Extensive Aqueous Alteration Research Articles

Phyllosilicates with embedded Fe‐based nanophases in Ryugu and Orgueil

AbstractSamples were recently collected from the carbonaceous asteroid (162173) Ryugu, by the Japan Aerospace Exploration Agency (JAXA) Hayabusa2 mission. They resemble CI chondrites material, thus showing clear evidence of extensive aqueous alteration attested by the widespread presence of a mixture of serpentine and saponite. We present here a scanning transmission electron microscopy study of the Ryugu dominant lithology of the phyllosilicate matrix at the nanometer scale, which we compare with that of the Orgueil CI chondrite. In both objects, the phyllosilicates are of comparable nature and texture, consisting of a mixture of small‐sized crystallites of serpentine and saponite. At the micrometer scale or less, the texture is an alternation of fine and coarse domains. The fine‐grained regions are dominated by saponite. In Ryugu, they enclose numerous Fe,Ni nanosulfides, whereas in Orgueil, S‐ and Ni‐rich ferrihydrite is abundant. The coarse‐grained regions contain more serpentine and no or little Fe,Ni sulfides or ferrihydrite. Scanning transmission x‐ray microscopy at the Fe‐L3 edge also reveals that iron valency of phyllosilicates is higher and more homogeneous in Orgueil (~70% Fe3+) than in Ryugu (<50% Fe3+). We interpret the observed textures as being mostly a consequence of aqueous alteration, likely resulting from the replacement by phyllosilicates of submicrometric components, initially agglomerated by a primary accretion. The fine‐grained domains may result from the replacement of GEMS (GEMS—glass with embedded metal and sulfides) objects or from other types of nanometric assemblages of silicate and Fe‐based nanophases. On the other hand, the coarse‐grained regions may correspond to the replacement of anhydrous crystalline silicates of the olivine and pyroxene type. The major difference is the presence of Fe,Ni sulfides in Ryugu and of ferrihydrite and higher iron valency of phyllosilicates in Orgueil. This might be due to long‐term terrestrial weathering that would have destabilized the nanosulfides. We also explore an alternative scenario involving more oxidizing hydrothermal conditions on the Orgueil parent body.

Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample

Studies of material returned from Cb asteroid Ryugu have revealed considerable mineralogical and chemical heterogeneity, stemming primarily from brecciation and aqueous alteration. Isotopic anomalies could have also been affected by delivery of exogenous clasts and aqueous mobilization of soluble elements. Here, we show that isotopic anomalies for mildly soluble Cr are highly variable in Ryugu and CI chondrites, whereas those of Ti are relatively uniform. This variation in Cr isotope ratios is most likely due to physicochemical fractionation between 54 Cr-rich presolar nanoparticles and Cr-bearing secondary minerals at the millimeter-scale in the bulk samples, likely due to extensive aqueous alteration in their parent bodies that occurred 5.2 − 1.4 + 1.8 Ma after Solar System birth. In contrast, Ti isotopes were marginally affected by this process. Our results show that isotopic heterogeneities in asteroids are not all nebular or accretionary in nature but can also reflect element redistribution by water.

The formation and aqueous alteration of CM2 chondrites and their relationship to CO3 chondrites: A fresh isotopic (O, Cd, Cr, Si, Te, Ti, and Zn) perspective from the Winchcombe CM2 fall

