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

摘要

We report a high-resolution Ar-40-Ar-39 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 (Ar-40/Ar-36)(trapped) ratios are found for argon in pyroxene melt inclusions (similar to 1500), maskelynite (similar to 1200), impact glass (similar to 1800) of Shergotty and impact glass of SaU 005 (similar to 1200). A high (Ar-40/Ar-36)(trapped) component-uually uniquely ascribed to Martian atmosphere-can also originate from the Martian interior, indicating a heterogeneous Martian mantle composition. As additional explanation of variable high (Ar-40/Ar-36)(trapped) ratios in shocked shergottites, we suggest argon implantation from a "transient atmosphere" during impact induced degassing. The best Ar-40-Ar-39 age estimate for Dhofar 019 is 642 +/- 72 Ma (maskelynite). SaU 005 samples are between 700-900 Ma old. Relatively high Ar-40-Ar-39 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 Ar-40 components. The youngest Ar-40-Ar-39 age of Shergotty separates (maskelynite) is similar to 370 Ma, that of Zagami is similar to 200 Ma. The Ar-40-Ar-39 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 Ar-40, because excess Ar-40 is supported by Ar-36(trapped). Young ages obtained by Ar-40-Ar-39 and other chronometers argue for primary rather than secondary events. The cosmic ray exposure ages calculated from cosmogenic argon are 15.7 +/- 0.7 Ma (Dhofar 0 19), 1.0-1.6 Ma (SaU 005), 2.1-2.5 Ma (Shergotty) and 2.2-3.0 Ma (Zagami).