Pressure-driven conductivity of lizardite-implication to the high conductive layers in craton lithosphere

The electrical transport behavior of lizardite was investigated by in-situ impedance measurements up to 22.6 GPa in a diamond anvil cell with comparation to its dehydrated counterpart. The conductivity of lizardite is found to increase one order of magnitude with increasing pressures from 0.2 to 1.9 GPa, due to pressure-activated ionic and electronic transportation. The proton hopping and hopping-created vacancy accounts for the conduction mechanisms. Compression initially promotes proton hopping at lower pressures and then impedes it at elevates pressure to make conduction purely electronic. Compared to the dehydrated specimen, the hydroxyl in lizardite enhances conductivity 4-7 times. The electronic resistivity at higher pressures gradually increases at a constant rate, except in the pressure range where pressure minimized the misfit structural disordering. The pressure-activated proton hopping in the lizardite and other phyllosilicates may ascribe the high conductive layer in the craton lithosphere and geoelectric anomalies related to earthquakes. Hosted file supporting information for public.doc available at https://authorea.com/users/560979/ articles/608715-pressure-driven-conductivity-of-lizardite-implication-to-the-highconductive-layers-in-craton-lithosphere


Introduction
fluids enrichment. However, the mechanism of electrical transport in the lower resistive domains 50 remains in debating. On the other hand, serpentine dehydration, a common physical performance of hydrated minerals at high pressure and high temperature, was claimed one of the major causes 52 of intermediate earthquakes (Obara, 2002). 53 Serpentine belongs to trioctahedral phyllosilicate, with 1:1 stacking of [SiO 4 ] tetrahedron 54 and [MgO 8 ] octahedron. The layer curvature and stacking variation lead to three main structural 55 varieties, namely, lizardite with a planar structure, chrysotile with cylindrically rolled layers, and 56 antigorite with periodic reversals of the layer's polarity (Wicks & O'Hanley, 1988). At ambient 57 conditions, serpentine crystallizes in the monoclinic or hexagonal structure (Mellini & Viti, 1994; 58 Capitani & Mellini, 2004). While the crystal structures of serpentine are quite stable, neither 59 amorphization nor other structural phase transition were resolved in natural serpentine up to 10 60 GPa at zero temperature, only its beta angles (β) were quite largely changed and reversed sign 61 from -0.27° GPa -1 to 0.43° GPa -1 at 5 GPa (Hilairet et al., 2006). 62 The hydroxyl OH-groups, being the central focus of present studies, are located at the center 63 of the six-fold tetrahedral ring (inner OH) and between the octahedral and tetrahedral layers 64 (outer OH) (Lemaire et al., 1999) in serpentine. Most studies in the past employed Raman 65 scattering technique to effectively reveal the hydroxyl performance. For instance, a third intense 66 Raman peak between 3730-3770 cm -1 was revealed upon compression to 6.7 GPa in lizardite, 8.7 67 GPa in antigorite, and 2.8 GPa in chrysotile, respectively, in addition to the two common strong 68 OH bands at frequencies range of 3550-3850 cm -1 (Auzende et al., 2004;Mizukami et al., 2007). 69 The new peak was ascribed to different origins such as the OH vibrational band with a new mode, 70 the LO modes of the in-phase vibrations associated with stacking disorder or structural defects 71 (Reynard & Wunder, 2006). The result that pressure promotion rate of the outer 72 OH vibrational band elevated slightly at about 6 GPa in contrast to the inner OH vibrational band 73 that had an almost constant promotion rate at all pressure ranges implies the anomalous 74 interaction between the hydrogen ion of the outer OH and neighboring basal oxygen upon 75 compression (Noguchi et al., 2012). This is concordant with the first-principle calculation result 76 of abrupt increase of the OH⋅⋅⋅O bond angle in outer hydroxyl at 7 GPa (Mookherjee & Stixrude,77 2009), and the experimental confirmation (Hilairet et al., 2006).

78
Measurements of the transportation behavior of a mineral directly reveal its dynamical, 79 electronic, and ionic properties. It would be particularly efficient in characterize the behavior of 80 hydroxyl in a hydrated mineral. Studies had been conducted with multi-anvil high-pressure 81 apparatuses with particularly focus on revealing the pressure effect on dehydration induced by 82 high temperature (Song et al., 1996) and the relation between the dehydration induced high 83 conductivity and the occurrence of a high-conductivity zone in the lower crust (Zhu et al., 2000).

