Magma Systems at Okataina Volcano, New Zealand: Insights From U-Th Model-age Spectra in Zircons
Abstract
The time scales over which large bodies of silicic magma are generated and stored in the crust are addressed here by studies of several key rhyolitic eruptives from Okataina volcano (Taupo Volcanic Zone, New Zealand). The eruption of the caldera-forming Rotoiti Tephra (>100 km3 magma) occurred at ca. 61 ka and was followed by 'Unit B', the first voluminous rhyolite (4.6 km3) in the post-caldera sequence at 45-50 ka. This was then followed by the Managone (32.8 ka, 19.9 km3) and the Rotorua (15.8 ka, >1km3). Here we report (238U-230Th) model age data from zircons as determined by SIMS to address (1) the time scales of crystallisation and storage of the Rotoiti rhyolite and the following post-caldera activity, and (2) the age and origin of zircon crystals in each of the studied units. The population density curves of SIMS model ages from zircons in the two types of Rotoiti pumice (crystal-rich, biotite-bearing and crystal-poor, biotite-free) are similar and range from 50±24 ka (within error of eruption age) to over 350 ka, with a pronounced peak at 70 to 90 ka. Zircon model age spectra for the ensuing post-caldera activity are complex, but to a large extent indicate a role for the recycling crystals from multiple older populations (i.e., antecrysts), which pre-date the caldera forming event. Superimposed on the spectrum of old crystal ages are those associated with crystallisation just prior to each individual eruption. The lack of commonality in these young peaks of zircon crystallisation in the post-caldera rhyolites, along with significant compositional and isotopic differences between each of the studied units, precludes the origin of these eruptives from a single, long-lived magma chamber. Rather, we favour a model in which sparsely porphyritic rhyolite magma can be generated in part by remobilisation of older, still-hot crystal mush and where magmas accumulate rapidly (0.4 to >2 m3/s) in a final holding chamber that is effectively emptied on each eruption.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V23E2171C
- Keywords:
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- 1036 Magma chamber processes (3618);
- 1120 Isotopic disequilibrium dating;
- 8428 Explosive volcanism;
- 8439 Physics and chemistry of magma bodies