1Nozomi Matsuda,2Naoya Sakamoto,1,3Shogo Tachibana,1,4Hisayoshi Yurimoto
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.07.006]
1Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
2Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
3UTokyo Organization for Planetary and Space Science (UTOPS), University of Tokyo, Tokyo 113-0033, Japan
4ISAS/JAXA, Sagamihara, Kanagawa, 252-210, Japan
Due to their common occurrence in various types of chondrites, igneous rims formed on pre-existing chondrules throughout chondrule-forming regions of the solar nebula. Although the peak temperatures are thought to reach similar values to those achieved during chondrule formation events, the heating duration in chondrule rim formation has not been well defined. We determined the two-dimensional chemical and oxygen isotopic distributions in an igneous rim of a chondrule within the Northwest Africa 3118 CV3oxA chondrite with sub-micrometer resolution using secondary ion mass spectrometry and scanning electron microscopy. The igneous rim experienced aqueous alteration on the CV parent body. The aqueous alteration resulted in precipitation of the secondary FeO-rich olivine (Fa40―49) and slightly disturbed the Fe-Mg distribution in the MgO-rich olivine phenocrysts (Fa11―22) at about a 1 µm scale. However, no oxygen isotopic disturbances were observed at a scale greater than 100 nm. The MgO-rich olivine, a primary phase of igneous rim formation, has δ17O = ―6 ± 3‰ and δ18O = ―1 ± 4 ‰, and some grains contain extreme 16O-rich areas (δ17O,□δ18O = ˜―30‰) nearly 10 µm across. We detected oxygen isotopic migration of approximately 1 µm at the boundaries of the extreme 16O-rich areas. Using oxygen self-diffusivity in olivine, the heating time of the igneous rim formation could have continued from several hours to several days at near liquidus temperatures (˜2000 K) in the solar nebula suggesting that the rim formed by a similar flash heating event that formed the chondrules.