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a Compilation of laboratory electrical conductivity measurements for lherzolite (Duba and ConsTable 1993), diopside (Wang et al. 1999), olivine (Wang et al. 2006), clinopyroxene (Yang et al. 2011), enstatite (Zhang et al. 2012), and tremolite (this study). The magenta striped area represents the high-electrical conductivity anomalies in the upper mantle. The red shaded area denotes the electrical conductivity of lherzolite with a tremolite content of 19–34%. The dotted blue line, solid blue line, dotted red line, and solid red line show the lower and upper bounds of lherzolite with 19 and 34% tremolite, respectively. These boundaries are calculated using the HS model (Hashin and Shtrikman 1963). b Tremolite conductivity variation with depth, superimposed on the thermal structure and P–T path of Venus’ interior (Aitta 2012) and the tremolite stability data (Welch and Pawley 1991; Jenkins 1991)
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We measured the electrical conductivity of tremolite over a range of pressures (1.0, 1.5, and 2.0 GPa) and temperatures (648–1373 K). At temperatures lower than 1173 K, the electrical conductivity of tremolite was ~ 0.001 S/m, but once the dehydroxylation reaction took place at 1173 K, we observed a significant increase in the electrical conductivi...
Citations
... Inspection of Table 1 shows that, despite the different methods used to determine the conductivity, the activation energies for all amphibole species with Fe/(Fe + Mg) higher than ~0.2 are below 1.0 eV. A notable exception is tremolite whose conduction mechanism is not based on electron hopping (Shen et al., 2020;see below). The absence of a clear correlation between E a and Fe/(Fe + Mg) indicates that the A-site occupancy as well as the nature of B and T cations are likely to affect the temperature of polaron formation and polaron mobility (see Mihailova et al., 2022). ...
... In contrast to Fe-amphiboles, polaron conductivity is not feasible in tremolite of close-to-ideal composition due to lack of Fe 2+ . Nonetheless, Shen et al. (2020) observed an abrupt rise in conductivity in single crystals of almost pure tremolite, triggered by breakdown and dehydrogenation at T > 900 • C (1173 K). This behaviour was also almost independent of pressure. ...
... This behaviour was also almost independent of pressure. The activation energy of the enhanced conductivity was 365 kJ/mol (3.79 eV), much higher that the values measured for Fe-bearing amphiboles, implying a different conduction mechanism such as ionic (Mg 2+ , Ca 2+ ) diffusion (Katsura et al., 2009;Shen et al., 2020). More recently, the appearance of RRS in actinolite (Rösche et al., 2022) revealed that even small amounts of Fe in an essentially magnesian amphibole are sufficient to trigger the formation of polaronic conduction, although at much higher temperatures than in Fe-rich amphiboles . ...
... The pressure calibrations were performed using the melting temperatures of sodium chloride at high pressures (Akella et al., 1969), and the uncertainty was calculated to be less than 0.11 GPa (K. Shen et al., 2020). The sample assembly for the high-pressure conductivity experiments was shown in Figure 1. ...
... The sample assembly for the high-pressure conductivity experiments was shown in Figure 1. This assembly has also been used in a previous study conducted by Shen et al. (2020). The pyrophyllite cube (length of 32.5 mm) was used as a pressure medium and two layers of stainless-steel foils were placed next to the pyrophyllite and were used as the furnace. ...
The dehydration of clinochlore may supply water for the creation of high‐conductivity anomalies and melting beneath volcanic arc. However, this process has not yet been constrained even though it is critical to understanding water cycling processes during subduction. The electrical conductivity of clinochlore was measured at pressures of 1.0–4.0 GPa and temperatures of up to 1273 K. The pressure weakly affected the electrical conductivity of clinochlore. In contrast, the electrical conductivity was significantly enhanced when the clinochlore was heated to temperatures beyond 1048 K, which was accompanied by decomposition into spinel, forsterite, enstatite and aqueous fluids. The elevated conductivity associated with the high activation energy may reflect the migration of Mg²⁺ and Al³⁺ during dehydration. We suggested that the aqueous fluids were released from both talc‐like and brucite‐like layer in the clinochlore, and the volumes of fluids released by samples post mortem determined using X‐ray computed tomography were 7.9–11.5 vol.%. Our results indicate that the dehydration of clinochlore results in a significant increase in conductivity of up to ∼1 S/m due to the interconnected network formed by the fluids. Combined with the geothermal gradient, the experimental data were used to interpret the high‐conductivity anomalies observed at depths of 75–120 km in hot subduction zones and 150–200 km in cold subduction zones. The updip migration of aqueous liquids liberated by clinochlore may act as a major water source for the melting at depths of 110 ± 20 km above the descending slab beneath a volcanic arc.
