Abstract-The Zagami shergottite experienced a complex, petrogenetic formation history (McCoy et al. 1992(McCoy et al. , 1999. Like several shergottites, Zagami contains excess 40 Ar relative to its formation age.To understand the origin of this excess 40 Ar, we made 39 Ar-40 Ar analyses on plagioclase and pyroxene minerals from two phases representing different stages in the magma evolution. Surprisingly, all these separates show similar concentrations of excess 40 Ar, ∼1 × 10 −6 cm 3 /g. We present arguments against this excess 40 Ar having been introduced from the Martian atmosphere as impact glass. We also present evidence against excess 40 Ar being a partially degassed residue from a basalt that actually formed ∼4 Gyr ago. We utilize our experimental data on Ar diffusion in Zagami and evidence that it was shock-heated to only ∼70 °C, and we assume this heating occurred during an ejection from Μars 3 Myr ago. With these constraints, thermal considerations necessitates either that its ejected mass was impossibly large, or that its shock-heating temperature was an order of magnitude higher than that measured. We suggest that this excess 40 Ar was inherited from the Zagami magma, and that it was introduced into the magma either by degassing of a larger volume of material or by early assimilation of old, K-rich crustal material. Similar concentrations of excess 40 Ar in the analyzed separates imply that this magma maintained a relatively constant 40 Ar concentration throughout its crystallization. This likely occurred through volatile degassing as the magma rose toward the surface and lithostatic pressure was released. These concepts have implications for excess 40 Ar in other shergottites.

We report 39 Ar- 40 Ar ages of whole rock (WR) and plagioclase and pyroxene mineral separates of nakhlites MIL 03346 and Y-000593, and of WR samples of nakhlites NWA 998 and Nakhla. All age spectra are complex and indicate variable degrees of 39 Ar recoil and variable amounts of trapped 40 Ar in the samples. Thus, we examine possible Ar-Ar ages in several ways. From consideration of both limited plateau ages and isochron ages, we prefer Ar-Ar ages of NWA 998 = 1334 ± 11 Ma, MIL 03346 = 1368 ± 83 Ma (mesostasis) and 1334 ± 54 Ma (pyroxene), Y-000593 = 1367 ± 7 Ma, and Nakhla = 1357 ± 11 Ma, (2r errors). For NWA 998 and MIL 03346 the Ar-Ar ages are within uncertainties of preliminary Rb-Sr isochron ages reported in the literature. These Ar-Ar ages for Y-000593 and Nakhla are several Ma older than Sm-Nd ages reported in the literature. We conclude that the major factor in producing Ar-Ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial 40 Ar on weathered grain surfaces that was degassed along with the first several percent of 39 Ar. A total K-40 Ar isochron for WR and mineral data from five nakhlites analyzed by us, plus Lafayette data in the literature, gives an isochron age of 1325 ± 18 Ma (2r). We emphasize the precision of this isochron over the value of the isochron age. Our Ar-Ar data are consistent with a common formation age for nakhlites. The cosmic-ray exposure (CRE) age for NWA 998 of $12 Ma is also similar to CRE ages for other nakhlites. Published by Elsevier Ltd.

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