  We present JWST/MIRI MRS spatially resolved $\sim 5-28\,\mu$m observations of
the central ~4-8kpc of the ultraluminous infrared galaxy and broad absorption
line quasar Mrk231. These are part of the Mid-Infrared Characterization of
Nearby Iconic galaxy Centers (MICONIC) program of the MIRI European Consortium
guaranteed time observations. No high excitation lines (i.e., [MgV] at
5.61$\mu$m or [NeV] at 14.32$\mu$m) typically associated with the presence of
an active galactic nucleus (AGN) are detected in the nuclear region of Mrk231.
This is likely due to the intrinsically X-ray weak nature of its quasar. Some
intermediate ionization potential lines, for instance, [ArIII] at 8.99$\mu$m
and [SIV] at 10.51$\mu$m, are not detected either, even though they are clearly
observed in a star-forming region ~920pc south-east of the AGN. Thus, the
strong nuclear mid-infrared (mid-IR) continuum is also in part hampering the
detection of faint lines in the nuclear region. The nuclear [NeIII]/[NeII]line
ratio is consistent with values observed in star-forming galaxies. Moreover, we
resolve for the first time the nuclear starburst in the mid-IR low-excitation
line emission (size of ~400pc, FWHM). Several pieces of evidence also indicate
that it is partly obscured even at these wavelengths. At the AGN position, the
ionized and warm molecular gas emission lines have modest widths
(W_80~300km/s). There are, however, weak blueshifted wings reaching velocities
v_02~-400km/s in [NeII]. The nuclear starburst is at the center of a large
(~8kpc), massive rotating disk with widely-spread, low velocity outflows. Given
the high star formation rate of Mrk231, we speculate that part of the nuclear
outflows and the large-scale non-circular motions observed in the mid-IR are
driven by its powerful nuclear starburst.
