TRANSMISSION ELECTRON-MICROSCOPE TEXTURE AND CRYSTAL-CHEMISTRY OF COEXISTING ORTHOPYROXENE AND CLINOPYROXENE IN THE ANTARCTIC UREILITE FRONTIER MOUNTAIN 90054 - IMPLICATIONS FOR THERMAL HISTORY

Frontier Mountain (FRO) 90054, from Antarctica, is a rare clino- and o rthopyroxene-bearing ureilite with a coarse equigranular oriented text ure (grains up to 3 mm); it is classified as a low-shock Ca-rich type. The crystal chemistry of its clinopyroxene (Wo(39.3)En(54.6)Fs(6.1)), orthopyroxene (En(84.2)Fs(11)Wo(4.8)) and olivine (Fa(12.6)Fo(86.9)) was investigated by single-crystal x-ray structural refinements and tr ansmission electron microscope (TEM) observations to obtain data on th e evolutionary history of the parent body. The M1 octahedron and unit cell volumes of the orthopyroxene and clinopyroxene are consistent wit h low-pressure crystallization. The closure temperatures for intracrys talline Mg-Fe2+ ordering yielded values of 674 degrees C and 804 degre es C for opx and 596 degrees C for cpx, which indicate high-temperatur e equilibration and fast cooling. Trasmission electron microscope inve stigations were performed on clinopyroxene, orthopyroxene and pigeonit e. The (100) twin lamellae in the clinopyroxene and intergrowth of cli no- and orthoenstatite lamellae in orthopyroxene most probably origina ted by deformation. Exsolution was not observed in any of the phases, which suggests rapid cooling. Analysis by TEM also revealed interstiti al Na-rich glass and pigeonite with sharp h+k odd reflections and rare stacking faults parallel to (100). Textural and crystal chemical data , obtained by TEM, indicated rapid cooling that was probably due to fa st radiative heat loss as a result of the disintegration of the parent body into small fragments, which subsequently reassembled into a larg er body. One or more collisional events caused fine-scale stacking fau lts and partial melting.