Trace element distributions in the Upper Bandelier Tuff, New Mexico: Zircon zoning and locations for magmatic evolution of the Valles system
Cindy Werner, James A. Stimac, and Donald. Hickmott
Trace element distributions in the upper Bandelier Tuff (UBT) are highly dependent on crystallization of minor minerals such as zircon and chevkinite. Our study concentrates on the systematics of U, Th, Y, Zr, Hf, and the rare earth elements (REE) in individual zircons and bulk-rock pumices. SEM backscattered imaging revealed zoning with signs of truncation and resorption in many but not all zircons from each cooling unit in the UBT. EMP analyses showed compositional changes across zone boundaries, ranging from 0.5 to 0.3 wt% U and Th oxide in the core regions respectively to 0 wt% in the rim regions. SIMS analysis gave corresponding values up to 4600 and 1640 ppm U and Th in the core regions, respectively. Uranium, Th and the heavy REEs for the zircon cores are all enriched with respect to zircon rims, whereas the light REEs show variable results. This data indicates change in the REE budget between the onset of zircon crystallization and eruption of the UBT. Whole-rock INAA data show systematic enrichments of U and Th toward the basal plinian fall unit, which indicates an increase of these elements toward the top of the UBT magma chamber. Using the combined data, we discuss several magmatic processes to explain the observed textural and compositional zoning in the UBT zircons. These include post-eruption vapor phase growth of zircon rims, inheritance of zircon cores from assimilated Precambrian basement, density separation and regrowth in a zoned magma chamber, and inheritance of zircon cores from ancestral Bandelier magmas. We prefer the model in which zoned zircons are related to an ancestral Bandelier magma body.
- Werner, Cindy; Stimac, James A.; Hickmott, Donald., 1996, Trace element distributions in the Upper Bandelier Tuff, New Mexico: Zircon zoning and locations for magmatic evolution of the Valles system, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 285-291.