CI Chondrites Orgueil Research Articles

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.

Nanoscale mineralogy and organic structure in Orgueil (CI) and EET 92042 (CR) carbonaceous chondrites studied with AFM‐IR spectroscopy

AbstractMeteorite matrices from primitive chondrites are an interplay of ingredients at the sub‐µm scale, which requires analytical techniques with the nanometer spatial resolution to decipher the composition of individual components in their petrographic context. Infrared spectroscopy is an effective method that enables the probing of vibrations at the molecule atomic scale of organic and inorganic compounds but is often limited to a few micrometers in spatial resolution. To efficiently distinguish spectral signatures of the different constituents, we apply here nano‐infrared spectroscopy (AFM‐IR), based on the combination of infrared and atomic force microscopy, having a spatial resolution beyond the diffraction limits. Our study aims to characterize two chosen meteorite samples to investigate primitive material in terms of bulk chemistry (the CI chondrite Orgueil) and organic composition (the CR chondrite EET 92042). We confirm that this technique allows unmixing the IR signatures of organics and minerals to assess the variability of organic structure within these samples. We report an investigation of the impact of the widely used chemical HF/HCl (hydrogen fluoride/hydrochloric acid) extraction on the nature of refractory organics (insoluble organic matter [IOM]) and provide insights on the mineralogy of meteorite matrices from these two samples by comparing to reference (extra)terrestrial materials. These findings are discussed with a perspective toward understanding the impact of post‐accretional aqueous alteration and thermal metamorphism on the composition of chondrites. Last, we highlight that the heterogeneity of organic matter within meteoritic materials extends down to the nanoscale, and by comparison with IOMs, oxygenated chemical groups are not affected by acid extractions.

Investigating the Relationship between (3200) Phaethon and (155140) 2005 UD through Telescopic and Laboratory Studies

Abstract The relationship between the near-Earth objects (3200) Phaethon and (155140) 2005 UD is unclear. While both are parents to meteor showers (the Geminids and Daytime Sextantids, respectively) and have similar visible-wavelength reflectance spectra and orbits, dynamical investigations have failed to find any likely method to link the two objects in the recent past. Here we present the first near-infrared reflectance spectrum of 2005 UD, which shows it to be consistently linear and red-sloped, unlike Phaethon’s very blue and concave spectrum. Searching for a process that could alter some common starting material to both of these end states, we hypothesized that the two objects had been heated to different extents, motivated by their near-Sun orbits, the composition of Geminid meteoroids, and previous models of Phaethon’s surface. We thus set about building a new laboratory apparatus to acquire reflectance spectra of meteoritic samples after heating to higher temperatures than available in the literature to test this hypothesis and were loaned a sample of the CI chondrite Orgueil from the Vatican Meteorite Collection for testing. We find that while Phaethon’s spectrum shares many similarities with different CI chondrites, 2005 UD’s does not. We thus conclude that the most likely relationship between the two objects is that their similar properties are only by coincidence as opposed to a parent-fragment scenario, though the ultimate test will be when JAXA’s DESTINY+ mission visits one or both of the objects later this decade. We also discuss possible paths forward to understanding Phaethon’s properties from dynamical and compositional grounds.

Primordial water and dust of the Solar System: Insights from in situ oxygen measurements of CI chondrites

