39Ar-40Ar dating of the Zagami Martian shergottite and implications for magma origin of excess 40Ar

The Zagami shergottite experienced a complex, petrogenetic formation history (McCoy et al. 1992, 1999). Like several shergottites, Zagami contains excess Ar-40 relative to its formation age. To understand the origin of this excess Ar-40, we made Ar-39-Ar-40 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 Ar-40, similar to 1 x 10(-6) cm(3)/g. We present arguments against this excess Ar-40 having been introduced from the Martian atmosphere as impact glass. We also present evidence against excess Ar-40 being a partially degassed residue from a basalt that actually formed similar to 4 Gyr ago. We utilize our experimental data on Ar diffusion in Zagami and evidence that it was shock-heated to only similar to 70 degrees C, and we assume this heating occurred during an ejection from Mars similar to 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 Ar-40 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 Ar-40 in the analyzed separates imply that this magma maintained a relatively constant Ar-40 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 Ar-40 in other shergottites.