Crystal-melt partitioning under high pressure hydrous conditions.

The presence of water inside the earth and on its surface has had the most profound influence on the evolution of our planet. The earth's crust, for example, is created from volcanic eruptions which are driven by water and which degas water to the atmosphere and oceans. Without water the crust would be of quite different composition, of low elevation and there would be no mountains. The degassing of water from inside the earth has created the oceans which were a major factor enabling the development of life. Despite its importance, however, we do not know how much water is left within the earth and we have only sketchy ideas about how water has influenced the compositions of the earth's different layers. The main aim of this project is to test a new hypothesis about the effect of water or the evolution of the earth's mantle, which is the primary source region of lavas arriving at the surface. The hypothesis is that, as material moves towards the surface, it releases water at 410 km depth (a pressure of 140000 atmospheres) and that this water causes melting to produce a thin-layer of water rich volcanic liquid. The remaining solid rises and becomes the main source region for the earth's crust. The liquid remains at depth. Our tests will involve determining the partitioning of large numbers of chemical elements between the upwelling solid minerals and the postulated melt layer. In this way we will be able to determine whether or not the chemical compositions of the outermost layers are consistent with the presence of substantial amounts of water at depth. The high pressure experiments and most of the detailed chemical analyses will be performed in the Earth Sciences department at the University of Bristol. This department has been developed, with NERC support, into a major European centre for high pressure research and microanalysis. Additional microanalysis will be performed at the NERC ion microprobe facility in the Department of Geosciences at the University of Edinburgh.