Heating duration of igneous rim formation on a chondrule in the Northwest Africa 3118 CV3(oxA) carbonaceous chondrite inferred from micro-scale migration of the oxygen isotopes

摘要

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 CV3(oxA) 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 (Fa(40-49)) and slightly disturbed the Fe-Mg distribution in the MgO-rich olivine phenocrysts (Fa(11-22)) at about a 1 mu m 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 delta O-17 = -6 +/- 3 parts per thousand and delta O-18 = -1 +/- 4 parts per thousand, and some grains contain extreme O-16-rich areas (delta O-17, delta O-18 = similar to -30 parts per thousand) nearly 10 mu m across. We detected oxygen isotopic migration of approximately 1 mu m at the boundaries of the extreme O-16-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 (similar to 2000 K) in the solar nebula suggesting that the rim formed by a similar flash heating event that formed the chondrules.