Phase equilibria and physical properties of core materials.

摘要

The physical properties and phase equilibria of materials suspected to be important for Earth's core have been investigated at high pressures using the diamond anvil cell. The optical properties of iron have been derived from reflectance measurements at room temperature up to 50 GPa. There are strong changes in the optical properties associated with the bcc to hcp phase transition in iron at &sim13 GPa. The spectral emissivity of iron is a particularly important property because it appears in the equations used to measure temperature in the laser heated diamond anvil cell, and shock wave experiments. The emissivity of iron in the near infrared is a strong function of wavelength below the bcc to hcp phase transition. Above this phase transition, the infrared emissivity of iron does not have wavelength dependence, it behaves as a greybody. Temperature measurements utilizing spectroradiometry on iron samples below &sim13 GPa need to take into account the wavelength dependent emissivity of iron, or accept errors on the measured temperature as large at &sim25% or more.Investigations into the high pressure binary Fe-FeO and Fe-Fe3S phase diagrams using the laser heated diamond anvil cell in conjunction with synchrotron x-ray diffraction have been used to place constraints on the temperature and composition of Earth's core. The data suggests that oxygen is likely to preferentially partition into the liquid at the inner core boundary, while sulfur does not have a strong preference for the liquid or solid. This is an important result because it suggests that oxygen may be required in the core in order to explain the different abundances of light elements in the inner and outer cores. The melting temperatures in these systems were also measured up to &sim100 GPa for the Fe-FeO system and up to &sim150 GPa for the Fe-Fe3S system. The melting point depression with respect to pure iron is negligible in the case of adding oxygen, but a depression of 100-800 K was observed for the sulfur system. These latter temperatures are so low, that it is hard to reconcile the data with thermal models of Earth's interior. In fact, if we assume that Earth's core is iron plus 10 wt. % sulfur, the melting temperatures compared to the geothermal gradient suggest that we should have a liquid inner core and a solid outer core, exactly the opposite of what is known to be true.It is not possible to rule out some small fraction of sulfur in the core, but all of the results together suggest that oxygen is a likely component of the core, and if there is sulfur in the core, it must be in small concentrations so not to affect the melting temperatures too dramatically.