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 17 O - 18 O space that can be explained by well understood fractionation processes. The 17 O - 18 O field for meteoric waters has almost no overlap with that of rocks. Globally, meteoric water defines a value of ~0.528, although a better fit to waters with 18 O values >-20 is ' 17 O = 0.52654 (0.00036) ' 18 O + 0.014 (0.003). Low temperature marine sediments define a unique and narrow band in 17 O - 18 O space with high 18 O and low 17 O values explained by equilibrium fractionation. Hydrothermal alteration shifts the rock composition to lower ' 18 O values at low fluid/rock ratios, and finally higher 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 ' 17 O is desirable. Adoption of RL = 0.528 ( RL = slope of 17 O - 18 O reference line, the 'Terrestrial Fractionation Line' or TFL) would bring the 'rock' community in line with well established hydrological reporting conventions.