Though two-thirds of Earth’s surface is covered by oceans, measurements of hydroxyl concentrations in upper mantle minerals, specifically in olivine, reportedly provide surprisingly low values. This has been interpreted to mean that there is little dissolved H2O in the Earth’s mantle. By inference, when Earth formed, there might not have been able enough water to fill the oceans through volcanic degassing. It has therefore been proposed that the missing water was delivered to Earth from space, through comets and other impacting bodies. However, the reported low hydroxyl concentrations in olivine and similar mineralsis probably based on a profound misunderstanding of a solid state reaction that converts hydroxyls into something more difficult to detect. There is indeed a redox reaction that converts, during cooling, solute hydroxyls in the matrix of minerals into peroxy plus H2. This widely overlooked redox conversion takes place under thermodynamic non-equilibrium conditions. Its significance is that any mineral and any rock available for collection at the Earth surface has gone through a process that causes hydroxyls, the telltale sign of dissolved H2O, to change into peroxyplusH2. The H2 molecules are diffusively mobile and may leave even structurally dense mineral grains. The remaining peroxy thus become the memory of the “true” solute H2O content, besides a few residual hydroxyls. Though first described over 30 years ago, this redox conversion has been largely ignored. As a result it is unknown how much H2O is contained in the Earth’s upper mantle but it is certainly much more than has been assumed until now on the basis of analysis of residual hydroxyls.
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