Thermally stable diamond-bonded compacts include a diamond-bonded body having a thermally stable region extending a distance below a diamond-bonded body surface. The thermally stable region comprises a matrix first phase of bonded together diamond crystals, and a second phase interposed within the matrix phase. At least some population of the second phase comprises a reaction product formed between an infiltrant material and the diamond crystals at high pressure/high temperature conditions. The diamond bonded body further includes a polycrystalline diamond region that extends a depth from the thermally stable region and has a microstructure comprising a polycrystalline diamond matrix phase and a catalyst material disposed within interstitial regions of the matrix phase. The compact includes a substrate attached to the diamond-bonded body.