... The starting material used in this study was almost pure natural tremolite crystals, which were picked out of a natural tremolite sample containing minor amounts of apatite (<1%) and diopside (<1%) obtained from Henan Province, China, and also used by Shen et al. (2020). The chemical composition of the samples determined using an electronic probe microanalyzer (JXA-8230; located at Hefei University of Technology) (Table 1) was reported by Shen et al. (2020). ...
... The starting material used in this study was almost pure natural tremolite crystals, which were picked out of a natural tremolite sample containing minor amounts of apatite (<1%) and diopside (<1%) obtained from Henan Province, China, and also used by Shen et al. (2020). The chemical composition of the samples determined using an electronic probe microanalyzer (JXA-8230; located at Hefei University of Technology) (Table 1) was reported by Shen et al. (2020). The density of tremolite was measured to be 2.964 kg/m 3 using the Archimedes principle. ...
Thermal conductivity (κ) and thermal diffusivity (D) of tremolite were measured at up to 2.5 GPa and 1,373 K using the transient plane‐source method in a multi‐anvil apparatus. Thermal conductivity and thermal diffusivity of tremolite decrease monotonically before dehydration (<1,173 K) and increase significantly after dehydration. Tremolite exhibits positive pressure dependence before dehydration. Heat capacity (C) of tremolite calculated from κ and D shows a positive pressure dependence and is controlled by an almost constant thermal expansion coefficient (α) with temperature. Conductive heat transport and radiative heat transport dominate the heat transport process before dehydration, and the significant increase in thermal conductivity after dehydration is attributed to convective heat transport. A compositional model of the Venusian lithosphere composed of a basaltic crust and peridotite mantle with or without tremolite was established. The thickness of the Venusian lithosphere with or without tremolite for Venus was calculated by combining the heat flow (from 20 to 80 mW/m²) at a certain depth (from 5 to 25 km) of crust, ranging from 24.4 to 184.6 km.
... Thus, our results on the elasticity and metastability of tremolite can provide a direct comparison with Hugoniot results, and are hence relevant to the properties of tremolite-bearing asteroids and planetary crusts under shock during accretionary events. Metastable tremolite has also been invoked as playing a primary role in both intermediate-depth earthquakes and in generating conductivity anomalies at depths substantially deeper than its equilibrium stability within subduction zones (Scambelluri et al., 2017;Shen et al., 2020). These results mirror recent results on glaucophane, which indicate its persistence at low temperatures to depths (240 km, or ~8 GPa) well beyond its formal thermodynamic stability limit (~3 GPa: Cheng et al., 2020) (Bang et al., 2021). ...
The high-pressure structure and stability of the calcic amphibole tremolite (Ca2Mg5Si8O22(OH)2) was investigated to ~40 GPa at 300 K by single-crystal X-ray diffraction using synchrotron radiation. C2/m symmetry tremolite displays a broader metastability range than previously studied clinoamphiboles, exhibiting no first-order phase transition up to 40 GPa. Axial parameter ratios a/b and a/c, in conjunction with finite strain versus normalized pressure trends, indicate that changes in compressional behavior occur at pressures of ~5 and ~20 GPa. An analysis of the finite strain trends, using third-order Birch-Murnaghan equations of state, resulted in bulk moduli (!") of 72(7), 77(2), and 61(1) GPa for the compressional regimes from 0-5 GPa (regime I), 5-20 GPa (II), and above 20 GPa (III), respectively, and accompanying pressure-derivatives of the bulk moduli (′ !") of 8.6(42), 6.0(3), and 10.0(2). The results are consistent with first-principle theoretical calculations of tremolite elasticity. The axial compressibility ratios of tremolite, determined as # : $ : % = 2.22:1.0:0.78 (regime I), This is the peer-reviewed, Always consult and cite the final, published document. See http:/www.minsocam.org or GeoscienceWorld 2 2.12:1.0:0.96 (II), and 1.03:1.0:0.75 (III), demonstrate a substantial reduction of the compressional anisotropy of tremolite at high pressures, which is a notable contrast with the increasingly anisotropic compressibility observed in the high-pressure polymorphs of the clinoamphibole grunerite. The shift in compression-regime at 5 GPa (I-II) transition is ascribed to stiffening along the crystallographic a-axis corresponding to closure of the vacant A-site in the structure, and a shift in the topology of the a-oriented surfaces of the structural I-beam from concave to convex. The II-III regime shift at 20 GPa corresponds to an increasing rate of compaction of the Ca-polyhedra and increased distortion of the Mg-octahedral sites, processes which dictate compaction in both high-pressure compression-regimes. Bond-valence analyses