Nesquehonite, a hydrous magnesium carbonate, occurs as a weathering product on the surface of the Antarctic meteorite LEW 85320(H5 chondrite). Antarctic meteorites have resided on the earth for periods of 104 to 106 years, but the time needed for weathering products to form has been uncertain. Isotopic measurements of [delta]13C and [delta]18O indicate that the nesquehonite formed at near freezing temperatures by reaction of meteoritic minerals with terrestrial water and carbon dioxide. Results from carbon-14 dating suggest that, although the meteorite has been in Antarctica for at least 3.2 x 104 to 3.3 x 104 years, the nesquehonite formed after A.D.1950.