# A comprehensive study of apatite grains in Ryugu rock fragments

B. J. Tkalcec, P. Tack, E. de Pauw, B. Bazi, B. Vekemans, M. Lindner, L.

Vincze, M. Di Michiel, J. Garrevoet, G. Falkenberg, et al.

► To cite this version:

B. J. Tkalcec, P. Tack, E. de Pauw, B. Bazi, B. Vekemans, et al. A comprehensive study of apatite grains in Ryugu rock fragments. Meteoritics and Planetary Science, 2024, 59 (8), pp.2149-2165. 10.1111/maps.14177. hal-05118266

HAL Id: hal-05118266

https://hal.science/hal-05118266v1

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A comprehensive study of apatite grains in Ryugu rock fragments

B. J. TKALCEC<sup>1,*</sup>, P. TACK<sup>2</sup>, E. DE PAUW<sup>2</sup>, B. BAZI<sup>2</sup>, B. VEKEMANS<sup>2</sup>, M. LINDNER<sup>1</sup>, L. VINCZE<sup>2</sup>, M. DI MICHIEL<sup>3</sup>, J. GARREVOET<sup>4</sup>, G. FALKENBERG<sup>4</sup>, T. NAKAMURA<sup>5</sup>, T. MORITA<sup>5</sup>, K. AMANO<sup>5</sup>, D. NAKASHIMA<sup>5</sup>, F. LANGENHORST<sup>6</sup>, K. POLLOK<sup>6</sup>, H. YURIMOTO<sup>7</sup>, T. NOGUCHI<sup>8</sup>, R. OKAZAKI<sup>9</sup>, H. YABUTA<sup>10</sup>, H. NARAOKA<sup>9</sup>, K. SAKAMOTO<sup>11</sup>, S. TACHIBANA<sup>12</sup>, T. YADA<sup>11</sup>, M. NISHIMURA<sup>11</sup>, A. NAKATO<sup>11</sup>, A. MIYAZAKI<sup>11</sup>, K. YOGATA<sup>11</sup>, M. ABE<sup>11</sup>, T. OKADA<sup>11</sup>, T. USUI<sup>11</sup>, M. YOSHIKAWA<sup>11</sup>, T. SAIKI<sup>11</sup>, S. TANAKA<sup>11</sup>, F. TERUI<sup>13</sup>, S. NAKAZAWA<sup>11</sup>, S. WATANABE<sup>14</sup>, Y. TSUDA<sup>11</sup>, and F. E. BRENKER<sup>1</sup>

*Correspondence

B. J. Tkalcec, Department of Geoscience, Goethe University, Altenhoeferallee 1, Frankfurt am Main 60438, Germany.

Email: tkalcec@em.uni-frankfurt.de

(Received 08 July 2023; revision accepted 11 April 2024)

**Abstract**—Apatite is present as an accessory phase in many meteorites and is often formed as a secondary product of aqueous alteration. Its propensity to incorporate rare earth elements (REE) results in apatite usually being the main REE-bearing phase in hydrously altered meteorites. Asteroid Ryugu is thought to have experienced pervasive aqueous alteration and material collected from the surface of Ryugu is expected to provide insight into asteroidal aqueous alteration processes without influence by terrestrial weathering. Morphologies and mineral associations of apatite grains from five rock fragments collected from the asteroid Ryugu by the Hayabusa2 spacecraft were examined and their REE concentrations were measured by synchrotron X-ray fluorescence (SXRF) spectroscopy. The main minerals associated with apatite are dolomite, magnetite, and pyrrhotite. Grain boundary corrosion of the interfaces between apatite assemblages and the surrounding matrix suggest that paragenetic formation on the asteroid was followed by a later episode of hydrous alteration. Light REE (LREE) concentration levels recorded at 20–150 times those of bulk CI levels together with a steady increase from LREE toward enrichment of medium REE (MREE, up to Er) at 50–400 times bulk CI levels may suggest postgenetic removal of LREE from Ryugu apatite grains by late-stage circulation of a hydrothermal fluid.

<footnote n="1"><sup>1</sup>Department of Geoscience, Goethe University, Frankfurt am Main, Germany</footnote>

<footnote n="2"><sup>2</sup>Department of Chemistry, Ghent University, Ghent, Belgium</footnote>

<footnote n="3"><sup>3</sup>ESRF—The European Synchrotron, Grenoble, France</footnote>

<footnote n="4"><sup>4</sup>Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany</footnote>

<footnote n="5"><sup>5</sup>Tohoku University, Sendai, Japan</footnote>

<footnote n="6"><sup>6</sup>Institute of Geoscience, Friedrich Schiller University Jena, Jena, Germany</footnote>

<footnote n="7"><sup>7</sup>Hokkaido University, Sapporo, Japan</footnote>

<footnote n="8"><sup>8</sup>Kyoto University, Kyoto, Japan</footnote>

<footnote n="9"><sup>9</sup>Kyushu University, Fukuoka, Japan</footnote>

<footnote n="10"><sup>10</sup>Hiroshima University, Higashi-Hiroshima, Japan</footnote>

<footnote n="11"><sup>11</sup>Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Japan</footnote>

<footnote n="12"><sup>12</sup>The University of Tokyo, Tokyo, Japan</footnote>

<footnote n="13"><sup>13</sup>Kanagawa Institute of Technology, Atsugi, Japan</footnote>

<footnote n="14"><sup>14</sup>Nagoya University, Nagoya, Japan</footnote>
INTRODUCTION

First results from the initial analysis of material brought to Earth by the Hayabusa2 sample return mission from the surface and subsurface of the asteroid 162173 Ryugu have shown it to be mineralogically and chemically similar to that of CI carbonaceous chondrites (Ito et al., 2022; Nakamura, Kobayashi, et al., 2022; Nakamura, Matsumoto, et al., 2022; Yokoyama et al., 2023). CI carbonaceous chondrites are thought to be the chemically most primitive meteorites of the Solar System and hold invaluable clues as to the earliest Solar System processes (Alfing et al., 2019; Anders & Ebihara, 1982; Anders & Grevesse, 1989; Barrat et al., 2012). However, to date, only nine meteorites have been officially classified as CI chondrites (Alais, Orgueil, Tonk, Ivuna, Revelstoke, as well as Yamato-86029, Yamato-86737, Yamato-980115, and Yamato-980134). These nine CI chondrites have all been on the Earth for many years, exposed to long-term terrestrial environmental conditions. Five of the nine CI chondrites (Alais, Orgueil, Tonk, Ivuna, and Revelstoke) were observed falls, arriving on the Earth between 1806 (Alais) and 1965 (Revelstoke). Although observed as falls and recovered almost immediately, these five CI chondrites have since been exposed to the terrestrial atmosphere for between 58 (Revelstoke) and 217 (Alais) years. Of the four Yamato pieces, two were found in 1986 and two in 1998 with no indication as to how long they had lain in the Antarctic before being discovered. Thus, the arrival of pristine material from the ~1 km wide, rubble-pile asteroid Ryugu (Watanabe et al., 2019) brings with it undisturbed information with regard to the infant years of the Solar System and postaccretional planetary processes. Results of the initial analysis of Ryugu material show that it has a similar level of brecciation to that of Orgueil and has experienced pervasive aqueous alteration that occurred at low temperatures of ~25–50°C (Nakamura, Matsumoto, et al., 2022). The major lithology found in Ryugu material consists of secondary phases, a phyllosilicate-rich matrix (serpentine with some saponite) with abundant iron sulfides (primarily pyrrhotite and some pentlandite), carbonates (primarily dolomite and some breunnerite), magnetite (in various morphologies), and hydroxyapatite (with <1 wt% fluorine), all products of aqueous alteration (Nakamura, Matsumoto, et al., 2022).

In particular, the calcium-phosphate phase, apatite (Ca₅[PO₄]₃[F,Cl,OH]), which occurs as accessory phase (≤1 vol%) in many chondritic and achondritic meteorite classes (Ward et al., 2017), can provide clues to the process of hydrothermal activity on the asteroidal parent bodies. Although in some differentiated meteorites, such as eucrites (Ward et al., 2017) and shergottites (McCubbin et al., 2012; Sarafian et al., 2013), the

calcium-phosphate phases apatite and merrillite are thought to have igneous origins, the apatite in most chondrites is generally thought to have formed through metasomatic processes (Brearley & Jones, 1998; Jones et al., 2014; Piralla et al., 2021; Ward et al., 2017). Many different elements, including rare earth elements (REE), can be substituted into the apatite structure, either during its initial formation or during a later hydrothermal alteration stage (Owens et al., 2019). Particularly its propensity for REE incorporation means that apatite is often the dominant REE-bearing phase in aqueously altered chondritic meteorites (Ward et al., 2017) and enrichment levels of REE within apatite can, thus, be used to reflect the early asteroidal environment and the postaccretional processes undergone on the chondrite parent body.

