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Introduction

A key objective of ongoing and future missions to Mars is determining when the planet’s hydrological cycle was active in the geologic past. For much of the Amazonian period (2.9 Ga to present), Mars’s surface was cold and arid with a thin atmosphere, making liquid water unstable at the surface (e.g., Carr and Head, 2010

Carr, M.H., Head III, J.W. (2010) Geologic history of Mars. Earth and Planetary Science Letters 294, 185–203. https://doi.org/10.1016/j.epsl.2009.06.042

). However, minerals that formed by aqueous alteration of Amazonian-aged rocks, which have travelled to Earth as meteorites, show that liquid water was available at some points during this time period (Gooding et al., 1991

Gooding, J.L., Wentworth, S.J., Zolensky, M.E. (1991) Aqueous alteration of the Nakhla meteorite. Meteoritics 26, 135–143. https://doi.org/10.1111/j.1945-5100.1991.tb01029.x

; Treiman et al., 1993

Treiman, A.H., Barrett, R.A., Gooding, J.L. (1993) Preterrestrial aqueous alteration of the Lafayette (SNC) meteorite. Meteoritics 28, 86–97. https://doi.org/10.1111/j.1945-5100.1993.tb00251.x

).

The nakhlite meteorites are a group of igneous rocks that crystallised between 1416 ± 4 and 1322 ± 5 Ma, and were subsequently ejected by an impact event at 10.7 ± 0.4 Ma (Cohen et al., 2017

Cohen, B.E., Mark, D.F., Cassata, W.S., Lee, M.R., Tomkinson, T., Smith, C.L. (2017) Taking the pulse of Mars via dating of a plume-fed volcano. Nature Communications 8, 640. https://doi.org/10.1038/s41467-017-00513-8

). Several nakhlites contain aqueous alteration products (e.g., Treiman, 2005

Treiman, A.H. (2005) The nakhlite meteorites: Augite-rich igneous rocks from Mars. Geochemistry 65, 203–270. https://doi.org/10.1016/j.chemer.2005.01.004

) that (1) are crosscut by fusion crusts that formed upon atmospheric entry to Earth (e.g., Gooding et al., 1991

Gooding, J.L., Wentworth, S.J., Zolensky, M.E. (1991) Aqueous alteration of the Nakhla meteorite. Meteoritics 26, 135–143. https://doi.org/10.1111/j.1945-5100.1991.tb01029.x

; Treiman et al., 1993

Treiman, A.H., Barrett, R.A., Gooding, J.L. (1993) Preterrestrial aqueous alteration of the Lafayette (SNC) meteorite. Meteoritics 28, 86–97. https://doi.org/10.1111/j.1945-5100.1993.tb00251.x

) and (2) have D/H ratios indicative of fluid equilibration with the Martian atmosphere (Leshin et al., 1996

Leshin, L.A., Epstein, S., Stolper, E.M. (1996) Hydrogen isotope geochemistry of SNC meteorites. Geochimica et Cosmochimica Acta 60, 2635–2650. https://doi.org/10.1016/0016-7037(96)00122-6

), requiring that these secondary phases formed via interaction of the igneous rocks with liquid water on Mars (e.g., Leshin et al., 1996

Leshin, L.A., Epstein, S., Stolper, E.M. (1996) Hydrogen isotope geochemistry of SNC meteorites. Geochimica et Cosmochimica Acta 60, 2635–2650. https://doi.org/10.1016/0016-7037(96)00122-6

).

Lafayette is one such member of the nakhlites. It is a 0.8 kg olivine-rich pyroxenite find with negligible evidence for terrestrial weathering (Graham et al., 1985

Graham, A.L., Hutchison, R., Bevan, A.W.R. (1985) Catalogue of meteorites. Fourth Edition, University of Arizona Press, Tucson.

