Chassignite

A chassignite is a rare class of coarse-grained, olivine-rich dunite Martian meteorites that forms the "C" subgroup of the historic SNC (shergottite–nakhlite–chassignite) clan. Named after the Chassigny meteorite—a ~4-kilogram stone that fell with thunderous detonations at midday on October 3, 1815, in the village of Chassigny (Haute-Marne, Champagne, France)—chassignites are the rarest of the three SNC groups: out of more than 380 recognized Martian meteorites, only three distinct chassignite stones have been identified to date (Chassigny, Northwest Africa [NWA] 2737, and NWA 8694). Composed of ~90–92 vol% cumulus olivine alongside minor clinopyroxene, chromite, plagioclase, and hydrous melt inclusions, chassignites share an identical crystallization age of 1,340 ± 40 million years (1.34 Ga) (Middle Amazonian period) and an identical cosmic-ray ejection age of ~10.7–11.4 million years (11 Ma) with the augite-rich nakhlites, proving that both rock types were excavated by the same asteroid impact from a single stratified Martian volcano-plutonic complex. Furthermore, because the Chassigny meteorite is virtually free of shock-implanted modern Martian atmospheric gas, its olivine crystals trap the pristine interior volatile reservoir of the Martian mantle (Chass-S)—preserving solar-type krypton and xenon isotope ratios that revolutionized planetary models of how Mars accreted its water and volatiles from the solar nebula and chondritic planetesimals (McSween, 1994; Beck et al., 2006; Udry & Day, 2018; Péron et al., 2022).

For 185 years after the 1815 fall of Chassigny (examined originally by Jean-Baptiste Biot and Louis-Nicolas Vauquelin in 1816), Chassigny remained a unique solo specimen until Saharan meteorite hunters recovered two additional chassignites in Morocco and Algeria:

Petrographically, chassignites are orthocumulate to mesocumulate dunites formed by crystal settling and compaction at the floor of a shallow Martian magma chamber or thick sill at ~1.34 Ga (Udry & Day, 2018):

• Cumulus Olivine Framework (85–92 vol%, 1–4 mm grains): Equant, subhedral olivine crystals showing 120° triple-junction annealing textures, enclosing early-crystallizing euhedral chromite (FeCr₂O₄, 1.4–2.5 vol%) spinels whose ferric iron content records crystallization near the quartz–fayalite–magnetite (QFM) oxygen fugacity buffer (ΔQFM ≈ −0.5 to +0.5)—significantly more oxidized than most depleted shergottites.
• Poikilitic Pyroxenes and Intercumulus Plagioclase (6–10 vol%): Large oikocrysts of augite (Wo₃₄En₄₇Fs₁₉) and low-calcium pigeonite/orthopyroxene enclose rounded olivine chadacrysts, alongside interstitial shock-amorphized plagioclase glass (maskelynite, An_20–38), sanidine, chlorapatite, troilite, pentlandite, ilmenite, and baddeleyite.
• Polyphase Hydrous Melt Inclusions (Kaersutite and Biotite): Trapped inside the cumulus olivine crystals of Chassigny and NWA 2737 are spherical 30 to 150 μm polyphase melt inclusions containing kaersutitic Ti-amphibole (Ca₂Na(Mg,Fe)₄TiSi₆Al₂O₂₂(OH,F,Cl)₂), Ti-biotite (phlogopite), rhyolitic glass, and Cl-rich apatite. Ion-microprobe measurements of the D/H (δD) and Cl/OH ratios of these hydrous phases demonstrate that the parental magma crystallized at depth under P_H2O ≈ 100–200 MPa (4–8 km depth in the Martian crust) from an undegassed, LREE-enriched, chlorine- and fluorine-bearing mantle plume source (Johnson et al., 1991; McCubbin et al., 2010).

