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Trace element distributions in the upper Bandelier Tuff, New Mexico; relevance of zircon to the magmatic evolution of the Valles system


, : Trace element distributions in the upper Bandelier Tuff, New Mexico; relevance of zircon to the magmatic evolution of the Valles system. Abstracts with Programs - Geological Society of America 28(7): 213

The Upper Bandelier Tuff (UBT), New Mexico, is a compositionally and mineralogically zoned rhyolitic ignimbrite that erupted about 1.22 million years ago during the formation of the Valles caldera. Distributions of trace elements such as Zr, Hf, Th, U, and REEs in the UBT are dependent on the crystallization of accessory minerals such as zircon, chevkinite, apatite, and monazite. We studied zoned zircons and host pumice compositions to better understand the origin of trace element zonation and the chemical evolution of the Valles magmatic system. BSE images reveal textural characteristics such as truncation and resorption in some zircons from each UBT cooling unit. EMP traverses across cores of many zircons reveal values of 0.5 and 0.3 wt.% U and Th oxide respectively, while rim values approach greater than or equal to 0.1 wt.% U and Th oxide. EMP concentrations across unzoned zircon gave values similar to the rim values of zoned zircons. SIMS analysis gave U and Th values up to 4600 and 1640 ppm, respectively, in core regions. Pumice and glass show systematic increases of these elements toward the basal plinian fall unit, indicating an increase of these elements toward the top of the UBT magma chamber. Several magmatic processes could explain the complexity of zircon crystallization. We find fractionation and recycling of zircons from ancestral Bandelier magmas (greater than or equal to 1.5 Ma) consistent with the zircon and magma zoning, however further work is needed to fully refute or verify any of the proposed models.

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