; Treiman et al., 1993

Treiman, A.H., Barrett, R.A., Gooding, J.L. (1993) Preterrestrial aqueous alteration of the Lafayette (SNC) meteorite. Meteoritics 28, 86–97. https://doi.org/10.1111/j.1945-5100.1993.tb00251.x

; Lee et al., 2015

Lee, M.R., Tomkinson, T., Hallis, L.J., Mark, D.F. (2015) Formation of iddingsite veins in the martian crust by centripetal replacement of olivine: Evidence from the nakhlite meteorite Lafayette. Geochimica et Cosmochimica Acta 154, 49–65. https://doi.org/10.1016/j.gca.2015.01.022

). Olivine grains and the mesostasis of Lafayette host aqueous alteration products that include K-bearing hydrous silicates (e.g., Piercy et al., 2022

Piercy, J.D., Bridges, J.C., Hicks, L.J. (2022) Carbonate dissolution and replacement by odinite and saponite in the Lafayette nakhlite: Part of the CO₂-CH₄ cycle on Mars? Geochimica et Cosmochimica Acta 326, 97–118. https://doi.org/10.1016/j.gca.2022.02.003

, and references therein; see Supplementary Information). We refer to this alteration mineral assemblage as “iddingsite”. The focus of the present study is to determine the age of iddingsite in the olivine-hosted veins (Fig. 1), which comprise 2.3–2.7 volume percent of Lafayette (Lee et al., 2015

Lee, M.R., Tomkinson, T., Hallis, L.J., Mark, D.F. (2015) Formation of iddingsite veins in the martian crust by centripetal replacement of olivine: Evidence from the nakhlite meteorite Lafayette. Geochimica et Cosmochimica Acta 154, 49–65. https://doi.org/10.1016/j.gca.2015.01.022

).

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The question of when Lafayette and the other nakhlites were exposed to liquid water on Mars remains unresolved. The most widely cited constraint derives from the Rb-Sr systematics of acid-leachates from the meteorites Lafayette and Yamato (Y) 000593 (Shih et al., 1998

Shih, C.-Y., Nyquist, L.E., Reese, Y., Wiesmann, H. (1998) The Chronology of the Nakhlite, Lafayette: Rb-Sr and Sm-Nd Isotopic Ages. Lunar and Planetary Science Conference XXIX, abstract 1145.

; Misawa et al., 2005

Misawa, K., Shih, C.-Y., Wiesmann, H., Garrison, D.H., Nyquist, L.E., Bogard, D.D. (2005) Rb-Sr, Sm-Nd and Ar-Ar isotopic systematics of Antarctic nakhlite Yamato 000593. Antarctic Meteorite Research 18, 133–151.

). These experiments were designed to measure the isotopic systematics of the primary igneous minerals. The acid leach component was meant to remove the iddingsite and any terrestrial weathering products, but was not designed to extract chronological information. Nonetheless, these measurements have also been used to extract apparent, two-point isochron ages of 674 ± 68 Ma and 653 ±80 Ma (2σ, recalculated; see Supplementary Information for Rb-Sr systematics and ⁴⁰Ar/³⁹Ar methods) for Lafayette (Shih et al., 1998

Shih, C.-Y., Nyquist, L.E., Reese, Y., Wiesmann, H. (1998) The Chronology of the Nakhlite, Lafayette: Rb-Sr and Sm-Nd Isotopic Ages. Lunar and Planetary Science Conference XXIX, abstract 1145.

) and Y000593 (Misawa et al., 2005

Misawa, K., Shih, C.-Y., Wiesmann, H., Garrison, D.H., Nyquist, L.E., Bogard, D.D. (2005) Rb-Sr, Sm-Nd and Ar-Ar isotopic systematics of Antarctic nakhlite Yamato 000593. Antarctic Meteorite Research 18, 133–151.

