Early lithosphere formation on Mars from 87Rb-87Sr systematics of nakhlite and chassignite meteorites

Nakhlite and chassignite meteorites are important samples for understanding the evolution of Mars as they all derive from the same ∼1.34 Ga magmatic system. More than half of the nakhlite-chassignite meteorites have been found in hot deserts and leaching experiments reveal terrestrial weathering strongly affects bulk rock 87 Rb- 87 Sr systematics. Excluding these meteorites, nakhlites and chassignites have bulk rock measured 87 Sr/ 86 Sr from 0.704184 to 0.706155. Correcting for in-situ 87 Sr ingrowth from 87 Rb decay over 1.34 Ga, nakhlites and chassignites possess significant variability in their initial 87 Sr/ 86 Sr ratios. Radiogenic initial 87 Sr/ 86 Sr for more fractionated nakhlites correspond with more radiogenic initial 187 Os/ 188 Os and mass-independently fractionated S isotopes. These signatures of crustal assimilation can be modelled by adding <10% of ancient (at least 2.4 Ga) martian surface basalt. Uncontaminated chassignites and nakhlites define a long-lived (∼4.5 Ga) mantle reservoir with low 87 Rb/ 86 Sr (0.069), making it at least 50 to 60% more depleted in 87 Rb than bulk silicate Mars estimates. The low Rb/Sr of the nakhlite-chassignite mantle source, combined with published 142 Nd- 143 Nd and Hf-W isotope systematics, indicates formation coincident with early martian differentiation, requiring crust extraction processes similar to those that produce Earth’s depleted mid-ocean ridge basalt mantle. The similarity in highly siderophile element contents estimated for the nakhlite-chassignite source and for bulk silicate Mars based on shergottites indicates that post-core formation late accretion must have occurred prior to the formation of these reservoirs, less than 100 million years after the accretion of Mars. Trace element signatures, including low Rb/Sr in nakhlites are consistent with 0.1-0.5% carbonate metasomatism of their depleted lithospheric source <30 million years prior to magmatism. Metasomatism of the nakhlite-chassignite source was probably engendered by plume magmatism that preceded rejuvenated magmatism, because of volcanic loading of the martian lithosphere.