AbstractAs part of an integrated consortium study, we have undertaken O, Cd, Cr, Si, Te, Ti, and Zn whole rock isotopic measurements of the Winchcombe CM2 meteorite. δ66Zn values determined for two Winchcombe aliquots are +0.29 ± 0.05‰ (2SD) and +0.45 ± 0.05‰ (2SD). The difference between these analyses likely reflects sample heterogeneity. Zn isotope compositions for Winchcombe show excellent agreement with published CM2 data. δ114Cd for a single Winchcombe aliquot is +0.29 ± 0.04‰ (2SD), which is close to a previous result for Murchison. δ130Te values for three aliquots gave indistinguishable results, with a mean value of +0.62 ± 0.01‰ (2SD) and are essentially identical to published values for CM2s. ε53Cr and ε54Cr for Winchcombe are 0.319 ± 0.029 (2SE) and 0.775 ± 0.067 (2SE), respectively. Based on its Cr isotopic composition, Winchcombe plots close to other CM2 chondrites. ε50Ti and ε46Ti values for Winchcombe are 3.21 ± 0.09 (2SE) and 0.46 ± 0.08 (2SE), respectively, and are in line with recently published data for CM2s. The δ30Si composition of Winchcombe is −0.50 ± 0.06‰ (2SD, n = 11) and is essentially indistinguishable from measurements obtained on other CM2 chondrites. In conformity with petrographic observations, oxygen isotope analyses of both bulk and micromilled fractions from Winchcombe clearly demonstrate that its parent body experienced extensive aqueous alteration. The style of alteration exhibited by Winchcombe is consistent with relatively closed system processes. Analysis of different fractions within Winchcombe broadly support the view that, while different lithologies within an individual CM2 meteorite can be highly variable, each meteorite is characterized by a predominant alteration type. Mixing of different lithologies within a regolith environment to form cataclastic matrix is supported by oxygen isotope analysis of micromilled fractions from Winchcombe. Previously unpublished bulk oxygen isotope data for 12 CM2 chondrites, when combined with published data, define a well‐constrained regression line with a slope of 0.77. Winchcombe analyses define a more limited linear trend at the isotopically heavy, more aqueously altered, end of the slope 0.77 CM2 array. The CM2 slope 0.77 array intersects the oxygen isotope field of CO3 falls, indicating that the unaltered precursor material to the CMs was essentially identical in oxygen isotope composition to the CO3 falls. Our data are consistent with earlier suggestions that the main differences between the CO3s and CM2s reflect differing amounts of water ice that co‐accreted into their respective parent bodies, being high in the case of CM2s and low in the case of CO3s. The small difference in Si isotope compositions between the CM and CO meteorites can be explained by different proportions of matrix versus refractory silicates. CMs and COs may also be indistinguishable with respect to Ti and Cr isotopes; however, further analysis is required to test this possibility. The close relationship between CO3 and CM2 chondrites revealed by our data supports the emerging view that the snow line within protoplanetary disks marks an important zone of planetesimal accretion.

Hydrogen Isotopic Composition of Hydrous Minerals in Asteroid Ryugu

Rock fragments of the Cb-type asteroid Ryugu returned to Earth by the JAXA Hayabusa2 mission share mineralogical, chemical, and isotopic properties with the Ivuna-type (CI) carbonaceous chondrites. Similar to CI chondrites, these fragments underwent extensive aqueous alteration and consist predominantly of hydrous minerals likely formed in the presence of liquid water on the Ryugu parent asteroid. Here we present an in situ analytical survey performed by secondary ion mass spectrometry from which we have estimated the D/H ratio of Ryugu’s hydrous minerals, D/HRyugu, to be [165 ± 19] × 10−6, which corresponds to δDRyugu = +59 ± 121‰ (2σ). The hydrous mineral D/HRyugu’s values for the two sampling sites on Ryugu are similar; they are also similar to the estimated D/H ratio of hydrous minerals in the CI chondrites Orgueil and Alais. This result reinforces a link between Ryugu and CI chondrites and an inference that Ryugu’s samples, which avoided terrestrial contamination, are our best proxy to estimate the composition of water at the origin of hydrous minerals in CI-like material. Based on this data and recent literature studies, the contribution of CI chondrites to the hydrogen of Earth’s surficial reservoirs is evaluated to be ∼3%. We conclude that the water responsible for the alteration of Ryugu’s rocks was derived from water ice precursors inherited from the interstellar medium; the ice partially re-equilibrated its hydrogen with the nebular H2 before being accreted on the Ryugu’s parent asteroid.

