The elusive iron-sapping mechanism that results in Earth’s continental crust

The crust that forms the upper layer of Earth’s continents is forged from magma located deep below the surface in volcanic arcs, such as the Cascades in the western US, where dense oceanic crust slides beneath existing continental crust. Like the crust it forms once it erupts and solidifies, the molten rock in those arc magmas is deficient in iron compared with the lavas that form ocean crust, and a relatively large amount of the remaining iron takes the form of oxidized Fe³⁺. For decades researchers have wondered how all that magmatic iron gets depleted and oxidized.

The mineral garnet has recently emerged as a possible agent for sapping arc magmas of iron, particularly Fe²⁺. In a 2018 paper , researchers at Rice University proposed that garnet functions as a “redox filter,” in that the high-melting-point mineral preferentially incorporates Fe²⁺ as it gradually crystallizes in the magma. To put that proposal to the test, Megan Holycross (Cornell University) and Elizabeth Cottrell (Smithsonian Institution) exposed rock samples to a simulated magma chamber. The researchers loaded the samples into piston cylinders and subjected them to temperatures exceeding 1200 °C and pressures up to 30 000 times that at sea level. At Argonne National Laboratory’s Advanced Photon Source, they analyzed their analogue arc magmas via x-ray absorption spectroscopy to quantify the concentrations of Fe³⁺ and Fe²⁺.

The garnet in the piston-cylinder experiments did not show a strong affinity for Fe²⁺ over Fe³⁺. After plugging their results into a model similar to the one used by the Rice researchers, Holycross and Cottrell found that the ratio of Fe³⁺ to Fe²⁺ in the liquid remaining after garnet crystallization would be too low to account for the measured ratios in arc magmas. Additionally, the garnet did not remove enough iron to match the observed levels in continental crust. The findings reduce the likelihood that garnet is the primary iron extractor and oxidizer in arc magmas.

It’s possible that another mineral performs the redox-filter function in the molten rock or that melted material from the underlying mantle is involved in the oxidation process. Finding a cause would help explain the current continental crust production mechanism and perhaps provide clues as to the formation of the first iron-sapped crusts that rose above the surface of Earth’s oceans hundreds of millions of years ago. (M. Holycross, E. Cottrell, Science 380, 506, 2023 .)