A geochemical study on the behavior of rare earth and trace elements in the Otowi Member of the Bandelier Tuff, Jemez Mountains, New Mexico.
Winters, Roger Lee (39988) A geochemical study on the behavior of rare earth and trace elements in the Otowi Member of the Bandelier Tuff, Jemez Mountains, New Mexico.
| Authors | Winters, Roger Lee |
|---|---|
| Year | 39988 |
| Title | A geochemical study on the behavior of rare earth and trace elements in the Otowi Member of the Bandelier Tuff, Jemez Mountains, New Mexico. |
| Document type | Thesis-or-Dissertation |
| Degree | Master's Washington State University. Pullman, WA, United States. Pages: 75. 2001. |
| GeoRef | 2003-032034 |
| Keywords | Bandelier-Tuff; Cenozoic-; geochemistry-; igneous-rocks; magmatism-; metals-; mineral-composition; New-Mexico; Otowi-Member; Pleistocene-; pumice-; pyroclastics-; Quaternary-; rare-earths; trace-elements; tuff-; United-States; Valle-Grande-Mountains; volcanic-rocks |
Abstract
The Otowi lower Member of the Bandelier Tuff erupted from the Valles Caldera, NM, at 1.61 Ma. The Otowi consists of approximately 350 km super 3 DRE of high-silica rhyolitic pumice deposited as plinian fallout units and ignimbrites. After deposition of an initial plinian fallout unit unit a, eruption of fallout and pyroclastic flows was simultaneous, and the entire compositional range seen in the ignimbrite is duplicated in the fallout. Quartz and sanidine phenocryst contents in pumice range from less than 5 percent up to 20 percent. Minor and trace phases include magnetite, hedenbergite, fayalite, zircon, allanite, chevkinite and britholite. Fayalite is confined to unit a. The REE-Ti silicate mineral chevkinite, and britholite, a REE-rich silicophosphate of the apatite group, occur as inclusions in fayalite, hedenbergite and magnetite. Relative to whole pumice, allanite and chevkinite have high LREE/MREE, while britholite is enriched in MREE. Otowi britholites represent an extreme example of REE substitution into apatite via the mechanism REE+Si = Ca+P, with a temperature dependence consistent with the experimental results of Watson and Green 1988. Whole-pumice trace element behavior is consistent with separation of the observed phenocryst and trace phase assemblage from rhyolitic liquid. Cs, Rb, Th, U, Nb, Ta and heavy REE behave incompatibly throughout the system. REE show decreasing compatibility with increasing atomic number a well known effect in high-silica rhyolites, ascribed to the very high mineral/melt distribution coefficients for LREE in chevkinite and allanite. Plots of Otowi MREE vs. incompatible elements show an inflection among the most fractionated compositions. This inflection coincides which the point at which britholite joins the fractionating assemblage; hence britholite represents a new addition to the list of trace phases that control REE abundances in strongly differentiated magmas.