), respectively. With an isochron defined by two points, there is no way to test for contamination or isotopic disturbance, the latter being particularly important as the leaching procedure may produce Rb/Sr fractionation (Clauer et al., 1993

Clauer, N., Chaudhuri, S., Kralik, M., Bonnot-Courtois, C. (1993) Effects of experimental leaching on Rb-Sr and K-Ar isotopic systems and REE contents of diagenetic illite. Chemical Geology 103, 1–16. https://doi.org/10.1016/0009-2541(93)90287-S

). And while these two datasets have been aggregated to estimate a single ‘date’ for nakhlite alteration (Borg and Drake, 2005

Borg, L., Drake, M.J. (2005) A review of meteorite evidence for the timing of magmatism and of surface or near‐surface liquid water on Mars. Journal of Geophysical Research: Planets 110, E12S03. https://doi.org/10.1029/2005JE002402

), they are not consistent with a single isochron (see Supplementary Information), which raises questions about the overall chronological significance of these data. Previous work applying the ⁴⁰K-⁴⁰Ar chronometer to Lafayette is also ambiguous, suggesting that the nakhlites interacted with aqueous fluids sometime between 670 and 0 Ma (Swindle et al., 2000

Swindle, T.D., Treiman, A.H., Lindstrom, D.J., Burkland, M.K., Cohen, B.A., Grier, J.A., Li, B., Olson, E.K. (2000) Noble gases in iddingsite from the Lafayette meteorite: Evidence for liquid water on Mars in the last few hundred million years. Meteoritics & Planetary Science 35, 107–115. https://doi.org/10.1111/j.1945-5100.2000.tb01978.x

). These dates were likely impacted by inhomogeneous K distribution between the distinct aliquots of material measured for their ⁴⁰K and ⁴⁰Ar content.

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Methods

We revisited the age of iddingsite in Lafayette using the ⁴⁰Ar/³⁹Ar technique, a variant of the ⁴⁰K-⁴⁰Ar dating method used by Swindle et al. (2000)

Swindle, T.D., Treiman, A.H., Lindstrom, D.J., Burkland, M.K., Cohen, B.A., Grier, J.A., Li, B., Olson, E.K. (2000) Noble gases in iddingsite from the Lafayette meteorite: Evidence for liquid water on Mars in the last few hundred million years. Meteoritics & Planetary Science 35, 107–115. https://doi.org/10.1111/j.1945-5100.2000.tb01978.x

. We obtained 0.216 g of Lafayette from the Smithsonian Institution, sourced from an interior core >20 cm from the fusion crust. Iddingsite was physically separated from the host olivine (see Supplementary Information). During the neutron irradiation required for ⁴⁰Ar/³⁹Ar dating, the ³⁹Ar atoms produced recoil with an average distance of 0.08 μm in silicate minerals (Turner and Cadogan, 1974

Turner, G., Cadogan, P.H. (1974) Possible effects of ³⁹Ar recoil in ⁴⁰Ar-³⁹Ar dating. Proceedings of the Fifth Lunar Conference 2, 1601–1615.

). Thus, fine-grained materials with high surface area-to-volume ratios, like the iddingsite studied here, may lose ³⁹Ar during the irradiation process, resulting in spuriously old ages (Onstott et al., 1995

Onstott, T.C., Miller, M.L., Ewing, R.C., Arnold, G.W., Walsh, D.S. (1995) Recoil refinements: Implications for the ⁴⁰Ar/³⁹Ar dating technique. Geochimica et Cosmochimica Acta 59, 1821–1834. https://doi.org/10.1016/0016-7037(95)00085-E

). The dating of fine-grained materials using the ⁴⁰Ar/³⁹Ar technique thus requires a non-conventional approach, which consists of micro-encapsulation to prevent loss of recoiled isotopes (Dong et al., 1997

Dong, H., Hall, C.M., Halliday, A.N., Peacor, D.R., Merriman, R.J., Roberts, B. (1997) ⁴⁰Ar/³⁹Ar illite dating of Late Caledonian (Acadian) metamorphism and cooling of K-bentonites and slates from the Welsh Basin, U.K. Earth and Planetary Science Letters 150, 337–351. https://doi.org/10.1016/S0012-821X(97)00100-3

