In situ U–Pb dating of 4 billion-year-old carbonates in the martian meteorite Allan Hills 84001

In situ carbonate U–Pb dating studies have proliferated dramatically in recent years. Almost all these studies have targeted relatively young terrestrial calcite up to Carboniferous in age. To assess the robustness of the carbonate U–Pb chronometer in deep time, we carried out in situ U–Pb analyses in magnesite–ankerite–calcite carbonates in the martian meteorite Allan Hills (ALH) 84001. Carbonates in ALH 84001 formed at ca. 3.94 Ga, and there is little evidence that much happened to this rock since then, making it an ideal sample to test the robustness of the U–Pb system in old carbonates. We obtained a concordant date of 3941 ± 49/110 Ma (n=14, MSWD = 2.0), which is identical to the step-leaching $\mathrm{Rb}/\mathrm{Sr}$ date determined previously. These results thus confirm that old carbonates are amenable to U–Pb dating in samples that have had a relatively simple history post-carbonate formation.

1 Introduction

Analytical developments in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) over the last decade have driven important progress in in situ dating of carbonates – particularly of calcite and, occasionally, dolomite – using the radioactive decay of uranium (U) into lead (Pb) (see the recent review by Roberts et al., 2020, for example). Indeed, biogenic, diagenetic, and vein carbonates can typically incorporate up to ca. 10–20 µg g⁻¹ U and up to ca. 100 µg g⁻¹ U in speleothems (e.g. Roberts et al., 2020). Carbonates typically also incorporate initial Pb, meaning that multiple analyses on carbonate samples often yield linear arrays in a Tera–Wasserburg inverse concordia diagram, providing information on both the ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ composition of the initial Pb and the age of formation of the carbonates. Recent applications of carbonate U–Pb dating using LA-ICP-MS include constraining the timing of sedimentation, lithification, and diagenesis (e.g. Drost et al., 2018; Godeau et al., 2018; Mueller et al., 2020; Brigaud et al., 2021); faulting (e.g. Ring and Gerdes, 2016; Roberts and Walker, 2016; Goodfellow et al., 2017; Nuriel et al., 2017, 2019; Hansman et al., 2018; Beaudoin et al., 2018; Holdsworth et al., 2019; Smeraglia et al., 2019); aragonite-to-calcite conversion in ammonites (Li et al., 2014); alteration of oceanic crust (Coogan et al., 2016); veining, hydrothermalism, and mineralisation (Burisch et al., 2017, 2018; Parrish et al., 2018; Walter et al., 2018; Bertok et al., 2019; Drake et al., 2019, 2020; MacDonald et al., 2019); palaeoclimate reconstructions (Nicholson et al., 2020); and hominin dispersion (Scardia et al., 2019), for example.

All these in situ studies have targeted relatively young samples (younger than ca. 465 Ma), with two thirds of the dates being younger than 50 Ma and all but three being younger than 300 Ma (Table S1 in the Supplement). This is consistent with the accepted idea that carbonates are not very resistant to resetting of their U–Pb isotope systematics when thermal- and/or fluid-related alteration events take place after their formation (e.g. Roberts et al., 2020). However, a few studies have focused on dating older carbonate samples, using wet chemistry to isolate Pb isotopes, and obtained $\mathrm{Pb}/\mathrm{Pb}$ isochron dates ranging between ca. 1.60 and 2.84 Ga (Moorbath et al., 1987; Bau et al., 1999; Ray et al., 2003; Sarangi et al., 2004; Fairey et al., 2013). These $\mathrm{Pb}/\mathrm{Pb}$ dates have been interpreted as dating the deposition of these carbonates, suggesting that, in some settings, the Pb isotope systematics of carbonates can remain undisturbed for billions of years. To further assess the robustness of the carbonate U–Pb chronometer in deep time, we decided to attempt in situ LA-ICP-MS U–Pb dating of carbonates in the martian meteorite Allan Hills 84001 (ALH 84001). The formation of these carbonates has been dated at 3.94 ± 0.02 Ga (2σ) using Rb-Sr analyses on acid leachates via thermal ionisation mass spectrometry (Borg et al., 1999; Beard et al., 2013; date recalculated using a ⁸⁷Rb decay constant of 1.3972 × 10⁻¹¹ a⁻¹; Villa et al., 2015).

