Noble Gas Isotopic Dating Of Helium And Argon

Noble gas isotopic dating of Helium and argon is based on the law of radioactive decay,which are the decay of radioactive parent atoms and the accumulation of the daughter atoms with time.According to the half-life of the radioactive isotope and the determined ratio of parent/daughter,the apparent age of the sample can be calculated.Then the apparent age can be explained as the forming age,eruptive age or cooling age.For the case of?U-Th?/He dating?often called He dating?,the daughter isotope⁴He is produced by parent isotopes ²³⁸U,²³⁵U and ²³²Th.In contrast,K-Ar dating is based on the concentrations of daughter product ⁴⁰Ar*to parent ⁴⁰K contained with the rock or mineral.The ⁴⁰Ar/³⁹Ar technique is merely a specialized variant of the K-Ar dating by employing the invariability of ⁴⁰K/³⁹K ratio in nature.Noble gas isotopic dating is different from other dating means in experimental technique and method.Low extraction system blank and mineral standards are necessary for?U-Th?/He and ⁴⁰Ar/³⁹Ar dating methods.For ⁴⁰Ar/³⁹Ar dating,there exist many mineral standards with various mineral and ages.However,currently few?U-Th?/He age reference materials are available for zircon.The Fish Canyon Tuff?FCT?is the most widely used standard for zircon thermochronology.Because of its relatively small grain size,additional alpha ejection corrections are required for age calculation which will result in large uncertainty.Thus,a large-grained zircon reference material,similar to the Durango apatite,is significant for zircon?U-Th?/He dating.Besides,⁴⁰Ar/³⁹Ar dating also depends on air standard and irradiation in experimental method,and all the four factors will affect precision.Therefore,this thesis focus on two aims:?1?finding the potential?U-Th?/He age reference materials for zircon,and establishing experimental procedure for zircon dating in our new?U-Th?/He lab;?2?improving the aforementioned experimental methods to improve the ⁴⁰Ar/³⁹Ar dating precision,especially for the low-potassium basalts which are widely distributed in nature,and devoid of zircon and sanidine.For the aim?1?,we attempt to introduce a well-studied U-Pb age reference material,Penglai zircon megacryst,to?U-Th?/He dating.The outer 20?m of the four crystals were removed by mechanical abrasion and twenty internal fragments were chosen at random as the objects of single-crystal laser fusion He determinations and the U-Th isotope dilution?ID?method.After removal of two age outliers,eighteen?U-Th?/He age results are consistent,and yield a weighted mean age of 4.06±0.35Ma?2s,MSWD=1.79?.This age is systematically slightly younger than the preferred²⁰⁶Pb/²³⁸U age of 4.4±0.1 Ma?95%confidence interval?determined by ID-TIMS and subsequently published U-Pb results.Because the zircon megacrysts?mantle xenoliths?were brought rapidly to the surface by the host basaltic magma and because the zircon?U-Th?/He system has a low closure temperature of?¹90°C?,the?U-Th?/He age is considered to represent the eruption age.In contrast,because the zircon U-Pb isotopic system has a higher closure temperature?⁹00°C?,the preferred ID-TIMS U-Pb age of the Penglai megacrysts probably records a combination of the magma chamber residence time and the time since eruption.Thus,the systematically younger?U-Th?/He age relative to the U-Pb age is reasonable.Because of the unlimited quantity,coarseness of the crystals,mostly weak zoning,rapid cooling and consistent?UTh?/He ages presented in this study,we suggest that Penglai zircon is also suitable to be a zircon reference material for use in?U-Th?/He isotope geochronology.Meanwhile,it also suggests that the extraction and determination methods for He,U and Th are reliable and practical.Regarding to our target?2?,we try to improve the precision of ⁴⁰Ar/³⁹Ar dating by improving the blank,air standard,mineral standard,and irradiation,and two case studies of low-patassium vocanic rocks are here provided to illustrate the above efforts.In case one,we collected the low-patassium plagioclase phenocryst?K%:0.4-0.7%?from high-Ti basalts series of Qiaojia section?Yunnan province?in the Emeishan large igneous province?ELIP?.The ⁴⁰Ar/³⁹Ar step-heating conducted on five parallel plagioclase aliquots,and yielded statistically indistinguishable results of259.3±2.4 Ma,259.7±2.5 Ma,260.5±2.5 Ma,260.2±2.6 Ma and 261.7±3.4 Ma,respectively,with the weighted mean age of 260.1±1.2 Ma.Because this very fresh sample was collected at the base of high-Ti basalt sequence,the obtaining age marks the onset of the high-Ti volcanism at the ELIP.Combining this age with the previously published U-Pb CA-TIMS age of 259.1±0.5 Ma for the terminal felsic ignimbrites in the Binchuan section allows calculations of the duration of the high-Ti phase of the volcanism as approximately 1.0±1.3 Ma.Meanwhile,our new data are indistinguishable from the Guadalupian-Lopingian boundary age set to 259.1±0.5Ma by the latest International Chronostratigraphic Chart?Feb 2017,www.stratigraphy.org?,strengthen the idea that a temporal link between the ELIP volcanism and the end-Gaudalupian mass extinction.The low-patassium vocanic rocks in case two are collected from the volcanic-sedimentary sequence of the Cretaceous Continental Scientific Drilling borehole?CCSD-LK-I?in Jiaolai Basin.Six basalt samples were collected for⁴⁰Ar/³⁹Ar dating at the depths interval from 614 to 769 m,and the potassium of the samples are as low as 0.1%-0.5%.According to the lithostratigraphic division,this volcanic-sedimentary sequence belongs to Member S5 of the Shijiatun Formation.⁴⁰Ar/³⁹Ar dating of the basalts yielded a latest age of 73.5±1.1 Ma at 614 m depth and a basal age of 79.7±2.7 Ma at 757 m depth,corresponding to Campanian in age.The established chronological framework allows the interregional correlation of Member S5 of the Shijiatun Formation in Jiaolai Basin to the strata from other basins in northeastern China,northwestern China and central-southern China.In addition,the ⁴⁰Ar/³⁹Ar age of 73.5±1.1 Ma from the topmost basalt of the Shijiatun Formation indicates that the age of the overlying Jiaozhou Formation is younger than Campanian,and it also supports the location of the K/Pg boundary in borehole CCSD-LK-I.In the aforementioned case studies,the objects of ⁴⁰Ar/³⁹Ar dating are both low-potassium volcanic rocks.The concentrations of potassium are as low as0.4%-0.7%for plagioclase phenocryst in the Emeishan large igneous province?ELIP?,and 0.1%-0.5%for the basalts from the Cretaceous Continental Scientific Drilling borehole?CCSD-LK-I?in Jiaolai Basin.In this thesis,we improved the precision to0.9%-1%for individual measurement for plagioclase phenocryst in case one and1%-2%for basalts in case two,by improving the experimental methods,such as reducing the background,monitoring the air standard,preferring the mineral standard and optimising the irradiation.