  This study, placed in the context of the preparation for the Uranus Orbiter
Probe mission, aims to predict the bulk volatile compositions of Uranus and
Neptune. Using a protoplanetary disk model, it examines the evolution of trace
species through vapor and solid transport as dust and pebbles. Due to the high
carbon abundance found in their envelopes, the two planets are postulated to
have formed at the carbon monoxide iceline within the protosolar nebula. The
time evolution of the abundances of the major volatile species at the location
of the CO iceline is then calculated to derive the abundance ratios of the
corresponding key elements, including the heavy noble gases, in the feeding
zones of Uranus and Neptune. Supersolar metallicity in their envelopes likely
results from accreting solids in these zones. Two types of solids are
considered: pure condensates (Case 1) and a mixture of pure condensates and
clathrates (Case 2). The model, calibrated to observed carbon enrichments,
predicts deep compositions. In Case 1, argon is deeply depleted, while
nitrogen, oxygen, krypton, phosphorus, sulfur, and xenon are significantly
enriched relative to their protosolar abundances in the two planets. Case 2
predicts significant enrichments for all species, including argon, relative to
their protosolar abundances. Consequently, Case 1 predicts near-zero Ar/Kr or
Ar/Xe ratios, while Case 2 suggests these ratios are 0.1 and 0.5-1 times their
protosolar ratios. Both cases predict a bulk sulfur-to-nitrogen ratio
consistent with atmospheric measurements.
