Mass-dependent triple oxygen isotope variations in terrestrial materials

Abstract

High precision triple oxygen isotope analyses of terrestrial materials show distinct fields and trends in Δ′17O - δ′18O space that can be explained by well understood fractionation processes. The Δ′17O - δ′18O field for meteoric waters has almost no overlap with that of rocks. Globally, meteoric water defines a λ value of ∼0.528, although a better fit to waters with δ18O values >-20 ‰ is δ ′17O = 0.52654 (±0.00036) δ′18O + 0.014 (±0.003). Low temperature marine sediments define a unique and narrow band in Δ′17O - δ′18O space with high δ′18O and low Δ′17O values explained by equilibrium fractionation. Hydrothermal alteration shifts the rock composition to lower δ′18O values at low fluid/rock ratios, and finally higher Δ′17O when F/R ratios are greater than 1. In order to make the triple isotope data tractable to the entire geological community, consensus on a reporting scheme for Δ′17O is desirable. Adoption of λRL= 0.528 (λ RL = slope of δ′17O - δ′18O reference line, the 'Terrestrial Fractionation Line' or TFL) would bring the 'rock' community in line with well established hydrological reporting conventions.