The meteorite ALH 84001 is an orthopyroxenite – a cumulate rock mostly comprising orthopyroxene, olivine, and chromite (e.g. Mittlefehldt, 1994) – which formed 4.09 ± 0.03 Ga ago, as suggested by Lu-Hf and $\mathrm{Pb}/\mathrm{Pb}$ dating (Bouvier et al., 2009; Lapen et al., 2010). Carbonate-rich areas are irregularly scattered throughout ALH 84001, appearing as spherical or hemispherical globules, discs along fractures, and irregular fillings in orthopyroxene (see review by Treiman, 2021, and references therein). The patches of carbonates show strong compositional zoning, ranging from calcite-rich to magnesite–siderite solid solution compositions (e.g. Corrigan and Harvey, 2004; Holland et al., 2005). These carbonates likely formed at low temperature (ca. 10–20 ^∘C; Halevy et al., 2011; del Real et al., 2016) during fluid–rock interactions that were broadly contemporaneous with the main shock event recorded in ALH 84001 (e.g. Treiman, 2021). In order to test the robustness of the U–Pb system in ca. 4 Ga-old carbonates, it is essential to understand the geological history of those carbonates to assess whether they have been affected by any hydrous or other alteration event since their formation.

Dating carbonate-rich fractions in ALH 84001 using the Rb-Sr system yielded a precise formation age of 3.94 ± 0.02 Ga (2σ; Borg et al., 1999; Beard et al., 2013), which is consistent with a less precise $\mathrm{Pb}/\mathrm{Pb}$ isochron corresponding to a date of 4.045 ± 0.090 Ga (2σ; Borg et al., 1999). The formation of these carbonates occurred at a low temperature (< 20 ^∘C) and likely involved the mixing of two different water sources, one rich in Ca, the other rich in Fe (e.g. Halevy et al., 2011, del Real et al., 2016; Bridges et al., 2019, Treiman, 2021). These carbonate-forming fluids equilibrated with the atmosphere at the time, unlike igneous minerals in the matrix (Shaheen et al., 2015). A contemporaneous impact event raised the temperature of the surrounding plagioclase to ca. 1400 ^∘C, melting it to produce glass and faulting carbonate globules (Mittlefehldt, 1994). Following this, Treiman (2021) states “there is little evidence that anything had happened to ALH 84001 since 3.9 Ga” – until another impact event at ca. 14 Ma, which caused the progenitor material that formed ALH 84001 to be ejected from Mars (Eugster et al., 1997). After ca. 14 Ma in space, ALH 84001's parent meteoroid fell to Antarctica ca. 13 000 years ago (Eugster et al., 1997) and remained buried deep in the ice for millennia, only emerging at the surface of the Allan Hills ice field probably no more than 500 years ago (Krähenbühl et al., 1998). Finally, there is extensive olivine and glass of plagioclase composition in ALH 84001, with no evidence of any alteration to clays or phyllosilicates. Modelling of Ar diffusion within the constituent minerals implies that the progenitor material for ALH 84001 was not subjected to temperatures > 30 ^∘C for any “long duration” (Cassata et al., 2010; Shuster and Weiss, 2005). All these lines of evidence indicate that the minerals in ALH 84001 were not exposed to hydrous fluids or temperatures > 25–30 ^∘C during the last 3.9 Ga (e.g. Treiman, 2021).

Figure 1 Backscattered electron image (a) and composite X-ray map (b) of the target carbonate patches in ALH 84001. Mineral abbreviations are ank = ankerite, cal = calcite, chr = chromite, mgs = magnesite, msk = maskelynite, and opx = orthopyroxene.

2 Studied sample

The studied polished section (Fig. 1) was derived from a chip of the ALH 84001,287 allocation from the NASA Ancient Mars Meteorite Program. The section contains patches of carbonates, associated with chromite, in between larger orthopyroxene grains (Fig. 1). The carbonates display the range of compositions typical for ALH 84001 carbonates, from Mg-rich magnesite to Ca-rich calcite, with intermediate Fe-rich ankerite areas (Fig. 1). The Mg- and Fe-rich carbonates seem to be part of broken rosettes, while the Ca-rich carbonates appear to be associated with maskelynite (Fig. 1).

3 Laser ablation inductively coupled plasma mass spectrometer analyses

U–Pb analyses were carried out at the University of Manchester using a Teledyne Photon Machines Analyte Excite+ 193 nm ArF excimer laser ablation system equipped with a HelEx II active two-volume ablation cell coupled to an Agilent 8900 triple-quadrupole inductively coupled plasma mass spectrometer (ICP-MS) using a signal-smoothing device (see Table S2 for a summary of the analytical setup and data processing procedure).

