Among the elements exhibiting non-mass dependent isotopic variations in meteorites, chromium (Cr) has been central in arguing for an isotopic homogeneity between the Earth and the Moon, thus ...questioning physical models of Moon formation. However, the Cr isotopic composition of the Moon relies on two samples only, which define an average value that is slightly different from the terrestrial standard. Here, by determining the Cr isotopic composition of 17 lunar, 9 terrestrial and 5 enstatite chondrite samples, we re-assess the isotopic similarity between these different planetary bodies, and provide the first robust estimate for the Moon. In average, terrestrial and enstatite samples show similar ε54Cr. On the other hand, lunar samples show variables excesses of 53Cr and 54Cr compared to terrestrial and enstatite chondrites samples with correlated ε53Cr and ε54Cr (per 10,000 deviation of the 53Cr/52Cr and 54Cr/52Cr ratios normalized to the 50Cr/52Cr ratio from the NIST SRM 3112a Cr standard). Unlike previous suggestions, we show for the first time that cosmic irradiation can affect significantly the Cr isotopic composition of lunar materials. Moreover, we also suggest that rather than spallation reactions, neutron capture effects are the dominant process controlling the Cr isotope composition of lunar igneous rocks. This is supported by the correlation between ε53Cr and ε54Cr, and 150Sm/152Sm ratios. After correction of these effects, the average ε54Cr of the Moon is indistinguishable from the terrestrial and enstatite chondrite materials reinforcing the idea of an Earth–Moon–enstatite chondrite system homogeneity. This is compatible with the most recent scenarios of Moon formation suggesting an efficient physical homogenization after a high-energy impact on a fast spinning Earth, and/or with an impactor originating from the same reservoir in the inner proto-planetary disk as the Earth and enstatite chondrites and having similar composition.
•The first Cr isotopic estimate (ε53Cr and ε54Cr) of the Moon is provided.•Cosmic ray irradiation has modified the Cr isotopic composition of lunar samples.•Corrected lunar value is similar to that of the Earth and enstatite chondrites.•Results emphasize new high-energy impact models of Moon formation.
The Paris chondrite provides an excellent opportunity to study CM chondrules and refractory inclusions in a more pristine state than currently possible from other CMs, and to investigate the earliest ...stages of aqueous alteration captured within a single CM bulk composition. It was found in the effects of a former colonial mining engineer and may have been an observed fall. The texture, mineralogy, petrography, magnetic properties and chemical and isotopic compositions are consistent with classification as a CM2 chondrite. There are ∼45vol.% high-temperature components mainly Type I chondrules (with olivine mostly Fa0–2, mean Fa0.9) with granular textures because of low mesostasis abundances. Type II chondrules contain olivine Fa7 to Fa76. These are dominantly of Type IIA, but there are IIAB and IIB chondrules, II(A)B chondrules with minor highly ferroan olivine, and IIA(C) with augite as the only pyroxene. The refractory inclusions in Paris are amoeboid olivine aggregates (AOAs) and fine-grained spinel-rich Ca–Al-rich inclusions (CAIs). The CAI phases formed in the sequence hibonite, perovskite, grossite, spinel, gehlenite, anorthite, diopside/fassaite and forsterite. The most refractory phases are embedded in spinel, which also occurs as massive nodules. Refractory metal nuggets are found in many CAI and refractory platinum group element abundances (PGE) decrease following the observed condensation sequences of their host phases. Mn–Cr isotope measurements of mineral separates from Paris define a regression line with a slope of 53Mn/55Mn=(5.76±0.76)×106. If we interpret Cr isotopic systematics as dating Paris components, particularly the chondrules, the age is 4566.44±0.66Myr, which is close to the age of CAI and puts new constraints on the early evolution of the solar system. Eleven individual Paris samples define an O isotope mixing line that passes through CM2 and CO3 falls and indicates that Paris is a very fresh sample, with variation explained by local differences in the extent of alteration. The anhydrous precursor to the CM2s was CO3-like, but the two groups differed in that the CMs accreted a higherproportion of water. Paris has little matrix (∼47%, plus 8% fine grained rims) and is less altered than other CM chondrites. Chondrule silicates (except mesostasis), CAI phases, submicron forsterite and amorphous silicate in the matrix are all well preserved in the freshest domains, and there is abundant metal preserved (metal alteration stage 1 of Palmer and Lauretta (2011)). Metal and sulfide compositions and textures correspond to the least heated or equilibrated CM chondrites, Category A of Kimura et al. (2011). The composition of tochilinite–cronstedtite intergrowths gives a PCP index of ∼2.9. Cronstedtite is more abundant in the more altered zones whereas in normal highly altered CM chondrites, with petrologic subtype 2.6–2.0 based on the S/SiO2 and ∑FeO/SiO2 ratios in PCP or tochilinite–cronstedtite intergrowths (Rubin et al., 2007), cronstedtite is destroyed by alteration. The matrix in fresh zones has CI chondritic volatile element abundances, but interactions between matrix and chondrules occurred during alteration, modifying the volatile element abundances in the altered zones. Paris has higher trapped Ne contents, more primitive organic compounds, and more primitive organic material than other CMs. There are gradational contacts between domains of different degree of alteration, on the scale of ∼1cm, but also highly altered clasts, suggesting mainly a water-limited style of alteration, with no significant metamorphic reheating.
