TEM OBSERVATIONS OF AMORPHIZED SILICATE-PEROVSKITE, AKIMOTOITE AND Ca-RICH MAJORITE IN A SHOCK-INDUCED MELT VEIN IN THE TENHAM L6 CHONDRITE

Introduction: Tenham is a famous highly shocked L6 chondrite. Previous studies revealed a series of high-pressure minerals in the melt vein, including ringwoodite, majorite, wadsleyite, magnesiowüstite, akimotoite, silicate-perovskite, and hollanditestructured plagioclase [1-8]. These high-pressure minerals have been studied intensively in static highpressure experiments [9,10]. The presence of glassy plagioclase and polycrystalline ringwoodite in Tenham indicates a shock stage of S6 (P > 45-55 GPa) based on shock classification of ordinary chondrites [11]. However, shock pressure also can be inferred from crystallization pressure of the melt at ~25 GPa, based on the phase equilibrium data [12-14]. The duration of the melt-vein solidification can be estimated based on simple thermal models [15, 16]. The goal of this study is to characterize the mineral assemblages in the melt veins of Tenham, using TEM, to constrain crystallization and to see how the crystallization pressures are related to the shock pressure inferred from the calibration of Stöffler et al [11]. Methods: Field-emission scanning electron microscopy (FESEM) is used to characterize the textures of a melt vein in a thin section of Tenham. A 3-mm dist, including a 600μm wide melt vein, was cored from the thin section to make TEM sample. TEM imaging techniques were employed to characterize the micro-texture of the veins and the microstructures of the vein minerals. Mineral phases were identified on the basis of selected area electron diffraction (SAED) patterns and quantitative energy dispersive X-ray spectroscopy (EDS). Results: The mineralogy and the micro-texture of the crystallized silicate melt at the vein edge differ from those of the vein center. The vein-edge region is ~30 μm wide and consists of a mixture of equant grains, non-equant crystals, and a few rounded metalsulfide droplets (Fig. 1). TEM imaging and electron diffraction show that the equant grains are amorphous silicate. Quantitative EDS analysis gives the composition Na0.08Fe0.16Mg0.82Ca0.03Al0.05Si0.93O3, which is similar to the composition of matrix majorite and consistent with the composition of silicate perovskite. The morphology and composition are similar to the vitrified silicate-perovskite in the Acfer 040 chondrite [17] and in the Zagami achondrite [15]. Unlike the previously reported silicate-perovskite in Tenham [7], which formed by a solid-state transformation of enstatite, we interpret these amorphous grains as silicateperovskite that crystallized from silicate melt at high pressure. These perovskites vitrified after pressure release, probably under elevated post-shock temperature conditions or possibly during sample preparation. The principle non-equant crystal in the vein edge is ringwoodite. EDS analysis shows that the ringwoodite (Fe0.7Mg1.3Al0.02Si0.98O4) is rich in Al2O3 and FeO comparing to the host olivine (Fa26) and the polycrystalline ringwoodite, indicating that it crystallized from the melt. Akimotoite (Fe0.12Mg0.89Al0.05Si0.96O3), which occurs along with ringwoodite, forms long crystals that are partially vitrified and surrounded by silicate glass. Majorite was also found beside amorphous silicateperovskite.