CM Parent Body Research Articles

Petrographic and C & O isotopic characteristics of the earliest stages of aqueous alteration of CM chondrites

CM chondrites form the largest group of hydrated meteorites and span a wide range of alteration states, with the Paris meteorite being the least altered CM described to date. Ca-Carbonates are powerful proxies for the alteration conditions of CMs because they are direct snapshots of the chemical and isotopic compositions of the parent fluids. Here, we report a petrographic and a C isotope and O isotope survey of Ca-carbonates in Paris in order to better characterize the earliest stages of aqueous alteration. Petrographic observations show that Paris contains two distinct populations of Ca-carbonates: Type 1a Ca-carbonates, which are surrounded by rims of tochilinite/cronstedtite intergrowths (TCIs), and new Type 0 Ca-carbonates, which do not exhibit the TCI rims. The TCI rims of Type 1a Ca-carbonates commonly outline euhedral crystal faces, demonstrating that these Ca-carbonates were (i) partially or totally pseudomorphosed by TCI and (ii) precipitated at the earliest stages of aqueous alteration, before Type 0 Ca-carbonates. Isotopic measurements show that Paris' Ca-carbonates have δ13C values that range from 19 to 80‰ (PDB), δ18O values that range from 29 to 41%, and δ17O values that range from 13 to 24‰ (SMOW). According to the δ13C–δ18O values of Paris' Ca-carbonates, we developed a new alteration model that involves (i) the equilibration of a primordial 17,18O-rich water (PW) with 16O-rich anhydrous silicates and (ii) varying contribution of 12C- and 13C-rich soluble organic matter (SOMs). It also suggests that many parameters control the C and O isotopic composition of Ca-carbonates, the principles being the degree of isotopic equilibration between the PW and the anhydrous silicates, the respective contribution of 12C and 13C-rich SOMs as well as the thermal evolution of CM parent bodies. Consequently, we suggest that CM Ca-carbonates could record both positive and negative δ13C–δ18O relationships, but a systematic correspondence is probably absent in CM chondrites due to the large number of factors involved in generating the isotopic characteristics of Ca-carbonates. From recent reports of the C-isotopic compositions of SOM in CM chondrites, we propose that water-soluble organic compounds were the most probable source of 13C enrichment in the majority of CM carbonates.

Paris vs. Murchison: Impact of hydrothermal alteration on organic matter in CM chondrites

Unravelling the origin of organic compounds that were accreted into asteroids requires better constraining the impact of asteroidal hydrothermal alteration on their isotopic signatures, molecular structures, and spatial distribution. Here, we conducted a multi-scale/multi-technique comparative study of the organic matter (OM) from two CM chondrites (that originate from the same parent body or from identical parent bodies that accreted the same mixture of precursors) and underwent a different degree of hydrothermal alteration: Paris (a weakly altered CM chondrite – CM 2.8) and Murchison (a more altered one – CM 2.5). The Paris insoluble organic matter (IOM) shows a higher aliphatic/aromatic carbon ratio, a higher radical abundance and a lower oxygen content than the Murchison IOM. Analysis of the OM in situ shows that two texturally distinct populations of organic compounds are present within the Paris matrix: sub-micrometric individual OM particles and diffuse OM finely distributed within phyllosilicates and amorphous silicates. These results indicate that hydrothermal alteration on the CM parent body induced aromatization and oxidation of the IOM, as well as a decrease in radical and nitrogen contents. Some of these observations were also reported by studies of variably altered fragment of Tagish Lake (C2), although the hydrothermal alteration of the OM in Tagish Lake was apparently much more severe. Finally, comparison with data available in the literature suggests that the parent bodies of other chondrite petrologic groups could have accreted a mixture of organic precursors different from that accreted by the parent body of CMs.

The bulk valence state of Fe and the origin of water in chondrites

There is abundant petrologic evidence for the oxidation of Fe during the aqueous alteration of chondrites, and water must have been the oxidant for this process. The H2 lost from the chondrite parent bodies as a result of Fe oxidation would have been isotopically very light, enriching any residual water in D. The extents of the D enrichments will have depended on the fractions of water consumed and the temperatures during Fe oxidation. Here we have estimated the likely ranges of water consumed by Fe oxidation in the CI, CM, CR and LL parent bodies, as well as the likely range of changes in water H isotopic compositions this would have produced. We first used Fe XANES to determine the Fe valences of bulk meteorite powders in Orgueil (CI1), a number of CMs and CRs that experienced varying degrees of alteration, and Semarkona (LL3.00). The total ranges of bulk Fe valences we obtained were: Orgueil 2.77, CMs 2.40–2.63, CRs 1.46–2.54, and Semarkona 2.10. Combining previous estimates of the present water/OH contents of our samples with the present bulk Fe valences and an estimated range of initial bulk Fe valences, we estimate the likely ranges of fractional water losses to have been: Orgueil 15–26%, Semarkona 73–83%, CMs 23–48%, and CRs 39–62%. The associated maximum and minimum changes in the H isotopic compositions of the remaining water were estimated assuming the equilibrium H2-H2O isotopic fractionation factor, Rayleigh fractionation of the H2, and oxidation temperatures of 0–200°C. Using previous estimates of the water H isotopic compositions in the chondrites, the ranges of estimated δD values for the initial chondritic waters are: Orgueil −672‰ to −422‰, CMs −676‰ to −493‰, CRs −527‰ to −56‰, and Semarkona −527‰ to 154‰. The CI, CM, CR and ordinary chondrites all accreted water with similar H isotopic compositions that were distinct from the compositions of comets or Saturn’s moon Enceladus. Thus, the carbonaceous chondrites are unlikely to have come from comets or from bodies that were scattered into the Asteroid Belt from comet forming regions by orbital migration of the giant planets. If the carbonaceous chondrites did form in the outer Solar System, as some models predict, it was probably not beyond 7 AU. However, based on water isotopic compositions at present it is equally plausible that the carbonaceous chondrites formed in the inner Solar System.