Cr isotopic compositions have been measured on carbonaceous chondrites (CC): Tafassasset, Paris, Niger I, NWA 5958, NWA 8157 and Jbilet Winselwan. In bulk samples, the 54Cr/52Cr ratios (expressed as ...ε54Cr) range from 0.93 to 1.58ε units. These values are in agreement with values characteristic for distinct petrologic types. Despite this 54Cr heterogeneity, the variability in the 53Cr/52Cr ratios (expressed as ε53Cr) of 0.2ε units and the Mn/Cr ratios is consistent with the previous finding of an isochron in the Mn–Cr evolution diagram.
The Mn/Cr ratio in CC corresponds to variable abundances of high-T condensate formed and separated at the beginning of the solar system, thus the canonical 53Mn/55Mn ratio can be defined. Based on a consistent chronology for U–Pb and Mn–Cr between the earliest objects formed in the solar nebula and the D’Orbigny angrite we define a canonical 53Mn/55Mn ratio and ε53Cri of 6.8×10−6 and −0.177, respectively.
The internal Mn/Cr systematics in Tafassasset and Paris were studied by two approaches: leaching technique and mineral separation. Despite variable ε54Cr values (up to >30ε) linear co-variations were found between ε53Cr and Mn/Cr ratio. The mineral separates as well as the leachates of Tafassasset fall on a common isochron indicating that (1) cooling of the Tafassasset’s parent body occurred at 4563.5±0.25Ma, and that (2) 54Cr is decoupled from the other isotopes even though temperatures >900°C have been reached during metamorphism. In the case of Paris, the leachates form an alignment with a 53Mn/55Mn ratio higher than the canonical value. This alignment is not an isochron but rather a mixing line. Based on leachates from various CM and CI, we propose the occurrence of three distinct Cr reservoirs in meteoritic material: PURE54, HIGH53 and LOW53 characterized by a ε53Cr and ε54Cr of 0 and 25,000, −2.17 and 8, and 0.5 and −151, respectively. PURE54 has already been described and is carried by highly refractory nano-spinel; HIGH53 is Mn-rich and most probably carried by sulfides in the matrix, whereas LOW53 is characterized by low Mn/Cr ratios and it is sensitive to metamorphism. This component could correspond to mineral phases such as refractory oxides and carbide. Variable mixing proportions of HIGH53 and LOW53 would explain the larger-than-expected uncertainty (MSWD of 5.5) on the CC bulk regression line. A Monte Carlo simulation allows us to evaluate the impact of the dispersion of the initial Cr isotopic ratios (as a function of variable HIGH53). The co-variation of the Mn/Cr ratio and the ε53Cr defined by the mineral separates from Paris corresponds to an age of 4566.44 +0.66/−0.75Ma, while their ε54Cr still differ by at least 0.42ε. This age is likely to date the segregation of forsteritic olivines (most probably from type I chondrules) from fayalitic olivines (from type II chondrules) and, given the sampling procedure by handpicking of hundreds of grains, corresponds to the average age of chondrule formation.
