The crystal structures of synthetic Re- and PGE-bearing magnesioferrite spinels: Implications for impacts, accretion and the mantle.

Re- and PGE-bearing magnesioferrite spinels were synthesized at oxidizing c onditions between 0.1 MPa and 5.0 GPa, and 1150 to 1600 degreesC in equilib rium with Re- or PGE-bearing metals and/or oxides. Electron microprobe anal ysis and single crystal X-ray diffraction techniques were employed to deter mine the composition, crystal structures and site occupancies, indicating t hat the magnesioferrite structure can accommodate wt% levels of Re and the platinum group elements (PGE) at oxidized conditions. These results suggest that magnesioferrite spinels found in K-T boundary samples worldwide could he an important host phase for the Ir, Ru, Os, Re, and Rh anomalies found in the boundary layer. Higher Ru/Ir values in Pacific magnesioferrite-beari ng samples may be a reflection of higher condensation temperatures of the o xides. The distribution of PGE's in mantle and magmatic rocks may depend on the stability of spinel-structure oxides, especially those with a high mag nesioferrite component. Finally, magnetite-rich meteorite fusion crusts may hold PGEs in oxidized form.