Lead Diffusion In Iron Sulfides: Experimental Applications To Study The Differentiation Of Iron Meteorites And Early Global Melting

During the early formation of planetary bodies, iron meteorites, which are compositionally similar to these differentiated planetary bodies, and are considered to be remnant core material, formed due to separation of the iron-rich core and the silicate mantle. These planetesimal bodies accreted and melted in approximately 1.5 to 5 million years after the first materials condensed in the solar system. Important elements occur in these meteorites as part of radioisotope systems, which are useful in sequencing early planetary history, core formation and other melting and cooling events. However, remelting events or erosional processes have displaced or “reset” many of these key components used for radioisotopic dating. This study involved an extensive series of procedures that helped to prepare the experiments involving lead diffusion in pyrrhotite (FeS), which are compositionally reminiscent of iron meteorites. As part of our study, we used natural pyrrhotite crystals and synthesized our own pyrrhotite with an added trace amount of lead and utilized x-ray diffraction and electron microprobe analysis techniques to verify their pyrrhotite compositions. We applied a constant source diffusion method, which involved combining natural polished pyrrhotite crystals and synthesized pyrrhotites of the same composition with an added lead component into an evacuated capsule, which helped to emulate more a realistic schematic of cooling events and transport of different elements. Experiments were conducted at variable durations from 1 day to 1 week at temperatures that ranged between 500 °C to 750 °C. The results of these experiments were analyzed using Rutherford Backscattering techniques, and show that pyrrhotites of lower temperature experiments exhibited lead concentrations with small diffusion profiles. Pyrrhotites of higher-temperature experiments showed higher lead concentrations where lead has diffused throughout the pyrrhotite crystal. Results also show a positive correlation between the diffusion coefficients of lead and temperature.