Carbonaceous chondritic meteorites are primordial Solar System materials and a source of water delivery to Earth. Fluid flow on the parent bodies of these meteorites is known to have occurred very ...early in Solar System history (first <4 million years). We analyze short-lived uranium isotopes in carbonaceous chondrites, finding excesses of 234-uranium over 238-uranium and 238-uranium over 230-thorium. These indicate that the fluid-mobile uranium ion U
moved within the past few 100,000 years. In some meteorites, this time scale is less than the cosmic-ray exposure age, which measures when they were ejected from their parent body into space. Fluid flow occurred after melting of ice, potentially by impact heating, solar heating, or atmospheric ablation. We favor the impact heating hypothesis, which implies that the parent bodies still contain ice.
Abstract
Some of the fields of research that have captured the persistent interest of both scientists and the general public are meteor phenomena. The main goal in the study of meteoroid impacts into ...Earth’s atmosphere is the recovery of the remnant matter after the ablation in the form of meteorites. This is a complementary approach, yet cheap alternative, to a sample return mission. Meteoroids are messengers since the time of the formation of the solar system due to the fact that they have preserved the same composition. The study of meteorites provides information regarding the chemical composition from which the planets formed. The increasing number of all-sky camera networks in recent years has resulted in a large set of events available for study. Thus, it is very important to use a method that determines whether the meteoroid could produce a meteorite or not. In this paper we study the meteors detected by the FRIPON network in Romania with the use of all-sky cameras. We focus on the events with noticeable deceleration (
V
f
/
V
0
< 0.8). We determine the ballistic coefficient
α
and the mass-loss parameter
β
for the selected sample. Based on this analysis the events are classified in three categories: (1) meteoroids that are likely to produce meteorites; (2) meteoroids that can possibly produce meteorites; (3) meteoroids that are unlikely to produce meteorites. The entry and final mass are determined for each event. From the recorded fireballs, we identified one possible meteorite dropper, and we analyzed its dynamical evolution.
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 eucrites and diogenites are meteorites that probably originate from asteroid 4-Vesta. The upper part of the crust of this body is certainly composed of eucrites which are basaltic or gabbroic ...rocks. Diogenites are ultramafic cumulates whose relationships with eucritic lithologies are unknown. Here, we show that the orthopyroxenes of some diogenites display very deep negative Eu anomalies (Eu/Eu∗ close to 0.1 or lower). The contamination of the parental magmas of diogenites by melts derived by partial melting of the eucritic crust can satisfactorily explain the range of the Eu anomalies displayed by diogenites. Thus, these anomalies are the first firm indication that parental melts of diogenites have intruded the eucritic crust, and consequently are younger than eucrites.
Enstatite meteorites include the undifferentiated enstatite chondrites and the differentiated enstatite achondrites (aubrites). They are the most reduced group of all meteorites. The oxygen isotope ...compositions of both enstatite chondrites and aubrites plot along the terrestrial mass fractionation line, which suggests some genetic links between these meteorites and the Earth as well.
For this study, we measured the Zn isotopic composition of 25 samples from the following groups: aubrites (main group and Shallowater), EL chondrites, EH chondrites and Happy Canyon (impact-melt breccia). We also analyzed the Zn isotopic composition and elemental abundance in separated phases (metal, silicates, and sulfides) of the EH4, EL3, and EL6 chondrites. The different groups of meteorites are isotopically distinct and give the following values (‰): aubrite main group (−7.08
<
δ
66Zn
<
−0.37); EH3 chondrites (0.15
<
δ
66Zn
<
0.31); EH4 chondrites (0.15
<
δ
66Zn
<
0.27); EH5 chondrites (δ
66Zn
=
0.27
±
0.09;
n
=
1); EL3 chondrites (0.01
<
δ
66Zn
<
0.63); the Shallowater aubrite (1.48
<
δ
66Zn
<
2.36); EL6 chondrites (2.26
<
δ
66Zn
<
7.35); and the impact-melt enstatite chondrite Happy Canyon (δ
66Zn
=
0.37).
The aubrite Peña Blanca Spring (δ
66Zn
=
−7.04‰) and the EL6 North West Forrest (δ
66Zn
=
7.35‰) are the isotopically lightest and heaviest samples, respectively, known so far in the Solar System. In comparison, the range of Zn isotopic composition of chondrites and terrestrial samples (−1.5
<
δ
66Zn
<
1‰) is much smaller (
Luck et al., 2005; Herzog et al., 2009).
