Ehime University study proposes deep-mantle garnet explains the oxidation of Martian meteorites

Planetary scientists Hideharu Kuwahara and Ryoichi Nakada of Ehime University published a study on Oct. 27, 2026, proposing that deep-mantle majorite garnet, a silicate mineral, explains the unexpected oxidation of Martian meteorites.

Mars formed four and a half billion years ago as heavy metals sank to the center and formed an iron-rich core like Earth’s. Pure metal strips oxygen away, so the mantle above the core should be reduced. NASA’s InSight mission confirmed a 150-kilometer-deep layer of molten silicate rock directly above Mars’s core, which scientists typically call the mantle.

On Mars, majorite, a silicate crystal, forms at the base of the mantle above the heavily reducing core at pressures around 18 gigapascals, or 180,000 times atmospheric pressure at sea level. Using a massive hydraulic press, the team squeezed synthetic Martian rock between tungsten-carbide blocks inside diamond capsules, then heated it to around 2,150 to 2,200 degrees Celsius and saturated it with metallic iron to mimic reducing conditions near the core. After cooling, researchers took the samples to the SPring-8 synchrotron facility about 250 kilometers away and blasted them with X-rays to examine how iron bonded into mineral grains.

According to the paper, majorite behaved much like an iron sponge, drawing oxidized ferric iron into its crystal structure. Oxidized iron made up between 11% and 21% of the iron inside majorite crystals, well above the share in neighboring silicates formed in the same experiment. As Mars’s interior crystallized, majorite precipitated out of the molten rock and trapped ferric iron that would otherwise have remained dissolved in the reducing basal layer. The paper ties meteorite chemistry to how crystals react under pressure with molten metal, a deep-mantle link that can last billions of years.

Earth is about twice the diameter of Mars and has a much deeper mantle, so bridgmanite, a silicate mineral, dominates its lower reaches near the core and can hold more iron than majorite, leaving Earth’s deep mantle chemically stratified. As the mantle rises, bridgmanite transforms into majorite and releases ferric iron into surrounding rock, oxidizing that transition zone from below.

Tanya Kizovski, Assistant Professor of Earth Sciences at Brock University in Canada, led the international team that identified garnet in a fragment of the Martian meteorite NWA 8171, part of the Royal Ontario Museum’s collections. The study appeared in Geochemical Perspectives Letters on June 16, 2026.

At first, the team assumed the fragment contained pyroxene, a common mineral, but its chemical makeup seemed unusual, so they took a second look. Using the University of Portsmouth Electron Microscopy and Microanalysis Unit and the Royal Ontario Museum’s specialized laser equipment, they unexpectedly identified garnet. Researchers also used electron microprobes and scanning electron microscopes to map minerals, trace chemical fingerprints and capture backscatter electron images of NWA 8171.

NWA 8171 belongs to a rare group of 18 Martian regolith breccias, all fragments from the same ancient fall. The garnet-bearing clast divides into two parts: Andradite, with a diopside domain rich in calcium garnet and pyroxene; and K-feldspar, with an augite domain featuring a feldspar-pyroxene mix. Augites in the K-feldspar-rich zone match Martian values, but diopsides in the andradite-bearing zone vary more, and similarities to Earth and chondritic rocks suggest the garnet may not be a primary igneous mineral.

Kizovski says the rock’s origin remains unproven: the evidence does not show whether it formed on Mars or reached the surface inside another meteorite. Confirming whether the garnet formed on Mars or came from another planetary body requires isotopic analysis. Measuring oxygen isotopes – atoms of the same element with different numbers of neutrons – from the rock itself would help confirm if it is Martian or from an exotic meteorite impactor. The isotope test would destroy part of the sample, so researchers have avoided it because the specimen is rare and may be the only garnet-bearing Martian rock available.

The discovery could suggest an unknown magma source on Mars, may represent a process that changes minerals under different conditions, could be a component from a regolith impactor brought by cosmic collisions, or might suggest a metamorphic event deep within Mars’s crust. Heat and pressure to produce garnet through metamorphism could have come from a meteorite impact, rising magma, or both.

Garnet opens a Martian archive by preserving a detailed record of the temperatures, pressures and geological processes that shaped the planet billions of years ago. The finding could reveal new details about conditions that shaped Mars more than four billion years ago. Unlike Earth, plate tectonics, volcanism and erosion continually reshaped its surface, while Mars preserved much of its ancient geological record, making every rare Martian rock a valuable archive.

ROM curator Kim Tait and Research Assistant Jessica Tomacic, together with Professor Darling, are continuing to examine the sample. The team also includes scientists from the Universita di Trieste in Italy and the Open University in the U.K. Funding comes in part from Canada’s Natural Sciences and Engineering Research Council and the Killam Trusts Dorothy Killam fellowship, plus Science and Technology Facilities Council funding at Portsmouth.

James Darling, Professor of Earth and Planetary Science from the University of Portsmouth’s School of the Environment and Life Sciences, said, “The findings add a striking new dimension to our understanding of the geology of Mars.”

Tanya Kizovski, Assistant Professor of Earth Sciences at Brock University in Canada, said the discovery will expand knowledge of geologic processes possible on Mars and could give clues to how the planet changed throughout its history. “This little section of the meteorite looked really interesting, and the chemistry was a bit odd.”