Shergottite

Shergottites are the largest petrological group of Martian meteorites—constituting the "S" in the classic SNC (Shergottite, Nakhlite, Chassignite) meteorite clan and accounting for more than 80% (~300+ paired specimens) of all known Martian rocks recovered on Earth. Named after the Shergotty meteorite, a 5-kilogram basaltic achondrite that fell near Sherghati in the Gaya district of Bihar, India, on August 25, 1865, shergottites are mafic to ultramafic volcanic and subvolcanic igneous rocks dominated by pigeonite and augite clinopyroxenes, olivine, and shock-amorphized plagioclase glass (maskelynite) (McSween, 1994; Udry et al., 2020). Because the majority of shergottites have young igneous crystallization ages of 150 to 600 million years (Ma), they initially baffled meteoriticists—who expected asteroids to have cooled 4.5 billion years ago—until Donald D. Bogard and Pratt Johnson (1983) discovered that pockets of impact-melt glass inside the Antarctic shergottite EETA 79001 contain trapped noble gases and nitrogen (⁴⁰Ar/³⁶Ar, ¹²⁹Xe/¹³²Xe, and ¹⁵N/¹⁴N) matching the exact elemental and isotopic composition of the Martian atmosphere measured in situ by the Viking 1 and 2 landers in 1976.

Based on crystal texture, mineralogy, and modal abundance of mafic phenocrysts, shergottites are subdivided into three primary petrological sub-classes (Goodrich, 2002; Udry et al., 2020):

Independently of their crystal texture, shergottites fall into three distinct geochemical lineages based on their light-rare-earth-element (LREE, La/Yb) patterns, initial radiogenic strontium (⁸⁷Sr/⁸⁶Sr), initial neodymium (ε¹⁴³Nd), hafnium (ε¹⁷⁶Hf), and magmatic oxygen fugacity (fO₂) relative to the Iron–Wüstite (IW) or Quartz–Fayalite–Magnetite (QFM) buffer (Borg & Draper, 2003; Herd et al., 2002):

1. Depleted Shergottites (e.g., QUE 94201, Tissint, DaG 476): Strongly depleted in incompatible LREEs ([La/Yb]_CI ≈ 0.1–0.3), characterized by highly radiogenic ε¹⁴³Nd (+35 to +50), unradiogenic ⁸⁷Sr/⁸⁶Sr (~0.701), and reduced magmatic oxidation states (IW + 0 to QFM − 3.5).
2. Enriched Shergottites (e.g., Shergotty, Zagami, Los Angeles, LAR 06319): Flat to slightly LREE-enriched ([La/Yb]_CI ≈ 0.8–1.2), characterized by negative ε¹⁴³Nd (−6 to −8), radiogenic ⁸⁷Sr/⁸⁶Sr (~0.722), and oxidized magmatic states (QFM − 1 to QFM + 0.5)—up to 3 log units more oxidized than depleted shergottites.
3. Intermediate Shergottites (e.g., EETA 79001, ALHA 77005): Bridge the isotopic and fO₂ gap between the depleted and enriched endmembers, produced by binary mixing inside the Martian mantle or crust.

Crucially, short-lived ¹⁴⁶Sm–¹⁴²Nd (t_1/2 = 103 Myr) and ¹⁸²Hf–¹⁸²W (t_1/2 = 8.9 Myr) extinct-radionuclide systematics prove that the depleted and enriched shergottite reservoirs separated at ~4.50 to 4.46 Ga during the crystallization of Mars's primordial magma ocean—and remained physically isolated from one another for more than 4 billion years because Mars lacks Earth-style convective plate tectonics to homogenize its mantle!

For decades, planetary scientists debated the "Shergottite Age Paradox" (Nyquist et al., 2001): how could >80% of Martian meteorites yield Sm–Nd, Rb–Sr, Lu–Hf, and U–Pb crystallization ages of 150 to 600 Ma (Late Amazonian) when ~50% of Mars's surface consists of >3.5 Ga Noachian and Hesperian highlands?