The revelation of a small 182W terrestrial excess relatively to the 182Hf–182W systematics in meteorites has led some authors to claim that the Earth experienced a rapid accretion and an early ...differentiation, 30–40 Ma after the birth of the solar system at 4.567 Ga. This interpretation has since been moderated, but the idea of an early segregation of the core is still widely advocated. We challenge this interpretation with quantitative arguments that concern Hf–W, U–Pb and I–Xe systematics on the Earth.
The W isotopic composition of the bulk silicate Earth can be explained by an incomplete isotopic re-equilibration between primitive metal and silicate components during the segregation of the Earth's core. We consider that the primitive metal/silicate differentiation in planetesimals and the segregation of cores of planetary bodies occurred during the first million years and that the segregation of the major part of the Earth's core occurred late in respect to the 182Hf decay. Consequently, the non-equilibrated fraction of primitive silicate material is estimated to be small, between 6 and 14%, enough however to « open » the 182Hf–182W chronometer as is presently observed.
This significant, but incomplete, metal/silicate re-equilibration only slightly affects the U–Pb chronometer. A reappraisal of the Pb isotope composition of the bulk silicate Earth allows us to define the mean age of the Earth's core's segregation, between 4.46 Ga and 4.38 Ga. This evaluation overlaps the time of outgassing of the atmosphere based on the 129I–129Xe systematics, 4.46–4.43 Ga.
We consider that the period around 4.45 Ga relates to the major primitive differentiation of the Earth. This scenario coherently and quantitatively explains the 182Hf–182W, 235, 238U–207, 206Pb, 129I–129Xe and 146Sm–142Nd terrestrial records and it is compatible with the radiometric constraints for the formation of the Moon and coherent with the ∼102 Ma time scale for the accretion of the Earth, as evaluated by current numerical simulations for terrestrial planet formation.
► First continuous record of the isotopic lead signature for a megacity conurbation, Paris. ► Paris conurbation is crossed by the Seine River. ► Allows defining the “pre-leaded gasoline” signature in ...the Seine River sediments. ► Precise determination of the lead gasoline source contribution to the river contamination. ► Highlights previous unexplained findings on European environmental contamination.
Four sediment cores collected in the Seine River basin and dated between 1916 and 2003 were analyzed for lead concentrations and isotopic composition. In all four cores, the measured Pb concentration (up to 460mgkg−1) lies significantly above the natural background (27–40mgkg−1), although a significant decrease (down to 75mgkg−1) was observed during the second half of the 20th century which can be explained by the reduction of lead emissions. The 206Pb/207Pb ratio measured in these samples indicates that the main source of Pb used in the Paris conurbation is characterized by a “Rio Tinto” signature (defined as 206Pb/207Pb=1.1634±0.0001). A high contribution, up to 25%, from the leaded gasoline (characterized by 206Pb/207Pb=1.08±0.02) is revealed in the Seine River downstream Paris, indicating that lead from the leaded gasoline is preferentially released to the river.
The dominating Pb signature in the Paris conurbation that is currently sampled through incinerators fumes (206Pb/207Pb=1.1550±0.0005) and waste water treatment plant (206Pb/207Pb=1.154±0.002), represents a mixture of highly recycled lead from the Rio Tinto mine and lead from leaded gasoline (imprinted by the low 206Pb/207Pb of the Broken Hill mine). This signature is called “urban” rather than “industrial”, because it is clearly distinct from the Pb that is found in areas contaminated by heavy industry, i.e. the heavy industries located on the Oise River which used lead from European ores characterized by high 206Pb/207Pb ratios (∼1.18–1.19) and possibly a minor amount of North American lead (206Pb/207Pb ratios>1.20). The “urban” signature is also found in a rural area upstream of Paris in the 1970’s. At the Seine River mouth in 2003, Pb with an urban signature represents 70% of the total Pb sediment content, with the 30% remaining corresponding to natural Pb.
Th/U variability in Allende chondrules Blichert-Toft, Janne; Göpel, Christa; Chaussidon, Marc ...