Mineralogical, crystallographic and redox features of the earliest stages of fluid alteration in CM chondrites

The CM chondrites represent the largest group of hydrated meteorites and span a wide range of conditions, from less altered (i.e., CM2) down to heavily altered (i.e., CM1). The Paris chondrite is considered the least altered CM and thus enables the earliest stages of aqueous alteration processes to be deciphered. Here, we report results from a nanoscale study of tochilinite/cronstedtite intergrowths (TCIs) in Paris—TCIs being the emblematic secondary mineral assemblages of CM chondrites, formed from the alteration of Fe-Ni metal beads (type-I TCIs) and anhydrous silicates (type-II TCIs). We combined high-resolution transmission electron microscopy, scanning transmission X-ray microscopy and electron diffraction tomography to characterize the crystal structure, crystal chemistry and redox state of TCIs. The data obtained are useful to reconstruct the alteration conditions of Paris and to compare them with those of other meteorites. Our results show that tochilinite in Paris is characterized by a high hydroxide layer content (n=2.1–2.2) regardless of the silicate precursors. When examined alongside other CMs, it appears that the hydroxide layer and iron contents of tochilinites correlate with the degree of alteration experienced by the chondrites. The Fe3+/ΣFe ratios of TCIs are high: 8–15% in tochilinite, 33–60% in cronstedtite and 70–80% in hydroxides. These observations suggest that alteration of CM chondrites took place under oxidizing conditions that could have been induced by significant H2 release during serpentinization. Similar results were recently reported in CR chondrites (Le Guillou et al., 2015), suggesting that the process(es) controlling the redox state of the secondary mineral assemblages were quite similar in the CM and CR parent bodies despite the different alteration conditions.According to our mineralogical and crystallographic survey, the formation of TCIs in Paris occurred at temperatures lower than 100°C, under neutral, slightly alkaline conditions that favored the formation of both tochilinite and cronstedtite. During the course of alteration, the reduction in sulfur activity and/or the decrease of temperature prevented tochilinite crystallization and favoured the formation of cronstedtite and iron hydroxides. We suggest that iron hydroxides probably formed as ferrihydrite and then progressively converted to goethite between 50° and 80°C, a temperature range that is also favorable for cronstedtite formation. The presence of cronstedtite plays a key role in the reconstruction of the alteration history, demonstrating that the alteration of Paris took place by way of serpentinization processes similar to those described on the Earth.

Type 1 aqueous alteration in CM carbonaceous chondrites: Implications for the evolution of water‐rich asteroids

AbstractThe CM carbonaceous chondrite meteorites experienced aqueous alteration in the early solar system. They range from mildly altered type 2 to almost completely hydrated type 1 chondrites, and offer a record of geochemical conditions on water‐rich asteroids. We show that CM1 chondrites contain abundant (84–91 vol%) phyllosilicate, plus olivine (4–8 vol%), magnetite (2–3 vol%), Fe‐sulfide (<5 vol%), and calcite (<2 vol%). The CM1/2 chondrites contain phyllosilicate (71–88 vol%), olivine (4–20 vol%), enstatite (2–6 vol%), magnetite (2–3 vol%), Fe‐sulfides (1–2 vol%), and calcite (~1 vol%). As aqueous alteration progressed, the abundance of Mg‐serpentine and magnetite in the CM chondrites increased. In contrast, calcite abundances in the CM1/2 and CM1 chondrites are often depleted relative to the CM2s. The modal data support the model, whereby metal and Fe‐rich matrix were the first components to be altered on the CM parent body(ies), before further hydration attacked the coarser Mg‐rich silicates found in chondrules and fragments. Based on the absence of tochilinite, we suggest that CM1 chondrites experienced increased alteration due to elevated temperatures (>120 °C), although higher water/rock ratios may also have played a role. The modal data provide constraints for interpreting the composition of asteroids and the mineralogy of samples returned from these bodies. We predict that “CM1‐like” asteroids, as has been proposed for Bennu—target for the OSIRIS‐REx mission—will have a high abundance of Mg‐rich phyllosilicates and Fe‐oxides, but be depleted in calcite.

