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We present a chemical abundance analysis of the faint halo metal-poormain-sequence star J0023+0307, with [Fe/H]<-6.3, based on a high-resolution(R~35,000) Magellan/MIKE spectrum. The star was originally found to have[Fe/H]< -6.6 based on a Ca II K measurement in an R~2,500 spectrum. No ironlines could be detected in our MIKE spectrum. Spectral lines of Li, C, Na, Mg,Al, Si, and Ca were detected. The Li abundance is close to the Spite Plateau,A(Li) = 1.7, not unlike that of other metal-poor stars although in starkcontrast to the extremely low value found e.g., in HE~1327-2326 at a similar[Fe/H] value. The carbon G-band is detected and indicates strong C-enhancement,as is typical for stars with low Fe abundances. Elements from Na through Sishow a strong odd-even effect, and J0023+0307 displays the second-lowest known[Ca/H] abundance. Overall, the abundance pattern of J0023+0307 suggests that itis a second-generation star that formed from gas enriched by a massivePopulation III first star exploding as a fall-back supernova The inferreddilution mass of the ejecta is 10^(5+-0.5) Msun of hydrogen, stronglysuggesting J0023+0307 formed in a recollapsed minihalo. J0023+0307 is likelyvery old because it has a very eccentric orbit with a pericenter in theGalactic bulge.

Chemical Abundance Signature of J0023+0307 -- A Second-Generation Main-Sequence Star with [Fe/H]<-6