In this study, 19 apatite grains from five samples brought back from the asteroid Ryugu by the Hayabusa2 sample return mission (Morota et al., 2020; Tachibana et al., 2022) were analyzed by synchrotron X-ray fluorescence (SXRF) spectroscopy. Initial REE analyses of four apatite grains in two 1–3 mm sized samples, A0055 and C0076, performed by SXRF within the Hayabusa2 initial analysis stage (Nakamura, Matsumoto, et al., 2022; Tack et al., 2022) were subsequently complemented in the second Hayabusa2 analysis stage by 15 further apatite grains in two additional samples, A0186 and C0070, considerably smaller in size (100–500 µm) that were allocated within the framework of the Hayabusa2 First Announcement of Opportunity (AO1). Morphology and mineral associations of apatite grains were examined in all four Ryugu samples as well as in thin sections and focused ion beam (FIB) sections of a fifth Ryugu sample, C0033. Here, we examine and compare the morphology, the mineral associations as well as the REE concentrations of all 19 apatite grains from five Ryugu samples. The results shed light on REE partitioning during hydrothermal formation of apatite on the asteroid Ryugu.

METHODS

Ryugu samples from both the first and second touchdowns of the Hayabusa2 spacecraft on the surface of the asteroid, captured in the A and C chambers, respectively, of the sampling device, (Tachibana et al., 2022) were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and analyzed by synchrotron X-ray fluorescence (SXRF) spectroscopy. SXRF was performed at the high-energy (90 keV) beamline ID15A at the ESRF—The European Synchrotron (Tack et al., 2022; Vaughan et al., 2020) to detect and quantify high-Z elements (Z ≤ 82, Pb) including REEs at high spatial resolution
(~0.5 × 0.5 µm² X-ray beam focal spot at sample position) and low detection limits (DLs; ~1 µg g⁻¹; Table 1). Here, the incident beam is focused using a KB mirror system and fluorescence radiation is detected using a combined detection setup consisting of a Cryopulse5+ HPGe-CMOS detector (Mirion Technologies) and a Hitachi VortexEM SDD detector. SXRF was additionally performed at the Hard X-ray Micro/Nano-Probe beamline P06 (20.5 keV) at the Deutsches Elektronen-Synchrotron (DESY, Petra III; Bazi et al., 2022; Schroer et al., 2010) for the detection and quantification of lower-Z elements (S-Zr; 16 ≤ Z ≤ 40). Here, the primary X-ray beam is focused by KB mirror optics to a 0.24 × 0.2 µm² beam size at the sample position, and fluorescence radiation is detected using a Hitachi Vortex ME 4-element SDD detector. Both sets of SXRF data were processed using PyMCA5 (Solé et al., 2007) spectral deconvolution algorithms (Solé et al., 2007), and REE content was quantified using a reference-based quantification algorithm. For this purpose, the following reference glass materials were used: NIST SRM 611, NIST SRM 613, and MPI-DING ATHO-G. Error margins were determined by error propagation of the Poisson statistics-governed spectral integration uncertainty, as well as the reference material uncertainty (Borisova et al., 2010; Jochum et al., 2006, 2011). Potential reference material heterogeneities are accounted for by the implementation of a rastering acquisition approach, providing a homogenized sample response (Kempenaers et al., 2003). Where an element is present in multiple reference materials, the average fluorescence yield was used for further quantification, with an attributed error equal to the largest value of either the standard deviation of the obtained yields or the value obtained through standard error propagation calculations. REE quantifications and error margins were further normalized to bulk CI concentrations (Lodders, 2020). SEM was performed by a JEOL 6490 scanning electron microscope in the laboratories of the Goethe University, Frankfurt am Main, Germany at 15 kV beam and elements were detected and measured by means of energy-dispersive X-ray (EDX) spectroscopy with Inca software by Oxford Instruments Inc. TEM was performed at the Friedrich Schiller University of Jena in Germany using a Tecnai G2 field emission gun (FEG) TEM (Langenhorst et al., 2014). The TEM was operated at 200 keV electron energy and equipped with an Oxford SDD energy-dispersive X-ray spectroscopy detector.

## Samples and Sample Preparation

The two Ryugu particles (A0055 and C0076) analyzed by high-energy SXRF, during the initial analysis stage of the Hayabusa2 return mission, remained

in non-air-exposed condition throughout the SXRF measurements. Each sample had been mounted on a thin carbon pad on top of an aluminum rod (3 mm Ø) and protected from exposure to terrestrial air by a sealed 25-µm-thick polyimide cap. These two particles were each 1.5–3 mm in size and mounting was performed at the Tohoku University in Japan. In addition, four <200-µm-thick sections (C0033-01, C0033-04, C0076-06, and A0055-02) were cut and polished (using ethanol, with the exception of A0055-02 that was dry-polished; Nakashima et al., 2022) at the Tohoku University in Japan and then analyzed by medium-energy SXRF and by EDX. Following this, a total of nine FIB sections were prepared from the thick sections C0033-01 and C0033-04 using an FEI Quanta 3D FEG dual beam instrument and these were examined by TEM (Langenhorst et al., 2014). The FIB preparation was performed with a gallium ion gun operated at an acceleration voltage of 30 kV and a beam current between 0.1 and 30 nA. Final cleaning of the sections was carried out at 5 kV and reduced ion beam intensity of 48 pA. In the second analysis stage of the Hayabusa2 return mission (AO1), three 100–300 µm Ryugu particles (A0186, C0070-04 and C0070-08) were each mounted onto a glass capillary and analyzed by medium- and high-energy SXRF. For SXRF analyses of all Ryugu particles at either beamline, no further sample preparation was performed. For SEM analysis of the polished sections, a thin carbon coating was applied to avoid ionic charging.

## RESULTS

The Ca-phosphates measured in Ryugu samples were identified as apatite rather than merrillite, based primarily upon TEM (and also SEM) data. TEM results reveal selected-area electron-diffraction patterns that unambiguously identify the crystal lattice as that of apatite (e.g., Figure S1). This is further supported by EDX results (SEM) that reveal, for example, an atomic ratio P:Ca of 0.594 ± 0.023 in the measured Ca-phosphate grains, which perfectly matches the ~0.6 expected for apatite, as opposed to a P:Ca of ~0.8 expected for merrillite. Our identification as apatite is in line with previously reported studies confirming Ryugu Ca-phosphate grains to be hydroxylapatite (Nakamura, Kobayashi, et al., 2022; Nakamura, Matsumoto, et al., 2022; Nakato et al., 2022; Yamaguchi et al., 2023).

## Apatite Morphology and Mineral Associations

Apatite in Ryugu samples is observed as discrete grains and grain clusters with grain sizes ranging from 1 to 70 µm. Apatite morphologies are also varied, ranging from euhedral with clear and sharp crystal boundaries, to
anhedral with clear and sharp boundaries, to irregular with sharp yet convoluting grain boundaries to irregular with intensely corroded grain boundaries (Figure 1).

Apart from being observed within the general phyllosilicate matrix, apatite is also often seen in close proximity to dolomite and Fe-rich phases, predominantly magnetite, less frequently pyrrhotite and, in two observed cases, also ilmenite (see Figure 2). These mineral associations show a strong similarity to the mineral associations reported for apatite in other CI chondrites, such as Orgueil, Ivuna, Alais, and Tonk (Alfing et al., 2019). The apatite–pyrrhotite–magnetite assemblage in Figure 2a reveals a pyrrhotite lath sandwiched between two apatite occurrences that are themselves in direct further association with magnetite occurrences, suggesting a pyrrhotite–apatite paragenesis. It is interesting to note that the orientation of the apatite grain directly above the euhedral pyrrhotite lath in

<figure><img src="image_2.png" /></figure>

FIGURE 1. High-angle annular dark-field (HAADF) TEM (a) and backscattered electron (BSE) (b–f) images of apatite morphologies in Ryugu samples: (a) euhedral, with clearly defined, straight, sharp, and intact boundaries; (b) subhedral, with less clearly defined boundaries; (c) subhedral, with strongly embayed and corroded boundaries; (d) anhedral and irregular, with sharp, clearly defined boundaries; (e) anhedral, with heavily corroded boundaries; (f) extremely irregular form with intensely corroded boundaries. Fine cracks in A0055-02 (e, f) may be artifacts from dry polishing. ap, apatite; dol, dolomite; mt, magnetite; pyrr, pyrrhotite. Scale bars are 5 µm. (Color figure can be viewed at wileyonlinelibrary.com)

<figure><img src="image_3.png" /></figure>

FIGURE 2. BSE (a–f) and TEM (g, h) images of apatite mineral associations in Ryugu samples. (a) A0055-02 apatite sandwiched between magnetite (plaquette and framboids) and a pyrrhotite lath, with apatite, also present directly above the pyrrhotite lath; (b) A0055-02 apatite–ilmenite assemblage; Fine cracks in A0055-02 (a and b) may be artifacts from dry polishing. (c) C0033-04 apatite–dolomite assemblage in close proximity to a larger dolomite grain and tiny magnetite framboids; (d) C0033-01 apatite between and around magnetite grains; (e) C0033-01 apatite grain directly adjacent to pyrrhotite lath. Red lines mark the yz-profile shown in (g); (f) C0033-01 apatite sandwiched between magnetite and pyrrhotite. Red lines mark the yz-profile shown in (h); (g) HAADF TEM image of the FIB section cut down through yz-profile marked red in (e) showing adjacent euhedral apatite and pyrrhotite crystals beneath the sample surface; (h) HAADF TEM image of the FIB section cut down through yz-profile marked red in (f) showing apatite and pyrrhotite crystals in direct contact beneath the sample surface. All scale bars = 5 µm. (Color figure can be viewed at wileyonlinelibrary.com)
Figure 2a follows the NE–SW oriented long axis of the pyrrhotite. The lower boundary of the upper apatite grain not only shares the same sharp crystal boundary with the adjacent pyrrhotite lath but extends the same sharp elongated and intact appearance into the adjoining matrix. In contrast, the upper outer boundary of the same NE–SW oriented apatite grain is irregular and clearly eroded with fairly large embayments. This may indicate a certain protection at the pyrrhotite–apatite interface against alteration that has clearly affected the outer boundary of the apatite in contact with the matrix. Indeed, the apatite below the pyrrhotite lath (Figure 2a), embedded within the matrix between a magnetite plaquette and pyrrhotite similarly shows considerable grain boundary corrosion and may reflect a stage of alteration similar to that of the magnetite plaquette or the several magnetite framboids in close proximity (Figure 2a).

