39Ar-40Ar age and thermal history of martian dunite NWA 2737

We report an Ar-39-Ar-40 age determination of a whole rock sample of the olivine-rich, martian m-48% of the Ar-39 define an Ar-39-Ar-40 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 Ar-40 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 Ar-40 was only partially degassed by an impact event similar to 11 Ma ago (the CRE age), appears possible only if NWA 2737 was shock-heated to temperatures >600 degrees 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,, p earlier, more intense shock event, which left it residing in a warm ejecta layer, and a less intense event similar to 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 similar to 300500 degrees C and buried in ejecta to a depth of similar to 1-20 m. These conclusions are compared to model constraints on meteorite ejection from Mars reported in the literature. The second, Mars-ejection impact similar to 11 Ma ago probably heated NWA 2737 to no more than similar to 400 degrees C. NWA 2737 demonstrates that some martian meteorites probably experienced shock heating in events that did not eject them into space. Published by Elsevier B.V.