A XANES and Raman investigation of sulfur speciation and structural order in Murchison and Allende meteorites

AbstractInsoluble organic matter (IOM) and hydrothermally treated IOM extracted from two carbonaceous chondrites, Murchison and Allende, was studied using sulfur K‐edge XANES (X‐ray absorption near edge structure) and μ‐Raman spectroscopy, with the aim to understand their IOM's sulfur speciation and structural order, and how aqueous alteration or thermal metamorphism may have transformed these materials. We found that the sulfur‐functional group chemistry of both the Murchison IOM and hydrothermally treated IOM samples have a large chemical variability ranging from oxidation states of S−2 to S+6, and exhibit a transformation in their oxidation state after the hydrothermal treatment (HT) to produce thiophenes and thiol compounds. Sulfoxide and sulfite peaks are also present in Murchison. Sulfates considered intrinsic to Murchison are most likely preaccretionary in nature, and not a result of reactions with water at high temperatures on the asteroid parent body. We argue that the reduced sulfides may have formed in the CM parent body, while the thiophenes and thiol compounds are a result of the HT. Micro‐Raman spectra show the presence of aliphatic and aromatic moieties in Murchison's material as observed previously, which exhibits no change after HT. Because the Murchison IOM was modified, as seen by XANES analysis, absence of a change observed using micro‐Raman indicated that although the alkyl carbons of IOM were cleaved, the aromatic network was not largely modified after HT. By contrast, Allende IOM contains primarily disulfide and elemental sulfur, no organic sulfur, and shows no transformation after HT. This nontransformation of Allende IOM after HT would indicate that parent body alteration of sulfide to sulfate is not feasible up to temperatures of 300°C. The reduced sulfur products indicate extreme secondary chemical processing from the precursor compounds in its parent body at temperatures as high as 624°C, as estimated from μ‐Raman D band parameters. The Raman parameters in Allende IOM that was interpreted in terms of amorphous carbon with regions of large clusters of benzene rings, was transformed after the HT to those with fewer benzene rings.

Mass independent sulfur isotope signatures in CMs: Implications for sulfur chemistry in the early solar system

We have investigated the quadruple sulfur isotopic composition of inorganic sulfur-bearing phases from 13 carbonaceous chondrites of CM type. Our samples include 4 falls and 9 Antarctic finds. We extracted sulfur from sulfides, sulfates, and elemental sulfur (S0) from all samples. On average, we recover a bulk sulfur (S) content of 2.11±0.39wt.% S (1σ). The recovered sulfate, S0 and sulfide contents represent 25±12%, 10±7% and 65±15% of the bulk S, respectively (all 1σ). There is no evidence for differences in the bulk S content between falls and finds, and there is no correlation between the S speciation and the extent of aqueous alteration. We report ranges of Δ33S and Δ36S values in CMs that are significantly larger than previously observed. The largest variations are exhibited by S0, with Δ33S values ranging between −0.104±0.012‰ and +0.256±0.018‰ (2σ). The Δ36S/33S ratios of S0 are on average −3.1±1.0 (2σ). Two CMs show distinct Δ36S/33S ratios, of +1.3±0.1 and +0.9±0.1. We suggest that these mass independent S isotopic compositions record H2S photodissociation in the nebula. The varying Δ36S/Δ33S ratios are interpreted to reflect photodissociation that occurred at different UV wavelengths. The preservation of these isotopic features requires that the S-bearing phases were heterogeneously accreted to the CM parent body. Non-zero Δ33S values are also preserved in sulfide and sulfate, and are positively correlated with S0 values. This indicates a genetic relationship between the S-bearing phases: We argue that sulfates were produced by the direct oxidation of S0 (not sulfide) in the parent body. We describe two types of models that, although imperfect, can explain the major features of the CM S isotope compositions, and can be tested in future studies. Sulfide and S0 could both be condensates from the nebula, as the residue and product, respectively, of incomplete H2S photodissociation by UV light (wavelength <150nm). This idea requires that FeS formation and the S0 condensation co-occur. As an alternative, ice accretion to the CM parent body could allow the delivery of S-MIF in CMs. In that case, sulfides would have been the only S-bearing condensate in CM precursors, and S0 would have been derived from the oxidation of H2S trapped in ices, after its photodissociation at low temperature (<500K) in the nebula. In our models, the observations of H2S UV photodissociation is required to occur at the disk surface, and allowed in nebular environments with canonical C/O ratios. Vertical motions in the disk would redistribute phases that condensed at high altitude to the midplane, where they accreted in the phases that make up the chondritic matrix.

