WATER SOLUBILITY IN MG-PEROVSKITES AND WATER STORAGE CAPACITY IN THE LOWER MANTLE

Abstract

The water storage capacity of the major constituent of the lower mantle, Mg-perovskite, is a matter of debate. Here we report water solubility of Mg-perovskites with different compositions observed in peridotite and MORB systems. IR spectra of pure MgSiO3-perovskite show bands at 3397, 3423, 3448, and 3482 cm-1 and suggest about 100 ppm H2O. The H2O content in Al-Mg-perovskite (4-7 wt% Al2O3; Mg#=100) is 1000-1500 ppm (major band at 3448 cm-1), whereas Al-Fe-Mg-perovskite in MORB (Al2O3=13-17 wt%; Mg#=58-61) contains 40-110 ppm H2O (major band at 3397 cm-1). The H2O content in Al-Fe-Mg-perovskite observed in peridotite (Al2O3=5-6 wt%; Mg#=88-90) is 1400-1800 ppm (major band at 3397 cm-1). Al-Fe-Mg-perovskite from the MORB system has a high Fe3+ content, Fe3+/#Fe=0.6, determined by electron energy loss spectroscopy measurements. Water can enter into the perovskite structure with oxygen vacancies originating from the substitution of Si by Al and Fe3+. Oxygen vacancy incorporation is favored for aluminous perovskite synthesized from the MgO-rich peridotite system. The substitution of Si4++Mg2+=2(Al,Fe)3+ prevails however in the Al-Fe-Mg-perovskite from the MORB system (MgO-poor, Al- and Fe-rich), explaining its restricted water solubility. The maximum amount of water stored in the lower mantle is estimated to be 3.42x1021 kg, which is 2.5 times the present ocean mass. Comparison of the phase relations in hydrous pyrolite and hydrous MORB indicates that pyrolite is more important as water container and water carrier in the mantle. Pyrolite contains: (1) dense hydrous magnesium silicates, existing under conditions of subducting slabs, and (2) hydrous wadsleyite, hydrous ringwoodite and water-bearing perovskite under the normal mantle and hotter conditions. Distribution of water to the MORB is restricted at the conditions of the transition zone and lower mantle.