The trapped components previously measured in the nakhlites MIL 03346 (40Ar/36Ar = 1425 ± 230)26 and Yamato 000593 (40Ar/36Ar = 1502 ± 159)27, as well as Chassigny (40Ar/36Ar = 1452 ± 168)27 are commensurate with our analyses (Fig. 4). All of these meteorites crystallized at ca. 1300–1400 Ma (Fig. 5 and ref. 28), and therefore the common trapped component likely reflects the isotopic composition of the Martian atmosphere at that time. The weighted average of these measurements is 1511 ± 74 (2σ) for the 40Ar/36Ar trapped component—the most precise constraint yet obtained for the mid-Amazonian atmosphere of Mars. This value is more precise than, but within analytical uncertainty of, measurements by the Curiosity rover for the present-day Martian atmosphere (1900 ± 600, 2σ)29. It is, however, distinct from the paleo-atmospheric 40Ar/36Ar recorded by the shergottite meteorites (40Ar/36Ar of 1800 ± 100)30, which yield 40Ar/39Ar cooling ages of ca. 180–600 Ma21. Together, these data therefore suggest that the atmospheric 40Ar/36Ar of Mars has increased significantly (Δ 40Ar/36Ar of 300 ± 130) over the last 1300 Ma. The 40Ar/36Ar value of Earth’s atmosphere has likewise increased over time31, and reflects degassing of radiogenic 40Ar from the interior. In the case of Mars, the change in 40Ar/36Ar also provides constraints on atmospheric loss through time32.

The 40Ar/39Ar ages show the nakhlites were erupted between 1416 ± 7 and 1322 ± 10 Ma (2σ, i.e., mid-Amazonian), a period spanning 93 ± 12 Ma (Table 1 and Fig. 6). Our interpretation is that the nakhlites have sampled a layered volcanic sequence, with the 40Ar/39Ar ages defining stratigraphic position (Fig. 6). Two sets of meteorites (respectively: Nakhla and MIL 03346; NWA 5790 and Yamato 000583) are not temporally resolvable at the 2σ level (Fig. 6); however, geochemical and petrographic differences between them15 suggest they may also sample separate flows. A layered volcanic sequence is consistent with the subtle but significant mineralogical, petrological, geochemical and isotopic differences between nakhlites13,14,15, 33, which we interpret as due to changes in magma composition between related—but temporally distinct—extrusive units from the same volcano (Fig. 6). This scenario is unsurprising if one considers an analogy of a moderate-sized bolide hitting a plume-fed shield volcano on Earth (e.g., Mauna Kea, Hawai’i) and the number of chemically similar yet temporally distinct igneous units that could be ejected.

Table 1 40Ar/39Ar ages for the nakhlites Full size table

Fig. 6 Stratigraphic model for the nakhlite meteorites. a Summary of 40Ar/39Ar age data. Each meteorite has multiple aliquots with highly reproducible plateau ages (red squares). Bold black squares and horizontal grey bars represent weighted mean ages. The 40Ar/39Ar results indicate that the nakhlites were erupted in at least four temporally discrete eruptions, with volcanic activity spanning 93 ± 12 Ma. All uncertainties are 2σ. b Schematic cross-section for a layered lava flow sequence, with nakhlite stratigraphic relationships and outline of post-impact structure Full size image

Our stratigraphic model is consistent with the geology of Martian volcanoes. High-resolution satellite imagery has revealed sequences of lava flows, with individual layers typically 4–26 m thick34, 35, commensurate with the planet’s relatively low gravity that allows for the eruption of numerous thin lava flows that extend for long distances34. Our model of a layered volcanic sequence (Fig. 6) differs from a previous interpretation of the nakhlites that invoked sampling from a single thick flow/intrusive unit18, 36, 37. Such a model where the nakhlites are from a single thick flow or intrusion requires that all of the meteorites would have the same cooling age, which is inconsistent with our 40Ar/39Ar data (Fig. 6).

We also note that the two Yamato meteorites have 40Ar/39Ar ages that differ by 70 ± 10 Ma (2σ). This age difference is incompatible with the hypothesis that these were fall-paired stones38 (i.e., two parts of a formerly larger meteorite that have become separated, e.g., during atmospheric entry). Despite being found in the same Antarctic field season, there is, however, no a priori scientific reason for why these stones should be fall-paired, particularly when considering that the Yamato meteorite stranding surface is the largest in Antarctica, covering an area of 4000 km2, and that the glaciers which feed into the stranding area extend for a further 500 km upstream39. Yamato 000593 and Yamato 000749 could therefore have fallen anywhere in the catchment area, only to be brought to a similar part of the ice field by the action of Antarctic glaciers.

