Shergottites Dhofar 019, SaU 005, Shergotty, and Zagami: 40Ar-39Ar chronology and trapped Martian atmospheric and interior argon

Abstract

We report a high-resolution 40Ar-39Ar study of mineral separates and whole-rock samples of olivine-phyric (Dhofar 019, Sayh al Uhaymir [SaU] 005) and basaltic (Shergotty, Zagami) shergottites. Excess argon is present in all samples. The highest (40Ar/40Ar) trapped ratios are found for argon in pyroxene melt inclusions (∼1500), maskelynite (∼1200), impact glass (∼1800) of Shergotty and impact glass of SaU 005 (∼1200). A high (40Ar/ 36Ar)trapped component-usually uniquely ascribed to Martian atmosphere-can also originate from the Martian interior, indicating a heterogeneous Martian mantle composition. As additional explanation of variable high (40Ar/ 36Ar)trapped ratios in shocked shergottites, we suggest argon implantation from a "transient atmosphere" during impact induced degassing. The best 40Ar- 39Ar age estimate for Dhofar 019 is 642 ± 72 Ma (maskelynite). SaU 005 samples are between 700-900 Ma old. Relatively high 40Ar-39Ar ages of melt inclusions within Dhofar 019 (1086 ± 252 Ma) and SaU 005 olivine (885 ± 66 Ma) could date entrapment of a magmatic liquid during early olivine crystallization, or reflect unrecognized excess 40Ar components. The youngest 40Ar-39Ar age of Shergotty separates (maskelynite) is ∼370 Ma, that of Zagami is ∼200 Ma. The 40Ar-39Ar chronology of Dhofar 019 and SaU 005 indicate <1 Ga ages. Apparent ages uncorrected for trapped (e.g., Martian atmosphere, mantle) argon components approach 4.5 Ga, but are not caused by inherited 40Ar, because excess 40Ar is supported by 36Artrapped. Young ages obtained by 40Ar-39Ar and other chronometers argue for primary rather than secondary events. The cosmic ray exposure ages calculated from cosmogenic argonare 15.7 ± 0.7 Ma (Dhofar 019), 1.0-1.6 Ma (SaU 005), 2.1-2.5 Ma (Shergotty) and 2.2-3.0 Ma (Zagami).