Seiko Watanabe

A transmission electron microscope study of pyroxene chondrules in equilibrated L-group chondrites

Abstract The fine structures and chemical compositions of pyroxenes in L-group chondrites of types 3–4, 4–5, and 6 have been studied by analytical electron microscopy. The pyroxenes in the chondrules of L3-4 and L4-5 are different from those of L3 in that, (1) Ca-free regions of pyroxenes are clinopyroxene in L3 and are orthopyroxene in L3-4 and L4-5, and (2) the differences of Ca content among the different zones of pyroxene laths are less prominent in L3-4 and L4-5 than L3, while Fe content varies in L3-4 and L4-5 and is constant in L3. These differences suggest that the cooling rate of L3 was greater than those of L3-4 and L4-5. In the L6 chondrite textures due to the original thermal history of the chondrite have apparently been wiped out by later shock deformations. In the chondrules of all the L3, 3–4 and 4–5 chondrites, the Ca-rich region of pyroxenes shows spinodal decomposition textures with similar periodicities. This texture indicates that very little diffusion of Ca was possible after spinodal decomposition in all pyroxenes of the L chondrites. Thus the equilibrated chondrites can not have experienced a significant reheating event, but cooled more slowly than L3. This favours the “autometamorphism” model.

Cooling history of pyroxene chondrules in the Yamato-74191 chondrite (L3)—an electron microscopic study

Abstract Fine textures of clinopyroxene in an excentroradial pyroxene chondrule (EPC) and a comb-like pyroxene chondrule (CPC) in the Yamato-74191 chondrite (L3) have been studied by analytical electron microscopy. Both pyroxenes consist of three regions different in composition and texture; core, mantle and marginal regions, though the pyroxenes of the CPC are more Fe-rich than those of the EPC. The core region is the most Mg-rich with no Ca component and commonly shows polysynthetic (100) twins. The mantle region is slightly calcic, and the marginal region shows a rapid increase of Ca outward. The polysynthetic twins, cracks and subgrain boundaries in the core in the EPC and CPC must have formed during the transition from proto-type to clino-type pyroxenes. The exsolution textures in the mantle and marginal regions indicate initial crystallization of pigeonite- C followed by decomposition into pigeonite- P and augite. The decomposition must have taken place by nucleation growth in the mantle region and by spinodal decomposition in the marginal region. The periodicity of 15–20 nm in the spinodal decomposition textures indicates that the cooling rate of the pyroxenes, when passing through about 1000°C, was of the order of a few tens to several degrees centigrade per hour. The cooling history of the chondrules has been explained by a monotonous cooling controlled by the cooling rate of the surrounding medium.

Highly fractionated REE in the Hedjaz (L) chondrite: implications for nebular and planetary processes

Abundances of REE, Sr, Rb, K, Ca and Mg in two whole-rock fragments, two lithic fragments and three chondrules of the Hedjaz (L3) chondrite were determined by isotope dilution mass spectrometry. One whole-rock fragment shows a step pattern with the light-REE enriched, indicating that the meteorite contains components with highly fractionated REE abundances. The chondrules (PP, BO and one unknown type) show variable (0.8 ∼ 2 × CI) REE abundances and almost flat REE patterns with minor irregularities of Ce, Eu and Yb. Anomalous REE patterns were identified for two light-colored lithic fragments. A pyroxene-rich, glass-bearing subrounded clast I has low REE abundances (0.3 ∼ 0.8 × CI) together with depletion of other lithophiles (Ca, Al, Sr, Na and K). It shows a light-REE enhanced pattern with positive anomalies of Ce, Eu and Yb. This is a new REE pattern reported for lithic clasts from ordinary chondrites. It is suggested that the clast formed through melting processes (possibly under planetary conditions) from the condensates either from a later stage nebular gas or from gas vaporized from dust. The second clast II (Ca = 1.1 ∼ 1.9%) consists mainly of coarse-grained olivine, minor pyroxene and plagioclase, and trace amounts of glass; it shows an igneous texture with shock features. It has a strong positive correlation of plagiophile elements (Ca, Sr and Eu) but heterogeneous distributions of common REE from portion to portion. One of the chips indicates a remarkable REE fractionation (LREE = ∼ 40 ×,HREE = ∼ 1.7 × CI) similar to that of Group II CAIs of carbonaceous chondrites. This is the first identification of Group II REE pattern in lithic clasts from ordinary chondrites. It is suggested that the clast had formed by shock-induced melting of an inhomogeneous CV-like precursor assemblage carrying high-temperature nebular REE components on a grand-parent body and was then incorporated into the Hedjaz meteorite parent body. The presence of impact-related products with highly fractionated REE components in a chondritic breccia indicates the existence of related physico-chemical conditions in the formation of chondrules, clasts and their precursors with that of CAIs.

Fine-grained aggregates in L3 chondrites

Abstract The textures and chemical compositions of the constituent minerals of the fine-grained aggregates (FGAs) of L3 chondrites were studied by the backscattered electron image technique, electron probe microanalysis, and transmission electron microscopy. Plagioclase and glass in the interstices between fine grains of olivine and pyroxene indicate that the FGAs once partly melted. Compositional zoning and decomposition texture of pyroxenes are similar to those observed in chondrules, indicating a common cooling history of the FGAs and chondrules. Therefore, the mechanism that caused melting of the FGAs is considered to be the same as for chondrules. Bulk compositions of the FGAs are within the range of those of chondrules, so some chondrules probably were produced by complete melting of the same precursor materials as those of the FGAs. The precursor materials must have included fine olivine and other grains that probably are condensates.