Abstract
Amphiboles are essential components of the continental crust and subduction zones showing anomalous anisotropic conductivity. Rock properties depend on the physical properties of their constituent minerals, which in turn depend on the crystal phonon and electron density of states. Here, to address the atomic-scale mechanism of the peculiar rock conductivity, we applied in situ temperature-dependent Raman spectroscopy, sensitive to both phonon and electron states, to Fe2+-rich amphiboles. The observed anisotropic resonance Raman scattering at elevated temperatures, in combination with density-functional-theory modelling, reveals a direction-dependent formation of mobile polarons associated with coupled FeO6 phonons and electron transitions. Hence, temperature-activated electron-phonon excitations in hydrous iron-bearing chain and layered silicates are the atomistic source of anisotropic lithospheric conductivity. Furthermore, reversible delocalization of H+ occurs at similar temperatures even in a reducing atmosphere. The occurrence of either type of charge carriers does not require initial mixed-valence state of iron or high oxygen fugacity in the system.
Highlights
Amphiboles are essential components of the continental crust and subduction zones showing anomalous anisotropic conductivity
Dehydrogenation of amphiboles and other representative hydrous silicates is considered to play an important role for the high-temperature anomalies in subduction zones[22] Besides, the dehydrogenation of amphiboles and related layered silicates can contribute to the water cycling at depth or, depending on the local rock assemblages, to the formation of abiotic H2 and CH423
In our previous studies on riebeckite[13] we showed that the MO6 mode ~146 cm−1 in y0ðx0x0Þy0 spectra is sensitive to Fe oxidation; the fractional intensity of this mode increased in the temperature range of reversible oxidation and dropped abruptly on the verge of irreversible oxidation
Summary
Amphiboles are essential components of the continental crust and subduction zones showing anomalous anisotropic conductivity. A important issue, which has been achieved mainly via Raman spectroscopy, is the existence of a thermal state of reversible dynamic oxidation This state consists of delocalization of electrons e− from the octahedrally coordinated iron cations as well as of H+ from the W-type anions[13]. The possible development of transport and magnetic properties in iron-bearing amphiboles at elevated temperatures is of extreme interest in geophysics[17,18,19,20], especially when modeling lithospheric conductivity[16,21] For the latter, dehydrogenation of amphiboles and other representative hydrous silicates is considered to play an important role for the high-temperature anomalies in subduction zones[22] Besides, the dehydrogenation of amphiboles and related layered silicates can contribute to the water cycling at depth or, depending on the local rock assemblages, to the formation of abiotic H2 and CH423
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