As the chemical compositions of CI chondrites closely resemble that of the Sun’s photosphere, their oxygen isotopic compositions represent a powerful tool to constrain the origin and dynamics of dust and water ice grains in the protoplanetary disk. However, parent-body alteration processes make straightforward estimation of the primordial isotopic compositions of CI chondritic water and anhydrous minerals difficult. In this contribution, we used in situ SIMS measurements to determine the oxygen isotope compositions of mechanically isolated olivine and carbonate grains from the CI chondrite Orgueil and carbonates in a polished section of the CI chondrite Ivuna. Most CI olivine grains have Earth-like O isotopic compositions (Δ17O ≈ 0‰) plotting at the intersection of the terrestrial fractionation line and the primitive chondrule minerals line. Ca-carbonates from Orgueil and Ivuna define a trend with δ17O = (0.50 ± 0.05) × δ18O + (0.9 ± 1.4) that differs from mass-independent variations observed in secondary phases of other carbonaceous chondrites. These data show that CIs are chemically solar but isotopically terrestrial for oxygen isotopes. This supports models suggesting that primordial Solar System dust was 16O-poor (Δ17O ≈ 0‰) relative to the 16O-rich nebular gas. Based on results, mass balance calculations reveal that the pristine O isotopic compositions of carbonaceous chondrite matrices differ significantly from the CI composition, except for CR chondrites (calculated Δ17O values of CM, CO, CV and CR matrices being –3.97 ± 1.19‰, –4.33 ± 1.45‰, –7.95 ± 1.95‰, and –0.07 ± 1.16‰, respectively). This confirms an open chondrule-matrix system with respect to oxygen isotopes where chondrule compositions reflect complex processes of chondrule precursor recycling and gas-melt interactions. As the Mg-Si-Fe chondrule budget is also partially controlled by gas-melt interactions, the complementary formation of chondrules and matrix from a single solar-like reservoir −if it exists− require that (i) this reservoir must have been in a closed system with the gas or (ii) the gas had a CI composition to satisfy the elemental mass balance.

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)

For the bulk rocks of CI chondrites, various values are given for the modal abundance of matrix (95–100 vol%) and the accompanying mineral constituents. Here, we have determined the modal abundance of phases >5 μm in the CI chondrites Orgueil, Ivuna, Alais, and Tonk. Considering this cut-off grain-size to distinguish between matrix and coarse-grained constituents, then, we find the modal abundance of the minor phases magnetite, pyrrhotite, carbonate, olivine, and pyroxene to be 6 vol% in total, and these phases are embedded within the fine-grained, phyllosilicate-rich matrix, making up 94 vol%. The values vary slightly from meteorite to meteorite. Considering all four chondrites, the most abundant phase is - by far - magnetite (4.3 vol%) followed by pyrrhotite (∼1.1 vol%). All four CI chondrites are complex breccias, and their degree of brecciation decreases in the sequence: Orgueil > Ivuna > Alais ∼ Tonk. Because these meteorites contain clasts with highly variable modal abundances, we therefore also studied individual clasts with high abundances of specific coarse-grained phases. In this respect, in Orgueil we found a fragment with a 21.5 vol% of magnetite as well as a clast having 31.8 vol% phosphate. In Ivuna, we detected an individual clast with a 21.5 vol% of carbonates. Thus, since the CI composition is used as a geochemical standard for comparison, one also should keep in mind that sufficiently large sample masses are required to reveal a homogeneous CI composition. Small aliquots with one dominating lithology may significantly deviate from the suggested standard CI composition.

Adsorption and thermoanalytical studies on the CI chondrite Orgueil as case of early preplanetary matter

The thermal, adsorption, and, in particular, the water-bearing properties of the CI-type meteorite Orgueil as an example of early preplanetary matter have been studied by quantitative sorption experiments with H2O, CO2, N2, and n-heptane, thermogravimetry/differential scanning calorimetry as well as by coupled thermogravimetry/mass spectroscopy. The study contributes to a detailed examination of the behavior of volatile species in carbonaceous chondrites and could support future surface missions on asteroids. The results confirm the presence of phyllosilicates, sulfates, carbonates, and presumably carbon in Orgueil. The decomposition fragments observed during the thermoanalytical experiment indicate somewhat lower temperatures of decomposition for the sulfates. The nature of the distinct partial step around 970K interpreted in terms of a carbon release remains uncertain. The adsorption measurements and thermogravimetric investigations suggest the presence of water-bearing Earth-like minerals such as phyllosilicates. The specific N2 BET surface area is 30m2/g. The total water quantity present in Orgueil may exceed 10wt%. At least half of this amount (as a minimum) is very strongly bonded in the water/OH bearing phases of Orgueil.

