Electrical conductivity of OH-bearing omphacite and garnet in eclogite: the quantitative dependence on water content
Eclogite is potentially an important constituent in local regions in the deep crust and upper mantle. The electrical conductivity of omphacite and garnet in eclogite has been measured at 1 GPa and 350–800 °C with pre-annealed OH-bearing samples. The conductivities were determined using a piston–cylinder apparatus and a Solartron-1260 Impedance/Gain Phase Analyser in the frequency range of 106–1 Hz. The sample water contents show almost no change before and after the experimental runs. The conductivity of both omphacite and garnet increases with temperature, and the activation enthalpy is ~ 82 kJ/mol for omphacite and 90 kJ/mol for garnet, which is nearly independent of water content in each mineral. The conduction is probably dominated by protons, and for both minerals, the conductivity increases linearly with water content, with a water content exponent of ~ 1. These data are used to model the bulk conductivity of an eclogite with different water contents and modal compositions. In combination with reported data, the conductivity of the eclogite is similar to that of typical granulites above 600 °C, but is much larger than that of olivine, assuming small to moderate water contents. This would mean that the contribution of eclogites, if present, to the electrical structure of the deep continental crust cannot be easily separated from that of granulites, and that the regional enrichments of eclogites in the upper mantle may cause high electrical anomalies. The results also provide information for the electrical property of orogen-related thickened deep crust where eclogites may be locally abundant, e.g., in the Dabieshan region and the Tibet plateau. At mantle depths, eclogitized portions of subducted slabs are usually of very low conductivities as suggested by geophysical observations, implying small water contents in the constitutive omphacite and garnet and the limited ability of these minerals in recycling water into the deep mantle.
KeywordsElectrical conductivity Water Omphacite and garnet Eclogite Deep water cycling Experimental studies
X.Y. thanks Stefan Keyssner for providing the starting eclogite. Editorial handling by Jochen Hoefs and comments by Fabrice Gaillard and the anonymous reviewers helped to improve the manuscript. This work was supported by the National Science Foundation of China (41725008 and 41590622) and National Basic Research Program of China (973 Project, 2014CB845904).
- Anderson DL (2007) The eclogite engine: chemical geodynamics as a Galileo thermometer. Geol Soc Am Spec Pap 430:47–64Google Scholar
- Orozbaev RT, Takasu A, Bakirov AB, Tagiri M, Sakiev KS (2010) Metamorphic history of eclogites and country rock gneisses in the Aktyuz area, Northern Tien-Shan, Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision. J Metamorph Geol 28:317–339CrossRefGoogle Scholar
- Skogby H, Bell DR, Rossman GR (1990) Hydroxide in pyroxene—variations in the natural environment. Am Mineral 75:764–774Google Scholar
- Wendlandt RF, Huebner JS, Harrison WJ (1982) The redox potential of boron nitride and implications for its use as a crucible material in experimental petrology. Am Mineral 67:170–174Google Scholar
- Yaxley GM, Green DH (1998) Reactions between eclogite and peridotite: mantle refertilisation by subduction of oceanic crust. Schweiz Mineral Petrogr Mitt 78:243–255Google Scholar