11:30 AM - 11:45 AM
[SCG57-04] Development of a U–Pb dating method for monazite utilizing LA-MC-ICP-MS
Keywords:Monazite, Geochronology, U–Pb dating, LA-ICP-MS
Monazite is one of the most useful minerals in geochronological studies. It is used to determine the timing of metamorphic stages of variable grades [e.g., 1]. A critical advantage in the monazite geochronometer is the potential to derive chronological data of early-prograde stages, which is not likely to be preserved in zircon [e.g., 2]. For acquiring reliable age data, in-situ age determination combined with petrological insights is important [e.g., 3, 4], and analytical techniques with high sensitivity and spatial resolution are employed.
Early studies on monazite geochronology were conducted with secondary ionization mass spectrometry (SIMS) [e.g., 5]. Despite the importance of SIMS, due to the complicated ionization process and ionization efficiencies highly depending on the physicochemical properties of analyte elements and sample matrices, the accurate U–Pb age determination requires references, which are severely matrix-matched to unknown samples. This is a key issue especially for determining the ages of young minerals with a little age reference materials other than zircon.
An alternative method is laser ablation (LA) inductively coupled plasma-mass spectrometry (ICP-MS). The high sampling efficiency of LA and the high ionization efficiency in ICP are advantageous in the reduced elemental fractionation, resulting in the high accuracy of elemental-ratio analysis. The elemental fractionation can be further suppressed by utilising femtosecond laser ablation with reduced thermal loading [6]. A recent development in LA-ICP-MS for age determination of young mineral samples is a multiple-collection (MC) system equipped with three Daly detectors [7]. The simultaneous monitoring of 206Pb, 207Pb, and 235U improves the duty cycle, and high-precision analysis of trace radioisotopes can be conducted compared to a conventional single-collection system [e.g., 8]. Moreover, the Daly detectors have a wide dynamic range covering from 100 to 107 count-per-second, resulting in the improvement in the analytical accuracy of U–Pb age determination for young samples. In this study, the present system of MC-ICP-MS coupled with femtosecond laser ablation is applied to the U–Pb dating method for monazite.
In the measurements, 44069 USGS monazite (ca. 425 Ma, [9]) was used as the primary reference material for Pb/U calibration, and Namaqualand monazite (ca. 1033 Ma, [10]) and 16-F-6 monazite (ca. 2.8 Ga, [11]) were analyzed with the spot size of 10 µm as secondary reference materials to evaluate data quality. The resulting U–Pb dates are as follows: 238U– 206Pb age of 1032 ± 11 Ma (2s, n = 10, MSWD = 1.7), 235U–207Pb age of 1038.7 ± 8.3 Ma (2s, n = 10, MSDW = 0.86) for Namaqualand monazite, and the intercept U–Pb age of 2838 ± 55 Ma (2s, n = 10, MSWD = 73) for 16-F-6 monazite. The obtained U–Pb ages are consistent with the literature values, and the relative uncertainty per each spot data is 2–3%. For a proof-of-concept study, Sori 06 monazites from the Sori granodiorite (zircon U–Pb age: 93.9 ± 0.6 Ma, [12]) is dated by the present technique. Owing to the high spatial resolution, high accuracy, and high precision, the variation of 238U–206Pb ages from 93 Ma to 80 Ma corresponding to the core-rim structure inside monazites was revealed. This can reflect the duration of the magmatic activities in the relevant area. In the presentation, the potential application of the present technique to petrochronology will be further discussed.
References
[1] Rubatto et al., 2012. Contrib Mineral Petrol. [2] Warren et al., 2018. Geol Soc Spec Publ. [3] Iaccarino et al., 2015. Lithos. [4] Catlos et al., 2002. J Asian Earth Sci. [5] Harrison et al., 1995. Earth Planet Sci Lett. [6] Russo et al., 2002. J Anal At Spectrom. [7] Hattori et al., 2017. J Anal At Spectrom. [8] Kohn & Vervoort, 2008. Geochem Geophys Geosyst. [9] Aleinikoff et al., 2006. Geol Soc Am Bull. [10] Knoper et al., 2001. Earth Sci Cong of the GSSA abst. [11] Simonetti et al., 2006. Int J Mass Spectrom. [12] Ogasawara et al., 2013. Island Arc.
