Redox melting processes in the mantle under reducing conditions

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Partial melting of the mantle is fundamental to the differentiation and chemical evolution of the Earths interior. In addition to an increase in temperature or a lowering of pressure, melting can also develop through redox reactions via processes referred to as redox melting. In this case, reduced fluids containing CH4 interact with relatively oxidized peridotite, causing carbon precipitation (e.g. as diamond). The fluid becomes richer in H2O, which lowers the peridotite solidus and induces melting without any change in temperature. There is a variety of evidence that points to this process being a viable mechanism for melt generation throughout Earth history: 1) the juxtaposition of domains with contrasting oxidation states, 2) the presence of reduced conditions in the deeper upper mantle and 3) isotopic compositions of some diamonds indicating formation from CH4-bearing fluids. However, next to nothing is known about the composition and trace element characteristics of such redox melts. The goals of this project are: i) to determine the solidus temperature of peridotite coexisting with CH4-bearing fluids at pressures corresponding to ca.120-200 km depth, ii) to determine the melt composition, including the behavior of selected trace elements, iii) to monitor the composition of coexisting minerals and fluids and iv) investigate the link between redox melting and diamond formation. Experiments will be performed using the double capsule technique, allowing the control of oxygen fugacity, which will be monitored independently using Ir-Fe redox sensors. Microprobe and LA-ICP-MS will be employed to measure major and trace element compositions, respectively. Fluids will be analysed in selected samples using capsule piercing/mass spectrometry at the ANU, Canberra.