3 super(9)Ar- super(4) super(0)Ar age and thermal history of martian dunite NWA 2737

• Published in: Earth and planetary science letters 2008-09, Vol.273 (3-4), p.386-392
• Main Authors: Bogard, D D, Garrison, D H
• Format: Article
• Language: English
• ISSN: 0012-821X
• DOI: 10.1016/j.epsl.2008.07.003

We report an super(3) super(9)Ar- super(4) super(0)Ar age determination of a whole rock sample of the olivine-rich, martian m-48% of the super(3) super(9)Ar define an super(3) super(9)Ar- super(4) super(0)Ar isochron age of 160-190 Ma, when evaluated in various ways. Higher temperature extractions show increasing ages that eventually exceed the reported Sm-Nd age of 1.42 Ga. At least part of this excess super(4) super(0)Ar may have been shock implanted from the martian atmosphere. We considered two possible interpretations of the Ar-Ar isochron age, utilizing the measured Ar diffusion characteristics of NWA 2737 and a thermal model, which relates Ar diffusion to the size of a cooling object after shock heating. One interpretation, that super(4) super(0)Ar was only partially degassed by an impact event ~11 Ma ago (the CRE age), appears possible only if NWA 2737 was shock-heated to temperatures >600 super(o)C and was ejected from Mars as an object a few 10 s of cm in diameter. The second interpretation, which we prefer, is that NWA experienced an earlier, more intense shock event, which left it residing in a warm ejecta layer, and a less intense event ~11 Ma ago, which ejected it into space. Our evaluation would require NWA 2737 to have been heated by this first event to a temperature of ~300500 super(o)C and buried in ejecta to a depth of arrow right -20 m. These conclusions are compared to model constraints on meteorite ejection from Mars reported in the literature. The second, Mars-ejection impact ~11 Ma ago probably heated NWA 2737 to no more than ~400 super(o)C. NWA 2737 demonstrates that some martian meteorites probably experienced shock heating in events that did not eject them into space.