Beal Research On The Dating Method And Application Of ¹⁰Be And ²⁶Al In Cobalt-rich Crusts

The growth rate and chronology of oceanic cobalt-rich crust(CoRC)is important in study of paleo-oceanic circulation,paleo-environmental and paleo-climatic transition.Cosmogenic nuclides ¹⁰Be(T_1/2=1.386±0.012 Ma)and ²⁶Al(T_1/2=0.705±0.176 Ma),which has an advantage of long half-life and less influence by environment,are considered as one of the most effective methods to date CoRCs.Based on previous studies,this paper attempts to optimize a new method for the extraction,separation and purification of authigenic ¹⁰Be and ²⁶Al in CoRCs,which is for exploring a new method(¹⁰Be and ²⁶Al)to date CoRC.Continuous leaching experiment and subsequent chemical separation and purification were carried out on two CoRCs samples collected from the Pacific Ocean.Inductively Coupled Plasma Mass Spectrometry(ICP-MS)and Accelerator Mass Spectrometry(AMS)were used to measure the distribution of stable isotopes ⁹Be,²⁷Al and radionuclides ¹⁰Be and ²⁶Al in the profile of CoRCs,respectively.And the cosmogenic chronology method was used to date the average growth rates and formation ages of two CoRCs collected from the Pacific Ocean.Some results obtained in this study are mainly as follows:(1)The result of leaching experiment shows that when CoRCs are sequentially leached three times with 0.04 M NH₂O·HCl in HOAc(25%V/V),?99%authigenic phase Be and?85%authigenic phase Al in CoRCs were extracted.After leaching six times,extractions of?99.7%authigenic phase Be and?95%authigenic phase Al in CoRCs were achieved.(2)The average growth rate calculated by ¹⁰Be concentration and ¹⁰Be/⁹Be ratio of sample CXD08-1 in the depth range of 5-30 mm is(10.23_-0.47^+0.52)mm/Ma and(14.25_-0.89^+1.02)mm/Ma,respectively.As a result,the formation age of each layer in crust CXD08-1 is extrapolated.However,the reliable ²⁶Al chronology could be not obtained due to ²⁶Al prefertially decay and the addition of ²⁷Al carrier.Therefore,the no-carrier technique was used to the dating method in the study of sample MDD46-1.(3)The average growth rate calculated by ¹⁰Be concentration and the ratio of¹⁰Be/⁹Be of crust MDD46-1 in the depth range of 0-10.66 mm is(2.56_-0.12^0.13)mm/Ma and(2.12_-0.08^+0.09)mm/Ma,respectively.And that the average growth rate is calculated by²⁶Al concentration and ²⁶Al/²⁷Al ratio of sample MDD46-1 in the depth range of 0-8.48 mm is(5.17_-2.35^+0.09)mm/Ma and(6.22_-2.97^+6.28)mm/Ma,respectively.The calculation results of radionuclides and isotope ratio are consistent in the range of 1?.These results made it possible to extrapolate the formation age of each layer in sample CXD08-1.(4)The growth of sample MDD46-1 has shown three stages(0-1.2,1.2-3.5and>3.5 Ma),suggesting that the growth rate of crust is different at different historical period.By comparing with previous studies,it is found that the different growth rate of CoRC is related to the change of paleo-ocean events and oceanic bottom current.(5)The average growth rates of the cobalt-rich crust calculated by radionuclides¹⁰Be and ²⁶Al are consistent within the range of 1?error.Thus,this paper preliminarily built the profile dating method of ¹⁰Be and ²⁶Al in CoRC.