We combine asteroseismology, optical high-resolution spectroscopy, and kinematic analysis for 26 halo red giant branch stars in the Kepler field in the range of -2.5<[Fe/H]{2), which constrains the relative age dispersion to <18%, corresponding to }-1.7 into two [Mg/Fe] groups. While the [{alpha}/Fe] and [Eu/Mg] ratios are different between the two subsamples, [s/Eu], where s stands for Ba, La, Ce, and Nd, does not show a significant difference. These abundance ratios suggest that the chemical evolution of the low-Mg population is contributed by Type Ia supernovae, but not by low- to intermediate-mass asymptotic giant branch stars, providing a constraint on its star formation timescale as 100Myr<{tau}<300Myr. We also do not detect any significant mass difference between the two [Mg/Fe] groups, thus suggesting that their formation epochs are not separated by more than 1.5Gyr.

Cone search capability for table J/ApJ/912/72/stars (Observation log (Table 1) and adopted stellar parameters (Table 6))