Evidence for aqueous activity on comet 81P/Wild 2 from sulfide mineral assemblages in Stardust samples and CI chondrites

The discovery of nickel-, copper-, and zinc-bearing iron sulfides from comet 81P/Wild 2 (Wild 2) represents the strongest evidence, in the Stardust collection, of grains that formed in an aqueous environment. We investigated three microtomed TEM sections which contain crystalline sulfide assemblages from Wild 2 and twelve thin sections of the hydrothermally altered CI chondrite Orgueil. Detailed structural and compositional characterizations of the sulfide grains from both collections reveal striking similarities. The Stardust samples include a cubanite (CuFe 2S 3) grain, a pyrrhotite [(Fe,Ni) 1− x S]/pentlandite [(Fe,Ni) 9S 8] assemblage, and a pyrrhotite/sphalerite [(Fe,Zn)S] assemblage. Similarly, the CI-chondrite sulfides include individual cubanite and pyrrhotite grains, cubanite/pyrrhotite assemblages, pyrrhotite/pentlandite assemblages, as well as possible sphalerite inclusions within pyrrhotite grains. The cubanite is the low temperature orthorhombic form, which constrains temperature to a maximum of 210 °C. The Stardust and Orgueil pyrrhotites are the 4C monoclinic polytype, which is not stable above ∼250 °C. The combinations of cubanite and pyrrhotite, as well as pyrrhotite and pentlandite signify even lower temperatures. The crystal structures, compositions, and petrographic relationships of these sulfides constrain formation and alteration conditions. Taken together, these constraints attest to low-temperature hydrothermal processing. Our analyses of these minerals provide constraints on large scale issues such as: heat sources in the comet-forming region; aqueous activity on cometary bodies; and the extent and mechanisms of radial mixing of material in the early nebula. The sulfides in the Wild 2 collection are most likely the products of low-temperature aqueous alteration. They provide evidence of radial mixing of material (e.g. cubanite, troilite) from the inner solar system to the comet-forming region and possible secondary aqueous processing on the cometary body.

Extreme 54Cr-rich nano-oxides in the CI chondrite Orgueil – Implication for a late supernova injection into the solar system

Systematic variations in 54Cr/ 52Cr ratios between meteorite classes ( Trinquier et al., 2007; Qin et al., 2010a) point to large scale spatial and/or temporal isotopic heterogeneity in the solar protoplanetary disk. Two explanations for these variations have been proposed, with important implications for the formation of the Solar System: heterogeneous seeding of the disk with dust from a supernova, or energetic-particle irradiation of dust in the disk. The key to differentiating between them is identification of the carrier(s) of the 54Cr anomalies. Here we report the results of our recent NanoSIMS imaging search for the 54Cr-rich carrier in the acid-resistant residue of the CI chondrite Orgueil. A total of 10 regions with extreme 54Cr-excesses ( δ 54Cr values up to 1500‰) were found. Comparison between SEM, Auger and NanoSIMS analyses showed that these 54Cr-rich regions are associated with one or more sub-micron (typically less than 200 nm) Cr oxide grains, most likely spinels. Because the size of the NanoSIMS primary O − ion beam is larger than the typical grain size on the sample mount, the measured anomalies are lower limits, and we estimate that the actual 54Cr enrichments in three grains are at least 11 times Solar and in one of these may be as high as 50 times Solar. Such compositions strongly favor a Type II supernova origin. The variability in bulk 54Cr/ 52Cr between meteorite classes argues for a heterogeneous distribution of the 54Cr carrier in the solar protoplanetary disk following a late supernova injection event. Such a scenario is also supported by the O-isotopic distribution and variable abundances in different planetary materials of other presolar oxide and silicate grains from supernovae.

