Application and Prospect of Ba Isotopes in the Evolution of Granitic Magmas and Fluid Activities
Barium(Ba),as a non-traditional stable isotope,gradually reveals its unique advantage as a tracer for granite magma evolution and fluid activity.Currently,the geochemical reservoir of Ba isotopes has been established.Crustal remelting can fractionate Ba isotopes,resulting in the enrichment of heavy Ba isotopes in the produced melt.However,light Ba isotope δ138/134 Ba values(-1.32‰ to +0.12‰)observed in the Himalayan Kampa leucogranites are likely related to fluid exsolution from large deep-seated magma reservoirs.The fluid released from deep magma reservoirs has significant effects on the magma evolution of Kampa leucogranites,leading to low light Ba isotope compositions(＜0).These fluids not only alter the Ba isotope compositions of leucogranites but also transport some mobile metal elements,potentially contributing to the formation of rare metal deposits.In contrast to granite melts,potassium feldspar is enriched in light Ba isotopes.Fractional crystallization of potassium feldspar in highly fractionated granites result in significantly elevated δ138/134 Ba values(＞0),while highly fractionated granites with characteristics of magmatic-hydrothermal interaction exhibit considerably lower δ138/134 Ba values compared to continental crust(～0‰).In fluid processes within subduction zones,Ba isotopes can undergo extensive fractionation,with light Ba isotopes preferentially enriching in minerals,leading to much higher δ138/134 Ba values in metamorphic fluids than in subducted materials(e.g.,altered oceanic crust).Heavy Ba isotopes(＞0)in subduction zones are likely associated with fluid contributions from altered oceanic crust,while light Ba isotopes(＜0)may reflect an increased contribution of subducted volcaniclastic sediments.Ultra-high pressure metamorphic rocks and high-pressure veins can record Ba isotope fractionation during subduction zone metamorphism and fluid-rock interactions,while the metamorphic re-equilibration can alter the Ba isotope compositions of mantle wedge and arc volcanic rocks.However,careful consideration is needed when using Ba isotopes to quantify contributions from different sources,particularly in the context of metamorphic dehydration and fluid activities.In summary,this paper summarizes the application and progress of Ba isotopes in granite and fluid activity,and provides an outlook on future research directions and challenges.
non-traditional stable isotopesbarium isotopesgranitesfluid activitiestranscrustal magmatic system
国家科技期刊平台
NSTL
万方数据
维普
