FORMATION HISTORY OF CI-LIKE PHYLLOSILICATE-RICH CLASTS IN THE TSUKUBA METEORITE INFERRED FROM MINERALOGY AND NOBLE GAS SIGNATURES

Abstract

Fine-grained, angular black clasts with longest dimensions up to 800 μm were found, dispersed in a brecciated region in the Tsukuba meteorite. The clasts, much darker than the dark portions of the light-dark structure in the Tsukuba host, are characterized by the presence of plaquette, spherulitic, and framboidal magnetites and pseudo-hexagonal pyrrhotite. Synchrotron radiation X-ray diffraction analyses showed that the major silicate phases in the clasts are saponite and serpentine. Transmission electron microscopy (TEM) revealed that the phyllosilicates have compositions quite similar to those in the Orgueil CI carbonaceous chondrite. Therefore, mineralogically and chemically, the clasts resemble CI chondrites and are quite different from the host H5-6 chondrite Tsukuba. The clasts serve as an indicator of temperatures during regolith lithification of the Tsukuba parent body, because their mineralogy and noble gas compositions are subject to change during weak heating. The 001 basal spacing of saponite in the clast is 13.2 A, which indicates that saponite retains interlayer water molecules. This observation and the presence of serpentine suggest that the clasts, as a whole, have never been heated above 500°C. Noble gas analyses showed that the clasts contain large amounts of solar and primordial noble gases. The results of stepwise heating analyses indicate that no apparent thermal loss of noble gases from the clasts occurred, consistent with the mineralogical evidence. TEM observation revealed that the periphery of a clast was transformed into amorphous phases, indicating that only the periphery was heated to 700°C. Based on the results of our analyses, the formation history of the clasts can be evaluated as follows. Constituents of the clasts had been exposed to solar wind on the surface of the CI-chondrite-like parent body and formed the clasts. After ejection from the CI-like parent body, the CI-like meteoroid accreted to the Tsukuba parent body after early thermal metamorphism that reached 700-1000°C. The clasts must have been located in the regolith, because they were found among the light-dark structure in the meteorite and contain abundant solar noble gases. Impacts into the regolith have crushed the clasts into smaller pieces and probably induced lithification of the regolith material. During lithification, the peripheries of the clasts were heated briefly to 700°C, but the temperature of the interior of the clasts has never exceeded 500°C. This is the first evidence constraining temperature during lithification of the ordinary-chondrite parent-body regolith. Finally, the Tsukuba meteoroid, including the CI-like clasts, was ejected from the Tsukuba parent body and fell onto the Earth after a transit time of 8.1 Ma.