Brecciation and chemical heterogeneities of CI chondrites

Fragments in the size range from 40 μm to several hundred μm in the CI chondrites Orgueil, Ivuna, Alais, and Tonk show a wide range of chemical compositions with variations in major elements such as iron (10.4–42.4 wt% FeO), silicon (12.7–42.2 wt% SiO 2), and sulfur (1.01–15.8 wt% SO 3), but also important minor elements such as phosphorous (up to 5.2 wt% P 2O 5) or calcium (up to 6.6 wt% CaO). These variations are the result of the varying mineralogical compositions of these fragments. The distribution of phyllosilicates, magnetites, and possibly ferrihydrite, in particular, control the abundances of these elements. High REE contents—up to 150 times the solar abundances—were observed in phosphates, while matrix and sulfates are REE-depleted. The studied 113 fragments were subdivided into eight lithologies with similar mineralogical and thus chemical properties. The most common is the CGA lithology, consisting of a groundmass of Mg-rich, coarse-grained phyllosilicates and varying abundances of inclusions such as magnetite. The second most abundant lithology is the FGA lithology, consisting of a groundmass of fine-grained Fe-rich phyllosilicates. A rare, but important lithology consists of fragments with high contents of phosphates and other minerals. The proposed model for the evolution of these lithologies is based on a closed system alteration, where mineralogical differences in the lithologies reflect heterogeneities in the starting material. Comparison of our results with literature data indicates a general similarity of the four CI chondrites analyzed. Further comparison of bulk analyses suggests that the mass ‘threshold’ for chemical heterogeneities in CI chondrite samples is smaller than ∼1–2 g.

Oxygen, magnesium and chromium isotopic ratios of presolar spinel grains

Oxygen isotopic measurements of 20 spinel (MgAl 2O 4) grains from the CM2 meteorite Murray (average diameter 0.45 μm), seven spinel grains from ordinary chondrites (OC, 0.3 to 2 μm) and three spinel grains from the CI chondrite Orgueil (0.4 to 0.7 μm) have revealed large anomalies and thus established their presolar origin. Their O isotopic ratios fall into all four previously defined groups and indicate that most of the grains come from red giant branch (RGB) or asymptotic giant branch (AGB) stars. With the NanoSIMS, we measured the magnesium isotopic compositions of all 30 grains. We also measured the Cr isotopic ratios of the three Orgueil grains and one OC grain. Two of the Orgueil grains are very rich in Cr, with a composition representing a 1:1 solid solution of spinel and magnesiochromite (MgCr 2O 4). At least 13 of the 30 analyzed grains have substantial Mg isotopic anomalies. They have excesses in 26Mg, primarily from the decay of 26Al, as well as excesses and deficits in 25Mg. We present the results of new model calculations of the evolution of Mg and Al isotopic ratios in the envelopes of AGB stars for a range of masses (1.5, 2, 3, and 5 M ⊙), metallicities (1/6, 1/3, 1/2 and 1Z ⊙) and different prescriptions for mass loss by stellar winds. Comparisons of the grain data with these models show that inferred 26Al/ 27Al ratios of several grains are much larger than predicted and require an extra production process for 26Al, most likely the result of deep mixing in the star’s envelope, also called cool bottom processing (CBP). One grain is probably from an intermediate-mass (∼5 M ⊙) AGB star in which the envelope extended down to the H-burning shell (“hot bottom burning”). On average, 26Al/ 27Al ratios in oxide grains are larger than those in SiC grains from AGB stars (mainstream, Y and Z grains), whose 26Al/ 27Al ratios agree with AGB models. This indicates that the parent stars of oxide grains with high (>5 × 10 −3) 26Al/ 27Al ratios are fundamentally different from those of SiC grains. The CBP experienced by the former might have prevented these stars from becoming the carbon stars that would have produced presolar SiC grains.

Geochemistry of the ungrouped carbonaceous chondrite Tagish Lake, the anomalous CM chondrite Bells, and comparison with CI and CM chondrites

