Nakhlite

Nakhlites are the second-most abundant petrological group of Martian meteorites—constituting the "N" in the SNC (Shergottite, Nakhlite, Chassignite) meteorite clan and comprising ~30 paired specimens recovered from Egypt, Antarctica, Northwest Africa, and Brazil (Treiman, 2005; Udry & Day, 2018). Named after the Nakhla meteorite—a shower of ~40 stones totaling ~10 kilograms that fell around the village of El Nakhla El Bahariya in the Abu Hommos district of Alexandria Governorate, Egypt, on June 28, 1911—nakhlites are green-to-brown, unbrecciated, mafic-to-ultramafic igneous cumulate rocks (clinopyroxenites / wehrlites) composed of 70% to 85% sub-calcic augite clinopyroxene and 5% to 20% iron-rich olivine, set in a fine-grained mesostasis of plagioclase, alkali feldspar, and titanomagnetite. In planetary science and astrobiology, nakhlites are celebrated because their olivine crystals and mesostasis host pre-terrestrial hydrothermal alteration veins ("iddingsite")—containing ferric smectite clays, ferrihydrite, siderite carbonate, gypsum, and halite evaporites precipitated directly from liquid Martian groundwater brines at ~630 Ma (Bridges et al., 2001; Lee et al., 2015).

Unlike shergottites—which were shocked at pressures of 25–45 GPa (converting their plagioclase into amorphous maskelynite glass) and sample multiple volcanic sources across Mars—all known nakhlites experienced remarkably gentle shock pressures (5 to 20 GPa) that left their crystalline plagioclase feldspar intact, and share a uniform Sm–Nd, Rb–Sr, U–Pb, and ⁴⁰Ar/³⁹Ar igneous crystallization age of 1.34 ± 0.04 billion years (Ga) (Middle Amazonian) and an identical cosmic-ray exposure (CRE) ejection age of 10.7 ± 0.8 million years (Ma) (Korochantseva et al., 2011; Udry & Day, 2018).

Despite sharing the same 1.34 Ga volcanic source and 11 Ma impact launch, individual nakhlite specimens differ systematically in the thickness of the iron-rich rims on their augite and olivine crystals, the degree of olivine Fe–Mg chemical zoning, and the crystallinity vs. glassiness of their interstitial mesostasis. Mikouchi et al. (2003, 2012) and Treiman (2005) demonstrated that these differences record a vertical burial-depth stratigraphy (from ~1 meter at the top to >30 meters at the base) within a single thick basaltic lava flow or series of stacked flows on Mars:

Geochemically, nakhlites and the three known chassignites (Chassigny [1815 fall in France], NWA 2737, and NWA 8694—olivine-rich dunite cumulates that share the exact same 1.34 Ga crystallization age and 11 Ma cosmic-ray exposure age as the nakhlites) were derived from a long-lived, LREE-depleted, oxidized Martian mantle source:

• Coupled ¹⁴²Nd and ¹⁸²W Isotopic Anomalies: Nakhlites possess positive ε¹⁴²Nd (+0.6 to +0.9), high ε¹⁴³Nd (+16), and strong radiogenic ε¹⁸²W (+2 to +3) anomalies, proving that the nakhlite mantle source formed via garnet-fractionating silicate differentiation at ~4.50 Ga—distinct from all three shergottite mantle reservoirs (Foley et al., 2005; Debaille et al., 2009).
• Oxidized Parental Magma (QFM to QFM + 2): Unlike depleted shergottites (which crystallized under reduced conditions near the Iron–Wüstite buffer), nakhlite and chassignite parental magmas were highly oxidized (at or above the Quartz–Fayalite–Magnetite buffer) and enriched in incompatible LREEs ([La/Yb]_CI ≈ 3–6) and halogens (chlorine and fluorine in chlorapatite) due to low-degree partial melting of a metasomatized sub-lithospheric mantle plume beneath Elysium Mons or Syrtis Major (McSween, 1994; Udry & Day, 2018).
• Identification of the Candidate Launch Crater (Crater Kotka / Elysium Planitia): Using automated machine-learning crater detection across 90 million Martian impact craters combined with rayed-crater thermal inertia, 1.34 Ga volcanic surface ages, and the 11 Ma CRE age, planetary geologists narrowed the nakhlite–chassignite launch site to a few young rayed craters in the volcanic plains of Elysium or Tharsis (such as Kotka Crater, 39.5 km in diameter in Elysium Planitia) (Lagain et al., 2022).

