Using Th-U-Pb geochronology to extract crystallization ages of Paleozoic metamorphic monazite contaminated by initial Pb

Abstract

Geochronology of Th-rich minerals is advantageous as it allows use of three isotopic systems (i.e., 206Pb/238U, 207Pb/235U, and 208Pb/232Th) for accurate data assessment. The 208Pb/232Th system is especially advantageous in cases where the dated mineral includes an initial Pb component, as 208Pb/232Th is the least sensitive to the effects of initial Pb amongst the three systems. This benefit is demonstrated with monazite from a white mica schist of the Tsäkkok Lens, Scandinavian Caledonides, where three distinct generations of Paleozoic monazite (MnzI, Mnz-II, Mnz-III) are recognized and dated using laser ablation inductively coupled mass spectrometry. The generations are interpreted to represent monazite crystallization in high-pressure conditions (MnzI), followed by lower-pressure monazite growth (Mnz-II), and likely dissolution-reprecipitation of the pre-existing monazite (Mnz-III). The results are compared in Tera-Wasserburg, Wetherill, and Th-U-Pb concordia space for each monazite generation. In both Tera-Wasserburg and Wetherill space, the data are all discordant and indicate an initial Pb component in the monazite. The trend and magnitude of discordance due to initial Pb in Mnz-I and Mnz-II is generally controlled by UO2 content of the monazite, with higher UO2 equating to greater radiogenic Pb and a dampening of the initial Pb effect, which is most prominent in the 207Pb/235U system. For the same generations, initial Pb discordance of 206Pb/238U versus 208Pb/232Th is less apparent due to the insensitivity of 208Pb/232Th. Mnz-III does not follow the initial Pb trends, likely due to disturbance of the chemical and isotopic systems during recrystallization. Additional discordance in Mnz-I and Mnz-II, which is not related to initial Pb, is recognized and increases with actinide content. The additional discordance may be due to Pb-mobilization in Mnz-I and Mnz-II domains and is revealed when utilizing the 208Pb/232Th system due to its insensitivity to initial Pb effects. Consequently, relying only on the UPb systems can lead to significant initial Pb overcorrections in Tera-Wasserburg or Wetherill concordia space and to calculations of erroneously young concordia dates. The Th-U-Pb concordia method, incorporating all three systems, does not require an initial Pb correction and, therefore, can account for the additional discordance. The Th-U-Pb concordia dates are interpretated as accurate crystallization ages for Mnz-I (484.7 ± 1.1 Ma, MSWD: 1.4) and Mnz-II (474.7 ± 1.2 Ma, MSWD: 1.9). The timing for Mnz-III formation is not well-resolved as it formed via result of dissolution-reprecipitation of the pre-existing monazite, likely under lower amphibolite- to greenschist-facies conditions.

Topics & Concepts