In situ U-Pb dating of xenotime by laser ablation (LA)-ICP-MS

Abstract

Xenotime is an ideal mineral for U-Th-Pb isotopic dating because of its relatively high U and Th contents, but typically low concentration of common Pb. These characteristics, and the fact that it is widespread throughout various types of rocks, suggest that the U-Th-Pb dating of xenotime has broad applications. Studies of U-Pb dating on xenotime by ion microprobe (such as SHRIMP) have increased in recent years, whereas studies by laser ablation (LA)-ICP-MS are still rare. In this study, we developed a technique for U-Pb dating of xenotime using the 193 nm ArF laser-ablation system and Agilent 7500a Q-ICP-MS. To evaluate the reliability of our method, a xenotime standard, BS-1, was analyzed and calibrated against another xenotime standard, MG-1. The weighted mean 206 Pb/ 238 U ages of 510.1 ± 5.2 Ma (2 n = 21), 509.8 ± 4.3 Ma (2 n = 21) and 510.0 ± 4.6 Ma (2n = 21) were obtained using beam diameters of 16, 24 and 32 m, respectively. These ages are identical to those determined by ID-TIMS method (weighted mean 206 Pb/ 238 U age of 508.8 ± 1.4 Ma), which supports the reliability of our LA-ICP-MS method. We also analyzed xenotimes in leucogranites from South Tibet and granites from Xihuashan in southern China, and obtained accurate and precise ages. Nevertheless, we observed systematic differences in Pb/U fractionation among xenotime, monazite and zircon. The matrix-effect resulted in either under-correction or over-correction of fractionation, and thus led to inaccurate ages. Thus, a matrix-matched material is required for U-Pb dating of xenotime by LA-ICP-MS.

SA01 – A Proposed Zircon Reference Material for Microbeam U‐Pb Age and Hf‐O Isotopic Determination

“…Although the seller informed us that the SA01 zircon megacryst was collected from the KwaZulu‐Natal Province, South Africa, zircons with similar geochemical features have not been found in KwaZulu‐Natal so far. As shown in Table , SA01 is quite different from the now fairly large suite of similar aged Sri Lankan zircons that have been published [SL7 (Kinny et al ), CZ3 (Pidgeon et al , Xu et al , Cavosie et al ), M257 (Nasdala et al , Li et al , Hu et al ), BR266 (Stern , Woodhead et al , Liu et al ), BB9 (Santos et al ), GZ7/GZ8 (Nasdala et al ), M127 (Nasdala et al ) and LGC‐1 (Tian et al )]. In this regard, the genesis of the SA01 zircon megacryst may be different from those of the Sri Lankan zircon megacrysts.…”

Detrital zircon without detritus: a result of 496-Ma-old fluid–rock interaction during the gold-lode formation of Passagem, Minas Gerais, Brazil

“…Both minerals concentrate U, but exclude Pb on crystallising, making them amenable to precise dating. Both minerals have high-closure temperatures for the U-Th-Pb system, N900°C for zircon (Cherniak and Watson, 2001;Lee et al, 1997), and similar (N900°C) for xenotime (Cherniak, 2006;Liu et al, 2011); hence, their ages are likely to record the time zircon and xenotime crystallised during magmatic, metamorphic and hydrothermal events. However, interpretation of age data may be complicated because zircon and xenotime in single rocks can form in more than one geological event (Black et al, 1986;Gerdes and Zeh, 2009), and can be affected by different styles of alteration after their formation (Geisler et al, 2001(Geisler et al, , 2003a(Geisler et al, , 2003b(Geisler et al, , 2007Gerdes and Zeh, 2009;Hoskin, 2005;Pidgeon, 1992;Spear and Pyle, 2002).…”

Timing of hydrothermal activity associated with the Douzhashan uranium-bearing granite and its significance for uranium mineralization in northeastern Guangxi, China

“…Determination of U-Th-Pb isotopic ages on primary monazite and xenotime was performed with an Agilent 7500a LA-ICP-MS coupled to an ArF excimer laser operated at a wavelength of 193 nm, housed at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China [22][23][24][25]. We used matrix-matched external standards of monazite 4406 and xenotime MG-1.…”