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The Phoenix stellar stream has a low intrinsic dispersion in velocity andmetallicity that implies the progenitor was probably a low mass globularcluster. In this work we use Magellan/MIKE high-dispersion spectroscopy ofeight Phoenix stream red giants to confirm this scenario. In particular, wefind negligible intrinsic scatter in metallicity ($\sigma(\mathrm{[Fe~II/H]}) =0.04^{+0.11}_{-0.03}$) and a large peak-to-peak range in [Na/Fe] and [Al/Fe]abundance ratios, consistent with the light element abundance patterns seen inthe most metal-poor globular clusters. However, unlike any other globularcluster, we also find an intrinsic spread in [Sr II/Fe] spanning $\sim$1 dex,while [Ba II/Fe] shows nearly no intrinsic spread ($\sigma(\mathrm{[Ba~II/H]})= {0.03}^{+0.10}_{-0.02}$). This abundance signature is best interpreted asslow neutron capture element production from a massive fast-rotating metal-poorstar ($15-20 \mathrm{M}_\odot$, $v_\mathrm{ini}/v_\mathrm{crit} = 0.4$,$[\mathrm{Fe/H}] = -3.8$). The low inferred cluster mass suggests the systemwould have been unable to retain supernovae ejecta, implying that any massivefast-rotating metal-poor star that enriched the interstellar medium must haveformed and evolved before the globular cluster formed. Neutron capture elementproduction from asymptotic giant branch stars or magneto-rotationalinstabilities in core-collapse supernovae provide poor fits to theobservations. We also report one Phoenix stream star to be a lithium-rich giant($A(\mathrm{Li}) = 3.1 \pm 0.1$). At $[\mathrm{Fe/H}] = -2.93$ it is among themost metal-poor lithium-rich giants known.

Signature of a massive rotating metal-poor star imprinted in the Phoenix stellar stream