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[SCG51-01] Quaternary geochronology of monazite utilizing laser ablation-ICP-mass spectrometry
Keywords:monazite, LA-ICP-MS, geochronology
Geochronological studies are mainly dependent upon radiometric ages of zircon as a uranium-enriched mineral determined by the U–Pb isotopic system. U–Pb ages of zircon are commonly acquired based on in situ isotopic measurements, and multiple ages can be obtained from each zircon grain or zoning texture inside mineral grains. The multiple chronological data of zircon accumulated until today is essential for deciphering various geologic processes.
Despite the widespread use of zircon, accurate radiometric dating of Quaternary zircon is problematic. This is partially due to the difficulty in detecting a minor amount of the daughter isotopes containing in young zircon samples, and another main cause is a systematic error derived from radioactive disequilibria in the uranium decay series[1]. The uranium decay series has long-lived radioactive intermediate products, such as 230Th and 226Ra. Hence, especially, the initially incorporated 230Th containing in zircon at the timing of crystallization should be estimated to correct zircon U–Pb ages, and poor precision and accuracy of this estimation for the initial 230Th content can result in the large error of radiometric ages.
These problems with zircon are critical in constraining timescales of igneous activities. In pioneering studies based on the age distribution of zircon, although, timescales of pre-eruptive processes (e.g., residence times of magma) ranging from tens of thousands to hundreds of thousands of years are constrained[2], the higher time resolutions are required to understand detailed processes, such as generation, storage, cooling, and differentiation of magma.
Instead of zircon, in this study, radiometric dating methods using thorium isotopes for monazite, which is a typical thorium-enriched mineral, is developed for precise determination of the crystallization ages. Naturally occurring thorium isotopes are mainly 232Th and 230Th. The thorium decay series begin with 232Th and does not have long-lived intermediate products. This results in the great advantage of accurately calculable 232Th–208Pb ages with negligible contribution of radioactive disequilibria[3]. 230Th is an intermediate product of the uranium decay series and 230Th has its α-decay product of 226Ra with a half-life of about 1600 years. This 230Th–226Ra system can be applied to age determinations on Holocene samples. Monazite incorporates thorium into the crystal more predominantly than uranium, and monazite usually has thorium concentrations of more than 10000 µg g-1, which is about 100-times higher than a typical thorium concentration of zircon. Therefore, monazite is a promising candidate for radiometric dating utilizing thorium isotopes to derive high-resolution chronological information from Quaternary samples.
However, the radiometric dating of Quaternary monazite is challenging based on previous analytical techniques. The isotopic measurement on 208Pb, 226Ra, and 230Th of Quaternary monazite samples requires a high sensitivity to detect these trace isotopes, and suppression of interfering ions, such as polyatomic ions derived from monazite matrices and tailing of 232Th. These challenges are solved by triple-quadrupole-based ICP-MS (ICP-MS/MS) in this study. By utilizing the ICP-MS/MS, polyatomic ions are effectively removed with a collision cell technique[4], and high abundance sensitivity is achieved by two-step filtering of ions using the first and third quadrupole mass analyzers. The in situ sampling technique coupled with ICP-MS/MS is a femtosecond laser ablation system for rapid analysis and reduction of elemental fractionation.
In our presentation, the accuracy and precision of measurements for the present technique will be evaluated through radiometric dating of Quaternary monazite separated from the Toya ignimbrite whose eruption ages are about 110 ka[5]. The developed technique will be applied to a Holocene monazite separated from a rhyolite sample occurring in Kozushima.
[1] Schärer, U., 1984, EPSL. [2] Reid, M. R. et al., 1997, EPSL. [3] Sakata, S. et al., 2017, Quat. Geochronol. [4] Niki, S. et al., 2022, GGR. [5] Tomiya, A. and Miyagi, I., 2020, Kazan.
Despite the widespread use of zircon, accurate radiometric dating of Quaternary zircon is problematic. This is partially due to the difficulty in detecting a minor amount of the daughter isotopes containing in young zircon samples, and another main cause is a systematic error derived from radioactive disequilibria in the uranium decay series[1]. The uranium decay series has long-lived radioactive intermediate products, such as 230Th and 226Ra. Hence, especially, the initially incorporated 230Th containing in zircon at the timing of crystallization should be estimated to correct zircon U–Pb ages, and poor precision and accuracy of this estimation for the initial 230Th content can result in the large error of radiometric ages.
These problems with zircon are critical in constraining timescales of igneous activities. In pioneering studies based on the age distribution of zircon, although, timescales of pre-eruptive processes (e.g., residence times of magma) ranging from tens of thousands to hundreds of thousands of years are constrained[2], the higher time resolutions are required to understand detailed processes, such as generation, storage, cooling, and differentiation of magma.
Instead of zircon, in this study, radiometric dating methods using thorium isotopes for monazite, which is a typical thorium-enriched mineral, is developed for precise determination of the crystallization ages. Naturally occurring thorium isotopes are mainly 232Th and 230Th. The thorium decay series begin with 232Th and does not have long-lived intermediate products. This results in the great advantage of accurately calculable 232Th–208Pb ages with negligible contribution of radioactive disequilibria[3]. 230Th is an intermediate product of the uranium decay series and 230Th has its α-decay product of 226Ra with a half-life of about 1600 years. This 230Th–226Ra system can be applied to age determinations on Holocene samples. Monazite incorporates thorium into the crystal more predominantly than uranium, and monazite usually has thorium concentrations of more than 10000 µg g-1, which is about 100-times higher than a typical thorium concentration of zircon. Therefore, monazite is a promising candidate for radiometric dating utilizing thorium isotopes to derive high-resolution chronological information from Quaternary samples.
However, the radiometric dating of Quaternary monazite is challenging based on previous analytical techniques. The isotopic measurement on 208Pb, 226Ra, and 230Th of Quaternary monazite samples requires a high sensitivity to detect these trace isotopes, and suppression of interfering ions, such as polyatomic ions derived from monazite matrices and tailing of 232Th. These challenges are solved by triple-quadrupole-based ICP-MS (ICP-MS/MS) in this study. By utilizing the ICP-MS/MS, polyatomic ions are effectively removed with a collision cell technique[4], and high abundance sensitivity is achieved by two-step filtering of ions using the first and third quadrupole mass analyzers. The in situ sampling technique coupled with ICP-MS/MS is a femtosecond laser ablation system for rapid analysis and reduction of elemental fractionation.
In our presentation, the accuracy and precision of measurements for the present technique will be evaluated through radiometric dating of Quaternary monazite separated from the Toya ignimbrite whose eruption ages are about 110 ka[5]. The developed technique will be applied to a Holocene monazite separated from a rhyolite sample occurring in Kozushima.
[1] Schärer, U., 1984, EPSL. [2] Reid, M. R. et al., 1997, EPSL. [3] Sakata, S. et al., 2017, Quat. Geochronol. [4] Niki, S. et al., 2022, GGR. [5] Tomiya, A. and Miyagi, I., 2020, Kazan.