Transmission electron microscopy of chondrule minerals in the Allende meteorite: constraints on the thermal and deformational history of granular olivine-pyroxene chondrules

Abstract

The microstructures of minerals of three granular olivine-pyroxene chondrules from the Allende carbonaceous chondrite have been studied by methods of transmission electron microscopy, including electron diffraction and energy dispersive X-ray microanalysis. The main issue is to deduce the thermal and deformational history of the chondrules. The chondrules consist dominantly of olivine (Fa 3–36), mostly Mg-rich ones ( < Fa 15), and low-Fe ( < 3 mol%) pyroxenes along the join Mg 2Si 2O 6CaMgSi 2O 6. In addition, Ca-rich plagioclase (An 80–An 90), awaruite, magnetite, whitlockite, pentlandite, troilite, MgFeCrAl spinels, sodalite, nepheline and corundum were observed. Pyroxenes found are orthopyroxene (Opx), clinoenstatite (Cen), pigeonite (Pig), lamellar intergrowth of pigeonite and diopside (Pig/Di) on (001) and diopside (Di). The assemblages and chemical compositions of Pig, Pig/Di and Di indicate a subsolidus closure temperature of approximately 1345°C. The lamellar intergrowth Pig/Di was formed by exsolution. The average wavelength λ (periodicity) of the lamellae is 29 nm in chondrule 1, 25 nm in chondrule 2 and 33 nm in chondrule 3. A cooling rate between 25 and 0.4°C/h for the temperature interval 1350-1200°C is estimated from the wavelength λ of the exsolution lamellae. Independent information on the cooling rate is provided by the size of the b-antiphase domains (APDs) of Ca-rich plagioclase, which reflects the time-and temperature-dependent ordering of Al Si . The size (average diameter) of the domains is 10–30 nm. Comparison with isothermal annealing experiments on Ca-rich plagioclases (An 70–An 100) yields a cooling rate between 12 and 0.03°C/h. Cooling rates derived from the microstructure of pyroxene and plagioclase are at least one order of magnitude lower than those obtained from dynamic crystallization experiments (e.g. Hewins, R. H. Meteorites and the Early Solar System, pp. 660–679, 1988). However, our estimate is only valid in a small temperature interval. This period of slow cooling is followed by quenching, as indicated by the absence of orthoenstatite lamellae in clinoenstatite, suggesting high cooling rates ⩾ 10 4°C/h at a temperature of ∼ 1000°C (Brearley, A. J. and Jones, R. H., Lunar Planet. Sci. XXIV, 185–186, 1993). Olivine and pyroxene grains are frequently intersected by veins—interpreted as former cracks—filled with Fe-rich olivine and chromitic and hercynitic spinels. Formation of the veins occurred after exsolution of clinopyroxene into Pig/Di. Steep Fe and Mg concentration gradients between Fe-rich veins and the surrounding Mg-rich olivine were observed. Here, Fe Mg interdiffusion is limited to approximately 1 μm. This observation indicates fast cooling in the solar nebula and low temperature, not exceeding approximately 330°C, in the parent body. The few deformation effects demonstrate that the Allende meteorite in total was never subjected to shock waves with a peak pressure exceeding 5 GPa.