Compositional properties of coexisting orthopyroxene and spinel in some Antarctic diogenites: Implications for thermal history

We analyzed the compositional profiles of coexisting orthopyroxenes and spi nels in six diogenite samples from the Antarctic meteorite collection and u sed the data to constrain their thermal histories. The closure temperatures of Fe2+-Mg exchange between spinel and orthopyroxene in these samples vary between similar to 630 and 830 degreesC. However, those in other diogenite samples, for which the compositional data are available in the literature, extend up to similar to 1125 degreesC. This wide range of closure temperat ures suggests repeated excavation of the diogenites from their original sit es over a long time interval during cooling. The orthopyroxene grains were found to be homogeneous in composition while two of the relatively large sp inel grains in the samples Elephant Moraine (EET) 87530 and Thiel Mountains (TIL) 82410 showed compositional zoning near the rim. Modeling of the spin el zoning in TIL 82410 suggests that it developed during cooling under a re golith or ejecta blanket, possibly at a depth of similar to 80-120 m, and t hat the spinel composition was homogeneous at similar to 900 degreesC. A no nlinear cooling model in which the cooling rate is given by T-eta(K)(2), wi th eta = 5.8 x 10(-3) K(-1)Ma(-2), leads to simulated retrograde zoning pro file in spinel which match the observed profile in TIL 82410 very well.