Incorporation of 16O-rich anhydrous silicates in the protolith of highly hydrated asteroid Ryugu

The abundant phyllosilicate and carbonate minerals characterizing most of the returned particles from asteroid Ryugu suggest a history of extensive aqueous alteration on its parent body, similar to the rare mineralogically altered, but chemically primitive, CI (Ivuna-type) chondrite meteorites. Particle C0009 differs mineralogically from other Ryugu particles examined so far by containing anhydrous silicates at a level of ~0.5 vol%, and thus can help shed light on the unaltered original materials that constituted Ryugu’s protolith. In situ oxygen isotope measurements of the most Mg-rich olivine and pyroxene in C0009 reveal two populations of Δ17O: −25‰ to −15‰ and −8‰ to −3‰. The former and the latter populations correlate well with silicate morphologies similar to those seen in amoeboid olivine aggregates and chondrule phenocrysts, respectively, both of which are abundant in less aqueously altered carbonaceous chondrites. This result also highlights the presence of olivine with Δ17O close to the solar value in either a CI chondrite or an asteroid with CI-chondrite characteristics, and provides strong evidence that amoeboid olivine aggregates and Mg-rich chondrules accreted into Ryugu’s protolith. Our data also raise the possibility that the protoliths of CI and other carbonaceous chondrites incorporated similar anhydrous silicates.

Geologically rapid aqueous mineral alteration at subfreezing temperatures in icy worlds

The most active icy worlds such as Europa or Enceladus are predicted to host extensive aqueous alteration driven by water–rock interactions at elevated temperatures1–3. On the other hand, it is assumed that such alteration is kinetically inhibited at the subzero temperatures of other icy worlds, such as the mid-sized moons of Saturn and Uranus or trans-Neptunian objects1,4. Here we perform aqueous alteration experiments on a chondrite-analogue material (olivine) and find that chemical alteration processes are still efficient at temperatures as low as −20 °C, as the presence of an unfrozen water film still allows olivine to dissolve in partially frozen alkaline solutions. We infer that aqueous alteration may be enhanced by salts and ammonia present in icy worlds, and therefore remains a geologically rapid process even at subzero temperatures. Our results imply that the primary chondritic minerals in most icy bodies exceeding 400–500 km in diameter will be completely altered to hydrous secondary minerals early in their evolutionary histories. Alkaline conditions in icy worlds favour aqueous alteration processes even at subzero temperatures. As a consequence of geologically rapid alterations, the rocky cores of all icy bodies larger than 500 km in diameter should be largely composed by altered hydrated minerals formed within a few hundred million years after formation.

Mineral Matrix Effects on Pyrolysis Products of Kerogens Infer Difficulties in Determining Biological Provenance of Macromolecular Organic Matter at Mars.

Ancient martian organic matter is likely to take the form of kerogen-like recalcitrant macromolecular organic matter (MOM), existing in close association with reactive mineral surfaces, especially iron oxides. Detecting and identifying a biological origin for martian MOM will therefore be of utmost importance for life-detection efforts at Mars. We show that Type I and Type IV kerogens provide effective analogues for putative martian MOM of biological and abiological (meteoric) provenances, respectively. We analyze the pyrolytic breakdown products when these kerogens are mixed with mineral matrices highly relevant for the search for life on Mars. We demonstrate that, using traditional thermal techniques as generally used by the Sample Analysis at Mars and Mars Organic Molecule Analyser instruments, even the breakdown products of highly recalcitrant MOM are transformed during analysis in the presence of reactive mineral surfaces, particularly iron. Analytical transformation reduces the diagnostic ability of this technique, as detected transformation products of both biological and abiological MOM may be identical (low molecular weight gas phases and benzene) and indistinguishable. The severity of transformational effects increased through calcite < kaolinite < hematite < nontronite < magnetite < goethite. Due to their representation of various habitable aqueous environments and the preservation potential of organic matter by iron, it is not advisable to completely avoid iron-rich strata. We conclude that hematite-rich localities, with evidence of extensive aqueous alteration of originally reducing phases, such as the Vera Rubin Ridge, may be relatively promising targets for identifying martian biologically sourced MOM.

