The modal mineralogy of asteroid 162173 Ryugu and its relationship to carbonaceous chondrite meteorites

Introduction: Primitive asteroids that accreted beyond the snowline in the protoplanetary disk likely played a crucial role in the delivery of water and organic matter to Earth and other terrestrial planets. The surface of the Cb-type asteroid 162173 Ryugu has low overall reflectance and an absorption feature at ~2.7 µm consistent with the presence of carbonaceous materials and phyllosilicates [1]. The spectral characteristics of Ryugu’s surface are most similar to the highly altered CI (“Ivuna-like”) and/or dehydrated CY (“Yamato-like”) carbonaceous chondrites [2].

In December 2020, JAXA’s Hayabusa2 mission successfully returned to Earth with >5 g of sample collected from the surface of Ryugu. The initial investigation of the samples suggested a close affinity with the CI chondrites [3], which consist of abundant phyllosilicates (~80 vol.%), plus minor amounts of magnetite (~10 vol.%), and dolomite (<5 vol.%), having experienced low temperature (<100°C) aqueous alteration [4]. In contrast, the CY chondrites contain dehydrated phyllosilicates and/or recrystallized olivine (~70 vol.%), Fe-sulphides (up to ~20 vol.%), and sometimes metal (~1 vol.%) following post-hydration metamorphism at temperatures >500°C [5]. Here, as part of the Hayabusa2 “Stone” preliminary examination team, we have characterised the modal mineralogy of a Ryugu particle to further constrain its aqueous and thermal history.

Methods: A powdered sub-sample of Ryugu particle C0002 (plates 3 and 4) was analysed using position-sensitive-detector X-ray diffraction (PSD-XRD) at the Natural History Museum (NHM), London. XRD patterns were acquired from the Ryugu powder using a high-intensity micro X-ray source for 6 hours, with the sample rotated throughout the analysis. Pure standards of each phase detected in the Ryugu powder were analysed under exactly the same conditions for 15 minutes. The modal mineralogy of the Ryugu powder was then determined using an established peak fitting method [4, 5].

Results & Discussion: Ryugu particle C0002 (plates 3 and 4) contains a mixture of Mg-rich serpentines and smectites that are present at an abundance of ~84 (± 2) vol.%. Other phases identified from the XRD pattern include magnetite (~8 vol.%), pyrrhotite (~7 vol.%), and dolomite (~2 vol.%). This mineralogy is broadly consistent with our petrographic observations of polished Ryugu sections (C0025-01 and C0103-02). Diffraction peaks from anhydrous olivine and pyroxene were not observed suggesting that their abundance is ≤1 vol.% in the analysed fraction of C0002. If we assume a maximum anhydrous silicate abundance of 2 vol.%, then the phyllosilicate fraction (PSF = total phyllosilicate abundance / [total anhydrous silicate + total phyllosilicate abundance]) of the Ryugu powder is 0.98, which corresponds to a petrologic sub-type of 1.1 on the alteration scale of Howard et al. [6].

The XRD pattern and modal mineralogy of Ryugu particle C0002 (plates 3 and 4) is very similar to the CI chondrites [4]. However, the Ryugu powder does not contain sulphates and ferrihydrite, which are common in the CI chondrites and thought to be terrestrial weathering products [7]. The Ryugu powder also has a comparable mineralogy to the recent C2_ung fall Tarda, which appears to be related to the Tagish Lake (C2_ung) meteorite [8]. However, Tarda retains partially altered chondrules and has a relatively high abundance of anhydrous silicates (~10 vol.%) [9]. In addition, the XRD pattern and modal mineralogy of the Ryugu powder is clearly distinct from the CY chondrites, which contain dehydrated phyllosilicates that lack coherent diffraction, and abundant poorly crystalline troilite and secondary olivine [5].

The modal mineralogy of Ryugu particle C0002 (plates 3 and 4) is consistent with having formed through low temperature aqueous alteration. The fluid-rock reactions reached near-completion, resulting in a secondary assemblage of phyllosilicate, sulphide, magnetite, and Mg-carbonate that was not overprinted by a later episode of thermal metamorphism at temperatures >~400°C.

References: [1] Kitazato et al. (2019) Science. 364:272. [2] Kitazato et al. (2021) Nature Astronomy. 5:246. [3] Yada et al. (2022) Nature Astronomy. 6:214. [4] King et al. (2015) Geochimica et Cosmochimica Acta. 165:148. [5] King et al. (2019) Geochemistry. 79:125531. [6] Howard et al. (2015) Geochimica et Cosmochimica Acta. 149:206. [7] Gounelle & Zolensky (2001) Meteoritics & Planetary Science. 36:1321. [8] Marrocchi et al. (2021) The Astrophysical Journal Letters 913:L9. [9] King et al. (2021) 52^nd LPSC, abstract #1909.