Abstract— I have determined the composition via instrumental neutron activation analysis of a bulk pristine sample of the Tagish Lake carbonaceous chondrite fall, along with bulk samples of the CI chondrite Orgueil and of several CM chondrites. Tagish Lake has a mean of refractory lithophile element/Cr ratios like those of CM chondrites, and distinctly higher than the CI chondrite mean. Tagish Lake exhibits abundances of the moderately volatile lithophile elements Na and K that are slightly higher than those of mean CM chondrites. Refractory through moderately volatile siderophile element abundances in Tagish Lake are like those of CM chondrites. Tagish Lake is distinct from CM chondrites in abundances of the most volatile elements. Mean CI‐normalized Se/Co, Zn/Co and Cs/Co for Tagish Lake are 0.68 ± 0.01, 0.71 ± 0.07 and 0.76 ± 0.02, while for all available CM chondrite determinations, these ratios lie between 0.31 and 0.61, between 0.32 and 0.58, and between 0.39 and 0.74, respectively. Considering petrography, and oxygen isotopic and elemental compositions, Tagish Lake is an ungrouped member of the carbonaceous chondrite clan. The overall abundance pattern is similar to those of CM chondrites, indicating that Tagish Lake and CMs experienced very similar nebular fractionations.Bells is a CM chondrite with unusual petrologic characteristics. Bells has a mean CI‐normalized refractory lithophile element/Cr ratio of 0.96, lower than for any other CM chondrite, but shows CI‐normalized moderately volatile lithophile element/Cr ratios within the ranges of other CM chondrites, except for Na which is low. Iridium, Co, Ni and Fe abundances are like those of CM chondrites, but the moderately volatile siderophile elements, Au, As and Sb, have abundances below the ranges for CM chondrites. Abundances of the moderately volatile elements Se and Zn of Bells are within the CM ranges. Bells is best classified as an anomalous CM chondrite.

Heating experiments simulating atmospheric entry heating of micrometeorites: Clues to their parent body sources

Abstract— Depending on their velocity, entry angle and mass, extraterrestrial dust particles suffer certain degrees of heating during entry into Earth's atmosphere, and the mineralogy and chemical composition of these dust particles are significantly changed. In the present study, pulse‐heating experiments simulating the atmospheric entry heating of micrometeoroids were carried out in order to understand the mineralogical and chemical changes quantitatively as well as to estimate the peak temperature experienced by the particles during entry heating. Fragments of the CI chondrites Orgueil and Alais as well as pyrrhotites from Orgueil were used as analogue material. The experiments show that the volatile elements S, Zn, Ga, Ge, and Se can be lost from 50 to 100 μm sized CI meteorite fragments at temperatures and heating times applicable to the entry heating of similar sized cosmic dust particles. It is concluded that depletions of these elements relative to CI as observed in micrometeorites are mainly caused by atmospheric entry heating. Besides explaining the element abundances in micrometeorites, the experimentally obtained release patterns can also be used as indicators to estimate the peak heating of dust particles during entry. Using the abundances of Zn and Ge and assuming their original concentrations close to CI, a maximum heating of 1100–1200 °C is obtained for previously analyzed Antarctic micrometeroites. Thermal alteration also strongly influenced the mineralogy of the meteorite fragments. While the unheated samples mainly consisted of phyllosilicates, these phases almost completely transformed into olivine and pyroxene in the fragments heated to ≥800 °C. Therefore, dust particles that still contain hydrous minerals were probably never heated to temperatures ≥800 °C in the atmosphere. During continued heating, the grain size of the newly formed silicates increased and the composition of the olivines equilibrated. Applying these results quantitatively to Antarctic micrometeorites, typical peak temperatures in the range of 1100–1200 °C during atmospheric entry heating are deduced. This temperature range corresponds to the one obtained from the volatile element concentrations measured in these micrometeorites and points to an asteroidal origin of the particles.

Constraints on earth evolution from antimony in mantle-derived rocks

We have analyzed Sb in a variety of mantle-derived volcanic rocks, peridotites, and in the CI chondrite Orgueil by spark source mass spectrometry. Concentrations vary from 0.02 to 0.8 ppm in oceanic basalts (mid-ocean ridge basalts, MORB; oceanic island basalts, OIB). Antimony is a moderately siderophile element which behaves like the incompatible lithophile element Pr during igneous processes in the mantle. Both MORB and OIB samples have similar Sb/Pr ratios of about 0.02, which are different from those in continental-crustal rocks. Antimony resembles Pb in that it behaves like a highly incompatible element during formation of continental crust, whereas it behaves only moderately incompatible during formation of oceanic basalts (MORB or OIB). Consequently, Sb/Pb ratios of oceanic basalts agree within error limits with those of the continental crust and with the CI chondritic value, indicating that Sb/Pb is not strongly fractionated during crust-mantle-core differentiation. From the Sb/Pb ratios we estimate a Sb concentration of 11 ± 5 ppb for the primitive mantle. An alternative estimate for the primitive-mantle abundance is obtained by assuming the Sb/Pr ratio of the primitive mantle to be intermediate between the MORB-01B value (0.02) and that of the continental crust (0.05). This approach yields Sb = 8 ± 4 ppb for the primitive mantle. Antimony is depleted in the bulk silicate Earth by a factor of 45. The volatility-corrected depletion factor of 7 is similar to other moderately siderophile elements.