COMPOSITIONAL HOMOGENEITY OF CM PARENT BODIES

ABSTRACT CM chondrites are the most common type of hydrated meteorites, making up ∼1.5% of all falls. Whereas most CM chondrites experienced only low-temperature (∼0°C–120°C) aqueous alteration, the existence of a small fraction of CM chondrites that suffered both hydration and heating complicates our understanding of the early thermal evolution of the CM parent body(ies). Here, we provide new constraints on the collisional and thermal history of CM-like bodies from a comparison between newly acquired spectral measurements of main-belt Ch/Cgh-type asteroids (70 objects) and existing laboratory spectral measurements of CM chondrites. It first appears that the spectral variation observed among CM-like bodies is essentially due to variations in the average regolith grain size. Second, the spectral properties of the vast majority (unheated) of CM chondrites resemble both the surfaces and the interiors of CM-like bodies, implying a “low” temperature (&lt;300°C) thermal evolution of the CM parent body(ies). It follows that an impact origin is the likely explanation for the existence of heated CM chondrites. Finally, similarly to S-type asteroids and (2) Pallas, the surfaces of large (D &gt; 100 km)—supposedly primordial—Ch/Cgh-type main-belt asteroids likely expose the interiors of the primordial CM parent bodies, a possible consequence of impacts by small asteroids (D &lt; 10 km) in the early solar system.

Aqueous alteration of chondrules from the Murchison CM carbonaceous chondrite: Replacement, pore filling, and the genesis of polyhedral serpentine

AbstractForsterite and clinoenstatite in type IAB chondrules from the Murchison CM carbonaceous chondrite have been partially serpentinized, and the mechanisms of their alteration reveal crystallographic and microstructural controls on the reaction of silicate minerals with parent body aqueous solutions. Grains of forsterite were altered in two stages. Narrow veinlets of Fe‐rich serpentine formed first and by the filling of sheet pores. Most of these pores were oriented parallel to (010) and (001) and had been produced by earlier fracturing and/or congruent dissolution. In the second stage, the subset of veinlets that were oriented parallel to (001) was widened accompanying the replacement of forsterite by Mg‐Fe serpentine. This reaction proceeded most rapidly parallel to [001], and crystallographic controls on the trajectory of retreating vein walls created fine‐scale serrations. Murchison clinoenstatite grains have a skeletal appearance due to the presence of abundant veinlets and patches of phyllosilicate. Two alteration stages can again be recognized, with initial water–mineral interaction producing tochilinite‐rich veinlets by the filling of (001)‐parallel contraction cracks. Pores then formed by congruent dissolution that was guided principally by orthopyroxene lamellae, and they were subsequently filled by submicrometer‐sized crystals of polyhedral serpentine. This finding that Murchison forsterite and clinoenstatite grains have been altered demonstrates that aqueous processing of magnesium silicate minerals started much earlier in CM parent body history than previously believed. Our results also show that the occurrence of polyhedral serpentine can be used to locate former pore spaces within the parent body.

GEMS-like material in the matrix of the Paris meteorite and the early stages of alteration of CM chondrites

The Paris meteorite is a weakly altered CM chondrite that has been discovered recently (Hewins et al., 2014). Its matrix offers the opportunity to search for well-preserved pristine pre-accretional material, as well as to study the earliest stages of aqueous alteration in the CM parent body. The study of the matrix of Paris has been conducted by analytical transmission electron microscopy on focused ion beam sections extracted from matrix areas showing different degrees of aqueous alteration.The least altered matrix sample consists of amorphous silicate grains, a few hundreds of nm in size, separated from one another by an abundant porosity. The amorphous silicates enclose numerous Fe-sulfide nanograins and their average composition is close to the chondritic composition. They share many similarities with GEMS (glass with embedded metal and sulfides) grains present in chondritic-porous interplanetary dust particles and with primitive type 3.0 carbonaceous chondrites. This first discovery of GEMS-like texture in a CM chondrite suggests that GEMS grains could have been the building blocks of the CM matrices.In more aqueously altered samples, pronounced microstructural heterogeneities were detected at the micrometer scale. The matrix consists mostly of a mixture of amorphous material and Fe-rich, spongy to fine-fibrous, poorly crystalline phyllosilicates. The porosity fraction is significantly reduced and the mixed amorphous-fibrous material frequently forms a continuous groundmass. The close association between these two material types suggests a replacement mechanism due to aqueous alteration. Chemical compositions correlate strongly with the microstructure. The amorphous material has a composition close to the chondritic value while the fine-fibrous phyllosilicate material is Fe-enriched. This Fe enrichment is found to be continuous from weakly to more heavily altered areas, in which the fibrous morphology is coarser and better crystalline. Cronstedtite with intercalated tochilinite is also found, but in pore spaces. This chemical evolution, concomitant with the maturation of the phyllosilicates, demonstrates that the early aqueous fluids that interacted with silicates in the matrix were enriched in Fe. This composition is probably the consequence of the preferential dissolution of metal and iron sulfides during the first stages of alteration. The enrichment of phyllosilicates in Mg seen in more altered CM chondrites is not observed in Paris.