Our 40Ar/39Ar and cosmogenic exposure ages inform about the provenance of the nakhlites on the surface of Mars, including their ejection crater, and properties of their source volcano. More than seven different Martian craters have been suggested as potential sources for the nakhlites40,41,42. However, when we use the recent re-mapping of the Martian surface by NASA43, we find that only one of these craters is situated in a mid-Amazonian volcanic terrain compatible with our 40Ar/39Ar results (Fig. 1 and Supplementary Table 4). This crater, which is located at 130.799°E, 29.674°N, has preserved ejecta rays42 that are indicative of a recent impact event, which in turn is consistent with the cosmogenic exposure age (10.7 ± 0.8 Ma) obtained for the nakhlites (Fig. 3). This crater has a diameter of 6.5 km, which is large enough for the impact to have had sufficient energy to have excavated and ejected material beyond Mars’ orbit44.

We have investigated high-resolution satellite images from the walls of this crater, which provide clear evidence for multiple layers (Fig. 7); similar layers elsewhere on Mars are interpreted as lava flows35. During an impact event the ejecta that exceeds escape velocity comes from the near-surface of Mars, up to a maximum depth of 0.2 times the impactor’s radius17. The 6.5 km diameter of this crater42 requires the impacting bolide to have a radius of ca. 200 m (Supplementary Table 4). Thus, the spallation zone for this crater is from the near-surface to a depth of 40 m (i.e., 0.2 × 200 m). If the identified crater is not the source of the nakhlites, there are other rayed craters41 situated on Amazonian volcanic terrains that have diameters of up to 10 km, which increases the maximum spallation depth to 66 m (Supplementary Table 4).

Fig. 7 A potential nakhlite source crater. This crater is located on the Elysium lava plains, to the northwest of the Elysium shield volcano on Mars, at 130.799°E, 29.674°N (Fig. 1). a Overview of the crater, which is 6.5 km in diameter42, large enough to have ejected Martian rocks towards Earth44. THEMIS image V13713007, Band 3, NASA/ASU. Black rectangles indicate the locations for subsequent images. b–d Detail of the northwestern, northeastern and southern crater rim, respectively, representing parts of HiRISE image ESP 017997_2100, NASA/JPL/University of Arizona. These images show numerous sub-horizontal layers, which are interpreted as lava flows (e.g., ref. 35 for similar features elsewhere on Mars). White arrows indicate prominent layers; see inset for detailed view. Solar illumination is from the west in all images Full size image

If the nakhlite samples analysed span the full spallation depth (near-surface to 40–66 m), then considering the time taken to accumulate the lava sequence (93 ± 12 Ma), approximately 0.4–0.7 m of lava was extruded every 1 Ma. If the nakhlites analysed do not span the full spallation depth, this rate represents an upper bound, whereas if the source crater is located on the volcano flank, or on lava plains, then the calculated rate represents a lower bound, as eruptions are likely to be more voluminous and frequent closer to the centre of an edifice. In all scenarios, the growth rate is three orders of magnitude lower than is observed for a terrestrial plume-derived volcano (e.g., 8400 ± 2600 m Ma−1 at Mauna Kea, Hawai’i)10.

Extrapolating a growth rate of 0.4–0.7 m Ma−1 to the full 6–22 km thickness of the massive Tharsis and Elysium volcanoes7 would necessitate volcanic activity over a timespan exceeding the age of the solar system (>31,000 Ma). Therefore, the eruption rate must have been greater prior to the mid-Amazonian. Both crater counting and planetary heat-flow models support this conclusion, as they demonstrate that the rate of volcanism during the Noachian and Hesperian (4500–3000 Ma) was at least 2–10 times greater than in the Amazonian (<3000 Ma)3, 4, 7. Our data compare favourably with remote-sensing crater-counting studies of Martian volcanoes45,46,47, which indicate that volcanism occurred at much lower rates in the recent past, compared to early in the planet’s history. For example, over the last 300 Ma the maximum effusion rate at the Arisa Mons volcano47 was between 1 and 8 km3 Ma−1—well below the eruption rate of 270 km3 Ma−1 averaged over the 3400 Ma duration of the volcano (Supplementary Table 1).

Our data thus quantify the growth rate of a mid-Amazonian volcano, and robustly show via radioisotopic dating that the minimum lifespan of at least one Martian edifice (93 ± 12 Ma; Fig. 2) far exceeds that of any terrestrial counterpart. This work also resolves the temporal relationships between the nakhlite meteorites, which sample a series of lava flows (Fig. 6), rather than a single thick intrusion/flow18, 36, 37. The high-precision data also provide an ‘absolute’ temporal anchor for the surface of Mars that could underpin a Martian crater counting calibration model, providing the provenance of the nakhlites (i.e., location of their source crater, Fig. 7) can be confirmed.