Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs,as well as fluid and element transfer between the Earth’s mantle and crust.However,...Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs,as well as fluid and element transfer between the Earth’s mantle and crust.However,fluid-rock element exchange and serpentinization kinetics under shallow hydrothermal conditions is still largely unconstrained.Here we present two constant temperature(230℃)time-series of natural peridotite(77.5%olivine;13.7%enstatite;6.8%diopside;2%spinel)serpentinization experiments:at 13.4 MPa;and 20.7 MPa.Al-enriched lizardite was the main secondary mineral in all runs after olivine(olv)and orthopyroxene(opx)serpentinization(without any detectable brucite,talc or magnetite),while primary spinel and diopside partially dissolved during the experiments.Initial serpentinization stages comprises intrinsically coupled reactions between olivine and enstatite,as Al and Si are progressively transferred from orthopyroxene-derived to olivine-derived serpentine,while the opposite is true for Mg and Fe,with homogenization of serpentines compositions after 40 days.The Ni/Cr ratios of serpentines,however,remain diagnostic of the respective primary mineral.Estimated average serpentine content indicates fast serpentinization rates of 0.55 wt.%·day^(-1)(0.26 mmol·day^(-1))and 0.26 wt.%·day^(-1)(0.13 mmol·day^(-1))at 13.4 and 20.7 MPa,respectively.Approximately 2x faster serpentinization kinetics at lower pressure is likely linked to enhanced spinel dissolution leading to one order of magnitude higher available Al,which accelerates olivine serpentinization while delays orthopyroxene dissolution.Additionally,time-dependent increase in solid products masses suggests rock volume expands linearly 0.37%±0.01%per serpentine wt.%independently of pressure.Mass balance constrains suggests olv:opx react at~5:2 and~3:2 M ratios,resulting in Si-deficient and Si-saturated serpentines at the end of the low-pressure series(13.4 MPa)and high-pressure series(20.7 MPa),respectively.Elevated starting peridotite olv:opx ratio(7.94:1)therefore indicates orthopyroxene serpentinization is~3.3x and~5.4x faster than olivine at 13.4 MPa and 20.7 MPa,respectively.This contradicts previous assumptions that olivine should dissolve faster than orthopyroxene at experimental conditions.Finally,serpentinization-derived fluids develop pH>10 and become enriched in H_(2),CH_(4),Ca^(2+)and Si within 6 weeks.Aqueous silica concentrations are highest after 5 days(265.75 and 194.79µmol/kg)and progressively decrease,reaching 13.84 and 91.54µmol/kg at 13.4 and 20.7 MPa after 40 days,respectively.These concentrations are very similar to the low-silica(M6)and high-silica(Beehive)endmembers of the Lost City Hydrothermal Field(LCHF).Beyond fluid characteristics,serpentinization products and conditions analogous to the LCHF suggest similar mechanisms between our experiments and natural processes.Our results demonstrate constant temperature serpentinization of a common protolith leads to distinct serpentine and fluid compositions at different pressures.Although additional data is necessary,recent studies and our experiments suggest peridotite serpentinization rates at 230℃rapidly decrease with increasing pressures at least up to 35 MPa.Whether pressure directly influences olivine and orthopyroxene serpentinization kinetics or indirectly controls reaction rates due to spinel dissolution under hydrothermal conditions deserves further investigation.展开更多
Fractional crystallization of parental magmas of shoshonitic or silica-saturated,ultrapotassic affinity,with variable amount of concurrent crustal assimilation,may result in granitic and syenitic rocks.Typical plutoni...Fractional crystallization of parental magmas of shoshonitic or silica-saturated,ultrapotassic affinity,with variable amount of concurrent crustal assimilation,may result in granitic and syenitic rocks.Typical plutonic members of the shoshonitic series are monzonites and quartz monzonites,whilst syenites and quartz syenites are the dominant plutonic products of the ultrapotassic series.Lamprophyric magmas are commonly found in association with both series and are frequently part of coeval mingling/mixing systems.Ultrapotassic and shoshonitic primary magmas,including lamprophyric ones,are derived from amphibole-phlogopite-bearing mantle sources produced by previous,subduction-related metasomatism.Acidic and intermediate rocks can be derived from such parental magmas,generally through AFC processes.Shoshonitic-like granitoids,which have not clear relation with intermediate or basic shoshonitic rocks,or are produced dominantly by crustal melting,should be named high-Ba-Sr granitoids.This study focuses mainly on Neoproterozoic shoshonitic and silica-saturated ultrapotassic rock associations formed in post-collisional settings from southern Brazil and Uruguay.The source of magmas,their evolution,the role played by crustal contamination in modifying pristine geochemical signatures and their tectonic control are discussed based on elemental and Sr-Nd isotope geochemistry.The main features of plutonic rocks related to the shoshonitic series are their potassic,silica-saturated alkaline character,predominance of monzonitic to syenitic compositions,high Sr and Ba contents,monotonous,light REE-enriched patterns,and moderate HFSE contents.Syntectonic shoshonitic and high Ba-Sr granitoids within shear zones show lower