Space weathering of the 3-μm phyllosilicate feature induced by pulsed laser irradiation

Here we present results from pulsed laser irradiation of CI and CM simulant samples in an effort to simulate space weathering on airless bodies via micrometeorite impacts. For this study, we focused on determining what type of alteration occurs in the 3-μm absorption region, as this region will be critical to ascertain compositional information of the surface regolith on hydrated asteroids. Generally, using entirely in situ spectral analysis, we find that the laser produces similar effects in both samples. Specifically, irradiation causes the blue spectral slope to increase until it is relatively flat. Furthermore, the sample darkens initially with laser irradiation but brightens back to about half of its original level by the end of the irradiation. We also find that laser irradiation causes the band depth on the 3-μm absorption band to increase by as much as 30%, yet the shape of the entire absorption band does not change and the band minima of the 2.72 μm shifts less than 0.001 μm as a result of laser irradiation. The constancy of the latter two parameters, which will be most critical to compositional analysis, suggests that this spectral region could be very useful to determine the asteroid composition on surfaces of hydrated asteroids that have undergone extensive aqueous alteration even if the surface had been subject to a significant amount of space weathering. Whether the same conclusion will be generally applicable to other surfaces containing minerals with a wide range of aqueous alteration is currently unclear but will be tested in future studies.

Linking mineralogy and spectroscopy of highly aqueously alteredCMandCIcarbonaceous chondrites in preparation for primitive asteroid sample return

AbstractThe highly hydrated, petrologic type 1CMandCIcarbonaceous chondrites likely derived from primitive, water‐rich asteroids, two of which are the targets forJAXA's Hayabusa2 andNASA'sOSIRIS‐REx missions. We have collected visible and near‐infrared (VNIR) and mid infrared (MIR) reflectance spectra from well‐characterizedCM1/2,CM1, andCI1 chondrites and identified trends related to their mineralogy and degree of secondary processing. The spectral slope between 0.65 and 1.05 μm decreases with increasing total phyllosilicate abundance and increasing magnetite abundance, both of which are associated with more extensive aqueous alteration. Furthermore, features at ~3 μm shift from centers near 2.80 μm in the intermediately alteredCM1/2 chondrites to near 2.73 μm in the highly alteredCM1 chondrites. The Christiansen features (CF) and the transparency features shift to shorter wavelengths as the phyllosilicate composition of the meteorites becomes more Mg‐rich, which occurs as aqueous alteration proceeds. Spectra also show a feature near 6 μm, which is related to the presence of phyllosilicates, but is not a reliable parameter for estimating the degree of aqueous alteration. The observed trends can be used to estimate the surface mineralogy and the degree of aqueous alteration in remote observations of asteroids. For example, (1) Ceres has a sharp feature near 2.72 μm, which is similar in both position and shape to the same feature in the spectra of the highly alteredCM1MIL05137, suggesting abundant Mg‐rich phyllosilicates on the surface. Notably, bothOSIRIS‐REx and Hayabusa2 have onboard instruments which cover theVNIRandMIRwavelength ranges, so the results presented here will help in corroborating initial results from Bennu and Ryugu.

Formation of Ares Vallis (Mars) by effusions of low-viscosity lava within multiple regions of chaotic terrain

