  We present ALMA $^{12}$CO (J=1-0, 3-2 and 6-5), $^{13}$CO (J=1-0) and
C$^{18}$O (J=1-0) observations of the local Ultra Luminous Infrared Galaxy,
IRAS 13120-5453 (dubbed "The Yo-yo"). The morphologies of the three isotopic
species differ, where $^{13}$CO shows a hole in emission towards the center. We
measure integrated brightness temperature line ratios of $^{12}$CO/$^{13}$CO
$\geq$ 60 (exceeding 200) and $^{13}$CO/C$^{18}$O $\leq$ 1 in the central
region. Assuming optical thin emission, C$^{18}$O is more abundant than
$^{13}$CO in several regions. The abundances within the central 500 pc are
consistent with enrichment of the ISM via a young starburst ($<$7Myr), a
top-heavy initial mass function or a combination of both.
