Carbonates in asteroid Ryugu reveal the temporal change in oxygen fugacity

Samples from asteroid Ryugu show evidence for past aqueous activity. We deciphered the evolution of Ryugu by analysing carbonate minerals that formed during aqueous alteration.
Carbonates in asteroid Ryugu reveal the temporal change in oxygen fugacity
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The Japan Aerospace Exploration Agency (JAXA) Hayabusa2 spacecraft explored the near-Earth asteroid 162173 Ryugu and brought samples of its surface materials back to Earth. Ryugu has been classified spectroscopically as a member of the C-complex of asteroids that are thought to preserve water and organic matter. It is a rubble pile asteroid consisting of numerous rocky blocks that are the fragments resulting from the disruption of an original, larger parent body. The scientific objective of Hayabusa2 mission is to clarify the evolution of the solar system and the formation process of the asteroid, with special interests on the origin of Earth’s water and organic matter, and where the building blocks of life on Earth originally came from.

The analyses of Ryugu samples have revealed that they have chemical composition close to that of the bulk solar system and that they resemble the most chemically primitive meteorites, namely, Ivuna-type (CI) carbonaceous chondrites. Ryugu samples are not only chemically primitive but also pristine, i.e., the influence of alteration on Earth is minimal.

As mentioned, the Ryugu samples are primitive in terms of their chemical composition, while they are not primitive in terms of their petrology and mineralogy because they underwent extensive aqueous alteration as the result of water activity in the original parent body. They have abundant secondary minerals that formed during aqueous alteration: phyllosilicates, carbonates, sulphides, and oxides. These secondary minerals may obscure what kinds of primary minerals originally made up the parent body but instead provide us the opportunity to elucidate how Ryugu evolved to its present state through the solar system history. So far, little is known about the temporal changes in the conditions (such as temperature, redox state, and fluid composition) during aqueous alteration.

In the present study, we noted carbonate minerals. Carbonates are secondary minerals that formed by the aqueous alteration and provide clues for understanding the environment of the aqueous alteration. We developed an analytical technique using secondary ion mass spectrometry (SIMS) which enables us to measure carbon and oxygen isotope compositions of carbonates with a spatial resolution as small as 1 µm. Then, we systematically carried out isotope analyses on calcium carbonate (calcite) and calcium-magnesium carbonate (dolomite) grains in Ryugu and Ivuna.

Backscattered electron image of a calcite grain in the Ryugu C0002 sample.

We found contrasting isotope characteristics between calcite and dolomite which indicate their formation in different alteration settings. A large variation in oxygen isotope compositions of calcite (δ18O values from 24 to 46‰) suggests that they precipitated first over a wide range of temperatures during prograde alteration, while dolomite with homogeneous compositions (δ18O values from 31 to 36‰) formed later at higher temperatures. The calcite also shows a large variation in carbon isotope compositions (δ13C values from 65 to 108‰). The carbon-isotope variation of calcite can be explained by the temporal variation in oxygen fugacity (fO2: oxygen partial pressure corrected for nonideal gas behaviour) and the associated change in the proportion of gaseous CO2/CO/CH4 molecules if we assume that the carbon isotope compositions of carbonates were controlled by the isotopic fractionation between carbonates and these gaseous species. By contrast, the more homogeneous carbon isotope compositions of the dolomite (δ13C values from 67 to 75‰) formed under more oxidizing and thus CO2-dominated environment when the system was approaching equilibrium. We conclude that the carbon and oxygen isotope compositions of carbonates record the temporal shift of alteration settings such as temperatures, oxygen fugacity, fluid compositions, and molecular compositions of carbon-bearing gaseous species, which determined the evolution of asteroid Ryugu from the original to present states.

δ13C and δ18O values of the calcite and dolomite in Ryugu and Ivuna samples. The changes in C and O isotope compositions due to variable formation temperatures and O fugacity, and water-rock interaction are illustrated by arrows. The calcite shows much larger variations in both C and O isotope compositions than the dolomite. 

The characteristic isotope compositions of carbonates in Ryugu and Ivuna have not been observed for other hydrous meteorites, suggesting a unique evolutionary pathway for their parent bodies. Future studies may clarify what controlled the evolutionary pathway for small bodies. In particular, it is intriguing to investigate how the abundance and chemical species of volatiles (such as water, CO2, and organic matter) accreted by small bodies were related to their evolution.

Besides the isotope variation, the highly 13C-rch carbon isotope compositions of calcite in Ryugu and Ivuna are notable. Indeed, they are among the 13C-richest compositions observed for extraterrestrial materials except for exotic components like presolar grains. Such 13C-rich compositions can never be found for terrestrial carbonates. The preceding argument implies that Ryugu and Ivuna parent bodies accreted significant amounts of CO2/CO/CH4 contained in ice. If correct, these molecular ices are 13C-rich. Although the mechanism to produce 13C-rich composition is not well understood, photochemical reactions in the solar nebula or the parent molecular cloud of the solar system are a possible mechanism. Thus, the inferred 13C-rich composition of carbon-bearing molecular ices would have resulted from such physicochemical reactions, and we conclude that the Ryugu and Ivuna parent bodies accreted materials that originated from these cold environments.

 

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