THERMAL AND SHOCK METAMORPHISM OF THE TENHAM CHONDRITE: A TEM EXAMINATION

Abstract

During the early episode of the solar system, the L6 chondrite Tenham has been affected by intense thermal metamorphism. Microanalytical data reveal homogeneous compositions of olivine (Fo75Fa25), enstatite (En79Fs19Wo2), and diopside (En47Fs8Wo45). Using these data, empirical pyroxene thermometers yield temperature estimates for this thermal metamorphism, ranging from 810 to 870°C. Due to the presence of thin shock veins, which contain the high-pressure phases majorite and ringwoodite, the L6 chondrite Tenham is an instructive example for strong shock metamorphism. In contrast to previous transmission electron microscopy (TEM) studies, which concentrated on these shock veins, we also systematically characterized the shock signature of the silicates occurring in the bulk of Tenham.Plagioclase is either pervaded by thin (200 nm), amorphous lamellae, so-called planar deformation features (''PDFs''), or it is transformed to maskelynite, a diaplectic glass of feldspar composition. In olivine, shock deformation has caused the formation of irregular and planar fractures and the activation of numerous (2 x 1014 m-2) c dislocations in the glide planes (100) and {110}; energetically favorable but less mobilea dislocations are totally absent. Fracturing in olivine is interpreted as the cause of dislocation formation. A low dislocation density (<1012 m-2) and clinoenstatite lamellae have been detected in orthoenstatite. Although other formation mechanisms are known for the ortho-/clino-enstatite inversion, a shock origin is most reasonable in this case because of the presence of strong shock damage in the other silicates. Dipside displays the greatest diversity of shock defects: mechanical twins parallel to (100) and (001), numerous dislocations, and PDFs. The predominant glide system of dislocations is (100)[001], but the {110}[001] glide system is also present to a lesser extent. To our knowledge, we report here on the first evidence of thin (=<50 nm), amorphous lamellae in naturally shocked diopside. These PDFs are oriented parallel to {221} and {221}. Fine-grained (< 2-3 μm), polycrystalline aggregates of the high-pressure spinels, majorite and ringwoodite, were observed in a thin shock vein. Majorite is defect-free, whereas ringwoodite contains several stacking faults parallel to {110} planes. Microanalyses show that both phases are less homogeneous than olivines and pyroxenes. This and the small grain sizes suggest a rapid crystallization of majorite and ringwoodite from a high-pressure melt. The stacking faults in ringwoodite are, hence, interpreted as growth defects.The results of this study substantiate that the shock pressure in Tenham is heterogeneously distributed, ranging from approximately 25 to 45 GPa. The formation of shock veins is not only ascribed to pressure excursions, but mainly to the high shear stresses resulting from the different shock impedances of Tenham's mineral constituents.