GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 198-14
Presentation Time: 9:00 AM-6:30 PM


ALEXANDER, Ellen W.1, YAMAMOTO-HILLMAN, Casey A.2, WIELICKI, Matthew3 and HARRISON, Mark1, (1)Earth, Planetary, and Space Sciences, University of California, Los Angeles, 595 Charles E. Young D. E., Box 951567, Los Angeles, CA 90095, (2)Geological and Planetary Sciences, Caltech, Pasadena, CA 91125; Earth, Planetary, and Space Sciences, University of California, Los Angeles, 595 Charles E. Young D. E., Box 951567, Los Angeles, CA 90095, (3)Department of Geological Sciences, University of Alabama, 201 7th Avenue, Tuscaloosa, AL 35406

Oxygen isotopes in magmatic systems can record signatures of protolith composition, assimilants, hydrothermal fluid interaction, and deuteric alteration. Additionally, high temperature equilibrium fractionation between magmatic phases can constrain magmatic temperature and track departures from isotopic equilibrium between phases. Identification of equilibrium vs. disequilibrium relationships between phases informs the application of multiphase thermobarometers, whose utility can be confounded by disequilibrium behavior. Coupled use of the Ti-in-zircon thermometer and Ti-in-quartz thermobarometer shows its potential for obtaining accurate and precise thermobarometry of silicic melt generation conditions. Paired analyses of quartz inclusions in zircon minimize the potential for unconstrained differences in P, T, and a(TiO2) during crystallization of the two phases. However, quartz inclusions may have been exposed, via cracks, to fluid alteration subsequent to crystallization of the zircon host; identifying cases of secondary alteration that may not be texturally evident is paramount to understanding the petrogenetic relations of inclusion-host pairs. High spatial resolution oxygen isotope analyses with SIMS, along with detailed textural analysis of cathodoluminescence (CL) images of zircon-host pairs, provide a minimally destructive and individualized test for the preservation of magmatic equilibrium between zircons and their quartz inclusions. We present a case study of nearly 100 host-inclusion pairs from Jurassic to Tertiary southern Lhasa terrane Gangdese Batholith granitoids. ∆18Oquartz-zircon often corresponds to equilibrium kinetic fractionation (ca. +1‰ to +4‰ at granitic temperatures), with many notable exceptions ranging from -4‰ to +10‰. Results are compared with textural features observed in CL imaging; particular cases where alteration is not texturally visible will be explored to guide future use of oxygen isotope equilibria as a proxy for secondary alteration.