The ilmenite–apatite assemblage (Figure 2b) also reveals a clear ilmenite–apatite paragenetic intergrowth. Again, there is noticeably less corrosion visible at the inner ilmenite–apatite interfaces (e.g., Figure 2b red arrow), but equally strong corrosion of the outer grain boundaries of both apatite and ilmenite phases. In the dolomite–apatite assemblage shown in Figure 2c, the apatite is enveloped on three sides by the dolomite, suggesting dolomite–apatite paragenesis. Again, the inner interface between the two phases is relatively uncorroded, whereas outer dolomite boundaries show clear signs of alteration. In further magnetite–apatite–pyrrhotite assemblages (e.g., Figure 2d) the apatite boundaries are irregular but sharp often enveloping the magnetite grains. Figure 2e reveals an apatite–pyrrhotite assemblage within the phyllosilicate matrix, displaying different degrees of corrosion at the apatite grain boundaries. The left-hand apatite-matrix interface is relatively intact and sharp, as is the apatite–pyrrhotite interface, whereas the right-hand apatite-matrix interface has been clearly disrupted. The FIB section of this apatite–pyrrhotite assemblage (Figure 2g, cut through the profile marked in red in Figure 2e) shows beneath the surface a clearly unaltered and intact pyrrhotite lath adjacent to similarly sharp and unaltered apatite crystal faces, suggesting that both minerals had space beneath the surface to develop their crystal structure without compromise, or that corrosion of the grain boundary did not happen. A further apatite–pyrrhotite-magnetite assemblage (Figure 2f), observed within a magnetite–apatite-rich vein structure, has an interstitial fill appearance. The external apatite-matrix interface clearly shows a greater degree of corrosion than at the internal apatite–magnetite or apatite–pyrrhotite interfaces. The FIB section of this apatite assemblage (Figure 2h, cut through the profile marked in red in Figure 2f) reveals a sharp subsurface

interface between the apatite grain and the underlying phyllosilicate matrix that displays no wavy embayments, but instead jagged angular incursions. This is in clear contrast to the apatite-matrix interfaces shown in Figure 1c,e,f that display intensely wavy embayments and a more heavily altered appearance. This marked difference in stage of alteration may reflect local differences in fluid abundance or differences in duration of fluid exposure. Indication for chemical heterogeneity is also observed within the apatite grain shown in the FIB section (Figure 2h) that shows regions of brighter and darker material. The distribution of apatite within the magnetite-rich vein appears to be greater than its overall distribution within the analyzed samples, suggesting a co-genetic relationship of the apatite and magnetite within the vein.

## Synchrotron X-Ray Fluorescence

Two of the main strengths of applying SXRF spectroscopy on samples such as those brought to Earth in sample return missions, such as Hayabusa2, are: (1) the ability to see the elemental distribution within the sample interior prior to any invasive sample preparation, that is, before it is cut, polished, or affected in any way; and (2) the sensitivity to be able to detect and quantify trace elements, including REEs, that are present at extremely low concentrations at depth within millimeter-sized samples (De Pauw et al., 2022; Tack et al., 2022; Tkalcec et al., 2022). Achieving these two goals is controlled by several factors: Firstly, desired elements of interest for a specific task must be within the range of elements whose K-lines can be reliably detected by SXRF. At an incident beam energy of 20.5 keV reliable detection is constrained to those elements with atomic numbers $16 \le Z \le 41$ (from S to Nb). At 90 keV, higher atomic number elements ($\le 82$, Pb) are reliably detected. Secondly, the ability of the element-specific fluorescent signals, emitted by the elements excited by the SXRF beam, to travel through the sample without being self-absorbed by the sample material, in order to successfully emerge at the sample surface and reach the external detectors. This factor is controlled by the density, composition, and size of the sample material and the energy of the emitted fluorescence, the latter being specific to each element, based upon its atomic number (Tkalcec et al., 2022).

For a successful SXRF study of apatite (and its REE concentrations) within the Ryugu samples, for example, two challenges must be overcome: The first challenge is to detect (at depth within a millimeter-sized particle) a calcium-rich phase. Calcium that is present at shallow sample depths is easily detected by the Si-based detector at both medium- and high-energy SXRF beamlines. However, as demonstrated in a recent study (Tkalcec
et al., 2022), the fluorescence signals of calcium at material depths of >80 µm (depending upon the material density) are absorbed by the sample material before they reach the sample surface. To remedy this issue, a positive correlation between Ca and Sr (whose signal is less absorbed at depth and can travel further through sample material) revealed that, for SXRF spectroscopy, Sr can be used as a proxy for the presence of Ca at greater sample depths (Tkalcec et al., 2022). Having detected Sr as proxy for Ca, the second challenge is to distinguish between the two main Ca-rich phases within the sample: apatite and dolomite, the former being an accessory Ca-rich phase and the latter a ubiquitous, dominant Ca-rich phase. The challenge here is that the two most obvious distinguishing elements would be C and P, both of which lie with their atomic numbers ($Z_C = 6$ and $Z_P = 15$) below the range of elements detectable by SXRF at 20.5 and 90 keV due to low sample escape depths and absorption by air paths and the detector

beryllium window through which the fluorescence radiation must propagate before detection. Comparison of SXRF and SEM results (e.g., Figure 3), however, have revealed that two further elements, Y and Mn, could be determined as distinguishing elements between apatite and dolomite in SXRF spectroscopy, with apatite being Y-rich and Mn-poor and dolomite being Mn-rich and Y-poor (Bazi et al., 2022; Tack et al., 2022).

This is demonstrated in the elemental distribution maps of Ca, Mn, Sr, and Y (Figure 3a) that show two Ca-rich phases in close proximity to each other within the polished section C0033-04, identified as apatite and dolomite (in the left and right regions, respectively), distinguished by enrichments of Y and Mn, respectively. This distinction is more clearly visualized in the CaYMn tricolor RGB overlay map (Figure 3b).

A comparison with the BSE image (Figure 3c) confirms the identification of the two Ca-rich phases. It is noted that the visual difference between the distributions

<figure><img src="image_4.png" /><figcaption>FIGURE 3. SXRF element distribution maps (of the detected Kα-line fluorescence signals) of Ca, Sr, Mn, and Y demonstrating the identification of and distinction between Ca-rich phases dolomite (on the right in each image) and apatite (on the left in each image). (a) Ca is present as major element in both dolomite and apatite; Sr is present as trace element in apatite (stronger) and dolomite (weaker); Y is present as trace element only in apatite; Mn is present as major element in dolomite and in small ilmenite grains within the apatite cluster; (b) tricolor RGB map showing the distributions of Ca, Y, and Mn; (c) BSE image of the same area (yellow box), revealing only surface occurrences. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>
of apatite and dolomite in the SXRF maps (Figure 3a,b) and those in the BSE image (Figure 3c) reflects the 3D nature of the SXRF scans that reveal the elemental distribution throughout the depth of the sample, compared to the 2D nature of the surface-only SEM scan technique. This is demonstrated by the tricolor RGB image (Figure 3b) of the Ca, Y, and Mn distributions that shows an orange (Ca-Y-rich) area on the upper left of the image and a green (Y-rich) area in the center of the image where only the Y (and no Ca) fluorescence signals are received at the detectors. In the BSE image (Figure 3c), no apatite is shown on the surface of the sample at this position. However, a comparison of the Sr and Y distribution maps (Figure 3a) reveals that Sr (and therefore inferred Ca) is indeed present at this Y-rich location, indicating that this part of the apatite cluster is at a deeper depth below the sample surface that is not measurable by SEM. It can be inferred that this is a cluster of apatite grains that are closer to the sample surface in the upper left region (orange) but extend downwards below the surface in the central region of the RGB image, where Ca is difficult to detect at depth. Similarly, the Ca-Mn-rich (purple) area (Figure 3b, to the right of the center) reveals a dolomite cluster close to the sample surface that extends downwards beneath the surface in the (blue-only) area (Figure 3b, bottom center) to a level that is too deep for Ca to be detectable by SXRF and too deep for SEM detection (Figure 3c). In addition to the apatite and dolomite phases, a comparison of the Mn distribution (Figure 3a) and the Y-Ca-Mn distributions (Figure 3b) reveals numerous small Mn-rich ilmenite grains in association with the apatite cluster in the left-hand region of the images.

