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 (Lu-Hf) isotopic decay system can be used to determine the nature and formation timescale of its source reservoir. 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. These zircons are present in evolved lithologies interpreted to reflect re-melted primary Martian crust , 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 definePb/Pb 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, 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.

The composition of Mars

“…Therefore comparison of physical and chemical properties of Mars with those of the Earth can provide important insights into the origin and evolution of the rocky planets, especially conditions for a habitable planet formation. Radioisotope dating of Martian meteorites demonstrates that its accretion and evolution occurred earlier than that of the Earth (Dauphas and Pourmand, 2011;Kruijer et al, 2017b;Bouvier et al, 2018). The rapid formation of Mars is consistent with a pebble accretion and/or runaway and oligarchic growth model, depending upon model assumptions (Dauphas and Pourmand, 2011;Johansen et al, 2015;Levison et al, 2015).…”

The potential science and engineering value of samples delivered to Earth by Mars sample return

“…; Bouvier et al. ), and the mantle appears to have formed a series of isotopically distinct reservoirs from which subsequent magmas were derived with little mixing (Debaille et al. ).…”

Probabilities of Collisions of Planetesimals from Different Regions of the Feeding Zone of the Terrestrial Planets with the Forming Planets and the Moon

“…From the analysis of the ratio of lead isotopes, 207 Pb/ 206 Pb, in zircon crystals contained in the material of the Martian meteorite NWA 7034, Bouvier et al (2018) concluded that the core formation and the crystallization of a magma ocean on Mars was completed in less than 20 Myr after the formation of the Solar System. These results agree with those reported by Mezger et al (2013) concerning the studies of the decay of a system of short-lived isotopes 182 Hf-182 W, which point to an age not larger than 10 Myr after the formation of the Solar System.…”