Electrical conductivity of omphacite and garnet in eclogite: implications for water recycling in the mantle

Eclogite is an important constituent of subduction slabs and plays a critical role in transporting surface materials (e.g., water) into the deep Earth. Eclogite consists mainly of omphacite and garnet. Although nominally anhydrous, omphacite and garnet contain some amount of structural water (OH) in the lattice, which is up to >1500 ppm wt. H₂O. This is virtually the highest content in nominally anhydrous minerals (NAMs) derived from the crust and upper mantle (Ingrin and Skogby, 2000). The electrical property of NAMs is very sensitive to water content and a small amount of water could dramatically enhance the conductivity. Thus, laboratory measured conductivity data of omphacite and garnet may help to understand the deep water recycling by eclogitized slab.

In this study, we have systemically determined the conductivity of omphacite and garnet with different water contents. The experiments were carried out at 350-800 °C, 1 GPa (note that the effect of pressure itself on conductivity is very small) and Ni-NiO buffered conditions. The data show that the conductivity of both omphacite and garnet increases with water content or temperature. The bulk conductivity is then modeled for different mineral compositions and water contents over a range of conditions (Liu et al., 2019). In combination with the geophysically documented high resistivity of the crustal part in deep subducted slabs, we suggest that the water content in omphacite and garnet in the deep-subducted eclogites should not be high at mantle depths. This provides new insights into the deep water recycling by subducted eclogites.

 

References:

Ingrin, J., and Skogby, H., 2000, Hydrogen in nominally anhydrous upper-mantle minerals: Concentration levels and implications: European Journal of Mineralogy, 12, 543–570.

Liu, H., Zhu, Q., and Yang, X., 2019, Electrical conductivity of OH-bearing omphacite and garnet in eclogite: the quantitative dependence on water content: Contributions to Mineralogy and Petrology, 174, doi:10.1007/s00410-019-1593-3.