). A cooling rate
4 1
of 5 10 Ky has been calculated for the Type A low-silica Whakamaru
ignimbrites from the temperature difference at the average pre-eruptive conditions
(7901C, water content of ca 3 wt%, pressure of 150 MPa; Brown et al., 1998) and
the liquidus at the same conditions (9151C, as calculated by MELTS; Ghiorso and
Sack, 1995) over the residence time.

3.1.2. Eruptive products between 300 and 45 ka
Charlier et al. (2003) investigated the Rotoiti and Earthquake Flat events
3
(ca. 100 and 10 km , respectively, both erupted at ca. 62 ka) using a combination
of SIMS analyses of zircon and separated phases analysed by TIMS. They
determined model ages (isochrons obtained with only two points) using U-Th
series disequilibria. The results are complex, and the SIMS and TIMS ages on
the same material do not always overlap. Comparison of eruption ages of Rotoiti
with the TIMS zircon data gives 9 ky of residence time. The SIMS analyses
show more spread in ages from about 50 to W350 ka. This may indicate that
zircon crystallised up to eruption. Crystals older than 100 ky might be an inherited
fraction rather than part of a continuous growth of a single magma body, and
Charlier et al. (2003) concluded that the Rotoiti magma accumulated and
crystallised zircon for a period of ca. 50 ky. The data from the Earthquake Flat
eruption show residence times of about 100 ky and could mean that the magma was
a largely crystalline pluton which was partly remobilised shortly prior to eruption
(Charlier et al., 2003).

3.1.3. Eruptive products o45 ka
The ages of zircons from deposits erupted from the Taupo volcanic centre between
45 and 3.5 ka (including the Oruanui eruption) were investigated by Charlier et al.
(2005). They found no significant age variation between crystal centres and rims.