REE-Rich Apatite

High-energy SXRF results confirm apatite as the main REE-bearing phase in Ryugu material. Mn, Sr, Y, Ce, and Nd element maps (Figure 4a) from a coarse overview, full-sample scan of C0070-08, performed at a step size of 5 µm and an acquisition time of 1 s pixel⁻¹, show the position of an apatite grain (yellow box), identified by its Sr enrichment (indicating a Ca-rich grain) and the combination of Y-rich and Mn-poor that distinguishes it from a potential dolomite grain (e.g., the grain below the center of the image in Figure 4a, Mn and Sr). Despite the large step size of the full-sample map, enrichments in Ce and Nd are clearly visible, suggesting that these are the two REEs with the highest concentrations in this apatite grain. Element maps resulting from a higher resolution scan around the apatite grain, performed at a step size of 0.5 µm and an acquisition time of 3 s pixel⁻¹ (Figure 4b) confirm it to be a large apatite of ~27 µm in size with a subhedral

morphology with fairly well-defined boundaries. Relative to the surrounding matrix, several REE-rich (and Ba-rich) concentrations are clearly visible within the grain, further demonstrating visually that Ce and Nd are the strongest REE concentrations within the apatite grain. These observations were further confirmed in all apatite grains measured in all the Ryugu samples analyzed by SXRF in this study.

By rotating the sample through the beam at a rotation step-size of 2°, a cross section scan was performed through the sample at the height of the Y-REE-rich grain (marked by a dashed line on Figure 5a) at a step size of 2 µm and an acquisition time of 1 s pixel⁻¹. The 1-µm-thick virtual cross-sectional scan allows us to see in 3D the elemental distribution (Figure 5b), and thereby identify the location of other mineral phases, through the sample at the selected height. This information is particularly important for accurate quantification of the trace element concentrations of any point analyses performed at points of interest at this height, for example, the Y-rich grain, since the path of the beam would travel through the complete sample and excite all elements within its path. The results reveal that this apatite grain is located about 100 µm deep within the sample from the front surface of the sample facing the incident beam (Figure 5) and that no further (REE-bearing) apatite grains are located within the cross-sectional slice of the sample at this height.

Quantified REE Concentrations

We determined the REE concentrations of 19 different apatite grains from four different samples (see Figure S2, e.g., REE spectra of the different Ryugu rock fragments). The quantified REE concentrations of all 19 measured apatite grains are shown in Figure 6 and Table 1. Two of the studied samples (A0055 and A0186) were among those collected during the first touchdown (TD1) of the Hayabusa2 spacecraft on the surface of Ryugu, and two of the samples (C0070 and C0076) were among those collected during the second touchdown (TD2; Morota et al., 2020). The results of all measurements were normalized to mean CI concentrations (Lodders, 2020). Error bars reflect 1-sigma deviations, also CI-normalized, and any gaps in the data profiles indicate REE concentrations below the DL (Table 1). We note that the concentration values for Tm (1.2 ± 0.3 µg g⁻¹ in sample C76-1) and Lu (0.6 ± 0.2 µg g⁻¹ in sample A55-3) are very slightly below the given DLs of 1.24 µg g⁻¹ ± 0.03 µg g⁻¹ and 0.62 ± 0.01 µg g⁻¹, respectively (Table 1), despite these thresholds being very low. However, the error margins in both cases can bring these values above the given detection levels. Comparison is made to the CI-normalized REE concentrations of 10 calcium phosphates measured in the
<figure><img src="image_5.png" /><figcaption>FIGURE 4. High-energy SXRF element distribution maps of Ryugu sample C0070-08. (a) full sample maps of Sr, Mn, and Y (identifying an apatite grain, yellow box) and Ce and Nd. (b) high-resolution element maps of the apatite grain identified in (a) showing enrichments in Sr, Y, Ba, and several REEs. Intensity counts are shown in color scale bars to the right of the images. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

Orgueil CI chondrite (Morlok et al., 2006), depicted in Figures 6-8 as a gray-shaded area that marks for each REE the range between the minimum and maximum (CI-normalized) concentrations of the 10 calcium-phosphate grains in Orgueil (Morlok et al., 2006).

### Light REE

All of the measured Ryugu apatite grains show very flat profiles of light REE (LREE) up to and including Sm (Figure 6). With the exception of one apatite grain measured in Ryugu sample C0076, whose LREE concentrations are slightly lower, LREE concentrations of all other Ryugu apatite grains lie within the range of 30–150 times greater than the mean CI values (Lodders, 2020). A comparison with REE concentrations of calcium phosphates measured in the CI chondrite Orgueil (gray-shaded area, Morlok et al., 2006) reveals

that the LREE concentrations in Ryugu apatite grains are in good agreement with those of Orgueil. The Pr concentrations in all but three apatite grains are excluded (Figures 6–8) due to a strong overlap with the relatively high Ba-Kβ line, compared to the reference materials, negatively impacting the effective DL<sub>Pr</sub>.

### Medium REE to Heavy REE

With the exception of the three A0055 grains (that remain relatively flat from Nd-Dy), a trending increase in REE concentration levels is noticeable with increasing atomic number from Sm up to and including Er (Figure 6), plateauing from Tb to Er with REE levels in the range of 50–400 times mean CI values. In most cases (15 of the 19 apatite grains), this increase in concentration can be described as a fairly steady trend, whereas three of the measured apatite
<figure><img src="image_6.png" /><figcaption>FIGURE 5. (a) Y-distribution overview map of Ryugu sample C0070-08 demonstrating the height of the SXRF cross-sectional scan through the apatite grain. (b) Element maps reveal the position of the apatite grain at a depth of about 100 µm from the front of the sample and demonstrate the enrichments of Sr, Y, Ba, and several REEs within the apatite grain. Mn-enrichments reveal the positions of several large dolomite grains, also visible in the Sr distribution map. (c) Transmission CT image of the slice through the sample acquired simultaneously to the SXRF cross-sectional scan, showing the location of the apatite grain (yellow arrow), several carbonates, and a large pyrrhotite grain. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

grains (A0055-5, A0186-Y5, and A0186-Y6) display more fluctuation, for example, Eu, Tb, and Ho that are either below the DLs (Table 1) or lie within the overall range of the Ryugu concentrations. This rise in REE enrichments from LREE toward medium REE (MREE) and heavy REE (HREE) in Ryugu apatite grains is observed in five of the 10 calcium phosphates measured in Orgueil (Morlok et al., 2006), with the other five Orgueil grains remaining at a relatively constant level from LREE to HREE. After Er, the trend toward increased REE levels in Ryugu grains ceases and a slight drop is observed in Yb and Lu, the latter being above the DL<sub>Lu</sub> (Table 1) in only five of the Ryugu apatite grains.

## Anomalies

The data from Ryugu apatite grains differ with regard to anomalies in Eu or Gd (Figure 6), their consistency across all measured Ryugu grains, and their respective comparison with those reported in Orgueil. With the exception of one grain (A0186-Y6) that shows a clearly positive Eu, all other Ryugu grains (N = 18) show no clear Eu anomaly. This is similar to the results for calcium-phosphate grains in Orgueil that

display a lack of Eu anomaly in all grains (Morlok et al., 2006). On the other hand, Gd anomalies in the Ryugu apatite grains vary from sample to sample with A0186 and C0070 apatite grains generally showing no Gd anomaly, and C0076 and A0055 grains showing a small positive Gd anomaly. These results lie in contrast to the Orgueil data that show clear negative Gd anomalies in 6 of the 10 measured calcium-phosphate grains (Morlok et al., 2006).

