arXiv subjects

Naoya Sakamoto

Publications and source records attributed to Naoya Sakamoto.

8 recordsLinked to original sources

arXiv2026

High solubility of water in post-perovskite and bridgmanite in the Earth's deep lower mantle

Water in the Earth's mantle induces melting, giving rise to strong chemical heterogeneity, and alters its rheological and transport properties, which are keys to understanding seismic-wave speeds anomalies, convective motions and electrical conductivity. However, the abundance of water and its role in the deep mantle remain uncertain and controversial, particularly at conditions of the core-mantle boundary (CMB) region. Here we carry out melting experiments on a hydrous, natural mantle composition including both $H_2O$ and $D_2O$ over the entire pressure range of the Earth's lower mantle, in which melt coexists with bridgmanite (Bdg) and/or post-perovskite (PPv), the primary constituents of the respective lower and lowermost mantle. High-resolution secondary-ion mass spectroscopy (Cryo-SIMS) measurements reveal high water concentrations in Bdg (up to 5,500 ppm by weight) and PPv (up to 1.22 wt%) with strong enrichment in deuterium/hydrogen (D/H) relative to coexisting melt. The high solubilities of water in Bdg and PPv at CMB conditions suggest that subducting slabs do not release water when they reach the bottom of the mantle, and such a process cannot account for strong chemical heterogeneities inferred in the lowermost mantle and the topmost outer core, nor for ultralow-velocity zone (ULVZ)-like structures in seismically fast regions. Water-rich Bdg and PPv may be present in the large low shear velocity provinces and ULVZs, which may be the residue of a basal magma ocean and are predicted to host a deep low D/H water reservoir inherited from the early Earth that is occasionally sampled by plumes of a lowermost-mantle origin.

physics.geo-ph↗

arXiv2026

Direct imaging of enantiomer-specific orientation dynamics in unidirectionally rotating chiral molecules

Selectively controlling the dynamics of molecular enantiomers underlies advances across chemistry, biology, and physics, yet direct imaging of enantiomer-specific motion has so far remained elusive. Here, we image ultrafast enantioselective orientation dynamics in isolated chiral molecules. Unidirectional coherent rotation induced by a femtosecond laser-pulse pair generates equal and opposite out-of-plane orientations of the two enantiomers. Applying this scheme to 2-methyloxirane, we follow the rotational wave packets by time-resolved Coulomb explosion imaging with two orthogonally arranged detectors. The measured angular distributions reveal that the unidirectional rotation is identical for both enantiomers, while the out-of-plane orientations are mirror images that persist through both early-time quasi-classical and quantum dynamics regimes, in quantitative agreement with simulations. We demonstrate that full angular distributions provide richer dynamical information, with some qualitatively different distributions yielding similar orientation factors upon integration. Our approach opens a route to real-time observation and control of chiral dynamics in the gas phase.

physics.chem-ph↗

arXiv2025

Core-mantle partitioning and the bulk Earth abundances of hydrogen and carbon: Implications for their origins

We determined the metal/silicate partition coefficients of hydrogen and carbon, DH and DC, simultaneously under typical conditions of Earth's core formation. Experiments demonstrate that both DH and DC diminish in the presence of carbon and hydrogen, respectively, indicating their strong interactions in liquid metal. With these partitioning data, we investigated the core and bulk Earth abundances of hydrogen and carbon based on core formation scenarios that are compatible with the bulk silicate Earth composition and the mass fraction and density deficit of the core. The results of the single-stage core formation modelling are markedly different from those using DH and DC individually determined in earlier experiments, indicating that the Earth building blocks do not match enstatite chondrites in water abundance and require contributions by carbonaceous chondrites. The multi-stage core formation models combined with an Earth accretion scenario accounting for isotopic composition show 0.18-0.49 wt% H and 0.19-1.37 wt% C in the core, leading to 0.53-1.40 wt% H2O (present as H in the core) and 0.07-0.44 wt% C in the bulk Earth. Our modelling also demonstrates that up to 53% and 72% of Earth's water (hydrogen) and carbon, respectively, could have been derived from non-carbonaceous chondritic materials.

astro-ph.EP↗

arXiv2024

Formation of Iron-Helium Compounds under High Pressure

We report the formations of fcc and distorted hcp iron-helium compounds with x in FeHex up to 0.13 and 0.48, respectively, based on experiments at 5-54 GPa and ~1000-2820 K. Upon releasing pressure under room temperature, these fcc and distorted hcp FeHex were still observed by XRD and SIMS measurements. Our first-principles calculations indicate that fcc and hcp FeHex, with helium atoms occupying the tetrahedral and trigonal-planar interstitial sites (instead of the octahedral sites), are dynamically stable throughout 0-50 GPa. These results support that the Earth's core can be a large reservoir of primordial 3He.

cond-mat.mtrl-sci↗

arXiv2024

The Ni isotopic composition of Ryugu reveals a common accretion region for carbonaceous chondrites

The isotopic compositions of samples returned from Cb-type asteroid Ryugu and Ivuna-type (CI) chondrites are distinct from other carbonaceous chondrites, which has led to the suggestion that Ryugu and CI chondrites formed in a different region of the accretion disk, possibly around the orbits of Uranus and Neptune. We show that, like for Fe, Ryugu and CI chondrites also have indistinguishable Ni isotope anomalies, which differ from those of other carbonaceous chondrites. We propose that this unique Fe and Ni isotopic composition reflects different accretion efficiencies of small FeNi metal grains among the carbonaceous chondrite parent bodies. The CI chondrites incorporated these grains more efficiently, possibly because they formed at the end of the disk's lifetime, when planetesimal formation was also triggered by photoevaporation of the disk. Isotopic variations among carbonaceous chondrites may thus reflect fractionation of distinct dust components from a common reservoir, implying CI chondrites and Ryugu may have formed in the same region of the accretion disk as other carbonaceous chondrites.

