Garnet U-Pb ages reset during ultrahigh temperature melt-rock interaction

In situ garnet U-Pb geochronology by laser ablation-inductively coupled mass spectrometry (LA-ICPMS) is a powerful tool for rapid and high-spatial resolution dating of metamorphic pressure-temperature–time histories. Yet, the substitution of U and Pb  into the structure of common pyralspite garnet and its influence on diffusion and potential age-resetting is poorly constrained. Studies by Mezger et al. (1989), Burton et al. (1995), and Dahl (1997) estimate U-Pb system closure temperatures in garnet of >800 °C. Similarly, Shu et al. (2024) proposed a closure temperature of  >1100 °C but suggested that recrystallization may have reset some garnet U-Pb ages. However, there is little information on how dynamic processes, such as recrystallization, may impact the closure of the U-Pb system in garnet. 

Here we examine the impacts of garnet recrystallization on U-Pb ages by examining ultrahigh-temperature (UHT) crustal xenoliths from the southwestern USA and northern Mexico. These metapelitic granulites experienced isobaric heating at >900 °C between 36 and 30 Ma and UHT conditions were maintained until eruption at <1 Ma (Droubi et al., 2024; Cipar et al., 2020; 2024). Garnet trace element zoning shows diffusively reset growth zoning overprinted by island-moat structures, which formed via melt-driven loss of REE and HFSE along now-healed fractures (Droubi et al., 2024). Zircon HREE depletions and ages suggest garnet initially formed at ca. 30 Ma, whereas diffusion modeling indicates that garnet island-moat structures formed rapidly <1 Myr before eruption (Droubi et al., 2024; Cipar et al., 2020; 2024). Our in situ U-Pb garnet ages, as measured by split-stream LA-MC-ICPMS, range from 4.6 ± 2.3 Ma (2s) to 1.77 ± 0.32 Ma (2s).  The corresponding trace element data show decreasing U contents with decreasing LREE and HFSE, consistent with significant (but partial) resetting of U contents during the formation of the island-moat structures. In contrast, Pb contents show no trends with REE and HFSE. These data, combined with the overlap of our U-Pb ages with the eruption age of the magmas that entrained the xenoliths, are consistent with complete resetting of Pb contents during melt-garnet interaction. Overall, we demonstrate that garnet ages are susceptible to (near-)complete resetting via dynamic open-system processes.

 

Burton et al. (1995). Earth Plan. Sci. Lett. 133, 199–211.

Cipar et al. (2020). Nat. Geosci. 13(11), 758–763.

Cipar et al. (2024). Geochem. Geophys. Geosys. 25(7), e2023GC011177.

Dahl (1997). Earth Plan. Sci. Lett. 150, 277–290.

Droubi et al. (2024). JGR Solid Earth 129, e2024JB029138.

Mezger et al. (1989). Contrib. Min. Pet. 101, 136–248.

Shu et al. (2024). Contrib. Min. Pet. 179 (49).