We present a chemical abundance analysis of the faint halo metal-poor main-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 iron lines 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 stark contrast 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 Si show 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 it is a second-generation star that formed from gas enriched by a massive Population III first star exploding as a fall-back supernova The inferred dilution mass of the ejecta is 10^(5+-0.5) Msun of hydrogen, strongly suggesting J0023+0307 formed in a recollapsed minihalo. J0023+0307 is likely very old because it has a very eccentric orbit with a pericenter in the Galactic bulge.
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