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We combine asteroseismology, optical high-resolution spectroscopy, andkinematic analysis for 26 halo red giant branch stars in the \textit{Kepler}field in the range of $-2.5<[\mathrm{{Fe}/{H}}]<-0.6$. After applyingtheoretically motivated corrections to the seismic scaling relations, we obtainan average mass of $0.97\pm 0.03\,\mathrm{M_{\odot}}$ for our sample of halostars. Although this maps into an age of $\sim 7\,\mathrm{Gyr}$, significantlyyounger than independent age estimates of the Milky Way stellar halo, weconsiderer this apparently young age is due to the overestimation of stellarmass in the scaling relations. There is no significant mass dispersion amonglower red giant branch stars ($\log g>2$), which constrains a relative agedispersion to $<18\%$, corresponding to $<2\,\mathrm{Gyr}$. The precisechemical abundances allow us to separate the stars with [{Fe}/{H}]$>-1.7$ intotwo [{Mg}/{Fe}] groups. While [$\alpha$/{Fe}] and [{Eu}/{Mg}] ratios aredifferent between the two subsamples, [$s$/Eu], where $s$ stands for Ba, La,Ce, and Nd, does not show a significant difference. These abundance ratiossuggest that the chemical evolution of the low-Mg population is contributed bytype~Ia supernovae, but not by low-to-intermediate mass asymptotic giant branchstars, providing a constraint on its star formation timescale as$100\,\mathrm{Myr}<\tau<300\,\mathrm{Myr}$. We also do not detect anysignificant mass difference between the two [{Mg}/{Fe}] groups, thus suggestingthat their formation epochs are not separated by more than 1.5 Gyr.

Star Formation Timescales of the Halo Populations from Asteroseismology and Chemical Abundances