This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)
The purpose of this projects is to provide a new set of laboratory data to understand the deep mantle water cycling. Possible melting near the 410-km and its consequence on geochemical cycling have been speculated. However, there have been no definitive studies on the conditions for partial melting under the deep upper mantle conditions. Consequently there have been large uncertainties on the critical water content. In this proposal, our efforts are focused on determining the critical water content for partial melting near 410-km depth. The critical water content for partial melting is a critical parameter to understand the deep water cycling including the origin and evolution of the ocean mass and the processes to control the water content in the upper mantle. In this project, we will use electrical conductivity to determine the onset of partial melting and consequently, the results of this work will also provide a critical new data set to interpret the distribution of electrical conductivity in terms of temperature and water content in the mantle.
The conditions for partial melting involving water are difficult to determine experimentally. We will investigate the phase diagram of water + peridotite system by measuring electrical conductivity. we anticipate that the onset of partial melting will dramatically change the conductivity and consequently, by measuring the electrical conductivity, we should be able to detect a small degree of partial melting. Experiments will be conducted using an impedance spectroscopy with a multianvil apparatus under the conditions of P (pressure) =4-16 GPa, and T (temperature) = 1000-1900 K. The partial melting will be detected by a jump in conductivity as well as the change in the trace element concentrations. The results have an important implication for the deep mantle water cycling as well as the interpretation of electrical conductivity.
In this project, we have studied the electrical conductivity of minerals in Earth's deep interior. Electrical conductivity is one of the properties of minerals that we can infer from remote sensing techniques. Another remote sensing approach in geophsyics is to use seismic waves. In contrast to sesimic wave propagation, elecrical conductivity of minerals is highly sensitive to temperature and water content but less sensitive to major element chemistry. Consequently, when stdies of electrical conductivity of minerals in the lab are combined with remote sensing studies on electrical conductivity in Earth's interior, one can learn a lot on the distribution of temperature and water in Earth's interior. In this project, our focuse has been the relatively shallow part of Earth (the upper mantle), where a lot of geodynamic activities occur. We have measured the elecrical conductivity in various minerals as a function of tempersture, water content or the content of graphite. Our experimental studies on the upper mantle and lower crurst minerals olivine, talc, amphibole have provided a first data set on the influence of water and graphite on electrical conductivity. These results provide an important data base to infer temperature and water content in the upper mantle. For example, we showed that the reported high conductivity in some of the lower crust can be atributed to the dehydration of amphibole.
