U th he dating of apatite a potential thermochronometry
2009), e U and grain age should have a positive correlation if the dispersion is derived from radiation damage (cf Fig. Within samples, we adopted grain ages that overlapped with the other grain ages within ±3σ to calculate the weighted mean age of each sample (Table 3; Fig. For A02-ST03, grain ages present a cluster ranging 80–50 Ma, except for one grain yielding an age of ~225 Ma.However, all of the grain ages belonging to the cluster do not overlap within ±3σ.Zircon grains were removed from their Pt capsules, and transferred to Parr bombs where they were spiked with Th and digested in small volumes (0.3–0.5 ml) at 240 °C for 40 h in HF.Standard solutions containing the same spike amounts as those in the samples were treated identically, as were a series of unspiked reagent blanks.(2000) for laser extraction of helium from single grains for both apatite and zircon.Clear, non-fractured euhedral grains with average grain radii in a close size range were hand-picked under an Olympus SZX12 binocular microscope, then immersed in ethanol and checked under polarized light to detect and exclude grains with possible inclusions.
(U–Th–Sm)/He ages were calculated and corrected for α-emission following the approach of Farley et al. Analytical uncertainties at the Melbourne He facility were conservatively assessed to be ~6.2% (±1σ), including the α-ejection correction, an estimated 5 μm of uncertainty in grain dimensions, gas analysis (estimated as Zircon He and Apatite He (ZHe and AHe, respectively) dating results are given in Tables 2 and 3, respectively. 2007), implantation of He in apatite from the adjacent minerals (Spiegel et al. 2013), and trapping of radiometric He in fluid inclusions (Danišík et al. Thus, samples with significant age dispersion reflect slow cooling or a complicated cooling history rather than simple rapid cooling because effects of these parameters are magnified by slow or complicated cooling history (e.g., Gautheron et al. The two horizontal red lines indicate the weighted mean age 95% confidence interval (CI) and −95% CI, respectively Effective uranium (e U) versus (U–Th–Sm)/He grain age of apatite.
Zircon samples were outgassed by applying laser power of ~12.6 W (~1300 °C) for 20 min to ensure complete extraction of He yields were too low, aliquots of multiple grains were analyzed.
The outgassed grains were removed from the laser chamber, and dissolved and analyzed for parent isotopes by using an Agilent 7700X ICP Mass Spectrometer.
1), providing a typical island arc setting (e.g., Minoura and Hasegawa 1992; Yoshida et al. The mountains on the fore-arc side are relatively wide, have gentle slopes, and low-relief surfaces (Fig. The OBR is a narrow and long mountain range wedged between active faults (Fig. Quaternary volcanism is dominant in the OBR and in a few parts of the back-arc side (Fig. The NE Japan Arc generally shows arc-parallel structures, for example, mountain ranges, active faults, and volcanic chains, although across-arc mountains and chains of volcanoes are also observed on the back-arc side (e.g., Tamura et al. FT ages are from Ohira and Honda (1999), Goto (2001) and Ohtani et al. This figure was drawn by using Generic Mapping Tools (GMT) (Wessel and Smith 1991) and 30 arc-second grid of the General Bathymetric Chart of the Oceans (GEBCO).
b Topographic cross-sections across the study area.