EH and EL3 chondrites have the same Zn isotopic composition as the Earth, which is another example of the isotopic similarity between Earth and enstatite chondrites. The Zn isotopic composition and abundance strongly support that the origin of the volatile element depletion between EL3 and EL6 chondrites is due to volatilization, probably during thermal metamorphism. Aubrites show strong elemental depletion in Zn compared to both EH and EL chondrites and they are enriched in light isotopes (δ
66Zn down to −7.04‰). This is the opposite of what would be expected if Zn elemental depletion was due to evaporation, assuming the aubrites started with an enstatite chondrite-like Zn isotopic composition. Evaporation is therefore not responsible for volatile loss from aubrites. On Earth, Zn isotopes fractionate very little during igneous processes, while differentiated meteorites show only minimal Zn isotopic variability. It is therefore very unlikely that igneous processes can account for the large isotopic fractionation of Zn in aubrites. Condensation of an isotopically light vapor best explains Zn depletion and isotopically light Zn in these puzzling rocks. Mass balance suggests that this isotopically light vapor carries Zn lost by the EL6 parent body during thermal metamorphism and that aubrites evolved from an EL6-like parent body. Finally, Zn isotopes suggest that Shallowater and aubrites originate from distinct parent bodies.
We have investigated the Na distributions in Semarkona Type II chondrules by electron microprobe, analyzing olivine and melt inclusions in it, mesostasis and bulk chondrule, to see whether they ...indicate interactions with an ambient gas during chondrule formation. Sodium concentrations of bulk chondrule liquids, melt inclusions and mesostases can be explained to a first approximation by fractional crystallization of olivine
±
pyroxene. The most primitive olivine cores in each chondrule are mostly between Fa
8 and Fa
13, with 0.0022–0.0069
±
0.0013
wt.% Na
2O. Type IIA chondrule olivines have consistently higher Na contents than olivines in Type IIAB chondrules. We used the dependence of olivine–liquid Na partitioning on FeO in olivine as a measure of equilibration. Extreme olivine rim compositions are ∼Fa
35 and 0.03
wt.% Na
2O and are close to being in equilibrium with the mesostasis glass. Olivine cores compared with the bulk chondrule compositions, particularly in IIA chondrules, show very high apparent D
Na, indicating disequilibrium and suggesting that chondrule initial melts were more Na-rich than present chondrule bulk compositions. The apparent D
Na values correlate with the Na concentrations of the olivine, but not with concentrations in the bulk melt. We use equilibrium D
Na to find the Na content of the true parent liquid and estimate that Type IIA chondrules lost more than half their Na and recondensation was incomplete, whereas Type IIAB chondrules recovered most of theirs in their mesostases
.
Glass inclusions in olivine have lower Na than expected from fractionation of bulk composition liquids, and mesostases have higher Na than expected in calculated daughter liquids formed by fractional crystallization alone. These observations also require open system behavior of chondrules, specifically evaporation of Na before formation of melt inclusions followed by recondensation of Na in mesostases. Within this record of evaporation followed by recondensation, there is no indication of a stage with zero Na in the chondrules, which is predicted by models for shock wave cooling at canonical nebular pressures, suggesting high P
T.
The high Na concentrations in olivine and mesostases indicate very high P
Na while chondrules were molten. This may be explained by local, very high particle densities where Type II chondrules formed. The high P
T, P
Na and number densities of chondrules implied suggest formation in debris clouds after protoplanetary collisions as an alternative to formation after passage of shock waves through large particle-rich clumps in the disk. Encounters of partially molten chondrules should have been frequent in these dense swarms. However, in many ordinary chondrites like Semarkona, “cluster chondrites”, compound chondrules are not abundant but instead chondrules aggregated into clusters. Chondrule melting, cooling and clustering in dense swarms contributed to rapid accretion, possibly after collision, by fallback on the grandparent body and by reaccretion as a new body downrange.
