Volatiles (nitrogen, noble gases) in recently discovered SNC meteorites, extinct radioactivities and evolution

Abstract

We report noble gas and nitrogen analyses of newly discovered SNC meteorites, one nakhlite (NWA817) and four shergottites (NWA480, NWA856, NWA1068, and SaU 005). The K–Ar age (1.3 Ga) as well as the cosmic-ray exposure (CRE) age (10.0±1.3 Ma) of nakhlite NWA817 agree with data of Nakhla. The CRE ages of NWA480, NWA856, and NWA1068 (2.35±0.20, 2.60±0.21 and 2.01±0.65 Ma, respectively) are consistent, within uncertainties, with other basaltic shergottites, but the CRE age of SaU 005 (1.25±0.07 Ma) is distinct and indicates a different ejection event. Bulk K–Ar ages of all shergottites exceed the reported radiometric ages and reveal the presence of inherited radiogenic 40Ar in basaltic lavas. The isotopic composition of nitrogen trapped in these SNC meteorites is not homogeneous, since δ 15N values of either +15 to 20‰ or +45‰, indicate different nitrogen reservoirs. All shergottites contain fission xenon from 238U, and fission Xe of extinct ( T 1/2=82 Ma) 244Pu, previously identified in ALH84001, in Chassigny and in Nakhla is also present in at least one shergottite (NWA856). The shergottites contain less fissiogenic Xe than other SNC, suggesting that either their source was more degassed or that the magma source region closed at a later time. In nakhlites, fission xenon from 244Pu correlates with uranium, a geochemical proxy of plutonium. Thus it is possible that fissiogenic Xe was not inherited during magma differentiation, but rather was produced in situ and retained in refractory mineral assemblages. In this interpretation, the magma evolution that settled the mineralogy and geochemistry of nakhlites took place at a time when 244Pu was alive and pre-dated the (late) events recorded in their radiometric ages. Alternatively, fissiogenic xenon was trapped from a mantle source during closed system evolution of the parent magmas, in which case such evolution might have taken place at considerable depth (pressure) in order to inhibit magma degassing during the course of differentiation.