COOLING RATES OF DIOGENITES - A STUDY OF FE2-MG ORDERING IN ORTHO-PYROXENE BY SINGLE-CRYSTAL X-RAY-DIFFRACTION()

The iron-magnesium exchange between M1 and M2 sites in orthopyroxene i s a reversible reaction that records the latest event in the thermal e volution of the host rock. A kinetic analysis of this process has been applied to 16 orthopyroxene single crystals from 7 different diogenit es to constrain the cooling history of their parent body. The Fe2+-Mg ordering degrees have been determined by single-crystal x-ray diffract ion. The Fe2+-Mg ordering closure temperatures were very homogeneous w ithin each sample and ranged, for all diogenites studied, between 311 +/- 29 degrees C and 408 +/- 10 degrees C. Cooling rates at these clos ure temperatures were calculated using a numerical method developed by Ganguly (1982). These ranged between similar to 5 degrees C/10(4) yea r in Johnstown and similar to 0.8 degrees C/year in Roda. A comparison with other achondrites studied with the same method showed that incre asing closure temperatures correspond to increasing cooling rates and that meteorites from a same parent body exhibit similar closure temper ature and cooling rate values. The cooling rates obtained for these di ogenites, at their low closure temperatures, should probably be ascrib ed to a complex thermal history of their parent body, thus confirming Miyamoto and Takeda's theory (1994a) of excavation of deep crustal mat erial due to impact events. The differences on cooling rate values for different diogenites could be due to different burial depths in the f ragment ejected from the parent body.