The molybdenum(Mo)isotope system is pivotal in reconstructing marine redox changes throughout Earth’s history and has emerged as a promising tracer for igneous and metamorphic processes.Understanding its composition ...The molybdenum(Mo)isotope system is pivotal in reconstructing marine redox changes throughout Earth’s history and has emerged as a promising tracer for igneous and metamorphic processes.Understanding its composition and variation across major geochemical reservoirs is essential for its application in investigating high-temperature processes.However,there is debate regarding theδ^(98/95)Mo value of the Earth’s mantle,with estimates ranging from sub-chondritic to super-chondritic values.Recent analyses of global mid-ocean ridge basalt(MORB)glasses revealed significantδ^(98/95)Mo variations attributed to mantle heterogeneity,proposing a two-component mixing model to explain the observed variation.Complementary studies confirmed the sub-chondriticδ^(98/95)Mo of the depleted upper mantle,suggesting remixing of subduction-modified oceanic crust as a plausible mechanism.These findings underscore the role of Mo isotopes as effective tracers for understanding dynamic processes associated with mantle-crustal recycling.展开更多
A number of high-temperature processes(e.g.,melt-rock reactions,metasomatism,partial melting)can produce signifi cant Ca isotopic fractionation and heterogeneity in the mantle,but the mechanism for such fractionation ...A number of high-temperature processes(e.g.,melt-rock reactions,metasomatism,partial melting)can produce signifi cant Ca isotopic fractionation and heterogeneity in the mantle,but the mechanism for such fractionation remains obscure.To investigate the eff ect of mantle partial melting on Ca isotopic fractionation,we reported high-precision Ca isotopic compositions of depleted mid-ocean ridge basalts(MORBs)from the East Pacifi c Rise and Ecuador Rift in the northeastern Pacifi c.Theδ44/40 Ca of these MORB samples exhibit a narrow variation from 0.84‰to 0.88‰with an average of 0.85‰±0.03‰,which are similar to those of reported MORBs(0.83‰±0.11‰)and back-arc basin basalts(BABBs,0.80‰±0.08‰)in literature,but are lower than the estimate value for the bulk silicate Earth(BSE,0.94‰±0.05‰).The lowδ44/40 Ca signatures of MORB samples in this study cannot be caused by fractional crystallization,since intermediate-mafi c diff erentiation has been demonstrated having only limited eff ects on Ca isotopic fractionation.Instead,the off set ofδ44/40 Ca between MORBs and the BSE is most likely produced by mantle partial melting.During this process,the light Ca isotopes are preferentially transferred to the melt,while the heavy ones tend to stay in the residue,which is consistent with the fact thatδ44/40 Ca of melt-depleted peridotites increases with partial melting in literature.The behavior of Ca isotopes during mantle partial melting is closely related to the inter-mineral(Cpx and Opx)Ca isotopic fractionation and melting mineral modes.Mantle partial melting is one of the common processes that can induce lowerδ44/40 Ca values in basalts and Ca isotopic heterogeneity in Earth’s mantle.展开更多
Multichannel seismic studies performed at fastspreading mid-ocean ridges revealed the presence of a thin(tens to hundreds of meters high), narrow(< 1-2 km wide) axial melt lens(AML) in the mid-crust, which is under...Multichannel seismic studies performed at fastspreading mid-ocean ridges revealed the presence of a thin(tens to hundreds of meters high), narrow(< 1-2 km wide) axial melt lens(AML) in the mid-crust, which is underlain by crystal/melt mush that is in turn laterally surrounded by a transition zone of mostly solidified material. In order to shed light on the complexity of magmatic and metamorphic processes ongoing within and at the roof of axial melt lenses, we have focused on the petrological and geochemical record provided by fossilized AMLs. Of particular significance is Hole 1256D in the equatorial Pacific drilled by the International Ocean Discovery Program(IODP), where for the first time, the transition between sheeted dikes and gabbros in intact fast-spreading crust was penetrated, providing a drill core with a more or less continuous record of the upper part of an AML(Teagle et al., 2006;Koepke et al., 2008). This can be regarded as rosetta stone to answer longstanding questions on the complex magmatic evolution within an AML, as well as on metamorphic and anatectic processes ongoing at the roof of a dynamic AML, rising upward in the midcrust as a consequence of a replenishment event. The plutonic rocks drilled from Hole 1256D consist of quartz-bearing gabbros, diorites and tonalites, which might represent the upper part of a fossilized AML. The gabbros and diorites are consistent with modeled products of MORB fractional crystallization, composed of mixed melt and cumulate in varying ratios. Modeled trace elements support a model in which the tonalites originated from low-degree partial melting of the sheeted dikes overlying the AML, rather than extreme fractional crystallization(Erdmann et al., 2015;Zhang et al., 2017a). Therefore, the upper