The analysis of noble gases in meteorites provides constraints on the early solar system and the pre-solar nebula. This requires a better characterization and understanding of the capture, ...production, and release of noble gases in meteorites. The knowledge of transfer properties of noble gases for each individual meteorite could benefit from using radon-222, radioactive daughter of radium-226. The radon-222 emanating power is commonly quantified by the effective radium-226 concentration (ECRa), the product of the bulk radium-226 concentration and of the emanation coefficient E, which represents the probability of one decaying radium-226 to inject one radon-222 into the free porous network. Owing to a non-destructive, high-sensitivity accumulation method based on long photomultiplier counting sessions, we are now able to measure ECRa of meteorite samples, which usually have mass smaller than 15g and ECRa<0.5Bqkg−1. We report here the results obtained from 41 different meteorites, based on 129 measurements on 70 samples using two variants of our method, showing satisfactory repeatability and a detection limit below 10−2Bqkg−1 for a sample mass of 1g. While two meteorites remain below detection level, we obtain for 39 meteorites heterogeneous ECRa values with mean (min–max range) of ca. 0.1 (0.018–1.30)Bqkg−1. Carbonaceous chondrites exhibit the largest ECRa values and eucrites the smallest. Such values are smaller than typical values from most terrestrial rocks, but comparable with those from Archean rocks (mean of ca. 0.18Bqkg−1), an end-member of terrestrial rocks. Using uranium concentration from the literature, E is inferred from ECRa for all the meteorite samples. Values of E for meteorites (mean 40±4%) are higher than E values for Archean rocks and reported values for lunar and Martian soils. Exceptionally large E values likely suggest that the 238U-226Ra pair would not be at equilibrium in most meteorites and that uranium and/or radium are most likely not uniformly distributed. ECRa of meteorites is correlated with E and seems to mainly reflect the gas permeability of the meteorite, which could be one important property, preserved in the meteorite, of its parent body, characterizing its history in space, possibly modified by alteration, shock metamorphism, and eventually weathering on Earth. Larger radon emanation values are associated with larger concentrations of the heaviest noble gases (argon, krypton, xenon), and larger 20Ne/22Ne and 36Ar/38Ar ratios, suggesting Earth’s atmosphere contamination or solar wind implantation, and probably a similar carrier phase such as Q phase. An unclear correlation is observed with 40Ar, which may rule out a purely radiogenic effect on radon emanation. Thus, larger radon emanation suggests a larger capacity of collecting solar and terrestrial gases, which should imply higher loss of gases generated in the meteorite and larger dispersion of Pb/U ratios for age determination. This study provides the first quantification of natural radon-222 loss from meteorites and opens promising perspectives to quantify the relationship between pore space connectivity and the transfer properties for noble gases in meteorites and other extraterrestrial bodies.
Cooling rates are one of the few fundamental constraints on models of chondrule formation. In this study, we used Cu and Ga diffusion profiles in metal grains to determine the cooling rates of type I ...chondrules in the Renazzo CR2 chondrite. To improve previous estimations of cooling rates obtained using this method, we used CT scanning and serial polishing of our sections to analyze equatorial sections of large metal grains. Through the cores of these metal grains situated at the surface of chondrules, the cooling rates calculated range from 21 to 86 K h−1 for a peak temperature Tp ~ 1623–1673 K. A metal grain embedded in the core of a chondrule exhibits a cooling rate of 1.2 K h−1 at a Tp ~ 1573 K. We also measured Cu‐Ga diffusion profiles from nonequatorial sections of metal grains and calculated a lower range of cooling rates of 15–69 K h−1 for Tp ~ 1473–1603 K compared to our results from equatorial sections. The high cooling rates inferred from the lightning model (several thousand K h−1) are clearly at odds with the values obtained in this work. The X‐wind model predicts cooling rates (~6–10 K h−1) lower than most of our results. The cooling rates calculated here are in close agreement with those inferred from shock wave models, in particular for temperatures at which olivine crystallizes (from ~10 to several hundreds K h−1 between 1900 and 1500 K). However, the chemical compositions of metal grains in Renazzo are consistent with the splashing model, in which a spray of metal droplets originated from a partially molten planetesimal. Volatile siderophile element depletion is explained by evaporation before metal was engulfed within silicate droplets. Liquid metal isolated from the liquid silicate crystallized during cooling, reacted with the ambient gas, and then re‐accreted within partially molten chondrules.
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