Detailed Product Description Special Shaped Graphite products are used in various industrails,including graphite crucible and graphite seal Special Shaped Graphite products are used in various industrails.it can be used as sealing in machinery as carbon graphite material, such as the quiet ring or the moving ring, doctorblade, piston-ring, sealing expansionring and the raw materias. Send a message to us: Email: Telephone: Qty Required: Acre/Acres Ampere/Amperes Bag/Bags Barrel/Barrels Box/Boxes Bushel/Bushels Carat/Carats Carton/Cartons Case/Cases Centimeter/Centimeters Chain/Chains Cubic Centimeter/Cubic Centimeters Cubic Foot/Cubic Feet Cubic Inch/Cubic Inches Cubic Meter/Cubic Meters Cubic Yard/Cubic Yards Degrees Celsius Degrees Fahrenheit Dozen/Dozens Dram/Drams Fluid Ounce/Fluid Ounces Foot/Feet Forty-Foot Container Furlong/Furlongs Gallon/Gallons Gill/Gills Grain/Grains Gram/Grams Gross Hectare/Hectares Hertz Inch/Inches Kiloampere/Kiloamperes Kilogram/Kilograms Kilohertz Kilometer/Kilometers Kiloohm/Kiloohms Kilovolt/Kilovolts Kilowatt/Kilowatts Liter/Liters Long Ton/Long Tons Megahertz Meter/Meters Metric Ton/Metric Tons Mile/Miles Milliampere/Milliamperes Milligram/Milligrams Millihertz Milliliter/Milliliters Millimeter/Millimeters Milliohm/Milliohms Millivolt/Millivolts Milliwatt/Milliwatts Nautical Mile/Nautical Miles Ohm/Ohms Ounce/Ounces Pack/Packs Pair/Pairs Pallet/Pallets Parcel/Parcels Perch/Perches Piece/Pieces Pint/Pints Plant/Plants Pole/Poles Pound/Pounds Quart/Quarts Quarter/Quarters Rod/Rods Roll/Rolls Set/Sets Sheet/Sheets Short Ton/Short Tons Square Centimeter/Square Centimeters Square Foot/Square Feet Square Inch/Square Inches Square Meter/Square Meters Square Mile/Square Miles Square Yard/Square Yards Stone/Stones Strand/Strands Ton/Tons Tonne/Tonnes Tray/Trays Twenty-Foot Container Unit/Units Volt/Volts Watt/Watts Wp Yard/Yards Company: More: Remaining: 4000 characters - Self introduction - Required specifications - Inquire about price/MOQ Send
Special Shaped Graphite products are used in various industrails,including graphite crucible and graphite seal Special Shaped Graphite products are used in various industrails.it can be used as sealing in machinery as carbon graphite material, such as the quiet ring or the moving ring, doctorblade, piston-ring, sealing expansionring and the raw materias. Send a message to us: Email: Telephone: Qty Required: Acre/Acres Ampere/Amperes Bag/Bags Barrel/Barrels Box/Boxes Bushel/Bushels Carat/Carats Carton/Cartons Case/Cases Centimeter/Centimeters Chain/Chains Cubic Centimeter/Cubic Centimeters Cubic Foot/Cubic Feet Cubic Inch/Cubic Inches Cubic Meter/Cubic Meters Cubic Yard/Cubic Yards Degrees Celsius Degrees Fahrenheit Dozen/Dozens Dram/Drams Fluid Ounce/Fluid Ounces Foot/Feet Forty-Foot Container Furlong/Furlongs Gallon/Gallons Gill/Gills Grain/Grains Gram/Grams Gross Hectare/Hectares Hertz Inch/Inches Kiloampere/Kiloamperes Kilogram/Kilograms Kilohertz Kilometer/Kilometers Kiloohm/Kiloohms Kilovolt/Kilovolts Kilowatt/Kilowatts Liter/Liters Long Ton/Long Tons Megahertz Meter/Meters Metric Ton/Metric Tons Mile/Miles Milliampere/Milliamperes Milligram/Milligrams Millihertz Milliliter/Milliliters Millimeter/Millimeters Milliohm/Milliohms Millivolt/Millivolts Milliwatt/Milliwatts Nautical Mile/Nautical Miles Ohm/Ohms Ounce/Ounces Pack/Packs Pair/Pairs Pallet/Pallets Parcel/Parcels Perch/Perches Piece/Pieces Pint/Pints Plant/Plants Pole/Poles Pound/Pounds Quart/Quarts Quarter/Quarters Rod/Rods Roll/Rolls Set/Sets Sheet/Sheets Short Ton/Short Tons Square Centimeter/Square Centimeters Square Foot/Square Feet Square Inch/Square Inches Square Meter/Square Meters Square Mile/Square Miles Square Yard/Square Yards Stone/Stones Strand/Strands Ton/Tons Tonne/Tonnes Tray/Trays Twenty-Foot Container Unit/Units Volt/Volts Watt/Watts Wp Yard/Yards Company: More: Remaining: 4000 characters - Self introduction - Required specifications - Inquire about price/MOQ Send
Special Shaped Graphite products are used in various industrails.it can be used as sealing in machinery as carbon graphite material, such as the quiet ring or the moving ring, doctorblade, piston-ring, sealing expansionring and the raw materias.
Special Shaped Graphite products are used in various industrails.it can be used as sealing in machinery as carbon graphite material, such as the quiet ring or the moving ring, doctorblade, piston-ring, sealing expansionring and the raw materias.
The molar volume of glaucophane [Na2Mg3Al2Si8O22(OH)2] has been determined in this study by correcting synthetic glaucophane-rich amphiboles made in the system Na2OMgOAl2O3SiO2H2O for very small deviations from ideal glaucophane composition using recent volume data on key amphibole components. The derived unit-cell volume for end-member glaucophane is 862.71.63, which gives a molar volume of 259.80.5cm3/mol and a calculated density of 3.0160.006g/cm3. This value has been corroborated through an essentially independent method by correcting the volumes of natural sodic amphiboles reported in the literature for non-glaucophane components, particularly including calcium-rich components, to yield a value of 861.21.93. The unit-cell volume derived from the synthetic amphiboles, which is considered here to be more reliable, is somewhat smaller than that reported previously in the literature. A thermal expansion (V) at 298K of 1.880.06105/K was derived from unit-cell volumes measured in the range of 25500C for a synthetic glaucophane sample, which is noticeably smaller than previously reported.
Cmara F, Oberti R, Iezzi G, Della Ventura G (2003) The P21/m <-> C2/m phase transition in synthetic amphibole NaNaMgMg5Si8O22(OH)2: thermodynamic and crystal-chemical evaluation. Phys Chem Minerals 30:570581
Della Ventura G, Iezzi G, Redhammer GJ, Hawthorne FC, Scaillet B, Novembre D (2005) Synthesis and crystal-chemistry of alkali amphiboles in the system Na2OMgOFeOFe2O3SiO2H2O as a function of f O2. Am Mineral 90:13751383
Grevel KD, Gollerthan S, Rohling S (1998) In situ X-ray diffraction studies on tremolite at high pressures and temperatures. Berichte der Deutschen Mineralogischen Gesellschaft, Beih z Eur J Mineral 10:111
Iezzi G, Della Ventura G, Cmara F, Pedrazzi G, Robert J-L (2003) BNaBLi solid-solution in A-site-vacant amphiboles: synthesis and cation ordering along the ferri-clinoferroholmquistiteriebeckite join. Am Mineral 88:955961
Smelik EA, Jenkins DM, Navrotsky A (1994) A calorimetric study of synthetic amphiboles along the tremolite-tschermakite join and the heats of formation of magnesiohornblende and tschermakite. Am Mineral 79:1110-1122
Ungaretti L, Smith DC, Rossi G (1981) Crystal-chemistry by X-ray structure refinement and electron microprobe analysis of a series of sodic-calcic to alkai-amphiboles from the Nyb eclogite pod, Norway. Bull Minral 104:400412
Welch MD, Graham CM (1992) An experimental investigation of glaucophanic amphiboles in the system Na2OMgOAl2O3SiO2SiF2 (NMASF): some implications for glaucophane stability in natural and synthetic systems at high temperatures and pressures. Contrib Mineral Petrol 111:248259
Fibrous glaucophane is a mineral species analogous to carcinogenic amphibole asbestos.In vitro toxicity and genotoxicity of fibrous glaucophane has been demonstrated.Fibrous glaucophane exposure led to ROS release and cell injury.The precautionary approach when dealing with glaucophane-rich rocks is recommended.
The health hazard represented by the exposure to asbestos may also concern other minerals with asbestos-like crystal habit. One of these potentially hazardous minerals is fibrous glaucophane. Fibrous glaucophane is a major component of blueschist rocks of California (USA) currently mined for construction purposes. Dust generated by the excavation activities might potentially expose workers and the general public. The aim of this study was to determine whether fibrous glaucophane induces in vitro toxicity effects on lung cells by assessing the biological responses of cultured human pleural mesothelial cells (Met-5A) and THP-1 derived macrophages exposed for 24 h and 48 h to glaucophane fibres. Crocidolite asbestos was tested for comparison. The experimental configuration of the in vitro tests included a cell culture without fibres (i.e., control), cell cultures treated with 50 g/mL (i.e., 15.6 g/cm2) of crocidolite fibres and 25-50100 g/mL (i.e., 7.815.631.2 g/cm2) of glaucophane fibres. Results showed that fibrous glaucophane may induce a decrease in cell viability and an increase in extra-cellular lactate dehydrogenase release in the tested cell cultures in a concentration dependent mode. Moreover, it was found that fibrous glaucophane has a potency to cause oxidative stress. The biological reactivity of fibrous glaucophane confirms that it is a toxic agent and, although it apparently induces lower toxic effects compared to crocidolite, exposure to this fibre may be responsible for the development of lung diseases in exposed unprotected workers and population.
The blueschist to eclogite transition is one of the major geochemicalmetamorphic processes typifying the subduction zone, which releases fluids triggering earthquakes and arc volcanism. Although glaucophane is an index hydrous mineral for the blueschist facies, its stability at mantle depths in diverse subduction regimes of contemporary and early Earth has not been experimentally determined. Here, we show that the maximum depth of glaucophane stability increases with decreasing thermal gradients of the subduction system. Along cold subduction geotherm, glaucophane remains stable down ca. 240km depth, whereas it dehydrates and breaks down at as shallow as ca. 40km depth under warm subduction geotherm or the Proterozoic tectonic setting. Our results imply that secular cooling of the Earth has extended the stability of glaucophane and consequently enabled the transportation of water into deeper interior of the Earth, suppressing arc magmatism, volcanism, and seismic activities along subduction zones.
Plate tectonics such as subduction has been operating since the early Earth1,2,3,4,5,6, possibly since the Hadean or Eoarchean7. The Archean mantle temperature is estimated to be ca. 15001650C, which is higher than the present mantle temperatures of ca. 135050C (refs. 8,9). Thus, geothermal gradients in the Archean are similar or even higher than those of modern-day warm subduction zones (~812Ckm1)2,3,4. Under the PT conditions of warm subduction, oceanic crust may release two-thirds of its water <2GPa, equivalent to ~60km depth, while still delivering ca. 2wt.% H2O to the deeper Earth10,11,12. On the other hand, the Earth has undergone secular cooling from 2.53.0Ga ago by as much as 50100CGa1 over the last 3Ga, while 100150CGa1 in the present because the surface heat loss exceeded internal heating9,13. In turn, such a cooling in the average mantle temperature has affected the tectonic processes of the Earth by facilitating the modern-style subduction with low thermal gradient slabs (~58Ckm1)13,14,15,16. These cold subduction zones may allow the oceanic crust to lose only about one-third of its water by 2GPa or ~60km depth10,11,12, hence transport more water into the deeper Earth. The water (or fluid) released from subducting slabs buoyantly rises in the overlying mantle wedge or crust to lower the solidus temperature by ca. 200400C inducing partial melting and volcanism17,18. The H2O transport into the deep Earth is realized by subducting hydrous minerals, which exhibit a range of stability dictated by the PT regime of the subduction system10,11,19. It is therefore essential to investigate the stability of hydrous minerals as a function of diverse geothermal gradients in the past and present tectonic settings to fully understand the evolution of deep water cycling, and related geochemical and geophysical activities.
The major fluid carriers in subducting oceanic crust are hydrous minerals, such as lawsonite, chlorite, and amphiboles, where H2O is contained in the form of molecules and/or structural hydroxyl20. Lawsonite (CaAl2Si2O7(OH)2H2O) contains as much as ca. 11.2wt.% H2O in both molecular and hydroxyl forms, whereas structural hydroxyls account for ca. 1013wt.% H2O in chlorites ((Mg,Fe)5Al2Si3O10(OH)8). Amphiboles carry ca. 13wt.% H2O, much smaller than that of lawsonite or chlorite, but represent the greatest H2O sink because amphiboles may account for a large portion of the metamorphosed oceanic crust by as much as 2060wt.% for basaltic (MORB) compositions21. Glaucophane (Na2(Mg,Fe)3Al2Si8O22(OH)2) is a sodic amphibole (Supplementary Fig.1 and Supplementary Table1), diagnostic for the blueschist facies together with either lawsonite or epidote. The role of amphiboles in basaltic slabs for the generation of arc magma has been experimentally studied for decades to determine whether and how the dehydration of amphiboles provides water to the overlying mantle wedge22. Such dehydration reactions responsible for the blueschist to eclogite transition indeed release a considerable amount of H2O, which triggers intermediate-depth earthquakes via the dehydration embrittlement23,24,25 and induces partial melting in the overlying mantle wedge leading to arc magmatism18,26. In general, the blueschist to eclogite transition is characterized by a suite of dehydration reactions involving the breakdown of amphiboles into pyroxenes27,28, and lawsonite into the garnetkyanitecoesite assemblage29 at elevated PT conditions27,30. The so-called absence of blueschist is thus linked to the global dehydration and breakdown of the blueschist in the Precambrian plate tectonic settings, where warm subduction was a predominant process for recycling of H2O (ref. 31).
In order to understand the present-day subduction process of oceanic crust and gain insights into the evolution of deep water cycle as a function of the Earths secular cooling, we have investigated the stability of glaucophane under PT conditions mimicking cold and warm subduction geotherms together with the high thermal gradients for the Proterozoic tectonic setting3 (Fig.1 and see Methods section inSupplementary information). Using the thermal models of global subduction system19, our experimental PT conditions followed the geotherms of the North Cascadia and South Chile subduction zones, and the Tonga and Kermadec subduction zones, representing warm and cold subduction systems, respectively. The Proterozoic thermal gradients of 2550Ckm1 in T/ Depth were based on the recent compilation of PT data estimated from 456 localities encompassing the Eoarchean to Cenozoic Eras3. We have used both resistive-heated and laser-heated (LH) diamond-anvil cell (DAC) techniques for in situ and ex situ high-pressure and high-temperature (HPHT) synchrotron X-ray powder diffraction (XRD) experiments up to 7.8(3)GPa and 139030C. We have also utilized a Paris-Edinburgh cell (PEC)32 to perform in situ reversal experiments on a mixture of reactants containing glaucophane and a 1000-ton multi-anvil press to retrieve the dehydration products of glaucophane from 3GPa and 9505C. A modified Griggs apparatus was also employed to extend the observation of glaucophane to a natural epidote blueschist rock up to 2GPa and 73010C condition. In order to ensure that our experiments represent equilibrium conditions, samples in DAC and large volume press runs were held at selected pressure and temperature conditions for 1h or up to several hours. Here, we show that the dehydration of glaucophane strongly depends on the thermal gradients of the subduction zone, so that arc magmatism, volcanism, and seismic activities would have been suppressed by secular cooling and subsequent generation of cold subduction system, where water is transported deeper into the Earth.
Curved lines represent individual subduction geotherms from Syracuse et al.19. Continuous and dashed curves denote the PT paths of subducting slab surfaces and corresponding slab Moho, respectively. The Tonga and Kermadec represent cold subducting slab, whereas the North Cascadia and South Chile represent warm subducting slabs. Black lines represent the upper- and lower-pressure stability of glaucophane from previous studies33,34,39, while black dashed lines represent related reactions40,61,62,63. The colored trilateral regions from top to bottom represent high, intermediate, and low geothermal gradients (T/Depth), as defined by Brown and Johnson3. The forbidden zone is a PT region of ultrahigh pressures, where many numerical models predict slab-top geotherms of <5Ckm1 (ref. 64). High-temperature experiments at ambient pressure were performed as a reference (Supplementary Fig.9). Phase abbreviations: glaucophane (Gl), jadeite (Jd), enstatite (En), albite (Ab), talc (Tlc), quartz (Qtz), coesite (Cs), and fluid (F).
In situ HPHT XRD experiments on glaucophane were performed up to 7.8(3)GPa and 76045C for the slab surface, and up to 5.6(3)GPa and 45030C for the slab Moho under dry and wet with 4wt.% of H2O conditions to follow the cold subduction geotherm of the Tonga and Kermadec thermal model19 (Fig.1, Supplementary Fig.2, and Supplementary Table2). Glaucophane under such cold subduction geotherm conditions remained stable up to 7.6(3)GPa and 66040C conditions, equivalent to ca. 240km depth. This extends the stability of glaucophane compared to 3.1(1)GPa at 70010C or 2.5(1)GPa at 84010C, as estimated in previous studies33,34,35. Our result is in agreement with the estimation that oceanic crust within cold subduction zone holds more water by ca. 2wt.% than in warm subduction zone10,11,12. Subsequently, at higher pressure and temperature conditions of 7.8(3)GPa and 76045C, equivalent to ca. 245km depth, we found that glaucophane dehydrates and breakdowns into pyroxenes and coesite as described below (Fig.2 and Supplementary Fig.5):
Representative X-ray powder diffraction patterns of glaucophane along the PT conditions of a cold subduction zone, b warm subduction zone, and c high thermal gradients in the Proterozoic tectonic settings. The experimental data and profile fits using the LeBail method65,66 are shown in black symbols and red lines, respectively, with difference curves in gray lines. The backgrounds of the X-ray diffraction patterns have been subtracted prior to the data being fitted (see Supplementary Fig.3 for the original patterns). Phase abbreviations: glaucophane (Gl), enstatite (En), jadeite (Jd), albite (Ab), coesite (Cs), gold (Au), and neon (Ne).
In order to confirm the enhanced stability and provide a link to interpret seismic low-velocity layer along cold subduction zones, we determined the bulk modulus and linear compressibility of glaucophane at ambient and high temperature at 620C under diverse pressure media (Supplementary Fig.4). Our derived bulk moduli are the same for the different pressure media within 2 and agree well with the data reported in previous studies36,37. We identify that under high-temperature conditions, the anisotropy in linear compressibility is significantly modulated, i.e., at ambient temperature, a=5.1 (kbar1104), while b=2.3 and c=2.2 (kbar1104), whereas at high temperature, the a-axis compressibility is reduced to a=3.9 (kbar1104), while b- and c-axes compressibilities are maintained to b=2.3 and c=2.4 (kbar1104), respectively.
Ex situ and in situ HPHT XRD experiments on glaucophane were performed up to 5.5(1)GPa and 109050C for the slab surface and up to 5.5(1)GPa and 81040C for the slab Moho to follow the warm subduction geotherms of the North Cascadia and South Chile thermal models by Syracuse et al.19 (Fig.1 and Supplementary Table2). The XRD data of the quenched samples after heating at the slab surface conditions above ca. 5.5(1)GPa and 109050C, equivalent to ca. 170km depth, were indexed to identify the breakdown products of glaucophane as in Eq. (1) (Fig.2 and Supplementary Fig.5). The same dehydration scheme, though different in depths, under cold and warm subduction conditions agrees well with the thermodynamic calculation38. Structural hydroxyls of glaucophane are released as fluid when it decomposes into jadeite-bearing assemblages of the eclogite facies. Our experimental results thus demonstrate that blueschist to eclogite transition can be simulated by the dehydration breakdown of glaucophane at different depths depending on the subduction geotherms.
In order to complement our experimental results using a single mineral phase in a DAC, we have further investigated the dehydration of glaucophane on a macroscopic scale using a natural epidote blueschist rock containing ca. 55vol.% glaucophane. A 3mm diameter core-drilled sample of blueschist rocks were heated up to 73010C at 2GPa for 9h, using a modified Griggs apparatus (Fig.1 and Supplementary Table2). The recovered sample showed that glaucophane has partially been dehydrated, and a new dehydration product, omphacite pyroxene ((Ca,Na)(Mg,Fe,Al)Si2O6), has formed, leaving trails of fluid inclusions in glaucophane crystals (Fig.3 and Supplementary Fig.7). Energy-dispersive spectroscopy confirmed the compositions of the recovered glaucophane and the new dehydration product omphacite, as shown in Supplementary Fig.7. This result establishes that the dehydration of glaucophane in a natural blueschist rock begins near 67010C at 2GPa conditions, which corresponds to ca. 60km depth along warm subduction zone.
A modified Griggs apparatus was used for the dehydration of the natural epidote blueschist. Back-scattered electron (BSE) images showing a no dehydration of epidote blueschist after experiment at 2GPa and 57010C and b partially dehydrated glaucophane (Gl) with rugged grain boundaries and fluid inclusion trails (white arrows) after experiment at 2GPa and 73010C; consequently, omphacite (om) has formed as the dehydration product (Supplementary Fig.7). Phase abbreviations: epidote (Ep), plagioclase (Pl), and titanite (Ttn).
Furthermore, we have carried out reversal experiments along the PT conditions of the warm subduction zones using a mixture of reactant minerals, i.e., glaucophane, jadeite, and talc, in the PEC. The mixture was heated above the breakdown condition of glaucophane up to 1000100C at 3.1(3)GPa over 4h, and then cooled down to 550100C at 2.1(3)GPa over 6h. At these PT conditions, we observed the regrowth of glaucophane (130) peak, attesting the reaction boundary would be between ca. 50 and 100km depths along the warm subduction geotherms (Supplementary Fig.8).
Additional LH-DAC experiments were conducted to mimic the high thermal gradients model in the Proterozoic tectonic setting. At conditions of 1.4(1)GPa after heating between 1095100 and 139030C, corresponding to a depth around ~40km, glaucophane breaks down to albite and enstatite, and releases fluid H2O (ref. 39; Fig.2 and Supplementary Fig.5), giving rise to the breakdown scheme:
This scheme is different from the Eq. (1) observed for the cold and warm subduction zones, but in the PT relationship of the reaction, albite=jadeite+quartz, albite is known to be stable at lower pressures than jadeite40. The onset depth of dehydration breakdown of glaucophane thus appears to be inversely proportional to the thermal gradients (T/Depth) of the subduction system. We, however, note that the established depths for the dehydration could change when reaction rates are considered, which is beyond our current experimental capability. Johnson and Fegley reported that partial dehydration can be initiated in amphibole tremolite over several months at temperatures between 750 and 965C (refs. 41,42).
At the same depth, temperature difference between cold and warm subduction zones ranges from ca. 175C to ca. 400C. According to the PT conditions studied here (up to 7.8(3)GPa and 76045C), glaucophane would persist to depths of ca. 240km in cold subduction zones with a geothermal gradient of ~58Ckm1. Along such cold subduction zones, the fully hydrated oceanic crust with the initial H2O content of ca. 6wt.% may lose ca. 2wt.% H2O by 2GPa or ~60km depth, while the rest would be transported deeper into the Earth10,11,12. This is in line with our observed stability of glaucophane under cold subduction conditions. On the other hand, glaucophane decomposes into pyroxenes. i.e., transition to eclogite, at shallower depths between 50 and 100km in warm subduction zones with a geothermal gradient of ~812Ckm1. Upon dehydration, hydroxyls of glaucophane are released to form aqueous fluid, which would migrate upward to induce partial melting of the overlying mantle wedge or cause the lowering of solidus temperature in the subducting slab itself43,44. We estimate the average H2O contents of glaucophane (and amphiboles) in the global oceanic crust to be in the range of 1.13.5104g H2Om3 or 0.391.22wt.% H2O, which accounts for 720% of the total water content in the hydrated oceanic crust with overall 56wt.% H2O (Supplementary Table4); such an amount, when released via dehydration reactions, would be sufficient to induce mantle melting and arc magmatism. In Fig.4, we present the models of glaucophane stability together with the observed seismic frequencies and established mineral assemblages in two contrasting geothermal gradient settings. In cold subduction zones, glaucophane remains stable and enables water transport to deeper mantle. The amount of H2O transported by glaucophane in global cold subduction zones is estimated to be as much as ca. 0.72.11019kg, which is approximately the amount of water in the Arctic ocean (Supplementary Table4).
Subducting slab models representing a the Tonga (cold slabs) and b the Chile (warm slabs). The geometry and temperature profiles were adopted using the information available from the International Seismological Center (ISC-EHB)67,68,69 and the thermal models by Syracuse et al.19. Glaucophane remains stable along the cold subduction zone down to ca. 240km depth (blue arrow), whereas it dehydrates and decomposes into jadeite and enstatite along the warm subduction zone between 50 and 100km depths (white arrow). Based on experiments on a natural blueschist rock, the onset depth of dehydration breakdown is estimated to be ~60km depth. In the middle 2D projection layer, the earthquake frequencies are shown as filled circles using the data from the ISC-EHB. In the upper 2D projection layer, subducting hydrous minerals from Hacker et al.70 and Magni et al.71 are shown in colors on each thermal model (slab dip, slab age, and convergence rate of 50, 110Ma, 165kmMa1 for the cold slab and 45, 50Ma, 60C km1 for the warm slab). Phaseabbreviations: blueschist (BS), eclogite (EC), amphibole (Am), chlorite (Chl), chloritoid (Cld), lawsonite (Lws), epidote (Ep), zoisite (Zo), talc (Tlc), serpentine (Srp), garnet (Grt), and pyroxene (Prx).
Our observation bears some implications for the distribution of seismic low-velocity layers, as well as seismic activities along the subducting slabs. According to our compressibility data (Supplementary Fig.4), glaucophane behaves anisotropic even at high temperature, indicating strong mechanical resistance along the (100) plane, while  direction is relatively weak. Hydrous minerals have been suggested to be related to the seismic anisotropy and delayed seismic travel times along subduction zones in the depth range of 100250km (refs. 45,46,47,48,49,50,51,52). The observed anisotropy and stability of glaucophane could, therefore, account for such seismic anomalies distributed, which would be deeper in the colder and older slabs than in the warmer and younger slabs53. Furthermore, seismic observations reveal that the low-velocity layers spatially coincide with the zones of intermediate-depth earthquakes54, which is in turn related to the dehydration of hydrous minerals45,55. With this regard, we show the correlation between the seismic frequencies along subduction zones and the stability range of glaucophane, i.e., the maximum depth of intraslab earthquakes ranges between 50 and 70km in warm subduction zones, whereas it extends down to over 200km in cold subduction zones56 (Fig.4 and Supplementary Fig.11). As dehydration embrittlement of serpentine was previously proposed as a possible mechanism for the intermediate-depth earthquakes23, subduction geotherm-dependent breakdown of glaucophane would provide another venue to explain the distribution of intermediate-depth earthquakes57,58,59. Our results would therefore serve as an experimental evidence to support the recent observation that the double seismic zone in the Tonga subduction system extends to deeper depths down to ca. 300km (ref. 60).
Among the 56 subduction geotherm data from Syracuse et al.19, we could categorize 16 subduction zones as cold subduction system on the basis of the thermal parameter with average values of 48.9 , 119.7Ma, 74.8kmMa1 for slab dip, age, and convergence rate, respectively (Supplementary Table5 and Supplementary Fig.10). Inferred from our results, sodic amphiboles, e.g., glaucophane, would be stable to deeper depths in ca. 28.5% of the global subduction system in the present Earth. Recent studies indicate that plate tectonics based on subduction-related processes has been initiated during the Proterozoic eon2,3,4,5,6 or as early as 3.8 or 4.4Ga (refs. 1,7), when the majority of subduction zones would be categorized as warm or intermediate-to-high thermal gradients system3. According to our experimental results, the dehydration depth of glaucophane has increased with decreasing thermal gradients hence with secular cooling, which would translate to transportation of water into deeper Earth (Supplementary Table4). We, therefore, conjecture that arc magmatism would have been more effective via ubiquitous transformation of blueschist to eclogite in the high thermal gradients system of the early Earth. As the Earth undergoes secular cooling, progressively colder subduction zones have emerged and resulted in ca. 28.5% of the whole subduction system in the present Earth. Consequently, arc magmatism, volcanism, and related seismic activities linked to the dehydration of amphiboles have been globally suppressed, enabling blueschist to persist and be preserved in todays geodynamic system. The absence of blueschist from the Precambrian rocks might thus be explained by the subduction geotherm-dependent dehydration of glaucophane. On the other hand, subduction efficiency, i.e., the proportional amount of subducted H2O passing through the subduction zone filter11, would have increased with the generation of low thermal gradients system toward the present Earth, as observed in our study.
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This work was supported by the Leader Researcher program (NRF-2018R1A3B1052042) of the Korean Ministry of Science and ICT (MSIT). We also thank the partial supports by NRF-2016K1A4A3914691, NRF-2019K1A3A7A09033395, and NRF-2020R1A2C2003765 grants of the MSIT. Synchrotron experiments were performed at the beamlines 3D and 5A at PLS-II, HPCAT and GSECARS at APS, and ECB P02.2 at PETRA-III. HPCAT operations are supported by DOE NNSAs Office of Experimental Sciences. GSECARS is supported by the NSF-Earth Sciences (EAR-1634415) and Department of Energy (DOE)-GeoSciences (DE-FG02-94ER14466). The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. H.C. thanks the support bythe U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.The authors thank Xueyan Du and Yuyong Xiong at HPSTAR for assisting laser-heating experiment, Youmo Zhou and Toru Shinmei at Ehime University for multi-anvil press experiment, Guoyin Shen, Rostislav Hrubiak and Curtis Kenney-Benson at HPCAT for supporting Paris-Edinburgh Cell experiment, G. Diego Gatta at the University of Milan for providing a natural sample of jadeite, and Moonsup Cho at Chungbuk National University and Sang-Heon Dan Shim at Arizona State University for valuable discussions.
Y.B. contributed to the experiments and data analysis with the help from H.H, T.K., Y.P., C.P., D.P., V.P., and H.-P.L. Y.L. designed the research, discussed the results with H.C., H.J., L.W., T.I., and H.-K.M. and worked on the manuscript with all authors.
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