SiO-silicate interactions explain oxygen isotope variation of rocky inner planetary bodies

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Major precursor components of enstatite chondrites (EC), differentiated planetesimals, Mars, and the Earth are thought to have been formed at similar heliocentric distances. The unequilibrated EC preserve records of nebular conditions in each constituent component, each of which has not been heavily overprinted by post-accretionary thermal processes. Thus, these components are considered reasonable analogs of the major source materials for the inner planets. Oxygen isotopes of chondrules record distinct characteristics of the localized nebular environment, thus they are considered a key tracer of the nebular conditions at which planetesimal precursors formed during the earliest stages of the protoplanetary disk. Oxygen isotopic compositions for most of the bulk EC and EC chondrules resemble those of the Earth and Moon, clustered around the terrestrial fractionation line (TFL), whereas some pyroxenes and olivines have more ¹⁶O-poor and ¹⁶O-rich compositions, respectively. Although these data show no clear trend on the three-oxygen isotope diagram, they have been interpreted as the result of mixing processes between precursors solids including ¹⁶O-rich components and nebular gas with oxygen isotopic compositions near the TFL. However, yet uncertain are the physical and chemical parameters for the chondrule formation process, including the oxygen isotopic compositions of the nebular gas, solid/gas ratios, temperature, and the exact mixing process involved.

In this research we show that, for chondrules in unequilibrated enstatite chondrites, high-precision Δ¹⁷O values (deviation of δ¹⁷O value from a terrestrial silicate fractionation line) vary significantly (ranging from -0.49 to +0.84‰) and fall on an array with a steep slope of 1.27 on a three oxygen isotope plot. This array can be explained by reaction between an olivine-rich chondrule melt and a SiO-rich gas derived from vaporized dust and nebular gas. This study also has revealed that the Δ¹⁷O values of EC chondrules span not only the ranges of for Earth and the Moon but also the entire range of differentiated meteorites. Because oxygen constitutes >30 wt.% of rocky planetary materials, this isotopic coincidence strongly constrains their close genetic linkage in obtaining their major components.

Oxygen isotopic compositions of the CAI and olivine-rich chondrules formed at the earliest stage of Solar system were primarily controlled by fractionation of ¹⁶O-rich and ¹⁶O-poor reservoirs in the inner and the outer regions of the solar nebula, respectively. Water removal from the inner protoplanetary disk that occurred within a few Myr after the formation of Solar system led to decreased oxygen fugacity in this region. This study infer that the variation in the oxygen isotopic compositions of EC chondrules could reflect this decreased oxygen fugacity. With increase in SiO/H₂O in the innermost region of the protoplanetary disk, silicate vaporization and melt-SiO reaction proceeded, resulting in the formation of enstatite-rich and silica-oversaturated chondrules. Over the same period, accretion of achondrite precursor bodies, followed by whole body melting, could have proceeded in the same region. Dust vaporization and re-condensation also elevated the volatile elements (e.g., alkaline elements and halogens) without increasing the amount of H₂O in the reactant, a characteristic of chondrules in EH relative to other chondrite groups.

References:

• Ryoji Tanaka, Eizo Nakamura. Silicate-SiO reaction in a protoplanetary disk recorded by oxygen isotopes in chondrules, Nature Astronomy, 1, 0137 (2017). doi:10.1038/s41550-017-0137, dream/20200325111340-443154.