). To achieve this, we encapsulated twelve ∼1 μg aliquots of the hand-picked iddingsite in evacuated, flame-sealed quartz glass capsules prior to neutron irradiation. Following irradiation, we measured the Ar isotopic composition of the iddingsite in two stages. First, the glass capsule was cracked under vacuum to measure any recoiled ³⁷Ar and ³⁹Ar. Second, the samples recovered from the cracked glass tubes were then fused with a diode laser. The ³⁷Ar and ³⁹Ar measurements from the cracked tubes were added to the total fusion isotope measurements (³⁶Ar, ³⁷Ar, ³⁸Ar, ³⁹Ar, and ⁴⁰Ar). Data were corrected for backgrounds, mass discrimination, radioactive decay since irradiation, cosmogenic Ar, and trapped Martian atmospheric Ar. Full analytical procedure details are provided in the Supplementary Information.

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Results

Upon cracking the quartz capsules, we observed that significant ³⁹Ar and ³⁷Ar recoil occurred during neutron irradiation into the quartz capsule of 16 ± 2 % and 13 ± 2 %, respectively. When the Ar isotopic data from the two-stage analytical procedure are combined, the data define a normal age distribution with a mean ⁴⁰Ar/³⁹Ar age of 741.8 ± 15.0/15.2 Ma (analytical precision/full external precision at 2σ). There is no statistically significant difference between the individual aliquot ages, defining a mean square weighted deviation (MSWD) of 1.2 and a p-value of 0.3 (Fig. 2). This age for the iddingsite in Lafayette agrees within uncertainty with the previously estimated age from Rb-Sr acid-leachates (Shih et al., 1998

Shih, C.-Y., Nyquist, L.E., Reese, Y., Wiesmann, H. (1998) The Chronology of the Nakhlite, Lafayette: Rb-Sr and Sm-Nd Isotopic Ages. Lunar and Planetary Science Conference XXIX, abstract 1145.

) but is significantly more precise. It is slightly older than the wide age range previously estimated from K-Ar measurements (Swindle et al., 2000

Swindle, T.D., Treiman, A.H., Lindstrom, D.J., Burkland, M.K., Cohen, B.A., Grier, J.A., Li, B., Olson, E.K. (2000) Noble gases in iddingsite from the Lafayette meteorite: Evidence for liquid water on Mars in the last few hundred million years. Meteoritics & Planetary Science 35, 107–115. https://doi.org/10.1111/j.1945-5100.2000.tb01978.x

).

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Discussion

Potential for diffusive loss of Ar. Because we conducted total fusion ⁴⁰Ar/³⁹Ar measurements on the iddingsite aliquots, we did not recover information about the spatial distribution of radiogenic ⁴⁰Ar in the constituent phases. Therefore, we must evaluate the possibility that the aqueous alteration in Lafayette occurred earlier than 742 Ma, and that heating events at 742 Ma, or later, caused partial loss of radiogenic ⁴⁰Ar. The maximum possible ⁴⁰Ar loss is 44 %, based on the difference between the measured iddingsite age and Lafayette’s igneous crystallisation age (∼1322 Ma; Cohen et al., 2017

Cohen, B.E., Mark, D.F., Cassata, W.S., Lee, M.R., Tomkinson, T., Smith, C.L. (2017) Taking the pulse of Mars via dating of a plume-fed volcano. Nature Communications 8, 640. https://doi.org/10.1038/s41467-017-00513-8

).

