Martian fluid and Martian weathering signatures identified in Nakhla, NWA 998 and MIL 03346 by halogen and noble gas analysis

We report argon (Ar) noble gas, Ar–Ar ages and halogen abundances (Cl, Br, I) of Martian nakhlites Nakhla, NWA 998 and MIL 03346 to determine the presence of Martian hydrous fluids and weathering products. Neutron-irradiated samples were either crushed and step-heated (Nakhla only), or simply step-heated using a laser or furnace, and analysed for noble gases using an extension of the ^(40)Ar–^(39)Ar technique to determine halogen abundances. The data obtained provide the first isotopic evidence for a trapped fluid that is Cl-rich, has a strong correlation with ^(40)Ar_(XS) (^(40)Ar_(XS) = ^(40)Ar_(measured) − ^(40)Ar_(radiogenic)) and displays ^(40)Ar_(XS)/^(36)Ar of ∼1000 – consistent with the Martian atmosphere. This component was released predominantly in the low temperature and crush experiments, which may suggest a fluid inclusion host. For the halogens, we observe similar Br/Cl and I/Cl ratios between the nakhlites and terrestrial reservoirs, which is surprising given the absence of crustal recycling, organic matter and frequent fluid activity on Mars. In particular, Br/Cl ratios in our Nakhla samples (especially olivine) are consistent with previously analysed Martian weathering products, and both low temperature and crush analyses show a similar trend to the evaporation of seawater. This may indicate that surface brines play an important role on Mars and on halogen assemblages within Martian meteorites and rocks. Elevated I/Cl ratios in the low temperature NWA 998 and MIL 03346 releases may relate to in situ terrestrial contamination, though we are unable to distinguish between low temperature terrestrial or Martian components. Whilst estimates of the amount of water present based on the ^(36)Ar concentrations are too high to be explained by a fluid component alone, they are consistent with a mixed-phase inclusion (gas and fluid) or with shock-implanted Martian atmospheric argon. The observed fluid is dilute (low salinity, but high Br/Cl and I/Cl ratios), contains a Martian atmospheric component, and may represent a mixture of a shallow fluid with one derived from fluid circulation within the nakhlite cumulate pile – with heat for fluid circulation supplied by either the igneous intrusion or by an impact event.