Strategies towards robust interpretations of in situ zircon oxygen isotopes

Abstract

Oxygen isotopes are a versatile tool to address a wide range of questions in the Earth sciences. Applications include geothermometry, paleoclimatology, tracing of geochemical reservoirs, fluid-rock interaction, magmatic petrogenesis, and identification of extra-terrestrial materials. Zircon arguably provides one of the most robust records of primary magmatic O isotope ratio due to low diffusion rates in crystalline grains. The ability to correlate zircon O isotopes with temporal and petrogenetic information (e.g. U-Pb geochronology, Lu-Hf isotopes, and trace elements) makes this mineral a key archive for understanding Earth’s crustal evolution. Consequently, zircon O isotope geochemistry has found widespread usage to address fundamental questions across the earth and planetary sciences. The general apparent ease of O isotopic acquisition through the advancement of rapid in situ techniques (i.e. secondary ion mass spectrometry; SIMS) and associated dedicated national laboratories has led to the generation of large O isotopic data sets of variable quality, highlighting the importance of a coherent workflow for data collection, reduction, and presentation. This paper presents a set of approaches for measurement, assessment, and reporting of zircon O isotope data. The focus in this contribution is on in situ analysis via secondary ion mass spectrometry using large geometry instruments, but other commonly used techniques are briefly reviewed for context. This work aims to provide an analytical framework necessary for geologically meaningful interpretation of O isotope data. In addition, we describe inherent geological (e.g. radiation-induced disturbance of the zircon O isotopic system) and analytical (e.g. fractionation due to sample topography effects) challenges and outline means to identify and avoid such issues as a prerequisite to the generation of robust primary O isotopic signatures for geological interpretation.