Accumulation of magnetite by flotation on bubbles during decompression of silicate magma

Magnetite (Fe 3 O 4 ) is an iron ore mineral that is globally mined especially for steel production. It is denser (5.15 g/cm 3 ) than Earth’s crust (~2.7 g/cm 3 ) and is expected to accumulate at the bottom of melt-rich magma reservoirs. However, recent studies revealed heterogeneous fluid bubble nucleation on oxide minerals such as magnetite during fluid degassing in volcanic systems. To test if the attachment on fluid bubbles is strong enough to efficiently float magnetite in silicate magma, decompression experiments were conducted at geologically relevant magmatic conditions with subsequent annealing to simulate re-equilibration after decompression. The results demonstrate that magnetite-bubble pairs do ascend in silicate melt, accumulating in an upper layer that grows during re-equilibration. This outcome contradicts the paradigm that magnetite must settle gravitationally in silicate melt.