The material ablated from target carbonates was carried to the ICP-MS by high purity He, which was mixed with Ar before injection into the plasma source. High purity N₂ was added to the He stream at a flow rate of 2 mL min⁻¹ to enhance sensitivity. Tuning of the ICP-MS and mass calibration were performed at the start of the analytical session by optimising the ion signals during ablation of the NIST SRM 612 reference glass while maintaining ${}^{\mathrm{238}}{\mathrm{U}}^{+}{/}^{\mathrm{232}}{\mathrm{Th}}^{+}$ close to unity and minimising the ${}^{\mathrm{232}}{\mathrm{Th}}^{\mathrm{16}}{\mathrm{O}}^{+}{/}^{\mathrm{232}}{\mathrm{Th}}^{+}$ ratio (ca. 0.3 %). Glass and carbonates were ablated using a 25 μm laser beam size, a fluence of 4 J cm⁻², and a repetition rate of 5 Hz. Each analysis lasted 50 s and was preceded by 30 s counting time of the gas blank (background). The masses analysed and the corresponding dwell times are reported in Table S2.

The reference glass NIST614 (0.823 µg g⁻¹ U, 2.32 µg g⁻¹ Pb; Jochum et al., 2011) was used to correct for ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ fractionation, while mass bias correction of the measured ${}^{\mathrm{238}}\mathrm{U}{/}^{\mathrm{206}}\mathrm{Pb}$ ratios was carried out using repeated analyses of the reference calcite WC-1, which has a thermal ionisation mass spectrometry (TIMS) age of 254.4 ± 6.4 Ma (Roberts et al., 2017). To ensure accuracy, the Duff Brown Tank (DBT) calcite (64.0 ± 0.7 Ma; Hill et al., 2016) and AUG-B6 calcite (43.0 ± 1.0 Ma; Pagel et al., 2018) were also analysed and used as secondary reference materials. Since there is no $\mathrm{U}/\mathrm{Pb}$ magnesite–ankerite carbonate reference material, we have to rely on using a calcite reference material to correct for $\mathrm{U}/\mathrm{Pb}$ fractionation in ALH 84001 carbonates.

Data processing was carried out using Iolite v4.5, using the NIST614 glass as a primary reference material to remove instrument baseline contributions, the mass bias of Pb isotopes, and the downhole fractionation and instrumental drift of ${}^{\mathrm{206}}\mathrm{Pb}{/}^{\mathrm{238}}\mathrm{U}$ ratios (Paton et al., 2011). The reproducibility obtained on NIST614 for ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ (± 1.7 %, n=9, 95 % confidence level) and ${}^{\mathrm{206}}\mathrm{Pb}{/}^{\mathrm{238}}\mathrm{U}$ (± 1.6 %, n=9, 95 % confidence level) ratios were propagated by quadrature addition into ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ and ${}^{\mathrm{206}}\mathrm{Pb}{/}^{\mathrm{238}}\mathrm{U}$ individual uncertainties for each analysis. Repeated analyses of the NIST612 glass yielded an average ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ ratio of 0.871 ± 0.044 (n=8, 2 standard deviation), which is within error of its known ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ ratio of 0.90745 ± 0.00004 (Baker et al., 2004).

The data obtained for the reference calcite WC-1 were then plotted in a Tera–Wasserburg diagram using IsoplotR (Vermeesch, 2018) and yielded a lower intercept uncorrected date of 255.2 ± 5.9 Ma (95 % confidence level, MSWD = 1.1, n=10) for a discordia anchored at the common ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ ratio of 0.85 ± 0.04, determined by Roberts et al. (2017). To obtain the known intercept age of 254.4 ± 6.4 Ma for the WC-1 calcite, we applied a linear correction factor of 1.0031 to the measured ${}^{\mathrm{206}}\mathrm{Pb}{/}^{\mathrm{238}}\mathrm{U}$ ratios, which we also applied to all the samples analysed in the session, as is commonly done for carbonate U–Pb dating by LA-ICP-MS (e.g. Roberts et al., 2017; Drost et al., 2018; Kylander-Clark, 2020). All calculated dates are associated with two uncertainties: the first one includes the random uncertainties for each analysis (internal uncertainties on measured ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ and ${}^{\mathrm{206}}\mathrm{Pb}{/}^{\mathrm{238}}\mathrm{U}$ ratios and reproducibility on repeated NIST614 analyses), while in the second one, systematic uncertainties (2.5 % uncertainty on the WC-1 age and 0.14 % and 0.11 % on the ²³⁵U and ²³⁸U decay constants, respectively; Jaffey et al., 1971) are propagated by quadratic addition.