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Systematic investigation for the crucial physical behavior of hydroxyl is still lacking. In this 85 work, the recently developed technique of micro-electrical circuit on a diamond anvil facet 86 (Wang et al., 2016) was adopted to in-situ measure the impedance spectra of natural lizardite 87 crystal in a diamond anvil cell to explore the transportation properties of its physical excitons 88 (i.e., hydroxyl, proton, and vacancies), in the attempt to investigate the possible connection to 89 high conduction zone at depth of the crust as well as to shallow earthquakes. Synchrotron X-ray 90 diffraction and Raman scattering measurements were also accomplished in revealing the physical 91 mechanism associated with the transportation performance.   A symmetric diamond anvil cell was adopted in the in-situ high-pressure measurements.

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The ruby fluorescence method was used in all high-pressure measurements for the pressure 113 calibration (Mao et al., 1986). In the in-situ impedance spectra measurements, an electrode in the

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The characterization of the samples used in the experiments, the X-ray and Raman 138 measurements to high pressures, as well as the impedance measurements of dehydrated 139 specimen, are first presented and discussed separately for convenient description purpose.     The axial compressibility showed that the a-axis was linearly compressed in the whole 165 pressure range with a rate of 1.7%/GPa except that at 17.6 GPa. It is worth to note that a 166 discontinuity at 5.3 GPa was observed in compressibility along c-axis, namely, the slope variated 167 from 3.6% to 2.2%/GPa. Such variation indicated the change of Si-O bond length, which was 168 concordant with that revealed by the Raman measurement (Fig. 4). Compressibility of the c-axis 169 was much higher than that of a-axis in the whole pressure range even after the transition.

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In the quantitative analysis, an RC (resistance-capacitance) equivalent circuit (Fig. 6f) was 202 routinely introduced to simulate the impedance spectra in order to deduce the resistance, 203 capacitance, and the relaxation frequency when the spectra just consist of semicircles. 204 Furthermore, with ascription of the low-frequency straight line to the Debye equivalence, a 205 Warburg element was introduced in addition to an RC circuit (Fig. 6e inset), allowing us to 206 retrieve both ionic and electronic resistance (Wang et al., 2016). Comparison between the fitting 207 curves and the measured results ( Fig. 6e and 6f) showed a precisely match. Consequently, the 208 electronic and ionic resistances and the relaxation frequencies acquired from the fitting can 209 precisely reflect the conductive characteristics of the samples.  closing dumped conductivity at the pressures will be discussed further.

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The magnitude of resistivity of dehydrated specimen remained at the order of 10 9 ·m 243 above 3.2 GPa, showing a general trend of gently positive slop (Fig.7). Extrapolation of the

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The pressure driven conduction reversals (Fig. 8)   ionic conduction (Fig. 8). Simultaneously, the proton run-away produced the proton vacancies the relaxation frequency of the natural lizardite increased with initial pressure increase (Fig.8).   316 The transition of the effective charge carrier from vacancy type to hydroxyl dipole type crystalline to compression (Fig. 3).

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The electronic resistivity elevated exponentially one order of magnitude to 10 8 ·m when 326 pressure was increased from 1.3 to 2.0 GPa (Fig. 7), which was attributed to the transition of the 327 effective charge carrier from vacancy type to hydroxyl dipole type. Then pressure increased to 5 328 GPa, the resistivity gradually increased. However, the resistivity decreased rapidly from 5 to 11 329 GPa (the maximum reduction reached 20% at around 7 GPa), which was caused by another 330 resistivity reversal. After then, the resistivity increased with pressure elevation with a rate similar 331 to that from 2 to 5 GPa.

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The gap narrowing affected the resistivity under compression. The mild pressure promotion 333 of resistivity in the pressure range of 2-5 and above 15 GPa was directly related to the gap 334 narrowing. The pressure coefficient of resistivity was proportional to the crystal lattice's c-axial 335 reduction. The rate of the crystal lattice's c-axial reduction indicated that the gap became 336 narrowed much faster than other directions (Fig. 3)