Carbonate abundances and isotopic compositions in chondrites

AbstractWe report the bulk C abundances, and C and O isotopic compositions of carbonates in 64 CM chondrites, 14 CR chondrites, 2 CI chondrites, LEW 85332 (C2), Kaba (CV3), and Semarkona (LL3.0). For the unheated CMs, the total ranges of carbonate isotopic compositions are δ13C ≈ 25–75‰ and δ18O ≈ 15–35‰, and bulk carbonate C contents range from 0.03 to 0.60 wt%. There is no simple correlation between carbonate abundance and isotopic composition, or between either of these parameters and the extent of alteration. Unless accretion was very heterogeneous, the uncorrelated variations in extent of alteration and carbonate abundance suggests that there was a period of open system behavior in the CM parent body, probably prior to or at the start of aqueous alteration. Most of the ranges in CM carbonate isotopic compositions can be explained by their formation at different temperatures (0–130 °C) from a single fluid in which the carbonate O isotopes were controlled by equilibrium with water (δ18O ≈ 5‰) and the C isotopes were controlled by equilibrium with CO and/or CH4 (δ13C ≈ −33‰ or −20‰ for CO‐ or CH4‐dominated systems, respectively). However, carbonate formation would have to have been inefficient, otherwise carbonate compositions would have resembled those of the starting fluid. A quite similar fluid composition (δ18O ≈ −5.5‰, and δ13C ≈ −31‰ or −17‰ for CO‐ or CH4‐dominated systems, respectively) can explain the carbonate compositions of the CIs, although the formation temperatures would have been lower (~10–40 °C) and the relative abundances of calcite and dolomite may play a more important role in determining bulk carbonate compositions than in the CMs. The CR carbonates exhibit a similar range of O isotopes, but an almost bimodal distribution of C isotopes between more (δ13C ≈ 65–80‰) and less altered samples (δ13C ≈ 30–40‰). This bimodality can still be explained by precipitation from fluids with the same isotopic composition (δ18O ≈ −9.25‰, and δ13C ≈ −21‰ or −8‰ for CO‐ or CH4‐dominated systems, respectively) if the less altered CRs had higher mole fractions of CO2 in their fluids. Semarkona and Kaba carbonates have some of the lightest C isotopic compositions of the meteorites studied here, probably because they formed at higher temperatures and/or from more CO2‐rich fluids. The fluids responsible for the alteration of chondrites and from which the carbonates formed were almost certainly accreted as ices. By analogy with cometary ices, CO2 and/or CO would have dominated the trapped volatile species in the ices. The chondrites studied are too oxidized for CO‐dominated fluids to have formed in their parent bodies. If CH4 was the dominant C species in the fluids during carbonate formation, it would have to have been generated in the parent bodies from CO and/or CO2 when oxidation of metal by water created high partial pressures of H2. The fact that the chondrite carbonate C/H2O mole ratios are of the order predicted for CO/CO2‐H2O ices that experienced temperatures of &gt;50–100 K suggests that the chondrites formed at radial distances of &lt;4–15 AU.

53Mn‐53Cr dating of aqueously formed carbonates in the CM2 lithology of the Sutter's Mill carbonaceous chondrite

AbstractRadiometric dating of secondary minerals can be used to constrain the timing of aqueous alteration on meteoritic parent bodies. Dolomite is a well‐documented secondary mineral in CM chondrites, and is thought to have formed by precipitation from an aqueous fluid on the CM parent body within several million years of accretion. The petrographic context of crosscutting dolomite veins indicates that aqueous alteration occurred in situ, rather than in the nebular setting. Here, we present 53Mn‐53Cr systematics for dolomite grains in Sutter's Mill section SM51‐1. The Mn‐Cr isotope data show well‐resolved excesses of 53Cr correlated with 55Mn/52Cr ratio, which we interpret as evidence for the in situ decay of radioactive 53Mn. After correcting for the relative sensitivities of Mn and Cr using a synthetic Mn‐ and Cr‐bearing calcite standard, the data yield an isochron with slope corresponding to an initial 53Mn/55Mn ratio of 3.42 ± 0.86 × 10−6. The reported error includes systematic uncertainty from the relative sensitivity factor. When calculated relative to the U‐corrected Pb‐Pb absolute age of the D'Orbigny angrite, Sutter's Mill dolomites give a formation age between 4564.8 and 4562.2 Ma (2.4–5.0 Myr after the birth of the solar system). This age is contemporaneous with previously reported ages for secondary carbonates in CM and CI chondrites. Consistent carbonate precipitation ages between the carbonaceous chondrite groups suggest that aqueous alteration was a common process during the early stages of parent body formation, probably occurring via heating from internal 26Al decay. The high‐precision isochron for Sutter's Mill dolomite indicates that late‐stage processing did not reach temperatures that were high enough to further disturb the Mn‐Cr isochron.

