Chapter 1 Residence Times of Silicic Magmas Associated with Calderas

Abstract

Abstract This paper reviews the times that silicic magmas related to major caldera systems spend in the crust prior to eruption. The significance of the time information is evaluated and combined with magma volumes and temperatures to quantify the mass and thermal fluxes associated to calderas. The data discussed includes the largest explosive eruptions on Earth: Taupo Volcanic Zone (New Zealand), the Youngest Toba Tuff (Indonesia), Yellowstone system (USA), Long Valley (USA), Carter Lake (USA), Valles-Toledo complex (USA), La Garita caldera (USA), La Pacana (Chile) and Kos (Greece). Magma residence times are calculated from the difference between the eruption age and the age obtained by radioactive clocks and minerals that are a closed system at high magmatic temperatures (e.g., U–Pb system in zircon). Large ranges of residence times between different systems are found. The shortest residences (4–19 ky) are those of some magmas from the Taupo Volcanic Zone (Oruanui and Rotoiti) and Yellowstone (Dry Creek and Lava Creek). There is not a good correlation between magma volume and residence time, although most eruptions 3 have residence times 100 km 3 have longer residences, some up to 300–500 ky (Fish Canyon, La Pacana). The residence times of some small ( 3 ) pre- and post-caldera magmas indicate that they were extracted from the same reservoir as the caldera-forming magma (e.g., Long Valley, Taupo). However, the time information from most small-volume magmas seems to reflect the recycling of crystals from previous cycles of caldera-forming magmas (Yellowstone), from plutonic rocks of the same caldera cycle with or without erupted equivalents on the surface (Crater Lake, Taupo, Long Valley), or from a partially solidified magma reservoir (Taupo). These interpretations are in agreement with cooling rates and solidification times obtained from simple thermal models of magma reservoirs. Magma production rates were calculated from the ratio of erupted volume and residence time, and they vary between 3 y −1 for small deposits ( 3 ) and ca. 0.1 km 3 y −1 for the Oruanui eruption (530 km 3 ). Estimates for most eruptions >500 km 3 are within 2±2×10 −2 km 3 y −1 . These high magma production rates are probably transient and comparable to global eruptive fluxes of basalts (e.g., Hawaii). Magma cooling rates for deposits >100 km 3 were calculated from the difference between the liquidus and pre-eruptive temperatures over their residence times, and they vary between 2×10 −4 and 3×10 −3 K y −1 . Integration of the calculated residence times and magma fluxes with a simple rheological model of the crust is not possible and should be a main topic of research if we are to understand the mechanisms and rates which permit large amounts of silicic magma to be stored below calderas.