The oxygen isotopic composition of olivine and pyroxene from CI chondrites

The CI chondrites are taken to represent average solar system material based on the similarity of their elemental compositions to that of the solar photosphere. However, their oxygen isotope geochemistry is dominated by secondary minerals that formed during aqueous alteration on the CI parent body. Precursors to this alteration, namely olivine and pyroxene, are extremely rare in CI chondrites, precluding previous measurements of their oxygen isotopic composition. We report ion microprobe analyses of oxygen isotopes in single olivine and pyroxene grains separated from CI chondrites Orgueil and Ivuna. The CI chondrite olivine and pyroxene grains most likely represent liberated chondrule phenocrysts, based on petrographic, chemical, and isotopic evidence consistent with crystallization from a melt. The oxygen isotope data form an array that falls nearly along the carbonaceous chondrite 16O mixing line with δ 18O values ranging from −9.3%, to + 12.3%o and δ 17O from −11.3%o to +7.8%o, consistent with nebular processes being the source of the oxygen isotopic compositions. The degree of 16O-enrichment in Orgueil olivines is negatively correlated with FeO content, but the exact nature and timing of the process that introduced this variation remains unknown. The pyroxene oxygen isotopic compositions are similar to those of olivines with >5 mol% fayalite. The oxygen isotopic analyses of the olivine and pyroxene in CI chondrites have been used to revise previous models for the isotopic evolution of CI materials. Our data require more complete gas-solid equilibration in the nebula and constrain the initial aqueous fluids on the CI parent body to have lower Δ 17O values than previously postulated. The refined model indicates that the temperature of aqueous activity on the CI parent body was no higher than ∼50°C, and the fluid:rock ratio was significantly less than previously estimated. Even prior to alteration and formation of secondary minerals, the CI chondrites were the most 16O-depleted carbonaceous chondrites and thus the solids originally contained in the CI chondrites are the most equilibrated nebular materials represented in the carbonaceous chondrites. The data suggest the oxygen isotopic composition of average solar system to be approximately equivalent to average terrestrial oxygen as recorded in the compositions of terrestrial and lunar basalts.

Rhodium and other platinum-group elements in carbonaceous chondrites

Five carbonaceous chondrites (including the CI chondrites Orgueil and Ivuna) were analyzed by spark source mass spectrometry (SSMS) for the platinum-group elements Ru, Rh, Os, Ir, Pt, as well as W, Re, An, Th, and U. Conventional photoplate detection and a recently developed multi-ion counting system were used for ion detection.Results obtained for CI chondrites agree with compiled values within 6%. This study contains the first Rh analyses for this chondrite group. Rhodium concentrations for Orgueil and Ivuna agree well, implying a Solar System abundance of 0.140 ± 0.004 ppm and corresponding to 0.359 ± 0.010 atoms relative to 1 × 106 Si atoms.Concentrations in CM2, CV3, and CK4 chondrites are enriched compared to those of CI chondrites. However, the abundances of the refractory siderophile elements Re, Os, Ir with condensation temperatures above 1600 K (at 10−4 atm) are higher by about 15–20% compared to the less refractory elements Ru, Pt, and Rh. Elements with similar condensation temperatures correlate very well resulting in uniform concentration ratios of RhPt = 0.136 ± 0.006, ReOs = 0.0821 ± 0.0019, and IrOs = 0.949 ± 0.039.

Early aqueous activity on primitive meteorite parent bodies.

