The regolith history of 14307

Noble gas and trace element analyses of matrix and a clast from breccia 14307 are reported. This sample was exposed to a large neutron fluence, as seen by an elevated Sm-150/Sm-149 ratio and by noble gases, particularly Xe-136 from neutron fission of U-235. Strong constraints on the exposure history result from combined consideration of Sm-150, Xe-136, and spallation noble gases. Both clast and matrix were irradiated for about 1 AE under substantial shielding beginning at least 2 AE ago, probably more than 3 AE ago. The manifestations of soil exposure seen in the matrix - solar wind gases, glass formation, etc. - thus must have been acquired in an ancient epoch. The matrix has had a longer exposure to cosmic rays than the clast, presumably during its prebrecciation history as a soil. Brecciation probably occurred more than 1 AE ago, perhaps more than 3 AE ago, but at least 0.4 AE after the formation of the matrix constituents.