Ar-Ar ages and trapped Ar components in Martian shergottites RBT 04262 and LAR 06319

We made Ar-39-Ar-40 (Ar-Ar) analyses of whole rock (WR) and mineral samples of two Martian shergottites, RBT 04262 (RBT) and LAR 06319 (LAR), in order to determine their Ar-Ar ages and the Ar-40/Ar-36 ratios of the trapped Martian Ar they contain. All samples released trapped (excess) Ar-40 and Ar-36 and suggested Ar-Ar ages older than their formation ages. Because trapped Ar components having different Ar-40/Ar-36 were released at different extraction temperatures, we utilized only a portion of the data to derive preferred Ar-Ar ages. We obtain Ar-Ar ages of 171 +/- 8 Ma for RBT plagioclase and 163 +/- 13 Ma for LAR whole rock. We identify two trapped Ar components. At low temperatures, particularly for plagioclase, Trapped-A with Ar-40/Ar-36 285 +/- 3 was released, and we believe this is most likely absorbed terrestrial air. At high extraction temperatures, particularly for pyroxene, Trapped-B with Ar-40/Ar-36 1813 +/- 127 was released. The poikilitic/non-poikilitic texture of RBT and the presence of large pyroxene oikocrysts allowed a clear definition of Trapped-B. This Ar component is Martian, and its isotopic similarity to the Martian atmospheric composition suggests that it may represent Martian atmospheric Ar incorporated into the shergottite melt via crustal rocks. Trapped-B partitioned into pyroxene at a constant molar ratio of K/Ar-36(Tr) = 33.2 +/- 9.5 x 10(6) for RBT 04262, and 80 +/- 21 x 10(6) for LAR 06319. Trapped-A mixed in different proportions with Trapped-B could give apparently intermediate trapped Ar-40/Ar-36 compositions commonly observed in shergottites. (c) 2013 Elsevier Ltd. All rights reserved.