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

We analyzed the compositional profiles of coexisting orthopyroxenes and spinels in six diogenite samples from the Antarctic meteorite collection and used the data to constrain their thermal histories. The closure temperatures of Fe²⁺-Mg exchange between spinel and orthopyroxene in these samples vary between ∼630 and 830 °C. However, those in other diogenite samples, for which the compositional data are available in the literature, extend up to ∼1125 °C. This wide range of closure temperatures suggests repeated excavation of the diogenites from their original sites over a long time interval during cooling. The orthopyroxene grains were found to be homogeneous in composition while two of the relatively large spinel grains in the samples Elephant Moraine (EET) 87530 and Thiel Mountains (TIL) 82410 showed compositional zoning near the rim. Modeling of the spinel zoning in TIL 82410 suggests that it developed during cooling under a regolith or ejecta blanket, possibly at a depth of ∼80-120 m, and that the spinel composition was homogeneous at ∼900 °C. A nonlinear cooling model in which the cooling rate is given by ηT(K)², with η=5.8 X 10^-3 K^-1Ma^-1, leads to simulated retrograde zoning profile in spinel which match the observed profile in TIL 82410 very well.