Optimization Of U-Th-Pb Dating Conditions Of Monazite By LA-ICP-MS And Application

Monazite is a common light rare earth-enriched phosphate mineral,usually in the form of accessory minerals in some granites,metamorphic rocks,sedimentary rocks and hydrothermal veins.It is similar to the most commonly used zircon U-Pb dating system.Compared with the most commonly used zircon U-Pb dating system,the U-Th-Pb isotope dating system of monazite is more advantageous in the dating of some metamorphic rocks or highly differentiated rocks.and the closed temperature of U-Th-Pb system of monazite(720??750?)is second to that of zircon U-Pb system(750??900?).After the system is formed and closed,it is not easily affected by the later geological processes;the ordinary Pb content in monazite is also low,which can be ignored in dating.The monazite U-Th-Pb isotopic dating system has gradually developed in geochronology,and it complements the zircon U-Pb dating system.However,because monazite is easier to be denudated than zircon and contains more inclusions;and the content of ²³²Th is higher(up to 10%),which is close to the detection limit of ICP-MS(1000ppm),if the traditional large laser beam spot diameter and high laser frequency are used,inaccurate age data are easy to be produced.Moreover,the international standard sample 44069 of monazite is difficult to obtain,which causes the monazite U-Th-Pb isotopic dating to be restricted to a certain extent.This article is to find suitable LA-ICP-MS monazite dating experimental conditions,using 193nm Ar F excimer laser ablation system and Agilent7500a inductively coupled plasma mass spectrometer,first by changing the laser parameters(laser beam spot diameter and laser frequency),respectively the laser beam spot diameter is 24?m,16?m and 10?m,and the laser frequency is 3Hz,4Hz and 5Hz.The best laser parameters are obtained as the laser beam spot diameter of 16?m and laser frequency of 5Hz.On this basis,the ICP-MS is changed innovatively.The parameters(dwell time of ²³²Th)are10ms,6ms,3ms and 1ms for U-Th-Pb dating.Finally,the monazite RW-1 was used as the standard sample to eliminate the influence of the matrix effect between different minerals on the age results,and the monazite sample Bananeira was corrected.The results show that the laser beam spot diameter is 16?m,the ²³²Th dwell time is 3ms or1ma,the energy density is 4J/cm²,the laser frequency is 5Hz,the carrier gas He flow rate is 0.35L/min,and the carrier gas Ar flow rate is 0.95L/min.The conditions are suitable for monazite U-Th-Pb isotopic age determination.Under these conditions,Bananeira's weighted average age of ²⁰⁷Pb/²³⁵U is 510.7±8.6Ma(MSWD=0.88)(recommended age value 507.7±1.3Ma);²⁰⁸Pb/²³²Th weighted average age is496.9±8.6Ma(MSWD=0.596)(recommended age value 497.6±1.6Ma),the error is about 0.59%and-0.14%.In order to verify the reliability of monazite U-Th-Pb isotopic dating under the above conditions,this condition was applied to the Huangshan sample HS-1,and the zircon and monazite single minerals in HS-1 were selected for dating.The HS-1 age of the Huangshan intrusion is reported for the first time.The weighted average of²⁰⁶Pb/²³⁸U,²⁰⁷Pb/²³⁵U and ²⁰⁸Pb/²³²Th are 128.1±1.6Ma(MSWD=0.95),128.3±2.4Ma(MSWD=0.73)and 129.4±2.0Ma(MSWD=1.4),respectively.The zircon age data(about 127.8Ma)obtained in the Huangshan area and the zircon ²⁰⁶Pb/²³⁸U age(127.0±2.1Ma;MSWD=0.93)of the sample HS-1 obtained in this experiment are consistent within the error range.Generally speaking,the monazite dating data under this condition is reliable,and a set of monazite laboratory dating methods can be established.