part of AML, largely composed of low density and high-viscosity felsic magmas, may serve as a barrier to eruptible MORB melts in the lower part of AML. Zoning of apatites from three different lithologies, tonalites, diorites, and gabbros, is common and shows a consistent evolution trend with depletion in Cl and REEs from core to rim. The cores are usually homogenous in composition and interpreted as magmatic origin, whereas zones with lower Cl and REEs are disseminated with heterogeneous concentrations, indicating exchanges with hydrothermal fluids. The high-Cl apatite core indicates assimilation of high-Cl brines at a magmatic stage, which is interpreted as immiscibility product from cycling seawater-derived fluids at a high temperature(Zhang et al., 2017b). The variation of F/Cl and Br/Cl ratios of bull rocks may reflect the mixing between MORB magmas and seawater-derived fluids, crystallization of apatite and amphibole, and/or extraction of magmatic fluids(Zhang et al., 2017c).展开更多
The high-pressure metamorphosed Gridino dyke swarm comprises a major group of Mesoarchean 2.87-2.82 Ga mafic dykes intruded within the Mesoarchean continental crust of the Kola craton(the Belomorian tectonic province
The relation of heat flow and floor depth across the mid-ocean ridges versus lithosphere age can be described by linear functions of square root of age according to plate thermal conductive Half Space Models(HSM).Howe...The relation of heat flow and floor depth across the mid-ocean ridges versus lithosphere age can be described by linear functions of square root of age according to plate thermal conductive Half Space Models(HSM).However,one of the long-standing problems of these classical models is the discrepancies between predicted and observed heat flow and floor depth for very young and very old lithosphere.There have been several recent attempts to overcome this problem:one model incorporates temperature-and pressure-dependent parameters and the second model includes an additional low-conductivity crustal layer or magma rich mantle layer(MRM).Alternatively,in the current paper,the ordinary density of lithosphere in the plate conductive models is substituted with a reduction of lithosphere density towards axis that features the irregularity and nonlinearity of plates across the mid-ocean ridges.A new model is formulated incorporating the new form of density for predicting both peak heat flow and floor depth.Simple solutions of power-law forms derived from the model can significantly improve the predicting results of heat flow and floor depth over the mid-ocean ridges.Several datasets in the literature were reutilized for model validation and comparison.These datasets include both earlier datasets used for original model calibration and the more recently compiled high-quality datasets with both sedimentary and crustal loading corrections.The results indicate that both the heat flow and the slope(first orderderivative)of sea floor approach infinity(undifferentiability or singularities)around the mid-ocean ridges.These singularities are partially due to the boundary condition as it has been already known in the literature and partially to the reduction of density of lithosphere as discovered for the first time in the current research.展开更多
Mid-ocean ridge and oceanic transforms are among the most prominent features on the seafloor surface and are crucial for understanding seafloor spreading and plate tectonic dynamics,but the deep structure of the ocean...Mid-ocean ridge and oceanic transforms are among the most prominent features on the seafloor surface and are crucial for understanding seafloor spreading and plate tectonic dynamics,but the deep structure of the oceanic lithosphere remains poorly understood.The large number of microearthquakes occurring along ridges and transforms provide valuable information for gaining an indepth view of the underlying detailed seismic structures,contributing to understanding geodynamic processes within the oceanic lithosphere.Previous studies have indicated that the maximum depth of microseismicity is controlled by the 600-℃isotherm.However,this perspective is being challenged due to increasing observations of deep earthquakes that far exceed this suggested isotherm along mid-ocean ridges and oceanic transform faults.Several mechanisms have been proposed to explain these deep events,and we suggest that local geodynamic processes(e.g.,magma supply,mylonite shear zone,longlived faults,hydrothermal vents,etc.)likely play a more important role than previously thought.展开更多
