Water in the Earth’s Lower Mantle

Abstract

All major, rock-forming lower-mantle minerals (bridgmanite, CaSi-perovskite, ferropericlase and stishovite) are “nominally anhydrous minerals” (NAMs), in which hydrogen comprises less than 1 wt % and whose chemical formula would be normally written without hydrogen. In NAMs, hydrogen occupies various defects of the crystal lattice and is bonded to structural oxygen, forming hydroxyl groups. Currently, two main techniques can be used for water determination in the mantle minerals: Fourier transform infrared spectrometry (FTIR) and secondary ion mass spectrometry (SIMS). They produce different results: determinations by SIMS are usually higher than quantifications of FTIR. As a result, the estimates of water concentrations in lower-mantle minerals vary widely. Most reliable concentrations of water are 1400–1800 ppm in bridgmanite, 10–80 ppm in ferropericlase, and 20–150 ppm in stishovite. The average concentration of water in the lower mantle is ~1500 ppm. Despite such minor concentrations in lower-mantle minerals, water forms a great reservoir within the lower mantle, probably amounting ~45.45 × 10²³ grams H₂O, i.e., ~3.3 times the mass of the Earth’s oceans. Some amount of water is transported into the lower mantle by subducting lithospheric slabs; this amount is balanced by the water flux from the lower mantle to the transition zone. Within areas of partial melting in the lower and upper parts of the lower mantle, as well as in some local areas, stress and thermal increase initiate release of water from lower-mantle minerals into melt. The enrichment of partial melts with H₂O depends on the P–T conditions, oxygen fugacity values, and percentage of melting. It causes major geodynamic processes that are initiated within the deep Earth. The major source of the water reservoir in the lower mantle is primordial water stored early in the Earth’s evolution.