Ares Vallis is one of the largest outflow channels on Mars, extending northward >1500 km from the highlands of Margaritifer Terra into the Chryse impact basin. This outflow system developed primarily during the Hesperian and Amazonian as a result of voluminous effusions from the subsurface that took place within Iani Chaos, Aram Chaos, Margaritifer Chaos, and Hydaspis Chaos. Though Ares Vallis is widely interpreted as a product of catastrophic outbursts from aquifers, its basic attributes do not appear to support aqueous origins of any kind: aquifer outburst mechanisms lack meaningful solar system analogs, clear examples of fluvial or diluvial sedimentary deposits are apparently absent at Ares Vallis, and there is no mineralogical evidence along component channels and within terminal basins for extensive aqueous alteration or for deposition of thick evaporite units. Instead, as is the case at hundreds of ancient channels of the inner solar system, the nature of Ares Vallis is aligned with dry volcanic origins. Such origins are broadly consistent with the system's relatively pristine mineralogy, the widespread mantling of component channels by lava flows, and the apparent presence of voluminous mare-style flood lavas within terminal basins. With assumed lava viscosities of 1 Pa s and temperatures of 1350 °C, discharge rates of ~97 × 106 m3/s are estimated at Ares Vallis for 100-m-deep flows along channel reaches with widths of 25 km and slopes of only 0.2°. Mechanical and thermal incision are respectively estimated to be ~12.2 m/day and 2.3 m/day for 100-m-deep flows on these slopes. Formation of the main Ares Vallis channel and associated regions of chaos is likely to have required a minimum effused lava volume of ~1.8 × 106 km3, and still greater lava volumes would have been necessary to form associated channel reaches including those located south of Margaritifer Chaos.

An aqueously altered carbon-rich Ceres

The surface mineralogy of dwarf planet Ceres appears to be dominated by products of rock–fluid interactions, such as phyllosilicates—some of which are NH4-bearing—and carbonates1–3. Elemental concentrations derived from the inferred mineral mixing fractions, however, do not match measurements of H, C, K and Fe on Ceres4. A complicating factor in assessing Ceres’s unique surface composition is the secular accretion of asteroids typical of chondritic compositions. Here we show that Ceres’s mineral and elemental data can be explained by the presence of carbonaceous chondritic-like materials (~50–60 vol%), possibly due to infalling asteroids, admixed with aqueously altered endogenic materials that contain higher-than-chondritic concentrations of carbon. We find that Ceres’s surface may contain up to 20 wt% of carbon, which is more than five times higher than in carbonaceous chondrites. The coexistence of phyllosilicates, magnetite, carbonates and a high carbon content implies rock–water alteration played an important role in promoting widespread carbon chemistry. These findings unveil pathways for the synthesis of organic matter, with implications for their transport across the Solar System. Infrared and neutron spectroscopic observations by Dawn give contrasting results on the elemental composition of Ceres’s surface, which can be reconciled by assuming that Ceres’s surface contains ~20 wt% of carbon, coming from impacts by carbonaceous asteroids and/or generated by extensive aqueous alteration.

Chapter 14 - Remote Detection of Clay Minerals

Spectral remote sensing in the visible/near-infrared (VNIR) and mid-IR (MIR) regions has enabled detection and characterisation of multiple clays and clay minerals on Earth and in the Solar System. Remote sensing on Earth poses the greatest challenge due to atmospheric absorptions that interfere with detection of surface minerals. Still, a greater variety of clay minerals have been observed on Earth than other bodies due to extensive aqueous alteration on our planet. Clay minerals have arguably been mapped in more detail on the planet Mars because they are not masked by vegetation on that planet and the atmosphere is less of a hindrance. Fe/Mg-smectite is the most abundant clay mineral on the surface of Mars and is also common in meteorites and comets where clay minerals are detected.

Distribution of phyllosilicates on the surface of Ceres.

The dwarf planet Ceres is known to host phyllosilicate minerals at its surface, but their distribution and origin have not previously been determined. We used the spectrometer onboard the Dawn spacecraft to map their spatial distribution on the basis of diagnostic absorption features in the visible and near-infrared spectral range (0.25 to 5.0 micrometers). We found that magnesium- and ammonium-bearing minerals are ubiquitous across the surface. Variations in the strength of the absorption features are spatially correlated and indicate considerable variability in the relative abundance of the phyllosilicates, although their composition is fairly uniform. These data, along with the distinctive spectral properties of Ceres relative to other asteroids and carbonaceous meteorites, indicate that the phyllosilicates were formed endogenously by a globally widespread and extensive alteration process.

