In porphyritic chondrules, the cores of the vast majority of olivines are characterized by a relatively low but variable CL intensity showing convoluted zones with embayment. The inner core shows different zones with variable CL intensities. Unzoned low-Ca pyroxene CL features present exceptionally well-resolved striated microstructures of thin alternations of orthoenstatite and clinoenstatite in response to the ortho-clino inversion upon cooling.Įach single Mg-rich olivine is composed of a core region surrounded by successive concentric layers of olivine that terminate by a euhedral faceted shape when olivine is bathed in the glassy mesostasis ( Figs. Mg-rich olivine grains show several dissolution fronts. Notice the significant difference between CL features of the Mg-rich olivines and the low-Ca pyroxenes, and that low-Ca pyroxene growth postdates the Mg-rich olivine. Notice that the high CL intensity recorded by the olivine core is similar to Ch#8 in (B). Mg-rich olivine grain showing multiple fronts of dissolution and growth. These CL images also reveal that olivine overgrowth episodes proceed by epitaxial growth on preexisting remnant grains. Notice the several crystallization and dissolution fronts despite the euhedral shape of the whole crystal. Details of a euhedral Mg-rich olivine bathed in the glassy mesostasis of a porphyritic olivine chondrule. Notice, however, the small rotating shift between the two images. Inset: Elemental map Al Ks x-ray showing the remarkable correspondence between Al concentration and CL intensity map. Low-Ca pyroxenes are characterized by CL well-resolved striated microstructures of ortho-clino inversion. CL zoning textures in the small olivine grains are similar to those preserved at the edge of the large porphyritic grain. Porphyritic olivine pyroxene chondrule showing heterogeneous grain size with a large porphyritic grain. ![]() Barred olivine chondrule with high CL dendritic growth in olivine bars and the thick and complex multilayer Mg-rich olivine shell at chondrule edge. The unconformity between the shell and the olivine bars and their common crystallographic orientation suggest that the Mg-rich olivine shell formed after the olivine bars by an epitaxial growth mechanism. The olivine rim that partly covers the chondrule surface corresponds to a multilayer Mg-rich olivine shell, similar to those observed at the outer edge of porphyritic chondrule. Notice the notable absence of FeNi metal blebs in olivine bars. CL images allow us to resolve the first dendritic growth of each olivine bar in this chondrule. Low-Ca pyroxene (px) postdates the olivine growth. This porphyritic olivine pyroxene chondrule shows an example of the Mg-rich olivine core with asymmetric overgrowth toward the periphery of the chondrule, forming a more or less continuous shell of the Mg-rich olivine at the outer edge, formed by successive olivine overgrowth layers. Olivine cores always contain metal (met) and glassy (gl) inclusions. This porphyritic olivine (ol) pyroxene chondrule shows the complex CL figures in each olivine with a core and euhedral outer edges. See sketch in section S1 of the important features to be observed. The inferred crystallization of olivines, from stable melts approaching equilibrium with the surrounding gas, provides an explanation for the notable absence of large and systematic isotopic fractionations in chondrules. ![]() Chondrules are thus direct thermochemical sensors of their high-temperature gaseous environment, and high partial pressures of gaseous Mg and SiO are required in their solar protoplanetary disk-forming region to maintain olivine saturation in chondrules. ![]() We argue that this interaction with the surrounding gas, rather than various cooling histories, defined chondrule composition and texture. We show that these overlooked internal zoning structures provide evidence for high-temperature gas-assisted near-equilibrium epitaxial growth of olivines during chondrule formation. High-resolution cathodoluminescence (CL) survey of chondrules from various chondrite groups revealed changes of CL activator concentrations of magnesium-rich olivines. Chondrules, millimeter-sized igneous spherules comprising the major component of most chondritic meteorites, formed during the first 4 million to 5 million years of the evolution of the solar protoplanetary disk and, therefore, can potentially offer important constraints on the conditions in the disk, provided that the processes that led to their formation can be understood.
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