MSA AWARD LECTURE: CALIBRATING THE EFFECT OF WATER ON SEISMIC DISCONTINUITIES IN THE EARTH'S MANTLE

It is generally accepted that most mantle seismic discontinuities are caused by phase transformations in silicate minerals. The 410 km discontinuity, for example, is apparently caused by the transformation of (Mg,Fe)₂SiO₄ olivine to the higher pressure polymorph wadsleyite. There are a rapidly increasing number of seismic studies that provide detailed information on the depth, depth interval and amplitude of the 410 km discontinuity on both a regional and global scale. Using the results of laboratory experiments these data can be interpreted to give details on the physical and chemical state of the mantle.

Due to the high solubility of H₂O in the mineral wadsleyite it has been recognized that the presence of H₂O in the mantle could influence the depth and depth interval of the olivine to wadsleyite phase transformation. This depth interval or width arises because the transformation occurs in a multi-component system, and is therefore smeared out over a pressure interval. The width of the 410 appears to vary regionally in the mantle from values of <4 km to over 30 km. To quantify the expansion of the wadsleyite stability field as a result of the presence of H₂O, multianvil experiments were performed in the Mg₂SiO₄-H₂O system. At 1200^oC the wadsleyite stability field was found to expand to lower pressures by approximately 1 GPa under H₂O saturated conditions. At 1400^oC the expansion was approximately 0.4 GPa and no expansion was detected at 1600^oC. When these results are combined with phase relations in the Mg₂SiO₄-Fe₂SiO₄ system it can be shown that significant effects on the pressure and pressure interval of the olivine to wadsleyite transformation are only expected at lower than ambient mantle temperatures and for H₂O concentrations that are substantially greater than 0.2 weight %. Areas of the mantle where the 410-km discontinuity appears to occur over a depth interval of over 20 km can only be explained if the temperature in these regions is at or below 1200^oCC and water concentrations are close to the level where olivine would become H₂O saturated (i.e. >0.5 wt %). A broad discontinuity may not just arise from the presence of H₂O however. The presence of ferric Fe can also broaden the olivine-wadsleyite transformation and there is significant evidence to argue that ferric iron is more likely to be the cause of broad 410 observations.