The defining astrobiological treasure of the nakhlites is the network of reddish-brown secondary mineral veins—collectively termed "iddingsite"—that cut across cracked olivine grains and interstitial mesostasis in Lafayette, Nakhla, Y-000593, and MIL 090030 (Gooding et al., 1991; Bridges et al., 2001):

1. Proof of Pre-Terrestrial Martian Origin: High-resolution transmission electron microscopy (TEM) and K–Ar / Rb–Sr isotopic dating of the iddingsite veins prove that they formed on Mars at 633 ± 23 Ma (Late Amazonian, ~700 Myr after the lava cooled, likely when a small nearby impact melted subsurface ground ice and drove a brief hydrothermal circulation cell through the nakhlite rock pile)—because the veins are crosscut and offset by the black fusion crust formed when the meteorites entered Earth's atmosphere in 1911 and carry elevated Martian atmospheric D/H (δD up to +900‰) and Δ¹⁷O signatures (Swindle et al., 2000; Borg & Drake, 2005).
2. Sequential Evaporitic Brine Mineralogy (Bridges & Schwenzer, 2012): As the CO₂-charged hydrothermal fluid (T ≈ 50–150 °C, pH 6–9, water-to-rock ratio ~10–100) dissolved host olivine and mesostasis glass and progressively evaporated inside the fractures, it precipitated a textbook sequence of secondary minerals from the vein walls inward:
   • Outer Vein Margins: Crystalline Ca–Fe–Mg siderite–rhodochrosite carbonates (δ¹³C ≈ +10‰ to +50‰, equilibrated with Mars's CO₂ atmosphere).
   • Central Vein Fill: Ferric-iron trioctahedral smectite phyllosilicates (saponite and ferric serpentine) and amorphous nanoparticulate ferrihydrite / opal-A gels.
   • Terminal Evaporite Salts: Anhydrous and hydrated calcium sulfates (gypsum and bassanite), halite (NaCl), and potassium-iron sulfates (jarosite) in the shallowest specimens (MIL 090030).
3. Biomorphic Microtubules and Abiotic Organic Carbon in Y-000593 and Nakhla: Within the iddingsite clay veins of Nakhla and the Antarctic nakhlite Yamato 000593 (Y-000593), McKay et al. (2006, 2014) and Steele et al. (2012) documented microscopic curved tubular etch-pits inside olivine, spheroidal carbon-rich microstructures, and indigenous macromolecular reduced organic carbon (δ¹³C = −18‰ to −28‰, free of terrestrial radiocarbon ¹⁴C). While most geochemists interpret these carbon-enriched clay spherules as abiotic hydrothermal Fischer–Tropsch-type or magmatic condensates trapped during brine precipitation, the nakhlite iddingsite veins provide physical laboratory proof that warm, neutral-pH, clay-forming subsurface brines capable of supporting chemolithoautotrophic life periodically formed inside Mars's volcanic crust within the last billion years.

1. Gooding, J. L., Wentworth, S. J., & Zolensky, M. E. (1991). "Aqueous alteration of the Nakhla meteorite." Meteoritics, 26(2), 135–143. https://doi.org/10.1111/j.1945-5100.1991.tb01029.x
2. Bridges, J. C., Catling, D. C., Saxton, J. M., Swindle, T. D., Lyon, I. C., & Grady, M. M. (2001). "Alteration assemblages in Martian meteorites: implications for near-surface processes." Space Science Reviews, 96(1–4), 365–392. https://doi.org/10.1023/A:1011965826553
3. Treiman, A. H. (2005). "The nakhlite meteorites: Augite-rich igneous rocks from Mars." Chemie der Erde / Geochemistry, 65(3), 203–270. https://doi.org/10.1016/j.chemer.2005.01.004
4. Bridges, J. C., & Schwenzer, S. P. (2012). "The nakhlite hydrothermal brine on Mars." Earth and Planetary Science Letters, 359–360, 117–123. https://doi.org/10.1016/j.epsl.2012.09.044
5. Udry, A., & Day, J. M. D. (2018). "1.34 billion-year-old magmatism on Mars evaluated from the high-precision geochemistry of the nakhlite and chassignite meteorites." Geochimica et Cosmochimica Acta, 238, 292–315. https://doi.org/10.1016/j.gca.2018.07.006