alkali,LREE,HFSE,and Sr contents than those formed away from the main deformation sites.Plutonic rocks related to the extended silica-saturated ultrapotassic series are mostly syenites,alkalifeldspar granites and lamprophyres with K_(2)O/Na_(2)O ratios above 2.The typical values of^(87)Sr/^(86)Sri for shoshonitic plutonic rocks are 0.706–0.708,ranging from 0.704 to 0.710.Theε_(Nd)(t)values are negative and vary from 0 to-24.Crustal contribution tends to increase^(87)Sr/^(86)Sri and decreaseε_(Nd)(t)values,depending on protolith isotope signature,melting conditions and volume of assimilated material.Ultrapotassic rocks,on the other hand,show higher^(87)Sr/^(86)Sri ratios,from 0.709–0.711 up to 0.720.Geochemical evidence,including Sr-Nd isotope data,indicates that the shoshonitic and ultrapotassic rocks discussed in this study were formed from OIB-like sources with strong influence of previous subduction,probably a phlogopite,K-amphibole bearing veined mantle.Lithological variability in ultrapotassic-shoshonitic associations is interpreted to result from(i)variation of source composition,(ii)different melt fractions from similar sources,(iii)mixing-mingling,fractional crystallization,and assimilation processes.展开更多
Post-collisional settings have long been discussed, both in the aspect of nomenclature, tectonic significance, sources of magmatism, and their relations to pre-collisional and intraplate settings. To many authors, it ...Post-collisional settings have long been discussed, both in the aspect of nomenclature, tectonic significance, sources of magmatism, and their relations to pre-collisional and intraplate settings. To many authors, it should be synonymous with lateorogenic or late-collisional settings because the setting itself is defined as the period following the main impact of the colliding landmasses. On the other hand, terms like post-tectonic should be avoided when referring to post-collisional associations.展开更多
基金We thank Petrobras for sponsoring the grad studies of the main author and its financial support through project 8310-0 FAURGS-UFRGS-PETROBRAS.
文摘Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs,as well as fluid and element transfer between the Earth’s mantle and crust.However,fluid-rock element exchange and serpentinization kinetics under shallow hydrothermal conditions is still largely unconstrained.Here we present two constant temperature(230℃)time-series of natural peridotite(77.5%olivine;13.7%enstatite;6.8%diopside;2%spinel)serpentinization experiments:at 13.4 MPa;and 20.7 MPa.Al-enriched lizardite was the main secondary mineral in all runs after olivine(olv)and orthopyroxene(opx)serpentinization(without any detectable brucite,talc or magnetite),while primary spinel and diopside partially dissolved during the experiments.Initial serpentinization stages comprises intrinsically coupled reactions between olivine and enstatite,as Al and Si are progressively transferred from orthopyroxene-derived to olivine-derived serpentine,while the opposite is true for Mg and Fe,with homogenization of serpentines compositions after 40 days.The Ni/Cr ratios of serpentines,however,remain diagnostic of the respective primary mineral.Estimated average serpentine content indicates fast serpentinization rates of 0.55 wt.%·day^(-1)(0.26 mmol·day^(-1))and 0.26 wt.%·day^(-1)(0.13 mmol·day^(-1))at 13.4 and 20.7 MPa,respectively.Approximately 2x faster serpentinization kinetics at lower pressure is likely linked to enhanced spinel dissolution leading to one order of magnitude higher available Al,which accelerates olivine serpentinization while delays orthopyroxene dissolution.Additionally,time-dependent increase in solid products masses suggests rock volume expands linearly 0.37%±0.01%per serpentine wt.%independently of pressure.Mass balance constrains suggests olv:opx react at~5:2 and~3:2 M ratios,resulting in Si-deficient and Si-saturated serpentines at the end of the low-pressure series(13.4 MPa)and high-pressure series(20.7 MPa),respectively.Elevated starting peridotite olv:opx ratio(7.94:1)therefore indicates orthopyroxene serpentinization is~3.3x and~5.4x faster than olivine at 13.4 MPa and 20.7 MPa,respectively.This contradicts previous assumptions that olivine should dissolve faster than orthopyroxene at experimental conditions.Finally,serpentinization-derived fluids develop pH>10 and become enriched in H_(2),CH_(4),Ca^(2+)and Si within 6 weeks.Aqueous silica concentrations are highest after 5 days(265.75 and 194.79µmol/kg)and progressively decrease,reaching 13.84 and 91.54µmol/kg at 13.4 and 20.7 MPa after 40 days,respectively.These concentrations are very similar to the low-silica(M6)and high-silica(Beehive)endmembers of the Lost City Hydrothermal Field(LCHF).Beyond fluid characteristics,serpentinization products and conditions analogous to the LCHF suggest similar mechanisms between our experiments and natural processes.Our results demonstrate constant temperature serpentinization of a common protolith leads to distinct serpentine and fluid compositions at different pressures.Although additional data is necessary,recent studies and our experiments suggest peridotite serpentinization rates at 230℃rapidly decrease with increasing pressures at least up to 35 MPa.Whether pressure directly influences olivine and orthopyroxene serpentinization kinetics or indirectly controls reaction rates due to spinel dissolution under hydrothermal conditions deserves further investigation.
