A laser-heated diamond anvil cell was used in resistance measurements at high pressures and high temperatures. A thin film of alumina (Al 2O3) was coated on both diamond anvils for thermal insulation before fabricating the microcircuit for resistance measurements. This technique significantly improved the heating conditions in the sample chamber. Using this technique, we measured the resistivity of (Fe0.125,Mg 0.875)2SiO4 to 35GPa and 3450K, which reveals an increase in the activation energy of the carrier with increasing pressure. The activation volume and activation energy are both less than the values at lower pressures and temperatures.