Synchrotron Infrared Absorbance Measurements of Hydrogen in MgSiO 3 Perovskite

Phase relations of bridgmanite, the most abundant mineral in the Earth's lower mantle

The knowledge of phase relations of constitutive minerals is essential to investigate the structure, dynamics and evolution of the Earth and planetary interiors. This paper reviews the phase relations of bridgmanite, the most abundant mineral in the Earth's lower mantle, with an ideal composition of MgSiO3. Bridgmanite has an orthorhombic structure with larger dodecahedral A and smaller octahedral B cation sites. The A-sites can incorporate Mg2+, Fe2+, Fe3+, and Al3+, while the B-sites accommodate Si4+, Al3+ and Fe3+. The incorporation of hydrogen and large cations like Ca is likely limited, although these issues are still debated. Al3+ and Fe3+, respectively, can form the charge-coupled components, AlAlO3 and Fe3+Fe3+O3 occupying both A- and B-sites. When both Al3+ and Fe3+ are present, Al3+ occupies B-sites, and Fe3+ occupies A-sites, forming Fe3+AlO3. In systems with excess MgO, Al and Fe3+ also form the oxygen vacancy components MgAl3+O2.5□0.5 and MgFe3+O2.5□0.5. The phase relationships of bridgmanite with coexisting phases are discussed as a function of pressure, temperature, and oxygen fugacity from the simple MgSiO3 system to the complex MgO-Fe2+O-Fe3+2O3-Al2O3-SiO2 system.

Water in Earth's lower mantle

Secondary ion mass spectrometry measurements show that Earth's representative lower mantle minerals synthesized in a natural peridotitic composition can dissolve considerable amounts of hydrogen. Both MgSiO3-rich perovskite and magnesiowüstite contain about 0.2 weight percent (wt%) H2O, and CaSiO3-rich perovskite contains about 0.4 wt% H2O. The OH absorption bands in Mg-perovskite and magnesiowüstite were also confirmed with the use of infrared microspectroscopic measurements. Earth's lower mantle may store about five times more H2O than the oceans.

The Effect of H 2 O on the 410-Kilometer Seismic Discontinuity

The 410-kilometer seismic discontinuity is generally considered to be caused by a phase transformation of the main constituent of the upper mantle, olivine, alpha-(Mg,Fe)(2)SiO(4), to beta-(Mg,Fe)(2)SiO(4). Recent data show that H(2)O dissolves in olivine and other nominally anhydrous mantle minerals and that the partitioning of H(2)O between olivine and beta-(Mg,Fe)(2)SiO(4) is about 1:10. Such behavior strongly affects the region over which the alpha to beta phase transformation occurs and hence the seismic discontinuity that results. The observed width of the discontinuity constrains the maximum H(2)O content of upper mantle olivine to about 200 parts per million by weight.