Evolution of the Youngest Toba Tuff magma reservoir as recorded by Zircon Geochemistry and Crystallization Temperatures
Abstract
The ~74 ka eruption of the Youngest Toba Tuff (YTT) in Sumatra, Indonesia, was one of the largest single volcanic eruptions in geologic history. We are exploring the compositional and thermal evolution of the magma reservoir prior to eruption through spatially-sensitive analyses of the trace element characteristics of zircon. Stepwise serial sectioning of rims and interior domains (ID=5 µm below the rim) was performed on forty-four zircons from pumices with a compositional range of 70-76 wt% SiO2. These rims and IDs were analyzed on the UCLA Cameca ims 1270 ion microprobe. Both rims and IDs from individual pumices exhibit a range in Zr/Hf ratios indicative of growth from melts variously affected by zircon fractionation; zircons from the high silica rhyolite did, however, crystallize from more evolved melts. Eu/Eu*, Nd/Yb, and Th/U decrease with decreasing Zr, showing that the variation in zircon compositions may be related by co-precipitation of feldspar and allanite along with zircon, throughout the growth history represented by the interior to rim chemistry. Titanium contents also decrease with decreasing Zr/Hf, suggesting that crystallization was driven by magma cooling. There is no apparent trend from compositionally homogeneous interiors to more rim diverse compositions prior to eruption, contrary to what is observed for Toba allanite (Vazquez and Reid, 2004): compositional variability for the IDs is similar to that for the rims. It is possible that not all Toba allanite and zircon populations co-crystallized, given zircon crystallizion in compositionally diverse domains throughout a significant part of their growth. Rim-ID pairs provide evidence of both normal and reverse zoning within individual crystals, with more than half of the zircons from each pumice sample being normally zoned. Temperatures of crystallization calculated using the Ti-in-zircon geothermometer (Watson et al., 1996), assuming a melt aTiO2 of 0.5, yield median temperatures for zircon rims ranging from 640-667 °C, while zircon interiors show slightly higher median values, ranging from 682-701 °C. These temperatures neglect a possible temperature correction of -40 to -80 °C due to pressure. Virtually all of the zircon rims and interior domains record temperatures lower than eruption temperatures of 700-780 °C estimated for the YTT by Chesner (1998) using FeTi oxide equilibria. We Will be determining concentrations of Ti in coexisting quartz to better understand the effects of aTiO2 on the Ti-geothermometers. Regardless of the absolute temperatures calculated from the Ti geothermometer, relative temperature differences between rims and interiors, along with decreasing Zr/Hf ratios and normal chemical zoning suggest reservoir crystallization conditions driven dominantly by cooling, with no compelling evidence for heating in the lead-up to eruption.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V34A..08G
- Keywords:
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- 1036 GEOCHEMISTRY / Magma chamber processes;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 8439 VOLCANOLOGY / Physics and chemistry of magma bodies;
- 8440 VOLCANOLOGY / Calderas