The Provenance of Earth's Water

Water is everywhere on the surface of Earth.  Water is also a key ingredient in geochemical processing of Earth.  Through plate tectonics water is cycled through the mantle and behaves as a flux for production of magmas.  The provenance of water on Earth appears straightforward.  Hydrogen and oxygen are the two commonest elements capable of forming molecules and so water is an expected species in the molecular cloud forming the solar system.  Water on Earth has been ascribed to late stage infall (comets and asteroids) although there are details that are only now being revealed.  Water can be characterised through its abundance, but also through isotopic compositions (D/H, 18O/16O, 17O/16O).

Our instrument of choice for carrying out these analyses is a large-radius secondary ion mass spectrometer, SHRIMP-SI, that was designed and built with water and oxygen isotopes specifically in mind.  The large radius allows high mass dispersion for separation of 16OH from 17O allowing coupled measurements of OH abundance and oxygen isotope compositions.  Much of the development of this instrument centered around excluding atmospheric water contamination, through differentially pumped chambers.  In rocks, water can be present from trace quantities to major contributions (>10 wt%) and so the detection system must be capable of a large dynamic range.  The charge-mode Faraday cup system was developed for this type of analysis.  Finally, the water background from mounting systems had to be controlled.  Epoxy mounts, thin sections, and even indium metal mounts can all show contamination.  Mounting of samples in molten BiSn allows a robust mounting material capable of being repolished and low inherent water background.

Our analytical work has followed two different routes: terrestrial geochemistry and solar system cosmochemistry.  Work in terrestrial samples has ranged from water concentrations in volcanic glasses through to low level water analyses in mantle and lower crustal melts.  But it is in the analysis of extraterrestrial samples where the coupled isotopes and water concentrations allow the greatest insight.  The low-level water concentrations in high-temperature objects like chondrules shows distinct three-oxygen isotope compositions and water concentrations.  

The ultimate question of where did water on Earth come from appears to have been solved through the recent sample return missions of Hayabusa 2 and Osiris REx.  These missions visited C-type asteroids, which were thought to be related to the carbonaceous chondrites.  But the C-type asteroids dominate the main asteroid belt and carbonaceous chondrites are quite rare.  Both Hayabusa 2 and Osiris REx recovered CI chondrite-like material, amongst the rarest subtype of carbonaceous chondrite.  The resolution of the abundance paradox comes down to the friability of the asteroid-return samples.  These materials simply do not survive atmospheric entry.  As such, it is likely that Earth received a large contribution of carbonaceous chondrite material during its formation and ongoing accretion of extraterrestrial material.