The X-discontinuity,which appears at the depth of approximately 300 km,is an important seismic interface with positive velocity contrasts in the upper mantle.Detecting its presence and topography can be useful to unde...The X-discontinuity,which appears at the depth of approximately 300 km,is an important seismic interface with positive velocity contrasts in the upper mantle.Detecting its presence and topography can be useful to understand phase transformations of relevant mantle minerals under the high-temperature and high-pressure circumstance of the Earth's interior.In this study,we detect the X-discontinuity beneath the Ryukyu subduction zone using five intermediate-depth events recorded by the dense Alaska Regional Network(AK).The X-discontinuity is successfully revealed from the robust slant stacking of the secondary down-going and converting Sd P phases.From the depth distribution of conversion points,we find that the X-discontinuity's depth ranges between 269 km and 313 km,with an average depth of 295 km.All the conversion points are located beneath the down-dipping side of the Philippine Sea slab.From energy comparisons in vespagrams for observed and synthetic seismograms,the strong converted energy is more likely from a thin high-velocity layer,and the S-wave velocity jumps across the X-discontinuity are up to 5% to 8% with an average of 6.0%.According to previous petrological and seismological studies,the X-discontinuity we detected can be interpreted as the phase transformation of coesite to stishovite in eclogitic materials within the oceanic crust.展开更多
The water migration in subduction zones,primarily driven by the phase transition in hydrous minerals,can give rise to hydrated regions with reduced velocity.A fundamental element in comprehending and deciphering these...The water migration in subduction zones,primarily driven by the phase transition in hydrous minerals,can give rise to hydrated regions with reduced velocity.A fundamental element in comprehending and deciphering these low-velocity zones revolves around acquiring insights into the stability and elasticity of relevant hydrous minerals.As one of the main water carriers in shallow areas,antigorite can dehydrate to form talc,forsterite,and fluid(talc-bearing peridotites)in deep areas of subduction zones,and then the talc thus serves as one of the minerals that can bring water to the deep Earth.Here,the elasticity of talc up to 24 GPa and forsterite up to 12 GPa are calculated by using the first principles method.The result supposes that the talc structure transforming from talc I to talc II is at a pressure between 6 GPa and 8 GPa,impacting the trend of elastic wave velocity in response to pressure.Furthermore,the elastic wave velocity of forsterite can be significantly affected by iron concentration.Meanwhile,a variation velocity model with antigorite consumption and talc content is set up for talc-bearing serpentinized peridotite based on the elastic properties of talc and forsterite in this study,and antigorite in Wang et al.(2022).The results of our model demonstrate a decrease in the low-velocity anomaly in subduction zones,particularly in deep regions or areas with higher initial serpentinization degrees.The results also suggest that the mode of antigorite dehydration can diminish the estimation of water content transported to depths of subduction zones,such as the Mariana Trench and Northern Japan subduction zones.The mode of antigorite dehydration thus provides a useful tool for constraining the composition,seismic velocity structure,and water migration in subduction zones.展开更多
基金supported by the China Postdoctoral Science Foundation(119103S282)National Natural Science Foundation of China(41704090,41474040 and 41504050)
文摘The X-discontinuity,which appears at the depth of approximately 300 km,is an important seismic interface with positive velocity contrasts in the upper mantle.Detecting its presence and topography can be useful to understand phase transformations of relevant mantle minerals under the high-temperature and high-pressure circumstance of the Earth's interior.In this study,we detect the X-discontinuity beneath the Ryukyu subduction zone using five intermediate-depth events recorded by the dense Alaska Regional Network(AK).The X-discontinuity is successfully revealed from the robust slant stacking of the secondary down-going and converting Sd P phases.From the depth distribution of conversion points,we find that the X-discontinuity's depth ranges between 269 km and 313 km,with an average depth of 295 km.All the conversion points are located beneath the down-dipping side of the Philippine Sea slab.From energy comparisons in vespagrams for observed and synthetic seismograms,the strong converted energy is more likely from a thin high-velocity layer,and the S-wave velocity jumps across the X-discontinuity are up to 5% to 8% with an average of 6.0%.According to previous petrological and seismological studies,the X-discontinuity we detected can be interpreted as the phase transformation of coesite to stishovite in eclogitic materials within the oceanic crust.
基金supported by the National Natural Science Foundation of China(Grant No.42174114,41674089).
文摘The water migration in subduction zones,primarily driven by the phase transition in hydrous minerals,can give rise to hydrated regions with reduced velocity.A fundamental element in comprehending and deciphering these low-velocity zones revolves around acquiring insights into the stability and elasticity of relevant hydrous minerals.As one of the main water carriers in shallow areas,antigorite can dehydrate to form talc,forsterite,and fluid(talc-bearing peridotites)in deep areas of subduction zones,and then the talc thus serves as one of the minerals that can bring water to the deep Earth.Here,the elasticity of talc up to 24 GPa and forsterite up to 12 GPa are calculated by using the first principles method.The result supposes that the talc structure transforming from talc I to talc II is at a pressure between 6 GPa and 8 GPa,impacting the trend of elastic wave velocity in response to pressure.Furthermore,the elastic wave velocity of forsterite can be significantly affected by iron concentration.Meanwhile,a variation velocity model with antigorite consumption and talc content is set up for talc-bearing serpentinized peridotite based on the elastic properties of talc and forsterite in this study,and antigorite in Wang et al.(2022).The results of our model demonstrate a decrease in the low-velocity anomaly in subduction zones,particularly in deep regions or areas with higher initial serpentinization degrees.The results also suggest that the mode of antigorite dehydration can diminish the estimation of water content transported to depths of subduction zones,such as the Mariana Trench and Northern Japan subduction zones.The mode of antigorite dehydration thus provides a useful tool for constraining the composition,seismic velocity structure,and water migration in subduction zones.