of the tremolite structure under pressure show dramatic overbonding of the Ca-cations (75% at 30 GPa), with significant Mg-cation overbonding as well (40%). These imply that tremolite's notable metastability range hinges on the calcium cation's bonding environment. The 8-fold coordinated Ca-polyhedron accommodates significant compaction under pressure, while the geometry of the Ca-O polyhedron becomes increasingly regular and inhibits the reorientation of the tetrahedral chains that generate phase transitions observed in other clinoamphiboles. Peak/background ratio of diffraction data collected above 40 GPa and our equation of state determination of bulk moduli and compressibilities of tremolite in regime III, in concert with the results of our previous Raman study, suggest that C2/m tremolite may be approaching the limit of its metastability above 40 GPa. Our results have relevance for both the metastable compaction of tremolite during impact events, and for possible metastable persistence of tremolite within cold subduction zones within the Earth.
... Seismic and electromagnetic anomalies in subduction zones have triggered continuously growing interest on amphiboles and layer silicates, as well as of hydrogen-bearing nominally anhydrous minerals, as responsible for the electrical conductivity of the midcrust and subduction zones [16][17][18][41][42][43][44][45][46] . Metasomatic processes leading to the formation of hydrous fluids may considerably contribute to the anomalies in lithospheric conductivity 22,44 . ...
... However, recent studies have indicated that conductive aqueous fluids alone cannot explain the anisotropic conductivity observed in some locations 16,42 . Moreover, although the geophysical anomalies generally depend on both temperate and pressure as thermodynamical parameters 47,48 , temperature rather than pressure is the major factor influencing the electrical conductivity of rock-forming minerals 16,22,46 . Two main temperature-activated solid-state processes have been considered to cause the rock conductivity: (i) hopping of electrons/electron holes between ferrous and ferric iron and (ii) diffusion of H +44 . ...
... The former process presumes the presence of minerals with mixed-valent Fe state, that is, with coexisting Fe 2+ and Fe 3+ in the nominal formula, like riebeckite or arfvedsonite, leading to polaron conductivity 16,45 . An additional process, postulated for Fe-free minerals, involves primarily the ionic conduction: this mechanism is characterized by high activation energy (>2 eV) and has been inferred to explain the conductivity of tremolite at very high temperatures 46 . ...
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 Fe ²⁺ -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 FeO 6 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.
... The observed electrical conductivity anomalies in subduction zone settings are often linked with the presence of aqueous fluids . Support for this argument is further reinforced by numerous electrical conductivity studies based on laboratory experiments which demonstrate higher electrical conductivities for aqueous fluid compared to solid mineral phases (Manthilake et al., 2015(Manthilake et al., , 2021bReynard et al., 2011;Shen et al., 2020;Wang & Karato, 2013;Wang et al., 2012;Zhang et al., 2014). Upon dehydration, the released fluids may lead to the formation of an interconnected network of a conductive phase (fluid/melt). ...
Amphiboles are hydrous minerals that are formed in the oceanic crust via hydrothermal alteration. The partial substitution of halogens for OH⁻ makes amphibole one of the principal hosts of Cl and F in the subducting slab. In this study, we investigated the electrical conductivity of a suite of halogen bearing amphibole minerals at 1.5 GPa up to 1,400 K. The discontinuous electrical behavior indicates dehydration of amphibole at ∼915 K. This is followed by dehydration induced hydrous melting at temperatures above 1,070 K. We find that the released aqueous fluids have an electrical conductivity of ∼0.1 S/m. This high electrical conductivity is likely to explain anomalously high electrical conductivity observed in certain subduction zone settings. This high electrical conductivity of an order of magnitude greater than the electrical conductivity of pure aqueous fluids at similar conditions is likely due to the partitioning of the F and Cl into the aqueous fluids. We also noted that subsequent to the dehydration, secondary phases form due to the breakdown of the primary halogen bearing amphibole. Chemical analyses of these secondary phases indicate that they are repositories of F and Cl. Hence, we infer that upon dehydration of the primary halogen bearing amphibole, first the F and Cl are partitioned into the aqueous fluids and then the halogens are partitioned back to the secondary mineral phases. These secondary minerals are likely to transport the halogen to the deep Earth and may in part explain the halogen concentration observed in ocean island basalt.
