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

Abstract

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 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.

Introduction

It is believed that chondrules formed during flash heating events in the solar nebula (e.g., Gooding et al., 1980; Grossman and Wasson, 1982; Hewins, 1996). Chondrules are often surrounded by rims that formed in the nebula after the host chondrules. Chondrule rims are divided into two types: (1) fine-grained or matrix-like rims that are similar in chemical composition and grain size to the host chondrite matrix (e.g., Ashworth, 1977; Allen et al., 1980; King and King, 1981; Scott et al., 1984), and (2) coarse-grained or igneous rims that show evidence of a high degree of melting (e.g., Rubin, 1984; Rubin and Wasson, 1987; Krot and Wasson, 1995). Igneous rims surround ∼50%, ∼10% and <1% of chondrules in the CV3, H-L-LL3, and CO3 chondrites, respectively (Rubin, 1984). The ubiquitous occurrence of igneous rims across the chondrite groups suggests that igneous rim formation commonly occurred in the solar nebula and was related to chondrule formation. However, the rim formation process is not as well understood compared with chondrule formation. For example, even the heating duration has not yet been evaluated.

Olivine phenocrysts within igneous rims sometimes contain very ¹⁶O-rich areas in their interiors (e.g., Takeda et al., 2002; Nagashima et al., 2003, 2011, 2013, 2015). The ¹⁶O-rich composition is clearly distinct from that of most olivine in igneous rims and the phenocrysts in the host chondrules, and is similar to that of amoeboid olivine aggregates. Such ¹⁶O-rich olivine cannot form during chondrule formation because the oxygen isotopic compositions of the minerals crystallized from chondrule melts are typically close to those of the rocky planets (Yurimoto et al., 2008; Tenner et al., 2018). Therefore, these ¹⁶O-rich olivines are the igneous rim feedstocks and heating process survivors (i.e. relict grain) from the igneous rim formation, and the rim formation process could be traced using these olivines.

To constrain the rim formation process, we studied the two-dimensional micro-distribution of the chemical compositions and oxygen isotopes in an igneous rim from the Northwest Africa (NWA) 3118 CV_oxA chondrite intercorrelated with petrography.