Redox melting processes in the mantle under reducing conditions

地幔的部分熔融是地球内部分化和化学演化的基础。除了温度升高或压力降低外，还可以通过氧化还原反应进行熔化，这一过程称为氧化还原熔化。在这种情况下，含有CH4的还原流体与相对氧化的橄榄岩相互作用，导致碳沉淀（如钻石）。水的含量变得更丰富，降低了橄榄岩的固相，在温度不变的情况下导致了熔融。有各种证据表明，这一过程是贯穿地球历史的熔体生成的可行机制：1)氧化状态不同的区域并置，2)上地幔深处存在还原条件，3)一些钻石的同位素组成表明是由含ch4的流体形成的。然而，对这种氧化还原熔体的组成和微量元素特征几乎一无所知。该项目的目标是：1)确定与含ch4流体共存的橄榄岩在对应约120-200 km深度压力下的固相温度；2)确定熔体成分，包括选定微量元素的行为；3)监测共存矿物和流体的成分；4)研究氧化还原熔融与金刚石形成之间的联系。实验将使用双胶囊技术进行，允许控制氧逸度，将使用Ir-Fe氧化还原传感器独立监测。微探针和LA-ICP-MS将分别用于测定主要元素和微量元素的组成。将在堪培拉澳大利亚国立大学使用胶囊穿刺/质谱法分析选定样品中的液体。

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.