The interstellar material from which the solar system formed has been modified by many processes: evaporation and condensation in the solar nebula, accretion into protoplanetary bodies and post-accretion processes within these bodies. Meteorites provide a record of these events and their chronology. Carbonaceous CI chondrites are among the most primitive, undifferentiated meteorites, but nevertheless show evidence of post-accretion alteration; they contain carbonates that are believed to have formed by reactions between anhydrous CI precursor materials and circulating fluids in the meteorite parent body (or bodies), yet little is known about the nature of these reactions or the timescale on which they occurred. Here we report measurements of excess 53Cr--formed by the decay of short-lived 53Mn--in five carbonate fragments from the CI chondrites Orgueil and Ivuna. Our results show that aqueous alteration on small protoplanetary bodies must have begun less than 20 Myr after the time of formation of the oldest known solar-nebula condensates (Allende refractory inclusions). This upper limit is much shorter than that of 50 Myr inferred from previous studies, and clearly establishes aqueous alteration as one of the earliest processes in the chemical evolution of the solar system.

Carbonates in CI chondrites: Clues to parent body evolution

All CI chondrites are regolith breccias consisting of various types of chemically and mineralogically distinct mineral and lithic fragments (or units). In the CI chondrite Ivuna, for example, four different lithological units were identified and are referred to as lithology I, II, III, and IV. So far, lithologies III and IV have been identified in Orgueil as well. It appears that at least Ivuna and Orgueil consist of the same basic lithologies in different proportions. Carbonates in CIs occur as individual grains within such lithic units or exist as large fragments between them. Carbonate fragments are renmants of former carbonate veins and, in contrast to individual carbonate grains, are not genetically linked to lithological units. Four different types of carbonates (dolomite, breunnerite, calcite, siderite) occur in CIs and they constitute, on average, 5 vol% of each studied section. In this study, carbonates in the CI chondrites Orgueil, Ivuna, Alais, and Tonk were studied petrographically, mineralogically, and chemically. The results clearly indicate that, in contrast to most previous studies, compositional differences exist between dolomites within and among CI chondrites. From these differences it can be derived that (1) several episodes of alteration occurred on the CI parent body, (2) physicochemical conditions during carbonate formation must have been different among CIs, and (3) CI carbonates obviously were formed at low temperatures in equilibrium with surrounding fluid(s). While local compositional changes in fluid(s) on a micrometer to millimeter scale (as reflected by dolomite compositions within CI chondrites) were most likely controlled by the availability of Ca2+, Mg2+, Fe2+, and especially Mn2+ ions in the aqueous solutions, more widespread compositional changes on a meter (or even larger) scale were controlled by variable pH, Eh, CO2 partial pressure, and, especially, temperature conditions (as reflected by the compositional variability of dolomites among CIs).

Tin in mantle-derived rocks: Constraints on Earth evolution

We have analyzed tin together with other trace elements in a variety of mantle-derived rocks by isotope dilution-spark source mass spectrometry. Tin concentrations in oceanic basalts (mid-ocean ridge basalts, or MORB; oceanic island basalts, or OIB; island arc volcanics, or IAV) vary from 0.4 to 6 ppm. Tin is a moderately siderophile element which behaves like the moderately incompatible and lithophile element Sm during igneous processes in the mantle. The Sn Sm ratio is therefore similar in various reservoirs of the silicate portion of the Earth at the present day. To investigate possible secular variations of siderophile/chalcophile element abundances, we have also analyzed komatiites and basalts with ages ranging from Archean to Tertiary. Rocks having ages up to 3.4 Ga yield Sn Sm ratios comparable to those of the oceanic basalts. These results indicate that core growth during the last 3.4 Ga was negligibly small. In addition, the Sn Sm ratios in different mantle reservoirs are identical within the uncertainty of the data scatter. This indicates that tin was homogenized within the mantle after core formation. We conclude that the value of Sn Sm = 0.32 found in mantlederived rocks is representative of the primitive mantle because the estimated continental-crustal abundances of Sn and Sm yield a similar Sn Sm ratio. This results in Sn = 0.12 ± 0.01 ppm for the primitive mantle abundance, in agreement with that determined from fertile peridotite xenoliths (0.15 ± 0.02 ppm). To determine an absolute depletion factor for the volatile siderophile element tin in the primitive mantle, we have also analyzed four carbonaceous chondrites, including the CI chondrites Orgueil and Ivuna. We obtain a CI-value of tin of 1.62 ± 0.03 ppm. Tin is depleted in the Earth's primitive mantle by a factor of 33 ± 3; the volatility corrected depletion factor is similar to other moderately siderophile elements ( Newsom, 1990).

