Abstract:

This article presents an overview on progress in the studies of water in mantle transition zone during the past two decades. It has been shown that water in the Earths deep interior is stored in nominal anhydrous minerals (NAMs) in the form of OH-(hydroxyl). HP/HT experiments revealed high solubility of hydroxyl in the minerals of mantle transition zone, which decreases with increasing temperature. The solubility of hydroxyl is up to 2%3% in β and γolivine, and about 01％ in majorite. So we can estimate that the mantle transition zone can dissolve 12%191％ water, about 3962 times of the water in ocean. The mantle transition zone is potentially the largest water reservoir in the mantle because the water solubility of the upper mantle (excluding the transition zone) and the lower mantle is not more than 01％. However, high water solubility may not lead to high water contents in the transition zone. A hydrous and dry transition zone have been argued and supported by various pieces of geological and geophysical evidence. If mantle transition zone is hydrous, many processes in deep Earth can be influenced. Under hydrous conditions, the pressure will be lower for α to β and postgarnet transitions and higher for postspinel transition compared with anhydrous conditions, resulting in a thicker mantle transition zone. Water can also lower the temperature of partial melting and increase the degree of melting. A hydrous mantle transition zone can also well explain the sharp “410 km” seismic discontinuity, the chemical difference between ocean island basalts (OIB) and midocean ridge basalts (MORB). The studies of the water in transition zone are of great importance for a better understanding of many geodynamic processes in the deep interior of the Earth including the deep subduction in Eastern China.

Key words: Nominal Anhydrous Minerals (NAMs), water solubility, mantle transition zone