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

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The iron-magnesium exchange between M1 and M2 sites in orthopyroxene is a reversible reaction which records the latest event in the thermal evolution of the host rock. A kinetic analysis of this process has been applied to sixteen orthopyroxene single-crystals from seven different diogenites to constrain the cooling history of their parent body. The Fe2+-Mg ordering degrees have been determined by X-ray single-crystal diffraction. The Fe2+-Mg ordering closure temperatures were very homogeneous within each sample and ranged, for all diogenites studied, between 311(29 degC and 408(10 degC. Cooling rates at these closure temperatures were calculated using a numerical method developed by Ganguly (1982). These ranged between ~5 degC/104y in Johnstown and ~0.8 degC/y in Roda. A comparison with other achondrites studied with the same method showed that increasing closure temperatures 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, 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.

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