基金Brazilian Research Council(CNPq)for the productivity grants to Lauro Valentim Stoll Nardi(No.306605/2018-0)and Maria de Fátima Bitencourt(No.311486/2015-0)funding through Universal Project(No.481841/2012-1)。
文摘Fractional crystallization of parental magmas of shoshonitic or silica-saturated,ultrapotassic affinity,with variable amount of concurrent crustal assimilation,may result in granitic and syenitic rocks.Typical plutonic members of the shoshonitic series are monzonites and quartz monzonites,whilst syenites and quartz syenites are the dominant plutonic products of the ultrapotassic series.Lamprophyric magmas are commonly found in association with both series and are frequently part of coeval mingling/mixing systems.Ultrapotassic and shoshonitic primary magmas,including lamprophyric ones,are derived from amphibole-phlogopite-bearing mantle sources produced by previous,subduction-related metasomatism.Acidic and intermediate rocks can be derived from such parental magmas,generally through AFC processes.Shoshonitic-like granitoids,which have not clear relation with intermediate or basic shoshonitic rocks,or are produced dominantly by crustal melting,should be named high-Ba-Sr granitoids.This study focuses mainly on Neoproterozoic shoshonitic and silica-saturated ultrapotassic rock associations formed in post-collisional settings from southern Brazil and Uruguay.The source of magmas,their evolution,the role played by crustal contamination in modifying pristine geochemical signatures and their tectonic control are discussed based on elemental and Sr-Nd isotope geochemistry.The main features of plutonic rocks related to the shoshonitic series are their potassic,silica-saturated alkaline character,predominance of monzonitic to syenitic compositions,high Sr and Ba contents,monotonous,light REE-enriched patterns,and moderate HFSE contents.Syntectonic shoshonitic and high Ba-Sr granitoids within shear zones show lower alkali,LREE,HFSE,and Sr contents than those formed away from the main deformation sites.Plutonic rocks related to the extended silica-saturated ultrapotassic series are mostly syenites,alkalifeldspar granites and lamprophyres with K_(2)O/Na_(2)O ratios above 2.The typical values of^(87)Sr/^(86)Sri for shoshonitic plutonic rocks are 0.706–0.708,ranging from 0.704 to 0.710.Theε_(Nd)(t)values are negative and vary from 0 to-24.Crustal contribution tends to increase^(87)Sr/^(86)Sri and decreaseε_(Nd)(t)values,depending on protolith isotope signature,melting conditions and volume of assimilated material.Ultrapotassic rocks,on the other hand,show higher^(87)Sr/^(86)Sri ratios,from 0.709–0.711 up to 0.720.Geochemical evidence,including Sr-Nd isotope data,indicates that the shoshonitic and ultrapotassic rocks discussed in this study were formed from OIB-like sources with strong influence of previous subduction,probably a phlogopite,K-amphibole bearing veined mantle.Lithological variability in ultrapotassic-shoshonitic associations is interpreted to result from(i)variation of source composition,(ii)different melt fractions from similar sources,(iii)mixing-mingling,fractional crystallization,and assimilation processes.
文摘Post-collisional settings have long been discussed, both in the aspect of nomenclature, tectonic significance, sources of magmatism, and their relations to pre-collisional and intraplate settings. To many authors, it should be synonymous with lateorogenic or late-collisional settings because the setting itself is defined as the period following the main impact of the colliding landmasses. On the other hand, terms like post-tectonic should be avoided when referring to post-collisional associations.
