The Chagos–Laccadive ridge (CLR) is an aseismic ridge in the Indian Ocean that formed by the action of the Réunion mantle plume. We use modeling of bathymetry and gravity data to constrain the crustal thickness variations at three locations from 0°S to 6°S where the plume was likely interacting directly with the Carlsberg spreading center. Ambiguities in the modeling were reduced by requiring both the density and thickness of the crust to be consistent with a single set of mantle melting conditions, in this case using a passive upwelling model in which mantle potential temperature is the controlling factor. Mean crustal thicknesses beneath the ridge at the three locations predicted from the gravity modeling were 16.6, 12.5 and 27.0 km; these thicknesses are up to 50% greater than would be obtained had we not required self-consistency in the modeling. If the thick crust is produced entirely by elevated potential temperatures in the melting region, mantle temperatures would have to be 100–250 °C higher than normal. Abrupt decreases in crustal thickness at a large-offset fracture zone are consistent with previous models for plume-spreading center interactions.