• High-spatial-resolution baddeleyite (ZrO₂) in-situ U–Pb dating and Pb-isotope modeling resolved the debate in favor of true Late Amazonian igneous crystallization (Moser et al., 2013), while the discovery of NWA 7635 and NWA 8159—depleted augite-rich shergottites with Sm–Nd crystallization ages of 2.40 ± 0.14 Ga (Early Amazonian)—demonstrated that a single long-lived depleted mantle plume erupted repeatedly at the same volcanic province in Tharsis or Elysium for over 2 billion years (Lapen et al., 2017).
• Cosmic-Ray Exposure (CRE) Launch Clusters: Measuring cosmogenic nuclides (³He, ²¹Ne, ³⁸Ar, ¹⁰Be, and ³⁶Cl) produced while the rocks traveled in interplanetary space between Mars (v_esc = 5.03 km s⁻¹) and Earth shows that all 300+ shergottites were ejected by just 5 to 7 discrete impact events over the last 20 million years—clustered at ~0.7 Ma (EETA 79001), ~1.1 Ma (Tissint + depleted olivine-phyric cluster, launched from a single crater such as Mojave or Tooting in Tharsis), ~2.5–3.0 Ma (Shergotty/Zagami), ~4.0 Ma, and ~19 Ma (NWA 8159). Because young Amazonian lava flows in Tharsis and Elysium are coherent, unbrecciated, high-acoustic-impedance crystalline basalts, they launch intact blocks above 5.03 km s⁻¹ inside the near-surface spallation zone (shock pressures of 25–45 GPa, converting plagioclase to maskelynite and high-pressure stishovite/seifertite without melting the rock) far more efficiently than weak, porous Noachian regolith (Head et al., 2002).

When the Tissint olivine-phyric shergottite (crystallized at 574 ± 20 Ma, ejected at 0.7–1.1 Ma) fell in the Oued Drâa valley of Morocco on July 18, 2011, its fragments were recovered within weeks before terrestrial rain or soil microbes could contaminate it:

• Within Tissint's black shock-melt glass veins and micro-fractures, geochemists discovered deuterium-enriched water (δD up to +4,000‰), oxidized sulfate and carbonate salts, and complex organomagnesium and macromolecular carbon compounds (δ¹³C = −13‰ to −33‰) (Lin et al., 2014; Steele et al., 2012).
• NanoSIMS and Raman spectroscopy proved that while the primary macromolecular reduced carbon precipitated abiotically from magmatic C–O–H fluids during lava cooling, the impact that launched Tissint at ~1.1 Ma simultaneously injected near-surface Martian groundwater brines into fractures of the rock—demonstrating that transient liquid water and abiotic organic carbon persisted in the shallow Martian subsurface into the latest Amazonian epoch.

1. Bogard, D. D., & Johnson, P. (1983). "Martian gases in an Antarctic meteorite?" Science, 221(4611), 651–654. https://doi.org/10.1126/science.221.4611.651
2. Head, J. N., Melosh, H. J., & Ivanov, B. A. (2002). "Martian meteorite launch: High-speed ejecta from small craters." Science, 298(5599), 1752–1756. https://doi.org/10.1126/science.1077483
3. Moser, D. E., Chamberlain, K. R., Tait, K. T., Schmitt, A. K., Darling, J. R., Barker, I. R., & Hyde, B. C. (2013). "Solving the Martian meteorite age conundrum using micro-baddeleyite and launch-generated zircon." Nature, 499(7459), 454–457. https://doi.org/10.1038/nature12341
4. Lapen, T. J., Righter, M., Andreasen, R., Irving, A. J., Satkoski, A. M., Beard, B. L., et al. (2017). "Two billion years of magmatism recorded from a single Mars meteorite ejection site." Science Advances, 3(2), e1600921. https://doi.org/10.1126/sciadv.1600921
5. Udry, A., Howarth, G. H., Herd, C. D. K., Day, J. M. D., Lapen, T. J., & Filiberto, J. (2020). "What Martian meteorites reveal about the interior and surface of Mars." Journal of Geophysical Research: Planets, 125(12), e2020JE006523. https://doi.org/10.1029/2020JE006523