Synthesis of a quenchable high-pressure form of magnetite (h-Fe3O4) with compositionFel(Fe2+0.75Mg0.26)Fe2(Fe3+0.70Cr0.15Al0.11Si0.04)2O4

We report the synthesis of h-magnetite, ideally h-Fe3O4with considerable amounts of substitutional cations (Cr, Mg, Al, Si) and quenchable to ambient conditions. Two types of experiments were performed at 18 GPa and 1800 °C in a multi-anvil press. In one, we used an oxide mixture with a majoritic stoichiometry Mg1.8Fe1.2(Al1.4Cr0.2Si0.2Mg0.2)Si3O12, with Si and Mg in excess as starting material (MA-367, MA-380). In the second type of experiment (MA-376), we started from an oxide mixture on the composition of the Fe-oxide phase obtained in MA-367. The Fe-oxide phases of both experiments were investigated by electron microprobe and transmission electron microscopy including electron diffraction tomography. Our investigations show that the Fe-oxide phases crystallize in the structure-type of h-magnetite. However, electron diffraction data show that keeping the cell setting from literature, this phase crystallizes in space group Amam and not in space group Bbmm as previously proposed. In the experiment MA-367, the Fe-oxide phase are mutually intergrown with majorite, the major phase of the run products. The formula for h-magnetite in this run was calculated as Fe1(Fe2+ 0.75Mg0.26)Fe2(Fe3+ 0.70 Cr0.15Al0.11Si0.04)2O4. In the experiment on the bulk composition of the Fe-oxide, the main phase was h-magnetite with composition Fe1(Fe2+ 1.02)Fe2(Fe3+ 0.65Cr0.19Al0.13Si0.03)2O4and traces of nearly pure endmember wadsleyite and stishovite. Our results indicate that the substitution of 20 to 30% of Fe (0.7 to 0.9 atoms per formula unit) by smaller cations favored the preservation of the high-pressure form to ambient conditions. We prove that the h-magnetite-type oxide is also stable in chemical systems more complex than Fe-O. Based on our results, which were obtained at 18 GPa and 1800 °C in a system (MA-367) that is closely related to Fe-enriched oceanic lithospheric material, we suggest that a Fe3O4-rich phase may be present in environments connected to deeply subducted slabs and possibly associated with deep carbonatitic melting. Our observations show that Cr strongly partitions in the oxide phase such that the coexisting silicates are depleted in Cr compared to Fe3O4-free assemblages. This may significantly affect the chemical signature of melts produced in the deep mantle.