The bulk ⁴⁰Ar/³⁹Ar systematics of the nakhlites reported by Cohen et al. (2017)

Cohen, B.E., Mark, D.F., Cassata, W.S., Lee, M.R., Tomkinson, T., Smith, C.L. (2017) Taking the pulse of Mars via dating of a plume-fed volcano. Nature Communications 8, 640. https://doi.org/10.1038/s41467-017-00513-8

preclude protracted heating of Lafayette during its residence in Mars’s crust. Therefore, we consider three aspects of Lafayette’s history that could have induced ⁴⁰Ar loss: (1) heating during an impact event, such as the impact that ejected Lafayette from Mars ca. 10.7 Ma; (2) heating during Lafayette’s transit in space before falling to Earth; and (3) heating during entry into Earth’s atmosphere. To assess the influence of heating during these events on the ⁴⁰Ar/³⁹Ar age, we modelled the diffusive loss of Ar from iddingsite. Following others who have applied ⁴⁰Ar/³⁹Ar dating to authigenic clays (e.g., Clauer et al., 2003

Clauer, N., Zwingmann, H., Gorokhov, I.M. (2003) Postdepositional Evolution of Platform Claystones Based on a Simulation of Thermally Induced Diffusion of Radiogenic ⁴⁰Ar from Diagenetic Illite. Journal of Sedimentary Research 73, 58–63. https://doi.org/10.1306/061002730058

), we assume the diffusion kinetics of Ar in muscovite are comparable to those in the K-bearing phases comprising iddingsite and use the kinetic parameters for Ar diffusion in muscovite reported by Harrison et al. (2009)

Harrison, T.M., Célérier, J., Aikman, A.B., Hermann, J., Heizler, M.T. (2009) Diffusion of ⁴⁰Ar in muscovite. Geochimica et Cosmochimica Acta 73, 1039–1051. https://doi.org/10.1016/j.gca.2008.09.038

. The grain size of the iddingsite in Lafayette is not well constrained, but it cannot be greater than 10–20 μm based on the width of the olivine-hosted veins. We therefore explored models with diffusion radii between 0.01 and 10 μm, assuming the grain size defines the diffusion length scale.

In the case of the 10.7 Ma impact event and/or atmospheric entry, we use approximations for fractional loss during a square pulse heating event (Fig. 3a; see Supplementary Information). The duration of heating for both events is geologically brief: up to several hours after ejection (e.g., Fritz et al., 2005

Fritz, J., Artemieva, N., Greshake, A. (2005) Ejection of Martian meteorites. Meteoritics & Planetary Science 40, 1393–1411. https://doi.org/10.1111/j.1945-5100.2005.tb00409.x

) or hundreds of seconds during atmospheric entry (e.g., Parnell et al., 2008

Parnell, J., Mark, D., Brandstätter, F. (2008) Response of sandstone to atmospheric heating during the STONE 5 experiment: Implications for the palaeofluid record in meteorites. Icarus 197, 282–290. https://doi.org/10.1016/j.icarus.2008.04.014

). For these scenarios, we therefore assume radiogenic ⁴⁰Ar production during heating is negligible. However, in the case of Lafayette’s 10.7 Myr transit in space, the production of radiogenic ⁴⁰Ar is nontrivial. We therefore use a solution for radiogenic ⁴⁰Ar production and diffusive loss during an isothermal heating event, which incorporates the initial age prior to heating. Although Lafayette’s temperature would have varied as it transited the inner Solar System, we use the isothermal heating solution as an end member to estimate the minimum temperatures that would yield diffusive loss (Fig. 3b; see Supplementary Information).

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For an impact and atmospheric entry, temperatures >∼250 °C would need to be sustained for several days, or longer, for Ar loss to exceed 1 % (Fig. 3a). Lower temperatures are required for Ar loss during space transit due to its longer duration (Fig. 3b). For example, for a grain radius of 0.01 μm, temperatures would need to exceed ∼160 °C for >1 % diffusive loss of Ar to take place. Higher temperatures during space transit are permitted without diffusive loss over durations <10.7 Myr.

