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SHRIMP U-Pb dating of the preeruption growth history of zircons from the 340 ka Whakamaru Ignimbrite, New Zealand: Evidence for >250 k.y. magma residence times

Authors :
Brown, Stuart J.A.
Fletcher, Ian R.
Source :
Geology. Nov, 1999, Vol. 27 Issue 11, p1035, 4 p.
Publication Year :
1999

Abstract

The Whakamaru group ignimbrites are products of one of the largest caldera-forming eruptions from the central Taupo Volcanic Zone, New Zealand, ca. 340 ka. Imaging of zircons separated from low-silica rhyolite pumice from Whakamaru group ignimbrite reveals a complex magmatic crystallization history; some grains contain resorbed cores. Cores and rims of individual zircons were dated using the sensitive high-resolution ion microprobe. Due to the high U contents of the zircons, (super 206)Pb count rates were sufficiently high to obtain (super 206Pb/(super 238)U ages with precisions of 5%-10% (1 sigma) using longer than usual (super 206)Pb(super +) count times, and this has allowed zircon core and rim ages to be distinguished. Zircon rim and outer growth zone ages range from within 1 (sigma) error of the eruption age to 449 +/- 20 ka, whereas zircon cores produced ages to 608 +/- 20 ka, indicating magmatic residence times exceeding 250 k.y. for some zircons. The data are consistent with a prolonged evolution of the magma system over hundreds of thousands of years, involving magma replenishment and mixing with residual phases of older silicic magma chambers, in addition to crystal fractionation processes, prior to eruption. The study has shown that (super 206)Pb/(super 238)U dating using high-resolution ion microprobe can be used to determine zircon preemptive growth histories in Quaternary lavas and pyroclastic rocks.

Details

ISSN :
00917613
Volume :
27
Issue :
11
Database :
Gale General OneFile
Journal :
Geology
Publication Type :
Academic Journal
Accession number :
edsgcl.62137859