... However, a recent experimental study found that the electrical conductivity of tremolite amphibole is much lower than previously thought, about 10 3 -10 5 lower than that of hornblendite. It only exceeds the conductivity of olivine by one order of magnitude at temperatures <1123 K, i.e., the melting temperature (Shen et al. 2020). Therefore, the role of amphiboles in the electrical conductivity of the mantle is ambiguous, and it certainly requires further study. ...
... The release of aqueous fluids often causes the change of slope, i.e., activation enthalpy in the temperature-dependent electrical conductivity of hydrous phases. The activation enthalpy after dehydration obtained in our study, i.e., the T 2 segment between 775 and 1000 K, is very similar to the activation enthalpy ~0.5 eV in the T 3 segment of the tremolite sample, where the conduction mechanism is dominant by the flow of aqueous fluids (Shen et al. 2020) (Fig. 3; Table 2). ...
... We also noted that the electrical conductivity of our diopsidetremolite-albite sample is higher than that of pure clinopyroxene (Yang et al. 2011) or pure tremolite (Shen et al. 2020) by a factor of 10 3 -10 4 , especially when the temperature is lower than 1000 K (Fig. 3). These electrical conductivity experiments on clinopyroxene, plagioclase, and amphibole were conducted at different pressures. ...
A plausible origin of the seismically observed mid-lithospheric discontinuity (MLD) in the subcontinental lithosphere is mantle metasomatism. The metasomatized mantle is likely to stabilize hydrous phases such as amphiboles. The existing electrical conductivity data on amphiboles vary significantly. The electrical conductivity of hornblendite is much higher than that of tremolite. Thus, if hornblendite truly represents the amphibole varieties in MLD regions, then it is likely that amphibole will cause high electrical conductivity anomalies at MLD depths. However, this is inconsistent with the magnetotelluric observations across MLD depths. Hence, to better understand this discrepancy in electrical conductivity data of amphiboles and to evaluate whether MLD could be caused by metasomatism, we determined the electrical conductivity of a natural metasomatized rock sample. The metasomatized rock sample consists of ~87% diopside pyroxene, ~9% sodium-bearing tremolite amphibole, and ~3% albite feldspar. We collected the electrical conductivity data at ~3.0 GPa, i.e., the depth relevant to MLD. We also spanned a temperature range between 400 to 1000 K. We found that the electrical conductivity of this metasomatized rock sample increases with temperature. The temperature dependence of the electrical conductivity exhibits two distinct regimes. At low temperatures <700 K, the electrical conductivity is dominated by the conduction in the solid state. At temperatures >775 K, the conductivity increases, and it is likely to be dominated by the conduction of aqueous fluids due to partial dehydration. The main distinction between the current study and the prior studies on the electrical conductivity of amphiboles or amphibole-bearing rocks is the sodium (Na) content in amphiboles of the assemblage. Moreover, it is likely that the higher Na content in amphiboles leads to higher electrical conductivity. Pargasite and edenite amphiboles are the most common amphibole varieties in the metasomatized mantle, and our study on Na-bearing tremolite is the closest analog of these amphiboles. Comparison of the electrical conductivity results with the magnetotelluric observations constrains the amphibole abundance at MLD depths to <1.5%. Such a low-modal proportion of amphiboles could only reduce the seismic shear wave velocity by 0.4–0.5%, which is significantly lower than the observed velocity reduction of 2–6%. Thus, it might be challenging to explain both seismic and magnetotelluric observations at MLD simultaneously.
... water; epidote and Fe-bearing amphibole contain~12 wt.% and~8 wt.% iron, respectively, all of which have higher electrical conductivity than talc (Figure 8b). Dehydration also plays an important role in the electrical conductivity of antigorite (Wang et al., 2017), lawsonite (Manthilake et al., 2015;Pommier et al., 2019), epidote (Hu et al., 2017), and tremolite (Shen et al., 2020). In addition to fluid generation, hydrous minerals decomposition into new minerals can also cause electrical conductivity changes, such as interconnected magnetite after chlorite decomposition (Manthilake et al., 2016) and Fe-bearing amphibole oxidation-dehydrogenation (Hu et al., 2018). ...