## Dolomite Associations

On account of the ubiquitous nature of Fe distribution within the Ryugu samples, it was not possible to reliably determine magnetite–apatite associations with high-energy SXRF for REE measurements. However, eight of the Ryugu apatite grains measured by SXRF were observed to be in close association with dolomite grains. Although in high-energy SXRF, the identification of and distinction between dolomite and apatite can be achieved by enrichments of Ca-Sr-Mn and Ca-Sr-Y-REE, respectively, reliable identification of dolomite–apatite associations requires more than the juxtaposition of the two phases in the scan image.
<figure><img src="image_7.png" /><figcaption>FIGURE 6. REE concentrations in 19 apatite grains from four Ryugu samples (A0055 green; C0076 purple; A0186 red; C0070 blue) measured by high-energy SXRF. REE concentrations and error margins (s1 standard deviation) are normalized to CI bulk values (Lodders, 2020). *Gray-shaded area depicts REE concentrations in 10 calcium-phosphate grains measured in Orgueil (Morlok et al., 2006). **Results from Nakamura, Matsumoto, et al. (2022) and Tack et al. (2022). (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

On account of the 2D visualization of a 3D object, which is common for SXRF results (whereby fluorescence signals are detected from elements throughout the full depth of the samples and projected onto a 2D map viewed from the beam direction), two phases that appear adjacent in the 2D element distribution maps can lie at different depths within the sample, thereby potentially having significant spatial separation at depth. Thus, in order to correctly identify apatite-dolomite associations (i.e., where apatite and dolomite are in direct contact with each other) in SXRF, only those cases were selected where a larger Ca-Sr-Mn-enriched region conjointly displays a localized Mn-depletion pocket that exactly coincides spatially with a localized Y-Ba-Ce-Nd enrichment. This was the case for eight of the 18 measured apatites (Figure 7a). The quantified REE concentrations of those Ryugu apatite grains that clearly lie in direct association with dolomite grains (Figure 7a) all display distinctly greater MREE-HREE enrichments relative to the calcium-phosphate grains in Orgueil (Morlok et al., 2006; Figure 7a). This difference appears less consistent in the Ryugu apatite grains not directly associated with dolomite (Figure 7b) which also include measured compositions with lower MREE-HREE, similar to the Orgueil phosphates (Morlok et al., 2006). One should note here that all the

dolomite-associated apatite grains (Figure 7a) are from one sample (A0186).

TD1 and TD2 Comparison

A comparison of the apatite REE levels in the two Ryugu A-samples from TD1 (Figure 8a) and the two C-samples from TD2 (Figure 8b) reveals a similar LREE-to-HREE increase in enrichment in both groups. A possible difference might be discernible in the smoothness and consistency of the profiles, with the C samples displaying fairly smooth and consistent trends with less omissions and less excursions, whereas the A samples display more fluctuation in the MREE-to-HREE region, including a greater number of elements below the DL. Tb and Ho, for example, are detected in all apatite grains from the two C samples but are below the DL in 8 (Tb) and 5 (Ho) of the 14 apatite grains measured in the two A samples (Table 1). However, this observation is to be considered with caution, since only 5 apatite grains were measured in the two C samples.

DISCUSSION

The wide variety of apatite morphologies and apatite-mineral associations is consistent with the heterogeneous distribution of a circulating hydrothermal
<table><caption>TABLE 1. REE concentrations (µg g⁻¹) of apatite grains in Ryugu samples.ª</caption><thead><tr><th></th><th>La K<br>µg g<sup>-1</sup></th><th>Ce K<br>µg g<sup>-1</sup></th><th>Pr K<br>µg g<sup>-1</sup></th><th>Nd K<br>µg g<sup>-1</sup></th><th>Sm K<br>µg g<sup>-1</sup></th><th>Eu K<br>µg g<sup>-1</sup></th><th>Gd K<br>µg g<sup>-1</sup></th><th>Tb K<br>µg g<sup>-1</sup></th><th>Dy K<br>µg g<sup>-1</sup></th><th>Ho K<br>µg g<sup>-1</sup></th><th>Er K<br>µg g<sup>-1</sup></th><th>Tm K<br>µg g<sup>-1</sup></th><th>Yb K<br>µg g<sup>-1</sup></th><th>Lu K<br>µg g<sup>-1</sup></th></tr></thead><tbody><tr><td>A55-2</td><td>31.8</td><td>1.2</td><td>91.4</td><td>3.0</td><td>11.9</td><td>0.6</td><td>64.3</td><td>2.2</td><td>19.8</td><td>0.8</td><td>6.6</td><td>0.4</td><td>30.7</td><td>1.4</td><td>4.6</td><td>0.3</td><td>34.3</td><td>1.4</td><td>16.0</td><td>0.7</td><td>2.0</td><td>0.3</td></tr><tr><td>A55-3</td><td>7.6</td><td>0.3</td><td>24.0</td><td>0.8</td><td>3.9</td><td>0.2</td><td>19.7</td><td>0.7</td><td>6.4</td><td>0.3</td><td>2.1</td><td>0.2</td><td>11.0</td><td>0.5</td><td>1.4</td><td>0.2</td><td>14.2</td><td>0.6</td><td>8.2</td><td>0.4</td><td>0.6</td><td>0.2</td></tr><tr><td>A55-5</td><td>14.8</td><td>0.9</td><td>47.7</td><td>1.7</td><td></td><td></td><td>37.4</td><td>1.4</td><td>10.0</td><td>0.7</td><td>3.2</td><td>0.7</td><td>17.6</td><td>1.1</td><td>2.8</td><td>0.7</td><td>25.9</td><td>1.3</td><td>20.3</td><td>1.6</td><td>41.2</td><td>3.1</td></tr><tr><td>C76-1</td><td>4.0</td><td>0.2</td><td>12.8</td><td>0.4</td><td>1.8</td><td>0.1</td><td>11.2</td><td>0.4</td><td>3.9</td><td>0.2</td><td>1.4</td><td>0.1</td><td>7.8</td><td>0.4</td><td>1.0</td><td>0.1</td><td>11.3</td><td>0.5</td><td>2.7</td><td>0.2</td><td>10.4</td><td>0.5</td></tr><tr><td>A186-Y1</td><td>8.2</td><td>0.5</td><td>20.4</td><td>0.6</td><td></td><td></td><td>13.9</td><td>0.4</td><td>5.8</td><td>0.4</td><td></td><td></td><td>12.9</td><td>0.6</td><td>4.8</td><td>0.7</td><td>18.3</td><td>0.7</td><td>4.2</td><td>0.6</td><td>14.4</td><td>0.8</td></tr><tr><td>A186-Y2</td><td>17.3</td><td>0.7</td><td>51.3</td><td>1.1</td><td></td><td></td><td>36.5</td><td>0.8</td><td>16.6</td><td>0.6</td><td>8.0</td><td>0.7</td><td>31.1</td><td>1.1</td><td>4.8</td><td>0.7</td><td>45.9</td><td>1.5</td><td>8.1</td><td>0.9</td><td>31.1</td><td>1.3</td></tr><tr><td>A186-Y3</td><td>19.2</td><td>1.3</td><td>56.3</td><td>1.6</td><td></td><td></td><td>38.4</td><td>1.2</td><td>16.4</td><td>1.1</td><td>8.0</td><td>1.3</td><td>35.9</td><td>1.9</td><td>8.0</td><td>1.5</td><td>52.9</td><td>2.3</td><td>12.0</td><td>1.9</td><td>30.2</td><td>2.2</td></tr><tr><td>A186-Y4</td><td>27.7</td><td>1.4</td><td>77.3</td><td>1.9</td><td></td><td></td><td>54.4</td><td>1.4</td><td>20.9</td><td>1.1</td><td>12.2</td><td>1.4</td><td>41.1</td><td>2.0</td><td>8.0</td><td>1.5</td><td>67.8</td><td>2.5</td><td>13.7</td><td>2.0</td><td>51.4</td><td>2.6</td></tr><tr><td>A186-Y5</td><td>9.3</td><td>0.9</td><td>28.4</td><td>1.0</td><td></td><td></td><td>18.0</td><td>0.8</td><td>7.7</td><td>0.8</td><td>8.2</td><td>1.0</td><td>15.3</td><td>1.2</td><td>10.9</td><td>1.1</td><td>26.8</td><td>1.5</td><td>7.3</td><td>1.4</td><td>18.3</td><td>1.6</td></tr><tr><td>A186-Y6</td><td>8.5</td><td>0.9</td><td>26.6</td><td>1.0</td><td></td><td></td><td>20.3</td><td>0.8</td><td>9.7</td><td>0.8</td><td>6.5</td><td>1.0</td><td>22.6</td><td>1.3</td><td></td><td></td><td>40.9</td><td>1.7</td><td>10.0</td><td>1.5</td><td>31.0</td><td>1.8</td></tr><tr><td>A186-Y7</td><td>12.5</td><td>1.0</td><td>36.7</td><td>1.1</td><td></td><td></td><td>25.5</td><td>0.8</td><td>8.9</td><td>0.8</td><td>4.6</td><td>1.0</td><td>22.5</td><td>1.3</td><td></td><td></td><td>75.7</td><td>5.6</td><td></td><td></td><td></td><td></td></tr><tr><td>A186-M3</td><td>35.4</td><td>3.9</td><td>82.1</td><td>4.0</td><td></td><td></td><td>53.7</td><td>3.0</td><td>21.8</td><td>3.0</td><td></td><td></td><td>59.3</td><td>5.0</td><td></td><td></td><td>29.7</td><td>2.2</td><td></td><td></td><td></td><td></td></tr><tr><td>A186-M4</td><td>11.9</td><td>1.5</td><td>30.0</td><td>1.5</td><td></td><td></td><td>22.1</td><td>1.2</td><td>12.0</td><td>1.2</td><td></td><td></td><td>19.2</td><td>1.9</td><td></td><td></td><td>66.7</td><td>4.3</td><td></td><td></td><td></td><td></td></tr><tr><td>A186-M8</td><td>22.7</td><td>2.8</td><td>62.7</td><td>2.9</td><td></td><td></td><td>44.1</td><td>2.2</td><td>22.0</td><td>2.2</td><td></td><td></td><td>34.0</td><td>3.5</td><td></td><td></td><td>56.0</td><td>4.2</td><td></td><td></td><td></td><td></td></tr><tr><td>A186-Sr1</td><td>28.9</td><td>2.7</td><td>68.9</td><td>2.8</td><td></td><td></td><td>48.8</td><td>2.1</td><td>21.5</td><td>2.1</td><td></td><td></td><td>33.4</td><td>3.4</td><td></td><td></td><td>8.0</td><td>1.2</td><td>4.1</td><td>0.8</td><td>48.5</td><td>4.8</td></tr><tr><td>C70-4-Y1</td><td>12.1</td><td>0.8</td><td>30.2</td><td>0.9</td><td></td><td></td><td>21.3</td><td>0.7</td><td>9.3</td><td>0.6</td><td></td><td></td><td>16.2</td><td>1.0</td><td></td><td></td><td>24.7</td><td>1.2</td><td>8.0</td><td>1.1</td><td>16.0</td><td>1.2</td></tr><tr><td>C70-4-Y3</td><td>16.4</td><td>0.7</td><td>42.5</td><td>1.0</td><td></td><td></td><td>28.2</td><td>0.7</td><td>11.2</td><td>0.6</td><td></td><td></td><td>21.8</td><td>1.0</td><td></td><td></td><td>33.9</td><td>1.3</td><td>8.7</td><td>1.0</td><td>20.5</td><td>1.2</td></tr><tr><td>C70-4-Y7</td><td>35.1</td><td>1.6</td><td>88.7</td><td>2.1</td><td></td><td></td><td>57.4</td><td>1.5</td><td>21.4</td><td>1.2</td><td></td><td></td><td>11.6</td><td>1.7</td><td></td><td></td><td>70.1</td><td>2.7</td><td>16.9</td><td>2.2</td><td>44.5</td><td>2.7</td></tr><tr><td>C70-8-Y1</td><td>26.7</td><td>1.1</td><td>61.1</td><td>1.4</td><td></td><td></td><td>47.8</td><td>1.1</td><td>18.3</td><td>0.8</td><td></td><td></td><td>35.0</td><td>1.5</td><td></td><td></td><td>52.2</td><td>1.8</td><td>12.5</td><td>1.3</td><td>34.6</td><td>1.7</td></tr><tr><td>DL 600<sup>b</sup></td><td>0.98</td><td>0.02</td><td>1.05</td><td>0.02</td><td>1.01</td><td>0.02</td><td>0.84</td><td>0.02</td><td>0.96</td><td>0.02</td><td></td><td></td><td>1.34</td><td>0.04</td><td></td><td></td><td>1.56</td><td>0.04</td><td>1.36</td><td>0.03</td><td>1.59</td><td>0.04</td></tr></tbody></table>

