Electrical conductivity of tremolite and actinolite amphiboles

Abstract:

Amphiboles are commonly found in crustal and mantle rocks. In this study, we explored the electrical conductivity of natural tremolite and actinolite amphiboles. We conducted the electrical conductivity measurements at ~377-1403 K and ~1.5 GPa. We found that the electrical conductivity of amphiboles increases with temperature and exhibits three distinct regimes. At temperatures up to 793 K, the conductivity of tremolite is likely to be dominated by solid-state conduction. At temperatures between than 892 K and 1175 K, tremolite exhibits a discontinuous increase in electrical conductivity and likely undergoes partial dehydration. At temperatures greater than 1223 K and up to 1403 K, tremolite exhibits another discontinuous increase in electrical conductivity up to 10^0.3 S/m and likely undergoes partial melting. Actinolite also shows similar discontinuous increases at slightly different temperatures. Our electrical conductivity results are crucial for evaluating the role of amphiboles in the electrical structures of different geological settings, including subduction zones and metasomatized continental lithosphere.

We used our results to evaluate if mantle metasomatism could be a potential cause of the seismologically observed mid lithospheric discontinuity (MLD). MLD is characterized by a 2-6% seismic shear wave velocity reduction at depths of 60-160 km in the sub cratonic lithosphere. Such lowering of seismic shear wave velocity could be partly attributed to mantle metasomatism because it often stabilizes amphiboles. However, existing results on electrical conductivity of amphiboles have failed to explain available magnetotelluric observations in MLD regions. We used our electrical conductivity results to explain both seismological and magnetotelluric observations at MLD depths by placing constraints on the degree of mantle metasomatism.

Acknowledgement: We acknowledge NSF funding EAR 1763215 and 1753125.