New Mexico Geological Society Annual Spring Meeting — Abstracts

The Cerro Toledo Rhyolite (CTR) comprises small rhyolite domes (<1 km³) and a well-preserved sequence oftephra deposits which are stratigraphically between the major caldera-forming eruptions ofthe upper Bandelier Tuff (UBT ) and the lower Bandelier Tuff (LBT). The chemical compositions of the tephra record the re-establishment of a compositional zonation in the Bandelier magma chamber following recharge after the LBT eruption (Stix and Gorton, 1993).

Laser fusion ⁴⁰Ar/³⁹Ar dating (relative to an age of 27.9 Ma for Fish Canyon Tuft) and new geochemical data document the temporal changes in the magma composition within the magma chamber. New single crystal laser fusion ⁴⁰Ar/³⁹Ar isochron dates for the UBT and LBT are 1.225 ± 0.008 Ma and 1.608 ± 0.010 Ma, respectively. Seven CTR tephra from two sections in Pueblo Canyon (north of the Los Alamos airport) have dates ranging from 1.212 ± 0.009 Ma to 1.593 ± 0.009 Ma. The dates indicate that ~80% ofthe CTR tephra section was erupted within 250 ka of the LBT, and the lowest unit in the section has an age indistinguishable from the LBT. A break of 140 ka occurs near the top ofthe tephra sequence .before renewed eruptions just prior to the plinian fall deposits of the UBT.

Compatible trace element concentrations (Zr, Sr, LREE, Eu) and compatible/incompatible element ratios (Zr/Nb, La/Lu) decline steadily for 130 ka and then remain relatively uniform. Suppression of zircon and allanite crystallization and decreasing partition of Sr and Eu in sanidine due to increasing volatile contents may have stabilized the Zr, La, Sr,and Eu concentrations. The incompatible element concentrations (Rb, Y, Nb, Cs, Th, U) exhibit a regular near linear systematic increase with time. These data suggest that differentiation ofthe Bandelier magma chamber occurred in a systematic manner over the 380 ka between the eruptions ofthe LBT and UBT.