ABSTRACT

Determination of isotopic composition of extraterrestrial liquid water provides important information regarding the origin of water on Earth and the terrestrial planets. Fluid inclusions in halite of ordinary chondrites are the only direct samples of extraterrestrial liquid water available for laboratory measurements. We determined H and O isotopic compositions of this water by secondary ion mass spectrometry equipped with a cryogenic apparatus for sample cooling. Isotopic compositions of the fluid inclusion fluids (brines) were highly variable among individual inclusions, -400 < δD < +1300‰; -20 < Δ¹⁷O < +30‰, indicating that these aqueous fluids were in isotopic disequilibrium before trapping in halite on asteroids. The isotopic variation of fluids shows that various degrees of water-rock interaction had been underway on the asteroids before trapping between D-rich-¹⁶O-poor aqueous fluid, D-poor-¹⁶O-rich aqueous fluid, and asteroidal rock by delivery of cometary water onto hydrous asteroids. This may be a fundamental mechanism in the evolution of modern planetary water.