astro-ph.EP↗

arXiv2022

Presolar stardust in asteroid Ryugu

We have conducted a NanoSIMS-based search for presolar material in samples recently returned from C-type asteroid Ryugu as part of JAXA's Hayabusa2 mission. We report the detection of all major presolar grain types with O- and C-anomalous isotopic compositions typically identified in carbonaceous chondrite meteorites: 1 silicate, 1 oxide, 1 O-anomalous supernova grain of ambiguous phase, 38 SiC, and 16 carbonaceous grains. At least two of the carbonaceous grains are presolar graphites, whereas several grains with moderate C isotopic anomalies are probably organics. The presolar silicate was located in a clast with a less altered lithology than the typical extensively aqueously altered Ryugu matrix. The matrix-normalized presolar grain abundances in Ryugu are 4.8$^{+4.7}_{-2.6}$ ppm for O-anomalous grains, 25$^{+6}_{-5}$ ppm for SiC grains and 11$^{+5}_{-3}$ ppm for carbonaceous grains. Ryugu is isotopically and petrologically similar to carbonaceous Ivuna-type (CI) chondrites. To compare the in situ presolar grain abundances of Ryugu with CI chondrites, we also mapped Ivuna and Orgueil samples and found a total of SiC grains and 6 carbonaceous grains. No O-anomalous grains were detected. The matrix-normalized presolar grain abundances in the CI chondrites are similar to those in Ryugu: 23 $^{+7}_{-6}$ ppm SiC and 9.0$^{+5.3}_{-4.6}$ ppm carbonaceous grains. Thus, our results provide further evidence in support of the Ryugu-CI connection. They also reveal intriguing hints of small-scale heterogeneities in the Ryugu samples, such as locally distinct degrees of alteration that allowed the preservation of delicate presolar material.

astro-ph.EP↗

arXiv2019

Nebular history of an ultrarefractory phase bearing CAI from a reduced type CV chondrite

Ultrarefractory (UR) phases in CAIs could have formed at higher T compared to common CAI minerals and thus they potentially provide constraints on very high-T processes in the solar nebula. We report a detailed characterization of an UR phase davisite bearing CAI from a reduced type CV chondrite. Absence of secondary iron- and/or alkali-rich phases and occurrence of LIME olivine indicate that primitive chemical and isotopic compositions are preserved in the CAI. Davisite occur only in one lithological unit that consists of three chemically and isotopically distinct parts: i) $^{16}$O-poor regions with reversely-zoned melilite and davisite; ii) $^{16}$O-rich regions consisting of unzoned, gehlenitic melilite, diopside and spinel; and iii) spinel framboids composed of $^{16}$O-rich spinel and $^{16}$O-poor melilite. Random distribution of chemical and isotopic heterogeneities with sharp boundaries in the CAI indicates its formation by an aggregation of mineral assemblages formed and processed separately at different time and/or space. Although isotope exchange prior to the final agglomeration of the CAI cannot be ruled out, we suggest that modification of chemical and isotopic composition of porous CAI precursors or aggregation of isotopically distinct mineral assemblages are alternative scenarios for the origin of O-isotopic heterogeneity in CAIs. In either case, coexistence of spatially and/or temporally distinct $^{16}$O-rich and -poor gaseous reservoirs at the earliest stage of the solar system formation is required. The grain-scale oxygen isotopic disequilibrium in the CAI indicate that post-formation heating of the CAI was short, which can be achieved by rapid outward transport of the CAI. High Ti$^{3+}$/Ti$^{\mathrm{tot}}$ ratios of pyroxene and presence of LIME olivine document that the entire CAI formation process took place under highly reducing conditions.

astro-ph.EP↗

arXiv2018

Fast and Accurate Tumor Segmentation of Histology Images using Persistent Homology and Deep Convolutional Features

Tumor segmentation in whole-slide images of histology slides is an important step towards computer-assisted diagnosis. In this work, we propose a tumor segmentation framework based on the novel concept of persistent homology profiles (PHPs). For a given image patch, the homology profiles are derived by efficient computation of persistent homology, which is an algebraic tool from homology theory. We propose an efficient way of computing topological persistence of an image, alternative to simplicial homology. The PHPs are devised to distinguish tumor regions from their normal counterparts by modeling the atypical characteristics of tumor nuclei. We propose two variants of our method for tumor segmentation: one that targets speed without compromising accuracy and the other that targets higher accuracy. The fast version is based on the selection of exemplar image patches from a convolution neural network (CNN) and patch classification by quantifying the divergence between the PHPs of exemplars and the input image patch. Detailed comparative evaluation shows that the proposed algorithm is significantly faster than competing algorithms while achieving comparable results. The accurate version combines the PHPs and high-level CNN features and employs a multi-stage ensemble strategy for image patch labeling. Experimental results demonstrate that the combination of PHPs and CNN features outperforms competing algorithms. This study is performed on two independently collected colorectal datasets containing adenoma, adenocarcinoma, signet and healthy cases. Collectively, the accurate tumor segmentation produces the highest average patch-level F1-score, as compared with competing algorithms, on malignant and healthy cases from both the datasets. Overall the proposed framework highlights the utility of persistent homology for histopathology image analysis.

cs.CV↗