Timescales associated with the opening phase of large caldera forming eruptions

Abstract:

We present a geochemical application for determining the decompression history prior to caldera formation through analysis of the first-erupted fractions of fall deposits for two voluminous eruptions: the 2.1 Ma Huckleberry Ridge Tuff (HRT; 2500 km³) and the 767 ka Bishop Tuff (BT; 650 km³). The BT is an archetypical example of a continuous explosive eruption, with the whole of the eruption inferred to have lasted ~6 days [1]. In contrast, the fall deposits at the base of the HRT contain evidence for intra-eruption reworking, interpreted as representing eruption breaks on the order of weeks to months [2]. We have analyzed volatiles in melt inclusions (MI) from the first stages of the fall deposits to track initial magma movement and conduit development, as diffusive loss of hydrogen species occurs on timescales ranging from hours to weeks [3]. MI from the initial 3 cm of the HRT deposit show considerable variability in H₂O concentrations (4.4-1.3 wt.%) which we attribute to diffusive loss during syn-eruptive decompression. Using a diffusion model [4], the timescales for H₂O loss by diffusion from the majority of MIs (T~800 °C) are on the order of 1-6 days, with some values approaching several weeks. However, fitting of diffusion profiles to transects of H₂O and CO₂ measured along reentrants (unsealed inclusions) provide timescales of final ascent between 1 and 3 hours [3]. These timescales suggest a two-stage depressurization history preceding, and in the earliest stages of, the HRT eruption. MI from the first 4 cm of the BT, however, present a narrower spread in H₂O concentrations, from 4.3-5.9 wt.%. If the lower values are the result of diffusive loss, then the decompression timescales required (T~720°C) are as much as 1-2 months, suggesting that some parcels of magma experienced extended decompression before the start of the eruption. Current work incorporating BT reentrants and MIs over the full initial BT stratigraphy will show whether a two-phase decompression model is also necessary.

[1] Wilson & Hildreth (1997), J Geol 105, 407.

[2] Wilson (2009), AGU Fall Meeting, #V23C-2085.

[3] Liu et al. (2007), JGR, 112, B06204.

[4] Cottrell et al. (2002), G³, 3, 1-26.