... The light orange region corresponds to the high electrical conductivity of subduction zones from MT observations. The purple solid line represents the electrical conductivity of amphibole at 2.0 GPa(Hu et al., 2018), the magenta dotted line represents the conductivity of epidote(Hu et al., 2017), the light blue hatched region and light blue dashed line indicate the conductivity of antigorite(Reynard et al., 2011;Wang et al., 2017), the red solid line denotes the conductivity of talc(Wang & Karato, 2013), the green dashed line and orange hatched region represent the conductivity of lawsonite(Manthilake et al., 2015;Pommier et al., 2019), the green solid line is the electrical conductivity of chlorite(Manthilake et al., 2016), the gray and pink hatched regions are the electrical conductivity of talc rocks and serpentinite at 3.0 GPa, respectively (X.Guo et al., 2011), and the wine red dashed line is the electrical conductivity of tremolite(Shen et al., 2020). Note that Amp = amphibole, Atg = antigorite, Chl = chlorite, Epi = epidote, Law = lawsonite, Srp = serpentinite, Tr = tremolite, and Tlc = talc. ...
The dehydration of hydrous minerals is one of the causes of high‐conductivity anomalies in subduction zones. To determine the origin of these anomalies, the trade‐off between the dehydration and conduction mechanisms of hydrous minerals at high pressures and temperatures should be clarified. Talc is a typical hydrous mineral in hot subduction zones, and previous studies may have underestimated its contribution to high‐conductivity anomalies. We report the new electrical conductivity results for talc, which were measured at 1.0–4.0 GPa and 523–1293 K using impedance spectroscopy. The pressure effect on conductivity of talc is obvious in the different heating stages. The pressure decreased the conductivity prior to dehydration and remarkably increased the conductivity during dehydration. A sharp conductivity increase was observed beyond the dehydration temperature, and the maximum conductivity was 0.1 S/m. The increase in conductivity associated with a high activation energy of 284.5 ± 11.8 kJ/mol and an activation volume of −6.2 ± 0.6 cm³/mol was attributed to an inhomogeneous dehydration model involving cation migration. The talc dehydration temperatures at different pressures derived from the conductivity inflection points are 1023–1093 K. The increased electrical conductivity produced by talc ongoing dehydration provides an explanation for the high‐conductivity anomalies observed at deep depths in hot subduction zone. The silica‐rich fluid released by talc may contribute to the silica deposition in plate interface and induce the high‐conductivity anomalies observed at shallow depths in the hot subduction zones.
... In a first approximation, by neglecting the contribution of the low-frequency features and any deviation of the experimental data from the semicircular shape, a single R-CPE equivalent circuit could be roughly used to model the mica samples and derive their total (bulk) conductivity as a function of temperature. Oversimplified approaches as the aforementioned one and even the simpler R-C circuit in parallel have often been used to estimate the bulk conductivity of minerals [50][51][52]. However, as the recorded impedance spectra of the mica samples exhibit more complicated spectral features, we are obliged to analyze them in more detail, in order to separate different contributions to the overall bulk conductivity. ...
The unique physicochemical, electrical, mechanical, and thermal properties of micas make them suitable for a wide range of industrial applications, and thus, the interest for these kind of hydrous aluminosilicate minerals is still persistent, not only from a practical but also from a scientific point of view. In the present work, complex impedance spectroscopy measurements were carried out in muscovite and biotite micas, perpendicular to their cleavage planes, over a broad range of frequencies (10 −2 Hz to 10 6 Hz) and temperatures (473-1173 K) that have not been measured so far. Different formalisms of data representation were used, namely, Cole-Cole plots of complex impedance, complex electrical conductivity and electric modulus to analyze the electrical behavior of micas and the electrical signatures of the dehydration/dehydroxylation processes. Our results suggest that ac-conductivity is affected by the structural hydroxyls and the different concentrations of transition metals (Fe, Ti and Mg) in biotite and muscovite micas. The estimated activation energies, i.e., 0.33-0.83 eV for biotite and 0.69-1.92 eV for muscovite, were attributed to proton and small polaron conduction, due to the bound water and different oxidation states of Fe.
