53-3 From terrestrial systems to planetary bodies: Silician magnetite as a nanoscale record of fluid–rock interactions

Abstract:

Magnetite is a common and highly refractory mineral that preserves evidence of formation and alteration conditions that may otherwise be overprinted or obliterated in complex geological systems. Silician magnetite is especially valuable in this regard, as it records mineral reactions and fluid-rock interaction in which silicon is incorporated into magnetite as lattice-bound Si, Si-rich defects, or nanoscale silicate inclusions.

Magnetite is also widespread in planetary materials, including Martian meteorites, lunar regolith, interplanetary dust particles, and returned asteroid samples. Magnetite inclusions in zircon from a Martian meteorite provide critical evidence for early hydrothermal activity on Mars, while returned samples from asteroid Ryugu contain a variety of magnetite textures, including framboidal, plaquette, and spherical, that are closely associated with phyllosilicates. Some spherulitic grains contain abundant Si-Mg-rich domains and pores with unresolved Fe-Si interactions during aqueous alteration. These occurrences indicate that extraterrestrial magnetite may retain mineral-scale records comparable to those already recognised in terrestrial hydrothermal systems. However, interpreting these records requires a process-based framework grounded in the detailed characterisation of comparable analogues.

Terrestrial studies provide this process-based foundation. In banded iron formations, skarns, and iron oxide-copper-gold systems, integrated micron- to atomic-scale investigations have shown that Si-rich defects in magnetite can act as nucleation sites for silicate minerals, while inclusions, replacement textures, and crystallographic relationships record successive stages of overprinting. Silician magnetite can therefore fingerprint multistage processes, including protolith inheritance, metasomatism, deformation, redox evolution, and silicate growth. In magmatic–hydrothermal systems, for example, the transition from inherited trellis textures to silicate-mottled domains with amphibole and non-classical pyribole inclusions in magnetite preserve distinct stages of fluid-rock interaction and overprinting.

Building on these results, we present a proof-of-concept for transfer of magnetite-characterisation approaches to planetary materials. This framework integrates micro- to atomic-scale mineral textures, inclusions, crystallographic relationships and chemistry to determine whether extraterrestrial magnetite preserves process signatures analogous to those recognised on Earth. Identifying comparable silician defects, nanoinclusions, and reaction textures could help distinguish primary mineral formation from later overprinting and constrain the roles of aqueous fluids, thermal evolution, and changing redox conditions in asteroid parent bodies and other primitive Solar System objects. More broadly, this approach establishes the potential of magnetite as a process-sensitive archive for fingerprinting aqueous alteration and geological evolution in planetary materials.

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