Extensive aqueous deposits at the base of the dichotomy boundary in Nilosyrtis Mensae, Mars

Thermal emission imaging system (THEMIS) and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectral datasets were used to identify high bulk SiO2 and hydrated compositions throughout the Nilosyrtis Mensae region. Four isolated locations were identified across the region showing short wavelength silicate absorptions within the 8–12μm spectral region, indicating surfaces dominated by high Si phases. Much more extensive exposures of hydrated compositions are present throughout the region, indicated by a spectral absorption near 1.9μm in CRISM data. Although limited in spatial coverage, detailed spectral observations indicate that the hydrated materials contain Fe/Mg-smectites and hydrated silica along with minor exposures of Mg-carbonates and an unidentified hydrated phase. The high SiO2 and hydrated materials are present in layered sediments near the base of topographic scarps at the hemispheric dichotomy boundary, typically near or within low albedo sand deposits. The source of the high SiO2 and hydrated materials appears to be from groundwater discharge from Nili Fossae and Syrtis Major to the south, where there is evidence for extensive aqueous alteration of the subsurface. Although discontinuous, the exposures of high SiO2 and hydrated materials span a wide area and are present in a similar geomorphological context to previously identified deposits in western Hellas Basin. These regional deposits may reflect aqueous conditions and alteration within the adjacent crust of the martian highlands.

Amino acid compositions in heated carbonaceous chondrites and their compound-specific nitrogen isotopic ratios

A novel method has been developed for compound-specific nitrogen isotope compositions with an achiral column which was previously shown to offer high precision for nitrogen isotopic analysis. We applied the method to determine the amino acid contents and stable nitrogen isotopic compositions of individual amino acids from the thermally metamorphosed (above 500 °C) Antarctic carbonaceous chondrites Ivuna-like (CI)1 (or CI-like) Yamato (Y) 980115 and Ornans-like (CO)3.5 Allan Hills (ALH) A77003 with the use of gas chromatography/combustion/isotope ratio mass spectrometry. ALHA77003 was deprived of amino acids due to its extended thermal alteration history. Amino acids were unambiguously identified in Y-980115, and the δ15N values of selected amino acids (glycine +144.8 ‰; α-alanine +121.2 ‰) are clearly extraterrestrial. Y-980115 has experienced an extended period of aqueous alteration as indicated by the presence of hydrous mineral phases. It has also been exposed to at least one post-hydration short-lived thermal metamorphism. Glycine and alanine were possibly produced shortly after the accretion event of the asteroid parent body during the course of an extensive aqueous alteration event and have abstained from the short-term post-aqueous alteration heating due to the heterogeneity of the parent body composition and porosity. These carbonaceous chondrite samples are good analogs that offer important insights into the target asteroid Ryugu of the Hayabusa-2 mission, which is a C-type asteroid likely composed of heterogeneous materials including hydrated and dehydrated minerals.

Organic molecules in meteorites

AbstractThe analysis of the organic content of meteorites provides a window into the conditions of the early solar system, such as the extension of aqueous alteration or thermal metamorphism on the meteorite parent bodies. The analysis of the soluble organic content of CM chondrites indicates that extensive aqueous alteration on their meteorite parent body may result on 1) the decomposition of α-amino acids; 2) synthesis of β- and γ-amino acids; 3) higher relative abundances of alkylated polycyclic aromatic hydrocarbons (PAHs); and 4) higher L-enantiomer excess (Lee) value of isovaline. Exogenous delivery of organic matter by meteorites may have contributed to the organic inventory of the early Earth, providing a diversity of resources to the first living organisms on Earth and on other places of our solar system where life could have potentially originated.

Mineralogy and morphology of geologic units at Libya Montes, Mars: Ancient aqueously derived outcrops, mafic flows, fluvial features, and impacts

AbstractThere is ample evidence of both ancient and long‐lasting fluvial activity and chemical alteration in the Libya Montes region south of Isidis Basin. The region hosts Noachian to Amazonian aged surface rocks with extensive outcrops of olivine‐ and pyroxene‐bearing material. Libya Montes also features surface outcrops and/or deposits hosting Fe/Mg‐smectite, Fe/Mg‐smectite mixed with carbonate and/or other Fe/Mg‐rich phyllosilicates, and Al‐smectite. These units likely formed through chemical alteration connected with hydrothermal activity resulting from the formation of the Isidis Basin and/or the pervasive fluvial activity throughout this region. The morphology and stratigraphy of the aqueous and mafic minerals are described using High Resolution Imaging Science Experiment and High Resolution Stereo Camera derived digital terrain models. Analyses of the Compact Reconnaissance Imaging Spectrometer for Mars spectra show variations in the chemistry of the Fe/Mg‐smectite from nontronite‐like exposures with spectral features near 2.29 and 2.4 µm more consistent with Fe3+2OH groups in the mineral structure, and saponite‐like outcrops with spectral features near 2.31 and 2.38 µm characteristic of Mg2+3OH groups. These Fe/Mg‐smectite bearing materials also have bands near 1.9 µm due to H2O and near 2.5 µm that could be due to the smectite, other phyllosilicates, and carbonates. All regions exhibiting carbonate features near 3.4–3.5 µm also have features consistent with the presence of olivine and Fe/Mg‐smectite, indicating that the carbonate signatures occur in rocks likely containing a mixture of these minerals. The Al‐smectite‐bearing rocks have bands near 1.41, 1.91, and 2.19 µm that are more consistent with beidellite than other Al‐phyllosilicates, indicating a higher‐temperature or diagenetically processed origin for this material. Our interpretation of the geologic history of this region is that ancient Noachian basaltic crustal materials experienced extensive aqueous alteration at the time of the Isidis impact, during which the montes were also formed, followed by emplacement of a rough olivine‐rich lava or melt, and finally the smooth pyroxene‐bearing caprock unit.

Highly 15N-enriched chondritic clasts in the CB/CH-like meteorite Isheyevo

The metal-rich carbonaceous chondrites (CB and CH) have the highest whole-rock 15N-enrichments (δ 15N up to 1500‰) among planetary materials. They are also characterized by the absence of interchondrule fine-grained matrix. The only fine-grained material is present as lithic clasts, which experienced extensive aqueous alteration in contrast to the surrounding high-temperature components (chondrules, refractory inclusions, metal grains). Hence, the clasts are foreign objects that were incorporated at a late stage into the final parent body of Isheyevo. Their origin is poorly constrained. Based on mineralogy, petrography, and thermal processing of the aromatic carbonaceous component, different types of clasts have been previously identified in the CB/CH-like chondrite Isheyevo. Here, we focus on the rare lithic clasts characterized by the presence of anhydrous silicates (chondrules, chondrule fragments, and CAIs). Their mineralogy and oxygen isotopic compositions reveal them to be micro-chondrules, fragments of chondrules, and refractory inclusions related to those in the Isheyevo host, suggesting accretion in the same region. In contrast to previously studied IDPs or primitive chondritic matrices, the fine-grained material in the clasts we studied is highly and rather uniformly enriched in heavy nitrogen, with bulk δ 15N values ranging between 1000‰ and 1300‰. It is also characterized by the presence of numerous 15N hotspots (δ 15N ranging from 1400‰ to 4000‰). No bulk (δD <–240‰) or localized deuterium enrichments were observed. These clasts have the highest bulk enrichment in heavy nitrogen measured to date in a fine-grained material. They represent a unique material, of asteroidal or cometary origin, in our collection of cosmomaterials. We show that they were 15N-enriched before their incorporation in the final parent body of Isheyevo. They experienced an extensive aqueous alteration that most likely played a role in redistributing 15N over the whole fine-grained material and may have significantly modified its initial hydrogen isotopic composition. Based on a review of isotopic fractionation models, we conclude that the nitrogen isotopic fractionation process, its timing, and its location are still poorly constrained. The 15N-rich clasts may represent the surviving original carrier of the 15N anomaly in Isheyevo whole-rock.

