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doi: 10.7185/geochemlet.1815 High precision triple oxygen isotope analyses of terrestrial materials show distinct fields and trends in Δʹ 17 O -δʹ 18 O space that can be explained by well understood fractionation processes. The Δʹ 17 O -δʹ 18 O 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 δ 18 O values >-20 ‰ is δ' 17 O = 0.52654 (±0.00036) δ' 18 O + 0.014 (±0.003). Low temperature marine sediments define a unique and narrow band in Δʹ 17 O -δʹ 18 O space with high δʹ 18 O and low Δʹ 17 O values explained by equilibrium fractionation. Hydrothermal alteration shifts the rock composition to lower δ' 18 O values at low fluid/rock ratios, and finally higher Δʹ 17 O 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' 17 O is desirable. Adoption of λ RL = 0.528 (λ RL = slope of δʹ 17 O -δʹ 18 O reference line, the 'Terrestrial Fractionation Line' or TFL) would bring the 'rock' community in line with well established hydrological reporting conventions.

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