Modern atmospheric signatures in 4.4 Ga Martian meteorite NWA 7034

The NWA 7034 Martian basaltic breccia, dated at ∼4.4 Ga∼4.4 Ga, represents an entirely new type of Martian meteorite. However, due to the unique make-up of NWA 7034 compared to other Martian meteorite types (including its anomalous oxygen isotope ratios), noble gas analyses – a key tool for Martian meteorite identification – are important to confirm its Martian origin. Here, we report the first noble gas results for NWA 7034, which show the presence of a trapped component that resembles the current Martian atmosphere. This trapped component is also similar in composition to trapped gases found in the much younger shergottites (∼150–600 Ma∼150–600 Ma). Our formation ages for the sample suggest events at ∼1.6 Ga (K–Ar), and ∼170 Ma (U–Th/He), which are considerably younger than those observed by Rb–Sr (2.1 Ga), and Sm–Nd (4.4 Ga; zircons ∼4.4 Ga). However, our K–Ar age is similar to a disturbance in the U–Pb zircon data at ∼1.7 Ga, which could hint that both chronometers have been subjected to disturbance by a common process or event. The U–Th/He age of ∼170 Ma could relate to complete loss of radiogenic ^4He at this time, and is a similar age to the crystallisation age of most shergottites. While this may be coincidental, it could indicate that a single event is responsible for both shergottite formation and NWA 7034 thermal metamorphism. As for cosmic ray exposure ages, our favoured age is ∼5 Ma, which is outside the ranges for other Martian meteorite groups, and may suggest a distinct ejection event. NWA 7034 shows evidence for neutron capture on Br, which has caused elevations in Kr isotopes ^(80)Kr and ^(82)Kr. These elevated abundances indicate significant shielding, and could relate to either a large meteoroid size, and/or shielding in relation to a regolithic origin. We have also applied similar neutron capture corrections to Ar and Xe data, which further refine the likelihood of a modern atmospheric component, though such corrections remain speculative. Cosmogenic production rates and noble gas data are consistent with a meteoroid radius of >50 cm. Fission contributions are clear in the Xe data, with evidence to suggest that NWA 7034 contains both ^(238)U and ^(244)Pu derived fission Xe components. If the gas in NWA 7034 was trapped at its ancient igneous formation, this would suggest little evolution of the Martian atmosphere between ∼4.4 Ga and present day. However, as NWA 7034 is a regolith breccia with multiple lithologies and a strong compositional similarity to Gusev soils, the timing and incorporation of trapped atmospheric gases is unclear. With hints of resetting events at ∼1.5–2.1 Ga∼1.5–2.1 Ga, the atmospheric component may have been incorporated during breccia formation – possibly in the Amazonian, though it could also have been incorporated on ejection from the surface.