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

During the early episode of the solar system, the L6 chondrite Tenham has been affected by intense thermal metamorphism. Microanalytical dat a reveal homogeneous compositions of olivine (Fo(75)Fa(25)), enstatite (En(79)Fs(19)Wo(2)), and diopside (En(47)Fs(8)Wo(45)). Using these da ta, empirical pyroxene thermometers yield temperature estimates for th is thermal metamorphism, ranging from 810 to 870 degrees 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 ex ample for strong shock metamorphism. In contrast to previous transmiss ion electron microscopy (TEM) studies, which concentrated on these sho ck veins, we also systematically characterized the shock signature of the silicates occulting in the bulk of Tenham. Plagioclase is either p ervaded by thin (200 nm), amorphous lamellae, so-called planar deforma tion features (''PDFs''), or it is transformed to maskelynite, a diapl ectic glass of feldspar composition. In olivine, shock deformation has caused the formation of irregular and planar fractures and the activa tion of numerous (2 x 10(14) m(-2)) c dislocations in the glide planes (100) and {110}; energetically favorable but less mobile a dislocatio ns are totally absent. Fracturing in olivine is interpreted as the cau se of dislocation formation. A low dislocation density (<10(12) m(-2)) and clinoenstatite lamellae have been detected in orthornstatite. Alt hough other formation mechanisms are known for the ortho-/clino-enstat ite inversion, a shock origin is most reasonable in this case because of the presence of strong shock damage in the other silicates. Diopsid e displays the greatest diversity of shock defects: mechanical twins p arallel to (100) and (001), numerous dislocations, and PDFs. The predo minant 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 (less than or equal to 50 nm ), amorphous lamellae in naturally shocked diopside. These PDFs are or iented parallel to {$$($) over bar 221} and {221}. Fine-grained (<2-3 mu m), polycrystalline aggregates of the high-pressure spinels, majori te and ringwoodite, were observed in a thin shock vein. Majorite is de fect-free, whereas ringwoodite contains several stacking faults parall el to {110} planes. Microanalyses show that both phases are less homog eneous than olivines and pyroxenes. This and the small grain sizes sug gest a rapid crystallization of majorite and ringwoodite from a high-p ressure melt, The stacking faults in ringwoodite are, hence, interpret ed 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 a scribed to pressure excursions, but mainly to the high shear stresses resulting from the different shock impedances of Tenham's mineral cons tituents.