The data obtained on the DBT calcite yielded a lower intercept date of 64.9 ± 2.2/2.8 Ma (95 % confidence level, MSWD = 2.9, n=12) for a discordia anchored at a common ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ ratio of 0.74 ± 0.02, calculated based on isotope dilution and multi-collector ICP-MS analyses (Hill et al., 2016) (Fig. S1 in the Supplement). Because of its lower U abundance and younger age, the data obtained on the AUG-B6 calcite are less precise, yielding a concordia date of 40.8 ± 2.0/2.2 Ma (95 % confidence level, MSWD concordance + equivalence = 2.2, n=8) (Fig. S1), which is identical to a ${}^{\mathrm{238}}\mathrm{U}{/}^{\mathrm{206}}\mathrm{Pb}$ weighted average date of 41.6 ± 2.1/2.3 Ma (95 % confidence level, MSWD = 0.4, n=8). All results are available in Table S3.

Table 1 LA-ICP-MS results for ALH 84001 carbonates.

^a Background-corrected ²⁰⁴Pb intensity, calculated using the measured ²⁰⁴(Pb+Hg) and ²⁰²Hg, and a ${}^{\mathrm{204}}\mathrm{Hg}{/}^{\mathrm{202}}\mathrm{Hg}$ of 0.22932.
^b Concordance (100 × ${}^{\mathrm{206}}\mathrm{Pb}{/}^{\mathrm{238}}\mathrm{U}$ date $/$ ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ date).

Figure 2 Tera–Wasserburg ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ vs. ${}^{\mathrm{238}}\mathrm{U}{/}^{\mathrm{206}}\mathrm{Pb}$ diagram (a) and ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ dates (b) obtained on ALH 84001 carbonates. Ellipses (a) and error bars (b) correspond to 2σ standard errors. The bold black ellipse in (a) corresponds to the calculated concordia date and associated uncertainty.

4 Results

The carbonates analysed in ALH 84001 contain ca. 0.1–0.4 µg g⁻¹ U and 0.1–0.5 µg g⁻¹ Th (Table 1). When plotted in a Tera–Wasserburg ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ vs. ${}^{\mathrm{238}}\mathrm{U}{/}^{\mathrm{206}}\mathrm{Pb}$ diagram, ALH 84001 carbonates yield a concordant date of 3941 ± 49/110 Ma (n=14, MSWD = 2.0; Fig. 2a), which is identical to a weighted mean ${}^{\mathrm{207}}\mathrm{Pb}{/}^{\mathrm{206}}\mathrm{Pb}$ date of 3967 ± 56/113 Ma (n=14, MSWD = 1.9; Fig. 2b). In detail, the Mg-rich and Ca-rich carbonate analyses yield concordia dates of 3890 ± 72 Ma (2σ, MSWD = 1.5, n=8) and 3995 ± 69 Ma (2σ, MSWD = 2.4, n=6), respectively; U–Pb dates for these two carbonate compositions are, therefore, indistinguishable when uncertainties are considered. Carbonate analyses plot on the concordia curve, indicating that they do not contain appreciable amounts of common Pb. This is consistent with the measured ²⁰⁴Pb intensities that are within error of 0 counts per second, although it is fair to point out that this observation is qualitative, considering the large uncertainties associated with the ²⁰²Hg, ²⁰⁴(Hg+Pb), and calculated ²⁰⁴Pb count rates (Tables 1 and S3). It is noteworthy that ²⁰²Hg intensities in ALH 84001 carbonates are about an order of magnitude higher than in the terrestrial carbonate standards (Table S3). This could indicate that martian carbonates contain higher Hg abundances than terrestrial carbonates. Alternatively, we think that this extra Hg likely originates from contamination of the ALH 84001 section by Au coating applied in the past for secondary ion mass spectrometry studies.

