Cooling rates of diogenites: A study of Fe2+-Mg ordering in orthopyroxene by single-crystal x-ray diffraction

The iron-magnesium exchange between Ml and M2 sites in orthopyroxene is a reversible reaction that records the latest event in the thermal evolution of the host rock. A kinetic analysis of this process has been applied to 16 orthopyroxene single crystals from 7 different diogenites to constrain the cooling history of their parent body. The Fe2+-Mg ordering degrees have been determined by single-crystal x-ray diffraction. The Fe2+-Mg ordering closure temperatures were very homogeneous within each sample and ranged, for all diogenites studied, between 311 ± 29 °C and 408 ± 10 °C. Cooling rates at these closure temperatures were calculated using a numerical method developed by Ganguly (1982). These ranged between ∼5 °C/104 year in Johnstown and ∼0.8 °C/year in Roda. A comparison with other achondrites studied with the same method showed that increasing closure tem-peratures correspond to increasing cooling rates and that meteorites from a same parent body exhibit similar closure temperature and cooling rate values. The cooling rates obtained for these diogenites, at their low closure temperatures, should probably be ascribed to a complex thermal history of their parent body, thus confirming Miyamoto and Takeda's theory (1994a) of excavation of deep crustal material due to impact events. The differences on cooling rate values for different diogenites could be due to different burial depths in the fragment ejected from the parent body.