No abstract

No abstract

Recent development in laser-ablation Lu-Hf dating has opened a new opportunity to rapidly obtain apatite ages that are potentially more robust to isotopic resetting compared to traditional U-Pb dating. However, the robustness of the apatite Lu-Hf system has not been systematically examined. To address this knowledge gap, we conducted four case studies to determine the resistivity of the apatite Lu-Hf system compared to the zircon and apatite U-Pb system. In all cases, the apatite U-Pb system records a secondary (metamorphic or metasomatic) overprint. The apatite Lu-Hf system, however, preserves primary crystallisation ages in unfoliated granitoids at temperatures of at least ∼660 °C. Above ∼730 °C, the Lu-Hf system records isotopic resetting by volume diffusion. Hence, in our observations for apatite of ‘typical’ volumes in granitoids (∼0.01-0.03 mm 2 ), the closure temperature of the Lu-Hf system is between ∼660 and ∼730 °C, consistent with theoretical calculations. In foliated granites, the Lu-Hf system records the timing of recrystallisation, while the apatite U-Pb system tends to record younger cooling ages. We also present apatite Lu-Hf dates for lower crustal xenoliths erupted with young alkali basalts, demonstrating that the Lu-Hf system can retain a memory of primary ages when exposed to magmatic temperatures for a relatively short duration. Hence, the apatite Lu-Hf system is a new insightful addition to traditional zircon (or monazite) U-Pb dating, particularly when zircons/monazites are absent or difficult to interpret due to inheritance or when U and Pb isotopes display open system behaviour. The laser-ablation based Lu-Hf method allows campaign-style studies to be conducted at a similar rate to U-Pb studies, opening new opportunities for magmatic and metamorphic studies. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6365962

The continental crust grew and matured compositionally during the Palaeo-to Neoarchean through the addition of juvenile tonalite-trondhjemite-granodiorite (TTG) crust. This change has been linked to the start of global plate tectonics, following the general interpretation that TTGs represent ancient analogues of arc magmas. To test this, we analysed B concentrations and isotope compositions in 3.8-2.8 Ga TTGs from different Archean terranes. The 11 B/ 10 B values and B concentrations of the TTGs, and their correlation with Zr/Hf, indicate differentiation from a common B-poor mafic source that did not undergo addition of B from seawater or seawater-altered rocks. The TTGs thus do not resemble magmatic rocks from active margins, which clearly reflect such B addition to their source. The Band 11 B-poor nature of TTGs indicates that modern style subduction may not have been a dominant process in the formation of juvenile continental crust before 2.8 Ga.

No abstract