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Shergottites Dhofar 019, SaU 005, Shergotty, and Zagami: 40 Ar‐ 39 Ar chronology and trapped Martian atmospheric and interior argon
Author(s) -
KOROCHANTSEVA Ekaterina V.,
TRIELOFF Mario,
BUIKIN Alexei I.,
HOPP Jens
Publication year - 2009
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2009.tb00735.x
Subject(s) - geology , martian , argon , mantle (geology) , geochemistry , olivine , isotopes of argon , basalt , mineralogy , mars exploration program , astrobiology , chemistry , physics , organic chemistry
— We report a high‐resolution 40 Ar‐ 39 Ar 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 ( 40 Ar/ 36 Ar) 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 ( 40 Ar/ 36 Ar) 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 ( 40 Ar/ 36 Ar) trapped ratios in shocked shergottites, we suggest argon implantation from a “transient atmosphere” during impact induced degassing. The best 40 Ar‐ 39 Ar age estimate for Dhofar 019 is 642 ± 72 Ma (maskelynite). SaU 005 samples are between 700–900 Ma old. Relatively high 40 Ar‐ 39 Ar 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 40 Ar components. The youngest 40 Ar‐ 39 Ar age of Shergotty separates (maskelynite) is ˜370 Ma, that of Zagami is ˜200 Ma. The 40 Ar‐ 39 Ar 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 40 Ar, because excess 40 Ar is supported by 36 Ar trapped . Young ages obtained by 40 Ar‐ 39 Ar 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 019), 1.0–1.6 Ma (SaU 005), 2.1–2.5 Ma (Shergotty) and 2.2–3.0 Ma (Zagami).