The fate of magmas in planetesimals and the retention of primitive chondritic crusts

High abundances of short-lived radiogenic isotopes in the early solar system led to interior melting and differentiation on many of the first planetesimals. Petrologic, isotopic, and paleomagnetic evidence suggests that some differentiated planetesimals retained primitive chondritic material. The preservation of a cold chondritic lid depends on whether deep melts are able to ascend and breach the chondritic crust. We evaluate the likelihood of melt ascent on a range of chondritic parent bodies. We find that, due to the efficient ascent of free volatiles in the gas and supercritical fluid phases at temperatures still below the solidus for silicates and metals, mobile silicate melts on planetesimals were likely volatile-depleted. By calculating the densities of such melts, we show that silicate melts likely breached crusts of enstatite chondrite compositions but did not ascend in the CV and CM parent bodies. Ordinary chondrite melts represent an intermediate case. These predictions are consistent with paleomagnetic results from CV and CM chondrites as well as spectral observations of large E-type asteroids.

Micrometeorite flux on Earth during the last ~50,000 years

AbstractFlux of micrometeorites is estimated by using cosmic spherule counts from a seafloor area of 2.50 m2 from the Indian Ocean. The spherules are recovered from sediment samples in close‐spaced locations from the Indian Ocean after sieving 293 kg of sediment. The terrestrial age of the spherules has a range of 0–~50,000 years. The spherules have a size range of 57–750 µm (average size 265 ± 92 µm). The diameter of the spherules increases from scoriaceous‐barred‐cryptocrystalline‐glassy types. The time‐averaged flux of the spherules is 160 t/yr, a sizeable mass (&gt;60%) resides in the &gt;300 µm fraction; the slope of distribution is similar to that of Deep‐Sea Spherules but significantly different from other collections which have lower average diameters. It is observed here, a significant population of cosmic dust resides in the larger sizes which can be recovered by sampling large areas in time and space. The spherule textures are similar to that of unbiased collections from the polar regions, indicating that the textural types of cosmic dust that have been raining on the Earth during the last 50 kyr have been constant regardless of size. Major element chemistry of a majority of the spherules show elemental ratios that are close to a CM or CI chondritic parent body; a single spherule (0.2% of the population) suggests an achondritic parent body. Unbiased collections spanning large areas temporally and spatially enlarge the inventory of the Earth‐crossing meteoroid complex and provide valuable inputs for models on cosmic dust accretion.

Spectral reflectance properties of carbonaceous chondrites: 2. CM chondrites

We have examined the spectral reflectance properties and available modal mineralogies of 39 CM carbonaceous chondrites to determine their range of spectral variability and to diagnose their spectral features. We have also reviewed the published literature on CM mineralogy and subclassification, surveyed the published spectral literature and added new measurements of CM chondrites and relevant end members and mineral mixtures, and measured 11 parameters and searched pair-wise for correlations between all quantities. CM spectra are characterized by overall slopes that can range from modestly blue-sloped to red-sloped, with brighter spectra being generally more red-sloped. Spectral slopes, as measured by the 2.4:0.56 μm and 2.4 μm:visible region peak reflectance ratios, range from 0.90 to 2.32, and 0.81 to 2.24, respectively, with values <1 indicating blue-sloped spectra. Matrix-enriched CM spectra can be even more blue-sloped than bulk samples, with ratios as low as 0.85. There is no apparent correlation between spectral slope and grain size for CM chondrite spectra – both fine-grained powders and chips can exhibit blue-sloped spectra. Maximum reflectance across the 0.3–2.5 μm interval ranges from 2.9% to 20.0%, and from 2.8% to 14.0% at 0.56 μm. Matrix-enriched CM spectra can be darker than bulk samples, with maximum reflectance as low as 2.1%. CM spectra exhibit nearly ubiquitous absorption bands near 0.7, 0.9, and 1.1 μm, with depths up to 12%, and, less commonly, absorption bands in other wavelength regions (e.g., 0.4–0.5, 0.65, 2.2 μm). The depths of the 0.7, 0.9, and 1.1 μm absorption features vary largely in tandem, suggesting a single cause, specifically serpentine-group phyllosilicates. The generally high Fe content, high phyllosilicate abundance relative to mafic silicates, and dual Fe valence state in CM phyllosilicates, all suggest that the phyllosilicates will exhibit strong absorption bands in the 0.7 μm region (due to Fe 3+–Fe 2+ charge transfers), and the 0.9–1.2 μm region (due to Fe 2+ crystal field transitions), and generally dominate over mafic silicates. CM petrologic subtypes exhibit a positive correlation between degree of aqueous alteration and depth of the 0.7 μm absorption band. This is consistent with the decrease in fine-grained opaques that accompanies aqueous alteration. There is no consistent relationship between degree of aqueous alteration and evidence for a 0.65 μm region saponite-group phyllosilicate absorption band. Spectra of different subsamples of a single CM can show large variations in absolute reflectance and overall slope. This is probably due to petrologic variations that likely exist within a single CM chondrite, as duplicate spectra for a single subsample show much less spectral variability. When the full suite of available CM spectra is considered, few clear spectral–compositional trends emerge. This indicates that multiple compositional and physical factors affect absolute reflectance, absorption band depths, and absorption band wavelength positions. Asteroids with reflectance spectra that exhibit absorption features consistent with CM spectra (i.e., absorption bands near 0.7 and 0.9 μm) include members from multiple taxonomic groups. This suggests that on CM parent bodies, aqueous alteration resulted in the consistent production of serpentine-group phyllosilicates, however resulting absolute reflectances and spectral shapes seen in CM reflectance spectra are highly variable, accounting for the presence of phyllosilicate features in reflectance spectra of asteroids across diverse taxonomic groups.

