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

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 as it can be accurately dated with the U-Pb system and is resistant to subsequent alteration. Moreover, the high concentration of Hf in zircon allow for the utilization of the (176)Lu-(176)Hf decay system to determine the nature and formation timescale of its source reservoir1–3. Ancient igneous zircons with ages of ~4430 Ma have been reported in martian meteorites believed to represent regolith breccias from the southern highlands of Mars4,5. These zircons are present in evolved lithologies interpreted to reflect re-melted primary martian crust4 thereby potentially providing unique insights 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 (207)Pb/(206)Pb dates ranging from 4476.3±0.9 Ma to 4429.7±1.0 Ma, 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 4547 Ma. Thus, a primordial crust existed on Mars by this time and survived for ~100 Myr before it was reworked, possibly by impacts4,5, 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 was completed <20 Myr after Solar System formation. These timescales support models suggesting rapid magma ocean crystallization leading to a gravitationally unstable stratified mantle, which subsequently overturns resulting in decompression melting of rising cumulates and extraction of a primordial basaltic to andesitic crust6,7.