Th, U and other trace elements in carbonaceous chondrites: Implications for the terrestrial and solar-system Th/U ratios

We have analyzed Th, U and several other trace elements in eleven carbonaceous chondrites (including the two CI chondrites Orgueil and Ivuna) by mass spectrometric isotope dilution techniques. In contrast to concentrations of Th and other refractory trace elements, U concentrations are highly variable, both among and within different meteorites. Th/U ratios range from about 1 to 4 and correlate inversely with U concentrations. Surprisingly, CI chondrites display by far the largest variation in Th/U. It is shown that the negative correlation between Th/U and U cannot be explained by terrestrial contamination or systematic fractionation within the solar nebula; rather it reflects low-temperature mobilization and redistribution of U on the chondrite parent bodies or within the meteorites. The idea of alteration-induced Th/U variations in chondrites is supported by the covariation of U in Orgueil with other elements which are known to be mobile under aqueous conditions (e.g., Sb, Cs, Ba) and the fact that the analytical data of Th, U lie on binary mixing lines. Our approach to estimating the solar-system Th/U ratio is based on analyses of carbonaceous chondrites of all types, the combination of data for Th, U with those for other trace elements and on published Pb isotopes in carbonaceous chondrites. The resulting value of Th/U= 3.9 ± 0.2 (weight ratio) is higher than previous estimates. The corresponding CI chondritic abundances are Th= 28.9 ± 1.5 ppb and U= 7.4 ± 0.5 ppb. This translates into solar-system abundances of Th= 0.0329 and U= 0.0082 (atomic abundances relative to 10 6 Si atoms). Assuming that refractory lithophile elements in the primitive mantle have chondritic relative abundances, but are enriched by a factor of 2.5 relative to CI, we obtain Th= 72.2 ppb and U= 18.5 ppb as primitive mantle abundances. If the early Earth's mantle possessed a Th/U weight ratio of greater than 4.1 as suggested by the lead isotopes of Archean rocks, this would reflect very early terrestrial Th U fractionation.

Mantle heterogeneities: Isotopic and trace element characterisation of a major OIB source component interpreted as representative of oceanic crust segments recycled into the mantle through its historical evolution

We have analyzed tin together with other trace elements in a variety of mantle-derived rocks by isotope dilution-spark source mass spectrometry. Tin concentrations in oceanic basalts (mid-ocean ridge basalts, or MORB; oceanic island basalts, or OIB; island arc volcanics, or IAV) vary from 0.4 to 6 ppm. Tin is a moderately siderophile element which behaves like the moderately incompatible and lithophile element Sm during igneous processes in the mantle. The SnSmratio is therefore similar in various reservoirs of the silicate portion of the Earth at the present day.To investigate possible secular variations of siderophile/chalcophile element abundances, we have also analyzed komatiites and basalts with ages ranging from Archean to Tertiary. Rocks having ages up to 3.4 Ga yield SnSm ratios comparable to those of the oceanic basalts. These results indicate that core growth during the last 3.4 Ga was negligibly small. In addition, the SnSm ratios in different mantle reservoirs are identical within the uncertainty of the data scatter. This indicates that tin was homogenized within the mantle after core formation. We conclude that the value of SnSm = 0.32found in mantlederived rocks is representative of the primitive mantle because the estimated continental-crustal abundances of Sn and Sm yield a similar SnSm ratio. This results in Sn = 0.12 ± 0.01 ppm for the primitive mantle abundance, in agreement with that determined from fertile peridotite xenoliths (0.15 ± 0.02 ppm).To determine an absolute depletion factor for the volatile siderophile element tin in the primitive mantle, we have also analyzed four carbonaceous chondrites, including the CI chondrites Orgueil and Ivuna. We obtain a CI-value of tin of 1.62 ± 0.03 ppm. Tin is depleted in the Earth's primitive mantle by a factor of 33 ± 3; the volatility corrected depletion factor is similar to other moderately siderophile elements (Newsom, 1990).