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Based on high-resolution spectra obtained with Magellan/MIKE, we present achemo-dynamical analysis for 27 near main-sequence turnoff metal-poor stars,including 20 stars analyzed for the first time. The sample spans a range in[Fe/H] from -2.5 to -3.6, with 44% having [Fe/H] <-2.9. We derived chemicalabundances for 17 elements, including strontium and barium. We derive Liabundances for the sample, which are in good agreement with the ``SpitePlateau'' value. A dozen of stars are carbon-enhanced. The lighter elements(Z<30) generally agree well with those of other low-metallicity halo stars.This broadly indicates chemically homogeneous gas at the earliest times. Of theneutron-capture elements, we only detected strontium and barium. We used the[Sr/Ba] vs. [Ba/Fe] diagram to classify metal-poor stars into five populationsbased on their observed ratios. We find HE0232-3755 to be a likely mainr-process star, and HE2214-6127 and HE2332-3039 to be limited-r stars.CS30302-145, HE2045-5057, and CD-24 17504 plausibly originated inlong-disrupted early dwarf galaxies as evidenced by their [Sr/Ba] and [Ba/Fe]ratios. We also find that the derived [Sr/H] and [Ba/H] values for CD-24 17504are not inconsistent with the predicted yields of the s-process in massiverotating low-metallicity stars models. Further theoretical explorations will behelpful to better understand the earliest mechanisms and time scales of heavyelement production for comparison with these and other observational abundancedata. Finally, we investigate the orbital histories of our sample stars. Mostdisplay halo-like kinematics although three stars (CS29504-018, HE0223-2814,and HE2133-0421) appear to be disk-like in nature. This confirms theextragalactic origin for CS30302-145, HE2045-5057, and, in particular, CD-2417504 which likely originated from a small accreted stellar system as one ofthe oldest stars.

Metal-poor stars observed with the Magellan Telescope. IV. Neutron-capture element signatures in 27 main-sequence stars