Geochimica et cosmochimica acta,
07/2020, Volume:
280
Journal Article
Peer reviewed
Open access
Lead isotope compositions were measured on both single and combined chondrules from the CV3 carbonaceous chondrite Allende with the goal of determining the range of Th/U implied by the radiogenic ...208Pb*/206Pb* values. All samples were aggressively acid step-leached to separate radiogenic from primordial lead. It is found that apparent Th/U varies both between individual chondrules and between the different leaching fractions of each chondrule or group of chondrules. Specifically, the apparent Th/U ratio deviates from the planetary value (3.876), varying spectacularly from 0.65 to 14.6. Variations between leachates and residues disclose the existence of internal heterogeneities, while inter-chondrule variations reveal the presence of external heterogeneities. Three main explanations for the observed Th-U fractionation that are not mutually exclusive prevail: (1) uranium species, notably UO and UO2, coexisted in the nebular gas at high temperature, whereas Th existed exclusively as ThO2; (2) chondrules interacted with an exotic oxidized vapor; and (3) chondrules represent melt of dust of different origins, a hypothesis dictated by the evidence of internal heterogeneity. The extent to which the measured apparent Th/U variability is due to each of these particular processes is difficult to assess, but the existence of substantial Th/U heterogeneity, especially within, but also among, single (or pooled) chondrules from the same chondrite calls for caution when Pb-Pb linear arrays, or mixing lines, are assigned isochronous significance.
Non-mass dependent chromium isotopic signatures have been successfully used to determine the presence and identification of extra-terrestrial materials in terrestrial impact rocks. Paleoproterozoic ...spherule layers from Greenland (Grænsesø) and Russia (Zaonega), as well as some distal ejecta deposits (Lake Superior region) from the Sudbury impact (1849±0.3 Ma) event, have been analyzed for their Cr isotope compositions. Our results suggest that 1) these distal ejecta deposits are all of impact origin, 2) the Grænsesø and Zaonega spherule layers contain a distinct carbonaceous chondrite component, and are possibly related to the same impact event, which could be Vredefort (2023±4 Ma) or another not yet identified large impact event from that of similar age, and 3) the Sudbury ejecta record a complex meteoritic signature, which is different from the Grænsesø and Zaonega spherule layers, and could indicate the impact of a heterogeneous chondritic body.
•Identification of impactors in paleoproterozoic ejecta deposits.•Chromium isotope study of the Sudbury, Grænsesø and Zaonega ejecta layers.•Links between paleoproterozoic ejecta, and the Sudbury and Vredefort impact events.
Cr isotopic compositions have been measured on carbonaceous chondrites (CC): Tafassasset, Paris, Niger I, NWA 5958, NWA 8157 and Jbilet Winselwan. In bulk samples, the Cr-54/Cr-52 ratios (expressed ...as epsilon Cr-54) range from 0.93 to 1.58 epsilon units. These values are in agreement with values characteristic for distinct petrologic types. Despite this 54 Cr heterogeneity, the variability in the Cr-54/Cr-52 ratios (expressed as epsilon Cr-53) of 0.2 epsilon units and the Mn/Cr ratios is consistent with the previous finding of an isochron in the Mn-Cr evolution diagram.The Mn/Cr ratio in CC corresponds to variable abundances of high-T condensate formed and separated at the beginning of the solar system, thus the canonical Mn-53/Mn-55 ratio can be defined. Based on a consistent chronology for U-Pb and Mn-Cr between the earliest objects formed in the solar nebula and the D'Orbigny angrite we define a canonical Mn-53/Mn-55 ratio and epsilon Cr-53(i) of 6.8 x 10(-6) and -0.177, respectively.The internal Mn/Cr systematics in Tafassasset and Paris were studied by two approaches: leaching technique and mineral separation. Despite variable epsilon Cr-54 values (up to >30 epsilon) linear co-variations were found between epsilon Cr-53 and Mn/Cr ratio. The mineral separates as well as the leachates of Tafassasset fall on a common isochron indicating that (1) cooling of the Tafassasset's parent body occurred at 4563.5 +/- 0.25 Ma, and that (2) Cr-54 is decoupled from the other isotopes even though temperatures >900 degrees C have been reached during metamorphism. In the case of Paris, the leachates form an alignment with a Mn-53/Mn-55 ratio higher than the canonical value. This alignment is not an isochron but rather a mixing line. Based on leachates from various CM and CI, we propose the occurrence