Orthopyroxene as a Geospeedometer: Thermal History of Kapoeta, Old Homestead 001 and Hughes 002 Howardites

Single crystals of orthopyroxene from small fragments of the Kapoeta, Old Homestead 001, and Hughes 002 howardites were studied by x-ray diffraction and microprobe analyses. The Fe-Mg equilibrium distribution coefficients k~ of the crystals were used to calculate the closure temperatures (T,) using the calibration by Stimpfl et al. (1 999). The compositions, the presence of exsolved augite lamellae, and the T, values (from 365 to 385 "C) obtained for Kapoeta orthopyroxenes suggest that our fragment comes from a diogenitic cumulate clast. The more Fe-rich composition, the absence of exsolved lamellae, and the higher T, values (from 583 to 605 "C) measured in the Old Homestead 001 orthopyroxenes suggest that this fragment comes from a cumulitic clast affected by fast cooling at high temperature. For the Hughes 002 orthopyroxenes, close in composition to Old Homestead 001, the different T, values (339, 358, and 607 "C) recorded by the various crystals and the presence of augite lamellae in the crystals with the lowest T, support the hypothesis that this howardite sample is an unheated breccia containing a mixture of cumulitic orthopyroxenes with different thermal histories.