  We report and discuss the discovery of a comoving pair of bright solar-type
stars, HD 240430 and HD 240429, with a significant difference in their chemical
abundances. The two stars have an estimated 3D separation of $\approx 0.6$ pc
($\approx 0.01$ pc projected) at a distance of $r\approx 100$ pc with nearly
identical three-dimensional velocities, as inferred from Gaia TGAS parallaxes
and proper motions, and high-precision radial velocity measurements. Stellar
parameters determined from high-resolution Keck HIRES spectra indicate that
both stars are $\sim 4$ Gyr old. The more metal-rich of the two, HD 240430,
shows an enhancement of refractory ($T_C>1200$ K) elements by $\approx 0.2$ dex
and a marginal enhancement of (moderately) volatile elements ($T_C<1200$ K, C,
N, O, Na, and Mn). This is the largest metallicity difference found in a wide
binary pair yet. Additionally, HD 240430 shows an anomalously high surface
lithium abundance ($A(\mathrm{Li})=2.75$), higher than its companion by $0.5$
dex. The proximity in phase-space and ages between the two stars suggests that
they formed together with the same composition, at odds with the observed
differences in metallicity and abundance patterns. We therefore suggest that
the star HD~240430, "Kronos", accreted 15 $M_\oplus$ of rocky material after
birth, selectively enhancing the refractory elements as well as lithium in its
surface and convective envelope.
