Mineralogy of augite‐bearing ureilites and the origin of their chemical trends

• Published in: Meteoritics
• Main Authors: Takeda, Hiroshi, Mori, H., Ogata, H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1989
• Subjects: Clayton; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1111/j.1945-5100.1989.tb00947.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1945-5100.1989.tb00947.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1945-5100.1989.tb00947.x

Abstract— Three augite‐bearing ureilites from Antarctica, Y74130, MET78008 and ALH82106, have been studied by electron microprobe, scanning electron microscope (SEM), and analytical transmission electron microscope (TEM). The first two belong to the low‐ 16 O subgroup of Clayton and Mayeda (1988) and are closely related; ALH82106 belongs to the high‐ 16 O subgroup. MET78008 is an augite‐olivine ureilite, similar to the augite‐bearing part of the Y74130. Augites poikilitically include ellipsoidal pigeonites, and low‐Ca pyroxene poikilitically encloses ellipsoidal olivine and augite. The temperature of last equilibration deduced from an orthopyroxene‐pigeonite‐augite assemblage is above 1200 °C. The ALH82106 pigeonite contains irregular augite inclusions produced by decomposition. Augites in Y74130, MET78008 and ALH82106 (decomposed one) all show similar spinodal decomposition textures on the TEM scale. Cooling rates estimated from an experimentally calibrated diagram for the wavelengths of spinodal decomposition versus cooling rates are about 20 °C/hr. The calcic trend for the low‐ 16 O subgroup including Y74130 and MET78008 shows large Ca variations in pyroxene with similar Mg/Fe ratio; differentiation involving reduction was not an important process. Augite in ALH82106 is a minor phase and contains lower Na 2 O contents than Y74130 and MET78008, confirming that ALH82106 is not directly related to them.