ANTARCTIC FRO90011 LODRANITE - COOLING HISTORY FROM PYROXENE CRYSTAL-CHEMISTRY AND MICROSTRUCTURE

The Antarctic FR090011 lodranite shows an achondritic coarse granoblastic texture with an approximately chondritic composition. The crystal chemistry of Pbca and C2/c pyroxene single crystals and the TEM texture of Pbca pyroxene were investigated to reconstruct the evolutionary history of the meteorite parent body. The structural behaviour of orthopyroxene (En(85)Fs(12.4)Wo(2.6)) is well constrained in the En-Fs trend. Orthopyroxene Fe2+-Mg ordering in the M1 and M2 sites yields a closure temperature (T-c) of 570 +/- 20 degrees C and a cooling rate 10 degrees C/day near the T-c. Clinopyroxene is a low-iron augite (En(51)Fs(5)Wo(44)). Clinopyroxene Fe2+-Mg ordering in the M1 and M2 sites yields a T-c of 513 +/- 50 degrees C, which is compatible with the coaling rate derived from the orthopyroxene crystals. TEM investigation on orthopyroxene showed a few thin lamellae not larger than 20 Angstrom parallel to (100) and rare one unit cell Guinier-Preston zones. TEM-EDS analysis did not reveal significant chemical differences between lamella-rich and lamella-free zones. The above data are interpreted as being due to fast cooling resulting from radiative heat loss of small fragments ejected from a parent body, which were subsequently reassembled into a larger body.