Based on 230Th-238U disequilibrium and major element data from mid-ocean ridge basalts(MORBs) and ocean island basalts(OIBs),this study calculates mantle melting parameters,and thereby investigates the origin of 230Th...Based on 230Th-238U disequilibrium and major element data from mid-ocean ridge basalts(MORBs) and ocean island basalts(OIBs),this study calculates mantle melting parameters,and thereby investigates the origin of 230Th excess.(230Th/238U) in global MORBs shows a positive correlation with Fe8,Po,Na8,and Fmelt(Fe8 and Na8 are FeO and Na2O contents respectively after correction for crustal fractionation relative to MgO = 8 wt%,Po=pressure of initial melting and Fmelt=degree of melt),while 230Th excess in OIBs has no obvious correlation with either initial mantle melting depth or the average degree of mantle melting.Furthermore,compared with the MORBs,higher(230Th/238U) in OIBs actually corresponds to a lower melting degree.This suggests that the 230Th excess in MORBs is controlled by mantle melting conditions,while the 230Th excess in OIBs is more likely related to the deep garnet control.The vast majority of calculated initial melting pressures of MORBs with excess 230Th are between 1.0 and 2.5 GPa,which is consistent with the conclusion from experiments in recent years that DU】DTh for Al-clinopyroxene at pressures of 】1.0 GPa.The initial melting pressure of OIBs is 2.2-3.5 GPa(around the spinel-garnet transition zone),with their low excess 226Ra compared to MORBs also suggesting a deeper mantle source.Accordingly,excess 230Th in MORBs and OIBs may be formed respectively in the spinel and garnet stability field.In addition,there is no obvious correlation of K2O/TiO2 with(230Th/238U) and initial melting pressure(Po) of MORBs,so it is proposed that the melting depth producing excess 230Th does not tap the spinel-garnet transition zone.OIBs and MORBs in both(230Th/238U) vs.K2O/TiO2 and(230Th/238U) vs.Po plots fall in two distinct areas,indicating that the mineral phases which dominate their excess 230Th are different.Ce/Yb-Ce curves of fast and slow ridge MORBs are similar,while,in comparison,the Ce/Yb-Ce curve for OIBs shows more influence from garnet.The mechanisms generating excess 230Th in MORBs and OIBs are significantly different,with formation of excess 230Th in the garnet zone only being suitable for OIBs.展开更多
基金the National Natural Science Foundation of China(Nos.42176087,42322605)the Laoshan Laboratory(No.LSKJ202204100)the Youth Innovation Promotion Association,Chinese Academy of Sciences(No.2021206)。
文摘The molybdenum(Mo)isotope system is pivotal in reconstructing marine redox changes throughout Earth’s history and has emerged as a promising tracer for igneous and metamorphic processes.Understanding its composition and variation across major geochemical reservoirs is essential for its application in investigating high-temperature processes.However,there is debate regarding theδ^(98/95)Mo value of the Earth’s mantle,with estimates ranging from sub-chondritic to super-chondritic values.Recent analyses of global mid-ocean ridge basalt(MORB)glasses revealed significantδ^(98/95)Mo variations attributed to mantle heterogeneity,proposing a two-component mixing model to explain the observed variation.Complementary studies confirmed the sub-chondriticδ^(98/95)Mo of the depleted upper mantle,suggesting remixing of subduction-modified oceanic crust as a plausible mechanism.These findings underscore the role of Mo isotopes as effective tracers for understanding dynamic processes associated with mantle-crustal recycling.
基金Supported by the National Natural Science Foundation of China(Nos.41773009,41873002)the Stake Key Laboratory of Geological Processes and Mineral Resources(No.GPMR201708)+2 种基金the National Science Foundation for Post-doctoral Scientists of China(No.2018M640660)the Taishan Scholar Program of Shandong(No.TS201712075)the AoShan Talents Cultivation Program Supported by Qingdao National Laboratory for Marine Science and Technology(No.2017ASTCP-OS07)。
文摘A number of high-temperature processes(e.g.,melt-rock reactions,metasomatism,partial melting)can produce signifi cant Ca isotopic fractionation and heterogeneity in the mantle,but the mechanism for such fractionation remains obscure.To investigate the eff ect of mantle partial melting on Ca isotopic fractionation,we reported high-precision Ca isotopic compositions of depleted mid-ocean ridge basalts(MORBs)from the East Pacifi c Rise and Ecuador Rift in the northeastern Pacifi c.Theδ44/40 Ca of these MORB samples exhibit a narrow variation from 0.84‰to 0.88‰with an average of 0.85‰±0.03‰,which are similar to those of reported MORBs(0.83‰±0.11‰)and back-arc basin basalts(BABBs,0.80‰±0.08‰)in literature,but are lower than the estimate value for the bulk silicate Earth(BSE,0.94‰±0.05‰).The lowδ44/40 Ca signatures of MORB samples in this study cannot be caused by fractional crystallization,since intermediate-mafi c diff erentiation has been demonstrated having only limited eff ects on Ca isotopic fractionation.Instead,the off set ofδ44/40 Ca between MORBs and the BSE is most likely produced by mantle partial melting.During this process,the light Ca isotopes are preferentially transferred to the melt,while the heavy ones tend to stay in the residue,which is consistent with the fact thatδ44/40 Ca of melt-depleted peridotites increases with partial melting in literature.The behavior of Ca isotopes during mantle partial melting is closely related to the inter-mineral(Cpx and Opx)Ca isotopic fractionation and melting mineral modes.Mantle partial melting is one of the common processes that can induce lowerδ44/40 Ca values in basalts and Ca isotopic heterogeneity in Earth’s mantle.
