金丸 礼

Abstract Silica polymorphs occur under various pressures and temperature conditions, and their characteristics can be used to better understand the complex metamorphic history of planetary materials. Here, we conducted isothermal heating experiments of silica polymorphs in basaltic eucrites to assess their formation and stability. We revealed that each silica polymorph exhibits different metamorphic responses: (1) Quartz recrystallizes into cristobalite when heated at ≥ 1040 °C. (2) Monoclinic (MC) tridymite recrystallizes into no other polymorphs when heated at ≤ 1070 °C. (3) Silica glass recrystallizes into quartz when heated at 900–1010 °C, and recrystallize into cristobalite when heated at ≥ 1040 °C. These results suggest that MC tridymite in eucrites does not recrystallize into other polymorphs during the reheating events, nor does it recrystallize from other silica phases below the solidus temperature of eucrite (~ 1060 °C). Additionally, we found that pseudo-orthorhombic (PO) tridymite crystallizes from quenched melts in the samples heated at ≥ 1070 °C. Previously, cristobalite has been considered as the initial silica phase, which crystallizes from eucritic magma. Our findings suggest that the first crystallizing silica minerals may not always be cristobalite. These require a reconsideration of the formation process of silica minerals in eucrites.

We performed a cathodoluminescence (CL) study of Ca-rich plagioclase (An_85-86Ab₁₄Or_<1) in Stillwater gabbronorite experimentally shocked at 20.1, 29.8, and ∼41 GPa, for characterization of the shock effects. Chroma CL image of unshocked plagioclase showed the homogeneous red CL emission. In contrast, experimentally shocked plagioclase showed the heterogeneous CL emission colors in red and blue. The Raman spectra analysis identified that the red and blue portions correspond to plagioclase and maskelynite, respectively. In our observation, plagioclase experimentally shocked at 20 GPa was partially converted into maskelynite. At 30 GPa, most of plagioclase were converted into maskelynite. At 40 GPa, plagioclase was fully converted into maskelynite. Our observations of Ca-rich plagioclase indicated that the maskelynization starts at a slightly lower pressure and completes at a higher pressure than those in the previous studies (∼24 GPa and ∼28 GPa, respectively). These pressure differences may be due to the high sensitivity of CL, which allows for the detection of small (a few µm in size) and rare phases that may have been overlooked in the traditional methods. The CL spectra of plagioclase showed a continuous change with increasing shock pressure. Hence, the CL imaging method using plagioclase and maskelynite is found to be very effective to estimate precisely shock pressure. In particular, there was a marked decrease in the CL intensity of Mn²⁺ and Fe³⁺ centers. Furthermore, the shock-induced center around the UV region was observed in experimentally shocked plagioclase and maskelynite. These CL features reflect the destruction of the framework structure to varying extents depending upon shock pressure. Combined with the FTIR analysis in the present study, the transition of plagioclase to maskelynite was clearly illustrated in spectra. The reflectivity decreased continuously with increasing shock pressures during maskelynization. Additionally, the absorption at ∼8.6 µm observed in plagioclase was absent in maskelynite. This feature can be used as a diagnostic feature to characterize plagioclase and maskelynite by FTIR. The combination of detailed petrology using CL and FTIR spectra provides valuable insights into the shock scale for achondrites and planetary materials rich in shock-experienced plagioclase.

The bulk density of grains from a celestial body is a fundamental property related to its composition and structure, contributing to the understanding of its evolutionary history. In this study, we provide the bulk density of 637 grains returned from the C-type near-Earth asteroid 162173 Ryugu. This is the largest number of grains to date for the curation activity, corresponding to 38 wt.% of the total returned samples (approximately 5.4 g). Although several densities of the Ryugu grains were reported, the volume estimation of some samples showed uncertainties. Therefore, we applied a new volume estimation model calibrated by X-ray micro-computed tomography (XCT) to the Ryugu grains to more accurately estimate their bulk density. The obtained average bulk density of 637 Ryugu grains was 1.79 ± 0.31 g/cm3 (1σ variation) for weights of 0.5‒100 mg (sub-mm ‒to 10 mm) irrespective of their 3D shapes characterized by three axial length ratios, considered to be a representative of the returned samples. The bulk density distributions of the grains in Chambers A and C were statistically distinguishable, with mean values of 1.81 ± 0.30 and 1.76 ± 0.33 g/cm3 (1σ variations), respectively. Despite the small difference, bulk density may have differed by sampling site. The obtained average bulk density value of A + C samples was almost the same as that of 16 Ryugu grains estimated based on CT scanned data, and was consistent with the densities of CI chondrites (1.57–1.91 g/cm3). The axial ratios of the grains in Chambers A and C were similar and those of the 724 returned samples and the flying particles ejected during the sampling operations were also similar, suggesting that relatively small Ryugu materials (mm‒cm in size) are similar in shape. The minor difference between the Rygu grains and flying particles could be attributed to events such as scraping during sampling operations and transportation. Graphical Abstract: [Figure not available: see fulltext.]

<p id="p1">The elastic property of asteroids is one of the paramount parameters forunderstanding their physical nature. For example, the rigidity enablesus to discuss the asteroid’s shape and surface features such as cratersand boulders, leading to a better understanding of geomorphological andgeological features on small celestial bodies. The sound velocity allowsus to construct an equation of state that is the most fundamental stepto simulate the formation of small bodies numerically. Moreover, seismicwave velocities and attenuation factors are useful to account forresurfacing caused by impact-induced seismic shaking. The elasticproperty of asteroids thus plays an important role in elucidating theasteroid’s evolution and current geological processes. The Hayabusa2spacecraft brought back the rock samples from C-type asteroid (162173)Ryugu in December 2020. As a part of the initial analysis of returnedsamples, we measured the seismic wave velocity of the Ryugu samplesusing the pulse transmission method. We found that P- and S-wavevelocities of the Ryugu samples were about 2.1 km/s and 1.2 km/s,respectively. We also estimated Young’s modulus of 6.0 – 8.0 GPa. Acomparison of the derived parameters with those of carbonaceouschondrites showed that the Ryugu samples have a similar elastic propertyto the Tagish Lake meteorite, which may have come from a D-typeasteroid. Both Ryugu and Tagish Lake show a high degree of aqueousalteration and few high-temperature components such as chondrules,indicating that they formed in the outer region of the solar system.</p>