ªBy acquisition time of 600 s.

ᵇDL: detection limit by acquisition time of 600 s.
<figure><img src="image_8.png" /><figcaption>FIGURE 7. REE concentrations in Ryugu apatite grains that are: (a) clearly associated with an occurrence of dolomite; and (b) not clearly associated with an occurrence of dolomite. REE concentrations and error margins (s1 standard deviation) are normalized to CI bulk values (Lodders, 2020). Colors of the symbols are the same as in Figure 6. *Gray-shaded area depicts REE concentrations in 10 calcium-phosphate grains measured in Orgueil (Morlok et al., 2006). **Results from Nakamura, Matsumoto, et al. (2022) and Tack et al. (2022). (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

fluid (Figure 1). Apatite appears both in paragenesis with
several secondary minerals, such as pyrrhotite, magnetite,
and carbonate, and as postgenetic interstitial fill
(Figure 2). This is consistent with observations in other
chondrites, regardless of chondrite class, with

Ca-phosphates predominantly reported with two main
mineral associations, metal-sulfide interfaces and, for
higher petrologic types, in paragenesis with silicates
(Alfing et al., 2019; Ward et al., 2017). No spatial
correlations could be observed regarding the various
<figure><img src="image_9.png" /><figcaption>FIGURE 8. REE concentrations in apatite grains from: (a) two Ryugu A-samples (A0055 and A0186) collected during the first touchdown (TD1); (b) two Ryugu C-samples (C0076 and C0070), collected during the second touchdown (TD2). REE concentrations and error margins (s1 standard deviation) are normalized to CI bulk values (Lodders, 2020). \*Gray-shaded area depicts REE concentrations in 10 calcium-phosphate grains measured in Orgueil (Morlok et al. 2006). \*\*Results from Nakamura, Matsumoto, et al. (2022) and Tack et al. (2022). (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

samples (or brecciation regions and lithologies), morphology nor mineral association. Furthermore, there appears to be no clear correlation between mineral association and the condition of the apatite grain boundaries. However, the apatite-mineral associations

reveal that the external boundaries of both the apatite and the associated mineral appear to reflect a similar degree of corrosion. The inner apatite-mineral interface boundaries are often less corroded than the external boundaries. This could be explained by a scenario
involving several stages of hydrothermal fluid interaction. The aqueous alteration of primary anhydrous phases such as olivine, pyroxene, and amorphous silicates would result in the formation of secondary hydrous phases, for example, phyllosilicates such as serpentine and saponite, which are reported to be the main phases of the Ryugu matrix (Nakamura, Matsumoto, et al., 2022). The circulating fluid would subsequently have given rise to the precipitation of carbonates (Endreß & Bischoff, 1996) and localized aqueous alteration of phyllosilicates that formed secondary alteration products such as Fe-sulfides, with later-stage oxidation then forming magnetite (Zolensky et al., 2002). In a non-igneous environment such as Ryugu, the circulation of hydrothermal fluid through these resulting minerals would lead to metasomatic reactions that enable the formation of apatite. The oxidation of P, originally dissolved in magnetite, pyrrhotite, Fe, and Ni-phosphides (such as schreibersite and allabogdanite observed in less-altered Ryugu clasts, described in Nakamura, Matsumoto, et al., 2022) or possibly in chondrules or CAIs, as well as the reaction with Ca from the carbonates and phyllosilicates (Brearley & Jones, 1998; Jones et al., 2014; Nakamura, Matsumoto, et al., 2022; Piralla et al., 2021; Ward et al., 2017) would result in the formation of apatite and the mineral associations observed in this study. Further exposure to aqueous fluids in the surrounding matrix would have then left these mineral associations vulnerable to corrosion at their external boundaries.

All SXRF maps reveal Nd and Ce to be the strongest concentrations of REE in Ryugu apatite (e.g., Figure 4). The CI-normalized REE concentrations (Figures 6–8) show that the Nd and Ce concentrations, like those of all other LREEs, are very similar to those of Orgueil (Morlok et al., 2006) and lie at levels 20–150 times those of bulk CI values (Lodders, 2020). However, the full CI-normalized REE profiles of Ryugu apatite are not as flat as most of the CI-normalized Orgueil calcium phosphates (Morlok et al., 2006) and show an increasing trend toward MREE that results in MREE-HREE some 50–400 times those of bulk CI and some noticeably greater than those of Orgueil (Figures 6–8). Furthermore, this MREE-HREE enrichment trend in Ryugu samples is in clear contrast to the steady LREE-HREE decreasing trend reported in eight ordinary chondrites as well as in the Karoonda CK4 carbonaceous chondrite (Ward et al., 2017). This MREE-HREE enrichment in Ryugu samples may be an indication for a secondary hydrous alteration of the apatite after its formation. As a late-stage phase, Ca-phosphates often incorporate the vast majority of the host meteorite's bulk REE budget (Ward et al., 2017), and this has been clearly observed in Ryugu material (Nakamura, Matsumoto, et al., 2022; Tack

