...V.R. and Aggarwal, H. (1993) Comet Impacts and Chemical...

来自 : www.oalib.com/references/15201 发布时间：2021-03-24

ABSTRACT: Thoughtwo-thirds of Earth’s surface is covered by oceans, measurements of hydroxyl concentrationsin upper mantle minerals, specifically in olivine, reportedly providesurprisingly low values. This has been interpreted to mean that there is littledissolved H2O in the Earth’s mantle. By inference, when Earthformed, there might not have been able enough water to fill the oceans throughvolcanic degassing. It has therefore been proposed that the missing water wasdelivered to Earth from space, through comets and other impacting bodies.However, the reported low hydroxyl concentrations in olivine and similar mineralsisprobably based on a profound misunderstanding of a solid state reaction thatconverts hydroxyls into something more difficult to detect. There is indeed aredox reaction that converts, during cooling, solute hydroxyls in the matrix ofminerals into peroxy plus H2. This widely overlooked redox conversiontakes place under thermodynamic non-equilibrium conditions. Its significance isthat any mineral and any rock available for collection at the Earth surface hasgone through a process that causes hydroxyls, the telltale sign of dissolved H2O,to change into peroxyplusH2. The H2 molecules are diffusivelymobile and may leave even structurally dense mineral grains. The remaining peroxy thus become the memory of the “true” solute H2Ocontent, besides a few residual hydroxyls. Though first described over 30 yearsago, this redox conversion has been largely ignored. As a result it is unknownhow much H2O is contained in the Earth’s upper mantle but it iscertainly much more than has been assumed until now on the basis of analysis ofresidual hydroxyls.