基金supported by the DFG(Deutsche Forschungsgemeinschaft)project KO 1723/17
文摘Multichannel seismic studies performed at fastspreading mid-ocean ridges revealed the presence of a thin(tens to hundreds of meters high), narrow(< 1-2 km wide) axial melt lens(AML) in the mid-crust, which is underlain by crystal/melt mush that is in turn laterally surrounded by a transition zone of mostly solidified material. In order to shed light on the complexity of magmatic and metamorphic processes ongoing within and at the roof of axial melt lenses, we have focused on the petrological and geochemical record provided by fossilized AMLs. Of particular significance is Hole 1256D in the equatorial Pacific drilled by the International Ocean Discovery Program(IODP), where for the first time, the transition between sheeted dikes and gabbros in intact fast-spreading crust was penetrated, providing a drill core with a more or less continuous record of the upper part of an AML(Teagle et al., 2006;Koepke et al., 2008). This can be regarded as rosetta stone to answer longstanding questions on the complex magmatic evolution within an AML, as well as on metamorphic and anatectic processes ongoing at the roof of a dynamic AML, rising upward in the midcrust as a consequence of a replenishment event. The plutonic rocks drilled from Hole 1256D consist of quartz-bearing gabbros, diorites and tonalites, which might represent the upper part of a fossilized AML. The gabbros and diorites are consistent with modeled products of MORB fractional crystallization, composed of mixed melt and cumulate in varying ratios. Modeled trace elements support a model in which the tonalites originated from low-degree partial melting of the sheeted dikes overlying the AML, rather than extreme fractional crystallization(Erdmann et al., 2015;Zhang et al., 2017a). Therefore, the upper part of AML, largely composed of low density and high-viscosity felsic magmas, may serve as a barrier to eruptible MORB melts in the lower part of AML. Zoning of apatites from three different lithologies, tonalites, diorites, and gabbros, is common and shows a consistent evolution trend with depletion in Cl and REEs from core to rim. The cores are usually homogenous in composition and interpreted as magmatic origin, whereas zones with lower Cl and REEs are disseminated with heterogeneous concentrations, indicating exchanges with hydrothermal fluids. The high-Cl apatite core indicates assimilation of high-Cl brines at a magmatic stage, which is interpreted as immiscibility product from cycling seawater-derived fluids at a high temperature(Zhang et al., 2017b). The variation of F/Cl and Br/Cl ratios of bull rocks may reflect the mixing between MORB magmas and seawater-derived fluids, crystallization of apatite and amphibole, and/or extraction of magmatic fluids(Zhang et al., 2017c).
文摘The high-pressure metamorphosed Gridino dyke swarm comprises a major group of Mesoarchean 2.87-2.82 Ga mafic dykes intruded within the Mesoarchean continental crust of the Kola craton(the Belomorian tectonic province
基金supported by National Natural Science Foundation of China(grant number 42050103)Guangdong Research Team Development Grant(grant number 2021ZT09H399)。
文摘The relation of heat flow and floor depth across the mid-ocean ridges versus lithosphere age can be described by linear functions of square root of age according to plate thermal conductive Half Space Models(HSM).However,one of the long-standing problems of these classical models is the discrepancies between predicted and observed heat flow and floor depth for very young and very old lithosphere.There have been several recent attempts to overcome this problem:one model incorporates temperature-and pressure-dependent parameters and the second model includes an additional low-conductivity crustal layer or magma rich mantle layer(MRM).Alternatively,in the current paper,the ordinary density of lithosphere in the plate conductive models is substituted with a reduction of lithosphere density towards axis that features the irregularity and nonlinearity of plates across the mid-ocean ridges.A new model is formulated incorporating the new form of density for predicting both peak heat flow and floor depth.Simple solutions of power-law forms derived from the model can significantly improve the predicting results of heat flow and floor depth over the mid-ocean ridges.Several datasets in the literature were reutilized for model validation and comparison.These datasets include both earlier datasets used for original model calibration and the more recently compiled high-quality datasets with both sedimentary and crustal loading corrections.The results indicate that both the heat flow and the slope(first orderderivative)of sea floor approach infinity(undifferentiability or singularities)around the mid-ocean ridges.These singularities are partially due to the boundary condition as it has been already known in the literature and partially to the reduction of density of lithosphere as discovered for the first time in the current research.