et al., 2022). It has been reported that, less compatible than HREE, LREE can be further dissolved more easily than HREE into a circulating F- and Cl-bearing fluid (Broom-Fendley et al., 2016). Preferential LREE redissolution into a later-stage hydrothermal fluid could result in LREE mobilization away from an initial apatite phase, resulting in a passively-induced enrichment of HREE in the original apatite (Broom-Fendley et al., 2016). The occurrence of a preferential mobilization of LREE from apatite in a later hydrous alteration process would likely have resulted in the LREE enrichment of another mineral. This has been reported to occur more frequently in connection with the presence of carbonate (Broom-Fendley et al., 2016). However, the comparison of carbonate-associated apatite and non-carbonate-associated apatite in the studied Ryugu samples (Figure 7) does not reveal any clear difference in CI-normalized LREE or HREE values. It is worth noting, however, that the CI-normalized REE concentrations of one dolomite grain in Ryugu sample A0055 have been reported to be enriched by 4–10 times CI values in LREE-MREE but with concentrations below the DL (Table 1) for all HREE heavier than Dy (Nakamura, Matsumoto, et al., 2022; Tack et al., 2022). This may suggest a secondary LREE enrichment that may have resulted from secondary preferential mobilization of LREE from original apatites, but further REE measurements of carbonates would be necessary to assess this hypothesis. It should further be noted that the SXRF point analysis measurements of apatite grains were performed in the center of each apatite grain. It is, thus, feasible that the apatite cores and rims may differ in REE enrichment relative to CI bulk values. LREE mobilization out of the apatite cores may be reprecipitated in late apatite formation at the apatite rims, resulting in HREE-enriched cores and LREE-enriched rims. This feature has been observed and studied in apatite grains within quartz–apatite rocks of carbonatite complexes in Malawi, and is thought to have resulted from the combined process of dissolution–reprecipitation of original apatite and the preferential transport of the LREE complexes by F- and Cl-bearing hydrothermal fluids (Broom-Fendley et al., 2016), whereby successive pulses of these fluids transport the LREE out of the original apatite and preferentially reprecipitate it on the rim. Hydrothermal-fluid-induced dissolution–reprecipitation of apatite has also been observed in several further natural terrestrial samples (Engvik et al., 2009; Harlov, Andersson, et al., 2002; Harlov et al., 2005; Krause et al., 2013) and has also been recreated experimentally over a range of temperatures (300–900°C) and pressures (500–1000 MPa) and with a range of fluid compositions (e.g., H₂O; NaCl-, KCl-, and CaCl₂-bearing brines; H₂O/CO₂ mixtures; HCl;
and H₂SO₄; Harlov, Förster, et al., 2002; Harlov & Förster, 2003; Harlov et al., 2005). Although these experiments were conducted at higher temperatures and pressures than those prevailing during the aqueous alteration of Ryugu (~25°C, Nakamura, Matsumoto, et al., 2022), it is notable that Harlov, Förster, et al. (2002) concluded from these experiments that the composition of the infiltrating fluid was the primary constraining factor, and to a much lesser extent temperature and pressure. More insight into the possibility of preferential dissolution-reprecipitation of Ryugu apatite could be provided by further studies focusing, with improved spatial resolution, on any REE-zoning within apatite grains in Ryugu samples and further experiments at low temperatures and pressures would help to test the feasibility of this process having occurred during aqueous alteration on Ryugu.

## CONCLUSIONS

Nineteen apatites from five Ryugu samples were investigated within the framework of the Hayabusa2 initial analysis stage and the subsequent Hayabusa2 AO1 analysis stage. The samples were imaged by SEM and TEM and examined by medium (20.5 keV) and high (90 keV) synchrotron radiation to observe their morphology and mineral associations and investigate their REE concentrations. The wide variety of apatite morphologies and apatite-mineral associations (mainly dolomite, magnetite, and/or pyrrhotite) observed is consistent with those reported in other chondrites (Alfing et al., 2019; Ward et al., 2017) and suggests multi-phase episodes of hydrothermal alteration. The lack of spatial correlation between morphology, mineral association, and (regional or sample-wide) location is consistent with the heterogeneous distribution of hydrothermal fluid circulation (Figure 1). LREE concentrations in Ryugu apatite grains are 30–150 times those of bulk CI values (Lodders, 2020) and in good agreement with those recorded in CI-chondrite Orgueil (Morlok et al., 2006; Figure 6). However, a steady increase in MREE-HREE concentration levels in Ryugu apatite grains, up to 50–400 times CI bulk values (Figures 6–8) and four times higher than those in Orgueil, may suggest a later-stage preferential mobilization of LREE away from the original apatite within the Ryugu material, resulting from interaction with later episodes of hydrothermal fluid.

**Acknowledgments**—Some of these experiments were performed on beamline ID15A at the ESRF—The European Synchrotron, Grenoble, France. We are grateful to the ESRF for providing beamtime under LTP ES-1018. Parts of this research were carried out at DESY (Hamburg, Germany), a member of the Helmholtz

Association HGF, and we are grateful for the local support at the beamline P06 at Petra III and for the provision of experimental facilities. The authors acknowledge E. Kagawa and M. Kikuiri for the preparation of the sample particles studied. These experiments were made possible thanks to funding from German Research Foundation DFG grant BR2015/38-1, Dr. Rolf M. Schwiete Stiftung, Ghent University Special Research Fund grants BOF20/PDO/037, BOF17-GOA-015 and Research Foundation Flanders grants G0D5221N, 1205322N. FL is grateful to the Deutsche Forschungsgemeinschaft for funding the FIB and TEM facilities at the University of Jena via the Gottfried Wilhlem Leibniz program (LA830/14-1). Open Access funding enabled and organized by Projekt DEAL.

*Data Availability Statement*—Data that support the findings of this study are available in the supplementary material of this article. Raw data used for the findings of this study are available on request from the corresponding author.

*Editorial Handling*—Dr. Michael Zolensky

## REFERENCES

Alfing, J., Patzek, M., and Bischoff, A. 2019. Modal Abundances of Coarse-Grained (>5 µm) Components within CI-Chondrites and their Individual Clasts—Mixing of Various Lithologies on the CI Parent Body(ies). *Chemie der Erde* 79: 125532.

Anders, E., and Ebihara, M. 1982. Solar-System Abundances of the Elements. *Geochimica et Cosmochimica Acta* 46: 2363–80.

Anders, E., and Grevesse, N. 1989. Abundances of the Elements: Meteoritic and Solar. *Geochimica et Cosmochimica Acta* 53: 197–214.

Barrat, J. A., Zanda, B., Moynier, F., Bollinger, C., Liorzou, C., and Bayon, G. 2012. Geochemistry of CI Chondrites: Major and Trace Elements, and Cu and Zn Isotopes. *Geochimica et Cosmochimica Acta* 83: 79–92.

Bazi, B., Tack, P., Lindner, M., Vekemans, B., DePauw, E., Tkalcec, B. J., Brenker, F. E., et al. 2022. Trace-Element Analysis of Mineral Grains in Ryugu Rock Fragment Thin Sections by Synchrotron Based Confocal X-Ray Fluorescence. *Earth, Planets and Space* 74: 161.

Borisova, A. Y., Freydier, R., Polvé, M., Jochum, K. P., and Candaudap, F. 2010. Multi-Elemental Analysis of ATHO-G Rhyolitic Glass (MPI-DING Reference Material) by Femtosecond and Nanosecond LA-ICP-MS: Evidence for Significant Heterogeneity of B, V, Zn, Mo, Sn, Sb, Cs, W, Pt and Pb at the Millimetre Scale. *Geostandards and Geoanalytical Research* 34: 245–255.

Brearley, A. J., and Jones, R. H. 1998. Chondritic Meteorites. In *Reviews in Mineralogy*, edited by J. J. Papike, 36th ed., 3-001-3-398. Washington, D.C.: Mineralogical Society of America.

Broom-Fendley, S., Styles, M. T., Appleton, J. D., Gunn, G., and Wall, F. 2016. Evidence for Dissolution-Reprecipitation of Apatite and Preferential LREE
Mobility in Carbonatite-Derived Late-Stage Hydrothermal Processes. *American Mineralogist* 101: 596-611.

De Pauw, E., Tkalcec, B. J., Tack, P., Vekemans, B., Di Michiel, M., Brenker, F. E., and Vincze, L. 2022. High Energy Synchrotron X-Ray Fluorescence Trace Element Study of a Millimeter-Sized Asteroidal Particle in Preparation for the Hayabusa2 Return Sample Analyses. *Spectrochimica Acta—Part B Atomic Spectroscopy* 188: 106346.

Endreß, M., and Bischoff, A. 1996. Carbonates in CI Chondrites: Clues to Parent Body Evolution. *Geochimica et Cosmochimica Acta* 60: 489-507.

Engvik, A. K., Golla-Schindler, U., Berndt, J., Austrheim, H., and Putnis, A. 2009. Intragranular Replacement of Chlorapatite by Hydroxy-Fluor-Apatite during Metasomatism. *Lithos* 112: 236-246.

Harlov, D. E., Andersson, U. B., Förster, H. J., Nyström, J. O., Dulski, P., and Broman, C. 2002. Apatite-Monazite Relations in the Kiirunavaara Magnetite-Apatite Ore, Northern Sweden. *Chemical Geology* 191: 47-72.

Harlov, D. E., and Förster, H.-J. 2003. Fluid-Induced Nucleation of (Y+REE)—Phosphate Minerals within Apatite: Nature and Experiment. Part II. Fluorapatite. *American Mineralogist* 88: 1209-29.

Harlov, D. E., Förster, H. J., and Nijland, T. G. 2002. Fluid-Induced Nucleation of (Y + REE)-Phosphate Minerals within Apatite: Nature and Experiment. Part I. Chlorapatite. *American Mineralogist* 87: 245-261.

Harlov, D. E., Wirth, R., and Förster, H. J. 2005. An Experimental Study of Dissolution-Reprecipitation in Fluorapatite: Fluid Infiltration and the Formation of Monazite. *Contributions to Mineralogy and Petrology* 150: 268-286.

Ito, M., Tomioka, N., Uesugi, M., Yamaguchi, A., Shirai, N., Ohigashi, T., Liu, M. C., et al. 2022. A Pristine Record of Outer Solar System Materials from Asteroid Ryugu's Returned Sample. *Nature Astronomy* 6: 1163-71.

Jochum, K. P., Stoll, B., Herwig, K., Willbold, M., Hofmiann, A. W., Amini, M., Aarburg, S., et al. 2006. MPI-DING Reference Glasses for In Situ Microanalysis: New Reference Values for Element concentrations and isotope ratios.

Jochum, K. P., Weis, U., Stoll, B., Kuzmin, D., Yang, Q., Raczek, I., Jacob, D. E., et al. 2011. Determination of Reference Values for NIST SRM 610-617 Glasses Following ISO Guidelines. *Geostandards and Geoanalytical Research* 35: 397-429.

Jones, R. H., McCubbin, F. M., Dreeland, L., Guan, Y., Burger, P. V., and Shearer, C. K. 2014. Phosphate Minerals in LL Chondrites: A Record of the Action of Fluids during Metamorphism on Ordinary Chondrite Parent Bodies. *Geochimica et Cosmochimica Acta* 132: 120-140.

Kempenaers, L., Janssens, K., Jochum, K. P., Vincze, L., Vekemans, B., Somogyi, A., Drakopoulos, M., and Adams, F. 2003. Micro-Heterogeneity Study of Trace Elements in USGS, MPI-DING and NIST Glass Reference Materials by Means of Synchrotron Micro-XRF. *Journal of Analytical Atomic Spectrometry* 18: 350-57.

Krause, J., Harlov, D. E., Pushkarev, E. V., and Brügmann, G. E. 2013. Apatite and Clinopyroxene as Tracers for Metasomatic Processes in Nepheline Clinopyroxenites of Uralian-Alaskan-Type Complexes in the Ural Mountains,

Russian Federation. *Geochimica et Cosmochimica Acta* 121: 503-521.

Langenhorst, F., Harries, D., Pollok, K., and Van Aken, P. A. 2014. Mineralogy and Defect Microstructure of an Olivine-Dominated Itokawa Dust Particle: Evidence for Shock Metamorphism, Collisional Fragmentation, and LL Chondrite Origin. *Earth, Planets and Space* 66: 118.

Lodders, K. 2020. Solar Elemental Abundances. In *Page the Oxford Research Encyclopedia of Planetary Science*. Oxford: Oxford University Press.

McCubbin, F. M., Hauri, E. H., Elardo, S. M., Vander Kaaden, K. E., Wang, J., and Shearer, C. K. 2012. Hydrous Melting of the Martian Mantle Produced both Depleted and Enriched Shergottites. *Geology* 40: 683-86.

Morlok, A., Bischoff, A., Stephan, T., Floss, C., Zinner, E., and Jessberger, E. K. 2006. Brecciation and Chemical Heterogeneities of CI Chondrites. *Geochimica et Cosmochimica Acta* 70: 5371-94.

Morota, T., Sugita, S., Cho, Y., Kanamaru, M., Tatsumi, E., Sakatani, N., Honda, R., et al. 2020. Sample Collection from Asteroid (162173) Ryugu by Hayabusa2: Implications for Surface Evolution. *Science* 368: 654-59.

Nakamura, E., Kobayashi, K., Tanaka, R., Kunihiro, T., Kitagawa, H., Potiszil, C., Ota, T., Sakaguchi, C., et al. 2022. On the Origin and Evolution of the Asteroid Ryugu: A Comprehensive Geochemical Perspective. *Proceedings of the Japan Academy. Series B, Physical and Biological Sciences* 98: 227-282.

Nakamura, T., Matsumoto, M., Amano, K., Enokido, Y., Zolensky, M. E., Mikouchi, T., Genda, H., et al. 2022. Formation and Evolution of Carbonaceous Asteroid Ryugu: Direct Evidence from Returned Samples. *Science* 379: eabn8671. https://doi.org/10.1126/science.abn8671.

Nakashima, D., Fujioka, Y., Katayama, K., Nakamura, T., Morita, T., Kikuiri, M., Amano, K., et al. 2022. Preparation Methods of Polished Sections of Returned Samples from Asteroid Ryugu by the Hayabusa2 Spacecraft. *53rd Lunar and Planetary Science Conference*, 16, 1-16.

Nakato, A., Inada, S., Furuya, S., Nishimura, M., Yada, T., Abe, M., Usui, T., et al. 2022. Ryugu Particles Found outside the Hayabusa2 Sample Container. *Geochemical Journal* 56: 197-222.

Owens, C. L., Nash, G. R., Hadler, K., Fitzpatrick, R. S., Anderson, C. G., and Wall, F. 2019. Apatite Enrichment by Rare Earth Elements: A Review of the Effects of Surface Properties. *Advances in Colloid and Interface Science* 265: 14-28.

Piralla, M., Tartèse, R., Marrocchi, Y., and Joy, K. T. 2021. Apatite Halogen and Hydrogen Isotope Constraints on the Conditions of Hydrothermal Alteration in Carbonaceous Chondrites. *Meteoritics & Planetary Science* 56: 809-828.

Sarafian, A. R., Roden, M. F., and Patiño-Douce, A. E. 2013. The Volatile Content of Vesta: Clues from Apatite in Eucrites. *Meteoritics & Planetary Science* 48: 2135-54.

Schroer, C. G., Boye, P., Feldkamp, J. M., Patommel, J., Samberg, D., Schropp, A., Schwab, A., et al. 2010. Hard X-Ray Nanoprobe at Beamline P06 at PETRA III. *Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment* 616: 93-97.

Solé, V. A., Papillon, E., Cotte, M., Walter, P., and Susini, J. 2007. A Multiplatform Code for the Analysis of Energy-
Dispersive X-Ray Fluorescence Spectra. *Spectrochimica Acta—Part B Atomic Spectroscopy* 62: 63–68.

Tachibana, S., Sawada, H., Okazaki, R., Takano, Y., Sakamoto, K., Miura, Y. N., Okamoto, C., et al. 2022. Pebbles and Sand on Asteroid (162173) Ryugu: In Situ Observation and Particles Returned to Earth. *Science* 375: 1011–16.

Tack, P., De Pauw, E., Tkalcec, B. J., Lindner, M., Bazi, B., Vekemans, B., Brenker, F. E., et al. 2022. Rare Earth Element Identification and Quantification in Millimetre-Sized Ryugu Rock Fragments from the Hayabusa2 Space Mission. *Earth, Planets and Space* 74: 146.

Tkalcec, B. J., Tack, P., De Pauw, E., Vekemans, B., Nakamura, T., Garrevoet, J., Falkenberg, G., Vincze, L., and Brenker, F. E. 2022. Sr Distribution as Proxy for Ca Distribution at Depth in SXRF Analysis of Mm-Sized Carbonaceous Chondrites: Implications for Asteroid Sample Return Missions. *Meteoritics & Planetary Science* 13: 1–13.

Vaughan, G. B. M., Baker, R., Barret, R., Bonnefoy, J., Buslaps, T., Checchia, S., Duran, D., et al. 2020. ID15A at the ESRF-a Beamline for High Speed Operando X-Ray Diffraction, Diffraction Tomography and Total Scattering. *Journal of Synchrotron Radiation* 27: 515–528.

## SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article.

**Figure S1.** Selected-area electron-diffraction (SAED) patterns of Ca-phosphate grains in Ryugu sample C0033-

Ward, D., Bischoff, A., Roszjar, J., Berndt, J., and Whitehouse, M. J. 2017. Trace Element Inventory of Meteoritic Ca-Phosphates. *American Mineralogist* 102: 1856–80.

Watanabe, S., Hirabayashi, M., Hirata, N., Hirata, N., Noguchi, R., Shimaki, Y., Ikeda, H., et al. 2019. Hayabusa2 Arrives at the Carbonaceous Asteroid 162173 Ryugu-A Spinning Top-Shaped Rubble Pile. *Science* 364: 268–272.

Yamaguchi, A., Tomioka, N., Ito, M., Shirai, N., Kimura, M., Greenwood, R. C., Liu, M. C., et al. 2023. Insight into Multi-Step Geological Evolution of C-Type Asteroids from Ryugu Particles. *Nature Astronomy* 7: 398–405.

Yokoyama, T., Nagashima, K., Nakai, I., Young, E. D., Abe, Y., Aléon, J., Alexander, C. M. O’D., et al. 2023. Samples Returned from the Asteroid Ryugu Are Similar to Ivuna-Type Carbonaceous Meteorites. *Science* 379: eabn7850.

Zolensky, M. E., Nakamura, K., Gounelle, M., Mikouchi, T., Kasama, T., Tachikawa, O., and Tonui, E. 2002. Mineralogy of Tagish Lake: An Ungrouped Type 2 Carbonaceous Chondrite. *Meteoritics and Planetary Science* 37: 737–761.

04 FIB7 (left) and C0033-01 FIB1 (right), confirming their identification as apatite in both cases.

**Figure S2.** REE spectra from example apatite grains from the four Ryugu rock fragments A0055, C0076, A0186, and C0070 (the latter consisting of two fragments, 04 and 08) measured by high-energy SXRF. The acquisition time in each measurement was 600 s.