^1,2,3Alan E. Rubin,^1,3Bastian Baecker
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13725]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, 90095–1567 USA
²Maine Mineral & Gem Museum, 99 Main Street, P.O. Box 500, Bethel, Maine, 04217 USA
³Baker Hughes, Baker-Hughes-Str. 1, Celle, 29229 Germany
Published by arrangement with John Wiley & Sons

Phosphorus X-ray maps of olivine phenocrysts in many type II (FeO-rich) porphyritic chondrules in LL3.00 Semarkona and CO3.05 Y 81020 reveal multiple sets of thin dark/bright (P-poor/P-rich) layers that resemble oscillatory zoning. Such discrete layers are generally not evident in BSE images or in Fe, Cr, Ca, Al, Mg, or Mn X-ray maps because rapid diffusion of these cations in olivine at high temperatures smoothed out their initial distributions, thereby mimicking normal igneous zoning. In contrast, the relatively slow diffusion of P in olivine preserves original dendritic or hopper morphologies of olivine crystals; these skeletal structures formed during quenching after initial chondrule melting. The skeletal olivine crystals were filled in with low-P olivine during cooling after one or more subsequent heating events, mainly involving the melting of mesostasis. Crystallization of mafic silicates depleted the mesostasis in FeO and MgO and enriched it in silico-feldspathic components. Sectioning of the olivine grains at particular orientations can produce apparent oscillatory zoning in P. Strong evidence of a secondary melting event is evident in Semarkona chondrule H5k. Phenocryst H5k-2 in this chondrule has a relict core (with rhythmic P zoning layers) that was fractured and severed; it is overlain by a set of differently oriented subparallel P-poor olivine layers. Chondrule C6f from Y 81020 contains a large multi-lobed olivine phenocryst that still preserves hopper cavities, partially outlined by P-poor/P-rich olivine layers. The thin P-rich rims surrounding many olivine phenocrysts could reflect a short period of rapid grain growth after a late-stage chondrule reheating event.

¹H. Palme,¹J. Zipfel
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13720]
¹Senckenberg Forschungsinstitut und Naturmuseum Frankfurt, Senckenberganlage 25, Frankfurt, 60325 Germany
Published by arrangement with John Wiley & Sons

Between 1973 and 1994, 15 samples of CI chondrites were analyzed by neutron activation analysis at the Max-Planck-Institute for Chemistry, Department of Cosmochemistry in Mainz, Germany. The analyses comprise nine Orgueil samples and three samples of Ivuna, two of Alais and one of Tonk. Samples came from various sources and had masses between 5 and 600 mg. Most data are published here for the first time. The results for the nine Orgueil samples demonstrate the essentially homogeneous chemical composition of Orgueil at a level of a few milligrams. The analytical results of Ivuna, Alais, and Tonk agree, with only few exceptions, with the results of Orgueil analyses. All samples agree within ±3% in their contents of Sc, Ir, Cr, Fe, Co, Zn, and Se. The elements Sc and Ir represent the refractory component; Cr, Fe, and Co the main component; and Zn and Se the volatile component. Thus, in all CI chondrites there are essentially the same fractions of the fundamental cosmochemical components. The essentially identical chemical composition of all samples shows that their water contents are constant at about 20 ± 5 wt%. There is excellent agreement between the data listed here with data reported in the relevant literature. There is no doubt that the CI composition is a well-defined entity, which is thought to represent the non-gaseous compositions of the solar nebula and the photosphere of the Sun. In addition, we conclude that the recently proposed new CI chondritic chlorine and Br values are too low, when compared to earlier measurements.