Indicators of parent-body processes: Hydrated chondrules and fine-grained rims in the Mokoia CV3 carbonaceous chondrite

A petrographic and electron microscopic study of the Mokoia CV3 carbonaceous chondrite shows that all of the chondrules and inclusions (>400 μm in diameter) and most of their fine-grained rims studied (referred to as chondrules/rims) contain various amounts of hydrous phyllosilicates (mostly saponite) formed by aqueous alteration of anhydrous silicates. The rims mainly consist of fine-grained olivine and saponite in varying proportions and contain crosscutting veins of Fe-rich olivine. The boundaries between the chondrules and their rims are irregular and show abundant evidence of aqueous alteration interactions between them. In contrast, the host matrix contains very minor amounts of saponite and shows no evidence of such extensive aqueous alteration. The boundaries between the chondrules/rims and the matrix are sharp and show no traces of the matrix having been involved in the alteration of the chondrules/rims. These observations indicate that the aqueous alteration in the chondrules/rims did not occur in the present setting. We suggest that the chondrules/rims are actually clasts transported from a location on the meteorite parent body different from where the Mokoia meteorite was from. The aqueous alteration of the chondrules/rims probably occurred there. The veins in the rims were originally fractures produced in an interchondrule matrix by impacts; these were later filled by Fe-rich olivine during aqueous activity. This location was then involved in impact brecciation, and individual chondrules were ejected as clasts with remnants of the matrix surrounding them. During the continuing brecciation, those chondrule/rim clasts were transported, mixed with anhydrous matrix grains, and finally lithified to the present meteorite. Therefore, the rims are fragmented remnants of a former matrix. Textures characterized by fine-grained rims surrounding chondrules in chondrites have been widely thought to have formed in the solar nebula before they accreted into their parent bodies. However, our results suggest that some textures may not be explained by such an accretionary model; instead, the multi-stage parent-body process modeled for the Mokoia rim formation may be a more plausible explanation.

Chondritic lithic clasts in the CB/CH-like meteorite Isheyevo: Fragments of previously unsampled parent bodies

The CB/CH-like chondrite Isheyevo is characterized by the absence of fine-grained interchondrule matrix material; the only present fine-grained material is found as chondritic lithic clasts. In contrast to the pristine high-temperature components of Isheyevo, these clasts experienced extensive aqueous alteration in an asteroidal setting. Hence, the clasts are foreign objects that either accreted together with the high-temperature components or were added later to the final Isheyevo parent body during regolith gardening. In order to constrain the origin and secondary alteration of the clasts in Isheyevo, we studied their mineralogy, petrography, structural order of the polyaromatic carbonaceous matter, and oxygen isotopic compositions of carbonates. Three main groups of clasts were defined based on mineralogy and petrology. Group I clasts consist of phyllosilicates, carbonates, magnetite, and lath-shaped Fe,Ni-sulfides. Group II clasts contain different abundances of anhydrous silicates embedded in a hydrated matrix; sulfides, magnetite, and carbonates are rare. With only a few exceptions, groups I and II clasts did not experienced significant thermal metamorphism. Group III clasts are characterized by the absence of magnetite and the presence of Fe,Ni-metal. In addition to aqueous alteration, they experienced thermal metamorphism as reflected by the structure of their polyaromatic carbonaceous matter. While there are some similarities between the Isheyevo clasts, CI chondrites, and the matrices of CM and CR chondrites, on the whole, the characteristics of the clasts do not match those of any of these aqueously altered meteorite classes. Nor do they match those of similar material in various types of chondritic clasts present in other meteorite groups. We conclude that the Isheyevo clasts represent fragments of previously unsampled parent bodies.