HR: 12:05h
AN: V11F-08    [PDF]
TI: The Protracted History of Magmatic Evolution Recorded by Zoning in Allanites
AU: * Vazquez, J A
EM: jvazquez@ess.ucla.edu
AF: UCLA, Dept of Earth and Space Sciences, Los Angeles, CA 90095-1567 United States
AU: Reid, M R
AF: UCLA, Dept of Earth and Space Sciences, Los Angeles, CA 90095-1567 United States
AU: Reid, M R
AF: Northern Arizona University, Dept of Geology, Flagstaff, AZ 86011 United States
AB: Compositional zoning in crystals provides a detailed but nevertheless ambiguous record of melt differentiation in magma chambers because the duration and absolute timing represented by the zoning stratigraphy are essentially unknown. Using a novel marriage of {\it in situ} compositional and isotopic analyses of zoning in allanite, we unravel the magmatic history recorded by single crystals and resolve the duration and ages of differentiation in the voluminous rhyolitic magma chamber that produced the 75 ka Youngest Toba Tuff (YTT), Indonesia. Detailed electron probe traverses reveal that allanites from the most-evolved portion of the YTT magma chamber (75 wt.% SiO$_{2}$) are strongly zoned in composition and single crystals may zone to compositions that match those for allanites from the {\it least-evolved} (69 wt.% SiO$_{2}$) rhyolite reported by Chesner and Ettlinger (1989). Low MnO/MgO, high La/Nd, and greater concentrations of Mg, La, \& Ce are characteristic of less-evolved allanite zones, and normal zoning produces trends to higher MnO/MgO, lower La/Nd, and higher concentrations of Mn \& Th. Most allanites have similar patterns of oscillatory zoning punctuated by resorbed boundaries and mantled by a near-rim section of normal zoning recording episodic mixing with hotter, less-evolved melts and subsequent growth from more-evolved melts. The core-to-rim differentiation history revealed by these single allanites represents a time scale of up to 150 ky that continued up to the time of eruption, as determined by ion probe $^{238}$U-$^{230}$Th dating. Nonetheless, no single allanite composition is associated with a specific time interval in the magmatic evolution. In fact, compositional variability increases by up to a factor of three within ca. 35 ky of eruption, suggesting an episode of increased crystal and/or melt mixing relatively close to eruption. The magnitude of MnO/MgO and La/Nd variations, as well as parental melt compositions predicted by the zoning, fall within the ranges reported for erupted YTT glasses, suggesting the allanites record interactions between a diversity of melts related by the 40-50% fractionation calculated by Chesner (1998). Our results demonstrate that minerals like allanite can record a complex differentiation history of fractionation and episodic mixing, which in the case of the YTT allanites represents protracted residence and crystallization in a voluminous magma chamber.
DE: 1035 Geochronology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting