Dehydration Kinetics Of Amphibole And It's Geophysical Implication

The subduction zone is an important recycling system on the earth.Intermediate-deep earthquakes in the subduction zone,especially the circum-Pacific Ocean subduction zones,occur in two distinct layers,forming different focal mechanisms of seismic zone.The simultaneous and discovery studies of Double Seismic Zones(DSZs)in the intermediate-depth earthquake(30-300 km)greatly contribute to the dehydration embrittlement.As an important hydrous mineral with abundant content and complex structure in the subduction zone,the study of the dehydration kinetics of amphibole will be of great significance to understand the transport and local accumulate of water in the subduction zone and exploring the focal mechanisms of intermediate-depth earthquakes.Thermogravimetric experiments were carried out on hornblende and tremolite with a particle size of 5?10 ?m under the air and nitrogen and the experimental temperature was 303 K to 1573 K.The model method and the free-model method(Friedman method)are used to calculate the kinetic parameters and mostly kinetics equation in the dehydration process.The pressure effect on the dehydration of hornblende is measured by differential thermal analysis(DTA)under the 1.0 GPa,2.0 GPa and 3.0 GPa.The main conclusions of the study are as follows:1.The thermal decomposition of hornblende occurs in the range of 848?1473 K.The most likely reaction mechanism of hornblende dehydration under the free-model fitting method(Friedman method)could be expressed as f(?)=1-? and G(?)=-ln(1-?).As the reaction progress(a)increases,the reaction activation energy is distributed in a wide interval(437?812 kJ/mol).It's impossible to fit the dehydration process of hornblende by the single kinetics mechanism.For comparison,The dehydration of hornblende under the model method can be divided into three stages of 848?1058 K,1058?1243 K and 1243?1473 K,and all three stages could be expressed as f(?)=(1-a)n and G(?)=(1-(1-?)(1-n))/(1-n).In the first stage,the derived values of 416.6 kJ/mol,1.58×1022 s-1 and 4.23 for E,A and n,respectively.In the second stage,the derived values of 411.4 kJ/mol,2.82×1015 s-1 and 2.3 for E,A and n,respectively.In the third stage,the derived values of 720.9 kJ/mol,2.51×1027 s-1 and 1.95 for E,A and n,respectively.The correlation coefficient is 99.953%.In the first stage,the hydroxyl group attached to Fe2+ is removed from the crystal structure of hornblende.In the second and third stage,the hydroxyl group attached to Mg2+ and the hydroxyl group that is attached to ferrous but does not undergo dehydroxylation reaction in the first stage.Fluid release rates of hornblende is 5.4×10-4 to 1.4×10-5 m3 fluid m-3 rock s-1.Compared with the fluid release rates of antigorite,the rapid dehydration of hornblende could induced the seismicity in the subduction.2.The dehydration reaction of tremolite occurs at 1130?1340 K.As the reaction progress increases(?=0.15?0.85),the reaction activation energy is distributed in a wide interval(831.6?1311 kU/mol).It's impossible to fit the dehydration process of tremolite by the single kinetics mechanism.3.The initial and peak temperature for dehydration of hornblende at 1.0 GPa is 1073 K,1133 K and 1281 K,respectively.The initial and peak temperature for dehydration of hornblende at 2.0 GPa is 1058 K,1098 K and 1318 K,respectively.The initial and peak temperature for dehydration of hornblende at 3.0 GPa is 1073 K,1131 K and 1263 K,respectively.Compared with the Thermogravimetric experiments of hornblende under air atmosphere,the dehydration of hornblende under high temperature and high pressure still has two stages.