Evidence for extremely rapid magma ocean crystallization and crust formation on Mars

Abstract

The formation of a primordial crust is a critical step in the evolution of terrestrial planets but the timing of this process is poorly understood. The mineral zircon is a powerful tool for constraining crust formation because it can be accurately dated with the uranium-to-lead (U-Pb) isotopic decay system and is resistant to subsequent alteration. Moreover, given the high concentration of hafnium in zircon, the lutetium-to-hafnium (¹⁷⁶Lu-¹⁷⁶Hf) isotopic decay system can be used to determine the nature and formation timescale of its source reservoir^1-3. Ancient igneous zircons with crystallization ages of around 4,430 million years (Myr) have been reported in Martian meteorites that are believed to represent regolith breccias from the southern highlands of Mars^4,5. These zircons are present in evolved lithologies interpreted to reflect re-melted primary Martian crust ⁴ , thereby potentially providing insight into early crustal evolution on Mars. Here, we report concomitant high-precision U-Pb ages and Hf-isotope compositions of ancient zircons from the NWA 7034 Martian regolith breccia. Seven zircons with mostly concordant U-Pb ages define ²⁰⁷Pb/²⁰⁶Pb dates ranging from 4,476.3 ± 0.9 Myr ago to 4,429.7 ± 1.0 Myr ago, including the oldest directly dated material from Mars. All zircons record unradiogenic initial Hf-isotope compositions inherited from an enriched, andesitic-like crust extracted from a primitive mantle no later than 4,547 Myr ago. Thus, a primordial crust existed on Mars by this time and survived for around 100 Myr before it was reworked, possibly by impacts^4,5, to produce magmas from which the zircons crystallized. Given that formation of a stable primordial crust is the end product of planetary differentiation, our data require that the accretion, core formation and magma ocean crystallization on Mars were completed less than 20 Myr after the formation of the Solar System. These timescales support models that suggest extremely rapid magma ocean crystallization leading to a gravitationally unstable stratified mantle, which subsequently overturns, resulting in decompression melting of rising cumulates and production of a primordial basaltic to andesitic crust^6,7.

Extended Data Figure 1

Photomicrographs of the NWA 7034 zircons analysed in this study taken under natural light. Given the limited number of zircons recovered from the crushing process and their small sizes, it was deemed preferable not to conduct additional imaging (i.e. cathodoluminescence) as this necessitate extra manipulation of the individual grains thereby increasing the risk of losing zircons. The fact that the zircons have mostly concordant U-Pb ages confirms their simple igneous history and, therefore, additional imagining to investigate potential zoning is not required here.

Fig. 1. U-Pb concordia diagram for seven zircon grains from the NWA 7034 meteorite.

Labels on concordia curve represent time before present in millions of years. Data-point error ellipses are 2σ. Data used in this figure are reported in full in Table S1.

Fig. 2. Hf isotope evolution diagrams.

Shown in (A) are the initial εHf values for the seven individual NWA 7034 zircons calculated with their corresponding ²⁰⁷Pb-²⁰⁶Pb ages using a λ¹⁷⁶Lu value of 1.867±0.008×10^-11 year^-1 (ref. 30) and Chondritic Uniform Reservoir parameters of ref. . The upper boundary of the forbidden region represents a reservoir with a ¹⁷⁶Lu/¹⁷⁷Hf = 0 and a formation age defined by the age of the Solar System at 4567 Ma. In (B), we show the time evolution of basaltic and andesitic crustal reservoirs required to account for the average initial Hf isotope compositions of the three concordant 4475 Ma zircons (S24b4, S24b7 and S25b10) using ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.020 and 0.011 for the basaltic and andesitic crusts, respectively,. Considering the upper uncertainty of the zircon average εHf value (–1.35±0.22), it is not possible to account for the initial Hf isotope composition of these grains if they formed from the reworking of a basaltic crust since extraction ages older than the Solar System are required. In contrast, using a more evolved, andesite-like ¹⁷⁶Lu/¹⁷⁷Hf ratio returns a minimum extraction age of 4547 Ma. Using the mean of the concordant grains at face value and a ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.011 yields an extraction age of ${4562}_{-15}^{+5}$ Ma. Note that the time evolution of this reservoir can account for the Hf isotope composition of the younger ~4450 Ma and ~4430 Ma zircons. Indeed, a regression of the mean of the ~4475 Ma, ~4450 Ma and ~4430 Ma zircons yields a slope corresponding to an andesite-like ¹⁷⁶Lu/¹⁷⁷Hf ratio of 0.011. Uncertainty on the εHf values reflect the internal precision (2SE) or the external reproducibility of 22 ppm, whichever is larger. Uncertainty on the ²⁰⁷Pb/²⁰⁶Pb ages (2σ) are smaller than symbols.