Modal mineralogy of CM chondrites by X-ray diffraction (PSD-XRD): Part 2. Degree, nature and settings of aqueous alteration

Within 5 million years after formation of calcium aluminium rich inclusions (CAI), high temperature anhydrous phases were transformed to hydrous phyllosilicates, mostly serpentines, which dominate the matrices of the most primitive carbonaceous chondrites. CMs are the largest group of meteorites to provide samples of this material. To understand the nature of the availability, and role of H 2O in the early solar system – as well as the settings of aqueous alteration – defining CM petrogenesis is critical. By Position Sensitive Detector X-ray Diffraction (PSD-XRD), we determine the modal abundance of crystalline phases present in volumes >1% for a suite of CMs – extending Part 1 of this work that dealt only with CM2 falls ( Howard et al., 2009) to now include CM2 and CM1 finds. CM2 samples contain 13–31% Fe,Mg silicates (olivine + pyroxene) and from 67% to 82% total phyllosilicate (mean 75% ± 1.3 2 σ). CM1 samples contain 6–10% olivine + pyroxene and 86–88% total phyllosilicate. Magnetite (0.6–5.2%), sulphide (0.6–3.9%), calcite (0–1.9%) and gypsum (0–0.8%) are minor phases across all samples. Since phyllosilicate forms from hydration of anhydrous Fe,Mg silicates (olivine + pyroxene), the ratio of total phyllosilicate to total anhydrous Fe,Mg silicate defines the degree of hydration and the following sequence results (in order of increasing hydration): QUE 97990 < Y 791198 < Murchison < Murray < Mighei < ALHA 81002 < Nogoya ⩽ Cold Bokkeveld ⩽ Essebi < QUE 93005 < ALH 83100 < MET 01070 < SCO 06043. High activities of Al (mostly from reactive mesostasis) and Si help to explain the composition and structure of CM serpentines that are distinct from terrestrial standards. Our data allows inference as to CM mineralogy at the point of accretion and challenges the conceptual validity of progressive alteration sequences. Modal mineralogy also provides new insights into CM petrogenesis and hints at a component of aqueous alteration occurring in the nebula, in addition to on the CM parent body(ies).

Tetrad effects in REE abundance patterns of chondrules from CM meteorites: Implications for aqueous alteration on the CM parent asteroid

Lanthanide tetrad effect in bulk chondrules from two moderately altered CM chondrites, Murchison and Yamato-793321 (Y-793321), are reported for the first time. Twenty-three chondrules were petrographically characterized and analyzed for 10 rare earth elements (REE) and other trace and major elements (Ba, Sr, Rb, K, Ca, Mg and Fe) using the precise isotope dilution technique. The results indicate systematic depletion (several times) of alkali and alkaline earths compared to CV and CO chondrules. Most of the porphyritic olivine (8 PO) and olivine-pyroxene (4 POP), porphyritic and radial pyroxene (2 PP, 1 RP), and granular olivine (1 GO) chondrules show a light-REE (L-REE) depleted, heavy-REE (H-REE) smoothly fractionated pattern composed of four (upward convex) segments possessing a relatively large negative Eu anomaly (CI-normalized La/Sm, Lu/Er and Eu/Eu* ratios = 0.3–1: Eu*, normal value). On the other hand, all barred-olivine (5 BO) chondrules, a few PO and POP indicate almost a flat L-REE pattern. In addition, regardless of their textural types, nearly half of the chondrules have a variable degree of Ce and Yb anomalies, and/or L/H-REE discontinuity, which is similar to CV and CO chondrules. The observed L- and H-convex REE patterns accompanied with the negative Eu anomaly is the first known case for chondrules as well as meteoritic materials, but have been previously reported for geological samples such as sedimentary rocks, late stage igneous and metamorphic rocks, and are explained as the lanthanide tetrad effect, which plausibly results from fluid–rock interaction. We suggest that the marked REE fractionations occurred by the selective incorporation of L-, H-REEs and Eu into alteration products in the matrix during alteration processes on the CM parent body, but that the gas/solid REE fractionation characteristics established in the nebula have basically remained unchanged. We suggest that the tetrad effects observed here represent a new index of physico-chemical conditions of fluid–rock interactions prevalent on the CM parent body.