基金Supported by the State Key Program of National Natural Science of China(No.42330308)the Project of Donghai Laboratory(No.DH-2022ZY0005)+4 种基金the Scientific Research Fund of the Second Institute of OceanographyMinistry of Natural Resources(No.QHXZ2301)the National Science Foundation for Distinguished Young Scholars of China(No.42025601)for Young Scientists of China(No.41906064)the Zhejiang Provincial Natural Science Foundation of China(No.LDQ24D060001)。
文摘Mid-ocean ridge and oceanic transforms are among the most prominent features on the seafloor surface and are crucial for understanding seafloor spreading and plate tectonic dynamics,but the deep structure of the oceanic lithosphere remains poorly understood.The large number of microearthquakes occurring along ridges and transforms provide valuable information for gaining an indepth view of the underlying detailed seismic structures,contributing to understanding geodynamic processes within the oceanic lithosphere.Previous studies have indicated that the maximum depth of microseismicity is controlled by the 600-℃isotherm.However,this perspective is being challenged due to increasing observations of deep earthquakes that far exceed this suggested isotherm along mid-ocean ridges and oceanic transform faults.Several mechanisms have been proposed to explain these deep events,and we suggest that local geodynamic processes(e.g.,magma supply,mylonite shear zone,longlived faults,hydrothermal vents,etc.)likely play a more important role than previously thought.
基金supported by National Natural Science Foundation of China (Grant No.40830849)Special Foundation for the Eleventh Five Plan of COMRA (Grant No.DYXM-115-02-1-03)National Natural Science Foundation of China (Grant No.40906029)
文摘Based on 230Th-238U disequilibrium and major element data from mid-ocean ridge basalts(MORBs) and ocean island basalts(OIBs),this study calculates mantle melting parameters,and thereby investigates the origin of 230Th excess.(230Th/238U) in global MORBs shows a positive correlation with Fe8,Po,Na8,and Fmelt(Fe8 and Na8 are FeO and Na2O contents respectively after correction for crustal fractionation relative to MgO = 8 wt%,Po=pressure of initial melting and Fmelt=degree of melt),while 230Th excess in OIBs has no obvious correlation with either initial mantle melting depth or the average degree of mantle melting.Furthermore,compared with the MORBs,higher(230Th/238U) in OIBs actually corresponds to a lower melting degree.This suggests that the 230Th excess in MORBs is controlled by mantle melting conditions,while the 230Th excess in OIBs is more likely related to the deep garnet control.The vast majority of calculated initial melting pressures of MORBs with excess 230Th are between 1.0 and 2.5 GPa,which is consistent with the conclusion from experiments in recent years that DU】DTh for Al-clinopyroxene at pressures of 】1.0 GPa.The initial melting pressure of OIBs is 2.2-3.5 GPa(around the spinel-garnet transition zone),with their low excess 226Ra compared to MORBs also suggesting a deeper mantle source.Accordingly,excess 230Th in MORBs and OIBs may be formed respectively in the spinel and garnet stability field.In addition,there is no obvious correlation of K2O/TiO2 with(230Th/238U) and initial melting pressure(Po) of MORBs,so it is proposed that the melting depth producing excess 230Th does not tap the spinel-garnet transition zone.OIBs and MORBs in both(230Th/238U) vs.K2O/TiO2 and(230Th/238U) vs.Po plots fall in two distinct areas,indicating that the mineral phases which dominate their excess 230Th are different.Ce/Yb-Ce curves of fast and slow ridge MORBs are similar,while,in comparison,the Ce/Yb-Ce curve for OIBs shows more influence from garnet.The mechanisms generating excess 230Th in MORBs and OIBs are significantly different,with formation of excess 230Th in the garnet zone only being suitable for OIBs.