When paired with other observations, these first-order thermal constraints suggest that an age for the Lafayette iddingsite significantly older than 742 Ma age is highly unlikely. For heating during impact ejection, independent estimates based on the shock petrography of Lafayette indicate that shock and post-shock temperatures were at most several tens of degrees Celsius above ambient Martian surface temperatures (e.g., Fritz et al., 2005

Fritz, J., Artemieva, N., Greshake, A. (2005) Ejection of Martian meteorites. Meteoritics & Planetary Science 40, 1393–1411. https://doi.org/10.1111/j.1945-5100.2005.tb00409.x

; Daly et al., 2019

Daly, L., Lee, M.R., Piazolo, S., Griffin, S., Bazargan, M., Campanale, F., Chung, P., Cohen, B.E., Pickersgill, A.E., Hallis, L.J., Trimby, P.W., Baumgartner, R., Forman, L.V., Bendix, G.K. (2019) Boom boom pow: Shock-facilitated aqueous alteration and evidence for two shock events in the Martian nakhlite meteorites. Science Advances 5, eaaw5549. https://doi.org/10.1126/sciadv.aaw5549

). Following ejection, the maximum temperatures that Lafayette could have experienced during space transit are 200–250 °C, which would occur if its orbit approached Mercury (Butler, 1966

Butler, C.P. (1966) Temperatures of Meteoroids in Space. Meteoritics 3, 59–70. https://doi.org/10.1111/j.1945-5100.1966.tb00355.x

). However, orbital dynamics calculations demonstrate that a prolonged duration near Mercury is extremely unlikely for a Martian meteorite that falls to Earth (e.g., Gladman, 1997

Gladman, B. (1997) Destination: Earth. Martian Meteorite Delivery. Icarus 130, 228–246. https://doi.org/10.1006/icar.1997.5828

). Therefore, Lafayette likely spent most, or all, of its 10.7 Myr space transit at temperatures well below those required to cause Ar loss (150–200 °C; Fig. 3b). Finally, we expect heating during atmospheric entry to be insignificant, considering that our samples were sourced ∼20 cm from Lafayette’s fusion crust. The Stone 5 experiments, which placed rocks on the nose of a spacecraft to simulate atmospheric entry, only documented re-entry heating >100 °C at <2 cm depth below the rocks’ fusion crust (Parnell et al., 2008

Parnell, J., Mark, D., Brandstätter, F. (2008) Response of sandstone to atmospheric heating during the STONE 5 experiment: Implications for the palaeofluid record in meteorites. Icarus 197, 282–290. https://doi.org/10.1016/j.icarus.2008.04.014

). Collectively, these observations and our calculations indicate that while partial Ar loss from Lafayette iddingsite is possible under certain conditions, it is very unlikely given other constraints about Lafayette’s history.

Significance of the ⁴⁰Ar/³⁹Ar age. Given that (1) the ⁴⁰Ar/³⁹Ar data show no significant scatter beyond what is expected from the measurement precision, (2) radiogenic ⁴⁰Ar loss from the Lafayette iddingsite is highly unlikely, and (3) the new ⁴⁰Ar/³⁹Ar date is broadly consistent with previous but significantly less precise age estimates, we interpret the ⁴⁰Ar/³⁹Ar age of 742 ± 15 Ma as recording the time when the iddingsite in Lafayette formed as a result of water–rock interaction close to the surface of Mars.

Our data show that the impact event recorded by Nakhla at ∼913 Ma (Cassata et al., 2010

Cassata, W.S., Shuster, D.L., Renne, P.R., Weiss, B.P. (2010) Evidence for shock heating and constraints on Martian surface temperatures revealed by ⁴⁰Ar/³⁹Ar thermochronometry of Martian meteorites. Geochimica et Cosmochimica Acta 74, 6900–6920. https://doi.org/10.1016/j.gca.2010.08.027

) was not coeval with the aqueous activity that led to iddingsite formation in Lafayette. The impact event at 913 Ma is the only evidence that the nakhlites have experienced impact-induced shock following cooling of the parent lavas and prior to their ejection at 10.7 Ma. The impact event at 913 Ma was likely responsible for producing the shock features in Lafayette documented by Daly et al. (2019)

