Oxygen isotopes as tracers and stopwatches for rock-fluid interaction in the crust: New analytical and experimental developments
Creators: name: Scicchitano, Maria Rosa
Publisher: name: The Australian National University
Year: 2018
Subjects: rock-fluid interactions
oxygen isotopes
ion microprobes
serpentinisation AND serpentinization
SIMS reference materials
antigorite
chrysotile
lizardite
olivine
matrix bias effects
oxygen diffusion
garnet
experiments

description: Metamorphism is the dominant transformation mechanism of rocks in most of the Earth’s lithosphere and it is driven by variations in temperature, pressure, deformation and influx of chemically active fluids. Fluids play a crucial role in affecting many physical and chemical processes. It is therefore essential to unravel their nature and source(s) and the extent of their infiltration to fully understand crustal evolution. Oxygen isotopes are one of the most robust tools used to obtain such information. Because fluid-rock interactions can occur at different stages of metamorphism, metamorphic rocks often preserve complex textures. A thorough understanding of rock-fluid interactions is however limited by the ability to resolve oxygen isotopic variations recorded at the microscale and by the robustness of oxygen isotopic signatures measured in nature. This challenge is addressed in this study by presenting new analytical and experimental developments concerning oxygen isotopes at the microscale and with implications for serpentinisation processes and oxygen diffusion in garnet. Serpentinised peridotites are important in recycling fluids and elements from ocean floor to mantle through seafloor metamorphism and subduction. This project represents the first investigation of (i) oxygen isotopes in serpentine minerals by ion microprobes and (ii) matrix bias effects related to Mg/Fe variations in olivine during oxygen isotope analysis by SHRIMP. Serpentine isotopic measurements were calibrated against the oxygen compositions of chemically homogeneous samples of antigorite, chrysotile and lizardite analysed by laser fluorination. Each serpentine mineral was analysed by SHRIMP to assess its homogeneity compared to San Carlos olivine, and potential matrix bias and crystal orientation effects. Matrix bias between antigorite/olivine and antigorite/lizardite was up to +3‰ and -1‰, respectively. Crystal orientation effects were identified only in chrysotile. Matrix bias effects in olivine were investigated by referencing isotopic measurements of ten chemically homogenous olivine standards (Fo74 to Fo100) to San Carlos olivine (Fo91). A matrix bias up to -2.0‰ was observed in olivine with Fo &gt; Fo92. The new standards and protocols were used to unravel the nature and source(s) of serpentinising fluid(s) in (i) a UHP serpentinite with multiple serpentine generations, (ii) a serpentinised dunite with zoned olivine grains surrounded by lizardite. Variations in the oxygen isotopic composition of garnet can be used to unravel the P-T-t-fluid evolution of numerous crustal rocks. However, the correct interpretation of measured oxygen isotopic heterogeneities and the understanding of rates and timescales of rock-fluid interactions require the knowledge of oxygen diffusion rates in garnet, which are yet poorly constrained. This work presents new high-P piston cylinder experiments performed in dry and wet conditions (T = 900-1600°C, P = 1.0-2.5 GPa) and 1 atm dry experiments (T = 1500-1600°C) using YAG and pyrope garnets. The recovered crystals were analysed in step-scanning and depth profiling mode by SHRIMP and in step-scanning mode by nanoSIMS. Mixing and edge effects during SHRIMP depth profiling were evaluated. The obtained results suggest no effects of chemical composition and pressure on oxygen diffusivity in garnet. The Arrhenius relation determined in this study predicts oxygen diffusivities faster than any previous calibration.