of three distinct Cr reservoirs in meteoritic material: PURE54, HIGH53 and LOW53 characterized by a epsilon Cr-53 and epsilon Cr-54 of 0 and 25,000, -2.17 and 8, and 0.5 and -151, respectively. PURE54 has already been described and is carried by highly refractory nano-spinel; HIGH53 is Mn-rich and most probably carried by sulfides in the matrix, whereas LOW53 is characterized by low Mn/Cr ratios and it is sensitive to metamorphism. This component could correspond to mineral phases such as refractory oxides and carbide. Variable mixing proportions of HIGH53 and LOW53 would explain the larger-than-expected uncertainty (MSWD of 5.5) on the CC bulk regression line. A Monte Carlo simulation allows us to evaluate the impact of the dispersion of the initial Cr isotopic ratios (as a function of variable HIGH53). The co-variation of the Mn/Cr ratio and the epsilon Cr-53 defined by the mineral separates from Paris corresponds to an age of 4566.44(+0.66)/(-0.75) Ma, while their epsilon Cr-54 still differ by at least 0.42 epsilon. This age is likely to date the segregation of forsteritic olivines (most probably from type I chondrules) from fayalitic olivines (from type II chondrules) and, given the sampling procedure by handpicking of hundreds of grains, corresponds to the average age of chondrule formation.
Northwest Africa 7533, a polymict Martian breccia, consists of fine‐grained clast‐laden melt particles and microcrystalline matrix. While both melt and matrix contain medium‐grained ...noritic‐monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast‐laden melt rocks contain clump‐like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene‐plagioclase clump‐aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic‐monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned‐pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr‐bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti‐bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni‐poor pyroxene clasts which are from pristine rocks. Clast‐laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ± 0.03 Ga (Humayun et al. ) and a 147Sm‐143Nd isochron for NWA 7034 yielding 4.42 ± 0.07 Ga (Nyquist et al. ) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher Δ17O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. ). This record of protolith interaction with atmosphere‐hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. ), and formation of Ni‐bearing pyrite veins during or shortly after this disturbance (Lorand et al. ).
Tafassasset is a primitive meteorite, the origin of which is still debated. Its possible relationship to either the CR chondrites – considered among the most primitive meteorites – or the brachinites ...– complex primitive achondrites – makes it an interesting sample for studying the initial stages of planetary accretion and differentiation in the early solar system. Here, we report tungsten (W) isotope data for bulk rock samples as well as for mineral fractions from Tafassasset, along with micro-computed tomography of a piece of the meteorite. Silicates show mass-independent W isotope anomalies, while the metal phase does not. These nucleosynthetic anomalies are interpreted as reflecting the presence of SiC presolar grains in the matrix of the meteorite, carrying s-process184W. After correction of the nucleosynthetic anomalies, a correlation is observed between the 182W/184W isotope compositions and the Hf/W ratios of the different fractions. A 182Hf–182W age of ca. 2.9 Ma after CAIs is inferred from the 182Hf–182W chronometer, slightly older than other estimates based on the 53Mn–53Cr, 26Al–26Mg, and Pb/Pb chronometers, but consistent with the difference in closure temperatures of the different isotopic systems. Numerical modeling of the thermal evolution of Tafassasset indicates accretion of a parent-body less than ∼50 km in diameter, ≤1 Ma after the formation of CAIs, at a time when short-lived radio-nuclides induced metal–silicate separation and partial melting of the silicates with extraction of a basaltic component. According to our new data, Tafassasset may represent an inner part of a CR-like parent body, with a differentiation history similar to, but less severe than, that of brachinites.
•We analyzed tungsten isotopes in separates from Tafassasset.•We show that Tafassasset presents strong nucleosynthetic anomalies.•With micro-CT, we show that the metal distribution in Tafassasset was misunderstood.•We propose a thermal evolution model for Tafassasset's parent body.•Incipient melting already took place on small bodies early in the solar system.
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