5 Discussion and implications

5.1 Comparison with previous ALH 84001 carbonate dating studies

In situ U–Pb dating of carbonates in ALH 84001 using LA-ICP-MS yields a concordia date of 3941 ± 49/110 Ma, which is identical to the carbonate step-leaching Rb-Sr isochron date of 3.94 ± 0.02 Ga (Borg et al., 1999; Beard et al., 2013) and the less precise $\mathrm{Pb}/\mathrm{Pb}$ isochron date of 4.045 ± 0.090 Ga (Borg et al., 1999). Before propagating the ± 2.5 % uncertainty associated with the age of the primary U–Pb reference carbonate WC-1 and the uncertainties associated with the ²³⁸U and ²³⁵U decay constants, the carbonate U–Pb concordia date is associated with a fairly precise 2σ uncertainty of ± 1.2 %, which increases to ± 2.8 % when all uncertainties are propagated. This suggests that in situ U–Pb dating of carbonates has the potential to yield precise dates, but it also highlights the need to reduce uncertainties on reference materials. Our LA-ICP-MS results also indicate that using a calcite primary reference material for correcting $\mathrm{U}/\mathrm{Pb}$ fractionation in Mg- and Fe-rich carbonate matrices, such as magnesite and ankerite, produce accurate dates (within the obtained uncertainties). A final point worth highlighting is the fact that carbonates in ALH 84001 do not contain appreciable amounts of common Pb, as indicated by their concordant U–Pb date of ca. 3.94 Ga. This is unusual, as most examples in terrestrial system carbonates do contain common Pb, incorporated during their crystallisation (e.g. Roberts et al., 2020). This observation suggests that the fluids from which ALH 84001 carbonates formed contained very little Pb.

5.2 Robustness of the carbonate U–Pb chronometer and further applications

Our in situ LA-ICP-MS analyses confirm that carbonates in ALH 84001 formed ca. 3.94 Ga ago and that the U–Pb chronometer in these carbonates has remained closed to any disturbance event since they formed. This is consistent with the suggestion that not much happened to ALH 84001 between 3.9 Ga and its launch from Mars 14 Ma ago (Treiman, 2021), and indicates that this latter event did not reset the carbonate U–Pb chronometer. From the evidence summarised in Sect. 1, Treiman (2021) concluded that minerals in ALH 84001, including the carbonates, have experienced neither temperatures in excess of ca. 25 ^∘C nor exposure to any hydrous fluids since 3.9 Ga, leading to the conclusion that the climate of Mars has remained globally cold and dry since then. Any aqueous events occurring on Mars due to a globally warmer and wetter climate, therefore, took place before 3.9 Ga. Heating and aqueous alteration events due to meteoroid impacts could have occurred at any time, but these would have been strictly localised. Our U–Pb data back up the view that the carbonates in ALH 84001 have not experienced any alteration since they formed, supporting the conclusion that the progenitor material of ALH 84001 was not exposed to hydrous fluids or high temperature events for the last 3.9 Ga, suggesting a globally cold and dry Mars since then.

On the other hand, carbonates in terrestrial Archean samples are probably not the best suited for U–Pb dating, as most Archean formations would have been heated up to at least low greenschist metamorphic conditions and/or would have been affected by hydrothermal alteration, because the Earth is geologically active and harbours a complex hydrological cycle. On the other hand, the results of this study open up opportunities for dating old carbonates in samples that have had a relatively simple history post-carbonate formation. For example, volatile-rich carbonaceous chondrites (e.g. CI and CM chondrites) typically contain carbonates formed during fluid–rock interactions on their parent asteroids ca. 4563–4561 Ma-ago (e.g. Lee et al., 2014; Jilly-Rehak et al., 2017, and references therein), which is within 10 Myr of the formation of the first solids in the solar system. Bulk CI and CM chondrites contain ∼ 10 ng g⁻¹ U (e.g. Braukmüller et al., 2018; Turner et al., 2021), a significant proportion of which is potentially hosted in labile phases such as carbonates (e.g. Burkhardt et al., 2019; Turner et al., 2021). As they make up ∼ 1 vol. %–2 vol. % of CM chondrites (e.g. Lee et al., 2014), these carbonates could host a few 100's ng g⁻¹ U, which is similar to the U abundance in the ALH 84001 carbonates analysed here. After this phase of early hydrothermal alteration, during which carbonates formed, it is thought that not much happens to volatile-rich carbonaceous chondrites on their parent asteroids until they end up on the Earth as meteorite fragments. Carbonates in carbonaceous chondrites could thus be prime targets to further constrain the timing of hydrothermal alteration on volatile-rich asteroids using the U–Pb dating chronometer.