HR: 0800h
AN: V51C-0713    [Abstracts]
TI: Integrating Trace Element Abundances and Derived Temperatures for Zircon and Titanite to Elucidate Petrogenesis of a Cretaceous Granodiorite, Southeastern California
AU: * Mazdab, F K
EM: fmazdab@usgs.gov
AF: U.S.G.S-Stanford Ion Probe Laboratory, 367 Panama Mall, Stanford, CA 94305, United States
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: U.S.G.S-Stanford Ion Probe Laboratory, 367 Panama Mall, Stanford, CA 94305, United States
AU: Barth, A P
EM: ibsz100@iupui.edu
AF: I.U.P.U.I. Department of Geology, 723 W. Michigan St. SL-118, Indianapolis, IN 46202, United States
AB: Zircon and titanite are among the more important accessory phases useful for geochronology and geochemistry. With the advent, calibrations, and subsequent refinements of the Ti-in-zircon and Zr-in-titanite geothermometers, two powerful new methods have become available to track and integrate the compositional variations of these minerals during magmatic processes. Using the SHRIMP-RG, we measured REE, U, Th and other trace elements in co-existing zircon and titanite from a Cretaceous granodiorite from Joshua Tree National Park, California. Concurrent measurements of Ti in zircon and Zr in titanite, coupled with independent assessments of a_TiO2, a_SiO2} and pressure, allowed the estimation of derived crystallization temperatures T_Ti and T_Zr in zircon and titanite, respectively. Data indicate that crystallization of zircon and titanite occurred largely simultaneously, from close to 800 °C (the calculated zircon saturation temperature) down to ~700 °C, with continuing zircon crystallization down to ~650 °C. In zircon, concentrations of both Th and U vary somewhat irregularly with T_Ti, but with cooling the Th/U ratio decreases steadily from 4 to 0.2. In contrast, Th concentrations in titanite decrease sharply with decreasing T_Zr, while U contents show two separate trends that intersect at the lowest T_Zr (~700 °C). Above T_Zr = 750±10 °C, titanite Th/U decreases steadily from 14 to 4 with decreasing T_Zr, but below this temperature there is an abrupt change in Th/U to nearly constant values of 0.1-0.5. Total REE concentrations decrease in both zircon and titanite with decreasing temperature, but the observed europium anomalies (Eu/Eu*) illustrate complex, dissimilar trends for each mineral. In zircon, interior zones show constant or decreasing Eu/Eu* (at values of Eu/Eu* <1) with decreasing T_Ti while outer rims show an increase, but still never attaining Eu/Eu* >1. In titanite, hotter zones have Eu/Eu* <1, but Eu anomalies decrease in magnitude and become increasingly positive (i.e. to values of Eu/Eu* >1) in cooler zones. In addition, an abrupt change in slope observed in Eu/Eu* vs. T_Zr occurs with cooling at 750±10 °C, roughly the same temperature at which the zircon interior-to-rim Eu/Eu* transition is also observed. These data suggest a major physiochemical change occurred in the magma around 750 °C which affected multiple compositional parameters and was manifested in both zircon and titanite. Additional changes in chemical trends in zircon occur below ~700 °C, likely due in part to the absence of competing titanite crystallization. The evaluation of additional geochemical data from the other accessory and major minerals will help elucidate the nature of these and other magmatic processes.
DE: 1065 Major and trace element geochemistry
DE: 1094 Instruments and techniques
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting