The ∼74 ka eruption of the Youngest Toba Tuff in Sumatra, Indonesia, was one of the largest single volcanic eruptions in geologic history. The compositional and thermal evolution of the magma reservoir prior to eruption was investigated 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 71–76 wt% SiO2. These rims and IDs were analyzed on the UCLA Cameca ims 1270 ion microprobe. Zircon chemistry provided robust insights into magmatic conditions. Chemical covariations of Zr/Hf, Eu/Eu*, Nd/Yb, Th/U, and Ti show that the melts in which zircon crystallized are related by temperature dependent fractionation of zircon + feldspar + allanite + quartz assemblage. These chemical covariations are qualitatively mirrored in YTT whole rock, matrix glass, and quartz melt inclusion compositions, suggesting that zircon compositions accurately reflect magmatic chemical changes. Titanium-in-zircon crystallization temperatures estimated for crystallization at 1 GPa range from just above to well below the probable YTT solidus (∼650°C), and these conflict with most previously established estimates for the thermal state of the YTT magma. Regardless of uncertainties in the Ti-in-zircon geothermometer, the observed normal Ti zoning suggests a down-temperature crystallization history. Thus, YTT zircons show no evidence for thermal diversification of magma in the lead-up to eruption, contrary to what was expected based on evidence from previous studies. The majority of the zircons show a mixture of both normal and reverse zoning with respect to Zr/Hf, Nd/Yb, Eu/Eu*, and Th/U. Melts responsible for the rims of zircons that display chemical reversals lie on the same fractionation trend as defined by the other zircon domains and therefore do not suggest intrusion of unrelated melt compositions into the magma chamber. Overall, if zircon rims are representative of near-eruption magma conditions, then the zircon compositions are characterized by a lack of evidence for significant increases in the chemical—as well as thermal—diversity of YTT magmas during the lead-up to eruption.
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|Advisor:||Reid, Mary R.|
|Commitee:||Vazquez, Jorge A., Wittke, James|
|School:||Northern Arizona University|
|Department:||School of Earth Sciences and Environmental Sustainability:|
|School Location:||United States -- Arizona|
|Source:||MAI 50/04M, Masters Abstracts International|
|Keywords:||Magmatic chemistry, Ti-in-zircon, Volcanic eruptions, Youngest Toba Tuff, Zircon|
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