Geological Society of America

198 D25. Recent Advances in Petrology, Igneous (Posters)

Tuesday, 24 September 2019
9:00 AM - 6:30 PM
Hall AB, North Building

UNDERSTANDING OXYGEN ISOTOPES OF ZIRCON INCLUSIONS: A CASE STUDY OF QUARTZ INCLUSIONS IN TIBETAN GRANITOID ZIRCONS

Paper number: 198-14
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Ellen W. Alexander Presenting Author
University of California, Los Angeles, Los Angeles, CA
Casey A. Yamamoto-Hillman
Caltech, Pasadena, CA
Casey A. Yamamoto-Hillman
University of California, Los Angeles, Los Angeles, CA
Matthew Wielicki
University of Alabama, Tuscaloosa, AL
Timothy Harrison
University of California, Los Angeles, Los Angeles, CA

Abstract

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. ∆¹⁸O_{quartz-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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-339224

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