Martian mantle signatures in Yamato nakhlites

We report nitrogen and heavy noble gas isotopic abundances in the Yamato nakhlites Y000593 and Y000749 and infer isotopic signatures for the magma source region in Mars. The nitrogen signature 15N=13±1 appears in broad plateau releases of both nakhlites and also has previously been observed in Nakhla and in Chassigny. This signature differs from the primitive indigenous nitrogen signature (15N=30) and, strikingly, from nitrogen in the modern Martian atmosphere. It demonstrates that exchanges between the solid planet and its modern atmosphere have been very limited. The xenon isotopic records support this conclusion. In nakhlite Y000749 radiogenic 129Xe is observed in one temperature fraction, while a five times larger component is observed in several steps of Y000593. The lack of an association with fission Xe or with indigenous Xe from the mantle source region suggests a crustal source. A crustal storage of the extinct 129I was previously invoked to account for radiogenic 129Xe in the Martian atmosphere. Fission Xe components due to extinct 244Pu were identified in both nakhlites. These records show that the magma source region had assimilated and retained fission gas since the early differentiation of Mars. The measured concentrations of 40Ar exceed the amounts produced from the decay of 40K in 1.3Ga and document that the magma inherited some radiogenic 40Ar, together with fission gas. CRE ages based on spallation 15Nc and 38Ar are consistent with a reported 81Kr-83Kr age and CRE ages for other nakhlites, confirming identical ejection ages for all nakhlites.