We discuss the problems associated with the measurement of temperature and the determination of melting in the laser-heated diamond-anvil cell. We also briefly summarize the main developments of the last three decades in simultaneous high-P-T measurements and review practical aspects of the technique. The melting curve of argon as a function of pressure has been determined up to 47 GPa, and within experimental error, agrees with a Simon relation extrapolation of the melting curve from previous data to only 1.1 GPa. On the basis of currently debated upper and lower limits for the melting curve of pure iron, the argon melting curve can intersect the melting curve of iron in the range 40-65 GPa. Our experiments suggest that the melting points of iron and argon are equal at 47$\pm $1.0 GPa and 2750$\pm $200 K. Above this pressure, pure iron would be expected to melt at a lower temperature than solid argon. In a separate experiment, the melting temperature of iron in an argon medium at 57$\pm $1 GPa appears greater than 2650$\pm $200 K. The intersection of the melting curves for non-interacting materials could serve as standard P-T fixedpoints and offer a route to the standardization of different methods of temperature measurement.