RESEARCH PROGRESS AND APPLICATION OF CARBONATE U-TH/HE ISOTOPE DATING
U-series dating(Uranium series disequilibrium dating)is one of the most widely used dating methods in radioisotope geochronology,mainly based on the disequilibrium relationship between radionuclide 238 U and its decay daughters 235 U/234 U and 230 Th to measure the age of rocks,minerals,and other geological bodies.U-Th/He isotope dating is based on the decay of radioactive elements such as U and Th in mineral particles to form stable 4 He isotopes.By measuring the cumulative content of these radioactive element decay products,The U-Th/He dating method has a large applicable time range for many minerals(such as apatite and zircon)and most geological periods,and can be used as a thermal timer to explain the thermal history of rocks,and can also be used as a geological timer to constrain the crystallization age of minerals and different geological events.Carbonate minerals,including calcite,dolomite,magnesite and aragonite,are widely distributed in the earth's crust and formed in the processes of sedimentation,magma,metamorphism and hydrothermal fluid metasomatism.In recent years,with the development of closing temperature theory,the recognition of He diffusion behavior,and new progress in He measurement technique,it has been found that helium can be retained in the lattice of carbonate minerals,and the diffusion activation energy and low closure temperature are close to those of apatite.Carbonate U-Th/He isotope dating technology has been greatly developed and applied in the fields of geochronology and thermochronology,which attracted wide attention in the field of geology.Due to the large particle size and extremely low closure temperature,the application of low-temperature dating of carbonate minerals has received increasing attention.Ideally,a mineral crystal dating with U-TH/He should contain all helium from the decay of the U and Th radioisotopes inside the mineral,that is,there is no inheritance of previously existing helium,and there is no loss of helium after.Any factor that breaks the closure of the U-Th/He dating system will affect the accuracy of the dating results.Helium has a small atomic mass and no charge,and when the temperature is high enough,it easily diffuses out of the mineral lattice.This article mainly analyzes the influencing factors of He diffusion behavior and the new progress of He gas extraction and measurement technology.The study of the diffusion behavior of helium in carbonates is a key theoretical link in the development and application of U-TH/He dating methods for carbonates.Research methods,diffusion domain,crystal size,alpha particle and grain boundaries have different degrees of influence on helium diffusion behavior and helium retention.Accurate activation energy and diffusion coefficient of helium diffusion are needed to understand the mineral age of carbonates under certain geological conditions.The development of extraction and measurement technology for He gas is a key technical link in carbonate U-Th/He dating.Due to the low content of He,U and Th in carbonate samples,relatively large samples and advanced He measurement equipment such as vacuum furnaces and mass spectrometers are required.In-situ laser U-Th/He isotope dating,which has the advantages of high precision and non-destructive,has made a breakthrough in measuring carbonate ages and has gradually established a standard experimental testing process.Carbonate U-Th/He isotope dating technology has broad application prospects and research value in archaeology,brittle structure,oil and gas accumulation,oceanic crust evolution,metallogenic mechanism,and ore-forming fluid tracing,and will play an important role in solving earth science problems.In this paper,the progress of methods and techniques for carbonate U-Th/He dating in the last two decades is reviewed,the methods and basic principles of U-Th/He dating are summarized,the uncertainties affecting helium dating are analyzed,and the future development direction prospects.
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