The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subduc...The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low(?5?C/km), lawsonite may carry water into great depths of ?300 km. In the hot subduction zone where the geothermal gradient is high(>25?C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of <80 km to form felsic melts, into which water is highly dissolved. In this case, only a minor amount of water can be transported into great depths. A number of intermediate modes are present between these two end-member dehydration modes, making subduction-zone dehydration various. Low-T/low-P hydrous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of ?60–80 km to release large amounts of water. In contrast, the low-T/low-P hydrous minerals are replaced by low-T/high-P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80–160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultrahigh-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately.展开更多
A continuous flow method,by a combination of thermal conversion elemental analyzer(TC/EA)with isotope ratio mass spectrometry(MS),was developed to determine both H isotope composition and H2O concentration of ultrahig...A continuous flow method,by a combination of thermal conversion elemental analyzer(TC/EA)with isotope ratio mass spectrometry(MS),was developed to determine both H isotope composition and H2O concentration of ultrahigh-pressure(UHP)metamorphic rocks in the Dabie-Sulu orogenic belt.By using the developed step-heating technique,we have studied H2O concentration and H isotope composition of the different forms of water(structural OH and molecular H2O)in garnet.The quantitative measurements of H2O concentration and H isotope composition of minerals in UHP metamorphic rocks from several typical outcrops indicate that the gneisses can release more amounts of water than the eclogites during exhumation of the deeply subducted continental crust.Therefore,by decompression dehydration at the contact between eclogite and gneiss,the released water could flow from the gneiss to the eclogite and result in significant hydration of the eclogite adjacent to the gneiss.The measured maximum water contents of minerals in eclogites indicate that garnet and omphacite have the maximum water solubilities of 2500and 3500 ppm,respectively,under the peak UHP metamorphic conditions.展开更多
基金supported by funds from the National Natural Science Foundation of China(Grant No.41590620)the Chinese Ministry of Science and Technology(Grant No.2015CB856100)
文摘The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low(?5?C/km), lawsonite may carry water into great depths of ?300 km. In the hot subduction zone where the geothermal gradient is high(>25?C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of <80 km to form felsic melts, into which water is highly dissolved. In this case, only a minor amount of water can be transported into great depths. A number of intermediate modes are present between these two end-member dehydration modes, making subduction-zone dehydration various. Low-T/low-P hydrous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of ?60–80 km to release large amounts of water. In contrast, the low-T/low-P hydrous minerals are replaced by low-T/high-P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80–160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultrahigh-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately.
基金supported by the Ministry of Science and Technology of China(2009CB825004)the National Natural Science Foundation of China(41273006 and 41221062)the Chinese Academy of Sciences(KZCX2-EW-QN502)
文摘A continuous flow method,by a combination of thermal conversion elemental analyzer(TC/EA)with isotope ratio mass spectrometry(MS),was developed to determine both H isotope composition and H2O concentration of ultrahigh-pressure(UHP)metamorphic rocks in the Dabie-Sulu orogenic belt.By using the developed step-heating technique,we have studied H2O concentration and H isotope composition of the different forms of water(structural OH and molecular H2O)in garnet.The quantitative measurements of H2O concentration and H isotope composition of minerals in UHP metamorphic rocks from several typical outcrops indicate that the gneisses can release more amounts of water than the eclogites during exhumation of the deeply subducted continental crust.Therefore,by decompression dehydration at the contact between eclogite and gneiss,the released water could flow from the gneiss to the eclogite and result in significant hydration of the eclogite adjacent to the gneiss.The measured maximum water contents of minerals in eclogites indicate that garnet and omphacite have the maximum water solubilities of 2500and 3500 ppm,respectively,under the peak UHP metamorphic conditions.
