Experimental Studies of Hydrous Mantle Melting

This project focuses on understanding how the Earth?s mantle melts in subduction zones. H2O is a key ingredient in the formation of subduction zone magmas that erupt at arc volcanoes, volcanoes that form above subducted oceanic plates. This study will use experimental petrology to understand how magmas are formed when H2O is involved in the melting process. The experiments will consist of melting rock (the Earth?s mantle) in the presence of H2O at the temperatures and pressures equivalent to those experienced at depth below the volcanic chains. Very little is known about how H2O influences the temperature and chemical compositions produced by mantle melting. By understanding water?s influence on the temperature, melt composition and chemical reactions of this magma generation process, we will gain new fundamental knowledge of where and how the Earth?s continental crust has been generated through geologic time. This new knowledge will advance basic science by providing a quantitative framework for understanding the recycling of water through the Earth over the 4.5 billion years of our planet?s history.

This Accomplishment Based Renewal requests support for a program of experimental petrology studies that will quantify the influence of water on melting processes in subduction zones. The experiments will systematically investigate (1) H2O-undersaturated mantle melting from 1.6 to 2.8 GPa and (2) H2O-undersaturated reaction of deeper mantle melts with shallower mantle wall rock at 1.2 GPa. Very little is known about hydrous melting in the pressure range of 1.6-2.8 GPa, so the experiments will provide new information on the process of hydrous melting in this pressure range. The proposed reaction experiments between deeper mantle melt and shallower mantle wall rock will be the first hydrous reaction experiments ever performed at pressures between 0.5 and 2 GPa. These experiments will allow a quantitative understanding of the effects of reaction within the mantle wedge, a process that is widely acknowledged but one that currently lacks experimental exploration. For both sets of experiments, the goal is to assess the influence of H2O on melt composition and to use the new data to develop quantitative models for mantle melting as well as models of melt-rock reaction in the sub-arc mantle. The magmatic processes that flux H2O through the mantle and crust in subduction zones are of fundamental importance for understanding the chemical evolution of the Earth over the last 4.5 billion years. The proposed work will provide a quantitative basis for understanding how H2O has influenced the chemical composition of the Earth?s mantle and crust. The experimental data will be used to develop models for predicting temperature, H2O content and pressure of melting for natural primitive lavas, which will be shared with the geosciences community. The results of the experimental study will also allow development of better geodynamic models of flow coupling between the asthenospheric mantle and the subducting slab, as well as providing constraints on the physical process of melt transport through the mantle wedge.