High-precision Sm–Nd, Rb–Sr, Lu–Hf, and Ar–Ar isochrons yield a crystallization age of 1,340 ± 40 Ma for both Chassigny and NWA 2737, indistinguishable from the 1,340 ± 20 Ma crystallization age of the nakhlites, while cosmogenic ³He, ²¹Ne, and ³⁸Ar abundances yield a shared space-exposure duration of 11.0 ± 1.5 Ma—confirming that a single 11 Ma impact crater (likely ~6.5 km Kotka Crater on the flanks of Elysium Mons or in northern Syrtis Major) excavated deeper cumulate dunite layers (chassignites) simultaneously with shallower augite-rich lava flows and sills (nakhlites) (Nyquist et al., 2001; Udry & Day, 2018; Lagain et al., 2021).

One of Chassigny's greatest contributions to planetary science lies in its noble-gas and volatile isotope geochemistry (Ott, 1988; Mathew & Marti, 2001; Mukhopadhyay, 2012; Péron et al., 2022):

• Whereas shergottite impact-melt glasses (such as EETA 79001) trap modern Martian atmospheric gas (characterized by high ¹²⁹Xe/¹³²Xe ≈ 2.6 and ⁴⁰Ar/³⁶Ar ≈ 1,900), Chassigny preserves the unfractionated interior mantle volatile reservoir of Mars (Chass-S), with a low, primordial ¹²⁹Xe/¹³²Xe ratio of 1.03 and solar-wind-like Xenon (Xe-S).
• For decades, planetary accretion models assumed that because Mars formed rapidly within 2 to 4 million years of Calcium–Aluminum-rich Inclusions (CAIs)—while the solar protoplanetary nebula was still present—the Martian interior ingassed solar nebular hydrogen, neon, and krypton, which later outgassed to form the modern solar-like Martian atmosphere, whereas chondritic asteroids arrived only after magma-ocean solidification as a "late veneer."
• In 2022, Sandrine Péron, Sujoy Mukhopadhyay, and colleagues performed high-precision step-crushing noble-gas mass spectrometry on Chassigny and made a surprising discovery: the krypton isotope ratios inside Chassigny's mantle inclusions (⁸⁶Kr/⁸⁴Kr, ⁸³Kr/⁸⁴Kr, ⁷⁸Kr/⁸⁴Kr) are purely chondritic (matching carbonaceous and ordinary chondrites), NOT solar—whereas the modern Martian atmosphere has solar krypton! This inverted the classic planetary accretion sequence: it proved that chondritic volatile-rich planetesimals were delivered to Mars very early during its main magma-ocean accretion phase (< 4 Myr) to seed the interior mantle preserved in Chassigny, whereas Mars subsequently captured a primary solar-nebula atmosphere after mantle crystallization that was partially stripped by solar extreme-ultraviolet hydrodynamic escape.

1. Ott, U. (1988). Noble gases in SNC meteorites: Shergotty, Nakhla, Chassigny. Geochimica et Cosmochimica Acta, 52(7), 1937–1948. https://doi.org/10.1016/0016-7037(88)90017-890017-8)
2. McSween, H. Y., Jr. (1994). What we have learned about Mars from SNC meteorites. Meteoritics, 29(6), 757–779. https://doi.org/10.1111/j.1945-5100.1994.tb01092.x
3. Mathew, K. J., & Marti, K. (2001). Early evolution of Martian volatiles: Nitrogen and noble gas components in ALH84001 and Chassigny. Journal of Geophysical Research: Planets, 106(E1), 1401–1422. https://doi.org/10.1029/2000JE001282
4. Beck, P., Gillet, P., El Goresy, A., & Mostefaoui, S. (2006). Timescales of shock processes in chondritic and martian meteorites. Nature, 442(7104), 784–787. https://doi.org/10.1038/nature05025
5. Treiman, A. H., Dyar, M. D., McCanta, M., Noble, S. K., & Pieters, C. M. (2007). Martian dunite NWA 2737: Petrographic constraints on geological history, shock events, and olivine color. Journal of Geophysical Research: Planets, 112(E4), E04002. https://doi.org/10.1029/2006JE002777
6. 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
7. Péron, S., Mukhopadhyay, S., Kurz, M. D., & Graham, D. W. (2022). Mars's interior krypton isotopic composition points to early accretion of chondritic volatiles before nebular gas capture. Science, 377(6603), 320–324. https://doi.org/10.1126/science.abk1175