Early studies on monazite geochronology were conducted with secondary ionization mass spectrometry (SIMS) [e.g., 5]. Despite the importance of SIMS, due to the complicated ionization process and ionization efficiencies highly depending on the physicochemical properties of analyte elements and sample matrices, the accurate U–Pb age determination requires references, which are severely matrix-matched to unknown samples. This is a key issue especially for determining the ages of young minerals with a little age reference materials other than zircon.
An alternative method is laser ablation (LA) inductively coupled plasma-mass spectrometry (ICP-MS). The high sampling efficiency of LA and the high ionization efficiency in ICP are advantageous in the reduced elemental fractionation, resulting in the high accuracy of elemental-ratio analysis. The elemental fractionation can be further suppressed by utilising femtosecond laser ablation with reduced thermal loading [6]. A recent development in LA-ICP-MS for age determination of young mineral samples is a multiple-collection (MC) system equipped with three Daly detectors [7]. The simultaneous monitoring of 206Pb, 207Pb, and 235U improves the duty cycle, and high-precision analysis of trace radioisotopes can be conducted compared to a conventional single-collection system [e.g., 8]. Moreover, the Daly detectors have a wide dynamic range covering from 100 to 107 count-per-second, resulting in the improvement in the analytical accuracy of U–Pb age determination for young samples. In this study, the present system of MC-ICP-MS coupled with femtosecond laser ablation is applied to the U–Pb dating method for monazite.
In the measurements, 44069 USGS monazite (ca. 425 Ma, [9]) was used as the primary reference material for Pb/U calibration, and Namaqualand monazite (ca. 1033 Ma, [10]) and 16-F-6 monazite (ca. 2.8 Ga, [11]) were analyzed with the spot size of 10 µm as secondary reference materials to evaluate data quality. The resulting U–Pb dates are as follows: 238U– 206Pb age of 1032 ± 11 Ma (2s, n = 10, MSWD = 1.7), 235U–207Pb age of 1038.7 ± 8.3 Ma (2s, n = 10, MSDW = 0.86) for Namaqualand monazite, and the intercept U–Pb age of 2838 ± 55 Ma (2s, n = 10, MSWD = 73) for 16-F-6 monazite. The obtained U–Pb ages are consistent with the literature values, and the relative uncertainty per each spot data is 2–3%. For a proof-of-concept study, Sori 06 monazites from the Sori granodiorite (zircon U–Pb age: 93.9 ± 0.6 Ma, [12]) is dated by the present technique. Owing to the high spatial resolution, high accuracy, and high precision, the variation of 238U–206Pb ages from 93 Ma to 80 Ma corresponding to the core-rim structure inside monazites was revealed. This can reflect the duration of the magmatic activities in the relevant area. In the presentation, the potential application of the present technique to petrochronology will be further discussed.
References
[1] Rubatto et al., 2012. Contrib Mineral Petrol. [2] Warren et al., 2018. Geol Soc Spec Publ. [3] Iaccarino et al., 2015. Lithos. [4] Catlos et al., 2002. J Asian Earth Sci. [5] Harrison et al., 1995. Earth Planet Sci Lett. [6] Russo et al., 2002. J Anal At Spectrom. [7] Hattori et al., 2017. J Anal At Spectrom. [8] Kohn & Vervoort, 2008. Geochem Geophys Geosyst. [9] Aleinikoff et al., 2006. Geol Soc Am Bull. [10] Knoper et al., 2001. Earth Sci Cong of the GSSA abst. [11] Simonetti et al., 2006. Int J Mass Spectrom. [12] Ogasawara et al., 2013. Island Arc.