In Nili Planum, the olivine-bearing unit covering large portions of this region is variably altered to produce the most expansive carbonate detection on Mars. The mechanism of carbonation is unknown. Here we test the conditions necessary to form the olivine-carbonate unit using the reactive transport model (RTM), CruchFlow. The continued presence of olivine requires a short duration of alteration, and/or limited fluid-rock ratios. Furthermore, temperatures must have been low to form a layer of the olivine-carbonate unit consistent with the observed thicknesses. Water availability must have been insufficient to prevent significant amounts of unobserved phases from forming, (e.g., talc, serpentine, and brucite) which would be consistent with alteration pathways on Earth that form carbonate from olivine. The Perseverance rover can make measurements of this unit, providing the data necessary to constrain its formation conditions further.