Hydrogen isotope evidence for the origin and evolution of the carbonaceous chondrites

We present new hydrogen isotope data for separated matrix, hydrated chondrules, and other hydrated coarse silicate fragments from nine carbonaceous chondrites. These data were generated using a micro-analytical method involving stepped combustion of tens to hundreds of micrograms of hydrous solids. We also re-evaluate hydrogen isotope data from previous conventional stepped combustion experiments on these and other carbonaceous chondrites.Hydrogen isotope compositions of matrix and whole-rock samples of CM chondrites are correlated with oxygen isotope indices, major and minor-element abundances, and abundance and isotope ratios of other highly volatile elements. These correlations include a monotonic decrease in δD with increasing extent of aqueous alteration and decreasing abundances of highly volatile elements (including C, N and Ar), between extremes of ∼0‰ (least altered, most volatile rich) and −200‰ (most altered, least volatile rich). In plots involving only abundances and/or isotope ratios of highly volatile elements, CI chondrites fall on the high-δD, volatile rich end of the trends defined by CM chondrites; i.e., CI chondrites resemble the least altered CM chondrites in these respects. These trends suggest the protoliths of the CM chondrites (i.e., before aqueous alteration) contained an assemblage of volatiles having many things in common with those in the CI chondrites. If so, then the volatile-element inventory of the CI chondrites was a more widespread component of early solar system objects than suggested by the scarcity of recognized CI meteorites. Differences in volatile-element chemistry between the CI and average CM chondrites can be attributed to aqueous alteration of the latter.Previous models of carbonaceous chondrite aqueous alteration have suggested: (1) the protoliths of the CM chondrites are volatile poor objects like the CO or CV chondrites; and (2) the CI chondrites are more altered products of the same process producing the CM chondrites. Both suggestions appear to be inconsistent with hydrogen isotope data and other aspects of the volatile-element geochemistry of these rocks. We present a model for aqueous alteration of the CM chondrites that reconciles these inconsistencies and suggests revised relationships among the major subtypes of carbonaceous chondrites. Our model requires, among other things, that the water infiltrating CM chondrites had a δD value of ∼−158‰, consistent with initial accretion of CM parent bodies at ∼4 AU.

Carbonates in CM2 chondrites: constraints on alteration conditions from oxygen isotopic compositions and petrographic observations

High-precision measurements of the oxygen isotopic compositions of carbonates (calcite and dolomite) from five CM2 chondrites are presented and put into context of the previously determined mineralogic alteration index (MAI), which places these meteorites into an alteration sequence. The carbonate oxygen isotopic compositions range from +20.0 to +35.7‰ for δ 18O, +8.0 to +17.7‰ for δ 17O, and −0.7 to −2.7‰ for Δ 17O. Carbonate Δ 17O values are inversely correlated with MAI and track the evolution of fluid composition from higher to lower Δ 17O values with increasing alteration on the CM parent body. Similar Δ 17O values for calcite and dolomite fractions from the same splits of the same meteorites indicate that calcite and dolomite in each split precipitated from a single fluid reservoir. However, reversed calcite dolomite fractionations (δ 18O dol − δ 18O cc) indicate that the fluid was subject to processes, such as freeze–thaw or evaporation, that fractionated isotopes in a mass-dependent way. Consideration of the carbonate isotopic data in the context of previously proposed models for aqueous alteration of carbonaceous chondrites has provided important insights into both the evolving alteration conditions and the utility of the models themselves. The data as a whole indicate that the isotopic evolution of the fluid was similar to that predicted by the closed-system, two-reservoir models, but that a slightly larger matrix–water fractionation factor may apply. In the context of this model, more altered samples largely reflect greater reaction progress and thus probably indicate more extended times of fluid exposure. Petrographic observations of carbonates reveal a trend of variable carbonate morphology correlated with alteration that is also consistent with changes in the duration of fluid–rock interaction. The data can also be reconciled with fluid-flow models in a restricted region of the parent body, which is consistent with assertions that the different types of carbonaceous chondrites derive from different regions of their parent bodies. In this case, the model results for a 9-km-radius body, and our data place the location of the CM chondrite formation in a 100-m-thick zone 1 km from the surface. The size of this zone could be increased if the model parameters were adjusted.

The oxygen isotopic composition of water from Tagish Lake: Its relationship to low‐temperature phases and to other carbonaceous chondrites

Abstract— The fall and recovery of the Tagish Lake meteorite in British Columbia in January 2000 provided a unique opportunity to study relatively pristine samples of carbonaceous chondrite material. Measurements of the oxygen isotopic composition of water extracted under stepped pyrolysis from a bulk sample of this meteorite have allowed us to make comparisons with similar data obtained from CI and CM chondrites and so further investigate any relationships that may exist between these meteorites. The much lower yield of water bearing a terrestrial signature in Tagish Lake is indicative of the pristine nature of the meteorite. The relationship between the isotopic composition of this water and reported isotopic values for carbonates, bulk matrix and whole rock have been used to infer the extent and conditions under which parent‐body aqueous alteration occurred. In Tagish Lake the difference in Δ17O isotopic composition between the water and other phases is greater than that found in either CM or CI chondrites suggesting that reaction and isotopic exchange between components was more limited. This in turn suggests that in the case of Tagish Lake conditions during the processes of aqueous alteration on the parent body, which ultimately controlled the formation of new minerals, were distinct from those on both CI and CM parent bodies.