Page 29 - Caldera Volcanism Analysis, Modelling and Response

4 Fidel Costa

crystallisation age, simply because the K–Ar system becomes closed at much lower
temperatures (e.g., on quenching of the magma upon eruption). Most of the age
data used in this manuscript were obtained using zircon and the U–Th–Pb decay
system and thus reflect the time since the beginning of zircon crystallisation and
final eruption. It is worth mentioning that there might be a systematic bias between
the radioactive clocks of the K–Ar and that of U–Pb systems, the latter giving
slightly older ages (o1%; e.g., Renne et al., 1998; Min et al., 2000; Renne, 2000;
Villeneuve et al., 2000; Schmitz and Bowring, 2001; Schoene et al., 2006). Since
the issue is not resolved at the time of writing it has not been considered for
calculating residence times. Recent reviews of the methods and time scales of
magmatic processes can be found in Condomines et al. (2003), Reid (2003), Turner
et al. (2003), Hawkesworth et al. (2004) and Peate and Hawkesworth (2005).

1.2. Magma production and cooling rates

Aside from compiling residence times and rates of processes, two other parameters
were calculated. One is a ‘magma production rate,’ which is the ratio of the erupted
volume over the residence time (e.g., Christensen and DePaolo, 1993; Davies et al.,
1994). It is not sensu stricto a magma production rate because it only accounts for the
erupted magma. It should be called ‘erupted magma production rate’ but this would
be very cumbersome. These rates are different from the ‘average magma eruption
rate’ (or output rate) calculated using the total erupted volume and time span of
magmatic activity at a given volcanic system (Crisp, 1984; White et al., 2006). They
are also different from the rates obtained from the erupted volume divided by the
time interval between two subsequent eruptions (Bacon, 1982). A magma cooling
3
rate has also been calculated for eruptions W100 km . This is the difference
between the magma temperature at pre-eruptive conditions and its liquidus
calculated by MELTS (Ghiorso and Sack, 1995), over the residence time. The
significance of such cooling rates is debatable: it could be a maximum if the magma
delivered to the reservoir was crystal-rich, or a minimum if the magma was reheated
prior to eruption. The magma cooling rates reported here should be considered as
first-order estimates and are applicable only to the erupted magma rather than to
the entire magmatic system at depth (which likely has many cooling rates).

1.3. Choice of caldera systems and organisation of the manuscript
The systems described are those that have produced large silicic eruptions for which
residence time data exists (Figure 1): Taupo Volcanic Zone (New Zealand), Toba
caldera (Indonesia), Yellowstone (USA), Long Valley (USA), Valles-Toledo
complex (USA), and La Garita (USA). The time information from Crater Lake
(USA), Kos Tuff (Greece) and La Pacana (Chile) are also briefly discussed.
A summary of residence times and rates of processes for major eruptions are shown
in Tables 1 and 2. The rest of this manuscript is organised in four more sections:
Section 2 discusses the methods used to obtain time information. Section 3 contains
the time information and other basic geological features of each caldera system.
Section 4 is a general discussion of what the residence times mean in the context of