Laser‐Heating Through a Diamond‐Anvil Cell: Melting at High Pressures

A laser-heating system was constructed that can stably heat through a diamond-anvil cell, probe regions as small as 6.7 μm in diameter, and accurately measure visible spectral radiation at frequencies up to 30 Hz. Spectra can be analyzed to measure temperature and to identify phenomena of interest such as melting. Reflecting optics were used where dispersion is critical. Temperature is stabilized by feedback from the thermal emissions of laser-heated samples and by attenuating the laser beam using a liquid-crystal variable waveplate and a fixed polarizer. The response time of the stabilizer is estimated to be 1.25 ms. The laser-heating system is suitable for experiments at the pressures and temperatures of the Earth's lower mantle and core. With this apparatus, a magnesium-iron-silicate perovskite, (Mg .88 Fe .12 ) SiO 3 , was melted in a diamond-anvil cell at pressures between 25 GPa and 85 GPa. Natural bronzite and synthetic enstatite were the starting materials. Polycrystalline samples with no pressure medium and single crystals in a NaCl pressure medium were melted. Melting was determined in situ by thermal analysis and corroborated by the appearance of glass in temperature-quenched samples. Corrections for radial temperature gradients in the samples were obviated by aperturing the collecting optics of the spectrometer. Corrections for axial temperature gradients were estimated from a simple model. The slope of the melting curve between 35 GPa and 85 GPa is 5.2 ± 0.8 K/GPa and sub-adiabatic. The melting temperature extrapolated to the core-mantle boundary is 4502 ± 176 K. Both are significantly less than recent estimates.