Exposure history of the Mocs (L6) chondrite: A study of strewn field samples

We measured cosmogenic radionuclides (Be-10, Al-26, and Cl-36) and noble ga ses (He, Ne, and Ar) in 10 specimens of the Mocs L6 chondrite to determine the exposure history and preatmospheric relationship among fragments from k nown locations in the strewn field. Cosmogenic noble gas contents alone are consistent with a simple irradiation exposure of 15.2 Ma. However, Mocs ha s very low Ne-22/Ne-21 ratios indicative of deep burial in a large meteoroi d, but radionuclide levels at saturation values typical for much smaller me teoroids: this paradox suggests a possible complex exposure. For the latter case, we propose a two-stage exposure history in which Mocs initially was deeply buried in a large object for 110 Ma, followed by exposure in a 65 cm object for 10.5 Ma. Relative shielding was inferred from the measured 22Ne /21Ne ratios assuming constant Ne-22/Ne-21 production for all samples durin g the first stage. These shielding levels, which are supported by estimates based on Cl-36 production by neutron capture, indicate a possible relation ship between depth of samples in the Mocs meteoroid and fall location in th e strewn field.