Daly, L., Lee, M.R., Piazolo, S., Griffin, S., Bazargan, M., Campanale, F., Chung, P., Cohen, B.E., Pickersgill, A.E., Hallis, L.J., Trimby, P.W., Baumgartner, R., Forman, L.V., Bendix, G.K. (2019) Boom boom pow: Shock-facilitated aqueous alteration and evidence for two shock events in the Martian nakhlite meteorites. Science Advances 5, eaaw5549. https://doi.org/10.1126/sciadv.aaw5549

, and for increasing the porosity and permeability for later aqueous solutions to infiltrate grain interiors within Lafayette at 742 Ma (Lee et al., 2015

Lee, M.R., Tomkinson, T., Hallis, L.J., Mark, D.F. (2015) Formation of iddingsite veins in the martian crust by centripetal replacement of olivine: Evidence from the nakhlite meteorite Lafayette. Geochimica et Cosmochimica Acta 154, 49–65. https://doi.org/10.1016/j.gca.2015.01.022

), but did not induce the aqueous alteration of Lafayette or the nakhlites directly. The data do not preclude the occurrence of a third impact event at 742 Ma. However, such an event would have to be low enough energy to not disturb the Ar isotope systematics of primary phases in the nakhlites, but high enough energy to induce subsurface melting of localised permafrost, providing the heat source for water–rock interaction that led to alteration of the nakhlites (e.g., Changela and Bridges, 2010

Changela, H.G., Bridges, J.C. (2010) Alteration assemblages in the nakhlites: Variation with depth on Mars. Meteoritics & Planetary Science 45, 1847–1867. https://doi.org/10.1111/j.1945-5100.2010.01123.x

). Moreover, it is unlikely that three large impact events occurred at the same place on the Martian surface during the last 1 Ga (ca. 913 Ma, 742 Ma, 10.7 Ma).

An alternate and simpler explanation is that magmatism acted as a localised heat source for melting of subsurface ice and water–rock interaction during the Amazonian on Mars. On Earth, iddingsite is commonly observed in olivine grains of igneous rocks that experience post-emplacement hydrothermal activity not linked to impact cratering (e.g., Carlson and Rodgers, 1975

Carlson, J.R., Rodgers, K.A. (1975) The petrology and alteration of tertiary basalts of the coalgate area, Northwest Canterbury. Journal of the Royal Society of New Zealand 5, 195–205. https://doi.org/10.1080/03036758.1975.10419372

), so it is reasonable to infer a similar process happening on Mars. The spatial variability of alteration within the nakhlites (Lee et al., 2015

Lee, M.R., Tomkinson, T., Hallis, L.J., Mark, D.F. (2015) Formation of iddingsite veins in the martian crust by centripetal replacement of olivine: Evidence from the nakhlite meteorite Lafayette. Geochimica et Cosmochimica Acta 154, 49–65. https://doi.org/10.1016/j.gca.2015.01.022

) is consistent with a model of heat from magmatic eruptions or intrusions at ∼742 Ma, inducing localised, short-lived melting of permafrost. Such spatial variability is diagnostic of a small-scale hydrologic system, whereas higher water to rock ratios, and more pervasive alteration, are more commonly associated with an allochemical, impact-driven hydrothermal cell (Bridges and Schwenzer, 2012

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

). A model for a short-lived heat source of magmatic origin is thus congruent with the low temperature models for nakhlite alteration proposed previously (e.g., Treiman et al., 1993

Treiman, A.H., Barrett, R.A., Gooding, J.L. (1993) Preterrestrial aqueous alteration of the Lafayette (SNC) meteorite. Meteoritics 28, 86–97. https://doi.org/10.1111/j.1945-5100.1993.tb00251.x

). Fluid–rock interaction at ∼742 Ma selectively widened and extended the fractures within olivine through carbonation (e.g., Tomkinson et al., 2013

