Early evolution of Martian volatiles: Nitrogen and noble gas components in ALH84001 and Chassigny

Abstract

Studies on SNC meteorites have permitted the characterization of modern Martian atmospheric components as well as indigenous Martian nitrogen and solar‐type xenon. New isotopic and elemental abundances of noble gases and nitrogen in ALH84001 and Chassigny provide important constraints on the early evolution of the planet. A primitive solar Xe component (Chass‐S) and an evolved Xe component (Chass‐E), augmented with fission Xe are identified in Chassigny. Both components represent interior reservoirs of Mars and are characterized by low 129Xe/132Xe (<1.07) and by distinct elemental ratios 36Ar/132Xe <5 and >130, respectively. Light nitrogen (δ15N = −30‰) is associated with the Chass‐S component and is enriched in melt inclusions in olivine. An ancient (presumably incorporated ∼4 Gyr ago) evolved Martian atmospheric component is identified in ALH84001 and has the following signatures: 129Xe/132Xe = 2.16, 36Ar/38Ar ≥ 5.0, 36Ar/132Xe = ∼50, 84Kr/132Xe = ∼6, and δ15N = 7‰. The trapped Xe component in ALH84001 is not isotopically fractionated. We observe major shifts in nitrogen signatures due to cosmogenic N component in both Chassigny and ALH84001. A heavy nitrogen component of comparable magnitude (δ15N>150‰) has previously been interpreted as (heavy) Martian atmospheric N. In situ produced fission Xe components, due to 244Pu in ALH84001 and due to 238U in Chassigny, are identified. The ALH84001 data strongly constrain exchanges of Martian atmospheric and interior reservoirs. Mars retained abundant fission Xe components, and this may account for the low observed fission Xe component in the modern Martian atmosphere. Chronometric information regarding the evolution of the early Martian atmosphere can be secured from the relative abundances of radiogenic and fission Xe, as ∼80% of the Martian 129Xer is observed in the atmospheric 129Xe/132Xe ratio ∼4 Gyr ago.