TEM texture and crystal chemistry of co-existing ortho- and clinopyroxene in the Antarctic ureilite FRO90054: Implications for thermal history

Details TEM texture and crystal chemistry of co-existing ortho- and clinopyroxene in the Antarctic ureilite FRO90054: Implications for thermal history TEM texture and crystal chemistry of co-existing ortho- and clinopyroxene in the Antarctic ureilite FRO90054: Implications for thermal history

Sep 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..671t&link_type=abstract

Meteoritics, vol. 32, pages 671-678

Physics

11

Scientific paper

FRO90054, from Antarctica, is a rare clino- and ortho-pyroxene bearing ureilite with a coarse equigranular oriented texture (grains up to 3 mm), classified as a low-shock Ca-rich type. The crystal chemistry of its clinopyroxene (Wo39.3En 54.6Fs6.1), orthopyroxene (En84.2Fs11Wo4.8) and olivine (Fa12.5Fo86.55) was investigated by single-crystal X-ray structural refinements and TEM observations to obtain data on the evolutionary history of the parent body. The M1 octahedron and unit cell volumes of the ortho-pyroxene and clinopyroxene are consistent with low-pressure crystallization. The closure temperatures for intracrystalline Mg-Fe2+ ordering yielded values of 674 deg and 804 deg C for opx and 596 deg C for cpx, indicating high-temperature equilibration and fast cooling. Transmission electron microscopic (TEM) investigations were performed on clinopyroxene, orthopyroxene and pigeonite. The (100) twin lamellae in the clinopyroxene and intergrowth of clino-and ortho-enstatite lamellae in orthopyroxene most probably originated by deformation. Exsolution was not observed in any of the phases, suggesting rapid cooling. TEM analysis also revealed interstitial Na-rich glass and optically overlooked pigeonite with sharp h+k odd reflections and rare stacking faults parallel to (100). TEM textural and crystal chemical data indicated rapid cooling, probably due to fast radiative heat loss as a result of the disintegration of the parent body into small fragments, which subsequently reassembled into a larger body. One or more collisional events caused fine scale stacking faults and partial melting.

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