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JOHN STIX, MICHAEL P. GORTON, Changes in Silicic Melt Structure Between the Two Bandelier Caldera-Forming Eruptions, New Mexico, USA: Evidence from Zirconium and Light Rare Earth Elements, Journal of Petrology, Volume 31, Issue 6, December 1990, Pages 1261–1283, https://doi.org/10.1093/petrology/31.6.1261
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Abstract
The Cerro Toledo Rhyolite is a group of high-silica rhyolite domes and tephras which were erupted during the period between the Lower Bandelier Tuff (LBT) at 1·45 Ma and the Upper Bandelier Tuff (UBT) at 1·12 Ma. The tephra sequence reflects the changing composition of the most fractionated liquids at the top of the magma chamber or chambers owing to crystallization during this 0·33-Ma interval, over which it is possible to trace magma evolution in detail. Incompatible elements such as Cs and Lu generally increase in concentration upsection through the tephra sequence. By contrast, Zr initially decreases upsection from 153 ppm to 134 ppm in the middle, then increases upward to 320 ppm in the most evolved rhyolites. The Zr/Cs and Hf/Cs ratios initially decline, then become constant. The dramatic rise in Zr at nearly constant Zr/Cs suggests suppression of zircon crystallization and a change from a high degree of zircon oversaturation to one of marginal oversaturation in the most evolved magmas. The observed Zr trends in the Cerro Toledo Rhyolite are opposite to those (1) predicted by experimental studics of Zr solubility in silicic magmas and (2) observed in the Bishop Tuff. LREE show broadly similar relations to Zr and Hf.
We have examined two parameters—iron content and volatile content of the magma—which may have controlled the crystallization behavior of zircon and the LREE-rich phase(s). The dramatic increase in Zr and LREE in the most evolved Cerro Toledo Rhyolites and basal UBT plinian tephras is accompanied by similar increases in iron and halogen contents. If changa in the halogen contents of matrix glasses are indicative of changes in overall volatile levels at the top of the magma chamber(s), the increase in volatiles and iron may have modified the structural state of the magma and increased the solubilities of zircon and the LREE-rich phase(s), thereby raising the saturation levels of Zr and LREE and stabilizing these elements in the melt. Disruption of the melt structure may have resulted from (1) Fe as a network modifier or quasi-molecular complex, (2) breaking of bridging oxygens by anions such as OH− and F−, and/or (3) complexing of Al and alkalis by OH, F, and/or Cl.
