Stable isotopes of authigenic minerals in variably-saturated fractured tuff

Abstract

Identifying stable isotope variation and mineralogical changes in fractured rock may help establish the history of climatic and geomorphological processes that might affect the isolation properties of a waste repository. This study examines the use of the stable isotope ratios of oxygen (¹⁸0/¹⁶0) and carbon (¹³C/¹²C) in authigenic minerals as hydrogeochemical tools tracing low-temperature rock-water interaction in variably-saturated fractured tuff. Isotopic compositions of fracture filling and rock matrix minerals in the Apache Leap tuff, near Superior, Arizona were concordant with geothermal temperatures and in equilibrium with water isotopically similar to present-day meteoric water and groundwater. Oxygen and carbon isotope ratios of fracture-fillings, in unsaturated fractured tuff, displayed an isotopic gradient probably resulting from near-surface isotopic enrichement due to evaporation rather than the effects of rock-water interaction. Oxygen isotope ratios of rock matrix opal samples exhibited an isotopic gradient due to leaching and reprecipitation of silica at depth. Results can be used to further define primary flowpaths and the movement of water in variably saturated fractured rock.