Storage and recycling of water in the Earth’s mantle

Abstract:

Most natural samples originating from the mantle contain traces of water. It can be observed that water content varies laterally as a function of the geodynamic context, but also with depth in cratons. Basalts from mid-ocean ridges, which sample the convecting upper mantle, contain generally below 0.6 wt% H₂O leading to 50-330 parts per million by weight in the source. Oceanic Islands Basalts are more hydrated with contents ranging from 0.6 to 1.1 wt%, leading to 350-1100 ppm wt H₂O in the source. Arc basalts are even more hydrated with water contents ranging from 0.2 to 5-6 wt% H₂O testifying of the recycling of water by subduction. Kimberlite magmas are also the proof that local saturation in volatiles is possible.

Among xenoliths, the samples from cratons are very interesting because they may provide a depth profile of water. However, the variation of water content in olivine with depth differs from craton to craton, and is the result of a complex geological history. Also, olivine inclusions in diamond and olivine from peridotite xenoliths do not give the same message regarding to water activity.

The water storage capacity of the mantle is defined as the maximum water or hydroxyl that can be incorporated in its constitutive minerals before a free fluid phase appears. It can be determined experimentally and confronted to geophysical observations, such as low seismic velocities, and electrical conductivity. In this talk we will review our current knowledge of water incorporation in NAMs as determined experimentally and compare it with available observations. New data concerning clinopyroxenes will be shown. The aim being to understand the deep water cycle.