Tomkinson, T., Lee, M.R., Mark, D.F., Smith, C.L. (2013) Sequestration of Martian CO₂ by mineral carbonation. Nature Communications 4, 2662. https://doi.org/10.1038/ncomms3662

). The dissolution of the host olivine provides many of the cations required to precipitate the secondary minerals observed in the nakhlites (Lee et al., 2015

Lee, M.R., Tomkinson, T., Hallis, L.J., Mark, D.F. (2015) Formation of iddingsite veins in the martian crust by centripetal replacement of olivine: Evidence from the nakhlite meteorite Lafayette. Geochimica et Cosmochimica Acta 154, 49–65. https://doi.org/10.1016/j.gca.2015.01.022

)—another key requirement of a highly localised, isochemical system. Similarly, the potassium found within the iddingsite (Fig. 1) was likely sourced locally from dissolution of feldspar and glass in the mesostasis (Bridges and Schwenzer, 2012

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

).

The timing of iddingsite formation in Lafayette coincides with a volcanically active period of Mars’s history, albeit eruption frequency was decreasing. Amazonian-age volcanism is restricted to the Tharsis and Elysium regions of Mars and their peripheries (Carr and Head, 2010

Carr, M.H., Head III, J.W. (2010) Geologic history of Mars. Earth and Planetary Science Letters 294, 185–203. https://doi.org/10.1016/j.epsl.2009.06.042

); the nakhlites are most likely sourced from these regions (Herd et al., 2024

Herd, C.D.K., Hamilton, J.S., Walton, E.L., Tornabene, L.L., Lagain, A., Benedix, G.K., Sheen, A.I., Melosh, H.J., Johnson, B.C., Wiggins, S.E., Sharp, T.G., Darling, J.R. (2024) The source craters of the martian meteorites: Implications for the igneous evolution of Mars. Science Advances 10, eadn2378. https://doi.org/10.1126/sciadv.adn2378

). Crater ages of tens of millions of years for volcanic surfaces in Tharsis and Elysium, including the crater that formed when the nakhlites were ejected, suggest that Mars is still episodically volcanically active.

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Conclusions

The new ⁴⁰Ar/³⁹Ar geochronology presented here demonstrates that the timing for aqueous alteration of Martian volcanic rocks by water was unrelated to their emplacement or an impact event, but most likely related to ongoing magmatic activity on Mars ca. 742 Ma. Our proposed model is consistent with previous models for the alteration environment of the nakhlites. Considering the low eruption rate for Amazonian volcanoes and the prevalence of permafrost across Mars, our data support interpretations that activation of localised hydrological cycles on Amazonian Mars by magmatic activity was infrequent but not unusual.

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Acknowledgements

Funding from UK Science and Technology Facilities Council (ST/H002472/1 and ST/H002960/1) is gratefully acknowledged. The sample we obtained from the Smithsonian was Lafayette USNM 1505. We thank R. Dymock and J. Imlach for assistance with micro-encapsulation ⁴⁰Ar/³⁹Ar dating, P. Chung for support with SEM observations, A. Fallick for advice on experiment design, J. Bridges and S. Schwenzer for fruitful discussion on nakhlite alteration, and D. Minton for discussion on meteorite orbits. Finally, we thank M. Martínez, A. Treiman, and two anonymous reviewers for their helpful feedback, as well as F. McCubbin for editorial handling.

Editor: Francis McCubbin

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References

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Supplementary Information

The Supplementary Information includes:

• Alteration Phases in Lafayette
• Recalculation and Reinterpretation of Rb-Sr Systematics in Martian Meteorites Lafayette and Yamato (Y) 000593
• Micro-encapsulation ⁴⁰Ar/³⁹Ar Methods
• Argon Diffusion Calculations
• Figures S-1 to S-3
• Tables S-1 to S-3
• Supplementary Information References

Download the Supplementary Information (PDF)

Download Table S-1 (.xlsx)

Download Table S-3 (.xlsx)